GB2581203A

GB2581203A - Vacuum pump

Info

Publication number: GB2581203A
Application number: GB1901814.2A
Authority: GB
Inventors: Miles Christopher; Jonathan Grant Nicolas; James Patey Alexander; Lucchetta Emiliano; David Smith Paul
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2019-02-11
Filing date: 2019-02-11
Publication date: 2020-08-12
Also published as: WO2020165564A1; GB201901814D0

Abstract

A vacuum pump comprising rotor assembly which is rotatable supported by a first bearing 28 and a second bearing 30, wherein at least one bearing is a ball bearing, the vacuum pump comprises a preload force compensator unit 38, which generates a force to adjust the preload force on the at least one ball bearing, a control unit 36 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. The preload force generator may be mechanical, electromagnetic or smart material. The controller may consider more than one parameter of the pump when determining the appropriate preload force. The controller may operate a machine learning module to determine the appropriate preload force.

Description

Vacuum DUMP The present invention relates to a vacuum pump and in particular to a turbo-5 molecular vacuum pump.

Known vacuum pumps comprise a housing with an inlet and an outlet. A rotor assembly is disposed in the housing. The rotor assembly comprises a rotor shaft rotated by an electric motor and at least one rotor element. The rotor element 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 turbomolecular pumps 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 magnetic bearing by slight misalignment between the rotated part and the static part of that bearing, both in a passive way. Thereby the magnetic forces are axially shifted in order to gen-erate 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. In common vacuum pumps after assembly the introduced preload force cannot be changed while the pump is being operated. However, additional forces that may cause additional misalignment 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 normal limits can cause conditions such as ball skidding or fatigue of the main bearing. Such conditions can greatly reduce the service life of the product caused by the operating life of the main bearing.

It is an object of the present invention to provide a vacuum pump with an improved service life. -2 -

The vacuum pump in accordance to the present invention which is in particular a turbomolecular vacuum pump 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 pump and comprises at least one rotor element. Therein, the rotor assembly may comprise a plurality of pump elements built as vanes, if the vacuum pump is a turbomolecular vacuum pump. The at least one rotor element is interacting with a stator of the vacuum pump wherein the stator is connected to the hous-ing of the vacuum pump. Due to the interaction of the rotor element of the rotor assembly with the stator of the vacuum pump, a gaseous medium is conveyed from an inlet to an outlet due to rotating of the rotor assembly by an electric motor. 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 of the at least one ball bearing. Further, the vacuum pump comprises a control unit connected to at least one sensor to detect a pump parameter. Further, the control unit is connected to the preload force compensator unit to generate a compensating force dependent on the detected pump parameter.

The vacuum pump in accordance to the present invention may provide the beneficial technical effect that due to the preload force compensator 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 con-dition may arise in which the preload force to the at least one ball bearing is outside the normal limits, a force is generated by the preload force compensator unit to compensate for this undesired preload force such that the preload force is transferred back to within the normal limits which are for example set during the assembly process of the vacuum pump. Thus, ball skidding or fa- tigue of the main bearing is prevented and service life of the product is pro-longed. -3 -

Preferably, the preload force compensator unit comprises at least one electromagnet and an armature wherein in particular the armature is connected to the rotor assembly. This may provide the beneficial effect that with the electromagnet a force can be applied to the armature in order to compensate the undesirable preload force to the at least one ball bearing. Thereby, the elec-tromagnetic force can be controlled to compensate for the undesired preload force. Thereby, the force of the electromagnet to the armature 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 armature. Alternatively, the electromagnet is placed radially to the armature.

Preferably, the electromagnet is connected to an electric motor of the vacuum pump. More preferably the electromagnet is an integral part of the motor sta-tor. Thus, the electromagnet can be arranged in close proximity within the stator potted stator assembly or can be built as a modified motor unit that in addition to the driving torque can also exert an axial force on the shaft via the armature.

Alternatively, the armature is built from a permanent magnet. Thus, it is possible to apply positive as well as negative forces to the armature by the electromagnet, simply by switching polarity of the supply current. Preferably, the armature is built from a material that can be magnetized by the electromagnet in order to generate an attractive force.

