Proportional with Integral and Derivative (PID) control

PID loop control can be difficult to tune and (often for this reason) is seldom used. However, in certain cases, PID control may be needed. An example is the control of a process with a long reaction time, such as temperature control of a large mass. For such a lag-oriented system, the derivative component of the PID loop output can help prevent overshoot that might otherwise result from PI control.

The derivative gain (KD) exerts an anticipating braking effect on the loop output, based on the rate-of-change of the process.

PID loop output is calculated as follows:

Output = KP x (ES + KI x ErrorSum + KD x ((ES - LastES) / deltaT)) if action = direct) or

Output = -(KP x [ES + KI x ErrorSum) + KD x ((ES - LastES) / deltaT))) (if action = reverse)

where: ES = [PV - setpt]

ErrorSum = Sum of ES over time

deltaT = time between samples

In the LoopPoint, the Derivative Constant property specifies the derivative gain (KD) directly in seconds (note this differs from some systems using derivative in minutes).

If using PID control, follow the same PI configuration as for Proportional with Integral (PI) control, with the addition of defining a positive value as the Derivative Constant. In general, a Derivative Constant less than 10 seconds should be tried first, and only then increased (if necessary), providing that the loop output remains stable at steady-state conditions.

Following is an example of a wire sheet for a loop point.

Figure 9. Example LoopPoint in a wire sheet, linked to other components

The following is the property sheet for the above example.

Figure 10. Example LoopPoint property sheet