Actuator assembly

Abstract

An actuator assembly comprises first 10 and second 20 parts that are relatively moveable along at least one movement path M in at least one degree of freedom of movement. An actuator 40a applies an a

Description

Field The present application generally relates to an actuator assembly, for example an actuator assembly comprising shape memory alloy elements as actuators. Background There are a variety of apparatuses in which it is desired to control movement of a movable component. Shape memory alloy (SMA) elements (such as SMA wires) may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA actuator required to apply a given force to the movable element can be relatively small. It may be desirable to provide an anti-centring force to the movable component to urge the movable component away from a central position. Such an anti-centring force may, for example, counteract a centring force applied by an external load to the movable component. Summary According to the present invention, there is provided an actuator assembly comprising first and second parts that are movable relative to each other along at least one movement path in at least one degree of freedom of movement; an actuator arrangement comprising at least one actuator configured, on actuation, to cause an actuating force to be applied to the second part so as to move the second part relative to the first part along the at least one movement path within a range of movement; a biasing arrangement configured to: apply, when the second part is positioned relative to the first part within the range of movement and at positions that are located along the at least one movement path and on opposite sides from a set position, a lateral force to the second part acting in a direction along the at least one movement path and away from the set position; wherein at least one common component is configured to apply to the second part both i) the lateral force and ii) the actuating force or a normal force for loading a bearing arrangement that guides the movement of the second part relative to the first part along the at least one movement path. The lateral force may be an anti-centring force. The lateral force may compensate for a centring force that is expected to be applied to the movable part by an external load that is to be coupled to the movable part. The net centring force may thus be at least reduced, improving the effectiveness of the actuator arrangement to effect relative movement of the first and second parts. Providing a common component to apply the lateral force, instead of providing an additional dedicated component to apply the lateral force, may improve compactness and reduce the number of parts of the actuator assembly. When the second part is at the set position, the biasing arrangement may be configured not to apply a lateral force along the at least one movement axis. The biasing arrangement may apply the lateral force when the second part is positioned at any position within the range of movement other than the set position. The lateral force may increase with increasing displacement of the second part from the set position. The common component may apply both the lateral force and the actuating force to the movable component. Alternatively, the common component may apply both the lateral force and the normal force that loads the bearing arrangement. The normal force may be perpendicular to the lateral force. The actuator assembly may comprise the bearing arrangement. The at least one movement path may comprise movement along at least one movement axis or along a curved movement path. The at least one movement path may comprise movement along two orthogonal axes, so as to allow translational movement in a plane, or may generally comprise movement in any subset of or in all of six degrees of freedom of movement. Movement described herein comprises translational movement, rotational movement and combinations of translational and rotational movement. Positions described herein comprise any translational positions, rotational positions (such as orientations) or combinations thereof (such as poses). The range of movement is the set of positions of the second part relative to the first part that is achievable during normal operation of the actuator assembly, i.e. due to actuation of the actuator arrangement. In some embodiments, the at least one common component comprises a common rigid element. The rigid element is capable of transmitting a force to the second part. The rigid element may thereby apply both a normal force and the lateral force or both the actuating force and the lateral force to the second part. The rigid element may, directly or indirectly, engage the second part so as to apply the force. In some embodiments, the common rigid element is configured to be in compression when applying the lateral force. The lateral force, the normal force and / or the actuating force may place the common rigid element under compression. In some embodiments, the common rigid element is elongate between first and second ends, wherein the first end engages the second part so as to move with the second part and wherein the second end is constrained from moving with the second part. In some embodiments, the common rigid element is a pivot arm configured to pivot relative to the second part upon movement of the second part relative to the first part, i.e. upon actuation of the actuator causing such movement. The pivot arm may be pivotable about an axis that is perpendicular to the movement path, e.g. to the instantaneous movement direction of the second part. The pivot arm may be elongate. The pivot arm may apply a force to the second part in a direction along the length of the pivot arm. The angle of the length of the pivot arm relative to the movement path may change on movement of the second part relative to the first part. Some embodiments comprise a bearing arrangement configured to guide movement of the second part relative to the first part. The biasing arrangement is further configured to apply the normal force for loading the bearing arrangement. The normal force is perpendicular to the lateral force. Optionally, the at least one common component comprises a common biasing element configured to apply both the normal force and the lateral force to the second part. The bearing arrangement may comprise a plain bearing or sliding bearing, comprising a bearing surface on the first part and another bearing surface on the second part that are slidingly engaged. Alternatively, the bearing arrangement may comprise a rolling bearing, comprising a rolling bearing element (such as a ball bearing) between the bearing surfaces, or a flexure bearing comprising one or more flexures. In some embodiments, a bearing arrangement is not provided and the second part is suspended relative to the first part by the actuator arrangement. The common biasing element may be a resilient element, such as a spring element. In some embodiments, the common biasing element applies a biasing force to the common rigid element, wherein the biasing force comprises a first component perpendicular to a direction of movement of the second part relative to the first part, the first component corresponding to the normal force, and wherein the biasing force comprises a second component parallel to the direction of movement of the second part relative to the first part, the second component corresponding to the lateral force. In some embodiments, the common biasing element and the common rigid element overlap when viewed in a direction perpendicular to the normal force. The common biasing element and the common rigid element may overlap when viewed in a direction perpendicular to the length of the elongate common rigid element. In some embodiments, the common biasing element is arranged in mechanical series with the common rigid element. The common biasing element may be directly or indirectly connected between one of the first and second parts and the common rigid element, and the common rigid element may be directly or indirectly connected between the other of the first and second parts and the common rigid element. In some embodiments, the common biasing element engages the first part at one end and the common rigid element at another end, and wherein the common rigid element engages the common biasing element at one end and the second part at another end. In some embodiments, the second part is arranged between the common biasing element and the common rigid element. The second part may be sandwiched between the common biasing element and the common rigid element, optionally indirectly via one or more other parts (such as a sprung part of the first part). In some embodiments, the common biasing element engages the first part at one end and a part comprising a first bearing surface at another end, wherein the second surface is arranged on the second part, and wherein the second part is arranged between the part comprising the first bearing surface and the common rigid element, and wherein the common rigid element engages at one end the second part and at another end the first part. The second part may be sandwiched between the common biasing element and the common rigid element. The first bearing surface may be comprised by the bearing arrangement and directly or indirectly (via a bearing element) engage a second bearing surface provided on the second part. In some embodiments, the biasing arrangement comprises a primary biasing element configured to apply a normal force to the second part, wherein the actuator assembly further comprises at least one secondary biasing element configured to apply the lateral force, wherein the at least one common component is configured to apply