Limiting PID Output

[b7031719]

I am asked if it is possible to limit the output of a PID controller by putting a software restriction on the output signal to the actuator. In this case it is a temperature controller on a fired heater and is regulating heat input into the system and controlling output temperature.

The assumption here is that a reasonable performance will be achieved with this output restriction in steady state. I am concerned that integrating action may make any control response very poor.

Has anybody had any experience with these "hard limits" on controller output and are there any tests that can be performed to get an idea of how the controller might cope under these conditions?

Is there anything else that might need to be considered before we implement this change?

 

[cswilson]

This limiting is a common, even standard, feature of PID controllers.

The phenomenon you are (correctly) worried about is known as "integrator windup". So "anti-windup" is also a standard feature of PID controllers.

The simplest technique for anti-windup protection is simply to disable additions to the integrator when the output is saturated on the limit. It is usually good enough.

Curt Wilson
Omron Delta TAu

 

[roydm]

Anti Reset Windup is another term used, as Curt says a standard feature of most controllers to be implemented by an interlock action, it could clamp the output at any point.
Not the same as an output limit which would fix it at some max or min value.