P564-GB01x 1 ACTUATOR ASSEMBLY Field The present application relates to an actuator assembly comprising an actuator arrangement configured to move a movable part relative to a support structure, and comprising a biasing arrangement configured to assist the actuator arrangement move the movable part. Background It is known to use an actuator assembly to drive movement of a movable part relative to a support structure. Such actuator assemblies are applied in a variety of devices including handheld devices, such as cameras and mobile phones. Such actuator assemblies are used for example in optical devices such as cameras for driving translational movement of a camera lens element along its optical axis, for example to effect focussing (autofocus, AF), or zoom. An example of such an actuator assembly is disclosed in WO 2008/099155 A1, which discloses a miniature camera lens actuation apparatus which provides an auto-macro function in which a camera lens element has two focus positions. Such actuator assemblies are also used for example in cameras for driving relative movement between a lens assembly and an image sensor, for example to effect optical image stabilisation (OIS). An example of such an actuator assembly is disclosed in WO 2013/175197 A1, which discloses an SMA (shape memory alloy) actuation apparatus that moves a movable element relative to a support structure in two orthogonal directions using a total of four SMA actuator wires each connected at its ends between the movable element and the support structure and extending perpendicular to the primary axis. Such actuator assemblies are also used for example in pop-out cameras (also known as retractable lens cameras or telescoping cameras) for driving translational movement of a camera lens assembly along its optical axis, so as to move the lens assembly between a retracted/collapsed state (e.g. in which the lens assembly is not capable of focussing an image on an image sensor, and/or in which the pop-out camera is in a non-operative state) and an extended/popped-out state (e.g. in which the lens assembly is capable of focussing an image on the image sensor, and in which the pop-out camera is in an operative state). Such actuator assemblies are also used in cameras comprising variable apertures, for example to change the aperture size defined by the variable aperture. An example of such an actuator assembly is disclosed in WO 2018/206768 A1, which discloses, broadly speaking, an actuator that comprises segments of shape memory alloy (SMA) actuator wire that can be used to deliver a relatively large output stroke. P564-GB01x 2 It is desirable for such actuator assemblies to have a biasing arrangement configured to help the actuator arrangement move the movable part relative to the support structure so as to, for example, increase stroke of the actuator arrangement, i.e. increase the amount the movable part can be moved by the actuator arrangement, and/or improve efficiency of the actuator arrangement. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: a movable part, a support structure, a primary axis defined with reference to the support structure, an actuator arrangement, and a biasing arrangement. The actuator arrangement is configured to (directly or indirectly) move the movable part relative to the support structure in at least one predetermined direction. The biasing arrangement is configured to (directly or indirectly) assist the actuator arrangement move the movable part in the at least one predetermined direction when moving the movable part away from an intermediate position. The primary axis may correspond or be parallel to the longitudinal axis of the actuator assembly. The at least one predetermined direction may be along, around, and/or perpendicular to the primary axis. By providing the biasing arrangement, the stroke of the actuator arrangement may be increased. When moving the movable part away from the intermediate position, the movable part may move a greater distance in the at least one predetermined direction for a given level of actuation of the actuator arrangement compared to when the biasing arrangement is absent. The stroke and/or gain of the actuator assembly may be increased without unduly decreasing the accuracy of movement. By providing the biasing arrangement, the efficiency of the actuator arrangement may be increased. When moving the movable part away from the intermediate position, less energy may be required for the actuator arrangement to move the movable part in the at least one predetermined direction by a certain distance compared to when the biasing arrangement is absent. Additionally or alternatively, less or no energy may be required for the movable part to be held at certain positions within its range of movement, for example at the ends of the range of possible movement of the movable part. Optionally, the intermediate position is a position between (i.e. not at any of) the ends of the range of possible movement of the movable part relative to the support structure, and a position in which the movable part is capable of being in equilibrium. This position may be referred to as the intermediate equilibrium position. P564-GB01x 3 Optionally, the biasing arrangement is configured to enable the movable part to move to positions beyond the range of positions the movable part can be moved over by the actuator arrangement without assistance from the biasing arrangement. In other words, the range of positions the movable part can be moved to by the actuator arrangement with support from the biasing arrangement may be greater than the range of positions the movable part can be moved to by the actuator arrangement without said support from the biasing arrangement. Optionally, the biasing arrangement is configured to provide said assistance to the actuator arrangement (i.e. assist the actuator arrangement move the movable part in the at least one predetermined direction) by exerting a force on the movable part. The force may be at least partly perpendicular to the at least one predetermined direction, and/or at least partly co-directional with the predetermined direction. The force may be directly exerted on the movable part. The force may be indirectly exerted on the movable part via an intermediate part or a further intermediate part, which are discussed in more detail below. In other words, the biasing arrangement may be configured to apply a force to the movable part with at least a component in the predetermined direction. Optionally, the actuator arrangement comprises at least one SMA element, e.g. configured to, upon contraction, drive said movement of the movable part. Optionally, the actuator assembly comprises a bistable arrangement which comprises the biasing arrangement. The bistable arrangement may be configured to (directly or indirectly) cause the movable part to have a first stable equilibrium position, a second stable equilibrium position, and an unstable equilibrium position between the first and second stable equilibrium positions; wherein the unstable equilibrium position corresponds to the intermediate position. The first stable equilibrium position may be a first position along or around the primary axis. The second stable equilibrium position may be a second position along or around the primary axis The unstable equilibrium position may be a third position along or around the primary axis between the first and second stable equilibrium positions. Optionally, the first and second stable equilibrium positions correspond to ends of the range of possible movement of the movable part relative to the support structure. Optionally, the at least one predetermined direction comprises a direction at least partly along the primary axis. P564-GB01x 4 Optionally, the primary axis corresponds to, or is parallel to, the longitudinal axis of the actuator assembly. Optionally, the movable part comprises a lens assembly configured to focus an image on an image sensor; and the primary axis corresponds to the optical axis of the lens assembly. The lens assembly may comprise one or more lenses. Optionally, the actuator assembly is configured such that the distance between the lens assembly and the image sensor along the primary axis in the first stable equilibrium position is less than the distance between the lens assembly and the image sensor