Actuator assembly

Abstract

An actuator assembly comprises a first part 10 (not shown), a second part 20 which is moveable relative to the first part, and a total of four actuating units 30. A primary axis Z is defined with reference to the first part. Each actuating unit can apply a force to the second part 20 to move the second part 20. At least one of the four actuating units comprises: a body portion 31, an SMA element 34, and a force-modifying element such as a force modifying flexure 32. The SMA element is connected between the body portion and the first part to apply an input force to the body portion. The force-modifying element is connected between the body portion and the first part, and modifies the input force so as to give rise to the actuating force. The four actuating units 30 are configured, on selective actuation, to tilt the second part relative to the first part about two non-parallel tilt axes that are each perpendicular to the primary axis.

Description

Field of the disclosure The present application relates to an actuator assembly with one or more actuating units, each of which includes a shape memory alloy (SMA) element. Background SMA actuator assemblies may be used in a variety of applications for moving a movable part relative to a support structure. For example, WO 2013 / 175197 A1 describes a camera in which four SMA wires are arranged to move a lens element relative to an image sensor in a plane that is perpendicular to the optical axis of the lens element, thereby effecting optical image stabilization (OIS). WO 2010 / 029316 A1 describes SMA actuator wires used to provide OIS in a camera by tilting a camera module. WO 2011 / 104518 A1 describes an actuator assembly having eight SMA wires capable of effecting positional control of a movable element with multiple degrees of freedom. Typically, the range of movement (also known as “stroke”) of such SMA actuator assemblies is limited by the maximum contraction of the SMA wires, and the actuating force is limited by the maximum force that can be generated by the SMA wires. To increase the movement range or the actuating force, longer or thicker SMA wires can be used, but this may be at the expense of increased cost, size and / or power, which may not be practical in miniature applications. WO 2022 / 084699 A1 discloses an actuator assembly comprising at least one actuating unit (incorporating an SMA wire) that, on actuation, moves a movable part relative to the support structure. The actuating unit may be configured to amplify the movement range of the movable part, to amplify the actuating force acting on the movable part, or to re-direct the force applied by the SMA wire. Summary of the disclosure According to an aspect of the present invention, there is provided an actuator assembly comprising: a first part, wherein a primary axis is defined with reference to the first part; a second part that is movable relative to the first part; and a total of four actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part; wherein at least one of the four actuating units comprises: a body portion; an SMA element connected between the body portion and the first part, and configured, on actuation, to apply an input force to the body portion; and a force-modifying element connected between the body portion and the first part, and configured to modify the input force so as to give rise to the actuating force; and wherein the four actuating units are configured, on selective actuation, to: tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; and tilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are non-parallel axes. Optionally, the actuating units are each configured to apply the actuating force with a major force component that is parallel to the primary axis. Optionally, a first pair of the four actuating units are each configured to apply an actuating force with a force component in a first direction, and a second pair of the four actuating units are each configured to apply an actuating force with a force component in a second direction; wherein the first and second directions are opposite directions parallel to the primary axis. Optionally, each actuating unit of the first pair of actuating units are provided on opposite sides of the actuator assembly, and wherein each actuating unit of the second pair of actuating units are provided on opposite sides of the actuator assembly different from the sides the first pair of actuating units are provided on. Optionally, the actuator assembly comprises a bearing arrangement defining the first and second tilt axes; and each of the four actuating units are configured to apply an actuating force with a force component in a first direction parallel to the primary axis (which drives the second part into engagement with the bearing arrangement). In other words, all four actuating units are configured to apply actuating forces with force components in the same direction parallel to the primary axis (which drive the second part into engagement with the bearing arrangement). The bearing arrangement may be provided between the first and second parts. The bearing arrangement may be configured to allow rotation of the second part relative to the first part about any axis substantially perpendicular to the primary axis and optionally about the primary axis. Such a bearing arrangement may restrict other types of movement such as translational movement of the second part relative to the first part, and therefore may help improve performance of the actuator assembly. The bearing arrangement may include, for example, one or more gimbals. The bearing arrangement may include, for example, a pivot bearing. Optionally, the actuating force produced by each actuating unit is at least generally perpendicular to the primary axis. Optionally, the four actuating units are configured, on selective actuation, to rotate the second part relative to the first part about the primary axis. Optionally, a first pair of the four actuating units are each configured to apply an actuating force capable of driving rotation of the second part in a first sense about the primary axis relative to the first part; and a second pair of the four actuating units are each configured to apply an actuating force capable of driving rotation of the second part in a second sense about the primary axis relative to the first part, wherein the first sense is opposite to the second sense. Optionally, each actuating unit of the first pair of actuating units are provided on opposite sides of the actuator assembly, and wherein each actuating unit of the second pair of actuating units are provided on opposite sides of the actuator assembly different from the sides the first pair of actuating units are provided on. Optionally, the actuator assembly comprises a bearing arrangement configured to guide the relative movement between the first and second parts. Optionally, the first and second tilt axes are defined by the bearing arrangement. Optionally, the bearing arrangement is configured to guide rotation of the second part relative to the first part about the primary axis. Optionally, when viewed perpendicular to the primary axis, the actuating forces are offset from the tilt axes. Optionally, the force-modifying element comprises or is a force-modifying flexure. Optionally, the force-modifying element is elongate and is stiff along its length and compliant in a direction perpendicular to its length. Optionally, the at least one of the four actuating units further comprises: a coupling link connected between the body portion and the second part, wherein the coupling link is configured to transmit the actuating force from the body portion to the second part, and wherein the coupling link is compliant in a direction perpendicular to the actuating force. Optionally, the coupling link comprises or is a coupling flexure. Optionally, the coupling link is elongate and is stiff along its length and compliant in a direction perpendicular to its length. Optionally, the coupling link comprises or is an SMA element. Optionally, the body portion, the coupling link and / or the force-modifying element are integrally formed. Optionally, the actuating forces are arranged on four sides of the actuator assembly around the primary axis, wherein the four sides extend in a loop around the primary axis. Optionally, when viewed along the primary axis, none of the actuating forces produced by the four actuating units are collinear. Optionally, the first part or second part comprises one or more lenses and / or an image sensor. Optionally, the primary axis is parallel to the optical axis of the one or more lenses and / or is perpendicular to a light-sensitive region of the image sensor. Optionally, the first part or second part comprises an emitter, a display, or a part thereof. Optionally, the primary axis is perpendicular to a plane defined by the display and / or is parallel to the general direction in which radiation is emitted from the emitter. According to another aspect of the present invention, there is provided an actuator assembly comprising: a first part, wherein a primary axis is defined with reference to the first part; a second part that is movable relative to the first part; a bearing arrangement configured to guide the relative movement between the first and second parts; and a plurality of actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part, wherein at least one of the actuating units comprises: a body portion; an SMA element connected between the body portion and the first part, and configured, on actuation, to apply an input force to the body portion; and a force-modifying element connected between the body portion and the first part, and configured to modify the input force so as to give rise to the actuating force; and wherein the actuating units are configured, on selective actuation, to: tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; and tilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are non-parallel axes. Brief description of the drawings A specific embodiment of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic cross-sectional view of an arrangement of a first part and a second part of an actuator assembly according to an embodiment of the present disclosure. Figure 2 shows a schematic perspective view of a first part and a second part of an actuator assembly according to an embodiment of the present disclosure. Figures 3A to 3E show schematic cross-sectional views of different variations of a camera module incorporating an actuator assembly according to an embodiment of the present disclosure. Figure 4 shows a schematic perspective view of a second part of an actuator assembly according to an embodiment of the present disclosure, indicating directions of actuating forces that may be applied to the second part. Figure 5 shows an actuating unit of an actuator assembly according to an embodiment of the present disclosure. Figure 5A shows a schematic perspective view of the actuating unit. Figure 5B shows a schematic side view of the actuating unit. Figure 6 shows a schematic side view of an actuator assembly according to an embodiment of the present disclosure, showing the actuating unit. Figure 7A shows a schematic perspective view of an actuator assembly according to an embodiment of the present disclosure, showing the actuating units. Figure 7B shows magnified plan view of an actuating unit. Figure 8A shows a schematic perspective view of an actuator assembly according to an embodiment of the present disclosure, showing the actuating units. Figure 8B shows an actuating unit comprising an anti-translational flexure mechanism. Figure 9 shows an actuating unit comprising an anti-translational flexure compliance mechanism. Figure 10 