Actuator assembly

Abstract

An actuator assembly comprises a support structure 116, a moveable part 104 and a bearing arrangement 112 arranged to guide movement of the moveable part in a plane of movement. At least one actuator

Description

Field The present application relates to an actuator assembly. Background It is known to use an actuator, for example a shape memory alloy, SMA, element, to drive translational movement of a movable element with respect to a support structure. SMA has particular advantages in miniature devices and may be applied in a variety of devices including handheld devices, such as cameras and mobile phones. Such SMA elements may be used for example in an optical device such as a camera for driving translational movement of a camera lens element along its optical axis, for example to effect focussing (autofocus, AF), zoom and / or to account for thermal variations in the device. Some examples of an SMA actuation apparatuses which are cameras of this type are disclosed in WO 2007 / 113478 Al. Herein, the movable element is a camera lens element supported on a support structure by a helical bearing arrangement comprising flexures that guide translational movement along the optical axis. In one example described herein, the SMA element is a piece of SMA wire connected at its ends to a support structure and hooked over a hook on a camera lens element for driving the translational movement. The straight SMA wires formed by the portions of the piece of SMA wire on either side of the hook extend at an acute angle of greater than 0 degrees to the movement direction parallel to the optical axis. Angling the SMA wires in this way increases the amount of movement compared to an SMA wire extending along the movement direction and also reduces the extent of the actuator in the movement direction. In this way, a relatively higher stroke is achieved. It is desirable in some circumstances to drive the movable element to make relatively small movements and to accurately control the position of the movable component. It is also desirable to reduce the power and / or energy required to control the movement and / or position of the movable element. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: a support structure; a movable part; a bearing arrangement arranged to guide movement of the movable part relative to the support structure in a plane of movement; a loading arrangement for loading the bearing arrangement; and at least one actuator component arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement. By applying the unloading torque, the load on the bearing arrangement can be controlled. As one example, this allows the load to be made lower when movement of the movable part is desired and made higher when movement is not desired. By applying the unloading torque with the actuator components, the number of parts may be minimised. By providing a bearing arrangement, the effect of lateral forces on the support structure or movable part may be reduced. This can help to increase the accuracy of control of the position of the movable part By reducing the load on the bearing arrangement by applying an unloading torque using at least one actuator component, the friction in the bearing arrangement can be varied between driving and non-driving of movement of the movable part. The advantage of variable friction for such a system is that when movement of the movable part is being driven, the driving component (in some examples the at least one actuator component) will have less friction compared to non-variable friction designs holding the same amount of force when powered off. This lessened friction assists in increasing gearing and hence stroke, as the stress is overall less. This can assist in combating stick slip behaviour, as the moving friction is lower and it can also improve the resolution of the actuator. By arranging the at least one actuator component to provide a force with components that oppose the (pre)-loading force of the loading arrangement (or any frictional surface's (pre)-load force) in the overall system, when the system is unpowered, the full preload is pressing on the bearing arrangement and generating the maximum frictional force. When the at least one actuators are powered, a component of the force of the actuator is generated which opposes the preload force. This lowers the overall force in the bearing arrangement and hence lowers the frictional force when the movable component is moved. Optionally, a component of the unloading torque is in a different degree of freedom to a component of the movement of the movable part relative to the support structure in the plane of movement. In this way, the force or torque to change the friction in the system (that is, the force in the bearing arrangement) is in a different degree of freedom as the movement of the moving portion. Optionally, the axis of the unloading torque has a component in the plane of movement. Optionally, the axis of the unloading torque has a component perpendicular to the plane of movement. Optionally, the movable part comprises a plurality of sides extending in a loop around a primary axis and the plane of movement has a component parallel to the primary axis. Optionally, the bearing arrangement is arranged to guide one dimensional movement of the movable part in the plane of movement relative to the support structure. In this way, the movable part may move along an axis within the plane of movement, for example along the primary axis. Optionally, the movable part comprises a