Internal models in regulation, stabilization, and tracking

In linear control systems, internal models have been described in terms of poles in the unstable (bad) region of the complex plane which contain the needed information for the control system to attain the desired objective. It has been erroneously believed for some time that such internal models did not exist in the nonrobust regulation problem when a generalized plant was considered. It is shown that such internal models always exist not only in regulation, but also in stabilization, and in tracking if the appropriate physically meaningful maps are considered, thus completely resolving the existing discrepancy between abstract results and intuition on one hand, and linear regulation results in the literature on the other. Both robust and nonrobust control problems are considered, and a complete treatment of the internal models in all basic problems is presented