Design of Robust PID Controllers with Constrained Control Signal Activity

This thesis presents a new method for design of PI and PID controllers with the level of control signal activity taken into consideration. The main reason why the D-part is often disabled in industrial control loops is because it leads to control signal sensitivity of measurement noise. A frequently varying control signal with too high amplitude will very likely lead to actuator wear and tear. For this reason it is extremely important for any PID design method to take this into account. The proposed controllers are derived using a newly developed design software that solves an IAE minimization problem with respect to H∞ robustness constraints on the sensitivityand complementary sensitivity function. The software is shown to be fast, easy to use and robust in giving well-performing controllers. By extracting measurement noise from the process value of a real plant, one can estimate its effect on the control signal variance. The time constant of the low-pass filter, through which measurements are fed, is varied to design controllers with constrained control signal activity. By comparing control signal variance and IAE, the user is also able to weigh actuator wear to estimated performance. The proposed PID design method has shown to give very promising results both on simulated examples and real plants such as a recirculation flow process. Optimal Youla parametrized controllers are used both as a quality check of the designed PI and PID controllers and as a tool for determining when these are valid choices compared to more advanced controllers.