Actuator assembly

Abstract

An actuator assembly comprises first and second parts that are movable and / or tiltable relative to each other along and / or about two orthogonal axes that are perpendicular to a primary axis, and four

Description

Field The present application relates to an actuator assembly with at least one actuating unit including a shape memory alloy (SMA) element. Background SMA actuator assemblies may be used in a variety of applications for moving a movable part relative to a support structure. For example, WO 2013 / 175197 Al describes a camera in which four SMA wires are arranged to move a lens element relative to an image sensor in a plane that is perpendicular to the optical axis of the lens element, thereby effecting optical image stabilization (OIS). WO 2010 / 029316 Al describes SMA wires used to provide OIS in a camera by tilting a camera module. WO 2011 / 104518 Al describes an actuator assembly having eight SMA wires capable of effecting positional control of a movable element with multiple degrees of freedom. Typically, the range of movement (also known as "stroke") of such SMA actuator assemblies is limited by the maximum contraction of the SMA wires, and the actuating force is limited by the maximum force that can be generated by the SMA wires. To increase the movement range or the actuating force, longer or thicker SMA wires can be used, but this may be at the expense of increased cost, size and / or power, which may not be practical in miniature applications. WO 2022 / 084699 Al discloses an actuator assembly comprising at least one actuating unit (incorporating an SMA wire) that, on actuation, moves a movable part relative to the support structure. The actuating unit may be configured to increase the stroke or the actuating force and / or to re-direct the force applied by the SMA wire. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: first and second parts that are movable and / or tiltable relative to each other along and / or about two orthogonal axes that are perpendicular to a primary axis; and a total of four actuating units each configured, on actuation, to apply a respective actuating force to the second part capable of moving the second part relative to the first part, wherein the four actuating units are arranged such that none of the four actuating forces are colinear and are capable of moving and / or tilting the second part relative to the first part to any position within a range of movement along and / or about the two orthogonal axes; wherein each actuating unit comprises: a body portion; a force-modifying flexure connected between the body portion and the first part; an SMA wire connected by connection elements between the body portion and the first part, wherein the SMA wire is arranged, on actuation, to apply an input force to the body portion, thereby causing the force-modifying flexure to modify the input force such that the actuating force is applied to the second part, wherein the length of the SMA wire between the connection elements is less than half of the extent of the actuator assembly in a direction parallel to the SMA wire, and wherein the four actuating units do not overlap when viewed along the primary axis. In some embodiments, the length of the SMA wire between the connection elements is less than 5mm, preferably less than 4mm, further preferably less than 3mm. In some embodiments, the length of the SMA wire between the connection elements is less than a third of the extent of the actuator assembly in a direction parallel to the SMA wire. In some embodiments, the none of the actuating units overlap when viewed along the primary axis. In some embodiments, the each actuating unit further comprises a coupling link connected between the body portion and the second part, wherein the coupling link is configured to transmit the actuating force from the body portion to the second part, and wherein the coupling link is compliant in a direction perpendicular to the actuating force. In some embodiments, the coupling link comprises a coupling flexure. In some embodiments, the the axis through the length of the force-modifying flexure, the axis through the length of the coupling flexure, and the axis through the length of the SMA wire all intersect at the same point. In some embodiments, the the force-modifying flexure is arranged between the SMA wire and the coupling flexure when viewed parallel to the primary axis. In some embodiments, the actuating units are arranged on four sides of the actuator assembly, wherein two actuating units on opposite sides are arranged such that the SMA wires of the two actuating units face each other when viewed along the primary axis, and the other two actuating units are arranged such that the coupling flexures of the other two actuating units face each other when viewed along the primary axis. In some embodiments, the the actuating units are arranged, when viewed along the primary axis, not to be mirror symmetric about an axis perpendicular to the primary axis. According to an aspect of the present invention, there is provided an actuator assembly comprising: first and second parts that are movable relative to each other along; and at least one actuating unit configured, on actuation, to apply a respective actuating force to the second part capable of moving the second part relative to the first part, wherein each actuating unit comprises: a body portion; a forcemodifying flexure connected between the body portion and the first part; an SMA wire connected between the body portion and the first part, wherein the SMA wire is arranged, on actuation, to apply an input force to the body portion, thereby causing the force-modifying flexure to flex such that the body portion rotates about an effective pivot point such that the actuating force is applied to the second part, and wherein the angle between vectors representing the input force and the actuating force is greater than 100 degrees, wherein the SMA wire is connected to the body portion at a location that is lateral to the force-modifying flexure, and wherein the three lines extending along the force-modifying flexure, along the input force and along the actuating force are concurrent lines. It will be appreciated that a reference to a component being "connected between" two other components means, for example, that the component is directly or indirectly connected to each of the other components. Such an indirect connection may involve a connection via further component(s) (e.g. a connector) with fixed position(s) relative to one of the other components. Such an indirect connection may involve a connection via further component(s) which is / are movable relative to the other components. For example, an SMA element may be connected to the one of the first and second parts via a further flexure, e.g. as described in WO 2022 / 144541 (which is herein incorporated by reference). 