Actuator assembly

Abstract

An SMA actuator assembly comprises a support structure 141, a movable part 130, and a bearing arrangement arranged to guide movement of the movable part relative to the support structure parallel to a

Description

Field The present application relates to actuator assemblies with one or more actuating units, in particular actuator assemblies with one or more actuating unit each of which includes a shape memory alloy (SMA) element. Background It is known to use one or more actuating assemblies comprising SMA elements to effect a range of motions of a movable part relative to a support structure. For example, an actuator assembly may be used in a camera assembly for effecting motion of one more optical elements, such as a lens carriage and / or an image sensor. Typically, a movement range and direction of a movable part is defined by the extent and direction of contraction in the SMA elements. The actuating force is typically defined by the input force achievable by the SMA wires. WO 2022 / 084699 Al discloses an actuator assembly comprising at least one actuating unit (incorporating an SMA wire) that, on actuation, moves a movable part relative to the support structure. The actuating unit may be configured to amplify the movement range of the movable part, to amplify the actuating force acting on the movable part, or to re-direct the force applied by the SMA wire. A bearing arrangement may be configured to guide movement of the movable part such that the movable part is movable in one degree of freedom. It may be beneficial to have an actuating assembly with an SMA element that is not parallel to that degree of freedom, such as for space-saving reasons or to allow a longer SMA element. However, typically re-directing a force applied by an SMA element would require two or more intermediate parts, which may introduce mechanical backlash and / or assembly challenges. For example, one option is to use a wedge drive to re-direct a direction of the force. It is an object of the present disclosure to provide an actuator assembly that is capable of re-directing an input force so as to effect movement of a movable part in one degree of freedom. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: a support structure, wherein a primary axis is defined with reference to the support structure; a movable part that is movable relative to the support structure; a bearing arrangement arranged to guide movement of the movable part relative to the support structure parallel to the primary axis; and at least one actuating unit configured to apply an actuating force to a connection point of either the movable part or the support structure, wherein the actuating force is capable of moving the movable part relative to the support structure and has a major component that is parallel to the primary axis, the actuating unit comprising: a force-modifying unit; a coupling link connected between the force-modifying unit and the connection point; and an SMA element connected between the force-modifying unit and the support structure; wherein on actuation of the SMA element: the SMA element is configured to apply an input force to the force-modifying unit; the force-modifying unit is configured to modify the input force so as to give rise to the actuating force applied to the connection point; and the actuating unit is configured to move the movable part in a direction parallel to the primary axis. In this way, the actuator assembly may be capable of moving the movable part in one degree of freedom, and may do so with an actuating force that is different in direction and / or magnitude from the input force. This allows, for example, a larger change in actuation of the SMA element to result in a smaller movement of the movable part, increasing accuracy of the movement of the movable part. This is achieved with fewer intermediate parts than would typically be needed. Furthermore, the SMA element may be at an angle to the primary axis, allowing a longer SMA element to be used while retaining a compact actuator assembly in a direction along the primary axis. The actuator assembly may comprise at least two actuating units, wherein a first actuating unit configured to move the movable part in a direction parallel to the primary axis and a second actuating unit is configured to move the movable part in a second direction opposite to the first direction. In this way, the actuator assembly may be used to move the movable part in two opposite directions parallel to the primary axis. Or, actuating units may be connected to more than one side of the movable part. The force-modifying unit may comprise a body portion connected to the coupling link; and a forcemodifying portion connected between the body portion and the support structure. The body portion and the force-modifying portion may be integrally formed. Advantageously, this may make manufacturing simpler and cheaper, and may make the resulting forcemodifying unit more compact and resilient to use. The actuator assembly may comprise at least two actuating units each comprising a body portion and a force-modifying potion, wherein the body portion and force-modifying portion of each actuating unit are integrally formed with each other and with the body portion and force-modifying portion of at least one other actuating unit. Two actuating units may be connected at a single point. In this way, a single movable connection point may be moved in two opposite directions parallel to the primary axis. The force-modifying portion may comprise a flexure. The force-modifying portion may comprise a pivot. The SMA element may be in a plane parallel to the primary axis in the absence of actuation of the SMA element. In this way, the actuating force may be parallel to the primary axis. The SMA element may be at an acute, non-zero angle to a plane parallel to the primary axis in the absence of actuation of the SMA element. Advantageously, this may result in a minor component of the actuating force that is perpendicular to the primary axis. If, for example, there is friction between the movable part and another surface, the minor component may reduce the friction such that the major component of the actuating force acts against a lower friction. The support structure may comprise a first surface and the movable part is arranged to move relative to the support structure across the surface, wherein the actuator assembly further comprises a biasing arrangement arranged to bias the movable part into contact with the first surface so as to generate frictional forces therebetween for retaining the movable part in position on the first surface in the absence of power to the at least one actuating unit. Advantageously, this may save power. Once the movable part has been moved by the actuating unit to a particular position, movable part may be retained in position even when the actuating unit is unpowered. The biasing arrangement may comprises a magnetic biasing arrangement; or a resilient biasing arrangement connected between the movable part and the support structure. The bearing arrangement may comprise: one or more pin bearings configured to constrain movement of the movable part to be parallel to the primary axis; and / or one or more plain bearings configured to constrain movement of the movable part to be parallel to the primary axis; and / or one or more rolling elements configured to constrain movement of the movable part to be parallel to the primary axis. The coupling link may comprise a flexure. A first end of the force-modifying unit may be connected to the SMA element; a second end of the force-modifying unit may be connected to the coupling link; and the force-modifying unit may be connected to the support structure between the first end and the second end of the actuating unit. A first end of the force-modifying unit may be connected to the support structure; a second end of the force-modifying unit may be connected to the coupling link; and the force-modifying unit may be connected to the SMA element between the first end and the second end of the actuating unit. According to another aspect of the present invention, there is provided an apparatus comprising the actuator assembly of any preceding claim, wherein the movable part comprises one or both of: an image sensor; and a lens assembly. The actuating unit may be used to move the image sensor and / or the lens assembly to adjust a focus of a camera comprising the image sensor and the lens assembly. The support structure, or a component that is static relative to the support structure, may comprise one of the image sensor and the lens assembly. 