Apparatus and method for evaluating a magnetic bearing

Description

[0001] The invention relates to a device and a method for evaluating a magnetic bearing. Furthermore, the invention relates to a vacuum pump whose magnetic bearing is optimized using this device and / or this method.

[0002] In vacuum pumps, and particularly in turbomolecular pumps, magnetic bearings are commonly used to mount a rapidly rotating pump rotor on a stator. Turbomolecular pumps often have a hybrid bearing in which the rotor is supported by a combination of a rolling element bearing on the low-pressure side of the pump and a magnetic bearing with permanent magnets arranged on the high-vacuum side of the pump. Such a permanent magnet bearing comprises a stator part and a rotor part, each with several pairs of ring magnets. The ring magnets are magnetized in the axial direction, i.e. parallel to a rotational axis of the rotor of the turbomolecular pump, arranged concentrically to one another, and have a repulsive design. This means that the magnetization of the ring magnets in the stator part and in the rotor part of the magnetic bearing is opposite to one another in successive rings.

[0003] The rotor of a vacuum pump, like any rotating object, always exhibits a certain degree of residual imbalance, which leads to radial forces. These forces can be transferred to the vacuum pump's housing via the roller bearing and permanent magnet bearing, causing vibrations and noise. However, certain applications, such as electron microscopes or ion mobility spectrometers, require a very low vibration level from the turbomolecular pump used.

[0004] Therefore, for these and other applications, it is necessary to minimize vibrations and noise in the turbomolecular pump. This can be achieved, on the one hand, by minimizing the residual imbalance of the rotor through optimized balancing, and, on the other hand, by analyzing and improving the bearing of the rotor relative to the stator or the housing of the vacuum pump. The present application is directed to such an analysis and improvement of a magnetic bearing for a rotor of a vacuum pump.

[0005] An ideally magnetized ring magnet has, on the one hand, a magnetization axis that exactly corresponds to the axis of symmetry of the ring magnet or the rotor to be supported, and on the other hand, the magnetic material of such an ideal ring magnet is perfectly homogeneous. In reality, however, the magnetization of a ring magnet deviates from the magnetization of an ideal ring magnet due to various factors or magnetization properties. For example, an angular error can occur in which the magnetic preferred direction of the ring magnet deviates from its axis of symmetry or the axis of symmetry of the rotor, and the magnetic material of the ring magnet can exhibit a certain inhomogeneity. In addition, a rotationally symmetric tilt of the actual magnetization can occur along the circumference of the ring magnet, which is referred to as north / south asymmetry.In addition, a ring magnet can have mechanical deviations from the shape of an ideal ring magnet, for example with regard to the desired dimensions as well as due to microcracks or chipping in or on the respective ring magnet.

[0006] The individual magnetization properties of the ring magnets create additional forces in the permanent bearing, particularly radial forces. In the case of radial forces, the rapid rotation of the rotor of a turbomolecular pump leads to a harmonic or periodic radial force, also known as a magnetic bearing error. The magnetic bearing error, like this radial force, is therefore a vector with a magnitude and angular direction. Such a magnetic bearing error affects the running characteristics of the rotor within the turbomolecular pump, especially at low speeds.

[0007] In addition, heating of the rotor during operation of the turbomolecular pump can cause an axial displacement between the rotor's ring magnets and the stator's ring magnets, leading to an additional change in the axial forces in the rotor. It has been shown that the rotating radial force and the corresponding magnetic bearing error are strongly dependent on such axial displacement. This can affect the start-up of the turbomolecular pump's rotor, and the pump may be vibratory and acoustically noticeable during operation, disrupting the aforementioned applications. Furthermore, an increased magnetic stray field can be caused by such a turbomolecular pump, which also disrupts applications that instead require the lowest possible stray field.

[0008] From US 2014 / 0035412 A1, a device and a method with the features according to the respective preamble of claim 1 and 6 are known.

[0009] DE 103 21 925 A1 describes a device and a method with features according to a related technology.

[0010] Furthermore, EP 3 708 843 A2 describes a method for producing an electric motor or a vacuum device with such a motor, in which a magnetic field of a permanent magnet of the electric motor is detected and optimized such that a deviation in the symmetry and / or a magnetic radial force between a rotor and a stator of the electric motor is as small as possible.

[0011] An object of the invention is to provide a device and a method that enable an evaluation of a magnetic bearing in order to thereby reduce its magnetic bearing errors and / or its magnetic stray field in order to optimize the magnetic bearing.

[0012] This object is achieved by a device and a method having the features of the independent claims. Advantageous developments of the invention are specified in the subclaims, the description, and the drawings.

[0013] The device is intended for evaluating a magnetic bearing, in particular a turbomolecular pump. The magnetic bearing has a plurality of magnetic elements for rotatably supporting a rotor on a stator. The device comprises a measuring device configured to acquire measured values ​​of the magnetic field strength at a plurality of predetermined positions of each of the plurality of magnetic elements, and an evaluation device communicatively connected to the measuring device.

[0014] The evaluation device is configured to receive the measured values ​​of the magnetic field strength acquired by the measuring device for each of the magnetic elements and to determine magnetization properties for each of the magnetic elements based on the measured values ​​of the magnetic field strength. The evaluation device is further configured to receive predefined geometric data of the magnetic bearing and, based on the predefined geometric data and the magnetization properties of each of the magnetic elements, to determine at least one resulting force that the magnetic elements exert on the rotor. Furthermore, the evaluation device is configured to determine and output an evaluation of the magnetic bearing based on the resulting force.

[0015] Detecting the magnetic field strength at several predetermined positions of the respective magnetic elements can involve detecting the magnetic field strength, for example, along the respective circumference of the magnetic elements at evenly spaced measuring points. Additionally, the magnetic field strength for ring segments of the respective magnetic element, which may extend, for example, over an angle of 45° or 90°, can be summarized, for example, by averaging over those measuring points located within the respective ring segment. The measuring device can, for example, comprise one or more Hall probes.

[0016] The magnetization properties of the respective magnetic element can be represented by one or more magnetization vectors, which are determined either for segments or sections of the respective magnetic element or as a resulting vector for the entire magnetic element. The magnetization properties can also include or represent deviations from an ideal magnetization of the respective magnetic element. These deviations from the ideal magnetization can include an angular error of the resulting magnetization vector with respect to an axis of symmetry of the magnetic element, a magnetization inhomogeneity, and a north / south asymmetry. The north / south asymmetry is caused by the fact that the respective local magnetization direction along the circumference of the respective magnetic element is inclined with respect to an axis of symmetry of the magnetic element.Deviations from the ideal magnetization can also be caused by a local deviation relative to the mean value of the magnetization of the entire magnetic bearing, with the local magnetization being described by a location-dependent magnetization vector. Deviations from the ideal magnetization can also be caused by mechanical defects in the respective magnetic element, which can include deviations from the specified dimensions of the magnetic element, microcracks, and chipping.

