ON THE DESIGN OF ROBUST INTERNAL MODEL-BASED CONTROLLERS

We consider methods for synthesizing robust controllers that can be di rectly implemented in the Internal Model Control (IMC) scheme. We show how to put H-infinity design methods into the IMC scheme to improve i ts robustness. These methods include one degree-of-freedom (one d.o.f. ) and two degrees-of-freedom (two d.o.f.) H-infinity designs. Explicit state-space formulae for the H-infinity one d.o.f. and two d.o.f. IMC -based controllers are derived in discrete time. We show that the IMC controller solves a special H-infinity control problem, namely a distu rbance feedforward control problem, and it proves robust stability aga inst coprime factor uncertainty and with a degree of robust performanc e for the two d.o.f. case, in the sense of making the closed-loop syst em match a pre-specified reference model. A design for a glass tube pr oduction process is presented as an illustrative example.