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 view of an actuating unit of an actuator assembly according to an embodiment of the present disclosure, in a neutral position. Figure 2 is a schematic view of the actuating unit of Figure 1, in a first position. Figure 3 is a schematic view of the actuating unit of Figure 1, in a second position. Figure 4 is a schematic view of an actuating unit of an actuator assembly according to an embodiment of the present disclosure. Figure 5 is a schematic view of the actuating unit of Figure 4, in a first position. Figure 6 is a schematic view of the actuating unit of Figure 4, in a second position. Figure 7 is a schematic view of a first and second actuating unit of an actuator assembly according to an embodiment of the present disclosure, each of the first and second actuating units comprising a forcemodifying unit comprising a flexure. Figure 8 is a schematic view of the first and second actuating units of Figure 7, in a first position. Figure 9 is a schematic view of the first and second actuating units of Figure 7, in a second position. Figure 10 is a schematic view of a first and second actuating unit of an actuator assembly according to an embodiment of the present disclosure each of the first and second actuating units comprising a force-modifying unit comprising a pivot. Figure 11 is a schematic view of the first and second actuating units of Figure 10, in a first position. Figure 12 is a schematic view of the first and second actuating units of Figure 10, in a second position. Figure 13 is a schematic view of a first and second actuating unit of an actuator assembly according to an embodiment of the present disclosure, each of the first and second actuating units comprising a force-modifying unit comprising a flexure. Figure 14 is a schematic view of the first and second actuating units of Figure 13, in a first position. Figure 15 is a schematic view of the first and second actuating units of Figure 13, in a second position. Figure 16 is a schematic view of an actuator assembly comprising the first and second actuating units of Figure 13. Figure 17 is a side view of the actuator assembly of Figure 16. Figure 18 shows a schematic of a magnetic biasing arrangement of an actuator assembly according to an embodiment of the present disclosure. Detailed description An actuator assembly is provided, comprising a support structure and a movable part. A primary axis is defined with reference to the support structure. The movable part is movable relative to the support structure. The actuator assembly further comprises a bearing arrangement that is arranged to guide movement of the movable part relative to the support structure, parallel to the primary axis. The bearing arrangement may, in certain embodiments, further be arranged to constrain movement of the movable part relative to the support axis in directions that are not parallel to the primary axis. The bearing arrangement may comprise a first bearing surface and a second bearing surface. In certain embodiments, the first and second bearing surfaces may be surfaces of the support structure and the movable part, respectively. The actuator assembly further comprises at least one actuating unit configured to apply an actuating force to a connection point of either the movable part or the support structure. The actuating force is capable of moving the movable part relative to the support structure, and has a major component that is parallel to the primary axis. The actuating unit comprises a force-modifying unit. The actuating unit further comprises a coupling link connected between the force-modifying unit and the connection point, and an SMA element connected between the force-modifying unit and the support structure. On actuation of the SMA element of the actuating unit, the SMA element is configured to apply an input force to the force-modifying unit. The force-modifying unit is configured to modify the input force so as to give rise to the actuating force applied to the connection point. The actuating unit is configured to move the movable part in a direction parallel to the primary axis. The input force may, in certain embodiments, be at an acute, non-zero angle to the primary axis. The movable part may comprise the connection point, such that on actuation of the SMA element of the actuating unit, the actuating unit is configured to move the connection point in a direction parallel to the primary axis. The support structure may comprise the connection point, such that on actuation of the SMA element of the actuating unit, the force-modifying unit is configured to modify the input force so as to give rise to the actuating force applied to the connection point of the support structure. The actuating unit may be connected to the movable part via a secondary connection point, such that applying the actuating force to the connection point of the support structure effects movement of the secondary connection point and, therefore, the movable part in a direction parallel to the primary axis. In certain embodiments, the force-modifying unit comprises a body portion and a force-modifying portion. The body portion is connected to the coupling link. The force-modifying portion is connected between the body portion and the support structure. The body portion may be further connected to the SMA element. The body portion may be rigid. The force-modifying portion may be flexible, or may be rotatable relative to the body portion and / or the support structure, or may be some other component. In certain embodiments, the body portion and the force-modifying portion are integrally formed. Zero Hold Power In certain embodiments, the actuator assembly may be configured to retain the movable part in position in the absence of power to the actuating unit. This may be known as "zero hold power" (ZHP). In use, the movable part may have a neutral position. A change to the actuation of the actuating unit may move the movable part away from its neutral position to a first position. A change to the actuation of the actuating unit may be achieved by altering power provided to the actuating unit. The first position may depend on the power provided to the actuating unit. The actuating unit may relax towards the neutral position in the absence of power to the actuating unit, for example as a result of power, a biasing force, tension, or some other force. Therefore, in the absence of a ZHP arrangement, to retain the movable part in the first position power would need to keep being provided to the actuating unit. However, if a ZHP arrangement is present, the movable part may be retained in the first position even when power is no longer provided to the actuating unit. In an example, the neutral position may be a position of the movable part that would be achieved in the absence of any power to the one or more actuating units, and in the absence of any ZHP arrangement. In use, the movable be moved to a particular position by actuation of the actuating unit. To actuate the actuating unit, power to the actuating unit may be applied and / or changed. Once the movable part reaches that particular position, power may stop being applied to the actuating unit. In the absence of any ZHP arrangement, the movable part would not be retained in the first position, and may return to the neutral position. For example, the movable part and / or actuating unit may relax back to the neutral position, or may return to the neutral position due to a biasing arrangement. In certain embodiments, the support structure may comprise a first surface. The movable part may be arranged to move relative to the support structure across the surface. The actuator assembly may further comprise a biasing arrangement arranged to bias the movable part into contact with the first surface so as to generate frictional forces therebetween for retaining the movable part in position on the first surface in the absence of power to the at least one actuating unit. The biasing arrangement may be configured to apply a force on the movable part such that the movable part bears against the support structure in a direction perpendicular to the primary axis. The biasing arrangement may comprise a magnetic biasing arrangement, a resilient biasing arrangement connected between the movable part and the support structure, or other biasing arrangement. In certain embodiments, the frictional forces may be constant, irrespective of whether (or the extent to which) an actuating unit is actuated. An actuating force of the actuating unit works against the frictional force to move the movable part. In other embodiments, actuation of an actuating unit may reduce the frictional forces between the first surface and the movable part. The actuating force of the actuating unit therefore works against a lower friction to move the movable part parallel to the primary axis. Bearing arrangement The bearing arrangement is configured to guide movement of the movable part parallel to the primary axis. The bearing arrangement may be further configured to constrain movement of the movable part to be parallel to the primary axis. In certain embodiments, the bearing arrangement may comprise one or more pin bearings configured to The bearing arrangement may comprise a bearing surface of each pin bearing and, for each pin bearing, a corresponding bearing surface of the movable part. The corresponding bearing surface of the movable part may be configured to move over the bearing surface of each pin bearing in a direction parallel to the primary axis. The support structure may comprise the one or more pin bearings. In certain embodiments, the bearing arrangement may comprise one or more plain bearings. The bearing arrangement may comprise a bearing surface of each plain bearing and the movable part may comprise a corresponding bearing surface of each plain bearing, wherein the corresponding bearing surface of the movable part is configured to move over the bearing surface of the plain bearing. The support structure may comprise a bearing surface of each plain bearing. In certain embodiments, the bearing arrangement may comprise one or more rolling elements configured to guide movement of the movable part to be