[0017] The specified geometric data of the magnetic bearing can include the dimensions of the respective magnetic elements of the magnetic bearing, the number of magnetic elements in the magnetic bearing, and their respective distances from one another. To determine the at least one resulting force exerted by the magnetic elements on the rotor, a coefficient matrix can be created that describes the influence of the respective magnetization properties of the magnetic elements present in the magnetic bearing on the respective forces exerted by the respective magnetic element. The resulting force can be determined by vectorially adding these individual forces of the respective magnetic elements.

[0018] The method according to the invention thus allows a prediction of the at least one resultant force exerted by the magnetic elements on the rotor before the magnetic bearing is assembled from the multiple magnetic elements using the specified geometric data. The prediction of the resultant force enables the evaluation of the magnetic bearing composed of the multiple magnetic elements, for which this resultant force should, for example, be as low as possible in order to achieve optimal mounting of the rotor on the stator. The device is therefore characterized in that it provides the basis for the optimization of the magnetic bearing by predicting the resultant force and evaluating it based on it.

[0019] The evaluation device is further designed to determine the at least one resultant force as a function of an axial displacement of the rotor relative to the stator. It has been shown that for optimal running properties of a magnetic bearing, for example in a turbomolecular pump, the predicted resultant force should exhibit as little dependence as possible on the axial displacement of the rotor relative to the stator. Therefore, determining the resultant force as a function of the axial displacement of the rotor relative to the stator represents a further criterion for evaluating the magnetic bearing. This makes it possible to take into account, when optimizing the magnetic bearing, that the resultant force should exhibit as little dependence as possible on the axial displacement between the rotor and stator, and the magnetic elements for the magnetic bearing can be selected or designed accordingly.In addition, the specified geometry data of the magnetic bearing can be adjusted to achieve the lowest possible dependence of the resulting force on the axial displacement between rotor and stator.

[0020] The evaluation device can further be designed to form a coefficient matrix that describes the dependence of individual forces exerted by the respective magnetic elements on the rotor on the respective magnetization properties of the individual magnetic elements, and to determine the resulting force by vectorially adding the individual forces of the respective magnetic elements. The coefficient matrix can therefore serve to establish the connection between the respective magnetization properties of the individual magnetic elements, which result from a respective measurement of the magnetic field strength by means of the measuring device, and the resulting force. The use of the coefficient matrix can further facilitate the replacement of an individual or multiple magnetic elements if the magnetic bearing is to be optimized with regard to the resulting force by means of an iterative application of the device.

[0021] The evaluation device can further be designed to determine and output a magnetic bearing error based on the at least one resulting force and / or a magnetic stray field of the magnetic bearing based on the magnetization properties of each of the magnetic elements and based on the predetermined geometric data of the magnetic bearing. As already explained above, the magnetic bearing error is caused by a harmonic rotating radial force that results from the magnetization properties of the individual magnetic elements and is caused by the rotation of the rotor. Thus, the magnetic bearing error can be derived from the at least one resulting force and, like the force, is a vector with magnitude and angular direction or angular position. The magnetic bearing error and the magnetic stray field of the magnetic bearing can represent alternative or additional criteria for evaluating and optimizing the magnetic bearing.Whether the magnetic bearing error, the magnetic stray field of the magnetic bearing, or both are used to evaluate the magnetic bearing may depend on the specific environment in which the magnetic bearing or, for example, a turbomolecular pump incorporating such a bearing is to be used. For example, the intended use of a turbomolecular pump may determine whether its magnetic bearing is optimized for the lowest possible magnetic bearing error or the lowest possible magnetic stray field of the magnetic bearing.

[0022] The device can further comprise an optimization device configured to vary the predefined geometric data of the magnetic bearing and / or parameters of the magnetic elements such that the magnetic bearing error and / or the magnetic stray field of the magnetic bearing satisfy a predefined optimization condition. Since the evaluation device can output a prediction for the magnetic bearing error and / or the stray field of the entire magnetic bearing, it can additionally be provided that the device, using the optimization device, determines and provides information on how the predefined geometric data of the magnetic bearing and / or the parameters or properties of the magnetic elements are to be changed in order to improve or optimize the magnetic bearing, specifically with regard to the magnetic bearing error and / or the magnetic stray field.

[0023] The optimization condition may, for example, include that the magnetic bearing error and / or the magnetic stray field should be as small as possible in terms of magnitude. Varying the geometric data of the magnetic bearing may, for example, include changing the number of magnetic elements, the dimensions of the magnetic elements, and / or the distance between the magnetic elements within the magnetic bearing in order to subsequently determine at least one resulting force or the magnetic bearing error and the magnetic stray field again, and to compare these values ​​with the previously determined values ​​of the force, the magnetic bearing error, and / or the magnetic stray field using an iterative process.

[0024] In a similar way, the parameters of the magnetic elements can be changed, for example, by changing their magnetization and / or dimensions. Additionally, one or more magnetic elements can be replaced by other magnetic elements for which the measuring device has also previously acquired measured values ​​of the magnetic field strength at several predetermined positions. In this way, a magnetic bearing can be configured that is optimized for a group of provided magnetic elements and their database with regard to the magnetic bearing error and / or the stray field.

[0025] According to a further embodiment, the magnetic bearing can be designed as a permanent magnet bearing, in which the magnetic elements comprise a predetermined number of ring magnets. The predetermined number of ring magnets can comprise inner and outer ring magnets, each assigned to the rotor or the stator, for example, within a turbomolecular pump. The use of a permanent magnet bearing with a ring magnet can enable a cost-effective and robust design of the magnetic bearing.

[0026] For such a permanent magnet bearing, the measuring device can further be designed to determine the magnetic field strength along a circumference of each of the ring magnets of the magnetic bearing. Hall probes can be used for such a determination. Along the circumference of the respective ring magnets, the magnetic field strength can be determined for a predefined number of measuring points, and the magnetic field strength can be summarized for segments of the ring magnets, for example by averaging over the measuring points present in the respective segment. The segments can extend over an angle of 45° or 90°, for example. By summarizing measurement data for the magnetic field strength, the calculation time required by the device to determine the results, i.e., to determine the at least one resultant force, the magnetic bearing error, and / or the magnetic stray field, can be shortened.

