A control system involves a plant and a controller. Plants are objects as diverse as a satellite, a distillation column, a robot arm, and a colony of bacteria. After measuring actual outputs of the plant, the controller computes signals that are applied at the inputs to the plant to achieve desired outputs. The design of controllers must be based upon mathematical models of plants, which are in most realistic situations composed of nonlinear differential and difference equations. A standard approach is to linearize the equations and use the powerful methods available for the design of linear control systems.