WO2020165564A1

WO2020165564A1 - Vacuum pump

Info

Publication number: WO2020165564A1
Application number: PCT/GB2020/050298
Authority: WO
Inventors: Christopher Miles; Nicolas Jonathan GRANT; Alexander James PATEY; Emiliano LUCCHETTA; Paul David Smith; Christian Berger
Original assignee: Edwards Limited; Leybold Gmbh
Priority date: 2019-02-11
Filing date: 2020-02-10
Publication date: 2020-08-20
Also published as: GB2581203A; GB201901814D0

Abstract

Vacuum pump, in particular turbomolecular pump comprising rotor assembly rotatable supported by a first bearing and a second bearing, wherein at least one bearing is a ball bearing. Further, the vacuum pump comprises a preload force compensator unit, wherein the preload force compensator unit is built to generate a force to adjust the preload force on the at least one ball bearing. A control unit is connected to at least one sensor detect a pumping parameter, then further connected to the preload force compensator unit to generate a force dependent on the detected pump parameter.

Description

Vacuu m pu m p

The present invention relates to a vacuum pum p and in particular to a turbom olecular vacuum pump.

Known vacuum pum ps com prise a housing with an inlet and an outlet. A rotor assembly is disposed in the housing. The rotor assem bly com prises a rotor shaft rotated by an electric motor and at least one rotor element. The rotor elem ent is interacting with a stator connected to the housing in order to convey a gaseous medium from the inlet to the outlet. Thereby, the rotor shaft is supported by bearings.

The majority of turbom olecular pum ps with oil fed or grease bearings use at least one ball bearing. During assembly, the preload force to the ball bearing is adjusted to the intended use of the vacuum pump by either a spring applying an axial force to the rotor assembly or for a m agnetic bearing by slight m isalignm ent between the rotated part and the static part of that bearing, both in a passive way. Thereby the m agnetic forces are axially shifted in order to generate an axial force. The amount of preload force can be adjusted by the amount of axial shift between the rotated part and the static part of the respective magnetic bearing. I n com m on vacuum pumps after assembly the introduced preload force cannot be changed while the pum p is being operated. However, additional forces that may cause additional m isalignm ent of at least one of the bearings may lead to additional forces being added to the bearing preload force. Additional negative or positive preload forces that are outside the norm al lim its can cause conditions such as ball skidding or fatigue of the m ain bearing. Such conditions can greatly reduce the service life of the product caused by the operating life of the m ain bearing.

It is an object of the present invention to provide a vacuum pump with an improved service life. The vacuum pum p in accordance to the present invention which is in particular a turbom olecular vacuum pum p comprises a rotor assembly rotatably supported by a first bearing and a second bearing, wherein at least the first bearing is a ball bearing. The rotor assembly is disposed in a housing of the vacuum pum p and com prises at least one rotor elem ent. Therein, the rotor assembly may com prise a plurality of pump elem ents built as vanes, if the vacuum pum p is a turbom olecular vacuum pump. The at least one rotor elem ent is interacting with a stator of the vacuum pum p wherein the stator is connected to the housing of the vacuum pum p. Due to the interaction of the rotor elem ent of the rotor assem bly with the stator of the vacuum pum p, a gaseous m edium is conveyed from an inlet to an outlet due to rotating of the rotor assem bly by an electric motor. Further, the vacuum pump com prises a preload force com pensator unit, wherein the preload force com pensator unit is built to generate a force to adj ust the preload force of the at least one ball bearing. Further, the vacuum pum p comprises a control unit connected to at least one sensor to detect a pum p parameter. Further, the control unit is connected to the preload force compensator unit to generate a compensating force dependent on the detected pum p parameter.

The vacuum pum p in accordance to the present invention may provide the beneficial technical effect that due to the preload force com pensator unit any additional forces to the at least one ball bearing are compensated. Thus, if during start up of the vacuum pump or operation of the vacuum pump a condition m ay arise in which the preload force to the at least one ball bearing is outside the normal lim its, a force is generated by the preload force compensator unit to com pensate for this undesired preload force such that the preload force is transferred back to within the norm al lim its which are for example set during the assem bly process of the vacuum pum p. Thus, ball skidding or fatigue of the m ain bearing is prevented and service life of the product is prolonged.

