Tammi K., Hötönen J., Daley S.

The paper describes experimental work that demonstrates the use of repetitive control to attenuate rotor vibrations. The experiments were performed on a rotor test rig having a 3-kg rotor supported by journal bearings and with a critical speed of approximately 50 Hz. The objective was to control the 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 control scheme comprises of two algorithms, an inner loop collocated feedback system and an outer loop repetitive controller. The inner loop controller is utilised to provide favourable characteristics for the repetitive control loop that compensates for the periodic excitation at the rotor midpoint. A novel aspect of the controller design is that the length of the control output vector of the repetitive controller was updated as a function of the rotational speed. This results in an adaptive repetitive control algorithm that could be also applied to a variable-speed machine. The repetitive control method used a time reversed FIR model of the plant to make the loop-gain function positive real in the frequency band of interest. A non-causal band-pass filter was applied on the control output vector to restrict high-frequency control actions to avoid the excitation of non-linearities in the control path. The experimental results illustrate that the repetitive controller successfully compensated for the excitation at the frequency of rotation. The operating range of the rotor was extended beyond the critical speed. The results obtained were comparable to those achieved in earlier studies with feedforward compensation methods. The best results were achieved when the frequency of rotation enables an integer ratio between disturbance period and sample rate.