along the primary axis in the second stable equilibrium position. In other words, in the first stable equilibrium position, the lens assembly is in a retracted/collapsed position, and in the second stable equilibrium position, the lens assembly is in a popped-out/extended position. Optionally, the lens assembly comprises one or more lenses, and the actuator assembly comprises a focus (e.g. an AF) actuator configured to drive relative movement between the one or more lenses and the image sensor along the primary axis. Optionally, the lens assembly is arranged such that it is not capable of focusing an image onto the image sensor when the movable part is in the first equilibrium position, and configured such that it is capable of focusing an image onto the image sensor when the movable part is in the second equilibrium position. Optionally, the actuator arrangement is configured to rotate the movable part relative to the support structure around the primary axis. The rotation of the movable part may be driven directly by the actuator arrangement, or indirectly driven by the actuator arrangement via an intermediate part. Optionally, the actuator assembly comprises a helical bearing arrangement supporting the movable part on the support structure and arranged to guide helical movement of the movable part relative to the support structure around the primary axis; wherein the helical bearing arrangement converts the rotation of the movable part around the primary axis into said helical movement of the movable part. Alternatively, the actuator assembly comprises a helical bearing arrangement supporting the movable part on the intermediate part and arranged to guide helical movement of the movable part relative to the intermediate part; wherein the helical bearing arrangement converts the rotation of the intermediate part around the primary axis into said helical movement of the movable part. P564-GB01x 5 Optionally, the movable part forms part of a variable aperture (e.g. an iris diaphragm) assembly configured such that rotation of the movable part relative to the support structure around the primary axis changes the (size of the) aperture of the variable aperture assembly; and wherein the primary axis corresponds to the optical axis of the variable aperture assembly. The aperture of the variable aperture assembly is the opening through which light travels to reach an image sensor. Optionally, the bistable arrangement (e.g. a biasing element of the bistable arrangement) is configured to (directly or indirectly) bias the movable part towards the first stable equilibrium position when the movable part is between the first stable equilibrium position and the unstable equilibrium position, and configured to bias the movable part towards the second stable equilibrium position when the movable part is between the second stable equilibrium position and the unstable equilibrium position. Optionally, the actuator arrangement (e.g. with assistance from the biasing arrangement) is configured to (directly or indirectly) drive movement of the movable part from the first stable equilibrium position to the second stable equilibrium position, and, optionally, to (directly or indirectly) drive movement of the movable part from the second stable equilibrium position to the first stable equilibrium position. A first SMA element of the actuator arrangement may be configured to drive movement of the movable part from the first stable equilibrium position to the second stable equilibrium position. A second SMA element of the actuator arrangement may be configured to drive movement of the movable part from the second stable equilibrium position to the first stable equilibrium position. Optionally, the actuator arrangement (e.g. the first SMA element) is configured to (directly or indirectly) drive movement of the movable part from the first stable equilibrium position to a first tipping position, wherein the first tipping position is a position between the unstable equilibrium position and the second stable equilibrium position; and wherein the bistable arrangement (e.g. the biasing element) is capable of driving (and configured to drive) movement of the movable part from the first tipping position to the second stable equilibrium position without (drive/force) assistance from the actuator arrangement. The actuator arrangement (e.g. the first SMA element) may be configured to (directly or indirectly) drive movement of the movable part from the first stable equilibrium position to the first tipping position against the biasing force of the bistable arrangement/biasing element up to the unstable equilibrium position. Optionally, the actuator arrangement (e.g. the second SMA element) is configured to (directly or indirectly) drive movement of the movable part from the second stable equilibrium position to a second tipping position, wherein the second tipping position is a position between the unstable equilibrium position and the first stable equilibrium position; and wherein the bistable arrangement (e.g. the biasing P564-GB01x 6 element) is capable of driving (and configured to drive) movement of the movable part from the second tipping position to the first stable equilibrium position without (drive/force) assistance from the actuator arrangement. The actuator arrangement (e.g. the second SMA element) may be configured to (directly or indirectly) drive movement of the movable part from the second stable equilibrium position to the second tipping position against the biasing force of the bistable arrangement/biasing element up to the unstable equilibrium position. Optionally, the actuator arrangement comprises one or more shape memory alloy (SMA) elements (e.g. two or four SMA elements) configured to (directly or indirectly) move the movable part relative to the support structure in the at least one predetermined direction (e.g. along, around, and/or perpendicular to the primary axis). Optionally, (each of) the one or more SMA elements are (directly or indirectly) coupled to the movable part and to the support structure. Some or all of the one or more SMA elements may be directly coupled to the movable part and the support structure via coupling components such as crimps. Some or all of the one or more SMA elements may be indirectly coupled to the movable part and the support structure via e.g. the bistable arrangement. Optionally, the one or more SMA elements comprise: a first SMA element configured to, upon contraction, drive the relative movement of the movable part in a first direction (at least partly along, around, and/or perpendicular to the primary axis); and a second SMA element configured to, upon contraction, drive the relative movement of the movable part in a second opposite direction (at least partly along, around, and/or perpendicular to the primary axis), wherein the first and second directions are opposite directions. Optionally, the actuator assembly comprises an intermediate part that is movable relative to the support structure and the movable part; and wherein the actuator arrangement (e.g. the one or more SMA elements) is configured to (indirectly via the intermediate part) move the movable part relative to the support structure by (directly or indirectly) moving the intermediate part relative to the support structure and the movable part. Optionally, (each of) the one or more SMA elements are (directly or indirectly) coupled to the intermediate part and to the support structure. Some or all of the one or more SMA elements may be directly coupled to the intermediate part and the support structure via coupling components such as crimps. Some or all of the one or more SMA elements may be indirectly coupled to the intermediate part and the support structure via e.g. the bistable arrangement. P564-GB01x 7 Optionally, the one or more SMA elements comprise: a first SMA element configured to, upon contraction, (indirectly) drive the relative movement of the movable part in a first direction (at least partly along, around, and/or perpendicular to the primary axis); and a second SMA element configured to, upon contraction, (indirectly via the intermediate part) drive the relative movement of the movable part in a second opposite direction (at least partly along, around, and/or perpendicular to the primary axis). Optionally, the intermediate part is configured