shows an anti-translational flexure compliance mechanism. Figure 10A shows a side view of the anti-translational flexure compliance mechanism. Figure 10B shows a top view of the anti-translational flexure compliance mechanism. Figure 11 shows a schematic perspective view of an actuator assembly according to an embodiment of the present disclosure, showing the actuating units. Figure 12A shows a schematic top view of an actuator assembly according to an embodiment of the present disclosure, indicating forces that may be applied by four actuating units. Figure 12B shows a schematic perspective view of an actuator assembly according to an embodiment of the present disclosure, indicating forces that may be applied by four actuating units. Figures 13A to 13F show a schematic perspective view of an actuator assembly according to an embodiment of the present disclosure, indicating the movement of the second part achieved by applying actuating forces using one or more of the four actuating units. Figure 14 shows a perspective side view of an actuating unit of an actuator assembly according to an embodiment of the present disclosure. Figure 15 shows an actuator assembly according to an embodiment of the present disclosure, showing the actuating units. Figure 15A shows a schematic top view of the actuator assembly. Figure 15B shows a schematic side view of the actuator assembly. Figure 16 shows a gimbal of an actuator assembly according to an embodiment of the present disclosure. Figure 16A shows a schematic cross-sectional view of the gimbal in the actuator assembly, in the x-y plane. Figure 16B shows a schematic cross-sectional view of the gimbal, in the z-x plane. Figure 17 shows a gimbal of an actuator assembly according to an embodiment of the present disclosure, wherein the movable part is rotated about the z axis. Figure 17A shows a schematic cross-sectional view of the gimbal in the actuator assembly, in the x-y plane, wherein the movable part of the actuator assembly is rotated about the z axis. Figure 17B shows a schematic cross-sectional view of the gimbal in the z-x plane, wherein the movable part of the actuator assembly is not tilted with respect to the x or y axes. Figure 18 shows a gimbal of an actuator assembly according to an embodiment of the present disclosure, wherein the movable part is rotated about the z axis. Figure 18A shows a schematic cross-sectional view of the gimbal in the actuator assembly, in the x-y plane, wherein the movable part of the actuator assembly is not rotated about the z axis. Figure 18B shows a schematic cross-sectional view of the gimbal in the z-x plane, wherein the movable part of the actuator assembly is tilted about the y axis. Figure 19 shows a schematic perspective view of a second part of an actuator assembly according to an embodiment of the present disclosure, indicating directions of actuating forces that may be applied to the second part. Figure 20 shows a schematic perspective view of a second part of an actuator assembly according to an embodiment of the present disclosure, indicating directions of actuating forces that may be applied to the second part. Detailed description According to an embodiment of the present disclosure, an actuator assembly comprises a first part, a second part and a total of four actuating units. A primary axis is defined with reference to the first part. The second part is movable relative to the first part. The actuating units are each configured to apply an actuating force to the second part, wherein the actuating force is capable of moving the second part relative to the first part. The first part may also be referred to herein as a support structure. The second part may also be referred to herein as a movable part. In certain examples, the actuator assembly may be incorporated in an apparatus such as a camera module. The second part of the actuator assembly may comprise an optical element and / or an imaging element. The apparatus may be incorporated in a portable electronic device, such as a smartphone. With reference to Figure 1, a cross section of an exemplary arrangement of a first part 10 (also referred to as a support structure 10) and a second part 20 (also referred to as a movable part 20) of an actuator assembly 2 is illustrated. The second part 20 is shown in a central position and orientation with respect to the first part. This may be the position of the second part 20 when no actuating forces are applied. In certain arrangements, an apparatus 1 may incorporate the actuator assembly 2. In this example, the primary axis A passes through the centre of the first part 10. The primary axis A 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 A. In other words, the extent of the actuator assembly 2, the support structure 10 and / or the movable part 20 along the primary axis A is less than the extent thereof along any direction perpendicular to the primary axis A. The primary axis A 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 A. Alternatively or additionally, in examples in which the apparatus 1 includes an optical element (such as a lens assembly) with an optical axis, or an imaging element (such as an imager sensor) with an imaging axis, the primary axis A 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. A primary axis A can be defined with reference to the actuator assembly 2 and / or the support structure 10. Figure 2 schematically shows a perspective view of an actuator assembly 2. The actuator assembly 2 includes a support structure (or first part) 10 and a movable part (or second part) 20. The movable part 20 is movable relative to the support structure 10. When the actuator assembly 2 is included, for example, 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 may not be included in the apparatus 1. The actuator assembly 2 of certain embodiments of the present disclosure includes a total of four actuating units 30. Two actuating units are shown in Figure 2, for simplicity. 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. 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 A may also be referred to as the z axis, and two further axes that are perpendicular to the primary axis A and to each other may be referred to as the x and y axes. The directions of these axes are illustrated in Figure 2. The origin may be at the centre of the movable part 20. 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 A. 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 A1 and WO 2017 / 072525 A1, 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 A1, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 30, similarly to WO 2011 / 104518 A1 which discloses an actuator assembly with 8 SMA wires connected between the support structure 10 and the movable part 20. WO 2011 / 104518 A1 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 A1, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 30, similarly to WO 2011 / 104518 A1. 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, Tx, 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 A1, 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 Fa between the movable part 20 and the support structure 10. Selectively varying the actuating forces Fa 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 Fa. In simple examples of this, one component of each actuating force Fa causes the movement of the movable part 20, and another component of each actuating force Fa acts against the bearing forces produced by the bearing arrangement 40. Figure 3 illustrates various arrangements of the first part 10 and second part 20 of an actuator assembly 2. These arrangements are exemplary, and are not intended to be limiting. The arrangements shown in Figure 3 show an actuator assembly 2 that, in use, is incorporated into a camera module 1. The actuator assembly comprises a support structure (first part) 10. The actuator assembly illustrated in Figure 3 comprises an optical element, such as a lens assembly 3, and an imaging element, such as 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. In the example shown in Figure 3, the optical axis coincides with the primary axis A, which is defined with reference to the first part 10. 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 module 1 may be a compact camera module in which each lens has a diameter of 20mm or less, for example of 12mm or less. The movable part 20 of the actuator assembly 2 may comprise one or both of the optical element 3 and the imaging element 4. The actuator assembly 2 may further comprise a controller 8, wherein the controller 8 is configured to generate drive signals for each actuating unit. In each of Figures 3A to 3E, the movable part 20 is illustrated by a dashed box. Possible directions of motion are indicated by arrows 5. With reference to Figure 3A, a variation of the camera module 1 is shown, wherein the movable part 20 includes the image sensor 4. This variation may be known as a “sensorshift” variation of the camera module 1. 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 an example, the image sensor 4 may be movable in a horizontal plane, perpendicular to the primary axis A. As illustrated by arrows 5, the image sensor 4 may be movable parallel to the x axis in the ±x directions. The image sensor 4 may additionally or alternatively be movable parallel to the y axis, or in any direction in the x-y plane where the x-y plane is perpendicular to the primary axis A. In the (“lens-shift”) variation shown in Figure 3B, 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 an example, the lens assembly 3 may be movable in a horizontal plane, perpendicular to the primary axis A. As illustrated by arrows 5, the lens assembly 3 may be movable parallel to the x axis in the ±x directions. The lens assembly 3 may additionally or alternatively be movable parallel to the y axis, or in any direction in the x-y plane where the x-y plane is perpendicular to the primary axis A. 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 A 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 module 1. In the sensor-shift variation, the movable part 20 may also be rotatable about the primary axis A so as to also enable compensation for roll. In the (“module-tilt”) variation shown in Figure 3C, 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 A and to each other, and optionally rotate the movable part 20 about the primary axis A, enabling OIS to be implemented in the camera module 1. Possible directions of tilt are indicated by arrows 5. In the (“autofocus”) variation shown in Figure 3D, 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 primary axis A and hence the optical axis O. Such movement has the effect of adjusting the focus of the image on the image sensor 4. So, auto-focus (AF) or zoom functionality can be implemented in the camera module 1. In some examples (not shown), the camera module 1 may include a first actuator assembly for providing OIS as illustrated in Figures 3A-C, and a second actuator assembly for providing AF as illustrated in Figure 3D. 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 3E, 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 module 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 module 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 the actuating assembly of the present disclosure, at least one actuating unit 30 comprises an SMA element.. 