WO2003067109A1

WO2003067109A1 - Bearing arrangement for linear slide

Info

Publication number: WO2003067109A1
Application number: PCT/NL2003/000086
Authority: WO
Inventors: Eduardus Gerardus Maria Holweg; Alexander Molenaar; Erik Zaaijer; Davy Camiel Cornelia Verstraeten
Original assignee: Ab Skf
Priority date: 2002-02-08
Filing date: 2003-02-07
Publication date: 2003-08-14
Also published as: AU2003206263A1; NL1019926C2

Abstract

Bearing arrangement for a linear slide (1) moveable in a first direction (x) with respect to a base plate (6). Each magnetic bearing unit (10) comprises a permanent magnet (13) and an electromagnet (12) and co-operates with a rail (7) of magnetically conducting material extending in the first direction (x). The magnetic bearing unit (10) comprises a double C-yoke configuration, the electromagnet being formed by the double C-yoke (11). Each C-yoke (11) is provided with a coil (12), and the permanent magnet (13) is positioned between the C-yokes (11). The rail (7) co-operates with the magnetic bearing unit (10) and can be placed between the end-faces of the C-yokes (11). The bearing arrangement further comprises at least two further electromagnets (20) cooperating with the at least one rail (7).

Description

Bearing arrangement for linear slide

The present invention relates to a bearing arrangement for a linear slide, which may e.g. be used in a wafer transport system. More specifically, the present invention relates to a bearing arrangement for a linear slide moveable in a first direction with respect to a base plate, comprising a plurality of magnetic bearing units for slideably mounting the linear slide with respect to the base plate, each magnetic bearing unit comprising a permanent magnet and an electromagnet and co-operating with at least one rail of magnetically conducting material extending in the first direction, a linear motor for driving the linear slide in the first direction.

Such a bearing arrangement is known from American patent US-A-5,360,740, which discloses a vehicle transport arrangement which may be used as a wafer transport system. The system uses magnetic bearings as these allow a dust free operation which is necessary in wafer transport systems. The system comprises four magnetic units for suspending the transport table and a linear motor for driving the transport table in a predetermined direction. The magnetic units comprise a permanent magnet and an electromagnet in series, in which the electromagnet and permanent magnet are designed such that in a normal operational situation (the table being a predetermined distance away from the rail comprising the linear motor), the current through the electromagnet coil is substantially zero. To allow the current through all four electromagnets to be substantially zero, one of the magnetic units is equipped with a piezo actuator to adjust the distance between this magnetic unit and the rail.

This known system has a number of disadvantages, e.g. in that precise control in all directions is not possible, or at least not addressed in the patent (only the distance between the table and the rail, and the displacement by the linear motor can be controlled). The system described cannot accommodate dissimilar loads on the table. Also, the electromagnets are positioned on the moving table, necessitating a power supply on the moving table, either using batteries or feed cables, which may hamper proper operation of the system. The present invention seeks to provide a bearing arrangement for moving a linear slide, e.g. in a wafer transport system, with a high degree of control in all directions. The system should also be able to cope with dissimilar loads on the linear slide. According to the present invention, a bearing arrangement is provided according to the preamble defined above, in which the magnetic bearing unit comprises a double C-yoke configuration, the electromagnet being formed by the double C-yoke, in which each C-yoke is provided with a coil, the at least one permanent magnet being positioned between the C-yokes, and the at least one rail co-operating with the magnetic bearing unit being placeable between the end- faces of the C-yokes, the bearing arrangement further comprising at least two further electromagnets cooperating with the at least one rail for controlling the position of the linear slide in a second direction perpendicular to the first direction. The double C-yoke configuration of the magnetic bearing unit allows full control of the position of the rail with respect to the bearing unit in two axes, i.e. the z-direction (perpendicular to the first direction (x-axis), and representing the distance between the end faces of the C-yokes and the rail) and the ψ-direction (i.e. the rotational axis around the y-axis, which is perpendicular to both the x- and z-axes, and describes the degree of tilt or pitch of the sliding table with respect to the rail). When two or more magnetic bearing units are used with two parallel rails, control in the ζ-direction is also possible (rotational axis around the x-axis).

