Abstract:
Stray magnetic fields created by non-symmetrical magnetisation of the permanent magnets in a rotating magnet array of a magnetic bearing assembly, when used in turbo molecular pumps, can cause significant problems to devices such as scanning electron microscopes. In order to minimise the stray fields, at least the dipole and quadrupole moments of each magnet in the array is first measured. The magnets in the array are then arranged relative to one another such that both the dipole and quadrupole moments are minimised, thus minimising the stray magnetic field when the magnet array rotates in the turbomolecular pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This Application is a Section 371 National Stage Application of International Application No. PCT/GB2012/050975, filed May 4, 2012, which is incorporated by reference in its entirety and published as WO 2012/153109 A2 on Nov. 15, 2012 and which claims priority to British Application No. 1107587.6, filed May 6, 2011. 
     BACKGROUND 
     The present disclosure relates to the field of magnetic bearings. In particular an apparatus and method for reducing stray transverse magnetic fields caused by permanent magnetic bearing arrangements in high speed rotary machines such as turbomolecular pumps. 
     Turbomolecular pumps are employed as part of the vacuum system used to create the high vacuum environment required for devices such as scanning electron microscopes (SEMS) and lithography devices. 
     It is common for turbomolecular pumps to comprise an oil free passive permanent magnetic bearing arrangement in the high vacuum end of the pump. 
     A cross section of a passive permanent magnetic bearing arrangement  10  for a turbomolecular pump (not shown) is illustrated in  FIG. 1 . In this example the bearing arrangement  10  comprises an array  12  of four outer rotating permanent magnet rings  12   a,    12   b,    12   c  and  12   d  and an array  14  of four inner non-rotating permanent magnetic rings  14   a,    14   b,    14   c  and  14   d  arranged such that the outer, rotating, array  12  surrounds the inner, static, array  14  in a concentric manner. The magnets are all formed of rare earth magnetic material. The outer array  12  is attached to the rotor of a turbomolecular pump (not shown) with the static array  14  attached to the stator of said pump. For reasons of mechanical strength and practical construction, it is normal for the outer array of rings to form the rotating part of the bearing arrangement and the inner rings to form the stationary part. 
     In this example the magnetisation of the magnetic rings  12   a  to  12   d  and  14   a  to  14   d  in each array  12 ,  14  respectively is substantially aligned with the axis of rotation  4  of the pump rotor (not shown). The direction of magnetisation has been indicated by the arrows, with the head of each arrow indicating the north pole. 
     The magnets are arranged within each array such that they are in mutual repulsion with each other; that is proximate magnets in an array meet their nearest neighbouring magnet in the same array with the same pole (e.g. magnets  12   a  and  12   b  meet each other with their south poles). The outer magnetic rings  12   a,    12   d,    14   a,    14   d  in each array have their north poles facing outermost. 
     The magnets  12   a  to  12   d  and  14   a  to  14   d  in each array  12 ,  14  of the arrangement  10  are orientated to provide a mutual repulsion between the arrays  12 ,  14  and therefore create an almost frictionless bearing. 
     A great many other configurations are possible, using different numbers of rings, with axial or radial magnetisation, and arranged for either repulsive or attractive forces between rotor and stator. Although a variety of configurations are possible, they all perform optimally when the direction of magnetisation in the rings is perfectly symmetrical with respect to their geometric axis. 
     The magnetisation in the rings  12   a  to  12   d  of the rotating array  12  is shown in  FIG. 1  as perfectly symmetrical with respect to their geometric axis  4 . However, in reality, the magnetisation of each magnetic ring  12   a  to  12   d  (and, similarly, for magnets  14   a  to  14   d ) is imperfect due to the practical limitations of their manufacturing process. This is illustrated in  FIGS. 2 a  and 2 b   . The largest magnetic asymmetry observed in axially magnetised permanent magnetic rings is usually a small angular error such that the magnet&#39;s axis is displaced from the geometric axis  4  by an angle of a few degrees as indicated in  FIG. 2 a   . Depending on the quality, or grade, of the magnet the angular error, θ, can be as much as 3°. This error may be regarded as a small perturbation from the ideal axial magnetisation; in effect a transverse magnetic dipole moment  8  superimposed on the intended axial dipole moment  6  as illustrated in  FIG. 2   b.    
     In addition to the transverse dipole (first order) asymmetry, higher order asymmetries exist, for example quadrupole and hexapole asymmetries. The magnitude, or magnetic field strength, of the asymmetry usually decreases as the number of poles increases. 
     Where these small asymmetries occur in any of the rings  12   a  to  12   d  of the rotating magnet array  12 , a time varying magnetic field is generated (the magnetic field is constant for the static magnets  14   a  to  14   d ). These 2, 4 and 6 pole asymmetries generate time varying magnetic fields at frequencies of 1, 2, 3 times the rotational speed of the pump rotor respectively. 
     The performance of scanning electron microscopes is highly susceptible to mechanical vibrations or stray magnetic fields emitted from turbomolecular pumps. The stray fields are known to directly interfere with the electron beam or with the instruments&#39; electrical circuits. 
     Although it common to use ferromagnetic shielding to reduce such magnetic field emissions, such shielding is costly and is only of limited effectiveness. 
     Therefore it is desirable to reduce the effect of these time-varying stray magnetic fields by alternative means. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     SUMMARY 
     A first aspect provides a method of assembling a rotating magnet array for a permanent magnet bearing arrangement, said rotating array comprising at least four permanent ring magnets, the method comprising the steps of:
         measuring the size and phase (vectors) of at least the first and second order transverse stray magnetic fields, namely the transverse dipole and quadrupole stray fields, of at least four ring magnets individually in relation to a reference point on said ring magnets;   calculating, or computing, the relative angular orientation and relative magnetic polarity direction of each of said at least 4 magnets within the array that, when the array is assembled, will provide the minimum time-varying magnetic field; and   assembling the array of magnets according to said calculation.       

