Assembly

Abstract

An actuator assembly 1 comprises a support structure 2, 4 and a movable part 8 with at least one SMA wire 10, 16, 17, 18 having first and second lengths 11, 12. The first and second lengths 11, 12 are arranged over or around an engagement member 19, so that they are able to apply a force, on contraction, in a force direction to rotate the movable part relative to the support structure. An angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute. This arrangement provides an SMA actuator assembly for increasing wire stroke without unduly increasing the size of the assembly.

Description

Field The present application generally relates to an assembly, and in particular to an assembly comprising a shape memory alloy (SMA) actuator assembly. Background Miniature actuators, such as SMA actuators, may be used in camera assemblies for effecting a range of motions of a lens carriage and / or an image sensor. For example, WO 2013 / 175197 Al describes a camera with an SMA actuator assembly including a support structure, a movable part that supports a lens assembly, plural SMA wires connected between the support structure and the movable part, and bearings to bear the movable part on the support structure. This actuator assembly also includes two flexure arms extending between the support structure and the movable part for providing a lateral biasing force that biases the lens assembly towards a central position. The SMA wires are configured to, on contraction, move the movable part in directions perpendicular to an optical axis to provide optical image stabilization (OIS). Typically, the movement range of a movable part is defined by the extent of contraction in the SMA wires, where the actuating force is also dictated by the input force achievable by the SMA wires. More specifically, the maximum displacement in the movable element in any movement direction may be limited by the relatively small amount of contraction (about 2-3%) in the SMA wires. To increase the maximum stroke, or the actuating force, longer or thicker strands of SMA actuator wire are required. However, such solutions increase the size of the actuator or reduce the response time. Thus, they may not be practical when used in miniaturised SMA actuator assemblies in mobile devices. Summary The present techniques provide an SMA actuator assembly for increasing wire stroke without unduly increasing the size of the assembly. Advantageously the displacement of a movable part may be amplified. Advantageously, such an arrangement may allow a greater stroke (i.e. displacement) to be achieved, thus improving actuator performance and, for example, OIS capability. According to a first aspect of the present invention, there is provided an assembly comprising: - a support structure defining a primary axis; - a movable part; and - an actuator assembly comprising at least one shape memory alloy, SMA, wire arranged, on contraction, to rotate the movable part relative to the support structure about the primary axis. The at least one SMA wire comprises: - a first length of SMA wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part; and - a second length of SMA wire extending between the engagement member and the other of the support structure and the movable part. The first and second lengths are arranged, on contraction, to apply a force in a force direction to the engagement member so as to rotate the movable part relative to the support structure. The actuator assembly is arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute. By arranging the SMA wire in this way (i.e. at an acute, non-zero angle to the force direction), the movement of the movable part is amplified. There is a gearing effect compared to a situation in which a length of wire is arranged along the force direction. This is because the angled orientation of the SMA wire effectively provides gearing, as the change in length of the SMA wire causes the orientation of the SMA wire to change so that the degree of movement along the force direction is greater than the actual change in length of the wire resolved along the force direction. The angling of the SMA wires may also mean that a greater length of wire may be used within a given space constraint, thus further increasing the amount by which the movable part can be driven to move. The assembly may be arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute across the full range of movement of the movable part. As described above, the first and second lengths are arranged, on contraction to apply a force in a force direction to the engagement member of the movable part. This force may be described as a resultant force. The first and second lengths may each apply a force to the engagement member comprising a first component along the force direction and a second component along a direction perpendicular to the force direction. The respective components along a direction perpendicular to the force direction applied by the first and second lengths of SMA wire may oppose each other and completely or partially cancel each other out. The actuator assembly is arranged to rotate the movable part relative to the support structure about the primary axis. The actuator assembly may be arranged so as to also be capable of driving translational motion of the movable part, for example in a plane perpendicular to the primary axis and / or along the primary axis. In some embodiments, the other of the support structure and the movable part may comprise at least one guide element and at least one of the lengths of SMA wire may be arranged around a respective one of the at least one guide element. The at least one guide element may be a pin (e.g. a cylindrical element) or some other feature of either the support structure or the movable part around which at least one of the lengths of SMA wire is arranged. The guide element may change the direction of the SMA wire such that the lengths of SMA wire either side of the guide element are angled with respect to one another (i.e. are not parallel to each other). The angle may be 90° (or approximately 90°) or may be acute or obtuse. A length of SMA wire on a first side of the guide element may extend in a first direction and a length of SMA wire on a second side of the guide element may extend in a second direction, where a component of the first direction may be opposite to a component of the second direction. In other words, the guide element may cause the SMA wire to turn back on itself. The one or more guide elements may be disposed on a corner of the other of the support structure and the movable part. In some embodiments the actuator assembly may comprise a plurality of SMA wires arranged, on contraction, to rotate the movable part relative to the support structure in opposite senses around the primary axis. For example, the actuator assembly may comprise one SMA wire arranged to drive rotation of the movable part in a first sense about the primary axis and another SMA wire arranged to drive rotation of the movable part in a second sense, opposite to the first sense, about the primary axis. In other embodiments, the actuator assembly may comprise an SMA wire arranged to drive rotation of the movable part in a first sense about the primary axis and a resilient element such as a spring to oppose the contraction of the SMA wire, i.e. to urge rotation of the movable part in the opposite sense. According to a second aspect of the present invention, there is provided an assembly comprising: - a support structure defining a primary axis; - a movable part; and - an actuator assembly comprising at least one shape memory alloy, SMA, wire arranged, on contraction, to move the movable part relative to the support structure. The at least one SMA wire comprises: - a first length of SMA wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part; and - a second length of SMA wire extending between the engagement member and the other of the support structure and the movable part. The first and second lengths are arranged, on contraction, to apply a force in a force direction to the engagement member so as to move the movable part relative to the support structure. The other of the support structure and the movable part comprises at least one guide element, wherein at least one of the lengths of SMA wire is arranged around a respective one of the at least one guide element. The actuator assembly is arranged such that an angle between the force direction and at least a portion of the first length of SMA wire between the engagement member and the other of the support structure and the movable part is non-zero and acute. The actuator assembly thus provides amplified motion of the movable part by virtue of the angled arrangement of SMA wire. The advantages, explanation and additional, optional features described above with reference to the angling of the first and second lengths of SMA wire also apply here. The actuator assembly also comprises at least one guide element around which an SMA wire is arranged. This may facilitate the use of a longer length of SMA wire, as compared to an assembly without a guide element, thus further increasing the range of motion of the movable part (as a longer length of SMA wire can contract more than a shorter length). The actuator assembly may be arranged to drive rotation of the movable part about any axis (e.g. the primary axis or an axis perpendicular to the primary axis) and / or translation of the movable part along any axis (e.g. the primary axis or one or more axes perpendicular to the primary axis). Unless specified otherwise, the following optional features may be applied to any of the aspects described herein. In some embodiments, for each of the first and second lengths of SMA wire an angle between the force direction and at least a portion of the respective length of SMA wire between the engagement member and the other of the support structure and the movable part (e.g. a portion adjacent the engagement member) is non-zero and acute. In other words, both the first and second lengths of SMA wires may be angled as described above. Advantageously, this may facilitate balancing of forces between the first and second lengths of SMA wire. The assembly may be arranged such that for each of the first and second lengths of SMA wire, an angle between the force direction and at least a portion of the respective length of SMA wire between the engagement member and the other of the support structure and the movable part is non-zero and acute across the full range of movement of the movable part. In some embodiments, the other of the support structure and the movable part comprises a plurality of guide elements and the first and second lengths of SMA wire are each arranged around respective guide elements. Both the first and second lengths of SMA wire may thus be arranged around guide elements, which may facilitate a longer length of both the first and second SMA wires (and hence increased stroke) arranged in an advantageous way. In some embodiments in which at least one guide element is present, for at least one of the lengths of SMA wire arranged around a respective one of the at least one guide element, a reflexive angle formed by the first and second lengths of SMA wire may be on the opposite side of the length of SMA wire arranged around the guide element compared to a reflexive angle formed by segments of the length of SMA wire on either side of the guide element when viewed along the primary axis. In other words, the SMA wire forms a 'zig-zag' shape and turns back on itself both at the guide element and at the engagement member. In some embodiments, this is true for both the first and second lengths of SMA wire and in this case, the first and second lengths together form an 'M' shape (with the guide elements at the two outer corners of the 'M' and the engagement member at the centre of the 'M'). In some embodiments, the first and second lengths of SMA wire may be arranged along adjacent edges of the movable part when viewed along the primary axis. In other embodiments, the first length may be arranged along two adjacent sides of the movable part (e.g. a first and second side) and the second length may be arranged along two adjacent sides of the movable part (e.g. the second side and a third side). Two of those sides may be the same side of the movable part such that overall, the first and second lengths of SMA wire together are arranged along three adjacent sides of the movable part. In some embodiments in which the actuator assembly is configured to drive rotation of the movable part about the primary axis with respect to the support structure, the assembly may be configured such that the force direction does not intersect the primary axis. In other words, the first and second