Actuator assembly

Abstract

An actuator assembly comprises first 10 and second 20 parts that are movable relative to each other, each comprising a mutually engageable friction surface 10f, 20f, and a biasing arrangement 30 confi

Description

Field The present application generally relates to an actuator assembly, for example an actuator assembly comprising shape memory alloy elements as actuators. The present invention also relates to a method for driving actuation of the actuator assembly. 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 element required to apply a given force to the movable element can be relatively small. The movable component may be retained in position by friction. The movable component is biased against a surface of a support structure by a biasing element to hold the movable part in any given position when SMA wires are not energised. Zero hold power functionality may thus be achieved. Upon actuation, the SMA wires act against the biasing element to reduce frictional forces, thereby enabling the movable component to be driven to a new position. WO2020 / 120997 Al and WO2023 / 094813 Al, the disclosures of which are herein incorporated by reference, disclose various actuator assemblies with zero hold power functionality. Summary According to the present invention, there is provided an actuator assembly comprising first and second parts that are movable relative to each other, the first part comprising a first friction surface and the second part comprising a second friction surface; a biasing arrangement configured to apply a normal force biasing the first and second friction surfaces against each other, thereby giving rise to a static frictional force that is capable of constraining relative movement of the first and second parts; an intermediate part configured selectively to engage the second part or to disengage from the second part; one or more SMA elements configured, when the intermediate part engages the second part, on actuation to cause actuation forces to be applied to the intermediate part to thereby i) cause reduction of the normal force between the first and second friction surfaces so as to reduce the static frictional force and ii) move the second part within a sub-range of movement relative to the first part; wherein, when the intermediate part is disengaged from the second part, the intermediate part is movable relative to the second part. The second part may be held in position by the static frictional force when the SMA elements are unpowered, thereby decreasing power consumption of the actuator assembly compared to an actuator assembly that does not enable such holding. The second part may further be moved relative to the first part via the intermediate part, and the intermediate part may be positioned relative to the second part. Successive movement of the second part and repositioning of the intermediate part may extend the range of movement of the second part compared to an actuator assembly in which the SMA elements act directly on the second part or in which the intermediate part can not disengage from the second part. The second part may be movable within an overall range of movement relative to the first part. The overall range of movement may be greater than the sub-range of movement, for example by a factor greater than 2 or greater than 4. The second part may be movable within the overall range of movement by repeatedly moving the second part within the sub-range of movement and moving or allowing movement of the intermediate part relative to the second part. Movement of the intermediate part relative to the second part may effectively reposition the sub-range of movement of the second part relative to the first part. The sub-range of movement within which the second part is movable on actuation of the SMA wires, when the intermediate part engages the second part, changes on repositioning of the intermediate part relative to the second part. The intermediate part may be actively moved relative to the second part by the SMA elements or may be passively moved relative to the second part by a biasing element when the SMA elements cease actuating. The intermediate part may engage with the second part by contacting the second part with a contact force that is above a threshold force, thereby giving rise to frictional forces between the intermediate part and the second part that constrain relative movement between the intermediate part and the second part. The intermediate part may disengage from the second part by reducing the contact force below the threshold force such that the intermediate part is movable, e.g. slidingly movable, relative to the second part. The intermediate part, when disengaged from the second part, may remain in contact with the second part or may be brought out of contact with the second part. In some embodiments, the one or more SMA elements are configured, when the intermediate part engages the second part, on actuation to increase a contact force between the intermediate part and the second part. Engagement between the intermediate part and the second part may thus be strengthened on actuation of the SMA element, reducing the risk of undesirable movement of the intermediate part relative to the second part when in engagement. Some embodiments further comprise one or more biasing elements configured, when the one or more SMA elements are not actuated, to urge the intermediate part to disengage from the second part. The SMA elements, on actuation, may urge the intermediate part into engagement with the second part. The SMA elements may thus cause engagement of the intermediate part with the second part on actuation and may cause disengagement of the intermediate part with the second part on ceasing actuation. Some embodiments further comprise one or more biasing elements configured to urge the intermediate part into engagement with the second part. The intermediate part may be caused to disengage from the second part due to actuation of the SMA elements or due to actuation of an additional actuator, for example. The one or more biasing elements may be any elements capable of applying a biasing force to the second part. The one or more biasing elements may, for example, comprise compression or tension springs, flexures or other resilient or elastic elements. Alternatively, the one or more biasing elements may comprise a magnet and a ferromagnetic material or another magnet. In some embodiments, the one or more biasing elements are further configured, when the one or more SMA elements are not actuated, to position the intermediate part at a set position relative to the first part. The biasing elements may thereby reposition the intermediate part relative to the first part. The set position may be a central position within the range of movement of the intermediate part relative to the first part. The same biasing element may be configured to position the intermediate part at the set position and to urge the intermediate part into or out of engagement with the second part. Alternatively, separate biasing elements may be used i) to position the intermediate part at the set position and ii) to urge the intermediate part into or out of engagement with the second part. Some embodiments further comprise an additional actuator configured, on actuation, to disengage the intermediate part from the second part. The additional actuator may be an additional SMA element. The intermediate part may be brought into engagement with the second part due to one or more biasing elements or due to actuation of the SMA elements. In some embodiments, the one or more SMA elements are configured, when the intermediate part is disengaged from the second part, on actuation to move the intermediate part relative to the second part. Some embodiments comprise one or more endstops configured to limit the range of movement of the intermediate part relative to the first part, wherein the one or more SMA elements are capable on actuation to drive movement of the intermediate part to the limit of the range of movement of the intermediate part relative to the first part so as to engage the one or more endstops. The one or more endstops may be between the intermediate part and the first part. Each endstop may comprise an endstop surface on the intermediate part opposing a corresponding endstop surface on the first part, wherein opposing endstop surfaces are brought into contact on engagement so as to limit the range of movement of the intermediate part relative to the first part. Some embodiments comprise one or more endstops configured to limit the range of movement of the intermediate part relative to the second part, wherein the one or more SMA elements are capable on actuation to drive movement of the intermediate part to the limit of the range of movement of the intermediate part relative to the second part so as to engage the one or more endstops. The one or more endstops may be between the intermediate part and the second part. Each endstop may comprise an endstop surface on the intermediate part opposing a corresponding endstop surface on the second part, wherein opposing endstop surfaces are brought into contact on engagement so as to limit the range of movement of the intermediate part relative to the second part. Some embodiments comprise a controller configured to generate drive signals for driving actuation of the one or more SMA elements. The controller may further be configured to generate drive signals for driving actuation of the additional actuator. In some embodiments, the controller is configured to: a) obtain a target position of the second part relative to the first part; b) determine if the target position is within a current sub-range of movement of the second part relative to the first part; c) in a case where the target position is within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part to the target position; d) in a case where the target position is not within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part in a first direction towards a limit of the current sub-range of movement and towards the target position, and then selectively disengage the intermediate part from the second part and control actuation of the SMA elements to allow or effect movement of the intermediate part relative to the second part in a second direction that is opposite to the first direction; and e) repeat steps b) to d) until the second part is at the target position. In some embodiments, the controller is configured to: a) obtain a target sub-range of movement of the second part relative to the first part; b) determine if the centre of the target sub-range of movement is within a current subrange of movement of the second part relative to the first part; c) in a case where the centre of the target sub-range of movement is within a current subrange of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part to the centre of the target sub-range of movement; and d) in a case where the centre of the target sub-range of movement is not within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part in a first direction and towards a limit of the current sub-range of movement that is towards the centre of the movement sub-range of movement, and then selectively disengage the intermediate part from the second part and control actuation of the SMA elements to allow or effect movement of the intermediate part relative to the second part in a second direction that is opposite to the first direction, and e) repeat steps b) to d) until the second part is at the centre of the target sub-range of movement. In some embodiments, the controller is configured to, in step d) in the case where the target position or the centre of the target sub-range of movement is not within the current sub-range of movement, selectively disengage the intermediate part from the second part and control actuation of the SMA elements to move intermediate part relative to the second part in the second direction beyond a centre of the range of movement of the intermediate part relative to the first part and to or towards a limit of the range of movement of the intermediate part relative to the first part. In some embodiments, the controller is configured to control actuation of the SMA elements so as to move the intermediate part such that one of the one or more endstops engages. Some embodiments further comprise a position sensor for measuring the actual position of the second part relative to the first part, wherein the controller is configured to generate the drive signals for driving actuation of the SMA elements in part based on a difference between the measured actual position of the second part relative to the first part and a target position of the second part relative to the first part. The position sensor may be a Hall sensor, for example. In some embodiments the second part is translationally movable relative to the first part. In some other embodiments the second part is rotatable relative to the first part. Some embodiments comprise a bearing arrangement arranged to guide the movement of the second part relative to the first part, wherein the bearing arrangement is separate from the first and second friction surfaces. The bearing arrangement may comprise a low-friction bearing. For example, the bearing arrangement may comprise a bearing guiding translation of the second part or guiding rotation of the second part. The bearing arrangement may be a rolling bearing, such as a ball bearing or roller bearing. The rolling bearing may comprise a first surface on the first part and a second surface on the second part and a rolling bearing element (such as a ball or roller) that is configured to roll across the first and second surfaces on relative movement thereof. Alternatively, the bearing arrangement may comprise the first and second friction surfaces such that the first and second friction surfaces guide movement of the second part relative of the first part. In some embodiments the intermediate part is translationally movable (i.e. linearly movable) relative to the first part. In some other embodiments the intermediate part is rotatable relative to the first part. The second part may be translationally movable (i.e. linearly movable) relative to the first part and the intermediate part may be rotatable relative to the first part. The engagement between the intermediate part and the second part may convert rotation of the intermediate part into translation of the second part. The second part may be rotatable relative to the first part and the intermediate part may be translationally movable relative to the first part. The engagement between the intermediate part and the second part may convert translation of the intermediate part into rotation of the second part. Some embodiments comprise at least two SMA elements configured to apply forces to the intermediate part with components in opposite directions so as to cause the second part to be moved in opposite directions. The position of the intermediate part (and the second part when engaged with the intermediate part) may more accurately be set compared to an actuator assembly with only one SMA element. Alternatively, the actuator assembly may comprise one SMA element configured to apply a force to the intermediate part with a component in a first direction and a resilient element, such as a spring, configured to apply a force opposing the force applied by the SMA element in the first direction. Some embodiments comprise a bearing arranged between the biasing arrangement and the second part so as to decouple the biasing force applied by the biasing arrangement from the movement of the second part. The bearing