Can we view the controller or the environment as a separate machine with its own state variables?
Environmental factors are sometimes called exogenous variables (the state of the environment) and they affect the dynamics of the system under consideration. A controller is also an input-output subsystem that has its own state. However, in this class, we will focus mainly on the state variables of the plant (the process that we want to control). --Soto 17:11, 3 October 2007 (PDT)