parallel to the primary axis. The rolling elements may comprise ball bearings, roller bearings or other rolling elements. Example of an actuating unit Figures 1 to 3 show an example of an actuating unit 100 of an actuator assembly according to an embodiment of the present disclosure. The actuating unit 100 comprises a force-modifying unit 110. In the example shown, the force-modifying unit 110 comprises a body portion 111 and a force-modifying portion 112. The force-modifying portion 112 is connected between the body portion 111 and the support structure via a support connection point 113. The actuating unit 100 further comprises a coupling link 120 connected between the force-modifying unit 110 and a movable connection point 130. In this example, the coupling link 120 is connected to the body portion 111 of the force-modifying unit 110. The movable connection point 130 may be connected to the movable part or may be integral to the movable part, such that movement of the movable connection point 130 effects corresponding movement of the movable part. Movement of the movable connection point 130 may be parallel to the primary axis, as indicated by arrows 131 and 132 The actuating unit further comprises an SMA element 140 connected between the force-modifying unit 110 and the support structure. In this example, a first end of the SMA element 140 is connected to the support structure via a support SMA connection 141, and a second end of the SMA element 140 is connected to the body portion 111 of the force-modifying unit 110 via a movable SMA connection 142. The support SMA connection 141 may be configured to connect the first end of the SMA element 140 to the support structure such that on actuation of the SMA element 140, there is no translation of the first end of the SMA element 140 relative to the support structure. On actuation of the SMA element 140 (or on a change to the actuation of the SMA element 140), the SMA element 140 may expand or contract as indicated by arrow 143. On actuation of the SMA element 140, the movable SMA connection 142 may be moved relative to the support structure. The first end of the SMA element 140 may be able to rotate relative to the support structure, either by rotation of the first end of the SMA element 140 relative to the support SMA connection 141, or by rotation of the support SMA connection 141 relative to the support structure. The support SMA connection 141 and / or the movable SMA connection 142 may, for example, comprise a crimp. In Figures 1 to 3 and in subsequent figures, connections that are movable relative to the support structure (such as the movable connection point 130 and the movable SMA connection 142) are shown as shapes with a dotted pattern. Connections that are static relative to the support structure or that are not translatable relative to the support structure (such as the support connection point 113 and the support SMA connection 141) are shown as shapes with a diagonal stripe pattern. The coupling link 120 and the force-modifying portion 112 of the force-modifying unit 110 may each comprise a flexure. Each flexure may be flexible in the plane of the force-modifying unit 110. The flexures may be flexible out of the plane of the force-modifying unit 110, or may be rigid (or less flexible than in the plane of the force-modifying unit) out of the plane of the force-modifying unit 110. Alternatively, one or both of the coupling link 120 and the force-modifying portion 112 of the forcemodifying unit 110 may comprise a different component. For example, one or both of the coupling link 120 and the force-modifying portion 112 of the force-modifying unit 110 may comprise a pivot. The coupling link 120 may comprise a rigid linkage connected rotatably to the body portion 111 of the forcemodifying unit 110. The rigid linkage of the coupling link 120 may be connected rotatably to the movable connection point 130, or may be fixed to the movable connection point 130. The forcemodifying portion 112 of the force-modifying unit 110 may comprise a rigid linkage connected rotatably to the support connection point 113 and connected rotatably to the body potion 111 of the forcemodifying unit 110. The body portion 111 of the force-modifying unit 110 may be rigid. Operation of a single actuating unit Figure 1 illustrates the actuating unit 100 in a neutral position. In certain examples, a neutral position may be achieved when the SMA element is at its unpowered length. Contracting the SMA element by providing power or increasing power to the actuating unit 100 may result in movement of the movable part in a first direction parallel to the primary axis. The actuator assembly may further comprise a biasing element configured to apply a force on the movable part in a second direction opposite to the first direction, such that expanding the SMA element by reducing or removing power to the actuating unit may result in movement of the movable part in the second direction parallel to the secondary axis. The movable connection point 130 may be movable only in the first direction away from the neutral position. Movement of the movable connection point 130 in the second direction may be only towards the neutral position. It is noted that, as will be described later, in the absence of power to the actuating unit 100, the actuating unit 100 may not be in a neutral position. This is because the actuator assembly may be configured to retain the movable part in position in the absence of power to the actuating unit. This may be known as "zero hold power" (ZHP). For the purposes of the description of Figures 1 to 3, the neutral position is taken to be a position that the actuating unit 100 would be in, in the absence of power to the actuating unit and when the movable part is free to move relative to the support structure (i.e. when there is no ZHP to retain the movable part in position). The neutral position may, of course, also be achieved when there is ZHP. In other examples, a neutral position may be achieved when there is power provided to the actuating unit 100, such that the SMA element 140 is partially contracted. Increasing power to the actuating unit 100 (resulting in contraction of the SMA element 140) may result in movement of the movable part in a first direction parallel to the primary axis. Decreasing power to the actuating unit (resulting in an increase in length of the SMA element 140) may result in movement of the movable part in the second direction parallel to the primary axis. The movable connection point 130 may be movable away from the neutral position in the first direction or in the second direction. The movable connection point 130 may be movable towards the neutral position in the first direction or in the second direction. The actuator assembly may further comprise a biasing element configured to apply a force on the movable part in the second direction. Figure 2 shows the actuating unit 100 of Figure 1 in a first position, with the SMA element contracted relative to the neutral position shown by Figure 1. The actuating unit 100 in the first position is overlayed over an indication of the actuating unit 100 in the neutral position, where like reference numerals with Figure 1 indicate elements that are in the neutral position. This is simply to show the changes between the neutral position and the first position. For the actuating unit in the neutral position in Figure 2, the movable connection point 130, the body portion 111 of the force-modifying unit, the force-modifying portion 112 of the force-modifying unit and the movable SMA connection 142 are indicated by white (unfilled) shapes and the SMA element 140 is indicated by a dotted line. For the actuating unit in the first position in Figure 2: the movable connection point 230 and the movable SMA connection 242 are indicated by shapes with a dotted fill pattern; the body portion 211 of the forcemodifying unit is indicated by a black (solid fill) shape; the SMA element 240, the coupling link 220 and the force-modifying portion 212 of the force-modifying unit are indicated by solid lines; and the support SMA connection 241 and the support connection point 213 are shown by shapes with a diagonal stripe pattern. The support SMA connection 241 and the support connection point 213 are in the same location for the actuating unit in the first position as in the neutral position. The SMA element 240 in the first position is contracted relative to the SMA 140 in the neutral position as indicated by arrow 243, pulling the movable SMA connection 242 towards the support SMA connection 241. The body portion 211 of the force-modifying unit 210 moves with a rotational component, bending the force-modifying portion 212 of the force-modifying unit 210. As shown in Figure 2, the rotational component of the movement of the force-modifying portion 212 is clockwise. The coupling link 221 bends and is pulled downwards, such that the movable connection point 230 moves down relative to the movable connection point 130 in the neutral position, as indicated by arrow 232. The primary axis in this example is vertical, so the movable connection point 230 is moved in a first direction (down) parallel to the vertical primary axis. Figure 3 shows the actuating unit 100 of Figure 1 in a second position, with the SMA element expanded relative to the neutral position shown by Figure 1. Similarly to Figure 2, the actuating unit 100 is shown in the