Preferably, the preload force compensator unit com prises at least one electrom agnet and an armature wherein in particular the arm ature is connected to the rotor assembly. This m ay provide the beneficial effect that with the electrom agnet a force can be applied to the arm ature in order to compensate the undesirable preload force to the at least one ball bearing. Thereby, the electrom agnetic force can be controlled to com pensate for the undesired preload force. Thereby, the force of the electrom agnet to the arm ature can be tailored to maintain under all operation conditions of the vacuum pump an optimal preload force to the at least one ball bearing.

Preferably, the electromagnet is placed in axial direction relative to the arm ature. Alternatively, the electromagnet is placed radially to the arm ature.

Preferably, the electromagnet is connected to an electric motor of the vacuum pum p. More preferably the electromagnet is an integral part of the motor stator. Thus, the electrom agnet can be arranged in close proxim ity within the stator potted stator assem bly or can be built as a m odified m otor unit that in addition to the driving torque can also exert an axial force on the shaft via the arm ature.

Alternatively, the armature is built from a perm anent m agnet. Thus, it is possible to apply positive as well as negative forces to the arm ature by the electrom agnet, sim ply by switching polarity of the supply current. Preferably, the arm ature is built from a m aterial that can be m agnetized by the electrom agnet in order to generate an attractive force.

Additionally or alternatively to the electromagnet, the preload force compensator unit preferably com prises a smart material elem ent to adjust the preload force of the ball bearing. Therein, a sm art m aterial is any reactive m aterial which properties can be changed by exposure to stim uli such as electrom agnetic fields, stress, moisture and tem perature, e. g. a m aterial which can be made to expand or contract by applying a voltage. Examples for a smart m aterial are electro active polymers (EAP) . This may provide the advantage that the smart m aterial elem ent can be implem ented easily and fine-controlled to adapt the preload force to the at least one ball bearing. Further, with im plem enting a smart m aterial the num ber of magnetic coils m ight be reduced to enhance the electrom agnetic com patibility of the vacuum pump.

Additionally or alternatively to the electromagnet, the preload force compensator unit preferably comprises a piezo element to adj ust the preload force of the ball bearing. This may provide the advantage that the piezo elem ent can be im plem ented easily and fine-controlled to adapt the preload force to the at least one ball bearing. Further, piezo-electric materials of the piezo elem ent are long-life materials which provide a reliable effect to maintain the optim al preload force to the at least one ball bearing. Further, with implementing a piezo element the number of magnetic coils m ight be reduced to enhance the electrom agnetic compatibility of the vacuum pum p.

Preferably, the smart m aterial element and/or the piezo elem ent is connected to the outer race of the at least one ball bearing in order to adjust the position of the ball bearing and thereby adj ust also the preload force of the ball bearing. This may provide the advantage that the position of the outer race of the ball bearing can be adjusted in order to m aintain the optim al preload force to the at least one ball bearing. Therein, the smart material elem ent or the piezo elem ent can be implemented alone to compensate for the preload force. Alternatively, the sm art m aterial element and the piezo element can act together in order to compensate the preload force.

Preferably, m ore than one sm art m aterial elem ent or piezo elem ent are implemented, wherein more preferably each smart m aterial element or piezo elem ent is configured to compensate substantially the m ass of the rotor. This m ay provide the advantage that, if the vacuum pum p is constructed to be operated in an upright orientation than upon rotation of the vacuum pump in a horizontal orientation, the preload to the bearing is com pensated by a first smart m aterial elem ent or piezo element. If the vacuum pump is operated upside down, the preload can be further com pensated by a second sm art m aterial element or piezo elem ent. Preferably, the second bearing is built as permanent m agnetic bearing. Wherein, more preferably, the second bearing is placed towards the direction of the inlet of the vacuum pum p, i.e. towards the region of lower pressures. Since the perm anent magnetic bearing can be operated contactless, no grease or lubricant is necessary in the regions of low pressures, i. e. high vacuums. Consequently, the first bearing which is built as ball bearing is disposed towards the outlet of the vacuum pump, towards the regions of high pressures, i.e. low vacuum .

