Improved independent design of robust decentralized controllers

The procedure for independent design of robust decentralized controllers proposed by Skogestad and Morari is improved by requiring that each individual controller is of the internal model control (IMC) type. It is shown how to find bounds on the magnitude of the IMC filter time constants such that robust stability or performance is guaranteed. In contrast, Skogestad and Morari found bounds on the sensitivity functions and complementary sensitivity functions for the individual loops, and therefore allowed a much larger class of designs, resulting in more donservative conditions. A property called robust decentralized detunability (RDD) is introduced. If a system has this property, any subset of the loops can be detuned independently and to an arbitrary degree without endangering robust stability. A simple test for RDD is developed for systems controlled by decentralized IMC controllers.