may be a rolling bearing, such as a ball bearing or roller bearing, for example. The rolling bearing may comprise a first surface fixed relative to the biasing arrangement and a second surface on the second part and a rolling bearing element (such as a ball or roller) that is configured to roll across the first and second surfaces on relative movement thereof. In some embodiments, the biasing arrangement is arranged between two portions of the first part, and the actuator assembly may further comprise one or more endstops between the two portions of the first part configured to limit relative movement of the two portions of the first part on actuation of the SMA elements. In some embodiments, the biasing arrangement is configured to apply a force couple in a first sense to the second part relative to the first part, thereby biasing the first and second friction surfaces against each other so as to give rise to the static frictional force, and the one or more SMA elements are configured, on actuation, to cause a force couple to be applied in a second sense to the second part relative to the first part, thereby urging the first and second friction surfaces apart so as to reduce the static frictional force. Some embodiments comprise a counterbalance arrangement configured, when the second part is positioned relative to the first part at positions that are located on opposite sides from a set position of the second part relative to the first part, to apply a lateral force to the second part acting in a direction away from the set position. Some embodiments comprise a common component configured to apply to the second part both the lateral force and the normal force. The actuator assembly may thus comprise fewer components compared to an actuator assembly in which the lateral force and the normal force are applied by separate components. The common component may be a common rigid element. The common component may be placed under compression by the biasing arrangement. The common component may be 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. The common rigid element may be a rod or a pivot arm configured to pivot relative to the second part on movement of the second part relative to the first part. The second part may optionally be coupled to an external load that applies a force to the second part urging the second part towards the set position. The lateral force may at least partially compensate for or counteract the force applied by the external load on the second part. In some embodiments, the intermediate part comprises a first engagement surface and the second part comprises a second engagement surface, wherein the first and second engagement surfaces are constrained from sliding relative to each other by frictional force when the intermediate part engages the second part. According to another aspect of the present invention, there is provided a method of driving actuation of the actuator assembly of any one of the preceding claims, the method comprising: engaging the second part with the intermediate part; when the intermediate part engages the second part, generating drive signals for controlling actuation of the SMA elements so as to cause actuation forces to be applied to the intermediate part to thereby i) cause reduction of the normal force between the first and second friction surfaces so as to reduce the static frictional force and ii) move the second part within a sub-range of movement relative to the first part; disengaging the intermediate part from the second part; and when the intermediate part is disengaged from the second part, generating drive signals for controlling actuation of the SMA elements so as to cause actuation forces to be applied to the intermediate part to thereby move the intermediate part relative to the second part. 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 achieving zero hold power and having an intermediate part for selectively engaging and driving movement of a movable part; Figure 2 schematically shows an actuator assembly of the type of Figure 1 with an additional decoupling bearing; Figure 3 schematically shows another actuator assembly of the type of Figure 1 with an additional counterbalance arrangement; Figure 4 schematically shows another actuator assembly achieving zero hold power and having an additional actuator for disengaging the intermediate part from the movable part; Figures 5a to 5f schematically show further actuator assemblies of the type of Figure 4; the actuator assemblies of Figures 5d to 5f additionally have endstops limiting the range of movement of the intermediate part; Figures 6a and 6b schematically show other actuator assemblies of the type of Figure 1 with an additional bearing separate from friction surfaces; Figures 7a to 7c schematically show other actuator assemblies achieving zero hold power and having a rotatable intermediate part for selectively engaging and driving movement of a movable part; Figures 8a to 8c schematically show other actuator assemblies with an intermediate part for selectively engaging a movable part and implementing torque loading of friction surfaces for achieving zero hold power; Figures 9a to 9e schematically show other actuator assemblies achieving zero hold power and having an intermediate part for selectively engaging and driving rotation of a rotatable part; Figure 10 schematically shows another actuator assembly achieving zero hold power and with an intermediate part selectively engaging the support structure for driving movement of a movable part; and Figures 11a to 11c schematically show other actuator assemblies achieving zero hold power and having an additional actuator coupled to the movable part for selectively disengaging an intermediate part. Detailed description Actuator assembly Figure 1 schematically shows 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 (unless stated otherwise), 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 comprises two SMA elements 40, in particular two SMA wires 40. The SMA elements 40 are configured, on actuation, to apply actuating forces to the movable part 20 capable of moving the movable part 20 relative to the support structure 10. In the depicted embodiment, the SMA elements 40 are connected between the support structure 10 and an intermediate part 50, and the SMA elements 40 apply actuating forces to the movable part 20 via the intermediate part 50. The SMA elements 40 are, directly or indirectly, connected between the support structure 10 and the intermediate part 50. The SMA elements 40 may be connected, directly or indirectly, to the support structure 10 and the intermediate part 50 by connection elements, such as crimps (not shown). Optionally, one or more additional intermediate parts may be connected between the SMA elements 40 and the intermediate part 50 and / or support structure 10. The SMA elements 40 may thereby, either directly or indirectly via one or more additional intermediate parts, cause actuating forces to be applied to the intermediate part 50. Selectively varying the actuating forces applied by the SMA elements 40 may cause the intermediate part 50 to move relative to the support structure 10. The SMA elements 40 are further capable, due to selective coupling of the intermediate part 50 and the movable part 20, of driving movement of the movable part 20 relative to the support structure 10. The actuator assembly 1 may include a bearing arrangement (shown in some embodiments as bearing arrangement 42, for example in Figures 6a, 6b or lib). Such a bearing arrangement may support the movable part 20 on the support structure 10 and guide movement of the movable part 20. The bearing arrangement 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). The bearing arrangement may cause the movable part 20 to move in directions which differ from the directions of the forces acting on the movable part 20 due to actuation of the SMA elements 40. Figure 1 shows an embodiment in which the movable part 20 is movable in one DOF, in particular along a movement axis M (horizontally in Figure 1). A bearing arrangement 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. A biasing arrangement 30 may load the bearing arrangement. The biasing arrangement 30 may include any element capable of applying a loading force to the bearing arrangement. In general, any of the biasing arrangements or biasing elements may comprise components capable of applying a biasing force, such as a resilient element such as a spring or flexure, an elastic material such as rubber, or a set of magnetic elements such as a magnet and a ferromagnetic material. The biasing arrangement 30 may load the bearing arrangement by biasing bearing surfaces of the bearing arrangement against each other (typically for a rolling bearing or a plain bearing) or by placing the bearing arrangement in tension (typically for a flexure bearing). In Figure 1, the bearing arrangement is embodied by surfaces lOf, 20f of the support structure 10 and the movable part 20, for example. The bearing arrangement may thus be a plain bearing. The biasing arrangement 30 is embodied by a compression spring that is connected between the movable part 20 and the support structure 10 so as to urge the movable part 20 against the surface lOf of the support structure 10. The movable part 20 may be slidingly movable across the surface lOf of the support structure 10. The bearing arrangement may include additional components (not shown) for constraining movement of the movable part 20, for example bearings that constrain movement into or out of the page of Figure 1. The actuator assembly 1 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 SMA elements 40 so as to actuate the SMA elements 40, for example by generating drive signals for contracting the SMA elements 40. 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 elements 40, thereby heating the SMA elements 40 by causing an electric current to flow, will cause the SMA elements 40 to contract and thus apply the actuating force so as to drive 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. Such a component to be moved may generally give rise to a load force (also referred to as an external load) acting on the movable part 20. 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, to move a projector or display in an augmented reality (AR) or virtual reality (VR) device or to move blades of a variable aperture (VA) assembly. 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 at any position within a range of movement of the movable part 20 relative to the support structure 10 when the SMA elements 40 are not actuated, e.g. when the SMA elements 40 are unpowered. The movable part 20 may remain in position without needing to power the SMA elements 40. 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 SMA elements 40 are continuously powered so as to keep a desired position of the movable part 20 relative to the support structure 10. In the actuator assembly 1 of Figure 1, the support structure 10 comprises a first friction surface lOf and the movable part 20 comprises a second friction surface 20f. The actuator assembly 1 further comprises the biasing arrangement 30 that biases the first and second friction surfaces lOf, 20f against each other with a normal force. For illustrative purposes, the first and second friction surfaces lOf, 20f are shown slightly spaced apart in Figure 1, but in practice the first and second friction surfaces lOf, 20f are in contact with each other when the SMA elements 40 are not actuated. 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 the range of movement of the movable part 20 relative to the support structure 10, for example when the SMA elements 40 are not actuated or when the intermediate part 50 is disengaged from the movable part 20. In the actuator assembly 1 of Figures 1, the SMA elements 40 are arranged such that, on actuation, the normal force between the first and second friction surfaces lOf, 20f is reduced. For example, when the two opposing SMA elements 40 are equally actuated, the movable part 20 may be urged (via the intermediate part 50) in a direction perpendicular to the first and second friction surfaces lOf, 20f (upward in Figure 1), such that the normal force between the first and second friction surfaces lOf, 20f is reduced. The force applied by the SMA elements 40 (e.g. the tension in the SMA elements 40) comprises a component that is normal to the first and second friction surfaces lOf, 20f and thereby reduces the normal force between the first and second friction surfaces lOf, 20f. 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 SMA elements 40 may thus be reduced, such that the SMA elements 40 may more effectively move the movable part 20 and / or a larger normal force may be applied when the SMA elements 40 are not actuated. Friction surfaces may generally be provided, for example in addition, between any two components of the actuator assembly 1 that are in engagement and move relative to each other. For example, friction surfaces may be arranged between any other intermediate part and the support structure 10, between any other intermediate part and the movable part 20, or between any two other intermediate parts that are coupled between the movable part 20 and the support structure 10. In general, the normal force is reduced on actuation of the SMA elements 40. WO 2020 / 120997 Al and WO 2023 / 094813 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 SMA elements 40 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. Intermediate part for movement of movable part The intermediate part 50 may selectively engage or disengage the movable part 20. Figure 1 shows a situation in which the intermediate part 50 engages the movable part 20. In the actuator assembly 1 of Figure 1, the SMA elements 40 are actuated to thereby urge the intermediate part 50 against the movable part 20 (upwards in Figure 1). The movable part 20 comprises an engagement surface 20e that is configured to engage with a corresponding engagement surface 50e of the intermediate part 50. The engagement surfaces 20e, 50e may be complementary surfaces, i.e. the engagement surfaces 20e, 50e may be parallel to each other so as to be in direct contact. In the depicted embodiment, the engagement surfaces 20e, 50e are flat (extending into the page of Figure 1), although in general the engagement surfaces 20e, 50e may be curved in complementary manner or have an any other shape allowing engagement of the engagement surfaces 20e, 50e. On actuation of the SMA elements 40, the engagement surfaces 20e, 50e engage and a contact force between the engagement surfaces 20e, 50e increases. When engaged, the engagement surfaces 20e, 50e may remain in position relative to each other. In the depicted embodiment, the engagement surfaces 20e, 50e remain in position relative to each other due to friction between the engagement surfaces 20e, 50e, allowing engagement at any relative position of the engagement surfaces 20e, 50e. In general, however, the engagement surfaces 20e, 50e may comprise any other features, such as complementary teeth, for constraining movement between the engagement surfaces 20e, 50e when in engagement. When in engagement, the movable part 20 is effectively coupled to the intermediate part 50 so as to move with the intermediate part 50. As the contact force between intermediate part 50 and movable part 20 increases, the movable part 20 is urged away from the support structure 10 (upward in Figure 1) such that the normal force and thus the static frictional force between the friction surfaces lOf, 20f decreases. The force required to move the movable part 20 relative to the