lengths of SMA wires may be arranged to apply a force to the engagement member in a direction which is offset from (i.e. does not pass through) the primary axis. In some embodiments, the first and second lengths of SMA wire may be provided as separate SMA wires. For example, the separate SMA wires may each be attached (e.g. crimped or otherwise attached) to the engagement member. In such embodiments, the assembly may need to be arranged to allow compliance in the connection to the engagement member to allow the movable part to move in two different, e.g. orthogonal directions. Otherwise, any motion in a first direction would place strain on the wires arranged to drive motion in a second, orthogonal direction (for example). In some embodiments, one of the support structure and the movable part may comprise a main body and an elongate flexure extending between the main body and the engagement member. The flexure may be arranged to flex when subjected to a force perpendicular to a direction of elongation of the flexure. In other words, an elongate flexure may connect the engagement member of the movable part to a main body of the movable part, for example. A force may therefore be applied to the movable part in a first direction (for example by a first SMA wire) which is transmitted to the main body of the movable part via the elongate flexure. However, when a force in a perpendicular direction is applied to the movable part (for example by a second SMA wire), the flexure flexes to prevent undesirable strain in the first SMA wire. Without this flexure connection, undesirable forces may be imparted by the second wire on the first wire (and vice versa). In some embodiments, the assembly may be arranged such that the flexure is in compression during use of the assembly. In some embodiments the assembly may be arranged such that the flexure is in tension during use of the assembly. Tension may be advantageous as an arrangement in which the flexure is under compression may risk buckling of the flexure during use. In some embodiments the first and second lengths may be provided as a single SMA wire arranged around the engagement member. The single SMA wire may be hooked around the engagement member. In some embodiments, the single SMA wire may be in sliding engagement with the engagement member. In some embodiments, for each of the first and second lengths of SMA wire the angle between the force direction and the respective length of SMA wire may be the same. This may be the case when the movable part is in a neutral position, e.g. when the tension in all of the SMA wires is the same. Advantageously, this may facilitate simple control of the SMA. In some embodiments, the length of the first length of SMA wire may be different to the length of the second length of SMA wire. This may be the case when the movable part is in a neutral position, e.g. when the tension in all of the SMA wires is the same. This difference in length may facilitate the use of a maximum length of SMA wire (overall in the assembly) within a given footprint in situations in which the engagement members are not centrally located along respective sides of the movable part. In some embodiments, the engagement member may be located at a corner of the movable part. According to a third aspect of the present invention, there is provided an assembly comprising: - a support structure defining a primary axis; - a movable part; and - an actuator assembly comprising at least one shape memory alloy, SMA, wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part and arranged, on contraction, to rotate the movable part relative to the support structure about the primary axis. The other of the support structure and the movable part comprises at least one guide element. At least one of the lengths of SMA wire is arranged around a respective one of the at least one guide element. Advantageously, by providing a guide element the length of SMA wire in the system may be increased (or maximized) within a given space constraint compared to a situation in which no guide element is provided. The guide element may change the direction of the SMA wire such that an angle between the lengths of SMA wire either side of the guide element are angled with respect to one another (i.e. are not parallel to each other). The angle may be 90° (or approximately 90°) or may be acute or obtuse. The one or more guide elements may be disposed on a corner of the support structure or movable part. Unless specified otherwise, the following optional features may be applied to any of the aspects described herein. In some embodiments, the movable part comprises a lens. The primary axis may be an optical axis of the lens or may be parallel to an optical axis of the lens. According to a fourth aspect of the present invention, there is provided an assembly comprising: - a support structure defining a primary axis; - a movable part; and an actuator assembly comprising at least one shape memory alloy, SMA, wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part and arranged, on contraction, to move the movable part relative to the support structure. The other of the support structure and the movable part comprises at least one guide element. At least one of the lengths of SMA wire is arranged around a respective one of the at least one guide element. The at least one SMA wire overlaps the movable part when viewed along the primary axis. By arranging the SMA wire to overlap with the movable part when viewed along the primary axis the available stroke of the SMA wire may be increased as compared to a situation in which the SMA wire does not overlap the movable part when viewed along the primary axis. Firstly, it may be possible to include in the assembly and make use of a longer length of SMA wire, thus increasing the overall amount by which the SMA wire can contract. Secondly, it may be possible to arrange the at least one SMA wire to be angled with respect to a given movement direction, e.g. as described above. In some embodiments, the at least one SMA wire may be configured to drive rotation of the movable part about the primary axis. In such embodiments, the at least one SMA wire may engage with the movable part at a point close to the primary axis, thus resulting in a larger amount of rotation for a given amount of SMA wire contraction compared to a situation in which the SMA wire engages with the movable part at a point further from the primary axis. The at least one SMA wire may engage with the movable part at a point which is closer to the primary axis (about which the movable part is driven to rotate) than to an outer edge (when viewed along the primary axis) of the movable part. In some embodiments the assembly may comprise: - a first length of SM A wire extending between the engagement member and the other of the support structure and the movable part; and - a second length of SMA wire extending between the engagement member and the other of the support structure and the movable part. The first and second lengths may be arranged, on contraction, to apply a force in a force direction to the engagement member so as to move the movable part relative to the support structure. The actuator assembly may be arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent to the engagement member is non-zero and acute. In this way, two SMA wires may be provided, as described above, at least one of which may be at an acute, non-zero angle to the force direction (which may be a direction in which the resultant force applied by the first and second lengths of SMA wire is applied, as described above). In some embodiments, the actuator assembly may comprise a plurality of SMA wires arranged, on contraction, to move the movable part relative to the support structure in different directions or senses. In some embodiments the different directions may oppose each other. In some embodiments the different directions may be perpendicular to each other. For example, the actuator assembly may comprise one SMA wire arranged to drive rotation of the movable part in a first sense about the primary axis and another SMA wire arranged to drive rotation of the movable part in a second sense, opposite to the first sense, about the primary axis. In other embodiments, the actuator assembly may comprise an SMA wire arranged to drive rotation of the movable part in a first sense about the primary axis and a resilient element such as a spring to drive rotation of the movable part in the opposite sense. Additionally or alternatively, the actuator assembly may comprise one SMA wire arranged to drive movement of the movable part along a first direction and another SMA wire arranged to movement of the movable part along a second direction, opposite to the first direction. In other embodiments, the actuator assembly may comprise an SMA wire arranged to drive movement of the movable part along a first direction and a resilient element such as a spring to oppose the SMA wire, i.e. to bias the movable part in an opposite direction. In some embodiments, two or more SMA wires may cross over each other when viewed along the primary axis. For example, two SMA wires associated with movement in different directions or senses respectively may cross over each other when viewed along the primary axis. In some embodiments the assembly may comprise an intermediate carriage arranged to support the movable part. The at least one SMA wire may be supported by the intermediate carriage and arranged to drive movement of the movable part relative to the intermediate carriage and another at least one SMA wire may be arranged to drive movement of the intermediate carriage relative to the support structure. The at least one SMA wire may be arranged to drive movement of the movable part along a first axis with respect to the support structure (by moving the movable part relative to the intermediate carriage) and the another at least one SMA wire may be arranged to drive movement of the movable part along a second axis, perpendicular to the first axis, relative to the support structure (by moving the intermediate carriage with respect to the support structure). In some embodiments the actuator assembly may comprise a first pair of SMA wires which are arranged to drive movement of the movable part in opposing directions along a first axis and a second pair of SMA wires which are arranged to drive movement of the movable part in opposing directions along a second axis which is perpendicular to the first axis. In some embodiments the assembly may comprise a bearing arrangement arranged to support movement of the movable part relative to the support structure. The bearing arrangement may comprise a plain bearing or a rolling bearing, for example. In some embodiments the bearing arrangement may be arranged to support movement of the movable part relative to the support structure in a plane and the at least one SMA wire may be inclined relative to the plane so as to bias the movable part against the bearing when actuated. In some embodiments the movable part may comprise an electronic component that extends laterally across the primary axis. In other words, the primary axis may pass through the electronic component. The electronic component may extend in a plane (or substantially in a plane) and the primary axis may be perpendicular to that plane. The electronic component may be an image sensor or a display. In some embodiments, the electronic component may comprise a light-sensitive region (e.g. in the case of an image sensor) or a light-emitting region (e.g. in the case of a display) and the SMA wire(s) may overlap with the light-sensitive or light-emitting region when viewed along the primary axis. In some embodiments, at least one SMA wire is coupled to the movable part at a point which lies within a footprint of the movable part when viewed along the primary axis. In some embodiments at least one SMA wire is coupled to the movable part at a point which lies within a footprint of the light-sensitive or light-emitting region when viewed along the primary axis. The at least one SMA wire may be disposed on a first side of the movable part, opposite to a second side of the movable part on which the light sensitive or light-emitting region is disposed. In some embodiments the actuator assembly may comprise a controller comprising a control circuit electrically connected to the at least one SMA wire for supplying drive signals thereto. In some embodiments, the controller may be arranged to implement optical image stabilisation of an image on an image sensor. The movable part may comprise a lens and / or an image sensor and / or a display and / or a light-source. In some embodiments, the at least one SMA wire may be arranged to drive translational motion of the