treated where proportional plus sum (discrete-time integral) control is employed.

     Other than the case of constant reference and disturbance signals,

it appears that there were no satisfactory results for the nonlinear servomechanism problem.   One could attempt to linearize the nonlinear system and design a controller based on linear servomechanism theory. However, such an approach will usually lead to steady-state tracking

error.

     In this dissertation, the servomechanism problem is solved for a class of multi-input, multi-output, nonlinear systems. Here the results are valid for reference and disturbance signals which belong to a much wider class of signals than simply those which tend to constants.

     The major contributions of this research are:

1.   Conditions are given for a solution to the nonlinear servomechanism

     problem.     When  these  conditions  are   not  satisfied  exactly, employing the type    of controller developed    here  still makes

     intuitive sense.

2.   The problem is solved in the time-domain using a completely new

     approach.   A time-domain approach is necessary because standard

     techniques (i.e., frequency domain analysis) used for solving the

     linear problem are not applicable to nonlinear systems.

3.   It becomes apparent that the idea of an internal model system which

     contains the dynamics of only the reference and disturbance signals

     is not complete.    It is shown that actually, the internal model system should include the dynamics found in both the input and in

     the state which must be present during successful tracking.