[0027] According to a further aspect, a method for evaluating a magnetic bearing, in particular a turbomolecular pump, is provided, which has a plurality of magnetic elements for rotatably supporting a rotor on a stator. The method comprises: a) recording measured values ​​of the magnetic field strength at a plurality of predetermined positions of each of the plurality of magnetic elements; b) determining magnetization properties for each of the magnetic elements, for example, averaged for segments of ring magnets, based on the measured values ​​of the magnetic field strength; c) receiving predetermined geometric data of the magnetic bearing; d) determining at least one resulting force exerted by the magnetic elements on the rotor based on the magnetization properties of each of the magnetic elements and the predetermined geometric data of the magnetic bearing; and e) determining and outputting an evaluation of the magnetic bearing based on the resulting force.

[0028] The device described above is therefore intended to carry out steps a) to e) of the method using the measuring device and the evaluation device. The above statements regarding the device according to the invention therefore also apply mutatis mutandis to the method according to the invention, in particular with regard to the disclosure, the advantages, and the preferred embodiments.

[0029] The at least one resulting force is determined as a function of an axial displacement of the rotor relative to the stator. A coefficient matrix can be formed that describes the dependence of individual forces exerted by the respective magnetic elements on the rotor on the respective magnetization properties of the individual magnetic elements. The resulting force can be determined by vectorially adding the individual forces of the respective magnetic elements. Furthermore, a magnetic bearing error can be determined based on the at least one resulting force, and / or a magnetic stray field of the magnetic bearing can be determined based on the magnetization properties of each of the magnetic elements and based on the specified geometric data of the magnetic bearing.

[0030] The specified geometric data of the magnetic bearing and / or parameters of the magnetic elements can also be varied such that the magnetic bearing error and / or the magnetic stray field of the magnetic bearing fulfill a predetermined optimization condition.

[0031] According to a further embodiment, the magnetic bearing can be designed as a permanent magnet bearing, and the magnetic elements can comprise a predetermined number N of ring magnets. The above-mentioned varying of the parameters of the magnetic elements can comprise providing a predetermined plurality M of ring magnets that is greater than the predetermined number N of ring magnets required to form or assemble the magnetic bearing. A combination of N ring magnets can be selected from the predetermined plurality M of ring magnets, wherein for this combination, the magnetic bearing error and / or the magnetic stray field of the magnetic bearing satisfy the predetermined optimization condition.

[0032] The plurality M can be 50, for example, so that the ring magnets for the magnetic bearing are selected from a buffer or supply of 50 ring magnets. The predetermined number N of ring magnets for forming or assembling the magnetic bearing can be 10, for example, for a turbomolecular pump, with 5 ring magnets being provided for the rotor and a further 5 ring magnets for the stator. However, it is also possible for 5 ring magnets to be initially selected for the stator, for example, and for a combination of 5 ring magnets for the rotor to be selected from the remaining ring magnets in the buffer according to the method such that the magnetic bearing error and / or the magnetic stray field of the magnetic bearing as a whole, i.e. for the assembled magnetic bearing, satisfy the predetermined optimization condition, according to which, for example, both are minimized.The method therefore allows in both cases to determine the best combination of N ring magnets for the M ring magnets of the buffer.

[0033] According to a further embodiment, for the predetermined plurality M of ring magnets, any possible combination of N ring magnets for forming the magnetic bearing can be determined. For each of these determined combinations of N ring magnets, steps a) to e) of the method can be carried out in order to determine, for the respective combination, the respective magnetic bearing error based on the respective at least one resultant force and / or a respective magnetic stray field of the magnetic bearing, based on the magnetization properties of the N ring magnets of the respective combination and based on the predetermined geometric data of the magnetic bearing. Finally, the combination of N ring magnets for which the respective magnetic bearing error and / or the respective magnetic stray field best fulfill the predetermined optimization condition can be selected.

[0034] The possible combinations of N ring magnets for forming the magnetic bearing can be determined as permutations without repetition. Additionally, the order of the ring magnets in the magnetic bearing can be taken into account. Steps a) to e) of the method for predicting the at least one resultant force in the rotor can thus be performed iteratively for each of the determined combinations. By selecting the "best" combination for which the magnetic bearing error and / or the magnetic stray field most closely approximate the predetermined optimization condition, it can be ensured that, for the plurality M of ring magnets, no better combination of N ring magnets corresponding to the predetermined optimization condition can actually be found.

[0035] The predetermined optimization condition may include the magnetic bearing error and / or the magnetic stray field remaining substantially constant along an axial direction of the magnetic bearing. "Substantially constant" in the present context means that a maximum relative deviation for the magnitude and / or angular direction or angular position of the magnetic bearing error and / or the magnetic stray field along the axial direction of the magnetic bearing is not exceeded and amounts, for example, to a maximum of 1-5%. The optimization condition therefore specifies that both the magnitude and the angular position or phase of the magnetic bearing error may change only slightly along the axial direction. The predetermined optimization condition with the magnetic bearing error and / or stray field as constant as possible can therefore ensure that the magnetic bearing error and / or the magnetic stray field exhibits the flattest possible profile in the axial direction.In turbomolecular pumps, for example, it has been shown that such a flat curve of the magnetic bearing error or the radial force can result in optimized and smooth running behavior of the magnetic bearing or the rotor of the turbomolecular pump.

[0036] Alternatively or additionally, the predetermined optimization condition may include the respective magnitude of the magnetic bearing error and / or the magnetic stray field being less than a predetermined threshold value. According to this alternative optimization condition, the magnetic bearing error and / or the stray field should be as small as possible. This may be specified, for example, for the use of the magnetic bearing, e.g., for a turbomolecular pump intended for use in a specific environment.

[0037] According to a further aspect, a vacuum pump is provided, which is in particular a turbomolecular pump. The vacuum pump has a permanent magnet bearing with N ring magnets, which are selected from a predetermined plurality M of ring magnets according to the method described above such that the magnetic bearing error and / or the magnetic stray field of the permanent magnet bearing satisfy a predetermined optimization condition. The predetermined optimization condition can include that the magnetic bearing error and / or the magnetic stray field remains substantially constant along an axial direction or upon displacement along the axial direction of the magnetic bearing and / or that the respective magnitude of the magnetic bearing error and / or the magnetic stray field is less than a predetermined threshold value.