movable part in a direction perpendicular to the primary axis. In some embodiments the assembly may comprise a plurality of SMA wires which are arranged to drive translation motion of the movable part in two degrees of freedom in a plane perpendicular to the primary axis. According to a fifth aspect of the present invention, there is provided camera comprising an assembly as described herein. In some embodiments, the camera may comprise folded optics. Such a camera may otherwise be referred to as a periscope camera. In a periscope camera, light enters the camera along a first axis and is then deflected through 90° by an optical element such as a mirror or prism to impinge on an image sensor. Brief Description of the Drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a camera comprising an assembly according to an embodiment of the present invention; Figure 2 is a schematic plan view of an assembly according to an embodiment of the present invention; Figure 3 is a schematic side view of the assembly shown in Figure 2; Figures 4-11 are schematic plan diagrams of assemblies according to embodiments of the present invention; Figure 12 is a schematic side view of the assembly shown in Figure 11; Figures 13-17 are schematic plan diagrams of assemblies according to embodiments of the present invention; and Figure 18 is a schematic plan diagram of an assembly according to an embodiment of the present invention. Detailed Description An assembly 1 in accordance with the present invention is shown in Figure 1, which is a cross-sectional view taken along the primary axis O. A camera may comprise the assembly 1. The primary axis 0 may be an optical axis of the lens or lenses of the camera. Such a camera may incorporate an OIS sensor shift function or an OIS lens shift function. The assembly 1 may be incorporated in a portable electronic device such as a mobile telephone, or tablet computer. Thus, miniaturisation is an important design criterion. The assembly 1 comprises a support structure 2 and a movable part 8 configured to move relative to the support structure 2. Optionally, the movable part 8 comprises an image sensor 6 having a light-sensitive region 7. The image sensor 6 may be mounted on an area of a printed circuit board, PCB. The primary axis 0 is orthogonal to the light-sensitive region 7. The image sensor 6 is configured to capture an image and may be of any suitable type, for example a CCD (charge-coupled device) or a CMOS (complementary metal-oxide-semiconductor) device. As is conventional, the image sensor 6 has a rectangular light-sensitive region 7. Optionally, the light-sensitive region 7 of the image sensor 6 has a diagonal length of at most 12mm. Optionally, the movable part 8 comprises a moving plate. The moving plate may be formed from sheet material, which may be a metal for example steel such as stainless steel. Although the movable part 8 comprises a single moving plate in this example, optionally the movable part 8 may comprise other layers which may be attached to or laminated with the moving plate. Optionally, the support structure 2 comprises a support plate 4 which may be formed from sheet material, which may be a metal for example steel such as stainless steel. Although the support structure 2 comprises a single support plate 4 in this example, optionally the support structure 2 may comprise other layers which may be attached to or laminated with the support plate 4. Optionally, the support structure 2 further comprises a rim portion fixed to the front side of the support plate 4 and extending around the support plate 4. The rim portion has a central aperture. Optionally, the support structure 2 further comprises an IC (integrated circuit) chip and a gyroscope sensor fixed on the rear side of the support plate 4. A control circuit is implemented in the IC chip. The movable part 8 is suspended on the support structure 2 in a manner allowing movement of the movable part 8, optionally in any direction laterally to the plane of the movable part 8 (i.e. laterally of the primary axis 0 and parallel to the plane in which the light-sensitive region 7 extends) and optionally (but not necessarily) further allowing rotation of the image sensor about the primary axis O. Optionally, the movable part 8 is suspended on the support structure 2 by a suspension system in the form of a plain bearing provided between the support plate 4 and the movable part 8. Optionally, the assembly 1 comprises a lens assembly 20 that is assembled with the assembly 1 by being mounted to the support structure 2, for example to the rim portion. The lens assembly 20 comprises a lens carriage 21 in the form of a cylindrical body that is mounted to the rim portion 4 of the support structure 2. The lens carriage 21 supports at least one lens 22 arranged along the primary axis O. In general, any number of one or more lenses 22 may be provided. Optionally, at least one lens 22 (i.e. each lens 22 if plural lenses are provided) typically has a diameter of at most 10mm. The at least one lens 22 of the lens assembly 20 is arranged to focus an image onto the image sensor 6. In this example, at least one lens 22 is supported on the lens carriage 21 in a manner in which at least one lens 22 is movable along the primary axis O relative to the lens carriage 21, for example to provide focussing or zoom, although that is not essential. In particular, the at least one lens 22 is fixed to a lens holder 23 which is movable along the primary axis 0 relative to the lens carriage 21. Where there are plural lenses 22, any or all of the lenses 22 may be fixed to the lens holder 23 and / or one or more of the lenses 22 may be fixed to the lens carriage 21 and so not movable along the primary axis O relative to the lens carriage 21. An axial actuator arrangement 24 provided between the lens carriage 21 and the lens holder 23 is arranged to drive movement of the lens holder 21 and lenses 22 along the primary axis O relative to the lens carriage 21. The axial actuator arrangement 24 may be any suitable type, for example being a voice coil motor (VCM) or an arrangement of SMA actuator wires, such as is described in WO2007 / 113478A1 which is incorporated herein by reference. In addition, the assembly 1 may comprise a can 15 fixed to the support structure 2 and protruding therefrom to encase and protect the other components of the assembly 1. As mentioned above, optionally, the movable part 8 comprises an image sensor 6. Alternatively, the movable part 8 may comprise one or more lenses. In operation SMA actuator wires are selectively driven to move the movable part 8 in any direction laterally and / or to rotate the movable part 8 about the primary axis O. This is used to provide OIS (either sensor shift or lens shift), compensating for image movement of the camera, caused by for example hand shake. The invention is described primarily in the context of sensor shift so as to avoid redundant description but it will be appreciated that the assemblies described herein may be used to move any moving part, whether an optical component or otherwise. Movement of the image sensor 6 relative to the support structure 2 and hence also relative to the lens assembly 20 may be used to stabilise the image against tilting of the assembly 1, i.e. rotation about axes extending laterally to the light-sensitive region 7 of the image sensor 6. This occurs in a similar manner to a camera apparatus providing OlS-lens shift of the type disclosed in WO 2013 / 175197 and WO 2014 / 083318 (both of which are incorporated herein by reference) which also involves relative lateral movement of the image sensor 6 and lens assembly 20. In addition, rotation of the images sensor 6 may be used to stabilise the image against rotation of the assembly 1 around the primary axis O. The SMA actuator wires are driven by the control circuit implemented in the IC chip. In particular, the control circuit generates drive signals for each of the SMA actuator wires and supplies the drive signals to the SMA actuator wires. The control circuit receives the output signals of the gyroscope sensor which acts as a vibration sensor. The gyroscope sensor detects the vibrations that the assembly 1 is experiencing and its output signals represent those vibrations, specifically as the angular velocity of the camera lens element 20 in three dimensions. The gyroscope sensor is typically a pair of miniature gyroscopes, for detecting vibration around three axes, being two axes laterally of the light-sensitive region 7 of the image sensor 6 and also the primary axis O. More generally, larger numbers of gyroscopes or other types of vibration sensor could be used. The drive signals are generated by the control circuit in response to the output signals of the gyroscope sensor so as to drive movement of the camera lens element 20 to stabilise an image focused by the camera lens element 20 on the image sensor 6, thereby providing OIS. The drive signals may be generated using a resistance feedback control technique for example as disclosed in any of International Patent Application No. PCT / GB2013 / 051325; International Patent Application No. PCT / GB2013 / 052959; WO2012 / 066285; WO2012 / 020212; WO2011 / 104518; WO2012 / 038703; WO2010 / 089529 or WO2010 / 029316, each of which is incorporated herein by reference. Figure 2 schematically depicts a plan view of an assembly 1 according to an embodiment of the present invention. The view of Figure 2 is a view along the primary axis 0. The primary axis 0 is defined with reference to the support structure 2. The support structure 2 is not shown in Figure 2. The assembly 1 comprises an actuator assembly. The actuator assembly is configured to move the movable part 8 relative to the support structure 2. The actuator assembly comprises at least one SMA wire. For example, the arrangement shown in Figure 2 comprises at least four SMA wires 10,16-18. As will be explained further below, each SMA wire is arranged in an 'M' shape: one 'M' which is the right way up as seen in Figure 2 (SMA wire 16), one upside down 'M' (SMA wire 17) and two 'M's on their sides (SMA wires 10 and 18). Each of the M-shaped wires is arranged around two guide elements 3 respectively. The locations of the guide elements 3 are indicated by dotted lines in Figure 2 but the guide elements can be seen in Figure 3. Optionally, the assembly 1 is arranged such that the movable part 8 can rotate relative to the support structure 2. For example, the movable part 8 may rotate relative to the support structure 2 about the primary axis 0. Optionally, the at least one SMA wire of the actuator assembly is arranged, on contraction, to rotate the movable part 8 relative to the support structure 2 about the primary axis O. However, it is not essential for the movable part 8 to be rotatable relative to the support structure 2. Elsewhere, embodiments are described in which the movable part 8 is configured to move laterally relative to the support structure 2 but not necessarily rotate (e.g. see Figure 11-17 and their associated description). As shown in Figure 2, optionally the at least one SMA wire 10 comprises a first length 11 of SMA wire. The first length 11 extends between an engagement member 19 of the movable part 8 and the support structure 2. As shown in Figure 2, optionally the movable part 8 comprises the engagement member 19. In an alternative arrangement, the support structure may comprise the engagement member 19. When the support structure 2 comprises the engagement member 19, the first length 11 of SMA wire extends between the engagement member 19 and the movable part 8. As shown in Figure 2, optionally the at least one SMA wire 10 comprises a second length 12 of SMA wire. The second length 12 extends between the engagement member 19 and the support structure 2. Alternatively, when the engagement member 19 is part of the support structure 2, then the second length 12 extends between the engagement member 19 and the movable part 8. During use of the assembly 1, the division of the at least one SMA wire 10 into the first length 11 and the second length 12 may vary, for example due to the SMA wire 10 sliding over the engagement member 19. The contact between the SMA wire 10 and the engagement member 19 defines the division between the first length 11 and the second length 12 of the SMA wire 10. Optionally, the first and second lengths 11,12 are arranged, on contraction, to apply a force in a force direction to the engagement member 19 so as to move the movable part 8 relative to the support structure 2. In the arrangement shown in Figure 2, the movable part 8 is arranged to rotate relative to the support structure 2. The first and second lengths 11,12 are arranged, on contraction to apply the force in the force direction