both the actuating force and the lateral force to the second part. The first biasing element may be a resilient element, such as a spring element. The second biasing element may be a resilient element, such as a spring element. The second biasing element may apply a force to the second part that is opposite to the normal force. The first biasing element may apply, when the second part is positioned within the range of movement away from the set position relative to the first part, a lateral force in a direction perpendicular to the normal force urging the second part towards the set position. In some embodiments, the actuator arrangement comprises first and second actuators and first and second drive members that are rotatable relative to the second part, and wherein the first actuator is configured to apply a respective input force to the first drive member so as to drive rotation of the first drive member, thereby applying a respective actuating force for moving the second part in a first direction and the second actuator is configured to apply a respective input force to the second drive member so as to drive rotation of the second drive member, thereby applying a respective actuating force for moving the second part in a second direction that is opposite to the first direction. In some embodiments, the first drive member comprises a first pair of drive members and wherein the second drive member comprises a second pair of drive members, wherein the first actuator is connected between first ends of the first pair of drive members and wherein the second actuator is connected between first ends of the second pair of drive members, wherein the second ends of the first pair of drive members are rotatably coupled to each other and to the second part and wherein the second ends of the second pair of drive members are rotatably coupled to each other and to the second part. The first and second pairs of drive members may be arranged in a scissor jack configuration. Some embodiments comprise two secondary biasing elements, wherein one of the two secondary biasing elements is configured to apply a respective biasing force to the first drive member (or first pair of drive members) and another of the two secondary biasing elements is configured to apply a respective biasing force to the second drive member (or second pair of drive members). In some embodiments, the biasing arrangement is configured to apply a normal force to the second part so as to bias first and second friction surfaces against each other to thereby generate a static frictional force between the first and second friction surfaces that constrains movement between the first and second parts at any position within the range of movement when the actuator is not actuated. In some embodiments, the actuator arrangement is configured, on actuation, to reduce the normal force between the first and second friction surfaces, thereby reducing the static frictional force therebetween. In some embodiments, the actuator arrangement comprises at least two actuators configured, on actuation, to cause opposing actuating forces to be applied to the second part so as to move the second part in opposite directions relative to the first part. In some embodiments, the or each actuator of the actuator arrangement comprises a shape memory alloy, SMA, element. The SMA element may be elongate. The SMA element may be an SMA wire. In some embodiments, the actuator arrangement comprises an intermediate part arranged between the actuator and the second part, wherein the intermediate part is configured to amplify the actuation amount of the actuator to a relatively greater amount of movement of the second part relative to the first part. The intermediate part may be the common rigid element. The intermediate part may be a drive member, a drive rod or a pivot arm. In some embodiments, the biasing arrangement comprises a resilient element, such as a spring element. In some embodiments, the first and second parts are translationally movable relative to each other. The first and second parts may be translationally movable relative to each other in one degree of freedom (e.g. along an axis or movement path), or in multiple degrees of freedom (e.g. within a movement plane. In some embodiments, the first and second parts are rotationally movable relative to each in at least two degrees of freedom about at least two orthogonal axes. Some embodiments comprise an external load coupled to the second part, wherein the external load is configured to apply a load biasing force urging the second part towards the set position when the second part is positioned away from the set position, and wherein the biasing arrangement is configured at least partially to compensate for the load biasing force by opposing the load biasing force at any position of the second part within the range of movement. Further aspects of the present invention are set out in the dependent claims and in the detailed description. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 schematically shows an actuator assembly; Figure 2 schematically shows another actuator assembly; Figure 3 schematically shows another actuator assembly; Figures 4A and 4B schematically show actuator assemblies including a common component for applying a counterbalance force and a normal force, included in the actuator assembly of Figure 1; Figure 4C is a graph showing the net force applied by a biasing arrangement and pivot arm on a movable part for a range of lengths of the pivot arm and a range of positions of the movable part; Figure 5 schematically shows another actuator assembly including a common component for applying a counterbalance force and a normal force, included in the actuator assembly of Figure 2; Figure 6 schematically shows an actuator assembly including a common component for applying a counterbalance force and an actuating force, included in the actuator assembly of Figure 3; Figure 7 schematically shows another actuator assembly with one degree of freedom of movement and including a common component for applying a counterbalance force and an actuating force; Figure 8 schematically shows another actuator assembly with two translational degrees of freedom of movement and including a common component for applying a counterbalance force and an actuating force; and Figure 9 schematically shows another actuator assembly with two rotational degrees of freedom of movement including a common component for applying a counterbalance force and an actuating force. Detailed Description Actuator assembly Figures 1 to 3 schematically show examples of an actuator assembly 1. The actuator assembly 1 includes a support structure 10 and a movable part 20. The movable part 20 is movable relative to the support structure 10. When the actuator assembly 1 is included in an apparatus, such as portable electronic device, the support structure 10 may be fixed relative to the main body of such an apparatus. However, in general, the support structure 10 need not be stationary and may be movable relative to or within such an apparatus. The support structure 10 herein serves as a reference structure relative to which movement of other components of the actuator assembly 1 is described, but it will be appreciated that the support structure 10 may equally move relative to the movable part 20, for example if the movable part 20 is fixed within a larger device. The support structure 10 and movable part 20 may also be referred to simply as a first part and a second part, or vice versa. The actuator assembly 1 includes an actuator arrangement 40. The actuator arrangement comprises one or more actuators 40a, embodied in Figures 1 to 3 as two SMA elements 40a, in particular two SMA wires 40a. The actuator arrangement 40 is configured, on actuation, to apply actuating forces F to the movable part 20 capable of moving the movable part 20 relative to the support structure 10. Purely for illustrative reasons, the actuating force F is only shown for one SMA wire 40a in the figures. The movable part 20 may be supported (i.e. suspended) on the support structure 10 exclusively by the actuator arrangement 40. Alternatively, the actuator assembly 2 may include a bearing arrangement 50 and a biasing arrangement 30. The bearing arrangement 50 supports the movable part 20 on the support structure 10 and guides movement of the movable part 20. The biasing arrangement 30 loads the bearing arrangement 50 with a normal force, thereby applying a force that helps guide movement of the movable part 20. The actuator arrangement 40 may support the movable part 20 on the support structure 10 together with the bearing arrangement 50 and the biasing arrangement 30. The bearing arrangement 50 may have any suitable form for allowing movement of the movable part 20 with respect to the support structure 10 with one or more degrees of freedom (DOFs). In general, the movable part 20 may be movable relative to the support structure 10 with up to six degrees of freedom (DOFs). The movable part 20 may be movable relative to the support structure 10 in all or in any subset (including only one) of three translational DOFs (so movement along three orthogonal axes, herein the x, y and z axes) and three rotational DOFs (so rotation about three orthogonal axes, in particular the x, y and z axes). Figures 1 to 3 show embodiments in which the movable part 20 moves in one DOF, in particular along a movement path such as a movement axis M. The actuator arrangement 40 and / or the bearing arrangement 50 and biasing arrangement 30 may constrain, i.e. reduce or prevent, other DOFs of movement of the movable part 20 relative to the support structure 10. For this purpose, the bearing arrangement 50 may, for example, include