Electric actuator

Abstract

An electric actuator (20) includes a fixed support (21) and a guide system having a single fixed guide (28) coupled to the fixed support (21), the single fixed guide having an axis (27). The actuator (20) further includes a fixed assembly (24) secured to the fixed support (21). A moving assembly (26) is movable relative to the fixed support (21) on the guide (28), wherein the moving assembly (26) and the fixed assembly (24) provide at least two sets of interacting magnetic fields that are arranged at equal angular intervals about the guide (28). A test sample support (76) is coupled to the moving assembly (26) and is disposed on one side of the fixed support (21) so as to move along the axis (27) as the moving assembly (26) moves, the axis (27) extending through the test sample support (76).

Description

Background Art

[0001] The following discussion is provided for general background information only and is not intended to be used as an aid in determining the scope of claimed subject matter.

[0002] Testing machines or equipment are used to test parameters and / or performance of materials, components, consumer products, electronic devices, and medical and other devices (ie, test specimens). Typically, a testing machine includes one or more actuators to provide input loads and displacements. Summary of the Invention

[0003] The Summary and Abstract herein are intended to introduce a selection of concepts in a simplified form that are further illustrated in the Detailed Description below. Neither the Summary nor the Abstract is intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all of the disadvantages identified in the Background.

[0004] One general aspect includes an electric actuator for applying a load to a test sample. The electric actuator includes a fixed support and a guide system having a single fixed guide coupled to the fixed support, the single fixed guide having an axis. The actuator also includes a fixed assembly secured to the fixed support. A moving assembly is movable relative to the fixed support on the guide, wherein the moving assembly and the fixed assembly provide at least two sets of interacting magnetic fields that are arranged at equal angular intervals around the guide. A test sample support is coupled to the moving assembly and is disposed on one side of the fixed support so as to move along an axis extending through the test sample support as the moving assembly moves.

[0005] Implementations of the foregoing aspects may include one or more of the following features. In one embodiment, the moving assembly may include a coil and the fixed assembly includes a magnet. The moving assembly includes a bearing guided on a guide and preferably a first bearing and a second bearing, the bearings being spaced apart from each other. The moving assembly may include a flat coil supported by a support track, the first bearing and the second bearing being coupled to the support track. Each of the bearing and the second bearing may include a bearing surface, a bearing support, and a flexure, the bearing surface being in direct or indirect contact with the guide, the flexure coupling the bearing surface to the bearing support, the flexure being compliant with respect to moments about an axis orthogonal to the guide and rigid with respect to forces along an axis orthogonal to the guide.

[0006] The electric actuator may include an auxiliary force generating device coupled to the moving assembly. The guide may include a hole, and the auxiliary force generating device may include a driven member that may be in the hole. The auxiliary force generating device may be a spring, such as a mechanical spring (compression spring, tension spring, etc.). Additionally or alternatively, the hole and the driven member form a sealed chamber filled with gas.

[0007] The anti-rotation bearing assembly may be coupled to the moving assembly and the fixed assembly. The anti-rotation bearing assembly may include a guide surface structure engaged to the moving assembly for movement therewith and a bearing element engaged to the fixed component, or vice versa.

[0008] Another general aspect includes an electric actuator having a fixed support and a fixed guide coupled to the fixed support. A fixed assembly is secured to the fixed support. A first bearing and a second bearing spaced apart from the first bearing are each guided on the guide. A movable assembly is secured to the first and second bearings and movable relative to the fixed support along with the first and second bearings on the guide, wherein each bearing may include a bearing surface, a bearing support, and a movable coupling, the bearing surface directly or indirectly contacting the guide, the movable coupling coupling the bearing surface to the bearing support, and the movable assembly is secured to each of the bearing supports.

[0009] Implementations of the aforementioned aspects may include one or more of the following features: In the electric actuator, the movable link is compliant to moments about an axis orthogonal to the guide and rigid to forces along an axis orthogonal to the guide. The movable link may include a flexure. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a perspective view of an electric actuator.

[0011] Figure 2 is a perspective view of the electric actuator in a first position.

[0012] Figure 3 is a perspective view of the electric actuator in the second position.

[0013] Figure 4 is a partial perspective view of an electric actuator with some components removed.

[0014] Figure 5 is another partial perspective view of the electric actuator with some components removed.

[0015] Figure 6 It is a three-dimensional diagram of the mobile component.

[0016] Figure 7is a perspective view of the bearing of the moving assembly with some parts removed.

