Soft Implementation of Cascaded Control Architectures Using the Youla Parameterization

Implementation can often be a major barrier for the adoption of new control strategies. Issues can stem from computational complexity to difficulties in the tuning process. In previous work, nested PID controllers have shown several benefits for building heating and air-conditioning controls including low order linearization and multi-input, multi-output decoupling. Tuning procedures for such cascaded control designs have also been developed to help select inner and outer loop gains for specific benefits. This paper proposes to use a Youla parameterization of nominal PI control to provide a soft implementation of cascaded control. Tuning of cascaded controllers can be done smoothly using a simple continuous switching variable, thereby creating a window in which test gains can be evaluated before the onset of unrecoverable behavior. The Youla framework also provides inherent stability guarantees at each frozen point during the transition.