one or more of the following bearings: a rolling bearing (such as a ball bearing), a flexure bearing (i.e. an arrangement of flexures or other resilient elements that guide movement), or a plain (i.e. sliding contact) bearing. The biasing arrangement 30 may include any element capable of applying a loading force to the bearing arrangement 50, for example a resilient element such as a spring or flexure, or a set of magnetic elements such as a magnet and a ferromagnetic material. The biasing arrangement 30 may load the bearing arrangement 50 by biasing bearing surfaces of the bearing arrangement 50 against each other (typically for a rolling bearing or a plain bearing) or by placing the bearing arrangement 50 in tension (typically for a flexure bearing). The actuators 40a are, directly or indirectly, connected between the support structure 10 and the movable part 20. In embodiments with an SMA wire 40a, the SMA wire 40a may be connected, directly or indirectly, to the support structure 10 and the movable part 20 by connection elements, such as crimps (not shown). Optionally, one or more intermediate parts may be connected between the SMA wires 40a and the movable part 20 and / or support structure 10. The SMA wires 40a or other actuators 40a may thereby, either directly or indirectly via one or more intermediate parts, apply actuating forces F to the movable part 20. Selectively varying the actuating forces F applied by the actuators 40a may cause the movable part 20 to move relative to the support structure 10, for example within the DOFs allowed by the bearing arrangement 50. The actuators 40a are thus capable of driving movement of the movable part 20 relative to the support structure 10. The bearing arrangement 50 may cause the movable part 20 to move in directions which differ from the directions of the actuating forces F. In simple examples of this, one component of each actuating force F causes the movement of the movable part 20, and another component of each actuating force F acts against or with the bearing forces produced by the bearing arrangement 50. The actuator assembly 2 may also include a controller (not shown). The controller may be implemented in an integrated circuit (IC) chip. The controller generates drive signals for the actuators 40a so as to actuate the actuators 40a. In embodiments in which the actuators 40a comprise SMA wires 40a, the controller generates drive signals for contracting the SMA wires 40a. SMA material has the property that, on heating, it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA wires 40a, thereby heating the SMA wires 40a by causing an electric current to flow, will cause the SMA wires 40a to contract and thus apply the actuating force so as to drive relative movement of the movable part 20. The drive signals are chosen to drive relative movement of the movable part 20 in the desired manner. The movable part 20 may comprise or be coupled to a component to be moved, such as a lens or an image sensor. Moving a lens or an image sensor may effect optical image stabilization and / or autofocus or zoom functionality in a camera apparatus, for example. In general, the actuator assembly 1 may be used in any device in which movement of a movable part 20 relative to a support structure 10 is desired, e.g. to provide haptic feedback in a haptic feedback device or to move a projector or display in an augmented reality (AR) or virtual reality (VR) device. Zero hold power The actuator assembly 1 may be configured such that the movable part 20 does not move relative to the support structure 10 when the actuators 40a are not actuated. The movable part 20 may remain in position without needing to power the actuators 40a. This is also referred to as zero hold power. Power consumption of the actuator assembly 1 is reduced compared to an actuator assembly 1 in which the actuators 40a are continuously powered so as to keep a desired position of the movable part 20 relative to the support structure 10. In the actuator assemblies 1 of Figures 1 to 3, the support structure 10 comprises a first friction surface lOf and the movable part 20 comprises a second friction surface 20f. The first and second friction surfaces lOf, 20f may form part of the bearing arrangement 50 (as in Figures 1 and 3) or be separate from the bearing arrangement 50 (as in Figure 2). The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force. A static frictional force is thus generated between the first and second friction surfaces lOf, 20f. The magnitude of the static frictional force may be sufficient to constrain movement of the movable part 20 relative to the support structure 10 at any position within a range of movement of the movable part 20 relative to the support structure 10 when the actuators 40a are not actuated. In the actuator assemblies 1 of Figures 1 to 3, the actuators 40a may be arranged such that, on actuation, the normal force between the first and second friction surfaces lOf, 20f is reduced. For example, when two opposing actuators 40a are equally actuated, the movable part 20 may be urged in a direction perpendicular to the first and second friction surfaces lOf, 20f, such that the normal force between the first and second friction surfaces lOf, 20f is reduced. The first and second friction surfaces lOf, 20f may remain in contact or may even disengage on reduction of the normal force. As a result, the static frictional force between the first and second friction surfaces lOf, 20f is reduced. The resistance to movement of the movable part 20 on differential actuation of the actuators 40a may thus be reduced, such that the actuators 40a may more effectively move the movable part 20 and / or a larger normal force may be applied when the actuators 40a are not actuated. The first and second friction surfaces lOf, 20f may generally be provided between any two components of the actuator assembly 1 that are in engagement and move relative to each other. For example, first and second friction surfaces lOf, 20f may be arranged between any intermediate part and the support structure 10, between any intermediate part and the movable part 20, or between any two intermediate parts that are coupled between the movable part 20 and the support structure 10. In general, the normal force may be reduced on actuation of the actuators 40a or the normal force may remain constant (or even increase) on actuation of the actuators 40a. WO 2020 / 120997 Al, WO 2023 / 094813 Al and WO 2023 / 084251 Al, which are herein incorporated by reference, describe various ways of arranging the first and second friction surfaces lOf, 20f for achieving zero hold power. Movement of the movable part 20 at any position within the range of movement may be constrained, in particular, when the actuators 40a are not actuated and when acceleration of the actuator assembly 1 is less than or equal to a hold threshold. The hold threshold may be at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2), where g is the acceleration due to Earth's gravity. Examples of actuator assemblies With particular reference to Figure 1, the actuator assembly 1 may comprise actuators 40a in the form of two SMA wires 40a, a bearing arrangement 50 comprising a plain bearing and a biasing arrangement 30 comprising a compression spring. Each SMA wire 40a is connected between the support structure 10 and the movable part 20. So, one end of the SMA wire 40a is connected to the support structure 10 and the other end of the SMA wire 40a is connected to the movable part 20. The plain bearing comprises a first surface lOf on the support structure 10 and a second surface 20f on the movable part 20. The spring is arranged to load the bearing arrangement 50 by biasing the first and second surfaces lOf, 20f against each other. The movable part 20 is arranged to be movable relative to the support structure 10 along a movement axis M (the horizontal axis in Figure 1). The spring applies a force to the movable part 20 in a direction that is perpendicular to the movement axis M (downwards in Figure 1). The SMA wires 40a apply actuating forces F to the movable part 20. In Figure 1, the SMA wires 40a are arranged at an angle a relative to the movement axis M. The actuating forces F typically act in a direction along the length of the SMA wires 40a. In Figure 1, the actuating forces F thus have a component along the movement axis M and a component perpendicular to the movement axis M that acts against the biasing force of the biasing arrangement 30. The SMA wires 40a are arranged in opposition, i.e. the SMA wires 40a apply actuating forces F to move the movable part 20 with force components in opposite directions along the movement axis M. On actuation of one SMA wire 40a (the left SMA wire 40a in Figure 1), the movable part 20 moves in one direction (leftwards) and on actuation of the other SMA wire 40a (the right SMA wire 40a in Figure 1), the movable part 20 moves in an opposite direction (rightwards). The first and second surfaces lOf, 20f of the bearing arrangement 50 correspond to first and second friction surfaces lOf, 20f. The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force, thereby giving rise to a static frictional force capable of holding the movable part 20 in position so as to achieve zero hold power. The component of the actuating force F that is perpendicular to the movement axis M acts against the biasing arrangement 30, and so reduces the normal force. On actuation of the SMA wires 40a, the static frictional force between the first and second friction surfaces lOf, 20f is thus reduced. The actuator assembly 1 of Figure 2 is similar to the actuator assembly 1 of Figure 1, except that the bearing arrangement 50 is embodied by a rolling bearing and that the first