GB2616298A

GB2616298A - SMA actuator assembly

Info

Publication number: GB2616298A
Application number: GB2202987.0A
Authority: GB
Inventors: Benjamin Simpson Brown Andrew; Foote Will
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2022-03-03
Filing date: 2022-03-03
Publication date: 2023-09-06
Also published as: GB202202987D0

Abstract

An SMA actuator assembly comprises a support structure 10 supporting a movable part 20 in a manner allowing tilt of the movable part relative to the support structure about two orthogonal tilt axes x, y which may be perpendicular to a principle axis z. Two or more SMA wires 30 are arranged to tilt the movable part 20 about the two orthogonal tilt axes, x, y, and are driven by a control circuit which applies drive signals to the SMA wires so as to drive tilting of the movable part between predetermined positions in a repeated pattern. These repeated movements may allow super-resolution imaging in a camera, or may achieve wobulation of a projected image, to increase the effective resolution.

Description

SMA ACTUATOR ASSEMBLY

Field

The present application relates to an SMA actuator assembly, in particular for tilting a movable part relative to a support structure between predetermined positions in a repeated pattern, for example for the purpose of achieving super-resolution imaging or wobulation.

Background

There is a desire to improve the quality of images from miniature cameras, such as those incorporated in laptops. The space available for such miniature cameras is very constrained and the size of such miniature cameras is limited in size to only a few millimetres in a lateral direction (e.g. along a y axis) that is perpendicular to the optical axis of the camera.

There is also a desire to improve the resolution of projected images and to improve the fill of pixels in projected images through wobulation. Small size and low power consumption are key requirements for many applications such as AR light engines.

Summary

According to an aspect of the present invention, there is provided an SMA actuator assembly comprising a support structure; a movable part supported on the support structure in a manner allowing tilt of the movable part relative to the support structure about two orthogonal tilt axes; two or more SMA wires arranged to tilt the movable part relative to the support structure about the two orthogonal tilt axes; a control circuit configured to apply drive signals to the two or more SMA wires so as to drive tilting of the movable part relative to the support structure between predetermined positions in a repeated pattern.

The SMA actuator assembly is compact in size. The high energy density of SMA wires allows rapid actuation with high actuation force combined with comparably small space requirements. Additionally, tilting in a repeated pattern allows the SMA actuator assembly to achieve super-resolution imaging or wobulation, depending on the device or optical element combined with the SMA actuator assembly.

The control circuit may be relatively simple and thus compact by limiting the drive scheme of the SMA wires to a predetermined pattern. Compared to translational motion, the tilt motion driven by the SMA wires reduces the size of a movement envelope required for effective super-resolution imaging or wobulation using SMA wires, thus working in concert with the compact SMA wires to allow provision of an even more compact SMA actuator assembly.

Further aspects of the present invention are set out in the dependent claims.

Brief description of the drawings

Certain embodiments of the present invention will now be described; by way of example only, with reference to the accompanying drawings in which: Figures 1A and 1B are schematic perspective and cross-sectional views of an SMA actuator assembly in accordance with embodiments of the present invention; Figures 2A and 2B are perspective and cross-sectional views of an SMA actuator assembly in accordance with embodiments of the present invention; and Figures 3A-D are schematic diagrams illustrating a particular method of achieving super-resolution imaging or wobulation.

Detailed description

Figures 1 and 2 schematically show SMA actuator assemblies 1 in accordance with the present invention. The SMA actuator assemblies 1 comprise a support structure 10 and a movable part 20. The movable part 20 is supported on the support structure 10 in a manner allowing tilt of the movable part 20 relative to the support structure 10 about two orthogonal tilt axes (the x and y axes in the depicted embodiments). In some embodiments, the SMA actuator assembly 1 comprises a bearing arrangement 40 that allows tilt of the movable part 20 relative to the support structure 10 about the two orthogonal tilt axes x,y.