plurality of sides extending in a loop around a primary axis and the plane of movement has a component perpendicular to the primary axis. Optionally, the bearing arrangement is arranged to guide two dimensional movement of the movable part in the plane of movement relative to the support structure. In this way, the movable part may move in any direction within the plane of movement, for example in the xy plane. Optionally, the bearing arrangement is arranged to guide rotational movement of the movable part in the plane of movement relative to the support structure. In this way, the movable part may rotate within the plane of movement, around an axis perpendicular to the plane of movement, for example around the z axis. Optionally, the at least one actuator component further arranged, on actuation, to drive the movement of the movable part relative to the support structure in the plane of movement. In this way, the same at least one actuator component may be arranged to both apply an unloading torque for reducing load on the bearing arrangement and drive the movement of the movable part relative to the support structure in the plane of movement. In this way, the number of actuator components needed for the system can be reduced. Different force components of the same actuator component may provide different functions on actuation. Optionally, the at least one actuator component comprises at least one pair of actuator components, and the actuator components of each pair of the at least one pair of actuator components are arranged, on actuation, to drive the movement of the movable part relative to the support structure in opposite directions in the plane of movement. By providing the forces in opposite directions, the total force applied in the plane of movement may be reduced. This can help to improve the accuracy of control of the movable part, particularly towards the extremes of the stroke. Optionally, the at least one actuator component is arranged to apply force components to the movable part relative to the support structure, wherein a first component of the force components applies the unloading torque for reducing load on the bearing arrangement and a second component of the force components drives the movement of the movable part relative to the support structure in the plane of movement. Different force components of the same actuator component may provide different functions on actuation. In this way, the number of actuator components needed for the system can be reduced. Optionally, the at least one actuator component is arranged to reduce the loading of the bearing arrangement by less than the loading applied by loading arrangement. By providing that the unloading is less than the loading, the bearing arrangement can remain loaded during use of the actuator assembly. In this way, the bearing arrangement may guide movement of the movable part relative to the support structure in a plane of movement more accurately. Also in this way, the load of the bearing arrangement (and hence the friction in the system) may be varied such that the full load is applied when the system is unpowered and the load is reduced when the system (and actuator components) are powered. In this way, the movement and position of the movable component can be accurately and precisely controlled with reduced power and / or energy requirements. Optionally, the loading arrangement is arranged to load the bearing arrangement so as to generate frictional force components therein that constrain the movement of the movable part relative to the support structure at any position within a range of movement when the actuator components are not actuated. In this way, the movable element may be constrained at a chosen position when the actuator components are not actuated, for example when the system is unpowered. In this way, the power and / or energy required to hold the movable element in position is zero. For example, the bearing arrangement may be unloaded by actuating the at least one actuator component. The at least one actuator component may then, on actuation, drive movement of the movable part in the plane of movement. When the movable part is in the desired position, the at least one actuator component may be turn off, such that the bearing arrangement is reloaded and the movable part is retained in the desired position by the friction in the bearing arrangement produced by the loading arrangement. In this way, the movable element may be driven to make relatively small movements and to accurately control the position of the movable component. This way also enables reduction of the power and / or energy required to control the movement and / or position of the movable element. Optionally, the at least one actuator component is arranged, on actuation, to apply the unloading torque so as to reduce the frictional force components in the bearing arrangement. By applying the unloading torque, the motion of the movable part can be made easier when required. This can help to reduce the possibility of the movable part undesirably sticking. The force required to move the movable part in the plane of movement may be reduced compared to a situation in which the frictional forces are not reduced by the unloading torque. Optionally, the loading arrangement is arranged to apply a loading torque for loading the bearing arrangement. Optionally, the axis of the loading torque has a component in the plane of movement. Optionally, the axis of the loading torque has a component perpendicular to the plane of movement. By