and second friction surfaces lOf, 20f are separate from the bearing arrangement 50. As shown in Figure 2, the support structure 10 comprises a first portion 10a and a second portion 10b. The first and second portions 10a, 10b are coupled by a compression spring forming the biasing arrangement 30. The first and second portions 10a, 10b of the support structure are movable relative to each other in a direction perpendicular to the movement axis M. However, in practice, movement of the first and second portions 10a, 10b relative to each other need not take place during operation of the actuator assembly 1. The first and second portions 10a, 10b may remain static relative to each other on actuation of the SMA wires 40a, and so are considered as a unit forming the support structure 10. Alternatively, the first portion 10a may be considered to correspond to the support structure 10 and the second portion 10b may be considered to correspond to another part 10b (such as a sprung part 10b) on which the second friction surface lOf is provided. In general, movement of the first and second portions 10a, 10b in a direction along the movement axis M is constrained. The bearing arrangement 50 comprises a ball bearing arranged between the movable part 20 and the support structure 10, in particular the first portion 10a thereof. The ball bearing guides movement of the movable part 20 along the movement axis M. The biasing arrangement 30, in the form of the compression spring, is arranged between the first and second portions 10a, 10b of the support structure 10. The compression spring loads the ball bearing via the second portion 10b and the movable part 20, i.e. the biasing force of the spring is transmitted to the ball bearing by the second portion 10b and the movable part 20. The biasing force acts in a direction perpendicular to the movement axis M. The SMA wires 40a are connected between the movable part 20 and the second portion 10b of the support structure 10. The SMA wires 40a are connected in opposition, so as to move the movable part 20 in opposite directions along the movement axis M. On actuation, the SMA wires 40a are arranged to further load the bearing arrangement 50. The first and second friction surfaces lOf, 20f are arranged on the movable part 20 and the second portion 10b of the support structure 10. The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force. The SMA wires 40a are angled relative to the movement axis M so as to reduce the normal force on actuation, thereby reducing the frictional force between the first and second friction surfaces lOf, 20f. The actuator assembly 1 of Figure 3 is similar to the actuator assembly 1 of Figure 1, except that the actuator arrangement comprises drive members 41, herein also referred to as drive rods 41, in addition to SMA wires 40a. The bearing arrangement 50, first and second friction surfaces lOf, 20f and biasing arrangement 30 are as described with reference to Figure 1. The SMA wire 40a is connected between the support structure 10 and a connection part 45. The connection part 45 may, for example, comprise a crimp or other connection element. The SMA wire 40a applies an input force Fi to the connection part 45. The SMA wire 40a, on actuation, drives movement of the connection part 45 along a direction that is not parallel to the movement axis M (such as perpendicular to the movement axis M, in particular upward in Figure 3). The connection part 45 may be constrained to move in the direction by a bearing (not shown), for example. Such a bearing may comprise a rolling bearing or a plain bearing, for example. The drive rod 41 is rotatably coupled between the connection part 41 and the movable part 20. So, the drive rod 41 is rotatable relative to the connection part 45 and relative to the movable part 20. In Figure 3, a pivot connection couples the drive rod 41 to the movable part 20 and to the connection part 45, although in general any mechanism guiding rotation of the drive rod 41 relative to the movable part 20 and relative to the connection part 45 may be used. The drive rod 41 is arranged at an angle a relative to the movement axis M. On actuation of the SMA wire 40a, the drive rod 41 is placed under compression so as to apply the actuating force F to the movable part 20. The actuating force F acts in a direction at the angle a relative to the movement axis M, and so effects movement of the movable part 20 and reduction of the frictional force between the first and second friction surfaces lOf, 20f as described in relation to Figure 1. The arrangement of the connection part 45 and drive rod 41 may amplify the movement achievable by the SMA wire 40a. On actuation of the SMA wire 40a, the connection part 45 may move by a first amount and the movable part 20 may move by a second amount, where the second amount is greater than the first amount. The ratio of second amount to first amount depends on the angle of the drive rod 41 relative to the movement axis M and relative to the direction of movement of the connection part 45. In the arrangement of Figure 3, arranging the drive rod 41 at an angle a relative to the movement axis M that is greater than 45 degrees leads to an amplification in the movement amount (also referred to as stroke). A greater angle a achieves a greater stroke amplification factor. Common component for counterbalance The actuator assembly 1 may generally be used to move an external load (not shown) that is coupled to the movable part 20. The external load may have a spring-like component that applies a centring force to the movable part 20, thereby urging the movable part 20 to a central position relative to the support structure 10. For example, when an electrical component is coupled to the movable part 20, any electrical connections (e.g. FPCs) to the electrical component may apply such a centring force to the movable part 20. Centring forces may also arise due to any other flexures or spring components or due to any magnetic arrangements (e.g. for parking of the movable part 20 at a central position on the support structure). The centring force may increase with increasing displacement from a central position. Such a centring force may affect the performance of the actuator assembly 1, particularly at the edges of the range of movement of the movable part 20 relative to the support structure 10 where the centring force is typically largest. It is thus desirable to counteract any centring forces from external loads. A counterbalance spring may be included in the actuator assembly 1 to counteract centring forces that are expected to act on the movable part 20. A counterbalance spring is a type of non-linear spring with a region of negative spring constant (such as a bistable spring) that can be used to cancel a centring force of an external load. Centring forces may not arise in the actuator assembly 1 itself but may arise only once the actuator assembly 1 is incorporated in a device for movement of a component, such as an electrical component. The counterbalance spring can be pre-emptively included in the actuator assembly 1. However, a counterbalance spring may be relatively large, and so adding such a counterbalance spring to the actuator assembly 1 as an extra component may add to the size, increase the number of components and increase cost of the actuator assembly 1. The present invention is concerned with using existing parts of the actuator assembly 1, such as components of the actuator assemblies 1 described in relation to Figures 1 to 3, as part of the counterbalance spring to reduce the impact of adding the counterbalance spring on the size and cost of the actuator assembly 1. Figures 4 to 9 schematically show embodiments of the actuator assembly 1 with an integrated counterbalance mechanism. In general, a common component is used in these actuator assemblies 1 to apply both the counterbalance force and another force, such as the normal force or the actuating force. Common component for counterbalance and normal force Figure 4A shows an actuator assembly 1 that is similar to the actuator assembly 1 of Figure 1, except that a pivot arm 31 is arranged in mechanical series with the biasing arrangement 30. The actuator 40a drives movement of the movable part 20 and achieves zero hold power functionality as described in relation to Figure 1. The friction surfaces lOf, 20f and the actuating force F are not illustrated in Figure 4A but may be present as shown in Figure 1. The actuator assembly 1 of Figure 4A comprises a pivot arm 31, which may generally be embodied by a beam or rod. The pivot arm 31 is typically a rigid element. The pivot arm 31 is elongate and may be rotatably coupled to other components at its ends. The pivot arm 31 is arranged between the biasing arrangement 30 and the movable part 20, in particular sandwiched between the biasing arrangement 30 and the movable part 20. The pivot arm 31 is rotatably coupled to the movable part 20 at one end and is rotatably coupled to the biasing arrangement 30 at the other end. The pivot arm 31 may thereby rotate or pivot relative to the movable part 20 and relative to the biasing arrangement 30. The biasing arrangement 30 is arranged to apply the normal force via the pivot arm 31. The biasing arrangement 30, for example in the form of a compression spring, applies a biasing force to the pivot arm 31. The pivot arm 31 is thereby placed under compression. The pivot arm 31 transmits the biasing force to the movable part 20, such that the biasing force is applied to the movable part 20 relative to the support structure 10. When the movable part 20 is at a set position relative to the support structure 10, such as at the central position or starting position, the pivot arm 31 may be arranged