Although movement of the movable part 20 is described herein with reference to a static support structure 10, it will be appreciated that this movement is merely relative. The support structure 10 need not be static, and may itself move either within a device into which the SMA actuator assembly 1 is incorporated or as part of such a device.

The SMA actuator assembly 1 comprises two or more SMA wires 30. The SMA wires 30 are arranged to tilt the movable part 20 relative to the support structure 10 about the two orthogonal tilt axes x,y.

The SMA actuator assembly 1 further comprises a control circuit (not shown) configured to apply drive signals to the two or more SMA wires 30 so as to drive tilting of the movable part 20 relative to the support structure 10. The control circuit drives such tilting between predetermined positions in a repeated pattern. One example drive scheme that may be implemented by the control circuit is described with reference to Figure 3.

For example, the control circuit may apply PWM (pulse width modulated) control signals to the SMA wires 30. Supplying electrical power to the SMA wires 30 heats the SMA wires 30. Upon reaching a transition temperature, the SMA wire 30 starts contracting. The amplitude and/or frequency of SMA wire contraction can be controlled by supplying appropriate PWM control signals, thus allowing selective and targeted contraction of the SMA wires 30 in the arrangement so as to accurately move/tilt the movable part 20 relative to the support structure 10.

Movement in a repeated pattern The control circuit drives tilting between predetermined positions in a repeated pattern. Such tilting between predetermined positions in a repeated pattern may be used in a variety of applications, for example to achieve super-resolution imaging or wobulation.

The predetermined positions in the repeated pattern may be at least two positions. So, the movable part 20 may repeatedly move between two predetermined positions.

Preferably, the predetermined positions in the repeated pattern may be arranged in two degrees of freedom. The predetermined positions may form a square, for example. In general, the predetermined positions may form a triangle, pentagon, hexagon, or any other regular shape. The predetermined positions may be positioned along a circular path.

The predetermined positions may be stationary positions, so the movable part 20 may stop at each of the predetermined positions before moving on to the next of the predetermined positions. Alternatively, the movable part 20 may move continuously between the predetermined positions, for example along a predetermined continuous path.

Super-resolution imaging In some embodiments, the movable part 20 comprises an optical device. The optical device may, for example, comprise a lens assembly defining an optical axis. The two orthogonal tilt axes x-y are perpendicular to the optical axis.

With reference to Figure 1, the optical device may be a camera module 21. The camera module 21 comprising an image sensor and the lens assembly. The lens assembly is arranged to focus an image onto the image sensor.

The control circuit may be configured to apply the drive signals to the SMA wires 30 for the purpose of achieving super-resolution imaging of the camera module 21. In particular, the control circuit drives movement of the movable part between predetermined positions in a repeated pattern, which in turn moves the image on the image sensor between predetermined image positions in the repeated pattern. The image on the image sensor may, for example, move by a sub-pixel distance between the predetermined image positions. Super-resolution imaging may then be achieved, for example, by combining two or more images that are captured at positions offset from one another by a sub-pixel distance.

For this purpose, the movable part 20 may be controllably moved/tilted between predetermined positions that are offset from each other such that the image on the image sensor moves, in a direction parallel to the light-sensitive region of the image sensor, by a sub-pixel distance. Light that falls onto a center of a pixel at one position (and so may be used to capture an image) thus falls between pixels at another position. The control circuit may drive the SMA wires 30 so as to controllably move the movable part 20 in this manner. A sub-pixel distance is a distance that is less than a pixel pitch of the light-sensitive region of the image sensor. The pixel pitch refers to the distance between the centers of two adjacent pixels.

The movable part may be controllably moved such that the image on the image sensor moves to a positional accuracy of 0.5p.m or smaller. Particular advantage is achieved in the case that the actuator arrangement comprises plural SMA wires, as SMA provides a high actuation force compared to other forms of actuator. This may assist in accurate positioning of the movable part relative to the support structure.

