Actuator assembly

Abstract

An actuator assembly comprises a first part 10, a rotatable part 31 arranged to be rotatable relative to the first part, and a shape memory alloy (SMA) element 28 connected between first and second co

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 the present invention, there is provided an actuator assembly comprising a first part; a rotatable part arranged to be rotatable relative to the first part; a shape memory alloy, SMA, element connected between first and second connection parts, wherein the first connection part is coupled to the first part and the second connection part is coupled to the rotatable part so that the SMA element is arranged, on actuation, to apply an input force to the rotatable part, thereby causing the rotatable part to rotate relative to the first part; and a connector coupling configured to couple the second connection part to the rotatable part and to transmit the input force from the SMA element to the rotatable part and to allow rotation of the second connection part relative to the rotatable part. The amount of rotation of the second connection part relative to the rotatable part, for a given amount of actuation of the SMA wire, is increased compared to an actuator assembly in which the connector coupling is omitted. The amount of rotation of the SMA element relative to the second connection part, for a given amount of actuation of the SMA element, is reduced compared to an actuator assembly in which the connector coupling is omitted. On actuation of the SMA element, the rotatable part may rotate in a first sense relative to the first part and the second connection part may rotate in a second sense, opposite to the first sense, relative to the rotatable part. The rotatable part need not undergo pure rotation and may undergo simultaneous rotational and translational movement. The rotatable part may also be referred to as an intermediate part or a body portion. In some embodiments, the connector coupling is configured such that the second connection part, on actuation of the SMA element, moves translationally relative to the first part. The second connection part may move translationally in a direction along or at an acute angle to the input force. Some embodiments further comprise a bearing arrangement configured to guide translational movement of the second connection part relative to the first part. In some embodiments, the bearing arrangement comprises a plain bearing or a rolling bearing arranged to guide translational movement of the second connection part relative to the first part. In some embodiments, the second connection part comprises a main portion and a connector portion fixedly connected to the main portion, wherein the SMA element is held by the connector portion and wherein the bearing arrangement is arranged between the main portion and the first part, and wherein the main portion is elongate between first and second ends, and wherein the connector portion is arranged on the first end of the main portion and the bearing arrangement is arranged on the second end of the main portion. In some embodiments, the three lines comprising i) an imaginary line along a force exerted by the SMA element to the second connection part, ii) an imaginary line exerted by the connector coupling to the second connection part and iii) an imaginary line exerted by the bearing arrangement to the second connection part intersect, in particular at a common point. So, the three lines are concurrent lines. The bearing arrangement may comprise a plain bearing (sliding bearing) or a rolling bearing. The plain bearing or rolling bearing constrains rotation of the second connection part relative to the first part. The bearing arrangement may comprise first and second bearing surfaces that are translationally movable relative to each other. The first bearing surface may be fixed relative to the first part. The second bearing surface may be fixed relative to the second connection part. The first and second bearing surfaces that may be arranged to slidably engage. Alternatively, a bearing element, such as a ball bearing or roller bearing, may be arranged between the first and second bearing surfaces. The bearing element may roll relative to the first and / or second bearing surfaces on relative movement of the first and second bearing surfaces. The bearing arrangement may alternatively comprise a flexure bearing. The flexure bearing comprises one or more flexures arranged to guide movement of the second connection part relative to the first part. The flexure bearing constrains rotation of the second connection part relative to the first part. In some embodiments, the connector coupling comprises a coupling flexure allowing the rotation of the second connection part relative to the rotatable part. The coupling flexure may be elongate in a direction along to the input force. The coupling flexure may be compliant in a direction perpendicular to the input force. In some embodiments, a length of the coupling flexure is collinear with or at an acute angle to a force applied by the SMA element to the second connection part. The SMA element may be elongate. The force applied by the SMA element to the second connection part may be along a length of the SMA element. So, the coupling flexure may be collinear with or at an acute angle to a length of the SMA element. In some embodiments, the coupling flexure is placed under tension on actuation of the SMA element. In some embodiments, the coupling flexure overlaps with the SMA element when viewed perpendicularly to a length of the coupling flexure. The coupling flexure may be coupled to the body portion of the second connection part at a point along the length of the body portion between the first and second ends of the body portion, so generally at a point between the bearing arrangement and the connector portion. The coupling flexure may be coupled to the body portion of the second connection part at a point, relative to the connector portion, that lies in a direction along the force applied by the SMA element to the second connection element. In some embodiments, the coupling flexure is formed integrally with the rotatable part and / or with the second connection part. In some embodiments, the connector coupling comprises a rotation bearing arranged between the rotatable part and the second connection part, the rotation bearing allowing the rotation of the second connection part relative to the rotatable part about a pivot axis. The rotation bearing may comprise a first bearing surface that is fixed relative to the rotatable part and a second bearing surface fixed relative to the second connection part. The first and second bearing surfaces may be pivotable relative to each other about a pivot axis. The rotation bearing may be a pin bearing. In some embodiments, the pivot axis lies on an imaginary line along a force applied by the SMA element to the second connection part. In some embodiments, the pivot axis lies beyond an end the SMA element extending from the second connection part when viewed from the first connection part. In some embodiments, the distance between the pivot axis and an end the SMA element extending from the second connection part is within a