Design of robust QFT controllers and prefilters for 3×3 distillation column

Abstract

Design of robust controller for multivariable systems is very challenging and their automation is a key concern in control system design. In this paper, a fixed structure multivariable Quantitative Feedback Theory (QFT) controller has been synthesized by solving QFT quadratic inequalities of robust stability and tracking specifications. The quadratic inequalities (constraints) are posed as Interval Constraint Satisfaction Problem (ICSP). The constraints are solved by constraint solver - RealPaver. The synthesis problem consists of obtaining a controller that ensures stability and meets a given set of performance specifications, in spite of disturbance and model uncertainties. In addition to perform the above tasks, a MIMO controller also has to perform the difficult task of minimizing interaction between the various control loops. In QFT, the prefilter is designed to achieve the desired tracking specifications. The synthesis of prefilter in QFT is usually carried out manually, and largely depends on the designer's skill and experience. Thus, a good design is not always assured by the manual design process. Thus, it is necessary to have methodology for automated synthesis of prefilter for multivariable systems. In the design method, we pose the prefilter design problem as an ICSP and solve it using the well-established constraint solver. To validate the above design approach, the designed controllers and prefilters are simulated on 3×3 distillation column system.