Preferably, the perm anent m agnetic bearing comprises a first m agnetic elem ent connected to the rotor assembly and preferably connected to the rotor shaft, and a static second m agnetic elem ent connected to the stator or housing of the vacuum pum p and interacting with the first m agnetic element to support the rotor assembly. Thereby, the position of the second m agnetic element is adj usted by the sm art material element or the piezo element in order to adj ust the preload force to the first bearing, i.e. the ball bearing. Therein, the smart m aterial elem ent or the piezo element can be implem ented alone to compensate for the preload force. Alternatively, the smart m aterial element and the piezo elem ent can act together in order to compensate the preload force.

Preferably, the detected pum p parameter includes but is not lim ited to at least one or m ore of the following parameters: the tem perature in particular the tem perature of the rotor assem bly, the tem perature difference between the rotor assembly and the stator, orientation of the vacuum pump, rotational speed of the rotor assembly, pressure at the inlet and/or the outlet, pressure change over tim e, power consum ption of the electric m otor, force to the ball bearing such as strain, position of the rotor assembly or im pedance value of the at least one ball bearing. This m ight have the following advantages:

• due to the temperature difference between the rotor assem bly and the stator, the rotor assem bly may undergo a larger therm al expansion than the stator leading to an increase of the preload force to the at least one ball bearing. Thus, upon m easuring the tem perature or the temperature difference between the rotor assem bly and the stator, the preload force introduced by this pump parameter can be com pensated by the preload force compensator unit.

• Further, the orientation of the vacuum pump may lead to additional forces to the rotor assem bly of the vacuum pump. Thus, if the preload force is set with the vacuum pum p in the vertical position, and for exam ple then operated in the horizontal or inverted position, the changed gravitational force acting on the rotor assem bly will apply an addition force to the at least one ball bearing as preload force. Thus, upon detecting the orientation of the vacuum pum p undesired preload force to the at least one ball bearing can be com pensated by the preload force com pensator unit.

• Further, in dependence on the rotational speed of the rotor assem bly due to the Poisson-effect, the preload force m ight be reduced with increasing rotational speed. Thus, upon detecting the rotational speed of the rotor assem bly, the preload force to the at least one ball bearing can be compensated by the preload force compensator unit.

• Further, a change of rotational speed or deceleration rate m ight be used for detection of venting that m ight cause an additional preload force to the at least one ball bearing that is compensated by the preload force compensator unit.

• Further, upon detecting pressure or pressure change and more preferably the inlet pressure and the outlet pressure, preload force to the at least one ball bearing can be compensated. I n particular, during harsh venting axial forces to the rotor m ay occur which lead to an increase of the preload force to the at least one ball bearing which can be compensated by the preload force compensator unit.

• Further, power consumption of the electric m otor can be detected by the sensor. The power consum ption depends on the load of the vacuum pump, i.e. current volum e flow, and m ight give also a hint to the inlet pressure. During high volum e flow or at the beginning of a pump cycle at high pressures at the inlet or low vacuum , an increased preload force can be occurring to the at least one ball bearing which can be compensated by the preload force compensator unit. • Further, the sensor can be built as strain sensor in order to directly m easure the preload force to the at least one ball bearing such that the preload force com pensator unit can compensate the preload force to the at least one ball bearing in dependence on the m easured force to the at least one ball bearing preload force.

• Further, the sensor can be built as position sensor in order to detect the axial position of the rotor assem bly. The axial position of the rotor assem bly m ight depend on an axial force applied to the rotor assembly resulting in additional preload forces. Thus, by detecting the axial position of the rotor assembly excessive or additional preload force to the at least one ball bearing can be com pensated by the preload force com pensator unit. Preferably, the position of the positioning sensor m ight be in the vicinity of or next to the at least one ball bearing. This m ight have the advantage that deviations for example due to therm al expansion are m inim ized and a direct connection between the axial position and the preload force can be established.