[0038] The invention is described below by way of example using advantageous embodiments with reference to the accompanying figures. They show, schematically: Fig. 1a perspective view of a turbomolecular pump, Fig. 2a view of the underside of the turbomolecular pump of Fig. 1 , Fig. 3 a cross-section of the turbomolecular pump along the Fig. 2 shown section line AA, Fig. 4 a cross-sectional view of the turbomolecular pump along the Fig. 2 shown section line BB, Fig. 5 a cross-sectional view of the turbomolecular pump along the Fig. 2 shown section line CC, Fig. 6 an illustration of different magnetization properties of ring magnets of a permanent magnet bearing, Fig. 7 a block diagram of a device for evaluating a magnetic bearing, Fig. 8 a block diagram of an algorithm that is executed by the device for evaluating the magnetic bearing, Fig. 9 exemplary diagrams for the course of the magnetic bearing error with the axial displacement between rotor and stator for a non-optimized and for an optimized permanent magnet bearing and Fig. 10 method steps for optimizing a permanent magnet bearing.

[0039] The Fig. 1 The turbomolecular pump 111 shown comprises a pump inlet 115 surrounded by an inlet flange 113, to which a recipient (not shown) can be connected in a manner known per se. The gas from the recipient can be sucked out of the recipient via the pump inlet 115 and conveyed through the pump to a pump outlet 117, to which a backing pump, such as a rotary vane pump, can be connected.

[0040] The inlet flange 113 forms the vacuum pump alignment according to Fig. 1 the upper end of the housing 119 of the vacuum pump 111. The housing 119 comprises a lower part 121, on which an electronics housing 123 is arranged laterally. Electrical and / or electronic components of the vacuum pump 111 are housed in the electronics housing 123, e.g., for operating an electric motor 125 arranged in the vacuum pump (see also Fig. 3 ). Several connectors 127 for accessories are provided on the electronics housing 123. In addition, a data interface 129, e.g., according to the RS485 standard, and a power supply connector 131 are arranged on the electronics housing 123.

[0041] There are also turbomolecular pumps that do not have such an attached electronics housing, but are connected to external drive electronics.

[0042] On the housing 119 of the turbomolecular pump 111, a flooding inlet 133, in particular in the form of a flooding valve, is provided, via which the vacuum pump 111 can be flooded. In the area of ​​the lower part 121, a sealing gas connection 135, which is also referred to as a purge gas connection, is also arranged, via which purge gas is supplied to protect the electric motor 125 (see e.g. Fig. 3 ) can be admitted into the motor compartment 137, in which the electric motor 125 is housed in the vacuum pump 111, before the gas delivered by the pump. Furthermore, two coolant connections 139 are arranged in the lower part 121, one of which serves as an inlet and the other as an outlet for coolant, which can be fed into the vacuum pump for cooling purposes. Other existing turbomolecular vacuum pumps (not shown) are operated exclusively with air cooling.

[0043] The lower side 141 of the vacuum pump can serve as a base, so that the vacuum pump 111 can be operated standing on the underside 141. However, the vacuum pump 111 can also be attached to a recipient via the inlet flange 113 and thus operated in a suspended position. Furthermore, the vacuum pump 111 can be designed so that it can also be operated when oriented in a different manner than in Fig. 1 As shown. Embodiments of the vacuum pump can also be realized in which the underside 141 is arranged facing either sideways or upwards, rather than downwards. In principle, any angle is possible.

[0044] Other existing turbomolecular vacuum pumps (not shown), which are particularly larger than the pump shown here, cannot be operated in an upright position.

[0045] On the underside 141, which is in Fig. 2 As shown, various screws 143 are arranged, by means of which components of the vacuum pump (not further specified here) are fastened together. For example, a bearing cover 145 is attached to the underside 141.

[0046] Mounting holes 147 are also arranged on the underside 141, through which the pump 111 can be attached, for example, to a support surface. This is not possible with other existing turbomolecular vacuum pumps (not shown), which are particularly larger than the pump shown here.

[0047] In the Figuren 2 bis 5 a coolant line 148 is shown in which the coolant introduced and discharged via the coolant connections 139 can circulate.

[0048] As the sectional views of the Figuren 3 bis 5 show, the vacuum pump comprises several process gas pumping stages for conveying the process gas present at the pump inlet 115 to the pump outlet 117.

[0049] In A rotor 149 is arranged in the housing 119, which has a rotor shaft 153 rotatable about a rotation axis 151.

[0050] The turbomolecular pump 111 comprises several turbomolecular pump stages connected in series for pumping purposes, with several radial rotor disks 155 attached to the rotor shaft 153 and stator disks 157 arranged between the rotor disks 155 and secured in the housing 119. A rotor disk 155 and an adjacent stator disk 157 each form a turbomolecular pump stage. The stator disks 157 are held at a desired axial distance from one another by spacer rings 159.

[0051] The vacuum pump also includes Holweck pump stages arranged radially one inside the other and connected in series for pumping efficiency. Other turbomolecular vacuum pumps (not shown) exist that do not have Holweck pump stages.

[0052] The rotor of the Holweck pump stages comprises a rotor hub 161 arranged on the rotor shaft 153 and two cylindrical-shell-shaped Holweck rotor sleeves 163, 165 attached to and supported by the rotor hub 161, which are oriented coaxially to the rotational axis 151 and nested within one another in the radial direction. Furthermore, two cylindrical-shell-shaped Holweck stator sleeves 167, 169 are provided, which are also oriented coaxially to the rotational axis 151 and nested within one another in the radial direction.

[0053] The pumping surfaces of the Holweck pump stages are formed by the lateral surfaces, i.e., the radial inner and / or outer surfaces, of the Holweck rotor sleeves 163, 165 and the Holweck stator sleeves 167, 169. The radial inner surface of the outer Holweck stator sleeve 167 lies opposite the radial outer surface of the outer Holweck rotor sleeve 163, forming a radial Holweck gap 171, and together with the latter forms the first Holweck pump stage following the turbomolecular pumps. The radial inner surface of the outer Holweck rotor sleeve 163 lies opposite the radial outer surface of the inner Holweck stator sleeve 169, forming a radial Holweck gap 173, and together with the latter forms a second Holweck pump stage. The radial inner surface of the inner Holweck stator sleeve 169 lies opposite the radial outer surface of the inner Holweck rotor sleeve 165, forming a radial Holweck gap 175 and together forming the third Holweck pumping stage.

[0054] At the lower end of the Holweck rotor sleeve 163, a radially extending channel can be provided, via which the radially outer Holweck gap 171 is connected to the central Holweck gap 173. Furthermore, at the upper end of the inner Holweck stator sleeve 169, a radially extending channel can be provided, via which the central Holweck gap 173 is connected to the radially inner Holweck gap 175. This connects the nested Holweck pump stages in series. A connecting channel 179 to the outlet 117 can also be provided at the lower end of the radially inner Holweck rotor sleeve 165.