[0017] Figure 8 Another perspective view of the bearing of the moving assembly with some parts removed.

[0018] Figure 9 It is a three-dimensional diagram of the bearing.

[0019] Figure 10 yes Figure 9 A perspective view of a bearing with some parts removed.

[0020] Figure 11 yes Figure 9 Another perspective view of the bearing with some parts removed.

[0021] Figure 12 is a schematic diagram of a bearing that is movable on a guide without providing compensation to the bearing for bending movements of the guide.

[0022] Figure 13 is a schematic diagram of a bearing and guide, where the bearing compensates for the bending movement of the guide.

[0023] Figure 14 It is another partial perspective view of the electric actuator.

[0024] Figure 15 is a partial top plan view of an electric actuator with some components removed.

[0025] Figure 16 is a schematic diagram of an electric actuator having a second force generating device.

[0026] Figure 17 It is a partial perspective view of an electric actuator having a second force generating device. DETAILED DESCRIPTION

[0027] The present disclosure provides an electric actuator 20 capable of linear displacement. The actuator 20 can be used in many different applications. In a non-limiting but advantageous embodiment, the electric actuator 20 is provided in a test machine to impart a desired load or displacement to a test specimen (such as, but not limited to, a shock absorber). Such test machines are well known, and in one embodiment, the test machine includes a transverse head that is movable on vertical columns via a transverse head drive (motor, gear reducer, drive belt, etc.), wherein these columns are supported by a base. Figures 1 to 3The electric actuator 20 shown in FIG. 1 and having aspects described herein is typically, but not limited to, mounted in the base of a test machine, but may also be mounted in a lateral load carrier. In alternative embodiments, the test machine may be sized to be supported by a tabletop. It will be appreciated that the actuator 20 may be oriented as desired depending on the application in which it is used. As will be appreciated by those skilled in the art, the components of the electric actuator 20 described below may be sized as needed to impart the desired load.

[0028] Typically, actuator 20 includes a fixed mount or frame 21, a fixed assembly 24 mounted to fixed mount 21, and a moving assembly 26 that moves relative to fixed assembly 24 through the interaction of magnetic fields, with moving assembly 26 moving along an axis 27 defined by guides 28. In actuator 20, guide 28 comprises, and preferably comprises, a single guide member, such as a rod secured to mount 21 at each end. By using only a single guide rod 28 (on which at least one bearing is guided), the guide system (guide rod / bearing(s)) does not overly constrain linear movement along axis 27. In conventional actuators, two or more spaced-apart guide assemblies move along spaced-apart guide paths or axes. Because two or more guide paths are provided for a single linear guided motion of a moving member in an actuator, the spaced-apart guide paths achieve an overly constrained guide system. The guide paths or axes are often not completely parallel to one another or may easily become skewed or non-parallel, for example due to shipping the actuator, thermal expansion of actuator components, and / or normal wear, thus making operation of such actuators difficult. Due to the use of a single guide rod 28 these problems are minimized.

[0029] Preferably, actuator 20 includes at least two sets of interacting magnetic fields, wherein a first set of interacting magnetic fields is disposed on one side of axis 27 and guide 28, and a second set of interacting magnetic fields is disposed on the opposite side (i.e., rotated 180 degrees about axis 27). Each set of interacting magnetic fields includes a fixed component 34 forming part of fixed assembly 24 and a moving component 36 forming part of moving assembly 26. At least one of the fixed component 34 and moving component 36 in each associated pair is a coil having turns that can be selectively energized, while the other of the fixed component 34 and associated moving component 36 can be formed from a permanent magnet or other coil. In the illustrated embodiment, each of the moving components 36 is a coil, while each of the fixed components 34 includes a plurality of magnets arranged in a continuous pattern parallel to axis 27 and guide 28. Because the moving component coils 36 have less mass than the fixed component magnets 34, actuator 20 can have greater responsiveness, but this arrangement of coils and magnets should not be considered limiting.

[0030] Reference Figure 6and Figure 14 , the coil of each moving part 36 is in the form of a generally planer blade 40, wherein the coil is supported on a planar support 41 having a support rail 42 that is stiffer than the planer support 41. In one embodiment, the planar support 41 and the support rail 42 are separate parts that may be made of different materials that are joined together. However, in alternative embodiments, such as Figure 14 As best shown in FIG. 4 , the planar support member 41 and the track 42 are integrally formed together from a single unitary body.