to the engagement member 19 so as to rotate the movable part 8 relative to the support structure 2. As can be seen from Figure 2, when the first and second lengths 11,12 of the SMA wire 10 are contracted, then the SMA wire 10 applies a force in a direction substantially from left to right in the orientation shown in Figure 2. This force will tend to rotate the movable part 8 anticlockwise relative to the support structure 2 in the orientation shown in Figure 2. Meanwhile, other SMA wires 16-18 may be arranged to apply forces in different directions. For example, the SMA wire 18 is arranged to apply a force in the right to left direction. This tends to rotate the movable part 8 in the anti-clockwise rotational direction. The SMA wire 16 engaged with the engagement member 19 shown at the top of Figure 2 is arranged to apply a force in the down to up direction. This tends to urge the movable part 8 in the clockwise rotational direction. The SMA wire 17 is arranged to apply a force in the up to down direction on the engagement member 19 at the bottom of Figure 2. This tends to urge the movable part 8 in the clockwise rotational direction about the primary axis O relative to the support structure 2. Optionally, the actuator assembly is arranged such that during use of the assembly 1, for each of the lengths 11,12 of SMA wire an angle is formed by the force direction and the length 11,12 of SMA wire between the engagement member 19 and the support structure 2 is acute and non-zero (i.e. is not equal to zero). The angle is therefore greater than 0° and less than 90°. In the arrangement shown in Figure 2, the force direction is substantially from left to right for the first and second lengths 11,12 of the SMA wire 10. The angle between the force direction and the first length 11 is greater than 45° during use of the assembly 1. Similarly, the angle between the force direction (i.e. an imaginary horizontal line through the engagement member 19) and the second length 12 is greater than 45°. The angle between the length of SMA wire and a direction perpendicular to the force direction is less than 45°. For example, the angle between the first length 11 and the edge of the movable part 8 on which the engagement member 19 is provided is less than 45°. Similarly, the angle between the second length 12 and that edge of the movable part 8 may be less than 45° during use of the assembly 1. The first and second lengths 11,12 may be referred to as shallow-angle wires. Optionally, for each of the lengths 11, 12 of SMA wire an angle formed by the force direction and the length of SMA wire is at least 60°, optionally at least 70°, optionally at least 80° and optionally at least 85°. The angle formed by the force direction and the edge of the movable part 8 may be at most 30°, optionally at most 20°, optionally at most 10°, and optionally at most 5°. By providing shallow-angle wires, the stroke of the movement of the movable part 8 relative to the support structure 2 is amplified. The stroke may be increased for a given size of the actuator assembly. As shown in Figure 2, optionally the actuator assembly does not have a significantly larger footprint than the movable part 8 when viewed along the primary axis O. By providing the shallow-angle wires for rotation of the movable part 8, the rotational stroke of the movable part 8 may be amplified. This may allow for greater OIS stroke particularly for sensor shift OIS. In general, rotational movements for OIS may not be so important in a lens shift apparatus. This is because in general the lens may be rotationally symmetric such that rotation does not have a significant effect. However, some lenses may not be rotationally symmetric such that the invention may be employed for lens shift OIS. Optionally, a camera comprising the assembly 1 may comprise folded optics. The camera may be a periscope camera. By providing the folded optics, the effective focal length of the camera may be relatively long. By amplifying the stroke, a higher OIS stroke may be provided which may be required in view of the longer effective focal length of the camera. By providing a camera that is a periscope camera, the effective focal length of the camera can be increased without unduly increasing the thickness of the camera, i.e., the dimension of the camera in a direction perpendicular to the image sensor 6. This may allow the camera to be incorporated into a thinner electronic device while providing a greater effective focal length. Figure 3 schematically depicts a side on view of the assembly 1 shown in Figure 2. Figure 3 shows the support structure 2. As can be most clearly seen in Figure 3, optionally the support structure 2 comprises at least one guide element 3. Two guide elements 3 are visible in Figure 3. In an embodiment the support structure 2 comprises four guide elements 3. The guide elements 3 may be provided at the four corners of the support structure 2. In plan view, the support structure may have a generally similar shape to the movable part 8. For example, the movable part 8 may have a rectangular shape in plan view. Optionally, the support structure may have a generally rectangular shape in plan view. As shown in Figure 2, optionally at least one of the lengths 11,12 of SMA wire 10 is arranged around a respective one of the at least one guide element 3. In the arrangement shown in Figure 2, the first length 11 of the SMA wire 10 is arranged around the guide element 3 shown in the top-right corner of Figure 2. The first length 11 may comprise two portions 11,13 on either side of the guide element 3. One portion extends between the engagement member 19 and the guide element 3. The other portion 13 extends between the guide element and an attachment 5 to the support structure 2. The attachment 5 may be, for example, a crimp. The first length 11 may be crimped to the support structure 2 at the attachment 5. As shown in Figure 2, optionally the two portions of the first length 11 may generally extend along adjacent edges of the movable part 8 (or support structure 2). In corresponding fashion, as shown in Figure 2 the second length 12 of the SMA wire 11 is arranged around the guide element 3 depicted in the bottom right corner of Figure 2. One portion of the second length 12 extends between the engagement member 19 and the guide element 3. The other portion 14 of the second length 12 extends between the guide element and an attachment 5 to the support structure 2. Each length 11, 12 of the SMA wire 10 may be arranged around the guide element 3 by being hooked around the guide element 3. By providing a wire around a corner, the length of the SMA wire may be increased. By providing the SMA wire around the guide elements 3, a higher stroke of movement of the movable part 8 relative to the support structure 2 may be achieved. Optionally, the guide element 3 is rotatable. The guide element 3 may be rotatable relative to the main body of the support structure 2. For example, the guide element may be configured as a rotatable pin or as a pulley wheel. By providing that the guide element 3 is rotatable, the engagement between the guide element 3 and the SMA wire does not significantly decrease the effect of the contraction of the SMA wire on the movement of the movable part 8. However, it is not essential for the guide element 3 to be rotatable. In an alternative arrangement, the guide element 3 may have a fixed orientation relative to the main body of the support structure 2. In an embodiment the guide element 3 and the SMA wire are arranged such that there is a relatively low coefficient of friction between them. The SMA wire may slide over the surface of the guide element 3. As shown in Figure 2, optionally the lengths 11,12 on either side of the engagement member 19 are arranged around (e.g. hooked around) respective guide elements 3. This gives the SMA wire 10 an M-shape. The arrangement shown in Figure 2 has four M-shaped SMA wires 10,16-18. The SMA wire 16 is an M-shape the right way up. The SMA wire 17 appears as an upside down M-shape. The other two SMA wires 10, 18 appear as sideways M-shapes. The M-shape results from combining the shallow-angle wires with the wires being arranged around corners. These features combine to significantly amplify the stroke of movement of the movable part 8 relative to the support structure 2. As shown in Figure 2, the M-shape of each SMA wire may comprise four portions of SMA wire. The four portions are separated by three bends. Two of the bends are at guide elements 3. One of the bends is at the engagement member 19. The bend at the engagement member 19 is in the middle of the M- shape. The ends of each of the M-shaped wires are attached to the support structure 2. The four portions of the M-shape generally form a zig-zag pattern. Where the first length 11 is arranged around the guide element 3, the first length 11 forms an acute angle between the two portions on either side of the guide element 3, as well as a reflexive angle (i.e. 360° minus the acute angle). Where the two lengths 11,12 are arranged around the engagement member 19, an obtuse angle is formed between the two lengths 11,12, as well as a reflexive angle (i.e. 360° minus the obtuse angle). For at least one of the lengths 11, 12 arranged around a guide element, the reflexive angle formed by the first and second lengths 11, 12 (i.e. at the engagement member bend) is on the opposite side of the length of SMA wire arranged around the guide element compared to the reflexive element formed by the segments / portions of the length of SMA wire on either side of the guide element when viewed along the primary axis 0. This means that a general zig-zag shape is formed by the segments / portions of SMA wire, rather than a generally polygonal shape being formed. In such a polygonal shape, all of the reflexive angles are on the same side of the SMA wire. In other words, at each bend, the SMA wire turns back on itself. As shown in Figure 2, optionally the actuator assembly comprises a plurality of SMA wires 10,16-18 arranged, on contraction, to rotate the movable part 8 relative to the support structure 2 in opposite rotational directions (also referred to as 'senses') around the primary axis O. For example, the SMA wires 10,18 are configured to rotate the movable part 8 in an anti-clockwise sense. The SMA wires 16, 17 are arranged to rotate the movable part 8 in a clockwise sense. As shown in Figure 2, optionally the location of engagement between the SMA wires and the engagement members 19 is offset from the optical axis 0 and also from the central position in the horizontal or vertical directions (in the orientation shown in Figure 2). By providing such an offset from the optical axis, about which the movable part is configured to rotate, the movable part 8 may be configured to rotate as a result of the forces applied by the SMA wires 10, 16-18. For example, the engagament location on the movable part 8 may be offset in X or Y (where X is the horizontal direction and Y is the vertical direction in the orientation shown in Figure 2). This helps to provide rotational control of the movable part 8. However, it is not essential to provide SMA wires that rotate the movable part 8 in opposite rotational directions around the primary axis O. In an alternative arrangement, one or more SMA wires may be provided to rotate the movable part 8 relative to the support structure 2 in one rotational direction around the primary axis O. A resilient member may be provided to urge the movable part 8 in the opposite rotational direction. For example, a spring may be provided to apply a force urging the movable part 8 in the opposite rotational direction. For example, the arrangement shown in Figure 2 could be modified by replacing the SMA wires 16,17 with a resilient member such as a spring configured to urge the movable part 8 in the clockwise direction relative to the support structure 2. When the torque applied by the SMA wires 10,18 in the anti-clockwise direction is less than the torque applied by the resilient member, then the movable part 8 may rotate in the clockwise direction. When the torque applied by the SMA wires 10,18 in the anti-clockwise direction is greater than the torque applied by the resilient member, then the movable part 8 may rotate in the anti-clockwise direction. When the torque applied by the SMA wires 10, 18 in the anti-clockwise direction is equal to the torque applied by the resilient member, then the movable part 8 may have a stable rotational position relative to the support structure 2. It is not essential for the SMA wires to be arranged around guide elements 3. Figure 4 schematically depicts a plan diagram of an assembly 1 according to an embodiment of the present invention. Features of the