applying the loading torque around an axis with a component perpendicular to the plane of movement, the lateral forces imposed by the loading arrangement may be reduced. This can help to increase the accuracy of control of the position of the movable part. Optionally, the at least one actuator component comprises at least one pair of actuator components and the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure for applying an unloading torque for reducing load on the bearing arrangement, wherein the force components are offset from each other along an axis perpendicular to the axis of the unloading torque. In this way, the forces may be combined to form the unloading torque about an axis. By offsetting the forces, a torque can be generated by the actuator components. This can help to provide the unloading function without unduly generating unwanted forces that may affect the movement of the movable part. Optionally, the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged on either side of the plane of movement. In this way, the actuator components may be separated by the movable component. Optionally, the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged on the same side of the plane of movement. In this way, the actuator components may be on the same side of the movable component. Optionally, the at least one actuator component comprises an actuator unit comprising a shape memory alloy, SMA, element. By providing an SMA element, the actuation may be effected particularly accurately and simply. SMA, due to its high energy density, may also provide for a particularly compact actuator component, allowing the actuator assembly to be used in miniature applications, such as miniature cameras. Optionally, the resultant force applied by the actuator unit is applied at an acute, non-zero angle to the plane of movement. An advantage of such an angled resultant force is that, in the case of an SMA wire for example, a longer length of wire may be able to be used within a given space. A longer length of wire provides a greater level of accuracy in controlling the length of the SMA. Angling the SMA element in this way, however, may provide a gearing up effect (i.e. the intermediate component would move by an amount which is greater than the contraction of the SMA element). Accordingly, in the case where such angled SMA elements are used, other aspects of the assembly (e.g. the angle of the first feature with respect to the primary axis) may be selected in order to over-compensate for the gearing up to achieve an overall gearing-down effect (if desired). In some embodiments, the resultant force is at an angle to the primary axis which is less than 45 degrees. In other embodiments, the angle may be greater than 45 degrees. Optionally, the loading arrangement comprises at least one pair of loading components; and the loading components of each pair of the at least one pair of loading components are arranged to apply force components to the movable part relative to the support structure in opposite directions for loading the bearing arrangement. By providing a pair of loading elements, the forces applied may at least partly cancel each other out in directions other than the desired rotational direction for the loading torque. Optionally, the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure in opposite directions for applying an unloading torque for reducing load on the bearing arrangement. A force component applied by the loading components for loading the bearing arrangement and a force component applied by the actuator components for applying an unloading torque are parallel with each other. Optionally, the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure in opposite directions for applying an unloading torque for reducing load on the bearing arrangement. A force component applied by the loading components for loading the bearing arrangement and a force component applied by the actuator components for applying an unloading torque are perpendicular with each other. Optionally, the at least one actuator component comprises at least one pair of actuator components; the bearing arrangement comprises at least one pair of bearings; and a distance between the actuator components of each pair of the at least one pair of actuator components is less than a distance between the bearings of each pair of the at least one pair of bearings. By providing a greater preload distance, it can be ensured that the loading arrangement applies a loading force over the entire possible range of movement. The bearing arrangement can thus reliably be held together by the loading arrangement at any position along the range of movement. This can help to increase the accuracy of control of the position of the movable part. Optionally, the bearing arrangement comprises at least one bearing that is a plain bearing comprising bearing surfaces on the support structure and the movable part arranged to slide against each other. By providing a plain bearing, the friction may be increased so that it is easier for the position of the movable part to be maintained with reduced power / energy requirements. Optionally, the loading arrangement comprises a magnetic loading arrangement. By providing a magnetic arrangement, the lateral forces on the movable part may be reduced. This can help to increase the accuracy of control of the position of the movable part. Optionally, the loading arrangement comprises a resilient loading arrangement for resiliently loading the bearing arrangement. By providing a resilient