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-E are schematic cross-sectional views of different variations of a camera assembly incorporating an actuator assembly; Figure 2 is a schematic perspective view of the actuator assembly; Figures 3A and 3B are perspective and plan views of an actuating unit forming part of the actuator assembly, and Figure 3C is a plan view of another such actuating unit; Figure 4 is a schematic plan view of an arrangement of four actuating units; Figure 5 is a schematic perspective view of an arrangement of eight actuating units; Figure 6 is a schematic plan view of a compact arrangement of four actuating units; Figure 7 is a schematic plan view of another compact arrangement of four actuating units; Figure 8 is a schematic plan view of a compact arrangement of four actuating units with a rectangular footprint; Figure 9 is a schematic plan view of another compact arrangement of four actuating units with a rectangular footprint; Figure 10 is a schematic plan view of another compact arrangement of four actuating units; and Figures 11 to 14 are schematic plan views of actuating units and arrangements of actuating units comprising 3rd class levers. Detailed description Camera assembly Figures 1A-E schematically show different variations of an apparatus 1 incorporating an actuator assembly 2. The apparatus 1 is, for example, a camera assembly 1. Generally, the apparatus 1 is to be incorporated in a portable electronic device such as a smartphone. Thus, miniaturisation can be an important design criterion. Figure 2 schematically shows the actuator assembly 2. The actuator assembly 2 includes a support structure 10 and a movable part 20. The movable part 20 is movable relative to the support structure 10. When the actuator assembly 2 is included e.g. in the apparatus 1, the support structure 10 may be fixed relative to the main body of the apparatus 1. However, in general, the support structure 10 need not be stationary and may be movable relative to or within the apparatus 1. The actuator assembly 2 includes one or more actuating units 30. Each actuating unit 30 is configured to apply an actuating force to the movable part 20 capable of moving the movable part 20 relative to the support structure 10. The movable part 20 may be supported (i.e. suspended) on the support structure 10 exclusively by the actuating units 30. Alternatively, the actuator assembly 2 may include a bearing arrangement 40 that supports the movable part 20 on the support structure 10. The actuating units 30 and the bearing arrangement 40 may together support the movable part 20 on the support structure 10. The bearing arrangement 40 may have any suitable form for allowing movement of the movable part 20 with respect to the support structure 10 with one or more degrees of freedom (DOFs). The actuating units 30 and / or the bearing arrangement 40 may constrain, i.e. reduce or prevent, other DOFs of movement of the movable part 20 relative to the support structure 10. For this purpose, the bearing arrangement 40 may, for example, include one or more of the following bearings: a rolling bearing (such as a ball bearing), a flexure bearing (i.e. an arrangement of flexures or other resilient elements that guide movement), or a plain (i.e. sliding contact) bearing. A primary axis P can be defined with reference to the actuator assembly 2 and / or the support structure 10. The primary axis P may extend through the actuator assembly 2, e.g. through the centre of the actuator assembly 2. In some examples, the actuator assembly 2, the support structure 10 and / or the movable part 20 extends predominantly in a direction perpendicular to the primary axis P. In other words, the extent of the actuator assembly 2, the support structure 10 and / or the movable part 20 along the primary axis P is less than the extent thereof along any direction perpendicular to the primary axis P. The primary axis P may be the longitudinal axis of the actuator assembly 2 and / or the support structure 10. Alternatively or additionally, the support structure 10 and / or movable part 20 may include a planar component that extends perpendicularly to the primary axis P. Alternatively or additionally, in examples in which the apparatus 1 includes an optical element (such as a lens assembly 3) with an optical axis, or an imaging element (such as an imager sensor 4) with an imaging axis, the primary axis P may be parallel to such an axis and / or may coincide with such an axis when the movable part 20 is in a central position or orientation (for example, see Figure 1A). In general, the movable part 20 may be movable relative to the support structure 10 with up to six degrees of freedom (DOFs). In the context of describing the DOFs of movement, the primary axis P may also be referred to as the z axis, and two further axes that are perpendicular to the primary axis P and to each other may be referred to as the x and y axes. The movable part 20 may be movable relative to the support structure 10 in all or in any subset (including only one) of the following DOFs: • TxandTy: Translational movement in the x-y plane. In other words, the movable part 20 may be independently movable along the x and y axes. The movable part 20 may be movable to any position in the x-y plane within a range of movement. Instead of such planar movement, the movable part 20 may be movable linearly, e.g. along the x or y axis. • Rx and Ry: Rotational movement (or simply rotation or tilting) about the x and y axes. In other words, the movable part 20 may be rotated about any line perpendicular to the primary axis P. The movable part 20 may be rotatable to any rotational position (i.e. to any orientation) within a range of movement. Instead of such two-axis rotation, the movable part 20 may be rotatable about a single axis, e.g. about the x or y axis. • Tz: Translational movement along the z axis. The movable part 20 may be movable to any translational position along the z axis within a range of movement. • Rz: Rotational movement (or simply rotation) about the z axis. The movable part 20 may be rotatable to any rotational position (i.e. to any orientation) within a range of movement. In some examples, the movable part 20 may be supported, e.g. by the bearing arrangement 40, so as to allow translational movement in the x-y plane (Tx, Ty) and / or rotational movement about the z axis (Rz). Translational movement along the z axis (Tz) and rotational movement about the x and y axes (Rx, Ry) may be constrained. Such support may be provided, for example, with a bearing arrangement 40 with a suitable arrangement of ball bearings or plain bearings which produce bearing forces in the +z direction and a biasing arrangement which produces a biasing force in the -z direction. Examples of actuator assemblies with such a bearing arrangement are disclosed in WO 2013 / 175197 Al and WO 2017 / 072525 Al, each of which is herein incorporated by reference. In some examples, the movable part 20 may be supported so as to allow tilting about the x and y axes (Rx, Ry) and optionally rotation about the z axis (Rz). The other DOFs of movement (i.e. Tx, Ty, Tz, Rz, or Tx, Ty, Tz) may be constrained. Such support may be provided by the bearing arrangement 40, for example in the form of a gimbal. Examples of such a bearing arrangement 40 are disclosed in WO 2021 / 209770 Al, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 30, similarly to WO 2011 / 104518 Al which discloses an actuator assembly with 8 SMA wires connected between the support structure 10 and the movable part 20. WO 2011 / 104518 Al is herein incorporated by reference. In some examples, the movable part 20 may be supported so as to allow three-dimensional translational movement (Tx, Ty, Tz), while rotational movement (Rx, Ry, Rz) may be constrained. Such support may be provided by the bearing arrangement 40, for example in the form of nested linear bearings. Examples of such a bearing arrangement 40 are disclosed in WO 2021 / 209769 Al, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 30, similarly to WO 2011 / 104518 Al. The movable part 20 may, alternatively or additionally, move in other DOFs. The movable part 20 may move in DOFs that are a combination of any two or more of Tx, Ty, Tz, Rx, Ry and Rz. For example, the movable part 20 may move along a helical path (i.e. move helically) about the z axis, and so concurrently move along the z axis and rotate about the z axis. In other words, Tz and Rz movement may be coupled. An example of such a helical actuator assembly is disclosed in WO 2019 / 243849 Al, which is herein incorporated by reference. The actuating units 30 are connected between the support structure 10 and the movable part 20. The actuating units 30 are arranged to apply actuating forces F (see e.g. Figs. 4 and 5) between the movable part 20 and the support structure 10. Selectively varying the