assembly 1 shown in Figure 4 may be the same as those features of Figure 2, except for differences described below. As shown in Figure 4, optionally the SMA wires 10, 16-18 are V-shaped. The SMA wires are arranged around the engagement members 19. However, there are no guide elements around which the SMA wires are arranged. As a result, the SMA wires have a V-shape instead of an M-shape. This reduces the length of SMA wire required to manufacture the assembly 1. Optionally, the first and second lengths 11,12 of the SMA wire 10 are provided as separate SMA wires. The first and second lengths 11, 12 may be controlled independently of each other. This may help to increase the force applied by the combination of the first and second lengths 11, 12 on the movable part 8. Optionally, the separate SMA wires (i.e. the first and second lengths 11,12) are attached to the engagement member 19. One end of each of the first and second lengths 11, 12 may be fixed to the engagement member 19. For example, the first and second lengths 11,12 may be crimped to the engagement member 19. The positions of the crimps may be similar to each other. However, it is not essential for the first and second lengths 11, 12 to be provided as separate SMA wires. In an alternative arrangement, the first and second lengths 11, 12 are provided as a single SMA wire. The single SMA wire 10 may be arranged around the engagement member 19. For example, the SMA wire 10 may be hooked around the engagement member 19. The SMA wire 10 may be configured to slide relative to the engagement member 19. By allowing the SMA wire 10 to slide relative to the engagement member 19, the engagement between the SMA wire 10 and the engagement member 19 is less likely to undesirably affect the effect of the SMA wires 16, 17 applying forces and torques on the movable part 8 in different directions. This may make it easier to control the movable part 8 relative to the support structure 2 in a plurality of degrees of freedom. As shown in Figure 4, optionally the angle formed by the force direction and the length of SMA wire between the engagement member 19 and the support structure 2 is substantially the same for both of the lengths of SMA wire. This angle is indicated by the Greek letter a in Figure 4. For the SMA wire 16 shown at the top of Figure 4, the force direction applied to the engagement member 19 when the two lengths of SMA wire are contracted is down to up. As shown in Figure 4, the angles for both lengths (i.e. on either side of the engagement member 19) are the same as each other. The lengths of the two lengths of wire may be different to eachother. Providing substantially the same angles of the wire on different sides of the engagement member 19 may make it easier to control movement of the movable part 8 relative to the support structure 2. In particular, by providing the same angles on both sides, the amplification of the two sides may be substantially the same as each other. However, it is not essential for the angles to be the same on both sides of the engagement member 19. For example, as shown in Figure 2 the angle may be different on different sides of the M-shape. This may allow longer lengths of SMA wire to be used because the lengths of SMA wire may extend more fully to the corners of the support structure 2. As shown in Figure 4, optionally the lengths 11,12 of SMA wire engage with the engagement member 19 at a location offset from the axis of rotation, i.e. the primary axis O (in the horizontal and vertical directions) when viewed along the primary axis O. In other words, the force direction does not intersect the primary axis. As a result, the lengths 11,12 of SMA wire are arranged, on contraction, to rotate the movable part 8 relative to the support structure 2 about the primary axis O. As shown in Figure 4, optionally the movable part 8 comprises a main body 8a and a plurality of elongate flexures 9. In the arrangement shown in Figure 4, the movable part 8 comprises a main body 8a and four elongate flexures 9. Each elongate flexure 9 extends between the main body and a respective engagement member 19. Each respective engagement member 19 corresponds to a pair of lengths of SMA wire engaged with the engagement member 19. As shown in Figure 4, optionally the elongate flexures 9 extend in different directions away from the main body. The elongate flexures 9 may protrude radially outward from the main body of the movable part 8. As shown in Figure 4, optionally the elongate flexures 9 extend from a corner of the main body of the movable part 8. The main body of the movable part 8 may be substantially polygonal in shape, for example rectangular. Alternatively, the elongate flexures 9 may protrude from a mid-point along edges of the main body (e.g. as shown in Figure 5 or Figure 6, for example). As shown in Figure 4, optionally a plurality of elongate flexures 9 extend from substantially the same point of the main body of the movable part 8. For example, two elongate flexures 9 protrude from the top left corner of the main body of the movable part 8. Another two elongate flexures protrude from the bottom right corner of the main body of the movable part 8. Pairs of elongate flexures 9 may protrude from opposite corners of the main body. Alternatively, the elongate flexures 9 may all extend from different points of the main body (e.g. as shown in Figure 5 or Figure 6, for example). When the SMA wires are contracted, the SMA wires apply a force on the engagement member 19. The engagement member 19 is fixed to the elongate flexure 9. The elongate flexure 9 shown in Figure 4 is in tension during use of the assembly 1. The elongate flexure 9 applies a torque to the main body of the movable part 8 so as to urge the movable part 8 to rotate relative to the support structure 2. Optionally, the flexure is arranged to flex when subjected to a force perpendicular to a direction of elongation of the flexure 9. For example, the flexure labelled 9a in figure 4 extending between the main body of the movable part 8 and the engagement member 19 engaged with the SMA wire 10 has a direction of elongation (otherwise referred to as a longitudinal axis) in the horizontal direction in Figure 4. The flexure may be stiff with respect to forces applied in the direction of elongation, i.e. along the x direction. In particular when the first and second lengths 11,12 of the SMA wire 10 are contracted, the elongate flexure 9 remains stiff when it undergoes the resulting increase in tension. However, when the elongate flexure 9 is subjected to a force in a perpendicular direction, i.e. a force in the up-down direction of the diagram of Figure 4, then the flexure 9 may be configured to flex. For example, when the SMA wires 16,17 are contracted so as to apply a torque to the main body of the movable part 8, then the flexure 9 associated with the SMA wire 10 may undergo a force in the up-down direction of the diagram. At this time, the elongate flexure 9 may flex, for example bend. By allowing the flexure 9 to flex, the movement of the movable part 8 is made compliant in different directions. This may help to reduce any undesirable cross talk between forces for moving the movable part 8 in different degrees of freedom (for example in different directions). This may be particularly desirable when the first and second lengths 11,12 are provided as separate SMA wires each fixed to the engagement member 19. In such an arrangement, the first and second lengths 11, 12 may not be free to slide relative to the engagement member 19. By allowing the flexure 9 to flex, compliance to other movements is improved. This may help to improve the accuracy with which the movements of the movable part 8 relative to the support structure 2 may be controlled. As shown in Figure 4, optionally the flexure 9 joins the main body of the movable part 8 at a location distanced from a centre of a side of the main body when viewed along the primary axis O. For example, as shown in Figure 4, the flexures may be arranged to join the main body at the corners of the main body of the movable part 8. However, it is not essential for the flexures to be arranged to flex. In an alternative arrangement, cross talk between different SMA wires may be compensated for or corrected for by the circuit that applies the drive signals to the SMA wires. Additionally or alternatively, the first and second lengths 11,12 of SMA wire may be provided as a single SMA wire that can slide relative to the engagement member 19. This may allow for increased compliance to different movements of the movable part 8 without requiring flexing flexures 9. It is not essential for the flexure 9 to join the main body at a location distanced from a centre of a side of the main body. In an alternative arrangement, flexures 9 may join the main body of the movable part 8 at the centre of the side of the main body when viewed along the primary axis O. This position may be aligned with the primary axis. This is shown in Figures 13 and 14, for example. As shown in Figure 4, in an embodiment the flexure 9 is in tension during use of the assembly 1. By providing that the flexure 9 is in tension during use of the assembly 1, the possibility of the flexure 9 undesirably buckling during use is reduced. However, it is not essential for the flexure to be in tension during use. In an alternative arrangement, the flexure is arranged to be in compression during use of the assembly 1. Figure 5 is a schematic plan diagram of an assembly 1 according to an alternative embodiment of the present invention. The arrangement shown in Figure 5 may have features the same as those shown in Figure 2 or Figure 4, for example, except where differences are described below. It is not essential for the SMA wires 10,16-18 in the arrangement of Figure 4 to be V-shaped. In an alternative embodiment, the SMA wires are arranged to be M-shaped. In particular, guide elements may be provided around which the lengths 11,12 of the SMA wires may be arranged. This may increase the overall length of SMA wires so as to improve amplification of the stroke. In an alternative arrangement, one of the lengths 11, 12 may be arranged around a guide element while the other of the lengths 11,12 may not be arranged around a guide element. This would result in a zig-zagged shape but not an M-shaped wire. As shown in Figure 5, the flexures 9 are offset from the centres of the sides of the main body of the movable part 8. This allows for a rotational control of the movable part 8 relative to the support structure 2. The SMA wires are hooked around the engagement members 19. The SMA wires are provided as V-shapes. In the arrangement shown in Figure 5, the flexures 9 are slightly offset from the corners of the main body of the movable part 8. As shown in Figure 5, optionally the first and second lengths 11,12 of SMA wire may be substantially equal in length to each other. Each engagement member 19 may be engaged with two lengths of SMA wire of substantially equal length. This may make it easier to control the rotation of the movable part 8 relative to the support structure 2. It is desirable for the first and second lengths 11, 12 to combined, when contracted, to apply the force direction substantially in the direction of elongation of the flexure 9 to which the engagement member 19 is secured. It is desirable for the first and second lengths 11,12 to apply forces in the perpendicular direction to the force direction that balance each other out. This may be easier to achieve when the first and second lengths 11, 12 have the same length as each other. Figure 6 is a schematic plan diagram of an assembly 1 according to an alternative embodiment of the present invention. The assembly 1 shown in Figure 6 may have the same features as described above, except for the differences described below. As shown in Figure 6, the flexures 9 are used to provide the rotation control by translating the force of the SMA wires from a central position to an offset position. In particular, the engagement members 19 are provided at substantially a central position along the edges of the main body of the movable part 8. However, the flexures 9 are arranged to join to the main body of the movable part 8 at a position that is offset from the axis of rotation (in the x and y directions). The force of the SMA wires on the engagement member 19 is at a central position. The flexures 9 translate this central force to a force at an offset position so as to rotate the movable part 8 relative to the support structure 2. By providing that the engagement members 19 are at the central position, the full lengths of the sides of the actuator assembly may be used, whilst maintaining that the lengths of SMA wire either side of the engagement members are equal