loading arrangement, the loading may be provided without increasing power requirements. Optionally, the movable part is a lens element comprising at least one lens, wherein the primary axis is the optical axis of the lens element and wherein the lens element is arranged to focus light emitted from an. By providing a lens assembly, the control of the position of the movable part may be implemented in the context of an optical focusing system or optical athermilization system, for example. Optionally, the movable part is a lens element comprising at least one lens, wherein the primary axis is the optical axis of the lens element and wherein the lens element is arranged to focus light on an image sensor mounted on the support structure. By providing an image sensor, the actuator assembly may be implemented as a camera, for example. Optionally, the bearing arrangement is arranged to have sufficient friction when loaded that the movable part remains in position, when the actuator components are not applying an unloading torque and / or when the actuator components are not driving movement of the movable part. Optionally, the bearing arrangement is arranged to have sufficient friction when loaded that the movable part, over a continuum of positions, remains in position, when the actuator components are not applying an unloading torque and / or when the actuator components are not driving movement of the movable part. The frictional forces in the bearing arrangement, when the actuator components are not applying an unloading torque, may be greater than the weight of the movable part (optionally including a lens assembly when such a lens assembly is fixed relative to the movable part). The frictional forces in the bearing arrangement, when the actuator components are not applying an unloading torque, may be greater than 1.5 times, or 2 times, the weight of the movable part (optionally including a lens assembly when such a lens assembly is fixed relative to the movable part). By providing sufficient friction, the power and / or energy requirements to maintain the position of the movable part may be reduced. By providing sufficient friction, the power and / or energy requirements to maintain an arbitrary position of the movable part within a range of movement of the movable part may be reduced. The movable part may be held in position by the frictional forces in the bearing arrangement, without powering the actuator components. Optionally, the bearing arrangement is arranged to have sufficient friction when loaded that the movable part, over a continuum of positions, remains in position when the actuator components are not driving movement of the movable part. The frictional forces in the bearing arrangement, when the actuator components are not applying an unloading torque, may be greater than the weight of the movable part (optionally including a lens assembly when such a lens assembly is fixed relative to the movable part). The frictional forces in the bearing arrangement, when the actuator components are not applying an unloading torque, may be greater than 1.5 times, or 2 times, the weight of the movable part (optionally including a lens assembly when such a lens assembly is fixed relative to the movable part). By providing sufficient friction, the power and / or energy requirements to maintain the position of the movable part may be reduced. By providing sufficient friction, the power and / or energy requirements to maintain an arbitrary position of the movable part within a range of movement of the movable part may be reduced. The movable part may be held in position by the frictional forces in the bearing arrangement, without powering the actuator components. Optionally, the bearing arrangement comprises at least one bearing that is a rolling bearing comprising bearing surfaces on the support structure and the movable element and at least one rolling bearing element disposed between the bearing surfaces. By providing a rolling bearing the ease of movement of the movable part may be increased. The rolling bearing may advantageously reduce friction in the assembly (at interfaces where that is desired) but may also add to the manufacturing cost and complexity. Optionally, the bearing arrangement comprises at least one bearing comprising bearing surfaces comprising: a groove on one of the support structure and the movable part and a planar or convex surface on the other of the support structure and the movable part; or grooves on each of the support structure and the movable part. By providing grooves, the bearing may constrain the degrees of freedom of movement of the movable part. This may reduce the number of bearings required. Optionally, the resilient loading arrangement comprises at least one resilient element between the support structure and the movable part, wherein the resilient element is stressed in its mounted position connected between the support structure and the movable part so as to load the bearing arrangement, whereby parts of the resilient element that engage with the support structure and the movable part are less distanced in a direction along the plane of movement than if the resilient element were not stressed. By providing a stressed resilient element, the loading torque may be applied in a mechanically simple way that is relatively easy to manufacture. Optionally, the difference in how distanced along the plane of movement the parts of the resilient element that engage with the support structure and the movable part are is greater than a possible range