Read And Write Heads And Head Actuator Mechanisms Of Hard Disks 1n3n11

 Read/Write Heads i. A hard disk drive usually has one read/write head for each platter surface. Hence, each platter has two sets of read/write heads -- one for the top side and one for the bottom side. ii. All the heads mounted on a common carrier device or rack. The heads move radially across the disk in unison; they can’t move independently because they are mounted on the same carrier or rack, called an actuator. iii. Each head is on an actuator arm that is spring-loaded to force the head into with a platter.When the drive is at rest, the heads are forced into direct with the platters by spring tension, but when the drive is spinning at full speed, air pressure develops below the heads and lifts them off the surface of the platter. Heads float or ski on this air bearing. iv. On a drive spinning at full speed, the distance between the heads and the platter can be anywhere from 0.5 µ-inches to 5 µ-inches or more in a modern drive.

 Head Actuator Mechanisms i. All the heads mounted on a common carrier device or rack. The heads move radially across the disk in unison; they can’t move independently because they are mounted on the same carrier or rack, called an actuator. ii.This mechanism moves the heads across the disk and positions them accurately above the desired cylinder. Many variations on head actuator mechanisms are in use, but all fall into one of two basic categories:  Stepper motor actuators  Voice coil actuators

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Characteristics Relative Access Speed Temprature sensivity Positionally Sensivity Automatic Head parking Preventive Maintenance Relative Reliability Suited for

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Stepper Motor Actuator Slow Yes (very) Yes Not usually Periodic Reformat Poor Floppy Disk Drives(135 TPI ) , early hard drives with capacity 100MB or less

Voice Coil Actuator Fast No No Yes None Requirement Excellent Modern hard Drives(more than 5000TPI)

Stepper Motor Actuators

i. A stepper motor is a motor that rotates in steps, when you rotate an ordinary motor, it will turn any angle of rotation, but a stepper motor turns a fixed angle with each step.The smallest fixed angle that a stepper motor rotates with each pulse is called a "detent". ii. With each electrical pulse received, a stepper motor turns in a predefined angle .These mechanism are used to convert the rotational motion of the stepper motor in: the in-and-out motion required to move the read/write head over the disk surface. iii. Stepper motor actuators were commonly used on hard drives made during the 1980s and early 1990s with capacities of 100MB or less. iv. The stepper motor is connected to the read/write head actuator using the followin two method 1.Split metal band mechanism 2.Rack-and-pinion gear mechanism 1

1. Split metal band mechanism i. The split metal band mechanism uses two band made of special steel alloys, to convert the rotation of the stepper motor into the linear motion required to move the head over the disk surface. ii. One end of the band is coiled on the spindle of the stepper motor and the other e connected to the head Tack. iii. As the spindle rotates one band uncoils from the spindle and the other band coils around the spindle, making the head rack to move; moving the head with it; in-out on the disk surface. iv. A metal band mechanism finds a particular information on the disk surface by rotating the stepper motor to move the read/write head to that particular location where data is located. v. If the data is located on the 50th cylinder, then the stepper motor is rotated by 50 detends, this will move the read/write head to the 50th cylinder, and the head can read information from it. vi. One problem with this type of system is with time the bands may stretch, and the positioning systems accuracy may reduce. vii. As the drive platters heat and cool, they expand and contract, and the tracks on the platters move in relation to a predetermined track position.Hence ,the change in the head positioning can also occur due to the thermal expansion of the platter, head actuator assembly etc. This is the reason why some hard disks show error when cold and later after keeping on for some time, start working without any trouble. viii. To solve this problem the stepper band based hard disk drive requires periodic low level formatting. This will write the cylinder/sector address at the new position on platter surface.

Split metal band Head Actuator Mechanism

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Rack-and-pinion gear mechanism

Rack-and-pinon gear Head Actuator Mechanism

i. In this mechanism a pinion gear is provided on the spindle of the stepper motor, as the motor turns it will also Split rotate the gear. this pinion Mechanism gear rotates it moves a rack gear in and out. Metal Band As Head Actuator ii. The rack gear is connected to the read/write head assembly, so the in and out movement of the rack gear will make the head to move in and out on the disk surface. iii. This rack-and-pinion assembly is slightly better than the Split metal band mechanism, because it provides better thermal stability . iv. One problem with this system is, with time as the gears deteriorate, the wear will increase play in the gears. This will slowly make the complete actuator system useless.

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Voice Coil Actuators

i. The name "voice coil" is used for this system because the basic principle behind this system is taken from the voice coil used inside a typical audio speaker. ii. In a voice coil based head actuator mechanism the electromagnetic coil is connected to the head rack and placed near a stationary permanent magnet. There is no direct between the magnet and the coil. iii. As the coil is given current, electromagnetic field will be generated in the coil.The magnetic field of permanent magnet acting on the elctromagnetic field of the coil creates the magnetic force which makes the coil to move away from or towards the permanent magnet. As the coil moves it will move the head rack with read/write heads over the disk surface. iv. Systems like these are extremely quick, efficient, and usually much quieter than systems driven by stepper motors. v. The voice coil actuators use a signal mechanism called servo from the drive to accurately determine the head positions and adjust them, if necessary. This arrangement provides significantly greater performance, accuracy, and reliability than traditional stepper motor actuator designs. vi. The track-following capabilities of a servo-controlled voice coil actuator eliminate the positioning errors that occur over time with stepper motor drives. vii. The two main types of voice-coil positioner mechanisms are 1. Linear voice-coil actuators 2. Rotary voice-coil actuators The two types differ only in the physical arrangement of the magnets and coils.