Additionally or alternatively to the electromagnet, the preload force compensator unit preferably comprises a smart material to adjust the preload force of the ball bearing. Therein, a smart material is any reactive material which prop-erties can be changed by exposure to stimuli such as electromagnetic fields, stress, moisture and temperature, e. g. a material which can be made to expand or contract by applying a voltage. Examples for a smart material are -4 - piezo-electric materials or electro active polymers (EAP). This may provide the advantage that the smart material can be implemented easily and fine-controlled to adapt the preload force to the at least one ball bearing. Further, in particular piezo-electric materials are long-life materials which provide a relia-ble effect to maintain the optimal preload force to the at least one ball bearing.

Further, with implementing a smart material the number of magnetic coils might be reduced to enhance the electromagnetic compatibility of the vacuum pump.

Preferably, the smart material 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 adjust 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 maintain the optimal preload force to the at least one ball bearing.

Preferably, the second bearing is built as permanent magnetic bearing. Wherein, more preferably, the second bearing is placed towards the direction of the inlet of the vacuum pump, i.e. towards the region of lower pressures. Since the permanent 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 permanent magnetic bearing comprises a first magnetic ele-ment connected to the rotor assembly and preferably connected to the rotor shaft, and a static second magnetic element connected to the stator or housing of the vacuum pump and interacting with the first magnetic element to support the rotor assembly. Thereby, the position of the second magnetic element is adjusted by the smart material in order to adjust the preload force to the first bearing, i.e. the ball bearing.

Preferably, the detected pump parameter includes but is not limited to at least one or more of the following parameters: the temperature in particular the temperature of the rotor assembly, the temperature 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 time, power consumption of the electric motor, force to the ball bearing such as strain, position of the rotor assembly or impedance value of the at least one ball bearing. This might have the following advantages: * due to the temperature difference between the rotor assembly and the stator, the rotor assembly may undergo a larger thermal expansion than the stator leading to an increase of the preload force to the at least one ball bearing. Thus, upon measuring the temperature or the temperature difference between the rotor assembly and the stator, the preload force introduced by this pump parameter can be compensated by the preload force compensator unit.

* Further, the orientation of the vacuum pump may lead to additional forces to the rotor assembly of the vacuum pump. Thus, if the preload force is set with the vacuum pump in the vertical position, and for example then operated in the horizontal or inverted position, the changed gravitational force acting on the rotor assembly will apply an addition force to the at least one ball bearing as preload force. Thus, upon detecting the orientation of the vacuum pump undesired preload force to the at least one ball bearing can be compensated by the preload force compensator unit.

* Further, in dependence on the rotational speed of the rotor assembly due to the Poisson-effect, the preload force might be reduced with increasing rotational speed. Thus, upon detecting the rotational speed of the rotor assembly, 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 might be used for detection of venting that might cause an additional preload force to the at least one ball bearing that is compensated by the preload force compensator unit. -6 -

* 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. In particular, during harsh venting axial forces to the rotor may 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 motor can be detected by the sensor. The power consumption depends on the load of the vacuum pump, i.e. current volume flow, and might give also a hint to the inlet pressure. During high volume 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 meas-ure the preload force to the at least one ball bearing such that the preload force compensator unit can compensate the preload force to the at least one ball bearing in dependence on the measured 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 assembly. The axial position of the rotor assem-bly might 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 compensated by the preload force compensator unit.