range from 1% to 10% of a length of the SMA element. In some embodiments, the first and second connection parts comprise first and second crimps. Specifically, the connector portion of the second connection part may comprise or be a second crimp. In some embodiments, the rotatable part is movable relative to the first part within a range of movement. The SMA actuator assembly is configured to constrain movement of the rotatable part relative to the first part at any position within the range of movement when the SMA element is not actuated. The range of movement comprises the set of rotational positions (or orientations) to which the rotatable part is rotatable during normal operation of the actuator assembly, i.e. due to actuation of the SMA element. Some embodiments comprise first and second friction surfaces that are biased against each other with a normal force, thereby generating a static frictional force between the first and second friction surfaces that constrains movement of the rotatable part at any position within the range of movement when the SMA element is not actuated. In some embodiments, the SMA element is configured, on actuation, to reduce the normal force between the first and second friction surfaces, thereby reducing the static frictional force therebetween. Movement of the rotatable part at any position within the range of movement may be constrained, in particular, when the SMA element is not actuated and when acceleration of the actuator assembly is less than or equal to a hold threshold. The hold threshold may be at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2), where g is the acceleration due to Earth's gravity. The first friction surface may be fixed relative to the first part and the second friction surface may be fixed relative to the rotatable part. Alternatively, the first friction surface may be fixed relative to a second part and the second friction surface may be fixed relative to one of the first part or rotatable part. In general, the first and second friction surfaces may be fixed relative to any components of the SMA actuator assembly that undergo relative movement on actuation of the SMA element. Some embodiments comprise a bearing arrangement between the first part and the rotatable part, wherein the bearing arrangement is configured to guide rotation of the rotatable part relative to the first part about a or the respective pivot axis. Some embodiments further comprise a second part that is movable relative to the first part and one or more 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, each actuating unit comprising an intermediate part corresponding to the rotatable part, the SMA element and the connector coupling. There is thus provided an actuator assembly comprising first and second parts that are movable relative to each other; one or more 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, each actuating unit comprising: an intermediate part arranged to be rotatable relative to the first part and coupled to the second part so as to be capable of applying the respective actuating force to the second part; a shape memory alloy, SMA, element connected between first and second connection parts, wherein the first connection part is coupled to the first part and the second connection part is coupled to the intermediate part so that the SMA element is arranged, on actuation, to apply an input force to the intermediate part, thereby causing the intermediate part to rotate relative to the first part; and a connector coupling configured to couple the second connection part to the intermediate part and to transmit the input force from the SMA element to the intermediate part and to allow rotation of the second connection part relative to the intermediate part. In some embodiments, the rotatable part is arranged to rotate relative to the first part about an effective pivot axis, wherein the perpendicular distance between the effective pivot axis and the input force is smaller than the perpendicular distance between the effective pivot axis and the actuating force. The perpendicular distance is the length of an imaginary straight line that intersects the pivot axis and is orthogonal, respectively, to the input force or to the actuating force. The perpendicular distance between the pivot axis and the force applied by the SMA element to the second connection part may be smaller than the perpendicular distance between the pivot axis and the actuating force. Some embodiments comprise a bearing arrangement between the rotatable part and the second part, wherein the bearing arrangement is configured to allow rotation of the rotatable part relative to the second part about a respective pivot axis. In some embodiments, the first and second parts are translationally movable relative to each other. The first and second parts may be movable in one degree of freedom relative to each other. In some embodiments, the actuating unit comprises a force-modifying flexure arranged between the rotatable part and the first part, wherein the force-modifying flexure is arranged to guide rotation of the rotatable part relative to the first part. The actuating unit may comprise a coupling link arranged between the rotatable part and the second part, wherein the coupling link is configured to transmit the actuating force and to allow movement of the second part relative to the rotatable part in a direction that is perpendicular to the actuating force. Further aspects of the present invention are set out in the dependent claims and in the detailed description. 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 module 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; Figures 5 is a schematic perspective view of an arrangement of eight actuating units; Figure 6 is a schematic plan view of part of an actuating unit with a connector coupling in the form of a coupling flexure; Figure 7A is a plan view of an actuating unit with a connector coupling in the form of a coupling flexure; Figure 7B is a plan view of an arrangement of four of the actuating units of Figure 7A in the arrangement of Figure 4; Figure 8 is a plan view of an arrangement of four actuating units of another type in the arrangement of Figure 4; Figures 9A and 9B are views of other actuating units with a connector coupling in the form of a coupling flexure; Figure 9C is a perspective view of an arrangement of four of the actuating units of Figure 9B; Figure 10 is a view of another actuating units with a connector coupling in the form of a coupling flexure, suitable for the arrangement of actuating units of Figure 5; Figures 11A to 11D are schematic plan views of connector couplings in the form of rotation bearings; Figure 12 is a schematic plan view of an actuating unit with a connector coupling in the form of a rotation bearing; and Figures 13A to 13C are schematic plan views of parts of actuator assemblies with a connector coupling in the form of a bearing. 