State Variable Feedback Control Theory
In the previous two chapters, the classical control theory was discussed and the concept of transfer function was used to describe the dynamic behavior of various systems. In practice, the transfer function of higher order than three or four becomes tedious and it is better to use state variable control theory. It was stated that with PID control, three parameters can be adjusted to design control systems. With three parameters, all roots of characteristic equation cannot be adjusted to achieve desirable transient and steady state behavior. In practice, it is only possible to make one or two roots to become dominant in response and a compromise between the transient response and steady state error has to be made. The derivative term always amplifies the noise in the practical systems and is not recommended. Instead a lead-lag network produces a better response.