neutral position behind the actuating unit 100 in the second position, where like reference numerals with Figure 1 indicate elements that are in the neutral position. For the actuating unit in the second position: the movable connection point 330 and the movable SMA connection 342 are indicated by shapes with a dotted fill pattern; the body portion 311 of the force-modifying unit is indicated by a black (solid fill) shape; the coupling link 320 and the force-modifying portion 312 of the force-modifying unit are indicated by solid lines; and the support SMA connection 341 and the support connection point 313 are shown by shapes with a diagonal stripe pattern. The SMA element 340 is shown by a solid black line except where it overlaps with the body portion 311 of the force-modifying unit 310, where the SMA element 340 is shown as a dashed line. The support SMA connection 341 and the support connection point 313 are in the same location for the actuating unit in the first position as in the neutral position. The SMA element 340 in the second position is expanded relative to the SMA 140 in the neutral position as indicated by arrow 343, such that the movable SMA connection 342 is moved further from the support SMA connection 341. The body portion 311 of the force-modifying unit 310 moves with a rotational component, bending the forcemodifying portion 312 of the force-modifying unit 310. As shown in Figure 3, the rotational component of the movement of the force-modifying portion 312 is anti-clockwise. The coupling link 321 bends and is pushed upwards, such that the movable connection point 330 moves up relative to the movable connection point 130 in the neutral position, as indicated by arrow 331. The primary axis in this example is vertical, so the movable connection point 330 is moved in a second direction (up) parallel to the vertical primary axis. The actuating unit 100 illustrated in Figures 1 to 3 may be in a different orientation. For example, assuming that the primary axis remains vertical, the actuating unit may be oriented such that it is reflected about a vertical axis relative to the orientation shown in Figures 1 to 3, such that contracting the SMA element results in a downwards movement of the movable connection point. In other examples, again assuming that the primary axis remains vertical, the actuating unit may be oriented such that it is either reflected about a horizontal axis or rotated by 180° relative to the orientation shown in Figures 1 to 3, each of which result in in an upwards movement of the movable connection point on contraction of the SMA element. An example is illustrated in Figures 4 to 6, where the actuating unit 400 has similar components to the actuating unit 100 shown in Figures 1 to 3, but is rotated by 180° relative to the actuating unit 100. Similarly to actuating unit 100, the actuating unit 400 comprises a force-modifying unit 410 comprising a body portion 411 and a force-modifying portion 412. The force-modifying portion 412 is connected between the body portion 411 and the support structure via a support connection point 413. The actuating unit 400 further comprises a coupling link 420 connected between the force-modifying unit 410 and a movable connection point 430. In this example, the coupling link 420 is connected to the body portion 411 of the force-modifying unit 410. Movement of the movable connection point 430 may be parallel to the primary axis, as indicated by arrows 431 and 432. The actuating unit further comprises an SMA element 440 connected between the force-modifying unit 410 and the support structure. In this example, a first end of the SMA element 440 is connected to the support structure via a support SMA connection 441, and a second end of the SMA element 440 is connected to the body portion 411 of the force-modifying unit 410 via a movable SMA connection 442. Figure 4 shows the actuating unit 400 in a neutral position. Figure 5 shows the actuating unit 400 in a first position, overlayed with the actuating unit 400 in the neutral position. Components of the actuating unit 400 in the neutral position are shown with like reference numerals to Figure 4. For the actuating unit 400 in the first position in Figure 5: the movable connection point 530 and the movable SMA connection 542 are indicated by shapes with a dotted fill pattern; the body portion 511 of the forcemodifying unit is indicated by a black (solid fill) shape; the SMA element 540, the coupling link 520 and the force-modifying portion 512 of the force-modifying unit are indicated by solid lines; and the support SMA connection 541 and the support connection point 513 are shown by shapes with a diagonal stripe pattern. The support SMA connection 541 and the support connection point 513 are in the same location for the actuating unit in the first position as in the neutral position. The SMA element 540 in the first position is expanded relative to the SMA 440 in the neutral position as indicated by arrow 543, such that the movable SMA connection 542 moves away from the support SMA connection 541. The body portion 511 of the force-modifying unit 510 moves with a rotational component, bending the force-modifying portion 512 of the force-modifying unit 510. As shown in Figure 5, the rotational component of the movement of the force-modifying portion 512 is anti-clockwise. The coupling link 521 bends and is pulled downwards, such that the movable connection point 530 moves down relative to the movable connection point 430 in the neutral position, as indicated by arrow 531. The primary axis in this example is vertical, so the movable connection point 530 is moved in a first direction (down) parallel to the vertical primary axis. Figure 6 shows the actuating unit 400 in a second position, overlayed with the actuating unit 400 in the neutral position. Components of the actuating unit 400 in the neutral position are shown with like reference numerals to Figure 4. For the actuating unit 400 in the second position in Figure 6: the movable connection point 630 and the movable SMA connection 642 are indicated by shapes with a dotted fill pattern; the body portion 611 of the force-modifying unit is indicated by a black (solid fill) shape; the SMA element 640, the coupling link 620 and the force-modifying portion 612 of the forcemodifying unit are indicated by solid lines; and the support SMA connection 641 and the support connection point 613 are shown by shapes with a diagonal stripe pattern. The support SMA connection 641 and the support connection point 613 are in the same location for the actuating unit in the second position as in the neutral position. The SMA element 640 in the first position is contracted relative to the SMA 440 in the neutral position as indicated by arrow 643, pulling the movable SMA connection 642 moves towards the support SMA connection 641. The body portion 611 of the force-modifying unit 610 moves with a rotational component, bending the force-modifying portion 612 of the force-modifying unit 610. As shown in Figure 6, the rotational component of the movement of the force-modifying portion 612 is clockwise. The coupling link 621 bends and moves upwards, such that the movable connection point 630 moves up relative to the movable connection point 430 in the neutral position, as indicated by arrow 632. The primary axis in this example is vertical, so the movable connection point 630 is moved in a second direction (up) parallel to the vertical primary axis. More than one actuating unit An actuator assembly according to certain embodiments of the present disclosure may comprise at least two actuating units. A first actuating unit may be configured to move the movable part in a direction parallel to the primary axis. A second actuating unit may be configured to move the movable part in a second direction opposite to the first direction. Each actuating unit may be an actuating unit as described above. Each actuating unit comprises a forcemodifying unit, a coupling link connected between the force-modifying unit and a connection point, and an SMA element connected between the force-modifying unit and the support structure. In certain embodiments, the first and second actuating units may each apply the actuating force to the same connection point. The first actuating unit and the second actuating unit may be connected at a single point. In certain embodiments, each of the first and second actuating units may comprise a body portion and a forcemodifying potion. The body portion and force-modifying portion of the first actuating unit may be integrally formed with each other and with the body portion and force-modifying portion of the second actuating unit. An example of a first and second actuating unit wherein each force-modifying unit comprises a flexure With reference to Figures 7 to 9, an example of a first actuating unit and a second actuating unit of an actuator assembly according to an embodiment of the present disclosure is illustrated. The first actuating unit is similar to actuating unit 100 illustrated in Figures 1 to 3. The second actuating unit is similar to actuating unit 400 illustrated in Figures 4 to 6. The first and second actuating units are both connected to the same movable connection point. The first actuating unit and the second actuating unit comprise a first force-modifying unit 710 and a second force-modifying unit 750, respectively. The first force-modifying unit 710 