After dealing with some problems with a process control feedback loop or proportional-integral-derivative controller (PID) I thought it would be helpful for other people going through the same problems.
We had highly fluctuating pressure so using a Back Pressure Control Valve we set up a feedback look that reading the process variable would open and close the analog valve.
We choose a setpoint of 500 kpa, this is the pressure the valve will attempt to return to (under the Setpoint: Value field).
Problems
Gain
The first problem we encountered was the pressure control valve opening and shutting too quickly. It was almost behaving as a discrete control valve. In this case the Gain was set too high, we changed it from 4 to 1 and the control slowly returned to normal. If the opposite is true and the valve takes too long to reach it’s desired pressure you can increase the Gain to speed up the opening and closing of the valve.
Scale Factor
The second problem we had was the valve seemed unstable. Although the pressure would slowly drop we would see changes in the output valve from 50% to 150% with no logical reason. This was a problem due to the Scale Factor.
The Scale Factor should never be left at the default (1). Using the formula Output Range/Variable Range (in this case 100%/4000kpa) we can come up with the correct scale factor of .025. to be safe you can add 10% to variable.
After the correction was made the valve would remain at the 100% without too much deviation while it was trying to lower the pressure. There was less volatility in the readings
Negative Gain
Another common problem is when the pressure control valve was shuts in completely or remains 100% open instead of trying to equalize the pressure. This was quickly resolved by changing the Gain (under the Tuning pullout) from -1 to 1. Depending on the configuration of the valve you may have to negate the Gain to equalize the pressure in the opposite direction.
I hope this helps out other people that have had similar problems please leave feedback or questions below.
