Abstract:
A magnetic bearing arrangement ( 1 ) for a motion element, having the following features: the magnetic bearing arrangement has a stator. The magnetic bearing arrangement has a passive magnetic bearing ( 3, 8, 9 ) for lateral guidance of the motion element ( 2 ) and a controllable magnetic bearing ( 3, 5 ) for guidance of the motion element perpendicular to the guidance by way of the passive magnetic bearing. The controllable magnetic bearing has an electronic stabilization device; the stabilization device has an electrical conductor ( 6, 7 ) that can have an electrical control current applied to it by the stabilization device and that is associated with the stator element ( 5 ) in such a way that the magnetization of the stator element is influenced by the control current. The controllable magnetic bearing has a permanent magnet ( 3 ); the permanent magnet is arranged on the motion element opposite the stator element. The magnetic force between permanent magnet and stator element is dimensioned such that with a control current of zero, the motion element is held in the working position; and only upon deviation from that working position is a control current generated that influences the magnetization of the stator element in the direction of re-establishing the working position.

Description:
FIELD OF THE INVENTION 
   The invention concerns a magnetic bearing arrangement for a motion element, having the features of the preamble of claim  1 . 
   BACKGROUND OF THE INVENTION 
   DE 38 18 556 A1 discloses a magnetic bearing arrangement of this kind within a vacuum pump. It has as the motion element a rotor on which are mounted rotor blades that alternate with stator blades. Provided for lateral guidance of the rotor are two passive magnetic bearings, arranged at an axial spacing, that each comprise two concentric circular permanent magnets of which one is arranged on the rotor and the other on the stator, i.e. immovably on the unit, and which are magnetized in mutually repulsive fashion. 
   Arranged at the lower end of the rotor is a controllable magnetic bearing having an electromagnet that acts on an attraction disk attached to the rotor. The electromagnet comprises, in usual fashion, a magnetizable core constituting a stator element, and a coil comprising an electrical conductor. Excitation of the electromagnet is controlled by a stabilization device which has a position sensor to sense the axial position of the rotor. By means of the electromagnet, the rotor is held in a working position in which the permanent magnets of the passive magnetic bearings are displaced with respect to one another in such a way that they generate an axial force opposed to the attractive force of the electromagnet. The working position is provided in such a way that the electromagnet must always be excited in order to hold the working position. By varying the excitation, the stabilization device ensures that the rotor is always moved back into the working position in the event of an axial displacement 
   It is a disadvantage of this magnetic bearing arrangement that a control current is constantly required in order to hold the rotor in the working position by way of the electromagnets. In addition, stabilization is difficult because the magnetic force profile of the electromagnet is not linear. 
   German Unexamined Application DE-OS 29 19 236 discloses a magnetic bearing arrangement, for example for flow measurements, that has as the motion element a horizontally extending rotor. In order to hold the rotor in floating fashion, here again two passive magnetic bearings, each having a stator-side and a rotor-side permanent magnet that are magnetized in mutually repulsive fashion, are provided, the field components of the two passive magnetic bearings being directly oppositely. 
   Arranged in the gap between the two passive magnetic bearings is a magnet coil with which the magnetic fields of the passive magnetic bearings can be superimposed. Excitation of the magnet coil is controlled by a regulation device which includes position sensors that sense the axial displacement of the rotor in non-contact fashion and regulate the excitation current as a function of the axial displacement. The rotor is held in a specific working position depending on the direction and magnitude of the electrical currents in the magnet coil. 
   A disadvantage of this bearing is the large distance between the magnet coil and the permanent magnets attached to the motion element. The return forces achievable for stabilizing the working position of the motion element are correspondingly low. In addition, the magnet coil occupies the space in which, in the case of a rotor bearing arrangement, the rotary field stator for rotational drive would advantageously be placed. 
   It is the object of the invention to embody a magnetic bearing arrangement of the kind cited above in such a way that it is of the simplest possible configuration and consumes little power. 
   This object is achieved, according to the present invention, by way of the following features:
         l) the controllable magnetic bearing has a permanent magnet;   m) the permanent magnet is arranged on the motion element opposite the stator element;   n) the magnetic force between permanent magnet and stator element is dimensioned such that with a control current of zero, the motion element is held in the working position; and that only upon deviation from that working position is a control current generated that influences the magnetization of the stator element in the direction of re-establishing the working position.       