[0055] The above-mentioned pump-active surfaces of the Holweck stator sleeves 167, 169 each have a plurality of Holweck grooves extending spirally around the rotation axis 151 in the axial direction, while the opposite lateral surfaces of the Holweck rotor sleeves 163, 165 are smooth and propel the gas in the Holweck grooves for operating the vacuum pump 111.

[0056] For the rotatable mounting of the rotor shaft 153, a rolling bearing 181 is provided in the area of ​​the pump outlet 117 and a permanent magnet bearing 183 is provided in the area of ​​the pump inlet 115.

[0057] In the area of ​​the rolling bearing 181, a conical spray nut 185 with an outer diameter increasing toward the rolling bearing 181 is provided on the rotor shaft 153. The spray nut 185 is in sliding contact with at least one wiper of a fluid reservoir. In other existing turbomolecular vacuum pumps (not shown), a spray screw can be provided instead of a spray nut. Since different designs are thus possible, the term "spray tip" is also used in this context.

[0058] The operating fluid storage comprises several stacked absorbent discs 187, which are impregnated with an operating fluid for the rolling bearing 181, e.g. with a lubricant.

[0059] During operation of the vacuum pump 111, the operating fluid is transferred by capillary action from the operating fluid reservoir via the wiper to the rotating injection nut 185. As a result of centrifugal force, it is conveyed along the injection nut 185 in the direction of the increasing outer diameter of the injection nut 185 to the rolling bearing 181, where it fulfills a lubricating function, for example. The rolling bearing 181 and the operating fluid reservoir are enclosed in the vacuum pump by a trough-shaped insert 189 and the bearing cover 145.

[0060] The permanent magnet bearing 183 comprises a rotor-side bearing half 191 and a stator-side bearing half 193, each comprising a ring stack of several permanent magnet rings 195, 197 stacked one on top of the other in the axial direction. The ring magnets 195, 197 are arranged opposite one another, forming a radial bearing gap 199, with the rotor-side ring magnets 195 being arranged radially outward and the stator-side ring magnets 197 being arranged radially inward.

[0061] The magnetic field present in the bearing gap 199 creates magnetic repulsion forces between the ring magnets 195, 197, which effect a radial bearing of the rotor shaft 153. The rotor-side ring magnets 195 are supported by a support section 201 of the rotor shaft 153, which radially surrounds the ring magnets 195 on the outside. The stator-side ring magnets 197 are supported by a stator-side support section 203, which extends through the ring magnets 197 and is suspended from radial struts 205 of the housing 119. The rotor-side ring magnets 195 are fixed parallel to the rotation axis 151 by a cover element 207 coupled to the support section 201. The stator-side ring magnets 197 are fixed parallel to the rotation axis 151 in one direction by a fastening ring 209 connected to the carrier section 203 and a fastening ring 211 connected to the carrier section 203.A disc spring 213 may also be provided between the fastening ring 211 and the ring magnets 197.

[0062] Within the magnetic bearing, an emergency or backup bearing 215 is provided, which runs idle without contact during normal operation of the vacuum pump 111 and only engages upon excessive radial deflection of the rotor 149 relative to the stator, forming a radial stop for the rotor 149 to prevent collision of the rotor-side structures with the stator-side structures. The backup bearing 215 is designed as an unlubricated rolling bearing and forms a radial gap with the rotor 149 and / or the stator, causing the backup bearing 215 to be disengaged during normal pumping operation. The radial deflection at which the backup bearing 215 engages is large enough so that the backup bearing 215 does not engage during normal operation of the vacuum pump, yet small enough so that collision of the rotor-side structures with the stator-side structures is prevented under all circumstances.

[0063] The vacuum pump 111 comprises the electric motor 125 for rotating the rotor 149. The armature of the electric motor 125 is formed by the rotor 149, whose rotor shaft 153 extends through the motor stator 217. A permanent magnet arrangement can be arranged radially on the outside or embedded in the portion of the rotor shaft 153 extending through the motor stator 217. Between the motor stator 217 and the portion of the rotor 149 extending through the motor stator 217, an intermediate space 219 is arranged, which comprises a radial motor gap, via which the motor stator 217 and the permanent magnet arrangement can magnetically influence each other to transmit the drive torque.

[0064] The motor stator 217 is secured in the housing within the motor compartment 137 provided for the electric motor 125. A seal gas, also referred to as purge gas, which may be air or nitrogen, for example, can enter the motor compartment 137 via the seal gas connection 135. The seal gas can be used to protect the electric motor 125 from process gas, e.g., from corrosive components of the process gas. The motor compartment 137 can also be evacuated via the pump outlet 117, i.e., the vacuum pressure in the motor compartment 137 is at least approximately equal to the vacuum pressure created by the backing pump connected to the pump outlet 117.

[0065] Furthermore, a so-called labyrinth seal 223, which is known per se, can be provided between the rotor hub 161 and a wall 221 delimiting the motor compartment 137, in particular in order to achieve a better sealing of the motor compartment 217 with respect to the Holweck pump stages located radially outside.

[0066] In Fig. 6 Different magnetization properties of the ring magnets 195, 197 of the permanent magnet bearing 183 are schematically illustrated. The upper four illustrations show a cross-sectional view of one of the ring magnets 195, 197 together with a local magnetization vector 600 and an axis of symmetry 605 of the ring magnet 195, 197. The term "local magnetization vector 600" in this context means that the magnetization is not considered globally or summed for the entire ring magnet 195, 197, but locally along its circumference, for example, using cylindrical coordinates. The lower two illustrations each show a schematic top view of one of the ring magnets 195, 197.

[0067] The upper illustration shows an ideal magnetization 610 of the ring magnet 195, 197, in which the magnetization or the local magnetization vector 600 has a substantially constant magnitude over the circumference of the ring magnet 195, 197 and is always aligned parallel to the axis of symmetry 605 of the ring magnet 195, 197. In contrast, the second illustration below shows an angular error 620, in which the magnitude of the magnetization vector 600 is substantially constant over the circumference of the ring magnet 195, 197, but is inclined with respect to the axis of symmetry 605. This inclination or directional deviation of the local magnetization vector 600 of individual ring magnet segments from the axis of symmetry 605 is not constant along the circumference of the ring magnet 195, 197, but rather variable.

