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Frequency domain interpretation of the Smith predictor

Abstract:

Dealing with the control design problem for a given system, the presence of pure time-delays in the plant imposes interesting challenges from the control point of view. In fact, different techniques can be applied among them the well-known Smith predictor. In this paper, the use of the Smith predictor is analyzed and approximated in the frequency domain by means of a Taylor expansion. Additionally, this approximation can be interpreted as the introduction of a prediction in the loop to modify the Nyquist plot, such that stability margins are improved. As an alternative, a notch filter to reduce the gain at the crossover frequency is also considered. A control design example for an unstable time-delayed plant illustrates this approach. Some advantages and drawbacks of this control design approach are also outlined.