   SUMMARY OF THE INVENTION 
   The basic idea of the invention is to stabilize a motion element, passively stabilized by repulsive magnetic forces, in an unstable working position by way of an adjacent stator element having an attractive effect; and to apply a control current to the stator element only if the motion element leaves that working position. Since a control current flows only in that situation, power consumption is low. In addition, the magnetic bearing arrangement according to the present invention is characterized by a simple physical design, and is suitable for motion elements moving both in translation and in rotation. 
   When the word “a” or a word derived therefrom appears above, it is to be understood not as a numerical term but rather as the indefinite article. This applies consistently to all the claims. 
   The stator element preferably is made of a magnetically soft steel. It can also be embodied as a permanent magnet, or assembled from ferromagnetic and permanent-magnet parts. 
   The possibility exists, in principle, of assembling the permanent magnets from several sub-magnets that rest flush against one another or are at a spacing from one another. 
   To allow the working position of the motion elements to be better stabilized, the invention furthermore provides for the stator element to have several electrical conductors, and for each electrical conductor to be part of a separate stabilization device. Each electrical conductor then receives, via the respectively associated stabilization device, a separate control current whose magnitude and direction are determined by the respective position sensor. 
   A particularly simple embodiment of the magnetic bearing arrangement according to the present invention is obtained if the permanent magnet of the controllable magnetic bearing simultaneously also constitutes the motion-element-side permanent magnet of the associated passive magnetic bearing, so that the stator-side permanent magnet(s) of the passive magnetic bearing lies/lie adjacent to the permanent magnet of that controllable magnetic bearing. 
   For elongated motion elements, it is recommended to arrange several passive magnetic bearings for lateral guidance of the motion element, with mutually repulsive motion-element-side and stator-side permanent magnets being provided. The passive magnetic bearings should be arranged at the end regions of the motion element. 
   It is also possible to provide several controllable magnetic bearings each having a motion-element-side permanent magnet and a stator element. This arrangement is recommended in particular when at least one separate stabilization device is associated with each stator element. Also possible in this context is an embodiment in which controllable magnetic bearings are arranged at the ends of the motion element and their permanent magnets each have adjacently associated with them a stator-side permanent magnet of a passive magnetic bearing, in such a way that the permanent magnets of the controllable magnetic bearings simultaneously constitute the motion-element-side permanent magnets of the passive magnetic bearing. 
   The motion element can be embodied as a linearly movable member whose permanent magnet(s), like the permanent magnets of the passive magnetic bearing(s), extend in the motion direction. The motion element can instead, however, also be embodied as a rotational member whose permanent magnet(s), like the permanent magnets of the passive magnetic bearing(s), are also circular in shape. The motion element and stator element can have an annular shape, and several (preferably at least three) electrical conductors, each of which is part of an independent stabilization device, can be arranged in distributed fashion over the stator element. The permanent magnet of the controllable magnetic bearing should then simultaneously constitute the motion-element-side permanent magnet of the passive magnetic bearing, and should be surrounded by the stator-side permanent magnet of the passive magnetic bearing. As an alternative to this, the possibility exists for the permanent magnet of the controllable magnetic bearing to surround the stator-side permanent magnet of the passive magnetic bearing. Both possibilities are, in general, sufficient to hold the motion element in the working position. This does not, however, exclude an embodiment in which the permanent magnet of the controllable magnetic bearing is enclosed internally and externally by a respective circular stator-side permanent magnet of the passive magnetic bearing. 
   The invention is illustrated in more detail, with reference to exemplary embodiments, in the drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows, in an oblique view, a first embodiment of a magnetic bearing arrangement having a linearly movable motion element; 
       FIG. 2  shows, in an oblique view, a second embodiment of a magnetic bearing arrangement having a linearly movable motion element; 
       FIG. 3  shows, in an oblique view, a third embodiment of a magnetic bearing arrangement having a linearly movable motion element; 
       FIG. 4  shows, in a partially sectioned oblique view, a first embodiment of a magnetic bearing arrangement having a rotationally movable motion element; 
       FIG. 5  shows, in an oblique view, a fourth embodiment of a magnetic bearing arrangement having a linearly movable motion element; 
       FIG. 6  shows, in a partially sectioned oblique view, a second embodiment of a magnetic bearing arrangement having a rotationally movable motion element. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Magnetic bearing arrangement  1  depicted in  FIG. 1  has a motion element  2 , extending in a bar shape and rectangular in cross section, that comprises a permanent magnet  3 , the magnetization direction being indicated by the arrow on the end face of motion element  2 . Motion element  2  is movable in the directions of double arrow  4  and is suitable, for example, as a part of linear motors or linear guidance systems. 
