FRIT (Fictitious Reference Iterative Tuning) is a model free designing method and a practically useful method. In the method, the optimal controller's parameters are searched to minimize a performance index. If the index is expressed in a form of a quadratic function of the parameters, then the optimal parameter is derived by a least square method straightforwardly. This paper gives a quadratic form of the index for the case of the general form of controllers.
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