Magnetic bearing apparatus

Abstract: A radial magnetic bearing apparatus suspends a rotating shaft and stably operates at a level exceeding a bending intrinsic vibration value of the rotating shaft by controlling magnetic forces generated by control coils wound around control magnetic poles. Grooves (11,12) are provided on the surface of each control magnetic pole (31,32) facing the rotating shaft, and sensor coils (6,7) are wound to pass through the grooves (11,12) in order to detect the position of the rotating shaft in the radial direction. A part of each control magnetic pole may be used as magnetic poles of an inductance-type displacement sensor.

Description: The present invention relates to a radial magnetic bearing apparatus for suspending a magnetic object by controlling a magnetic force with electromagnets, and, particularly, to the structure of control magnetic poles of the electromagnets. Fig. 1 is a sectional view of a radial magnetic bearing apparatus of the prior art. A control magnetic pole core 1 is provided with control magnetic poles 31, 32, 35 and 36 around which control coils 51, 52, 55 and 56 are wound. An object made of a magnetic material, for example, a rotating shaft 8, is suspended in a direction X under a control force of electromagnets by control magnetic poles and control coils. The generated control magnetic poles 31 and 32 form a pair. The respective control coils 51 and 52 are wound to form different magnetic poles so that the magnetic fluxes formed by these coils are closed. Therefore, a pair of control magnetic poles 31 and 32 attract the rotating shaft 8 in the +X direction. On the other hand, a pair of control magnetic poles 35 and 36 provided opposite to the control poles 31 and 32 also attract the rotating shaft 8 in the -X direction. As a result, the rotating shaft 8 is positioned and supported in the direction X without any contact with the control magnetic poles 31, 32, 35 and 36 by balancing the magnetic force generated by the electromagnets. In the same manner, the rotating shaft 8 is also positioned and supported in the direction Y without any contact with the control magnetic poles 33, 34, 37 and 38 by balancing the magnetic force generated by the electromagnets comprising a pair of control magnetic poles 33 and 34 and a pair of control poles 37 and 38. A pair of control poles located at a position opposite to a pair of control poles provided with control coils supports the rotating shaft 8 in a balanced position under a magnetic force generated by these electromagnets. As such, it is necessary to control excitation of the control coils by detecting the position of the rotating shaft, in order to balance the magnetic forces generated by the electromagnets. To this end, an inductance-type displacement sensor is usually provided adjacent to each control magnetic pole to detect the radial position of the rotating shaft 8. This sensor has a magnetic pole similar to the control magnetic pole and around which a sensor coil is wound for detecting a change in inductance. Fig. 2 is a sectional view of inductance-type displacement sensors provided in the radial magnetic bearing apparatus shown in Fig. 1. The inductance-type displacement sensors comprise sensor magnetic poles 41 to 48 and sensor coils 61 to 68 wound around the sensor magnetic poles. The position of the rotating shaft 8 in the direction X can be detected by measuring a ratio of inductance of a pair of sensor coils 61 and 62 wound around the magnetic poles 41 and 42, respectively, and the inductance of a pair of sensor coils 65 and 66 wound around the magnetic poles 45 and 46 located opposite to the magnetic poles 41 and 42 with respect to the rotating shaft. The position of the rotating shaft 8 in the direction Y is detected by measuring a ratio of the inductances of the sensor coils 63, 64, 67, 68 in a similar manner. Fig. 3 is a sectional view of the radial magnetic bearing apparatus shown in Fig. 1. As shown in Fig. 3, displacement detecting points of the inductance-type displacement sensor comprising the sensor magnetic pole 41 around which the sensor coil 61 is wound and the inductance-type displacement sensor comprising the sensor magnetic pole 45 around which the sensor coil 65 is wound are located by ΔL below in the axial direction of the rotating shaft 8 from working points of the control magnetic pole 31 and the control magnetic pole 35, respectively. However, such a conventional magnetic bearing apparatus having control magnetic poles and magnetic poles of inductance-type displacement sensors, when operated at a level above the intrinsic value of bending vibration of the rotating shaft 8 results in oscillation which is impossible to control. Fig. 4 is a diagram explaining a bending mode vibration at the intrinsic value of bending vibration of the rotating shaft 8. In Fig. 4, positions AA' and CC' indicate the position of the working point of the control magnetic poles, while positions BB' and DD' indicate the position of poles of the displacement sensors. The control magnetic pole positions AA' and CC' are displaced by ΔL from the magnetic pole positions BB' and DD' of the displacement sensors as shown in Fig. 3. Under a normal control condition, the