Vacuum pump and permanent magnet bearing assembly for vacuum pump

Abstract

A passive magnetic bearing assembly (10) for a vacuum pump and a vacuum pump having such a bearing are disclosed. The invention relates to a passive magnetic bearing assembly having a plurality of pairs of ring magnets (20, 30) arranged in a stack, one of the ring magnet pairs being arranged on a stator side (4) of the bearing and the other of the ring magnet pairs being arranged on a rotor side (2). The ring magnets are radially magnetized and each rotor ring magnet comprises at least a first harmonic of magnetic inhomogeneity, where the magnetic inhomogeneity is measured as a ratio between a 0-peak variation of remanence [Delta] Bi and an average remanence BR0, the first harmonic of magnetic inhomogeneity being less than 3%.

Description

Technical Field

[0001] The field of this invention relates to vacuum pumps and permanent magnet bearing assemblies for such pumps. Background Technology

[0002] Vacuum pumps (e.g., turbomolecular pumps) may have rotors mounted on magnetic bearings to reduce friction and contamination from lubricants.

[0003] Typically, the permanent magnet bearings (PMBs) in these pumps are made using an axial magnetization array. Figure 1 Such a bearing is schematically illustrated. In certain applications such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and ion traps, stray magnetic fields induced by magnetic bearings can be a problem. Two types of stray magnetic fields exist in magnetic bearings: AC stray fields, which are time-varying magnetic fields generated during rotation, primarily due to magnetization errors in the bearing's magnet; and DC stray fields, which are time-invariant intrinsic magnetic fields generated by the magnet. Figure 2 The AC and DC fields of the magnetic bearing are schematically illustrated. Figure 3 Two types of magnetization errors are shown in the case of axially magnetized toroidal magnets: global skew and inhomogeneity.

[0004] Conventionally, AC stray magnetic fields in magnetic bearings are reduced by selectively aligning the magnetic bearings (e.g., see EP2705263) or by correcting the magnetization error of each magnet. Such techniques are time-consuming and can increase costs. While these techniques address the AC stray magnetic field problem, axially magnetized magnets, and therefore PMBs constructed using them, inherently possess high DC stray magnetic fields, which are more difficult to address.

[0005] The desired outcome is to provide PMBs with both low DC and AC stray magnetic fields. Additionally, designs that do not require selective assembly / alignment or correction of magnetization errors will be advantageous. Summary of the Invention

[0006] On one hand, a passive magnetic bearing assembly for a vacuum pump is provided, the passive magnetic bearing assembly comprising: multiple pairs of annular magnets arranged in a stacked manner, one annular magnet in one pair being disposed on the stator side of the bearing, and the other magnet in the pair being disposed on the rotor side; the annular magnets are radially magnetized, and each annular magnet on the rotor side includes at least a first harmonic of magnetic inhomogeneity, wherein the inhomogeneity is measured as a 0-peak variation ΔB of the remanence. i With average remanence B R0 The ratio between them, and wherein, for at least most of the annular magnets on the rotor side, and preferably for all of these annular magnets, the first harmonic of the magnetic non-uniformity is less than 3%.

[0007] In certain applications, it is important that the magnetic bearing has a low stray magnetic field. The DC stray magnetic field is the inherent magnetic field produced by a magnet in the surrounding space. The inventors have recognized that a radially magnetized annular magnet has a naturally low DC stray field, much lower than an axially magnetized annular magnet. In addition, it has been recognized that a pair of stator and rotor radially magnetized annular magnets, the building blocks of a radially magnetized PMB, have an even lower DC stray field than the same pairing built with axially magnetized annular magnets. However, for radially magnetized bearings, the AC stray field is more difficult to manage, as the process of selective alignment is particularly difficult for such magnets due to the attraction between adjacent magnets. The inventors have recognized not only that the stray DC field is low for radially magnetized magnets, but also that the AC stray field is manageable if magnets with low inhomogeneity are used on the rotor side. In addition, the inventors have recognized that the sensitivity of the AC stray magnetic field to inhomogeneity also depends on the shape of the inhomogeneity around the perimeter of the magnet, and is maximum for the first order harmonic, and rapidly decreases for higher order harmonics. This is particularly true in the case where the field is measured at a certain distance from the magnet, for example at a location where a low AC stray field is particularly important, such as outside the stator of a vacuum pump.

[0008] The inhomogeneity of a magnet is caused by the variation of its remanence BR in its volume, where remanence is a property of the magnet and is the magnetic flux density that remains after removal of an external field.

[0009] The inhomogeneity can be specified as the 0-peak variation AB of the remanence i to the average remanence B R0 The inhomogeneity can be determined by measuring the radial flux density of the magnetic field produced by the magnet in the space around the magnet. The average remanence is determined by averaging the radial flux density measured around the outer perimeter at the same set distance from the magnet, after removal of any background magnetic field (e.g. the earth’s magnetic field).