support structure 10 thus decreases, enabling faster and / or more reliable movement of the movable part 20 on actuation of the SMA elements 40. On differential actuation of the SMA elements 40, the intermediate part 50 (and so also the movable part 20 engaged by the intermediate part 50) may move relative to the support structure 10. With particular reference to Figure 1, for example, when the right SMA wire 40 is actuated (i.e. contracts) by a greater amount than the left SMA wire 40, the intermediate part 50 and the movable part 20 are moved rightward relative to the support structure 10. The movable part 20 may thus effectively and accurately be moved relative to the support structure 10 via engagement with the intermediate part 50. When the intermediate part 50 is disengaged from the movable part 20, the intermediate part 50 may move relative to the movable part 20. So, the movable part 20 may be decoupled from the intermediate part 50. In the embodiment of Figure 1, when actuation of the SMA elements 40 is reduced or ceases, the intermediate part 50 is urged away from the movable part 20 (downward in Figure 1) by one or more biasing elements 52. The contact force between the engagement surfaces 20e, 50e is reduced, for example to zero (i.e. the engagement surfaces 20e, 50e are no longer in contact) or to a relatively low magnitude allowing sliding or other movement of the engagement surfaces 20e, 50e relative to each other. In the particular embodiment of Figure 1, the biasing elements 52 further urge the intermediate part 50 to a set position relative to the support structure 10. The set position may be a central position within a range of movement of the intermediate part 50 relative to the support structure 10, for example. The range of movement of the intermediate part 50 may be defined by optionally provided endstops 54 or be due to a threshold actuation amount of the SMA elements 40 (e.g. due to a maximum strain of the SMA elements 40 or a maximum power that may be applied to the SMA elements 40). Such a threshold actuation amount may for example be predetermined and set so as to ensure reliable operation of the actuator assembly 1. The actuator assembly 1 may optionally comprise endstops 54 that limit the range of movement of the intermediate part 50 relative to the support structure 10. Each endstop 54 comprises an endstop surface on the intermediate part 50 and a corresponding endstop surface on the support structure 10. The endstop surfaces may be spaced apart, i.e. the endstop may be disengaged, when the intermediate part 50 is at the set position. The endstops 54 may for example define the limit of the range of movement of the intermediate part 50 relative to the support structure 10. The SMA elements 40 may, on actuation, drive movement of the intermediate part 50 to a limit of the range of movement so as to engage the respective endstop 54. The actuator assembly 1 may optionally comprise endstops between the movable part 20 and the support structure 10 or endstops between the movable part 20 and the intermediate part 50. In general, any such endstops comprise an endstop surface on one part and a corresponding endstop surface on the other part. The endstops may generally define the limit of the range of movement of the parts relative to each other. Operation of the actuator assembly The actuator assembly 1 may effect movement of the movable part 20 by the SMA elements 40 moving the intermediate part 50 when in engagement with the SMA elements 40. In particular, when the intermediate part 50 is engaged by the movable part 20, actuation of the SMA elements 40 may move the movable part 20 within a sub-range of movement relative to the support structure 10. In the actuator assembly 1 of Figure 1, the sub-range of movement has an extent equal to the range of movement of the intermediate part 50 relative to the support structure 10. In general, however, the sub-range of movement of the movable part 20 may have a different relation (e.g. be a fraction or multiple thereof) to the range of movement of the intermediate part 50, for example when the movable part 20 or the intermediate part 50 are rotatable (as for example in the actuator assemblies 1 of Figures 7 and 9). The movable part 20 may be movable relative to the support structure 10 within an overall range of movement that is greater than the sub-range of movement, in particular by successively actuating or stepping the movable part 20 through more than one sub-range of movement. When the intermediate part 50 is disengaged from the movable part 20, the intermediate part 50 may be repositioned relative to the movable part 20. Such repositioning of the intermediate part 50 relative to the movable part 20 effectively achieves a repositioning the sub-range of movement within which the movable part 20 is movable relative to the support structure 10. The sub-range of movement within which the movable part 20 is movable relative to the support structure 10 when engaged with the intermediate part 50 is herein also referred to as the current sub-range of movement. In the embodiment of Figure 1, in order to move the movable part 20, the SMA elements 40 may initially actuate (e.g. by equal amounts) so as to engage the movable part 20 with the intermediate part 50. The SMA elements 40 may then be differentially actuated so as to move the intermediate part 50 and the movable part 20 coupled thereto. On ceasing actuation of the SMA elements 40, the intermediate part 50 may disengage from the movable part 20 and the friction surfaces lOf, 20f may reengage so as to hold the movable part 20 in position (e.g. towards the right of the position in Figure 1) relative to the support structure 10. The intermediate part 50 may be repositioned relative to the movable part 20, for example to a central position by the biasing elements 52. On subsequent actuation of the SMA elements 40, the intermediate part 50 may reengage the movable part 20 and may for example move the movable part 20 further rightward. The sub-range of movement of the movable part 20 relative to the support structure 10 has thus been effectively moved rightward. The movable part 20 may be moved further than allowed by the sub-range of movement by such iterative movement of the movable part 20, such that the movable part 20 may be moved within an overall range of movement relative to the movable part 20. The overall range of movement may be a multiple of the sub-range of movement, for example a multiple greater than 1, preferably greater than 2 or greater than 4. The movable part 20 may thus, through repeated engagement and disengagement with the intermediate part 50, step across a larger overall range of movement than a movable part 20 that is directly driven by SMA elements 40. Movement of the movable part 20 may be effected, for example, in response to the controller obtaining a target position of the movable part 20 relative to the support structure 10. The target position may in general be any measure indicative of where the movable part 20 should be located, for example a specific position that achieves a desired degree of focus when the movable part 20 is used to move a lens along the optical axis. To move the movable part 20 to the target position, the intermediate part 50 may engage the movable part 20 and the SMA elements 40 may move the movable part 20 to the target position if the target position is within the current sub-range of movement, or else move the movable part 20 in a first direction to or close to the limit of the current sub-range of movement in the direction of the target position. The SMA elements 40 may then be unpowered so as to cease actuating and the intermediate part 50 may be disengaged from the movable part 20. The intermediate part 50 may then be moved (e.g. due to the biasing elements 52) in a second direction opposite to the first direction relative to the movable part 20. If the movable part 20 is not yet at the target position, the intermediate part 50 may re-engage the movable part 20 such that the movable part 20 is moved closer or to the target position. These steps may be repeated until the movable part 20 is at the target position. Alternatively or additionally, it may be desirable to select a sub-range of movement within which the movable part 20 is desirably movable relative to the support structure 10. The controller may determine that the movable part 20 is expected to be moved (e.g. in a selected operating mode) within a sub-range of movement that is different to a current sub-range of movement. Movement of the movable part 20 may thus be effected in response to the controller obtaining a target subrange of movement of the movable part 20 relative to the support structure 10. The target subrange of movement may in general be any measure indicative of a range within which the movable part 20 is expected to be moved, for example a range of movement that achieves a desired range of focus for a selected mode of operation of a camera when the movable part 20 is used to move a lens along the optical axis. To move the location of the sub-range within the overall range of movement of the movable part 20, the SM A elements 40 may be actuated so as to move the movable part 20 to the centre of the selected or desired sub-range if that is within the current sub-range, or else to or close to the limit of the current sub-range in the direction of the centre of the selected or desired sub-range. If the movable part 20 is not yet at the centre of the selected or desired sub-range, the intermediate part 50 may disengage from and be repositioned relative to the movable part 20, and the steps may be repeated until the movable part 20 is at the centre of the selected or desired sub-range. When the movable part 20 is at the centre of the new sub-range and the intermediate part 50 is also at the centre of the wire stroke (e.g. the set or known position relative to the movable part 20), the SMA elements 40 may drive movement of the movable part 20 within the desired sub-range without disengaging the engagement surfaces 20e, 50e. As such, a sub-range of movement can be selected and once the sub-range has been reached movements within that sub-range can be achieved without further disengagement of the intermediate part 50, thereby avoiding the risk of any positioning errors accumulating due to engagement and disengagement of the intermediate part 50. Decoupling biasing force from movement of movable part Figure 2 shows another actuator assembly 1. The actuator assembly 1 is similar to the actuator assembly 1 described in relation to Figure 1, except that the biasing arrangement 30 comprises a decoupling bearing 32, in particular a rolling bearing or ball bearing, via which the biasing force is applied to the movable part 20. The decoupling bearing 32 may optionally act as the bearing arrangement that guides movement of the movable part 20 relative to the support structure 10, thereby having the dual purpose of guiding movement of the movable part 20 and decoupling the biasing force from such movement. As also shown in Figure 2, the support structure 10 may be considered to comprise 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 the vertical direction in Figure 2) in a direction perpendicular to the movement of the movable part 20. However, in practice, movement of the first and second portions 10a, 10b relative to each other need not take place or may be negligible 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 an additional part 10b (such as a sprung part 10b) of the support structure 10. In general, movement of the first and second portions 10a, 10b in a direction along the movement of the movable part 20 is constrained. The decoupling bearing 32 comprises a ball bearing arranged between the movable part 20 and the support structure 10, in particular the second portion 10b thereof. The ball bearing guides movement of the movable part 20. 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, i.e. the biasing force is transmitted to the ball bearing by the second portion 10b. The biasing force acts in a direction perpendicular to the movement of the movable part 20. The biasing arrangement 30 is thus effectively decoupled from the movement of the movable part 20 by the bearing 32. Compared to the actuator assembly 1 of Figure 1, for example, the biasing arrangement 30 of Figure 2 does not apply any lateral force components to the movable part 20 even when the movable part 20 is displaced from a central position relative to the support structure 10. The biasing arrangement 30 in Figure 2 does not deform in a direction parallel to the movement of the movable part 20. The biasing arrangement 30 does not apply any force opposing movement of the movable part 20, such that the effectiveness of actuation of the SMA elements 40 is improved. The SMA elements 40 in either of the actuator assemblies 1 of Figures 1 and 2 may be arranged, on actuation, to disengage the friction surfaces lOf, 20f. In Figure 1, the SMA elements 40 may lift the movable part 20 off the bearing arrangement and so off the friction surface lOf on the support structure 10. The SMA elements 40 may thus suspend the movable part 20 from the support structure 10 when actuated. In Figure 2, the SMA elements 40 may similarly lift the movable part 20 off the friction surface lOf on the support structure 10. In Figure 2, the SMA elements 40 are arranged, on actuation, to further load the decoupling bearing 32, i.e. the SMA elements 40 are arranged to apply a force with a force component urging the movable part 20 towards the decoupling bearing 32. Optionally, the actuator assembly 1 of Figure 2 may comprise one or more endstops 34 for limiting the amount of movement of the second portion 10b of the support structure 10 relative to the first portion 10a thereof. Each endstop 34 comprises a pair of endstop surfaces that are spaced apart when the SMA elements 40 are not actuated. A first endstop surface is provided on the first portion 10a of the support structure 10 and a second endstop surface is provided on the second portion 10b of the support structure 10. The SMA elements 40 may be configured, on actuation, to disengage the first and second friction surfaces lOf, 20f and to engage the endstop 34. With particular reference to Figure 2, the movable part 20 may be moved upwards, i.e. in a direction towards the decoupling bearing 32, on actuation of the SMA elements 40, thereby moving the second portion 10b towards the endstop 34 so as ultimately to engage the endstop 34. Movement of the movable part 20 may then be guided by the decoupling bearing 32. Counterbalance force 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. 