Research On Unbalance Vibration Compensation For Active Magnetic Bearings System

In active magnetic bearings (AMB) system, unbalance vibrations synchronous with the rotationalspeed will be produced because of the rotor's mass unbalance. To reduce the bad influence of theseunbalance vibrations, force minimum compensation and displacement minimum compensation arestudied in this paper.Aiming at the inertial force minimum vibration compensation, the major solution is weakeningAMB system control current through eliminating the unbalance vibrations in feedback displacements.The decrease of the control currents helps to weaken the electromagnetic force of magnetic baeringstators and the radial constraint on the rotor, and make the rotor spin around its inertia axis under thecentrifugal force. Finally, the purposes of reducing the effects of rotor inertia force andelectromagnetic force of magnetic baerings are implemented. Three kinds of compensation schemesare adopted in this paper. Firstly, a scheme of switching compensation control in fixed frequencyusing adaptive filter based on norm LMS algorithm is presented. As a filter, the norm LMS algorithmis added into the primary feedback control loop, and the unbalance vibration at some fixed rotationalfrequencies in the feedback displacement can be filtered effectively through switching. Inertial forceminimum switching compensation is achieved at300Hz and400Hz, and the amplitudes of controlcurrent after compensation are attenuated90%and95%compared with the amplitudes beforecompensation, respectively. Secondly, in order to realize real-time inertial force minimumcompensation, implementing inertial force minimum compensation with changing rotationalfrequencies, a new variable step LMS algorithm is presented, and its core is providing feed-forwardsignal to compensate the unbalance vibration. The control current amplitudes after compensation isattenuated97.4%at300Hz, and the compensatory effects become better with the increase of the rotorrotational frequency. Thirdly, in view of the interference influence of the two proposed compensatoryschemes on the primary control system of AMB, another real-time inertial force minimumfeed-forward compensation scheme combined by H∞algorithm and a new modified variable stepLMS algorithm is proposed. This compensatory scheme also realizes inertial force minimumcompensation for AMB system at any frequency, and improves the anti-jamming capability andstability of the control system. The amplitudes of control current after compensation can alsoattenuate97.4%at300Hz, and the higher rotational frequency the better compensatory effects can beachieved.Aiming at the displacement minimum vibration compensation, a feed-forward displacement minimum compensatory scheme is proposed based on a new adaptive iterative learning controlalgorithm in consideration of the periodic variation of unbalance interference in the processes of rotorstart and shutoff. In the compensation scheme, this algorithm can reduce the interference to AMBcontrol system due to the inconformity of learning circle and can adaptively provide expectedunbalance compensatory signals. Study results show that the proposed scheme can achievedisplacement minimum compensation for AMB system in fixed rotation frequency, and observablyreduce the rotor radial runout. The radial displacement after compensation has reduced by93.1%.The beat vibration phenomenon existing in the AMB system is discovered, and the internalrelation between beat vibration and the parameters of AMB system is theoretically analyzed.Simulation and experiment results show the unfavorable effects of beat on the control system can beeffectively weakened by changing the generalized dynamical stiffness of AMB.