Electrical generator or motor structure – Dynamoelectric – Rotary
Patent
1990-03-19
1992-08-18
Skudy, R.
Electrical generator or motor structure
Dynamoelectric
Rotary
318649, H02K 524, F16L 3906, G05B 1132
Patent
active
051402092
DESCRIPTION:
BRIEF SUMMARY
This invention relates to magnetic bearings. It is known to use electromagnets to support a rotatable shaft without mechanical contact. It is also known to determine the position of the shaft relative to the supporting magnets and to use this determination to apply a correction for any unwanted displacement from a selected position.
However, absolute position of the shaft relative to the magnets is only one part of the problem, since it is generally insufficient to merely apply a restoring force proportional to the magnitude of the displacement. To illustrate this, it is possible to achieve bearing "stiffness" by applying large restoring forces in response to very small displacements, but this is at the expense of also achieving vulnerability to over-shoot, beyond the target or zero displacement attitude. Stiffness may thus be accompanied by unacceptable instability.
In an attempt to overcome this problem of instability it has been proposed that the determination of absolute position should be further processed to generate an indication of the rate of change of position and that this latter indication should be combined with the absolute position, the resultant combined indication being used to generate a restoring force whose magnitude/direction not only reflects absolute position but also rate of change of position. The latter introduces damping to the system, to counteract the stiffness developed in response to small changes of absolute position alone.
For example, a Hall-effect sensor may be employed to generate an electrical signal directly representative of position. This signal may then be differentiated to generate a second electrical signal, which is representative of the rate of change of absolute position. By applying these two signals, amplified as necessary, to a summing amplifier, a combined control signal for use in controlling the electromagnets of a magnetic bearing system can be developed. As stated above, the arrangement just described adds damping to counterbalance stiffness which has been achieved by correcting only for departures from absolute position. In practice however, the range of displacement over which both controlled stiffness and damping are achieved becomes too narrow. It will be appreciated that for satisfactory operation, damping must be effective throughout the full range of absolute displacement, since outside constraints may prevent the latter from being held indefinitely within the narrow range over which stiffness can be achieved.
It is an object of the present invention to overcome, or at least mitigate the aforementioned difficulty.
According to the present invention a method of controlling the electromagnets of a magnetic bearing comprises the steps of (1) deriving from a first sensor a first signal representative of the absolute position of a shaft supported in said bearing, (2) limiting said first signal to a first control range, (3) deriving a second signal substantially independent of the first signal and also representative of said absolute position, (4) processing said second signal to derive therefrom a third signal representing the rate of change of said absolute position over a second control range, (5) combining said first and third signals to produce a control signal, and (6) applying said control signal to control said electromagnets so as to restore the shaft to a desired position in said bearing.
The first control range is preferably limited to correspond to a small range of displacement from the desired position, such that the full magnetic restoring force is developed in response to said small displacement. The second control range is preferably selected to correspond to the full range of possible displacement. It will be seen that the arrangement provides maximum restoring force (in response to the limited first signal) over a small displacement range, thereby conferring maximal stiffness on the system. Using conventional techniques, the third signal would also be effective over the same range, thereby restricting the damping effect to that narrow
REFERENCES:
patent: 3215901 (1965-11-01), Carniol
patent: 4090745 (1978-05-01), Dohogne et al.
patent: 4379598 (1983-04-01), Goldowsky
patent: 4999534 (1991-03-01), Andrianos
Rebsch D. L.
Skudy R.
The Glacier Metal Company Limited
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