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 arrangement may be included in the actuator assembly 1 to counteract centring forces that are expected to act on the movable part 20. The counterbalance arrangement may be embodied by a counterbalance spring, for example, which 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 movementof a component, such as an electrical component. A counterbalance arrangement can be pre-emptively included in the actuator assembly 1. Figures 3, 7b, 8b and lib show examples of actuator assemblies 1 comprising a counterbalance arrangement. In general, a counterbalance arrangement may be provided in any of the actuator assemblies 1 disclosed herein. Figure 3 shows an actuator assembly 1 that is similar to the actuator assembly 1 of Figures 1 and 2, except that a pivot arm 31 is arranged in mechanical series with the biasing arrangement 30. The actuator assembly 1 drives movement of the movable part 20 via the intermediate part 50 and achieves zero hold power functionality as described in relation to Figure 1. The actuator assembly 1 of Figure 3 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 (e.g. via another rigid part) 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 urge the friction surfaces lOf, 2Of together. When the movable part 20 is at a set position relative to the support structure 10, such as at a central position or starting position (e.g. where an external load is zero), the pivot arm 31 may be arranged so as to be perpendicular to the movement of the movable part 20 relative to the support structure 10. With particular reference to Figure 3, the pivot arm 31 may be vertical in the set position, while the movable part 20 moves horizontally. When the movable part 20 is moved away from the set position, for example on differential actuation of the SMA elements 40, the pivot arm 31 pivots 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 30 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 3, 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 opposite senses on movement of the movable part 20 in opposite 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 (i.e. the biasing force of the biasing arrangement 30 transmitted by the pivot arm 31) has a component perpendicular to the movement axis M (corresponding to the normal force urging the friction surfaces lOf, 20f together) 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 20 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 3, 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. However, in general, the actuator assembly 1 may comprise a dedicated counterbalance spring that is separate from the biasing arrangement 30 and is used to provide the lateral force. Figure lib, for example, shows an example of a counterbalance spring that is separate from the biasing arrangement 30. Additional SMA element for disengaging intermediate part from movable part The intermediate part 50 of the actuator assemblies 1 of Figures 1 to 3 returns to a set position, due to the biasing elements 52, when the SMA elements 40 cease actuating. In some situations, it may be preferable to avoid such returning to a set position. For example, in some situations, returning the intermediate part 50 to the set position using biasing elements 52 may introduce uncertainty in the exact position of the intermediate part 50 and so introduce a position error in the position of the movable part 20. This may be because upon returning to the set position, the set position is dependent on the balancing of forces between the biasing elements, frictional forces and any external forces acting on the intermediate part 50. Variation in the set position may accumulate as positional errors of the movable part 20 if the controller cannot measure or otherwise determine or reset the actual position. Furthermore, the distance that the movable part 20 may be moved in a desired direction while engaged with the intermediate part 50 may be limited effectively to half of the range of movement of the intermediate part 50 relative to the support structure 10 (i.e. the distance from the centre of the range of movement of the intermediate part 50 to the limit of the range of movement thereof). Figure 4 shows an embodiment of the actuator assembly 1 in which the intermediate part 50 is not returned to a set position by a biasing element 52. The actuator assembly 1 may effect zero hold power functionality and movement of the movable part 20 via an intermediate part 20 generally as described in relation to Figure 1. In contrast to the actuator assemblies 1 of Figures 1 to 3, in Figure 4 the biasing element 52 urges the intermediate part 50 and the movable part 20 together, i.e. into engagement. The biasing element 52 thereby aids in or causes engagement of the engagement surfaces 20e, 50e. The biasing element 52 may hold the intermediate part 50 and the movable part 20 in engagement regardless of the actuation status of the SMA elements 40. In the depicted embodiment, the biasing element 52 is a compression spring connected between the intermediate part 50 and the movable part 20, although in general the biasing element 52 may alternatively be connected between the intermediate part 50 and the support structure 10. The actuator assembly 1 further comprises an additional actuator 45. The additional actuator 45 may be controlled to selectively disengage or engage the intermediate part 50 and the movable part 20. On actuation, the additional actuator 45 may disengage the intermediate part 50 from the movable part 20. On ceasing actuation, the intermediate part 50 may be brought into engagement with the movable part 20 by the biasing element 52. The additional actuator 45 is embodied by an additional SMA element 45 in the depicted embodiments, although in general any other type of actuator (e.g. a voice coil motor or a piezo electric actuator) may be used as the additional actuator 45. When the additional actuator 45 is not actuated, the intermediate part 50 engages the movable part 20. The movable part 20 may thus be moved with the intermediate part 50 on actuation of the SMA elements 40, and the static frictional force between the friction surfaces lOf, 20f may be reduced on actuation of the SMA elements 40. When the movable part 20 is at the desired position or when a range of motion limit of the intermediate part 50 has been reached, the SMA elements 40 may cease actuating so that the static frictional force increases and the movable part 20 is held in position relative to the support structure 10. The additional actuator 45 may be actuated so as to disengage the intermediate part 50 from the movable part 20. When the intermediate part 50 is disengaged, the SMA elements 40 may be actuated so as to move the intermediate part 50 relative to the movable part 20, thereby repositioning the sub-range within which the movable part 20 is movable on subsequent engagement of the intermediate part 50 with the movable part 20. The SMA elements 40 may actively move the intermediate part 50 relative to the movable part 20, thereby allowing more accurate positional control thereof than in an actuator assembly 1 relying solely on a biasing element 52 for moving the intermediate part 50 relative to the support structure 10. The SMA elements 40 may move the intermediate part 50 to a set position, such as a centre of a range of movement of the intermediate part 50. Alternatively, the SMA elements 40 may move the intermediate part 50 in a second direction to or towards a limit of the range of movement of the intermediate part 50 beyond the set position, for example, thereby increasing the possible movement of the movable part 20 in the first direction (opposite the second direction) in a subsequent cycle of movement thereof. The additional actuator 45 may then cease actuating such that the intermediate part 50 reengages the movable part 20. The movable part 20 may thereby be moved to a target position or target sub-range of movement in similar manner to that described in relation to Figure 1. Figures 5a to 5f show further actuator assemblies 1 comprising the additional actuator 45. Some of the components of the actuator assembly 1, such as the biasing arrangement 30 and the biasing element 52, are shown only conceptionally in Figures 5a to 5d, but may nonetheless be present. For illustrative purposes, the support structure 10 and the movable part 20 are shown as multiple separate blocks in some of the Figures, but it will be appreciated that these portions making up the support structure 10 and movable part 20 may be fixedly connected together and may form a single part in practice. The views of Figures 5a to 5f are rotated by 90 degrees compared to Figures 1 to 4, such that the movement axis M of the movable part 20 is in the vertical direction. The actuator assembly 1 of Figure 5a is generally as described in relation to Figure 4. The biasing element 52 is shown as comprising a first portion 52a and a second portion 52b. The first portion 52a biases the intermediate part 50 against the movable part 20. The second portion 52b provides a centring force to bias the intermediate part 50 to a set position relating to the support structure 10. The first and second portions 52a, 52b may in practice be embodied by a single element (such as the centring springs shown in Figures 1 to 3) or may be embodied by separate elements. The second portion 52b, i.e. the bias towards a set position, is optional and may be omitted in the actuator assembly 1. The additional actuator 45 is connected between the intermediate part 50 and the movable part 20. The additional actuator 45, on actuation, disengages the intermediate part 50 from the movable part 20. The SMA elements 40 effect movement of the intermediate part 50 and the movable part 20 if engaged by the intermediate part 50 as generally described in relation to Figures 1 and 4. The actuator assemblies 1 of Figures 5b and 5c are similar to the actuator assembly 1 of Figure 5a, except that the additional actuator 45 is connected between the intermediate part 50 and the support structure 10. The additional actuator 45 may thus aid in returning the intermediate part 50 to the set position, e.g. to the centre of the range of movement of the intermediate part 50. Figure 5c shows the additional actuator 45, embodied by an SMA wire 45, when the intermediate part 50 is not at a central position. The tension in the SMA wire 45 has a component parallel to the movement of the intermediate part 50 (i.e. downwards in Figure 5c) that is towards the set position. The additional actuator 45 may thus aid in returning the intermediate part 50 to the set position. The actuator assemblies 1 of Figures 5b and 5c do not comprise a biasing element that applies a force for returning the intermediate part 50 to the set position. Instead, the SMA elements 40 may return the intermediate part 50 to the set position or to another position within the range of movement of the intermediate part 50 relative to the support structure 10. The intermediate part 50 may thus be moved to a variable position, such as a limit of the range of movement of the intermediate part 50, when the intermediate part 50 is disengaged from the movable part 20, thereby increasing the step size of the movable part 20 when the movable part 20 is moved across multiple sub-ranges of movement within the overall range of movement. Active positioning of the intermediate part 50 when disengaged also allows measurement of the position of the intermediate part 50, thereby ultimately reducing the risk and / or magnitude of positional errors of the movable part 20. The actuator assembly 1 of Figure 5d is identical to that of Figure 5c, except for the provision of endstops 58a, 58b between the intermediate part 50 and the movable part 20. Although the endstops 58a, 58b are shown specifically in relation to an embodiment with the additional actuator 45, in general such endstops 58a, 58b between the intermediate part 50 and the movable part 20 may be provided in the actuator assembly 1 of Figure 1 or in any of the other embodiments described herein. The endstops 58a, 58b between the intermediate part 50 and the movable part 20 may be particularly useful in embodiments in which the SMA elements 40 actively drive movement of the intermediate part 50 relative to the movable part 20 when the intermediate part 50 is disengaged from the movable part 20. As shown in Figure 5d, the actuator assembly 1 may comprise two endstops 58a, 58b between the intermediate part 50 and the movable part 20. A first endstop 58a may engage when the intermediate part 50 is moved in a first direction (downward in Figure 5d) to a limit of the range of movement of the intermediate part 50 relative to the movable part 20 and a second endstop 58b may engage when the intermediate part 50 is moved in a second direction (upward in Figure 5d), opposite to the first direction, to a limit of the range of movement of the intermediate part 50 relative to the movable part 20. The first and second endstops 58a, 58b may thus define opposite limits of the range of movement of the intermediate part 50 relative to the movable part 20. The overall range of movement of the movable part 20 relative to the support structure 10 may be equal to the sum of the range of movement of the intermediate part 50 relative to the support structure 10 and the range of movement of the intermediate part 50 relative to the movable part 20, where the range of movement of the intermediate part 50 relative to the movable part 20 is the sum of the distances between endstop surfaces of the two endstops 58a, 58b. In Figure 5d, the endstop surfaces of the endstops 58a, 58b are for illustrative purposes shown to be relatively close to each other, but it will be appreciated that the distance between the endstop surfaces may be greater than shown. The endstops 58a, 58b between the intermediate part 50 and the movable part 20 may be configured such that the range of movement of the intermediate part 50 relative to the movable part 20 is equal to or greater than the range of movement of the intermediate part 50 relative to the support structure 10, for example. Alternatively, the endstops 58a, 58b between the intermediate part 50 and the movable part 20 may be configured such that the range of movement of the intermediate part 50 relative to the movable part 20 is equal to or less than the range of movement of the intermediate part 50 relative to the support structure 10. The actuator assembly 1 of Figure 5e comprises endstops 54a, 54b between the intermediate part 50 and the support structure 10. Figure 5f shows an actuator assembly 1 comprising such endstops 54a, 54b between the intermediate part 50 and the support structure 10 in addition to endstops 58a, 58b between the intermediate part 50 and the movable part 20. A first endstop 54a may engage when the intermediate part 50 is moved in a first direction (upward in Figures 5e and 5f) to a limit of the range of movement of the intermediate part 50 relative to the support structure 10 and a second endstop 54b may engage when the intermediate part 50 is moved in a second direction (downward in Figures 5e and 5f), opposite to the first direction, to a limit of the range of movement of the intermediate part 50 relative to the support structure 10. The first and second endstops 58a, 58b may thus define opposite limits of the range of movement of the intermediate part 50 relative to the support structure 10. Error reduction in position of movable part The movable part 20 may be positioned relative to the support structure 10 via movement in multiple separate steps or cycles, where after each step or cycle the intermediate part 50 is repositioned relative to the movable part 20. According to random walk theory, a typical error in the position of the movable part 20 may be determined by AxVn where n is the number of relative motions made between the intermediate part 50 the movable part 20 and Ax is the error introduced in each motion. This position error may be removed by driving the movable part 20 to an endstop 58a, 58b between the movable part 20 and the support structure 10. The actuator assembly 1 may thus comprise an endstop between the movable part 20 and the support structure 10 and the SMA elements 40 may be configured to move the movable part 20 against the support structure 10 so as to engage the endstop 58a, 58b, thereby moving the movable part 20 to a known position and removing any error in position of the movable part 20 relative to the support structure 10. However, moving the movable part 20 against an endstop to the support structure 10 may