What is PID tuning (valve tuning)?
PID tuning is a type of valve tuning that adjusts how the valve(s) on your Alicat flow or pressure controller responds to the setpoints that you command. “PID” stands for three variables that make up the control algorithm: proportional, integral and derivative. These variables can be applied by your Alicat as PD or PD2I control loop algorithms.
Alicat uses the operational parameters you provide (inlet and outlet pressures, volume being controlled) to tune the valve(s) on your controller before it ships, but you can also adjust the PID tuning in the field.
The variables: proportional, derivative, integral
The P, D and I values are gains that your Alicat applies to the raw proportional, derivative and integral variables. The greater the value for P, D and I, the greater the effect they will have within the control loop.
- Proportional (P). The proportional variable applies power to the valve as it tries to decrease the error between the set point and the process value position to achieve the setpoint. Your Alicat multiples the current error by the P value, so the higher P is, the faster it will race toward the new setpoint.
- Derivative (D). The derivative variable acts as a damper to reduce the rate of change of the valve position. Your Alicat multiples the current rate of change by the D value and then subtracts this from the calculation made with the P value.
- Integral (I). The integral variable determines the output of the valve as a function of the sum of all the errors as the valve works to achieve the setpoint. Whereas the P and D variables only take into account the current measurement and the one immediately preceding it, the I variable uses many previous readings to reduce the error and correct the process value to reach the setpoint. The way your Alicat uses the I value varies according to the control loop it is using (see below).
You can view the current settings for your P, D and I variables by going to MENU > CONTROL > PID on the front panel of your Alicat controller.
The control loops: PD, PD2I
- PD. If you have a single-valve flow controller or pressure controller, then we have most likely configured it with the PD control loop algorithm. Your Alicat multiples P by the current error between setpoint and actual flow reading, and then it subtracts D multiplied by the current rate of change. These calculations occur 1000 times every second.
- PD2I. If you have a dual-valve flow controller (MCD series) or pressure controller (PCD series), then we have configured it with the PD2I control loop algorithm. Unlike a traditional PID algorithm, Alicat’s PD2I algorithm incorporates a predictive function that makes dual-valve controllers in particular more responsive. It can also be applied to single-valve controllers, but the difference in control response is usually negligible compared to the standard PD control loop algorithm.
You can view your selected control loop by going to MENU > CONTROL > PID > LOOP TYPE on the front panel of your Alicat controller.