to at least partly move in directions perpendicular to the primary axis. Optionally, the intermediate part is configured to rotate about the primary axis. Optionally, the one or more SMA elements are not configured to move the intermediate part along the primary axis. Optionally, the bistable mechanism is configured to cause the intermediate part to have a first stable equilibrium position, a second stable equilibrium position, and an unstable equilibrium position between the first and second stable equilibrium positions. When the intermediate part is moved to its first stable equilibrium position, the movable part may also be moved to its first stable equilibrium position. When the intermediate part is moved to its second stable equilibrium position, the movable part may also be moved to its second stable equilibrium position. When the intermediate part is moved to its unstable equilibrium position, the movable part may also be moved to its unstable equilibrium position. The bistable mechanism may be configured to only cause the movable part to be bistable. Optionally, the intermediate part and the movable part are connected to each other via an intermediate bearing arrangement, wherein the intermediate bearing arrangement is configured such that, when the intermediate part is moved by the actuator arrangement, the movable part is also moved (relative to the support structure). Optionally, the actuator assembly comprises a further intermediate part between the intermediate part and the movable part, wherein the actuator arrangement (e.g. the one or more SMA elements) are P564-GB01x 8 configured to (indirectly) move the movable part relative to the support structure via the intermediate part and the further intermediate part. Optionally, the intermediate part and the further intermediate part are connected to each other via an intermediate bearing arrangement, wherein the intermediate bearing arrangement is configured such that, when the intermediate part is moved by the actuator arrangement, the further intermediate part is also moved (e.g. relative to the support structure and e.g. relative to the intermediate part). Optionally, the intermediate bearing arrangement comprises bearing surfaces (which e.g. form part of the intermediate part and the movable part, or form part of the intermediate part and the further intermediate part) angled relative to a plane normal to the primary axis. Optionally, there is an amount of backlash (i.e. play) between the movable part and the part of the actuator assembly configured to directly move the movable part (i.e. the intermediate part or the further intermediate part). Optionally, the amount of backlash is at least equal to the amount of displacement (e.g. linear or rotational displacement) the movable part is configured to undergo when moving from the unstable equilibrium position to the first or second stable equilibrium positions. Optionally, the support structure comprises a first endstop configured to engage the movable part when the movable part is in the first stable equilibrium position, and a second endstop configured to engage the movable part when the movable part is in the second stable equilibrium position. Optionally, the bistable arrangement is configured to hold the movable part in the first stable equilibrium position, the second stable equilibrium position, or the unstable equilibrium position when the actuator arrangement is not moving the movable part relative to the support structure. Optionally, the actuator assembly comprises at least one guide bearing configured to guide movement of movable part relative to the support structure. Optionally, the actuator assembly comprises at least one guide bearing configured to guide movement of the intermediate part relative to the support structure. Optionally, the actuator assembly comprises at least one guide bearing configured to guide movement of the further intermediate part relative to the support structure. P564-GB01x 9 Optionally, the actuator assembly comprises at least one pivoted lever (e.g. hinged lever) configured to guide movement of the movable part (and/or the intermediate part, and/or the further intermediate part) relative to the support structure. Optionally, (each of) the one or more SMA elements are parallel, perpendicular, or at an acute angle (e.g. inclined) relative to the primary axis. Optionally, the biasing arrangement comprises an elastic component (e.g. a spring or a flexure) and/or one or more magnets configured to provide said assistance to the actuator arrangement. Optionally, the bistable arrangement comprises a bistable compliant body. Optionally, the actuator arrangement comprises: a first SMA element coupled at its ends to a first side of the bistable compliant body, and a second SMA element coupled at its ends to a second side of the bistable compliant body; wherein the first and second sides of the bistable compliant body are opposite sides of the bistable compliant body. Optionally, the actuator arrangement comprises: a first SMA element and a second SMA element each coupled at one end to the support structure and at the other end to a first side of the bistable compliant body, and a third SMA element and a fourth SMA element each coupled at one end to the support structure and at the other end to a second side of the bistable compliant body; wherein the first and second sides of the bistable compliant body are opposite sides of the bistable compliant body. Optionally, the intermediate position is a stable equilibrium position. Optionally, the actuator assembly comprises a centering arrangement configured to bias the movable part towards the intermediate position. Optionally, the biasing arrangement comprises one or more magnets configured to provide said assistance to the actuator arrangement. Optionally, the movable part is movable relative to the support structure across a range of movement in two orthogonal directions perpendicular to the primary axis; and wherein the at least one predetermined direction comprises a direction (or directions) in the plane of the two orthogonal directions. P564-GB01x 10 Optionally, the actuator arrangement comprises (a total of) four SMA elements (e.g. SMA wires) connected between the movable part and the support structure in an arrangement wherein the SMA elements are capable of being selectively driven to move the movable part relative to the support structure to any position in said range of movement without applying any net torque to the movable element in the plane of the two orthogonal directions around the primary axis. Optionally, the movable part comprises an image sensor and/or a lens assembly. Optionally, the movable part comprises a display, an emitter, or a part thereof. The primary axis may be parallel to a general direction in which the display or emitter emits light. 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: Fig.1 is a schematic view of an actuator assembly with two opposing SMA elements arranged to move a movable part; Fig.2 is a schematic view of an actuator assembly with two opposing SMA elements arranged to move a movable part via an intermediate part; Fig.3 is a schematic view of an actuator assembly with two opposing SMA elements arranged to move a movable part via an intermediate part, in a first position; Fig.4 is a schematic view of the actuator assembly of Fig.3 in a second position; Fig.5 is a schematic view of the actuator assembly of Fig.3 in a third position; Fig.6 is a schematic view of the actuator assembly of Fig.3 in a fourth position; Fig.7 is a schematic view of the actuator assembly of Fig.3 in a fifth position; Fig.8 is a schematic view of the actuator assembly of Fig.3 in a sixth position; Fig.9 is a schematic view of an actuator assembly with two opposing SMA elements arranged to move a movable part via an intermediate part; Fig.10 is a schematic view of an actuator assembly with two opposing SMA elements arranged to move a movable part via an intermediate part and a further intermediate part; Fig.11 is a schematic view of an actuator assembly with two angled opposing SMA elements arranged to move a movable part; Fig.12 is a schematic view of a bearing arrangement; Fig.13 is a schematic view of a pivoted lever; Fig.14 is a schematic view of an actuator assembly with a bistable arrangement; P564-GB01x 11 Fig.15 