Preferably, the position of the positioning sensor might be in the vicinity of or next to the at least one ball bearing. This might have the advantage that deviations for example due to thermal expansion are minimized 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 impedance of the at least one ball bearing. By measuring the impedance of the ball bearing -7 -load of the bearing can be determined. In 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 can compensate the preload force to the at least one ball bearing in dependence on the measured 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 maintain an optimal preload to the at least one ball bearing. In 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 comprises a look-up table to determine the required force exerted by the preload force compensator unit from the detected pump parameter or pump parameters. Additionally or alternatively, the control unit comprises a mapping function between the detected pump parameter and the force required to maintain the optimal preload force to the at least one ball bearing. Additionally or alternatively, the control unit comprises a machine learning unit which has been trained to connect the detected pump parameter to the preload force on the at least one ball bearing. Thus, for training the vacuum pump may 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 parameter directly to the strain exerted on the ball bearing. If the machine learning algorithm is sufficiently trained, the machine learning unit can be transferred to same or similar vacuum pumps. 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 pump may be detected for -8 -other reasons as well and can be also used for the preload force adjustment by the control unit and the preload force compensator unit.

The following is a detailed description of the invention with reference to pre-5 ferred embodiments and to the accompanied drawings.

The figures: Figures la to le: the vacuum pump in accordance to the present invention with different configurations of the preload force compensator unit as electro-magnet, Figure 2a and 2b: a vacuum pump in accordance with the present invention with different configurations of the preload force compensator unit as smart material, and Figure 3: a block diagram in accordance with an embodiment of the present invention.

Figure la shows a vacuum pump in accordance to the present invention with a housing 10. The housing 10 defines an inlet 12 and an outlet 14. In the housing is disposed a rotor assembly 16. The rotor assembly 16 comprises a rotor shaft 18 and numerous rotor elements 20. The rotor assembly 16 is rotated by an electric motor 22. Thereby, the rotor elements 20 are interacting with stator elements 24 which are connected to the housing 10 of the vacuum pump. By rotating of the rotor assembly 16 a gaseous medium is conveyed from the inlet 12 to the outlet 14. In the present embodiment, the vacuum is built as turbomolecular pump with an additional molecule drag stage 26.

For rotation of the rotor assembly 16, the rotor assembly 16 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 -9 -bearing 30 is built as permanent magnetic bearing and arranged toward the inlet 12 of the vacuum pump in the range of low pressures, i.e. high vacuums. The permanent magnetic bearing 30 comprises a first magnetic element 32 which is connected to the rotor assembly 16 and rotated together with the rotor assembly 16 during operation. Further, the permanent magnetic bearing comprises a static second magnetic element 34 connected to an element of the housing 10 and arranged to interact with the first magnetic element 32 to contactless support the rotor assembly 16. Therein the second magnetic element 34 might be slightly displaced during assembly of the vacuum pump rel-ative to the first magnetic element in order to generate an axial force towards the ball bearing. By this axial force a passive and determined 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 pre- load force compensator unit 38. The preload force compensator unit 38 com-prises an electromagnet 40 in the example of figure la integrated into to the electric motor 22. The electromagnet 40 is controlled by the control unit 36 in order to exert a force to an armature 42, wherein the armature 42 is connected to the rotor assembly 16 of the vacuum pump. Thus, by the electromagnet 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 maintained in narrow limits around an optimum of the ball bearing. Thus, life time or service life of the ball bearing 28 can be enhanced.

In the following figures, the same or similar elements are indicated by identical reference signs.

In figure lb, the armature 42 is connected to the rotor shaft 18. The electro-magnets 40 are placed axially relative to the armature 42 in order to generate an electromagnetic force on the rotor assembly 16 to control the preload force -10 -on the first bearing 28. Thereby, the armature 42 can be built as permanent magnet in order to generate by the electromagnets 40 positive or negative forces on the rotor assembly 16. Alternatively, as indicated in figure lb, the preload force compensator unit 38 comprises a second electromagnetic coil 44 in order to generate a positive as well as a negative force on the rotor assembly 16.

In figure lc, the armature 42 is connected to the rotor shaft 18 while the electromagnet 40 of the preload force compensator unit 38 is connected to the coils of the electric motor 22.

In figure ld, the armature 42 is integrated into the shaft 18 of the rotor assembly 16. The electromagnet 40 of the preload force compensator unit 38 is integrated in the electric motor 22 and arranged radially to the armature 42.