be undesirable in some situations, and position errors may arise when the movable part 20 moves away from the endstop. The position error may be reduced or removed by providing a position sensor (not shown) for measuring the actual position of the movable part 20 relative to the support structure 10. Such a position sensor may, for example, be a Hall sensor capable of measuring the relative position of the movable part 20 and support structure 10. The SMA elements 40 may be driven to move the movable part 20 in dependence on a difference between the actual position measured by the position sensor and a target position or centre of a target sub-range of movement of the movable part 20. The controller may implement feedback control, such as closed loop control, using the actual position measured by the position sensor as a feedback measure for controlling actuation of the SMA elements 40. Providing such a position sensor may in certain situations not be desirable, due to the additional materials, cost and space requirements of such a position sensor. The accumulation of positional errors may also be reduced by reducing the number of steps n required to move the movable part 20 to a target position or target sub-range of movement. This may be achieved, for example, by moving the intermediate part 50 relative to the movable part 20, when disengaged therefrom, beyond a set position that is central to a range of movement of the intermediate part 50 relative to the movable part 20. Alternatively or additionally, the error Ax introduced between the intermediate part 50 and the movable part 20 can be reduced by driving the intermediate part 50 to an endstop 58a, 58b between the intermediate part 50 and the movable part 20 prior to engaging the movable part 20. The endstops 58a, 58b may be configured such that the range of movement of the intermediate part 50 relative to the movable part 20 is equal to or less than the range of movement of the intermediate part 50 relative to the support structure 10. So, the overall range of movement of the movable part 20 relative to the support structure 10 may be less than or equal to twice the range of movement of the intermediate part 50 relative to the support structure 10. In such actuator assemblies 1, the endstops 58a, 58b between the movable part 20 and the intermediate part 50 may be arranged such that the movable part 20 can be driven to any position relative to the support structure 10 within the overall range of movement while one of the endstops 58a, 58b is engaged. The SMA elements 40 may thus move the intermediate part 50, when disengaged from the movable part 20, relative to the movable part 20 such that one of the endstops 58a, 58b engages before the engagement surfaces 20e, 50e re-engage. The positional error between the movable part 20 and the intermediate part 50 is thus determined by the knowledge of or calibration of the endstop positions, such that the position error of the movable part 20 is not increased when the intermediate part 50 and the movable part 20 disengage and re-engage. Alternatively, the endstops 58a, 58b may be configured such that the range of movement of the intermediate part 50 relative to the movable part 20 is greater than the range of movement of the intermediate part 50 relative to the support structure 10. The overall range of movement of the movable part 20 relative to the support structure 10 may thus be larger than twice the range of movement of the intermediate part to relative to the support structure 10. In such actuator assemblies 1, there are some positions of the movable part 20 in the overall range of movement for which none of the endstops 58a, 58b can engage. Two or more cycles may thus be required to reach a target position or target sub-range of movement of the movable part 20 from a position in which one of the endstops 58a, 58b is engaged. The presence of the endstop 58a, 58b and knowledge of the position of the endstop 58a, 58b may nonetheless be used to decrease the position error of the movable part 20. In particular, if the target sub-range is not accessible (or the target position is not within a sub-range that is accessible) when one of the endstops 58a, 58b is engaged, the SMA elements 40 may first move the intermediate part 50 to so as to engage one of the endstops 58a, 58b (thereby removing the position error between movable part 20 and intermediate part 50) and then move the intermediate part 50 relative to the movable part 20. The position error of the movable part 20 may thus be increased only by the error introduced by the one or few steps from the endstop 58a, 58b, rather than by an accumulation of errors over the history of many steps. In similar manner, the error Ax can be reduced by driving the intermediate part 50 to an endstop 54a, 54b between the intermediate part 50 and the support structure 10 (such as the endstops 54a, 54b shown in Figure 5e) before the intermediate part 50 is moved or allowed to move into engagement with the movable part 20. In particular, the distance between opposing endstop surfaces of the endstops 54a, 54b may be calibrated or measured during assembly of the actuator assembly 1 and may thus be known. By disengaging the intermediate part 50 from the movable part 20, moving the intermediate part 50 such that one of the endstops 54a, 54b engages and then engaging the movable part 20 with the intermediate part 50, and the moving the intermediate part 50 so as to engage the other endstop 54a, 54b, the distance moved by the movable part 20 may accurately be determined to correspond to the sum of the distances between the endstops surfaces of the endstops 54a, 54b. When the movable part 20 is moved to a target position within the subrange of movement, the position error of the movable part 20 may be minimized by starting movement from a known position at which the intermediate part 50 engages one of the endstops 54a, 54b. The error Ax can be further reduced by driving movement of the intermediate part 50 so as to engage endstops 54a, 54b between the intermediate part 50 and the support structure 10 and to engage endstops 58a, 58b between the intermediate party 50 and the movable part 20, for example using the actuator assembly 1 of Figure 5f. In particular, the intermediate part 50 may be moved, when disengaged from the movable part 20, such that one of the endstops 58a, 58b between the intermediate party 50 and the movable part 20 engages. The intermediate part 50 may then engage the movable part 20 and move so as to engage one of the endstops 54a, 54b between the intermediate part 50 and the support structure 10. Both an endstop 54a, 54b between the intermediate part 50 and the support structure 10 and an endstop 58a, 58b between the intermediate party 50 and the movable part 20 may thereby be engaged at the same time. The position of the movable part 20 relative to the support structure 10 may be defined and known when both of these endstops engage. The movable part 20 may then be moved relative to the support structure 10 by repeatedly moving the intermediate part 50 between the endstops 54a, 54b between the intermediate part 50 and the support structure 10, in particular to the one endstop when engaged with the movable part 20 and back to the other endstop when disengaged from the movable part 20. The change in position of the movable part 20 relative to the support structure 10 for each such step is defined by the endstops 54a, 54b between the intermediate part 50 and the support structure 10 and may thus be known, such that the error Ax may not be increased on successively actuating the movable part 20 across sub-ranges of movement. Another way to reduce the position error is to actively move the intermediate part 50 relative to the movable part 20 using the SMA elements 40, compared to a situation in which the biasing elements 52 are solely responsible for such repositioning. When only relying on biasing elements 52 for returning to the set position, the set position is dependent on the balancing of forces between the biasing elements, frictional forces and any forces (e.g. due to actuators 40, 45) acting on the intermediate part 50. Such forces may be unknown and / or variable and thus lead to variation in the set position, which may accumulate as a positional error of the movable part 20 over the course of one or more steps if the controller cannot measure or otherwise determine the actual position. Actively moving the intermediate part 50 using the SMA elements 40 may more accurately position the intermediate part 50 at a known position (e.g. a set position or a position at or towards a limit of movement) relative to the movable part 20. The position error of the movable part 20 may thus be reduced. Separate bearing arrangement Figures 6a and 6b schematically depict further actuator assemblies 1, showing the bearing arrangement 42 that guides movement of the movable part 20 relative to the support structure 10. The bearing arrangement 42 may also be referred to as a guide bearing arrangement 42. The view of Figures 6a and 6b is different from those of Figures 1 to 5 and effectively shows a side view so as to show the bearing arrangement 42. The views of Figures 1 to 5 are viewed from the top downwards onto the view of the actuator assembly 1 of Figures 6a and 6b. The bearing arrangement 42 comprises a rolling bearing 42, in particular in the form of a rolling bearing element (such as a ball bearing) arranged in grooves on the movable part 20 and on the support structure 10. The rolling bearing 42 guides translational movement of the movable part 20 relative to the support structure 10 along the movement axis M, which is into and out of the page of Figures 6a and 6b. In the embodiment of Figure 6a, an additional biasing arrangement 44, depicted as a compression spring 44, loads the rolling bearing 42. The additional biasing arrangement 44 is separate from the biasing arrangement 30 loading the friction surfaces lOf, 20f. The biasing arrangement 44 may continuously load the bearing arrangement 42 so as to improve reliably guiding of the movable part 20 relative to the support structure 10. In the embodiment of Figure 6b, the biasing arrangement 30 applies the normal force to the friction surfaces lOf, 20f and loads the bearing arrangement 42. The bearing arrangement 42 is arranged between biasing arrangement 30 and the friction surfaces lOf, 20f, such that the bearing arrangement 42 effectively loads the friction surfaces lOf, 20f via the bearing arrangement 42. The bearing arrangement 42 further decouples the biasing force of the biasing arrangement 30 from movement of the movable part 20, and thereby may act as the decoupling bearing arrangement 32 described in relation to Figure 2. In Figures 6a and 6b, the support structure 10 comprises a first portion 10a and a second portion 10b. The biasing arrangement 30 is connected between the first and second portions 10a, 10b and biases the second portion 10b against the movable part 20. The biasing arrangement 30 thereby loads the friction surfaces lOf, 20f so as to achieve zero hold power functionality in the actuator assembly 1 of Figure 6. The first and second portions 10a, 10b need not move on actuation of the SMA elements 40 and may thus be considered as a unit forming part of the same support structure 10, similarly to the second portion 10b described in relation to Figure 2. The actuator assembly 1 further comprises the intermediate part 50 and SMA elements 40 connected between the second portion 10b of the support structure 10 and the intermediate part 50. Although not apparent from the views of Figures 6a and 6b, the SMA elements 40 may be angled relative to the movement axis M in the manner depicted in Figures 1 to 5, for example. On actuation of the SMA elements 40, the intermediate part 50 is urged into engagement with the movable part 20. In addition, the support structure 10 (in particular the second portion 10b thereof in Figure 6a and the first portion 10a thereof in Figure 6b) is urged away from the movable part 20, thereby reducing the normal force between the friction surfaces lOf, 20f and reducing the static frictional force. The actuator assembly 1 may thus effect movement of the movable part 20 along the movement axis M via the intermediate part 50 as described in relation to Figure 1, for example. Although not shown, a bearing arrangement 42 and dedicated biasing arrangement 44 may be present in any of the other actuator assemblies 1 described herein. A bearing arrangement 42 may, for example, be located in front of or behind the movable part 20 in the views of Figures 1 to 5. Rotating intermediate part The actuator assemblies of Figures 1 to 6 comprise intermediate parts 50 that are translationally movable on actuation of the SMA elements 40. Figures 7a to 7c show actuator assemblies with rotating intermediate parts 50. The actuator assembly 1 of Figure 7a comprises a rotation bearing 56 in the form of a flexure 56 that constrains translational movement of the intermediate part 50 and guides rotational movement of the intermediate part 50 about a pivot axis P. On actuation of the SMA elements 40, the intermediate part 40 is urged into engagement with the movable part 20 such that the engagement surfaces 20e, 50e directly contact each other. The friction surfaces lOf, 20f are urged apart so as to reduce the static frictional force therebetween. On differential actuation of the SMA elements 40, the intermediate part 50 rotates about the pivot axis P. When engaged, relative sliding between the engagement surfaces 20e, 50e is constrained such that the rotation of the intermediate part 50 is converted into translation of the movable part 20 along the movement axis M. When the SMA elements 40 cease actuating, the movable part 20 is held in position relative to the support structure 10 by the static frictional force between the friction surfaces lOf, 20f. The intermediate part 50 is urged away from the movable part 20 by biasing elements 52 and disengages from the movable part 20 such that relative movement (e.g. sliding) of the engagement surfaces 20e, 50e is allowed. The biasing elements 52 return the intermediate part 50 to a set rotational position, such as a rotational position that is central to the range of rotation of the intermediate part 50 relative to the support structure 10. The movable part 20 may then be moved again on reengagement of the intermediate part 50. The actuator assembly 1 of Figure 7b operates generally as described in relation to Figure 7a. The actuator assembly 1 of Figure 7b comprises a rotation bearing 56 in the form of a pivot, in particular a protrusion on the support structure 10 that is arranged in a corresponding groove on the intermediate part 50. The actuator assembly 1 of Figure 7b also comprises a counterbalance arrangement in the form of a pivot arm 31 arranged between the biasing arrangement 30 and the movable part 20. The counterbalance arrangement generally operates as described in relation to the counterbalance arrangement of Figure 3. The actuator assembly 1 of Figure 7c comprises a rotation bearing 56 in the form of a pin bearing. A pin or other protrusion from the support structure 10 engages a hole in the intermediate part 50 so as to guide rotation thereof. The actuator assembly 1 of Figure 7c further comprises two intermediate parts 50a, 50b. A first SMA element 40 is coupled to a first intermediate part 50a and a second SMA element 40 is coupled to a second intermediate part 50b. Each intermediate part 50a, 50b selectively engages and disengages the movable part 20 on actuation or ceasing actuation of the respective SMA element 40. A respective