[0031] In the illustrated embodiment, each associated fixing member 34 includes at least one, and preferably two, sets of magnets 43, wherein a first set of magnets 43A is arranged facing a first major surface of a planar support 41, and a second set of magnets 43B is arranged facing a second major surface of the planar support 41, the second major surface facing in an opposite direction from the first major surface. It should be noted that a single set of coils can be provided on the planar support 41 to magnetically couple with both the first set of magnets 43A and the second set of magnets 43B. Alternatively, the planar support 41 can support the first set of coils on one side of the planar support 41 facing the first set of magnets 43A, and support the second set of coils on the opposite side of the planar support 41 facing the second set of magnets 43B. Each set of magnets is mounted to extended support members 45A, 45B of a U-shaped support frame 45, wherein a central section 45C of the U-shaped support frame 45 is secured to the bracket 21. The “magnetic coupling unit” herein is a magnetic component(s) (magnets or coils) on each planar support member 41 that is magnetically coupled to its associated magnetic component(s) (magnets or coils) on the fixed member 34, as provided by each U-shaped support frame 45.

[0032] Like the planar support 41, the support members 45A, 45B and the central section 45C of the support frame 45 can be formed by joining together separate parts or integrally formed from a single unitary body. In the exemplary embodiment, the fixed components 34 and the corresponding movable components 36 are arranged in pairs on opposite sides of the axis 27, wherein, as shown in FIG. Figure 14, three pairs of fixed components 34 and mobile components 36 (or six independent magnetically coupled operating units, a magnetically coupled operating unit being a single fixed component 34 and mobile component 36) are presented as a first group of magnetically coupled operating units 50A generally on one side of the axis 27 and a second group of magnetically coupled operating units 50B generally on the opposite side of the axis 27 so as not to cause any unbalanced load relative to the axis; however, this should not be considered limiting, as more or less than three pairs may be given depending on the desired operating specifications. Furthermore, in yet another alternative embodiment, it should be noted that the groups of magnetically coupled operating units (such as group 50A) can also be arranged at the same angular spacing around the axis 27 so as not to cause any unbalanced load relative to the axis. For example, three groups of magnetically coupled operating units can be arranged around the axis 27 so as to be spaced approximately 120 degrees apart from each other. Typically, if there are multiple planar supports in each magnetically coupled operating unit, there will be multiple groups of equally spaced planar supports (each group comprising planar supports from each group of magnetically coupled operating units) equally spaced about axis 27, with the fixed components supporting the associated magnetic components in the same manner. Figure 15 , each having three planar supports 41 (blades 40). The plurality of equally spaced planar supports may be three groups, wherein the first group includes planar supports 57A, the second group includes planar supports 57B, and the third group includes planar supports 57C. The moving member 36 in the form of blades 40 and the fixed member 34 in the form of a U-shaped magnetic track are available from Aerotech, Inc., Pittsburgh, Pennsylvania, USA.

[0033] As described above, the moving assembly 26 is guided by the guide 28, which is embodied herein as a guide rod. Specifically, the moving assembly 26 includes at least one (and in one embodiment, two) bearing assemblies 70 that are movable relative to the guide rod 28. In a preferred embodiment, the bearing assemblies 70 are spaced apart from each other, with each bearing assembly being connected only to the support rails 42 of the moving component 36, rather than to a separate structure outside of the support rails 42. In this way, because no additional structure other than the blades 40 is connected to the bearings 70, the mass of the moving assembly 26 is advantageously reduced.

[0034] The moving assembly 26 further includes a test sample support 76 to which the sample is attached at its end 22. The test sample support 76 extends beyond the fixed assembly 24 or its support 21. In the exemplary embodiment, the sample support 76 includes two support rods 78 and a cross member 81 forming the end 22. At opposite ends of the cross member 81, the support rods 78 engage one of the bearing assemblies 70, while the support rods 78 extend through apertures 83 in the support 21. Because the sample support 76 and the bearing assembly 70 are secured to each other and move together, the side load capacity of the actuator 20 does not change as the sample support 76 extends from the support 21, as is the case with conventional actuators in which the bearing assemblies are secured to the corresponding support. In this way, the side load capacity is advantageously constant throughout the travel length of the moving assembly 26. Preferably, test specimen support 76 is configured such that axis 27 extends through test specimen support 76, such that a test specimen attached thereto is centered about axis 27, and support rods 78 are equally spaced about axis 27 (if multiple support rods 78 are used). In alternative embodiments, the support rods may be replaced by a single element (e.g., a rod, cylinder, etc.) centered about axis 27 and extending from one of the bearing assemblies 70 along axis 27 away from both bearing assemblies 70 or away from the fixed bracket 21.