[0068] The third illustration below shows an inhomogeneity 630, in which the magnetization vector 600 is aligned parallel to the symmetry axis 605 over the circumference of the ring magnet 195, 197, but the magnitude of the magnetization vector 600 changes over the circumference of the ring magnet 195, 197.

[0069] The illustration below shows the so-called north-south effect 640, in which the local magnetization vector 600 of the individual ring magnet segments has a constant magnitude over the circumference of the ring magnet 195, 197 and, when viewed locally in cylindrical coordinates, is inclined in the same direction with respect to the axis of symmetry 605 along the circumference of the ring magnet 195, 197 and is thus constant. This creates a conical alignment of the respective magnetization vectors 600 across the circumference of the ring magnet 195, 197 relative to the axis of symmetry 605.

[0070] The penultimate representation in Fig. 6 shows a top view of the ring magnet 195, 197, which has microcracks 650, while the bottom view of Fig. 6 a top view of the ring magnet 195, 197, which exhibits mechanical damage in the form of a so-called "chip" 660. The microcracks 650 and the chip 660 can lead to an inhomogeneity of the magnetization or the magnetization vector 600 along the circumference of the ring magnet 195, 197 and / or to an incorrect alignment of the magnetization vector 600 over the circumference of the ring magnet 195, 197 with respect to the axis of symmetry 605.

[0071] The above-described individual magnetization properties of the respective ring magnets 195, 197, ie the respective deviation from the ideal magnetization 610, cause additional forces, in particular radial forces, within the permanent magnet bearing 183. In the case of radial forces, rotation of the rotor 149 (cf. Fig. 3-5 ) harmonic rotating radial forces, which are also referred to as magnetic bearing errors. The magnetic bearing error, like each of the radial forces, is thus a vector with magnitude and angular direction or angular position. Such a magnetic bearing error influences or impairs the running properties of the rotor 149 of the turbomolecular pump 111, especially at low speeds, in a similar way to an imbalance of the rotor 149. As a result of the magnetic bearing error, vibrations and noises can occur within the turbomolecular pump 111, just as with an imbalance. The device according to the invention and the method according to the invention are provided for the magnetic bearing error and / or a magnetic stray field, which is also caused by the Fig. 6 The permanent magnet bearing 183 is evaluated based on the magnetic bearing error and / or the magnetic stray field in order to subsequently optimize it.

[0072] Fig. 7 shows a block diagram of a device 700 for evaluating the permanent magnet bearing 183 of the turbomolecular pump 111, which has the ring magnets 195 in the rotor-side bearing half 191 and the ring magnets 197 in the stator-side bearing half 193 (cf. Fig. 3 ). The device 700 comprises a measuring device 710, an evaluation device 720, and an optimization device 730.

[0073] The measuring device 710 is provided to record measured values ​​of the magnetic field strength at several positions along the circumference of the respective ring magnets 195, 197. The evaluation device 720 is communicatively connected to the measuring device 710 and is designed to execute an algorithm 725 which, in connection with Fig. 8 is explained in more detail and is provided for determining or predicting at least one resulting force which the ring magnets 195, 197 exert on the rotor 149. The evaluation device 720 is further configured to determine an evaluation of the permanent magnet bearing 183 based on the determined resulting force and to output this to the optimization device 730. The evaluation device 720 further determines the magnetic bearing error described above based on the at least one resulting force and / or the magnetic stray field of the permanent magnet bearing 183 in order to also output this to the optimization device 730. Based on the information provided by the evaluation device 720, the optimization device executes an algorithm 735 or method steps for optimizing the permanent magnet bearing 183, as described in connection with Fig. 10 are explained in more detail. The optimization is carried out before the assembly of the permanent magnet bearing 183 within the turbomolecular pump 111 using a plurality of ring magnets 195, 197, from which an optimal combination is selected by predicting at least one resulting force for a plurality of combinations of the ring magnets 195, 197 in order to evaluate the respective combination.

[0074] Fig. 8 shows a block diagram of the resulting force algorithm 725 used by the evaluation device 720 (cf. Fig. 7 ) is carried out. At 810, measured values ​​of the magnetic field strength are received for individual measuring points or at several predetermined positions along the circumference of the ring magnets 195, 197 of the permanent magnet bearing 183, wherein these measured values ​​have already been previously received, ie before the assembly of the permanent magnet bearing 183, by the measuring device 710 (cf. Fig. 7 ). The ring magnets 195, 197 are individually measured during this measurement process. Subsequently, at 820, the magnetization properties of the ring magnets 195, 197 are determined based on the received magnetic field strength values. The magnetization properties are determined by one or more magnetization vectors 600 (see Fig. 6 ), which are determined either for segments or sections of the respective ring magnet 195, 197 or as a resulting vector for the entire ring magnet 195, 197. The magnetization properties also include the deviations from the ideal magnetization 610 of the respective ring magnet 195, 197, as shown at 620 to 660 in Fig. 6 are shown.

[0075] At 830, geometric data of the permanent magnet bearing 183 is received. The geometric data of the permanent magnet bearing 183 are predetermined and include the dimensions of the respective ring magnets 195, 197, the number of ring magnets 195, 197 in the permanent magnet bearing 183, and their respective distances from one another.

[0076] Based on the magnetization properties of the ring magnets 195, 197 determined at 820 and based on the geometric data of the permanent magnet bearing 183 received at 830, the magnetic bearing error, i.e. the harmonic radial force acting on the rotor 149, and / or the magnetic stray field of the permanent magnet bearing 183 are calculated at 840. The calculation at 840 is performed using a coefficient matrix, as represented by block 845. The coefficient matrix describes the influence of the respective magnetization properties of the ring magnets 195, 197 present in the permanent magnet bearing on the respective forces exerted by the respective ring magnet 195, 197. The resulting force for the permanent magnet bearing 183 or the corresponding magnetic bearing error is then determined by a vectorial addition of the individual forces of the respective ring magnets 195, 197.

[0077] The calculation of the magnetic bearing error and / or the magnetic stray field for the permanent magnet bearing 183 is further performed iteratively, wherein an axial displacement between the rotor 149 and the stator is varied for each iteration step at 850. The magnetic bearing error and / or the magnetic stray field are thus calculated as a function of this axial displacement between the rotor 149 and the stator. This results in a curve of the magnetic bearing error for the permanent magnet bearing 183 as a function of the axial displacement, as shown in Fig. 9 is shown.

[0078] It has been shown that for optimal running properties of the permanent magnet bearing 183 or of the rotor 149 supported by it, the magnetic bearing error should have the smallest possible dependence on the axial displacement of the rotor 149 with respect to the stator or a flat curve in the case of such a displacement.

