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A necessary and sufficient condition for the existence of a servo controller is derived using the fractional representation approach for the plant, whose measured variables are not necessarily coincident with its controlled outputs. An independent parametrization of the servo controllers and the attainable transfer characteristics is proposed. The possible classes of all controllers with integrity in the error channel or measured output channel is also clarified using the derived parametrization.
Shinji Hara (Sun,) studied this question.