Copy Of Robotics Book 4z584j

ME1008

ROBOTICS

3 0 0

100 (Common to Mechanical, Automobile and Production - core) OBJECTIVES To introduce the basic concepts, parts of robots and types of robotsTo make the student familiar with the various drive systems for robot, sensors and their applications in robots, programming of robots To discuss about the various applications of robots, justification, implementation and safety of robot 1. FUNDAMENTALS OF ROBOT 7 Robot – Definition – Robot Anatomy – Co-ordinate Systems, Work Envelope, types and classification – Specifications – Pitch, Yaw, Roll, t Notations, Speed of Motion, Pay Load – Robot Parts and Their Functions – Need for Robots – Different Applications 2. ROBOT DRIVE SYSTEMS AND END EFFECTORS 10 Pneumatic Drives – Hydraulic Drives – Mechanical Drives – Electrical Drives – D.C. Servo Motors, Stepper Motor, A.C. Servo Motors – Salient Features, Applications and Comparison of all these Drives End Effectors – Grippers – Mechanical Grippers, Pneumatic and Hydraulic Grippers, Magnetic Grippers, Vacuum Grippers; Two Fingered and Three Fingered Grippers; Internal Grippers and External Grippers; Selection and Design Considerations 3. SENSORS AND MACHINE VISION 10 Requirements of a sensor, Principles and Applications of the following types of sensors – Position of sensors (Piezo Electric Sensor, LVDT, Resolvers, Optical Encoders, Pneumatic Position Sensors), Range Sensors (Triangulation Principle, Structured, Lighting Approach, Time of Flight Range Finders, Laser Range Meters), Proximity Sensors (Inductive, Hall Effect, Capacitive, Ultrasonic and Optical Proximity Sensors), Touch Sensors, (Binary Sensors, Analog Sensors), Wrist Sensors, Compliance Sensors, Slip Sensors Camera, Frame Grabber, Sensing and Digitizing Image Data – Signal Conversion, Image Storage, Lighting Techniques. Image Processing and Analysis – Data Reduction, Segmentation, Feature Extraction, Object Recognition, Other Algorithms. Applications – Inspection, Identification, Visual Serving and Navigation. 4. ROBOT KINEMATICS AND ROBOT PROGRAMMING 10 Forward Kinematics, Inverse Kinematics and Differences; Forward Kinematics and Reverse Kinematics of Manipulators with Two, Three Degrees of Freedom (In 2 Dimensional), Four Degrees of Freedom (In 3 Dimensional) – Deviations and Problems Teach Pendant Programming, Lead through programming, Robot programming Languages – VAL Programming – Motion Commands, Sensor Commands, End effecter commands, and Simple programs 5. IMPLEMENTATION AND ROBOT ECONOMICS 8 RGV, AGV; Implementation of Robots in Industries – Various Steps; Safety Considerations for Robot Operations; Economic Analysis of Robots – Pay back Method, EUAC Method, Rate of Return Method. TEXT BOOK 1. M.P.Groover, “Industrial Robotics – Technology, Programming and Applications”, McGrawHill, 2001 ReferenceS 1. Fu.K.S. Gonzalz.R.C., and Lee C.S.G., “Robotics Control, Sensing, Vision and Intelligence”, McGraw-Hill Book Co., 1987 2. Yoram Koren, “Robotics for Engineers”, McGraw-Hill Book Co., 1992

1

UNIT-1 FUNDAMENTALS OF ROBOT Robot – Definition Robot Anatomy Co-ordinate Systems Work Envelope Types and classification Specifications – Pitch, Yaw, Roll t Notations Speed of Motion Pay Load – Robot Parts and Their Functions Need for Robots Different Applications

2

1.1 Robot Definition The Robotics Industries Association (RIA) defines robot in the following way: “An industrial robot is a programmable, multi-functional manipulator designed to move materials, parts, tools, or special devices through variable programmed motions for the performance of a variety of tasks” An industrial robot consists of a number of rigid links connected by ts of different types, Controlled and monitored by a computer. To a large extend, the physical construction of a robot resembles a human arm.

1.2 Robot Anatomy (physicals structure of robot) Robot anatomy is concerned with the physical construction and characteristics of the body, arm, and wrist, which are the component of the robot manipulator. - Base.-fixed are mobile - The manipulator- arm which several degrees of freedom (DOF). - The end-effector or gripper- holding a part or tool - Drives or actuators – Causing the manipulator arm or end effector to move in a space. - Controller – with hardware & software for giving commands to the drives - Sensors - To feed back the information for subsequent action of the arm or grippers as well as to interact with the environment in which the robot is working. - Interface – Connecting the robot subsystem to the external world.

3

Which consist of a number of component that allowed be oriented in a verity of position .movements between the various components of the body, arm, and wrist are provided by a series of ts. These t movements usually involve either rotation or sliding motions.

Robot Classification Robots may be classified, based on: • Physical configuration or Co-ordinate Systems • control systems

Classification Based on Physical Configuration (or) Co-ordinate Systems: • Cartesian configuration • Cylindrical configuration • Polar configuration • t-arm configuration

Cartesian Configuration: Robots with Cartesian configurations consist of links connected by linear ts (L). Gantry robots are Cartesian robots (LLL).

Cylindrical Configuration: Robots with cylindrical configuration have one rotary (R) t at the base and linear (L) ts succeeded to connect the links.

4

The designation of the arm for this configuration can be TRL or TRR. Robots with the designation TRL are also called spherical robots. Those with the designation TRR are also called articulated robots. An articulated robot more closely resembles the human arm.

t-arm Configuration: The ted-arm is a combination of cylindrical and articulated configurations. The arm of the robot is connected to the base with a twisting t. The links in the arm are connected by rotary ts. Many commercially available robots have this configuration

Simple Comparison Configuration

Advantages

Disadvantages

Cartesian coordinates

3 linear axes, easy to visualize, Can only reach front of itself, rigid structure, easy to program requires large floor space, axes hard to seal

Cylindrical coordinates

2 linear axes +1 rotating, can reach all around itself, reach and height axes rigid, rotational axis easy to seal

Can’t reach above itself, base rotation axis as less rigid, linear axes is hard to seal, won’t reach around obstacles

5

SCARA coordinates

1 linear + 2 rotating axes, 2 ways to reach point, difficult to height axis is rigid, large work program off-line, highly area for floor space complex arm

Spherical coordinates

1 linear + 2 rotating axes, long Can’t reach around obstacles, horizontal reach short vertical reach

Revolute coordinates

3 rotating axes can reach above Difficult to program off-line, 2 or below obstacles, largest or 4 ways to reach a point, most work area for least floor space complex manipulator

Body of Robotics Arm Most robotic arms use the following t types:     

Prismatic or slider ts in which the link is ed on a linear slider bearing, and linearly actuated by ball screws and motors or cylinders. Revolute or rotary ts often driven by electric motors and chain/belt/gear transmissions, or by hydraulic cylinders and levers. Spherical ts, these are the third most utilized t and just slide causing a revolving movement. Screw ts, these just follow the thread of the axis in spiral to move along the axis. Cylindrical ts, these are very rare and are use in some equipment like Parallel Robots or Flying simulator Mechanism.

With the combination of these s, various robotic arms configuration are formed.The complete body of the robotic arms usually is attached to some form of structural frames, for examples, machine table top or factory floor. These structural frames in many cases are not movable.

6

Law Zero: A robot may not injure humanity, or, through inaction, allow humanity to come to harm. Law One: A robot may not injure a human being, or, through inaction, allow a human being to come to harm, unless this would violate a higher order law. Law Two: A robot must obey orders given it by human beings, except where such orders would conflict with a higher order law. Law Three: A robot must protect its own existence as long as such protection does not conflict with a higher order law.

Classification Based on Control Systems: 1. Point-to-point (PTP) control robot 2. Continuous-path () control robot 3. Controlled-path robot

Point to Point Control Robot (PTP): The PTP robot is capable of moving from one point to another point. The locations are recorded in the control memory. PTP robots do not control the path to get from one point to the next point. Common applications include: • Component insertion • Spot welding • hole drilling • Machine loading and unloading • Assembly operations

Continuous-Path Control Robot (): The robot is capable of performing movements along the controlled path. With from one control, the robot can stop at any specified point along the controlled path. All the points along the path must be stored explicitly in the robot's control memory. Applications Straight-line motion is the simplest example for this type of robot. Some continuous-path controlled robots also have the capability to follow a smooth curve path that has been defined by the programmer. In such cases the programmer manually moves the robot arm through the desired path and the controller unit stores a large number of individual point locations along the path in memory (teach-in). Typical applications include: • spray painting • finishing 7

• gluing • Arc welding operations

Controlled-Path Robot: In controlled-path robots, the control equipment can generate paths of different geometry such as straight lines, circles, and interpolated curves with a high degree of accuracy. Good accuracy can be obtained at any point along the specified path. Only the start and finish points and the path definition function must be stored in the robot's control memory. It is important to mention that all controlled-path robots have a servo capability to correct their path.

Robot Work Envelope or Reach: Robot reach, also known as the work envelope or work volume, is the space of all points in the surrounding space that can be reached by the robot arm. Reach is one of the most important characteristics to be considered in selecting a suitable robot because the application space should not fall out of the selected robot's reach. For a Cartesian configuration the reach is a rectangular-type space. For a cylindrical configuration the reach is a hollow cylindrical space. For a polar configuration the reach is part of a hollow spherical shape. Robot reach for a ted-arm configuration does not have a specific shape.

8

The Robotic ts A robot t is a mechanism that permits relative movement between parts of a robot arm. The ts of a robot are designed to enable the robot to move its end-effector along a path from one position to another as desired. The basic movements required for a desired motion of most industrial robots are:

1. Rotational movement: This enables the robot to place its arm in any direction on a horizontal plane. 2. Radial movement: This enables the robot to move its end-effector radially to reach distant points. 9

3. Vertical movement: This enables the robot to take its end-effector to different heights.

These degrees of freedom, independently or in combination with others, define the complete motion of the end-effector. These motions are accomplished by movements of individual ts of the robot arm. The t movements are basically the same as relative motion of ading links. Depending on the nature of this relative motion, the ts are classified as prismatic or revolute.

Prismatic ts are also known as sliding as well as linear ts. They are called prismatic because the cross section of the t is considered as a generalized prism. They permit links to move in a linear relationship.

Revolute ts permit only angular motion between links. Their variations include: • Rotational t (R) • Twisting t (T) • Revolving t (V) In a prismatic t, also known as a sliding or linear t (L), the links are generally parallel to one another. In some cases, ading links are perpendicular but one link slides at the end of the other link. The t motion is defined by sliding or translational movements of the links. The orientation of the links remains the same after the t movement, but the lengths of the links are altered.

A rotational t (R) is identified by its motion, rotation about an axis perpendicular to the ading links. Here, the lengths of ading links do not change but the relative position of the links with respect to one another changes as the rotation takes place.

A twisting t (T) is also a rotational t, where the rotation takes place about an axis that is parallel to both ading links.

A revolving t (V) is another rotational t, where the rotation takes place about an axis that is parallel to one of the ading links. Usually, the links are aligned perpendicular to one another at this kind of t. The rotation involves revolution of one link about another.

10

11

Robot Specification Accuracy: How close does the robot get to the desired point? When the robot's program instruct the robot to move to a specified point, it does not actaully perform as per specified. The accuracy measrue such variance. That is, the distance between the specified position that a robot is trying to achieve (programming point), and the actual X, Y and Z resultant position of the robot end effector. Repeatability: The ability of a robot to return repeatedly to a given position. It is the ability of a robotic system or mechanism to repeat the same motion or achieve the same position. Repeatablity is is a measure of the error or variability when repeatedly reaching for a single position. Repeatability is often smaller than accuracy.

Degree of Freedom (DOF) - Each t or axis on the robot introduces a degree of freedom. Each DOF can be a slider, rotary, or other type of actuator. The number of DOF that a manipulator possesses thus is the number of independent ways in which a robot arm can move. Industrial robots typically have 5 or 6 degrees of freedom. 3 of the degrees of freedom allow positioning in 3D space (X, Y, Z), while the other 2 or 3 are used for orientation of the end effector (yaw, pitch and roll). 6 degrees of freedom are enough to allow the robot to reach all positions and orientations in 3D space. 5 DOF requires a restriction to 2D space, or else it limits orientations. 5 DOF robots are commonly used for handling tools such as arc welders. Resolution: The smallest increment of motion or distance that can be detected or controlled by the robotic control system. It is a function of encoder pulses per revolution and drive (e.g. reduction gear) ratio. And it is dependent on the distance between the tool center point and the t axis.

12

Envelope: A three-dimensional shape that defines the boundaries that the robot manipulator can reach; also known as reach envelope.   

Maximum envelope: the envelope that encomes the maximum designed movements of all robot parts, including the end effector, workpiece and attachments. Restricted envelope is that portion of the maximum envelope which a robot is restricted by limiting devices. Operating envelope: the restricted envelope that is used by the robot while performing its programmed motions.

Reach: The maximum horizontal distance from the center of the robot base to the end of its wrist. Maximum Speed: A robot moving at full extension with all ts moving simultaneously in complimentary directions at full speed. The maximum speed is the theoretical values which does not consider under loading condition.. Payload: The maximum payload is the amount of weight carried by the robot manipulator at reduced speed while maintaining rated precision. Nominal payload is measured at maximum speed while maintaining rated preci-sion. These ratings are highly dependent on the size and shape of the payload due to variation in inertia.

Robot Selection In a survey published in 1986, it is stated that there are 676 robot models available in the market. Once the application is selected, which is the prime objective, a suitable robot should be chosen from the many commercial robots available in the market. The characteristics of robots generally considered in a selection process include: • Size of class • Degrees of freedom • Velocity • Drive type • Control mode 13

• Repeatability • Lift capacity • Right-left traverse • Up-down traverse • In-out traverse • Yaw • Pitch • Roll • Weight of the robot 1. Size of class: The size of the robot is given by the maximum dimension (x) of the robot work envelope. • Micro (x < 1 m) • Small (1 m < x < 2 m) • Medium (2 < x < 5 m) • Large (x > 5 m) 2. Degrees of freedom. The cost of the robot increases with the number of degrees of freedom.Six degrees of freedom is suitable for most works. 3. Velocity: Velocity consideration is effected by the robot’s arm structure. • Rectangular • Cylindrical • Spherical • Articulated 4. Drive type: • Hydraulic • Electric • Pneumatic 5. Control mode: • Point-to-point control(PTP) • Continuous path control() • Controlled path control 6. Lift capacity: • 0-5 kg • 5-20 kg • 20-40 kg and so forth 14

Application With different payload capability, reach and design, articulate robots are designed to employ in the following applications:                 

Arc welding Spot welding Assembly cleaning/spraying Cutting Deburring Die casting Gluing/sealing Grinding/polishing Injection moulding Machine tending Material handling Packing Palletizing Picking Pre-machining Press brake tending

Medical Robots Robots are used in medicine because they are highly precision machines. By tooling with surgical instruments, they have been used in the field of robotic surgery to perform closed-chest, beating-heart surgery. The first generations of surgical robots aren't true autonomous robots that can perform surgical tasks on their own, but they are lending a mechanical helping hand to surgeons. These machines still require a human surgeon to operate them and input instructions. Remote control and voice activation are the methods by which these surgical robots are controlled.

Military Robots Military robots are capable of replacing humans to perform many, if not most combat functions on the battlefield. Suggested by the U.S. t Forces Command, the presence of autonomous robots, networked and integrated, on the battlefield will take human out of the loop as early as 2025. Military robots may look like 15

vehicles, airplanes, insects or animals or other objects in an attempt to camouflage or to deceive the adversary. Remarkable success has been achieved with unmanned aerial vehicles like the Predator drone, which are capable of taking surveillance photographs, and even accurately launching missiles at ground targets, without a pilot. On the ground, robots have been deployed as mine sweepers and for bomb disposal. Defence contractors in the USA are developing autonomous "robot soldiers", though currently it looks more like tanks than humans.

Space Robots Space robotics is generally divided into two main areas: robotic manipulators - such devices are deployed in space or on planetary surfaces to emulate human manipulation capabilities, and Robotic Rovers - they are deployed on planetary surfaces to emulate human mobility capabilities.

Robots in Automobile Industries In the automobile industry, robotic arms are used in diverse manufacturing processes including assembly, spot welding, arc welding, machine tending, part transfer, laser processing, cutting, grinding, polishing, deburring, testing, painting and dispensing. Robots have proved to help automakers to be more agile, flexible and to reduce production lead times.

16

Electronics/Semi-Conductor Application of clean room Robots in semiconductor manufacturing results in the reduction in scrap from broken wafers and chips, which translate into major cost savings in wafer handling. The avoidance of contamination and the savings in scrap from dropped wafers in machine loading and unloading can exceed millions of dollars. Typically, clean Room robots are used predominantly in machine loading, unloading, and parts transfer in semiconductor industry, though, assembly, packaging, and testing processes are other application areas for clean room robots.

Food & Beverage While food and beverage applications represent a small fraction of industrial robotics installation, it is widely recognize as one of the fastest growing segments. The vast majority of robots in the Food & Beverage industry are found in the packaging area, with secondary functions such as case packing and palletizing dominating. High-speed Material Handling robotic arms and vision-guided systems are beginning to work alongside and-in many cases-instead of humans in food factories.

17

Ship Buiding Unlike the automobile industry where the use of robots is widespread, shipbuilding is more of a 'one-of-atype' production. This makes efficient and cost-effective robotic implementation extremely difficult to achieve.

Construction Construction robots aim to improve the efficiency of work at construction sites. With proper planning and development, robots are used in the applications like inner pipe crawling, excavation, load transport, mining and submersion, bricklaying, earth work, foundatio03-Oct-2006t/steel-framework, prefabrication of reinforcement, pavement work and many others. Generally, where there are dangerous conditions or accessibility and/or space limitations that persist, robots will be used.