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. Aprimary axis Pean be defined with referenceto 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 image 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: Tx and Ty: 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 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 an intermediate part 31, herein also referred to as 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 axis R. Although the effective pivot axis R is shown in Figure 3B as being positioned in the middle of force-modifying flexure 32, the effective pivot axis R 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 connection part 15, such as a crimp 15. The other end of the SMA wire 34 is connected to the body portion 31, e.g. by a connection part 35, such as 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 of 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 axis R. 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 force-modifying 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 deamplified 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 axis R to the line on which the SMA wire 34 lies and ii) the distance De from the effective pivot axis R 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 axis R than the line on which the coupling flexure 33 lies, then the input force Fi is deamplified. 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 axis R 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 force-modifying 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 axis R, 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 forcemodifying 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. Figure 6, for example, shows a force-modifying element 32 embodied by a rotation bearing 32. The rotation bearing 32 provides the pivot axis R about which the intermediate part 31 rotates so as to modify the input force Fi and give rise to the actuating force F (not shown in Figure 6). 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 10 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 10 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 Figure 5 schematically shows a perspective view of a version 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 downpulling actuating units 30 on the other two sides (e.g. the second and fourth sides). Zero hold power The actuator assemblies 2 described herein may generally be configured such that the movable part 20 is constrained from moving relative to the support structure 10 at any position within the range of movement when the SMA elements 34 are not actuated. The movable part 20 may remain in position without needing to power the SMA elements 34. This is also referred to as zero hold power. Power consumption of the actuator assembly 2 is reduced compared to an actuator assembly 2 in which the SMA elements 34 are continuously powered so as to keep a desired position of the movable part 20 relative to the support structure 10. Such zero hold power functionality may be achieved by providing first and second friction surfaces lOf, 20f on the actuator assembly 2. Such first and second friction surfaces lOf, 20f may generally be provided between any two components of the actuator assembly 2 that are in engagement and move relative to each other. For example, first and second friction surfaces lOf, 20f may be arranged between the support structure 10 and the movable part 20, between any intermediate part 31 (such as the body portion 31) and the support structure 10, between any intermediate part 31 and the movable part 20, or between any two intermediate parts 31 that are coupled between the movable part 20 and the support structure 10. The first and second friction surfaces lOf, 20f are typically biased against each other with a normal force, for example by a biasing arrangement. The biasing arrangement may comprise any components capable of applying a biasing force between two surfaces, such as a resilient element (e.g. a spring or a flexure) or a magnetic arrangement. Biasing the friction surfaces lOf, 20f against each other with the normal force gives rise to a static frictional force between the friction surfaces lOf, 20f. The magnitude of the static frictional force is sufficient to constrain movement of the movable part 20 relative to the support structure 10 when the SMA elements 34 are not actuated. The normal force may be reduced on actuation of the SMA elements 34, for example when both SMA elements 34 are actuated equally. For example, the SMA elements 34 may cause a force to be applied with a normal force component that opposes the normal force and with a lateral force component that effects movement of the movable part 20. The first and second friction surfaces lOf, 20f may remain engaged as the normal force is reduced, or the first and second friction surfaces lOf, 20f may disengage (e.g. the movable part 20 may lift off the support structure 10) so as to effectively reduce the normal force to zero. The resistance to movement of the movable part 20 on differential actuation of the SMA elements 34 may thus be reduced, such that the SMA elements 34 may more effectively move the movable part 20 and / or a larger normal force (and thereby larger frictional force) may be applied when the SMA elements 34 are not actuated. Alternatively, the normal force may remain constant (or even increase) on actuation of the SMA elements 34. WO 2020 / 120997 Al, WO 2023 / 094813 Al and WO 2023 / 084251 Al, which are herein incorporated by reference, describe various ways of arranging the first and second friction surfaces lOf, 20f for achieving zero hold power. Some other embodiments implementing zero hold power do not make use of friction surfaces. Instead, hysteretic properties of the SMA elements 34 themselves may be used to hold the movable part 20 in position when the SMA elements 34 are not actuated. The SMA elements 34 may act as super-elastic SMA elements, for example by having a phase transition temperature below the normal operating range of the actuator assembly 2. WO 2023 / 118880 Al, incorporated herein by reference, provides examples of super-elastic SMA elements that may be implemented in the actuator assembly 2 for providing zero hold power. Movement of the movable part 20 relative to the support structure 10 at any position within the range of movement may be constrained, in particular, when the SMA elements 34 are not actuated and when acceleration of the actuator assembly 2 is less than or equal to a hold threshold. The hold threshold may be at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2), where g is the acceleration due to Earth's gravity. Connector coupling Actuator assemblies 2 such as those described with reference to Figures 3A-C make use of a rotatable intermediate part 31 in the form of a body portion 31 to transfer a force from an SMA element 34 to the movable part 20. The stroke or force acting on the movable part 20 due to actuation of the SMA element 34 may thus be amplified and / or redirected. On actuation of the SMA element 34, the intermediate part 31 rotates. The angle between the length of the SMA element 34 and the second connection part 35, herein referred to as the wire exit angle, may change on such rotation of the intermediate part 31. Relatively large and repeated changes of the wire exit angle can result in increased localized stresses in the SMA element 34 at a portion proximal to the second connection part 35, thereby risking fatigue and damage to the SMA element 34 over time. According to the