[0010] By measuring the amplitude of the radial flux density of any given harmonic, the corresponding inhomogeneity harmonic can be determined. Thus, magnets on the rotor side with low first and second harmonic magnetic inhomogeneity are selected to provide low AC and DC stray fields.

[0011] Highly inhomogeneous magnets are increasingly readily available, and it is provided to have a magnetic bearing with radially magnetized magnets, where the rotor magnets are highly inhomogeneous radially magnetized magnets, which is an effective way to reduce both AC and DC stray fields while still providing an effective magnetic bearing.

[0012] In some embodiments, the stator magnets can also be highly uniform radial magnetization magnets, and in some embodiments they can have the same or similar requirements for uniformity as the rotor magnets. Although the problem of AC stray fields only arises with rotating magnets, it can be advantageous to make the stator magnets in the bearing highly uniform as well, for example to reduce bearing eccentricity caused by magnetization errors.

[0013] Although a first harmonic non-uniformity of less than 3% can provide an effective bearing, it can be more desirable for a bearing in which at least most and preferably all of the annular magnets on the rotor side have both a first harmonic and a second harmonic of less than 3%.

[0014] In some embodiments, it can be possible to provide less than 2% or preferably less than 1% first harmonic and (in some cases) second harmonic for at least most and preferably all of the annular magnets on the rotor side.

[0015] In some embodiments, at least some of the annular magnets comprise NdFeB magnets. In some embodiments, all of the rotor annular magnets, and in some embodiments all of the magnets, comprise NdFeB magnets.

[0016] NdFeB magnets have high performance, and with high uniformity can be particularly effective magnets for radial magnetized passive magnetic bearing assemblies. NdFeB magnets can contain other elements in addition to Nd, Fe and B. For example, praseodymium can be added in variable amounts as a substitute for neodymium. Other elements can include one or more of Dy, Ga, Co, Al and Cu.

[0017] In some embodiments, at least some of the annular magnets comprise hot deformed NdFeB magnets. In some embodiments, all of the rotor annular magnets, and in some cases all of the stator annular magnets, comprise hot deformed NdFeB magnets.

[0018] Some hot deformed NdFeB magnets can provide particularly uniform magnets, and thus be particularly effective in providing low AC fields.

[0019] For hot deformed radial magnetized NdFeB magnets, the attractive force is high, making assembly and in particular selective alignment more difficult. However, non-magnetic spacers between the magnets can be used to reduce the attractive force, with only negligible / moderate loss of bearing stiffness for the required thickness.

[0020] Radial magnetized magnets can be made with plastic bonded magnets, such as Smco, NdFeB, SmFeN or mixtures thereof.

[0021] In some embodiments, at least some of the ring magnets comprise hot deformed NdFeB without any heavy rare earth elements ("HREE-free"), having a minimum intrinsic coercivity of > 1400 kA / m at 20°C.

[0022] HREEs such as dysprosium (and / or other elements such as terbium) are often used in magnetic materials to help preserve their magnetism, and in turn, their high temperature performance. Even without HREEs such as dysprosium, hot deformed NdFeB magnets can have high coercivity, making them a particularly good choice for certain PMB applications. HREEs are expensive elements that are not readily available and can present ethical considerations in their sourcing, so it is advantageous to find magnets without or with reduced dysprosium content and still have reasonable coercivity.

[0023] In some embodiments, at least some of the ring magnets comprise hot deformed NdFeB with < 3% HREE content, the HREEs are non-grain boundary diffused within the magnet, and the ring magnets have a minimum intrinsic coercivity of > 1700 kA / m at 20°C. In some embodiments, with grain boundary diffusion of HREEs, the HREE content is less than < 1.5% with a minimum intrinsic coercivity of > 1700 kA / m at 20°C.

[0024] In other embodiments, magnets with low HREE content can be used, and these magnets can have higher coercivity and can be more effective in certain applications. It has been found that the grain boundary diffusion properties of HREEs provide effective temperature protection with reduced amounts of HREEs present.

[0025] In other embodiments, the magnets can comprise PrFeB magnets.

[0026] In some embodiments, the magnets comprise plastic bonded magnets.

[0027] Radially magnetized magnets can be made with plastic bonded magnets, such as Smco, NdFeB, SmFeN, or mixtures thereof.

[0028] Another way to form magnets with low inhomogeneity is to use plastic bonding to form the magnets. By controlling the manufacturing process, plastic bonded magnets can be manufactured with desired properties, and can provide magnets that are particularly suitable for forming radially magnetized passive magnetic bearings with low inhomogeneity.

[0029] In some embodiments, the bearing assembly further comprises a non-magnetic spacer arranged to axially separate the ring magnets in the stack.

