The present invention relates to a pump and method.

BACKGROUND

Pumps, such as turbomolecular pumps, are often employed as a component of a vacuum system for evacuating devices such as scanning electron microscopes (SEMs), lithography devices and other apparatus.

The performance of scanning electron microscope and other electrical equipment is highly susceptible to mechanical vibrations and stray magnetic fields emitted from such pumps. In particular, stray magnetic fields which vary with time as a result of rotation of magnetic components associated with the magnetic bearings and/or motor of the pump may cause issues in operation of apparatus around a vacuum pump. For example, stray fields are known to directly interfere with the electron beam or with the instruments’ electrical circuits.

It would be desirable to provide a means to mitigate stray magnetic fields emitted from pumps.

SUMMARY

The first aspect provides a method of reducing a stray magnetic field of a pump, said pump comprising a motor assembly and a magnetic bearing assembly, said method comprising: determining at least one component of a stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point; providing at least one magnet generated by additive manufacturing for mounting on said pump, said at least one magnet being configured with magnetic characteristics such that a magnetic field generated by said magnet counteracts said determined at least one component of said stray magnetic field; and mounting said magnet on said pump. The inventor of the present invention recognised that vacuum pumps such as turbomolecular pumps that are used to generate high vacuums often use high speed rotating magnets during operation in, for example, the permanent magnetic bearings used to support the shaft. These generate stray magnetic fields, that is magnetic fields that extend beyond their useful operational region and which may interfere with the operation of other components in the vicinity of the bearings and the pumps.

It would be desirable to be able to reduce the stray magnetic field and embodiments provide a method whereby at least one component of the stray magnetic field generated at least partly by the magnetic bearings, and in some embodiments solely by the magnetic bearings at a reference point or in a reference area or volume is determined or measured. The reference point may be selected as the point where it is important that the stray magnetic field is low. Thus, where the pump is to be used in conjunction with other devices that are sensitive to magnetic fields then this point may be at or close to such a device. In some embodiments, it may not be a point at or close to the device it may be an area such as a plane or a volume at or close to the device.

The stray magnetic field emitted by the pump when the rotor is not rotating is typically a static time invariant field known also as ‘DC’ field and can be determined as a vector. If the rotor is rotating and if the magnetisation of the bearing magnets is not uniform, the magnetic field measured in any position in space changes with time. Such a time varying field is also known as ‘AC’ field and can also be determined as a vector with time varying components. Thus, when the rotor is rotating, the stray magnetic field is essentially constituted of, the time varying and time invariant components. Typically, the amplitude of the timevarying (AC) field is much smaller than the amplitude of the static (DC) field.

The measuring may measure all, a subset or one of the vector components. Once the one or more components of the stray magnetic field have been determined then one or more magnets can be provided using at least one additive manufacturing technique whose magnetic field generated by rotation of the one or more magnets is configured to reduce or counteract and in some cases completely compensate for the determined stray magnetic field or stray magnetic field component of interest.

Additive manufacturing techniques provide an opportunity for generating permanent magnets with particular magnetic field characteristics. This allows a magnet that can counteract the stray magnetic field generated at least in part by the bearings to be manufactured and then mounted on the rotor to compensate for this stray magnetic field.

Where the vector component of interest is a time varying vector component then said step of determining is performed during rotation of said magnetic bearing assembly.

In some embodiments, said at least one component of said stray magnetic field comprises at least one time varying vector component generated by rotation of said magnetic bearing assembly, and said step of mounting comprises mounting said magnet on a rotor.

The magnet may be mounted in a predetermined angular orientation in a predetermined position on the rotor.

In some embodiments the method of reducing the stray magnetic field comprises a method of assembling or building a pump with a magnet mounted on the pump, in some embodiments on the rotor, configured to counteract said at least one component of the stray magnetic field. Alternatively, it may be a method of servicing a pump, wherein an additional magnet for counteracting the stray magnetic field is mounted on the pump, in some embodiments on the rotor, or an existing rotor mounted magnet of the pump is replaced by a magnet configured to counteract the stray field. ln some embodiments, said magnet is formed from bonded powder, at least some of said powder being magnetic powder, said magnetic characteristics of said magnet are produced by variations in density of said magnetic powder within a volume of said magnet.

Additive manufacturing techniques allow a magnet to be generated by bonding powder, some of the powder being magnetic powder, in a controlled fashion.

This allows the local density of the magnetic powder to be controlled and in this way a magnet with particular magnetic characteristics can be generated.

