Angol
PI controller are simple, easy to design and are cheap but they fail to operate when controlled object is uncertain and highly non linear . It is used when their is no requirement of fast response, disturbance and noise are acceptable and when large transport delays are there in the system PD controller increase stability of the system by improving control since it can predict future error of the system. To avoid effect of sudden change in error signal derivative is taken from output response of system variable instead of error signal. Hence to avoid sudden change in control output resulting from sudden change in error signal D mode is designed to be proportional to change of output variable. Other then this D mode modifies process noise so it is not used alone. This book emphasizes on a relatively new approach i.e. design of PI-PD controller which gave better performance in comparison to PI and PD controllers.