actuating forces F may cause the movable part 20 to move relative to the support structure 10, for example within the DOFs allowed by the bearing arrangement 40. The actuating units 30 are thus capable of driving movement of the movable part 20 relative to the support structure 10. The bearing arrangement 40 may cause the movable part 20 to move in directions which differ from the directions of the actuating forces F. In simple examples of this, one component of each actuating force F causes the movement of the movable part 20, and another component of each actuating force F acts against the bearing forces produced by the bearing arrangement 40. The camera assembly 1 also includes a lens assembly 3 and an image sensor 4. The lens assembly 3 includes one or more lenses configured to focus an image on the image sensor 4. The lens assembly 3 defines an optical axis O. The lens assembly 3 may include a lens carrier, for example in the form of a cylindrical body, supporting the one or more lenses. The image sensor 4 captures an image and may be of any suitable type, for example a charge coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) device. The camera assembly 1 may be a compact camera assembly in which each lens has a diameter of 20mm or less, for example of 12mm or less. In the ("sensor-shift") variation of the camera assembly 1 shown in Figure 1A, the movable part 20 includes the image sensor 4. The lens assembly 3 may be fixed relative to the support structure 10, or may be movable relative to the support structure 10 along the optical axis O, as described below. In the ("lens-shift") variation shown in Figure IB, the image sensor 4 is fixed relative to the support structure 10 and the movable part 20 includes the lens assembly 3. The lens assembly 3 may be movable relative to the movable part 20 along the optical axis O, as described below. In both of these variations, the actuator assembly 2 is configured to move the lens assembly 3 relative to the image sensor 4 in any direction in the plane perpendicular to the primary axis P and hence the optical axis O. Such movement has the effect of moving the image on the image sensor 4 and enables optical image stabilisation (OIS) to be implemented in the camera assembly 1. In the sensor-shift variation, the movable part 20 may also be rotatable about the primary axis P so as to also enable compensation for roll. In the ("module-tilt") variation shown in Figure IC, the movable part 20 includes both the lens assembly 3 and the image sensor 4. Again, the lens assembly 3 may be movable relative to the movable part 20 along the optical axis O, as described below. The actuator assembly 2 is configured to tilt the movable part 20 about two axes perpendicular to the primary axis P and to each other, and optionally rotate the movable part 20 about the primary axis P, enabling OIS to be implemented in the camera assembly 1. In the ("autofocus" or "zoom") variation shown in Figure ID, the movable part 20 includes the lens assembly 3, and the actuator assembly 2 moves the movable part 20 relative to the support structure 10 along the the optical axis O. Such movement has the effect of adjusting the focus of the image on the image sensor 4 or providing zoom functionality. So, auto-focus (AF) or zoom functionality can be implemented in the camera assembly 1. In some examples (not shown), the camera assembly 1 may include a first actuator assembly for providing OIS as illustrated in Figures 1A-C, and a second actuator assembly for providing AF or zoom as illustrated in Figure ID. One or both of the first and second actuator assemblies may correspond to actuator assemblies 2 as described herein. One of the first and second actuator assemblies may be another type of SMA actuator assembly or may be a non-SMA actuator assembly, e.g. a voice-coil motor (VCM) actuator assembly. As will be appreciated, in the lens-shift and module-tilt variations, the support structure 10 of the second actuator assembly 2 is fixed to (or corresponds to) the movable part 20 of the first actuator assembly 2. In the ("AF+OIS") variation shown in Figure IE, the movable part 20 includes the lens assembly 3, and the actuator assembly 2 produces three-dimensional translational movement of the movable part 20 relative to the support structure 10, enabling both AF and OIS to be implemented using one actuator assembly 2. Other variations are also possible. For example, in the autofocus variation or the AF+OIS variation, the movable part 20 may include the image sensor 4 rather than the lens assembly 3. The camera assembly 1 may include combinations of the above-described features, e.g. (a) lens shift and sensor shift, (b) module tilt and lens shift or sensor shift and autofocus, or (c) module tilt and AF+OIS. The camera assembly 1 also includes a controller 8. The controller 8 may be implemented in an integrated circuit (IC) chip. The controller 8 generates drive signals for the actuating units 30, in particular for SMA wires 34 forming part of the actuating units 30. SMA material has the property that, on heating, it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA wires 34, thereby heating the SMA wires 34 by causing an electric current to flow, will cause the SMA wires 34 to contract and thus actuate the actuating unit 30 so as to drive relative movement of the movable part 20. The drive signals are chosen to drive relative movement of the movable part 20 in a desired manner, for example so as to achieve OIS by stabilizing the image sensed by the image sensor 4 or to achieve AF by adjusting the focus of the image sensed by the image sensor 4. The controller 8 supplies the generated drive signals to the SMA wires 34. Optionally, the camera assembly 1 also includes a motion sensor (not shown), which may include a 3-axis gyroscope and a 3-axis accelerometer. The motion sensor can generate signals representative of the motion (specifically vibrations or "shake") of the camera assembly 1, which can be processed so as to produce signals representative of the required movement of the movable part 20 to compensate for such shake. The controller 8 receives such signals and can generate the drive signals for the SMA wires 34 to achieve OIS. Although the actuator assembly 2 is described in connection with a camera assembly 1, it will be appreciated that the actuator assembly 2 may be used in any device in which movement of a movable part 20 relative to a support structure 10 is desired, e.g. to provide haptic feedback in a haptic feedback device or to move a projector or display in an augmented reality (AR) or virtual reality (VR) device. Actuating unit Figure 3A shows a perspective view of an example of the actuating unit 30. Figure 3B shows part of the actuating unit 30 in plan view. A single actuating unit 30 is shown in Figures 3A and 3B, but it will be appreciated that the actuator assembly 2 generally has multiple actuating units 30, each of which may include the same components described with reference to Figures 3A and 3B. The actuating unit 30 includes a body portion 31 to which several other components of the actuating unit 30 are connected as described below. Typically, the body portion 31 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 30. In some examples, the body portion 31 is not a distinct part of the actuating unit 30. For example, the body portion 31 may be defined as part of one of the other components of the actuating unit 30 or simply as a connection point between other components of the actuating unit 30. The actuating unit 30 also includes a force-modifying flexure 32. The force-modifying flexure 32 is connected between the body portion 31 and the support structure 10. One end of the force-modifying flexure 32 is connected to the body portion 31. The other end of the force-modifying flexure 32 is connected to the support structure 10, e.g. via a foot portion 36. The foot