In some embodiments, the magnet may be formed of plastic bonded powder using for example FFF (fused filament fabrication) or FDM (fused deposition modelling) techniques while in other embodiments it may be formed at least partially from metallic powder perhaps by selective laser melting. In either case the powder is joined securely to form the magnet.

In some embodiments, said step of providing said at least one magnet comprises manufacturing a magnet with a predetermined volume and said magnetic characteristics using additive manufacturing.

In other embodiments, said step of providing said at least one magnet comprises selecting a magnet with said magnetic characteristics from a plurality of magnets of a same predetermined volume and with different magnetic characteristics, each of said plurality of magnets being manufactured using additive manufacturing.

The magnet that is used to counteract at least one component of the stray magnetic field may be generated following determination of the stray magnetic field and may be configured for this. In other embodiments a plurality of magnets with different magnetic characteristics may be generated using at least one additive manufacturing technique and the step of providing may be simply choosing one of the magnets with appropriate magnetic characteristics to counteract the determined stray magnetic field.

In some embodiments, said magnet is mounted within a protective shell.

The magnetic bearing generated by at least one additive manufacturing technique may not be as robust as a magnet formed in another way and in some embodiments, it is mounted within a protective shell. In some embodiments, the correction magnet may be mounted adjacent to a stack of magnets forming the magnetic bearing assembly and there may be a shell provided around the whole set of magnets. In this regard, the magnet may have a predetermined volume and be configured to fit into a predetermined space.

In some embodiments the magnet generated as the correction magnet may be a single magnet and in other embodiments it may be more than one magnet. In some embodiments, said correction magnet is one of a magnet pair.

In some embodiments, said magnetic bearing assembly comprises a stack of pairs of oppositely polarised ring magnets, a ring magnet of a pair being arranged on the stator side of the bearing and the other magnet of the pair on the rotor side; wherein said rotor magnets are mounted within an outer shell, said shell being sized to accommodate said stack of rotor magnets and said correction magnet, said step of mounting said correction magnet on said rotor comprises placing said correction magnet within said shell.

It should be noted that the magnets that form the stack of magnet pairs of the bearing assembly may also be generated by at least one additive manufacturing technique in which case mounting them within a protective shell helps to protect these magnets as well.

In some embodiments, said motor assembly comprises a permanent magnet motor, and said at least one magnet comprises a motor rotor magnet for said motor assembly, said motor rotor magnet being configured to provide a magnetic field for driving said motor and to provide a magnetic field to counteract said determined at least one component of said stray magnetic field.

In some cases, the motor that drives the rotor may be a magnetic motor with a permanent magnet mounted on the rotor. In such a case, it may be possible to form the motor rotor magnet using an additive manufacturing technique to provide not only the magnetic field required for the motor but also to provide the magnetic field for counteracting the stray magnetic field generated at least in part by the bearings. In effect the motor rotor magnet is configured to perform two functions one to provide the correct magnetic flux required for operation of the motor and one to provide the counteracting field to the overall stray field. This is in contrast to other embodiments where the magnet is a correction magnet and has only the function of counteracting the stray magnetic field.

In some embodiments, said motor rotor magnet is configured to extend beyond said motor stator in at least one direction.

In some embodiments, said motor rotor magnet is configured to extend beyond said motor stator in only one direction.

In some embodiments, said at least one or only direction is towards said magnetic bearing assembly.

In some embodiments, the motor rotor magnet may comprise a portion with a magnetic density configured primarily to generate the magnetic field required for the motor in the portion of the motor rotor magnet within the stator. The motor rotor magnet may extend beyond the stator in at least one direction and the magnetic density within the at least one extending portion may be configured primarily to provide the counteracting magnetic field. In some embodiments, it may extend beyond the stator in the direction of the magnetic bearings. A further aspect provides a method of manufacturing a magnet for mounting on a pump, said pump comprising a motor assembly and a magnetic bearing assembly, said method comprising: determining at least one component of a stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point; and determining magnetic characteristics of a magnet which when mounted on said pump would counteract said determined at least one component of said stray magnetic field; using additive manufacturing to manufacture said magnet with said determined magnetic characteristics, said magnet being formed from bonded powder at least some of said powder being magnetic powder, a local density of said magnetic powder being controlled to generate said magnetic characteristics.

A method of manufacturing a magnet using an additive manufacturing technique is provided, the method generating the required magnetic field by controlling the local density of the magnetic powder within the volume of the magnet. It should be noted that the magnet so generated may be a correction magnet for counteracting the determined stray field, or it may be both correction and functional magnet such that it is configured to perform one function required by the pump and is further configured with the additional function of providing this counteracting magnetic field. The magnet may be a single correction magnet configured to counteract the stray field alone or it may counteract a portion of the field and be used in conjunction with another magnet which counteracts a further portion of the field. Both magnets may be generated using one or more additive manufacturing techniques.