• Further, the sensor can be built in order to measure the im pedance of the at least one ball bearing. By measuring the impedance of the ball bearing load of the bearing can be determ ined. I n particular, together with temperature and rotational speed a preload force to the at least one ball bearing can be detected. The preload force com pensator unit can compensate the preload force to the at least one ball bearing in dependence on the m easured impedance to the at least one ball bearing.

Preferably, more than one pump parameter is considered by the control unit in order to control the preload force compensator unit to generate a force to compensate for any additional forces and m aintain an optim al preload to the at least one ball bearing. I n particular, counteracting effects or the direction of the effective forces are considered. If one effect leads to an increase of the preload force and another effect leads to a decrease of the preload force, only the difference of preload forces between these two effects are compensated for. Preferably, the control unit com prises a look-up table to determ ine the required force exerted by the preload force compensator unit from the detected pump param eter or pum p param eters. Additionally or alternatively, the control unit comprises a m apping function between the detected pump param eter and the force required to m aintain the optim al preload force to the at least one ball bearing. Additionally or alternatively, the control unit com prises a m achine learning unit which has been trained to connect the detected pump param eter to the preload force on the at least one ball bearing. Thus, for training the vacuum pum p m ay comprise a sensor to detect a pump parameter other than the force or strain sensor to the ball bearing and additionally strain sensor to detect the force or strain to the ball bearing directly. Then, the machine learning unit connects the detected param eter directly to the strain exerted on the ball bearing. If the m achine learning algorithm is sufficiently trained, the m achine learning unit can be transferred to sam e or sim ilar vacuum pum ps. This may have the advantage that costly and complex strain sensors to directly detect the strain on the ball bearing can be avoided. Temperature, pressure, rotational speed and the orientation of the vacuum pum p m ay be detected for other reasons as well and can be also used for the preload force adj ustm ent by the control unit and the preload force compensator unit.

The following is a detailed description of the invention with reference to preferred em bodim ents and to the accom panied drawings.

The figures:

Figures 1 a to 1 e: the vacuum pum p in accordance to the present invention with different configurations of the preload force compensator unit as electrom agnet,

Figure 2a and 2b: a vacuum pum p in accordance with the present invention with different configurations of the preload force compensator unit, and

Figure 3 : a block diagram in accordance with an embodiment of the present invention. Figure 1 a shows a vacuum pum p in accordance to the present invention with a housing 10. The housing 10 defines an inlet 12 and an outlet 14. I n the housing is disposed a rotor assembly 16. The rotor assem bly 16 comprises a rotor shaft 18 and numerous rotor elem ents 20. The rotor assembly 16 is rotated by an electric motor 22. Thereby, the rotor elem ents 20 are interacting with stator elem ents 24 which are connected to the housing 10 of the vacuum pum p. By rotating of the rotor assem bly 16 a gaseous medium is conveyed from the inlet 12 to the outlet 14. I n the present em bodim ent, the vacuum is built as turbomolecular pum p with an additional m olecule drag stage 26.

For rotation of the rotor assem bly 16, the rotor assem bly 1 6 is supported by a first bearing 28 and a second bearing 30. The first bearing 28 is built as ball bearing and arranged towards the outlet 14 of the vacuum pump. The second bearing 30 is built as perm anent magnetic bearing and arranged toward the inlet 12 of the vacuum pum p in the range of low pressures, i.e. high vacuums. The perm anent magnetic bearing 30 comprises a first m agnetic element 32 which is connected to the rotor assembly 16 and rotated together with the rotor assem bly 16 during operation. Further, the perm anent m agnetic bearing 30 comprises a static second magnetic elem ent 34 connected to an elem ent of the housing 10 and arranged to interact with the first m agnetic elem ent 32 to contactless support the rotor assembly 1 6. Therein the second m agnetic elem ent 34 m ight be slightly displaced during assem bly of the vacuum pump relative to the first m agnetic element in order to generate an axial force towards the ball bearing. By this axial force a passive and determ ined preload force is applied to the ball bearing necessary for proper operation of the ball bearing.