of movement of the movable part along the plane of movement. By providing a greater preload distance, it can be ensured that the loading arrangement applies a loading force over the entire possible range of movement. The bearing arrangement can thus reliably be held together by the loading arrangement at any position along the range of movement. This can help to increase the accuracy of control of the position of the movable part. Optionally, the resilient loading arrangement comprises at least one resilient element that engages with at least one of the support structure and the movable part via a bearing arrangement. Optionally, the movable part comprises an electronic component. Optionally, the movable part comprises an optical component. Optionally, the movable part comprises an image sensor. Optionally, the movable part comprises a light source or emitter. 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 1A is a schematic view of an actuator assembly. Figure IB is a different schematic view of the actuator assembly shown in Figure 1A. Figure IC is a different schematic view of the actuator assembly shown in Figure 1A. Figure ID is a different schematic view of the actuator assembly shown in Figure 1A. Figure 2A is a schematic view of an actuator assembly. Figure 2B is a different schematic view of the actuator assembly shown in Figure 2A. Figure 3A is a schematic side view of an actuator assembly. Figure 3B is a difference schematic view of the actuator assembly shown in Figure 3A. Figure 4 is a schematic side view of an actuator assembly. Figure 5 is a schematic view of an actuator assembly. Figure 6 is a schematic view of an actuator assembly. Figure 7 is a cross sectional view of an actuator assembly with a rolling bearing. Detailed description Actuator assembly An actuator assembly may comprise a support structure, a movable part, a bearing arrangement, a loading arrangement, and at least one actuator component. The bearing arrangement is arranged to guide movement of the movable part relative to the support structure in a plane of movement. The loading arrangement is for loading the bearing arrangement. The at least one actuator component is arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement. The actuator assembly may be a camera. The actuator assembly is described primarily in the context of the actuator assembly being a camera. However, the actuator assembly is not required to be a camera and may be embodied as a different type of apparatus. For example, an apparatus which requires controlled drive of movement of a movable element in a plane of movement. It may be desirable to reduce the power consumption of the actuator assembly. One way of achieving this is to reduce the amount of time during which power must be supplied to the at least one actuator component. In the example of Figures 1A-1D the at least one actuator component may be a pair of SMA elements. The configuration of the actuator assembly described with reference to Figures 1A-1D has the result that a normal force between the movable component and the support structure is reduced when the movable component is in motion and is increased when the movable component is stationary. In this way, frictional forces between the support structure and the movable component are lower during motion of the movable component and higher when the movable component is stationary. The actuator assembly can be thus configured such that when no power is supplied to the SMA elements the movable component remains stationary with respect to the support structure. Some of the present embodiments include a movable part and / or support structure comprising a lens having an optical axis, however the disclosure is not limited as such and it is understood that the movable part and / or support structures of the described embodiments may instead (or additionally) comprise any optical element having a primary axis (which may be an optical axis). The features outlined and discussed below would also apply equally to an embodiment including a movable component and / or support structure comprising an optical component other than a lens or any other component, whether optical or otherwise. In some embodiments, the movable part may instead comprise a part of an optical component, such as a part of a deformable optical element. Such a deformable optical element may be a deformable lens, such as a liquid lens, or deformable mirror, for example. The features outlined and discussed would also apply equally to an embodiment including a movable part comprising a part (e.g. a surface) of an optical component, optionally a deformable optical component. Movement of the movable part may thus be driven to deform a deformable optical component. This may be done to change a focal length or some other optical property of the optical component. Figures 1A to ID Figures 1A-1D are schematic views of an actuator assembly 100. Figures 1A, IB, IC are perspective views, Figures ID is a plan view. It will be appreciated that various features of the embodiments shown in the figures (e.g. bearing arrangements, biasing arrangements, actuator arrangements etc.) could be combined in various combinations. Support structure and movable part - Figures 1A to ID The actuator assembly 100 comprises a support structure 102 and a movable part 104. The movable part 104 comprises one or more lenses (not shown) and is configured to move relative to the support structure 102 in a plane of movement. The movable part comprises a plurality of sides extending in a