present invention, a connector coupling 38 is provided between the intermediate part 31 and the second connection part 35. The connector coupling 38 couples the second connection part 35 to the intermediate part 31. The connector coupling 38 thereby transmits the input force Fi from the SMA element 34 to the intermediate part 31. The connector coupling 38 allows rotation of the second connection part 35 element relative to the intermediate part 31. Thereby, rotation of the intermediate part 31 is effectively decoupled from rotation of the SMA element 34. Change in the wire exit angle on actuation of the SMA element 34 may thus be avoided or at least reduced. Figures 6 to 12 show various embodiments of the connector coupling 38. The connector coupling 38 may be embodied by a coupling flexure 38a, as shown in Figures 6 to 10, or by a rotation bearing 38b, as shown in Figures 11 and 12. Coupling flexure as connector coupling Figure 6 schematically shows a portion of an actuating unit 30, in particular part of the intermediate part 31, which is arranged to rotate about the pivot axis R, and part of the SMA element 34. The portion of the actuating unit 30 is shown at two rotational positions of the intermediate part 31, illustrating the effect of the connector coupling 38 on reducing the change in wire exit angle. The connector coupling 38 is embodied by a coupling flexure 38a. The coupling flexure 38a is elongate between two ends. One end of the coupling flexure 38a is connected to the second connection part 35. The other end of the coupling flexure 38a is connected to the intermediate part 31. The coupling flexure 38a transmits a force, specifically the input force Fi from the SMA element 34, along the length of the coupling flexure 38a. The input force Fi is thus applied by the SMA element 34 to the intermediate part 31 via the coupling flexure 38a. The coupling flexure 38a flexes in a direction perpendicular to the length thereof. On actuation, the SMA element 34 applies a bending moment to the coupling flexure 38a to thereby urge the coupling flexure 38a to remain in alignment with the length of the SMA element 34. The coupling flexure 38a thereby allows rotation of the second connection part 35 relative to the intermediate part 31. As shown in Figure 6, for example, the wire exit angle may remain substantially constant even during relatively large rotation of the intermediate part 31 relative to the support structure 10. The orientation of the second connection part 35 is substantially the same at both rotational positions of the intermediate part 31 shown in Figure 6. So, the second connection element 35 effectively moves translationally (in the top-left direction in Figure 6) on actuation of the SMA element 34 and on the resulting rotation of the intermediate part 31. Another way to put this is that the coupling flexure 38a allows the second connection element 35 to rotate relative to the intermediate part 31 in a sense that is opposite to the sense of rotation of the intermediate part 31 relative to the support structure 10. With particular reference to Figure 6, as the intermediate part 31 rotates in one sense (e.g. anti-clockwise) on actuation of the SMA element 34, the second connection element 35 rotates in the opposite sense (e.g. clockwise) relative to the intermediate part 31. These rotations in the opposite senses may cancel out such that the second connection element 35 does not rotate relative to the support structure 10, or at least rotates less relative to the support structure 10 than the intermediate part 31. The coupling flexure 38a, in particular a portion thereof that is proximal to the second connection part 35, aligns with the length of the SMA element 34 on rotation of the intermediate part 31. The coupling flexure 38a, or at least a portion thereof, is thus colinear with the length of the SMA element 34. Figure 7A schematically shows another actuating unit 30 with a connector coupling 38 embodied by a coupling flexure 38a. The actuator assembly 2 is generally as described in relation to the actuating unit of Figure 3B, except for the addition of the coupling flexure 38a, a translation bearing 37 and the provision of a relatively larger second connection part 35. The intermediate part 31 rotates about the effective pivot axis R to thereby effect movement of the movable part 20 (not shown in Figure 7 A, but coupled to the coupling flexure 33) relative to the support structure 10 (not shown in Figure 7A, but coupled to the first connection part 15, the foot portion 33 and the translation bearing 37) in the manner described in relation to Figure 3B. The second connection part 35 is embodied as an elongate part comprising both a connector portion 35a and an elongate main portion 35b. The main portion 35b is elongate in a direction along the length of the SMA element 34. The connector portion 35a is arranged on one end of the main portion 35b and holds the SMA element 34. The connector portion 35a may mechanically and electrically connect to the SMA element 34. The connector portion 35a may be a crimp, for example. A translation bearing 37 is arranged on the other end of the main portion 35b. The translation bearing 37 is arranged between the second connection part 35, in particular the main portion 35b thereof, and the support structure 10. The translation bearing 37 guides translational movement of the second connection part 35 relative to the support structure 10. The translational movement may be predominantly in the direction of the length of the SMA element 35, so the SMA element 34 and the translational movement may be at an acute angle or parallel relative to each other. Provision of the translation bearing 37 may reduce the risk of inadvertent rotation of the second connection part 35 on actuation of the SMA element 34 compared to the embodiment of Figure 6, in which no such translation bearing 37 is provided. The SMA element 34 is arranged, on actuation, to load the translation bearing 37. As shown in Figure 7A, the SMA wire 34 is offset from the coupling flexure 38a in a direction perpendicular to the SMA wire 34. The input force Fi exerted by the SMA wire 34 and the resultant force from the coupling flexure 38a on the second connection part 35 apply a force couple or torque urging the second connection part 35 to rotate, in particular in the anti-clockwise sense in Figure 7A. The translation bearing 37 is arranged to counteract that force couple so as to constrain the second connection part 35 from rotating. The translation bearing 37 is thereby loaded on actuation of the SMA element 34, i.e. bearing surfaces of the translation bearing 37 are biased against each other on actuation of the SMA element 34. As shown in Figure 7A, the translation bearing 37 is arranged laterally to a point of intersection between the SMA element 34 and a line 381 along the coupling flexure 38a. Generally, the translation bearing 37 is arranged such that a force exerted by the translation bearing 37 on the second connection part 35, the input force Fi exerted by the SMA element 34 on the second connection part 35 and a force exerted by the coupling flexure 38a (or, in general, by the connector coupling 38) intersect at a common point. The force