so as to be perpendicular to the movement axis M. With particular reference to Figure 4A, the pivot arm 31 may be vertical in the set position, while the movement axis M extends horizontally. When the movable part 20 is moved away from the set position, for example on actuation of the actuators 40a, the pivot arm 31 pivots so such that the angle of the pivot arm 31 relative to the movement axis M changes. The end of the pivot arm 31 coupled to the biasing arrangement 31 is constrained from moving along the movement axis M, while the end of the pivot arm 31 coupled to the movable part 20 moves with the movable part 20 along the movement axis M. With particular reference to Figure 4A, when the movable part 20 moves leftward, the pivot arm 31 rotates clockwise. When the movable part 20 moves rightward, the pivot arm 31 rotates anticlockwise. The pivot arm 31 thus rotates in opposing senses on movement of the movable part 20 in opposing directions. The pivot arm 31 transmits a force along its length, and so when the pivot arm 31 is arranged at an angle to the movement axis M, the force applied by the pivot arm 31 to the movable part 20 has a component perpendicular to the movement axis M (corresponding to the normal force) and a component parallel to the movement axis M (corresponding to a lateral force). The lateral force is directed away from the set position, so as to urge the movable part 20 away from the set position when the movable part is moved away from the set position. The pivot arm 31 thereby applies a counterbalance or anti-centring force to the movable part 20. A larger movement of the movable part 20 away from the set position leads to a larger change in angle of the pivot arm 31 relative to the movement axis M, such that a greater component of the biasing force transmitted by the pivot arm 31 to the movable part 20 acts in a direction along the movement axis M. So, the lateral force that acts in a direction away from the set position increases with distance of the movable part 20 from the set position. In the embodiment of Figure 4A, the biasing arrangement 30 and the pivot arm 31 both correspond to common components that apply the lateral force to the movable part 20 and apply the normal force to the movable part 20. The biasing arrangement 30 that is conventionally used to apply the normal force is used for the additional purpose of applying the lateral force. The implementation of a counterbalance force is thus simplified compared to providing an additional counterbalance spring for the sole purpose of providing the counterbalance force. Figure 4B shows an actuator assembly 1 that is similar to the actuator assembly 1 of Figure 4A, except that the pivot arm 31 overlaps with the biasing arrangement 30 when viewed perpendicularly to the length of the pivot arm 31. The pivot arm 31 may thus be made longer within a given space constraint compared to a situation where such overlap is not allowed, or the arrangement of pivot arm 31 and biasing arrangement 30 may be made more compact. The length of the pivot arm 31 may be set by varying the amount of overlap between the pivot arm 31 and the biasing arrangement 30. The length of the pivot arm 31 may vary between the extremes shown in Figure 4B (where the entire spring overlaps with the pivot arm 31) and Figure 4A (where none of the spring overlaps with the pivot arm 31). So, a portion of the biasing arrangement 30 (such as 50% of the biasing arrangement) may overlap with the pivot arm 31 when viewed orthogonally to the length of the pivot arm 31. Variation of the length of the pivot arm 31 allows control over the rate of change of the lateral force on movement of the movable part 20. Providing a relatively shorter pivot arm 31 allows the pivot arm 31 to undergo a relatively greater amount of rotation for a given amount of movement of the movable part 20, such that the angle of the pivot arm 31 relative to the movement axis M is reduced at the edges of the range of movement and such that the lateral force makes up a relatively greater component of the biasing force. The lateral force may thus be increased for a given amount of displacement, thereby increasing the counterbalance effect. However, a relatively shorter pivot arm 31 results in a greater movement (perpendicular to the movement axis M) of the end of the pivot arm 31 coupled to the biasing arrangement 30, and so a greater extension or compression of the biasing arrangement 30. The biasing force applied by the biasing arrangement 30 may thus undesirably be reduced. For a given spring rate of the biasing arrangement 30, there is an optimum length of the pivot arm 31 for effective counterbalancing. Figure 4C is a graph showing the net force applied by the biasing arrangement 30 and pivot arm 31 on the movable part 20 for a range of lengths of the pivot arm (coupling arm length in mm) and a range of positions of the movable part 20 (position). The graph is for a biasing arrangement 30 with a spring rate of 4N / mm and a pre-load force of IN. The graph of Figure 4C shows the net force acting on the movable part 20 for a range of positions of the movable part 20 relative to the support structure 10 (along the axis labelled "Position") and for a range of lengths of the pivot arm 31 (along the axis labelled "Pivot arm length"). The graph of Figure 4C thus illustrates that the length of the pivot arm 31 may be selected to achieve a desired rate of range of the counterbalance force with movement of the movable part 20. Figure 5 shows an actuator assembly 1 similar to the actuator assembly 1 of Figure 2, except that a pivot arm 31 replaces the bearing 50 between the support structure 10 and the movable part 20. The view of Figure 5 is up-side-down compared to the view of Figure 2. Generally, the actuator 40a effects movement of the movable part 20 and achieves zero hold power functionality as described in relation to Figure 2. The friction surfaces lOf, 20f and the actuating force F are not illustrated in Figure 5 but may be present as shown in Figure 2. The actuator assembly 1 comprises a pivot arm 31 arranged between the movable part 20 and the support structure 10, in particular the first portion 10a of the support structure 10. The pivot arm 31 is sandwiched between the movable part 20 and the first portion 10a of the support structure 10. The pivot arm 31 is configured generally as described in relation to Figure 4A. So, the pivot arm 31 is typically a rigid element, such as a beam or a rod. The pivot arm 31 is elongate and is rotatably coupled atone end to the movable part 20 and at the other end to the first portion 10a of the support structure 10. The end of the pivot arm 31 that is coupled to the first portion 10a of the support structure 10 is constrained from moving along the movement axis M, and the end of the pivot arm 31 that is coupled to the movable part 20 moves with the movable part 20. The biasing arrangement 30 is connected between the first portion 10a and the second portion 10b of the support structure 10. The biasing arrangement 30 thereby urges the first and second portions 10a, 10b apart with a biasing force. The movable part 20 engages the second portion 10b of the support structure 10, such that the biasing force is transmitted to the movable part 20 by the second portion 10b. The movable part 20 engages the pivot arm 31, thereby further transmitting the biasing force to the pivot arm 31. The pivot arm 31 is placed under compression. The pivot arm 31 generally carries the compressive force along its length. When the movable part 20 is at the set position (e.g. central position) relative to the support structure 10, the pivot arm 31 is perpendicular to the movement axis M. The force applied by the pivot arm 31 to the movable part 20 thus comprises a component that is perpendicular to the movement axis M, corresponding to the normal force that biases the movable part 20 against the second portion 10b of the support structure 10. The normal force is equal to the biasing force applied by the biasing arrangement 30 when the movable part 20 is at the set position. When the movable part 20 is moved away from the set position, the pivot arm 31 pivots so as to be angled relative to the movement axis M. The force applied by the pivot arm 31 to the movable part 20 thus has a component perpendicular to the movement axis M (corresponding to the normal force) and a component parallel to the movement axis M (corresponding to a lateral force). The lateral force is directed away from the set position, so as to urge the movable part 20 further away from the set position when the movable part 20 is moved away from the set position. The pivot arm 31 thereby applies the counterbalance or anti-centring force to the movable part 20. In the embodiment of Figure 5, the movable part 20 moves relative to the support structure 10 effectively along an arc centred about the pivot point of the coupling between the pivot arm 31 and the first portion 10a of the support structure 10. The length of the pivot arm 31 may be relatively large compared to the range of movement of the movable part 20 along the movement axis M, such that the arc approximates the movement axis M. Any movement of the movable part 20 that is orthogonal to the movement axis M may be insignificant compared to the movement along the movement axis M. In general, the movable part 20 may move along a movement path relative to the support structure 10. In the embodiment of Figure 5, similarly to the embodiments of Figures 4A and 4B, the biasing arrangement 30 and the pivot arm 31 both correspond to common components that apply the lateral force to the movable part 20 and apply the normal force to the movable part 20. The embodiments of Figures 4 and 5 thus make