A Multivariable QFT Design for a Paper Machine Benchmark

Abstract

This paper presents an alternative robust control system design for an uncertain non-linear multivariable paper machine benchmark problem. The aim is to control the output variables basis weight and ash contents with the control inputs thickstock and filler flow, such that time domain specifications are fullfilled for all plant cases. In this paper, a static non-diagonal non-linear precompensator is first designed to minimize steady state cross coupling. Then a set of equivalent linear transfer functions are identified by Least Squares. The time domain specifications are translated to approximative frequency domain specifications. Based on the frequency domain specifications, and the set of equivalent transfer functions, a diagonal dynamic feedback controller and a diagonal dynamic prefilter are designed according to the Horowitz robust, control design method, or QFT. Time domain simulations of the controlled non-linear system show that the specifications are satisfied. The paper illustrates a practical design method for multivariable paper machine control, whereby design trade-offs are made visible during the design process.