This paper enlarges the stability range of time-delay control systems with delay mismatch. First, various time-delay control systems are described as a canonical form on the basis of internal model parametrization of controller. Second, The mechanism of narrow stability range for delay mismatch is clarified for the canonical system. It is shown that the 1-order Padé approximation of delay is necessary for removing the defect and enlargement of the stability range. The sufficient conditions are derived for enlargement of robust stability range. The design method of the controller is proposed on the basis of the conditions. Third, it is clarified that the 1-order Padé approximations of delay in predictor and in the controlled system yield to the same structure about robust stability and the proposed method is less conservative than the conventional robust control. Finally, the numerical examples illustrate that the stability range can be enlarged considerably.