use of common components in the form of the biasing arrangement 30 and the pivot arm 31 to apply the lateral force and the normal force to the movable part 20. Figures 6 to 9 show an alternative approach in which drive rods 41 are used as a common component to apply the lateral force to the movable part 20 and to apply the actuating force to the movable part 20. Common component for counterbalance and actuating force Figure 6 shows an actuator assembly 1 that is similar to the actuator assembly 1 of Figure 3, except that biasing elements 42 are arranged to apply biasing forces to the drive rods 41, in particular via the connection parts 45. The actuator 40a effects movement of the movable part 20 and zero hold power functionality is achieved as described in relation to Figure 3. The friction surfaces lOf, 20f and the actuating force F are not illustrated in Figure 6 but may be present as shown in Figure 3. The actuator assembly 1 comprises biasing elements 42, each arranged to apply a biasing force to a respective connection part 45. Each biasing element 42 is arranged between the support structure 10 and the respective connection part 45. In Figure 6, each biasing element 42 is embodied by a compression spring connected at one end to the support structure 10 and at the other end to the connection part 45. In general, the biasing element 42 may comprise any element capable of applying a biasing force to the connection part 45. The biasing force acts to aid the input force Fi applied by the SMA element 40a to the connection part 45, and so may be in the same direction as the input force Fi, or at least comprise a force component that is in the same direction as the input force Fi. In Figure 6, the input force Fi acts in an upward direction and the biasing force also acts in an upward direction. The drive rod 41 is coupled to the connection part 45. The biasing force is transmitted to the drive rod 41 by the connection part 45. The drive rod 41 is thus placed under compression by the biasing force, even when the SMA element 40a is not actuated. When the movable part 20 is at the set position (e.g. central position) relative to the support structure 10, the drive rods 41 are angled in equal and opposite ways relative to the movement axis M. The biasing forces of the biasing elements 42 act on the movable part 20 in equal and opposite ways. With particular reference to Figure 6, the left drive rod 41 applies the biasing force to the movable part 20 in the top-right direction, whereas the right drive rod 41 applies the biasing force to the movable part 20 in the top-left direction. The components of the biasing forces along the movement axis M thus balance, such that the overall lateral force applied to the movable part 20 is zero when the movable part 20 is at the set position. When the movable part 20 is moved away from the set position, for example on actuation of the actuators 40a, the drive rods 41 rotate such that the angles of the drive rods 41 relative to the movement axis M change. The balance of the components of the biasing forces along the movement axis M thus changes, such that the component along the movement axis M of the biasing force of one biasing element 42 becomes greater than the component along the movement axis M of the biasing force of another biasing element 42. An overall lateral force, equal to the difference in the components along the movement axis M of the biasing forces applied by the biasing elements 42, is thus applied to the movable part 20. The overall lateral force urges the movable part 20 further away from the set position, thereby applying an anti-centring force to the movable part 20. For example, with particular reference to Figure 6, when the movable part 20 moves leftward, the angle of the left drive rod 41 relative to the movement axis M increases and the angle of the right drive rod 41 relative to the movement axis M decreases. The drive rods 31 apply respective biasing forces along their lengths. The angle relative to the movement axis M of the biasing force applied by the right drive rod 41 to the movable part 20 is thus smaller than the angle relative to the movement axis M of the biasing force applied by the left drive rod 41. The component along the movement axis M of the biasing force applied by the right drive rod 41 is greater than the component along the movement axis M of the biasing force applied by the left drive rod 41. Therefore, an overall lateral force acts on the movable part 20 in a leftward direction, urging the movable part 20 further away from the set position. The spring rate of the biasing elements 42 may be relatively small, in particular smaller than the rate of change of the component of the biasing force along the movement axis M on extension and contraction of the biasing elements 42. Considering the example of Figure 6, when the movable part 20 moves leftward and the drive rods 31 move accordingly, the left biasing element 42 is compressed and the right biasing element 42 extends. The biasing force of the left biasing element 42 may thus increase and the biasing force of the right biasing element 42 may decrease. Such increase and decrease of the biasing forces counteract the desired anti-centring force, and so the spring rate of the biasing elements 42 is set such that the increase or decrease of the biasing force for a given amount of movement of the movable part 20 (which is proportional the spring rate) is less than the change in the component of the biasing force along the movement axis M. The biasing force applied by the biasing arrangement 30 to the movable part 20 may be greater than the biasing force applied by the biasing elements 42. The force applied by the biasing elements 42 to the movable part 20, via the drive rods 41, also has a force component that is in a direction perpendicular to the movement axis M (upward in Figure 6). The force component applied by the biasing elements 42 in a direction perpendicular to the movement axis M is opposite to the biasing force applied by the biasing arrangement 30. The biasing force applied by the biasing arrangement 30 may be greater than the biasing forces applied by the biasing elements 42, for example by a factor of at least 2, or at least 5, so as to ensure that the friction surfaces lOf, 20f remain in engagement or the bearing arrangement 50 remains loaded. Figures 7 to 9 show further actuator assemblies 1 with a common component applying both the actuating force F and the lateral force to the movable part 20. The common component is a pair of drive members 41 of the actuator assembly 1. Each of the actuators assemblies 1 comprises the support structure 10 and the movable part 20, as well as an actuator arrangement 40 comprising a plurality of actuating units. Each actuating unit comprises an actuator 40a, embodied by an SMA wire 40a in the depicted embodiments, a pair of drive members 41 and a biasing element 42. The drive members 41 are elongate between first and second ends and may correspond to drive rods 41 as described in relation to Figures 3 and 6. The SMA wire 40a and the biasing element 42 are mechanically connected in parallel between first ends of the two drive members 41. The second ends of the two drive members 41 are pivotally connected to each other. The first ends of the drive members 41 are mechanically constrained to move translationally along a first axis (the horizontal axis in Figure 7), for example by any suitable bearing arrangement capable of guiding such translational movement. The SMA wire 40a may extend along the first axis and apply an input force between the drive members along the first axis. On actuation of the SMA wire 40a, the first ends of the two drive members 41 are urged towards each other along the first axis, thereby urging the second ends of the two drive members 41 in a direction along a second axis (the vertical axis in Figure 7) that is orthogonal to the first axis. The drive members 41 thus form a scissor jack arrangement. On actuation of the SMA wire 40a along the first axis, the actuating unit applies the actuating force F to the movable part along the second axis. The biasing element 42 is connected between the first ends of the drive members 41, thereby urging the first ends of the drive members 41 towards each other. The biasing element 42 is embodied by a tension spring in Figures 7 to 9, although in general any element capable of applying a biasing force urging the first ends of the drive members 41 towards each other may be used as the biasing element 42. The biasing element 42 applies the biasing force with a component along the input force applied by the SMA wire 40a. The biasing force thereby acts to aid the input force applied by the SMA element 40a. Figure 7 shows an actuator assembly 1 comprising two actuating units that are configured to apply actuating forces F to the movable part 20 in opposite directions. The movable part 20 may be constrained by a bearing arrangement 50 to translationally move along a movement axis M. The two actuating units are arranged on opposite sides of the movable part 20 along the movement axis M. The actuating units apply the actuating forces F in a direction parallel to the movement axis M. Figure 7 shows the movable part at the set position. The drive members 41 of the opposing actuating units are angled in equal and opposite ways relative to the movement axis M. The biasing forces of the biasing elements 42 thereby act on the movable part 20 in equal in opposite ways. The overall lateral force applied to the movable part 20 is zero when the movable part 20 is at the set position. When the movable part 20 is moved away from the set position, for example on actuation of the actuators 40a, the drive members 41 