**Subject :**Advance control system

## Improving Steady-State Error via Cascade Compensation

- We discuss two ways to improve the steady-state error of a feedback control system using cascade compensation.

- One objective of this design is to improve the steady-state error without appreciably affecting the transient response.

- The first technique is ideal integral compensation, which uses a pure integrator to place an open-loop, forward-path pole at the origin, thus increasing the system type and reducing the error to zero.

- The second technique does not use pure integration.

- This compensation technique places the pole near the origin, and although it does not drive the steady-state error to zero, it does yield a measurable reduction in steady-state error.

- While the first technique reduces the steady-state error to zero, the compensator must be implemented with active networks, such as amplifiers.

- The second technique, although it does not reduce the error to zero, does have the advantage that it can be implemented with a less expensive passive network that does not require additional power sources.

- The names associated with the compensators come either from the method of implementing the compensator or from the compensator's characteristics.

- Systems that feed the error forward to the plant are called proportional control systems. Systems that feed the integral of the error to the plant are called integral control systems.

- Finally, systems that feed the derivative of the error to the plant are called derivative control systems.

- Thus, in this section we call the ideal integral compensator a proportional-plus-integral (PI) controller, since the implementation, as we will see, consists of feeding the error (proportional) plus the integral of the error forward to the plant.

- The second technique uses what we call a lag compensator. The name of this compensator comes from its frequency response characteristics.

- Thus, we use the name PI controller interchangeably with ideal integral compensator, and we use the name lag compensator when the cascade compensator does not employ pure integration.