A yet further aspect provides a pump comprising a motor assembly and a magnetic bearing assembly, said pump further comprising: at least one correction magnet mounted on said pump, said at least one correction magnet being formed from bonded powder, at least some of said powder being magnetic powder; wherein said at least one correction magnet comprises variations in local density of said magnetic powder across a volume of said at least one correction magnet, said variations generating a magnetic field which counteracts at least one component of a stray magnetic field generated at least partly by said magnetic bearing assembly at at least one reference point.

In some embodiments said at least one correction magnet is mounted on said rotor and is configured such that on rotation of said at least one correction magnet, said at least one correction magnet counteracts said at least one component of said stray magnetic field generated by rotation of said magnetic bearing assembly.

In some embodiments, the pump comprises a vacuum pump. In some embodiments, the pump comprises a turbo molecular pump.

In some embodiments, said magnetic bearing assembly comprises a stack of pairs of oppositely polarised ring magnet pairs, a ring magnet of a pair being arranged on the stator side of the magnetic bearing assembly and the other ring magnet of the pair on the rotor side; said ring magnets on said rotor side being mounted within a protective outer shell, said outer protective shell being sized to accommodate said stack of ring magnets on said rotor side and said at least one correction magnet.

In some embodiments, said at least one correction magnet is one of a magnet pair.

The correction magnet may be one of a magnet pair in a magnetic bearing, the correction magnet being configured to provide the functionality of one of the rotor bearing magnets as well as counteracting the stray magnetic field.

In some embodiments, said at least one correction magnet comprises a rotor magnet for said motor assembly. ln some embodiments, said motor assembly comprises a motor stator and said motor rotor magnet, said motor rotor magnet being configured to extend beyond said motor stator at a side towards said magnetic bearing assembly.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 schematically shows a magnetic bearing with pairs of magnets in an axially magnetised radial array;

Figure 2 shows a ring magnet with transversal radial magnetisation;

Figure 3 shows a ring magnet with homopolar radial magnetisation;

Figure 4a shows a variable remanence shape in a radially magnetised homopolar magnet created by a variable density of magnetic powder in the ring such as that shown in Figure 3;

Figure 4b shows the stray magnetic field of a dipole shaped radial component at radial distance of 200mm, in the plane of the magnet, resulting from the magnetisation shape of Figure 4a;

Figure 5a shows a variable remanence shape in a radially magnetised homopolar magnet created by a variable density of magnetic powder in the ring such as that shown in Figure 3;

Figure 5b shows the stray magnetic field quadrupole shaped radial component at a radial distance of 200mm, in the plane of the magnet, resulting from the magnetisation shape of Figure 5a; Figure 6 schematically shows an example of rotor magnets of the bearings of Figure 1 with a correction magnet according to an embodiment;

Figure 7 shows a pump with a motor rotor magnet according to an embodiment;. Figure 8 schematically shows a flow diagram illustrating a method of reducing the stray magnetic field of a pump with magnetic bearings according to an embodiment; and

Figure 9 schematically shows a flow diagram illustrating a method of reducing the stray magnetic field of a pump with magnetic bearings according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Additive manufacturing of permanent magnetic bearings can be used to correct the magnetic stray field generated by a passive magnetic bearing (PMB) in a pump.

Each magnetic bearing has its stray field signature and additive manufacturing techniques can be used to manufacture a permanent magnet that has the required magnetic field characteristics to compensate or at least counteract or reduce this stray magnetic field.

Additive manufacturing can be used to provide a magnet with the required characteristics by varying the density of the magnetic active material in the magnet to create the desired effect. In this way magnetisation and density patterns are created to generate the magnetic field correcting effect that is required for a particular passive magnetic bearing. Mechanical strength issues can be addressed by printing or mounting the magnet inside a protective shell I support structure. The support structure may be a new part or may be an existing part of the passive magnetic bearing and or of the pump. Indeed the correction magnet could be one magnet in the array, 3D printed with the magnetisation characteristic required to provide an overall stray field below a certain threshold.

Imperfections in the magnetisation of the ring magnets used in passive magnetic bearings result in a non-uniform magnetic stray field in space and when the pump rotates this creates a time varying magnetic field in any given fixed position.