rotate such that the angles of the drive members 41 relative to the movement axis M change. With particular reference to Figure 7, when the top SMA wire 40a contracts so as to move the movable part 20 downwards away from the set position, the angle between the movement axis M and the drive members 41 of the top actuating unit decreases and the angle between the movement axis M and the drive members 41 of the bottom actuating unit increases. The balance of the components of the biasing forces along the movement axis M thus changes, such that the component along the movement axis M of the biasing force of one biasing element 42 (the top biasing element 42 in Figure 7) becomes greater than the component along the movement axis M of the biasing force of another biasing element 42 (the bottom biasing element 42 in Figure 7). An overall lateral force, equal to the difference in the components along the movement axis M of the biasing forces applied by the biasing elements 42, is thus applied to the movable part 20. The overall lateral force urges the movable part 20 further away from the set position, thereby applying an anti-centring force to the movable part 20. The lateral force is thus applied to the movable part 20 to urge the movable part 20 away from the set position in a manner similar to that described with reference to Figure 6. The biasing element 42 may thus be configured, for example the spring rate of the biasing force may be set, as described with reference to Figure 6. Figure 8 shows another embodiment of the actuator assembly 1, comprising four actuating units of the type described with reference to Figure 7. The movable part 20 is movable within a range of movement along two orthogonal axes, i.e. along the vertical and along the horizontal axes in Figure 7. The four actuating units are arranged to be coplanar with a plane parallel to the two orthogonal axes. The four actuating units are arranged in two pairs of opposing actuating units. One pair of actuating units (the left and right actuating units) apply actuating forces F to the movable part 20 so as to move the movable part 20 along one of the two orthogonal axes (the horizontal axis in Figure 7). The other pair of actuating units (the top and bottom actuating units) apply actuating forces F to the movable part 20 so as to move the movable part 20 along the other of the two orthogonal axes (the vertical axis in Figure 7). The arrangement of actuating units applies an anti-centring force to the movable part 20 as described in relation to Figure 7, except that the anti-centring force may be applied in any direction within the plane spanned by the two orthogonal axes. Figure 9 shows another embodiment of the actuator assembly 1, comprising four actuating units of the type described with reference to Figure 7. The movable part 20 may be suspended on the support structure 10 by the four actuating units. The actuator assembly 1 may not comprise a bearing arrangement between the support structure 10 and the movable part 20. The movable part 20 may be driven to translationally move along a movement axis M. When viewed along the movement axis M, the four actuating units are arranged in a loop around four sides of the movable part 20. The actuating units generally extend in planes that are parallel to the movement axis M. Two of the actuating units (e.g. two actuating units arranged on opposite sides of the movable part 20 when viewed along the movement axis M) apply the actuating force in a first direction along the movement axis M, and the other two of the actuating units (e.g. the other two actuating units arranged on opposite sides of the movable part 20 when viewed along the movement axis M) apply the actuating force in a second direction, opposite to the first direction, along the movement axis M. The actuator arrangement 40 comprising the four actuating units may thus effect movement of the movable part 20 along the movement axis M. The anti-centring force may be applied to the movable part 20 generally as described in relation to Figure 7. The movable part 20 of the embodiment shown in Figure 9 may, in addition or as an alternative to movement along the movement axis M, be rotatable about two orthogonal axes Rx, Ry. The two orthogonal axes Rx, Ry are perpendicular to the movement axis M in Figure 9. For example, actuation of a first actuating unit (e.g. the bottom right actuating unit) may drive rotation of the movable part in a first sense (anti-clockwise) about a first axis Rx and actuation of a second actuating unit (e.g. the top left actuating unit) may drive rotation of the movable part in a second sense (clockwise) that is opposite to the first sense about the first axis Rx. Actuation of a third actuating unit (e.g. the bottom left actuating unit) may drive rotation of the movable part in a first sense (anti-clockwise) about a second axis Ry and actuation of a fourth actuating unit (e.g. the top left actuating unit) may drive rotation of the movable part in a second sense (clockwise) that is opposite to the first sense about the second axis Ry. The anti-centring force may be applied to the movable part 20 generally as described in relation to Figure 7. In the embodiments of Figures 6 to 9, the drive members 41 acts as a common component that applies both the actuating force F and the lateral force to the movable part 20. The drive members 41 that are provided for the purpose of applying the actuating force F are used for the additional purpose of applying the lateral force. The implementation of a counterbalance force is thus simplified compared to providing an additional counterbalance spring for the sole purpose of providing the counterbalance force. Modifications and Alternatives The actuator assemblies 1 described in relation to Figures 4 to 7 allow movement of the movable part 20 relative to the support structure 10 in one degree of freedom, such as along a movement axis M or generally along a movement path. In general, the present invention may be implemented for movement of the movable part 20 relative to the support structure 10 in multiple degrees of freedom. For example, many of the embodiments described in relation to Figures 4 to 7 may additionally allow movement of a movable part 20 in a direction into and out of the page of the figures. With particular reference to Figure 4A, for example, the actuator assembly 1 may comprise a pair of SMA elements 40a in addition to the depicted pair of SMA elements 40a that on actuation drives movement of the movable part 20 along a second axis (into the page of Figure 4A) that is orthogonal to the movement axis M that is shown. The biasing arrangement 30 and the pivot arm 31 may be used to apply the lateral force (alongside the normal force) away from the set position also when the movable part 20 is arranged along the second axis. The embodiments of Figures 4B, 5, 6 and 7 may similarly allow movement of the movable part 20 along such a second axis. The specific embodiments of Figures 4A, B and 5 show a single pivot arm 31 and a biasing arrangement 30 comprising a single biasing element that apply both the normal force and the lateral force. In general, multiple pivot arms 31 and multiple biasing elements may be provided. For example, the single pivot arm 31 and a biasing arrangement 30 comprising a single biasing element of the embodiment of Figure 4A may be replaced with an arrangement of two pivot arms 31 and two biasing elements, arranged basically in the manner of the drive rods 41 and biasing elements 42 described in relation to Figure 6, except for arranging the drive rods 41 to apply the normal force to urge the first and second friction surfaces lOf, 20f together (rather than apart). So, the single pivot arm 31 in Figure 4A may be replaced with two pivot arms arranged, when the movable part 20 is at the set position, at equal and opposite angles to the movement axis M. A respective biasing element may apply a biasing force to each of the two pivot arms. The specific embodiment of Figure 6 shows a pair of drive rods 41 and a pair of biasing elements 42 to apply both the actuating force and the lateral force. In general, a single drive rod 41 and a single biasing element 42 may be provided. For example, the two drive rods 41 and the two biasing elements 42 of Figure 6 may be replaced by a single drive rod 41 and a single biasing element 42, arranged basically in the manner of pivot arm 31 and biasing arrangement 30 described in relation to Figure 4A, except for arranging the drive rod 31 to apply the normal force to urge the first and second friction surfaces lOf, 20f apart (rather than together). The SMA elements 40a or other actuator 40a may, on actuation, drive rotation of the drive rod 31 in opposite senses, so as to drive movement of the movable part 20 in opposite directions. Another way to put this is that in the embodiment of Figure 4A, the SMA elements 40a may be coupled (indirectly or directly) to the pivot arm 31 so as to drive rotation of the pivot arm 31, thereby effecting movement of the movable part 20. The present invention has been described in connection with SMA wires 40a. The term 'SMA wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. In general, actuators 40a other than SMA elements 40a may be used to drive movement of the movable part 20 relative to the support structure 10. For example, voice coil motors or piezoelectric actuators may be used to apply the actuating force F or input force Fi described as being applied by the SMA elements 40a in relation to the specific embodiments. The foregoing has described some embodiments of the present invention, but the present invention is not limited to these embodiments. The scope of the invention is defined in the appended claims.