is a schematic view of an actuator assembly with a bistable arrangement; Fig.16 is a schematic view of an actuator assembly with a bistable arrangement; Fig.17 is a schematic view of an actuator assembly with a bistable compliant body; Fig.18 is a schematic view of an actuator assembly with a bistable compliant body in a first position; Fig.19 is a schematic view of the actuator assembly of Fig.18 with the bistable compliant body in a second position; Fig.20 is a schematic view of an actuator assembly with two opposing SMA elements arranged to rotate a movable part, in a first position; Fig.21 is a schematic view of the actuator assembly of Fig.20, in a second position; Fig.22 is a schematic view of an actuator assembly with a movable part in a first position; Fig.23 is a schematic view of the actuator assembly of Fig.22 with a movable part in a second position; Fig.24 is a schematic view of an actuator assembly with four SMA elements. Detailed description A. Bistable actuator assemblies Fig.1 is a schematic view of an actuator assembly 100 with two opposing SMA elements 31, 32 arranged to, upon selective contraction, move a movable part 3 relative to a support structure 4. In Fig.1, the SMA elements 31, 32 (i.e. the actuator arrangement of Fig.1) are arranged to translationally move the movable part 3 in directions M, along a primary axis (not shown). However, the SMA elements 31, 32 may instead be arranged to move the movable part 3 around (i.e. rotate the movable part 3 around) the primary axis. The first SMA element 31 is provided on a first side the movable part 3, and the second SMA element 32 is provided on a second, opposite side of the movable part 3. Both SMA elements 31, 32 are coupled at a first end to the movable part 3 and at a second end to the support structure 4. However, one or both of the SMA elements 31, 32 may instead be coupled at both ends to the support structure 4 or the movable part 3, and have a central portion that is coupled (e.g. hooked) to the other of the support structure 4 and the movable part 3. The first SMA element 31 is configured to, upon contraction, drive the movable part 3 to move relative to the support structure 4 in a first direction. The second SMA element 32 is configured to, upon contraction, drive the movable part 3 to move relative to the support structure 4 in a second opposite direction. At least one guide bearing configured to guide movement of the movable part 3 relative to the support structure 4 may be provided. P564-GB01x 12 Where the SMA elements 31, 32 are arranged to rotate the intermediate part 40 around the primary axis, a helical bearing arrangement may be provided between the support structure 4 and the movable part 3. The helical bearing arrangement may support the movable part 3 on the support structure 4 and may be arranged to guide helical movement of the movable part 3 relative to the support structure 4 around the primary axis, i.e. the helical bearing arrangement may be arranged to convert the rotation of the movable part 3 around the primary axis into helical movement of the movable part 3 relative to the support structure 4. A bistable arrangement (not shown) is configured to cause the movable part 3 to behave in a bistable manner, i.e. cause the movable part 3 to have a first stable equilibrium position, a second stable equilibrium position, and an unstable equilibrium position between the first and second stable equilibrium positions. The bistable arrangement provides a biasing force that holds the movable part 3 in the first stable equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement. Similarly, the biasing force of the bistable arrangement holds the movable part 3 in the unstable equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement. Similarly, the biasing force of the bistable arrangement holds the movable part 3 in the second equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement. The biasing force is weak enough to allow the actuator arrangement to move the movable part 3 between the three equilibrium positions. Of course, given that the movable part 3 is unstable in the unstable equilibrium position, in practice, it may be difficult for the actuator assembly to move and hold the movable part 3 in this position. The first and second stable equilibrium positions may correspond to the ends of the range of movement that the movable part 3 is arranged to be moved in relative to the support structure 4 by the actuator arrangement. The unstable equilibrium position may correspond to the centre of said range of movement. The support structure 4 may comprise a first endstop configured to engage the movable part 3 when the movable part 3 is in the first stable equilibrium position. The support structure 4 may also comprise a second endstop configured to engage the movable part 3 when the movable part 3 is in the second stable equilibrium position. The bistable arrangement (which may be considered to be part of a biasing arrangement as it exerts a biasing force on the movable part 3) assists the actuator arrangement move the movable part 3 relative P564-GB01x 13 to the support structure 4 when moving the movable part 3 away from the unstable equilibrium position (which may also be referred to as an intermediate position) and e.g. towards the first or the second equilibrium positions. In other words, the bistable arrangement is configured to bias the movable part 3 towards the first stable equilibrium position when the movable part 3 is moved, e.g. by the actuator arrangement, to a position between the first stable equilibrium position and the unstable equilibrium position. And configured to bias the movable part 3 towards the second stable equilibrium position when the movable part 3 is moved, e.g. by the actuator arrangement, to a position between the second stable equilibrium position and the unstable equilibrium position. This helps, for example, increase the stroke of the actuator arrangement, i.e. increase the amount the movable part 3 can be moved by the actuator arrangement, and/or improve the efficiency of the actuator arrangement. The actuator arrangement (e.g. the first SMA element 31) may (e.g. only) be configured to drive movement of the movable part 3 from the first stable equilibrium position to a first tipping position, wherein the first tipping position is a position between the unstable equilibrium position and the second stable equilibrium position, and the bistable arrangement may be configured to drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. Similarly, the actuator arrangement (e.g. the second SMA element 32) may (e.g. only) be configured to drive movement of the movable part 3 from the second stable equilibrium position to a second tipping position, wherein the second tipping position is a position between the unstable equilibrium position and the first stable equilibrium position, and the bistable arrangement may be configured to drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. In other words, the bistable arrangement may be configured to enable the movable part 3 to move to positions beyond the range of positions the movable part 3 could be moved over by the actuator arrangement without assistance from the bistable arrangement. In other words, the range of positions the movable part 3 can be moved to by the actuator arrangement with support from the bistable arrangement may be greater than the range of positions the movable part 3 can be moved to by the actuator arrangement without said support from the bistable arrangement. Fig.2 is a schematic view of an actuator assembly 101 with two opposing SMA elements 41, 42 arranged to, upon selective contraction, move a movable part 3 relative to a support structure 4 via an intermediate part 40, i.e. indirectly move the movable part 3 via the intermediate part 40. In Fig.2, the SMA elements 41, 42 (i.e. the actuator arrangement of Fig.2) are arranged to translationally move the intermediate part 40 in directions S, perpendicular to the primary axis (i.e. the P564-GB01x 14 axis in which the movable part 3 is arranged to move along/around). However, the SMA elements 41, 42 may instead be arranged to move the intermediate part 40 around (i.e. rotate the intermediate part 40 around) the primary axis. The intermediate part 40 and the movable part 3 are connected to each