[0035] Figures 7 to 9 An advantageous structure of the bearing assembly 70 is shown, in particular the use of a flexure 84 to connect the bearing 80 (in direct or indirect contact with the guide rod 28) to a bearing support 82. The bearing support 82 is fastened to the support rail(s) 42, wherein the sample support 76 is also engaged to an upper portion of the bearing support 82.

[0036] The bearing 80 provides a bearing surface capable of reacting to radial loads, which is necessary to provide a guide system for a typical electric actuator used for characterization testing. However, the flexure 84 provides a mechanical means of keeping the bearing surface positioned collinear with the guide rod 28 even when the guide rod is bent.

[0037] Reference Figure 9, the flexure 84 is configured to be substantially rigid with respect to linear forces along axes 90 ("x") and 92 ("y"), which are orthogonal to axis 27, and along axis 94 ("z"). However, the flexure 84 is compliant with respect to moments about axes 90 and 92. This enables the bearing 80 to rotate as desired with any bending of the guide rod 28. In the illustrated embodiment, the flexure 84 is a planed face having a central aperture through which the guide rod 28 extends. Arms 85 mount the flexure to the bearing support 82, for example being positioned between joined portions 87 and 88 of the bearing support 82. A cutout 97 is provided between the arms 85 at each end of the flexure 84. Using the enlarged aperture for the guide rod 28, the cutout 97 provides a small portion 89 ("z") of the flexure 84 that acts as a pivot along axis 90. Figure 10 Similarly, the width of the flexure 84 is selected so that the enlarged aperture for guiding the rod 28 also forms a small portion 91 along the axis 92 that serves as a pivot member ( Figure 11 In a preferred embodiment, the bearings 80 are mounted to the flexures 84 through the center of mass of each bearing 80. The bending moment stiffness of the flexures 84 is significantly less than the pitch stiffness of the bearings 80.

[0038] exist Figure 13 The guide rod 28 and the bearing 80 are schematically shown in an exaggerated manner in FIG. When there is a side load from the sample loading, the side load is transmitted through the bearing 80 and reacts on the guide rod 28. The load acting on the guide rod 28 may cause lateral and angular deflection of the guide rod 28, such as Figure 13 However, for moments about axes 90 and 92, the moment stiffness of the flexure 84 is less than the pitch stiffness of the bearing 80, so the bearing surface that is in direct or indirect contact with the guide rod 28 is able to follow the angular deflection of the guide rod 28. This allows the bearing surface to remain generally collinear with the guide rod 28 along the length of the bearing 80, thereby providing optimal bearing load capacity to react to external radial side loads.

[0039] Figure 12 This diagram illustrates what might happen when there is no flexure 84. In this configuration, the guide rod 28 bends, but because the bearing 80 does not rotate, there may be bearing edge loads that can significantly reduce the side load rating of the actuator.

[0040] It should be noted that the bearing 80 may take any conventional form, such as, but not limited to, a (rotating or non-rotating) contact bearing surface with or without lubricant. However, in a preferred embodiment, the bearing 80 may comprise an air bearing that provides a frictionless, contactless air bearing surface. Figure 10 and Figure 11A cylindrical porous air bushing 110 is sized to be positioned adjacent the outer surface of the guide rod 28. An upper outer bearing housing 112 and a lower outer bearing housing 114 surround the air bushing 110 and are fastened together, sandwiching the annular portion of the flexure 84. Such bearings are sold by Newway Air Bearings of Aston, Pennsylvania, USA.

[0041] Return to reference Figure 2 and Figure 3 , an anti-rotation bearing assembly 120 is preferably provided to inhibit rotation of the moving assembly 26 about the axis 27 due to the guide 28 being a rod and the bearing 70 having a cylindrical surface. Generally, the anti-rotation bearing assembly 120 and the alternatives described below are radially offset from the axis 27 and do not create an overly constrained guide structure. In the illustrated embodiment, the bearing assembly 120 includes a bearing element 122 (such as a rolling element or bushing) in direct or preferably indirect (such as an air bearing bushing) contact with a guide surface parallel to the axis 27. Figure 2 and Figure 3 In FIG. 1 , the bearing element 122 is engaged to the moving component 26 to move therewith, while the fixed guide surface is present on the fixed component 24 .