portion 36 is fixed relative to the support structure 10. The force-modifying flexure 32 allows the body portion 31 to pivot relative to the support structure 10 about an effective pivot point P. Although the effective pivot point P is shown in Figure 3B as being positioned in the middle of force-modifying flexure 32, the effective pivot point P may have a different position and also need not lie on the force-modifying flexure 32. Such pivotal movement of the body portion 31 relative to the support structure 10 is initially in a direction that is substantially perpendicular to the force-modifying flexure 32. The actuating unit 30 also includes an SMA element 34. In this example, the SMA element 34 is an SMA wire 34. The SMA wire 34 is connected between the body portion 31 and the support structure 10. One end of the SMA wire 34 is connected to the support structure 10, e.g. by a crimp 15. The other end of the SMA wire 34 is connected to the body portion 31, e.g. by a crimp 35. The actuating unit 30 also includes a coupling link 33. In this example, the coupling link 33 is a coupling flexure 33. The coupling flexure 33 is connected between the body portion 31 and the movable part 20. One end of the coupling flexure 33 is connected to the body portion 31. The other end of the coupling flexure 33 is connected to the movable part 20. The coupling link 33 transfers or transmits an actuating force F from the body portion 31 to the movable part 20. The coupling link 33 is compliant (i.e. deformable) in a direction (or in multiple directions) perpendicular to the actuating force F. This allows the movable part 20 to move in directions other than the direction of the coupling flexure 33 and actuating force F. This can be needed, for example, where different actuating units 30 cause the movable part 20 to move in different directions. In this example, the body portion 31, the force-modifying flexure 32, the coupling flexure 33 and the foot portion 36 are integrally formed, for example from a single sheet of material (such as metal). In other examples, one or more or these features, if present, may be formed from different parts or materials. The SMA wire 34 is arranged, on contraction, to apply an input force Fi on the body portion 31. The input force Fi acts parallel to the length of the SMA wire 34. The force-modifying flexure 32 and the body portion 31 are arranged to modify the input force Fi so as to give rise to the actuating force F, which is transmitted from the body portion 31 to the movable part 20 by the coupling flexure 33. In particular, the input force Fi deforms the force-modifying flexure 32, thereby causing the body portion 31 to pivot about the effective pivot point P. In simple terms, the force-modifying flexure 32 and the body portion 31 act like a lever. The force-modifying flexure 32 and the body portion 31 may modify the direction and / or the magnitude of the input force Fi so as to give rise to the actuating force F. In the example illustrated in Figures 3A and 3B, the coupling flexure 33 is at an angle of -90° relative to the SMA wire 34. Also, in this example, the force-modifying flexure 32 is arranged at an angle a of -30° relative to the SMA wire 34, and the force-modifying flexure 32 is placed in tension on contraction of the SMA wire 34. Hence, on contraction of the SMA wire 34 and on resulting deformation of the forcemodifying flexure 32, the body portion 31 initially moves at an angle of -60° (90°-a) relative to the length of the SMA wire 34. Thus, it will be appreciated that, in this example, the force is de-amplified and the stroke is amplified, while the direction of the forces / movements is changed by an angle of -90°. More generally, the change in direction of the force depends on the angle between the SMA wire 34 and the coupling flexure 33. Also more generally, the change in magnitude of the force is dependent on the ratio of i) the distance Ds from the effective pivot point P to the line on which the SMA wire 34 lies and ii) the distance De from the effective pivot point P to the line on which the coupling flexure 33 lies. In particular, F / Fi is proportional to Ds / Dc. If the SMA wire 34 lies on a line that is closer to the effective pivot point P than the line on which the coupling flexure 33 lies, then the input force Fi is de-amplified. At the same time, the movement of the movable part 20 is amplified, i.e. increased relative to a change in length of the SMA wire 34. Alternatively, if the SMA wire 34 lies on a line that is further away from the effective pivot point P than the line on which the coupling flexure 33 lies, then the input force Fi is amplified. At the same time, the movement of the movable part 20 is de-amplified, i.e. decreased relative to a change in length of the SMA wire 34. The actuating unit 30 can thus be configured to amplify movement or to amplify force due to contraction of the SMA wire 34. The actuating unit 30 can also be configured to change the direction of the input force Fi. In some examples, the actuating unit 30 is configured to change the direction of the input force Fi without changing the magnitude of the force or movement. The ratio Ds / Dc is dependent on the location of the end of the SMA wire 34 that is connected to the body portion 31, and on the location of the end of the coupling flexure 33 that is connected to the body portion 31. By way of example, the distance De could be increased by connecting the coupling flexure 33 further to the left of body portion 31 shown in Figure 3B, thereby decreasing Ds / Dc and so increasing the amount of stroke amplification. The ratio Ds / Dc is also dependent on the orientation of the SMA wire 34, and on the orientation of the coupling flexure 33. Such orientations can be defined with reference to the forcemodifying flexure 32 (as above) or any suitable reference line. By way of example, the distance Ds could be decreased by angling the SMA wire 34 shown in Figure 3B so that it passes closer to the effective pivot point P, thereby decreasing Ds / Dc and so increasing the amount of stroke amplification. In summary, the amount by which the force-modifying flexure 32 amplifies or de-amplifies the force / stroke of the SMA wire 34 may be tailored by: • adjusting the orientation of the SMA wire 34 (and thus of the input force Fi); • adjusting the location of the connection point between the SMA wire 34 and the body portion 31 (and thus the location at which the input force Fi acts on the body portion 31); • adjusting the orientation of the coupling flexure 33 (and thus of the actuating force F); and / or • adjusting the location of the connection point between the coupling flexure 33 and the body portion 31 (and thus the location from which the body portion 31 applies the actuating force F). In some examples, at least one actuating unit 30 (preferably each actuating unit 30) is configured such that the force-modifying flexure 32 and the body portion 31 amplifies an amount of contraction of the SMA wire 34. Such amplification, for example, may be by a factor greater than 1.5, preferably greater than 2, further preferably greater than 3. For this purpose, in the example illustrated in Figures 3A and 3B, the angle a between the SMA wire 34 and the force-modifying flexure 32 may be in the range from 0 to 45 degrees, preferably from 13 to 40 degrees. However, in general, the angle a may have other values and the connection points of the SMA wire 34 and / or coupling flexure 33 to the body portion 31 may be adjusted to achieve a desired amount of amplification. As described above, in the example illustrated in Figures 3A and 3B, the coupling flexure 33 is at an angle of about 90 degrees relative to the SMA wire 34. This allows the actuating unit 30 to fold around a corner of the movable part 20 in a compact manner. The angle between the coupling flexure 33 and the SMA wire 34 may be in the range