loop around a primary axis. In this case, the movement is one-dimensional movement in a plane of movement which is parallel to a primary axis defined by the optical axis O of the one or more lenses. The support structure 102 comprises two bearing pins 112a, 112b which bear the movement of the movable part 104. The bearing pins 112a, 112b constrain movement of the movable component. As seen in Figure ID a portion of the movable component 104 has a cross-section having a V-shaped surface. This portion engages with a first bearing pin 112a at two surfaces (one on each side of the V). As seen in Figure ID a portion of the movable component 104 has a cross-section having a flat surface. This portion engages with a second bearing pin 112b with a single surface (a flat face in this example) of the movable part 104. Accordingly, a total of three surfaces of the movable component 104 contact the bearing pins 112a, 112b. These multiple contact surfaces ensure that the movable component 104 moves along the bearing pins 112a, 112b smoothly. A greater number of contact surfaces could over-constrain the movable component 104 and lead to less-smooth motion. The movable part 104 moves along or slides over the bearing pins 112a, 112b of the support structure 102. One or more surfaces of the movable part 104 and / or the bearing pins 112a, 112b are configured such that a frictional force between the bearing pins 112a, 112b and the movable component 104 is great enough to hold the movable part 104 in position with respect to the support structure 102 when the actuator component does not apply a force to the movable part 104. In the case of the present SMA element, the frictional force holds the movable part 104 in position when the SMA elements are unpowered (and hence not contracted). A mechanism by which frictional forces are reduced during motion of the movable component 104 and are increased again when movement is ceased is employed. The support structure may further comprise support portions which the SMA elements of the actuator components are attached to. The support portions may also constrain movement of the movable part 104 when the actuator component 110 is actuated. For example, support portions *** of Figure ID limit the rotation of the movable part 104 around the primary axis defined by the optical axis of the one or more lenses. Bearing arrangement - Figures 1A to ID The actuator assembly 100 also comprises a bearing arrangement 106. The bearing arrangement 106 supports the movable part 104 on the support structure 102. The bearing arrangement 106 is arranged to guide movement of the moveable part 104 relative to the support structure 102 in the plane of movement. The direction of movement is shown in Figure 1A by the arrow labeled "Movement direction". In this example, the movement is linear one dimensional movement in a plane which is parallel to the primary axis. In this example, the primary axis is defined by the optical axis O of the one or more lenses. The movement of the movable part 104 guided by the bearing arrangement 106 includes a translational movement parallel to the primary axis defined by assembly (optical axis O). The translational movement along an axis parallel to the optical axis O is the desired movement of the movable part 104, for example to change the focus of the image on an image sensor and / or to change the magnification (zoom) of the image on an image sensor. The bearing arrangement 106 of Figures 1A-1D comprises a pair of bearings. The pair of bearings are plain bearings comprising a bearing surface on the support structure 102 and a bearing surface on the movable part 104, arranged to slide against each other, as described above, with respect to the bearing pins 112a, 112b of the support structure 102. Loading arrangement - Figures 1A to ID The actuator assembly 100 also comprises a loading arrangement (not shown). The loading arrangement is arranged between the support structure 102 and the movable part 104. The loading arrangement is for loading the bearing arrangement 106. The loading arrangement may also be referred to as a biasing arrangement. Loading the bearing arrangement 106 means urging the different parts (e.g. bearing surfaces) of the bearing arrangement 106 towards each other. The loading arrangement is arranged to apply a loading torque for loading the bearing arrangement 106. The axis of the loading torque is parallel to the plane of movement. The loading arrangement comprises a pair of loading components. Each loading component is arranged to apply the force component to the movable part 104 relative to the support structure 102 in opposite directions for loading the bearing arrangement 106. The loading arrangement is arranged to load the bearing arrangement 106 so as to generate frictional force components therein that constrain the movement of the movable part 104 relative to the support structure 102 at any position within a range of movement when actuator components 110 are not actuated. If the bearing arrangement 106 is not loaded (i.e. is unloaded), then the bearing arrangement 106 may not be capable of guiding movement of the movable part 104 relative to the support structure 103 in the plane of movement. If the bearing arrangement 106 is not loaded, then the bearing surfaces of the bearing arrangement 106 may lose contact with each other. By loading the bearing arrangement 106, the bearing arrangement 106 may reliably guide movement of the movable part 104 relative to the support structure 102 in the plane of movement. Figure ID