Aircraft & Aerospace Today, aircraft manufacturers are seeking automated solutions, spurred by the need to fabricate highperformance weapon systems at lower costs, apply lean manufacturing concepts, and achieve Six Sigma quality. A range of new automated and robotic production tools and technologies can be adapted to aircraft fabrication processes and methods. Painting, drilling, and composite fabrication are the best candidates for automation. Some examples of the robotics system using in the aerospace industry are: Robotic coating system (e.g. fuselage, airframe and component) and robotic water-jet coating removal.

18

Welding Robots in Manufacturing Industrial

When following through the history of welding, in year 1988, matrix of robots are employed in the automobile industry to perform resistance spot welding on car bodies. Following that, more and more arc welding robots are being installed both in large and small manufacturing plants. Since then, welding robots are used in two ways in manufacturing -- as elements in a production line and as stand-alone units (or Flexible Robotic Manufacturing System) for batch production. Though, robots work well for repetitive tasks on similar pieces that involve welds in more than one axis or where access to the pieces is difficult, manufacturers who have automated their welding operations request to include more flexibility to process a variety of parts with the same robot system. This results in the creation of Flexible Robotic Manufacturing System. The Robot system here possesses flexibility to facilitate quick changeover by changing the robot end-effector and modifying the robot program. In addition, material handling robots are used within welding systems to position the part while welding or also to load/unload the part into a secondary operation such as an inspection station. Another emerging trend is the use of automated inspection of parts, including the inspection of welded ts and part dimensions. These inspection systems can be integrated into production equipment to automatically control flow of conforming and non-conforming material. Further enhancement includes the networking of factory equipments to allow sharing of information such as inspection data, production status to highlight the utilization of capital and deliver quick response to maintenance issues.

Why welding robots?

Cost Saving: Automated parts inspection dramatically reduces rework and repair costs that can occur from inconsistent or failed manual inspection. Labour Issues: Labour challenges (including retention and training) as well as ergonomic and safety issues are some reasons for manufacturers to automate their welding operations with robotic installations. Quality & Lead Time: New controller technology allows the motion of the welding robot to be coordinated with handling robots. This results in faster travel speeds and better quality. Because robot welding improves weld repeatability - robots will give precisely the same welds every time on work pieces of the same dimensions and specifications, many manufacturing plants use robotics welding systems to improve productivity and product quality.

Considerations when automate the welding operation Part Design 19

    

Welding positions required - if the part needs to rotate, needs to consider cost incur to install a positioner in contrast to manual operation. Welding t design - For instance - fillet welds work better than butt welds for automation. Manufacturing process used to produce parts to be welded - How repeatable are the parts to be welded. (stampings, laser cut vs. flame cut, etc. ) Welding sequences, (part build-up or distortion control) - Consider if the parts require some type of weld sequence to control distortion, this will interrupt the welding cycle. Weld fixture design/cost - Do manual fixtures exist? If so will they work for automation? How much will new tools cost?

Production Requirements   

Production volumes - What is the number of units? This determines the equipment utilization. Number of different assemblies you want to automate on a single cell - this controls the complexity of the tooling and control system. How often do you change-over to run different parts? This greatly effect the tooling design and cost.

Safety concerns (exposure to hazards) - Is the manual operation hazardous? For instance fumes produced by welding on galvanized material. This alone may be a reason to automate. Pinch points, welding radiation. Workforce  Operator / welder availability.  Workers skill level. Competitive issues     

Lead times - With automation it may reduce the lead time for manufacturing your product. Consistent quality Cost of assemblies - With automation you may be able to reduce labor, reduce scrap, reduce weld consumables, and, the process can be controlled and more consistent. Cycle time requirements - Produce more parts in less time. ISO type regulation - Process control may be easier to achieve with automation.

20

Material Handling Robots Material Handling and Logistics is the movement, protection, storage and control of materials and products throughout the process of their manufacture and distribution, consumption and disposal. By fitting the robot with an appropriate end-effector (e.g., gripper), the robot can grasp the object that needs to be moved. The robot may be mounted either stationary on the floor or on a traversing unit, enabling it to move from one workstation to another. The robot can also be ceiling mounted. Robots that are used for material handling in many cases can interface with other material handling equipment such as containers, conveyors, guided vehicles, monorails, automated storage/retrieval systems, and carousels. Robotic material-handling applications range from tending injection-molding machines and machine tools, to reorienting parts between processes, to packaging and palletizing.

Pick and Place

In the pick.place applications, objects move quickly and precisely, one at a time, from one location to another.

Packing

21

Packing application is about the use of robots to organize a variety of items into boxes. It includes case packing - smaller boxes packed into larger ones, and racetrack packing - many items picked simultaneously with the aid of grippers.

Machine Tending The robot provides both manipulative and transport capabilities. Robots can be used to grasp a work piece from a supply point (e.g., a conveyor belt), transport it to a machine, orient it, and then insert it into the machine work holder. This may require that the robot signal the machine tool when the work piece is in the correct position, so that the part can be secured in the work holder. The robot then releases the part and withdraws the arm so that machining can begin. Upon completion of the machining, the robot unloads the work piece and transfers it to another machine or conveyor. In a robotic cell, a single robot can service several machines. Examples of machine tending functions include the following:

      

Exchanging machine tools, such as lathe and machining centers; Stamping press loading and unloading; Tending plastic injection molding machines; Holding a part for a spot welding operation; Loading hot billets into forging presses; Loading auto parts for grinding; Loading gears onto CNC milling machines 22

Palletizing/de-palletizing Robotic palletizers and depalletizers usually utilize 4, 5 or 6-axis ted arm robots. Many palletizing applications handle more than one production line by building different loads at multiple positions inside the robot's work envelope. Cases, pails or bags can be picked individually, in rows or in layer quantities, depending on the application. Depalletizing frequently involves lifting complete layers of product and placing them on an unscrambler, which orients the product for the downstream conveyor system. Vision systems are frequently used to accurately locate layers in depalletizing applications.

Primary Benefits The primary benefits of using robots for material handling are:     

To reduce direct labor costs and remove humans from tasks that may be hazardous, tedious, or fatiguing. A material handling robot can work 24 hours a day without worries or fatigue. By using robotic system, outputs are consistent and higher quality. The use of robots for moving fragile objects results in less damage to parts during handling. Improves process flow due to predictability of cycle times; consistently runs at same speed with no breaks, shift changes, layoffs, or rehires.

23

UNIT-

2

ROBOT

DRIVE

SYSTEMS

AND

END

EFFECTORS

Pneumatic Drives – Hydraulic Drives – Mechanical Drives – Electrical Drives – D.C. Servo Motors, Stepper Motor, A.C. Servo Motors – Salient Features, Applications Comparison of All these Drives End Effectors – Grippers – Mechanical Grippers, Pneumatic Hydraulic Grippers, Magnetic Grippers, Vacuum Grippers; Two Fingered and Three Fingered Grippers; Internal Grippers External Grippers Selection and Design Considerations

24

2.1ROBOT DRIVE SYSTEMS The links of the robots move about the prescribed axis by receiving the power through, what are called the drive systems, also known as actuators. The movements produced may be translatsry in nature or rotary about a t. At the ts the actuators provide required force or torque for the movement of the links. The movements of all the links combined together form the arm end or wrist motion. The source of power for the actuators can be through the compressed air, pressurised fluid or the electricity, based on which they are classified as follow

2.2. BASIC PNEUMATIC SYSTEM Pneumatic systems use pressurized air to make things move. Basic pneumatic system consists of an air generating unit and an air-consuming unit. Air compressed in compressor is not ready for use as such, air has to be filtered, moisture present in air has to be dried, and for different applications in plant pressure of air has to be varied. Several other treatments are given to the air before it reaches finally to the Actuators. The figure 2.1 gives an overview of a pneumatic system. Practically some accessories are added for economical and efficient operation of system

25

Fig. 2.1. Basic Pneumatic System

Compressor. A device, which converts mechanical force and motion into pneumatic fluid power, is called compressor. Every compressed-air system begins with a compressor, as it is the source of airflow for all the downstream equipment and processes Electric Motor Electric motor is used to drive the compressor. Air Receiver It is a container in which air is stored under pressure .Pressure Switch. Pressure Switch is used to maintain the required pressure in the receiver; it adjusts the High Pressure Limit and Low Pressure Limit in the receiver. The compressor is automatically turned off when the pressure is about to exceed the high limit and it is also automatically turned on when the pressure is about to fall below the low limit. .Safety Valve. The function of the safety valve is to release extra pressure if the pressure inside the receiver tends to exceed the safe pressure limit of the receiver. .Auto Drain. Air condenses to give out water in the receiver and a device called Auto Drain directs this water out. Check Valve. The valve enables flow in one direction and blocks flow in a counter direction is called Check Valve. Once compressed air enters the receiver via check valve, it is not allowed to go back even when the compressor is stopped. .Pressure Gauge. Pressure gauge tells us the pressure inside the compressor receive L .Air Dryer.lt is a device for reducing the moisture content of the working compressed airAfter Filter filter, which follows the compressed air dryer and usually used for the protection of downstream equipment from desiccant dust etc. is called after filter. The name Filter refers to a device whose primary function is the removal of insoluble contaminants from a liquid or a gas with the help of porous media. 26

.Tapping (Gooseneck connection). Supply from Main/High pressure line can be taken by using a pipe t (Tee type). But again to ensure minimum amount of water in the line this connection should be taken from upside as shown in f 3.L This type of connection is always preferred in pneumatics to avoid water in the lowpressure lines and is called as Gooseneck connection. .Auto Drain. The function of auto drain is to let off all the water remaining in the tube; Drain should be placed at the lowest point of every tube. Air Service Unit. Filter, Regulator and lubricator combined in one device is popularly known as Air Service Unit or F.R.L unit. Its purpose is to supply air to other successive applications in the line. It provides clean air at required pressure with lubricant added to it to increase the life of equipment, which require lubrication. .Direction Control Valve. Directional-control valve are devices used to change the flow direction of fluid within a Pneumatic/Hydraulic circuit. They control compressed-air flow to cylinders, rotary actuators, grippers, and other mechanisms in packaging, handling, assembly, and countless other applications. These valves can be actuated either manually or electrically. .Pneumatic Actuator A device in which power is transferred from one pressurized medium to another without intensification. Pneumatic actuators are normally used to control processes requiring quick and accurate response, as they do not require a large amount of motive force. They may be reciprocating cylinders, rotating motors or may be a robot end effectors. .Speed Controller Most of the pneumatics applications require speed control circuits for the extraction and retraction of pneumatic cylinders slowly. For this a combination of flow control valves and check valves are used, this topic is discussed with enough details in the chapter “Valves Further we can add few more components, first Air intake filter and second air intercooler (if multistage compressor is used). Function of former is to prevent the entry of vast quantities dust and dirt along with air and latter is to cool the air again to room temperature after it is discharged from low-pressure compressor.

2.2.1. Features, The pneumatic actuators are characterised by the following features  •Lowest power to weight ratio.  •Highly compliant system.  •Drift under load constantly.  •Low, inaccurate response due to low stiffness.  •Less leakage of air and not susceptible to sparks.  •Uses low pressure compressed air, hence less actuation force or torque.  •Useful in on-off applications like pick and place robots.  •Simple and low cost components. 27

 •Reliable and easily available components.  •The exact positions of the actuators can be controlled by servo control valves by differential movements.

2.2.2 ADVANTAGES AND DISADVANTAGES OF PNEUMATICS • Lower cost • Inherently modulating actuators and sensors. • Explosion proof components. • High efficiency e.g., a relatively small compressor can fill a large storage tank to meet intermittent high demands for compressed air. • Ease of design and implementation. • High reliability, mainly because of fewer moving parts. • Easy installation and maintenance. Disadvantages • Need for a compressor producing clean and dry air. • Cost of air piping. • Need for regular component calibration.

APPLICATION OF PNEUMATICS •Operation of heavy or hot doors •Lifting and moving in slab moulding machines •Spray painting •Bottling and filling machines •Component and material conveyor transfer •Unloading of hoppers in building, mining and chemical industry •Air separation and vacuum lifting of thin sheets •Dental drills

28

2.3 BASIC HYDRAULIC SYSTEM

Fig. 2.2. Basic Hydraulic System

Electric Motor Electric motor is used to drive the compressor Hydraulic Pump. Hydraulic pumps convert mechanical energy from a prime mover (engine or electric motor) into hydraulic (pressure) energy. The pressure energy is used then to operate an actuator Pumps push on a hydraulic fluid and create flow. Strainers and Filters. To keep hydraulic components performing correctly, the hydraulic liquid must be kept as clean as possible. Foreign matter and tiny metal particles from normal wear of valves, pumps, and other components are going to enter a system. Strainers, filters, and magnetic plugs are used to remove foreign particles from a hydraulic liquid and are effective as safeguards against contamination Strainers. A strainer is the primary filtering system that removes large particles of foreign matter from a hydraulic liquid. Even though its screening action is not as good as a filter’s, a strainer offers less resistance to flow. Filters. A filter removes small foreign particles from a hydraulic fluid and is most effective as a safeguard against contaminates. They are classified as full flow or proportional flow:

29

(a) Full-Flow Filter In a full-flow filter, all the fluid entering a unit es through a filtering element. Although a full-flow type provides a more positive filtering action, it offers greater resistance to flow, particularly when it becomes dirty (b) Proportional-Flow Filters. This filter operates on the venturi principle in which a tube has a narrowing throat (venturi) to increase the velocity of fluid flowing through it. Flow through a venturi throat causes a pressure drop at the narrowest point. This, pressure decrease causes a sucking action that draws a portion of a liquid down around a cartridge through a filter element and up into a venturi throat. Pressure Gauge .5Pressure Regulator/Unloading valve. Pressure regulators, often referred to as unloading valves, are used in fluid power systems to regulate pressure. Pressure regulator as the name implies, regulates the pressure in the hydraulic system. When it senses a built-up in pressure in the lines to the selector valves, it acts so that the system automatically goes to by Valves. Valves are used in hydraulic systems to control the operation of the actuators. Valves regulate pressure by creating special pressure conditions and by controlling how much oil will flow in portions of a circuit and where it will go. Pressure Relief Valve. Relief valves are the most common type of pressure-control Valves. The relief valves’ function may vary; depending on a system’s needs. They can provide overload protection for circuit components or limit the force or torque exerted by a linear actuator or rotary motor. The internal design of all relief valves is basically similar. The valves consist of two sections: a body section containing a piston that is retained on its seat by a spring(s), depending on the model, and a cover or pilot-valve section that hydraulically controls a body piston’s movement. The adjusting screw adjusts this control within the range of the valves. Valves that provide emergency overload protection do not operate as often since other valve types are used to load and unload a pump. However, relief valves should be cleaned regularly by reducing their pressure adjustments to flush out any possible sludge deposits that may accumulate. Operating under reduced pressure will clean out sludge deposits and ensure that the valves operate properly after the pressure is adjusted to its prescribed setting .Direction Control Valve. Directional control valves are devices used to change the flow direction of fluid within a Pneumatic/Hydraulic circuit. They control compressed-air flow to cylinders, rotary actuators, grippers, and other mechanisms in packaging handling, assembly, and countless other applications. Hydraulic Actuator A hydraulic actuator receives pressure energy and converts it to mechanical force and motion. An actuator can be linear or rotary. A linear actuator gives force and motion outputs in a straight line. It is more commonly called a cylinder but is also referred to as a ram reciprocating motor, or linear motor. A rotary actuator produces torque and rotating motion. It is more commonly called a hydraulic motor or motor Load 30

Check Valve. Check valves are the most commonly used in fluid-powered systems they allow flow in one direction and prevent flow in the other direction. They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve. The valve element may be a Sleeve, cone, ball, poppet, piston, spool, or disc. Force of the moving fluid opens a check valve; backflow, a spring, or gravity closes the valve . Reservoir. A reservoir stores a liquid that is not being used in a hydraulic system. It has many other important functions too •:It also allows gases to expel and foreign matter to settle out from a liquid. •It functions as a cooler. •It functions as a “coarse strainer”, providing sedimentation of impurities. •It functions as an air and water separatot •It functions as a foundation for pumps etc. A properly constructed reservoir should be able to dissipate heat from the oil, separate air from the oil, and settle out contaminates that are in it. It should be high and narrow rather than shallow and broad. The oil level should be as high as possible above the opening to a pump’s suction line. This prevents the vacuum at the line opening from causing a vortex or whirlpool effect, which would mean that a system is probably taking in air. Most mobile equipment reservoirs are located above the pumps. This creates a flooded-pump-inlet condition. This condition reduces the possibility of pump cavitations—a condition where all the available space not filled and often metal parts will erode. Most reservoirs are vented to the atmosphere. A vent opening allows air to leave or enter the space above the oil as the level of the oil goes up or down. This maintains a constant atmospheric pressure above the oil.

2.3.1 Features,  Features of Hydraulic Actuators  Provide high power in small light components  Have flat load-speed or torque speed characteristics  Can operate safely and continuously under stall conditions  Provide stepless variation in speed  Have longer life and reliability due to the lubricating properties of the oil.  Can be easily built using readily available standard elements.  Have contaminant sensitive elements.  The operation is noisy. 31

 Higher inertia on the robot ts.  Power loss and unclean work area due to possibility of leak.  Less deflection due to low compliance of the elements.

2.3.2 ADVANTAGES AND DISADVANTAGES HYDRAULICS Advantages  Through the use of simple devices, an operator can readily start, stop, speed up, slow down, and control large forces with very close and precise tolerance.  High power output from a compact actuator.  Hydraulic power systems can multiply forces simply and efficiently from a fraction of an ounce to several hundred tons of output.  Force can be transmitted over distances and around corners with small losses of efficiency.  There is no need for complex systems of gears, cams, or levers to ojtain a large mechanical advantage.  Extreme flexibility of approach and control. Control of a wide range of speed and forces is easily possible.  Safety and reliability.  Hydraulic systems are smooth and quiet in operation. Vibration is kept to a minimum.

Disadvantages  System components must be engineered to minimize or preclude fluid leakage.  Protection against rust, corrosion, dirt, oil deterioration, and other adverse environment is very important.  Maintenance of precision parts when they are exposed to bad climates and dirty- atmospheres.  Fire hazard if leak occurs.  Adequate oil filtration must be maintained.

