Relay feedback of multivariable systems and its use for auto-tuning of multi-loop PI controllers

This paper presents an auto-tuning method for designing multiloop PI controllers for a class of multivariable plants based on their behaviour under closed loop relay feedback. Unlike relay feedback in single loop plants, multiloop relay feedback may exhibit a number of different behaviours depending on the strength of interactions. Hence, different design strategies catering to all the possible scenarios are proposed. The proposed technique uses ratios of some ultimate quantities to tune the required PI controllers, similar to single loop plants (Astrom - Hagglund (1988)). Simulation results are presented to illustrate the design methodology.