comprises a first body portion 711 and a first force-modifying portion 712 comprising a flexure. The first force-modifying portion 712 is connected between the first body portion 711 and the support structure via a support connection point 713. The second force-modifying unit 750 comprises a second body portion 751 and a second force-modifying portion 752 comprising a flexure. The second force-modifying portion 752 is connected between the second body portion 751 and the support structure via a second support connection point 753. The first actuating unit further comprises a first coupling link 720 comprising a flexure connected between the first force-modifying unit 710 and a movable connection point 730. The second actuating unit further comprises a second coupling link 760 comprising a flexure connected between the second force-modifying unit 750 and the movable connection point 730. In this example, the first coupling link 720 is connected to the first body portion 711 of the first force-modifying unit 710 and the second coupling link 760 is connected to the second body portion 751 of the second force-modifying unit 740. The first coupling link 720 and the second coupling link 760 may be connected to opposite sides of the movable connection point 730, or may be connected to the movable connection point 730 such that the first coupling link 720 extends from a first side of the movable connection point 730 and the second coupling link 760 extends from a second side of the movable connection point 760 opposite to the first side The movable connection point 730 may be connected to the movable part or may be integral to the movable part, such that movement of the movable connection point 730 effects corresponding movement of the movable part. Movement of the movable connection point 730 may be parallel to the primary axis, as indicated by arrows 731 and 732 The first actuating unit further comprises a first SMA element 740 connected between the first forcemodifying unit 710 and the support structure. In this example, a first end of the first SMA element 740 is connected to the support structure via a first support SMA connection 741, and a second end of the first SMA element 740 is connected to the first body portion 711 of the first force-modifying unit 710 via a first movable SMA connection 742. The second actuating unit further comprises a second SMA element 770 connected between the second force-modifying unit 740 and the support structure. In this example, a first end of the second SMA element 770 is connected to the support structure via a second support SMA connection 771, and a second end of the second SMA element 770 is connected to the second body portion 751 of the second force-modifying unit 750 via a second movable SMA connection 772. Figure 7 shows the movable connection point 730 and the first and second actuating units in a neutral position. Both the first SMA element 740 and the second SMA element 770 may be partially contracted, such that the first SMA element 740 and the second SMA element 770 are each at a length between their maximum length (i.e. their length at zero power) and their most contracted length. Figure 8 shows the first and second actuating units of Figure 7 in a first position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 7. In Figure 8, the first SMA element 840 has been extended as indicated by arrow 843, such that the first movable SMA connection 842 moves further from the first support SMA connection 841. The first body portion 811 of the first force-modifying unit 810 moves with a rotational component. In the view shown in Figure 8, the rotational component is anti-clockwise. The first force-modifying portion 812 of the first force-modifying unit 810 (connected between the first support connection point 813 and the first body portion 811) bends. The first coupling link 820 bends and moves down, such that a downwards actuating force is applied to the movable connection point 830. The second SMA element 870 has been contracted as indicated by arrow 873, such that the second movable SMA connection 873 moves closer to the second support SMA connection 871. The second body portion 851 of the second force-modifying unit 850 moves with a rotational component. In the view shown in Figure 8, the rotational component is clockwise. The second force-modifying portion 852 of the second force-modifying unit 850 (connected between the second support connection point 853 and the second body portion 851) bends. The second coupling link 860 bends and moves down, such that a downwards actuating force is applied to the movable connection point 830. The movable connection point 830 moves downwards as indicated by arrow 832. Figure 9 shows the first and second actuating units of Figure 7 in a second position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 7. In Figure 9 the first SMA element 940 has been contracted as indicated by arrow 943, such that the first movable SMA connection 942 moves closer to the first support SMA connection 941. The first body portion 911 of the first force-modifying unit 910 moves with a rotational component. In the view shown in Figure 9, the rotational component is clockwise. The first force-modifying portion 912 of the first force-modifying unit 910 (connected between the first support connection point 913 and the first body portion 911) bends. The first coupling link 920 bends and moves up, such that a upwards actuating force is applied to the movable connection point 930. The second SMA element 970 has been extended as indicated by arrow 973, such that the second movable SMA connection 973 moves further from the second support SMA connection 971. The second body portion 951 of the second force-modifying unit 950 moves with a rotational component. In the view shown in Figure 9, the rotational component is anti-clockwise. The second force-modifying portion 952 of the second force-modifying unit 950 (connected between the second support connection point 953 and the second body portion 951) bends. The second coupling link 960 bends and moves up, such that an upwards actuating force is applied to the movable connection point 930. The movable connection point 930 moves upwards as indicated by arrow 931. An example of a first and second actuating unit wherein each force-modifying unit comprises a pivot With reference to Figures 10 to 12, an example of a first actuating unit and a second actuating unit of an actuator assembly according to an embodiment of the present disclosure is illustrated. The first and second actuating units are similar to those illustrated in Figures 7 to 9 except that the force-modifying units each comprise a pivot instead of a flexure and the coupling links each comprise a pivot instead of a flexure. The first and second actuating units are both connected to the same movable connection point. The first actuating unit and the second actuating unit comprise a first force-modifying unit 1010 and a second force-modifying unit 1050, respectively. The first force-modifying unit 1010 comprises a first body portion 111 and a first force-modifying portion 1012. The first force-modifying portion 1012 is connected between the first body portion 1011 and the support structure via a support connection point 1013. The first force-modifying portion 1012 comprises a rigid linkage, wherein a first end of the linkage is rotatably connected to the first body portion 111 and a second end of the linkage is rotatably connected to the first support connection point 1013. The second force-modifying unit 1050 comprises a second body portion 1051 and a second force-modifying portion 1052. The second force-modifying portion 1052 is connected between the second body portion 1051 and the support structure via a second support connection point 1053. The second force-modifying portion 1052 comprises a rigid linkage, wherein a first end of the linkage is rotatably connected to the second body portion 1051 and a second end of the linkage is rotatably connected to the second support connection point 1053. The first actuating unit further comprises a first coupling link 1020 connected between the first forcemodifying unit 1010 and a movable connection point 1030. The first coupling link 1020 may comprise a rigid linkage, wherein the linkage is rotatably connected to the first body portion 1011 of the first forcemodifying unit 1010. The linkage of the first coupling link 1020 may be fixed to the movable connection point 1030 or may be rotatably connected to the movable connection point. The second actuating unit further comprises a second coupling link 1060 connected between the second force-modifying unit 1050 and the movable connection point 1030. The second coupling link 1060 may comprise a rigid linkage, wherein the linkage is rotatably connected to the second body portion 1051 of the second forcemodifying unit 1050. The linkage of the second coupling link 1060 may be fixed to the movable connection point 1030 or may be rotatably connected to the movable connection point. The first coupling link 1020 and the second coupling link 1060 may be connected to opposite sides of the movable connection point 1030, or may be connected to the movable connection point 1030 such that the first coupling link 1020 extends from a first side of the movable connection point 1030 and the second coupling link 1060 extends from a second side of the movable connection point 1060 opposite to