1. Field of the Invention
This invention relates generally to apparatus for balancing the rotor of a magnetic suspension system including a magnetic bearing and more particularly to apparatus for auto-balancing the rotor so that it spins about its principal axis of inertia rather than its central geometric axis when the two axes are not coincident.
2. Description of the Prior Art
In any suspension system for a rotating element such as the rotor of an electromagnetic machine, rotor balance poses a problem since it is virtually impossible to machine and mount a rotor in bearings so that the axis of inertia of the rotor exactly coincides with the axis of rotation defined by the bearings. The resulting non-coincidence results in undesired vibration and power loss. To alleviate this problem, great efforts have been expended to mechanically balance the rotor with high precision and great delicacy; however, it is virtually impossible to thereafter compensate for aging or thermal deformations without additional mechanical rebalancing requiring undesired shut-down of the equipment and furthermore such deformations cannot always be substantially compensated for all operating speeds.
With the development of magnetic bearings for the suspension of a rotor, the existence of any rotor unbalance results in the tendency of the rotor to rotate about the principal axis of inertia lying closest to a desired axis of rotation defined by the bearing rather than the axis of rotation. In the context of this application, the principal axis of inertia of the rotor is hereinafter referred to simply as the axis of inertia or inertial axis. Where such a condition exists, it can be compensated for by detecting the position of the rotor for any departure from its predetermined axial position from which energizing signals are generated and applied to the windings of the bearing to bring the axis of rotation back into proper alignment.
One known technique for compensating for synchronous disturbances of a rotor which is supported by a radial magnetic bearing is disclosed in U.S. Pat. No. 4,121,143, entitled, "Device For Compensating Synchronous Disturbances In The Magnetic Suspension Of A Rotor", issued to H. Habermann, et al. on Oct. 17, 1978. As disclosed in this patent, a two axis tracking notch filter implemented by a pair of lowpass integrators and two resolvers are connected in a two axis feedback loop which is coupled into the X and Y axis position control loops of the magnetic bearing control system. The tracking notch filter reduces the control loop gain at the rotor's frequency of rotation, thus allowing the rotor to spin about its inertial axis rather than its central geometric axis. This in turn reduces the reaction forces and vibration coupled to the stator and so reduces the power dissipated in the control system. An inherent limitation exists in such a system due to the fact that since resolvers multiply their respective inputs by the factors sin .omega.t and cos .omega.t, where .omega. is proportional to rotational speed of the rotor and generate therefrom a pair of output signals utilized in the feedback loop, the loop gain reduction can only occur at the fundamental rotational frequency .omega.. Accordingly, any imbalances and asymmetries which exhibit higher harmonics of the rotational fundamental frequency, will still transmit vibrations forces to the stator and thus cause the control system to necessarily dissipate power in trying to oppose these forces.
It is a primary object of the present invention, therefore, to provide an improvement in the control of a magnetic suspension system for a rotor which is supported by a radial electromagnetic bearing.
It is another object of the invention to provide an improvement in the auto-balancing a rotor in a magnetic bearing system.
And yet another object of the invention is to provide an improvement in a system for auto-balancing the magnetically suspended rotor so as to allow the rotor to spin about its inertial axis rather than its geometric axis.
And still a further object of the invention is to provide an improvement in an auto-balancing magnetic bearing system which operates to eliminate external vibration and reduce power consumption.