The multivariable servomechanism problem from the input-output viewpoint

The control system considered consists of a plant or actuator controlled by a compensator processing the error between a reference command input and the plant output. The plant and compensator are modeled in input-output form by linear convolution equations. Closed-loop stability and tracking are characterized, and the problem encountered when these two properties are insensitive to small errors in the plant and compensator models is considered. Finally, these results are compared with recent results on the corresponding state-space problem.