the first side. The movable connection point 1030 may be connected to the movable part or may be integral to the movable part, such that movement of the movable connection point 1030 effects corresponding movement of the movable part. Movement of the movable connection point 1030 may be parallel to the primary axis, as indicated by arrows 1031 and 1032 Similarly to the example illustrated in Figures 7 to 9, the first actuating unit further comprises a first SMA element 1040 connected between the first force-modifying unit 1010 and the support structure. In this example, a first end of the first SMA element 1040 is connected to the support structure via a first support SMA connection 1041, and a second end of the first SMA element 1040 is connected to the first body portion 1011 of the first force-modifying unit 1010 via a first movable SMA connection 1042. The second actuating unit further comprises a second SMA element 1070 connected between the second force-modifying unit 1040 and the support structure. In this example, a first end of the second SMA element 1070 is connected to the support structure via a second support SMA connection 1071, and a second end of the second SMA element 1070 is connected to the second body portion 1051 of the second force-modifying unit 1050 via a second movable SMA connection 1072. Figure 10 shows the movable connection point 1030 and the first and second actuating units in a neutral position. Both the first SMA element 1040 and the second SMA element 1070 may be partially contracted, such that the first SMA element 1040 and the second SMA element 1070 are each at a length between their maximum length (i.e. their length at zero power) and their most contracted length. Figure 11 shows the first and second actuating units of Figure 10 in a first position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 10. In Figure 11, the first SMA element 1140 has been extended as indicated by arrow 1143, such that the first movable SMA connection 1142 moves further from the first support SMA connection 1141. The first body portion 1111 of the first force-modifying unit 1110 moves with a rotational component. In the view shown in Figure 11, the rotational component is anti-clockwise. The first force-modifying portion 1112 of the first force-modifying unit 1110 rotates relative to the first support connection point 1113 and relative to the first body portion 1111. The first coupling link 1120 rotates relative to the first body portion 1111 and moves down, such that a downwards actuating force is applied to the movable connection point 1130. The second SMA element 1170 has been contracted as indicated by arrow 1173, such that the second movable SMA connection 1173 moves closer to the second support SMA connection 1171. The second body portion 1151 of the second force-modifying unit 1150 moves with a rotational component. In the view shown in Figure 11, the rotational component is clockwise. The second force-modifying portion 1152 of the second force-modifying unit 1150 rotates relative to the second support connection point 1153 and relative to the second body portion 1151. The second coupling link 1160 rotates relative to the second body portion 1151 and moves down, such that a downwards actuating force is applied to the movable connection point 1130. The movable connection point 1130 moves downwards as indicated by arrow 1132. Figure 12 shows the first and second actuating units of Figure 10 in a second position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 10. In Figure 12 the first SMA element 1240 has been contracted as indicated by arrow 1243, such that the first movable SMA connection 1242 moves closer to the first support SMA connection 1241. The first body portion 1211 of the first force-modifying unit 1210 moves with a rotational component. In the view shown in Figure 12, the rotational component is clockwise. The first force-modifying portion 1212 of the first force-modifying unit 1210 rotates relative to the first support connection point 1213 and the first body portion 1211. The first coupling link 1220 rotates relative to the first body portion 1211 and moves up, such that a upwards actuating force is applied to the movable connection point 1230. The second SMA element 1270 has been extended as indicated by arrow 1273, such that the second movable SMA connection 1273 moves further from the second support SMA connection 1271. The second body portion 1251 of the second force-modifying unit 1250 moves with a rotational component. In the view shown in Figure 12, the rotational component is anti-clockwise. The second force-modifying portion 1252 of the second force-modifying unit 1250 rotates relative to the second support connection point 1253 and the second body portion 1251. The second coupling link 1260 rotates relative to the second body portion 1251 and moves up, such that an upwards actuating force is applied to the movable connection point 1230. The movable connection point 1230 moves upwards as indicated by arrow 1231. The above examples either have both the force-modifying portion and the coupling link of each actuating unit having a flexure or have both the force-modifying portion and the coupling link of each actuating unit having a pivot. In other embodiments, one of the force-modifying portion and the coupling link of an actuating unit may be have a flexure and the other may have a pivot. Lever arrangements of the actuating unit In the foregoing, for each actuating unit a first end of the body portion of the force-modifying unit is connected to the SMA element, and a second end of the body portion of the force-modifying unit is connected to the coupling link. The body portion of the force-modifying unit is connected to the support structure between the first and second ends of the body portion, via the force-modifying portion of the force-modifying unit. Other lever configurations are also possible. For example, a first end of the force-modifying unit may be connected to the support structure and a second end of the force-modifying unit may be connected to the coupling link. The force-modifying unit may be connected to the SMA element between the first end and the second end of the actuating unit. An example is illustrated in Figures 13 to 15. With reference to Figures 13 to 15, an example of a first actuating unit and a second actuating unit of an actuator assembly according to an embodiment of the present disclosure is illustrated. The first and second actuating units are both connected to the same movable connection point. The first actuating unit and the second actuating unit comprise a first force-modifying unit 1310 and a second force-modifying unit 1350, respectively. The first force-modifying unit 1310 comprises a first body portion 1311 and a first force-modifying portion 1312 comprising a flexure. The first forcemodifying portion 1312 is connected between a first end of the first body portion 1311 and the support structure via a support connection point 1313. The second force-modifying unit 1350 comprises a second body portion 1351 and a second force-modifying portion 1352 comprising a flexure. The second force-modifying portion 1352 is connected between the a first end of the second body portion 1351 and the support structure via a second support connection point 1353. The first actuating unit further comprises a first coupling link 1320 comprising a flexure connected between the first force-modifying unit 1310 and a movable connection point 1330. The first coupling link 1320 is connected to a second end of the first body portion 1311. The second actuating unit further comprises a second coupling link 1360 comprising a flexure connected between the second forcemodifying unit 1350 and the movable connection point 1330. The second coupling link 1360 is connected to a second end of the second body portion 1351. The first coupling link 1320 and the second coupling link 1360 may be connected to opposite sides of the movable connection point 1330, or may be connected to the movable connection point 1330 such that the first coupling link 1320 extends from a first side of the movable connection point 1330 and the second coupling link 1360 extends from a second side of the movable connection point 1360 opposite to the first side The movable connection point 1330 may be connected to the movable part or may be integral to the movable part, such that movement of the movable connection point 1330 effects corresponding movement of the movable part. Movement of the movable connection point 1330 may be parallel to the primary axis, as indicated by arrow 1331. The first actuating unit further comprises a first SMA element 1340 connected between the first forcemodifying unit 1310 and the support structure. In this example, a first end of the first SMA element 1340 is connected to the support structure via a first support SMA connection 1341, and a second end of the first SMA element 1340 is connected to the first end of the first body portion 1311 of the first force-modifying unit 1310 via a first movable SMA connection 1342. The second actuating unit further comprises a second SMA element 1370 connected between the second force-modifying unit 1340 and the support structure. In this example, a first end of the second SMA element 1370 is connected to the support structure via a second support SMA connection 1371, and a second end of the second SMA element 1370 is connected