Therein the electromagnet 40 can be arranged in close proximity within the stator potted stator assembly or can be built as a modified motor unit that in addition to the driving torque can also exert an axial force on the shaft via the armature. Thus, by controlling the electromagnets 40 of the preload force compensator unit 38 a force can be generated on the rotor assembly 16 to corn-pensate undesired preload force on the first bearing 28.

In figure le, 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 46 which is connected to the control unit 36. By the smart material 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.

Figure 2b shows that the smart material 46 is connected to the outer race of the first bearing 28. Thus, by the smart material 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 opti-mal 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 as- sembly 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 con-veyed 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 may detect the orientation of the vacuum pump. Due to different orientations of the vacuum pump, the rotor assembly 16 may experience different forces due to the gravitational force. Thus, different orientations of the vacuum pump lead to different preload forces on the first bearing 28 which may be compensated by the preload force compensation unit 38.

A third sensor 54 may detect the rotational speed of the rotator assembly which also influences the preload force on the first bearing 28 due to the Poisson- -12 -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 pump 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 con-sumption of the electric motor 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 assembly 16 which might be compensated by the preload force compensation unit 38.

While in the above the first to fourth sensor are described as individual entities, they can be integrated into one or more sensors or the detection can be provided by different elements of the vacuum pump, i.e. the control unit controlling operation of the vacuum pump, which then are considered as sensors in the meaning 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 measures 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 measured and compensated if necessary. Additionally or alternatively a sensor can be used to measure the axial position of the rotor assembly 16. The axial position of the rotor assembly 16 might depend on an axial force applied resulting in additional preload forces. Thus, by detecting the axial position of the rotor assembly 16 excessive or additional preload force to the at least one ball bearing can be compensated 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 assembly 16 might be in the vicinity of -13 -or next to the at least one ball bearing. Additionally or alternatively a sensor can be used to measure the impedance of the at least on ball bearing. By measuring the impedance of the ball bearing the load of the bearing can be determined. In 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 impedance to the at least one ball bearing. Therein the sensor for measuring the impedance of the at least one ball bearing can be implemented in a closed loop detection circuit directly corn-any excessive preload force to the at least one ball bearing.

It is clear that the present invention is not limited to the specific combination of sensors as described above. One, more than one or all described pump parameter can be used to determine the necessary compensation of the preload force. Additionally, other pump parameters can be detected if these pump pa-rameters can be directly or indirectly connected to the preload force on the bearings of the vacuum pump. The above given list of possible pump parameters is not intended to be exhaustive or limiting.

For example, another sensor could be implemented detecting the power con-sumption of the electric motor 22 which is directly related to the pumping condition such as inlet pressure or volume flow through the vacuum pump. Increased inlet pressure or increased volume flow might cause additional preload forces to the at least one ball bearing which can be properly compensated by the preload force compensation unit 38.

Further, the control unit 36 comprises a correlation unit 60 which might be built as look-up table. Then, the control unit 36 connects the required force for compensating the preload force to the detected pump parameters by the look-

up table.

-14 -Alternatively, a mapping function between the detected pump parameters and the required force is stored in the correlation unit, whereby the control unit 36 uses this mapping function to determine the required force for compensating undesired preload force and maintain an optimal preload force on the at least one ball bearing. For example, a model can be applied describing the influence of the detected pump parameter to the resulting preload force such that a mathematical function is given receiving the respective pump parameter and outputting the required force.

Additionally or alternatively, the correlation unit 16 comprises a machine learn-ing 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 measurements 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 same or similar vacuum pumps. Then no direct detection circuit is necessary and a sensor for directly measuring the strain or preload force on the first bearing might be omitted.

-15 -Reference numbers: 10 housing 12 inlet 14 outlet 16 rotor assembly 18 rotor shaft rotor elements 22 electric motor 24 stator elements 26 molecule drag stage 28 first bearing second 32 first magnetic element 34 second magnetic element 36 control unit 38 preload force compensator unit electromagnet 42 armature 44 second electromagnetic coil 46 mart material 48 sensors first sensor 52 second sensor 54 third sensor 56 fourth sensor 58 fifth sensor correlation unit