An H∞ approach to two degree of freedom design

The authors introduce a novel design procedure for two-degree-of-freedom controllers. The technique will be used to design a control system for the distillation column design case study described by D. J. N. Limebeer (1991). The feedback controller and the prefilter are designed in a single step in an H∞ optimization framework. The feedback controller is used to meet the robust stability and disturbance rejection specifications, while the prefilter is used to shape the time responses of the closed loop. This approach has the advantage that no additional controller states are required for the prefilter; the feedback controller and prefilter share the same state space. It is proved that the state estimator part of the controller is a Kalman filter, which is a property inherited from the normalized coprime factorization design procedure given by D. McFarlane and K. Glover (1990)