Controlling flow rate with valve PID

[blackbird307]

Quick question. I have a PID controller that has the following parameters
Process: Flowrate (L/s)
Setpoint: FlowrateSP (L/s)
Output: ValvePIDOut (L/s)

I add the PID output to the setpoint to get a new variable. This variable is scaled from 0 L/s to 50 L/s to 0 to 65000 for modulating analog output. I don't know what the maximum flowrate possible is. It has never been to 50L/s.

If the error is negative, the PID controller output is negative. Otherwise PID controller output is positive. Controller is reverse acting. I have not touched the gains yet, I will when tuning.

I am using Connected Components Workbench

Does this make sense? I don't see a better way of doing it.

 

[gclshortt]

It sounds like you are on the right track. Depending on the response of the system you might just need PI control.

Here is a Micro800™ and Connected Components Workbench™ Application Guide. It gives a good overview of the PID.
http://literature.rockwellautomation.com/idc/groups/literature/documents/qr/2080-qr002_-en-p.pdf

Hope this helps,

 

[danw]

I don't know what Connected Components Workbench is, or its PID algorithm, but

a) >I add the PID output to the setpoint to get a new variable.
Interesting concept but I don't know what the resulting variable means or is.

b) > This variable is scaled from 0 L/s to 50 L/s to 0 to 65000 for modulating analog output.

A proportional controller's output is always 0-100%. What that translates to is a valve position in percentage:
0% = closed; 100% = open.
To the extent that valve position represents flow, there is an association between flow rate and output, but making that association is not part of PID.

c) The amount of error (PV (flowrate) minus SP (flowrate SP) (or SP minus PV, depending) has a gain factor applied to it that changes the output from what it was a moment earlier.

Zero error means "no change, hold the output".
Positive error drives the output in one direction, for instance, up, like from 35% output to 40% output.
Negative error drives the output in the opposite direction, down, for instance, from 35% to 30% output.

The direction up or down is determined by the control action, reverse or direct.

That action has to match the action for the valve and its actuator so the valve opens more when you need more flow, closes when you need less flow.