In most modern engineering systems, there is a need to control the evolution with

time of one or more of the system variables. Controllers are required to ensure

satisfactory transient and steady-state behavior for these engineering systems. To

guarantee satisfactory performance in the presence of disturbances and model

uncertainty, most controllers in use today employ some form of negative feedback.

A sensor is needed to measure the controlled variable and compare its behavior

to a reference signal. Control action is based on an error signal defined as the

difference between the reference and the actual values.