exerted by the coupling flexure 38a is a reaction force that results from the input force Fi that is actively applied by the SMA element 34. The three forces are concurrent, i.e. lines along the three forces are concurrent. The main portion 35b is elongate to allow the translation bearing 37 to be located so as to achieve the three concurrent forces. The three forces intersect when the movable part 20 is at a particular position of the movable part 20 relative to the support structure 10. The particular position may be a starting position or central position of the movable part 20, for example. In embodiments with multiple actuating units 30, for example at least two actuating units 30 comprising at least two opposing SMA wires 34 and corresponding second connection parts 35, the three forces may intersect for both of the at least two second connection parts 35 at the same position of the movable part 20 relative to the support structure 10. In the particular embodiment of Figure 7A, for example, the SMA element 34 and the coupling flexure 38a are at an acute angle relative to each other. The intersection point between the length of the SMA element 34 and a line along the coupling flexure 38a is thus relatively far away from the connector portion 35a, such that the translation bearing 37 is also located relatively far away from the connector portion 35a. The main portion 35b is therefore elongate in the depicted embodiment. The translation bearing 37 is a plain bearing in the embodiment of Figure 7A. The plain bearing comprises a first bearing surface on the second connection part 35 and a second bearing surface on the support structure 10. The bearing surfaces are in slidable engagement with each other, such that the bearing surfaces slide relative to each other to guide the translational movement of the second connection part 35 relative to the support structure 10. In general, other types of bearings may be used to guide the translational movement. The translation bearing 37 may, for example, be a rolling bearing that comprises a rolling element (such as a ball bearing) between the bearing surfaces or be a flexure bearing comprising one or more flexures arranged to guide the translational movement. Figure 9B shows an example of such a translation bearing 37 in the form of a flexure bearing. As further shown in Figure 7A, the actuating unit 30 may be configured such that the forces exerted on the intermediate part 31 intersect at a common point. In particular, a force exerted by the coupling flexure 38a (or, in general, by the connector coupling 38) on the intermediate part 31, a force exerted by the force-modifying flexure 32 on the intermediate part 31 and a force exerted by the coupling flexure 33 on the intermediate part intersect at a common point. In Figure 7 A, the coupling flexure 38a is arranged to overlap with the SMA element 34 when viewed in a direction perpendicular to the length of the SMA element 34. The coupling flexure 38a is connected to the main portion 35b of the second connection part 35 at a location that is between the connector portion 35a and the translation bearing 37. The coupling flexure 38a is connected to the second connection part 35 at a location that is, relative to the location of the connection of the SMA element 34 to the second connection part, in a direction along the input force Fi. The SMA element 34 and / or the coupling flexure 38a may thus be longer (while retaining a compact footprint) compared to a situation in which such overlap is not allowed. In the embodiments of Figures 6 and 7A, the coupling flexure 38a is placed in tension on actuation of the SMA element 34. The risk of buckling of the coupling flexure 38a may therefore be reduced. The coupling flexure 38a is arranged to be colinear with or at an acute angle to the length of the SMA element 34 so as to ensure reliable tensioning of the coupling flexure 38a, for example. In general, the coupling flexure 38a may alternatively be arranged to be under compression on actuation of the SMA element 34. In the embodiments of Figures 6 and 7A, the coupling flexure 38a is integrally formed with the intermediate part 31 and the second connection element 35, thereby reducing the number of parts of the actuator assembly 2 and making assembly of the actuator assembly 2 simpler. In general, the coupling flexure 38a may be formed integrally with one or both of the intermediate part 31 and the second connection element 35, or may be formed as a separate component that is attached, for example by welding or adhesive, to the intermediate part 31 and the second connection element 35. Figure 7B is a plan view of an arrangement of four actuating units 30 that are in the arrangement described in relation to Figure 4. The four actuating units 30 may thus effect movement of the movable part 20 relative to the support structure 10 along two orthogonal axes (in the plane of Figure 7B) and / or rotation of the movable part 20 relative to the support structure 10 about an axis that is perpendicular to the two orthogonal axes (out of the plane of Figure 7B). Zero hold power functionality may be achieved by biasing the movable part 20 against the support structure 10 in a direction into the page of Figure 7B, for example. The actuating units 30 shown in Figures 7A and 7B make use of a first-class lever system, in which the coupling flexure 38a (or generally the connector coupling 38) is arranged on an opposite side of the force-modifying flexure 32 (or generally the rotation axis R) compared to the coupling link 33. Figure 8 shows an alternative type of actuating unit 30 making use of a third-class lever system, in which the coupling flexure 38a (or generally the connector coupling 38) is arranged on the same side of the force-modifying flexure 32 (or generally the rotation axis R) as the coupling link 33. The connector coupling 38 may generally be implemented in actuating units 30 making use of any type of lever arrangement. The actuator assembly 2 of Figure 8 corresponds generally to the actuator assembly 2 described in relation to Figure 7B, except for the provision of a different type of actuating unit 30 making use of the third-class lever system. Figure 8 further shows another advantage of providing a relatively large main portion 35b as part of the second connection part 35. The main portion 35b may be used to constrain out-of-plane movement of the second connection part 35 by providing a set of three out-of-plane bearings 41 between the main portion 35b and the support structure 10. The three out-of-plane bearings 41 may span a triangle when viewed in a direction perpendicular to the plane of the actuating unit 30. The main portion 35b may comprise a protrusion 41p extending laterally away from the elongate main portion 35b within the plane of the actuating unit 30 so as to allow the provision of the three out-of-plane bearings 41. The three out-of-plane bearings 41 and protrusion 41p are labelled in Figure 8 for only one of the actuating units 30, but in general all of the actuating units 30 may comprise these features. Figures 9A and 9B show further embodiments of actuating units 30 comprising the coupling flexure 38a. The actuating units 30 are generally as described