A decoupling pole placement self‐tuning controller for a class of multivariable processes

This paper describes a new multivariable self-tuning controller that specifically handles different time delays between each of the input-output pairs (multiple delays), and deals with unknown or varying time delay (variable dead time compensation) without requiring an explicit estimate of each delay. The new algorithm can control unstable and/or non-minimum phase processes; it eliminates both set-point and load offsets without having to maintain integral action continuously; it decouples the loops, both dynamically and at steady state. The effective decoupling algorithm not only minimizes significant interactions among the control loops, it permits the use of a very simple design criterion, namely the approximate assignment of the primary closed-loop poles to prespecified locations. A straightforward autotuning technique locates the closed-loop poles on-line so as to optimize the system set-point step responses. One desirable side-effect is to account for inexact decoupling. The controller is demonstrated using a simulated paper machine head box having an unusually large amount of (off-diagonal) interaction, a simulated two-input, two-output distillation column with time-varying parameters (varying gains and time delays), and a highly interacting unstable and non-minimum phase system on which the autotuning technique is demonstrated. These three processes place stringent requirements on any control method, and two of them have been used extensively in the literature to test various multivariable and/or self-tuning algorithms. Hence, the success of the new technique indicates that it is a very promising candidate to deal with intransigent processes, i.e. those characterized as unstable, non-minimum phase, time-varying (non-linear), and interacting.