The at least two further electromagnets can be used to control the distance between the magnetic bearing unit to the rail in the y-direction. At least two further electromagnets are necessary as these can only generate attractive forces to the rail. When two parallel rails are used, four further electromagnets can be used to allow control in both the y-direction and in the φ-direction (rotational axis around z-axis). Together with the control in x-direction by the linear motor, this allows a full six degrees of freedom control of the linear slide with respect to the base plate. It is noted that the at least two further electromagnets could also comprise additional double C- yoke configured magnetic bearing units.

It should be noted that the double C-yoke configuration is known as such from the Dutch patent NL- 1005344.

The further electromagnets may be of the E-core type having a coil on the middle leg of the E-core. This type of electromagnets may be easily and economically produced, and allow sufficient control for the application in a wafer transport system. The normal operation condition would be to operate the further electromagnets at half of their current rating. Most effective control is then possible when the currents through the two further electromagnets on opposite sides of the linear slide are differentially controlled.

In a particular embodiment, the bearing arrangement further comprises at least one sensor for detecting the position of one of the plurality of magnetic bearing units relative to the at least one rail, and processing means which are connected to the at least one sensor and the electromagnets of the plurality of magnetic bearing units and the at least two further electromagnets for controlling the position of the linear slide with respect to the base plate. This allows five degrees of freedom to be controlled by the processing means, allowing a full array of operational conditions, including unequal loading of the linear slide.

In a first group of embodiments, the at least one rail is fixedly mounted to the base plate and the plurality of magnetic bearing units and the at least two further electromagnets are fixedly mounted on the linear slide. This allows to use e.g. only three magnetic bearing units to provide a full support and control of the linear slide, which provides a very cost-effective solution.

However, this group of embodiments would require a power source to be available on the linear slide, e.g. using a battery or feed cables. As an alternative, inductive coupling may be used to transfer power and sensor data to and from the linear slide. In a second group of embodiments, this problem is overcome by having the at least one rail fixedly mounted on the linear slide and the plurality of magnetic bearing units and at least two further electromagnets fixedly mounted on the base plate. This would require commutation of the magnetic bearing units when the distance to be traveled in the x-direction by the linear slide is larger than the length of the at least one rail. Because the magnetic bearing unit comprises a permanent magnet, the magnetic bearing unit will cause a reluctance force, when the rail exits the area between the C- yokes of the magnetic bearing unit (trying to pull back the rail between the end surfaces of the C-yokes). Such a peak disturbance force may hamper proper operation of the linear motor. This problem may be essentially eliminated when the at least one rail has a tapered end, as this causes the reluctance forces to decrease in a less peak wise manner.

In the second group of embodiments, the linear slide only comprises the rail, and no active elements, thus eliminating the need for a power supply to the linear slide. In a further embodiment, each of the plurality of magnetic bearing units has at least one, e.g. two, further electromagnet associated therewith, and positioned adjacent to the magnetic bearing unit in the first direction. This allows to combine the control of the linear slide in all axes in an efficient manner. The plurality of magnetic bearing units may be positioned such that at all times at least three magnetic bearing units are supporting the linear slide. This may be accomplished in a number of ways, by varying the distance between magnetic bearing units or the length of the at least one rail in the x-direction. A three-point support allows full control of movement of the linear slide in all directions. An even better control may be accomplished when the plurality of magnetic bearing units is positioned such that at all times at least four magnetic bearing units are supporting the linear slide.