from 70 to 110 degrees, preferably from 80 to 100 degrees. However, in general, the angle between coupling flexure 33 and SMA wire 34 may be outside these ranges. For instance, in the actuating unit 30 illustrated in Figure 3C, the force-modifying flexure 32, the coupling flexure 33 and the SMA wire 34 are substantially parallel to one another. In the above-described examples, the actuating unit 30 is arranged in a plane. In particular, the SMA wire 34, the coupling flexure 33 and the force-modifying flexure 32 are arranged so as to substantially extend in a common plane, at least when the actuator assembly 2 is in an initial configuration. This allows for a compact configuration of the actuating unit 30. The body portion 31, when embodied by a plate, may further be arranged to extend in the plane. However, in general, the components of the actuating unit 30 need not be arranged in a common plane. The SMA wire 34 and / or the coupling flexure 33 may be angled relative to the plane, for example. In the above-described examples, the force-modifying flexure 32 is placed in tension on contraction of the SMA wire 34. This reduces the risk of buckling of the force-modifying flexure 32, reducing the risk of damage to the actuator assembly 2 and making the actuator assembly 2 more reliable. However, the force-modifying flexure 32 could instead be arranged so as to be placed under compression on contraction of the SMA wire 34. With reference to Figure 3B, for example, the force-modifying flexure 32 could extend to the bottom-right from the connection point between the body portion 31 and the force-modifying flexure 32, and so be placed under compression on contraction of the SMA wire 34. An arrangement in which the force-modifying flexure 32 is placed under compression is disclosed in WO 2022 / 084699 Al, which is herein incorporated by reference. In the above-described examples, the force-modifying flexure 32 and the SMA wire 34 connect at one end to the support structure 10, and the coupling flexure 33 connects at one end to the movable part 20. In general, this arrangement may also be reversed, with the force-modifying flexure 32 and the SMA wire 34 connecting at one end to the movable part 20, and the coupling flexure 33 connecting at one end to the support structure 10. In the above-described examples, the actuating unit 30 includes a coupling link 33 in the form of a coupling flexure 33. The purpose of the coupling link 33 is to allow movement of the movable part 20 in directions perpendicular to the actuating force F. In general, however, the actuating unit 30 need not include a coupling link 33, e.g. in examples in which there is no movement of the movable part 20 in directions perpendicular to the actuating force F. Furthermore, the coupling link 33 may be embodied by components other than the coupling flexure 33, for example by a ball bearing or plain bearing configured to transmit the actuating force F to the movable part 20 while allowing movement of the movable part 20 in directions perpendicular to the actuating force F. Such alternative examples of the coupling link 33 are disclosed in WO 2022 / 084699 Al. The coupling link 33 may (or may not) be formed by an SMA wire, which may (or may not) be integral with the SMA wire 34 and may (or may not) be driven together with the SMA wire 34. Furthermore, instead of the force-modifying flexure 32, the actuator assembly may include a different type of force-modifying element configured to enable the above-described movement of the body portion 31 relative to the support structure 10. Such a force-modifying element may include, for instance, a rigid member with one end connected to the support structure 10 via a suitable pivoting connection (e.g. a pin joint) and the other end connected to the body portion 31. Arrangement of four actuating units Figure 4 schematically shows a plan view of an example of the actuator assembly 2, showing an arrangement of actuating units 30. In this example, the actuator assembly 2 includes a total of four actuating units 30. The four actuating units 30 may apply actuating forces F between the movable part 20 and the support structure 10. The actuating forces F are applied to the movable part 20 relative to the support structure 10. The arrangement of actuating units 30 of Figure 4 may be used, for example, in examples in which the movable part 20 is movable relative to the support structure 10 in a movement plane. So, Tx, Ty and optionally Rz movement of the movable part 20 may be allowed. The four actuating units 30 of Figure 4 are in an arrangement capable of applying actuating forces F so as to move the movable part 20 relative to the support structure 10 to any position within a range of movement. The range of movement may be within a movement plane that is perpendicular to the primary axis P. In particular, two actuating units 30 (e.g. the top and bottom actuating units in Figure 4) are arranged to apply actuating forces F in opposite directions parallel to a first axis (e.g. the x axis). The other two actuating units (e.g. the left and right actuating units in Figure 4) are arranged to apply actuating forces F in opposite directions parallel to a second axis (e.g. the y axis), perpendicular to the first axis. By appropriately varying the difference in actuation amount between the opposing actuating units 30, the movable part 20 may thus be moved independently along the first and second axes. The opposing actuating forces F are not colinear, but offset from each other in a direction perpendicular to the actuating forces F. Providing opposing actuating units 30 allows the tension in the SMA wires 30 of the respective actuating units 30 to be controlled, allowing for more accurate and reliable positioning of the movable part 20 compared to a situation in which actuating units 30 do not oppose each other. In some examples, none of the actuating forces F are collinear. This allows the arrangement of actuating units 30 to translationally move the movable part 20 without applying any net torque to the movable part 20. So, the movable part 20 can be moved translationally in the movement plane without rotating the movable part 20 in the movement plane. In general, the arrangement of actuating units 30 is capable of accurately controlling a torque or moment of the movable part 20 about the primary axis P. So, the actuating units 30 are capable of rotating (or not rotating) the movable part 20 relative to the support structure about the primary axis P. In particular, two actuating units 30 (e.g. the top and bottom actuating units in Figure 4) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 20 and the support structure 2 in a first sense (e.g. clockwise) around the primary axis P. The other two actuating units 30 (e.g. the left and right actuating units 30 in Figure 4) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 20 and the support structure 2 in a second, opposite sense (e.g. anti-clockwise) around the primary axis P. This allows the movable part 20 to be rotated by simultaneously increasing or decreasing the tension of SMA wires in any of the two actuating units 30. As shown, two actuating units 30 may be arranged to apply actuating forces F in a corner of the actuator assembly 2. The other two actuating units 30 may be arranged to apply actuating forces F in another, opposite corner of the actuator assembly 2. The actuator assembly 2, and in particular the movable part 20 and / or the support structure 10, may have a square or rectangular footprint. Each actuating unit 30 may be provided on one of the four sides of the actuator assembly 2. In particular, each actuating unit 30 may bend around a corner of the movable part 20 such that the SMA wire 34 and the coupling flexure 33 of each actuating unit 30 extend along adjacent edges of the movable part 20. So, the actuating unit 30 may be as configured in Figures 3A and 