in relation to Figure 7A. The actuating unit 30 of Figure 9A comprises a translation bearing 37 in the form of a plain bearing. The actuating unit 30 of Figure 9B comprises a translation bearing 37 in the form of a flexure bearing. The flexure bearing may comprise two flexures guiding the movement of the second connection part 35 along an axis. The flexures are connected between the support structure 10 and the main portion 35b of the second connection part 35. As also shown in Figure 9B, one of the flexures may be coupled to the foot portion 36 that is fixed to the support structure 10. The actuating units 30 shown in Figures 9A and 9B may be particularly suitable for incorporation in an actuator assembly 2 driving Tz movement of the movable part 20 along the primary axis P, for example for the variation of Figure ID, or for driving Rx and Ry rotation of the movable part 20 about the x and y axes, example for the variation of Figure IC. Figure 9C shows an actuator assembly 2 comprising four actuating units 30 for implementing such Tz and / or Rx and Ry movement. Figure 10 shows a further embodiment of an actuating unit 30 comprising the coupling flexure 38a. The actuating unit 30 is generally as described in relation to Figure 7A. The actuating unit 30 of Figure 10 may be particularly useful for an arrangement of eight actuating units 30 as described in relation to Figure 5. Rotation bearing as connector coupling Figures 11A to 11D show various embodiments of the connector coupling 38 as a rotation bearing 38b. The rotation bearing 38b allows rotation of the second connection part 35 relative to the intermediate part 31. The rotation bearing 38b of Figures 11A to 11D is specifically a pin bearing. The pin bearing comprises a pin protruding from the intermediate part 31 (in a direction into or out of the page) and a complementary hole formed in the second connection part 35 for receiving the pin. In general, the rotation bearing 38b may be formed by any other mechanism suitable for guiding rotation of the second connection part 35 relative to the intermediate part 31, for example including rolling bearing elements (such as ball bearings) or including an arrangement of flexures. Figure 11A shows a rotation bearing 38b between the second connection part 35 and the intermediate part 31 at the location of the connection point between the SMA element 34 and the second connection part 35. So, the rotation bearing 38b is located where the SMA element 34 extends from the second connection part 35. The rotation bearing 38b may be offset from the SMA element 34 in a direction perpendicular to the length of the SMA wire 34, in particular into or out of the page in Figure 11A. The rotation bearing 38b may be located away from the connection point between the SMA element 34 and the second connection part 35, as shown in Figures 11B to 11D, so as to make manufacture of the rotation bearing 38b simpler. Figure 11B shows the rotation bearing 38b arranged in-line (or at least close to in-line) with the length of the SMA element 34, or more generally in-line (or at least close to in-line) with the force exerted by the SMA element 34 on the second connection part 31. The rotation bearing 38b is further arranged beyond the connection point of the SMA element 34 on the second connection part 31. The rotation bearing 38b allows the second connection part 35 to remain aligned with the SMA element 34 on rotation of the intermediate part 31. The force exerted by the SMA element 34 on the second connection part 35 and the force exerted by the rotation bearing 38b on the second connection part 35 may be directed in opposite directions away from each other along a common line. The SMA element 34, on actuation, urges the second connection part 35 to rotate relative to the intermediate part 31 so as to remain in-line with the length of the SMA element 34. Due to the relatively high force achievable by the SMA element 34 on actuation, the separation between the rotation bearing 38b and the connection point of the SMA element 34 to the second connection part 35 may be relatively small, while still urging the rotation of the second connection part 35 to remain in-line with the SMA element 34. The distance between the rotation bearing 38b (in particular the pivot axis R provided by the rotation bearing 38b) and the connection point of the SMA element 34 to the second connection part 35 may be less than 20% of the length of the SMA element 34, for example less than 10% of the length of the SMA element 34. The distance may be greater than 1% of the length of the SMA element 34. Figure 11C shows the rotation bearing 38b arranged offset from a line along the length of the SMA element 34, or more generally offset from a line along the force exerted by the SMA element 34 on the second connection part 31. The rotation bearing 38b is further arranged beyond the connection point of the SMA element 34 on the second connection part 35. The force exerted by the SMA element 34 on the second connection part 35 and the force exerted by the rotation bearing 38b on the second connection part 35 may be directed in opposite directions away from each other, along lines that are offset from each other. The two forces thus may apply a force couple or torque to the second connection part 35. Figure 11D shows the rotation bearing 38b arranged in-line with the length of the SMA element 34, or more generally in-line with the force exerted by the SMA element 34 on the second connection part 31. The rotation bearing 38b is further arranged before the connection point of the SMA element 34 on the second connection part 31. The force exerted by the SMA element 34 on the second connection part 35 and the force exerted by the rotation bearing 38b on the second connection part 35 may be directed in opposite directions towards each other along a common line. In the embodiments of Figures 11C and 11D, an additional constraint may be provided to constrain rotation of the second connection part 35 relative to the SMA element 31. The additional constraint may take the form of a translation bearing 37 as described in relation to Figure 7A, for example. In this regard, it is noted that the forces acting on the second connection part 35 of Figure 11C are similar to those acting on the second connection part 35 of Figure 7A, and so a similar arrangement of a bearing 37 may be used to counteract the force couple applied by the SMA element 34 and rotation bearing 38b on the second connection part 35. Figure 12 shows an exemplary embodiment of an actuating unit 30 with the connector coupling 38 formed as a rotation bearing 38b. The actuating unit 30 may generally function in a manner similar to the actuator assembly 30 described with reference to Figure 3B. The force-modifying element 32 is embodied by a rotation bearing 32, such as a pin bearing. The intermediate part 31 rotates about the pivot axis R provided by the rotation bearing 32. The actuator assembly 2 of Figure 12 is similar to that of Figure 7A, except that the connector coupling 38 is embodied by a rotation bearing 38b, the translation bearing 37 is embodied by a flexure 37, and the coupling link 33 is embodied by a simple plain bearing. The rotation bearing 38b is