The vacuum pump comprises a control unit 36 which is connected with a preload force com pensator unit 38. The preload force compensator unit 38 comprises an electrom agnet 40 in the exam ple of figure 1 a integrated into to the electric m otor 22. The electromagnet 40 is controlled by the control unit 36 in order to exert a force to an arm ature 42, wherein the arm ature 42 is connected to the rotor assembly 16 of the vacuum pum p. Thus, by the electrom agnet 40 an attracting force to the armature 42 can be applied, thereby controlling the preload force on the first bearing 28 which is built as ball bearing. This may have advantage that by controlling the preload force on the ball bearing, actively the preload force to the ball bearing can be m aintained in narrow lim its around an optim um of the ball bearing. Thus, life time or service life of the ball bearing 28 can be enhanced.

I n the following figures, the same or sim ilar elem ents are indicated by identical reference signs.

I n figure 1 b, the arm ature 42 is connected to the rotor shaft 18. The electrom agnets 40 are placed axially relative to the armature 42 in order to generate an electrom agnetic force on the rotor assembly 16 to control the preload force on the first bearing 28. Thereby, the arm ature 42 can be built as perm anent magnet in order to generate by the electrom agnets 40 positive or negative forces on the rotor assem bly 16. Alternatively, as indicated in figure 1 b, the preload force com pensator unit 38 com prises a second electromagnetic coil 44 in order to generate a positive as well as a negative force on the rotor assembly 16.

I n figure 1 c, the armature 42 is connected to the rotor shaft 18 while the electrom agnet 40 of the preload force compensator unit 38 is connected to the coils of the electric motor 22.

I n figure 1 d , the armature 42 is integrated into the shaft 18 of the rotor assembly 16. The electrom agnet 40 of the preload force com pensator unit 38 is integrated in the electric motor 22 and arranged radially to the armature 42. Therein the electrom agnet 40 can be arranged in close proxim ity within the stator potted stator assem bly or can be built as a m odified m otor unit that in addition to the driving torque can also exert an axial force on the shaft via the arm ature. Thus, by controlling the electrom agnets 40 of the preload force compensator unit 38 a force can be generated on the rotor assembly 16 to com pensate undesired preload force on the first bearing 28. In figure 1 e, the armature 42 is connected to the rotor shaft 18, wherein the electromagnets 40 of the preload force compensator unit 38 are arranged radially and apart from the coil of the electric motor 22.

In figure 2a, the preload force compensator unit 38 comprises a smart material element 46 which is connected to the control unit 36. By the smart material element 46, the position of the second magnetic element 34 can be adapted in order to exert a force on the rotor assembly 16. Thereby, the preload force on the first bearing 28 can be controlled. In an alternative embodiment, instead of the shown smart material element 46, a piezo element is used to adjust the preload force.

Figure 2b shows that the smart material element 46 is connected to the outer race of the first bearing 28. Thus, by the smart material element 46 the position of the outer rays can be adapted and therefore additional force on the rotor assembly compensated to maintain the preload force on the first bearing 28 within the optimal limits.

Figure 3 shows a block diagram of the control unit 36, wherein the control unit 36 is connected to the preload force compensator unit 38 in order to control the preload force on the at least one ball bearing 28.

With the control unit 36, numerous sensors 48 are connected. The first sensor 50 may detect the temperature or temperature difference between the rotor assembly 16 and the stator 24 or housing 10. Due to the thermal expansion of the rotor assembly 16, the first bearing 28 may experience undesired preload force. This might occur during start-up of the vacuum pump, if the rotor assembly 16 has not yet its operation temperature. Thus, thermal expansion of the rotor assembly 16 might not be sufficient to ensure the optimal preload force to the first bearing 28. Alternatively, if the temperature of the rotor assembly is too high, for example due to an increased temperature of the conveyed gaseous medium, the thermal expansion might exceed the optimal value causing increased preload force to the first bearing 28. In both cases, the preload force compensator unit 38 is controlled by the control unit 36 to compensate for the undesired preload force.