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1. Linear voice-coil actuators i. The linear actuator coil moves in and out on a track surrounded by the stationary magnets which moves the heads in and out over the platters in a straight line. ii. The primary advantage of the linear design is that it eliminates the head azimuth variations that occur with rotary positioning systems. (Azimuth refers to the angular measurement of the head position relative to the tangent of a given cylinder.) A linear actuator does not rotate the head as it moves from one cylinder to another, thus eliminating this problem. iii. Although the linear actuator seems to be a good design, it has one fatal flaw. The components in this mechanism are very heavy. The lighter the mechanism, the faster it can accelerate and decelerate the heads from one cylinder to another. Hence ,this type of actuators have less seek time which reduces the drive’s overall performance.

2. Rotary voice-coil actuators i. The voice coils system that uses a rotatory arm to move the read/write head is called the rotary voice coil actuator. ii. Rotary actuators also use stationary magnets and a movable coil, but the coil is attached to the end of an actuator arm. As the coil moves relative to the stationary magnet, it swings the head arms in and out over the surface of the disk.

Rotary Voice Coil Actuator

iii. The primary advantage of this mechanism is its light weight, which means the heads can accelerate and decelerate very quickly, resulting in very fast average seek times. Because of the lever effect on the head arm, the heads move faster than the actuator, which also helps to improve access times. iv. The disadvantage of a rotary system is that as the heads move from the outer to the inner cylinders, they rotate slightly with respect to the tangent of the cylinders. This rotation results in an azimuth error and is one reason the area of the platter in which the cylinders are located is somewhat limited. By limiting the total motion of the actuator, the azimuth error is contained to within reasonable specifications.

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Servo Mechanisms

Voice coil actuators use a guidance mechanism called a servo to tell the actuator where the heads are in relation to the cylinders and to place the heads accurately at the desired positions. This positioning system often is called a closed loop mechanism. It works by sending the index (or servo) signal to the positioning electronics, which return a signal that is used to position the heads accurately. The system also is called servo-controlled, which refers to the index or servo information that is used to dictate or control head-positioning accuracy. No Temprature effect on the Servo System A voice coil actuator with servo control is not affected by temperature changes, as a stepper motor is. When temperature changes cause the disk platters to expand or contract, the voice coil system compensates automatically because it never positions the heads in predetermined track positions. Rather, the voice coil system searches for the specific track, guided by the prewritten servo information, and then positions the head rack precisely above the desired track, wherever it happens to be. Because of the continuous of servo information, the heads adjust to the current position of the track at all times. For example, as a drive warms up and the platters expand, the servo information enables the heads to “follow” the track. As a result, a voice coil actuator is sometimes called a track following system. Three servo mechanism designs have been used to control voice coil positioners over the years:  Wedge servo  Embedded servo  Dedicated servo The three designs are slightly different, but they accomplish the same basic task: They enable the head positioner to adjust continuously so it is precisely positioned above a given cylinder on the 5

disk. The main difference between these servo designs is where the gray code information is actually written on the drive.

Wedge Servo Early servo-controlled drives used a technique called a wedge servo. In these drives, the gray-code guidance information is contained in a “wedge” slice of the drive in each cylinder immediately preceding the index mark. The index mark indicates the beginning of each track, so the wedge-servo information was written in the PREINDEX GAP, which is at the end of each track. This area is provided for speed tolerance and normally is not used by the controller. Another problem was that the servo information appears only one time every revolution, which means that the drive often needed several revolutions before it could accurately determine and adjust the head position. Because of these problems, the wedge servo never was a popular design; it no longer is used in drives.

A Wedge Servo

Embedded Servo An embedded servo is an enhancement of the wedge servo. Instead of placing the servo code before the beginning of each cylinder, an embedded servo design writes the servo information before the start of each sector. This arrangement enables the positioner circuits to receive many times in a single revolution, making the head positioning much faster and more precise. Another advantage is that every track on the drive has its own positioning information, so each head can quickly and efficiently adjust position to compensate for any changes in the platter or head dimensions, especially for changes due to thermal expansion or physical stress. Most drives today use an embedded servo to control the positioning system. As in the wedge servo design, the embedded servo information is protected by the drive circuits and any write operations are blocked whenever the heads are above the servo information. Thus, it is impossible to overwrite the servo information with a low-level format, as some people incorrectly believe.

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Embedded Servo Dedicated Servo A dedicated servo is a design in which the servo information is written continuously throughout the entire track, rather than just once per track or at the beginning of each sector. The term “dedicated” comes from the fact that this platter side is completely dedicated to the servo information and can’t contain any data. When building a dedicated servo drive, the manufacturer deducts one side of one platter from normal read/write usage and records a special set of gray-code data there that indicates the proper track positions. Because the head that rests above this surface can’t be used for normal reading and writing, the gray code can never be erased and the servo information is protected— as in the other servo designs. When the drive moves the heads to a specific cylinder, the internal drive electronics use the signals received by the servo head to determine the position of the read/write heads. As the heads move, the track counters are read from the dedicated servo surface. When the servo head detects the requested track, the actuator stops. The servo electronics then fine-tune the position so the heads are precisely above the desired cylinder before any writing is permitted. Although only one head is used for servo tracking, the other heads are attached to the same rack so that if one head is above the desired cylinder, all the others are as well. The drawback to a dedicated servo is that dedicating an entire platter surface for servo information is wasteful. Virtually all drives today use a variation on the embedded servo technique instead. Some drives combined a dedicated servo with an embedded servo, but this type of hybrid design is rare.

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A dedicated servo, showing one entire head/side used for servo reading.

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