in length. The first and second lengths 11,12 may be longer (compared to in Figure 5) while still being symmetrical on either side of the engagement member 19. This helps to increase the stroke without reducing accuracy of control. Either of the arrangements shown in Figure 5 and Figure 6 may be modified to have M-shaped wires instead of V-shaped wires. This may help to increase the stroke of the movement of the movable part 8. Figure 7 is a schematic view of an assembly 1 according to an alternative embodiment of the invention. The assembly 1 may have the same features as described above, except where differences are described below. As shown in Figure 7, the SMA wires are arranged generally to have M-shapes. The SMA wires are bent at guide elements 3 as well as at the engagement members 19. As shown in Figure 7, optionally the actuator assembly is arranged to provide corner-to-corner stroke. This is achieved by making the M-shaped SMA wires diagonal. As shown in Figure 7, optionally the engagement member 19 is located at a corner of the movable part 8. By providing the engagement members 19 at the corners, the diagonal M-shaped SMA wires allow the SMA wires to be relatively long. This may help to increase the stroke of movement of the movable part 8. The SMA wires 10,16-18 are arranged around the guide elements 3, which may be fixed hook points. The lengths of SMA wire may be crimped or sliding at the engagement members 19 located at the corners of the movable part 8. As shown in Figure 7, wires are in opposite corners so as to provide rotation. The wires are arranged around corners. As shown in Figure 7, optionally the M-shaped wires are arranged along adjacent sides of the movable part 8. Figure 8 schematically depicts an assembly 1 according to an alternative embodiment of the present invention. Features of the assembly 1 may be the same as described above, except where differences are described below. As shown in Figure 8, optionally the assembly 1 comprises three M-shaped wires. This is different from the arrangement shown in Figure 7 which comprises four M-shaped SMA wires. In the arrangement shown in Figure 8, two M-shaped wires are arranged around engagement members 19 at adjacent corners of the movable part 8. These M-shaped wires may be configured to apply torques in opposite rotational directions around the primary axis O. as shown in Figure 8, a third M-shaped SMA wire 10 is arranged around an engagement member 19 that may be substantially centrally located along an edge of the movable part 8. This SMA wire 10 may be arranged similarly to that as shown in Figure 2, for example. The torque applied by the SMA wires 16, 17 may depend on the angle at which the corner crimps on the engagement members 19 are arranged. For example, if the angle of the corner crimps is 45° to the X and Y axis, then the movable part 8 may be rotated by actuation of the SMA wires 16, 17. Depending on how the crimps at the engagement members 19 are arranged, the SMA wires 10, 16, 17 may be arranged to apply torque in the same rotational direction as each other. Alternatively, they may be arranged to apply torques in different rotational directions. Optionally, the SMA wire 10 may be arranged around an engagement member 19 at an offset position from the centre of the edge of the movable part 8 such that the SMA wire 10 may apply a torque so as to rotate the movable part 8. When the SMA wires 10,16,17 all apply a torque in the same rotational direction, then a resilient member such as a torque spring may be provided so as to urge the movable part 8 in the opposite rotational direction. Figure 9 schematically depicts a plan view of an assembly 1 according to an alternative embodiment of the present invention. The assembly 1 may have the same features as described above, except where differences are described below. As shown in Figure 9, optionally the actuator assembly comprises V-shaped SMA wires 10, 16-18. As shown in Figure 9, optionally the V-shaped SMA wires are attached at opposite corners of the support structure 2. For example, the SMA wire 18 is attached at an attachment 5 at the top right corner as well as another attachment at the bottom left corner of the support structure 2 as viewed in Figure 9. This helps to increase the length of the SMA wire 18, so as to improve the stroke of movement of the movable part 8 relative to the support structure 2. As shown in Figure 9, the assembly 1 utilises tension flexures 9. The flexures 9 are kept in tension during use of the assembly 1. An outer end of the flexures 9 is attached to the movable part 8. An inner end of the flexures 9 is provided with an engagement member 19 around which the SMA wires is arranged or attached. Such an arrangement is expected to achieve high amplification of stroke with corner-to-corner wires. Optionally, the SMA wires may be located behind the image sensor 6. The image sensor 6 may be located between the SMA wires of the actuator assembly and the optical path between the image sensor 6 and the lenses. As shown in Figure 9, optionally the SMA wires 10, 16-18 are angled away from the middle (i.e. where the primary axis O intercepts the movable part 8). The flexures 9 may be attached in the corners of the support structure 2 to provide corner-to-corner motion. The flexures 9 may be arranged to deflect (i.e. flex) if any other force than pure tension is applied to them. Although not show in Figure 9, optionally the SMA wires 10, 16-18 are angled relative to the plane of the movable part 8. By angling the SMA wires, the SMA wires 10, 16-18 may be arranged such that they overlap each other when viewed along the primary axis 0 without significantly interfering with each other. In an embodiment, the SMA wires 10,16-18 are angled relative to the plane of the movable part 8 so as to apply a force onto a bearing (e.g. a planar bearing) which helps to constrain motion of the movable part 8 within the plane. This may help to make the assembly 1 more reliable and / or robust. As shown in Figure 9, optionally the assembly 1 has a rectangular footprint. The support structure 2 and / or the movable part 8 may be generally rectangular. By providing a rectangular shape, the movable part 8 may be arranged to rotate relative to the support structure 2 when the SMA wires 10,16-18 apply forces to the flexures 9. Figure 10 schematically shows a modified version of the assembly 1 shown in Figure 9. As shown in Figure 10, optionally the engagement members 19 may be located relatively close to the centre of the movable part 8. The engagement members 19 may be located closer to a centre point of the movable part 8 than to an edge of the movable part 8 when viewed along the primary axis O. Optionally, the engagement members 19 and the SMA wires 10,16-18 may be located behind the image sensor 6. The SMA wires and the engagement members 19 may be kept out of the optical pathway. Their presence does not negatively affect imaging using the assembly 1. As shown in Figure 10, optionally the flexures 9 are attached to the main body of the movable part 8 at joints 89. As shown in Figure 10, optionally the joints 89 may be located at one or more corners of the movable part 8. The arrangement shown in Figure 10 advantageously provides shallow-angle wires for rotation of the movable part 8. As explained elsewhere, optionally the movable part 8 is configured to rotate relative to the support structure 2. However, it is not essential for the movable part 8 to be rotatable relative to the support structure 2. In an alternative arrangement, the movable part 8 may be configured to move translationally relative to the support structure 2 without rotation around the primary axis O. For example, the movable part 8 may be configured to move within a plane relative to the support structure 2 while maintaining its rotational position with respect to the support structure 2. Figure 11 is a schematic plan view of an assembly 1 according to an embodiment of the invention. Optionally, the assembly 1 may be of the type in which the movable part 8 is not configured to rotate around the primary axis O. As shown in Figure 11, the assembly 1 may comprise a plurality of SMA wires 10, 16-18. Lengths 11,12 of SMA wire may be arranged around guide elements 3 as described elsewhere. Lengths 11,12 of SMA wire may be arranged at shallow-angles. These features help to amplify the stroke of the movement of the movable part 8 relative to the support structure 2. In the arrangement shown in Figure 11, The SMA wire 10 may be configured to urge the movable part in the +X direction. The opposing SMA wire 18 may be arranged to urge the movable part 8 in the -X direction. The SMA wire 16 may be arranged to urge the movable part 8 in the +Y direction. The SMA wire 17 may be arranged to urge the movable part 8 in the -Y direction. Optionally, the length of SMA wire on either side of each engagement member 19 are controlled so as to apply balancing forces in the direction perpendicular to the direction in which the SMA wire is configured to urge the movable part 8. As shown in Figure 11, optionally the engagement members 19 are in line with the axis of rotation (i.e. SMA wires 18 and 11 are at the same Y position as the axis of rotation and SMA wires 16 and 17 are at the same X position as the axis of rotation). This helps to reduce the possibility of undesirable rotation of the movable part 8 relative to the support structure 2. Figure 12 is a schematic side on view of the assembly 1 shown in Figure 11. Figure 12 shows the guide elements 3 around which lengths of SMA wire are arranged. Figure 13 schematically depicts a plan view of an assembly 1 according to an alternative embodiment of the present invention. Features of the assembly 1 may be the same as described elsewhere, except where differences are described below. As shown in Figure 13, lengths 11,12 of SMA wire are provided on either side of each engagement member 19. Each wire could be a single length hooked around the engagement member 19 of the movable part 8. Alternatively, two separate lengths 11, 12 which are attached (e.g. crimped) at the engagement member 19 may be provided. In the case of a single length, optionally the contact between the SMA wire 10 and the engagement member 19 allows for sliding motion in both directions. As shown in Figure 13, the assembly 1 may comprise flexures 9. The flexures 9 may be particularly advantageous in the case of two separate lengths 11,12 of wire being attached at the engagement member 19. The flexures 9 are each connected between an engagement member 19 and a main body 8a of the movable part 8. The flexures 9 are arranged as compliant flexures to provide compliance of movement of the movable parts 8 in one direction to movement of the movable part 8 in another (e.g. perpendicular) direction. The flexures 9 may be arranged to allow the attachments between the SMA wire 10 and the engagement members 19 to move. This helps to reduce cross talk between orthogonal motions. Any such cross talk would otherwise reduce the available motion of the movable part 8 from wire strain. The flexures 9 may be tension flexures, i.e. arranged to be in tension during ease of the assembly 1. Alternatively, the flexures 9 may be configured to be compression flexures. By providing tension flexures, the risk of the flexures 9 buckling during use may be reduced. This may help to improve the stability of the actuator assembly relative to the main body of the movable part 8 when the flexures 9 bend. Figure 14 schematically depicts a modified version of the assembly 1 of Figure 13. The assembly 1 may have features as described above, except where differences are described below. As shown in Figure 14, optionally the assembly 1 comprises electrical connectors 25. The electrical connectors may be flexible. The electrical connectors 25 are configured to connect the movable part 8 to ground potential. This may allow the engagement member 19 to function as a common electrode for different lengths 11,12 of SMA wire. Electrical potential is applied at other ends of the lengths of SMA wire may be independently controlled. The electrical connectors 25 may be particularly advantageous when the lengths 11,12 are provided as separate SMA wires attached to the engagement member 19. Optionally, the SMA wires may be arranged to load a bearing for supporting movement of the movable part 8 relative to the support structure 2. For example, the SMA wires may be inclined relative to the moving plane so as to pull the movable part 8 onto the bearing when the SMA wires are contracted. Figure 15 schematically depicts a plan view of an assembly 1 according to an alternative embodiment of the present invention. The assembly 1 may have features as described above, except where