Actuator assembly

Abstract

An actuator assembly comprises a first support part 10 and a second moveable part 20 that is movable from a reference (e.g. central) position. A biasing force urges the second part to the reference p

Description

Field The present application generally relates to an actuator assembly, for example an actuator assembly comprising shape memory alloy elements as actuators. Background There are a variety of apparatuses in which it is desired to control movement of a movable component. Shape memory alloy (SMA) elements (such as SMA wires) may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA actuator required to apply a given force to the movable element can be relatively small. The movable component is typically used to move an external load. The external load may apply a load biasing force urging the movable component to a central position. The presence of the load biasing force may affect the performance of actuation of the SMA elements, in particular at the edges of stroke where such load biasing force is typically largest. It may be desirable to improve the performance of an actuator assembly experiencing a load biasing force. Summary According to the present invention, there is provided an actuator assembly comprising a first part; a second part that is movable relative to the first part from a reference position, wherein a biasing force urges the second part to the reference position relative to the first part; and one or more shape memory alloy, SMA, elements configured, on actuation, to cause one or more actuation forces to be applied to the second part for moving the second part relative to the first part within a range of movement, wherein each SMA element is configured such that, when the SMA element on actuation effects movement of the second part away from the reference position, the ratio of the change in position of the second part to the change in actuation amount of the SMA element decreases with increasing displacement of the second part from the reference position. The ratio of the change in position of the second part to the change in actuation amount of the SMA element may also be referred to as stroke gearing. Reducing stroke gearing of the actuating SMA element may allow the actuating SMA element to oppose the biasing force more effectively. A reduction of stroke gearing of the actuating SMA element may lead to an increase in force gearing of the actuating SMA element, where force gearing is the ratio of the actuating force exerted on the second part to the input force provided by the SMA element. Such an increase in force gearing may more effectively overcome the biasing force. The variation in tension in the SMA elements due to the biasing forces as the second part moves within the range of movement may be reduced by the provision of stroke gearing reduction, thereby improving the performance of the actuator assembly. The actuating force is the component of the force on the second part that is due to actuation of the SMA element and that acts in a direction parallel to instantaneous movement of the second part. The input force is the force provided by the SMA element, for example tension in the SMA element. The SMA element may be elongate. The SMA element may be an SMA wire. The actuation amount of the SMA element may be the change in length of the SMA element on actuation of the SMA element. Actuation of the SMA element may be contraction of the SMA element. The position of the second part relative to the first part herein refers to both translational and rotational position of the second part, and so includes the orientation or pose of the second part. The reference position may correspond to a reference orientation or reference pose of the second part. Displacement of the second part from the reference position may include rotation of the second part from the reference orientation or reference pose. The range of movement is the set of positions that can be reached by the second part during normal operation of the actuator assembly, i.e. due to actuation of the SMA elements. The biasing force may urge the second part to the reference position relative to the first part when the second part is positioned relative to the first part within the range of movement and at positions that are located on opposite sides from the reference position. When the second part is at the reference position, the biasing force may be zero. The biasing force may urge the second part to the reference position relative to the first part when the second part is positioned relative to the first part within the range of movement and at any position other than the reference position. The biasing force may be the component of an overall spring force acting on the second part that is in a direction parallel to instantaneous movement of the second part effected by the biasing force. In some embodiments, the ratio of the maximum magnitude of the biasing force to the weight of the second part is greater than 2, preferably greater than 5. The biasing force may be greater than 50% of the sum of all forces (e.g. gravitational, frictional and spring / biasing forces) acting on the second part. The biasing force may not include a contribution due to gravitational or a frictional forces. The maximum magnitude of the biasing force may correspond to the largest magnitude of the biasing force at any position of the second part relative to the first part within a range of movement. The magnitude of the biasing force may change with changing position of the second part. The magnitude of the biasing force may be zero when the second part is at the reference position. The magnitude of the biasing force may be largest or maximum when the second part is at a limit or edge of the range of movement of the second part, for example. The weight of the second part corresponds to the average weight on Earth, so to the product of the mass of the second part and Earth's acceleration due to gravity (9.81 m / s2). The weight of the second part includes the weight of any components movable with the second part, for example when the external load is coupled to the second part. The weight of the second part is half of the variation of the gravitational force acting on the second part on changing the orientation of the actuator assembly relative to Earth by 180 degrees. In some embodiments, the maximum magnitude of the biasing force is in the range from 50% to 95% of the maximum actuation force applied by the SMA elements to the second part. The actuation force is the component of the force applied to the second part due to actuation of the SMA elements and acting a direction parallel to the instantaneous movement of the second part. The maximum actuation force may be generated by a stress in the SMA elements in the range from 100 to 500 MPa. Limiting the stress in the SMA elements reduces the risk of damage or failure of the SMA elements. In some embodiments, the biasing force increases with increasing displacement of the second part from the reference position. The biasing force may increase linearly or may increase non-linearly. In some embodiments, the one or more SMA elements are configured such that the rate of decrease of the ratio of the change in position of the second part to the change in actuation amount of the SMA element matches the rate of increase of the biasing force with displacement of the second part from the reference position. The rate of decrease and the rate of increase may be substantially equal, for example differ by less than 10%. Put another way, the rate of force gearing increase of the one or more SMA elements may match the rate of the biasing force increase. In some embodiments, the one or more SMA elements are configured such that the ratio of the change in position of the second part to the change in actuation amount of the SMA element is greater than 1, preferably greater than 2. So, the stroke of the SMA elements may be amplified so as to achieve stroke amplification. In some embodiments, the one or more SMA elements comprise two SMA elements configured, on actuation, to move the second part in opposite directions relative to the first part. The two SMA elements may apply actuation forces to the second part that oppose each other. In some embodiments, each of the two SMA elements is configured such that with increasing displacement of the second part from the reference position, when the SMA element on actuation effects movement of the second part away from the reference position, i) the ratio of the change in position of the second part to the change in actuation amount of the SMA element decreases and ii) the ratio of the change in position of the second part to the change in actuation amount of the other SMA element increases. In some embodiments, the two SMA elements are configured such that, at the reference position, the ratio of the change in position of the second part to the change in actuation amount of each SMA element is substantially equal. In some embodiments, the two SMA elements are configured, on actuation, to move the second part in opposite directions relative to the first part when the second part is within a first sub-range of movement relative to the first part, and wherein the two SMA elements are further configured, when the second part is beyond the first sub-range of movement and within a second sub-range of movement, on actuation, to apply actuation forces urging the second part in the same direction. In some embodiments, the second part is rotatable relative to the first part about a rotation axis, wherein the one or more