other via an intermediate bearing arrangement 43. The intermediate bearing arrangement 43 is configured such that when the intermediate part 40 is moved by the SMA elements 41, 42 relative to the support structure 4 (e.g. in directions S, perpendicular to the primary axis in this example), the movable part 3 is also moved relative to the support structure 4 (e.g. in directions M, along the primary axis in this example). The first SMA element 41 is provided on a first side the intermediate part 40, and the second SMA element 42 is provided on a second opposite side of the intermediate part 40. Both SMA elements 41, 42 are coupled at a first end to the intermediate part 40 and at a second end to the support structure 4. However, one or both of the SMA elements 41, 42 may instead be coupled at both ends to the support structure 4 or the intermediate part 40, and have a central portion that is coupled (e.g. hooked) to the other of the support structure 4 and the intermediate part 40. The first SMA element 41 is configured to, upon contraction, drive the intermediate part 40 to move relative to the support structure 4 in a first direction. The second SMA element 32 is configured to, upon contraction, drive the movable part 3 to move relative to the support structure 4 in a second opposite direction. The intermediate bearing arrangement 43 comprises bearing surfaces of the intermediate part 40 and the movable part 3 which are angled relative to a plane normal to the primary axis. The ramp interaction between the intermediate part 40 and the movable part 3 via the angled bearing surfaces provides an amplification in stroke. The intermediate bearing arrangement 43 may comprise ball bearings between the bearing surfaces as shown in Fig.2. Where the SMA elements 41, 42 are arranged to rotate the intermediate part 40 around the primary axis, the bearing surfaces may be helical bearing surfaces. At least one guide bearing configured to guide movement of the movable part 3 relative to the support structure 4 is provided. At least one guide bearing 33 configured to guide movement of the intermediate part 40 relative to the support structure 4 is provided. To ensure that the intermediate part 40 and the movable part 3 are directly mechanically coupled to each other (with no play/backlash between them) the intermediate part 40 and the movable part 3 are biased against each other by one or more biasing components (not shown), such as springs and/or magnets. P564-GB01x 15 A bistable arrangement (not shown) is configured to cause the intermediate part 40 to behave in a bistable manner, i.e. cause the intermediate part 40 to have a first stable equilibrium position, a second stable equilibrium position, and an unstable equilibrium position between the first and second stable equilibrium positions. Because the intermediate part 40 and the movable part 3 are directly mechanically coupled to each other (with no play/backlash between them), the bistable arrangement also, indirectly via the intermediate part 40, causes the movable part 3 to behave in a bistable manner, i.e. causes the movable part 3 to have a first stable equilibrium position (when the intermediate part 40 is in its first stable equilibrium position), a second stable equilibrium position (when the intermediate part 40 is in its second stable equilibrium position), and an unstable equilibrium position between the first and second stable equilibrium positions (when the intermediate part 40 is in its unstable equilibrium position). The bistable arrangement provides a biasing force that holds the intermediate part 40 and (indirectly) the movable part 3 in their first stable equilibrium positions when the intermediate part 40 and the movable part 3 are moved to their first stable equilibrium positions, for example, by the actuator arrangement. Similarly, the biasing force of the bistable arrangement holds the intermediate part 40 and (indirectly) the movable part 3 in their unstable equilibrium positions when the intermediate part 40 and the movable part 3 are moved to their unstable equilibrium positions, for example, by the actuator arrangement. Similarly, the biasing force of the bistable arrangement holds the intermediate part 40 and (indirectly) the movable part 3 in their second equilibrium positions when the intermediate part 40 and the movable part 3 are moved to their second equilibrium positions, for example, by the actuator arrangement. The biasing force is weak enough to allow the actuator arrangement to move the intermediate part 40 and (indirectly) the movable part 3 between their three equilibrium positions. The bistable arrangement may comprise one or more resilient or elastic components (e.g. one or more springs) and/or one or more magnets arranged to bias the intermediate part 40 along the direction indicated by the arrow S in Figure 2. The first and second stable equilibrium positions may correspond to the ends of the range of positions that the intermediate part 40 and the movable part 3 are capable of being moved to relative to the support structure 4 by the actuator arrangement. The unstable equilibrium positions may correspond to the centres of said ranges of positions. The support structure 4 may comprise an endstop configured to engage the intermediate part 40 when the intermediate part 40 is in its first stable equilibrium position. The support structure 4 may also comprise an endstop configured to engage the intermediate part 40 when the intermediate part 40 is in its second stable equilibrium position. Additionally or alternatively, the support structure 4 may P564-GB01x 16 comprise an endstop configured to engage the movable part 3 when the movable part 3 is in its first stable equilibrium position and, optionally, an endstop configured to engage the movable part 3 when the movable part 3 is in its second stable equilibrium position. The bistable arrangement (which may be considered to be part of a biasing arrangement as it exerts a biasing force) assists the actuator arrangement move the movable part 3 (indirectly via the intermediate part 40) relative to the support structure 4 when (indirectly) moving the movable part 3 away from its unstable equilibrium position (which may also be referred to as an intermediate position), and e.g. towards its first or second equilibrium position. In other words, the bistable arrangement is configured to (indirectly via the intermediate part 40) bias the movable part 3 towards its first stable equilibrium position when the movable part 3 is moved, e.g. (indirectly) by the actuator arrangement, to a position between its first stable equilibrium position and its unstable equilibrium position. It is also configured to (indirectly) bias the movable part 3 towards its second stable equilibrium position when the movable part 3 is moved, e.g. (indirectly) by the actuator arrangement, to a position between its second stable equilibrium position and its unstable equilibrium position. This helps, for example, increase the stroke of the actuator arrangement, i.e. increase the amount the movable part 3 can be moved by the actuator arrangement, and/or improve the efficiency of the actuator arrangement. The actuator arrangement (e.g. the first SMA element 41) may (e.g. only) be configured to (indirectly) drive movement of the movable part 3 from the first stable equilibrium position to a first tipping position, wherein the first tipping position is a position between the unstable equilibrium position and the second stable equilibrium position, and the bistable arrangement may be configured to (indirectly) drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. Similarly, the actuator arrangement (e.g. the second SMA element 42) may (e.g. only) be configured to (indirectly) drive movement of the movable part 3 from the second stable equilibrium position to a second tipping position, wherein the second tipping position is a position between the unstable equilibrium position and the first stable equilibrium position, and the bistable arrangement may be configured to (indirectly) drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. In other words, the bistable arrangement may be configured to (indirectly) enable the movable part 3 to move to positions beyond the range of positions the movable part 3 can be moved over (indirectly) by the actuator