2.3.3 Application of Hydraulic Actuators  Used to drive the spray coating robots  Used in heavy part loading robots  Useful in material handling robot system  Used to drive the ts of assembly (heavy) robots  Useful in producing translatory motion in Cartesian robot  Useful in robots operating in hazardous, sparking environments 32

 Useful in gripper mechanisms

2.4 MECHANICAL DRIVE MECHANISMS

When the various driving methods like hydraulic, pneumatic, electrical servo motors and stepping motors are used in robots, it is necessary to get the motion in linear or rotary fashion. When motors are used, rotary motion is converted to linear motion through rack and pinion gearing, lead screws, worm gearing or bail screws. Rack and Pinion Movement The pinion is in mesh with rack (gear of infinite radius). If the rack is fixed, the pinion will rotate. The rotary motion of the pinion will be converted to linear motion of the carriage. Ball Screws Sometimes lead screws rotate to drive the nut along a track. But simple lead screws cause friction and wear, causing positional inaccuracy. Therefore ball bearing screws are used in robots as they have low friction. The balls roll bet weens the nut and the screw. A cage is provided for recirculation of the balls. The rolling friction of the ball enhances transmission efficiency to about 90%. Gear Trains Gear trains use spur, helical and worm gearing. A reduction of speed, change of torque and angular velocity are possible. Positional errors are caused due to backlash in the gears. Harmonic Drive For speed reduction, standard gear transmission gives sliding friction and backlash. Moreover, it takes more space. Harmonic drive due to its natural preloading eliminates backlash and greatly reduces tooth wear. Harmonic drives are suitable for robot drives due to their smooth and efficient action The harmonic drive as shown in Fig. 3.109 is made up of three major elements: the circular spline, the wave generator and the flex spline. The circular spline is a rigid ring with gear teeth machined on the inside diameter. The flex spline is a flexible ring with the teeth cut on its outside diameter. The flex spline has fewer teeth (say 2 teeth less) than the circular spline. The wave generator is elliptical and is given input motion. The wave generator is assembled into the flex spline the entire assembly of. Wave generator and flex spline is placed into the circular spline such that the outer tooth of flex spline is in mesh with the internal teeth of circular spline

33

FIG. 2.3 Harmonic drive elements

If the circular spline has 100 teeth and the flex spline has 98 teeth, and if the wave generator makes one complete revolution, the flex spline will engage 98 teeth of the circular spline. Since circular spline has 100 teeth and only 98 teeth have been in engagement for one complete rotation, the circular saline's position has been shifted by 2 teeth. Thus after 50 revolutions of the wave generator, the circular spline will have made one full rotation. The ratio of harmonic drive is 2: 100 or 1: 50. The gear ratio is influenced by the number of teeth cut into the circular spline and the flex spline. The harmonic drive has high torque capacity.

2.5. ELECTRIC DRIVES Principle A rotational movement is produced in a rotor when an electric current flows through the windings of the armature setting up a magnetic field opposing the field set up by the magnets. The Main Components Rotor, stator, brush and commutator assembly. The rotor has got windings of armature and the stator has got the magnet. The brush and the commutator assemblies switch the current to the armature maintaining an opposed field in the magnets

Types of electric drive The most commonly .DC Servo motor .AC Servo motor .Stepper motor. Used electric drives in robotics are

34

STEPPER MOTORS The stepper motors are unique type of motors that produce rotational movement in the form of finite angular steps. The intermittent electrical pulses make the stepper motor shaft to rotate in steps The Fig. 2.14 shows the schematic arrangement of the stepper motor principle. The stator in this case is made up of fourelectromagnetic poles. The rotor is a permanent magnet with two poles N and S. When the excitation of pole 2 (P2) is changed to P3 pole the magnetic north of the rotates by 9Q0 clockwise. By continuous change in excitation in the order P2-P3-P4 the clockwise rotation is produced in the shaft of the rotor, which results in continuous movement

. Fig. 2.4. Schematic of Stepper Motor

35

36

2.5.1.Features,

37

2.6 COMPARISION OF CHARACTERISTICS OF ROBOT DRIVE SYSTEM

38

Robot End-Effectors The robot end-effector or end-of-arm tooling Is the bridge between the robot arm and the environment around it. Depending on the task, the actions of the gripper vary. What is the ideal gripper? Should it be like a human hand with fingers that have 22 degrees of freedom? The most striking thing about the human hand is that it can adapt to the task as it is a sensory and communicating organ. A human being decides the global position of his hand based on the analysis of his eye and memory and then determines his choice of grip and the necessary manipulation with the aid of sensors on his skin, arm and wrist (Fig. 4.1) A robotic end-eflector which is attached to the wrist of the robot arm is a device that enables the general-purpose robot to grip materials, parts and tools to perform a specific task. The endeffectors are also called the grippers. There are various types of end-effectors to perform the different work functions.

The various types of grippers can be divided into the following major categories: • Mechanical grippers • Hooking or lifting grippers • Grippers for scooping or ladling powders or molten metals or plastics • Vacuum cups • Magnetic grippers • Others: Adhesive or electrostatic grippers.

The grippers may be classified into Part handling grippers Tools handling grippers and Special grippers The part handling grippers are used to grasp and hold objects that are required to be transported from one point to another or placed for some assembly operations. The part handling applications include machine loading and unloading, picking arts from a conveyor and moving parts, etc.

There are grippers to hold tools like welding gun or spray painting gun to perform a specific task. The robot hand may hold a deburring tool. The grippers of the robot may be specialized devices like Remote Centre Compliance (RCC) to insert an external mating component into an internal member, viz, inserting a plug into a hole. 39

The other types of end-effectors employ some physical principle like magnetism or vacuum technology to hold the object securely

CLASSIFICATION OF END-EFFECTORS An end-effector of a robot can be designed to have several fingers, ts and degrees of freedom. Any combinations of these factors give different grasping modalities to the end-effector The general end-effectors can be grouped according to the type of grasping Modality as follows  Mechanical fingers  Special tools  Universal fingers Mechanical fingers are used to perform some special tasks. Gripping by mechanical type fingers is less versatile and less dexterous than holding by universal fingers as the grippers with mechanical fingers have fewer numbers of ts and lesser flexibility. However, they economize the device cost . The grippers can be sub grouped according to finger classifications, for example, the number of fingers, typically two-, three-, and five-finger types The two-finger gripper is the most popular 40

Mother classification is according to the single gripper and multiple grippers mounted on the wrist. Multigripper systems shown in Fig. 2.6 enable effective simultaneous execution of more than two different jobs. Design methods for each individual gripper in a multigripper system are subject to those of single grippers

FIG. 4.6 Multiple gripper

Robot end-effectors can be classified on the basis of the mode of gripping as external and internal gripping. The internal gripping system shown in Fig. 4.3(a) grips the internal surface of objects with open fingers whereas the external gripper shown in Ag. 4.3(b) grips the exterior surface of objects with closed fingers. Robot end-effectors are also classified according to the number of degrees of freedom (DOF) incorporated in the gripper structures. Typical mechanical grippers belong to the class of I DOF. A few grippers can be found with more than 2 DOFs. Using some special tooling action, robot grippers can be designed to retain objects by electromagnetic action or under the action of vacuum. E and vacuum cups are typical devices in this class. Usually, if the objects to be handled are too large and ferromagnetic in nature, electromagnetic grippers may be employed. In some applications where the objects are too thin to be handled, they can be held by vacuum grippers

41

Universal fingers usually comprise multipurpose grippers of more than three fingers and or more than one t on each finger which provide the capacity to perform a wide variety of grasping and manipulating assignments.

FIG. 2.7Internal and external grippers (a) Internal gripper (b) External gripper

DRIVE SYSTEM FOR GRIPPERS In typical robot gripper systems, there are three kinds of drive methods  Pneumatic  Electric  Hydraulic

In electric drive system, there are typically two kinds of actuators, dc motors and stepper motors. In general, each motor requires appropriate reduction gear systems to provide proper output force or torque. In the electric system, a servo power amplifier is also needed to provide a complete actuation system. The pneumatic system has the merit of being less expensive than other methods, which is the main reason for it being used in most of the industrial robots. Another advantage of the pneumatic system is the low-degree of stiffness of the air-drive system. This feature of the pneumatic system can be used effectively to achieve compliant grasping which is necessary for one of the most important functions of grippers; to grasp objects t4ith delicate surfaces carefully. On the other hand, the relatively limited stiffness of the system makes precise control difficult

42

Hydraulic drives used in robot gripping systems are usually electrohydraulic drive systems. They have almost the same configuration as pneumatic systems, though their features are different from each other. A typical hydraulic drive system consists of actuators, control valves and power units. There are three kinds of actuators in the system: piston cylinder, swing motor, and hydraulic motor. To achieve positional control using electric signals electrohydraulic conversion drives are used

MECHANICAL GRIPPERS A mechanical gripper is an end-effector that uses mechanical fingers actuated by a mechanism to grip an object. The fingers are the appendages of the gripper that actually makes with the object. The fingers are either attached to the mechanism or are an integral part of the mechanism. The use of replaceable fingers allows f or wear and interchangeability. Different sets of fingers for use with the same gripper mechanism can be designed to accommodate different parts models. A gripper with interchangeable fingers is shown in Fig. 2.8 .

FIG. 2.8Mechanical gripper with interchangeable fingers Several kinds of gripper functions can be realized using various mechanisms. From observations of the usable pair elements in the gripping device, the following kinds are identified: (i) linkage (ii) gear-and rack (iii) cam, (iv) screw (v) cable and pulley and so on. The selection of these mechanisms is influenced by the kind of actuators to be employed and the kind of grasping modality to be used. Another method of classifying the mechanical grippers is according to the type of finger movement used by the gripper. In this classification, the grippers can actuate the opening or dosing of the fingers by one of the following motions 43

 Pivoting or swinging movement  Linear or translational movement  In most applications, two fingers are sufficient to hold the workpiece. Grippers with three or more fingers are less common.

Mechanical Grippers with Two Fingers Pivoting or Swinging Gripper Mechanisms This is the most popular mechanical gripper for industrial robots. It can be designed for limited shapes of an object, especially cylindrical work piece. If actuators that produce linear movement are used, like pneumatic piston- cylinders, the device contains a pair of slider-crank mechanisms. In the slider crank mechanism shown in Hg. 4.6, when the piston 1 is pushed by pneumatic pressure to the right, the elements in the cranks 2 and 3, rotate counter clockwise with the fulcrum F and clockwise with the fulcrum F respectively, when B < 1800. These rotations make the grasping action at the extended end of the crank elements 2 and 3. The releasing action can be obtained by moving the piston to the left. An angle B ranging from 160° to is commonly used.

FIG. 2.9 Schematic diagram of a gripper using slider-crank mechanism

Figure 2.10 is another example of swinging gripper that uses the piston- cylinder. This is the swingblock mechanism. The sliding rod 1, actuated by the pneumatic piston transmits motion by way of the two symmetrically arranged swing-block linkages 1--2--3--4 and 1—2—3’—4’ to grasp or release the object by means of the subsequent swinging motions of links 4 and 4’ at their Pivots F and F

44

Fig2.10 Schematic diagram of a gripper using swing block mechanism Figure2.11 illustrates a typical example of a gripper using a rotary actuator in which the actuator is placed at the cross point of the two fingers. Each finger is connected to the rotor and the housing of the actuator, respectively. The actuator movement directly produces grasping and releasing actions.

FIG. 2.11 Gripper with a rotary actuator The cam actuated gripper includes a variety of possible designs, one of which is shown in F 2.12 A cam and follower arrangement, often using a spring-loaded follower, can provide the opening and closing action of the gripper. The advantage of this arrangement is that the spring action would accommodate different sized objects.

FIG. 2.12 Cam actuated gripper Figure 2.13 indicates an example of screw-type actuation used in the gripper design. The screw is turned by a motor, usually accompanied by a speed reduction mechanism. Due to the rotation of the screw, the threaded block moves, causing the opening and dosing of the fingers depending on the direction of rotation of the screw

45

FIG. 2.13 Screw type gripper

Translation Gripper Mechanisms Translational mechanisms are widely used in grippers of industrial robots. The simplest translational gripper uses the direct motion of the piston cylinder, shown in Fig. 2.15.. The finger motion corresponds to the piston movement without any connecting mechanisms between them. The drawback is that sometimes it is difficult to design the desired size of the gripper, because here the actuator size decides the gripper size

. FIG. 2.15 Translational gripper using cylinder piston Figure 2.16 shows a translational gripper using a hydraulic or pneumatic piston-cylinder, which includes a dual-rack gear mechanism and two pairs of symmetrically arranged parallel closing linkages. The pinion and sector gears are connected to the elements L and L respectively. When the piston rod moves towards left, the translation of the, rack causes the two pinions to rotate clockwise and anticlockwise respectively and produces the grasping action, keeping each finger direction constant. The release action occurs rod moves to the right in the same way when the piston

FIG. 2.16 Translational gripper using parallel bar linkages

46

Rotary actuators can also be used for translational gripper mechanisms as shown In Fig. 2.17

FIG. 2.17 Translational gripper using rotary actuators

Mechanical Grippers with Three Fingers The Increase of the number of fingers and degrees of freedom will greatly aid the versatility of grippers. The main reason for using the three-finger gripper is its capability of grasping the object in three spots, enabling both a tighter grip and the holding of spherical objects of different size keeping the centre of the object at a specified position. Three point chuck mechanisms are typically used for this purpose. Figure 2.18 gives an example of this gripper. Each finger motion is performed using ball-screw mechanism. Electric motor output is transmitted to the screws attached to each finger through bevel gear trains which rotate the screws. When each screw is rotated clockwise or anticlockwise, the translational motion of each finger will be produced, which results in the grasping-releasing action

47

FIG. 2.18 Gripper using three point chuck mechanism

MAGNETIC GRIPPERS Magnetic grippers are used extensively on ferrous materials. In general, magnetic grippers offer the following advantages in robotic handling operations  Variations in part size can be tolerated  Pickup times are very fast  They have ability to handle metal parts with holes  .Only one surface is required for gripping The residual magnetism remaining in the work piece may cause problems. Mother potential disadvantage is the problem of picking up one sheet at a time from a stack. The magnetic attraction tends to penetrate beyond the top sheet in the stack, resulting in the possibility that more than a single sheet will be lifted by the magnet. Magnetic grippers shown In Hg. 2.18can use either electromagnets or permanent magnets. Electromagnetic grippers (Fig. 2.18(b) are easier to control, but require a source of dc power and an appropriate controller. When the part is to be released, the control unit reverses the polarity at a reduced power level before switching off the electromagnet. This procedure acts to cancel the residual magnetism in the work plece ensuring a positive release of the part. The attractive force, P of an electromagnet is found from Maxwell’s equation given by

Permanent magnets do not require an external power and hence they can be used in hazardous and explosive environments, because there is no danger of sparks which might cause ignition in such environments. When the part is to be released at the end of the handling cycle, in case of permanent magnet grippers, some means of separating the part from the magnet must be provided. One such stripping device is shown in Fig. 2.18(a) 48

FIG. 2.18 Magnetic grippers (a) Permanent magnet type (to Electro magnet type

VACUUM GRIPPERS Large flat objects are often difficult to grasp. One solution to this problem is the use of vacuum gripper. Vacuum grippers are used for picking up metal plates, pans of glass, or large lightweight boxes. Since the vacuum cups are made of elastic materials, they are complaint. The gripper is tolerant of errors in the orientation of the part and is especially suited for pick-and-place work. For handling softer materials, cups made of harder material are used A typical vacuum cup gripper is shown in Fig. 2.19(a). It is used extensively for lifting fragile materials. A compressed air supply and a venturi are used to create a gentle vacuum that lifts the part The lift capacity of the suction cup (Hg. 2.19(b)) depends on the effective area of the cup and the negative air pressure between the cup and the object the relationship can be shown by the equation

49

Fig2.19..Vacuum gripper (a) Ventury device for flat surface gripping (b) Gripper for contoured surface

Instead of a venturi, a vacuum pump powered by an electrical motor may also be used

50

ADHESIVE GRIPPERS An adhesive substance can be used for grasping action in gripper design. The requirements on the items to be handled are that they must be gripped on one side only. The reliability of this gripping device is diminished with each successive operation cycle as the adhesive substance loses its tackiness on repeated use. To overcome this limitation, the adhesive material can be loaded In the form of a continuous ribbon into a feeding mechanism attached to the robot wrist . HOOKS, SCOOPS AND OTHER MISCELLANEOUS DEVICES Hooks can be used as end-effectors to handle containers and to load and unload parts hanging from overhead conveyors. The item to be handled by a hook must have some sort of handle to enable the hook to hold it Ladles and scoops can be used to handle certain materials in liquid orpowder form. One of the limitations is that the amount of material being scooped by the robot is sometimes difficult to control. Other types of grippers Include inflatable devices, in which an inflatable bladder is expanded to grasp the object. The inflatable bladder is fabricated out of some elastic material like rubber, which makes it appropriate for gripping fragile objects. In contrast to the typical mechanical grippers where a concentrated force is applied an the object, the gripper applies a uniform grasping pressure against the surface of the object. An example of this type of gripper is shown in Figs 4.17(a) and (b.)