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, thereby heating the SMA wires by causing an electric current to flow, will cause the SMA wires to contract and thus actuate the actuating unit 30 so as to move the movable part 20. The drive signals are chosen to drive 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 / zoom 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. Adjusting the tension of the SMA wire in this way allows precise control of the motion of the movable part 20. Depending on the arrangement of the actuating units and on any motion constraints, adjusting tension of the SMA wire(s) may be used to achieve one or more of Rx, Ry, Rz, Tx, Ty and Tz. Optionally, the camera module 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 module 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 module 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. An actuator assembly 2 according to an embodiment of the present disclosure comprises, as discussed above, a first part 10, a second part 20 and a total of four actuating units 30. Each of the four actuating units is configured to apply an actuating force to the second part wherein the actuating force is capable of moving the second part relative to the first part. The first and second parts may be arranged as described above, or in a different manner. The second part may be movable relative to the first part as described in one of the above examples, or in a different manner, At least one of the four actuating units comprises a body portion, an SMA element and a force-modifying element. The SMA element is connected between the body portion and the first part, and is configured, on actuation, to apply an input force to the body portion. The force-modifying element is connected between the body portion and the first part, and is configured to modify the input force so as to give rise to the actuating force. The four actuating units are configured, on selective actuation, to tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; and tilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are non-parallel axes. According to certain embodiments, the actuating units are each configured to apply the actuating force with a major force component that is parallel to the primary axis. The actuating force may be resolved into components parallel to the primary axis and another axis, wherein the major component is the largest of the two components. In certain embodiments, the major component parallel to the primary axis may be significantly larger than the other component, such that the actuating force is substantially parallel to the primary axis. A first pair of the four actuating units may each be configured to apply an actuating force with a force component in a first direction, and a second pair of the four actuating units may each be configured to apply an actuating force with a force component in a second direction; wherein the first and second directions are opposite directions parallel to the primary axis. Each actuating unit of the first pair of actuating units may be provided on opposite sides of the actuator assembly. Each actuating unit of the second pair of actuating units may be provided on opposite sides of the actuator assembly different from the sides the first pair of actuating units are provided on. With reference to Figure 4, actuating forces that can be applied to a second part 20 of an actuator assembly according to certain embodiments of the present disclosure are illustrated. The actuating forces are parallel to the primary axis A, which is parallel to the z axis. In the embodiment illustrated in Figure 4, a first pair of the four actuating units are each configured to apply an actuating force with a force component 411 and 412 in a first direction. A second pair of the four actuating units are each configured to apply an actuating force with a force component 421 and 422 in a second direction; wherein the first and second directions are opposite directions parallel to the primary axis. In the example illustrated in Figure 4, forces 411 and 412 are in the -z direction and forces 421 and 422 are in the +z direction. The forces are illustrated as applied to a second part 20 having a rectangular cuboid shape. The primary axis A is perpendicular to the top and bottom surfaces of the second part 20. The first pair of actuating forces may be applied to opposite sides 431 and 432 of the second part 20. The second pair of actuating forces may be applied to opposite sides 441 and 442 of the second part 20, wherein sides 441 and 442 are different from sides 431 and 432. With reference to Figure 4, the second part 20 may be configured to be tiltable about the x axis and about the y axis. In an event that only force 421 is applied, in the +z direction, side 441 of the second part 20 moves in the +z direction and side 442 of the second part 20 moves in the -z direction. The second part 20 tilts about the x axis. In an event that only force 422 is applied, in the +z direction, side 442 of the second part 20 moves in the +z direction and side 441 of the second part 20 moves in the -z direction. The second part 20 tilts about the x axis in the opposite sense. In an event that only force 411 is applied, in the -z direction, side 431 of the second part 20 moves in the -z direction and side 432 of the second part 20 moves in the +z direction. The second part 20 tilts about the y axis. In an event that only force 412 is applied, in the -z direction, side 432 of the second part 20 moves in the -z direction and side 431 of the second part 20 moves in the +z direction. The second part 20 tilts about the y axis, in the opposite sense. The second part 20 may be further configured to be movable along the primary axis. In an event that force 411 and force 412 are applied, sides 431 and 432 move in the -z direction and the second part 20 moves in the -z direction. In an event that force 421 and force 422 are applied, sides 441 and 442 move in the +z direction and the second part 20 moves in the +z direction. Each actuating force may be applied to any point on a side of the second part 20. The second part 20 may be a different shape to that illustrated in Figure 4. The actuating forces are each applied by an actuating unit 30. Each actuating unit 30 comprises a body portion; an SMA element connected between the body portion and the first part; and a force-modifying element connected between the body portion and the first part. The SMA element is configured, on actuation, to apply an input force to the body portion. The force-modifying element is configured to modify the input force so as to give rise to the actuating force. Modification of the input force may comprise modifying the magnitude of the input force and / or modifying the direction of the input force. Figure 5A shows a perspective view of an example of the actuating unit 30. Figure 5B shows part of the actuating unit 30 in plan view. A single actuating unit 30 is shown in Figures 5A and 5B, but it will be appreciated that the actuator assembly 2 has four actuating units 30, each of which may include the same components described with reference to Figures 5A and 5B. The actuating unit 30 includes a body portion 31 to which several other components of the actuating unit 30 are connected as described below. Typically, the body portion 31 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 30. In some examples, the body portion 31 is not a distinct part of the actuating unit 30. For example, the body portion 31 may be defined as part of one of the other components of the actuating unit 30 or simply as a connection point between other components of the actuating unit 30. The actuating unit 30 also includes a force-modifying flexure 32. The force-modifying flexure 32 is connected between the body portion 31 and the support structure 10. One end of the force-modifying flexure 32 is connected to the body portion 31. The other end of the force-modifying flexure 32 is connected to the support structure 10, e.g. via a foot portion 36. The foot portion 36 is fixed relative to the support structure 10. Typically, the foot portion 36 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 30. The force-modifying flexure 32 allows the body portion 31 to pivot relative to the support structure 10 about an effective pivot point P. Although the effective pivot point P is shown in Figure 5B as being positioned in the middle of force-modifying flexure 32, the effective pivot point P may have a different position and also need not lie on the force-modifying flexure 32. Such pivotal movement of the body portion 31 relative to the support structure 10 is initially in a direction that is substantially perpendicular to the force-modifying flexure 32. In this example, the force-modifying flexure 32 is connected to an end of the body portion 31. However, the force-modifying flexure 32 may be connected to another position on the body portion 31. The actuating unit 30 also includes an SMA element 34. In this example, the SMA element 34 is an SMA wire 34. The SMA wire 34 is connected between the body portion 31 and the support structure 10. One end of the SMA wire 34 is connected to the support structure 10, e.g. by a crimp 15. The other end of the SMA wire 34 is connected to the body portion 31, e.g. by a crimp 35. The crimp 35 is movable with relative to the support structure 10, such that contraction of the SMA wire 34 moves the crimp 35 and, in turn, causes the body portion 31 to pivot about the pivot point P. 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, e.g. by a crimp or other connection 37. The coupling link 33 transfers or transmits an actuating force Fa 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 Fa. This allows the movable part 20 to move in directions other than the direction of the coupling flexure 33 and actuating force Fa. This can be needed, for example, where different actuating units 30 cause the movable part 20 to move in different directions. The coupling link 33 may be connected to a different position of the body portion 31 than that shown in this example. In this example, the body portion 31, the force-modifying flexure 32, the coupling flexure 33 and the foot portion 36 are integrally formed, for example from a single sheet of material (such as metal). In other examples, one or more or these features, if present, may be formed from different parts or materials. The SMA wire 34 is arranged, on contraction, to apply an input force Fj on the body portion 31. The input force Fj 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 Fj so as to give rise to the actuating force Fa, 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 forcemodifying flexure 32, thereby causing the body portion 31 to pivot about the effective pivot point P. In simple terms, the force-modifying flexure 32 and the body portion 31 act like a lever. The force-modifying flexure 32 and the body portion 31 may modify the direction and / or the magnitude of the input force Fi so as to give rise to the actuating force Fa. Effectively, the movement of the SMA results in movement of the point at which the coupling flexure is attached to the movable part 20. The direction and / or magnitude of the movement is modified by modifying the direction and / or the magnitude of the input force Fj. 