   Arranged immovably on the unit below permanent magnet  3  and extending parallel to motion element  2  is a stator element  5  made of magnetizable material, preferably magnetically soft steel. Arranged on either side of stator element  5  are electrical conductors  6 ,  7  that belong to a stabilization device by way of which a control current can be delivered onto electrical conductors  6 ,  7 . 
   Arranged on either side of motion element  2  are two stator-side permanent magnets  8 ,  9  that, as indicated by the arrow symbols on the end faces, are magnetized in the same direction as permanent magnet  3  of motion element  2 . As a result, repulsive magnetic forces are created between permanent magnets  8 ,  9  and motion element  2 , thereby elastically holding motion element  2  in the center between permanent magnets  8 ,  9 . The three permanent magnets  3 ,  8 ,  9  thus constitute a passive magnetic bearing for lateral guidance of motion element  2 . Permanent magnets  8 ,  9  are part of the stator, which is not depicted here in more detail. 
   Two magnetic forces that compensate for one another act in vertical direction  10  on motion element  2 . The attractive force between stator element  5  and permanent magnet  3  is dimensioned so that motion element  2  is held in a working position offset slightly upward with respect to permanent magnets  8 ,  9 , in which permanent magnets  8 ,  9  exert an upwardly directed magnetic force. This working position is unstable, however, so that motion element  2  can break away upward or downward. This offset is sensed by a position sensor (not depicted in more detail) that scans in non-contact fashion on an inductive, galvanomagnetic, capacitative, or optical basis and causes the stabilization device to deliver through electrical conductors  6 ,  7  a current which influences the magnetization of stator element  5  in such a way that motion element  2  is moved back into the working position. 
   Once motion element  2  has again assumed its working position, the control current goes to zero, i.e. in this position, magnetic bearing arrangement  1  consumes no current. Permanent magnet  3  and stator element  5  thus constitute, with the stabilization device, a controllable magnetic bearing, permanent magnet  3  also being simultaneously part of the passive magnetic bearing. 
   Magnetic bearing arrangement  11  depicted in  FIG. 2  differs from magnetic bearing arrangement  1  according to  FIG. 1  only by having a different arrangement of stator element  12 , and by the fact that two stabilization devices are provided. Stator element  12  is now arranged above motion element  13 . The magnetic force acting between stator element  12  and permanent magnet  14  of motion element  13  is dimensioned such that motion element  13  is offset into the working position downward with respect to the two laterally extending permanent magnets  15 ,  16 , so that because of the repulsive magnetic forces, these permanent magnets  15 ,  16  not only guide motion element  13  laterally but also exert a downwardly directed force. 
   That force is opposed by the attractive force between permanent magnet  15  of motion element  13  and stator element  12 ; the two forces cancel one another out when permanent magnet  14  is located in the slightly downwardly offset working position described above. 
   So that tilting of motion element  13  about transverse axis  17  can be reliably prevented, magnetic bearing arrangement  11  comprises two stabilization devices that each have an electrical conductor  18 ,  19 . Electrical conductors  18 ,  19  are arranged at the ends of stator element  12  in the form of windings, and each have associated with them separate position sensors (not depicted here) which sense the position of motion element  13  in the regions of electrical conductors  18 ,  19 , so that depending on the motion of motion element  13  about transverse axis  17 , they have different control currents applied to them in order to align motion element  13  once again parallel to stator element  12 . 
   Here as well, motion element  13  is movable in longitudinal direction  20  and is therefore useful for applications for which magnetic bearing arrangement  1  shown in  FIG. 1  is also suitable. 