[0079] In Fig. 9 Three different diagrams are shown, each of which plots the magnetic bearing error 910 on the y-axis against the axial displacement 920 on the x-axis. Curves 930, 932, and 934 show an example of the magnetic bearing error 910 for a non-optimized permanent magnet bearing.

[0080] As can be seen, the magnitude of the magnetic bearing error 910 varies significantly depending on the axial displacement, such that, compared to a minimal value, the magnitude of the magnetic bearing error is several times greater as soon as the axial displacement increases or decreases sufficiently. Furthermore, a change in the phase direction or angular direction of the magnetic bearing error 910 can occur depending on the axial displacement, although this is not shown here.

[0081] For comparison, the diagram below shows Fig. 9 the course of the magnitude of the magnetic bearing error 910 as a function of the axial displacement for a permanent magnet bearing 183 is shown, which was measured using the Fig. 10 illustrated algorithm or method, which is explained in more detail below. As can be seen, the optimization of the permanent magnet bearing 183 results in a considerably flatter curve 940 as a function of the axial displacement 920 compared to the curve 934 for the non-optimized permanent magnet bearing, wherein the amount of the magnetic bearing error 910 is approximately one order of magnitude lower for small and relatively large values ​​of the axial displacement 920 compared to the curve 934 for the non-optimized permanent magnet bearing. The flat curve 940 for the amount of the magnetic bearing error 910 therefore significantly improves the running properties of the permanent magnet bearing 183 and the rotor 149.

[0082] In Fig. 10 The algorithm 735 for optimizing the permanent magnet bearing 183 and corresponding method steps are shown schematically. The algorithm 735 for optimizing the permanent magnet bearing 183 is executed by the optimization device 730 (see Fig. 7 ).

[0083] At 1010, a predetermined plurality M of ring magnets 195, 197 is provided as a buffer. The predetermined plurality M of ring magnets 195, 197 is, for example, 50 and is thus greater than a number N of ring magnets 195, 197 required to construct the permanent magnet bearing 183. In the example of Fig. 3 Five ring magnets 195 and five ring magnets 197 are present in the rotor-side and stator-side bearing halves 191, 193, respectively. Within the algorithm 735, the buffer of ring magnets 195, 197 provided at 1010 does not contain the ring magnets themselves, but rather their measured values ​​of the magnetic field strength, which are determined for each ring magnet 195, 197 of the buffer by means of the measuring device 710 (cf. Fig. 7 ) can be determined.

[0084] At 1020, all combinations of the M ring magnets 195, 197 present in the buffer provided at 1010 are determined, with which it is possible to form or assemble the permanent magnet bearing 183. For each of these combinations of ring magnets 195, 197, the at least one resulting force is predicted or calculated at 1030, as described above in connection with Fig. 8 In other words, the algorithm 725 described in Fig. 8 is shown, is carried out iteratively at 1030 for each of the possible combinations of ring magnets 195, 197 determined at 1020. This includes also determining the course or dependence of the resulting force or the magnetic bearing error and / or the stray field on the displacement between the rotor 149 and the stator of the turbomolecular pump 111.

[0085] Based on the results determined at 1030, optimized combinations of the ring magnets 195, 197 that most closely match a predetermined optimization condition are selected at 1040. The optimization condition includes the presence of a magnetic bearing error that is as constant as possible with respect to the displacement of the rotor 149 and the stator and / or a magnetic bearing error that is as small as possible and / or a magnetic stray field that is as small as possible for the respective combination of ring magnets 195, 197. Furthermore, at 1040, the best combination of the selected optimized combinations of ring magnets 195, 197 is determined, taking into account a limit value for the magnetic bearing error and / or for the magnetic stray field, in order to use this combination.

[0086] The unused ring magnets 195, 197 of the buffer with M ring magnets are sorted out at 1050 in order to subsequently refill the buffer of unused ring magnets 195, 197 with new ring magnets 195, 197 at 1060. This in turn means that the corresponding measured values ​​of the magnetic field strength for further ring magnets 195, 197 are additionally provided, whereby these measured values ​​of the magnetic field strength are determined by means of the measuring device 710 (cf. Fig. 7 ) are recorded.

[0087] Finally, the combination of ring magnets 195, 197 determined at 1040 is output at 1070 in order to form the optimized permanent magnet bearing 183 by means of this combination of ring magnets 195, 197. Bezugszeichenliste

[0088] 111Turbomolecular pump 113Inlet flange 115Pump inlet 117Pump outlet 119Housing 121Lower section 123Electronics housing 125Electric motor 127Accessory connection 129Data interface 131Power supply connection 133Flood inlet 135Seal gas connection 137Motor compartment 139Coolant connection 141Underside 143Screw 145Bearing cover 147Mounting hole 148Coolant line 149Rotor 151Rotation axis 153Rotor shaft 155Rotor disc 157Stator disc 159Spacer ring 161Rotor hub 163Holweck rotor sleeve 165Holweck rotor sleeve 167Holweck stator sleeve 169Holweck stator sleeve 171Holweck gap 173Holweck gap 175Holweck gap 179Connecting channel 181Rolling bearing 183Permanent magnet bearing 185Injection nut 187Disc 189Insert 191Rotor-side bearing half 193Stator-side bearing half 195Ring magnet 197Ring magnet 199Bearing gap 201Support section 203Support section 205Radial strut 207Cover element 209Support ring 211Fastening ring 213Disc spring 215Emergency orSafety bearing 217 Motor stator 219 Space 221 Wall 223 Labyrinth seal 600 Magnetization vector 605 Symmetry axis 610 Ideal magnetization 620 Angular error 630 Inhomogeneity 640 North-south effect 650 Microcracks 660 Chipping 700 Device for evaluating a magnetic bearing 710 Measuring device 720 Evaluation device 725 Algorithm for resultant force 730 Optimization device 735 Algorithm for optimizing the magnetic bearing 810 to 850 Steps of the resultant force algorithm 910 Magnetic bearing error 920 Axial displacement 930, 932, 934 Magnetic bearing error curve for a non-optimized permanent magnet bearing 940 Magnetic bearing error curve for an optimized permanent magnet bearing 1010 to 1070 Steps the algorithm for optimizing the magnetic bearing.