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 90°-a = -60° relative to the length of the SMA wire 34. The direction of the input force and, therefore, the movement is changed by an angle of -90°. The magnitude of the force depends on the ratio of Ds to De, wherein Ds is the perpendicular distance from the line on which the SMA wire 34 lies to the effective pivot point P and wherein De is the perpendicular distance from the line on which the coupling flexure 33 lies to the effective pivot point P. The moments around the effective pivot point P balance, so FiDs = FaDc and, therefore, the ratio Fa / Fj is proportional to the ratio Ds / Dc. In this example, the SMA wire 34 lies on a line that is closer to the effective pivot point P than the line on which the coupling flexure 33 lies. Ds is smaller than De, so the input force Fj is de-amplified. As the distance De from the coupling flexure to the effective pivot point is larger than the distance Ds from the SMA wire to the effective pivot point, the distance moved by the coupling flexure as it rotates about effective pivot point P is larger than the distance moved by the SMA wire. The angle by which the coupling flexure rotates is small, and so the major component of the movement of the coupling flexure is in a direction -90°. Thus, it will be appreciated that, in this example, the force is de-amplified and the stroke is amplified, while the direction of the forces / movements is changed by an angle of ~90°. More generally, the change in direction of the force depends on the angle between the SMA wire 34 and the coupling flexure 33. Also more generally, the change in magnitude of the force is dependent on the ratio of i) the distance Ds from the effective pivot point P to the line on which the SMA wire 34 lies and ii) the distance De from the effective pivot point P to the line on which the coupling flexure 33 lies. In particular, Fa / Fi is proportional to Ds / Dc. If the SMA wire 34 lies on a line that is closer to the effective pivot point P than the line on which the coupling flexure 33 lies, then the input force F, is de-amplified. At the same time, the movement of the movable part 20 is amplified, i.e. increased relative to a change in length of the SMA wire 34. Alternatively, if the SMA wire 34 lies on a line that is further away from the effective pivot point P than the line on which the coupling flexure 33 lies, then the input force Fi is amplified. At the same time, the movement of the movable part 20 is de-amplified, i.e. decreased relative to a change in length of the SMA wire 34. The actuating unit 30 can thus be configured to amplify movement or to amplify force due to contraction of the SMA wire 34. The actuating unit 30 can also be configured to change the direction of the input force Fj. In some examples, the actuating unit 30 is configured to change the direction of the input force Fj 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 point P, thereby decreasing Ds / Dc and so increasing the amount of stroke amplification. In summary, the amount by which the force-modifying flexure 32 amplifies or de-amplifies the force / stroke of the SMA wire 34 may be tailored by: adjusting the orientation of the SMA wire 34 (and thus of the input force Fj); 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 F, acts on the body portion 31); adjusting the orientation of the coupling flexure 33 (and thus of the actuating force Fa); 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 Fa). 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 5A and 5B, 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 5A and 5B, 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. For example, in an event that the actuating unit 30 is arranged adjacent to a side of the movable part 20, the SMA wire 34 may be parallel to and proximate to a first edge of the movable part 20, and the coupling link 33 may be parallel to an proximate to a second edge of the movable part 20 wherein the first and second edges meet at a corner of the side of the movable part 20. 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. With reference to Figure 6, a possible arrangement of an actuating unit 30 relative to a side of the second part 20 is illustrated. The actuating unit 30 may be arranged in a plane, such that 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 parallel to the side of the second part 20. In the example shown in Figure 6, the SMA wire 34 is connected to the support structure 10 (not shown) by crimp 15. The SMA wire 34 is parallel to the top surface of the movable part 20, i.e. parallel to the x-y plane. The SMA wire 34 is connected to the body portion 31 by a crimp 35. The force-modifying flexure 32 is connected between the body portion 31 and the support structure 10, wherein the force-modifying flexure 32 is connected to the support structure at 36. Coupling link 33 is connected to the body portion 31 and is connected to the movable part 20 at 37. When the SMA wire 34 contracts (an input force in the -x direction), the body portion rotates anti-clockwise about the effective pivot point, and the coupling link 33 moves downwards in the -z direction. Figure 7 illustrates a possible arrangement of four actuating units 30 relative to a movable part 20. In this example, the movable part 20 is shown as a rectangular cuboid. Figure 7A shows all four actuating units 30. Figure 7B shows a magnified section of an actuating unit 30. Each actuating unit 30 may be arranged in a plane, such that 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 parallel to the side of the second part 20. In the example shown in Figure 7A, the SMA wire 34 is connected to the support structure 10 (not shown) by crimp 15. The SMA wire 34 is parallel to the x-y plane, such that the SMA wire 34 of each actuating unit 30 is parallel to the top and bottom edges of the side of the movable part 20 to which the actuating unit is attached. Each SMA wire 34 is connected to a body portion 31 by a crimp 35. Each force-modifying flexure 32 is connected between the body portion 31 and the support structure 10, wherein the force-modifying flexure 32 is connected to the support structure at 36. Coupling link 33 is connected to the body portion 31 and is connected to the movable part 20 at 37. In this example, the coupling link 33 of each actuating unit 30 is connected to the movable part 20 proximate to a corner of a side of the movable part 20. However, the coupling link 33 may be connected to the movable part 20 in a different location. The opposite sides 711 and 712 are movable in the +z direction by the actuating units connected to sides 711 and 712. The opposite sides 721 and 722 are movable in the -z direction by the actuating units connected to sides 721 and 722. In the configuration shown in Figure 7A, a corner of each side of the movable part 20 may be movable parallel to the z axis, as each coupling link 33 is connected near to a corner of a side of the movable part 20. The corners of movable part 20 may be movable parallel to the z axis. Movement of the movable part 20 may constrained in one or more directions, and / or the movement of the movable part 20 may be guided (for example by bearings) such that the movement of one or more corners of the movable part results in a different motion of the movable part 20. The movable part 20 may be tiltable about one or more of the x axis, y axis and diagonal axis in the x-y plane. The movable part 20 may be movable parallel to the z axis. In the example shown in Figure 7, each coupling link 33 is connected proximate to the right hand edge of each side of the movable part 20. The coupling links 33 of each actuator may not be connected to the same edges of each side of the movable part 20. The coupling links 33 may be connected to a different location of the movable part 20. For example, the coupling links 33 may be connected to a different location proximate to an edge of the side of the movable part 20, or a location that is not proximate to an edge of the side of the movable part 20. Connecting the coupling links 33 to different locations of the movable part 20 may result in different motions of the movable part 20. With reference to Figure 8, another arrangement of the four actuating units is illustrated relative to the support structure 10 and the movable part 20. Figure 8A shows a perspective view. Figure 8B shows a side view. The support structure 10 is shown as a rectangular cuboid. The movable part 20 has four arms and a central portion. The movable part 20 may be configured to tilt about the central portion. Each arm of the movable part 20 is connected at 37 to a coupling link 33 of an actuating unit 30. Other shapes and arrangements of movable part 20 are possible, wherein the movable part 20 connects at 37 to coupling link 33 of the four actuating units 30. The body portion 31 has a bent shape, wherein the body portion 31 is connected to the flexure at the bend point of the body portion 31. The coupling link 33 is connected to an end of the body portion 31. With reference to Figure 8B, movable connections (such as crimps) are shown as circles. Static connections are shown as rectangles or squares. The SMA wire 34, which can contract in a direction parallel to its length, is shown as a dashed line. Rigid body portion 31 is shown as a bold solid line. Flexures 32, 33 and 81, which can bend or deform in a direction in the plane of the actuating unit, are indicated by dotted lines. On contraction of the SMA wire 34, the crimp 35 moves in the +x direction. Body portion 31 pivots about an effective pivot point P, such that the arm of the movable part 20 is moved in the -z direction. The body portion 31 rotates such that the end of the body portion 31 that is attached to the coupling link 33 rotates clockwise (as viewed in Figure 8B). At connection 37, the movable part 20 is pulled in the direction of motion of the end of the body portion 31 that is attached to the coupling link 33. Due to the small angle approximation, the major component of the movement of connection 37 and therefore the arm of the movable part 20 is in the -z direction. In certain embodiments, the connection 37 to the arm of the movable part may be constrained from moving in one or more directions perpendicular to the z axis. In certain embodiments, the arm of the movable part 20 may be further connected via 81 to a portion 82. For example, the arm of the movable part 20 may be connected to a static portion 82 by an anti-translation flexure 81. As in the examples above, a first pair of the four actuating units are each configured to apply an actuating force with a force component in a first direction (parallel to the +z direction), and a second pair of the four actuating units are each configured to apply an actuating force with a force component in a second direction (parallel to the -z direction). The first and second directions are opposite directions parallel to the primary axis. Each actuating unit of the first pair of actuating units are provided on opposite sides of the actuator assembly. Each actuating unit of the second pair of actuating units are provided on opposite sides of the actuator assembly different from the sides the first pair of actuating units are provided on. In the examples provided above, the actuating forces are parallel to the primary axis (in this case parallel to the z axis) such that the movement of the movable part 20 at the point at which it is connected to the actuating unit 30 is parallel to the primary axis. The actuating units may comprise one or more components to constrain translational motion of the movable part at the point at which it is connected to the actuating unit 30, i.e. to constrain translational motion of connection 37. Each actuating unit 30 may comprise an anti-translational component configured to constrain translation in the x-y plane of connection 37 (i.e. of the movable part 20 at the point at which it is connected to the actuating unit 30). As illustrated in Figure 8B, the anti-translational component 81 may be connected between the movable part 20 and a static component 82. The anti-translational component 81 may be under tension, such that when the end of the body portion 31 that is attached to the coupling link 33 rotates clockwise, the arm of the movable part 20 that is connected to the coupling link 33 can move only in the -z direction, and not in the -x direction. The anti-translational component 81 may comprise an anti-translational flexure. The anti-translational flexure 81 may minimise cross-talk during operation of the actuator assembly. The