biasing element 52 may return each intermediate part 50a, 50b to a set position on disengagement from the movable part 20. In use, the intermediate parts 50a, 50b may move generally in tandem so as to rotate together, although small variations in the rotational position of the intermediate parts 50a, 50b relative to each other may arise. Figures 7a to 7c show rotation bearings 56 in the form of a flexure, a pivot and a pin bearing, but in general any other type of rotation bearing that is capable of guiding rotation of the intermediate part 50 relative to the support structure 10 may be used in any of the actuator assemblies 1 of Figures 7a to 7c. Torque loading of friction surfaces The actuator assemblies 1 of Figures 1 to 7 provide a linear biasing force for loading the friction surfaces lOf, 20f and a linear force for unloading the friction surfaces lOf, 20f on actuation of the SMA elements 40. Figures 8a to 8c schematically depict actuator assemblies with torque loading and unloading of the friction surfaces lOf, 20f. Figure 8a shows three orthogonal views of an actuator assembly, in particular a plan view (the topleft view) and two orthogonal side views (the right view and the bottom view). The movable part 20 is movable along a movement axis M that extends into and out of the page in the plan view of Figure 8a. As shown in the plan view of Figure 8a, the actuator assembly 1 comprises two pairs of friction surfaces lOf, 20f. The biasing arrangement 30 comprises two compression springs that together apply a force couple or torque in a first sense (clockwise) to the movable part 20, thereby loading the two pairs of friction surfaces lOf, 20f and giving rise to the static frictional force that holds the movable part 20 in position relative to the support structure 10. The actuator assembly 1 of Figure 8a further comprises two intermediate parts 50, although in general a single intermediate part 50 that comprises two portions corresponding to the intermediate parts 50 shown in Figure 8a may also be provided. Two SMA elements 40 are connected between each intermediate part 50 (or each portion of a single intermediate part 50) and the support structure 10. The two SMA elements 40 connected to each intermediate part 50 oppose each other, so one of the two SMA elements 40 connected to each intermediate part 50 applies an actuation force with a force component in a first direction along the movement axis M and the other of the two SMA elements 40 connected to each intermediate part 50 applies an actuation force with a force component in a second direction along the movement axis M that is opposite to the first direction. Each pair of SMA elements 40 further applies the actuation force with a force component that is orthogonal to the movement axis M and urges the respective intermediate part 50 towards and into engagement with the movable part 20. The two intermediate parts 50 thereby, on engagement with the movable part 20, apply a force couple or torque to the movable part 20 in a second sense (anticlockwise in Figure 8a) that opposes the force couple or torque applied by the biasing arrangement 30. The SMA elements 40 may thus be actuated to unload the friction surfaces lOf, 20f so as to reduce the static frictional force between movable part 20 and support structure 10. When the intermediate parts 50 engage the movable part 20, the SMA elements 40 connected to each intermediate part 50 may be differentially actuated to move the movable part 20 along the movement axis M. In particular, with reference to the side view of the actuator assembly 1 at the bottom of Figure 8a, each pair of SMA elements 40 comprises one "up-pulling" SMA element 40 (extending from the bottom left to the top right) and one "down-pulling" SMA element 40 (extending from the top left to the bottom right). By increasing actuation of the "up-pulling" SMA elements 40 relative to actuation of the "down-pulling" SMA element 40, the intermediate parts 50 and the movable part 20 engaged thereby are moved in a first direction (upwards in the side view at the bottom of Figure 8a) along the movement axis M. Similarly, the movable part 20 may be moved in the opposite direction by preferentially actuating the "down-pulling" SMA elements 40. Actuation of the SMA elements 40 may cease, i.e. the SMA elements 40 may be unpowered, when the movable part 20 has reached a target position or when a limit of a current sub-range of movement of the movable part 20 (i.e. the limit of motion of the intermediate part 50) has been reached. The limit of movement of the intermediate part 50 may be reached when the intermediate part 50 engages endstops (not shown in Figure 8a) or when a threshold strain of the SMA elements 40 has been reached. When actuation of the SMA elements 40 ceases, the force coupled applied by the SMA elements 40 on the movable part 20 via the intermediate parts 50 is reduced such that the loading of the friction surfaces lOf, 20f increases and the static frictional force can hold the movable part 20 in position relative to the support structure 10. The intermediate parts 50 further disengage from the movable part 20. Also shown in Figure 8a are biasing elements 52 in the form of flexures 52 connected between each intermediate part 50 and the support structure 10. The flexures 52 urge the intermediate parts 50 away from the movable part 20 so as to disengage the intermediate parts 50 from the movable part 20 when the SMA elements 40 are not actuated. The flexures 52 further provide a force returning the respective intermediate part 50 to a set position, such as a central position within the range of movement of the intermediate part 50 relative to the support structure 10, when the SMA elements 40 are not actuated. If, following actuation of the SMA elements 40, the movable part 20 has not reached the target position, then the SMA elements 40 may be actuated again to move further towards or to the target position. Figure 8b shows three orthogonal views of another actuator assembly, in particular a plan view (the top-left view) and two orthogonal side views (the right view and the bottom view). The actuator assembly 1 is and operates generally as described in relation to Figure 8a. Compared to the actuator assembly 1 of Figure 8a, the actuator assembly 1 of Figure 8b further comprises a counterbalance arrangement. The counterbalance arrangement is generally as described in relation to Figure 3 and comprises a pivot arm 31 between each biasing element of the biasing arrangement 30. The pivot arms 31 are placed under compression by the biasing arrangement 30 and carry a compressive force generally along the length of the pivot arm. The side view on the right of Figure 8b shows a situation in which the movable part 20 is displaced from a set position (e.g. from a central position within an overall range of movement of the movable part 20) such that the pivot arm 31 is angled relative to the movement axis M. The force transferred by the pivot arm 31 to the movable part 20 therefore has a component that is parallel to the movement axis M, urging the movable part 20 further away from the set position. The pivot arms 31 may thus apply lateral forces to the movable part 20 that counteract any centring forces of an external load urging the movable part 20 towards the set position. Figure 8c shows a plan view of another actuator assembly 1. The side views are not shown in Figure 8c, but it will be appreciated that the actuator assembly 1 may comprise two pairs of SMA elements 40 that are arranged as described in relation to Figure 8a. The actuator assembly 1 comprises a single intermediate part 50. The actuator assembly 1 comprises biasing elements 52 that urge the intermediate part 50 into engagement with the movable part 20, as well as additional actuators 45 that actuate to disengage the intermediate part 50 from the movable part 20. The actuator assembly 1 may thus correspond to an actuator assembly 1 of the type described in relation to Figures 4 and 5 and may operate in similar manner. Two biasing elements 52 are connected between the intermediate part 50 and the movable part 20, and two pairs of engagement surfaces 20e, 50e are provided on the intermediate part 50 and the movable part 20. The two biasing elements 52 are arranged to apply a force couple or torque (clockwise in Figure 8c) to the intermediate part 50 relative to the movable part 20 that biases the two pairs of engagement surfaces 20e, 50e into engagement. The two additional actuators 45 are embodied by additional SMA elements 45 in Figure 8c. On actuation, the two additional actuators 45 apply a force couple or torque (anticlockwise in Figure 8c) to the intermediate part 50 relative to the movable part 20 that urges the two pairs of engagement surfaces 20e, 50e apart. The two additional actuators 45 thereby oppose the two biasing elements 52 so as to disengage the intermediate part 50 from the movable part 20 on actuation. The two additional actuators 45 are connected between the support structure 10 and the intermediate part 50 in Figure 8c, but alternatively the two additional actuators 45 may be connected between the movable part 20 and the intermediate part 50 (e.g. as described in relation to Figure 5a). The additional actuators 45 may thus selectively engage the intermediate part 50 with or disengage the intermediate part 50 from the movable part 20. The SMA elements 40 may move the movable part 20 relative to the support structure 10 when in engagement with the intermediate part 50 or may move the intermediate part 50 relative to the movable part 20 when the parts are not in engagement, generally in the manner described in relation to Figure 8a. Movable part is rotatable relative to support structure The actuator assemblies 1 of Figures 1 to 8 comprise a movable part 20 that is translationally movable relative to the support structure 10. Figures 9a to 9e show further actuator assemblies 1 comprising a movable part 20 that is rotatable relative to the support structure 10. The movable part 20 may also be referred to as a rotatable part 20 in the actuator assemblies 1 of Figures 9a to 9e. The rotatable part 20 is rotatable about a rotation axis R relative to the support structure 10. The rotatable part 20 may be continuously rotatable or be rotatable within a range of rotational positions. A bearing arrangement, for example embodied by the friction surfaces lOf, 20f may guide rotation of the rotatable part 20 relative to the support structure 10. The bearing arrangement may in general comprise any type of bearing for guiding such rotation and may, for example, comprise a rolling bearing, a plain bearing or an arrangement of flexures. The actuator assembly 1 may be used in any application in which rotational position control of a rotatable part 20 is desired, for example in a variable aperture assembly 1 in which blades are coupled between the support structure 10 and the rotatable part 20 so as to effect a change in size of a variable aperture on rotation of the rotatable part 20. The rotatable part 20 may also comprise a helical bearing to another movable part that is translationally movable along the rotation axis R, thereby effecting translational movement of such other movable part on rotation. The actuator assembly 1 of any of Figures 9a to 9e comprises a biasing arrangement 30 and friction surfaces lOf, 20f on the rotatable part 20 and on the support structure 10. The biasing arrangement 30 biases the friction surfaces lOf, 20f against each other with the normal force, thereby giving rise to the static frictional force for zero hold power functionality. The biasing arrangement 30 may comprise any component capable of applying such a biasing force between the rotatable part 20 and the support structure 10, for example springs, flexures or other resilient elements, or magnetic arrangements. The rotatable part 20 has a circular footprint when viewed along the rotation axis R and the friction surfaces lOf, 20f extend parallel to the rotation axis R and are curved when viewed along the rotation axis R. The actuator assembly 1 of any of Figures 9a to 9e further comprises biasing elements 52. The biasing elements 52 are coupled between the intermediate part 50 and the support structure 10 in the depicted actuator assemblies 1, although in general the biasing elements 52 may be arranged in any way allowing the intermediate part 50 to be biased relative to the movable part 20 (e.g. be coupled between the intermediate part 50 and the movable part 20). In Figures 9a, 9b and 9e, the biasing elements 52 urge the intermediate part 50 out of engagement with the movable part 20. In Figures 9c and 9d, the biasing elements 52 urge the intermediate part 50 into engagement with the movable part 20 and an additional actuator 45 for disengaging the intermediate part 50 from the movable part 20 is provided. The biasing elements 52 need not be provided in the actuator assemblies 1 of Figures 9c and 9d, for example in embodiments in which the intermediate part 50 is brought into engagement with the movable part 20 solely by the SMA elements 40. The actuator assembly 1 of Figure 9a is similar to the actuator assembly 1 of Figure 1, except that the movable part 20 is rotatable in Figure 9a. The intermediate part 50 is biased out of engagement with the rotatable part 20 when the SMA elements 40 are not actuated. On actuation of the SMA elements 40, the engagement surfaces 20e, 50e on the movable part 20 and intermediate part 50 engage with each other. On further actuation of the SMA elements 40, the contact force between the engagement surfaces 20e, 50e increases and the friction surfaces lOf, 20f are urged apart, thereby reducing the static frictional force therebetween. Differential actuation of the SMA elements 40 moves the intermediate part 50 on an arc around the rotation axis R, thereby rotating the rotatable part 20 when engaged with the intermediate part 20. The rotatable part 20 may thus be moved to or towards a target rotational position (such as a centre of a target sub-range of rotations or another target position) by the SMA elements 40. When the target rotational position has been reached or when a range of movement limit of the intermediate part 50 is reached, the SMA elements 40 may cease actuating such that the contact force between the engagement surfaces 20e, 50e decrease and the normal force between the frictional surfaces lOf, 20f (and thus the static frictional force therebetween) increases. The intermediate part 50 disengages from the movable part 20 and is returned to the set position shown in Figure 9a by the biasing elements 52. The actuator assembly 1 of Figure 9b is similar to the actuator assembly 1 of Figure 9a, except that a decoupling bearing 32 is arranged between the biasing elements of the biasing arrangement 30 and the rotatable part 20. The decoupling bearing 32 may generally be as described in relation to Figure 2. The support structure 10 comprises a first portion and a second portion 10b that is coupled to the first portion via the biasing arrangement 30. The decoupling bearing 32, depicted as a ball bearing in Figure 9b, is arranged between the second portion 10b of the support structure 10 and the rotatable part 20. The rotatable part 20 may move relative to the second portion 10b of the support structure 10, thereby avoiding lateral deformation of the biasing arrangement 30 as the rotatable part 20 rotates within the overall range of movement of the rotatable part 20 relative to the support structure 10. The overall range of movement of the rotatable part 20 may be greater compared to a situation in which a decoupling bearing 32 is