SMA elements are configured, on actuation, to rotate the second part relative to the first part, and wherein each SMA element is configured such that, when the SMA element on actuation effects movement of the second part away from the reference position, the distance to the rotation axis of the input force applied by the SMA element to the second part increases. In some embodiments, the two SMA elements are arranged on opposite sides of the rotation axis when the second part is within the first sub-range of movement, and wherein one of the SMA elements is moved beyond the rotation axis at the limit of the first sub-range of movement such that the two SMA elements are arranged on the same side of the rotation axis when the second part is within the second sub-range of movement. Some embodiments further comprise a third part arranged to be translationally movable relative to the first part, wherein the third part is coupled to the second part such that rotation of the second part relative to the first part effects translational movement of the third part relative to the first part. The biasing force may act indirectly on the second part via the third part, i.e. the biasing force may act on the third part so as to urge the third part to a reference position of the third part, thereby urging the second part to the reference position of the second part via the coupling of the third part to the second part. Some embodiments comprise first and second friction surfaces that are biased against each other with a normal force so as to give rise to a frictional force therebetween for constraining movement of the second part relative to the first part when the one or more SMA elements are not actuated. The static frictional force (for example before any reduction due to SMA element actuation) is thus greater than the biasing force at any position of the second part within the range of movement. Movement of the second part relative to the first part at any position within a 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 second part. Alternatively, the one of the friction surfaces may be fixed relative to the first part, the second part or a third part and the other friction surface may be fixed relative to another of the first part, the second part, the third part or another 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. In some embodiments, the one or more SMA elements are configured, on actuation, to reduce the normal force so as to reduce the static frictional force between the first and second friction surfaces. The one or more SMA elements may be configured, on actuation, to apply a friction-reducing force to reduce the normal force. The SMA element may be configured such that the ratio of its frictionreducing force to its actuation force increases with increasing displacement of the second part from the reference position in the direction opposite to the direction in which the SMA element is configured to move the second part. Some embodiments comprise two pairs of first and second friction surfaces, wherein one of the two SMA elements is arranged, on actuation, to reduce the normal force between a first pair of first and second friction surfaces so as to reduce the static frictional force between the first pair of first and second friction surfaces and the other of the two SMA elements is arranged, on actuation, to reduce the normal force between a second pair of first and second friction surfaces so as to reduce the static frictional force between the second pair of first and second friction surfaces. In some embodiments, the one or more SMA elements are configured, on actuation, to disengage the first and second friction surfaces. In some embodiments, the maximum magnitude of the biasing force is within the range from 80% to 95% of the magnitude of the static frictional force when the SMA elements are not actuated. The maximum magnitude of the biasing force may be within the range from 80% to 95% of the magnitude of the static frictional force when the SMA elements are not actuated and when the second part is positioned within the range of movement such that the biasing force is maximized. In some embodiments, the actuator assembly is configured such that the static frictional force decreases with increasing displacement of the second part from the reference position. Some embodiments an external load coupled to the second part, wherein the external load comprises a resilient element, such as a spring element, configured to apply the biasing force or wherein the external load comprising a magnetic arrangement configured to apply the biasing force. Some embodiments further comprise a counterbalance biasing arrangement configured, when the second part is positioned relative to the first part at positions that are located on opposite sides from the reference position, to apply a counterbalance force for urging the second part away from the reference position relative to the first part. In some embodiment, the magnitude of the counterbalance force is in the range from 25% to 75% of the magnitude of the biasing force, for example at any position of the second part within the range of movement. In some embodiments, the counterbalance biasing arrangement comprises one or more resilient elements or the counterbalance biasing arrangement comprises a magnetic arrangement. According to another aspect of the present invention, there is provided an actuator assembly as specified in claim 25. Such an actuator assembly may include any suitable features of the first aspect of the present invention as set out above, or any suitable features described in the detailed description. 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; Figures 4a to 4d show an actuator assembly with stroke gearing reduction of an actuating SMA element at various positions of a rotating movable part; Figures 5a and 5b show another actuator assembly with stroke gearing reduction of an actuating SMA element at different positions of a movable part; Figure 6 shows another actuator assembly with stroke gearing reduction of an actuating SMA element and having a translationally movable intermediate part; Figure 7 shows another actuator assembly with stroke gearing reduction of an actuating SMA element and having actuating units such as those shown in Figures 3A to 3C; Figures 8a and 8b show another actuator assembly with stroke gearing reduction of an actuating SMA element and having a counterbalance spring; Figures 9a and 9b show another actuator assembly with stroke gearing reduction of an actuating SMA element and having a counterbalance magnet; and Figure 10 shows another actuator assembly with frictional forces that vary on displacement of a movable part. 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 imager sensor 4) with an imaging axis, the primary axis P may be parallel to such an axis and / or may coincide with such an axis when the movable part 20 is in a central position or orientation (for example, see Figure 1A). In general, the movable part 20 may be movable relative to the support structure 10 with up to six degrees of freedom (DOFs). In the context of describing the DOFs of movement, the primary axis P may also be referred to as the z axis, and two further axes that are perpendicular to the primary axis P and to each other may be referred to as the x and y axes. The movable part 20 may be movable relative to the support structure 10 in all or in any subset (including only one) of the following DOFs: 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 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, which may also be referred to as a rotation 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 forcemodifying 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. Stroke gearing reduction The present invention generally relates to actuator assemblies 2 in which stroke gearing is reduced on actuation of SMA elements 34. Stroke gearing corresponds to the amount of stroke amplification or de-amplification. Stroke gearing is thus the ratio of the change in position of the movable part 20 to the change in actuation amount of the SMA element 34, for example the ratio of the amount of movement of the movable part 20 to the change in length of the SMA element 34. So, the SMA elements 34 are configured such that, on actuation of the SMA element 34, the ratio of the change in position of the movable part 20 to the change in actuation amount of the SMA element 34 decreases with increasing displacement of the movable part 20. Stroke gearing reduction may be implemented in actuator assemblies 2 such as those generally described with reference to Figure 2. For example, the actuating units 30 may be configured such that stroke gearing is reduced on actuation of the SMA element 34. With reference to Figure 3B, for example, the SMA element 34 may, on actuation, move away from the effective pivot axis R such that the distance Ds between the effective pivot axis R and the SMA element 34 (or more generally the input force Fi applied by the SMA element 34) increases. As a result of the increasing distance Ds, the stroke amplification factor Dc / Ds decreases. The actuating unit 30 may thus be arranged to achieve stroke gearing reduction. Figures 4a to 4d schematically show an embodiment of another actuator assembly 2 as generally described with reference to Figure 2. As shown, the movable part 20 is rotatable relative to the support structure 10 about a pivot axis R. The movable part 20 may also be referred to as a rotatable part. The actuator assembly 2 comprises a bearing arrangement in the form of a