Claims

1. An actuator assembly comprisingfirst and second parts that are movable relative to each other along at least one movement path in at least one degree of freedom of movement;an actuator arrangement comprising at least one actuator configured, on actuation, to cause an actuating force to be applied to the second part so as to move the second part relative to the first part along the at least one movement path within a range of movement;a biasing arrangement configured to:apply, when the second part is positioned relative to the first part within the range of movement and at positions that are located along the at least one movement path and on opposite sides from a set position, a lateral force to the second part acting in a direction along the at least one movement path and away from the set position;wherein at least one common component is configured to apply to the second part both i) the lateral force and ii) the actuating force or a normal force for loading a bearing arrangement that guides the movement of the second part relative to the first part along the at least one movement path.

2. An actuator assembly according to claim 1, wherein the at least one common component comprises a common rigid element.

3. An actuator assembly according to claim 2, wherein the common rigid element is configured to be in compression when applying the lateral force.

4. An actuator assembly according to claim 2 or 3, wherein the common rigid element is elongate between first and second ends, wherein the first end engages the second part so as to move with the second part and wherein the second end is constrained from moving with the second part.

5. An actuator assembly according to any one of claims 2 to 4, wherein the common rigid element is a pivot arm configured to pivot relative to the second part on movement of the second part relative to the first part.

6. An actuator assembly according to any one of the preceding claims, further comprising a bearing arrangement configured to guide movement of the second part relative to the first part, wherein the biasing arrangement is further configured to apply the normal force for loading the bearing arrangement, wherein the normal force is perpendicular to the lateral force, and wherein the at least one common component comprises a common biasing element configured to apply both the normal force and the lateral force to the second part.

7. An actuator assembly according to claim 6 when dependent on any one of claims 2 to 5, wherein the common biasing element applies a biasing force to the common rigid element, wherein the biasing force comprises a first component perpendicular to a direction of movement of the second part relative to the first part, the first component corresponding to the normal force, and wherein the biasing force comprises a second component parallel to the direction of movement of the second part relative to the first part, the second component corresponding to the lateral force.

8. An actuator assembly according to claim 7 or according to claim 6 when dependent on any one of claims 2 to 5, wherein the common biasing element and the common rigid element overlap when viewed in a direction perpendicular to the normal force.

9. An actuator assembly according to claim 7 or 8 or according to claim 6 when dependent on any one of claims 2 to 5, wherein the common biasing element is arranged in mechanical series with the common rigid element.

10. An actuator assembly according to claim 9, wherein the common biasing element engages the first part at one end and the common rigid element at another end, and wherein the common rigid element engages the common biasing element at one end and the second part at another end.

11. An actuator assembly according to claim 7 or 8 or according to claim 6 when dependent on any one of claims 2 to 5, wherein the second part is arranged between the common biasing element and the common rigid element.

12. An actuator assembly according to claim 11, wherein the common biasing element engages the first part at one end and a part comprising a first bearing surface at another end, wherein the second surface is arranged on the second part, and wherein the second part is arranged betweenthe part comprising the first bearing surface and the common rigid element, and wherein the common rigid element engages at one end the second part and at another end the first part.

13. An actuator assembly according to any one of claims 1 to 5, wherein the biasing arrangement comprises a primary biasing element configured to apply a normal force to the second part, and wherein the actuator assembly further comprises at least one secondary biasing element configured to apply the lateral force, wherein the at least one common component is configured to apply both the actuating force and the lateral force to the second part.

14. An actuator assembly according to any one of the preceding claims, wherein the actuator arrangement comprises first and second actuators and first and second drive members that are rotatable relative to the second part, and wherein the first actuator is configured to apply a respective input force to the first drive member so as to drive rotation of the first drive member, thereby applying a respective actuating force for moving the second part in a first direction and the second actuator is configured to apply a respective input force to the second drive member so as to drive rotation of the second drive member, thereby applying a respective actuating force for moving the second part in a second direction that is opposite to the first direction.

15. An actuator assembly according to claim 14 when dependent on claim 13, comprising two secondary biasing elements, wherein one of the two secondary biasing elements is configured to apply a respective biasing force to the first drive member and another of the two secondary biasing elements is configured to apply a respective biasing force to the second drive member.

16. An actuator assembly according to claim 14 or 15, wherein the first drive member comprises a first pair of drive members and wherein the second drive member comprises a second pair of drive members, wherein the first actuator is connected between first ends of the first pair of drive members and wherein the second actuator is connected between first ends of the second pair of drive members, wherein the second ends of the first pair of drive members are rotatably coupled to each other and to the second part and wherein the second ends of the second pair of drive members are rotatably coupled to each other and to the second part.

17. An actuator assembly according to any one of the preceding claims, wherein the biasing arrangement is configured to apply the normal force to the second part so as to bias first and second friction surfaces against each other to thereby generate a static frictional force between the first andsecond friction surfaces that constrains movement between the first and second parts at any position within the range of movement when the actuator is not actuated.

18. An actuator assembly according to claim 17, wherein the actuator arrangement is configured, on actuation, to reduce the normal force between the first and second friction surfaces, thereby reducing the static frictional force therebetween.

19. An actuator assembly according to any one of the preceding claims, wherein the actuator arrangement comprises at least two actuators configured, on actuation, to cause opposing actuating forces to be applied to the second part so as to move the second part in opposite directions relative to the first part.

20. An actuator assembly according to any one of the preceding claims, wherein the or each actuator of the actuator arrangement comprises a shape memory alloy, SMA, element.

21. An actuator assembly according to any one of the preceding claims, wherein the actuator arrangement comprises an intermediate part arranged between the actuator and the second part, wherein the intermediate part is configured to amplify the actuation amount of the actuator to a relatively greater amount of movement of the second part relative to the first part.

22. An actuator assembly according to any one of the preceding claims, wherein the biasing arrangement comprises a resilient element, such as a spring element.

23. An actuator assembly according to any one of the preceding claims, wherein the first and second parts are translationally movable relative to each other in one or more degrees of freedom.

24. An actuator assembly according to any one of the preceding claims, wherein the first and second parts are rotationally movable relative to each in at least two degrees of freedom about at least two orthogonal axes.

25. An actuator assembly according to any one of the preceding claims, comprising an external load coupled to the second part, wherein the external load is configured to apply a load biasing force urging the second part towards the set position when the second part is positioned away from the set position, and wherein the biasing arrangement is configured at least partially to compensate forthe load biasing force by opposing the load biasing force at any position of the second part within the range of movement.

Images