TABLE OF CONTENTS As a result of studying this chapter, and after having completed the relevant exercises, the student should be able to:
Indicate the concept and strategy of combined feedback and feedforward control
Demonstrate how to develop tuning procedures for combined feedback and feedforward control.
Chapter 10 illustrated the concepts of feedforward control and showed that one problem it gives us is drifting of the PV from the systems SP value. This is caused solely because the PV is not taken into account in feedforward control, if it was it would become a feedback (closed loop) controlled system.
Examination of the feedforward concept shows us that it is normally used to minimize the impact of disturbances on a process. This is achieved by detecting and measuring a process disturbance and changing a related manipulated variable before the disturbance has an adverse effect on the process itself.
It is important to remember that process disturbances constitute anything from unexpected changes in either: magnitude of pressure, flow, temperature and any other physical quantity associated with the process or changes in time, of any of the process responses.
This latter variable, time, is very often overlooked as a quantity that may need correcting in a process environment. This is illustrated in Figure 10.3 where we use feedforward to equalize the difference in heating and cooling times of a feed heater system.
This should make the process responses both in magnitude and time, the same, irrespective...