differences are described below. As shown in Figure 13 and Figure 14, for example, optionally the flexure arrangement may extend across a central part of the movable part 8 when viewed in plan view. Such a flexure arrangement may be arranged behind the image sensor 6 (i.e. on the opposite side of the image sensor 6 from the optical path) or a display or some other component. The flexure arrangement may therefore not interfere undesirably with the optics of a camera comprising the assembly 1. Alternatively, as shown in Figure 15, the flexure arrangement may be arranged around a peripheral part of the movable part 8. This may allow the flexure arrangement to be located on the same side of the movable part 8 as the optical path. The flexure arrangement may be arranged to surround the optical path without interfering with light incident on the image sensor 6. In the context of a lens shift OIS system, the flexure arrangement may be arranged around the optical path. The flexures may therefore not interfere with the light transmitted through the lenses. As shown in Figure 15, for example, the SMA wires 10, 16-18 may be arranged substantially around the edges of the movable part 8. This may reduce the possibility of heat generated by the image sensor 6, for example, from interacting undesirably with the SMA wires. By providing the SMA wires around the edge of the movable part 8, other undesirable effects of heat generated in the image sensor 6 such as control areas and noise issues, may be reduced. As shown in Figure 15, the flexure system may be arranged around the edge of the movable part 8. This may be applied to when the movable part 8 comprises either an image sensor 6 or a lens, for example. In the arrangement shown in Figure 15, the SMA wires 10, 18 may be arranged to move the movable part along the X-axis. When the movable part 8 is moved along the X-axis, it is desirable for the engagement members 19 and the SMA wires 16,17 to move together with the movable part 8 along the X-axis. As shown in Figure 15, optionally the assembly 1 comprises a flexure arrangement comprising a first flexure system 26, 27 and a second flexure system 28, 29. The first flexure system 26, 27 is arranged to allow the SMA wires 16,17 and their associated engagement members 19 to move along the X-axis when the SMA wires 10,18 are contracted to move the movable part 8 along the X-axis. In particular, as shown in Figure 15 optionally the first flexure system comprises a stiff component 26 and a flexible component 27. The stiff component 26 may be arranged substantially parallel to the X-axis. The flexible component 27 may be arranged substantially perpendicular to the X-axis. The flexible component 27 may be arranged to flex (e.g. bend) when the SMA wires 10,18 cause the movable part 8 to move along the X-axis. The stiff component 26 may be stiff enough that it does not significantly flex when it undergoes a force in a direction perpendicular to its direction of elongation. Similarly, as shown in Figure 15 optionally the second flexure system comprises a stiff component 28 and a flexible component 29. The stiff component 28 may extend substantially parallel to the Y-axis. The flexible component 29 may be arranged substantially perpendicular to the Y-axis. The SMA wires 16,17 are arranged to move the movable part 8 along the Y-axis when they are contracted. The flexible component 29 are arranged to flex (e.g. bend) so that the SMA wires 10,18 and their associated engagement members 19 move along the Y-axis together with the movable part 8. As shown in Figure 15, optionally the two flexure systems may be arranged substantially coplanar with each other. This may help to reduce the size of the assembly 11 in the Z direction. In an alternative arrangement, the flexure systems may be stacked relative to each other. For example, optionally the assembly 1 comprises an intermediate carriage (not shown). The intermediate carriage may be arranged to support the movable part 8. Optionally, at least one of the SMA wires is arranged to move the intermediate carriage relative to the support structure 2 in one direction. Another at least one of the SMA wires is supported by the intermediate carriage and is arranged to move the movable part 8 relative to the intermediate carriage in another (e.g. orthogonal) direction. By stacking the flexure systems, the footprint of the flexure system in the X-Y plane may be reduced. Figure 16 schematically depicts an assembly 1 according to an alternative embodiment of the present invention. Features of the assembly 1 may be as described elsewhere, except where differences are described below. As shown in Figure 16, the movable part 8 may comprise an engagement member 19. The engagement member 19 may be located centrally with respect to the movable part 8. For example, the engagement member 19 may be arranged as a post that is substantially concentric with the primary axis O. Different lengths of SMA wire may be arranged around different sides of the engagement member 19. For example, the arrangement shown in Figure 16 comprises two M-shaped SMA wires 10,18 configured to move the movable part 8 along the X-axis. For example, the SMA wire 10 may be arranged to move the movable part 8 in the +X direction. The SMA wire 18 may be arranged to move the movable part 8 in the -X direction. The SMA wire 10 may comprise a first section extending roughly parallel to the X-axis along the top edge (as viewed in Figure 16) of the assembly 1. The SMA wire 10 is arranged around the top guide element 3. The second section (part of the first length 11) extends from the top guide element 3 around the left side of the engagement member 19. The third section of the SMA wire extends from the left side of the engagement member 19 to the right side of the lower guide element 3. The fourth section 14 extends substantially parallel to the X-axis along the bottom edge of the assembly 1. The opposing SMA wire 18 has a similar form but is the mirror image of the SMA wire 10. As shown in Figure 16, the V-shaped wires may be provided for moving the movable part 8 along the Y-axis. A first V-shaped wire 16 is arranged around the bottom side of the engagement member 19 and is fixed to the corners of the support structure 2. The first V-shaped SMA wire 16 is arranged to move the movable part in the +Y direction when it is contracted. An opposing V-shaped SMA wire 17 is also provided. Optionally, the SMA wires 10,16-18 are arranged so as to slide over the engagement member 19, which may be arranged as a central pin. Figure 17 schematically depicts a plan view of an assembly 1 according to an alternative embodiment of the present invention. The assembly 1 may have features as described above, except where differences are described below. As shown in Figure 17, optionally the SMA wires are arranged diagonally across the movable part 8. The SMA wires may be secured to the support structure 2 at attachments 5. The attachments 5 may be provided at the corners of the support structure 2. This may help to increase the length of the SMA wires by making use of the X-Y plane footprint of the assembly 1. As shown in Figure 17, optionally the assembly 1 comprises tension flexures 9. The tension flexures 9 may be fixed to the movable part 8 at joints 89. The joints 89 may be provided at the corners of the movable part 8. The other ends of the flexures 9 may be provided with the engagement members 19. Optionally, the assembly 1 is arranged such that contraction of the SMA wires results in a diagonal movement of the movable part 8 relative to the support structure 2. A plurality of SMA wires may be contracted so as to control movement along the X-axis or along the Y-axis, for example. Optionally, the SMA wires may be angled slightly relative to a plane perpendicular to the primary axis 0. This may allow the SMA wires to overlap with each other (when viewed along the primary axis O) without interfering undesirably with each other. The SMA wires may be arranged to provide a force on the bearing which constrains motion of the movable part 8. As shown in Figure 17, optionally the assembly 1 has a square footprint. For example, the movable part 8 and / or the support structure 2 may be generally square when viewed along the primary axis 0. In the arrangement shown in Figure 17, the movable part 8 may be arranged to move translationally relative to the support structure 2 without rotation. Alternatively, the flexures 9 may be angled such that rotational control is allowed. For example, Figure 9 depicts an arrangement similar to that of Figure 17 but for an assembly 1 that allows for rotation of the movable part 8. Figure 18 schematically depicts an assembly 1 according to an alternative embodiment of the present invention. As shown in Figure 18, the movable part 8 may be arranged to rotate relative to the support structure 2. In particular, the lines of force on the movable part 8 may be distanced from the primary axis 0. As a result, contraction of the SMA wires may result in a torque being applied to the movable part 8. The assembly 1 is also capable of shifting the movable part 8 in the x-y plane. Movement along the Y-direction may be achieved by actuating wires 10 and 16 (for +Y movement) and wires 17 and 18 (for -Y movement). Movement along the X-direction may be achieved by actuating wires 18 and 16 (for -X movement) and wires 17 and 10 (for +X movement). Different combinations of wires may be actuated to move the movable part 8 to any position in the x-y plane. As shown in Figure 18, it is not essential for lengths of SMA wire to be arranged around a common engagement member 19 or crimped to the same point on a common engagement member 19. The arrangement of SMA wires shown in Figure 18 may be considered to be an arrangement of two M-shaped wires but where the M-shape has been divided into two separate wires. Each length of SMA wire 10,16-18 is arranged around a guide element 3. This helps to amplify the stroke of the movable part 8. By adjusting the positions at which the SMA wires are crimped to the movable part 8 and the positions of the guide elements 3, the torque applied to the movable part 8 for a given contraction of SMA wire may be adjusted. There may be a trade-off between the rotational stroke size and the accuracy with which the rotational position of the movable part may be controlled. As shown in Figure 18, the SMA wires 10 and 16-18 each overlap with the movable part 8 when viewed along the primary axis O. In particularly, the points at which each of the SMA wires engages with the movable part 8 (i.e. the engagement members 19 lie within a footprint of the movable part 8 when viewed along the primary axis 0. The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. Various modifications may be made to the specific examples described above. For example, as mentioned above, the SMA wires may be fixed to the support structure 2 at attachments 5, and either hooked around or fixed to the movable part 8. However, in an alternative arrangement, the SMA wires may be fixed to the movable part 8 at attachments, and either hooked around or fixed to the support structure 2. As another example, as mentioned above, the movable part 8 may comprise one of the lens assembly and the image sensor 6. However, in an alternative arrangement the movable part 8 may comprise both the lens assembly and the image sensor 6. Equally, the movable part may comprise any component, whether part of an optical device such as a camera, or any other type of component. In various embodiments described herein, the actuator assembly comprises a first length of SMA wire and a second length of SMA wire, each extending between (i) an engagement member of one of the movable part and the support structure and (ii) the other of the support structure and the movable part. Such embodiments may be modified so as to only include the first length of SMA wire (and not the second length). Such embodiments may comprise a bearing arrangement to guide movement of the movable part along a movement direction and the first length of SMA wire may, in some embodiments, form an acute, non-zero angle with the movement direction. In general, some embodiments have been described with respect to one type of movement of the movable part (e.g. rotation) but it will be appreciated that in some embodiments, the actuator assembly may also be capable of different types of movement of the movable part (e.g. translation along an axis or translational movement in two degrees of freedom in a plane). Some embodiments have been described with reference to use in a camera but it will be appreciated that the assemblies described herein may be used to move any component in any type of system or device, whether optical or not.