This is undesirable for certain applications (i.e. Scanning Electron Microscopes) using Turbo Molecular Pumps (TMP) and different methods are devised to reduce it. An established stray field reduction method, described in patent EP 3146222 B1 , is to measure the magnetisation error of each ring magnet in the rotor of the bearing and form a low stray field bearing rotor by aligning the ring magnets between each other to provide a cancelling effect. EP 2708753 B1 describes an alternative method where the magnetisation errors are corrected by lasering the magnets and thus altering their magnetisation locally.

Additive manufacturing of permanent magnets provides an alternative way of providing a magnet with predetermined characteristics that can be configured to counteract a stray magnetic field by modulating the density of the magnetic powder to generate the required magnetic field distribution.

Figure 1 schematically shows an axially magnetised radial array of ring magnets in a magnetic bearing, arranged around an axis of rotation 9. In this embodiment a stack of oppositely polarised pairs of magnets are mounted half on the stator as stator magnets 20 and half stacked on the rotor as rotor magnets 30.

Figure 2 shows a transversally magnetised ring magnet such as may be used as a correcting magnet in the stack of magnets in the bearing of Figure 1 .

The magnet of Figure 2 creates a rotating dipole that can correct for a given error. Conventionally the magnitude of the dipole generated by such a magnet can be trimmed by for example adjusting the diameter or thickness of the magnet. Alternatively, for a fixed size magnet it may be trimmed by adjusting the uniform density of the magnetic powder in the ring. Embodiments provide an alternative way of controlling the field of the magnetic ring using a spatially non- uniform magnetic powder density that is configured to provide the required variable stray magnetic field.

Embodiments may provide a magnetic ring with a spatially non-uniform magnet powder density to create a variable stray magnetic field.

Figure 3 shows an alternative magnetised homopolar ring magnet that may be used as a correcting magnet when the density of magnetic powder in the ring is non uniform.

Specifically, a powder density varying sinusoidally along the ring circumference creates a remanence Br also varying sinusoidally see figure 4a, which provides a variable stray field similar to that of a dipole see figure 4b, albeit with a non zero mean value.

If the magnetic powder density and thus remanence vary with a 2x special frequency along the ring magnet circumference see figure 5a, the resulting stray field (see figure 5b) is typical of a quadrupole and so forth. In principle the magnetic power density (and thus material remanence Br) can be changed to more complex profiles obtaining the combination of dipoles and quadrupoles within a single magnet or by the use of a combination of magnets magnetised in this way. A combination of correcting magnets can also be used to shift the DC value of the total correcting field to zero if so required. In this regard where it is the DC stray field that is being corrected, the correction magnet for correcting this may be mounted either on the rotor or on the stator.

Figure 6 shows the rotor magnets of a magnetic bearing assembly with an additional correction magnet 32 configured to reduce the stray magnetic field generated by the bearings. The correction magnet 32 can be encapsulated in protective shell 34 to provide additional support. Figure 6 shows an example, where the correction magnet 32 of a fixed size is mounted within the shell 34 that provides support and protection on top of the stack of rotor magnets 30.

The process for assembling the bearing with the correction magnet comprises the following steps:

1 . Form a rotor magnet using a random set of PMB (permanent magnetic bearing) magnets

2. Measure the stray field of the magnet array

3. 3D print a magnet with the required correcting magnetic field

4. Add it to the array

An alternative method comprises replacing step 3 with the step of selecting a magnet from a stock of 3D printed correction magnets with graded magnetic characteristics.

In some embodiments, the correction magnet could be merged with other existing components. In others as described above, it could be one magnet in the stack printed with a non uniform density to create the required correction.

In one embodiment the method comprises incorporating the correction magnetisation within the motor rotor magnet. Generally the motor rotor magnet is bonded in either the rotor and / or a containing sleeve attached to the rotor and thus mechanical strength and manufacturing tolerances of 3D printing are less of a concern.

In some embodiments a 3D printed motor rotor magnet to reduce stray magnetic field emitted by passive magnetic bearings in a turbo molecular pump is provided.

Such an embodiment provides a motor rotor magnet with variable magnetisation strength to provide the required magnitude of the stray magnetic field so that the total stray field can be zero (or thereabout) for each pump. A potential problem of this approach is that the magnetic moment of the motor rotor magnet as seen by the motor stator and its windings would also be changed resulting in inadequate motor performance.

This could be addressed in embodiments by providing a 3D printed motor rotor magnet of an increased length but fixed shape with the magnetic properties being changed within the volume by bonding powder at least some of it being magnetic powder using 3D printing and controlling the density of the magnetic powder, so that the magnetic stray field can be altered whilst keeping its magnetic moment as seen by the motor stator constant thus keeping the motor power and torque capability within required design limits.

