Feed Forward Feed Backward Process 453333

Assignment :Topic DIFFERENTIATE FEED BACKWARD AND FEED FORWARD CONTROL SYSTEMS . MONITOR THEIR ADVANTAGES AND .DISADVANTAGES TAKING SOME INDUSTRIAL EXAMPLES

Submitted To: Sir Tahir Ishfaq. Submitted By: M.Tanzeel UR Rehman. Roll NO: 10-CH-70

NFC IEFR FAISALABAD.

Contents

Feed backward control system

 Explanation and Example  Advantages of feed backward control system  Disadvantages of feed backward control system

Feed Forward control system

 Explanation and example  Combination with Feed backward control system  Advantages system

of

feed

forward

control

 Disadvantages of feed forward control system

Industrial Examples

Feed Backward Control System

Definition: In closed loop control system information about the controlled variable are fed back to the controller and final control element. It use direct measurement of the of controlled variables to adjust values of manipulated variables.

Explanation: A system in which information about the controlled variable is fed back to controller and final control element is known as feed backward control system. It is also called closed loop control system. It does not directly measure the disturbance.

The term comes direction in which the measured values travel in the block diagram. The signal

begins at the output of the controlled system. The output of the controller is the input of controlled system. Thus the measured value signal is fed back from the output of the controller system to the input. The term closed loop refers to the loop created by path. A closed loop or backward control system measure the difference between the actual value and desired value and uses the difference to the actual value towards the desired values. Example: Let us consider a example of a CSTR system. In this system feed is entered at s specific temperature and composition. The reaction is taken placed and product is obtained at a specific temperature which is required for the product. A temperature controller is installed at the outlet system. If the temperature is increased or decreased from the required value, a signal goes from the controller to the valve and the manipulated

variable

is

controlled.

Advantages:

 One of the advantages of control is that by analyzing the output of a system unstable process can be stabilize.  It does not require identification and measurement of any disturbance.  It is intensive to modelling errors.  It is insensitive to parameter changes.  The primary advantages feed backward control is the potential for more accurate control of process. Disadvantages:

 Time lag in a system is the main disadvantage in the system.  The feed backward system is more expensive than the open loop system.  The feature of closed loop control system makes it possible for the system to become unstable.  A unstable system make oscillation of the control variable, often with very large amplitude.  It waits until the disturbance effect the system.  It is not suitable for slow processes or with significant dead time.

Feed Forward control System Definition: In this process disturbance is measured in advance and compensate without waiting for a change in control variable. It is used for the direct measurement of disturbance to adjust values of manipulated variables. it measure the disturbance before they effect the system.

Explanation: Feed backward control is most common strategy in the process Industry. However control may not

provide required performance. For these processes other types of control systems are designed. One of such system is feed forward control system. It uses the measurement of an input disturbance to the plant as additional information for enhancing single loop PID control performance. This measure provides a early warning that the controller variable will be upset sometime in feature.

With this warning it has chance to adjust the manipulated variable before the control variable deviates from the set point. It does not use the output of the control system. A system which exhibits feed forward behaviour responds to a disturbance without having to be effected by it first. Sometimes feed backward control system is also use with combination of feed forward control to get best accurate results. An example of combination of feed forward and feed backward control system is given below:

In this example a heat exchanger with the combination of feed forward and feed backward control system is shown. Feed forward control system controls the flow before it effect the final product and backward system controls the flow when it affects the system and any change occur.

Advantages:  Feed backward system Is active, but this system is proactive.  It can respond more quickly to a known kind of disturbance.  It can control the disturbance before it effect the system.

 It does not have stability problems.  They have theoretical potential for perfect control.  It is good for slow systems.  Improve quality and accuracy, enhanced stability and faster feed of working.  It is advance type of controller.

Disadvantages:

 It requires the system to respond to disturbance in order to notice the deviation.

the

 It can do not anything with novel disturbance.  It cannot indicate the change in output of system.  It can control only a known disturbance.