arrangement without assistance from the bistable arrangement. In other words, the range of positions the movable part 3 can be (indirectly) moved to by the actuator arrangement with support from the bistable arrangement may be greater than the range of positions the movable part 3 can be moved to (indirectly) by the actuator arrangement without said support from the bistable arrangement. P564-GB01x 17 Bistable actuator assemblies with backlash Figs.3 to 8 are schematic views of an actuator assembly 102 with two opposing SMA elements 41, 42 arranged to, upon selective contraction, move a movable part 3 relative to a support structure 4 via an intermediate part 40, i.e. arranged to indirectly move the movable part 3 via the intermediate part 40. The actuator assembly 102 of Figs.3 to 8 and the actuator assembly 101 of Fig.2 are similar. The main differences are that the actuator assembly 102 of Figs.3 to 8 has: (a) an amount of backlash (i.e. play) between the movable part 3 and the intermediate part 40, (b) both the movable part 3 and the intermediate part 40 configured to move parallel to the primary axis, (c) no ramp interaction between the intermediate part 40 and the movable part 3 via angled bearing surfaces, and (d) the bistable arrangement 30 configured to directly apply the biasing force to the movable part 3, instead of the intermediate part 40. It is worth noting that the intermediate part 40 of Figs.3 to 8 does not need to be bistable. In Figs.3 to 8, the SMA elements 41, 42 (i.e. the actuator arrangement of Figs.3 to 8) are arranged to translationally move the intermediate part 40 and (indirectly) the movable part 3 in directions perpendicular to the primary axis. However, the SMA elements 41, 42 may instead be arranged to move the intermediate part 40 and/or the movable part 3 around (i.e. rotate the intermediate part 40 around) the primary axis. The intermediate part 40 and the movable part 3 are arranged to mechanically engage each other such that when the intermediate part 40 is moved by the SMA elements 41, 42 relative to the support structure 4 in a first direction (e.g. by contracting SMA element 41), the movable part 3 is also moved relative to the support structure 4 in a first direction. The intermediate part 40 and the movable part 3 are arranged to mechanically engage each other such that when the intermediate part 40 is moved by the SMA elements 41, 42 relative to the support structure 4 in a second opposite direction (e.g. by contracting SMA element 42), the movable part 3 is also moved relative to the support structure 4 in a second opposite direction. An amount of backlash (i.e. play) is provided between the movable part 3 and the intermediate part 40. The amount of backlash is at least equal to the amount of displacement the movable part 3 is configured to undergo when moving from its unstable equilibrium position to its first or second stable equilibrium position. P564-GB01x 18 The first SMA element 41 is provided on a first side the intermediate part 40, and the second SMA element 42 is provided on a second opposite side of the intermediate part 40. Both SMA elements 41, 42 are coupled at a first end to the intermediate part 40 and at a second end to the support structure 4. However, one or both of the SMA elements 41, 42 may instead be coupled at both ends to the support structure 4 or the intermediate part 40, and have a central portion that is coupled (e.g. hooked) to the other of the support structure 4 and the intermediate part 40. At least one guide bearing configured to guide movement of the movable part 3 relative to the support structure 4 may be provided. At least one guide bearing 33 configured to guide movement of the intermediate part 40 relative to the support structure 4 may be provided. The bistable arrangement 30 (which is illustrated as a compression spring 30 in Figs.3 to 8) is configured to cause the movable part 3 to behave in a bistable manner, i.e. cause the movable part 3 to have a first stable equilibrium position, a second stable equilibrium position, and an unstable equilibrium position between the first and second stable equilibrium positions. The bistable arrangement 30 may be configured to bias the movable part 3 and the intermediate part 40 against each other so as to ensure that the movable part 3 and the intermediate part 40 are kept mechanically coupled to each other, but this may not be required, for example, if the above-mentioned guide bearings are provided. The bistable arrangement 30 provides a biasing force that holds the movable part 3 in the first stable equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement (indirectly via the intermediate part 40). Figs.3 to 6, show the movable part 3 being moved to the first stable equilibrium position (i.e. the position in Fig.6) from the second stable equilibrium position (i.e. the position in Fig.3) by having the SMA element 42 contract and move the intermediate part 40 towards the right. The biasing force of the bistable arrangement 30 holds the movable part 3 in the unstable equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement (indirectly via the intermediate part 40). Fig.5 shows the movable part 3 in the unstable equilibrium position wherein the biasing force exerted by bistable arrangement 30 is perpendicular to the primary axis. The biasing force of the bistable arrangement 30 holds the movable part 3 in the second equilibrium position when the movable part 3 is moved to this position, for example, by the actuator arrangement P564-GB01x 19 (indirectly via the intermediate part 40). Figs.6 to 8 show the movable part 3 being moved towards the second stable equilibrium position from the first stable equilibrium position by having the SMA element 41 contract and move the intermediate part 40 towards the left. The first and second stable equilibrium positions correspond to the ends of the range of movement that the movable part 3 is arranged to be moved in relative to the support structure 4 by the actuator arrangement. The unstable equilibrium position may correspond to the centre of said range of movement. The support structure 4 comprises a first endstop configured to engage the movable part 3 when the movable part 3 is in the first stable equilibrium position. The support structure 4 also comprises a second endstop configured to engage the movable part 3 when the movable part 3 is in the second stable equilibrium position. The bistable arrangement 30 (which may be considered to be part of a biasing arrangement as it exerts a biasing force on the movable part 3) assists the actuator arrangement move the movable part 3 relative to the support structure 4 when moving the movable part 3 away from the unstable equilibrium position (which may also be referred to as an intermediate position) and e.g. towards the first or the second equilibrium positions. In other words, the bistable arrangement 30 is configured to bias the movable part 3 towards the first stable equilibrium position when the movable part 3 is moved, e.g. by the actuator arrangement, to a position between the first stable equilibrium position and the unstable equilibrium position. And configured to bias the movable part 3 towards the second stable equilibrium position when the movable part 3 is moved, e.g. by the actuator arrangement, to a position between the second stable equilibrium position and the unstable equilibrium position. This helps, for example, increase the stroke of the actuator arrangement, i.e. increase the amount the movable part 3 can be moved by the actuator arrangement, and/or improve the efficiency of the actuator arrangement. The actuator arrangement (e.g. the first SMA element 41) may (e.g. only) be configured to drive movement of the movable part 3 from the first stable equilibrium position to a first tipping position, wherein the first tipping position is a position between the unstable equilibrium position and the second stable equilibrium position, and the bistable arrangement 30 may be configured to drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. Similarly, the actuator arrangement (e.g. the second SMA element 42) may (e.g. only) be configured to drive movement of the movable part 3 from the second stable equilibrium position to a second tipping position, wherein the second tipping position is a position between the