TABLE OF CONTENTS As a result of studying this chapter and after having completed the relevant exercises, the student should be able to:
Describe the concept and strategy of feedforward control
Develop and then clearly describe the tuning procedures for feedforward control.
If, within a process control's feedback system, large and random changes to either the PV or lag time of the process occur, the feedback action becomes very ineffective in trying to correct these excessive variances. These variances usually drive the process well outside its area of operation, and the feedback controller has little chance of making an accurate or rapid correction back to the SP term.
The result of this is that the accuracy and standard of the process becomes unacceptable. Feedforward control is used to detect and correct these disturbances before they have a chance to enter and upset the closed or feedback loop characteristics.
It must be remembered that feedforward control does not take the process variable into account; it reacts to sensed or measurement of known or suspected process disturbances, making it a compensating and matching control to make the impact of the disturbance and feedback control equal.
The difference between feedforward and feedback control can be considered as:
Feedforward is primarily designed and used to prevent errors (process disturbances) entering or disturbing a control loop within a process system.
Feedback is used to correct errors, caused by process disturbances, that are detected within a closed loop control...
