TABLE OF CONTENTS As a result of studying this chapter, and after having completed the relevant exercises, the student should be able to:
Apply the procedures for open and closed loop tuning
Calculate the tuning constants according to Ziegler and Nichols and according to Pessen
Demonstrate how to perform fine tuning of closed loop control systems.
There are often many and sometimes contradictory objectives, when tuning a controller in a closed loop control system. The following list contains the most important objectives for tuning a controller:
Minimization of the integral of the error: The objective here is to keep the area enclosed by the two curves, the SP and PV trends, to a minimum. This is the aim of tuning, using the methods developed by Ziegler and Nichols as illustrated in Figure 8.1.
Minimization of the integral of the error squared: As Figure 8.2 shows, it is possible to have a small area of error but an unacceptable deviation of PV from SP for a start time. In such cases special weight must be given to the magnitude of the deviation of PV from SP. Since the weight given is proportional to the magnitude of the deviation, the weight is multiplied by the error. This gives us error squared (error squared = error weight). Many modern controllers with automatic and continuous tuning work on this basis.
Fast control: In most cases fast control is a principle requirement from an operational point of view;
