Robust controller design for time delay systems using H-infinity techniques

Using H∞ control, the design problem is formulated in terms of user defined weighting polynomials on the process closed-loop Sensitivity functions to achieve desired closed-loop performance and robust stability in the presence of process modelling error. In this paper stability conditions , in terms of the process sensitivity functions, are derived for processes containing a pure time delay for the following three design scenarios i) the time delay is neglected in the control design and is considered as part of the unmodelled process dynamics ii) A Pade approximation of the delay is included in the controller design and the effect of the residual time-delay modelling error on stability is considered iii) stability conditions for time delay mismatch using the Smith Predictor are derived.