arrangement of actuating units 30 illustrated in Figure 8A may result in rotation about the z axis during a change to the tilt of the actuator assembly. An anti-translational component may prevent rotation about the z axis. The actuating unit 30 shown in Figure 8B comprises an anti-translational component 81 that is under tension. In an event that the actuator assembly 2 comprising the actuating unit 30 is dropped or experiences a force or shock, the anti-translational component 81 may break. With reference to Figure 9, an actuating unit 30 comprising an anti-translational component 81 is illustrated, wherein the actuating unit 30 further comprises a compliance mechanism 91 connecting the anti-translational component 81 to the static component 82. The compliance mechanism 91 may be a separate component to the anti-translational component 81 or may be an extension of the anti-translational component 81. In the example shown in Figure 9, the compliance mechanism 91 comprises a spring. The actuating unit 30 shown in Figure 9 has a high enough stiffness to constrain translational motion of the movable part 20 at the point at which it is connected to the actuating unit 30, but has compliance that allows the actuating unit to elongate during a shock event. Actuating units such as that shown in Figure 9 may be used in the actuator assembly illustrated in Figure 8A. Figure 10 shows an alternative anti-translation mechanism of an actuating unit 30. Figure 10A shows a side view and Figure 10B shows a top view. The coupling link 33 of the actuating unit 30 connects to the movable part 20 at 37. The anti-translation flexure 1010 connects to the support structure 10 at 1011. The anti-translation component comprises a buckling arm 1012 configured to buckle out of plane in an event that the actuating unit 30 experiences a force or shock. In an event that the actuating unit 30 experiences a shock, the buckling arm buckles in the x-y plane and the anti-translation flexure moves to an endstop. Figure 11 shows an actuator assembly similar to that shown in Figure 8A, and comprising similar actuating units 30 to that shown in Figure 9. However, the orientation of the actuating units differs from Figure 8A. In Figure 8A, actuating units on adjacent sides of the actuator assembly are rotated by 180°. In other words, the static crimp 15 attached to the SMA wire 34 of one actuating unit is adjacent to the static crimp 15 attached to the SMA wire 34 of another actuating unit. Similarly, the movable crimp 35 attached to the SMA wire 34 of one actuating unit is adjacent to the movable crimp 35 attached to the SMA wire 34 of another actuating unit. For the actuator assembly of Figure 11, actuating units on adjacent sides of the actuator assembly are flipped about the horizontal axis. In other words, the static crimp 15 attached to the SMA wire 34 of one actuating unit is adjacent to the movable crimp 35 attached to the SMA wire 34 of another actuating unit. This creates a net torque. The net torque can be resisted by anti-translation flexures, which are held under higher tensions than for the actuator assembly of Figure 8A. This may ensure that the antitranslation flexures are always in tension, such that undesirable compression states of the anti-translation flexures are avoided. In the examples described with reference to Figures 4 to 11, the input force applied by contracting the SMA wire was modified to change both its magnitude and its direction, such that a horizontal input force is modified to a vertical actuating force. In other embodiments, a horizontal input force may be modified to change its magnitude, such that a component of the actuating force remains horizontal. In certain embodiments, the direction of the input force may be modified by 180°. In certain embodiments, the four actuating units 30 may be configured to apply actuating forces with major components in the x-y plane. The actuating force produced by each actuating unit 30 may be at least generally perpendicular to the primary axis. The four actuating units 30 may be configured, on selective actuation, to rotate the second part 20 relative to the first part 10 about the primary axis A. In certain embodiments, a first pair of the four actuating units 30 are each configured to apply an actuating force Fa capable of driving rotation of the second part 20 in a first sense about the primary axis A relative to the first part 10. A second pair of the four actuating units 30 are each configured to apply an actuating force capable of driving rotation of the second part 20 in a second sense about the primary axis A relative to the first part 10, wherein the second sense is opposite to the first sense. Each actuating unit 30 of the first pair of actuating units 30 may be provided on opposite sides of the actuator assembly 2. Each actuating unit 30 of the second pair of actuating units 30 may be provided on opposite sides of the actuator assembly 2 different from the sides the first pair of actuating units 30 are provided on. In certain embodiments, the first pair of actuating units 30 may each be configured to apply an actuating force with a major component parallel to the x axis. Each actuating unit 30 of the first pair of actuating unit 30 may apply an actuating force in opposite directions parallel to the x axis, such that in an event that both actuating units 30 of the first pair of actuating units apply an actuating force, a torque is applied about the z axis (wherein the primary axis A is parallel to the z axis). Each actuating unit 30 of the second pair of actuating unit 30 may apply an actuating force in opposite directions parallel to the y axis, such that in an event that both actuating units 30 of the second pair of actuating units apply an actuating force, a torque is applied about the z axis in the opposite sense to that applied by the first pair of actuating units. With reference to Figure 12, an example of an actuator assembly 2 is illustrated wherein the actuating force produced by each actuating unit 30 is at least generally perpendicular to the primary axis. A plan view of the actuator assembly 2 is shown in Figure 12A, and a perspective view of the positions of the actuating units 30 relative to the second part 20 is shown in Figure 12B. The four actuating units 30 are configured to apply actuating forces between the second part 20 and the first part 10. The actuating forces illustrated in Figure 12 are in a plane. This arrangement of actuating units 30 may be used, for example, in examples in which the second part 20 is movable relative to the first part 10 in a movement plane. Tx, Ty and optionally Rz movement of the second part 20 may be allowed. The four actuating units 30 of Figure 12 are in an arrangement capable of applying actuating forces Fa so as to move the second part 20 relative to the first part 10 to any position within a range of movement. The range of movement may be within a movement plane that is perpendicular to the primary axis A. In certain embodiments, two actuating units 30 are arranged to apply actuating forces Fa in opposite directions parallel to a first axis (e.g. the x axis). The other two actuating units are arranged to apply actuating forces Fa in opposite directions parallel to a second axis (e.g. the y axis), perpendicular to the first axis. By appropriately varying the difference in actuation amount between the opposing actuating units 30, the movable part 20 may thus be moved independently along the first and second axes. The opposing actuating forces F may not be colinear, but instead may be offset from each other in a direction perpendicular to the actuating forces Fa. Providing opposing actuating units 30 allows the tension in the SMA wires 30 of the respective actuating units 30 to be controlled, allowing for more accurate and reliable positioning of the movable part 20 compared to a situation in which actuating units 30 do not oppose each other. In some examples, none of the actuating forces Fa are collinear. This allows the arrangement of actuating units 30 to translationally move the movable part 20 without applying any net torque to the movable part 20. The movable part 20 can be moved translationally in the movement plane without rotating the movable part 20 in the movement plane. In an event that translational motion is constrained, the application of actuating forces that would otherwise result in translational motion of the movable part 20 may, instead, result in a tilt of the movable part 20. In general, the arrangement of actuating units 30 is capable of accurately controlling a torque or moment of the movable part 20 about the primary axis P. The actuating units 30 are capable of either rotating or not rotating the movable part 20 relative to the support structure about the primary axis A. The top and bottom actuating units 30 as viewed in Figure 12A are each configured to apply an actuating force Fa parallel to the x axis. The top and bottom actuating units are configured to apply actuating forces in opposite directions parallel to the x axis, and are offset from one another in a direction parallel to the x axis. Therefore, in an event that the top and bottom actuating units 30 are selectively actuated, a torque is applied in a first sense about the primary axis A. In the example indicated in Figure 12A, the first sense is a clockwise direction about the primary axis A (out of the page). The left and right hand actuating units 30 as viewed in Figure 12A are each configured to apply an actuating force Fa parallel to the y axis. The left and right actuating units are configured to apply actuating forces in opposite directions parallel to the y axis, and are offset from one another in a direction parallel to the y axis. Therefore, in an event that the left and right actuating units 30 are selectively actuated, a torque is applied in a second sense about the primary axis A, wherein the second sense is opposite to the first sense. In the example indicated in Figure 12A, the second sense is an anti-clockwise direction about the primary axis A (out of the page). This allows the movable part 20 to be rotated by simultaneously increasing or decreasing the tension of SMA wires in any of the two actuating units 30. In the example illustrated in Figure 12, the four actuating units are configured to apply forces to two opposite corners of the second part 20. Two of the actuating units 30 may be arranged to apply actuating forces Fa to a first corner of the actuator assembly 2. The two actuating units 30 may be connected to adjacent sides of the second part 20, such that each actuating force applied by each of the two actuating units is applied to a side of the second part 20 at a position proximate to the first corner of the actuator assembly. Two of the actuating units 30 may be arranged to apply actuating forces Fa to a second, opposite corner of the actuator assembly 2. The two actuating units 30 may be connected to adjacent sides of the second part 20, such that each actuating force applied by each of the two actuating units is applied to a side of the second part 20 at a position proximate to the second corner of the actuator assembly. The actuator assembly 2, and in particular the movable part 20 and / or the support structure 10, may have a square or rectangular footprint. Each actuating unit 30 may be provided on one of the four sides of the actuator assembly 2. In particular, each actuating unit 30 may bend around a corner of a side of the movable part 20 such that the SMA wire 34 and the coupling flexure 33 of each actuating unit 30 extend along adjacent edges of the movable part 20. For example, the SMA wire 34 and the coupling flexure 33 of each actuating unit 30 may extend along the lower and upper edges respectively