rotation bearing 42 between the movable part 20 and the support structure 10. The rotation bearing 42 guides rotation of the movable part 20 relative to the support structure 10, and so defines the pivot axis R about which the movable part 20 rotates. In the depicted embodiment, the rotation bearing 42 is embodied by a pin bearing, comprising a pin protruding from the support structure 10 and further comprising a surface on the movable part 20, in particular the inner surface of a hole in the movable part 20 that engages the pin. In general, the rotation bearing 42 may be implemented by any suitable mechanism for guiding rotation of the movable part 20 relative to the support structure 10, including for example a flexure, such as the force-modifying flexure 32 of Figures 3a to 3c. The actuator assembly 2 further comprises two SMA elements 34a, 34b, shown in the form of SMA wires 34a, 34b. The two SMA elements 34a, 34b comprise a first SMA element 34a and a second SMA element 34b. In the depicted embodiment, the SMA elements 34a, 34b are directly connected between the movable part 20 and the support structure 10, in particular by connection elements 35, 15 such as crimps. The two SMA elements 34a, 34b oppose each other. So, one SMA element 34a on actuation drives rotation of the movable part 20 in a first sense (clockwise in Figures 4a) and the other SMA element 34b on actuation drives rotation of the movable part 20 in a second sense, opposite to the first sense (anti-clockwise in Figure 4a). The two SMA elements 34a, 34b apply torques to the movable part 20 in opposite senses so as to drive rotation of the movable part 20 in opposite senses. Figure 4a shows the movable part 20 at a reference position relative to the support structure 10. The reference position may also be referred to as a central position or a set position of the movable part 20. In the depicted embodiment, the SMA elements 34 are arranged equidistant from the pivot axis R when the movable part 20 is at the reference position. The stroke gearing of the two SMA elements 34 is thus the same at the reference position. Figure 4a shows the input force Fi applied by the first SMA element 34a and the actuating force F applied to the movable part 20. The input force Fi typically acts along the length of an elongate SMA element 34, such as an SMA wire 34. The actuating force F herein acts along the direction of movement of the movable part 20, and so parallel to a tangent of a circle about the rotation axis R for a rotating movable part 20 as in Figure 4a. The actuating force F is a force component of the input force Fi in Figure 4. For illustrative reasons, the input force Fi and the actuating force F are only shown for the first SMA wire 34a and only in Figure 4a, but it will be appreciated that such forces are similarly present for the second SMA wire 34b and in the positions / orientations of the movable part 20 of the remaining Figures. The stroke gearing is the ratio of i) the perpendicular distance of the actuating force F to the pivot axis R (corresponding to distance De shown in Figure 3B) and ii) the perpendicular distance of the input force Fi to the pivot axis R (corresponding to distance Ds shown in Figure 3B). The perpendicular distance to the pivot axis R is generally the distance, along a line that is perpendicular to a force applied by the SMA element 34 or a length of the SMA element 34, between the pivot axis R and the force applied by the SMA element 34 or the length of the SMA element 34. In Figure 4a in particular, the stroke gearing is thus the ratio of i) the distance between the pivot axis R and the connection point of the SMA element 34 to the movable part 20 and ii) the perpendicular distance between the SMA element 34 and the pivot axis R. More generally, the stroke gearing is the ratio of i) the perpendicular distance to the pivot axis R of the actuating force applied to the movable part 20 and ii) the perpendicular distance to the pivot axis R of the input force Fi applied by the SMA element 34 (which force typically corresponds to tension in an elongate SMA element 34 and so acts along the length of an elongate SMA element 34). The force gearing is typically the reciprocal of the stroke gearing. Figure 4b shows the movable part 20 at a position away from the reference position. The first SMA element 34a has been actuated to move the movable part 20 away from the reference position. Purely for purposes of explanation, the first SMA element 34a may herein also be referred to as the actuating SMA element 34 and the second SMA element 34b may be referred to as the opposing SMA element 34, although in practice both SMA elements 34 may be controlled to be actuating, and so the same SMA element 34 may be considered to be either opposing or actuating depending on the position of the movable part 20. Compared to Figure 4a, the first SMA element 34a is positioned further away from the pivot axis R. As a result, the stroke gearing for the first SMA element 34a is less when the movable part 20 is at the position shown in Figure 4b compared to the position shown in Figure 4a. The stroke gearing of the first SMA element 34a decreases with increasing displacement of the movable part 20 from the reference position, in particular because the perpendicular distance to the pivot axis R of the first SMA element 34a (or generally of the input force Fi applied by the first SMA element 34a) increases with increasing displacement of the movable part 20 from the reference position. The force gearing of the first SMA element 34a increases on actuation of the first SMA element 34a. As also shown in Figure 4b, the stroke gearing of the second SMA element 34b increases on actuation of the first SMA element 34a. The perpendicular distance to the pivot axis R of the second SMA element 34b (or generally of the input force Fi applied by the second SMA element 34b) decreases with increasing displacement of the movable part 20 from the reference position. The force gearing of the second SMA element 34b decreases on actuation of the first SMA element 34a. Biasing force for returning to reference position The inventors have found that, advantageously, stroke gearing reduction may be used in actuator assemblies 2 to aid in overcoming biasing forces that urge the movable part 20 to the reference position. Biasing forces urging the movable part 20 to the reference position arise in a variety of applications of the actuator assembly 2. The actuator assembly 2 is typically used to move an external load. Such an external load may apply various forces to the movable part 20, such as i) gravitational forces due to the mass of the movable part 20 including any components to be moved, ii) frictional forces due to friction between any components that are in direct engagement and move or iii) biasing forces due to any resilient or elastic elements, such as spring components, or magnetic elements. The external load on the movable part 20 typically comprises a mixture of all of these forces. In some applications, the biasing forces dominate the overall load on the movable part 20. The biasing force acting on the movable part 20 may be a spring force or magnetic force that acts in a direction parallel to instantaneous movement of the movable part 20. Biasing forces that urge the movable part 20 to the reference position may, for example, be due to electrical connections to an electronic component fixed to the movable part 20. Other sources of the biasing force include any flexures (such as those described in relation to Figures 3a to 3c) or other springs, any magnetic arrangements (for example for parking the movable part 20 relative to the support structure 10), any force due to deforming components coupled to the movable part 20, or forces due to other SMA elements 34 (such as tension in an extended SMA wire 34). Any such biasing forces may bias the movable part 20 towards the reference position, for example a central position of the movable part 20 relative to the support structure 10. The reference position may be a central position of the movable part 20 within a range of movement of the movable part 20 relative to the support structure 10. The inventors have found that an actuator assembly 2 implementing stroke gearing reduction of an actuating SMA element 34 is particularly useful for applications in which the overall force on the movable part 20 is dominated by such biasing forces when the movable part 20 is moved away from the reference position. The maximum biasing force (when the movable part 20 is moved away from the reference position) may be greater than 50% of the overall force acting on the movable part 20 so as to effect movement of the movable part 20 when the SMA elements 34 are not actuated. The stroke gearing reduction may, for example, be used to reduce any variation in tension in the SMA elements 34 due to the biasing forces. The performance of the actuator assembly 2 may thus be improved. In some embodiments, the biasing force on the movable part 20 may increase with displacement of the movable part 20 from the reference position. The biasing force may be zero or negligible at the reference position and may increase in dependence on the amount of displacement of the movable part 20 from the reference position. The biasing force may for example increase linearly with displacement. The biasing force may thus be variable as the movable part 20 moves within the range of movement. Reference to an absolute or relative magnitude of the biasing force herein means the maximum magnitude of the biasing force for any position of