Combined feedback-feedforward control for rotor vibrations

The present experimental work demonstrates the use of feedback and feedforward control methods to attenuate rotor vibrations. The experiments were performed on a rotor test rig having a 3-kg rotor supported by journal bearings. The critical speed of the rotor was about 50 Hz. The objective was to control radial response at the rotor midpoint by using an actuator located outside the bearing span. Control forces were generated with an electro-magnetic actuator. The identification of the dynamic system was carried out experimentally. The transfer functions were defined with a new method taking into account the disturbance due to rotation. The feedback control method was approximately collocated: the feedback signal was provided by the displacement sensors at the end of the rotor. The feedback system was a derivative-type controller providing basic damping into the rotor system. A feedforward control system was constructed to work together with the feedback controller. So called higher harmonic control method was applied. The control objective of the feedforward system was to compensate the response at the rotor midpoint due to a mass imbalance in the rotor. The feedforward system used the displacement sensors at the rotor midpoint to adapt the compensating force and the rotational speed measurement to generate a reference signal. The control methods together provided over 90 % decrease in the displacement response of the rotor midpoint at sub-critical speeds. For super-critical speeds, the decrease in the response was from 10 % to 40 % due to the restricted control authority. The control authority was restricted because the amplitude was limited at the actuating point. In addition to these results, the paper also discusses the significance of sufficient damping when applying a feedforward compensation system. An example shows how modelling errors may lead to an unstable feedforward control system for lightly damped mechanical systems.