The predetermined positions may be offset from one another in a direction along a pixel row and/or along a pixel column of the light-sensitive region of the image sensor. The predetermined positions may comprise i) one or more positions that are offset from a starting position by a sub-pixel distance along a pixel row, and ii) one or more positions that are offset from a starting position by a sub-pixel distance along a pixel column. Optionally, the two or more positions may comprise one or more positions that are offset from a starting position by a sub-pixel distance along a pixel row and along a pixel column.

Images are captured at each of the predetermined positions using the image sensor. A controller may control the image sensor so as to capture the images. The controller may be implemented as part of the control circuit or as part of another circuit. Alternatively, the controller may be implemented as part of the processor that forms part of the portable electronic device.

The images may then be combined so as to form a super-resolution image, for example using the processor of the portable electronic device or the above-described controller. The super-resolution image has a resolution that is greater than the resolution of the individual images that are captured by the image sensor. For example, the two or more images may be combined by interleaving the two or more images.

Wobulation With reference to Figure 2, the SMA actuator assembly 1 may comprise a projector 22. The projector 22 may project an image. The movable part 20 comprises at least a portion of the projector 22, such that tilting of the movable part 20 relative to the support structure 10 moves the projected image. The movable part 20 may, for example, comprise a light-emitting element, such as a laser or laser array, or a light-modifying element, such as a diffraction grating or reflection element. In either case, movement of the movable part 20 may move the projected image.

The control circuit is configured to apply the drive signals to the two or more SMA wires 30 for the purpose of achieving wobulation of the projected image. In this regard, wobulation may be achieved in general in synonymous manner described with reference to super-resolution imaging. Instead of capturing an image with an image sensor at each predetermined position, the projector may project an image at each predetermined position. The image projected at each predetermined position may be a lower resolution image formed of a subset of pixels of a high-resolution image. The high-resolution image may thus be effectively split into multiple lower-resolution images that are projected in rapid succession. The movable part 20 may be moved between the predetermined positions in the repeated pattern at a frequency of greater than 30 Hz, preferably greater than 60 Hz, further preferably greater than 120 Hz, such that the succession of lower-resolution images is perceived by the human eye as one high-resolution image.

Instead of providing a projector, the movable part 20 may comprise a display panel for displaying an image. The control circuit may be configured to apply the drive signals to the two or more SMA wires for the purpose of achieving wobulation of the displayed image.

Arrangement of SMA wires 30 Figures land 2 depict two possible embodiments of arrangements of SMA wires 30 for driving tilt of the movable part 20 relative to the support structure 10. These arrangements of SMA wires 30 are particularly compact and so especially suitable for applications with limited space constraints. In general, any other arrangements of SMA wires 30 capable of tilting the movable part 20 relative to the support structure may be used.

In general, at least two SMA wires 30 are provided so as to allow tilting in at least two degrees of freedom. The two SMA wires 30 may be opposed by a resilient element, such as a spring.

Preferably, the SMA actuator assembly 30 comprises at least three SMA wires 30. This allows the tension in the SMA wires 30 to be controlled (as a third degree of freedom), allowing more accurate movement control of the movable part 20, which is particularly beneficial for the purposes of achieving super-resolution imaging and wobulation.

As shown in Figures land 2, the SMA actuator assembly 1 may comprise four SMA wires 30, preferably a total of four SMA wires 30. This allows for a more symmetric arrangement of SMA wires 30, allowing more accurate and reliable movement control with less complexity in the drive scheme.

The SMA wires 30 may be connected between the movable part 20 and support structure 10. One end of each SMA wire 30 may be connected to the movable part 20 by a connection element 23, such as a moving crimp. The other end of each SMA wire 30 may be connected to the support structure 10 by a connection element 13, such as a static crimp. Alternatively, the SMA wires 30 may be connected at both ends to the support structure 10 or movable part 20 and hooked about a contact part with the movable part 20 or support structure 10, for example. Further alternatively, an intermediate mechanism may be provided between the SMA wires 30 and the movable part 20 and/or support structure 10 so as to transfer the force imparted by contraction of the SMA wires 30.

