The case for the Smith-Åström predictor
Stefan L. Hölzl
Abstract
The general opinion on Åström et al.’s modification of the Smith predictor (which allows for plants with an integrator) appears to be quite poor: “good performance, yet (far) too complicated”. This opinion, however, is based on missing insight and – sadly enough – a printing error in the original publication. The present article argues the case for the Smith-Åström predictor to rehabilitate its general perception by (1) explaining its working principle (focusing more on practical than on theoretical insights) and (2) comparing it to other modifications of the Smith predictor. As in the original publication, the considered plant model is an integrator with dead time for which it is shown that some subsequent modifications yield the same disturbance-to-output behaviour as the Smith-Åström predictor, yet in contrast to some of them, the latter also provides an explicit disturbance estimate. To complete the plea, the original results are extended by addressing practical issues such as parameter tuning, actuator constraints, model uncertainties (including unmodelled dynamics), and disturbance reconstruction, all of which are illustrated by means of simulation; furthermore, the design procedure is given in a more general fashion to allow for different (and possibly unstable) plants. It is shown that the Smith-Åström predictor yields good results and is easy to use!