A second sensor 52 m ay detect the orientation of the vacuum pum p. Due to different orientations of the vacuum pump, the rotor assembly 16 m ay experience different forces due to the gravitational force. Thus, different orientations of the vacuum pum p lead to different preload forces on the first bearing 28 which m ay be com pensated by the preload force compensation unit 38.

A third sensor 54 m ay detect the rotational speed of the rotator assembly which also influences the preload force on the first bearing 28 due to the Poisson- effect. Thus, this effect can be also considered by the preload force compensation unit 38 while compensating the preload force on the first bearing 28. Further, by the third sensor detecting the rotational speed, change of rotational speed or deceleration rate of the rotor can be detected caused by venting of the vacuum pum p which causes additional preload force to the at least one ball bearing.

A fourth sensor 56 may detect the pressure at the inlet 12 and the pressure change at the inlet 12 over time. Alternatively, or additionally, the energy consumption of the electric m otor 22 is detected which can be connected to the pressure at the inlet. However, pressure and in particular pressure during harsh venting can lead to an additional force on the rotor assem bly 16 which m ight be com pensated by the preload force com pensation unit 38.

While in the above the first to fourth sensor are described as individual entities, they can be integrated into one or m ore sensors or the detection can be provided by different elem ents of the vacuum pum p, i.e. the control unit controlling operation of the vacuum pump, which then are considered as sensors in the m eaning of the present invention.

While the first to fourth sensor 50-56 describe an open loop detection and compensating system , it is also possible to have a fifth sensor 58 which directly m easures the preload force or strain on the first bearing 28 to build a closed loop detection circuit. Thus, the strain on the first bearing 28 can be directly m easured and com pensated if necessary. Additionally or alternatively a sensor can be used to m easure the axial position of the rotor assembly 16. The axial position of the rotor assembly 16 m ight depend on an axial force applied resulting in additional preload forces. Thus, by detecting the axial position of the rotor assembly 1 6 excessive or additional preload force to the at least one ball bearing can be com pensated by the preload force compensator unit 38 preferably in a closed loop detection circuit. Preferably, the position of the sensor for detecting the axial position of the rotor assem bly 16 m ight be in the vicinity of or next to the at least one ball bearing. Additionally or alternatively a sensor can be used to measure the im pedance of the at least on ball bearing. By measuring the impedance of the ball bearing the load of the bearing can be determ ined. I n particular, together with temperature and rotational speed a preload force to the at least one ball bearing can be detected. The preload force compensator unit 38 can compensate the preload force to the at least one ball bearing in dependence on the measured im pedance to the at least one ball bearing. Therein the sensor for m easuring the im pedance of the at least one ball bearing can be implemented in a closed loop detection circuit directly com pensating any excessive preload force to the at least one ball bearing.

It is clear that the present invention is not lim ited to the specific com bination of sensors as described above. One, m ore than one or all described pum p param eter can be used to determ ine the necessary com pensation of the preload force. Additionally, other pum p parameters can be detected if these pum p parameters can be directly or indirectly connected to the preload force on the bearings of the vacuum pump. The above given list of possible pum p param eters is not intended to be exhaustive or lim iting.

For exam ple, another sensor could be im plem ented detecting the power consumption of the electric m otor 22 which is directly related to the pum ping condition such as inlet pressure or volume flow through the vacuum pump. I ncreased inlet pressure or increased volum e flow m ight cause additional preload forces to the at least one ball bearing which can be properly compensated by the preload force com pensation unit 38.

Further, the control unit 36 comprises a correlation unit 60 which m ight be built as look-up table. Then, the control unit 36 connects the required force for com pensating the preload force to the detected pump param eters by the look up table.