     By optimal selection and assembly of the magnet rings, the asymmetrical magnetisation of the several rotating rings is substantially mutually counteracted. By calculating the effects of all permutations of the magnets in the “up” and “down” orientations, and all their relative angular orientations, it is possible to determine the optimum configuration of the magnets for minimising the majority of the stray magnetic field. 
     A measurement on the set of magnets in the preferred orientation and permutation according to the calculation to determine the stray field and/or the measurement of stray fields on the finished array can then be carried out. 
     Other preferred and/or optional aspects of the invention are defined in the accompanying claims. 
     The Summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be well understood, embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a cross sectional illustration of a passive magnetic bearing arrangement. 
         FIG. 2 a    is an illustration of the asymmetric magnetisation of a permanent ring magnet. 
         FIG. 2 b    is an illustration of the asymmetric magnetisation of a permanent ring magnet. 
         FIG. 3  is a schematic representation of an apparatus for carrying out the measurement according to various embodiments. 
         FIG. 4  is an illustration of a reduced stray transverse dipole moment magnetic field of a four ring magnet system 
         FIG. 5  is an illustration of a stray transverse dipole and quadrupole moment magnetic field of a four ring magnet system. 
         FIG. 6  is an illustration of a reduced stray transverse dipole and quadrupole moment magnetic field of a four ring magnet system. 
         FIG. 7  is a flow diagram of a four magnet array measurement according to various embodiments. 
         FIG. 8  is a flow diagram of a four magnet array measurement according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic of an apparatus for carrying out the measurement according to various embodiments. The magnet measuring station (or magnetic alignment rig)  50  comprises a magnet rotation section  58 ,  60 ,  62 ,  64 ,  66  and a magnetic field measuring section  56 ,  54 ,  68 ,  70 ,  72 . 
     The magnet rotation section comprises a motor  62 , for spinning the magnet. The motor  62  is driven by a speed control device  64 , such as an inverter, and a power supply  66 . A non magnetic spindle, or rotor,  60  attached to the motor  62  extends up through a non magnetic bench  52  to a magnet holder  58  (also non magnetic). The motor is positioned sufficiently far away from the magnet holder and measuring station that any magnetic fields emitted from said motor  62  do not interfere with any measurements taken. The rotor is preferably supported by a non-magnetic bearing arrangement (not shown) at the end distal from the motor, proximate to the magnet holder  58  to ensure the magnet is able to spin substantially free of vibration or movement. The magnet holder  58  comprises a base  58   a,  attached to the spindle  60 , and an axial clamping arrangement  58   b.  The base  58   a  comprises a substantially flat surface  58   c,  upon which the ring magnet  12   a  is received, preferably with a central shaft (not shown) extending upwardly from the base. The shaft is substantially the same diameter as the inner circumference of the ring magnet so that the magnet is prevented from moving radially during rotation. The axial clamping arrangement  58   b  comprises an inner bore, also substantially matching the diameter of the central shaft. The clamping arrangement  58   b  preferably attaches to the base  58   a  with a screw thread arrangement. A phase reference pickup device  80  is also provided to detect the angular position of the spindle during rotation. The pickup  80  may use, for example, an optical trigger such as a datum or reference line on the base  58   a.  The pickup is in communication with a computer  72 . 
     The magnetic field measuring section comprises a magnetic field measuring device  56  supported on the bench  52  by a non magnetic support  54 . The measuring device  56  may be a flux gate sensor, hall-effect sensor or a search coil with a sensitivity capable of resolving 1 nT with less than 500 pT noise. The sensor  56  is connected to a signal conditioning device  68 , a fast Fourier transform order tracking device  70  and a computer  72 . The computer  172  is provided with software to compute the preferred configuration and/or permutation of the magnets based on the measurements taken for each magnet. 
     In use, the individual magnet to be measured  12   a  is placed on the magnet holder base  58   a,  positioned with the north pole directed downwards. A reference point, which can simply be a mark made by a pen, is placed on the ring magnet  12   a  to match to a reference datum line on the base  58   a.  The magnet  12   a  is then fixed in this position to the magnet holder with clamping arrangement  58   b.    
     The magnet  12   a  and sensor  56  should be aligned such that the sensor axis  84  is substantially aligned with the equator of the outer circumference of the magnet  12   a  (i.e. midway between its north and south pole). 