The SMA wires 30 may be in an arrangement as described in WO 2013/175197 Al, which is herein incorporated by reference. So, the SMA wires 30 may be in an arrangement capable of applying a torque to the movable part 20 about an axis z perpendicular to the two orthogonal tilt axes x,y. This allows the torque about the axis z to be controlled by the SMA wires 30, for example so as to reduce or mitigate rotation about the axis z. A bearing arrangement constraining rotation about the axis z may not be required with such an SMA wire arrangement, allowing the SMA actuator assembly 30 to be made more compact.

Two of the four SMA wires may be arranged to apply a torque to the movable part 20 in a first sense, e.g. clockwise, like the SMA wires 30 connected to the top right and bottom left corners of the movable part 20 in Figure 1A. The other two SMA wires 30 may be arranged to apply a torque to the movable part 20 in a second, opposite sense, e.g. anti-clockwise, like the SMA wires 30 connected to the top left and bottom right corners of the movable part 20 in Figure 1A. Put another way, the four SMA wires 30 may be arranged in a loop at different angular positions around the axis, successive SMA wires being arranged to apply a force to the movable part in alternate senses.

Furthermore, the four SMA wires may arranged such that respective subsets of two SMA wires are arranged to apply a force to the movable part in four respective directions along the two orthogonal tilt axes x,y. For example, the two SMA wires 30 provided on the left of Figure 1A may apply a force in the -x direction and the two SMA wires provided 30 on the right of Figure 1A may apply a force in the +x direction. The two SMA wires 30 connected to the top of the movable part 20 in Figure 1A may apply a force in the -y direction and the two SMA wires 30 connected to the bottom of the movable part 20 in Figure 1A may apply a force in the +y direction. This provides for a symmetrically balanced arrangement of SMA wires, making movement control simpler and more accurate.

In general, the arrangement of SMA wires 30, when viewed perpendicularly to the two orthogonal tilt axes x-y (so when viewed along the z axis), may be rotationally symmetrical. This may achieve the benefits of a symmetrically balanced arrangement of SMA wires of making movement control simpler and more accurate.

As shown in Figure 1A, the four SMA wires 30 may comprise two pairs of SMA wires. Each pair of SMA wire may be arranged on opposite sides of the movable part 20 when viewed perpendicularly to the two orthogonal tilt axes x-y (so when viewed along the z axis). This allows the SMA actuator assembly 1 to be made more compact along one of the orthogonal axes (here along the y axis), which is particularly useful in applications such as laptop cameras. The SMA wires 30 of each pair of SMA wires may cross over or overlap when viewed perpendicularly to the two orthogonal tilt axes x-y. This allows the SMA wires 30 to be made longer compared to a situation in which such overlap is not allowed (for a given orientation of the SMA wires). The SMA wires in each pair may be offset along the z axis, to avoid the SMA wires coming into direct physical contact or rubbing against each other on contraction.

As shown in Figures lA and 2A, the four SMA wires 30 may be arranged to apply forces to the movable part that are in angular directions offset by 90 degrees from each other. The SMA wires 30 may extend in angular directions offset by 90 degrees from each other. This provides for a particularly balanced arrangement of SMA wires.

The SMA wires 30 may be arranged parallel to the two orthogonal tilt axes x-y, optionally in a plane. This may reduce the size of the SMA actuator assembly in a direction perpendicular to the orthogonal tilt axes x-y (so along the z axis). There may be no need for a bearing arrangement constraining movement of the movable part 20 along the z axis.

So, as shown in Figs 1A and 1B, the SMA wires 30 may be mounted at the top of the camera module 21. In this design, the SMA wires 30 lie in a plane normal to the optical axis and are at 90 degrees relative to each other. The wires are displaced slightly along the z axis so that they can cross without touching. The wires are at 90 degrees relative to each other so that the lines of force produced by the wires are all orthogonal. This is preferred (but not required) as it increases the symmetry of the motion of the actuator.

Similarly, as shown in Figs 2A and 28, the SMA wires 30 are mounted at the top of the projector 22. In this design the wires lie in a plane normal to the optical axis and are in the SMA wire arrangement described in WO 2013/175197 Al.