In some embodiments, the method comprises forming a PMB for the pump without screening or selectively aligning the magnets, and then measuring the resulting stray field before the motor rotor magnet is added. This may be done on a suitable rig I instrument.

Using this measurement a motor rotor magnet can be 3D printed with the desired stray field and added to the rotor in the required angular orientation to generate near zero field at the required location in space.

The advantage is the elimination of the magnet screening and alignment procedure that is labour intensive replacing it with the 3D printing process which can be automated and more effective.

Figure 7 illustrates schematically the arrangement of a pump assembly 10, according to such an embodiment. A motor assembly comprises a motor stator 12, comprising an annular ring housing a set of coils. The motor stator 12 is coaxially-aligned with a central axis 9 of the pump assembly 10. The motor assembly also comprises a rotor magnet 1 which is co-axially aligned with, and housed within, the motor stator 12. The rotor magnet 1 is typically a permanent cylindrical magnet, which is also co-axially located on the central axis 9. It may also be a hollow cylinder mounted round the rotor shaft, in this case there may also be an external shell for example a CFRP (carbon fibre reinforced plastic) sleeve to contain and protect the magnet. The rotor magnet 1 is received on a rotor 2, which is also co-axially aligned on the central axis 9. The rotor magnet 1 extends beyond the stator 12 in one direction, that is the direction towards the passive magnetic bearings 20, 30.

The rotor 2 has a stem portion which receives the rotor magnet 1 and a head or hub portion extending therefrom. The head portion defines a cylindrical, recessed, void portion on an end face of the rotor 2. A radially-inner surface 2B of the head portion which defines the void receives a first part of a magnetic bearing assembly formed by an outer stack of magnets 30. In this embodiment, the outer stack of magnets 30 comprises three ring magnets stacked atop one another, with adjacent ring magnet having opposing polarities. An upper stator 4 receives a second part of the magnetic bearing assembly formed by an inner stack of stator magnets 20. In this embodiment, the inner stack of stator magnets 20 comprises three ring magnets stacked atop one another, with adjacent ring magnet having opposing polarities. However, it will be appreciated that more or fewer magnets may be provided to form the magnetic bearing assembly.

In operation, the rotor 2 is rotated at one end by the motor rotor magnet 1 in response to switching by the motor stator 12 and the rotor 2 is supported at the other end by the magnetic bearing assembly. Such an arrangement is particularly suited to pumps such as turbomolecular pumps.

In this embodiment the motor rotor magnet 1 is formed by an additive manufacturing technique and is configured to provide the function of both motor rotor magnet and correction magnet. The motor rotor magnet 1 extends beyond the motor stator 12 in the direction of the bearings and the density of magnetic powder of the portion of the motor rotor magnet within the stator 12 is configured to provide the driving functionality, while the density in the portion extending beyond the motor stator 12 is configured to provide the correction of the stray magnetic field functionality.

Figure 8 schematically illustrates a method of reducing the stray magnetic field according to an embodiment. At step S10 a stray magnetic field generated by rotation of the magnetic bearings is determined at a point. This is done by measuring the stray magnetic field or a vector component of this field by arranging the permanent magnetic bearing in a measuring device, as if it were arranged on the rotor of the vacuum pump and rotating it while at least one component of the stray magnetic field is measured.

The measurement can alternatively be carried out on the pump itself after the permanent magnet bearing has been installed. Where the pump has a magnetic motor, then in this case the stray field will comprise components from both the bearings and the motor rotor magnet. Such a measurement has the advantage that in the measurement includes the contributions made by the electric drive of the vacuum pump or the rotary unit and I or also those from other components.

At step S20 the magnetic characteristics of a magnet which when mounted on the rotor would counteract at least one component of the determined stray magnetic field is determined, and such a magnet is provided using additive manufacturing at step S30. The correction magnet is then mounted on the rotor, at step S40, perhaps in a protective shell.

Figure 9 shows a flow chart schematically illustrating steps in a method of manufacturing a magnet to be used to counteract a stray magnetic field. The initial step S50 is the same as step S10 of the method of Figure 8, that is the stray magnetic field at a point is determined. At step S60 the magnetic characteristics of a magnet which when mounted on the rotor would counteract at least one component of the determined stray magnetic field is determined and then at step S70 a magnet with these characteristics is manufactured using additive manufacturing. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

1 motor rotor magnet

2 rotor shaft

2B inner diameter of rotor hub 4 pump stator

9 axis of rotation

10 pump

12 motor stator

20 stator bearing magnets 30 rotor bearing magnets

32 correction magnet

34 protective shell