rotating shaft 8 is straight as indicated by symbol 8a, but under conditions of vibration occurring at the bending intrinsic value, the shaft 8 is bent as indicated by symbol 8b. Therefore, since each displacement sensor detects a displacement at the position separated by ΔL from the position of the control magnetic pole under vibration occurring at the bending intrinsic value, the displacement sensor may erroneously indicate that a control force is required to be exerted upwardly, although a control force exerted downwardly is essentially required at the operating point of the control magnetic pole. In such a case, since an exciting current flows into the control magnetic pole to generate a control force exerted upwardly, the rotating shaft 8 is driven to an oscillating condition, resulting in a loss of control on the magnetic bearing apparatus. Such oscillating and no control conditions may be avoided by conforming the position of the operating point of each control magnetic pole with the position of a corresponding magnetic pole of the displacement sensor. However, conventional displacement sensors consist of magnetic sensors, except for optical sensors or electrostatic capacitance-type sensors. Accordingly, if an inductance-type displacement sensor is mounted near a control magnetic pole, the displacement sensor generates an error signal by magnetic fields generated by the control pole. On the other hand, an optical sensor is very expensive and is not desirable from the point of view of cost. Moreover, since control magnetic poles generate heat due to exciting currents, electrostatic capacitance-type sensors which are easily influenced by a change in temperature can not be employed. FR-A-2 163 152 which was used as a basis for drafting the preamble of claim 1 shows a magnetic bearing comprising a magnet coil and a position detector located inside the magnet coil. The position detector comprises two sensor coils which are connected in series and induce magnetic fields in opposite directions. It is therefore an object of the present invention to provide a magnetic bearing apparatus wherein inductance-type displacement sensors are not influenced by magnetic forces generated by control magnetic poles and leakage of magnetic fluxes to the outside of the sensor magnetic poles are reduced. In order to achieve the object described above, the present invention provides a radial magnetic bearing apparatus as set forth in claim 1. Preferred embodiments of the invention are shown in the dependent claims. The inductance-type displacement sensor having such sensor coils operates at an operating point of the corresponding control magnetic pole. If a magnetic bearing apparatus of the prior art is operated at a level exceeding the intrinsic value of bending vibration of the suspended object, a problem occurs that the position of a displacement sensor is separated from the position of a corresponding control magnetic pole. The present invention can overcome that problem. The position of the suspended object can be detected at the operating point of each control magnetic pole, and a control force required by each control magnetic pole can be determined accurately. As a result, the radial magnetic bearing apparatus can be operated stably at a level exceeding the intrinsic value of bending vibration of the suspended object. Adjacent two sensor coils are connected in series to generate magnetic fields in opposite directions. A plurality of pairs of control magnetic poles exert magnetic forces on the suspended object in the same direction and the sensor coils provided on the plurality of pairs of control magnetic poles are connected in series. Adjacent two sensor coils of adjacent two control magnetic poles are connected to form the same magnetic pole on the same side thereof. Grooves may be provided on end surface of a control magnetic pole in a circumferential direction or axial direction of the suspended object. One end surface of a control magnetic pole may be provided with a plurality of grooves. Sensor coils are wound passing through such grooves. Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings wherein: Fig. 1 is a sectional view of a radial magnetic bearing apparatus of the prior art; Fig. 2 is a sectional view of inductance-type displacement sensors provided in the magnetic bearing apparatus shown in Fig. 1; Fig. 3 is a sectional view of the radial magnetic bearing apparatus taken along the line III-III shown in Fig. 1; Fig. 4 is a diagram explaining a bending intrinsic vibration mode of a rotating shaft; Fig. 5 is a diagram illustrating control magnetic poles of a radial magnetic bearing apparatus as a first embodiment of the present invention; Fig. 6 is a diagram illustrating control magnetic poles of a radial magnetic bearing apparatus as a second embodiment of the present invention; and Fig. 7 is a diagram explaining electrical connection of the sensor coils in the embodiments of the present invention. The preferred embodiments of a radial magnetic bearing apparatus of the present invention will be explained hereunder with reference to the drawings. In Fig. 5 to Fig. 7, components like those in Fig. 1 to Fig. 4 are designated by like reference numerals or