offset from the length of the SMA element 34 in the manner described in relation to Figure 11C. The offset between the forces exerted by the rotation bearing 38b and by the SMA element 34 on the second connection part 35 gives rise to a force couple urging rotation of the second connection part 35. The translation bearing 37, here embodied as a flexure 37, counteracts the force couple so as to constrain rotation of the second connection part 35. The second connection part 35 may thus move translationally on actuation of the SMA element 34, such that the angle between the second connection part 35 and the SMA element 34 does not undergo large changes. With particular reference to Figure 12, the SMA element 34 is connected to the second connection part 35 that is coupled via the rotation bearing 38b to the intermediate part 31. The SMA element 34 and rotation bearing 38b apply a force couple to the second connection part 35, in particular in a clockwise sense in Figure 12. The force couple is resisted by a flexure 37 connected between the second connection part 35 and the support structure 10. The flexure 37 is placed under tension by the force couple acting on the second connection part 35. The flexure 37 thus acts to constrain rotation of the second connection part 35 relative to the support structure 10 and to guide translational movement of the second connection part 35 relative to the support structure 10. As described with reference to Figure 7A, the second connection part 35 is embodied as an elongate part comprising both a connector portion 35a and an elongate main portion 35b. The elongate main portion 35 extends in a direction parallel to the length of the SMA element 34. Generally, the flexure 37 is arranged such that a force exerted by the flexure 37 on the second connection part 35 (so preferably a length of the flexure), the input force Fi exerted by the SMA element 34 on the second connection part 35 (so preferably a length of the SMA element 34) and a force exerted by the rotation bearing 38b (or, in general, by the connector coupling 38) intersect at a common point. The three forces intersect at at least one position (such as a starting or central position) of the movable part 20 relative to the support structure 10. The three forces may intersect for both of the second connection parts 35 at the same position of the movable part 20 relative to the support structure 10. The rotation bearing 38b shown in Figure 12 comprises a rolling bearing element arranged in grooves on the second connection part 35 and the intermediate part 31. The SMA element 34, on actuation, loads the rotation bearing 38b by biasing the second connection part 35 against the rolling bearing element. In general, any other suitable rotation bearing 34b may be used, such as a pin bearing. Such a pin bearing comprises a pin or other protrusion extending from the second connection part 35 in a direction that is parallel to the effective pivot axis R. The intermediate part 31 may comprise a corresponding wedge-shaped recession configured to engage with the pin. The wedge-shaped recession comprises two flat surfaces that are angled relative to each other, for example at an angle of 90 degrees. The pin is in contact with both of the two flat surfaces. The SMA element 34, on actuation, may load the rotation bearing 38b by biasing the pin against the surfaces of the wedge-shaped recession. Figures 13A to 13C show further embodiments of an actuator assembly 2 comprising an actuating unit 30 with a connector coupling 38 in the form of a rotation bearing 38b. The actuator assembly 2 comprises the support structure 10 and the movable part 20. The bearing 40 guides movement of the movable part 20 relative to the support structure 10 and is embodied as a rolling bearing (in particular a ball bearing) in Figures 13A and 13C and as a plain bearing in Figure 13B. The movable part 20 may move translationally in one degree of freedom relative to the support structure 10. The SMA wire 34 is connected at one end to the support structure 10 via a first connection part 15 (not shown in Figures 13A to 13C). The other end of the SMA wire 34 is connected to a second connection part 35. A translation bearing 37 is arranged between the second connection part 35 and the support structure 10. The translation bearing 37 is embodied by a rolling bearing (in particular a ball bearing) in Figures 13A and 13C and by a plain bearing in Figure 13B. The translation bearing 37 constrains rotation of the second connection part 35 such that the angle between the SMA element 34 and the second connection part 35 does not change significantly on actuation of the SMA element 34. The second connection part 35 is coupled to the intermediate part 31 via the connector coupling 38. The connector coupling 38 is embodied by a rotation bearing 38b. The rotation bearing 38b may be a pin bearing similar to that described with reference to Figures 11 or 12, for example. The rotation bearing 38b may alternatively comprise a rolling contact bearing, with a rolling bearing element between bearing surfaces. In Figures 13A and 13C, the intermediate part 31 serves to redirect movement in a first direction generated by the SMA element 34 (acting in the horizontal direction in Figure 13A) to movement in a second direction (in a vertical direction in Figure 13A) that is perpendicular to the first direction. In Figure 13B, the first and second directions are at an acute angle relative to each other. In either case, the intermediate part 31 may rotate relative to the support structure 10 on actuation of the SMA element 31 about a rotation axis R. The rotation axis R need not lie on the intermediate part 31, as shown in Figures 13A to C. The rotation axis R is shown schematically only and may not be in the exact location indicated in Figures 13A to C. The connection point of the SMA element 34 to the second connection part 35 is decoupled from the rotation of the intermediate part 31 by the rotation bearing 38b. The intermediate part 31 of Figures 13A to 13C undergoes both rotation and translation on actuation of the SMA element 34, so the intermediate part 31 does not undergo purely rotational movement. Modifications and Alternatives The embodiments of Figures 6 to 13 each comprise an intermediate part 31 that is rotatable relative to the support structure 10. The intermediate part 31 is described as being intermediate to a movable part 20, such that a coupling between the intermediate part 31 effects movement of the movable part 20 on movement of the intermediate part 31. In general, however, the desired movement may be rotation of the intermediate part 31 and the movable part 20 may be omitted from the actuator assembly 2. The intermediate part 31 therefore need not be intermediate, and may also be referred to more generally as a rotatable part 31 that is rotatable relative to the support structure 10. The present invention has been described in connection with SMA wires 30. The term '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. The foregoing has described some embodiments of the present invention, but the present invention is not limited to these embodiments. The scope of the invention is defined in the appended claims.