not provided. The overall range of movement of the rotatable part 20 may be unlimited, for example, such that the rotatable part 20 is continuously rotatable relative to the support structure 10. The biasing arrangement 30 of Figure 9b comprises two biasing elements that are arranged to extend generally tangentially to a circle around the rotation axis R. Compared to the actuator assembly 1 of Figure 9a, for example, the biasing arrangement 30 does not comprise components extending radially towards the rotation axis R. Such a biasing arrangement 30 may be particularly suitable for applications in which a space around the rotation axis R is desirably empty of other components, for example when the rotatable part 20 is used in a variable aperture assembly or other optical assembly. The actuator assembly 1 of Figures 9c and 9d comprises the biasing elements 52 urging the intermediate part 50 into engagement with the rotatable part 20 and the additional actuator 45 that on actuation disengages the intermediate part 50 from the movable part 20. The actuator assembly 1 may operate generally as described in relation to the actuator assemblies 1 of Figures 4 and 5a to 5c. The actuator assembly 1 of Figure 9d further comprises an endstop 54 that limits the range of movement of the intermediate part 20 relative to the support structure 10. The endstop 54 may generally be as described in relation to Figure 4, and provide a known position to which the rotatable part 20 may be moved by the SMA elements 40, for example for calibration purposes. The actuator assembly 1 of Figures 9e comprises the rotatable part 20 as well as an intermediate part 50 that is rotatable relative to the support structure 10. The intermediate part 50 rotates in the opposite sense to the rotatable part 20 when the rotatable part 20 and the intermediate part 50 are in engagement with each other. In any of the actuator assemblies 1 of Figures 9a to 9e, the friction surfaces lOf, 20f may be provided elsewhere than indicated in the Figures. For example, the friction surfaces lOf, 20f may be applied close to the rotation axis R, for example when the actuator assembly 1 comprises an axle guiding rotation of the rotatable part 20. Intermediate part selectively engaging support structure The actuator assemblies 1 of Figures 1 to 9 comprise intermediate parts 50 that selectively engage and disengage the movable part 20. In general, in any of these actuator assemblies 1, the depicted movable part 20 may be referred to as the support structure 10 and vice versa. Figure 10 specifically shows an actuator assembly 1 in which the intermediate parts 50 selectively engages and disengages the support structure 10. In the particular actuator assembly 1 of Figure 10, the intermediate part 50 and the movable part 20 are both independently loaded against the support structure 10. The biasing arrangement 30 is arranged between two portions 20a, 20b of the movable part 20. The two portions 20a, 20b need not move relative to each other on actuation of the SM A elements 40 and so are considered herein as a unit corresponding to the same movable part 20. A bearing 42 is arranged between the support structure 10 and one portion 20b of the movable part 20 and the friction surfaces lOf, 20f are arranged between the support structure 10 and the other portion 20a of the movable part 20. The bearing 42 guides movement of the movable part 20 along the movement axis M. The biasing arrangement 30 loads the bearing 42 and further biases the friction surfaces lOf, 20f together so as to achieve zero hold power functionality. The movable part 20 is effectively wedged between two regions of the support structure 10. A biasing element 52 is arranged between two portions 50a, 50b of the intermediate part 50. The two portions 50a, 50b need not move relative to each other on actuation of the SM A elements 40 and so are considered herein as a unit corresponding to the same intermediate part 50. A bearing 53 is arranged between the support structure 10 and one portion 50b of the intermediate part 20 and the engagement surfaces lOe, 50e are arranged between the support structure 10 and the other portion 50a of the intermediate part 50. The bearing 53 guides movement of the intermediate part 50 relative to the support structure 10. The biasing element 52 loads the bearing 52 and further biases the engagement surfaces lOe, 50e into engagement. The intermediate part 50 is effectively wedged between two other regions of the support structure 10. The actuator assembly 1 comprises an additional actuator 45 between the two portions 50a, 50b of the intermediate part 50. The additional actuator 45 on actuation disengages the engagement surfaces lOe, 50e. The SMA elements 40 are connected between the intermediate part 50 (in particular a portion 50b thereof) and the movable part 20 (in particular a portion 20a thereof). When the engagement surfaces lOe, 50e are disengaged, the intermediate part 50 is movable relative to the support structure 10. The SMA elements 40 may thus on actuation move the intermediate part 20 relative to the support structure 10. When the engagement surfaces lOe, 50e are engaged, the intermediate part 50 is not movable relative to the support structure 10. On actuation of the SMA elements 40, the normal force (and thereby static frictional force) between the friction surfaces lOf, 20f is reduced and the movable part 20 is moved relative to the support structure 10. The actuation force of the SMA elements 40 (e.g. the tension in SMA wires) may be different depending on whether the engagement surfaces lOe, 50e are engaged. When the engagement surfaces lOe, 50e are disengaged, the actuation force of the SMA elements 40 may be relatively low such that the static frictional force between the friction surfaces lOf, 20f remains relatively high and movement of the movable part 20 relatively to the support structure 10 is constrained. When the engagement surfaces lOe, 50e are engaged, the actuation force of the SMA elements 40 may be relatively high such that the static frictional force between the friction surfaces lOf, 20f is sufficiently reduced to allow movement of the movable part 20 relatively to the support structure 10. Such relatively higher actuation forces may, for example, allow a greater mass or generally greater external load to be moved by the movable part 20. By selectively engaging and disengaging the engagement surfaces lOe, 50e and further driving actuation of the SMA elements 40 when the engagement surfaces lOe, 50e are disengaged and when the engagement surfaces lOe, 50e are engaged, the intermediate part 50 and the movable part 20 may both move in the same direction relative to the support structure 10. The intermediate part 50 may move relative to the support structure 10 within an overall range of movement that is relatively large compared to other actuator assemblies 1 disclosed herein. The intermediate part 50 may step across this overall range of movement in successive steps through sub-ranges of movement, similar to the manner described in relation to Figure 1, for example. In Figure 10, the movable part 20 moves along the movement axis M along a straight path relative to the support structure 10. In general, the movable part 20 and the intermediate part 50 may move along any path defined by the support structure 10. The support structure 10 may, for example, define a curved path (e.g. as in a track system) along which the movable part 20 and the intermediate part 50 are movable. The curved path may form a loop, such as a circle, allowing continual motion of the movable part 20 and intermediate part 50 along the curved path. Additional actuator connected to movable part The actuator assemblies 1 of Figures 4, 5a-d, 8c, 9c and 9d comprise an additional actuator 45 that is connected to the intermediate part 50 for disengaging the intermediate part 50 from the movable part 20. Figures 11a to 11c schematically show actuator assemblies 1 comprising such an additional actuator 45 that is connected to the movable part 20 instead of the intermediate part 50. The additional actuator 45 may thereby apply a force to the movable part 20 so as to allow the intermediate part 50 to be disengaged from the movable part 20 by the SMA elements 40. The biasing arrangement 30 may be connected between the support structure 10 and the intermediate part 50. The biasing arrangement 30 may urge the intermediate part 50 into contact with the movable part 20 so as to urge the engagement surfaces 20e, 50e into engagement. The biasing arrangement 30 may further, via the intermediate part 50, urge the movable part 20 against the support structure 10 so as to apply the normal force between the friction surfaces lOf, 20f for zero hold power functionality. The movable part 20 is sandwiched between the intermediate part 50 and the support structure 10. The actuator assembly 1 further comprises the biasing element 52. The biasing element 52 urges the engagement surfaces 20e, 50e into engagement. In Figure 11a, the biasing element 52 is connected between the intermediate part 50 and the movable part 20. The additional actuator 45 is connected between the movable part 20 and the support structure 10. The additional actuator 45, on actuation, may increase the normal force between the frictional surfaces lOf, 20f. On actuation of the additional actuator 45 and simultaneous actuation of the SMA elements 40, the movable part 20 is held in position relative to the support structure 10 and the intermediate part 50 is urged away from the movable part 20 so as to disengage the engagement surfaces 20e, 50. The additional actuator 45 and the SMA elements 40 thereby work together to disengage the engagement surfaces 20e, 50e. The force applied by the additional actuator 45 and the SMA elements 40 overcomes the biasing force of the biasing element 52 that urges the engagement surfaces 20e, 50e into engagement. When the additional actuator 45 is actuated such that the intermediate part 50 is disengaged from the movable part 20, the SMA elements 40 may on differential actuation move the intermediate part 50 relative to the movable part 20. When the additional actuator 45 is not actuated, the biasing element 52 holds the engagement surfaces 20e, 50e in engagement such that the movable part 20 is movable together with the intermediate part 20. The SMA elements 40 may thereby apply a force to the movable part 20, via the intermediate part 50 and the biasing element 52, that reduces the normal force (and thereby the static frictional force) between the friction surfaces lOf, 20f. On differential actuation of the SMA elements 40, the movable part 20 moves together with the intermediate part 50 relative to the support structure 10. When the SMA elements 40 cease actuating, the normal force between the friction surfaces lOf, 20f increases to achieve zero hold power. The movable part 20 and the intermediate part 50 are held in position relative to each other and relative to the support structure 10. The actuator assembly 1 may effectively constrain movement of the movable part 20 when the SMA elements 40 and the additional actuator 45 are not actuated, by the biasing element 30 being able to apply a relatively high normal force between the friction surfaces lOf, 20f and between the engagement surfaces 20e, 50e. However, when the SMA elements 40 are actuated for moving the movable part 20, the external load that may be moved by the movable part 20 is limited by the frictional force between the engagement surfaces 20e, 50e due to the biasing force applied by the biasing element 52. The biasing force of the biasing element 52 must be low enough to allow the SMA elements 40 to overcome the biasing force when the additional actuator 45 is actuated so as to allow the engagement surfaces 20e, 50e to disengage. The actuator assembly 1 of Figure 11a may comprise a counterbalance arrangement to assist in moving an external load that applies a centring force to the movable part 20. The counterbalance arrangement may generally be as described in relation to Figure 3. Figure lib shows an example of a counterbalance arrangement that may be used in the actuator assembly 1 of Figure 11a. Figure lib shows a cross-sectional side view of the actuator assembly 1 of Figure 11a, in particular a crosssection along a vertical line through the movable part 20 in Figure 11a. As shown in Figure lib, a bearing 42 may be provided between the movable part 20 and the support structure 10. A compression spring 33, also referred to as an over-centre spring, is connected between the movable part 20 and the support structure. The compression spring 33 loads the bearing 42. When the movable part 20 is moved away from a set position (such as a central position in a range of movement) relative to the support structure 10, the compression spring 33 may apply the biasing force at an angle to the movement axis M. The compression spring 33 thereby applies a force to the movable part 20 with a force component urging the movable part 20 away from the set position. In the actuator assembly 1 of Figure 11a, the range of movement of the movable part 20 relative to the support structure 10 may be limited by the additional actuator 45 being connected between the movable part 20 and the support structure 10. Figure 11c shows an alternative actuator assembly 1 that is similar to the actuator assembly 1 of Figure 11a, except that the additional actuator 45 is connected between two portions of the movable part 20. A portion of the support structure 10 is wedged between the two portions of the movable part 20, in particular when the additional actuator 45 is actuated. In Figures 11a and 11c, the movable part 20 moves along a movement axis M along a straight line. The movable part 20 may alternatively move along a curved path defined by the support structure 10. For example, the friction surface lOf along which the movable part 20 is movable may be curved around axis Q, and the intermediate part 50 may be pivotally movable about axis Q. Modifications or Alternatives The actuator assemblies 1 described above all comprise two SMA elements 40 that are arranged in opposition so as to move the movable part 20 in opposite directions or rotate the rotatable part 20 in opposite senses. Alternatively, one of the SMA elements 40 may be replaced by a resilient element such as a spring providing an opposing force to actuation of the other of the SMA elements 40. The present invention has been described in connection with SMA elements 40, for example in the form of SMA wires 40. 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. 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, the first part comprising a first friction surface and the second part comprising a second friction surface;a biasing arrangement configured to apply a normal force biasing the first and second friction surfaces against each other, thereby giving rise to a static frictional force that is capable of constraining relative movement of the first and second parts;an intermediate part configured selectively to engage the second part or to disengage from the second part;one or more SMA elements configured, when the intermediate part engages the second part, on actuation to cause actuation forces to be applied to the intermediate part to thereby i) cause reduction of the normal force between the first and second friction surfaces so as to reduce the static frictional force and ii) move the second part within a sub-range of movement relative to the first part;wherein, when the intermediate part is disengaged from the second part, the intermediate part is movable relative to the second part.