the movable part 20 within the range of movement, unless otherwise indicated. The range of movement of the movable part 20 relative to the support structure comprises the set of positions, orientations or poses that the movable part 20 reach under normal operation of the actuator assembly 2, i.e. due to actuation of the SMA elements 34. The SMA elements 34 may be configured such that the stroke gearing change (and the accompanying force gearing change) on displacement of the movable part 20 from the reference position matches the increase in biasing forces acting on the movable part 20 due to such displacement. Variations in the tension in the actuating SMA element 34 may thus be kept relatively small within the range of movement of the movable part 20 relative to the support structure 10, despite any increase in biasing force. The variation in the tension in the actuating SMA element 34 may be less than the variation of the biasing force, for example, less than 50% or less than 20% thereof. The performance of the actuator assembly 2 may thus be enhanced. Stroke gearing reduction may also be particularly advantageous in applications in which the biasing force is relatively large. The biasing force (e.g. the maximum biasing force within the range of movement) may be greater than 50%, for example greater than 80% or greater than 95%, of the actuating force that can applied by the SMA elements 34 to drive movement of the movable part 20. Stroke gearing reduction may beneficially be used in combination with such relatively large biasing forces. Conventionally, spring components of an external load may be avoided or minimized in the interest of improving performance of the actuator assembly 2. The inventors have found that effective use of stroke gearing reduction allows larger biasing forces to be applied to the movable part 20, thereby increasing the variety and complexity of external loads that can be driven by the movable part 20. The biasing force may be greater than the weight of the movable part 20. So, the biasing force may be a dominant force acting on the movable part 20 compared to any gravitational forces acting on the movable part 20. The ratio of the maximum magnitude of the biasing force to the weight of the movable part may be greater than 2, preferably greater than 5. In some embodiments, the maximum biasing force may become larger than the force that can be applied by the actuating SMA elements at the reference position. For example, the SMA elements 20 may be configured to apply an actuation force of a first magnitude to the movable part 20 when the movable part 20 is at the reference position, for example corresponding to the actuation force applied when the SMA elements 34 are at a pre-determined maximum stress. The actuation force applied by the actuating SMA element 34 to the movable part 20 may increase on displacement from the reference position. The SMA elements 34 may be configured to apply an actuating force of a second magnitude to the movable part 20 when the movable part 20 is at a position away from the reference position (corresponding to the actuating force applied when the SMA elements 34 are at the pre-determined stress). The first magnitude may be less than and the second magnitude may be greater than the magnitude of the biasing force at the position away from the reference position. Force gearing increase (and accompanying stroke gearing decrease) of the actuating SMA element 34 and force gearing decrease (and accompanying stroke gearing increase) of the opposing SMAS element 34 may thus effectively be used to move the movable part 20 despite a relatively large biasing force. Combined SMA element actuation at edges of range of movement Figures 4c and 4d show that, in some embodiments, the two SMA elements 34a, 34b may act together so as to move the movable part 20 when the movable part 20 is moved relatively far from the reference position within the range of movement. Conventionally, the range of movement may extend to a position of the movable part 20 such as shown in Figures 4b. In any of the positions of the movable part 20 between the positions shown in Figures 4a and 4b, the two SMA elements 34 oppose each other so as to effect movement of the movable part 20 in opposite directions. The inventors have realized that, particularly in situations in which the biasing force is relatively large so as to urge the movable part 20 to return to the reference position, the range of movement may be extended beyond positions such as the position shown Figure 4b. Figure 4c shows the movable part 20 at a position reached on further actuation of the first SMA element 34a compared to the position of Figure 4b. The second SMA element 34b intersects the pivot axis R in Figure 4c. The second SMA element 34b may in such a position not apply an actuation force opposing the actuation force of the second SMA element 34a. The second SMA element 34b may not be capable, on actuation, of returning the movable part 20 to the reference position. Instead, the biasing force may move the movable part 20 towards the reference position of Figure 4a from the position in Figure 4c. Extending the range of movement to the position of Figure 4c is thus enabled by providing a relatively large biasing force. Figure 4d shows the movable part 20 at a position reached on even further actuation of the SMA element 34b. As shown, the SMA element 34b has crossed over the pivot axis R. Both SMA elements 34a, 34b apply input forces Fi on the same side of the pivot axis R, such that the actuating forces F applied by the SMA elements 34a, 34b act in the same direction. The SMA wires 34a, 34b apply torques in the same sense when in the position in Figure 4d, so as together to drive movement of the movable part 20 further away from the reference position. Using both SMA wires 34a, 34b to urge the movable part 20 in the same direction allows the biasing force to be greater at the edges of the range of movement compared to a situation in which only one SMA wire 34 applies an actuation force. The range of movement of the movable part 20 may be further extended compared to providing a limit of the range of movement at the position shown in Figure 4c. The range of movement of the movable part 20 relative to the support structure 10 may thus comprise a first sub-range of movement and a second sub-range of movement that is different to the first sub-range of movement. The first sub-range of movement comprises positions of the movable part 20 between the reference position shown in Figure 4a and the position shown in Figure 4c. The second sub-range of movement comprises positions of the movable part 20 beyond the position shown Figure 4c, such as the position shown in Figure 4d. The SMA elements 34 move the movable part 20 in opposite directions (i.e. apply actuation forces in opposite directions) when the movable part 20 is within the first sub-range of movement and move the movable part 20 in the same direction (i.e. apply actuating forces in the same direction) when the movable part 20 is within the second sub-range of movement. Alternative actuator assemblies for stroke gearing reduction Figures 5a and 5b schematically show another actuator assembly 2 that achieves stroke gearing reduction on actuation of SMA elements 34. The actuator assembly 2 comprises two intermediate parts 31, each in the form of an elongate rod or beam. Two SMA elements 34 are arranged to oppose each other, each coupled to a respective intermediate part 31 via a respective connection part 35. In the depicted embodiment, the movable part 20 is translationally movable relative to the support structure 10 in one degree of freedom (in the up-down direction in Figures 5a and 5b). A bearing, such as a rolling bearing or ball bearing, is arranged between the support structure 10 and the movable part 20 so as to guide translational movement of the movable part 20 relative to the support structure 10. The SMA elements 34 are connected between the support structure 10 (at the left side of Figures 5a, 5b) and the connection part 35. The connection part 35 may comprise a crimp that mechanically holds the SMA element 34. A bearing, such as a rolling bearing or ball bearing, is arranged between the support structure 10 and the connection part 35 so as to guide translational movement of the connection part 35 relative to the support structure 10. In the depicted embodiment, the translational movement is in a direction parallel to the input force applied by the SMA element 34, but in general the translational movement may be at an angle to the input force. The translational movement of the connection part 35 is non-parallel to the translational movement of the movable part 20. In the depicted embodiments, the movement of the connection part 35 is perpendicular to the movement of the movable part 20, although in general movement of the connection part 35 may be at an angle to movement of the movable part 20. The intermediate part 31 is coupled between the connection part 35 and the movable part 20. The intermediate part 31 is rotatable relative to the connection part 35 and relative to the movable part 20. A rotation bearing is arranged between the intermediate part 31 and the connection part 35, and another rotation bearing is arranged between the intermediate part 31 and the movable part 20. The rotation bearing may generally be any mechanism capable of guiding relative rotation of two parts. Upon actuation of the SMA elements 34 and translational movement of the connection part 35 and movable part 20, the intermediate part 31 rotates about