of a side of the movable part 20. The four SMA wires 34 (each SMA wire 34 being part of one of the four actuating units 30) may extend along the four lower edges of the movable part 20. The arrangement of actuating forces F applied between movable part 20 and support structure 10 corresponds to the arrangement of SMA wires 30 described in WO2013 / 175197 A1, which is herein incorporated by reference. In this example, the actuating forces Fa are perpendicular to the primary axis A, and may be parallel to the movement plane. However, in general the actuating forces Fa may be angled relative to the movement plane. The actuating forces Fa may thus have a component along the primary axis A. This component along the primary axis A may be resisted by a bearing arrangement, for example, to provide movement of the movable part 20 in degrees of freedom allowed by the bearing arrangement. In some examples, it may even be desirable for actuating forces Fa to have a component in parallel to the primary axis A, for example so as to load plain or rolling bearings arranged between the movable part 20 and the support structure 10. Although, for illustrative purposes, the arrangement of actuating units 30 was described as moving the movable part 20 in the movement plane (e.g. translationally along the x and y axis, or rotationally about the primary axis A), in other examples the movable part 20 may be moved differently. For example, the same arrangement of actuating forces Fa may be used to tilt the movable part 20 relative to the support structure 10 about axes perpendicular to the primary axis A, due to appropriate movement constraints provided by the bearing arrangement. For example, the bearing arrangement may include a plurality of flexures for guiding tilting of the movable part 20 about the axes perpendicular to the primary axis A. Examples of such bearing arrangement are described in WO2022 / 029441 A1, which is herein incorporated by reference. Although the actuator assembly 2 is described herein in the context of four actuating units 30, in general the actuator assembly 2 may include fewer actuating units 30. For example, the actuator assembly 2 may include two actuating units 30, e.g. the two actuating units 30 depicted in the top left of Figure 12A. The forces applied to the movable part 20 by the two actuating units 30 may be opposed by a biasing force of one or more resilient elements, such as springs. With reference to Figure 12A, the two actuating units 30 in the bottom right corner may be replaced with springs applying biasing forces along the corresponding depicted arrows, for example. Other actuating forces may be applied by selectively actuating one or more actuating units 30. For example, a force parallel to either the x or y axis may be applied by selectively actuating only one actuating unit 30. In certain embodiments, the actuating forces are offset from the tilt axes when the actuator assembly is viewed perpendicular to the primary axis. In certain embodiments, the actuator assembly 2 may comprise a component or components configured to guide the relative movement between the first and second parts 10, 20. The actuator assembly 2 may comprise a bearing arrangement configured to guide the relative movement between the first and second parts. The first and second tilt axes may be defined by the bearing arrangement. In certain embodiments, the bearing arrangement is configured to guide the rotation of the second part 20 relative to the first part 10 about the primary axis. The bearing arrangement, or other component configured to guide the relative movement between the first and second parts 10, 20, may be configured to constrain the motion of the second part 20 in certain degrees of freedom. For example, the bearing arrangement may be configured to constrain translational movement in the x-y plane. The second part 20 may be able to rotate about the z axis, tilt about the x axis and tilt about the y axis. With reference to Figure 13, the motion of the second part 20 achieved by selective actuation of one or more actuating units 30 is illustrated, wherein translational movement in the x-y plane is constrained. In Figure 13A, opposite actuating units 30 are actuated, applying actuating forces in opposite directions parallel to the x axis. A torque about the z axis is applied, such that the movable part rotates clockwise about the z axis. In Figure 13B, opposite actuating units 30 are actuated, applying actuating forces in opposite directions parallel to the y axis. A torque about the z axis is applied, such that the movable part rotates anti-clockwise about the z axis. In Figure 13C, one actuating unit 30 is applied, such that an actuating force is applied in the -x direction. As translational motion in the ±x direction is prevented, the second part 20 tilts about the y axis in a first sense. In Figure 13D, one actuating unit 30 is applied, such that an actuating force is applied in the +x direction. As translational motion in the ±x direction is prevented, the second part 20 tilts about the y axis in a second sense, opposite to the first sense. In Figure 13E, one actuating unit 30 is applied, such that an actuating force is applied in the +y direction. As translational motion in the ±y direction is prevented, the second part 20 tilts about the x axis in a first sense. In Figure 13F, one actuating unit 30 is applied, such that an actuating force is applied in the -y direction. As translational motion in the ±y direction is prevented, the second part 20 tilts about the x axis in a second sense, opposite to the first sense. As described above, each actuating unit 30 comprises a force-modifying element. Whereas in the above examples the input force was modified such that the actuating force is at approximately 90° to the input force, in certain embodiments a horizontal input force may be modified to achieve a horizontal actuating force. With reference to Figure 14, a force actuating element is illustrated that is configured to modify an input force such that the actuating force is at 180° to the input force. Like reference numerals indicate like components with reference to Figure 5. The force actuating element comprises an SMA wire 34, configured to contract and apply a horizontal input force Fi. The SMA wire 34 is connected to the body portion 31 by crimp 35, such that on contraction of the SMA wire 34 the body portion 31 rotates about effective pivot point P. The body portion 31 is connected to the first part 10 via force-modifying flexure 32. The body portion 31 is connected to the coupling link 33, such that rotation of the body portion applies a horizontal actuating force by pulling the coupling link 33 horizontally (at small angles). In this example, the body portion 31 and the coupling link 33 are shown as being rigidly connected. The body portion 31 and the coupling link 33 may be integrally formed. The coupling link 33 and the SMA wire 34 may be substantially parallel to one another. As in the actuating unit 30 of Figure 5, the actuating unit 30 includes a body portion 31 to which several other components of the actuating unit 30 are connected as described below. Typically, the body portion 31 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 30. In the example shown in Figure 14, the body portion 31 and the coupling link 33 may be rigid and rigidly connected. In other embodiments, the body portion 31 may not be rigidly connected to the coupling link 33. The coupling link 33 may be rigid or deformable (as described in relation to Figure 5). 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. Typically, the foot portion 36 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 30. The force-modifying flexure 32 allows the body portion 31 to pivot relative to the support structure 10 about an effective pivot point P. Although the effective pivot point P is shown in Figure 5B as being positioned in the middle of force-modifying flexure 32, the effective pivot point P may have a different position and also need not lie on the force-modifying flexure 32. Such pivotal movement of the body portion 31 relative to the support structure 10 is initially in a direction that is substantially perpendicular to the force-modifying flexure 32. The actuating unit 30 also includes an SMA element 34. In this example, the SMA element 34 is an SMA wire 34. The SMA wire 34 is connected between the body portion 31 and the support structure 10. One end of the SMA wire 34 is connected to the support structure 10, e.g. by a crimp 15. The other end of the SMA wire 34 is connected to the body portion 31, e.g. by a crimp 35. The crimp 35 is movable with relative to the support structure 10, such that contraction of the SMA wire 34 moves the crimp 35 and, in turn, causes the body portion 31 to pivot about the pivot point P. 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 Fj so as to give rise to the actuating force Fa, which is transmitted from the body portion 31 to the movable part 20 by the coupling flexure 33. In particular, the input force Fj deforms the forcemodifying flexure 32, thereby causing the body portion 31 to pivot about the effective pivot point P. In simple terms, the force-modifying flexure 32 and the body portion 31 act like a lever. The force-modifying flexure 32 and the body portion 31 may modify the direction and / or the magnitude of the input force Fi so as to give rise to the actuating force Fa. In this example, the actuating force Fa is at 180° to the input force Fi, and the magnitude of the input force is modified. Effectively, the movement of the SMA results in movement of the point at which the coupling flexure is attached to the movable part 20. The direction and / or magnitude of the movement is modified by modifying the direction and / or the magnitude of the input force Fi. In this example, the direction of movement of the point at which the coupling flexure is attached to the movable part 20 is at 180° to the movement of the SMA, and the magnitude of movement of the point at which the coupling flexure is attached to the movable part 20 is larger than the movement of the SMA . As in the example shown in Figure 5, the moments about the effective pivot P balance, so FjDs = FaDc and, therefore, the ratio Fa / Fj is proportional to the ratio Ds / Dc. In this example, the SMA wire 34 lies on a line that is closer to the effective pivot point P than the line on which the coupling flexure 33 lies. Ds is smaller than De, so the input force Fj is deamplified. As the distance De from the coupling flexure to the effective pivot point is larger than the distance Ds from the SMA wire to the effective pivot point, the distance moved by the coupling flexure as it rotates about effective pivot point P is larger than the distance moved by the SMA wire. The SMA wire 34 is parallel to the coupling link 33, so the input and actuating forces are parallel but in opposite directions. With reference to Figure 15, an actuator assembly 2 is illustrated, configured to apply actuating forces as illustrated in Figure 12. The actuating units 30 of the actuator assembly 2 may each comprise the force-modifying flexure illustrated in Figure 14. The actuator assembly 2 may further comprise a gimbal and bearing arrangement configured to guide motion of the second part 20 such that the second part 20 is able to rotate about the z axis, tilt about the x axis and tilt about the y axis. The gimbal and bearing arrangement may be configured to prevent translational motion in the x-y plane. Figure 15A shows a top view of the actuator assembly 2. Figure 15B shows a side view of the actuator assembly 2. The coupling links 33 of the actuating units 30 are connected to the second part 20 at 1510, such that two actuating forces are applied