[0042] Figure 14 and Figure 15 Another anti-rotation bearing assembly 130 is shown. Anti-rotation bearing assembly 130 includes a guide surface structure 132 (such as a track or disc) coupled to moving assembly 26 for movement therewith, while bearing elements 134 are secured to fixed assembly 24, preferably on opposite sides of guide surface structure 132. Bearing elements 134 are preferably mounted on a rotating coupling 136, which in turn is secured to fixed assembly 24 via brackets 139. If implemented as an air bushing, an air supply hose 138 supplies air or other gas from a supply source (not shown) to bearing elements 134. Anti-rotation bearing assembly 130 may be preferred over anti-rotation bearing assembly 120 because air supply hose 138 does not have to move or flex with movement of moving assembly 26.

[0043] Air core electric linear actuators have a tendency to overheat if high continuous forces are generated. Therefore, in one embodiment, a Figure 15 and Figure 16, to cope with the desired static or continuous force from the actuator 20. Typically, the auxiliary force generating device 150 includes a spring or other force generating component to provide an upward force to the moving assembly 26 that overcomes gravity. In one embodiment, the guide rod 28 includes: a cavity or hole 152 in which the spring or other force generating component is located; and a follower element 154 that is operably connected to the spring or other force generating component and the moving assembly 26. The follower element 154 is implemented herein as a rod that extends through the aperture 28A in the guide rod 28 (and the aperture 158 in the bracket 21) and engages the sample support 76 in a fixed manner or via a coupling that allows at least orthogonally limited movement relative to the axis 27 so as to no longer produce an overly constrained guided structure. The driven element 154 can be permanently fastened to the sample support 76 so as to always move along the axis 27 with movement of the sample support, or be fastened to the sample support in a releasable manner so that during at least some movement, the moving assembly 26 does not include the mass of the driven element 154.

[0044] The auxiliary force generating device 150 may include a mechanical spring located in a cavity or hole 152, and / or the cavity or hole 152 may serve as a cylinder, with the driven element 154 serving as a piston with a seal 155 movable in the cylinder. The mechanical spring increases mass, friction, and may produce undesirable resonance. The piston-cylinder arrangement may be pneumatic, oil-over nitrogen, etc. Oil-over nitrogen systems increase oil inertia (moving mass), the need for an oil pressure control system, and the possibility of leakage. Air boost systems are typically only used up to about 100 pounds per square inch (psi) (to avoid spontaneous combustion in the event that a fuel source happens to be present). Systems based on nitrogen pressure allow for higher operating pressures, wherein leakage (if any) in a ventilated or unconstrained environment is generally not a problem. Although shown as a single-ended piston, it should be noted that the double-ended piston rod with springs effectively formed above and below the cylinder allows for bidirectional static support loads.

[0045] While various embodiments and examples have been shown and described herein, the present disclosure is illustrative only and changes in details of structure, arrangement, and method may be made without departing from the scope of the present disclosure.

Claims

1. An electric actuator for applying a load to a test specimen, the electric actuator comprising: Fixed support member; a guide system comprising a single fixed guide secured to the fixed support at opposite ends and having an axis extending therethrough; a fixing assembly, the fixing assembly being fastened to the fixing support; a moving assembly movable on the single fixed guide relative to the fixed support, wherein the moving assembly and the fixed assembly provide at least two sets of interacting magnetic fields, the at least two sets of interacting magnetic fields being arranged at equal angular intervals about the single fixed guide and the axis; and a test sample support coupled to the moving assembly and disposed on one side of the fixed support so as to move along the axis as the moving assembly moves, the axis extending through the test sample support; The moving assembly comprises a bearing guided on the single fixed guide, the bearing comprising a bearing surface, a bearing support, and a movable connecting member, the bearing surface being in direct or indirect contact with the single fixed guide, the movable connecting member coupling the bearing surface to the bearing support.

2. The electric actuator according to claim 1, wherein: The moving assembly includes a coil.

3. The electric actuator according to claim 1, wherein: The fixing assembly includes a magnet.

4. The electric actuator of claim 1, further comprising an auxiliary force generating device coupled to the moving assembly.

5. The electric actuator according to claim 4, wherein: The single fixed guide includes a hole, and the auxiliary force generating device includes a driven member movable in the hole.