a rotation axis R relative to the support structure. The rotation axis R is perpendicular to the movement of the connection part 35 and to the movement of the movable part 20. The SMA element 34, connection part 35 and intermediate part 31 may be arranged so as to amplify actuation of the SMA element 34 to a relatively greater amount of movement of the movable part 20. The combination of SMA element 34, connection part 35 and intermediate part 31 may thus function in a manner similar to the actuating unit 30 described in relation to Figures 3a to 3c and may thus effect stroke amplification. Figure 5a shows the movable part 20 at a reference position. Figure 5b shows the movable part 20 at a position away from the reference position due to actuation of one of the SMA elements 34 (the top SMA element in Figure 5b). On actuation of the SMA element 34, the connection part 35 moves translationally so as to cause rotation of the intermediate part 31 coupled to the connection part 35, thereby causing translational movement of the movable part 20. The rotation axis R about which the intermediate part rotates relative to the support structure 10 moves away from the SMA element on actuation of the SMA element 34. Stroke gearing of the SMA element 34 thereby decreases on actuation of the SMA element 34. Figure 6 schematically shows another actuator assembly 2 that achieves stroke gearing reduction on actuation of SMA elements 34. The actuator assembly 2 comprises an intermediate part 31 comprising a plurality of ramps. Two SMA elements 34 are arranged to oppose each other, each coupled to the intermediate part 31. The movable part 20 is translationally movable relative to the support structure 10 along a first axis (in the up-down direction in Figure 6) and the intermediate part 31 is translationally movable relative to the support structure 10 along a second axis that is not parallel to the first axis (in the leftright direction in Figure 6). In the particular embodiment of Figure 6 the first and second axes are perpendicular to each other, but in general the first and second axes may be angled relative to each other. Respective bearing arrangements (not shown) may be configured to guide the translational movement of the movable part 20 and / or intermediate part 31 relative to the support structure 10. The actuator assembly 2 comprises a bearing arrangement 44 between the intermediate part 31 and the movable part 20. The bearing arrangement 44 comprises a plain bearing, in particular formed by protrusions on the movable part 20 being in sliding engagement with bearing surfaces on the ramps of the intermediate part 31. The intermediate part 31 comprises bearing surfaces that are inclined relative to the movement direction of the intermediate part 31 and relative to the movement direction of the movable part 20. The inclination of the bearing surface relative to the movement direction of the intermediate part 31 varies along the movement direction. On movement of the movable part 20, the protrusions on the movable part 20 travel across the bearing surfaces on the intermediate part 31. At the reference position depicted in Figure 6, the protrusions are positioned on a portion of the bearing surfaces that has a relatively large angle relative to the movement direction of the intermediate part 31. As such, stroke amplification may be relatively large. On actuation of the SMA element 34, the protrusions move along the bearing surfaces to a portion of the bearing surfaces that has a relatively small angle relative to the movement direction of the intermediate part 31. As such, stroke amplification may be relatively small. The stroke gearing may thus be reduced on actuation of the SMA elements 34 and movement of the movable part 20 away from the reference position. Figure 7 schematically shows another actuator assembly 2 that achieves stroke gearing reduction on actuation of SMA elements 34. The actuator assembly 2 comprises two actuating units 30, for example actuating units 30 as generally described in relation to any of Figures 3a to 3c. The actuating units 30 oppose each other so as to drive movement of the movable part 20 in opposite directions. The actuating units 30 apply actuating forces F in opposite directions. Figure 7 shows the movable part 20 at the reference position. On actuation of an SMA element 34, the SMA element 34 moves away from the effective pivot axis R provided by the force-modifying element 32. With particular reference to Figure 7, on actuation of the top SMA element 34 the corresponding body portion 31 moves upward, thereby moving the end of the SMA element 34 upward and away from the force-modifying element 32. As a result, the stroke gearing of the actuating SMA element 34 is reduced, thereby increasing the force gearing thereof. The stroke gearing of the opposing SMA element 34 is increased, thereby reducing the force gearing thereof. Such stroke gearing reduction may generally be achieved in any of the actuator assemblies 2 described with reference to Figures 1 and 2 and having actuating units 30 such as those described with reference to Figure 3. The stroke gearing of the other SMA element 34 that opposes the actuating SMA elements 34 is increased on actuation of the actuating SMA element 34. 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 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 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. The normal force component may also be referred to as a friction-reducing force. 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 (i.e. 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. Movement of the movable part 20 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. The maximum magnitude of the biasing force may be within the range from 80% to 95% of the magnitude of the static frictional force when the SMA elements are not actuated. The maximum magnitude of the biasing force may be within the range from 80% to 95% of the magnitude of the static frictional force when the SMA elements are not actuated and when the second part is positioned within the range of movement such that the biasing force is maximized. A particular embodiment in which zero hold power functionality may be achieved is shown in Figure 7. As shown, the movable part 20 and the support structure 10 are in direct engagement with each other. A first friction surface lOf is provided on the support structure 10 and a second friction surface 20f is provided on the movable part 20. The first and second friction surfaces lOf, 20f are biased against each other by one or more biasing elements 52, which are implemented as two compression springs in Figure 7. In the specific embodiment of Figure 7, the support structure 10 comprises a first portion 10a and two second portions 10b. Each biasing element 52 is arranged between the first portion 10a and a respective second portion 10b. The first and second portions 10a, 10b of the support structure 10 are movable relative to each other in a direction perpendicular to the movement axis along which the movable part 20 moves. However, in practice, movement of the first and second portions 10a, 10b relative to each other need not take place during operation of the actuator assembly 2. The first and second portions 10a, 10b may remain static relative to each other on actuation of the SMA elements 34, and so are considered as a unit forming the support structure 10. Alternatively, the first portion 10a may be considered to correspond to the support structure 10 and each second portion 10b may be considered to correspond to another part 10b (such as a sprung part 10b). In general, movement of the first and second portions 10a, 10b in a direction along the movement axis of the movable part 20 is constrained. The movable part 20 is sandwiched between the first and second portions 10a, 10b of the support structure 10 by the biasing elements 52. The first and second friction surfaces lOf, 20f are thereby biased against each other by the normal force due to the biasing elements 52. The static frictional force is thus generated between the first and second friction surfaces lOf, 20f. The magnitude of the static frictional force may be sufficient to constrain movement of the movable part 20 relative to the support structure 10 at any position within a range of movement of the movable part 20 relative to the support structure 10 when the SMA elements 34 are not actuated. Figure 7 shows first and second friction surfaces lOf, 20f between the movable part 20 and the second portions 10b of the support structure 10. In general, the first and second friction surfaces lOf, 20f may additionally be provided elsewhere, for example between the movable part 20 and the first portion 10a of the support structure 10 (not labelled specifically in Figure 7 for illustrative purposes). Each of the first and second friction surfaces lOf, 20f may, in practice, comprise a plurality of disconnected regions, i.e. the first and second friction surfaces lOf, 20f need not be continuous. The first and second friction surfaces lOf, 20f may correspond to any of the surfaces of the support structure 10 and of the movable part 20 that are in direct engagement with each other so as to frictionally constrain movement of the movable part 20. Although Figure 7 shows direct engagement of both first and second portions 10a, 10b with the movable part 20, in general only one of the portions 10a, 10b may be provided with a friction surface lOf engaging a corresponding friction surface 20f of the movable part 20. For example, a rolling bearing may be