Limitations of PID controllers

The objective of the paper is to outline the limitations of classical PID controllers acting on the error signal for control of a plant modelled as a linear transfer function. It is shown how a modified form of controller, a PI-PD controller, which corresponds to PI control of a plant transfer function changed by the PD feedback can produce improved control in several situations. This implementation avoids the derivative kick problem associated with derivative action in the forward path, which still exists when a filter is included. Further, the PD in the inner feedback loop enables placement of the open loop poles in appropriate positions thereby providing good control for open loop system transfer functions having resonances, unstable, or integrating poles. The simulation results show significantly improved performance of the proposed control method particularly for the latter processes.