In a third group of embodiments of the present invention, the base plate is mounted on a fixed base to allow movement of the base plate with respect to the fixed base in the first direction using a secondary linear motor. The secondary linear motor may comprise a normal stator and rotor part, and be supported by normal mounting means, such as rails or bearings. The linear motor driving the position of the base plate relative to the linear slide may then be used to fine-tune the position of the linear slide. This embodiment allows to have the base plate and linear slide in a high vacuum environment (or even a very high vacuum environment) and the rest of the bearing arrangement in an environment having less strict vacuum requirements.

As the linear motor driving the linear slide with respect to the base plate now only has to move over very small distances, this linear motor may comprise a simple reluctance based actuator. These actuator are very reliable and cost-effective.

The linear motor alternatively may comprise two parallel operating linear motors. This allows a stable operation using two linear motors with diminished specifications, and allows to create space under the linear slide for cabling etc.

The present invention will now be described in further detail using a number of exemplary embodiments, with reference to the accompanying drawings, in which

Fig. 1 shows a perspective view of the magnetic bearing assembly according to a first embodiment of the present invention;

Fig. 2 shows a perspective view of a magnetic bearing unit as used in the present invention; Fig. 3 shows a sectional view of a table transport system according to a further embodiment of the present invention;

Fig. 4 shows a top view of the table transport system of Fig. 3; Fig. 5 shows a top view of an alternative arrangement of the table transport system of Fig. 3;

Fig. 6 shows a sectional view of a table transport system according to an even further embodiment of the present invention;

Fig. 7 shows an alternative arrangement for the table transport system of Fig. 6. Fig. 8 shows a block diagram of a control arrangement for the table transport system according to the present invention.

In Fig. 1, a perspective view is shown of a bearing assembly according to the present invention. The bearing assembly may be used in a wafer transport system, in which a linear slide or table 1 (see description of Fig. 3-5 below) is moveable in a first direction, indicated by the x-direction in Fig. 1. In Fig. 1, the other co-ordinate axes y and z are shown, as well as the rotational axes φ, ψ and _. In a wafer transport system, high environmental requirements apply, e.g. with respect to vacuum (high vacuum or even ultra high vacuum requirements) to allow dust free transport of a wafer on the linear slide 1.

The bearing assembly comprises one or two rails 7, a number of magnetic bearing units 10 and a number of further electromagnets 20. In the embodiment shown in Fig. 1, three magnetic bearing units 10 are shown and four further electromagnets 20, which co-operate with two rails 7. In Fig. 1, the two rails 7 are thought to be attached to the fixed world (stationary base) and the magnetic bearing units 10 and further electromagnets 10 to the linear slide 1. The magnetic bearing units 10 in the embodiment in Fig. 1 allow a three point support of the linear slide 1, and each allow control of the linear slide position in the z-direction and ψ-direction. Furthermore, the three-point support allows control in the ^-direction (control of vertical suspension and rotation in vertical planes).

The further electromagnets 20 each allow control of the position of the linear slide 1 in the y-direction and in combination allow control in the φ-direction (horizontal suspension). The further electromagnets 20 as shown comprise an E-type yoke 22 having a coil 21 on the center leg of the E-type yoke 22. As the disturbance forces in the horizontal direction are relatively small, such a configuration may be used in a cost- effective manner. The further electromagnets 20 may be produced using stacked E- shaped laminated transformer steel and a coil. One such an E-type electromagnet can only apply an attractive force, and thus at least two of these further electromagnets 20 need to be used to allow sufficient control. In the embodiment shown, a pair of further electromagnets 20 co-operating with the two rails 7, are given a bias current, typically of half the maximum current or less, allowing a control current to be added to the bias current of the first of the pair of electromagnets 20 and subtracted from the bias current of the other of the pair of electromagnets 20. To maximize the performance of these further electromagnets 20, they are positioned at the outmost corners of the linear slide 1.

The control in the x-direction may be implemented using a linear motor 2, 3 (see Fig. 2), comprising a rotor part 2 and a stator part 3. The linear motor used may be an off-the-shelf type.

