Understanding PID

[Garyhlucas]

We have a PID loop controlling pH which auto tuned really well despite a very slow system response. We were really impressed and pleased by this. The issue we are having is during a cleaning operation the pH probe sees very high or very low pH and we have to stop it for a couple hours.  What we have noticed is despite the rung being off it appears as if it continues to run but with no output.  So when we turn the rung back on it is as if it has accumulated a large error and then makes a huge output change. While we are cleaning the system pH changes very little so it should be able to come back on with minimal bump.  Is there something else we need to do?  In the past I have lied to the loop telling it that it is at setpoint and no change is needed.

[BobO]

The box leg switches between Auto and Manual. While in manual, the input and output scales are still running, but basically nothing else. On the rising edge of the input (initial switch into Auto) there are some things that get initialized internally, which could be causing what you are seeing. That still shouldn't matter much if the PV and SP are close. It points to the PV being considerably different than the SP.

If nothing is changing, you could just bypass execution of the loop by putting in a task that isn't being executed, rather than taking it out of Auto.

[Controls Guy]

It's the integrator running?  Doesn't sound like it should be from BobO's description of man mode.   Is there a bias lock input to the structure?

[BobO]

It's the integrator running?  Doesn't sound like it should be from BobO's description of man mode.   Is there a bias lock input to the structure?

Nothing is running except the PV and Output scales.