symbols. Fig. 5 illustrates the structure of a first embodiment of a radial magnetic bearing apparatus to which the present invention is applied. Fig. 5a is a plan view of a part including adjacent two control magnetic poles 31 and 32 of such magnetic bearing apparatus, while Fig. 5b illustrates a diagram of the control magnetic poles 31 and 32 viewed from the center of the rotating shaft 8 and Fig. 5c is a sectional view of the control magnetic pole 32 taken along a line C-C of Fig. 5b. In this embodiment shown in Fig. 5, two circumferential grooves are formed at one end of respective control magnetic poles whereby tongue members which function as the sensor magnetic poles of an inductance-type displacement sensor are provided. Around these tongue members sensor coils are wound. Specifically, as shown in Figs. 5b and 5c, on the inner end surface of the control magnetic pole 32 a couple of circumferential grooves 11 and 12 are formed to provide sensor magnetic poles 19 and 20 of the inductance-type displacement sensor. The sensor coils 6 and 7 are wound respectively around the sensor magnetic poles 19 and 20. As explained above, an inner end portion of the control magnetic pole 32 operates as the sensor magnetic poles 19 and 20 of the inductance-type displacement sensor. The control magnetic pole 31 is structured in the same manner as the control magnetic pole 32 and the sensor coils 4 and 5 are wound around corresponding sensor magnetic poles. As shown in Fig. 5a, the control coils 51 and 52 wound around the corresponding control magnetic poles 31 and 32 are connected to form a pair of poles so that control magnetic fluxes 9 are generated by these coils in opposite directions to each other to form a closed magnetic path passing through the rotating shaft 8. As shown in Fig. 5c, the sensor coils 6 and 7 are connected in series so that closed magnetic paths 10 passing through the corresponding sensor magnetic poles 19 and 20 are formed to generate magnetic fields in the opposite directions; namely to form magnetic fields having different poles on the same ends of the sensor magnetic poles. Accordingly, the control magnetic flux 9 formed by the control coil 52 serves to cancel the magnetic flux generated by the sensor coil 6 and to be added to the magnetic flux generated by the sensor coil 7. Therefore, any influence of the control magnetic flux 9 can be neglected by connecting the sensor coils 6 and 7 in series. As a result, the inductance-type displacement sensor can be operated without being influenced by the control magnetic flux 9 even if a part of the control magnetic pole 32 is used as the sensor poles 19 and 20. Fig. 6 is a diagram illustrating the structure of control magnetic poles of a radial magnetic bearing apparatus of a second embodiment according to the present invention. Fig. 6a is a sectional view of a part of the control magnetic pole core 1 and Fig. 6b is a sectional view of the control magnetic poles 31 and 32 viewed from the rotating shaft 8. In this embodiment, an inner end surface of each control magnetic pole 31 or 32 has two axial grooves 11 and 12 facing the rotating shaft 8, thereby forming tongue members which serve as the sensor magnetic poles 19 and 20 of the inductance-type displacement sensor and the sensor coils 6 and 7 are wound around the tongue members. As shown in Fig. 6, the sensor magnetic poles 19 and 20 are formed in the axial direction of the rotating shaft 8. These sensor magnetic poles operate in the same manner as those of the first embodiment shown in Fig. 5. Fig. 7 is a diagram explaining electrical connection of sensor coils shown in Fig. 5 and Fig. 6. The sensor coils 4, 5, 6, and 7 wound through respective grooves 11 and 12 formed on the control magnetic poles 31 and 32 are connected in series so that the magnetic fluxes are generated to pass through the corresponding control magnetic poles in the opposite directions with each other as indicated by the dotted lines, resulting in the generation of different magnetic poles on the same ends of the adjacent coils. On the other hand, the sensor coils 14, 15, 16 and 17 provided at a pair of control magnetic poles 35 and 36 located in the direction X opposite to the magnetic poles 31 and 32 are also connected in series so that the magnetic fluxes are generated in the opposite directions with each other at the corresponding control magnetic poles, resulting in the formation of different magnetic poles at the same ends of the adjacent coils. Further, the sensor coils 14 - 17 are connected in series with the sensor coils 4, 5, 6 and 7 provided at the control magnetic poles 31 and 32. A voltage generated by an AC oscillator 21 is applied across the terminals of the sensor coils 4 to 7 and 14 to 17 connected in series. A neutral point of the series-connected sensor coils are connected to a sensor amplifier circuit 22. Therefore, if the rotating shaft 8 moves, for example, in the +X direction, a composite inductance of a group of the sensor coils 4, 5, 6 and 7 increases, while a composite inductance of a group of the sensor coils 14, 15, 16 and 17 decreases and a signal corresponding to a ratio of those inductances is input to the sensor amplifier circuit 22. According to such a