Fig. 2 shows a detailed view of one of the magnetic bearing units 10 shown in Fig. 1. These magnetic bearing units 10 allow the vertical suspension of the linear slide 1, and the rotations in the vertical planes (z-, ψ- and f-direction). They combine permanent magnets 13 with electromagnets 12. This type of actuator shows a high performance to volume ratio and a low power consumption. The magnetic bearing unit 10 comprises a mount 15 for mounting the various elements. The magnetic bearing unit 10 comprises two C-type yokes 11, on to each of which a coil 12 is wound. The magnetic bearing unit 10 is furthermore provided with two permanent magnets 13, which are connected to the C-yokes 11 using connecting irons 14 to form a closed magnetic circuit. This type of actuator is also described in Dutch patent NL 1005344, which is included herein by reference. The rails 7 can then be suspended between the end faces of the C-yokes 11.

The function of the magnetic bearing unit 10 is twofold. The permanent magnets 13 provide a preload in both the upward and downward direction (z-direction), also called biasing. Because of the permanent magnets 13, a large force may be generated using only little current through the coils 12. This allows to use only a small number of windings for the coils 12 to already give a high performance.

It was found that for a linear slide load of 5 kg, it was sufficient to use three magnetic bearing units 10 having each two permanent magnets 13 of only 20 mm x 20 mm and a thickness of 4.5 mm. In this configuration, the rails 7 and C-yokes 11 were dimensioned to provide an air gap between them of only 0.5 mm having a pole area of 15 mm x 15 mm. The magnetic bearing units 10 and further electromagnets 20 were relatively positioned as shown in Fig. 1.

In the configuration shown in Fig. 1, it is necessary to have cable feeds to the linear slide 1 to supply power to the magnetic bearing units 10 and further electromagnets 20. Fig. 3 shows an alternative arrangement, in which the rails 7 are attached to the linear slide 1 in an ultra high vacuum space 4, and the magnetic bearing units 10 and further electromagnets 20 are attached to a fixed base in a high vacuum space 5. Preferably, also the linear motor rotor 2 (coils) is attached to a fixed base, and the linear motor stators 3 (magnets) to the linear slide 1. This allows to operate the linear slide 1 without any cabling or feeds running to it.

Fig. 4 shows a top view of a first alternative of the arrangement of Fig. 3. The rails 7 are dimensioned so as to extend beyond the linear slide or table 1, to allow the magnetic bearing units 10 to be positioned such that at all times three support points exist. To allow sufficient control in the y-direction and rotation around the z-axis, at all times four further electromagnets 20 should be adjacent to one of the rails 7. In the embodiment shown, each magnetic bearing unit 10 has two further electromagnets 20 adjacent to it. By proper dimensioning, it will also be possible to have only one further electromagnet 20 for each magnetic bearing unit 10. Movement of the linear slide 1 by the linear motor 2, 3, will necessitate the switching on and off of the various magnetic bearing units 10.

Fig. 5 shows an alternative arrangement of the magnetic bearing assembly, in which the linear slide is always supported by four magnetic bearing units 10. This allows a simpler control of the switching of the magnetic bearing units 10. In the embodiments shown in Fig. 3-5, the movement of the linear slide 1 relative to the magnetic bearing units 10 will result in a change of the weight distribution of the linear slide 1 over each of the magnetic bearing units 10. In the configuration of Fig. 4 (three point support) this may be as high as 30%, and in the configuration of Fig. 5 (four point support) this may be as high as 25%. The magnetic bearing units 10 should thus be dimensioned to handle these kind of load changes.

The presence of permanent magnets 13 in the magnetic bearing units 10 has another effect when the rail 7 moves out of the area between the C-yoke end faces. A reluctance force will try to pull the rail 7 back between the end faces of the C-yokes 11. This results in a peak force in the linear motion direction, which will be a disturbance factor for the linear motor 2, 3. This problem may be alleviated by carefully tuning the current through the coils 12 of the magnetic bearing unit 10, but this maybe a complex control task. It is also possible to adapt the shape of the ends of the rails 7 (at the longitudinal ends). When the end is changed from a flat straight end to a more slanted or tapered end, the reluctance force will be reduced when the rail 7 moves away from the magnetic bearing unit 10.

