Notch Filter
When the servo gain coefficient is relatively large, high-frequency vibration components can't be effectively attenuated, that would trigger system resonance. In order to maintain the servo response performance and drive the mechanical load smoothly without reducing the gain, it rquires to effectively suppress resonance. Generally, servo manufacturers will install notch filter in the front channel of control loop to attenuate loop gain amplitude at the resonance point by a fixed point so as to realize vibration suppression.
Transfer function of notch filter is