     To characterise the magnetic field of each magnet, the motor  62  is rotated, which spins the magnet  12   a  via the spindle  60  and magnet holder  58 . The motor runs between 21 Hz to 24 Hz. The spindle  60  and magnet holder  58  should repeatably orientate magnet(s) such that the axis of the magnetic field sensor  56  lies in the plane of the magnet (i.e. orthogonal to the nominal magnetisation axis) to &lt;1°. The (axial) run-out of the magnet abutment face on the rotating shaft  60  and holder  58   a  should be &lt;10 μm (1 μm run-out or 0.0019° magnet tilt is equivalent to 0.6 nT error). 
     The sensor  56 , in combination with the pickup  80 , a signal conditioner  68 , a fast Fourier transform analyser  70 , and a computer  72  detects and records the magnitude and phase data of the magnet&#39;s transverse magnetic fields. At least the first and second order transverse stray magnetic fields, namely the transverse dipole and quadrupole stray fields, should be measured and recorded for each magnet. However higher order multi pole transverse fields, such as hexapole, data may also be collected. 
     If the process is being carried out for a rotating magnet array  12  for a permanent magnetic bearing arrangement  10  comprising at least four magnets this process is repeated for at least three other magnets  12   b,    12   c,    12   d.  The data is collected for each magnet over at least 20 seconds of rotation. 
     The relative angular orientation (compared to the reference point made on each magnet during the measurement step) and relative magnetic polarity direction of each of the four magnets  12   a,    12   b,    12   c,    12   d  within the array  12  is then calculated that will provide an assembled array  12  with minimum time-varying magnetic field during rotation. 
     As illustrated in  FIG. 4 , if the calculation is confined to reducing just the stray transverse dipole, or first order, magnetic field, in an array of 4 rotating magnet rings, the vector addition of the 4 transverse dipole moments ( 21 ,  22 ,  23 ,  24 ) of the 4 magnet rings need only be considered. By rotating the 4 rings relative to each other in the array it is possible to create a relative angular orientation of the magnets in which the 4 individual dipole moments ( 21 ,  22 ,  23 ,  24 ) form a closed quadrilateral and thus have a zero vector sum (as long as no individual magnet has a dipole moment greater than the sum of the other three magnets). It is possible to form this quadrilateral in several relative angular orientations ways and still achieve the zero vector sum. The arrangement of 4 ring magnets to providing zero total dipole is said to have one degree of freedom. 
     A rotating magnet array of a magnetic bearing with 3 rotating magnets in each array can also be arranged to form a closed triangle (depending on the relative magnitude of the 3 vectors), but there is only one relative orientation, and so has no degrees of freedom. In general an N-magnet system can be assembled for minimum dipole moment with N-3 degrees of freedom. 
     When the time-varying field due to second order, quadrupole moments, stray fields are then considered in addition to dipole moments, it is found that there is no particular relationship between the direction of the quadrupole and dipole moments, so that satisfying a zero vector sum of dipole moments does not generally produce a zero sum of quadrupole moment. Whereas a 4-magnet configuration gave 1 degree of freedom for dipole only, there are less than zero degrees of freedom for achieving a zero sum of both dipole and quadrupole because the problem is over-constrained. This is illustrated in  FIG. 5 . The 4 dipoles  21 ,  22 ,  23 ,  24  are arranged for zero sum, but the corresponding 4 quadrupole vectors  31 ,  32 ,  33 ,  34  do not have a zero sum, but instead form a combined vector  100 . 
     However, because two of the magnets  12   a,    12   b,    12   c,    12   d  in the rotating array  12  are required to be oriented with the North magnetic pole upwards and two with North downward. Physically inverting two of the magnets increases the number of degrees of freedom and makes the phase of the quadrupole negative with respect to the dipole compared to the non-inverted arrangement. Thus, as illustrated in  FIG. 6  the total transverse field due to the magnets dipoles  21 ,  22 ,  23 ,  24  can be kept at zero, while reducing the total effect  200  of the quadruples  31 ,  32 ,  33 ,  34  by at least half. 
     The computer  72  calculates the relative angular orientation (compared to the reference point made on each magnet during the measurement step) and relative magnetic polarity direction of each of the four magnets  12   a,    12   b,    12   c,    12   d  within the array  12  that provides the minimum time-varying stray dipole and quadrupole transverse magnetic field during rotation. 
     It will be apparent that it would also be advantageous to measure the stray magnetic field on a large batch of magnets, say 20, and then carry out the above calculation to determine up to five different magnet arrays, each comprising four magnets, with optimum low stray magnetic fields emissions. 
     The calculation can be carried out by considering the time variant fields and using the principle of superposition:
         Total stray field=A+B+C+D where:
 