A further alternative embodiment of the magnetic bearing assembly of the present invention is shown in the figures 6 and 7. These embodiments can be advantageously used in ultra high vacuum environments. The third alternative embodiment as shown is a combination of two parts. An additional linear motor 27, 28 of a conventional type is used for a long linear stroke of a base plate 6. The conventional linear motor may e.g. use a number of magnets 28 attached to the fixed world and coils 27 attached to the base plate 6. The base plate 6 is supported by normal support means, such as rolling bearings 26, or alternatively air suspension bearings. On top of this linear motor, the base plate 6 is mounted having standard arrangement of magnetic bearing units, e.g. as discussed above with reference to Fig. 3-5. This means that the magnetic bearing units 10 travel the full length of the linear stroke. The magnetic bearing units 10 control the position of the slide located in the UHN environment 4 in five degrees of freedom as discussed above (all six degrees of freedom except in the direction of the long linear stroke, the x-direction) and hold the slide 1 in one position. To fix the relative position of the linear slide 1 to the additional linear motor 27, 28, the linear motor 2, 3 of the earlier described embodiments is used. The coil part 2 of the linear motor is fixed on the base plate 6 of the additional linear motor 27, 28; the magnets 3 of the linear motor are located in the UHN environment 4. Thus, the only function of linear motor 2, 3 is to constrain the slide located in the UHV environment 1 relative to the base plate 6 located in the normal environment (extra position correction is also possible with linear motor 2, 3 if required.)

The linear motor 2, 3 would in normal circumstances only require a very small stroke of movement in the x-direction. This may also be achieved using a reluctance- based actuator, comprising two magnetic parts with a plurality of teeth. Reluctance forces induced by a coil or magnet on one of the magnetic parts will create a reluctance force trying to align the teeth of the two magnetic parts allowing a sufficient position control.

In some applications, it may still be necessary to run cables to the linear slide 1 of the transport system shown in Fig. 6. This may be made easier when more space is available under the linear slide 1. This may be accomplished by e.g. turning the linear motor 2, 3 by 90° thereby creating extra space under the UHV space 4. Fig. 7 shows a further alternative that allows more space below the UHN space 4, by using two linear motors 2, 3; 2', 3'. The linear motors used may then be dimensioned smaller and with reduced performance. The advantages of the embodiments described with reference to Fig. 6 and 7 is that a conventional linear motor system may be used for the long linear stroke in x- direction, with permanent magnets 28 for the additional linear motor on the fixed base 25 and coils 27 on the base plate 6. This allows to use a conventional linear measurement system for the linear motor. The set of magnetic bearing units 10 allows control of five degrees of freedom, and the only function of the magnetic bearing units 10 is to hold the linear slide 1 in a position relative to the moving part (base plate 6) of the conventional linear motor 27, 28. An extra linear motor 2, 3, is added to fix the relative position in the long stroke linear direction. The only magnetic circuit parts for the magnetic bearing units 10 in UHV space 4 are two steel rails 7 and the permanent magnets 27 of the additional linear motor 27, 28. Although cables are present and required, they are not located in the UHV environment 4. Cables for the coils of the linear motor 27, 28, magnetic bearing units 10 and linear motor 2, 3 are connected to the moving part of linear motor 27, 28. However, the cables do not affect the behaviour of the magnetic bearings or cause disturbances forces on the magnetic bearings. Fig. 8 shows a schematic diagram of a control arrangement for use with the magnetic bearing assembly of the present invention. Processing means 8, such as a general purpose computer, or an industrial controller (PLC), receives input from sensors 16 associated with the magnetic bearing units 10. The sensors 16 may be used to accurately determine the position of the rail 7 between the end faces of the C-yokes 11 , e.g. using inductive sensors, or eddy-current type of sensors. The processing means 8 are arranged to control the coils 12 of the magnetic bearing units 10 and the coils 21 of the further electromagnets 21 to hold the linear slide 1 in its desired position in five control axes (y-, z-, φ-, ψ- and f-axes). Furthermore, the processing means 8 receive input from a further sensor 17 which detects the displacement of the linear slide 1 in the x-direction, and controls the rotor 2 of the linear motor 2, 3.

