PID with a pause


Thread Starter

Jake Thompson

Hello everyone! I have an application were we are controlling pH in a pipeline, the hard part is there are times when there is no flow in the pipe. I do not want the the metering pump to run and add chemical when there is no flow so we will shut it off automatically. I want the PID controller to hold its outputs and not change anything while there is no flow. I am worried when there is no flow the pH could drift causing the controller to increase error over time due to the I and D values, this would change the speed of the metering pump(it would be shut off) and when the flow comes back it may take a while for the controller to re-adjust itself. Thanks in advance to anyone who can help!!
The Moore Products (now part of Siemens) 353 loop controller has discrete inputs and outputs as well as analog ins and outs. It is function block
configurable with many blocks to choose from (timers, math, compare, switches, etc.). If your application is small and does not justify a full PLC this could be a good choice.

James Fountas

You are on the right track. But it goes a step further. A flow switch will give you an on/off condition. You wil also encounter bad
control on high flow verses low flow. This assumes variable flow rate.

Johan Bengtsson

Is the flow always the same when there is flow (ie two possible states some certain flow or no flow) or does it vary and can be anything between zero and whatever max flow you have? (I suppose the later)

Do you measure the flow? (I suppose so, if it can change)

Is it safe to assume that the pump adding chemicals is linear? (ie twice as big control signal results in twice the flow) If so then take the output of the controller, multiply it with the measured flow and feed the pump with that signal.

If this is not linear I would have put a flow controller here or made a rough linearisation fo it.

Then adjust the proportinal gain of your controller to be proportional to the flow. This is a simple parameter scheduling that I think will do good enough for your purposes.

/Johan Bengtsson

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There is a cheap, specialized controller that can be used to control the pump, take an input from a flow switch, and output a signal to the PLC or other monitoring station. This comes pre-programmed, prewired, and if it doesn't work right, you call the manufacturer.

Contrast this with doing the same thing with a PLC. You have to interface the sensor, write the algorithms, program the controls, document the code, and if it doesn't work right, the pH sensor manufacturer tells you which bridge.

Which costs more and takes more time to do? The PLC of course. Even if you already have a PLC.

This is an example of techblindness. We get so used to using PLCs for everything that we forget that there are absolutely better ways to do


Walt Boyes
Bumpless Transfer Auto/Manual will do the trick. Many controllers have this feature activated via contact input to the rear terminals. Others must use an A/M switch on the front.