provided between surfaces of the first portion 10a and the movable part 20, such that the friction surfaces are formed only on the interface between the movable part 20 and the second portion 10b. In the actuator assembly 2 of Figure 7, the SMA elements 34 are arranged such that, on actuation, the normal force between the first and second friction surfaces lOf, 20f is reduced. The SMA elements 34 may thus be configured to apply a friction-reducing force that reduces the normal force. For example, when two opposing SMA elements 34 are equally actuated, the two portions 10a, 10b of the support structure 10 may be urged apart so as to reduce the normal force between the first and second friction surfaces lOf, 20f. In Figure 7, the SMA elements 34 are connected between the second portion 10b and the body portions 31 of the actuating units 30, and so the second portion 10b may be urged in a leftward direction on actuation of the SMA elements 34. As a result, the normal force between the first and second friction surfaces lOf, 20f is reduced, thereby leading to a reduction in the static frictional force between the first and second friction surfaces lOf, 20f. Figure 7 also shows an endstop 55 arranged between the first and second portions 10a, 10b. The endstop 55 comprises surfaces (in particular on the first portion 10a and on the second portion 10b) that are spaced apart when the SMA elements 34 are not actuated. The SMA elements 34 may be configured, on actuation, to disengage the first and second friction surfaces lOf, 20f and to engage the endstop 55. In Figure 7, the second portion 10b may be urged in a leftward direction, thereby disengaging friction surfaces lOf, 20f between the movable part 20 and the second portion 10b until the endstop 55 engages, i.e. until the second portion 10b engages the first portion 10a of the support structure 10. First and second friction surfaces lOf, 20f may generally be provided in any of the actuator assemblies 2 described herein so as to implement zero hold power functionality. Purely by way of example, first and second friction surfaces lOf, 20f may be provided between the movable part 20 and the support structure 10 when viewed orthogonally to the views of Figures 4a to 4d, 5a, 5b or 6. The first and second friction surfaces lOf, 20f may be positioned behind the movable part 20 in the views of Figures 4a to 4d, 5a, 5b or 6. The first and second friction surfaces lOf, 20f may also or in addition be provided by any plain bearing described herein, or any rolling bearing described herein may be replaced by a plain bearing so as to provide the first and second friction surfaces lOf, 20f. In the embodiments of Figures 5a and 5b, for example, one or more of the rolling bearings guiding the translational movement of the connection part 35 or of the movable part 20 relative to the support structure 10 may be replaced by a plain bearing comprising first and second friction surfaces lOf, 20f. The inventors have found that stroke gearing reduction may be particularly advantageous in an actuator assembly 2 implementing zero hold power functionality and providing a biasing force urging the movable part 20 to the reference position. In actuator assemblies 2 such as the actuator assembly 2 shown in Figure 7, each SMA element 34 may modulate the static frictional force between a different pair of first and second friction surfaces lOf, 20f. One SMA element 34 (the upper SMA element 34 in Figure 7), on actuation, reduces the static frictional force between a first pair of friction surfaces lOf, 20f (the upper pair in Figure 7), and the opposing SMA element 34 (the lower SMA element 34 in Figure 7), on actuation, reduces the static frictional force between a second pair of friction surfaces lOf, 20f (the lower pair in Figure 7). So as to move the movable part 20, an actuating SMA element 34 (e.g. the upper SMA element 34 in Figure 7) applies an actuating force F greater than the combination of i) the external load L acting on the movable part, ii) the frictional force Ff between any pairs of friction surfaces, and iii) the actuating force Fe applied by the opposing SMA element 34 (e.g. the lower SMA element 34 in Figure 7) on the movable part 20, so F >L + Ff + Fe. It may be desirable to reduce the overall force that needs to be overcome by the actuating force F applied by the actuating SMA element 34 so as to operate efficiently and reduce the stress in the actuating SMA element 34. One way to reduce the overall load may be to reduce the actuating force Fe applied by the opposing SMA element 34. However, in embodiments in which the SMA elements 34 modulate the static frictional force between friction surfaces lOf, 20f, a reduction in the actuating force Fe applied by the opposing SMA element 34 may lead to an undesirable increase in the static frictional force between the friction surfaces lOf, 20f. Such an increase in the static frictional force may be especially undesirable in embodiments in which the opposing SMA element 34 modulates the static frictional force between one pair of friction surfaces lOf, 20f (and the actuating SMA element 34 does not modulate that frictional force), such as the embodiment shown in Figure 7. The inventors have found that stroke gearing reduction may advantageously applied to actuator assemblies 2 with the biasing force acting on the movable part 20 and with zero hold power functionality, especially where opposing SMA elements 34 modulate the frictional force between different pairs of friction surfaces lOf, 20f. The stroke gearing reduction of the actuating SMA element 34 and the stroke gearing increase of the opposing SMA element 34 may improve the ability of the actuating SMA element 34 to overcome the actuating force from the opposing SMA element 34 and / or the friction resulting from any diminished reduction in frictional forces due to any reduction in the actuating force from the opposing SMA element 34. In addition, the ratio of the friction-reducing force of the opposing SMA element 34 to its actuation force may increase with increasing displacement of the second part from the reference position due to actuating by the actuating SMA element 34, such that the opposing SMA element 34 may more effectively cause a reduction in frictional force Ff with a relatively lower actuating force Fe. Opposition to movement of the movable part 20 on actuation of the actuating SMA element 34 may thus be reduced. Stroke gearing reduction may also be particularly beneficial in actuator assemblies 2 with zero hold power functionality in which the SMA elements 34 are configured to disengage the friction surfaces lOf, 20f on actuation. In such actuator assemblies 2, it is desirable to reduce, ideally to zero, the force applied by the opposing SMA element 34 so as to improve the efficiency of the actuating SMA element 34. However, if only the input force applied by the actuating SMA element 34 disengage the friction surfaces lOf, 20f and the opposing SMA element 34 does not apply a force to the movable part 20, then on a reduction of the actuating force of the actuating SMA element 34 there is no increase in frictional force until the friction surfaces lOf, 20f reengage. The condition for holding the movable part 20 in position is that F + Ff >L + Fe. On reduction of the actuating force F of the actuating SMA element 34, the forces for holding the movable part 20 in position may thus be insufficient such that the movable part 20 moves towards the reference position and is not held in position at the edges of the range of movement. So as to operate an actuator assembly 2 with zero hold power functionality and disengaging friction surfaces lOf, 20f, both SMA elements 34 (i.e. the actuating SMA element 34 and the opposing SMA element 34) may be used to modulate the static frictional force between friction surfaces lOf, 20f. On ceasing actuation of the SMA elements 34 so as to maintain a position of the movable part 20, the opposing SMA element 34 may be controlled to cease actuation first (i.e. the opposing SMA element 34 may be powered off first) so as to allow the friction surfaces lOf, 20f to engage before the actuating SMA element 34 is controlled to cease actuation. The position of the movable part 20 may thus be maintained at any position within the range of movement, so as to achieve zero hold power functionality at the edges of the range of movement. Providing stroke gearing reduction of the actuating SMA element 34 and stroke gearing increase of the opposing SMA element 34 ensures that the actuating force F applied by the actuating SMA element 34 may overcome the increased overall force acting on the movable part 20 due to actuation of the opposing SMA element 34. Compensation mechanism to aid in overcoming biasing force Any of the actuator assemblies 2 may be provided with a compensation mechanism that compensates for the biasing force urging the movable part 20 to the reference position. Figures 8 to 10 schematically show three different implementations of such a counterbalance mechanism. The counterbalance mechanism may, when the movable part 20 is displaced from the reference position, apply a counterbalance force with a component parallel to the direction of movement of the movable part 20 that is in the range from 25% to 75%, for example about 50%, of the biasing force urging the movable part 20 towards the reference position. The net force urging the movable part 20 towards the reference position may thus be reduced. Figures 8a and 8b schematically show an actuator assembly 2 comprising a counterbalance spring 62. The actuator