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.