51

FIG. 4.17 Expanding bladder tot gripping internal surface (a) Bladder fully expanded (b) Bladder inside the container between the part surface and the finger surface, the acceleration of the part and the orientation of the direction of motion during acceleration with respect to the direction of the fingers. This may be demonstrated through the analysis of several examples given below

52

The purpose of the gripper mechanism is to convert input power into the required motion and force to grasp and retain an object. So the gripping force required must be calculated first. When the gripping force is established, the required actuator force or torque can be computed for a given gripper design. There are two ways of constraining the part in the gripper. In the first way, the gripper fingers may enclose the part to some extent, thereby constraining the motion of the part. This is accomplished by deg the ing surface of the fingers to be in the approximate shape of the part geometry as shown in Fig. 4.18. The second way of holding the part is by friction between the fingers and the object. In this approach, the fingers must apply a force that is sufficient for friction to retain the part against gravity, acceleration and any other force that might arise during the holding portion of the working cycle. The friction method of holding the parts results in a less complicated and therefore less expensive gripper design and it tends to be readily adaptable to a greater variety of workparts. However there is a problem with the friction method that is avoided with the physical constriction method. If a force of in sufficient magnitude is applied against the part in a direction parallel to the friction surfaces of the fingers as shown in Fig. 4.19, the part might slip out of the gripper. To resist this slippage, the gripper must be designed to exert a force that depends on the weight of the part, the coefficient of friction CONSIDERATIONS IN GRIPPER SELECTION AND DESIGN Most of this chapter has been concerned with grippers rather than tools as end effectors. As indicated in Sec. 5-4, tools are used for spot welding, arc welding, rotating spindle operations, and other processing applications. We will examine the tooling used with these operations when we discuss the corresponding applications in Chaps. Fourteen and Fifteen. In this section, let us summarize our discussion of grippers by enumerating some of the con siderations in their selection and design Certainly one of the considerations deals with determining the grasping requirements for the gripper Engel Berger [ defines many of the factors that should be considered in assessing gripping requirements. The following list is 53

Based on Engel Berger's discussion of these factors the part surface to be grasped must be reachable. For example, it must not be enclosed within a chuck or other holding fixture

.

54

55

UNIT-III

SENSORS AND MACHINE VISION

Requirements of a sensor, Principles and Applications of the following types of sensors Position of sensors (Piezo Electric Sensor, LVDT, Resolvers, Optical Encoders, Pneumatic Position Sensors Range Sensors Triangulation Principle, Structured, Lighting Approach, Time of Flight Range Finders, Laser Range Meters), Proximity Sensors Inductive, Hall Effect, Capacitive, Ultrasonic Optical Proximity Sensors Touch Sensors, Binary Sensors, Analog Sensor Wrist Sensors, Compliance Sensors, Slip Sensors Camera, Frame Grabber, Sensing and Digitizing Image Data Signal Conversion, Image Storage, 56

Lighting Techniques. Image Processing and Analysis Data Reduction, Segmentation, Feature Extraction, Object Recognition, Other Algorithms. Applications – Inspection, Identification, Visual Serving and Navigation.

Robot Sensors and Vision INTRODUCTION The interaction of the robot with the environment set-ups needs mechanisms known as Sensors

Need for sensors Robot guidance without obstruction. Object identification tasks. Handling the objects. The sensors that provide the information's like t position, velocity and acceleration known as internal state sensors. The robots are being guided by the help of vision and range sensors that are known as non- external state sensors. The task of object identification is done by proximity and touch sensors known as external state sensors. The information's of object handling are supplied as a from force and torque sensors termed as type internal state sensors

Principle of position sensors: Position sensors are used to monitor the position of ts. Information about the position is fed back to the control systems that are used to determine the accuracy of positioning.

57

In most cases in robots, a primary interest is to control the position of the arm. There is a large variety of devices available for sensing position. However, the most popular angular-position sensors are the following devices: a. Encoders b. Synchros c. Resolvers d. Potentiometers Encoders The three general types of angular-position encoders using digital out put are

tachometer incremental

optical absolute optical The tachometer encoder is a sin device that generates pulses in response to sensor rotation vice senses only increments of moo on and not direction a rotary pulse generator that outputs a signal every time a magnet es by. The angular resolution of this device in degrees is 1 where ii is the number of magnets If the device is energized or sampled for a fixed-time interval, the number of output pulses per unit time represents angular velocity. The incremental optical encoder is illustrated in Figure 6.7.2. It consists of a glass disk marked with alternating transparent and opaque strip aligned radially. A photo transmitter (light source) is located on one side of the disk, and a photo receiver is on the other. As the disk rotates, the light beam is alternately completed and broken. The output from the photo receivers a pulse train whose frequency is proportional to the speed of rotation of the disk. In a typical encoder there are two sets of photo transmitters and receivers aligned 90 out of phase. This phasing provides direction information; that is, if signal A leads signal B by 98’, the encoder disk is rotating in one direction; but if B leads A, then it is going in the other direction. By counting the pulses and by adding or subtracting based on the sign, it is possible to use the encoder to provide position in information with respect to a known starting location. The absolute optical encoder employs the same basic construction as incremental optical encoders except that there are more tracks of stripes and a corresponding number of receivers and transmitters. Usually, the stripes are arranged to provide a binary number proportional to the shaft angle. The first track might have two stripes, the second four, the third eight, and so on. In this way the angle can be read directly from the encoder without any necessary counting. Figure 3 illustrates an absolute optical encoder. The resolution of an absolute optical

58

59

LVDT The linear variable differential transformer (LVDT) The linear variable differential transformer (LVDT) is another type of position sensor, whose construction is shown in Fig. 5.5. It consists of a primary, two secondaries, and a movable core. The primary is excited with an ac source. When the core is in its exact central location, the amplitude of the voltage induced in secondary- will be the same as that in secondary-2. The secondaries

60

2. Range sensors: The distance between the object and the robot hand is measured using the range sensors Within it is range of operation. The calculation of the distance is by visual processing. Range sensors find use in robot navigation and avoidance of the obstacles in the path. the - location and the general shape characteristics of the part in the work envelope of the robot S done by special applications for the range sensors. There are several approaches like, triangulation method, structured lighting approach and time-of61

flight range finders etc. In these cases the source of illumination can be light-source, laser beam or based on ultrasonic.

Triangulation Method This is the simplest of the techniques, which is easily demonstrated in the Fig. 7.8. The object is swept over by a narrow beam of sharp light. The sensor focussed on a small spot of the object surface detects the reflected beam of light. If ‘8’ is the angle made by the illuminating source and ‘b’is the distance between source and the sensor, the distance ‘c of the sensor on the robot is given as

.

Structured Lighting Approach This approach consists of projecting a light pattern the distortion of the pattern to calculate the range. A pattern in use today is a sheet of light generated narrow slit. As illustrated in. Fig. 6.3, the intersection of the sheet with objects in the’ work space yields a light stripe which is viewed through a television camera displaced a distance B from the light source. The stripe pattern is 62

easily analyzed by a computer to obtain range information. For example, an inflection indicates a change of surface, and a break corresponds to a gap between surfaces. Specific range values are computed by first calibrating the system. One of the simplest arrangements is shown in Fig: 6.31’, which represents a top view of Fig. 6 .3a. In this, arrangement, the light source and camera are placed at the same height, and the sheet of light is perpendicular to the line ing the origin of the light sheet and the center of the camera lens. We call the vertical plane containing this line the reference plane. Clearly, the reference plane is perpendicular to the sheet of light, and any vertical flat surface that intersects the sheet Will produce a vertical stripe of light (are Fig. 6.3a) in which every point will have the same perpendicular distance to the reference plane. - The objective of. the arrangement shown in Fig. 6.31’ is to position the camera so that every such vertical stripe also appears vertical in the image plane. In this way, every point, the same column in the ‘image will be known to have the same distance to the ‘reference plane

Time-of-Flight Range Finders In this section we discuss three methods for determining range based on the time- of-flight concept introduced at the beginning of Sec. 6.2. Two of the methods utilize a laser, while the third is based on ultrasonic. 63

One approach for using a laser to determine range is to measure the time it takes an emitted pulse of light to return coaxially (i.e., along the same path) front a reflecting surface. The distance to the surface is given by the simple relationship D = cT/2, where T is the pulse transit time and e is the speed of light. ft is of interest to note that, since light travels at approximately I ft/as, the ing electronic instrumentation must be capable of 50-ps time resolution in order to achieve a ± ‘4-inch accuracy in range.

A pulsed-laser system described by larvis [produces a two-dimensional array with values proportional to distance. The two-dimensional scan is accom plished by deflecting the laser light via a rotating mirror. The 64

working range of this device is on the order of I to 4 m, with an accuracy of ± 0.25 cm. An exam ple of the output of this system is shown hi Fig. 6.4. Part (a) of this figure shows a collection of three-dimensional objects, and Fig. 6.41 is the corresponding sensed array displayed as art image in which the intensity at each point is proportional to the distance between the sensor and the reflecting surface at that point (darker is closer). The bright areas around the object boundaries represent discontinuity in range determined by post processing in a computer An alternative to pulsed light is to use a continuous-beam laser and measure the delay (i.e., phase shift) between the outgoing and returning beams. We illus treat this concept with the aid of Fig. 6.5. Suppose that a beam of laser light of wavelength X is split into two beams. One of these (called the reference beam) travels a distance L to a phase measuring device, and the other travels a distance I out to a reflecting surface. The total distance traveled by the reflected beam is =L + 2D. Suppose that D = 0. Under this condition U = L and both the reference and reflected beams arrive simultaneously at the phase measuring device, If we let D increase, the reflected beam travels a longer path and, therefore, a phase shift is introduced between the two beams at the point of measurement,

PROXIMITY SENSORS The output of the proximity sensors gives an indication of the presence of an object with in the vicinity job operation. In robotics these sensors are used to generate information of object grasping and obstacle avoidance. This section deals with some of the important proximity sensors used in robotics.

Inductive Sensors Principle The ferromagnetic material brought close to this type of sensor results in change in position of the flux lines of the permanent magnet leading to change in inductance of the coil. The induced current pulse in the coil with change in amplitude and shape is proportional to rate of change of flux line in magnet

65

Construction The proximity inductive sensor basically consists of a wound coil located in front of a permanent magnet encased inside a rugged housing. The lead from the coil, embedded in resin is connected to the display through a connector

Hall-Effect Sensors :Principle Hall-Effect deals with the voltage between the two points in a conductor which changes by the near field of the magnetized or ferromagnetic material. The sensor experiences a weakened magnetic field in the close proximity of a ferromagnetic material, due to the bending of the flux lines of the magnet through approaching object Construction: 66

A sensor element is stationed between the poles of a horse shoe magnet constructed inside a container. The principle of operation is as depicted in Fig. 7.5

. The decrease in the strength of the magnetic field resulting due to the proximity of the object field reduces the voltage across the sensor. The sensor gives binary output for the decision making devices of control fo further actions. The silicon makes the ideal selection for a semiconductor in of size, strength and capacity to electrical interference prevention.

Capacitive Sensors Unlike inductive and Hall-effect sensors which detect only ferromagnetic materials, capacitive sensors are potentially capable (with various degrees of sensitivity) of detecting all solid and liquid materials. As their name implies, these sensors are based on detecting a change in capacitance induced by a surface that is brought near the sensing element. The basic components of a capacitive sensor are shown in Fig. 6.12. The sensing element is a capacitor composed of a sensitive electrode and a reference electrode. These can be, for example, a metallic disk and ring separated by a dielectric material. A cavity of dry air is usually placed behind the capacitive ele ment to provide isolation. The rest of the sensor consists of electronic circuitry which can be included as an integral part of the unit, in which case it is normally embedded in a resin to provide sealing and mechanical . 67

There are a number of electronic approaches for detecting proximity based on a change in capacitance. One of the simplest includes the capacitor as part of an

Oscillator circuit designed so that the oscillation starts only when the capacitance of the sensor exceeds a predefined threshold value. The start of oscillation is then translated into an output voltage which indicates the presence of an object. This method provides a binary output whose triggering sensitivity depends on the threshold value. A more complicated approach utilizes the capacitive element as part of a cir cuit which is continuously driven by a reference sinusoidal waveform. A change in capacitance produces a phase shift between the reference signal and a signal derived from the capacitive element. The phase shift is proportional to the change in capacitance and can thus be used as a basic mechanism for proximity detection. Figure 6.13 illustrates how capacitance varies as a function of distance for a proximity sensor based on the concepts just discussed. It is of interest to note that sensitivity decreases sharply past a few millimeters, and that the shape of the response curve depends on the material being sensed. Typically. These sensors are operated in a binary mode so that a change in the capacitance greater than a preset threshold T indicates the presence of an object, while changes below the threshold indicate the absence of an object with respect to detection limits established by the value of T..

Ultrasonic Proximity Sensor :Principle The previously discussed proximity sensors are useful for detection of ferro-magnetic matter only. If the robot has to handle other type of materials ultrasonic sensors find the application. Construction The main part in this type of sensor is the transducer which can act both as transmitter and receiver. The sensor is covered by a resin block which protects from dust and humidity. For the acoustic damping, absorber material is provide as shown in Fig. 7.6 (a). Finally a metallic housing gives general protection 68

Operation The acoustic waves emitted by the sensors reach the object and get reflected and the receiver sensors the waves to generate the information about the presence of the object. This type of operation is the echomode, type. When the sensor acts only as the transmitter the waves get blocked by the presence of the object and the receiver gets no signal. This type is known as opposed mode. The .echomode type of operation is shown

Optical Sensors Optical Sensors are similar to ultrasonic sensors. The proximity of the object is detected by the action of the travelling light wave as it propogates from the transmitter and reflected by the object towards the receiver The Fig. 7.7 shows the constructional details of the optical sensor. The light emitted by a diode is focussed by the transmitter lens, on to the object surface. The reflected light waves travel back and received by the solidstate photo diode, through a receiver lens. When the object is within the range of the sensor it is possible to 69

detect the presense of the receiver. The range is defined by the position and orientation of the object and the focal length of the sensor lenses

TOUCH SENSORS The touch sensors gather the information's established by the between the parts to be handled and the fingers in the manipulator end effectors. The signals of touch information's are useful in  Locating the objects.  Recognizing the object type.  Force and torque control needed for task manipulation. The Types of Touch Sensors are Binary sensors detect the existence of the object to be handled. e.g. micro switches and limit switches. Analog sensors produce proportional output signal for the force exerted locally. e.g. a code wheel with a plunger A useful application of binary sensors is to use it on a robot engaged in inspection of the parts. A robot with six degree of freedom can provide higher manuarability compared to three axis co-ordinate measuring machine . BINARY SENSORS

70

The devices that deliver sensing signal by at two gripping points are termed the binary sensors. The fingers as shown in Fig. 7.1. accommodate the binary sensors. The with the parts results in deflection and this information is sufficient to determine the presence of the object between the fingers. The proper grasping and manipulation of the object in the work envelope can be easily achieved through centering of the fingers assisted by the information given by binary sensors.

. This type of sensors is featured by spring actuated plunger connected to a code wheel. The deflection of the plunger rod by the action of force, results in rotation of the wheel which gives an output proportional to the sensors force. The schematic arrangement of the analog sensor is as shown in Fig. 7.2.

ANALOG SENSORS This type of sensors is featured by spring actuated plunger connected to a code wheel. The deflection of the plunger rod by the action of force, results in rotation of the wheel which gives an output proportional to the sensors force. The schematic arrangement of the analog sensor is as shown in Fig. If k is the spring rate and S is the deflection of the plunger recorded, the force of is given by

71

Wrist sensors Force-sensing wrist the purpose of a force-sensing wrist is to provide in formation about the three components of force (F, F, and F and the three moments (Ms, M and M being applied at the end-of-the-arm. One possible construction of a force-sensing wrist is illustrated in Fig. 6-1. The device consists of a metal bracket fastened to a rigid frame. The frame is mounted to the wrist of the robot and the tool is mounted to the center of the bracket. The figure shows how the sensors might react to a moment applied to the bracket due to forces and moments on the tool Since the forces are usually applied to the wrist in combinations it is Necessary to first resolve the forces and moments into their six components The computation can be carried out by the robot controller (if it has ‘the required computational capability) or by a specialized amplifier designed for this purpose. Based on these calculations the robot controller can obtained the required e forces moments being applied at the wrist. This informer, applications. As an example. An It inserting a peg into, a hole in an assembly application requires that thereof side forces being applied to the peg. Another example is where the robot’s end effector is required to follow along an edge or contour of an irregular surface. This is called force accommodation. With this technique, certain forces arc set equal to zero while others are set equal to specific values. Using force accommodation, one could

COMPLIANCE Compliance is a special end effector that is neither a gripper nor a process tool but rather a sensor or device that fits between the robot wrist and end effector for special assembly applications. In general, a compliant

72

robot system is one that complies with externally genente4 forces to modify motion for the purpose of alignment between mating parts. If a robot uses a force sensor (piezoelectric, magnetic, or strain gauges) and modifies its control strategy based on sensor’s output, the term active compliance is used to describe the behavior. On the other hand, if the robot’s gripper is constructed in such a way that the mechanical structure debris to comply with those forces, the term ive compliance is used. Therefore, problems with mating-part alignment in assembly and other applications are resolved using active and ive compliance techniques

Active Compliance The general task of inserting a pin into a hole represents three types of during the process: (a) The chamfer occurs when the pin is not perfectly aligned with the hole; (b) if the pin is not rigid it will rotate slightly and start to slide and make a along one side of the hole; end (c) if the misalignment is severe, the pin will make a two-point with the base of the pin and the far wall of the hole. Figure 5.8.1 shows how a misalignment of a pin into whole results in an axial and lateral force, and a twisting moment by a force applied to the wrist sensor for correction. By moving in the correct direction with the compliance (Figure 5.8.2), the robot can reduce these forces on the pin Whitney (1983) has investigated the forces resulting from pin insertion in great detail and has arrived at a careful analysis of the forces acting on the pin being inserted into an unhampered hole. See Figure 5.8.3 Active compliance systems as indicated earlier measure the active force and torque when the robot performs the programmed task and often are called Ff1’- sensing systems. Force-sensing systems allow the robot to detect changes and variations in the work piece or tooling during the operation and adapt the program to correct them. F sensing uses an adaptor placed between the gripper and the robot tool plate to measure the force and torque caused by between mating

73

74

Slip Sensors One of the capabilities of the human hand certainly taken for granted is its ability to determine when an object that is being grasped is slipping. The biological control system associated with the hand utilizes the inputs from the appropriate slip receptors and causes the gripping force to be increased or decreased, as the case may be. Machine determination of slippage of a part or object when in the grasp of a robot or other electromechanical “hand” is still in the experimental stage. Of all the “external” robotic senses, slip detection is perhaps the least developed, and in fact, much of the research in the field has been oriented toward Perhaps applications. It is the firm belief of the authors, however, that this situation will change. Most certainly, in the next few years, as more complex and sophisticated assembly applications become commonplace, it will be necessary to detect slip rapidly and to adjust the gripping force “on the fly” to prevent the part from being damaged in a fall. Perhaps the simplest way to determine if a part is slipping (or has not been properly grasped) is to use what is often termed the lift-and-try technique (see Figure 5.9.18). This entails using the motor current of a particular t or set of ts on a robot as a measure of whether or not a part is slipping. In this respect, current monitoring can be performed either digitally or in an analog manner. Regardless of which technique is employed, the gripper is first oriented correctly, next placed over the particular part, and then a certain minimum grasping force Applied.