Alternatively, the two or more SMA wires may be arranged to be angled relative to the two orthogonal tilt axes. This may be particularly useful, for example, when a plain bearing or rolling bearing is provided, because the angled SMA wires may bias the movable part 20 against the plain bearing or rolling bearing. So, the SMA wires could be angled down from the moving part 20 to the support structure. In this case, a bearing constraint to prevent the camera module 21 moving in the -z direction is advantageous.

Bearing arrangement 40 The SMA actuator assembly 1 may further comprise a bearing arrangement 40. The bearing arrangement 40 allows tilt of the movable part 20 relative to the support structure 10 about the two orthogonal tilt axes x,y.

The bearing arrangement 40 is arranged to constrain translational movement of the movable part 20 along the two orthogonal tilt axes. So, translational movement of the movable part 20 along the x or y axes may be reduced or even prevented.

Optionally, the bearing arrangement is further arranged to constrain translational and/or rotational movement of the movable part 20 along and/or about an axis z perpendicular to the two orthogonal tilt axes x-y.

As shown in Figures 1B and 2B, the bearing arrangement 40 is arranged to constrain movement of the movable part 20 along the two orthogonal tilt axes at a position that is offset along an axis z perpendicular to the two orthogonal tilt axes x-y from the force applied by the two or more SMA wires 30 to the movable part 20.

Figures 1B and 2B show the bearing arrangement 40 in the form of a sheet material or flexure. The sheet material or flexure is connected between the support structure 10 and the movable part 20. The sheet material or flexure spans a gap between the support structure 10 and the movable part 20.

In particular, the sheet material or flexure extends along one or both of the two orthogonal tilt axes x-y.

The sheet material or flexure may be flat, with a smaller extent along the z axis compared to the x or y axes. The sheet material or flexure may extend generally in the plane defined by the two orthogonal tilt axes x-y.

So, in the design of Fig 1B, the camera module 21 in constrained from moving along the x and y axes by the sheet material or flexure that is connected to both the camera module 21 and the support structure at the bottom of the camera module 21. The sheet material or flexure could be the FPC or a PCB that the image sensor of the camera module 21 is mounted on, or it could be an additional component such as sheet stainless steel.

Similarly, as shown in Fig 2B, the projector 22 is constrained from moving along the x and y axes by the sheet material or flexure that is connected to both the projector 22 and the support structure 10 at the bottom of the projector 22. This sheet material or flexure could be embodied by the FPC or PCB that the light emitting element is mounted on or could be an additional component such as sheet stainless steel.

Although the bearing arrangement 40 in Figures 1B and 2B is embodied by a flexure, many other suitable bearing arrangement 40 are available. For example, the bearing arrangement 40 may comprise a pivot point about which the movable part pivots so as to tilt about the two orthogonal axes x-y. The bearing arrangement 40 may comprise a plain bearing, for example a spherical plain bearing such as a ball. The center of the sphere or ball may be at the intersection of the two orthogonal tilt axes x-y.

Further alternatively, the bearing arrangement 40 may comprises a compliant bulk material, such as rubber.

So, in a variant of any of the above designs, the flexure or sheet material could be replaced by a pivot or bearings providing lateral constraint. In a further variant, the flexure could be replaced by a rubber or compliant material that is placed under the camera module 21 or projector. This material could also be used to influence the resonant frequency.

The embodiments of the present invention have been described in combination with several examples of a bearing arrangement 40. However, in general, provision of a bearing arrangement 40 is not strictly required and the movable part 20 may be supported on the support structure 10 solely by the SMA wires 30. For example, the eight SMA wires of WO 2011/104518 Al, which is herein incorporated by reference, may support the movable part 20 and drive tilt of the movable part 20 relative to the support structure 10 in the absence of any bearing arrangement 40.

Resonance design In some embodiments of the present invention, the SMA wires not only drives tilt of the movable part 20, but also acts as a spring that causes resonant motion at the desired frequency.