[0030] As mentioned previously, radially magnetized magnets attract each other, making them difficult to selectively align to further reduce the AC magnetic field. Although the use of magnets with low inhomogeneity reduces the AC field to some extent, the AC field can be further reduced with selective alignment. Adjacent radially magnetized magnets are difficult to align because they strongly attract each other. This is a particular problem with hot deformed radially magnetized NdFeB magnets, the attraction is high and makes assembly and especially selective alignment more difficult. However, non-magnetic spacers (defined as materials with relative magnetic permeability less than 1.1) placed between the magnets can be used to reduce the attraction, with only negligible / moderate loss of bearing stiffness for the required thickness.

[0031] In some embodiments, the spacers preferably have a low coefficient of friction on at least some of their outer surfaces to facilitate angular alignment between the magnets.

[0032] In some embodiments, the spacers are made of a polymer or a fiber-reinforced polymer, such as PEEK or, for example, Torlon.

[0033] In some cases, the low coefficient of friction can be provided by the material of the spacer, in other cases, it can be provided by a low-friction coating.

[0034] Another aspect provides a vacuum pump comprising a passive magnetic bearing according to the first aspect, the vacuum pump comprising a rotor rotatably mounted within a stator, one of each pair of annular magnets being mounted to the rotor and the other annular magnet of the pair being mounted to the stator.

[0035] In some embodiments, the vacuum pump comprises a turbomolecular pump.

[0036] In some embodiments, the rotor annular magnets are selectively aligned to reduce stray AC fields.

[0037] In some embodiments, the annular magnets are selectively oriented vertically to reduce stray DC fields.

[0038] In addition to reducing stray AC fields by selectively aligning the rotor annular magnets, stray DC fields can also be addressed by selectively orienting the stator and rotor annular magnets vertically so that magnets with similar levels of unwanted magnetization components in the axial direction can be arranged with opposite orientation so that the axial fields point in different directions and reduce the overall stray DC magnetic field.

[0039] In some embodiments, the magnets can be selected so that they have similar levels of unwanted magnetization components in the axial direction so that with the appropriate orientation, the global sum of the DC field is particularly low.

[0040] Another aspect provides a method for manufacturing a passive magnetic bearing assembly according to the first aspect, the method comprising: measuring the inhomogeneity of at least one harmonic of a plurality of radially magnetized rotor annular magnets by: determining the radial component of a magnetic flux density harmonic measured at different points around the perimeter of the annular magnet at a predetermined distance from the rotor annular magnet; calculating the inhomogeneity of the at least one harmonic by dividing the 0-peak amplitude of the magnetic flux harmonic by the average flux; selecting rotor annular magnets such that, for at least most and preferably all of the annular magnets of the rotor, the calculated inhomogeneity of the at least one harmonic is below a threshold; and stacking the selected rotor annular magnets and a corresponding set of stator annular magnets to form the passive magnetic bearing assembly.

[0041] In some embodiments, the first and second harmonics are measured, and the non-uniformity of both the first and second harmonics should be below a threshold.

[0042] In some embodiments, the plurality of radially magnetized rotor ring magnets to which the step of measuring non-uniformity is performed are a subset of a batch of ring magnets to be used to form the PMB, and statistical analysis of the results is used to determine whether other rotor ring magnets from that batch should be tested, or whether the entire batch of ring magnets can be selected to form the passive magnetic bearing assembly.

[0043] Although rotor toroidal magnets are those that contribute to the AC stray field, it may be advantageous to match stator toroidal magnets with rotor toroidal magnets, and therefore it may be advantageous to test and select them in the same manner.

[0044] In some embodiments, the stacking step includes separating the annular magnets in the stack with non-magnetic spacers.

[0045] In some embodiments, the step of determining the magnetic flux includes determining the amplitude and relative phase of the flux density components of different harmonics, and the stacking step includes aligning the ring magnet such that the total variation in flux around the perimeter of the stack of the magnet is reduced for different harmonics, such as for the first and second harmonics.

[0046] The total variation in flux around the perimeter can include the vector sum of flux densities of different harmonics.

[0047] In some embodiments, the magnets forming the array are selected based on the amplitude of their non-uniformity harmonics (and / or the measured magnetic flux density) such that, when properly aligned, they compensate for each other and provide a very low AC stray magnetic field.

[0048] In some embodiments, the method further includes determining any unintended magnetization components in the axial direction of each toroidal magnet by measuring the axial DC field at a predetermined distance from the toroidal magnets, and the step of stacking the magnets includes orienting the toroidal magnets in the stack to reduce the total axial DC field of the magnetic bearing assembly. In some embodiments, the magnets forming the array are also selected based on the magnitude of the unintended magnetization components in the axial direction such that, when properly oriented, the axial components will substantially cancel each other out, providing a very low DC stray magnetic field.