Alternatively, a m apping function between the detected pump param eters and the required force is stored in the correlation unit, whereby the control unit 36 uses this m apping function to determ ine the required force for com pensating undesired preload force and m aintain an optim al preload force on the at least one ball bearing. For example, a model can be applied describing the influence of the detected pum p param eter to the resulting preload force such that a m athem atical function is given receiving the respective pump param eter and outputting the required force.

Additionally or alternatively, the correlation unit 16 comprises a m achine learning unit which is trained properly, wherein preferably during training at least one open loop detection circuit with one of the first sensors to fourth sensors are correlated to the m easurements of the closed loop detection circuit detecting the strain or force on the first bearing 28 directly. If the correlation unit 60 is properly trained, the correlation unit can be transferred to sam e or sim ilar vacuum pum ps. Then no direct detection circuit is necessary and a sensor for directly measuring the strain or preload force on the first bearing m ight be om itted. Reference numbers:

10 housing

12 inlet

14 outlet

16 rotor assembly

18 rotor shaft

20 rotor elements

22 electric motor

24 stator elements

26 molecule drag stage

28 first bearing

30 second

32 first magnetic element

34 second magnetic element 36 control unit

38 preload force compensator unit 40 electromagnet

42 armature

44 second electromagnetic coil 46 smart material element

48 sensors

50 first sensor

52 second sensor

54 third sensor

56 fourth sensor

58 fifth sensor

60 correlation unit

Claims

CLAI MS

1 . Vacuum pum p, in particular a turbomolecular pum p, comprising a rotor assembly rotatably supported by a first bearing and a second bearing, wherein at least one bearing is a ball bearing, and a preload force com pensator unit, wherein the preload force compensator unit is built to generate a force to control the preload force on the at least one ball bearing, a control unit connected to at least one sensor to detect a pum p param eter and further connected to the preload force com pensator unit to generate a force dependent on the detected pum p parameter.

2. Vacuum pum p according to claim 1 , characterized in that the preload force compensator unit comprises at least one electromagnet and an armature.

3. Vacuum pump according to claim 2, characterized in that the arm ature is connected to the rotor assem bly.

4. Vacuum pum p according to claim 2 or 3, characterized in that the electromagnet is placed in an axial direction relative to the arm ature or placed radially.

5. Vacuum pump according to any of claim s 2 to 4, characterized in that the electromagnet is connected to an electric motor of the vacuum pump.

6. Vacuum pump according to any of claim s 1 to 5, characterized in that the preload force compensator unit comprises at least one sm art m aterial elem ent to adj ust the preload force to the ball bearing.

7. Vacuum pump according to any of claim s 1 to 6, characterized in that the preload force compensator unit com prises at least one piezo elem ent to adj ust the preload force to the ball bearing.

8. Vacuum pump according to claim 6 or 7, characterized in that the position of an outer race of the ball bearing is adj usted by the sm art m aterial elem ent and/or the piezo elem ent in order to adj ust the preload force.

9. Vacuum pump according to any of claim s 1 to 8, characterized in that the second bearing is a permanent m agnetic bearing.

10. Vacuum pump according to claim 9, characterized in that the permanent m agnetic bearing comprises a first magnetic elem ent connected to the rotor assembly and a static second magnetic element, wherein the position of the second m agnetic element is adj usted by the sm art m aterial elem ent or piezo element in order to adj ust the preload force to the ball bearing.

1 1 . Vacuum pum p according to any of claim s 1 to 10, characterized in that the pump param eter com prises at least one of the following parameters: Temperature, tem perature difference between the rotor and a stator, orientation of the vacuum pum p, rotational speed, pressure, pressure change, power consumption of the electric m otor, strain on the ball bearing, position of the rotor, im pedance of the at least one ball bearing.

12. Vacuum pump according to claim 1 1 , characterized in that more than one pump param eter is considered by the control unit.

13. Vacuum pump according to any of claims 1 to 12, wherein the control unit comprises a look-up table or a functional relationship, or a m achine learning unit to determ ine the required force exerted by the preload force compensator unit in dependence on the detected pum p parameter.