unstable equilibrium position and the first stable equilibrium position, P564-GB01x 20 and the bistable arrangement 30 may be configured to drive movement of the movable part 3 from the first tipping position to the second stable equilibrium position by itself, i.e. without drive/force assistance from the actuator arrangement. In other words, the bistable arrangement 30 may be configured to enable the movable part 3 to move to positions beyond the range of positions the movable part 3 could be moved over by the actuator arrangement without assistance from the bistable arrangement. In other words, the range of positions the movable part 3 can be moved to by the actuator arrangement with support from the bistable arrangement 30 may be greater than the range of positions the movable part 3 can be moved to by the actuator arrangement without said support from the bistable arrangement 30. As shown in Fig.9, the intermediate part 40 may instead be arranged to move perpendicular to the primary axis (i.e. the axis the movable part 3 moves along/around). Where this is the case, the surfaces of the intermediate part 40 and/or the movable part 3 that are separated by the backlash and are configured to engage each other, e.g. surfaces 34 and 35, are each angled relative to a plane normal to the primary axis. As shown in Fig.10, a further intermediate part 50 may be provided between the intermediate part 40 and the movable part 3, and the actuator arrangement may be configured to (indirectly) move the movable part 3 relative to the support structure 4 via the intermediate part 40 and the further intermediate part 50. The further intermediate part 50 may be configured to move parallel to the primary axis (i.e. the axis the movable part 3 moves along/around). The backlash may be provided between the further intermediate part 50 and the movable part 3, instead of between the intermediate part 40 and the further intermediate part 50. The intermediate part 40 and the further intermediate part 50 may be connected to each other via an intermediate bearing arrangement 43’. The intermediate bearing arrangement 43’ may be configured such that, when the intermediate part 40 is moved by the actuator arrangement (i.e. SMA elements 41 and 42), the further intermediate part 50 is moved relative to the support structure. The intermediate bearing arrangement 43’ comprises bearing surfaces of the intermediate part 40 and the further intermediate part 50 which are each angled relative to a plane normal to the primary axis. The ramp interaction between the intermediate part 40 and the further intermediate part 50 via the angled bearing surfaces provides an amplification in stroke. The intermediate bearing arrangement 43 may comprise ball bearings between the bearing surfaces as shown in Fig.10. Where the SMA elements P564-GB01x 21 41, 42 are arranged to rotate the intermediate part 40 around the primary axis, the bearing surfaces may be helical bearing surfaces. At least one guide bearing configured to guide movement of the further intermediate part 50 relative to the support structure 4 may be provided. As shown in Fig.11, any of the above-mentioned SMA elements may be angled relative to the primary axis. The SMA element(s) may be at an actute, non-zero angle to the primary axis. For example, the SMA element(s) may be arranged to apply a force to the movable part 3 with a component perpendicular to a direction of motion of the movable part 3. For example, the force applied by the SMA may aid in loading the movable part onto a bearing, such as guide bearing 33. As shown in Fig.12, any of the above-mentioned bistable arrangements may comprise two oppositely inclined slopes forming a common peak; and may be configured to bias (see biasing force F) the movable part 3 against the inclined slopes. Such a biasing force F may be provided by one or more resilient (or elastic) components (e.g. springs) or magnets, for example. As shown in Fig.13, movement of the movable part 3 and/or the intermediate part 40 and/or the further intermediate part 50 relative to the support structure may be guided by a pivoted lever (e.g. hinged lever) where appropriate. As shown in Fig.14 and 15, any of the above-mentioned bistable arrangements may comprise one or more elastic components 30 configured to provide said assistance to the actuator arrangement. For example, as shown in Fig.14 (and Figs.3 to 8), the bistable arrangement may comprise a single compression spring 30. For example as shown in Fig.15, the bistable arrangement may comprise two opposing compression springs/flexures 30. Additionally or alternatively, as shown in Fig.16, any of the above-mentioned bistable arrangements may comprise one or more magnets 36, 37 configured to provide said assistance to the actuator arrangement. Bistable compliant body Any of the above-mentioned bistable arrangements may comprise a bistable compliant body 310, such as those illustrated in Figs.17 to 19, configured to (directly or indirectly) cause the movable part 3 to be bistable. The bistable compliant body 310 is fixed at its ends to the support structure 4. The bistable compliant body 310 comprises two protruding portions 311 and 312 (i.e. a first protruding portion 311 and a P564-GB01x 22 second protruding portion 312) provided on a first side of the bistable compliant body 310, and two protruding portions 313 and 314 (i.e. a third protruding portion 313 and a fourth protruding portion 314) provided on a second side of the bistable compliant body 310. The first and second sides are opposite sides of the bistable compliant body 310 which do not overlap. The bistable compliant body 310 is configured such that when the first and second protruding portions 311, 312 are moved towards each other, the bistable compliant body 310 is moved to its first stable equilibrium position. The bistable compliant body 310 is configured such that when the third and fourth protruding portions 313, 314 are moved towards each other, the bistable compliant body 310 is moved to its second stable equilibrium position. The bistable compliant body 310 is coupled to the movable part 3 such that when the bistable compliant body 310 is moved to its first stable equilibrium position, the movable part 3 is also (directly or indirectly) moved to its first stable equilibrium position, and when the bistable compliant body 310 is moved to its second stable equilibrium position, the movable part 3 is also (directly or indirectly) moved to its second stable equilibrium position. Similarly, when the bistable compliant body 310 is moved to its unstable equilibrium position, the movable part 3 (directly or indirectly) is also moved to its unstable equilibrium position. As shown in Fig.17, a first SMA element 321 (of the actuator arrangement) may be coupled at its ends to the first and second protruding portions 311, 312 so that it can drive the bistable compliant body 310 to its first stable equilibrium position by contracting. Similarly, a second SMA element 322 (of the actuator arrangement) may be coupled at its ends to the third and fourth protruding portions 313, 314 so that it can drive the bistable compliant body 310 to its second stable equilibrium position by contracting. Alternatively, as shown in Figs.18 and 19, a first SMA element 331 (of the actuator arrangement) may be coupled at one end to the support structure 4 and at the other end to the first protruding portion 311, a second SMA element 332 (of the actuator arrangement) may be coupled at one end to the support structure 4 and at the other end to the second protruding portion 312, a third SMA element 333 (of the actuator arrangement) may be coupled at one end to the support structure 4 and at the other end to the third protruding portion 313, and a fourth SMA element 334 (of the actuator arrangement) may be coupled at one end to the support structure 4 and at the other end to the fourth protruding portion 314. Contraction of both the first and second SMA elements 331, 332 would drive the bistable compliant body 310 to its first stable equilibrium position. Contraction of both the third and fourth SMA P564-GB01x 23 elements 333, 334 would drive the bistable compliant body 310 to its second stable equilibrium position. It is worth noting that, in the arrangement shown in Figs.18 and 19, the compliant body 310 may be driven