Response to Control Mode Switching

How to Run Level Control Loop Experiments Successfully, Even After Encountering Controller Performance Issues After Changing Control Mode?

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Al Pawlowski, PE

A: A very slow integral results in an equally long lag in the control loop. Most likely there will be a very long period oscillation. When the process changes, the operator will need to offset the setpoint to move the process to the desired value, and allow the controller to react to short term changes. The operator then has to watch the output drift slowly as the reset drifts. And we wonder why the loop gets set to manual?

Once upon a time, we had "manual reset," also called zero adjustment, but it would stay put and not drift. However, that was with the good old pneumatic controllers we knew and loved.

Cullen Langford

A: It very much depends on who wrote the PID computer algorithm. The Fieldbus Foundation specifies a standard PID algorithm by its parameters.

The implementation of the control algorithm is in the hands of the vendor.

I assume you are talking about software because you "delete" the integral term. That would be the only solution, given the constraints. However, the time constant should be very long—small repeats/min, large time constant in seconds or minutes.

Bill Hawkins

A: This sounds like an oddity in the algorithm. Perhaps removing the integral has dropped you back to proportional, plus manual reset, with a default value for manual reset, rather than the value stored as the integral term.

Ian H. Gibson

A: A well-designed PID should not bump/change output while any of PID parameters (K, Ti, Td) are changing. This is the basis for implementing in PID adaptive control, and adjusting parameters without affecting controller output.

Willy Wojsznis

A: When using P + I in the steady state, the output of the controller is wholly contributed to by the integral term; i.e., the integration of prior error signal over time. In the steady state, the P (proportional) section of that controller is subject to a zero-error signal and, hence, contributes nothing to the output.

When eliminating the integral (I) function while in automatic, you eliminate its output contribution and cause the process, via the valve, to be bumped. The level will change and no longer match the setpoint. The error signal internal to the controller will rise from zero to a new value that causes the level to rise toward the setpoint. The level will not restore to the setpoint because, in proportional control, an error or offset is necessary to have a controller output. 

Some controllers have a manual reset provision that can be used to eliminate the offset, but only for one load condition. Most processes have more than one load condition, and thus, P + I is the better solution. The control settings for typical controllers, since modern times, can be changed when in balance and while in the auto mode without causing a process upset other than that dictated by the function value change. But, eliminating or adding functions must be done while in manual.

I am not familiar with the detailed construction of the controllers you mentioned. The fact that you did not have the problem using other controllers suggests that the controller you are using is either malfunctioning or unsuitable for your level application.

In a level application, integral or reset action is a handicap during start-up or sudden, large load changes since, in its raw form, it causes the process variable to overshoot the setpoint. Competent controller or control algorithm manufacturers have devised a variety of techniques to overcome this problem. Perhaps the controller or control algorithm you are using is unsuitable in an application such as yours. I would make inquiries to the manufacturer to satisfy yourself on this point.

Otto Muller-Girard, PE

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