2. An actuator assembly according to claim 1, wherein the one or more SMA elements are configured, when the intermediate part engages the second part, on actuation to increase a contact force between the intermediate part and the second part.

3. An actuator assembly according to claim 1 or 2, further comprising one or more biasing elements configured, when the one or more SMA elements are not actuated, to urge the intermediate part to disengage from the second part.

4. An actuator assembly according to claim 1 or 2, further comprising one or more biasing elements configured to urge the intermediate part into engagement with the second part.

5. An actuator assembly according to claim 3 or 4, wherein the one or more biasing elements are further configured, when the one or more SMA elements are not actuated, to position the intermediate part at a set position relative to the first part.

6. An actuator assembly according to any one of the preceding claims, further comprising an additional actuator configured, on actuation, to disengage the intermediate part from the second part, optionally wherein the additional actuator is an additional SMA element.

7. An actuator assembly according to any one of the preceding claims, wherein the one or more SMA elements are configured, when the intermediate part is disengaged from the second part, on actuation to move the intermediate part relative to the second part.

8. An actuator assembly according to any one of the preceding claims, further comprising one or more endstops configured to limit the range of movement of the intermediate part relative to the first part and / or relative to the second part, wherein the one or more SMA elements are capable on actuation to drive movement of the intermediate part to the limit of the range of movement of the intermediate part relative to the first part and / or relative to the second part so as to engage the one or more endstops.

9. An actuator assembly according to any one of the preceding claims, comprising a controller configured to generate drive signals for driving actuation of the one or more SMA elements and optionally of the additional actuator.

10. An actuator assembly according to claim 9, wherein the controller is configured to:f) obtain a target position of the second part relative to the first part;g) determine if the target position is within a current sub-range of movement of the second part relative to the first part;h) in a case where the target position is within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part to the target position;i) in a case where the target position is not within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part in a first direction towards a limit of the current sub-range of movement and towards the target position, and then selectively disengage the intermediate part from the second part and control actuation of the SMA elements to allow or effect movement of the intermediate part relative to the second part in a second direction that is opposite to the first direction; andj) repeat steps b) to d) until the second part is at the target position.

11. An actuator assembly according to claim 9 or 10, wherein the controller is configured to: f) obtain a target sub-range of movement of the second part relative to the first part; g) determine if the centre of the target sub-range of movement is within a current subrange of movement of the second part relative to the first part;h) in a case where the centre of the target sub-range of movement is within a current subrange of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part to the centre of the target sub-range of movement; andi) in a case where the centre of the target sub-range of movement is not within the current sub-range of movement, selectively engage the second part with the intermediate part and control actuation of the SMA elements so as to move the second part in a first direction and towards a limit of the current sub-range of movement that is towards the centre of the movement sub-range of movement, and then selectively disengage the intermediate part from the second part and control actuation of the SMA elements to allow or effect movement of the intermediate part relative to the second part in a second direction that is opposite to the first direction, andj) repeat steps b) to d) until the second part is at the centre of the target sub-range of movement.

12. An actuator assembly according to claim 10 or 11, wherein the controller is configured to, in step d) in the case where the target position or the centre of the target sub-range of movement is not within the current sub-range of movement, selectively disengage the intermediate part from the second part and control actuation of the SMA elements to move intermediate part relative to the second part in the second direction beyond a centre of the range of movement of the intermediate part relative to the first part and to or towards a limit of the range of movement of the intermediate part relative to the first part.

13. An actuator assembly according to any one of claims 9 to 12 when dependent on claim 8, wherein the controller is configured to control actuation of the SMA elements so as to move the intermediate part such that one of the one or more endstops engages.

14. An actuator assembly according to any one of claims 9 to 13, further comprising a position sensor for measuring the actual position of the second part relative to the first part, wherein thecontroller is configured to generate the drive signals for driving actuation of the SMA elements in part based on a difference between the measured actual position of the second part relative to the first part and a target position of the second part relative to the first part.

15. An actuator assembly according to any one of the preceding claims, wherein the second part is translationally movable relative to the first part or wherein the second part is rotatable relative to the first part.

16. An actuator assembly according to any one of the preceding claims, comprising a bearing arrangement arranged to guide the movement of the second part relative to the first part, wherein the bearing arrangement is separate from the first and second friction surfaces.

17. An actuator assembly according to any one of the preceding claims, wherein the intermediate part is translationally movable relative to the first part or wherein the intermediate part is rotatable relative to the first part.

18. An actuator assembly according to any one of the preceding claims, comprising at least two SMA elements configured to apply forces to the intermediate part with components in opposite directions so as to cause the second part to be moved in opposite directions.

19. An actuator assembly according to any one of the preceding claims, comprising a bearing arranged between the biasing arrangement and the second part so as to decouple the biasing force applied by the biasing arrangement from the movement of the second part.

20. An actuator assembly according to any one of the preceding claims, wherein the biasing arrangement is arranged between two portions of the first part, and further comprising one or more endstops between the two portions of the first part configured to limit relative movement of the two portions of the first part on actuation of the SMA elements.

21. An actuator assembly according to any one of the preceding claims, wherein the biasing arrangement is configured to apply a force couple in a first sense to the second part relative to the first part, thereby biasing the first and second friction surfaces against each other so as to give rise to the static frictional force, andwherein the one or more SMA elements are configured, on actuation, to cause a force couple to be applied in a second sense to the second part relative to the first part, thereby urging the first and second friction surfaces apart so as to reduce the static frictional force.

22. An actuator assembly according to any one of the preceding claims, comprising a counterbalance arrangement configured, when the second part is positioned relative to the first part at positions that are located on opposite sides from a set position of the second part relative to the first part, to apply a lateral force to the second part acting in a direction away from the set position.

23. An actuator assembly according to claim 22, comprising a common component configured to apply to the second part both the lateral force and the normal force.

24. An actuator assembly according to any one of the preceding claims, wherein the intermediate part comprises a first engagement surface and the second part comprises a second engagement surface, wherein the first and second engagement surfaces are constrained from sliding relative to each other by frictional force when the intermediate part engages the second part.

25. A method of driving actuation of the actuator assembly of any one of the preceding claims, the method comprising:engaging the second part with the intermediate part;when the intermediate part engages the second part, generating drive signals for controlling actuation of the SMA elements so as to cause actuation forces to be applied to the intermediate part to thereby i) cause reduction of the normal force between the first and second friction surfaces so as to reduce the static frictional force and ii) move the second part within a sub-range of movement relative to the first part;disengaging the intermediate part from the second part; andwhen the intermediate part is disengaged from the second part, generating drive signals for controlling actuation of the SMA elements so as to cause actuation forces to be applied to the intermediate part to thereby move the intermediate part relative to the second part.IntellectualPropertyOfficeApplication GB2502748.3Search report under Section 17 of the Patents Act 1977Date search completed: 14 October 2025Claims searched: 1-25International classificationSubclass and subgroup Valid from F03G7 / 06 01 / 01 / 2006Field of searchWorldwide search of patent documents classified in the following areas of the IPC:F03GDatabases used in the preparation of this search report:SEARCH-PATENTDocuments considered to be relevantPatent literatureCategory Relevant claims Document of relevance X 1-5, 7-10, 12-25 JP H08165984A CASIO, see figures and e.g. paragraph 41: SMA coil adjusts friction and actuates moveable partX 1-5, 7-10, 12-25 JP H079384 A SHIMADZU, see whole document: SMA actuator with intermediate members X 1-10, 12-25 US 8316840 B2 KARIM, example of rotary SMA actuator with intermediate member X 1-10, 12-25 US 2002069941 A1 INDIAN SPACE RESEARCH, example of rotary actuator with intermediate members 2Non-patent literatureCategory Relevant claims Document of relevanceCategoriesLetter or DescriptionsymbolX Document indicating lack of novelty or inventive step.Y Document indicating lack of inventive step, if combined with anotherdocument of the same category.& Member of the same patent family. A Document indicating technological background. P Document published on or after the priority date but before the fling date of the present application.Letter or symbol Description E Earlier application published on or after the filing date of the present application.

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