to a first corner of the movable part 20 and two actuating forces are applied to a second corner of the movable part 20, wherein the first corner and second corner are opposite corners. The SMA wire 34 and coupling link 33 of each actuating unit 30 are parallel to the x-y plane. A gimbal 1520 is configured to guide the motion of the second part 20. The movable part 20 may be attached to an optical component 1530 such as a lens. Figure 16A shows a cross-section of the actuator assembly 2 of Figure 15, in the x-y plane. Figure 16A shows a cross-section of the actuator assembly 2 of Figure 15, in the z-x plane. Curved slots 1610 in the first part 10 allow rotation in the x, y and z axes. The gimbal 1520 may comprise balls or curved portions 1620 configured to rotate within the curved slots. The gimbal 1520 may comprise a flat sheet material with balls 1620 imbedded at the pivot points. The balls or curved portions 1620 could also be formed in the sheet material. Figure 17 shows the actuator assembly 2 illustrated in Figure 16, after rotation of the movable part 20 about the z axis. The movable part 20 is parallel to the x-y plane, but rotated about the z axis. Figure 17A shows a cross-section of the actuator assembly 2 of Figure 15, in the x-y plane, after rotation about the z axis. Figure 17A shows a crosssection of the actuator assembly 2 of Figure 15, in the z-x plane, with no tilt about the y axis. Figure 18 shows the actuator assembly 2 illustrated in Figure 16, after tilting of the movable part 20 about the y axis. The movable part 20 is not rotated about the z axis. Figure 18A shows a cross-section of the actuator assembly 2 of Figure 15, in the x-y plane, with no rotation about the z axis. Figure 18A shows a cross-section of the actuator assembly 2 of Figure 15, in the z-x plane, with tilt about the y axis. Tilt about the x axis is also possible in the arrangement shown in Figure 16. Figures 17 and 18 show tilt or rotation about one axis only. Tilt or rotation may occur about only one axis at a given time. Tilt or rotation about more than one axis at a given time may be possible. For example, the rotation about the z axis indicated in Figure 17A and the tilt about the y axis indicated in Figure 18B may occur simultaneously. In drop test, the sheet component would deform allowing the camera module to flex and contact end stops. The gimbal 1520 could be embodied as a stiff material and would flex in normal operation. Alternatively, the gimbal 1520 could be preloaded against the first part 10 and the second part 20. With sufficiently high preload, the gimbal would not flex in normal operation and stroke could be maintained. In other embodiments, an arrangement other than a gimbal may be used to guide the motion of the second part 20. The actuator assembly of certain embodiments may comprise a force-modifying element that comprises or is a force-modifying flexure. In certain embodiments, the force-modifying element is elongate and is stiff along its length and compliant in a direction perpendicular to its length. In certain embodiments, at least one of the four actuating units further comprises a coupling link connected between the body portion and the second part. The coupling link is configured to transmit the actuating force from the body portion to the second part. The coupling link may be compliant in a direction perpendicular to the actuating force. The coupling link may comprise or be a coupling flexure. The coupling link may be elongate and stiff along its length, and compliant in a direction perpendicular to its length. The coupling link may comprise or be an SMA element. In certain embodiments, the body portion, the coupling link and / or the force-modifying element are integrally formed. In certain embodiments, the actuating forces are arranged on four sides of the actuator assembly around the primary axis, wherein the four sides extend in a loop around the primary axis. When viewed along the primary axis, in certain embodiments none of the actuating forces produced by the four actuating units are collinear. In other embodiments, one or more pairs of actuating forces may be collinear. The first part or second part may comprise one or more lenses and / or an image sensor. The primary axis may be parallel to the optical axis of the one or more lenses and / or perpendicular to a light-sensitive region of the image sensor. The first part or second part may comprise an emitter, a display, or a part thereof. The primary axis may be perpendicular to a plane defined by the display and / or is parallel to the general direction in which radiation is emitted from the emitter. In the above-described examples, the actuating unit 30 is arranged in a plane. In particular, the SMA wire 34, the coupling flexure 33 and the force-modifying flexure 32 are arranged so as to substantially extend in a common plane, at least when the actuator assembly 2 is in an initial configuration. This allows for a compact configuration of the actuating unit 30. The body portion 31, when embodied by a plate, may further be arranged to extend in the plane. However, in general, the components of the actuating unit 30 need not be arranged in a common plane. The SMA wire 34 and / or the coupling flexure 33 may be angled relative to the plane, for example. In the above-described examples, the force-modifying flexure 32 is placed in tension on contraction of the SMA wire 34. This reduces the risk of buckling of the force-modifying flexure 32, reducing the risk of damage to the actuator assembly 2 and making the actuator assembly 2 more reliable. However, the force-modifying flexure 32 could instead be arranged so as to be placed under compression on contraction of the SMA wire 34. With reference to Figure 3B, for example, the force-modifying flexure 32 could extend to the bottom-right from the connection point between the body portion 31 and the forcemodifying 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 A1, 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 Fa. 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 Fa. 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 Fa to the movable part 20 while allowing movement of the movable part 20 in directions perpendicular to the actuating force Fa. Such alternative examples of the coupling link 33 are disclosed in WO 2022 / 084699 A1. The coupling link 33 may 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 forcemodifying 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. In certain embodiments, an actuator assembly comprises a first part, a second part, a bearing arrangement and a plurality of actuating units. A primary axis is defined with reference to the first part. The second part is movable relative to the first part. The bearing arrangement is configured to guide the relative movement between the first and second parts. The plurality of actuating units are each configured to apply an actuating force to the second part capable of moving the second part relative to the first part. At least one of the actuating units comprises a body portion, an SMA element connected between the body portion and the first part, and a force-modifying element connected between the body portion and the first part. The SMA element is configured, on actuation, to apply an input force to the body portion. The force-modifying element is configured to modify the input force so as to give rise to the actuating force. The actuating units are configured, on selective actuation, to tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; and tilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are non-parallel axes. The above-described SMA actuator assemblies comprise at least one SMA element. The term ‘shape memory alloy (SMA) element’ may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term ‘SMA element’ may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. It will be appreciated that there may be many other variations of the above-described examples. For example, the actuator assembly may include different types of actuating units to those described above. Examples of such actuating units include a folded SMA wire arrangement as disclosed in WO 2021 / 111131 A1, a V-shaped SMA wire with a compliant connector as disclosed in WO 2013 / 121225 A1, a scissor jack arrangement as disclosed in WO 2021 / 156458 A1, a two-stage arrangement as disclosed in WO 2021 / 111181 A1, or simply an SMA wire connected between the support structure 10 and the movable part 20. The documents referred to in the preceding sentence are each herein incorporated by reference. The actuator assembly may have any number of different types of actuating units, and may have any suitable number of actuating units of each type. It will be appreciated that the movable part 20 may be supported on the support structure 10 exclusively by the actuating units 30 in the embodiments of e.g. Figures 4 to 11. However, such embodiments may also have a bearing arrangement 100 between the support structure 10 and the movable part 20, as shown schematically in Figure 19. The bearing arrangement 100 may be configured to allow rotation of the movable part 20 relative to the support structure 10 about any axis substantially perpendicular to the primary axis A and optionally about the primary axis A. The bearing arrangement 100 may restrict other types of movement such as translational movement of the movable part 20 relative to the support structure 10, and therefore may help improve performance of the actuator assembly. The bearing arrangement 100 may include, for example, one or more gimbals. The bearing arrangement 100 may include, for example, a pivot bearing. The bearing arrangement 100 may include, for example, bearing arrangements 100 equivalent or similar to those described in WO 2021 / 209770 A1 (which is herein incorporated by reference) with reference to e.g. Figures 10, 16 or 17 thereof. As illustrated in Fig. 19 and as discussed above in relation to the embodiments of e.g. Figures 4 to 11, four actuating units 30 may be provided comprising: a first pair of actuating units 30 each configured to apply an actuating force with a force component in a first direction, and a second pair of actuating units 30 each configured to apply an actuating force with a force component in a second direction; wherein the first and second directions are opposite directions parallel to the primary axis A. Alternatively, as illustrated in Figure 20, all four actuating units 30 may each be configured to apply an actuating force with a major force component in a first direction parallel to the primary axis, which drives the second part into engagement with a bearing arrangement 100 defining the first and second tilt axes. In other words, all four actuating units 30 may be configured to apply actuating forces with major force components in the same direction along the primary axis A, which drive the second part into engagement with a bearing arrangement 100, which defines the first and second tilt axes. The bearing arrangement 100 may be provided between the support structure 10 and the movable part 20. The bearing arrangement 100 may restrict other types of movement such as translational movement of the movable part 20 relative to the support structure 10, and therefore may help improve performance of the actuator assembly. The bearing arrangement 100 may include, for example, one or more gimbals. The bearing arrangement 100 may include, for example, a pivot bearing which e.g. forms part of the support structure 10. The bearing arrangement 40 may include, for example, bearing arrangements 40 equivalent or similar to those described in WO 2021 / 209770 A1 (which is herein incorporated by reference) with reference to e.g. Figures 10, 16 or 17 thereof.