     FIG. 3  shows a further magnetic bearing arrangement  21  that represents a variation of magnetic bearing arrangement  1  according to  FIG. 1 . It has a plate-shaped motion element  22  standing on edge, which has, both on the underside and on the upper side, horizontally extending permanent magnets  23 ,  24  that are magnetized in the same direction. Adjacent to each of permanent magnets  23 ,  24  are two stator-side and therefore immovably unit-mounted permanent magnets  25 ,  26  and  27 ,  28 , respectively, which are magnetized in the same direction and therefore guide motion element  22  laterally by magnetic repulsion. Each three permanent magnets  23 ,  25 ,  26  and  24 ,  27 ,  28  constitute a respective passive magnetic bearing for lateral guidance of motion element  22 ; here, in contrast to magnetic bearing arrangement  1  according to  FIG. 1 , motion element  22  is guided in a manner stabilized against tilting about the motion axis. 
   Arranged below lower permanent magnet  23  of motion element  22  is a stator element  29  that is enclosed on either side by electrical conductors  30 ,  31  of a stabilization device. By way of the attractive force between permanent magnet  23  and stator element  29  made of magnetically soft material, motion element  22  is held (as in the case of magnetic bearing arrangement  1 ) in a working position offset slightly upward with respect to permanent magnets  25 ,  26  and  27 ,  28 , in which permanent magnets  25 ,  26 ,  27 ,  28  exert on motion element  22  an upwardly directed magnetic force that is compensated for by the attractive force between permanent magnet  23  and stator element  29 . If motion element  22  leaves this working position upward or downward, the stabilization device applies to electrical conductors  30 ,  31  a control current that influences the attractive force between permanent magnet  23  and stator element  29  in such a way that motion element  22  is returned to its working position. Permanent magnet  23  is thus part of the lower passive magnetic bearing, and also part of the controllable magnetic bearing. 
   It is understood that stator element  29  can also be equipped with two independent stabilization devices as in the exemplary embodiment shown in  FIG. 2 . In addition, motion element  22  shown here is also linearly movable in the directions indicated by double arrow  32 , and is thus also suitable for the applications for which magnetic bearing arrangements  1 ,  11  according to  FIGS. 1 and 2  are provided. 
   Magnetic bearing arrangement  41  depicted in  FIG. 4  constitutes a rotationally symmetrical variant of the magnetic bearing arrangement according to  FIG. 3 . Magnetic bearing arrangement  41  has a vertically extending rotor  42 , rotatable about the vertical axis, as the motion element, which has at each of its end faces a circular permanent magnet  43 ,  44 . Permanent magnets  43 ,  44  are each surrounded by annular permanent magnets  45 ,  46  which are parts of the stator and are joined by a circular sleeve  47 . Permanent magnets  45 ,  46  and sleeve  47  are depicted in section and therefore only partially, in order to make rotor  42  visible. Permanent magnets  45 ,  46  are magnetized in the same direction as permanent magnets  43 ,  44  of rotor  42 , so that repulsive magnetic forces act between permanent magnets  43 ,  45  and  44 ,  46  and radially center rotor  42 . The respectively adjacent pairs of permanent magnets  43 ,  45  and  44 ,  46  constitute passive magnetic bearings. 
   Arranged below the end face of lower permanent magnet  43  of rotor  42  is a cylindrical stator element  48  that is surrounded by an electrical conductor  49  which is depicted in section. Electrical conductor  49  is part of a stabilization device (not depicted here in more detail) that also includes a position sensor which senses the vertical position of rotor  42 . The magnetic force acting between lower permanent magnet  43  of rotor  42  and stator element  48  is dimensioned such that permanent magnets  43 ,  44  of rotor  42  are offset slightly upward with respect to permanent magnets  45  and  46  that surround them, so that an equilibrium of forces exists between the upwardly directed vertical force proceeding from permanent magnets  45 ,  46  and the attractive force between permanent magnet  43  and stator element  48 . If rotor  42  is deflected vertically, the position sensor senses this and causes the stabilization device to deliver onto electrical conductor  49  a control current such that the magnetization of stator element  48  is influenced in such a way that rotor  42  returns to its working position. 
     FIG. 5  shows a magnetic bearing arrangement which represents a variant of the magnetic bearing arrangement according to  FIG. 2 . Magnetic bearing arrangement  51  has a plate-shaped motion element  52 , extending horizontally, that is delimited on either side by bar-shaped permanent magnets  53 ,  54 . Extending parallel thereto are stator-side permanent magnets  55 ,  56  which are magnetized in the same direction as permanent magnets  53 ,  54  on motion element  52 . Repulsive magnetic forces are thereby created between the respective adjacent permanent magnets  53 ,  55  and  54 ,  56 , and guide motion element  52  laterally. Each pair of adjacent permanent magnets  53 ,  55  and  54 ,  56  constitutes a passive magnetic bearing. 