75

As the manipulator attempts to lift the object in question from the surface (e.g., a pallet, table, or conveyor) the motor current in one or more ts should increase due to the added load torque. If no increase is detected, the manipulator is commanded to return to the starting point. The force is then incremented by some predetermined amount and the robot “tries again.” The procedure is repeated until the monitored t current does increase, at which time it is assumed that the part is not slipping and is properly grasped There are obvious difficulties with this technique. The first is that even if the part is successfully raised above the resting surface, there is no guarantee that it will not slip out of the gripper as the manipulator moves. If, in fact this occurs the procedure outlined above will not detect the slippage while the robot is in motion. A second problem is that if a fragile part is to be lifted, the minimum applied gripping force should be small, to avoid crushing. On the other hand, a heavy, more robust part could easily handle a larger initial force. If the mix and order of parts to be lifted were not known a priori, it is possible that either damage could be done to some or that it would take far too long to acquire others. The last difficulty with the technique is that monitoring motor current is not always error free. Care would have to be taken to prevent spikes due to brush noise from being mistaken as a current above the “lifting threshold.” Obviously, the use of brushless motors would reduce this problem but would increase the cost, due to the need for electronic commutation. In addition to the lift-and-try procedure, a number of experimental devices based on optical, magnetic, or conductive sensing techniques have been developed. We now describe briefly several of these slip detectors, which were proposed by groups working in Japan and Yugoslavia

76

INTRODUCTION TO MACHINE VISION Machine vision is concerned with the sensing of vision data and its inter pretation by a computer. The typical vision system consists of the camera and digitizing hardware, a digita] computer, and hardware and software necessary to interface them. This interface hardware and software is often referred to as a preprocessor. The operation of the vision system consists of three functions: 1. Sensing and digitizing image data 2. Image processing and analysis 3. Application The relationships between the three functions are illustrated in the diagram of Fig. 7-1 The sensing and digitizing functions involve the input of vision data by means of a camera focused on the scene of interest. Special lighting techniques are frequently used to obtain an image of sufficient contrast for later process ing. The image viewed by the camera is typically digitized and stored in computer memory. The digital image is called a frame of vision data, and is frequently captured by a hardware device called a frame grabber. These devices are capable of digitizing images at the rate of 30 frames per second. The frames consist of a matrix of data representing projections of the scene sensed by the camera. The elements of the matrix are called picture elements, or pixels. The number of pixels are determined by a sampling process per formed on each image frame. A single pixel is the projection of a small portion of the scene which reduces that portion to a single value. The value is a measure of the light intensity for that element of the scene. Each pixel intensity is converted into a digital value. (We are ignoring the additional complexities involved in the operation of a color video camera.) We will examine the details of machine vision sensing in Sec. 7-2. The digitized image matrix for each frame is stored and then subjected to image processing and analysis functions for data reduction and interpretation of the image. These steps are required in order to permit the real-time application of vision analysis required in robotic applications. Typically an image frame will be thresholded to produce a binary image, and then various feature measurements will further reduce the data representation of the image. This data reduction can change the representation of a frame from several

77

78

CAMERAS · Cameras use available light from a scene. · The light es through a lens that focuses the beams on a plane inside the camera. The focal distance of the lens can be moved toward/away from the plane in the camera as the scene is moved towards/away. · An iris may also be used to mechanically reduce the amount of light when the intensity is too high. · The plane inside the camera that the light is focused on can read the light a number of ways, but basically the camera scans the plane in a raster pattern. · An electron gun video camera is shown below. - The tube works like a standard CRT, the electron beam is generated by heating a cathode to eject electrons, and applying a potential between the anode and cathode to accelerate the electrons off of the cathode. The focussing/deflecting coils can focus the beam using a similar potential change, or deflect the beam using a differential potential. The significant effect occurs at the front of the tube. The beam is scanned over the front. Where the beam is incident it will cause electrons to jump between the plates proportional to the light intensity at that point. The scanning occurs in a raster pattern, scanning many lines left to right, top to bottom. The pattern is repeated some number of times a second - the typical refresh rate is on the order of 30Hz

· Charge Coupled Device (CCD) - This is a newer solid state video capture technique. An array of cells are laid out on a semiconductor chip. A grid like array of conductors and insulators is used to move a collection of charge through the device. As the charge moves, it sweeps across the picture. As photons strike the semiconductor they knock an electron out of orbit, creating a negative and positive charge. The positive charges are then accumulated to determine light intensity. The mechanism for a single scan line is seen below.

79

80

Color video cameras simply use colored filters to screen light before it strikes a pixel. For an RGB scan, each color is scanned 3 times.

FRAME GRABBER · A simple frame grabber is pictured below,

In the system. The frame grabber, representing the third step, is an image storage and computation device which stores a given pixel array. The frame grabber can vary in capability from one which simply stores an image to significant computation capability. 81

In the more powerful frame grabbers, threshold, windowing, and histogram modification calculations can be carried out under computer control. The stored image is then subsequently processed and analyzed by the combination of the frame grabber and the vision controller

SENSING AND DIGITIZING FUNCTION IN MACHINE VISION Our description of the typical machine vision system in the preceding section identified three functions: sensing and digitizing, image processing and analysis, and application. This and the following sections in the chapter will elaborate on these functions. The present section is concerned with the sensing and digitizing aspects of machine vision. Image sensing requires some type of. Image formation device such as a camera and a digitizer which stores a video frame in the computer memory. We divide the sensing and digitizing functions into several steps. The initial step involves capturing the image of the scene with the vision camera. The image consists of relative light intensities corresponding to the various portions of the scene. These light intensities are continuous analog values which must be sampled and converted into digital form.

The second step, digitizing, is achieved by an analog-to-digital (AID) converter. The AID converter is either a part of a digital video camera or the front end of a frame grabber. The choice is dependent on the type of hardware

IMAGE STORAGE Following AID conversion, the image is stored in Computer memory, typically called a frame buffer. This buffer may be part of the frame grabber or in the computer itself. Various techniques have been developed to acquire and access digital images. Ideally, one would want to acquire a single frame of data in real time. The frame grabber is one example of a video data acquisition device that will store a digitized picture and acquire it in -,s. Digital frames are typically quantized to 8 bits per pixel. However a 6—bit buffer is adequate since the average camera system cannot produce 8 bits of noise-free data. Thus the lower-order bits are dropped as a means of noise cleaning. In addition the human eye can only resolve about 26 = 64 gray levels. A combination of row and column counters are used in the frame grabber which are synchronized with the scanning of the electron beam in the camera. 82

Thus, each position on the screen can be uniquely addressed. To read the in formation stored in the frame buffer, the data is “grabbed” via a signal sent from the computer to the address corresponding to a row— column combination. Such frame grabber techniques have become extremely popular and are used frequently in vision systems LIGHTING TECHNIQUE AND DEVICES The fundamental types of lighting devices used in robot vision are classed into the following groups a) Diffuse surface Devices are exemplified by the fluorescent lamps and lighted tables. (b) Condenser projectors : transforms an diverging light source into a focusing light source. (c) Flood or spot projector: used to illuminate object surface areas from all angles. (d) Collimator : is a device which produces parallel beam of light on the object whose image is to be captured. (e) Imagers example slide projectors and optical enlargers produce at the object plane real form of an image. The illumination techniques are many. Some such special cases are listed in table 7.1. along with the application. The objective of illumination is to provide a suitable environment for the camera to provide the realistic images of the object in the work-space. There are basically two major type of illumination techniques: Front lighting and rear lighting. In the front lighting, source of light is on the same side of the camera and in rear lighting technique, the source of light is on the opposite side of the camera

.

83

Illumination Techniques

IMAGE PROCESSING AND ANALYSIS In the industrial applications the algorithms and programs are developed to process the images captured, digitized and stored in the computer memory. The size of data to be processed is huge, of the order of 106 which is to be substantially executed in seconds. The difficult and time consuming task of processing is handled effectively by the following techniques (1) Image data reduction (2) Segmentation (3) Feature extraction (4) Object recognition. Image Data Reduction The purpose of image data reduction is to reduce the volume of data either by ellimination of some or part processing, leading to the following sub-techniques.

84

(a) Digital conversion Digital conversion is characterized by reduction in number of gray levels. For a 8-bit each pixel would have 28=256 gray levels. When fewer bits are used to represent pixel intensity the digital conversion is reduced, to suit the requirements.

SEGMENTATION · An image can be broken into regions that can then be used for later calculations. In effect this method looks for different self contained regions, and uses region numbers instead of pixel intensities.

· A simple segmentation algorithm might be, 85

1. Threshold image to have values of 1 and 0. 2. Create a segmented image and fill it with zeros (set segment number variable to one). 3. Scanning the old image left to right, top to bottom. 4. If a pixel value of 1 is found, and the pixel is 0 in the segmented image, do a flood fill for the pixel onto the new image using segment number variable. 5. Increment segment # and go back to step 3. 6. Scan the segmented image left to right, top to bottom. 7. If a pixel is found to be fully contained in any segment, flood fill it with a new segment as in steps 4 and 5.

EDGE DETECTION · An image (already filtered) can be checked to find a sharp edge between the foreground and background intensities. · Let's assume that the image below has been prefiltered into foreground (1) and background (0). An edge detection step is then performed.

86

OBJECT RECOGNITION Form Fitting · It can sometimes help to relate a shape to some other geometric primitive using compactness, perimeter, area, etc. - ellipse - square - circle - rectangle

Decision Trees · In the event that a very limited number of parts is considered, a decision tree can be used. The tree should start with the most significant features first, then eventually make decisions on the least significant. Typical factors considered are, - area - hole area - perimeter - maximum, minimum and average radius - compactness · An example of a decision tree is given below. (Note: this can be easily implemented with if-then rules or Boolean equations)

87

Bar Codes · Bar codes are a common way to encode numbers, and sometimes letters. · The code is sequential left to right, and is characterized by bars and spaces of varied widths. The bar widths corresponds to a numerical digits. These are then encoded into ASCII characters. · To remain noise resistant there are unused codes in the numerical sequence. If any value scanned is one of the unused values the scan is determined to be invalid. · There are different encoding schemes. Code 39/Codabar - these use bars of two different widths for binary encoding Code 128 - these use different bar widths uses proportional widths to encode a range of values UPC (Universal Product Code) EAN (European Article Numbering) · The example below shows how a number is encoded with a bar code. six distinct categories:

MACHINE VISION APPLICATIONS The reason for interpreting the vision system is to achieve some practical objective in robot application. Machine vision systems are being used increasingly in manufacturing and robot automation to perform various tasks and can be divided into

1. Inspection 2. Part identification 3. Part orientation 4. Part location 5. Visual guidance and control 6. Safety monitoring

88

Inspection is the biggest category for quality control of parts. According to M. P. Groover (1996) it is estimated that inspection constitutes as much is 90 percent of machine vision. Machine vision installations in industry perform a variety of automated in spection tasks, most of which are either on-line/in-process or on-line/postprocess. The applications are almost always in mass production, where the time required to program and set up the vision system c be spread over many thousands of units. One hundred percent inspection is done almost exclusively. Typical industrial inspection tasks include the following: Dimensional measurement or gauging is used for accuracy and geometrical integrity. The parts are measured or gauged by the camera, and the dimensions are calculated and compared with a computer-stored model to determine the size values. Verification of product is used in flexible automated assembly systems to the presence of components in an assembled product. Verification of holes is checked by the vision system for missing holes, location, and number of holes in a part. Identification of flaws, defects, or printed label is checked on the stir- face of a part, which often reveals a change in reflecting light. The vision system can identify the deviation from an ideal model of the surface. In addition to optical methods, various no optical techniques are used in inspection. These include sensor techniques based on electrical fields, radiation, and ultrasonic.

Part identification is used commercially in many applications where the vision system stores data for different pairs in active memory and uses it to recognize or distinguish between parts so that some action can be taken as they enter the work cell. The applications include counting, pan sorting, and character recognition.

Part orientation is used in parts to be gripped in a specified manner by the end effector. The vision system supplies the information and data to drive the gripper into, the correct orientation for pick-or-place action.

Part location is used to locate randomly placed parts on x, y axes. The vision system measures the x and y distances and identifies the center of the camera to coincide with the center of the randomly placed part.

Visual guidance and control involves applications in which a vision system is teamed with a robot or other devices in the robot cell to control the movement of a machine. Examples of these applications include Sean tracking in continuous arc welding, part positioning and/or reorientation, and bin picking.

Safety monitoring involves applications of machine vision in which the vision system is used to monitor the operation of a production cell. Its purpose is to detect irregularities that indicate a condition that is hazardous to equipment or people working in the cell. It can also be used to detect human intruders which might beat risk by wandering into the robot cell.

89

UNIT-IV Robot kinematics and Robot programming Forward Kinematics, Inverse Kinematics and Differences; Forward Kinematics and Reverse Kinematics of Manipulators with Two, Three Degrees of Freedom (In 2 Dimensional), Four Degrees of Freedom (In 3 Dimensional) – Deviations and Problems Teach Pendant Programming, Lead through programming, Robot programming Languages – VAL Programming – Motion Commands, Sensor Commands, End effecter commands, And Simple programs.

Forward kinematics The transformation of coordinates of the end-effector point from the t space to the world space is known as forward kinematic transformation

Reverse kinematics The transformation of coordinates from world space to t space is known as backward or reverse kinematic transformation

Robot Kinematics INTRODUCTION Robot arm kinematics deals with the analytic study of the motion of a robot arm with respect to a fixed reference coordinate system as a function of time. The mechanical manipulator can be modelled as an open loop articulated chain with several rigid links connected in series by either ‘revolute’ or ‘prismatic’ ts driven by the actuators. 90

For a manipulator, (the position and orientation of the end-effector are derived from the given t angles and link parameters, the scheme is called the forward kinematics problem If, on the other hand, the t angles and the different configuration of the manipulator are derived from the position and orientation of the endeffector, the scheme is called the reverse kinematics problem. F 2.1 illustrates the scheme of forward and reverse kinematics.

INPUTS:1 AND_3 :2

Fig: 4.1Forward and inverse kinematics scheme Representing the Position Considering the revolute type of t only, the position of the end-effector can be represented by the t angles, 0 02,...,. 0,, as,

The position of the end-effector can also be defined in world space as,

91

Forward and Reverse Kinematics of Manipulators with 2 Degrees of freedom

92

93

Adding Orientation: A 3-Degree of Freedom Arm in (2D) Two Dimensions The arm we have been modeling is very simple; a two-ted robot arm has little practical value except for very simple tasks. Let us add to the manipulator a modest capability for orienting as well as positioning a part or tool. Accordingly, we will incorporate a third degree of freedom into the previous configuration to develop the RR:R manipulator shown in Fig. 4-5. This third degree of

94

freedomwillrepresent a wrist t. The world space coordinates for the wrist end would

be.

A 4-Degree of Freedom Manipulator in (3D) Three Dimensions Figure 4-6 illustrates the configuration of a manipulator in three dimensions. The manipulator has 4 degrees -of freedom: t I (type T t) allows rotation about the z axis; t 2 (type R) allows rotation about an axis that is perpendicular to the z axis; t 3 is a linear t which is capable of sliding over a certain range; and t 4 is a type R t which allows rotation about an axis that is parallel to the t 2 axis. Thus, we have a TRL: R manipulator. 95

Let us define the angle of rotation of t I to be the base rotation 0; the angle of rotation of t 2 will be called the elevation angle 4.; the length of linear t 3 will be called the extension L (L represents a combination of links 2 and 3); and the angle that t 4 makes with the x — y plane will be called the pitch angle 4.. These features are shown in Fig. 4-6. The position of the end of the wrist, P, defined in the world coordinate system for the robot, is given by

96

One facet of our approach in the preceding analysis which should be noted by the reader is that we separated the orientation problem from the positioning problem. This approach of separating the two problems greatly simplifies the task of arriving at a solution.

97

Robot Programming INTRODUCTION Robots are becoming more powerful, with more sensors, more intelligence, and cheaper components. As a result robots are moving out of controlled industrial environments and into uncontrolled service environments such as homes, hospitals, and workplaces where they perform tasks ranging from delivery services to entertainment. It is this increase in the exposure of robots to unskilled people that requires robots to become easier to program and manage. The flexibility of a robot system comes from its ability to be programmed. How the robot is programmed is a main concern of all robot s. A good mechanical arm can be underutilized if it is too difficult to program. A robot program can be defined as a path in space to be followed by the manipulator, combined with peripheral actions that the work cycle. Examples of the peripheral actions include opening and closing the gripper, performing logical decision making, and communicating with other pieces of equipment in the robot cell. The methods of entering the commands are:  On-line programming  Lead-through programming  Walk-through programming  Off-line programming  Task programming.

ON-LINE PROGRAMMING On-line programming systems (Figure 16.1) uses a teach pendant to direct the robot’s movement which allows trained personnel physically to lead the robot through the desired sequence of events by activating the appropriate pendant button or switch. Position data and functional information are “taught” to the robot, and a new program is written. Taught data is stored in the pendant’s memory then transferred to the robot’s controller. The teach pendant can be the sole source by which a program is established, or it may be used in conjunction with an additional programming console and/or the robot’s controller. When using this technique of teaching or programming, the person performing the teach function can be within the robot’s working envelope, with operational safeguarding devices deactivated or inoperative. There are a number of teach pendant types available, depending on the type of application for which they will be used. If the goal is simply to monitor and control a robotics unit, then a simple control box style is suitable. If additional capabilities, such as on the fly programming are required, more sophisticated boxes should be used.