3B, for example. The four SMA wires 32 of the four actuating units 32 may extend along the four different edges of the movable part 20. The arrangement of actuating forces F applied between movable part 20 and support structure 10 corresponds to the arrangement of SMA wires 30 described in WO2013 / 175197 Al, which is herein incorporated by reference. In this example, the actuating forces F are perpendicular to the primary axis P, and may be parallel to the movement plane. However, in general the actuating forces F may be angled relative to the movement plane. The actuating forces F may thus have a component along the primary axis P. This component along the primary axis P may be resisted by the bearing arrangement 40, for example, to provide movement of the movable part 20 in degrees of freedom allowed by the bearing arrangement 40. In some examples, it may even be desirable for actuating forces F to have a component in parallel to the primary axis P, for example so as to load plain or rolling bearings arranged between the movable part 20 and the support structure 10. Although, for illustrative purposes, the arrangement of actuating units 30 was described as moving the movable part 20 in the movement plane (e.g. translationally along the x and y axis, or rotationally about the primary axis P), in other examples the movable part 20 may be moved differently. For example, the same arrangement of actuating forces F may be used to tilt the movable part 20 relative to the support structure 10 about axes perpendicular to the primary axis P, due to appropriate movement constraints provided by the bearing arrangement 40. For example, the bearing arrangement 40 may include a plurality of flexures for guiding tilting of the movable part 20 about the axes perpendicular to the primary axis P. Examples of such bearing arrangement 40 are described in WO2022 / 029441 Al, which is herein incorporated by reference. Although the actuator assembly 2 is described herein in the context of four actuating units 30, in general the actuator assembly 2 may include fewer actuating units 30. For example, the actuator assembly 2 may include two actuating units 30, e.g. the two actuating units 30 depicted in the top left of Figure 4. The forces applied to the movable part 20 by the two actuating units 30 may be opposed by a biasing force of one or more resilient elements, such as springs. With reference to Figure 4, the two actuating units 30 in the bottom right corner may be replaced with springs applying biasing forces along the corresponding depicted arrows, for example. Arrangement of eight actuating units Figures 5a-c schematically show perspective views of different versions of an actuator assembly 2 with a total of eight actuating units 30. The eight actuating units 30 may apply actuating forces F between the movable part 20 and the support structure 10. The actuating forces F are applied to the movable part 20 relative to the support structure 10. The arrangement of actuating units 30 of Figure 5 may be used, for example, in examples in which the movable part 20 is movable relative to the support structure 10 in three translational degrees of freedom (Tx, Ty, Tz) (see Figure IE) or in two or three rotational degrees of freedom (Rx, Ry or Rx, Ry, Rz) (see Figure IC). The eight actuating units 30 may be arranged such that their actuating forces F are oriented or arranged in a manner equivalent to the orientation or arrangement of the forces applied by the eight SMA wires in the actuator assemblies disclosed in WO 2011 / 104518 Al. More specifically, the actuating forces F (e.g. when visualised as vectors at particular positions in space) are arranged on each of four sides (i.e. a first side, a second side, a third side and then a fourth side) around the primary axis P. The two actuating forces F on each side are inclined in opposite senses relative to a plane perpendicular to the primary axis P, when viewed perpendicular from the primary axis P. The four sides on which the actuating forces F are arranged extend in a loop around the primary axis P. In this example, adjacent sides are perpendicular to each other, and the sides form a square when viewed along the primary axis P, but alternatively the sides could take a different e.g. quadrilateral shape. In this example, the actuating forces F are parallel to the outer faces of the square envelope of the movable part 20 but this is not essential. Four actuating forces F, including one actuating force F on each of the sides, form a 'first' group that have a component in one direction ('upwards' or +z) and the other four actuating forces F form a 'second' group that have a component in the opposite direction ('downwards' or -z). Herein, 'up' and 'down' refer to opposite directions along the primary axis P. The actuating forces F have a symmetrical arrangement in which their magnitudes and inclination angles are the same, so that both the first group of actuating forces F and the second group of actuating forces F are each arranged with two-fold rotational symmetry about the primary axis P. As a result of this symmetrical arrangement, different combinations of the actuating forces F are capable of driving movement of the movable part 20 with multiple degrees of freedom, as follows. The first group of actuating forces F, when generated together, drive upwards (+z) movement, and the second group of actuating forces F, when generated together, drive downwards (-z) movement. Within each group, adjacent pairs of actuating forces F, when differentially generated, drive tilting about a lateral axis perpendicular to the primary axis P (Rx or Ry). Tilting in any arbitrary direction may be achieved as a linear combination of tilts about the two lateral axes. Sets of four actuating forces F, including two actuating forces F from each group, when generated together, drive movement along a lateral axis perpendicular to the primary axis P (Tx or Ty). Movement in any arbitrary direction perpendicular to the primary axis z may be achieved as a linear combination of movements along the two lateral axes. The actuator assembly 2 may have other specific arrangements of actuating units 30 to those shown in Figure 5. For example, strict symmetry is not required. Furthermore, instead of there being an up-pulling actuating unit 30 and a down-pulling actuating unit 30 on each side, there may be two up-pulling actuating units 30 on each of two opposite sides (e.g. the first and third sides) and two down-pulling actuating units 30 on the other two sides (e.g. the second and fourth sides). Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, the actuator assembly may include different types of actuating units to those described above. Examples of such actuating units include a folded SMA wire arrangement as disclosed in WO 2021 / 111131 Al, a V-shaped SMA wire with a compliant connector as disclosed in WO 2013 / 121225 Al, a scissor jack arrangement as disclosed in WO 2021 / 156458 Al, a two-stage arrangement as disclosed in WO 2021 / 111181 Al, or simply an SMA wire connected between the support structure 10 and the movable part 20. The documents referred to in the preceding sentence are each herein incorporated by reference to the maximum extent permissible by law. The actuator assembly may have any number of different types of actuating units, and may have any suitable number of actuating units of each type. Compact arrangement of four actuating units The SMA wire arrangement described in WO 2013 / 175197 Al allows the implementation of OIS by lateral movement of a lens assembly and an image sensor relative to each other. The stroke achievable by the four SMA wires may be sufficient for existing OIS applications. So, there may not be a need to amplify the stroke using the actuating units 30. However, the inventors have found that the use of actuating units 30 may be advantageous even when there is no