In the last chapter we considered the use of state feedback to modify the dynamics of a system through feedback. In many applications, it is not practical to measure all of the states directly and we can measure only a small number of outputs (corresponding to the sensors that are available). In this chapter we show how to use output feedback to modify the dynamics of the system, through the use of state estimators (also called ``observers). We introduce the concept of observability and show that if a system is observable, itis possible to recover the state from measurements of the inputs andoutputs to the system.
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