98

This is a convenient and easy method of programming when tasks are simple and revisions or adjustments can be made on the spot. However, the production line must be stopped during the programming and there are safety issues to consider, as the programmer must work within the robot’s work envelope. Advantages • Easily accessible. Disadvantages • Slow movement of the robot while programming

.

• Program logic and calculations are hard to program • Suspension of production while programming • Cost equivalent to production value • Poorly documented.

99

The Teach Pendant (or Manual Control Pendant) The teach pendant has the following primary functions: • Serve as the primary point of control for initiating and monitoring operations. • Guide the robot or motion device, while teaching locations. • application programs. The Teach Pendant (Figure 16.2) is used with a robot or motion device primarily to teach Robot locations for use: in application programs. The Teach Pendant is also used with custom Applications that employ “teach routine's that pause execution at specified points and allow an Operator to teach * re-teach the robot locations used by the program. There are two styles of Teach Pendants: the programmer’s pendant, which is designed for use while an application is being written and debugged, and the operator’s pendant, which is designed for use during normal system operation. The operator’s pendant has a palm-activated switch, which is connected to the remote emergency stop circuitry of the controller. Whenever this switch is released, arm power is removed from the motion device. To operate the Teach Pendant left hand is put through the opening on the left-hand side of the pendant and the left thumb is used to operate the pendant speed bars. The right hand is used for all the other function buttons. The major areas of the Teach Pendant are: 1. Liquid Crystal Display (LCD) 2. Data Entry Buttons. The data entry buttons are used to input data, normally in response to prompts that appear on the pendant display The data entry buttons include YES/NO, DEL, the numeric buttons, the decimal point and the REC/DONE button, which behaves like the Return or Enter key on a normal keyboard. In many cases, application programs have s press the REC/DONE button to signal that they have completed a task. 3. Emergency Stop Switch. The emergency stop switch on the Teach Pendant immediately halts program execution and turns off arm power. 100

4. LED. The pendant is in background mode when the LED is in not lit and none of the predefined functions are being used. The LED is lit whenever an application program is making use of the Teach Pendant. 5. Mode Control Buttons. The mode control buttons change the state being used to move the robot, switch control between the Teach Pendant and the application programs and enable arm power when necessary. 6. Manual Control Buttons. When the Teach Pendant is in manual mode, these buttons select which robot t will move, or the coordinate axis along which the robot will move. 7. Manual State LEDs. The manual state LEDs indicates the type of manual motion that has been selected. 8. Speed Bars. The speed bars are used to control the robot’s speed and direction. Pressing the speed bar near the outer ends will move the robot faster, while pressing the speed bar near the center will move the robot slower. 9. Slow Button. The slow button selects between the two different speed ranges of the speed bars. 10. Predefined Function Buttons. The predefined function buttons have specific, system- wide functions assigned to them, like display of coordinates, clear error, etc. 11. Programmable Function Buttons. The programmable function buttons are used in custom application programs, and their functions will vary depending upon the program being run. 12. Soft Buttons. The “soft” buttons have different functions depending on the application program being run, or the selection made from the predefined function buttons.

101

Lead-Through Programming or Teaching. This method of teaching uses a proprietary teach pendant (the robot's control is placed in a "teach" mode), which allows trained personnel physically to lead the robot through the desired sequence of events by activating the appropriate pendant button or switch. Position data and functional information are "taught" to the robot, and a new program is written the teach pendant can be the sole source by which a program is established, or it may be used in conjunction with an additional programming console and/or the robot's controller. When using this technique of teaching or programming, the person performing the teach function can be within the robot's working envelope, with operational safeguarding devices deactivated or inoperative.

FIGURE IV:4-3. ROBOT LEAD-THROUGH PROGRAMMING OR TEACHING.

1.Walk-Through Programming or Teaching. A person doing the teaching has physical with the robot arm and actually gains control and walks the robot's arm through the desired positions within the working envelope (Figure IV:4-4). FIGURE IV:4-4. WALK-THROUGH PROGRAMMING OR TEACHING.

102

During this time, the robot's controller is scanning and storing coordinate values on a fixed time basis. When the robot is later placed in the automatic mode of operation, these values and other functional information are replayed and the program run as it was taught. With the walk-through method of programming, the person doing the teaching is in a potentially hazardous position because the operational

safeguarding

devices

are

deactivated

or

inoperative.

3.Off-Line Programming. The programming establishing the required sequence of functional and required positional steps is written on a remote computer console (Figure IV:4-5). Since the console is distant from the robot and its controller, the written program has to be transferred to the robot's controller and precise positional data established to achieve the actual coordinate information for the robot and other equipment. The program can be transferred directly or by cassette or floppy discs. After the program has been completely transferred to the robot's controller, either the lead-through or walk-through technique can be used for obtaining actual positional coordinate information for the robot's axes. FIGURE IV:4-5. OFF-LINE PROGRAMMING OR TEACHING.

103

When programming robots with any of the three techniques discussed above, it is generally required that the program be verified and slight modifications in positional information made. This procedure is called program touch-up and is normally carried out in the teach mode of operation. The teacher manually leads or walks the robot through the programmed steps. Again, there are potential hazards if safeguarding devices are deactivated or inoperative. Advantages • Effective programming of program logics and calculations with state-of-the-art debugging facilities. • Locations are built according to models and this can mean that programmers will have to fine tune programs on-line or utilize sensors. . Effective programming of locations. • Verification of program through simulation and visualization. • Well documented through simulation model with appropriate programs. • Reuse of existing CAD data. • Cost independent of production. Production can continue while programming. • Process tools for instance selection of welding parameters. Disadvantages • Demands investing in an off-line programming system. • Needs extensive training.

Robot Languages and Programming ROBOT LANGUAGES  Robot languages have been developed for ease of control of motions of robots having different structures and geometrical capabilities.  Some of the robot languages have been developed by modifying the existing general- purpose computer languages and some of them are written in a completely new style. 104

 Programming languages have been developed by the pioneer efforts of various researchers at Stanford Artificial Intelligence Laboratory; research laboratories of IBM Corporation, under U.S. Air Force sponsorship, General Electric Co., Unimation and many other robot manufacturers. Some of the programming languages are: WAVE, AL, VAL, AML, MCL, RAIL, HELP, RPL, PAL and ADA WAVE and AL  WAVE, developed at Stanford, demonstrated a robot hand—eye coordination while it was implemented in a machine vision system.  Later a powerful language AL was developed to control robot arms. WAVE incorporated many important features.  Trajectory calculations through coordination of t movements, end-effector positions and touch sensing were some of the new features of WAVE. But the algorithm was too complex and not friendly.  They could not be run in real-time and on-line. On the other hand, trajectory calculations are possible at compile time and they can be modified during run time.  AL has a source language, a translator to generate run ablecode and a run time system for effecting various motions of the robot manipulator.  The syntax of the language can implement various subroutines, invoking activities between the robot and its surroundings, various statements concerning SIGNALS and WAIT to carry on tasks in sequence.  Different sensors can be incorporated and programming can take care of some condition monitoring statements. The robot manipulator movement-commands include various motions, velocities, forces, torques, etc. POINTY is another interactive system and a part of AL system VAL  VAL is a popular textual robot language developed by Unimation Inc. for the PUMA series of robots. VAL has been upgraded to VAL II system with more interlocking facilities.

105

 Victor Sheinman developed VAL language. VAL is very -friendly. It provides aim movement in t, world and tool coordinates, gripping and speed control.  WAIT and SIGNAL commands can be given to implement a specific task. The commands are subroutines written in BASIC and translated with the aid of an interpreter. Compiled BASIC has more flexibility. AML  A manufacturing language, AML was developed by IBM. AML is very useful for assembly operations as different —robot programming interfaces are possible.  The programming language AML is also used in other automated manufacturing systems.  The advantage of using AML is that integers, real numbers and strings can be specified in the same aggregate which is said to be an ordered set of constants or variables.  An aggregate can be used to specify coordinate values of the robot’s ts or wrist position and orientation. MCL  US Air force ICAM project led to the development of another manufacturing control language known as MCL by McDonnel—Douglas.  This is a modification of the popular APT (Automatically Programmed Tooling) language used in CNC machine tools as many similar commands are used to control machine tools in CAM applications.  Lines, circles, planes, cylinders and many other complex geometrical features can be defined in MCL. RAIL  RAIL was developed by Automatic for robotic assembly, inspection, arc welding and machine vision. A variety of data types as used in PASCAL can be used.  An interpreter is used to convert the language into machine language commands. It uses Motorola 68000 type microcomputer system; It s many commands and control of the vision system. 106

HELP  HELP was developed by General Electric Company. It acts more or less like RAIL.  It has the capability to control two robot arms at the same time. The structure of the language is like PASCAL. JARS  JARS was developed by NASA JPL. The base of the language is PASCAL. JARS can be interfaced with PUMA 6000 robot for running robotic programs. 6.1.8 RPL RPL was developed at SRI International. A compiler is used to convert a program into the codes that can be interpreted by an interpreter. Unimation PUMA 500 can be controlled with the help of RPL. The basic ideas of LISP (an Al language) have been organized into a FORTRAN-like syntax in RPL. It is modular and flexible. Besides these, there are some other languages like PAL, ADA etc. PAL has been written by Richard Paul by modifying WAVE and incorporating features of PASCAL. But the representations of syntaxes used in the program are difficult to handle. ADA developed by the Department of Defense (DOD) in USA is a real-time system that can be run on several microcomputers like Zilog, VAX, Motorola 68000, etc. ADA is convenient for controlling the robots used in a manufacturing cell. Different textual robot languages have different attributes. Far example, VAL, HELP and MC though powerful for many simple tasks, do not have the same structured modular programming capability like AL, AML, JARS and ADA or VAL II. In a manufacturing cell, multiple robots or robotic equipment work in unison. Control of two or more operations done by the robots in a coordinated manner is complex. Synchronizing the motions of the robots requires necessary software commands. AL, ADA, AML, MCL have the capability of controlling multiple arms. The programming language must be capable of expressing various geometric features like t angles, coordinate transformations such as rotation, translation, and vector quantities. Homogeneous matrices are used to specify the rotation. Rotation can also be specified by Euler angles. AML, RAIL and VAL use Euler angles while AL manipulates homogeneous matrix for control. AL is very suitable for assembly tasks wherein many sensors are employed, though other languages like AML and

107

HELP are flexible enough to run various subroutines. Slewing and straight- line motions control are available with most of the languages. However, in all the robot languages, features like editor, interpreter, compiler, data management, debugging are common. 6.2 CLASSIFICATION OF ROBOT LANGUAGES Robot languages can be grouped broadly into three major classes: 1. First generation language 2. Second generation language 3. World modelling and task-oriented object level language• The first generation language provides an of f-line programming in combination with the programming through robot pendant teaching. VAL is an example of a first generation robot programming language. The capability of a first generation language is limited to the handling of sensory data (except ON/OFF binary signals) and communication with other computers. However, branching, input/output Interfacing and commands.

108

109

UNIT-5 IMPLIMENTATION AND ROBOT ECONOMICS 110

-

RGV,AGV; Implementation of Robots in Industries Various Steps Safety Considerations for Robot Operations Economic Analysis of Robots

1. Pay back Method 2. EUAC Method 3. Rate of Return Method.

Rail-Guided

Vehicle

[

RGV

]

Daifuku’s Sorting Transfer Vehicle (STV) is a fast, flexible and easily installed material transport system. STVs can be used to move loads of all sizes in a warehouse. For example, a STV may be used as a sorting tool for goods coming out of storage and heading to shipping. STV features sorting and collecting capabilities for multiple AS/RS aisle conveyor stations. It enables picking by order line and sorting by destination to one

111

Fast Moving

STV load transfer

STV picking

STV vs Conveyor

STV

System Stimulation

Separate input/output stations enable the STV to perform multiple tasks at one time. STVs are compact and move agilely over a track system, enabling higher throughput than conveyors. The STV track can be arranged in a loop or straight line to accommodate a variety of applications, such as mixed SKU pallet picking, cycle 112

counting, quality inspection, load sorting and truck loading. Advantages of STVs include: fewer motors, no single point of failure, high-speed, high-throughput and expansion flexibility to handle future growth. Daifuku's STVs are designed for seamless integration with mini load and unit load AS/RS to effectively transport raw materials, work-in-process and/or finished goods. STVs can also integrate with conveyor or ergonomic workstations to facilitate efficient order picking for consolidation and shipping. Features  High speed  Close order traffic control  Multiple layout options (multi-vehicle loop or single-vehicle shuttle)  Variety of models (carton and pallet handling)  Microprocessor control with RF (Radio Frequency) communication  Expandable by adding vehicles  Easy maintenance  Quiet operation  Sort loads to different delivery stations without complex conveyor intersections  Accommodate a variety of building layout, space and performance requirements  Bi-directional movement on single straight track for load pickup and delivery  Modular layout simplifies system modification/expansion. Increase system capacity by simply adding vehicles Benefits  Improved inventory control  Increased accuracy  Increased throughput  Eliminates of single-point failure; system can continue operating if a vehicle fails  Enhanced product protection - smooth transport of items or goods Specifications The STV family includes product variations such as load handling (tote/box vs. pallet), layout (shuttle or loop), input/output/transfer mechanism (roller conveyor, chain conveyor, fork top) and number of vehicles. STV track can be arranged in a loop or straight line to accommodate a variety of uses such as mixed SKU pallet picking, cycle counting, quality inspections, load sorting and truck loading. STV's advantages over conveyor delivery systems include: fewer motors, no single point of failure, high speed, high throughput and flexibility to expand to handle future growth.    

Standard load weight capacity: 50-1,500 kg (110-3,300 lbs.) Standard horizontal speed: 100-200 m/min. (323-656 ft./min.) Standard transfer speed: 12-30 m/min. (39-98 ft./min.) Standard load footprint: 300-1500 mm x -350-1500 mm (11.8-59.1 in. x 13.8-59.1 in.)

Primary Problems Addressed: 

Floor space o



Need more space for production

Productivity 113





o o

Limited capacity in storage and production areas Lack of responsiveness to product orders

o o

Poor product accessibility No real-time product tracking (WIP buildup)

Inventory

Ergonomics o o o o o





Workers in hostile environments Workers walking long distances Excessive noise levels from material handling equipment Safety hazards Labor-intensive processes Disted Operations o Multiple staging areas o Product is often moved from one staging area to another o Many Non-Value Added operations Labor o High employee turnover o High labor costs o Small labor pool o Low job satisfaction

Applicable Industries:           

Apparel Automotive Beauty & Cosmetics Book & Publication Distribution Electronics Food & Beverage Household Appliances Pharmaceuticals & Healthcare Retail Semiconductor Textiles

MATERIAL HANDLING AND AUTOMATED GUIDED VEHICLES [

AGV ]

.1 INTRODUCTION The flow of material through the processes is an important element in the manufacturing system. Some type of automation is essential for handling of materials intra-shop and inter-shop in order to reduce labour cost and fulfil many other functions. It is also required to carry raw materials from stores to various work cells and 114

the finished product to the warehouse. The material handling element needs to be synchronized with work stations and different operations under a hierarchical computer control system capable of applying decision logic to the movement of materials. Automated guided vehicles (AGV’s) may be considered as one of the solutions for effective and economical material handling.

.2 MATERIAL HANDLING

Material handling may be defined as the functions and systems associated with the transportation, storage, and physical control to work-in-process material in manufacturing. From a system approach, it can be defined as “using the right method to provide safely the right amount of the right material at the right place, at the right time, in the right sequence, in the right position, in the right condition, and the right cost.”

The purpose of material handling in a factory is to move raw materials, work-in-process, finished parts, tools, and supplies from one location to another to facilitate the overall operations of manufacturing. The material control function is concerned with the identification of the various materials in the handling system, their routings, and the scheduling of their moves. In most factory o it is important that the origin, current location, and future destination of materials

be known. This function is achieved through the shop floor control system by maintaining accurate, complete, and current records of all materials in the factory. This control is sometimes augmented by means of automatic identification system whose purpose is to identify parts as they are moved or stored. Material Handling Systems The material handling equipment may be categorized in many ways. One of the methods is: the source of the motive force and the synchronization of the load movements. In path motive handling equipment, the force applied to move the loads is provided by the transport path itself, e.g. an escalator and cable car system. In vehicle motive equipment, the force applied to move the loads is provided by a vehicle or load platform, e.g. trucks, trains, buses, and automobiles. In synchronous transport system, all loads moving on the transport network or on a self-con tained segment of the network move simultaneously, at the same speed, and with a constant space between loads. In a non synchronous transport system, the loads can move independently of one another, upto the physical limits of the system, e.g. a system of trucks, trolley cars, and taxis. The synchronous v non-synchronous classification is of most concern in assembly systems.

