In this paper the following three control systems for first-order time-delayplants are studied and compared: the feedback proportional-integral controller(PI), the Smith Predictor (SP) and a proposed variable structure consisting oftwo blocks. This structure acts as an open-loop proportional controller, aftera setpoint change, and as a closed-loop integrating controller, when the errorenters in a preset band. A chart, provided with the borderlines of thestability zone and with the curves of two design parameters, is implemented foreach controller. The first parameter is the overshoot of the controlledvariable, evaluated during a step change of the setpoint and made equal to apreset value. The second parameter, only for the PI and SP controllers, is theintegral of the squared error (ISE), which must have the minimum allowablevalue. The ISE is also assumed as comparison index and the proposed controllerappears as the best.
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