A Real-Time Approach to Process Control, 3rd Edition

5

Tuning Feedback Controllers

There is no absolute right way or, for that matter, absolute wrong way to tune a controller. Controller settings depend on what the engineer/operator deems to be good performance in terms of the desired response to process upsets. The type of process, the process gain, the time constant and dead time all play a role in determining the controller settings.

The settings also depend on the anticipated type of disturbances that the process will encounter. A controller would be tuned differently for stability due to set point changes (servo control) than for load disturbances (regulatory control). In process control systems load disturbances are most frequently encountered and, hence, most systems are optimized for regulatory control.

This chapter will discuss control quality and optimization, including the performance criteria that need to be considered when tuning a controller. A number of methods that can be used to determine controller settings in order to achieve the desired control are also described.

5.1 Quality of Control and Optimization

Controller tuning can be defined as an optimization process that involves a performance criterion related to the form of controller response and to the error between the process variable and the set point. When tuning a controller some of the questions that may be asked include

• Can offset be tolerated?
• Is no overshoot desired?
• Is a certain decay ratio required?
• Is a fast rise time needed?

These questions address ...