Material Handling Equipment 115

The material handling equipment can be divided as: (a) Manually operated devices—hand trucks, powered trucks, cranes, monorails and hoists. (b) Automated systems—conveyors, AGV’s. (c) Miscellaneous systems—industrial robots, transfer mechanisms, elevators, pipelines, containers, dial indexing tables, etc. 1. Hand trucks— These are platforms with wheels for manual movement of items, unit loads, and bulk materials, e.g. wheelbarrows, two and four wheeled trucks, hand lift or manually operated fork—lift trucks. 2. Powered trucks— These are powered vehicles with platform for mechanized movement of items, unit loads, and bulk materials. They are driven by human beings, powered by battery, diesel, or petrol, e.g. walkie trucks, riding trucks, forklift trucks, side loaders, tractor trailer trains, and industrial crane trucks. 3. Cranes, monorails, and hoists— These handling devices are usually manually operated, designed for lifting, lowering, and transporting heavy objects, e.g. bridge cranes, gantry cranes, jib cranes, overhead monorails, hand and powered hoists. 4. Conveyors—. It is a large family of handling devices, often mechanized, sometimes automated, designed to move materials between specific locations over a fixed path, generally in large quantities or volumes. Examples include gravity conveyors (chutes, rollers) and powered conveyors (rollers, belt, chain, overhead, in-floor tow, and cart-on-track). 5. Automated Guided Vehicles (AGV’s) Systems— These are battery-powered, auto matically steered vehicles designed to follow defined pathways. Some are capable of automat ically loading and unloading unit loads. They are usually interfaced with other automated systems to achieve full benefits of integrated automation. Examples include driverless trains, pallet trucks, and unit load carriers. Principles of Material Handling The principles for material handling are: 1. Unit load principle Materials to he moved should be aggregated into a larger unit size, and the unit size should be the same for all materials. The materials are typically placed on a pallet or other standard-sized container for convenience in handling. The materials and container are referred to as the unit load. The unit load should be as large as practical. 2. Avoid partial loads : Transport the full unit load whenever possible rather than partial loads. Load the material handling equipment to its maximum safe limit. 3. Shortest distance principle : Movements of materials should be over the shortest distances possible. This depends on the plant layout design. 116

4. Straight-line flow rule : The material handling path should be in a straight line from the point of origination to point of destination: This rule is consistent with the shortest distance principle. 5. Minimum terminal time principle: Movement of a unit load consists of the move time plus the time required for loading, unloading, and other activities that do not involve actual tnznsport of the materials. Minimize these nonmove times. 6. Gravity principle : Use gravity to assist the movement of materials to the extent possible, at the same time giving consideration to safety and risk of product damage. 7. Carry loads both ways : The handling system should be designed and scheduled, to the extent possible, to carry loads in both directions. Return trips with empty loads are wasteful. 8. Mechanization principle Manual handling of materials should be avoided. The handling process should be mechanized where possible to increase efficiency and economy. 9. Systems principles Integrate the materials handling system with other systems in the facility, including receiving, inspection, storage, production and assembly, packaging, warehousing, shipping, and transportation. 10. Systems flow principle Integrate the flow of materials with the flow of information in handling and storage systems. The information for each item moved should include identifica tion, origination (pick up) point, and destination point. 11. Part orientation principle : In automated production systems, the orientation of the workpart should be established and maintained throughout the material handling process. Selection of Material Handling Equipment When selecting material handling equipment, due regard should be given to the follotng 1. Low maintenance costs— Few moving parts, sealed-for-life bearings and gear boxes, accessibility for maintenance, controlled diagnostics for electrical and mechanical components, use of standard, readily available components and equipment. 2. Compliance with safety standards— Adequate guarding, provision of emergency stop facilities, incorporation of mechanical and control safety locks together with all necessary audio/visual warnings. a. Operational environment— In addition to temperature and humidity, consider the functional impact on equipment of such things as abrasive dust, excessive oils and grease, swarf, fire, floods and explosive hazards. 4. Buffer storage— Consideration must be given to the possible need for “buffer storage” in order to maximize production equipment utilisation and ensure continuous operation. 5. Effective use of operating time— The maximization of operating time of high cost dedicated units, e.g. robots, is an important factor in the development of an economic manufacturing system. Material Storage 117

Material storage is inescapable in the manufacturing system. Storage is required for work-in-process parts and delivery of material taken in excess of our immediate needs. As the complexity of manufactured products increases and as the number of options needed increases due to competitive factors, storage needs increase geometrically. Storage may be classified as static storage or live storage. Static storage systems provide only a storage medium, without any provision for moving or handling the loads to be stored. Line storage systems, on the other hand, integrate the storage medium with internal material handling devices and possibly an interface to the work-in-process transport system. Generally speaking, live line storage systems are more appropriate when space is at a andior when the amount of work-inprocess and the storage/retrieval transaction rates are large. (a) Static storage systems. Static storage systems provide only a storage space, thus requiring that the items to be stored are inserted and retrieved by other means. The static storage systems are: load location on the floor, load stand, shelves, bins, etc. (b) Live storage systems. Live storage systems provide not only storage locations, but also the mechanism for inserting and retrieving the storage loads. Some examples of live storage are : automatic work changer, carousel conveyor, automated storage/retrieval systems (AS/RS), microload AS/RS, flow rack. (c) Automated storage/Retrieval systems (AS/RS). In the AS/ material is delivered to the door of the factory, checked, entered into the inventory control system through readers, physically transferred to the storage area by machine, and stored in the appropriate location without ever having been touched by a human. When a shop order reaches the storage area, the proper quantities of each subassembly or part are removed from their individual locations, conglomerated (picked) and transported to the desired location, again with automated machinery. The process of taking the individual parts or subassemblies and package them into assembly units, is called kitting. Automation Equipment For the modern factory we need programmable equipment. The easiest challenge is to make equipment to transport the same part to the same place forever. To satis& the requirements for different configurations and options, it is necessary to build versatility and flexibility into the system. For this purpose, we rely on the computers to do much of the job. The equipment must be versatile, programmable, and must be able to fit into the computer network. AUTOMATED GUIDED VEHICLES - AGVs An AGV is a computer controlled, driverless vehicle used for transporting materials from point-to-point in a manufacturing setting. They represent a major category of automated materials handling devices. They are guided along defined pathways in the floor. The vehicles are powered by means of on-board batteries that allow operation for several hours between recharging. The definition of the pathways is generally 118

accomplished using wire embedded in the floor or reflective paint on the floor surface. Guidance is achieved by sensors on the vehicles that can follow the guide wires or paint. When it arrives at the proper destination, the material is off loaded onto another conveyor or the workstation. The vehicle is then dispatched to the next location or to home to await further orders. A computer controls its motion. The key in AGV are Guide path — The term guide path refers to the actual path the AGV follows in making its rounds through manufacturing plant. The guide path may be of the embedded wire type or optical devices. Routing — It is the ability of the AGV to make decisions that allow it to select the appropriate route as it moves across the shop floor. Traffic management — This is the method to prevent collisions and to optimize traffic flow and traffic patterns of the AGV. The devices used for this purpose are: shop signs, yield signs, caution lights, and stop lights. Types of AGV’s

Driver tractor tram. 1. Towing vehicles — These are the most widely used type of AG V’s and are called the work horse. They are most commonly used for transporting large amounts of bulky and heavy materials from the warehouse to various locations in the manufacturing plant, e.g. driverless train

2. Unit

load

vehicles

—

They are used in settings with short guide paths, high volume, and need for independent movement and versatility. Warehouses and distribution centres are the most likely settings for these vehicles. They can operate in an en vironinent where there is not much room and movement is restricted. Fig. 13.2 shows a unit load type AGV

119

High lift guided vehicle. 3. Pallet trucks —

They can

be operated manually and are used most frequently for material handling and distribution systems. They are driven along a guide path from location-to-location and are unloaded as they go. Their frequent use is to move palletized loads. 4. Fork trucks — They are used when it is necessary to pick materials up at the shop floor level and move it to a location at a higher level and vice versa. They travel along a guide path. Fig. 13.3 shows a high lift guided vehicle. 5. Light load Vehicles— They are used in manufacturing settings where the material to be moved is neither heavy nor bulky (

1 kN). The most common application of the light load vehicle AGV is in electronics

manufacturing settings. 6. Assembly line vehicles — This type of AGV’s are used in conjunction with an assembly line process, like automobiles, to transport major subassemblies. Guidance of AGV’s The guidance of AGV’s may be of the following types: 1. Manual guidance. 2. Computer/optical guidance. 3. Self guidance. The optically guided AGV’s rely on sensing devices that follow a highly reflective line along the shop floor. The line may be painted on the floor or a special reflective tape may be used. In either case, the line is liable to be obstructed by dirt, debris, or normal wear, and the system becomes unreliable. Self-guidance represents the optimum in AGV technology. When self-guidance of AGV’s is accomplished, this technology will have achieved its fullest potential. There are two approaches used for steering AGV’s: 1. Differential speed steer control. 2. Steered-wheel steer control. An AGV with a differential speed steer control system uses two independent fixed wheel drives on either side of the vehicle. This allows the drive on one side to operate at a faster speed than the drive on the other side, thereby causing the vehicle to turn. The amplitude detection guidance sensor feeds required data to the drive systems so that they know when to turn. An AGV using the differential speed steer control system has left and 120

right sensing devices mounted on the front of the vehicle. These sensors receive information for left or right turns. The amplitude detection guidance sensor balances the signals received from the left and right sensing devices . This is used with assembly line vehicles, unit load vehicles, fork trucks, and light load vehicles.

MATERIAL HANDLING AND AUTOMATED GUIDED VEHICLES The AGV’s using steered-wheel steer control have a single front wheel that rides along the paint or tape strip that marks the guide path along the shop floor. A phase detection guidance sensor detects whether the wheel is on course, steering to the left or right of the guide path, sends data back to AGV, and the fron wheel turns to compensate so that the AGV is in phase. This is used with assembly-line vehici t towing vehicles, unit load vehicles, pallet trucks, fork trucks, and light load vehicles. AGV System Management Fig. 13.4 illustrates an AGV system.

An AGV system.

There are three key issues in AGV system management. These are traffic control, vehicle dispatch, and system monitoring. 1. Traffic Control There are three methods for traffic control. (a) Zone control - The zone control system (Fig. 13.5) is the most widely used in which the guide path areas of the shop floor are divided into zones. Only one AGV is allowed in a given zone at a time. (b) Forward sensing control - This is a traffic control system that works well when most of the guide paths are straight. With this traffic control system (Fig. 13.6), each AGV has one or more sensing devices mounted on its front. There is a specified range or distance that must be maintained between AGV’s. The sensing devices continuously “look” forward to sense any object that falls within the specified range. If one vehicle gets too close to another vehicle, the sensors will direct that vehicle and cause the one in the rear to stop.

121

Forward sensing control system. (c) Combination control - It is used when the guide path system contain3 long stretches of straight guide path but still has intermittent curves and/or intersections. 2. Vehicle Dispatch There are five methods for dispatching an AGV (a) Onboard dispatch - In this approach, each AGV has a control mounted on it. A human operator uses this control to program the AGV stops along the guide path. The control can also be used to program the AGV activities at each stop. (b) Ofiboard Call Systems - These systems are used in situations where the process of transferring materials from the AGV to a stop location is automated. (c) Remote terminal - In this method, a human operator moves the AGV’s around the shop floor, using the computer as an aid in doing so. (d)Central terminal

vehicle dispatch is called central computer dispatch. In this method, the M are

programmed by computer to move along the guide path and make the necessary stops. (e) Combination dispatch - In this method, two or more methods described above are combined to dispatch the AGV. 3. System Monitoring An ideal monitoring system is one that gives human operators instant, real-time , on the following 1. Location of all vehicles within the system. 2. Location of malfunctioning or inoperative vehicles. 3. Movement of vehicles. 4. Amount of time vehicles spend at each stop and enroute between stops. 5. Status of all vehicles in the system, loaded or unloaded. 6. Destination of all vehicles within the system. 7. Status of the batteries in all vehicles within the system : charged, charging, or week. There are three types of systems widely used for monitoring AGV systems ; as described below (a) Local Monitoring The is a series of lights within zones on the that correspond with zones on the shop floor. When an AGV enters a given zone, the light on the corresponding to that zone illuminates. When the AGV 122

moves out of that zone, the light goes out and the light for the next zone illuminates. This monitoring method does not tell which AGV, whether the vehicle is loaded or unloaded, what the vehicle destination is, or whether the battery of the vehicle is charged or getting week. (b) Computer Display Monitoring In the computer display system, the monitoring data can be displayed on a computer terminal in two forms : graphic or alphanumeric. This method gives the human operator all the information needed to take necessary action when a breakdown occurs. (c) Performance Reports Monitoring Performance reports are logged records of the type of information displayed on the computer terminal with computer display monitoring of AGV systems. These reports give the historical record of the performance of each AGV within the system.

Components of an AGV The essential components of an AGV are:  Mechanical structure  Driving and steering mechanism actuators  Servo controllers  On board computing facility  Servo amplifiers  Feed back components  On board power system. Applications of AGV’S The applications of AGV’s are in the following categories: 1. Driverless train operations—for movement of large quantities of materials over relatively large distances. 2. Storage/distribution systems—unit load carriers and pallet trucks are used in these applications by interfacing with AS/RS in a distribution system. This can also be applied in light manufacturing and assembly operations. 3. Assembly live operations. Between the workstations, components are kitted and placed on the vehicle for the assembly operations that are to be performed on the partially completed product at the next station. 4. Flexible manufacturing systems. The AGV’s are used as the materials handling system in the FMS. The vehicles deliver work from the staging area to the individual work stations in the system and between stations in the manufacturing system.

123

5. Miscellaneous applications—such as mail delivery in office buildings and hospital material handling operations. Advantages of AGV’s The important advantages of AGV’s are:  AGV’s represent a flexible approach to materials handling as they can be computer controlled.  They decrease labour costs by decreasing the amount of human involvement in materials handling.  They can operate in hazardous environments.  They are compatible with production and storage equipment.  They can handle and transport hazardous materials safely.  6. Reduction in downtime of machines due to timely availability of materials.  7. Improvement in productivity and profit.  8. Continuous work without intruptions.

INDUSTRIAL ROBOTS AND ROBOT SYSTEM SAFETY

I.

INTRODUCTION. Industrial robots are programmable multifunctional mechanical devices designed to move material, parts, tools, or specialized devices through variable programmed motions to perform a variety of tasks. An industrial robot system includes not only industrial robots but also any devices and/or sensors required for the robot to perform its tasks as well as sequencing or monitoring communication interfaces. Robots are generally used to perform unsafe, hazardous, highly repetitive, and unpleasant tasks. They have many different functions such as material handling, assembly, arc welding, resistance welding, machine tool load and unload functions, painting, spraying, etc. See Appendix IV:4-1 for common definitions. Most robots are set up for an operation by the teach-and-repeat technique. In this mode, a trained operator (programmer) typically uses a portable control device (a teach pendant) to teach a robot its task manually. Robot speeds during these programming sessions are slow. This instruction includes safety considerations necessary to operate the robot properly and use it automatically in conjunction with other peripheral equipment. This instruction applies to fixed industrial robots and robot systems only. See Appendix IV:4-2 for the systems that are excluded. A.

ACCIDENTS: PAST STUDIES. 1.

Studies in Sweden and Japan indicate that many robot accidents do not occur under normal operating conditions but, instead during programming, program touch-up or refinement, maintenance, repair, testing, setup, or adjustment. During many of these operations the operator, programmer, or corrective maintenance worker may 124

temporarily be within the robot's working envelope where unintended operations could result in injuries. 2.

Typical accidents have included the following: 

B.

II.

A robot's arm functioned erratically during a programming sequence and struck the operator.  A materials handling robot operator entered a robot's work envelope during operations and was pinned between the back end of the robot and a safety pole.  A fellow employee accidentally tripped the power switch while a maintenance worker was servicing an assembly robot. The robot's arm struck the maintenance worker's hand. ROBOT SAFEGUARDING. 1.

The proper selection of an effective robotic safeguarding system should be based upon a hazard analysis of the robot system's use, programming, and maintenance operations. Among the factors to be considered are the tasks a robot will be programmed to perform, start-up and command or programming procedures, environmental conditions, location and installation requirements, possible human errors, scheduled and unscheduled maintenance, possible robot and system malfunctions, normal mode of operation, and all personnel functions and duties.

2.

An effective safeguarding system protects not only operators but also engineers, programmers, maintenance personnel, and any others who work on or with robot systems and could be exposed to hazards associated with a robot's operation. A combination of safeguarding methods may be used. Redundancy and backup systems are especially recommended, particularly if a robot or robot system is operating in hazardous conditions or handling hazardous materials. The safeguarding devices employed should not themselves constitute or act as a hazard or curtail necessary vision or viewing by attending human operators.

HAZARDS. The operational characteristics of robots can be significantly different from other machines and equipment. Robots are capable of high-energy (fast or powerful) movements through a large volume of space even beyond the base dimensions of the robot (see Figure IV:4-6). The pattern and initiation of movement of the robot is predictable if the item being "worked" and the environment are held constant. Any change to the object being worked (i.e., a physical model change) or the environment can affect the programmed movements.

A ROBOT'S WORK ENVELOPE.

125

Some maintenance and programming personnel may be required to be within the restricted envelope while power is available to actuators. The restricted envelope of the robot can overlap a portion of the restricted envelope of other robots or work zones of other industrial machines and related equipment. Thus, a worker can be hit by one robot while working on another, trapped between them or peripheral equipment, or hit by flying objects released by the gripper. A robot with two or more resident programs can find the current operating program erroneously calling another existing program with different operating parameters such as velocity, acceleration, or deceleration, or position within the robot's restricted envelope. The occurrence of this might not be predictable by maintenance or programming personnel working with the robot. A component malfunction could also cause an unpredictable movement and/or robot arm velocity. Additional hazards can also result from the malfunction of, or errors in, interfacing or programming of other process or peripheral equipment. The operating changes with the process being performed or the breakdown of conveyors, clamping mechanisms, or process sensors could cause the robot to react in a different manner. I.

II.

TYPES OF ACCIDENTS. Robotic incidents can be grouped into four categories: a robotic arm or controlled tool causes the accident, places an individual in a risk circumstance, an accessory of the robot's mechanical parts fails, or the power supplies to the robot are uncontrolled. 1.

Impact or Collision Accidents. Unpredicted movements, component malfunctions, or unpredicted program changes related to the robot's arm or peripheral equipment can result in accidents.

2.

Crushing and Trapping Accidents. A worker's limb or other body part can be trapped between a robot's arm and other peripheral equipment, or the individual may be physically driven into and crushed by other peripheral equipment.

3.

Mechanical Part Accidents. The breakdown of the robot's drive components, tooling or end-effector, peripheral equipment, or its power source is a mechanical accident. The release of parts, failure of gripper mechanism, or the failure of end-effector power tools (e.g., grinding wheels, buffing wheels, deburring tools, power screwdrivers, and nut runners) are a few types of mechanical failures.

4.