to the first end of the second body portion 1351 of the second forcemodifying unit 750 via a second movable SMA connection 1372. Figure 13 shows the movable connection point 1330 and the first and second actuating units in a neutral position. Both the first SMA element 1340 and the second SMA element 1370 may be partially contracted, such that the first SMA element 1340 and the second SMA element 1370 are each at a length between their maximum length (i.e. their length at zero power) and their most contracted length. Figure 14 shows the first and second actuating units of Figure 13 in a first position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 13. In Figure 14, the first SMA element 1440 has been extended as indicated by arrow 1443, such that the first movable SMA connection 1442 moves further from the first support SMA connection 1441. The first is connected to the first end of the body portion 1411 of the first force-modifying unit 1410, so the body portion 1411 of the first force-modifying unit 1410moves with a rotational component. In the view shown in Figure 14, the rotational component is anti-clockwise. The first force-modifying portion 1412 of the first force-modifying unit 1410 (connected between the first support connection point 1413 and the first end of the first body portion 1411) bends. The first coupling link 1420 bends and moves down, such that a downwards actuating force is applied to the movable connection point 1430. The second SMA element 1470 has been contracted as indicated by arrow 1473, such that the second movable SMA connection 1473 moves closer to the second support SMA connection 1471. The second movable SMA connection 1473 is connected to the second body portion 1451 of the second force-modifying unit 1450, so the second body portion 1451 of the second force-modifying unit 1450 moves with a rotational component. In the view shown in Figure 14, the rotational component is clockwise. The second forcemodifying portion 1452 of the second force-modifying unit 1450 (connected between the second support connection point 1453 and the first end of the second body portion 1451) bends. The second coupling link 1460 bends and moves down, such that a downwards actuating force is applied to the movable connection point 1430. The movable connection point 1430 moves downwards as indicated by arrow 1431. Figure 15 shows the first and second actuating units of Figure 13 in a second position, overlayed over the first and second actuating units in the neutral position. The first and second actuating units in the neutral position are shown with like reference numerals to Figure 13. In Figure 15 the first SMA element 1540 has been contracted as indicated by arrow 1543, such that the first movable SMA connection 1542 moves closer to the first support SMA connection 1541. The first movable SMA connection 1542 is connected to the first end of the first body portion 1511 of the first force-modifying unit 1510, so the first body portion 1511 moves with a rotational component. In the view shown in Figure 15, the rotational component is clockwise. The first force-modifying portion 1512 of the first force-modifying unit 1510 (connected between the first support connection point 1513 and the first end of the first body portion 1511) bends. The first coupling link 1520 bends and moves up, such that a upwards actuating force is applied to the movable connection point 1530. The second SMA element 1570 has been extended as indicated by arrow 1573, such that the second movable SMA connection 1573 moves further from the second support SMA connection 1571. The second movable SMA connection 1573 is connected to the second body portion 1551 of the second force-modifying unit 1550, so the second body portion 1551 moves with a rotational component. In the view shown in Figure 15, the rotational component is anti-clockwise. The second force-modifying portion 1552 of the second force-modifying unit 1550 (connected between the second support connection point 1553 and the second body portion 1551) bends. The second coupling link 1560 bends and moves up, such that an upwards actuating force is applied to the movable connection point 1530. The movable connection point 1530 moves upwards as indicated by arrow 1531. Example of a movable part With reference to Figures 16 and 17, an example of an actuator assembly 1600 is illustrated, comprising the first and second actuating units of Figure 13. Figure 16 shows a perspective view, and Figure 17 shows a side view. The actuator assembly comprises a movable part 1610, and a support structure 1620 comprising a first pin bearing 1630 and a second pin 1640 bearing. The first and second pin bearings 1630 and 1640 are configured to guide movement of the movable part 1610 parallel to a primary axis that parallel to an axis of each pin bearing (in the view of Figure 17, the primary axis is vertical). The movable part 1610 may, in certain examples, comprise a lens carriage. The movable connection point 1330 is connected to the movable part 161. The first and second support SMA connections 1341, 1371 are connected to the first and second pin bearings 1630, 1640, respectively. The first and second support connection points 1313,1353 are connected to the first and second pin bearings 1630, 1640, respectively. As described above, the support structure may comprise a first surface and the movable part may be arranged to move relative to the support structure across the surface. The actuator assembly may further comprise a biasing arrangement arranged to bias the movable part into contact with the first surface so as to generate frictional forces therebetween for retaining the movable part in position on the first surface in the absence of power to the at least one actuating unit. In the example illustrated in Figures 16 and 17, each pin bearing may comprise a first surface, such that a biasing arrangement biases the movable part into contact with the bin bearings so as to generate frictional forces between the movable part and the pin bearings. In the absence of power to the first and second actuating units, the movable part 1610 is retained in position relative to the support structure 1620. The biasing arrangement may, in an example, comprise a magnetic biasing arrangement. An example is illustrated in Figure 18, where a first magnet 1810 is located on or in the movable part 1610 a second magnet 1820, attracted to the first magnet 1810, is located in or on a force-modifying unit 1830, and a ball bearing 1890 or a multiple ball bearings positioned between the first magnet 1810 and the second magnet 1820. The ball bearing allows the lateral movement of the flexure (in or out of the page) to be decoupled from the vertical movement (in the axis of the magnets). Angles ofSMA elements Each SMA element of an actuating unit may be in a plane parallel to the primary axis. The SMA element may be in a plane parallel to the primary axis in the absence of actuation of the SMA element and may remain in the same plane when the SMA element is actuated. In the example illustrated in Figures 16 and 17, this would mean that only vertical forces are applied to the movable part. The frictional forces between the movable part and the pin bearings may be constant, such that the actuating force acts against the frictional forces to move the movable part. In other examples, the SMA element may be at an acute, non-zero angle to a plane parallel to the primary axis in the absence of actuation of the SMA element. In the example shown in Figures 16 and 17, actuation of the SMA elements may result in a minor component of the actuating force that is perpendicular to the primary axis. This may reduce the frictional forces between the movable part and the pin bearings arising from the biasing arrangement, such that on actuation of one or both SMA elements, the friction reduces. The major component of the actuating force (parallel to the primary axis) therefore acts against a lower friction than exists when the SMA elements are not actuated, resulting in an effective "zero hold power" but lowering the power needed to move the movable part. Other examples The examples above are not considered limiting. In particular, other combinations of the features described above may be possible. For example, the actuator units illustrated in Figure 16 may comprise a pivot rather than a flexure for one or more components. The actuator assembly may comprise a different movable part and support structure than the example shown in Figure 16. For example, the bearing arrangement may differ. Where two actuating units are described above, they are connected to the same movable connection point. However, they may be connected to different movable connection points from one another. In an example, two or more actuating units may be connected to two or more sides of a movable part. Applications An apparatus may be provided, comprising the actuator assembly of any preceding claim, wherein the movable part comprises an optical element. In particular, the movable part may comprise an image sensor and / or a lens carriage or lens assembly. The actuator assembly may be used to move one or both of the image sensor and lens assembly, in order to adjust the autofocus of a camera. SMA The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. Other variations It will be appreciated that there may be many other variations of the above-described examples.

