  The internal model principle states that all robustly regulating controllers
must contain a suitably reduplicated internal model of the signal to be
regulated. Using frequency domain methods, we show that the number of the
copies may be reduced if the class of perturbations in the problem is
restricted. We present a two stage design procedure for a simple controller
containing a reduced order internal model achieving robust regulation. The
results are illustrated with an example of a five tank laboratory process where
a restricted class of perturbations arises naturally.