[0049] In some embodiments, instead of determining the unintended axial magnetization component of all magnets, measurements are taken only for a subset of toroidal magnets for this characteristic. These measurements are used in statistical quality analysis to confirm high uniformity and low components of axial magnetization in the toroidal magnets, and further measurements are taken only if the statistical analysis indicates a lower-than-expected magnet quality.

[0050] In some embodiments, after the PMB is partially assembled or after the pump is fully assembled, the stray field of the PMB is tested while the pump or PMB is rotating at a given speed, and in particular the AC stray field is tested to determine whether the stray magnetic field is below a predetermined threshold. In some cases, all pumps / PMBs may be tested, while in others only a subset is tested, and statistical analysis of the results is used to determine whether the pump meets the expected requirements or requires further testing or improvement.

[0051] Further specific and preferred aspects are set forth in the appended independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims, and may be presented in combinations different from those expressly set forth in the claims.

[0052] When a device feature is described as operable to provide a certain function, it will be understood that this includes providing that function or a device feature adapted or configured to provide that function. Attached Figure Description

[0053] Embodiments of the invention will now be described in further detail with reference to the accompanying drawings, in which: Figure 1 An axially magnetized passive magnetic bearing assembly is schematically shown. Figure 2 The AC and DC stray magnetic fields caused by the rotating axially magnetized PMB are schematically shown. Figure 3 The diagram schematically illustrates global skewness and inhomogeneity in an axially magnetized toroidal magnet. Figure 4 A magnetized passive magnetic bearing assembly according to an embodiment is schematically shown; Figure 5The DC field emitted by a radially and axially magnetized toroidal magnet at a radial distance of 100 mm is schematically shown. Figure 6 The diagram schematically illustrates global skewness and inhomogeneity in a radially magnetized toroidal magnet. Figure 7 Examples of first and second inhomogeneous harmonics in a toroidal magnet are shown; Figure 8 A portion of a vacuum pump according to an embodiment is schematically shown; Figure 9 A portion of a magnetic bearing assembly according to an embodiment is schematically shown; Figure 10 The axial stiffness of bearings with different shim thicknesses according to embodiments is shown; and Figure 11 A flowchart illustrating the steps of a method according to an embodiment is shown. Detailed Implementation

[0054] Before discussing the embodiments in more detail, an overview will first be provided.

[0055] Passive magnetic bearings used in vacuum pumps are typically made using axially magnetized toroidal magnets. The advantages of axially magnetized toroidal magnets include ease of magnetization and assembly, and the variety of materials that can be used to form the magnet. Disadvantages include an inherently high DC stray field and the need for selective alignment or correction to reduce the AC stray field.

[0056] Radial magnetized arrays are more difficult to magnetize and assemble, have fewer materials available for fabrication, and possess stiffness comparable to their axially magnetized array counterparts. However, they do exhibit an inherently low DC stray field. This low DC stray field is particularly important in certain applications such as ion traps and TEMs.

[0057] Bearings made with axially magnetized rings are very sensitive to skew and less sensitive to inhomogeneities, while bearings made with radially magnetized rings are much more sensitive to a lack of uniformity. The AC field can be reduced by selectively aligning different rings. However, this is more difficult in radially aligned bearings than in axially aligned bearings because the rings attract each other.

[0058] Passive magnetic bearings using radially magnetized magnet arrays can provide reduced leakage time-invariant or DC stray magnetic fields. Furthermore, time-varying or AC stray magnetic fields generated during rotation can also be reduced, provided that rotor magnets with at least the required level of uniformity are used. The use of high-coercivity HREE or dysprosium-free (“Dy”) or low-HREE or Dy NdFeB magnetic alloys in radially magnetized magnets produced by hot-deformation processes results in materials with a very fine crystal structure that is very uniformly oriented in the radial direction and therefore has very uniform magnetization, resulting in a much lower AC stray field.

[0059] Another way to reduce the AC field is by using selective alignment of magnets, allowing the AC field from one magnet to compensate for the AC field from another. This can be problematic in radially magnetized arrays due to the strong attraction between adjacent magnets. Using non-magnetic spacers inserted between adjacent layers of a magnet pair can reduce the attraction, thus making magnet assembly and selective alignment easier. Magnets with similar levels of inhomogeneity can be selected so that compensation between properly aligned magnets is effective.

[0060] Figure 4 A schematic diagram of a radially magnetized PMB according to an embodiment is shown. It shows a cross-section of one side of the bearing assembly, revealing the stator side 20 and the rotor side 40 of the bearing assembly. The rotor side is composed of a stack of annular magnets 24, while the rotor side is composed of a stack of annular magnets 34. The magnetization direction is indicated by arrows.

