PID Loop Simulator: Modelling the Process

Description| How it works| Derivative on PV| Process Model| Robustness| PID Tuning| PID Equations

There are 2 aspects of loop tuning - performance and robustness. Performance means that when the set point is changed, the controller should bring the process value to the new set point within the desired time. Robustness means that the controller should maintain control even if there are changes to the process. Performance is often given importance, while robustness is ignored.

To illustrate the importance of robustness, consider a heating process where a vessel is heated by steam. The process described by K=1, T = L= 30 sec, and is controlled by a PI controller C = 1 and I = 40 sec. The controller controls the steam inlet. The controller is tuned in summer, it is agressive, and performs very well, as shown in the figure below.

Stable loop

This process is sensitive to external temperature and the dynamics are likely to change with the seasons. In winter, the heating is not that effective, and there is likely to be a larger delay in the process. If the delay changes increases to 60 sec, the same controller causes the process instability as shown below.

UnStable loop

This example illustrates the importance of robustness. The controller must be tuned to accomodate reasonable changes in the process. Typically, a gain and phase margin of 2 is used (Gain margin of 2 means that the system will be stable even if the process gain changes by a factor of 2).