FAQ: Why is it better to use a lead controller than a PD controller?

(Contributed by Elisa Franco, 3 Dec 07)

The lead controller helps us in two ways: it can increase the gain of the open loop transfer function, and also the phase margin in a certain frequency range. But why could we not use just a PD controller, which increases the phase over a wider range of frequencies?

The use of the derivative action should always be limited: it helps improve the control performance when the error signal changes rapidly, but for the same reason it could amplify unwanted disturbances on the measured output. Hence, a lead compensator rolls off the gain at high frequency. The same transfer function can be realized by coupling a PD controller with a low pass filter (whose break frequency is chosen to be above the frequency of the zero from the PD controller).