[0061] Figure 5 The diagram schematically illustrates the DC fields emitted by radially and axially magnetized toroidal magnets at a distance of 100 mm, and as can be seen, the DC field of the axially magnetized toroidal magnet is significantly higher than that of the radially magnetized toroidal magnet. As previously mentioned, the DC field is the time-invariant intrinsic magnetic field in the space surrounding the magnet, while the AC field is caused by magnetization errors, with the radially magnetized magnet being highly sensitive to uniformity and insensitive to global skew. Therefore, if a radial rotor magnet with low non-uniformity is sampled, a PMB with both low AC and DC stray fields can be fabricated.

[0062] Figure 6 The global skew and inhomogeneity in a radially magnetized toroidal magnet are schematically illustrated.

[0063] Figure 7The first and second harmonics of the inhomogeneity in the toroidal magnet are shown. The remanence of these two harmonics is plotted against the circumference angle of the toroid. For these two harmonics, the 0-peak amplitude ΔBi is 0.1 T, while the average remanence Bro is 1.35 T; therefore, the inhomogeneity is 0.1 / 1.35 = 0.074 (7.4%). Therefore, according to the embodiment, such a magnet would not be suitable for use in the rotor of a PMB.

[0064] Figure 8 A vacuum pump including a passive magnetic bearing 10 according to an embodiment is schematically shown. The PMB 10 includes a stack 30 of rotor magnets forming either side of the passive magnetic bearing and a stack 20 of stator magnets. The stator bearing magnets 20 are mounted on a stator 4, while the rotor bearing magnets are mounted on a rotor portion with a shaft 2. The pump also includes a motor 1 mounted on the shaft 2 for rotating the shaft about an axis.

[0065] Figure 9 A schematic cross-section of a stack of annular magnets 34 is shown, which may form the rotor portion 30 or stator portion of a passive magnetic bearing. In this embodiment, a non-magnetic shim 32 exists between each magnet 34. This allows the magnets to be stacked and rotated more easily to selectively align them, thereby further reducing the AC field when needed. A potential problem with the shims is that they may reduce the axial stiffness of the bearing. In some embodiments, these shims are made of a plastic material, while in other embodiments they may be a non-magnetic metal. Plastic materials are easier to manufacture and can have a low coefficient of friction, making alignment easier, but may not be as rigid or robust as metal shims. Preferred plastic materials may include fiber-reinforced PEEK.

[0066] Figure 10 The axial stiffness for different shim thicknesses is shown, and how the axial stiffness decreases with increasing shim thickness. As can be seen, thinner shims do not cause a significant difference in the axial stiffness of the magnet array.

[0067] Figure 11 The steps of the method according to an embodiment are illustrated. In the initial step at S10, the magnetic flux at different points and predetermined distances around the perimeter of a plurality of toroidal magnets is determined. At step S20, the average flux based on these measurements is determined. At step S30, the first and second harmonic inhomogeneities of the different toroidal magnets are calculated, which are evaluated using the 0-peak flux density amplitude of the corresponding harmonics, and divided by the average flux around the perimeter, which is obtained after substantially removing the background magnetic field. This inhomogeneity is defined as ΔBi / Br0.

[0068] The background field can be removed by measuring the background field and then subtracting the measured field from the measurement. Alternatively, shielding can be used to reduce / eliminate the background field, or the background field can be reduced / or even eliminated by using a set of coils to generate an opposite field.

[0069] Step S10, which measures the magnetic flux, may involve a magnetic probe measuring the components of the magnetic flux density, such as Brad AC [0-pk] and DC around the magnet at a given radius R, typically R>r. o (External r of the magnet). The measurement is performed when the probe axis is located in a plane perpendicular to the axis of the magnet and cuts the magnet into two equal parts ('z=0). If the measuring device includes processing circuitry that performs, for example, a fast Fourier transform or order spectrum, the harmonics of the flux density can be calculated, or it can be the direct output of the measuring device.

[0070] For a given harmonic ΔBi / Br0, the ratio Brad[o-pk] / Brad[DC] will depend on the magnet size and the location where the magnetic flux density is measured.

[0071] The coefficients can be calculated or determined experimentally to allow the inhomogeneity of the toroidal magnet for each harmonic to be determined based on measurements of the flux density around its perimeter. In practice, Brad DC varies little between different magnets with set materials and geometries, meaning that measuring only the AC flux component, such as Brad [0-pk], may suffice.

[0072] If needed, the 0-pk amplitudes of different 0-pk harmonics and their corresponding phases relative to a reference position on the magnet can be measured; these measurements can be performed as quality control, but can also be used to provide the information needed for selective alignment of the magnet to obtain even lower AC fields when performing this step.

[0073] In addition to Brad 0-pk, the axial flux density Bz 0-pk (and phase) of the harmonics can also optionally be measured using the same setup to ensure that the field generated by the global skew error is negligible as a quality control, or, if necessary, to provide the information needed for selective alignment of the magnet (if required) to obtain even lower AC fields.