requirement for increased stroke. The present invention relates, in particular, to using actuating units with relatively short SMA wires so as to achieve the same or similar amounts of stroke as the actuator assembly of WO 2013 / 175197 Al. Using relatively short SMA wires has the benefit of reducing power consumption of the actuator assembly, thus making the actuator assembly for efficient. In addition, the use of relatively short SMA wires in an actuating unit may allow for more a compact layout of the actuator assembly 2. Figure 6 schematically depicts an arrangement of four actuating units 30. The four actuating units 30 are arranged to apply forces to the movable part 20 as described in relation to Figure 4. The length of the SMA wire 34 of the actuating unit 30 may be relatively short. The length of the SMA wire 34 is, in particular, the length of SMA wire 34 extending between the connection elements 15, 25, e.g. the length of SMA wire 34 extending between crimps. In the embodiment of Figure 6, the length of the SMA wire 34 is less than half of the extent of a side of the actuator assembly 2, i.e. less than half of the largest extent of the actuator assembly 2 along an axis parallel to the SMA wire 34. In some embodiment, the length of the SMA wire 34 maybe less than a third of the extent of a side of the actuator assembly 2, i.e. less than a third of the largest extent of the actuator assembly 2 along an axis parallel to the SMA wire 34. So, the SMA wire 34 is shorter than an SMA wire 34 in an arrangement of SMA wires disclosed in WO 2013 / 175197 Al. The SMA wire 34 may have a length of less than 5mm, preferably less than 4mm, further preferably less than 3mm. The lateral extent of the actuator assembly 2 may be more than 10mm, or more than 15mm. The SMA wire 34 may thus be much shorter than the lateral extent of the actuator assembly 2. The actuating unit 30 may be configured so as to amplify contraction of the SMA wire 34 to a relatively greater movement of the movable part 20. The actuating unit 30 may be configured to amplify the contraction of the SMA wire 34 by an amplification factor, where the amplification factor is the ratio of i) movement of the movable part 20 (in a direction parallel to the actuating force) to ii) the amount of contraction of the SMA wire 34. The amplification factor may be equal to or similar to (i.e. within 50% of, preferably within 20% of) the ratio of i) the largest extent of the actuator assembly 2 along an axis parallel to the SMA wire 34 to ii) the length of the SMA wire 34. As such, the stroke of the actuating unit 30 may be similar to that achievable by a single SMA wire 34 arranged along the entire extent of the actuator assembly 2. The plural actuating units 30 may generally be co-planar. The actuating planes within which the plural actuating units 30 actuate (i.e. within which the body portion 31 of the plural actuating units 30 may move on actuation) may coincide. The body portion 31, force-modifying flexure 32, coupling flexure 33 and SMA wire 34 of the plural actuating units 30 may overlap when viewed perpendicularly to the primary axis P. None of the plural actuating units 30 may overlap when viewed parallel to the primary axis P. This allows the height of the actuator assembly 2 to be reduced compared to a situation in which the actuating units 30, or any components of different actuating units 30, are allowed to overlap. Furthermore, assembly of the actuator assembly 2 is easier, because (due to the lack of overlap) all actuating units 30 may be assembled concurrently in one step. The SMA wire 34, the coupling flexure 33 and the force-modifying flexure 32 may extend in similar directions. For example, the largest angle between SMA wire 34, the coupling flexure 33 and the forcemodifying flexure 32 may be less than 45 degrees, preferably less than 30 degrees, further preferably less than 20 degrees. As a result, the extent of the actuating unit 30 in a direction orthogonal to the lengths of the SMA wire 34, the coupling flexure 33 and the force-modifying flexure 32 may be reduced. The actuating unit 30 is thus compact, allowing for a more compact actuator assembly 2. As shown in Figure 6, the four actuating units 30 may be arranged with mirror symmetry. Such an arrangement of actuating units 30 is particularly preferably when the actuator assembly 2 (and in particular the movable part) has a square footprint, for example when the movable part 20 is configured to receive a lens assembly. The coupling flexures 33 of the four actuating units 30 may be equidistant from a centre of the movable part 20 of the actuator assembly 2. Figure 7 shows an embodiment in which the four actuating units 30 are not arranged with mirror symmetry. The four actuating units 30 may be arranged with two-fold rotational symmetry. Such an arrangement allows for a more compact actuator assembly when the actuating units 30 surround an asymmetrically shaped space, such as a rectangle. For example, in may be desirable to move a rectangular image sensor using the actuator assembly 2. The coupling flexures 33 of the four actuating units 30 may be equidistant from a centre of the movable part 20 of the actuator assembly 2. This ensures that the torque applied by the two pairs of opposite actuating units 30 may naturally balance, making control of the actuator assembly 2 simpler. Figure 8 shows another preferably embodiment in which the four actuating units 30 surround a rectangular space, when viewed along the primary axis. As shown, two actuating units 30 that are arranged on opposite sides of the actuator assembly 2 (in particular the top and bottom actuating units 30, i.e. the actuating units 30 arranged on the longer sides of the rectangular space) are arranged such that the SMA wires 30 face each other. The other two actuating units 30 that are arranged on opposite sides of the actuator assembly 2 (in particular the left and right actuating units 30, i.e. the actuating units 30 arranged on the shorter sides of the rectangular space) are arranged such that the coupling flexures 33 face each other. The actuating units 30 may thus be packed more compactly around the rectangular space, while keeping the coupling flexures 33 of the four actuating units 30 equidistant from the centre of the movable part 20 of the actuator assembly 2. Figure 9 shows another preferably embodiment in which the four actuating units 30 surround a rectangular space, when viewed along the primary axis. The embodiment of Figure 9 is similar to the embodiment described in relation to Figure 8, except that the actuating units 30 are arranged closer to the centre of the actuator assembly 2. The actuator assembly 2 may thus be made even more compact. Figure 10 shows another embodiment of the actuator assembly 2 with a compact arrangement of actuating units 30. The actuator assembly 2 is similar to that described in relation to Figure 6, expect that the actuating units 30 are arranged closer to the corners of the actuator assembly 2. The space in the corners of the actuator assembly 2 may thus be used more efficiently. Figures 11 to 14 schematically depict actuating units making use of a 3rd class lever arrangement. In particular, the angle between vectors representing the input force and the actuating force is greater than 100 degrees, preferably greater than 135 degrees, further preferably greater than 160 degrees. In addition, the SMA wire 34 is connected to the body portion 331 at a location that is lateral to the forcemodifying flexure 32. The three lines extending along the force-modifying flexure, along the input force and along the actuating force are concurrent lines. So, these three lines intersect at a common point. SMA The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element 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 element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. 06 11 25