In this regard, the control circuit may be configured to apply drive signals to the two or more SMA wires so as to drive tilting of the movable part between the predetermined positions in the repeated pattern at a predetermined frequency.

The resonance frequency of tilting the movable part about the two orthogonal tilt axes may be substantially equal to the predetermined frequency. So, the resonance frequency may differs by less than 10%, preferably less than 5% or less than 1%, from the predetermined frequency.

The resonance frequency of the SMA actuator assembly may be influenced by the diameter, length of the SMA wires, height of the wires from the orthogonal tilt axes, the stiffness of the bearing arrangement (e.g. flexure) and the moment of inertia of the movable part (in use, so with camera module or portion of a projector, etc). By appropriately selecting these parameters, the SMA actuator assembly may be at or near resonant frequency at the desired frequency of motion. This may reduce power consumption of the SMA actuator assembly, because reduced force input is needed to drive movement of the movable part between the predetermined positions.

Example of a model of repeated movement for super-resolution Figures 3A-D show an example method of repeated movement enabling super-resolution imaging.

The basic concept of super resolution imaging is that a number of images are captured each with the image offset on the image sensor by a different amount. The images are then combined and the additional information afforded by the additional images is used to create an image of higher resolution (and/or possibly higher quality) than that of the original images.

This simple model assumes that there is a gap between collecting the images and when the image can be moved to a new position. However, if a rolling shutter is used there is often no gap between frames when the image can be moved over the image sensor. This means that using this simple approach a frame needs to be dropped when the image is moved, and this decreases the rate at which images can be collected.

The image may thus be moved relative to the image sensor continuously in a circular manner while four images are collected per cycle. This allows images to be collected for super resolution while a rolling shutter is used.

The image may generally be moved over the image sensor in a continuous manner (motion has a constant velocity) by moving the image sensor, other optical elements or both (e.g. in the camera module described herein).

In the first manifestation the motion is circular and the SMA wires (or other actuator) move the image over the image sensor in a complete circle once every four exposures. This is shown in Figures 3A-D.

The timing of the actuator motion is locked to the exposures so that at the middle of the image an exposure starts when the image is at the top of the circle. In this case each of the four exposures can be directly equated to one of the super resolution pixels indicated by the grid shown in Figures 3A-D. Having the exposures correlated in this manner gives the best quality of image and so is done at the centre of the image.

At the top and the bottom of the image, four independent samples of light are still taken, but they are not arranged in a uniform square grid. This data is converted to a square grid to provide a super resolution image.

The same method can be applied to creating a super-resolution image from images displayed on a display panel or projected by a projector, thus implementing wobulation.

