Design of internal model control dead-time compensation scheme for first order plus dead-time systems

Internal model control (IMC) is a well-known model-based control structure that has additional dead-time compensation (DTC), while PID is the popularly-implemented control structure due to its simple structure, ease of implementation, and satisfactory performance at a wide range of operating conditions. Therefore, IMC-based PID tuning methods are introduced to include the benefits of IMC in PID. Approximations involved in the existing IMC-based PID tuning methods for specific types of transfer function models result in performance degradation. The IMC structure can be rearranged to the form of a standard PID type controller without approximation, which retains good servo performance for time delay processes. On the other hand, the presence of a time delay element in the controller structure makes the loop highly sensitive to dead-time variations. Thus, the sensitivity of the IMC scheme realized in the PID structure is studied based on the conventional Nyquist stability criterion. It is observed that the high sensitivity is due to the occurrence of multiple interaction points of the Nyquist curve of the loop transfer function and unit circle. Besides, the number of the interaction points is relative to the ratio of loop dead-time to closed loop time constant. Further, it is shown that the occurrence of multiple interaction points can be avoided by the right choice of closed loop time constant. The performance and robustness of the proposed design approach is confirmed via simulation analysis.