Claims

1. An actuator assembly comprisinga first part;a rotatable part arranged to be rotatable relative to the first part;a shape memory alloy, SMA, element connected between first and second connection parts, wherein the first connection part is coupled to the first part and the second connection part is coupled to the rotatable part so that the SMA element is arranged, on actuation, to apply an input force to the rotatable part, thereby causing the rotatable part to rotate relative to the first part; and a connector coupling configured to couple the second connection part to the rotatable part and to transmit the input force from the SMA element to the rotatable part and to allow rotation of the second connection part relative to the rotatable part.

2. An actuator assembly according to claim 1, wherein the connector coupling is configured such that the second connection part, on actuation of the SMA element, moves translationally relative to the first part.

3. An actuator assembly according to claim 1 or 2, further comprising a bearing arrangement configured to guide translational movement of the second connection part relative to the first part.

4. An actuator assembly according to claim 3, wherein the bearing arrangement comprises a plain bearing or a rolling bearing arranged to guide translational movement of the second connection part relative to the first part.

5. An actuator assembly according to claim 3 or 4, wherein the second connection part comprises a main portion and a connector portion fixedly connected to the main portion, wherein the SMA element is held by the connector portion and wherein the bearing arrangement is arranged between the main portion and the first part, and wherein the main portion is elongate between first and second ends, and wherein the connector portion is arranged on the first end of the main portion and the bearing arrangement is arranged on the second end of the main portion.

6. An actuator assembly according to any one of claims 3 to 6, wherein the three lines comprising i) an imaginary line along a force exerted by the SMA element to the second connection part, ii) an imaginary line exerted by the connector coupling to the second connection part and iii) animaginary line exerted by the bearing arrangement to the second connection part intersect at a common point.

7. An actuator assembly according to any one of the preceding claims, wherein the connector coupling comprises a coupling flexure allowing the rotation of the second connection part relative to the rotatable part.

8. An actuator assembly according to claim 7, wherein a length of the coupling flexure is collinear with or at an acute angle to a force applied by the SMA element to the second connection part.

9. An actuator assembly according to claim 7 or 8, wherein the coupling flexure is placed under tension on actuation of the SMA element.

10. An actuator assembly according to any one of claims 7 to 9, wherein the coupling flexure overlaps with the SMA element when viewed perpendicularly to a length of the coupling flexure.

11. An actuator assembly according to any one of claims 7 to 10, wherein the coupling flexure is formed integrally with the rotatable part and / or with the second connection part.

12. An actuator assembly according to any one of claims 1 to 6, wherein the connector coupling comprises a rotation bearing arranged between the rotatable part and the second connection part, the rotation bearing allowing the rotation of the second connection part relative to the rotatable part about a pivot axis.

13. An actuator assembly according to claim 12, wherein the pivot axis lies on an imaginary line along a force applied by the SMA element to the second connection part.

14. An actuator assembly according to claim 12 or 13, wherein the pivot axis lies beyond an end the SMA element extending from the second connection part when viewed from the first connection part.

15. An actuator assembly according to any one of claims 12 to 14, wherein the distance between the pivot axis and an end the SMA element extending from the second connection part is within a range from 1% to 10% of a length of the SMA element.

16. An actuator assembly according to any one of the preceding claims, wherein the first and second connection parts comprise first and second crimps.

17. An actuator assembly according to any one of the preceding claims, wherein the rotatable part is movable relative to the first part within a range of movement, and wherein the SMA actuator assembly is configured to constrain movement of the rotatable part relative to the first part at any position within the range of movement when the SMA element is not actuated.

18. An actuator assembly according to claim 17, comprising first and second friction surfaces that are biased against each other with a normal force, thereby generating a static frictional force between the first and second friction surfaces that constrains movement of the rotatable part at any position within the range of movement when the SMA element is not actuated.

19. An actuator assembly according to claim 18, wherein the SMA element is configured, on actuation, to reduce the normal force between the first and second friction surfaces, thereby reducing the static frictional force therebetween.

20. An actuator assembly according to any one of the preceding claims, comprising a bearing arrangement between the first part and the rotatable part, wherein the bearing arrangement is configured to guide rotation of the rotatable part relative to the first part about a or the respective pivot axis.

21. An actuator assembly according to any one of claim 1 to 19, comprising a force-modifying flexure arranged between the rotatable part and the first part, wherein the force-modifying flexure is arranged to guide rotation of the rotatable part relative to the first part.

22. An actuator assembly according to any one of the preceding claims, further comprising a second part that is movable relative to the first part; andone or more 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, each actuatingunit comprising an intermediate part corresponding to the rotatable part, the SMA element and the connector coupling.

23. An actuator assembly according to claim 22, wherein the rotatable part is arranged to rotate relative to the first part about an effective pivot axis, wherein the perpendicular distance between the effective pivot axis and the input force is smaller than the perpendicular distance between the effective pivot axis and the actuating force.

24. An actuator assembly according to claim 22 or 23, further comprising a bearing arrangement between the intermediate part and the second part, wherein the bearing arrangement is configured to allow rotation of the intermediate part relative to the second part about a respective pivot axis.

25. An actuator assembly according to any one of the preceding claims, wherein the first and second parts are translationally movable relative to each other.

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