Claims

1. An assembly comprising:a support structure, wherein a primary axis is defined with reference to the support structure;a movable part; andan actuator assembly comprising at least one shape memory alloy, SMA, wire arranged, on contraction, to rotate the movable part relative to the support structure about the primary axis, wherein the at least one SMA wire comprises:a first length of SMA wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part; anda second length of SMA wire extending between the engagement member and the other of the support structure and the movable part, wherein the first and second lengths are arranged, on contraction, to apply a force in a force direction to the engagement member so as to rotate the movable part relative to the support structure, andwherein the actuator assembly is arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute.

2. An assembly according to claim 1, wherein the other of the support structure and the movable part comprises at least one guide element, wherein at least one of the first and second lengths of SMA wire is arranged around a respective one of the at least one guide element.

3. An assembly according to any preceding claim, wherein the actuator assembly comprises a plurality of SMA wires arranged, on contraction, to rotate the movable part relative to the support structure in opposite senses around the primary axis.

4. An assembly comprising:a support structure, wherein a primary axis is defined with reference to the support structure;a movable part; andan actuator assembly comprising at least one shape memory alloy, SMA, wire arranged, on contraction, to move the movable part relative to the support structure, wherein the at least one SMA wire comprises:a first length of SMA wire extending between an engagement member of one of thesupport structure and the movable part and the other of the support structure and the movable part; anda second length of SMA wire extending between the engagement member and the other of the support structure and the movable part,wherein the first and second lengths are arranged, on contraction, to apply a force in a force direction to the engagement member so as to move the movable part relative to the support structure, wherein the other of the support structure and the movable part comprises at least one guide element, wherein at least one of the first and second lengths of SMA wire is arranged around a respective one of the at least one guide element, andwherein the actuator assembly is arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute.

5. An assembly according to any preceding claim wherein for each of the first and second lengths of SMA wire an angle between the force direction and at least a portion of the respective length of SMA wire between the engagement member and the other of the support structure and the movable part is non-zero and acute.

6. An assembly according to any preceding claim, wherein the other of the support structure and the movable part comprises a plurality of guide elements, wherein the first and second lengths of SMA wire are each arranged around respective guide elements.

7. An assembly according to any of claims 4 to 6, wherein for at least one of the lengths of SMAwire arranged around a respective one of the at least one guide element, a reflexive angle formed by the first and second lengths of SMA wire is on the opposite side of the length of SMA wire arranged around the guide element compared to a reflexive angle formed by segments of the length of SMA wire on either side of the guide element when viewed along the primary axis.

8. An assembly according to any preceding claim, wherein the first and second lengths of SMA wire are arranged along adjacent edges of the movable part when viewed along the primary axis.

9. An assembly according to any preceding claim, wherein the actuator assembly is configured to drive rotation of the movable part about the primary axis with respect to the support structure and wherein the assembly is configured such that the force direction does not intersect the primary axis.

10. An assembly according to any preceding claim, wherein the one of the support structure and the movable part comprises a main body and an elongate flexure extending between the main body and the engagement member.

11. An assembly according to claim 10, wherein the flexure is arranged to flex when subjected to a force perpendicular to a direction of elongation of the flexure.

12. An assembly according to claim 10 or claim 11, wherein the flexure is in tension during use of the assembly.

13. An assembly according to claim 10 or claim 11, wherein the flexure is in compression during use of the assembly.

14. An assembly according to any preceding claim, wherein the length of the first length of SMA wire is different to the length of the second length of SMA wire.

15. An assembly according to any preceding claim, wherein the engagement member is located at a corner of the movable part.

16. An assembly according to any preceding claim, wherein the first and second lengths are provided as separate SMA wires.

17. An assembly according to claim 16, wherein the separate SMA wires are each attached to the engagement member.

18. An assembly according to any of claims 1-15, wherein the first and second lengths are provided as a single SMA wire arranged around the engagement member.

19. An assembly comprising:a support structure, wherein a primary axis is defined with reference to the support structure;a movable part; andan actuator assembly comprising at least one shape memory alloy, SMA, wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part and arranged, on contraction, to drive rotation of the movable part relative to the support structure about the primary axis,wherein the other of the support structure and the movable part comprises at least one guideelement, wherein the at least one SMA wire is arranged around a respective one of the at least one guide element.

20. An assembly according to any preceding claim, wherein the movable part comprises a lens.

21. An assembly comprising:a support structure, wherein a primary axis is defined with reference to the support structure;a movable part; andan actuator assembly comprising at least one shape memory alloy, SMA, wire extending between an engagement member of one of the support structure and the movable part and the other of the support structure and the movable part and arranged, on contraction, to move the movable part relative to the support structure,wherein the other of the support structure and the movable part comprises at least one guide element, wherein the at least one SMA wire is arranged around a respective one of the at least one guide element andwherein the at least one SMA wire overlaps the movable part when viewed along the primary axis.

22. An assembly according to claim 21, wherein the at least one SMA wire is configured to drive rotation of the movable part about the primary axis.

23. An assembly according to claim 21 or 22, comprising:a first length of SMA wire extending between the engagement member and the other of the support structure and the movable part; anda second length of SMA wire extending between the engagement member and the other of the support structure and the movable part,wherein the first and second lengths are arranged, on contraction, to apply a force in a force direction to the engagement member so as to move the movable part relative to the support structure, andwherein the actuator assembly is arranged such that an angle between the force direction and at least a portion of the first length of SMA wire adjacent the engagement member is non-zero and acute.

24. An assembly according to any preceding claim, wherein the actuator assembly comprises a plurality of SMA wires arranged, on contraction, to move the movable part relative to the supportstructure in different directions or senses.

25. An assembly according to claim 24, wherein the different directions oppose each other.

26. An assembly according to claim 24, wherein the different directions are perpendicular to eachother.

27. An assembly according to any of claims 24 to 26, wherein the SMA wires associated with movement in different directions or senses cross over each other when viewed along the primary axis.

28. An assembly according to any preceding claim comprising an intermediate carriage arranged to support the movable part, wherein the at least one SMA wire is supported by the intermediate carriage and is arranged to drive movement of the movable part relative to the intermediate carriage and another at least one SMA wire is arranged to drive movement of the intermediate carriage relative to the support structure.

29. An assembly according to any preceding claim, wherein the actuator assembly comprises a first pair of SMA wires which are arranged to drive movement of the movable part relative to the support structure in opposing directions along a first axis and a second pair of SMA wires which are arranged to drive movement of the movable part relative to the support structure in opposing directions along a second axis which is perpendicular to the first axis.

30. An assembly according to any preceding claim, comprising a bearing arrangement arranged to support movement of the movable part relative to the support structure.

31. An assembly according to claim 30, wherein the bearing arrangement is arranged to support movement of the movable part relative to the support structure in a plane and wherein the at least one SMA wire is inclined relative to the plane so as to bias the movable part against the bearing when actuated.

32. An assembly according to any preceding claim, wherein the movable part comprises an electronic component that extends laterally across the primary axis.

33. An assembly according to claim 32, wherein the electronic component is an image sensor or a display.

34. An assembly according to claim 33, when dependent on claim 21 or any claim dependent thereon, wherein the electronic component comprises a light-sensitive region or a light-emitting region and wherein the at least one SMA wire overlaps with the light-sensitive or light-emitting region when viewed along the primary axis.

35. An assembly according to any preceding claim, wherein the at least one SMA wire is coupled to the movable part at a point which lies within a footprint of the movable part when viewed along the primary axis.

36. An assembly according to claim 34, wherein the at least one SMA wire is coupled to the movable part at a point which lies within a footprint of the light-sensitive or light-emitting region when viewed along the primary axis.

37. An assembly according to any preceding claim, wherein the actuator assembly comprises: a controller comprising a control circuit electrically connected to the at least one SMA wire for supplying drive signals thereto.

38. An assembly according to any preceding claim, wherein the at least one SMA wire is arranged to drive translational motion of the movable part in a direction perpendicular to the primary axis.

39. An assembly according to any preceding claim comprising a plurality of SMA wires which are arranged to drive translational motion of the movable part in two degrees of freedom in a plane perpendicular to the primary axis.

40. A camera comprising the assembly of any preceding claim.

41. A camera according to claim 40 comprising folded optics.

Images