[0074] At step S40, toroidal magnets with calculated first and second harmonic non-uniformity below a threshold are selected, and then at S50, pairs of selected toroidal magnets are stacked to form a passive magnetic bearing assembly according to the embodiment.

[0075] In some embodiments, steps S10 to S30 are not performed for each magnet, but rather a subset of measurable magnets are used in statistical quality analysis, and measurements of other toroidal magnets are performed only if the number of magnets that do not have the required uniformity rises above a threshold.

[0076] In some embodiments, an additional step is performed to determine the stray field of the assembled passive magnet, wherein the bearing is rotated at a given speed to check whether it is below a desired threshold. Alternatively and / or additionally, the stray field caused by the assembled bearing within a vacuum pump operating at full speed may be tested to determine whether it is below the desired threshold. As for the testing of the magnet, this may be performed on each bearing or pump, or it may be performed as a statistical analysis, wherein a subset is tested and further testing is performed only if the failure rate can reach a certain level.

[0077] In some embodiments, when magnets are stacked, non-magnetic pads may be placed between the magnets, and in some embodiments, toroidal magnets may be selectively aligned to reduce the global AC field by aligning a portion of each magnet such that the minimum flux density harmonic of one magnet coincides with the maximum flux of another magnet. In some embodiments, the DC field may be reduced by oriented magnets such that the axial component of the magnetic field density of the magnet array is minimized, eliminated, or at least reduced, including the axial component of the magnetic field density generated by undesired magnetization components in the axial direction.

[0078] The undesired component of axial magnetization in the axial direction can be determined using the same setup used to determine non-uniformity. Using the same setup (z = 0), Bz DC can also be measured at a set distance from the center of the magnet. For a fully radially magnetized magnet, at z = 0, Bz DC = 0 once the background (i.e., Earth's) magnetic field is subtracted. If it is non-zero, an axial magnetization component may be present, which would result in an increase in the DC magnetic field. Therefore, if a very low DC field is desired, the magnets in the stack can be oriented (in the sense of flipping them over or vice versa) and selectively assembled to form an array with a particularly low DC field.

[0079] Below is a summary and examples of the sensitivity of toroidal magnets with different magnetizations (radial and axial magnetizations) to stray fields with different magnetization errors (global skew and inhomogeneity).

[0080] Global skew can be quantified by the misalignment angle between the actual magnetization direction and the expected direction. Magnetization inhomogeneity can be measured by the 0-peak value variation ΔB of the remanence for different harmonics. i With average remanence B R0 The ratio between them is used for quantification. For example, in a toroidal magnet, the following equation describes the first harmonic of the remanence around its perimeter of the magnet bearing: BR = B R0 + ΔB i cosθ (1).

[0081] When the magnet is magnetized radially and axially, the stray field variation around a magnet with a given geometry can be evaluated by calculating the stray field variation at a given distance from the magnet's centerline and at a given height z above the plane that contains the magnet and cuts it in half. In the case of inhomogeneity, the sensitivity can be calculated as the ratio of the different components (e.g., radial, axial, tangential) ΔB of the AC stray field amplitude to the remanence 0-peak change ΔB i as defined in Equation 1 above. In the case of skewness, the sensitivity can be calculated as the ratio of the different components ΔB of the AC stray field amplitude to the skewness angle (in degrees).

[0082] Tables 1 and 2 show the results for examples of radially and axially magnetized magnets with a given geometry for a representative distance R from the magnet's centerline: Table 1: Sensitivity of radially magnetized toroidal magnets; Table 2: Sensitivity of axially magnetized toroidal magnets.

[0083] The results show that arrays with radially magnetized magnets are much less sensitive to global skewness, which is good. Conversely, they are more affected by inhomogeneity. Therefore, it is important to have a very high magnetization uniformity in order to reduce the AC stray magnetic field from radially magnetized magnets.

[0084] For radially magnetized toroidal magnets, the sensitivity of the AC stray magnetic field to inhomogeneity varies with the shape of the inhomogeneity around the perimeter of the magnet and is maximum for the first harmonic and decreases rapidly for higher harmonics, especially when the field is not measured very close to the magnet, such as at positions where it is important to obtain a low AC stray field outside the stator of a vacuum pump.

[0085] The sensitivity to different harmonics can be demonstrated by calculating the amplitude of the radial flux density harmonics generated by harmonics of inhomogeneity of the same magnitude.

[0086] Table 3 shows the calculation results for the first four harmonics for z = 0 (as in the previous example) and at three different radii R1 < R2 < R3: Table 3: AC stray field sensitivity to different harmonics of inhomogeneity.

[0087] Therefore, magnetic rings with low first- and second-order harmonic inhomogeneities are particularly suitable for these designs.