Claims

1. An apparatus (700) for assessing a magnetic bearing (183) of a vacuum pump, in particular a turbomolecular pump (111), said magnetic bearing (183) having a plurality of magnetic elements (195, 197) to rotatably support a rotor (149) at a stator, wherein the apparatus (700) comprises: a measurement device (710) which is configured to acquire measurement values of the magnetic field strength at a plurality of predetermined positions of a respective one of the plurality of magnetic elements (195, 197), and an evaluation device (720) which is communicatively connected to the measurement device (710) and which is configured: to receive, for each of the magnetic elements (195, 197), the measurement values of the magnetic field strength acquired by the measurement device (710), to determine magnetization properties for each of the magnetic elements (195, 197) based on the measurement values of the magnetic field strength, to receive predefined geometric data of the magnetic bearing (183), to determine at least one resulting force, which is exerted by the magnetic elements (195, 197) on the rotor (149), based on the magnetization properties of each of the magnetic elements (195, 197) and based on the predefined geometric data of the magnetic bearing (183), and to determine and output an assessment of the magnetic bearing (183) based on the resulting force, characterized in that the evaluation device (720) is further configured to determine the at least one resulting force in dependence on an axial displacement of the rotor (149) with respect to the stator and to assess the magnetic bearing (183) based on the dependence of the resulting force on the axial displacement of the rotor (149) with respect to the stator.

2. An apparatus (700) according to claim 1, wherein the evaluation unit (710) is further configured: to form a coefficient matrix which describes the dependence of individual forces, which are exerted by the respective magnetic elements (195, 197) on the rotor (149), on the respective magnetization properties of the individual magnetic elements (195, 197), and to determine the resulting force by a vectorial addition of the individual forces of the respective magnetic elements (195, 197).

3. An apparatus according to claim 1 or 2, wherein the evaluation device (710) is further configured to determine and output a magnetic bearing error based on the at least one resulting force and / or to determine and output a magnetic stray field of the magnetic bearing (183) based on the magnetization properties of each of the magnetic elements (195, 197) and based on the predefined geometric data of the magnetic bearing.

4. An apparatus (700) according to claim 3, wherein the apparatus (700) comprises an optimization device (730) which is configured to vary the predefined geometric data of the magnetic bearing (183) and / or parameters of the magnetic elements (195, 197) such that the magnetic bearing error and / or the magnetic stray field of the magnetic bearing (183) satisfies / satisfy a predetermined optimization condition.

5. An apparatus (700) according to any one of the claims 1 to 4, wherein the magnetic bearing is configured as a permanent magnet bearing (183) in which the magnetic elements comprise a predefined number of ring magnets (195, 197), and the measurement device (710) is configured to determine the magnetic field strength along a circumference of every single one of the ring magnets (195, 197) of the magnetic bearing (183).

6. A method of assessing a magnetic bearing (183) of a vacuum pump, in particular a turbomolecular pump (111), said magnetic bearing (183) having a plurality of magnetic elements (195, 197) to rotatably support a rotor (149) at a stator, wherein the method comprises that: a) measurement values of the magnetic field strength are acquired at a plurality of predetermined positions of a respective one of the plurality of magnetic elements (195, 197), b) magnetization properties are determined for each of the magnetic elements (195, 197) based on the measurement values of the magnetic field strength, c) predefined geometric data of the magnetic bearing (183) are received, d) at least one resulting force, which is exerted by the magnetic elements (195, 197) on the rotor (149), is determined based on the magnetization properties of each of the magnetic elements (195, 197) and based on the predefined geometric data of the magnetic bearing (183), and e) an assessment of the magnetic bearing (183) is determined and output based on the resulting force, characterized in that the at least one resulting force is determined in dependence on an axial displacement of the rotor (149) with respect to the stator, and the magnetic bearing (183) is assessed based on the dependence of the resulting force on the axial displacement of the rotor (149) with respect to the stator.

7. A method according to claim 6, wherein a coefficient matrix is formed that describes the dependence of individual forces, which are exerted by the respective magnetic elements (195, 197) on the rotor (149), on the respective magnetization properties of the individual magnetic elements (195, 197), and the resulting force is determined by a vectorial addition of the individual forces of the respective magnetic elements (195, 197).

8. A method according to claim 6 or 7, wherein a magnetic bearing error is further determined based on the at least one resulting force and / or a magnetic stray field of the magnetic bearing (183) is determined based on the magnetization properties of each of the magnetic elements (195, 197) and based on the predefined geometric data of the magnetic bearing (183).

9. A method according to claim 8, wherein the predefined geometric data of the magnetic bearing (183) and / or parameters of the magnetic elements are varied such that the magnetic bearing error and / or the magnetic stray field of the magnetic bearing (183) satisfies / satisfy a predetermined optimization condition.

10. A method according to claim 9, wherein the magnetic bearing is configured as a permanent magnet bearing (183) and the magnetic elements comprise a predefined number N of ring magnets (195, 197), wherein the varying of the parameters of the magnetic elements (195, 197) comprises that: a predetermined plurality M of ring magnets (195, 197) is provided that is greater than the predefined number N of ring magnets (195, 197), a combination of N ring magnets (195, 197) is selected from the predetermined plurality M of the ring magnets (195, 197) for which the magnetic bearing error and / or the magnetic stray field of the magnetic bearing satisfies / satisfy the predetermined optimization condition.

11. A method according to claim 10, wherein, for the predetermined plurality M of ring magnets (195, 197), every possible combination of N ring magnets (195, 197) is determined to form the magnetic bearing, for each determined combination of N ring magnets (195, 197), steps a) to e) are performed to determine, for the respective combination, the respective magnetic bearing error based on the respective at least one resulting force and / or to determine a respective magnetic stray field of the magnetic bearing (183) based on the magnetization properties of the N ring magnets (195, 197) of the respective combination and based on the predefined geometric data of the magnetic bearing (183), and that combination of N ring magnets (195, 197) is selected for which the respective magnetic bearing error and / or the respective magnetic stray field best satisfies / satisfy the predetermined optimization condition.

12. A method according to any one of the claims 9 to 11, wherein the predetermined optimization condition comprises that the magnetic bearing error and / or the magnetic stray field is / are substantially constant along an axial direction of the magnetic bearing (183), and / or wherein the predetermined optimization condition comprises that the respective magnitude of the magnetic bearing error and / or of the magnetic stray field is smaller than a predetermined threshold value.

13. A vacuum pump, in particular a turbomolecular pump (111), comprising: a permanent magnet bearing (183) comprising N ring magnets (195, 197) which are selected from a predetermined plurality M of ring magnets (195, 197) in accordance with the method according to any one of the claims 9 to 12 such that the magnetic bearing error and / or the magnetic stray field of the permanent magnet bearing (183) satisfies / satisfy a predetermined optimization condition.