assembly 2 of Figures 8a and 8b is generally as described with reference to Figures 4a to 4d. The movable part 20 overlaps with the SMA elements 34 and with the connection elements 15, 35 connecting the SMA elements 34 to the support structure 10 and to the movable part 20. The SMA elements 34, connection elements 15, 35 and counterbalance spring 62 may be arranged between the support structure 10 and the movable part 20 when viewed along the pivot axis R. The SMA elements 34, connection elements 15, 35 and counterbalance spring 62 are thus arranged behind the movable part 20 in the view of Figures 8a and 8b, and the movable part 20 is transparent in the views of Figures 8a and 8b for illustrative purposes. The counterbalance spring 62 is connected at a first end 62a to the support structure 10 and at a second end 62b to the movable part 20. The counterbalance spring 62 applies a counterbalance force Feb to the movable part 20. When the movable part 20 is at the reference position as shown in Figure 8a, the counterbalance force FCb is on a line intersecting the pivot axis R. The counterbalance force Feb thus does not have a component along the direction of movement of the movable part 20 when at the reference position. In the reference position, the counterbalance spring does not apply a torque to the movable part 20. Figure 8b shows the movable part 20 after being moved away from the reference position, for example due an input force Fi applied by the bottom SMA element 34. When the movable part 20 is not at the reference position, the counterbalance force FCb applied by the counterbalance spring 62 is on a line that is offset from the pivot axis R. The counterbalance force FCb aids movement of the movable part 20 due to actuation of the SMA element 34. The counterbalance force FCb opposes the biasing force acting on the movable part 20, for example due to an external load. The component of the counterbalance force FCb acting in a direction parallel to the movement direction of the movable part 20 may increase with displacement of the movable part 20 from the reference position. The overall biasing force acting on the movable part 20 when positioned away from the reference position may thus effectively be reduced, by the counterbalance force FCb cancelling at least part of the biasing force. Figures 9a and 9b show another actuator assembly 2. The actuator assembly 2 corresponds to the actuator assembly 2 described in relation to Figures 8a and 8b, except that a magnetic arrangement 64 is provided instead of the counterbalance spring 62. The magnetic arrangement comprises two magnets fixed to the support structure 10 and two ferromagnetic elements fixed to the movable part 20, arranged in two pairs of a magnet and a ferromagnetic element. At the reference position shown in Figure 9a, the distance between the magnet and the ferromagnetic element is the same for each pair. When the movable part 20 is displaced from the reference position, as shown in Figure 9b, the distance between the magnet and the ferromagnetic element of a first pair is reduced and the distance between the magnet and the ferromagnetic element of a second pair is increased. The counterbalance force FCb applied by the magnetic arrangement to the movable part 20 aids movement of the movable part 20 due to actuation of the SM A element 34. The counterbalance force Feb increases on displacement of the movable part 20 from the reference position, in particular non-linearly. The counterbalance force FCbthus opposes the biasing force, effectively reducing the overall biasing force acting on the movable part 20 when displaced from the reference position. Such a magnetic arrangement for providing the counterbalance force FCb may be particularly beneficial when the biasing force is non-linear, with the biasing force increasing mainly at the edges of the range of movement. Figure 10 schematically shows an actuator assembly 2 comprising a variable friction arrangement 66. The variable friction arrangement 66 may be applied in any of the actuator assemblies 2 described above, in particular in any actuator assemblies 2 with zero hold power functionality. The actuator assembly 2 comprises first and second friction surfaces lOf, 20f provided respectively on the support structure 10 and on the movable part 20. A biasing element 52 loads the friction surfaces lOf, 20f with the normal force, thereby giving rise to the static frictional force. The biasing element 52 is connected between the support structure 10 and a biasing part 66a. The movable part 20 is sandwiched between the biasing part 66a and the support structure 10. The biasing element 52 thus loads the friction surfaces lOf, 20f via the biasing part 66a and the movable part 20. The actuator assembly 2 comprises a variable friction arrangement 66 embodied by inclined surfaces on the biasing part 66a and on the movable part 20. The inclined surfaces are angled relative to a movement axis (in the left-right direction in Figure 10) along which the movable part 20 moves. Figure 10 shows the movable part 20 at the reference position. At the reference position, the normal force that loads the friction surfaces lOf, 20f may be largest. As the movable part 20 moves away from the reference position (in the left-right direction in Figure 10), the biasing element 52 extends due to the inclined surfaces on the biasing part 66a and the movable part 20. The normal force loading the friction surfaces lOf, 20f is thus reduced with displacement of the movable part 20 away from the reference position. In addition, in the particular embodiment of Figure 10, the inclined surfaces impart a component of force from the biasing element 52 on the movable part 20 that urges the movable part 20 away from the reference position when the movable part 20 is displaced from the reference position. The biasing element 52 thus effectively applies a counterbalance force to the movable part 20 via the inclined surfaces of the biasing part 66a. Although the particular embodiment of the actuator assembly 2 of Figure 10 does not make use of stroke gearing reduction of the actuating SMA element 34, stroke gearing reduction may be particularly advantageous in combination with the variable frictional forces. The stroke gearing reduction (and consequential force gearing increase) of the actuating SMA element 34 may help maintain the ability of the actuator assembly 2 to hold the movable part 20 in position or move the movable part 20 at the edges of the range of movement, where the variable frictional forces are reduced. Modifications and Alternatives The actuator assemblies 2 above have been described with two opposing SMA elements 34. In general, the actuator assembly 2 may comprise one SMA element 34 that is opposed by a resilient element, such as a spring. The resilient element may urge the movable part 20 against an endstop so as to define the reference position. With reference to Figures 4a to 4d, for example, the opposing SMA element 34b may be replaced by a tension spring that applies a force opposing the actuating force F caused by the SMA element 34a. Similarly, one of the SMA elements 34 in the remaining embodiments may be replaced by a resilient element applying a constant opposing force. The reference position is described above in relation to positions of the movable part 20 in which the arrangement of opposing SMA elements 23 is symmetrical. So, stroke gearing of the opposing SMA elements 34 is equal in the above-described embodiments. In general, the stroke gearing of opposing SMA elements 34 need not be equal at the reference position, and the arrangement of SMA elements 34 need not be symmetrical at the reference position. The embodiments of Figures 4 to 10 show a movable part 20 that is movable in a single degree of freedom, either a rotational degree of freedom or a translational degree of freedom. In general, the stroke gearing reduction may be used in actuator assemblies 2 allowing movement of the movable part 20 in any sub-set of the six degrees of freedom described with reference to Figure 2. For example, the actuator assembly 2 may be configured to allow rotation of the movable part 20 about two orthogonal axes, such as in the "module-tilt" variation shown in Figure IC. Four actuating units 30 may be arranged to drive rotation of the movable part 20 about the two orthogonal axes. Optionally, a first pair of actuating units 30 may be arranged to drive rotation about a first axis of an intermediate part relative to the support structure 10, and a second pair of actuating units 40 may be arranged to drive rotation about a second axis (orthogonal to the first axis) of the movable part 20 relative to the intermediate part. The present invention has been described in connection with SMA wires. 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 second part that is movable relative to the first part from a reference position, wherein a biasing force urges the second part to the reference position relative to the first part; andone or more shape memory alloy, SMA, elements configured, on actuation, to cause one or more actuation forces to be applied to the second part for moving the second part relative to the first part within a range of movement, wherein each SMA element is configured such that, when the SMA element on actuation effects movement of the second part away from the reference position, the ratio of the change in position of the second part to the change in actuation amount of the SMA element decreases with increasing displacement of the second part from the reference position.

