Improved controller design for two-input-two-output (TITO) unstable processes

Controller design for unstable processes is relatively difficult when compared to stable processes. The complexity increases further for
multivariable unstable processes. In this work, simplified tuning rules are proposed to design PID controller for unstable multivariable processes.
Decouplers are applied to make the loops independent and diagonal elements of equivalent transfer function are used to design controllers. For this,
the decoupler design procedure proposed by Hazarika and Chidambaram [10] is used. Two theoretical examples of TITO unstable processes with
time delays are considered for simulation. Comparative analysis has been carried out with the recently reported methods in the literature and
observed that the proposed method provides improved closed loop performances. Robustness studies are also carried out with various perturbations
in the processes.