Claims

1. An actuator assembly comprising:a first part, wherein a primary axis is defined with reference to the first part;a second part that is movable relative to the first part; anda total of four actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part;wherein at least one of the four actuating units comprises:a body portion;an SMA element connected between the body portion and the first part, and configured, on actuation, to apply an input force to the body portion; anda force-modifying element connected between the body portion and the first part, and configured to modify the input force so as to give rise to the actuating force; andwherein the four actuating units are configured, on selective actuation, to:tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; andtilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are non-parallel axes.

2. The actuator assembly of claim 1, wherein the actuating units are each configured to apply the actuating force with a major force component that is parallel to the primary axis.

3. The actuator assembly of claim 2 wherein a first pair of the four actuating units are each configured to apply an actuating force with a force component in a first direction, and a second pair of the four actuating units are each configured to apply an actuating force with a force component in a second direction; wherein the first and second directions are opposite directions parallel to the primary axis.

4. The actuator assembly of claim 3 wherein each actuating unit of the first pair of actuating units are provided on opposite sides of the actuator assembly, and wherein each actuating unit of the second pair of actuating units are provided on oppositesides of the actuator assembly different from the sides the first pair of actuating units are provided on.

5. The actuator assembly of claim 2, comprising a bearing arrangement defining the first and second tilt axes; and wherein each of the four actuating units are configured to apply an actuating force with a force component in a first direction parallel to the primary axis.

6. The actuator assembly of claim 1 wherein the actuating force produced by each actuating unit is at least generally perpendicular to the primary axis.

7. The actuator assembly of claim 6 wherein the four actuating units are configured, on selective actuation, to rotate the second part relative to the first part about the primary axis.

8. The actuator assembly of claim 6 or 7 wherein a first pair of the four actuating units are each configured to apply an actuating force capable of driving rotation of the second part in a first sense about the primary axis relative to the first part; and a second pair of the four actuating units are each configured to apply an actuating force capable of driving rotation of the second part in a second sense about the primary axis relative to the first part, wherein the first sense is opposite to the second sense.

9. The actuator assembly of claim 8 wherein each actuating unit of the first pair of actuating units are provided on opposite sides of the actuator assembly, and wherein each actuating unit of the second pair of actuating units are provided on opposite sides of the actuator assembly different from the sides the first pair of actuating units are provided on.

10. The actuator assembly of any of claims 6 to 9 wherein the actuator assembly comprises a bearing arrangement configured to guide the relative movement between the first and second parts.

11. The actuator assembly of claim 10 wherein the first and second tilt axes are defined by the bearing arrangement.

12. The actuator assembly of claim 10 or 11 wherein the bearing arrangement is configured to guide rotation of the second part relative to the first part about the primary axis.

13. The actuator assembly of any of claims 6 to 12 wherein, when viewed perpendicular to the primary axis, the actuating forces are offset from the tilt axes.

14. The actuator assembly of any preceding claim wherein the force-modifying element comprises or is a force-modifying flexure.

15. The actuator assembly of any preceding claim wherein the force-modifying element is elongate and is stiff along its length and compliant in a direction perpendicular to its length.

16. The actuator assembly of any preceding claim wherein the at least one of the four actuating units further comprises: a coupling link connected between the body portion and the second part, wherein the coupling link is configured to transmit the actuating force from the body portion to the second part, and wherein the coupling link is compliant in a direction perpendicular to the actuating force.

17. The actuator assembly of claim 16 wherein the coupling link comprises or is a coupling flexure.

18. The actuator assembly of claim 16 or 17 wherein the coupling link is elongate and is stiff along its length and compliant in a direction perpendicular to its length.

19. The actuator assembly of claim 16 wherein the coupling link comprises or is an SMA element.

20. The actuator assembly of any of claims 16 to 19 wherein the body portion, the coupling link and / or the force-modifying element are integrally formed.

21. The actuator assembly of any preceding claim wherein the actuating forces are arranged on four sides of the actuator assembly around the primary axis, wherein the four sides extend in a loop around the primary axis.

22. The actuator assembly of any preceding claim wherein, when viewed along the primary axis, none of the actuating forces produced by the four actuating units are collinear.

23. The actuator assembly of any preceding claim wherein the first part or second part comprises one or more lenses and / or an image sensor.

24. The actuator assembly of claim 23 wherein the primary axis is parallel to the optical axis of the one or more lenses and / or is perpendicular to a light-sensitive region of the image sensor.

25. The actuator assembly of any preceding claim wherein the first part or second part comprises an emitter, a display, or a part thereof.

26. The actuator assembly of claim 25 wherein the primary axis is perpendicular to a plane defined by the display and / or is parallel to the general direction in which radiation is emitted from the emitter.

27. An actuator assembly comprising:a first part, wherein a primary axis is defined with reference to the first part; a second part that is movable relative to the first part;a bearing arrangement configured to guide the relative movement between the first and second parts ; anda plurality of actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part, wherein at least one of the actuating units comprises:a body portion;an SMA element connected between the body portion and the first part, and configured, on actuation, to apply an input force to the body portion; anda force-modifying element connected between the body portion and the first part, and configured to modify the input force so as to give rise to the actuating force; andwherein the actuating units are configured, on selective actuation, to:tilt the second part relative to the first part about a first tilt axis perpendicular to the primary axis; andtilt the second part relative to the first part about a second tilt axis perpendicular to the primary axis, wherein the first and second tilt axes are5 non-parallel axes.

Images