The servomechanism problem has been successfully dealt with for linear, time-invariant systems. Many results are available [1-11] and an excellent summary is provided by Desoer and Wang [2]. A more
abstract discussion is give by Wonham [11]. It has been shown that an essential ingredient in a controller designed to solve the servomechanism problem is an internal model system. This internal model
system is a system which replicates the dynamics of the exogenous signals (i.e., reference and disturbance) in the feedback loop.
Because any real system is seldom linear, it is important to consider the servomechanism problem for nonlinear systems. Some results
exist for the nonlinear problem [12-15]; but for the most part, the results apply only when the reference and disturbance signals are constant.
Desoer and Wang [12] have approached the problem using input-output techniques. They have considered a linear system with nonlinearities
both preceeding it (input channel nonlinearities) and following it (output channel nonlinearities). They first treat the case of input channel nonlinearities (such as a sensor nonlinearity). Conditions are given as to when tracking (disturbance rejection is not considered) will occur. Although conditions are given, no method is provided which will enable one to construct a suitable controller nor is discussion given as
to ways of testing the conditions. The main results derived by Desoer and Wang, however, are for memoryless nonlinearities (both input and output channel). These results are valid only for reference and disturbance signals which tend to constants. The conditions given for a solution to the problem are precise, however, it appears that the
algorithm recommeded for selecting the control law is useful only for