Multiple-model adaptive estimators: Open problems and future directions

Over the past decades there have been a considerable research effort on the subject of adaptive observers. In simple terms, an adaptive observer or estimator can be defined as a process that provides in real time the estimate of the state (or some function of it) of the plant from partial and possibly noise measurements of the inputs and outputs, inexact knowledge of the initial condition and in the presence of unknown parameters in the description of the plant. In many cases, adaptive observers also identifies the unknown parameters.