Other Accidents. Other accidents can result from working with robots. Equipment that supplies robot power and control represents potential electrical and pressurized fluid hazards. Ruptured hydraulic lines could create dangerous high-pressure cutting streams or whipping hose hazards. Environmental accidents from arc flash, metal spatter, dust, electromagnetic, or radio-frequency interference can also occur. In addition, equipment and power cables on the floor present tripping hazards.

SOURCES OF HAZARDS. The expected hazards of machine to humans can be expected with several additional variations, as follows. 1.

Human Errors. Inherent prior programming, interfacing activated peripheral equipment, or connecting live input-output sensors to the microprocessor or a peripheral can cause dangerous, unpredicted movement or action by the robot from 126

human error. The incorrect activation of the "teach pendant" or control is a frequent human error. The greatest problem, however, is overfamiliarity with the robot's redundant motions so that an individual places himself in a hazardous position while programming the robot or performing maintenance on it.

III.

2.

Control Errors. Intrinsic faults within the control system of the robot, errors in software, electromagnetic interference, and radio frequency interference are control errors. In addition, these errors can occur due to faults in the hydraulic, pneumatic, or electrical subcontrols associated with the robot or robot system.

3.

Unauthorized Access. Entry into a robot's safeguarded area is hazardous because the person involved may not be familiar with the safeguards in place or their activation status.

4.

Mechanical Failures. Operating programs may not for cumulative mechanical part failure, and faulty or unexpected operation may occur.

5.

Environmental Sources. Electromagnetic or radio-frequency interference (transient signals) should be considered to exert an undesirable influence on robotic operation and increase the potential for injury to any person working in the area. Solutions to environmental hazards should be documented prior to equipment start-up.

6.

Power Systems. Pneumatic, hydraulic, or electrical power sources that have malfunctioning control or transmission elements in the robot power system can disrupt electrical signals to the control and/or power-supply lines. Fire risks are increased by electrical overloads or by use of flammable hydraulic oil. Electrical shock and release of stored energy from accumulating devices also can be hazardous to personnel.

7.

Improper Installation. The design, requirements, and layout of equipment, utilities, and facilities of a robot or robot system, if inadequately done, can lead to inherent hazards.

INVESTIGATION GUIDELINES. .

MANUFACTURED, REMANUFACTURED, AND REBUILT ROBOTS. 1.

All robots should meet minimum design requirements to ensure safe operation by the . Consideration needs to be given to a number of factors in deg and building the robots to industry standards. If older or obsolete robots are rebuilt or remanufactured, they should be upgraded to conform to current industry standards.

2.

Every robot should be designed, manufactured, remanufactured, or rebuilt with safe design and manufacturing considerations. Improper design and manufacture can result in hazards to personnel if minimum industry standards are not conformed to on mechanical components, controls, methods of operation, and other required information necessary to insure safe and proper operating procedures. To ensure that robots are designed, manufactured, remanufactured, and rebuilt to ensure safe operation, it is recommended that they comply with Section 4 of the ANSI/RIA R15.06-1992 standard for Manufacturing, Remanufacture, and Rebuild of Robots. 127

I.

INSTALLATION. 1.

A robot or robot system should be installed by the s in accordance with the manufacturer's recommendations and in conformance to acceptable industry standards. Temporary safeguarding devices and practices should be used to minimize the hazards associated with the installation of new equipment. The facilities, peripheral equipment, and operating conditions which should be considered are:       

2.

IV.

Installation specifications; Physical facilities; Electrical facilities; Action of peripheral equipment integrated with the robot; Identification requirements; Control and emergency stop requirements; and Special robot operating procedures or conditions.

To ensure safe operating practices and safe installation of robots and robot systems, it is recommended that the minimum requirements of Section 5 of the ANSI/RIA R15.061992, Installation of Robots and Robot Systems be followed. In addition, OSHA's Lockout/Tagout standards (29 CFR 1910.147 and 1910.333) must be followed for servicing and maintenance.

CONTROL AND SAFEGUARDING PERSONNEL. For the planning stage, installation, and subsequent operation of a robot or robot system, one should consider the following. 0.

RISK ASSESSMENT. At each stage of development of the robot and robot system a risk assessment should be performed. There are different system and personnel safeguarding requirements at each stage. The appropriate level of safeguarding determined by the risk assessment should be applied. In addition, the risk assessments for each stage of development should be documented for future reference.

1.

SAFEGUARDING DEVICES. Personnel should be safeguarded from hazards associated with the restricted envelope (space) through the use of one or more safeguarding devices:     

Mechanical limiting devices; Nonmechanical limiting devices; Presence-sensing safeguarding devices; Fixed barriers (which prevent with moving parts); and Interlocked barrier guards.

128

2.

AWARENESS DEVICES. Typical awareness devices include chain or rope barriers with ing stanchions or flashing lights, signs, whistles, and horns. They are usually used in conjunction with other safeguarding devices.

3.

SAFEGUARDING THE TEACHER. Special consideration must be given to the teacher or person who is programming the robot. During the teach mode of operation, the person performing the teaching has control of the robot and associated equipment and should be familiar with the operations to be programmed, system interfacing, and control functions of the robot and other equipment. When systems are large and complex, it can be easy to activate improper functions or sequence functions improperly. Since the person doing the training can be within the robot's restricted envelope, such mistakes can result in accidents. Mistakes in programming can result in unintended movement or actions with similar results. For this reason, a restricted speed of 250 mm/§ or 10 in/§ should be placed on any part of the robot during training to minimize potential injuries to teaching personnel. Several other safeguards are suggested in the ANSI/RIA R15.06-1992 standard to reduce the hazards associated with teaching a robotic system.

4.

OPERATOR SAFEGUARDS. The system operator should be protected from all hazards during operations performed by the robot. When the robot is operating automatically, all safeguarding devices should be activated, and at no time should any part of the operator's body be within the robot's safeguarded area. For additional operator safeguarding suggestions, see the ANSI/RIA R15.06-1992 standard, Section 6.6.

5.

ATTENDED CONTINUOUS OPERATION. When a person is permitted to be in or near the robots restricted envelope to evaluate or check the robots motion or other operations, all continuous operation safeguards must be in force. During this operation, the robot should be at slow speed, and the operator would have the robot in the teach mode and be fully in control of all operations. Other safeguarding requirements are suggested in the ANSI/RIA R15.06-1992 standard, Section 6.7.

6.

MAINTENANCE AND REPAIR PERSONNEL. Safeguarding maintenance and repair personnel is very difficult because their job functions are so varied. Troubleshooting faults or problems with the robot, controller, tooling, or other associated equipment is just part of their job. Program touchup is another of their jobs as is scheduled maintenance, and adjustments of tooling, gages, recalibration, and many other types of functions. While maintenance and repair is being performed, the robot should be placed in the manual or teach mode, and the maintenance personnel perform their work within the safeguarded area and within the robots restricted envelope. Additional hazards are present during this mode of operation because the robot system safeguards are not operative. To protect maintenance and repair personnel, safeguarding techniques and procedures as stated in the ANSI/RIA R15.06-1992 standard, Section 6.8, are recommended.

7.

MAINTENANCE. Maintenance should occur during the regular and periodic inspection program for a robot or robot system. An inspection program should include, but not be limited 129

to, the recommendations of the robot manufacturer and manufacturer of other associated robot system equipment such as conveyor mechanisms, parts feeders, tooling, gages, sensors, and the like. These recommended inspection and maintenance programs are essential for minimizing the hazards from component malfunction, breakage, and unpredicted movements or actions by the robot or other system equipment. To ensure proper maintenance, it is recommended that periodic maintenance and inspections be documented along with the identity of personnel performing these tasks. 8.

SAFETY TRAINING. Personnel who program, operate, maintain, or repair robots or robot systems should receive adequate safety training, and they should be able to demonstrate their competence to perform their jobs safely. Employers can refer to OSHA's publication 2254 (Revised), "Training Requirements in OSHA Standards and Training Guidelines."

9.

GENERAL REQUIREMENTS. To ensure minimum safe operating practices and safeguards for robots and robot systems covered by this instruction, the following sections of the ANSI/RIA R15.06-1992 must also be considered:

ECONOMIC ANALYSIS FOR ROBOTICS In addition to the technological considerations involved in applications engineering for a robotics project, there is also the economic issue. Will the robot justify itself economically? The economic analysis for any proposed engineering project is of considerable importance in most companies because management usually decides whether to install the project on the basis of this analysis. In the present chapter, we consider the economic analysis of a robot project. We discuss the various costs and potential benefits associated with the robot installation, and we describe several methods for analyzing these factors to determine the economic merits of the project.

ECONOMIC ANALYSIS: BASIC DATA REQUIRED To perform the economic analysis of a proposed robot project, certain basic information is needed about the project. This information includes the type of project being considered, the cost of the robot installation, the production cycle time, and the savings and benefits resulting from the project. Type of Robot Installation There are two basic categories of robot installations that are commonly encountered. The first involves a new application. This is where there is no existing facility. Instead, there is a need for a new facility, and a robot 130

installation represents one of the possible approaches that might be used to satisfy that need. In this case, the various alternatives are compared and the best alternative is selected, assuming it meets the company’s investment criteria. The second situation is the robot installation to replace a current method of operation. The present method typically involves a production operation that is performed manually, and the robot would be used somehow to substitute for the human labor. In this situation, the economic justification of the robot installation often depends on how inefficient and costly the manual method is, rather than the absolute merits of the robot method. In either of these situations, certain basic cost information is needed in order to perform the economic analysis. The following subsection discusses the kinds of cost and operating data that are used to analyze the alternative investment projects. The methods by which the analysis is accomplished are explained later in the chapter Cost Data Required for the Analysis The cost data required to perform the economic analysis of a robot project divide into two types: investment costs and operating costs. The investment Table 12-1 Direct costs associated with robot project A. Investment cotts 1. Robot purchase cost—The basic price of the robot equipped from the manufacturer with the proper options (excluding end effector) to perform the application. 2. Engineering costs— The costs of planning and design by the company’s engineering staff to install the robot. 3. Installation costs— This includes the labor and materials needed to prepare the installation site (provision for utilities, floor preparation, etc.). 4. Special tooling— This includes the cost of the end eflector, parts position and other fixtures and tools required to operate the work cell, 5. Miscellaneous costs—This covers the additional investment costs not included by any of the above categories (e.g., other equipment needed for the cell). B. Operating costs and savings 6. Direct labor cost—The direct labor cost associated with the operation of the robot cell. Fringe benefits are usually included in the calculation of direct labor rate, but other overhead costs are excluded. 7. Indirect labor cost—The indirect labor costs that can be directly allocated to thc operation of the robot cell. These costs include supervision, setup, programming, and other personnel costs not included in category 6 above.

131

8. Maintenance—This covers the anticipated costs of maintenance and repair for the robot cell. These costs are included under this separate heading rather than in category 7 because the maintenance costs involve not only indirect labor (the maintenance crew) but also materials (replacement parts) and service calls by the robot manufacturer. A reasonable ‘rule of thumb” in the absence of better data is that the annual maintenance cost for she robot will be approximately 10 percent of the purchase price (category I). 9. Utilities—this includes the cost of utilities to operate the robot cell (e.g., electricity, air pressure, gas). These are usually minor costs compared to the above items. 10. Training—Training might be considered to be an investment cost because much of the training required for the installation will occur as a first cost of the installation. However, training should he a continuing activity, and so it is included as an operating cost. Costs include the purchase cost of the robot and the engineering costs associated with its installation in the work cell. In many robot application projects. the engineering costs can equal or exceed the purchase cost of the robot. Table 12—I presents a list of the investment costs typically encountered in robot projects. The operating costs include the cost of any labor needed to operate the cell, maintenance costs, and other expenses associated with the robot cell operation. lhe table lists most of the major operating costs for a robot application project. In the case of the operating costs, it is often convenient to identify the cost savings that will result from the use of a robot as compared to an existing method, rather than to separately identity the operating costs of the alternative methods. Material savings, scrap reductions. and advantages resulting from more consistent quality arc examples of these savings. Items 6 through loin Table 12-I should he interpreted to allow for this possible method of declaring cost savings between the alternatives. The manner in which these investment costs and operating costs play out over the life of the robot installation can he conceptualized as illustrated in Fig. 12-1. At the beginning of the project, the investment costs are being paid into the project with no immediate return. When the installation is completed and the project begins operation, the operating costs begin. I leveler, there is also a compensating cash 11 representing revenues to the company which should exceed the amount of the operating cost. The difference between the revenues and the operating costs is the net cash flow. At the beginning of operations, there are usually startup problems to be solved and “hugs” to he worked out of the system. These difficulties often prevent the net cash flow from immediately reaching the steady-state value anticipated for the project. If the robot project is a good investment, the net cash flow will allow the company to recover its inves costs in the project in a relatively short period of time. The point at which the investment is recovered is displayed in Fig. 12-1 as the payhack period, and this pavhack period represents one of several methods for evaluating investment alternatives The pa period method, as well as several other methods for analyzing the economics of rohot projects are discussed in the following section. 132

METHODS OF ECONOMIC ANALYSIS We shall descrihe three methods for analyzing investments and comparing investment alternatives that are in common use in industry. The three methods are: 1. Payback(or payback period) method

2. Equivalent uniform annual cost (FUAC) method 3. Return on investment (ROl) method Each of the methods accomplishes the analysis using a slightly different twist. Ideally, the same decision should be reached no matter which method is used; however, this is not always the case. We assume that the reader has some familiarity with the principles of engineering economy.

Payback Method The payback method uses the concept of the payback period as shown in Fig. 12 The payback period is the length of time required for the net ac- cumulated cash flow to equal the initial investment in the project. Under the assumption that the net annual cash flows are equal from year to year. This notion can be reduced to the following simple formula

where

n = the payhack period

IC = the investment cost NACF = the net annual cash flow In most investment projects it would be unlikely that the cash flows would be exactly equal each year. When there are year-to-year differences in the cash flows, Eq. (12-t) must he altered slightly to for the differences. The subscript i is used to identify the year in the following. 133

In this equation, the value of n is determined so that the sum of the annual cash flows is equal to the initial investment cost. In the special case when the net annual cash flows are equal, Eq. (12-2) can he recast as

This is equivalent to Eq. (12-1). The reader should note that we have adopted the logical convention that costs are treated as negative values and revenues or Savings (as well as profits) are treated as positive values in these equations. The NACF is assumed to be a positive cash flow since revenues derived from the robot project would he greater than the operating costs (we hope). We have also assumed that all cash flows occur either at the beginning of the year or at the end of the year. Any investments are assumed to be transactions that occur at the beginning of the year, while the net annual cash flows ate assumed to be end-of-year transactions. Most companies today require paybacks of no more than two or three years. An investment whose cash flow pays back the investment in less than one year is considered excellent. Let us illustrate the payback method by means of the following example. Example 12-1 Suppose that the total investment cost is estimated to he $100,000 for a particular robot project The total operating costs (labor, maintenance, and other annual expenses) are expected to be $20,000 per year, and the anticipated revenues from the robot installation are $65,000 annually. It is expected that the robot project will have a service life of 5 years. Determine the payhack period that is expected of the investment. The net annual cash flow for the robot project is $65,000— $20,000 = $45,000. Using Eq.

134

One of the disadvantages of the payback period method is that it ignores the time value of money. It does not consider the objective of the company to derive a certain minimum rate of return from its investments. The other two methods to be discussed do include this consideration.

Equivalent Uniform Annual Cost Method The equivalent uniform annual cost (EUAC) method converts all of the present and future investments and cash flows into their equivalent uniform cash flows over the anticipated life of the project. It does this by making use of the various interest factors associated with engineering economy calculations. We present a tabulation of these interest factors in an appendix to this chapter, and we save a considerable amount of explanation by assuming that the reader is familiar with their use. To begin with, the company must select a minimum attractive rate-of-return (MARR) which is used as a criterion to decide whether a potential investment project should be funded. Today, MARR values of 20 to 50 percent are not unusual for robot projects. Using the interest factors for the MARR to make the conversions, the uniform annual cost method then sums up the EUAC values for each of the various investments and cash flows associated with the project. If the sum of the L is greater than zero, this is interpreted to mean that the actual rate of return associated with the investment is greater than the MARR used by the company as the criterion. If the FUAC sum is less than zero, then the project is considered unattractive Example 12-2 We will illustrate the EUAC method using the same data from Example 12-1. The company uses a 30 percent MARR as a criterion for selecting its investment projects. As mentioned in Example 12-1, the robot project is expected to have a 5-year service life, and that is what we shall use in determining the values for any interest factors required in our calculations. The annual operating cost ($20,000) and the annual revenues ($65,000) are already expressed as uniform annual cash flows. The initial investment cost ($100,000) must he converted to its equivalent uniform annual cash value using the capital recovery factor from the appendix. The sum of the annual cash flows would he figured as follows.

135

Since the resulting uniform annual cost value is positive, this robot project would be a good investment.

Return on Investment Method The return on investment (ROl) method determines the rate of return for the proposed project based on the estimated costs and revenues. This rate of return is then compared with the company’s minimum attractive rate of return to decide whether the investment is justified. The determination of the rate of return involves setting up an equivalent uniform annual cost equation similar to the one used in Example 12-2. The difference is that the EUAC sum on the left-hand side of the equation is made equal to zero. Then the values of the interest factors (and correspondingly, the interest rates) are found that make the right-hand side of the equation sum to zero. The following example will illustrate this procedure. Example 12-3 Again the same data are used from our previous two examples. The company’s MARR is 30 percent as before. The EUAC equation would be set up as follows.

Looking through the interest factor tables for a match of the A/P factor for n = 5 years, we find the following values:

By interpolation, the rate of return for our problem turns out to he = 34.94 percent. It can he seen that our calculated value of (A/P. i,5) = 0.15 is very close to (A/P. 35°/o. 5) = (1.45046, so it stands to reason that the rate of return for our problem should he close to 35 percent. In Example 12-3, the determination forward computation. However, many calculation procedures because there is must be used in the FUAC equation. We will demonstrate this procedure In Example 12-4. Indeed, there are several complications that are encountered in the economic analysis of robot applications problems.

136

These complications are not necessarily unique to robotics problems. But we will discuss them in the context of robotics.

.

.

137