2. An actuator assembly according to claim 1, wherein the ratio of the maximum magnitude of the biasing force to the weight of the second part is greater than 2, preferably greater than 5.

3. An actuator assembly according to any one of the preceding claims, wherein the maximum magnitude of the biasing force is in the range from 50% to 95% of the maximum actuation force applied by the SMA elements to the second part.

4. An actuator assembly according to any one of the preceding claims, wherein the biasing force increases with increasing displacement of the second part from the reference position5. An actuator assembly according to any one of the preceding claims, wherein the one or more SMA elements are configured such that the rate of decrease of the ratio of the change in position of the second part to the change in actuation amount of the SMA element matches the rate of increase of the biasing force with displacement of the second part from the reference position.

6. An actuator assembly according to any one of the preceding claims, wherein the one or more SMA elements are configured such that, at the reference position, the ratio of the change in position of the second part to the change in actuation amount of the SMA element is greater than 1, preferably greater than 2.

7. An actuator assembly according to any one of the preceding claims, wherein the one or more SMA elements comprise two SMA elements configured, on actuation, to move the second part in opposite directions relative to the first part.

8. An actuator assembly according to claim 7, wherein each of the two SMA elements is configured such that with increasing displacement of the second part from the reference position, when the SMA element on actuation effects movement of the second part away from the reference position, i) the ratio of the change in position of the second part to the change in actuation amount of the SMA element decreases and ii) the ratio of the change in position of the second part to the change in actuation amount of the other SMA element increases.

9. An actuator assembly according to claim 7 or 8, wherein the two SMA elements are configured such that, at the reference position, the ratio of the change in position of the second part to the change in actuation amount of each SMA element is substantially equal.

10. An actuator assembly according to any one of claims 7 to 9, wherein the two SMA elements are configured, on actuation, to move the second part in opposite directions relative to the first part when the second part is within a first sub-range of movement relative to the first part, and wherein the two SMA elements are further configured, when the second part is beyond the first sub-range of movement and within a second sub-range of movement, on actuation, to apply actuation forces urging the second part in the same direction.

11. An actuator assembly according to any one of the preceding claims, wherein the second part is rotatable relative to the first part about a rotation axis, wherein the one or more SMA elements are configured, on actuation, to rotate the second part relative to the first part, and wherein each SMA element is configured such that, when the SMA element on actuation effects movement of the second part away from the reference position, the distance to the rotation axis of the input force applied by the SMA element to the second part increases.

12. An actuator assembly according to claims 10 and 11, wherein the two SMA elements are arranged on opposite sides of the rotation axis when the second part is within the first sub-range of movement, and wherein one of the SMA elements is moved beyond the rotation axis at the limit of the first sub-range of movement such that the two SMA elements are arranged on the same side of the rotation axis when the second part is within the second sub-range of movement.

13. An actuator assembly according to claim 11 or 12, further comprising a third part arranged to be translationally movable relative to the first part, wherein the third part is coupled to the second part such that rotation of the second part relative to the first part effects translational movement of the third part relative to the first part.

14. An actuator assembly according to any one of the preceding claims, comprising first and second friction surfaces that are biased against each other with a normal force so as to give rise to a frictional force therebetween for constraining movement of the second part relative to the first part when the one or more SMA elements are not actuated.

15. An actuator assembly according to claim 14, wherein the one or more SMA elements are configured, on actuation, to reduce the normal force so as to reduce the static frictional force between the first and second friction surfaces.

16. An actuator assembly according to claim 15 and any one of claims 7 to 10, comprising two pairs of first and second friction surfaces, wherein one of the two SMA elements is arranged, on actuation, to reduce the normal force between a first pair of first and second friction surfaces so as to reduce the static frictional force between the first pair of first and second friction surfaces and the other of the two SMA elements is arranged, on actuation, to reduce the normal force between a second pair of first and second friction surfaces so as to reduce the static frictional force between the second pair of first and second friction surfaces.

17. An actuator assembly according to any one of claims 14 to 16, wherein the one or more SMA elements are configured, on actuation, to disengage the first and second friction surfaces.

18. An actuator assembly according to any one of claims 14 to 17, wherein the maximum magnitude of the biasing force is within the range from 80% to 95% of the magnitude of the static frictional force when the SMA elements are not actuated.

19. An actuator assembly according to any one of claims 14 to 18, wherein actuator assembly is configured such that the static frictional force, when the SMA elements are not actuated, decreases with increasing displacement of the second part from the reference position.

20. An actuator assembly according to any one of the preceding claims, comprising an external load coupled to the second part, wherein the external load comprises a resilient element, such as a spring element, configured to apply the biasing force or wherein the external load comprising a magnetic arrangement configured to apply the biasing force.

21. An actuator assembly according to any one of the preceding claims, further comprising a counterbalance biasing arrangement configured, when the second part is positioned relative to the first part at positions that are located on opposite sides from the reference position, to apply a counterbalance force for urging the second part away from the reference position relative to the first part.

22. An actuator assembly according to claim 21, wherein the magnitude of the counterbalance force is in the range from 25% to 75% of the magnitude of the biasing force.

23. An actuator assembly according to claim 21 or 22, wherein the counterbalance biasing arrangement comprises one or more resilient elements or wherein the counterbalance biasing arrangement comprises a magnetic arrangement.

24. An actuator assembly according to any one of the preceding claims, wherein the second part is movable in one degree of freedom relative to the first part, wherein the one degree of freedom comprises translation or rotation of the second part relative to the first part or wherein the second part is rotatable about two orthogonal axes relative to the first part.

25. An actuator assembly comprisinga first part;a second part that is movable relative to the first part, wherein a biasing force urges the second part to the reference position relative to the first part;at least two shape memory alloy, SMA, elements configured, on actuation, to cause opposing actuation forces to be applied to the second part for moving the second part relative to the first part in opposite directions within a range of movement;first and second friction surfaces that are biased against each other with a normal force so as to give rise to a frictional force therebetween for constraining movement of the second part relative to the first part when the at least two SMA elements are not actuated;wherein each of the at least two SMA elements is configured, on actuation, to apply a friction-reducing force to reduce the normal force so as to reduce the static frictional force between the first and second friction surfaces; andwherein, for each of the at least two SMA elements, the SMA element is configured such that the ratio of its friction-reducing force to its actuation force increases with increasing displacement of the second part from the reference position in the direction opposite to the direction in which the SMA element is configured to move the second part.A

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