Improving PID Recovery from Limit Conditions

On the second day of the IFAC PID’12 conference, the paper that Dan Coyne, BP, and I wrote Improving PID Recovery from Limit Conditions was presented in the poster session. The paper addresses the performance of the PID under startup or during normal operation when the PID output becomes limited. Common techniques that have been utilized to reduce the time required to get to setpoint during process startup are reviewed in the paper. The response of the PID to conditions that limit PID operation during normal operating conditions is discussed for different implementation approaches. The recovery from a process saturation condition may be improved by modifying the PID operation to use a variable pre-load under limit conditions. The improvement that may be achieved is illustrated using a compressor anti-surge control application example. Variable pre-load is a feature of the PID block in DeltaV v11 and can be enabled on applications that commonly encounter limit conditions.

In the poster session on Thursday, March 29th, 2012 we presented key ideas from the paper. The reset component is realized in many commercial products using a positive feedback network as illustrated below.

When control operates at a limit condition for an extended period of time, the control parameter can overshoot the setpoint. The traditional approach to improve recovery is to provide a specified “Preload value” that is applied under a limit condition.

As addressed in the paper, rather than use a fixed pre-load value, the point at which control action should be taken to avoid overshoot can be calculated as illustrated for below for PI control.

The effectiveness of providing a variable preload to determine when control action is taken is illustrated in the paper using a simulated compressor surge control application.

Control Response for 60% reduction in Load with no Pre-load Applied in the PI Control is shown below.

Control response for 60% reduction in load with variable pre-load automatically applied in the PI Control

If you would like to learn more about this new PID functionality then the paper Improving PID Recovery from Limit Conditions may be freely downloaded from a web site created by IFAC.