 A=A 1*COS(θ1+φ1)+ A 2*COS(2* θ1+ψ1)
 
 B=B 1*COS(θ2+φ2)+ B 2*COS(2*θ2+ψ2)
 
 C=C 1*COS(θ3+φ3)+ C 2*COS(2*θ3+ψ3)
 
 D=D 1*COS(θ+φ4)+ D 2*COS(2*θ+ψ4)
       

     Where:
 
θ1=θ+ T 1; θ2=θ+ T 2; θ3=θ+ T 3;
         φn and ψn are the phase angle of the dipole and quadrupole moments respectively   T 1 , T 2 , and T 3  are rotational adjustments which can be made to individual magnets   According to the conditions mentioned, two of the pairs of φn and ψn must be negative (to represent inverted magnets) and two positive.   The objective is to minimise the peak to peak value of A Total for 0≦θ≦2π by varying T 1 , T 2 , T 3  and the signs of the combinations of pairs of φn and ψn
 
The resulting output from the calculation by the computer  72  is required to indicate:
   a) The order of assembly, i.e. which magnet is to occupy which position in the array  12 .   b) The rotation of each magnet with respect to a reference (angular) zero.   c) The predicted stray magnetic field in nT peak-peak of the array  12         

     If though any of the four magnets  12   a  to  12   d  is unsuitable for use in a set, the software on the computer  72  should indicate which magnet is to be replaced with another candidate magnet and the cycle to re-start from the beginning. 
     Once the relative angular orientation and relative magnetic polarity direction for a set of four magnets  12   a  to  12   d  has been calculated, the array  12  can be arranged according to the calculation and the array checked on the magnet measuring station. 
     If this further step is carried out, the sensor axis  84  should be substantially aligned with the middle of the outer circumference of the magnet array (i.e. between magnets  12   b  and  12   c  in  FIG. 1 ). The target time varying maximum transverse stray field measured at 200 mm from the axis for a set of four magnets is 180 nT peak-peak. 
     The flow diagram for the individual magnet and array measurements is shown in  FIG. 7  and  FIG. 8 . 
     The arrays  12  assembled according to the present invention can then be assembled into bearing arrangements to be fitted into a high speed rotating machine such as a turbomolecular pump. 
     The optimised bearing assembly may be used on its own, or combined with passive shielding or active cancellation to reduce stray fields still further. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.