Automatic Tuning of Decentralized PID Controllers for TITO Processes

Abstract

The paper presents a new automatic tuning algorithm for Decentralized PID Control for two inputs - two outputs (TITO) plants. The tuning procedure, which is an extension of the Ziegler-Nichols method requires identification of a critical point consisting of critical gains of the various loops and a critical frequency. Unlike SISO plants there are infinitely many such points and knowledge of the desired one is essential to the tuning procedure. The auto-tuner identifies the desired critical point with no apriori information of the process. During the identifiction phase all controllers are replaced by relays, thus generating limit cycles with the same period in both loops. It is shown that this limit cycle corresponds to a single critical point of the process. By varying the relays parameters different critical point can be determined. The auto-tuner contains a procedure which converges rapidly to the desired critical point while maintaining the amplitudes of the process variables within prespecified ranges. The steady-state process gains, which are required for the appropriate choice of the desired critical point, are determined in closed-loop fashion simultaneously with the identification of the critical points. It is shown that the proposed auto-tuner is more efficient and reliable than a recently published algorithm and works well also in cases where the latter algorithm fails to converge and even destabilizes the system