[0088] In the case of plastic-bonded magnets, the uniformity of magnetization is mainly determined by the uniformity of the magnetic powder within the plastic matrix, especially for magnets produced using isotropic powders.

[0089] Therefore, to minimize or at least reduce the AC stray field, the flow and final density of the powder must be controlled very precisely. Compression-molded magnets typically exhibit higher uniformity than injection-molded magnets.

[0090] When the powder is anisotropic, the uniformity of magnetization will also be controlled by the orientation of the powder during processes such as injection molding.

[0091] In some cases, plastic-bonded magnets can have low strength, as defined by their BHmax product, resulting in low bearing stiffness, and therefore they are not commonly used as PMBs in fast rotating pumps such as turbomolecular pumps.

[0092] For these faster-rotating pumps, there is more interest in PMBs made of sintered magnets; for example, sintered NdFeB for axial magnetization.

[0093] Sintered NdFeB magnets exhibit strong magnetism, high BHmax product, and high stiffness can be achieved with PMB.

[0094] Radially magnetized NdFeB magnetic rings are produced using a different process than sintering, in which the magnetic orientation of the material is not imparted by an external magnetic field, but rather by thermomechanical action. Therefore, this material is referred to as “thermally deformed” NdFeB, rather than sintered, although these materials are very similar in physical properties (e.g., density) and magnetic properties (e.g., BHmax product).

[0095] Radially magnetized NdFeB magnetic rings are available from various sources at different grades, defined by the material's magnetic strength (BHmax product) and temperature resistance. Temperature resistance is typically improved by adding and / or increasing the dysprosium (Dy) content. Higher Dy content results in higher intrinsic coercivity (Hcj) and therefore greater resistance to demagnetization; magnets without Dy or with low Dy content typically have an Hcj below 1000 kA / m.

[0096] When pumps (such as turbomolecular pumps) operate at temperatures below or above 120°C, a minimum Hcj of 1400 kA / m is generally preferred to prevent extensive demagnetization during assembly and use. This may require a dysprosium content of approximately 3% (or approximately half that if grain boundary diffusion techniques are used), corresponding to the standard H grade for such magnets. Depending on the application, higher coercivity may be required, i.e., 1600 kA / m corresponding to the SH grade, or sometimes, for other materials, approximately 2000 kA / m corresponding to the UH grade, with a dysprosium content of approximately 9%.

[0097] Tests show that the AC stray magnetic field generated by the thermally deformed NdFeB magnet may be higher than expected. The results confirm that the inhomogeneity of the first harmonic is the largest contributor to the AC stray field, with a contribution exceeding 4% ΔBi / Bro for the first harmonic.

[0098] The AC stray field generated by each magnet may necessitate selective alignment to keep the total AC stray field from the PMB below the limits required for critical applications.

[0099] However, in-depth research into how to adjust process parameters in the manufacture of hot-deformed magnets to obtain this material has inspired efforts to significantly reduce or eliminate the use of dysprosium (an element in high demand and short supply) while maintaining high intrinsic coercivity and thus being able to operate at sufficiently high temperatures.

[0100] By lowering the heat deformation temperature during manufacturing, Dy-free materials with high coercivity can be obtained, as described in Dr. K. Hioki's technical publication "High performance hot deformed NdFeB magnets". The magnetic strength of Dy-free materials is comparable to that of normal-grade materials tested with intrinsic coercivity of 1500 kA / m or higher, while the typical intrinsic coercivity of Dy-free NdFeB is slightly below 1000 kA / m.

[0101] The resulting material is a full-density nanocrystalline material with the highest level of radial orientation, resulting in higher uniformity and high intrinsic coercivity when tested with radially magnetized toroidal magnets made of Dy-free material. The stray magnetic field provided by the resulting material is lower than that of standard thermally deformed NdFeB. As determined by measurements available on the website of suppliers of such magnets, typical magnetization uniformity for the first harmonic is ~2% ΔBi / Bro or lower.

[0102] Using these magnets in a PMB made with radially magnetized magnets allows for low stray AC fields and very low stray DC fields without requiring selective alignment of the magnets. If selective alignment of the magnets is used, even lower AC stray field PMBs can be obtained.

[0103] Similarly, hot-deformed Nd-Fe-B magnets with a Dy content of <3% and a minimum intrinsic coercivity of >1700 kA / m at 20°C can be obtained using similar process parameters, compared to <1400 kA / m for standard hot-deformed NdFeB materials with similar Dy composition.

[0104] These low-dysprosium, high-heat-resistant materials can be used for the same effect.

[0105] Although illustrative embodiments of the invention have been disclosed in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to the specific embodiments, and various changes and modifications may be made therein by those skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0106] Reference Signs 1. Motor 2-axis 4. Stator 9. Rotation axis 10 Passive magnetic bearings 20 Stator Magnet Stacks 24 Stator toroidal magnet 30 Rotor magnet stacks 32 Non-magnetic gaskets 34. Rotor ring magnet.