Claims

1. An actuator assembly comprising:first and second parts that are movable and / or tiltable relative to each other along and / or about two orthogonal axes that are perpendicular to a primary axis; anda total of four actuating units each configured, on actuation, to apply a respective actuating force to the second part capable of moving the second part relative to the first part, wherein the four actuating units are arranged such that none of the four actuating forces are colinear and are capable of moving and / or tilting the second part relative to the first part to any position within a range of movement along and / or about the two orthogonal axes;wherein each actuating unit comprises:a body portion;a force-modifying flexure connected between the body portion and the first part;an SMA wire connected by connection elements between the body portion and the first part, wherein the SMA wire is arranged, on actuation, to apply an input force to the body portion, thereby causing the force-modifying flexure to modify the input force such that the actuating force is applied to the second part, wherein the length of the SMA wire between the connection elements is less than half of the extent of the actuator assembly in a direction parallel to the SMA wire, and wherein the four actuating units do not overlap when viewed along the primary axis.

2. An actuator assembly according to claim 1, wherein the length of the SMA wire between the connection elements is less than 5mm, preferably less than 4mm, further preferably less than 3mm.

3. An actuator assembly according to claim 1 or 2, wherein the length of the SMA wire between the connection elements is less than a third of the extent of the actuator assembly in a direction parallel to the SMA wire.

4. An actuator assembly according to any one of claims 1 to 3, wherein none of the actuating units overlap when viewed along the primary axis.

5. An actuator assembly according to any one of claims 1 to 4, wherein each actuating unit further comprises a coupling link connected between the body portion and the second part,wherein the coupling link is configured to transmit the actuating force from the body portion to the second part, andwherein the coupling link is compliant in a direction perpendicular to the actuating force.

6. An actuator assembly according to claim 5, wherein the coupling link comprises a coupling flexure.

7. An actuator assembly according to claim 6, wherein the axis through the length of the forcemodifying flexure, the axis through the length of the coupling flexure, and the axis through the length of the SMA wire all intersect at the same point.

8. An actuator assembly according to claim 6 or 7, wherein the force-modifying flexure is arranged between the SMA wire and the coupling flexure when viewed parallel to the primary axis.

9. An actuator assembly according to any one of claims 6 to 8, wherein actuating units are arranged on four sides of the actuator assembly, wherein two actuating units on opposite sides are arranged such that the SMA wires of the two actuating units face each other when viewed along the primary axis, and the other two actuating units are arranged such that the coupling flexures of the other two actuating units face each other when viewed along the primary axis.

10. An actuator assembly according to any one the preceding claims, wherein the actuating units are arranged, when viewed along the primary axis, not to be mirror symmetric about an axis perpendicular to the primary axis.

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