Final paragraphs The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

Claims

Claims 1. An SMA actuator assembly comprising a support structure; a movable part supported on the support structure in a manner allowing tilt of the movable part relative to the support structure about two orthogonal tilt axes; two or more SMA wires arranged to tilt the movable part relative to the support structure about the two orthogonal tilt axes; a control circuit configured to apply drive signals to the two or more SMA wires so as to drive tilting of the movable part relative to the support structure between predetermined positions in a repeated pattern.
2. An SMA actuator assembly according to claim 1, wherein the movable part comprises an optical device comprising a lens assembly defining an optical axis, wherein the two orthogonal tilt axes are perpendicular to the optical axis.
3. An SMA actuator assembly according to claim 2, wherein the optical device is a camera module comprising an image sensor, wherein the lens assembly is arranged to focus an image onto the image sensor.
4. An SMA actuator assembly according to claim 3, wherein the control circuit is configured to apply the drive signals to the two or more SMA wires for the purpose of achieving super-resolution imaging of the camera module.
5. An SMA actuator assembly according to claim 1, wherein the SMA actuator assembly comprises a projector for projecting an image, wherein the movable part comprises at least a portion of the projector such that tilting of the movable part relative to the support structure moves the projected image.
6. An SMA actuator assembly according to claim 5, wherein the control circuit is configured to apply the drive signals to the two or more SMA wires for the purpose of achieving wobulation of the projected image.
7. An SMA actuator assembly according to claim 1, wherein the movable part comprises a display panel for displaying an image.
8. An SMA actuator assembly according to claim 7, wherein the control circuit is configured to apply the drive signals to the two or more SMA wires for the purpose of achieving wobulation of the displayed image.
9. An SMA actuator assembly according to any preceding claim, wherein the predetermined positions in the repeated pattern are arranged in two degrees of freedom.
10. An SMA actuator assembly according to any preceding claim, wherein the two or more SMA wires comprise a four SMA wires, optionally a total of four SMA wires in an arrangement capable of applying a torque to the movable part about an axis perpendicular to the two orthogonal tilt axes.
11. An SMA actuator assembly according to claim 10, wherein two of the four SMA wires are arranged to apply a torque to the movable part in a first sense and the other two SMA wires are arranged to apply a torque to the movable part in a second, opposite sense.
12. An SMA actuator assembly according claim 10 or 11, wherein the four SMA wires are arranged in a loop at different angular positions around the axis, successive SMA wires being arranged to apply a force to the movable part in alternate senses.
13. An SMA actuator assembly according any one of claims 10 to 12, wherein the four SMA wires are arranged such that respective subsets of two SMA wires are arranged to apply a force to the movable part in four respective directions along the two orthogonal tilt axes.
14. An SMA actuator assembly according any one of claims 10 to 13, wherein the four SMA wires comprise two pairs of SMA wires, each pair of SMA wire is arranged on opposite sides of the movable part when viewed perpendicularly to the two orthogonal tilt axes.
15. An SMA actuator assembly according to claim 14, wherein the SMA wires of each pair of SMA wires cross over or overlap when viewed perpendicularly to the two orthogonal tilt axes.
16. An SMA actuator assembly according any one of claims 10 to 15, wherein the four SMA wires are arranged to apply forces to the movable part that are in angular directions offset by 90 degrees from each other.
17. An SMA actuator assembly according to any preceding claim, wherein the two or more SMA wires are arranged parallel to the two orthogonal tilt axes, optionally in a plane.
18. An SMA actuator assembly according to any one of claims 1 to 16, wherein the two or more SMA wires are arranged to be angled relative to the two orthogonal tilt axes.
19. An SMA actuator assembly according to any preceding claim, wherein the control circuit is configured to apply drive signals to the two or more SMA wires so as to drive tilting of the movable part between the predetermined positions at a predetermined frequency.
20. An SMA actuator assembly according claim 18, wherein the resonance frequency of tilting the movable part about the two orthogonal tilt axes is substantially equal to the predetermined frequency, in particular differs by less than 10%, preferably less than 5% or less than 1%, from the predetermined frequency.
21. An SMA actuator assembly according to any preceding claim, further comprising a bearing arrangement allowing tilt of the movable part relative to the support structure about the two orthogonal tilt axes.
22. An SMA actuator assembly according to claim 21, wherein the bearing arrangement is arranged to constrain translational movement of the movable part along the two orthogonal tilt axes, optionally wherein the bearing arrangement is further arranged to constrain translational and/or rotational movement of the movable part along and/or about an axis perpendicular to the two orthogonal tilt axes.
23. An SMA actuator assembly according to claim 21 or 22, wherein the bearing arrangement is arranged to constrain movement of the movable part along the two orthogonal tilt axes at a position that is offset along an axis perpendicular to the two orthogonal tilt axes from the force applied by the two or more SMA wires to the movable part.
24. An SMA actuator assembly according to any one of claims 21 to 23, wherein the bearing arrangement comprises a sheet material or flexure connected between the support structure and the movable part, optionally wherein the sheet material or flexure extends in the plane defined by the two orthogonal tilt axes.
25. An SMA actuator assembly according to any one of claims 21 to 23, wherein the bearing arrangement comprises a pivot or a plain bearing, in particular a spherical plain bearing, or wherein the bearing arrangement comprises a compliant bulk material, such as a rubber.