Generalized error coefficients for the multivariable servomechanism problem

The steady-state tracking and regulation errors are considered for a controlled linear time-invariant system which includes either a feedforward compensator [1] or a robust servo-compensator [2]. The controlled system is subjected to classes of reference and disturbance inputs larger than and including the class of inputs for which the compensator was designed, and the resulting steady-state error is parameterized in terms of generalized error coefficients. These define the steady-state error in terms of plant and controller parameters, and so represent a generalization of the error coefficients of classical control theory. It is then shown that exact steady-state error regulation may be achieved in the robust servomechanism controller for a class of disturbance/reference-input signals which is larger than the nominal class of input signals for which the controller is designed.