PID Question

When using a PID instruction to control a valve which will open to let enough flow through to obtain a user entered flowrate that is monitored by a flowmeter. Should the cv output go to 100% after a few minutes if the desired flowrate is not reached? I've got a idea for trying to control flow from one of our holding ponds back into the treatment process at the wastewater plant i currently work at. I set up a test on my work bench and i've got the loop operating but it doesn't keep applying an increasing cv output if the setpoint isn't reached. Never worked with a PID instruction before so i figure i'm doing something wrong.

You will get more help if you provide more info on your system.
What PLC? Flow meter?

Make sure you aren't limiting the CV output.
If you don't have enough P or I there may not be enough push to get the CV output to 100%

Something that helped me understand a PID is to think of how the cruise control in your car works. How it reacts to hills,wind...

cirtcele said:

When using a PID instruction to control a valve which will open to let enough flow through to obtain a user entered flowrate that is monitored by a flowmeter. Should the cv output go to 100% after a few minutes if the desired flowrate is not reached?

Click to expand...

It will because the integrator will wind up if the set point is not reached. So why doesn't the flow increase quickly and reach the flow set point? A common error is to ask the PV to go to a set point it can not reach. In this case the integrator will wind up and the CV will go to 100%.

I've got a idea for trying to control flow from one of our holding ponds back into the treatment process at the wastewater plant i currently work at. I set up a test on my work bench and i've got the loop operating but it doesn't keep applying an increasing cv output if the setpoint isn't reached.

Click to expand...

This doesn't make sense. Now it sounds like the integrator is not working. If the set point isn't reached the integrator will wind up as it attempts to get the PV to the SP but if the integrator isn't running valve will stop opening as the error starts to decrease.

I added some integral gain to get the increasing cv output i was looking for, now when the flow does not increase to the SP the valve will keep opening and when the flow is greater than the SP it will continue to close. The next problem will be keeping the change slow enough so it doesen't overshoot the setpoint and start oscillating. The change in flow will lag the gate movement. The gate is 60" restricting flow from a 42" pipe feeding a 60 MGD screw pump which dumps into 42" pipe where the flowmeter is located. I know from what i have read on this website i'm a longggg way from understanding how to set this up.

Is the valve a motorized valve? How fast does it respond?

If the integrator time constant is too small (fast) compared to the time it takes the valve to respond the integrator will wind up and over shoot or under shoot.

Another approach you can take is to ramp the set point up and down instead of changing the set point in jumps or steps. If you ramp the set point up at just a little lower rate than the valve responds the control output will not saturate.

Its motorized and moves about 12" a minute. The way the system used to work with the 1970's era fischer porter unit is the operator would wait until the incoming flow at the plant would drop below 60 MGD lets say it dropped to 30 MGD he would then spin the dial on the F&P unit to 60
the unit would open the gate with a 4-20mA to reach the desired 60MGD setpoint it worked fine until it died a couple weeks ago. So now they want it to be adjusted the same way from scada through the plc's. The old Fischer Porter unit was operating without the plc's the plc only monitored the gate position and flow for display on the scada system.

It might help someone help you if you tell the forum what PLC you have and post its program if possible. ( zip it first)

A/B SLC500. I've got some rungs setup on a rack on my workbench when i think i have the logic close i'll intergrate it into the main plc program. There are alot of safety features that have to be installed to keep the gate from opening at the wrong time and flooding the plant. So right now its still a rough draft.