   Arranged above each of permanent magnets  53 ,  54  of motion element  52  is a stator element  57 ,  58  that extends parallel to permanent magnets  53 ,  54  and is made of magnetically soft steel. Stator elements  57 ,  58  are connected by a bridge  59 . 
   As in the exemplary embodiment shown in  FIG. 2 , stator elements  57 ,  58  each have two electrical conductors  60 ,  61  and  62 ,  63 , which are respectively arranged in the region of the ends of stator elements  57 ,  58 . Each electrical conductor  60 ,  61 ,  62 ,  63  belongs to a separate stabilization device having a respective position sensor, so that electrical conductors  60 ,  61 ,  62 ,  63  can have different control currents applied to them. 
   Motion element  52  is held by the magnetic forces acting in vertical direction  64  in a working position that is offset slightly downward with respect to the planes occupied by permanent magnets  55 ,  56 . Permanent magnets  55 ,  56  thus exert on motion element  52  a downwardly directed magnetic force that is compensated for by the magnetic forces acting between the motion-element-side permanent magnets  53 ,  54  and the relevant stator elements  57 ,  58 . Permanent magnets  53 ,  54  are not only part of the passive magnetic bearings (respectively comprising the two adjacent permanent magnets  55 ,  57  and  56 ,  58 ), but also are part of two controlled magnetic bearings comprising permanent magnet  53  and stator element  57  on the one hand, and permanent magnet  56  and stator element  58  on the other hand. Any deflection of motion element  53  in vertical direction  64  is sensed by the position sensors and results in an application of control current to electrical conductors  60 ,  61 ,  62 ,  63  that turns the deflection back in the direction toward the working position by appropriate magnetization of stator elements  57 ,  58 . Any tilting of motion element  52  about horizontal axis  65  is also sensed by the position sensors, in which case electrical conductors  60 ,  61 ,  62 ,  63  receive different control currents depending on the tilting motion of motion element  52 , thus bringing motion element  52  back into the working position. 
   Motion element  52  is horizontally linearly movable in the direction of double arrow  66 , and is therefore suitable for linear motors or linear guidance systems. 
     FIG. 6  depicts a magnetic bearing arrangement  71  which has as the motion element an annular rotor  72  that is depicted in partially cutaway fashion. Rotor  72  is embodied as a permanent magnet  73 , and surrounds a stator-side permanent magnet  74 . The two permanent magnets  73 ,  74  are magnetized in the same direction, so that they repel one another and rotor  72  is held in horizontally centered fashion with respect to permanent magnet  74 . 
   Arranged below rotor  72  is an annular stator element  75  made of magnetically soft steel. Three electrical conductors  76  guided in a coil shape, of which only one is shown here, are distributed over the periphery of stator element  75 . Each electrical conductor  76  is coupled to a separate stabilization device having a respective position sensor that senses the vertical position of rotor  72  at certain points. 
   Rotor  72  is held in a working position in which it is offset slightly upward with respect to stator-side permanent magnet  74 . As a result, an upwardly directed magnetic force acts on rotor  72 ; this is compensated for by the attractive force between rotor  72  and stator element  75  in such a way that in the working position, an equilibrium of forces exists in the vertical direction. In the event of a deflection of rotor  72  out of that working position—by parallel displacement upward or downward and/or by tilting about a horizontal axis—electrical conductors  76  have applied to them control currents corresponding to the positional changes sensed by the position sensors in such a way that stator element  75  is magnetized, in sections, in such a way that rotor  72  is returned to the working position. 
   Any kind of structure whose purpose is to be caused to rotate can be mounted on rotor  72 , for example blade wheels or disks for measuring flow velocities, or blade wheels for use in pumps, if magnetic bearing arrangement  71  is combined with a motor that causes rotor  72  to rotate. The same applies to magnetic bearing arrangement  41  according to  FIG. 4 . 
   It is understood that magnetic bearing arrangement  71  according to  FIG. 6  can also be varied in that stator-side permanent magnet  74  is arranged externally so that it surrounds rotor  72 , as is the case in principle in the embodiment shown in  FIG. 4 .