Model based design of decentralized control configurations

by Schmidt, Henning

Decentralized control is almost the de-facto standard for control of large scale systems, and in particular for systems in the process industry. An important task in the design of a decentralized control system is the selection of the control configuration, the so-called input-output pairing. Previous research addressing this problem has primarily focused on stability issues. In this thesis the problem of selecting control configurations that can deliver a desired performance is addressed. It is assumed that the various subsystems are to be tuned independently, and hence the issue of interactions between the subsystems is central. It is shown that existing measures of interactions, such as the Relative Gain Array (RGA), are poor for selecting configurations for performance due to their inherent assumption of perfect control. A new interaction measure, the decentralized relative gain (dRG), which takes finite bandwidth and decentralized control into account is proposed. The dRG, as the RGA, measures effects of interactions on the open/loop properties. To select configurations for specific closed-loop properties, measures for the effect of interactions on the overall closed-loop performance are also derived in this thesis. Model based tools for selecting control configurations with respect to setpoint tracking as well as attenuation of specific disturbances are proposed. The usefulness of the proposed tools are demonstrated on several examples from the process industry.