Limitations and countermeasures of PID controllers

It is well-known that PID controllers show poor control performances for an integrating process and a large time delay process. Moreover, it cannot incorporate ramp-type set-point change or slow disturbance. We discuss the above-mentioned limitations of the PID controller. Through the analysis of the limitations, simple techniques are developed to avoid these poor control performances and incapabilities. We introduce a control structure composed of two PID controllers to manipulate both the set-point change process and the input disturbance rejection process. A simple feedforward and feedback loop are proposed to treat a ramp-type set point and disturbance (low-frequency set point and disturbance). In addition, an internal feedback loop is introduced to incorporate integrating processes, and a tuning strategy using the model reduction is proposed to efficiently tune the controller for high-order processes. A dead-time compensator is used to manipulate a large time delay process more efficiently.