circuit structure, since the position of the rotating shaft 8 can be determined by a ratio of an average inductance of the sensor coils 4, 5, 6 and 7 and that of the sensor coils 14, 15, 16 and 17, accurate displacement of the rotating shaft 8 can be detected. In the present invention, it is noted that the sensor coils are electrically connected so that the magnetic fluxes generated by one sensor coil provided at one control magnetic pole and the adjacent sensor coil provided at the adjacent control magnetic pole form the same polarity on the same sides of the sensor magnetic poles. For example, as shown in Fig. 5b, in the adjacent control magnetic poles 31 and 32, the sensor coils 5 and 7 are connected so that the sensor magnetic pole around which the sensor coil 5 is wound and the sensor magnetic pole around which the sensor coil 7 is wound form the pole N on the sides facing the rotating shaft 8. Similarly, in Fig. 6b, the adjacent sensor coils 5 and 6 wound at the ends of the control magnetic poles 31 and 32 are connected so that the pole N is formed on the same side of the respective sensor magnetic poles. As explained previously, the sensor coils provided at the same control magnetic pole are wound to generate magnetic fluxes in the reverse directions (different polarities). It is noted that leakage of magnetic fluxes to the outside of the sensor magnetic poles are reduced by causing the adjacent sensor coils of the adjacent control magnetic poles to form the magnetic fluxes in the same direction, thereby forming the same polarity on the same side of the sensor magnetic poles. In the first and second embodiments, a pair of grooves are provided on the inner end surfaces of control magnetic poles, but the number of such grooves may be reduced to one or increased to three or more. A more average position detection of an object can be realized by providing a plurality of grooves on the inner end surfaces of control magnetic poles and dispersing sensor coils. As described heretofore, according to a magnetic bearing apparatus of the present invention, since the position of operating point of an inductance-type displacement sensor and that of a corresponding control pole are matched in the axial direction of a rotating shaft, namely, since the displacement sensor can detect the position of the rotating shaft at the operating point of the corresponding control pole, even if the rotating shaft is subjected to bending vibration, the magnetic bearing apparatus does not give rise to a problem of oscillation and the position of the rotating shaft can be controlled stably. While the present invention has been described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail can be made without departing from the scope of the invention as defined in the claims.

Claims: A radial magnetic bearing apparatus for suspending an object (8) by means of magnetic forces generated by control magnetic poles (31-38) provided facing said object (8), wherein sensor coils (4-7; 14-17) of displacement sensors are wound to pass through grooves (11, 12) provided on end surfaces of said control magnetic poles (31-38) to detect a radial position of said object (8) thereby using a portion of said control magnetic poles as magnetic poles (19, 20) of the displacement sensor, adjacent sensor coils (4-7 ; 14-17) provided on the same control magnetic pole (31-38) being connected in series to generate magnetic fields (10) in opposite directions to each other, said magnetic bearing apparatus being characterized in that each control magnetic pole (31 - 38) is provided with sensor coils (4-7 ; 14-17), said control magnetic poles (31-38) are provided as a plurality of pairs of adjacent poles (31-38), whereby in each pair of poles (31-38) magnetic fluxes (9) are generated in opposite directions to each other to form a closed magnetic path, said sensor coils (4-7; 14-17) provided at said plurality of pairs of said control magnetic poles (31-38) are connected in series, and that any adjacent two of said sensor coils (4,6; 5,7; 5,6) provided at corresponding adjacent two control magnetic poles (31-38) are connected to generate the same polarity on the same side of said adjacent two sensor coils (4,6; 5,7; 5,6). A magnetic bearing apparatus according to claim 1, wherein said grooves (11,12) are provided on the inner end surfaces of said control magnetic poles (31-38) in the rotational direction of said object. A magnetic bearing apparatus according to claim 1, wherein said grooves (11,12) are provided on the inner end surfaces of said control magnetic poles (31-38) in the axial direction of said object. A magnetic bearing apparatus according to claim 2 or 3, wherein the inner end surface of each control magnetic pole (31,38) is provided with a plurality of grooves (11,12), sensor coils (4-7; 14-17) are respectively wound to pass through said plurality of grooves (11,12) and any adjacent two of said sensor coils are connected in series to generate magnetic fields (10) in opposite directions to each other.

Inventors: MARUI EISHI, SHIRAO YUJI, SUZUKI MAMORU, MARUI, EISHI, SHIRAO, YUJI, SUZUKI, MAMORU
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1993
Main classification: F16C39
IPC codes: F16C39, F16C32