to two further stable equilibrium positions between the first and stable equilibrium positions, so the compliant body 310 of Figs.18 and 19 could instead be referred to as a multi-stable compliant body 310. One of the two further stable equilibrium positions being between the unstable equilibrium position and the first stable equilibrium position, and the other of the two further stable equilibrium positions being between the unstable equilibrium position and the second stable equilibrium position. Fig.19 shows the compliant body 310 in one of these two further stable equilibrium positions. The term ‘bistable compliant body’ is used herein broadly to refer to any compliant body capable of having at least two stable equilibrium positions, including multi-stable compliant bodies. Use with variable aperture mechanism As shown in Figs.20 and 21, the actuator arrangement (in this example SMA elements 41, 42) may be configured to (in this example, indirectly) rotate the movable part 3 (in this example, via the intermediate part 40) relative to the support structure 4 around the primary axis P. The movable part 3 may form part of a variable aperture (e.g. an iris diaphragm) assembly 60 configured such that rotation of the movable part 3 relative to the support structure 4 around the primary axis P changes the (size of the) aperture of the variable aperture assembly 60. The primary axis P may correspond to the optical axis of the variable aperture assembly 60. Use with pop-out cameras The above-described actuator assemblies may be used in pop-out cameras (also known as retractable lens cameras or telescoping cameras) for example to move one or more lenses between two stable equilibrium positions. The first equilibrium position corresponding to a retracted/collapsed state of the pop-out camera, e.g. in which the one or more lenses are configured such that they are not capable of focusing an image on the image sensor 3 of the camera (i.e. a state in which the camera is non- operative). The second equilibrium position corresponding to an extended/popped-out state, e.g. in which the one or more lenses are configured such that they are capable of focusing an image on the image sensor 3 of the camera (i.e. a state in which the camera is operative). Where this is the case, the movable part 3 would be the part comprising the one or more lenses (or a lens assembly comprising the one or more lenses) configured to focus an image on the camera’s image P564-GB01x 24 sensor. Also, the primary axis (i.e. the axis in which the movable part 3 is moved along/around) would coincide with to the optical axis of the one or more lenses/the lens assembly. When the movable part 3 is in the first equilibrium position (i.e. in the retracted/collapsed state), the distance between the lens assembly/one or more lenses and the image sensor along the primary axis is less than when the movable part 3 is in the second equilibrium position (i.e. in the extended/popped- out state). A focus (e.g. AF) actuator configured to drive relative movement between the one or more lenses and the image sensor along the primary axis may be provided within the pop-out camera. B. Actuator assemblies with magnets for countering centring forces Figs.22 and 23 are schematic views of an actuator assembly comprising a movable part 3 which is movable relative to a support structure 4. A centring arrangement 81, 82 is configured to bias the movable part 3 towards an intermediate/central stable equilibrium position. The actuator assembly comprises an actuator arrangement (not shown) configured to drive the movable part 3 relative to the support structure 4 away from the intermediate position in directions perpendicular to the primary axis which, in this example only, is not the axis in which the movable part 3 is moved along/around, but e.g. the longitudinal axis of the actuator assembly. The biasing arrangement comprises two magnets 72, 72’ (i.e. a first magnet 72 and a second magnet 72’) fixed to the support structure 4 and two target bodies 71, 71’ (i.e. a first target body 71 and a second target body 71’) fixed to the movable part 3. The first magnet 72 is configured to attract the first target body 71, e.g. when the movable part 3 is moved away from the intermediate position in a first direction D. The second magnet 72’ is configured to attract the second target body 71’, e.g. when the movable part 3 is moved away from the intermediate position in a second opposite direction. In alternative embodiments, the magnets 72, 72’ may instead be fixed to the movable part 3 and the target bodies 71, 71’ may instead be fixed to the support structure 4. The forces of magnetic attraction between the magnets 72, 72’ and the target bodies 71, 71’ help counteract the centring force of the centring arrangement 81, 82 when the movable part 3 is being moved away from the intermediate position. For example, the further the movable part 3 is moved from P564-GB01x 25 intermediate position in direction D, as shown in Fig.23, the centring force on the movable part 3 increases but this is counteracted by the increase in magnetic force between the first target body 71 and the magnet 72 which results from the first target body 71 and the magnet 72 getting closer to each other. Thus, the biasing arrangement of Figs.22 and 23 assist the actuator arrangement move the movable part 3 when moving the movable part 3 away from the intermediate position. This helps, for example, increase the stroke of the actuator arrangement, i.e. increase the amount the movable part 3 can be moved by the actuator arrangement, and/or improve the efficiency of the actuator arrangement (e.g. by reducing the energy required to move the movable part 3 or reducing the energy required to hold the movable part 3 at a particular position away from the intermediate position). The magnets 72, 72’ and/or the target bodies 71, 71’ may be electromagnets. The movable part 3 may be movable relative to the support structure 4 across a range of movement in two orthogonal directions perpendicular to the primary axis. Fig.24 shows an example of an actuator arrangement 10 that may be used with the actuator assembly of Figs.22 and 23. This actuator arrangement 10 comprises (a total of) four SMA elements (e.g. SMA wires) 11, 12, 13, 14 connected between the movable part 3 and the support structure 4 in an arrangement wherein the SMA elements 11, 12, 13, 14 are capable of being selectively driven to move the movable part 3 relative to the support structure 4 to any position in said range of movement without applying any net torque to the movable element 3 in the plane of the two orthogonal directions around the primary axis. The movable part 3 may comprise an image sensor and/or a lens assembly with an optical axis O which may be parallel to the primary axis. The movable part 3 may comprise a display, an emitter, or a part thereof. The primary axis may be parallel to a general direction in which the display or emitter emits light. SMA element The above-described SMA actuator assemblies comprise at least one SMA element. The term ‘shape memory alloy (SMA) element’ may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the P564-GB01x 26 SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term ‘SMA element’ may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. Variations It will be appreciated that there may be many other variations of the above-described examples. The actuator arrangement may only be arranged to exert a force on the movable part in a first direction, for example, where the movable part can be manually moved in a second opposite direction. Where required, the SMA elements mentioned above may be provided with significant amounts of slack. The herein-mentioned actuator arrangements may, for example, be voice coil motor (VCM) actuator arrangements or piezoelectric actuator arrangements, instead of SMA actuator arrangements. Any suitable actuator arrangement, whether SMA or otherwise, may be used. For example, the SMA wires shown in any of Figures 1 to 11 may be replaced with a different actuator configured to apply a force in the same direction as those applied by the SMA. P564-GB01x 27 The herein-described actuator assemblies can be used in many applications such as: AF, zoom, haptics, optical image stabilisation (OIS), valves, augmented reality (AR), etc.