AUTOMATIC TUNING OF DECENTRALIZED PID CONTROLLERS FOR MIMO PROCESSES

An automatic tuning algorithm for decentralized PID control in multipl e-input multiple-output (MIMO) plants is presented. This algorithm gen eralizes the authors' recent auto-tuner for two-input two-output syste ms to any number of inputs and outputs. The algorithm consists of two stages. In the first, the desired critical point, which consists of th e critical gains of all the loops and a critical frequency, is identif ied. The auto-tuner identifies the desired critical point with almost no a priori information about the process. During the identification p hase all controllers are replaced by relays, thus generating limit cyc les with the same period in all loops. It is shown that each limit cyc le corresponds to a single critical point of the process. By varying t he relays parameters different points can be determined. The auto-tune r contains a procedure which converges rapidly to the desired critical point while maintaining the amplitudes of the process variables as we ll as of the manipulated variables within prespecified ranges. In the second stage, the data of the desired critical point is used to tune t he PID controllers by the Ziegler-Nichols rules or their modifications . This paper focuses on the first stage. The steady-state process gain s, which are required for the appropriate choice of the desired critic al point, are determined by the auto-tuner in closed-loop fashion simu ltaneously with the identification of the critical points. The identif ication of the process gains is achieved at no extra plant time. Based upon a large number of simulated cases, the proposed auto-tuner seems to be efficient and robust. The paper discusses the underlying princi ples of the auto-tuner and its properties and capabilities are demonst rated via examples. (C) 1997 Elsevier Science Ltd. All rights reserved .