PID tuning and features determine process performance but the relationship is not well understood leading to a divergence of opinions and a multitude of rules. This seminar unifies major tuning rules to a simpler set that when used with key PID options can achieve a diverse spectrum of process objectives.
In part 3 we start a list of the essential concepts needed to understand what is most important and what to do to help make a loop meet process objectives. The concepts are presented in the broadest possible terms to provide a perspective that can be used in a wide spectrum...
In part 2 we evaluate a misleading statement about the amount of derivative to use and provide some better guidance. We take a look at how mechanical and process design and operating conditions affect the need for derivative action.
The mechanical, piping, and process design determines the steady state and integrating process gains and the process deadtimes and lags. The process engineer usually sets the project basis for the control system in the development of the Process Flow Diagram (PFD) and in the writing of the operating and process descriptions.
The PID structures with proportional on error cause a step change in the PID output for a large setpoint change. For structures with derivative on error there is also a sharp bump almost looking like a spike unless you zoom in.
What are the relative merits of different PID structures, a setpoint (SP) filter, and analog output (AO) setpoint rate (velocity) limits? Should I seek a general solution I can use all the time and each knob fits a particular purpose, or a controller with fewer knobs that does exactly what...
Anti-reset windup (ARW) protection is a standard feature of industrial PID controllers. In some DCS, ARW limits are adjustable besides output limits. The ARW limits may not be at their best values. ARW default values may not match up with output limits as output scale and engineering units change.
All of the major tuning methods end up with the same expressions for PID gain, reset time, and rate time when the tuning objective is maximum disturbance rejection. Differences come down to tweaking of the a, b, and c coefficients.
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