Claims

1. Bearing arrangement for a linear slide (1) moveable in a first direction with respect to a base plate (6), comprising a plurality of magnetic bearing units (10) for slideably mounting the linear slide (1) with respect to the base plate (6), each magnetic bearing unit (10) comprising a permanent magnet (13) and an electromagnet (12) and co-operating with at least one rail (7) of magnetically conducting material extending in the first direction (x), a linear motor (2, 3) for driving the linear slide (1) in the first direction, characterised in that, the magnetic bearing unit (10) comprises a double C-yoke configuration, the electromagnet being formed by the double C-yoke (11), in which each C-yoke (11) is provided with a coil (12), the at least one permanent magnet (13) being positioned between the C-yokes (11), and the at least one rail (7) co-operating with the magnetic bearing unit (10) being placeable between the end-faces of the C-yokes (11), the bearing arrangement further comprising at least two further electromagnets (20) cooperating with the at least one rail (7) for controlling the position of the linear slide (1) in a second direction (y) perpendicular to the first direction (x).

2. Bearing arrangement according to claim 1, in which the further electromagnets

(20) are of the E-core type having a coil (21) on the middle leg of the E-core (22).

3. Bearing arrangement according to claim 1 or 2, in which the bearing arrangement further comprises at least one sensor (16) for detecting the position of one of the plurality of magnetic bearing units (10) relative to the at least one rail (7), and processing means (8) which are connected to the at least one sensor (16) and the electromagnets (12) of the plurality of magnetic bearing units (10) and the at least two further electromagnets (20) for controlling the position of the linear slide (1) with respect to the base plate (6).

4. Bearing arrangement according to one of the claims 1-3, in which the at least one rail (7) is fixedly mounted to the base plate (6) and the plurality of magnetic bearing units (10) and the at least two further electromagnets are fixedly mounted on the linear slide (1).

5. Bearing arrangement according to claim 5, in which the plurality of magnetic bearing units (10) comprises three magnetic bearing units (10).

6. Bearing arrangement according to one of the claims 1-3, in which the at least one rail (7) is fixedly mounted on the linear slide (1) and the plurality of magnetic bearing units (10) and at least two further electromagnets (20) are fixedly mounted on the base plate (6).

7. Bearing arrangement according to claim 6, in which the at least one rail (7) has a tapered end.

8. Bearing arrangement according to claim 6 or 7, in which each of the plurality of magnetic bearing units (10) has at least one, e.g. two, further electromagnet (20) associated therewith, positioned adjacent to the magnetic bearing unit (10) in the first direction (x).

9. Bearing arrangement according to one of the claims 6-8, in which the plurality of magnetic bearing units (10) are positioned such that at all times at least three, e.g. four, magnetic bearing units (10) are supporting the linear slide (1).

10. Bearing arrangement according to one of the claims 1-9, in which the base plate (6) is mounted on a fixed base (25) to allow movement of the base plate (6) with respect to the fixed base (25) in the first direction (x) using a secondary linear motor (27, 28).

11. Bearing arrangement according to claim 10, in which the linear motor (2, 3) driving the linear slide (1) with respect to the base plate (6) comprises a reluctance based actuator.

12. Bearing arrangement according to claim 10 or 11, in which the linear motor comprises two parallel operating linear motors (2, 3; 2', 3').

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