Claims

1. A passive magnetic bearing assembly for a vacuum pump, the passive magnetic bearing assembly comprising: Multiple pairs of ring magnets are arranged in a stacked manner, with one ring magnet in each pair arranged on the stator side of the bearing and the other magnet in each pair arranged on the rotor side. The annular magnets are radially magnetized, and each annular magnet on the rotor side includes at least a first harmonic of magnetic inhomogeneity, wherein the inhomogeneity is measured as the 0-peak variation ΔB of the remanence. i With average remanence B R0 The ratio between, and wherein, for at least most of the annular magnets on the rotor side, the first harmonic of the magnetic inhomogeneity is less than 3%.

2. The passive magnetic bearing according to claim 1, wherein, For at least most of the annular magnets on the rotor side, each of the first and second harmonics of the magnetic non-uniformity is less than 3%.

3. The passive magnetic bearing assembly according to claim 2, wherein, For at least most of the annular magnets on the rotor side, each of the first and second harmonics of the magnetic inhomogeneity is less than 2%.

4. The passive magnetic bearing assembly according to claim 2 or 3, wherein, For at least most of the annular magnets on the rotor side, each of the first and second harmonics of the magnetic inhomogeneity is less than 1%.

5. The passive magnetic bearing assembly according to any of the preceding claims, wherein, Each toroidal magnet on the stator side includes at least a first harmonic of magnetic inhomogeneity, wherein the inhomogeneity is measured as the 0-peak variation ΔB of the remanence. i With average remanence B R0 The ratio between, wherein, for at least most of the toroidal magnets on the stator side, the first harmonic of the magnetic inhomogeneity is less than 3%.

6. The passive magnetic bearing assembly according to any of the preceding claims, wherein, At least some of the ring magnets include NdFeB magnets.

7. The passive magnetic bearing assembly according to claim 6, wherein, At least some of the ring magnets include thermally deformable NdFeB magnets.

8. The passive magnetic bearing assembly according to any one of claims 1 to 5, wherein, The magnet includes a plastic-bonded magnet.

9. The passive magnetic bearing assembly according to any of the preceding claims further includes a nonmagnetic spacer arranged to axially separate the annular magnets in the stack.

10. The passive magnetic bearing assembly according to claim 9, wherein, The non-magnetic pad is formed of a material having a low coefficient of friction on the surface in contact with the magnet.

11. The passive magnetic bearing assembly according to claim 9 or 10, wherein, The non-magnetic pad is made of plastic material.

12. A vacuum pump comprising a passive magnetic bearing assembly according to any of the preceding claims, the vacuum pump comprising a rotor rotatably mounted within a stator, one annular magnet in each pair of annular magnets being mounted to the rotor, and the other annular magnet in the pair being mounted to the stator.

13. The vacuum pump according to claim 12, wherein, The vacuum pump includes a turbomolecular pump.

14. The vacuum pump according to claim 12 or 13, comprising the passive magnetic bearing assembly according to any one of claims 8 to 10, wherein, The ring magnet is selectively aligned to reduce stray AC fields.

15. The vacuum pump according to any one of claims 12 to 14, wherein, The ring magnet is selectively oriented vertically to reduce stray DC fields.

16. A method for manufacturing a passive magnetic bearing assembly according to any one of claims 1-11, the method comprising: The first harmonic inhomogeneity of at least several radially magnetized rotor annular magnets is measured using the following method: Determine the radial magnetic flux density at a predetermined distance from the magnetic ring, measured at different points around the perimeter of the magnetic ring; and After removing the radial flux density of the background field, at least the first harmonic inhomogeneity is calculated by dividing the 0-peak amplitude of the corresponding magnetic flux harmonic by the average flux around the perimeter. The toroidal magnets of the rotor are selected as follows, wherein, for at least a majority of the toroidal magnets of the rotor, the first harmonic of the calculated magnetic inhomogeneity has at least a first harmonic inhomogeneity below a threshold; and The selected rotor annular magnet is stacked with a corresponding set of stator annular magnets to form the passive magnetic bearing assembly.

17. The method according to claim 16, wherein, The stacking step includes separating the ring magnets in the stack with non-magnetic spacers.

18. The method according to claim 16 or 17, wherein, The step of determining the magnetic flux includes determining the amplitude and phase of the flux density harmonics, and the stacking step includes aligning and optionally selecting the toroidal magnets such that the total variation of the flux density harmonics around the perimeter of the stack of the magnets is reduced.

19. The method of claim 18, further comprising determining the axial DC field of each annular magnet by measuring the field at an axial distance from the annular magnet, and the step of stacking the magnets comprising orienting and optionally selecting the annular magnets in the stack to reduce the total axial DC field of the magnetic bearing assembly.

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