6. The electric actuator according to claim 5, wherein: The auxiliary force generating device is a spring.

7. The electric actuator according to claim 6, wherein: The single fixed guide and the bore comprise a cylinder, and the driven member is a piston movable in the cylinder.

8. The electric actuator of claim 1 further comprising an anti-rotation bearing assembly coupled to the moving assembly and the stationary assembly.

9. The electric actuator according to claim 8, wherein: The anti-rotation bearing assembly includes a guide surface structure engaged to the moving assembly to move therewith and a bearing element engaged to the fixed assembly.

10. The electric actuator according to claim 1, wherein The movable link is compliant with respect to moments about an axis normal to the single fixed guide and is rigid with respect to forces along an axis normal to the single fixed guide.

11. The electric actuator of claim 10, wherein the movable coupling comprises a flexure.

12. An electric actuator for applying a load to a test specimen, the electric actuator comprising: Fixed support member; a guide system comprising a single fixed guide secured to the fixed support at opposite ends and having an axis extending therethrough; a fixing assembly, the fixing assembly being fastened to the fixing support; a moving assembly movable on the single fixed guide relative to the fixed support, wherein the moving assembly and the fixed assembly provide at least two sets of interacting magnetic fields, the at least two sets of interacting magnetic fields being arranged at equal angular intervals about the single fixed guide and the axis; and a test sample support coupled to the moving assembly and disposed on one side of the fixed support so as to move along the axis as the moving assembly moves, the axis extending through the test sample support; wherein the moving assembly comprises a bearing guided on the single fixed guide; wherein the moving assembly comprises a second bearing guided on the single fixed guide, the second bearing being spaced apart from the first bearing; and Each of the bearing and the second bearing includes a bearing surface, a bearing support, and a flexure, the bearing surface being in direct or indirect contact with the single fixed guide, the flexure coupling the bearing surface to the bearing support.

13. The electric actuator according to claim 12, wherein: The moving assembly includes a coil supported by a support rail, the bearing and the second bearing being coupled to the support rail.

14. The electric actuator according to claim 12, wherein: The flexure is compliant to moments about an axis normal to the single fixed guide and is rigid to forces along an axis normal to the single fixed guide.

15. An electric actuator comprising: Fixed support member; a fixed guide secured to the fixed support at opposite ends, the fixed guide having an axis extending therethrough; a fixing assembly, the fixing assembly being fastened to the fixing support; a first bearing and a second bearing spaced apart from the first bearing, each bearing being guided on the fixed guide; and a moving assembly secured to the first and second bearings and movable with the first and second bearings relative to the fixed support on the fixed guide, wherein each bearing comprises a bearing surface, a bearing support, and a movable coupling, the bearing surface being in direct or indirect contact with the fixed guide, the movable coupling coupling the bearing surface to the bearing support, the moving assembly being secured to each of the bearing supports.

16. The electric actuator according to claim 15, wherein: The movable link is compliant with respect to moments about an axis normal to the fixed guide and is rigid with respect to forces along an axis normal to the fixed guide.

17. The electric actuator according to claim 16, wherein: The movable coupling includes a flexure.

18. The electric actuator according to claim 15, wherein: The movable coupling includes a flexure.

19. An electric actuator for applying a load to a test specimen, the electric actuator comprising: Fixed support member; a guide system comprising a single fixed guide secured to the fixed support at opposite ends and having an axis extending therethrough; a fixing assembly, the fixing assembly being fastened to the fixing support; a moving assembly movable on the single fixed guide relative to the fixed support, wherein the moving assembly and the fixed assembly provide at least two sets of interacting magnetic fields, the at least two sets of interacting magnetic fields being arranged at equal angular intervals about the single fixed guide and the axis; and a test sample support coupled to the moving assembly and disposed on one side of the fixed support so as to move along the axis as the moving assembly moves, the axis extending through the test sample support; wherein the moving assembly comprises a bearing guided on the single fixed guide, the bearing comprising a bearing surface, a bearing support, and a movable coupling, the bearing surface being in direct or indirect contact with the single fixed guide, the movable coupling coupling the bearing surface to the bearing support, the movable coupling being compliant with moments about an axis orthogonal to the single fixed guide and rigid with respect to forces along an axis orthogonal to the single fixed guide, such that the bearing surface in direct or indirect contact with the single fixed guide can follow angular deflections of the single fixed guide, so that the bearing surface remains approximately collinear with the single fixed guide along the length of the bearing.

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