Claims

1. An actuator assembly comprising:a support structure, wherein a primary axis is defined with reference to the support structure;a movable part that is movable relative to the support structure;a bearing arrangement arranged to guide movement of the movable part relative to the support structure parallel to the primary axis; andat least one actuating unit configured to apply an actuating force to a connection point of either the movable part or the support structure, wherein the actuating force is capable of moving the movable part relative to the support structure and has a major component that is parallel to the primary axis, the actuating unit comprising:a force-modifying unit;a coupling link connected between the force-modifying unit and the connection point; andan SMA element connected between the force-modifying unit and the support structure;wherein on actuation of the SMA element:the SMA element is configured to apply an input force to the force-modifying unit;the force-modifying unit is configured to modify the input force so as to give rise to the actuating force applied to the connection point; andthe actuating unit is configured to move the movable part in a direction parallel to the primary axis.

2. An actuator assembly according to claim 1 comprising at least two actuating units, wherein a first actuating unit is configured to move the movable part in a direction parallel to the primary axis and a second actuating unit is configured to move the movable part in a second direction opposite to the first direction.

3. An actuator assembly according to claim 1 or 2, wherein the force-modifying unit comprises:a body portion connected to the coupling link; anda force-modifying portion connected between the body portion and the support structure.

4. An actuator assembly according to claim 3, wherein the body portion and the force-modifying portion are integrally formed.

5. An actuator assembly according to claim 3 or 4, wherein the actuator assembly comprises at least two actuating units each comprising a body portion and a force-modifying potion and wherein the body portion and force-modifying portion of each actuating unit are integrally formed with each other and with the body portion and force-modifying portion of at least one other actuating unit.

6. An actuator assembly according to any of claims 2 to 5, wherein two actuating units are connected at a single point.

7. An actuator assembly according to any preceding claim wherein the force-modifying portion comprises a flexure.

8. An actuator assembly according to any of claims 1 to 6 wherein the force-modifying portion comprises a pivot.

9. An actuator assembly according to any preceding claim wherein the SMA element is in a plane parallel to the primary axis in the absence of actuation of the SMA element.

10. An actuator assembly according to any of claims 1 to 8 wherein the SMA element is at an acute, non-zero angle to a plane parallel to the primary axis in the absence of actuation of the SMA element.

11. An actuator assembly according to any preceding claim, wherein the support structure comprises a first surface and the movable part is arranged to move relative to the support structure across the surface, wherein the actuator assembly further comprises a biasing arrangement arranged to bias the movable part into contact with the first surface so as to generate frictional forces therebetween for retaining the movable part in position on the first surface in the absence of power to the at least one actuating unit.

12. An actuator assembly according to claim 11, wherein the biasing arrangement comprises: a magnetic biasing arrangement; ora resilient biasing arrangement connected between the movable part and the support structure.

13. An actuator assembly according to any preceding claim, wherein the bearing arrangement comprises:one or more pin bearings configured to constrain movement of the movablepart to be parallel to the primary axis; and / orone or more plain bearings configured to constrain movement of the movablepart to be parallel to the primary axis; and / orone or more rolling elements configured to constrain movement of the movable part to be parallel to the primary axis.

14. An actuator assembly according to any preceding claim, wherein the coupling link comprises a flexure.

15. An actuator assembly according to any preceding claim, wherein:a first end of the force-modifying unit is connected to the SMA element;a second end of the force-modifying unit is connected to the coupling link; andthe force-modifying unit is connected to the support structure between the first endand the second end of the actuating unit.

16. An actuator assembly according to of claims 1 to 14, wherein:a first end of the force-modifying unit is connected to the support structure;a second end of the force-modifying unit is connected to the coupling link; andthe force-modifying unit is connected to the SMA element between the first end andthe second end of the actuating unit.

17. An apparatus comprising the actuator assembly of any preceding claim, wherein the movable part comprises one or both of:an image sensor; anda lens assembly.

Images