shows loading force arrows but does not show the loading arrangement itself. The loading force arrows show the forces applied to the movable part 104 by the loading arrangement. For example, the loading arrangement may urge the movable part by a force that acts generally perpendicular to the plane of movement. As shown in Figure ID, the loading arrangement is arranged to apply a loading torque about an axis parallel to the plane of movement for loading the bearing arrangement 106. The loading torque providing by the loading arrangement 108 may be in a sense opposite to the unloading torque provided by the actuator components 110, in embodiments in which both the loading and unloading torques are provided. As shown in Figure ID, the forces applied by the loading arrangement on the movable part 104 act generally in the directions perpendicular to the plane of movement. However, the two forces applied by the two loading elements of the loading arrangement act on either side of the plane of movement. The plane of movement is between the loading force arrows. This creates a loading torque. The axis about which the loading torque is applied is an axis that extends into and out from the drawing sheet. By providing that the loading of the bearing arrangement 106 is achieved by a loading torque about an axis parallel to the plane of movement, the possibility of the forces that load the bearing arrangement 106 undesirably affecting the movement of the movable part 104 is reduced. It is desirable for the forces that load the bearing arrangement 106 not to act in a direction that could cause movement of the movable part 104 relative to the support structure along the plane of movement when this is not desired. For example, as shown in Figure ID the two loading force arrows for the loading arrangement (not shown) act generally in opposite directions to each other. As a result, the overall force in the direction of the plane of movement may be small or even zero. As a result, the loading arrangement itself may not significantly drive movement of the movable part 104 relative to the support structure 102. This may help the movement of the movable part 104 in the plane of movement to be controlled more accurately by controlling movement of the movable part 104 by the actuator components 110. Actuator component - Figures 1A to ID The actuator assembly 100 also comprises a pair of actuator components 110. The pair of actuator components are SMA elements, which comprise SMA wires. Only one of the SMA wires is visible from the angle of Figure 1A. As shown in Figure 1A, the SMA wire is connected between the support structure 102 and the movable part 104. The SMA wire is connected to the support structure 102 via a connection element such as a static crimp 122. The SMA wire is connected to the movable part 104 via a connection element such as a moving crimp 124. The second SMA wire which is not visible in Figure 1A is provided on the opposite side of the actuator assembly 100. The static crimp 122 for connecting the second SMA wire to the support structure can be seen in Figure 1A. The crimps described herein may be any type of coupling elements or connectors suitable for mechanically (and electrically) connecting SMA wires to a component of the SMA actuator assembly. During use, power is supplied to each SMA element to cause resistive heating of the SMA wire which in turn causes the SMA wire to contract and actuate. The pair of actuator components 110 are arranged, on actuation, to apply an unloading torque for reducing a load on the bearing arrangement 106. The unloading torque is applied by pair of actuator components 110 (e.g. SMA wires) by driving rotation of the movable part 104 in the same sense around an axis in the plane of movement. The two SMA wires can be actuated to cause rotational movement around the axis in the same sense. The SMA wires may be controlled (i.e. actuated) so as to control the position of the movable part 104 around the axis, for example within a range of rotational movement of the movable part 104 relative to the support structure 102. Preferably, the movable part 104 does not actually rotate, the rotation of the movable part for applying the unloading torque merely reduces the load on the bearing arrangement such that the normal force is reduced, without any actual rotational movement of the movable part 104. The pair of actuator components 110 are further arranged, on actuation, to drive the movement of the movable part 104 relative to the support structure 102 in a plane of movement. The driving force is applied by the pair of actuators to drive movement of the movable part 104 in the plane of movement in opposite directions in the plane of movement. The two SMA wires can be actuated to cause translational movement in the plane of movement in opposite directions. The SMA wires are controlled (i.e. actuated) so as to control the position of the movable part 104 along the axis, for example within a range of translational movement of the movable part 104 relative to the support structure 102. The SMA wires are angled with respect to the plane of movement in order to both apply the unloading torque and also drive movement of the movable part in the plane of movement. The angling of the SMA wires also enables a longer length of wire to fit into the space. Figure IB is a schematic side view of the actuator assembly 100 shown in Figure 1A. Figure IC is a schematic side view of the opposite side of the actuator assembly 100 shown in Figure 1A and IB. The two different SMA