A New Design Method for Discrete Equivalents of Analog Controllers

This paper considers the design method of discrete equivalents of analog controllers. Firstly, existing methods are reviewed with numerical examples and it is shown that they introduce extra phase-lags at high frequencies compared to the original controller. Secondly, a new design method for discrete equivalents of analog controllers is proposed. The proposed design method is an improved pre-warped bilinear transformation and is simple. The resulting digital controller does not have any extra phase-lag compared with the original analog controller in a frequency range under the maximum absolute value of poles and zeros of the analog controller, even when it is lower but comparable with the Nyquist frequency. Validity of the proposed method is shown by numerical examples.