A compensator is an additional component that is incorporated into a system to equalize or compensate for performance insufficiency. There are several basic types of compensators used in many classical SISO control applications. Most of these compensators are normally placed in series with the system to be compensated with feedback applied around the combination. This configuration is often called cascade compensation and is illustrated below:

The system to be compensated is defined as
*GH*(*s*) and the compensator as *C*(*s*). Negative feedback
is applied around the series combination. The signal *R* is a reference
signal, and the signal *Y* is the output signal.

The basic types of compensators designed here are:

• Gain or Proportional (P)

• Proportional - Derivative (PD)

• Lead

• Proportional - Integral (PI)

• Lag