wires that are the actuator components can be seen in Figures IB and IC, which show opposite sides of the same actuator assembly 100. Figures IB and IC show force arrows indicating the direction of forces applied by the actuator components. Two of the arrows are wire force components and one of the arrows is the wire force resultant. These are forces that are applied to the movable part 104. The upper force component arrow shown in Fig. IB shows the force applied to the movable part 104 to urge the movable part in the indicated direction. This force is applied when the first SMA wire is contracted. The bottom force component arrow shown in Figure IB shows the force applied to the movable part 104 by contraction of the first SMA wire to apply an unloading torque for reducing load on the bearing arrangement. Figure IC shows force arrows indicating the direction of forces applied by the SMA wires. Two of the arrows are wire force components and one of the arrows is the wire force resultant. These are forces that are applied to the movable part 104. The lower force component arrow shown in Fig. IC shows the force applied to the movable part 104 to urge the movable part in the indicated direction. This force is applied when the second SMA wire is contracted. The upper force component arrow shown in Figure IC shows the force applied to the movable part 104 by contraction of the second SMA wire to apply an unloading torque for reducing load on the bearing arrangement. Figures IB and IC further show a resultant force arrow. The resultant force arrow indicates the general direction of the force formed by a combination of the component force arrows applied by the first and second SMA wires as actuator components. - Unloading torque - In order to reduce the load on the bearing arrangement by providing the unloading torque around an axis, at least one of the pair of SMA wires are contracted. The axis about which the unloading torque acts is referred to as the axis of the unloading torque. In this example, the axis of the unloading torque is parallel to the primary axis. When at least one of the SMA wire contracts, the force it applies on the movable component 104 has components along the X and Z axes. The component of force along the X axis acts to pull the movable component 104 along the X axis. This in turn reduces the normal force acting between the movable component 104 and the bearing pins, along the x direction. This means that when the first SMA wire is contracted, the normal force (and hence the frictional forces) between the movable component 104 and the bearing pins is reduced. Friction is therefore lower during motion of the movable component 104 and relatively higher when the movable component is stationary. The assembly can be configured such that when the movable component 104 is stationary the friction is high enough to hold it in position with respect to the support structure 102. Power therefore does not need to be supplied to the SMA wires to hold the movable component still. The power consumption of the device is therefore reduced. The component force of the first SMA wire along the Z axis acts to pull the movable component 104 along the Z axis in a first direction. The second SMA wire also acts in the same way to reduce the normal force of the movable component 104 on the bearing pins during motion. The component force of the second SMA wire along the Z axis acts to put the movable component 104 along the Z axis in a second direction. The second SMA wire pulls the movable component 104 in the opposite direction along the z axis to the first SMA wire. As shown in Figure ID, the pair of actuator components (e.g. SMA wires) are arranged to apply an unloading torque about an axis parallel to the primary axis defined by the assembly so as to reduce loading of the bearing arrangement. In the actuator assembly shown in Figure ID, the axis about which the unloading torque is applied is an axis that extends into and out from the drawing sheet. The axis is generally perpendicular to the length of the SMA wires and parallel to the primary axis defined by the assembly. In this example, the primary axis defined by the assembly may be that the primary axis is defined by multiple sides of the movable part extending in a loop around the primary axis. As shown in Figure ID, the pair of actuator components (e.g. SMA wires) are arranged to apply forces to the movable part 104 relative to the support structure 102 that are offset from each other along an axis perpendicular to the plane of movement. This offset along an axis perpendicular to the plane of movement allows the forces to combine to form the unloading torque about the an axis which is parallel to the plane of movement, that is the axis of the unloading torque. In the example shown in Figure ID, the actuator components 110 are SMA wires. In such a case, the SMA wires may be arranged to be offset from each other along an axis perpendicular to the plane of movement. The axis about which the unloading torque is applied is between the forces applied by the actuator components 110, for example between the SMA wires when the SMA wires are the actuator components 110. The forces applied by the SMA wires act in the direction of the SMA wires. As shown in Figure ID, the pair of actuator components 110 are arranged to apply forces in opposite directions perpendicular to the plane of movement such that the unloading torque can be applied without applying an overall force perpendicular to the plane of movement. The force arrows shown in Figure ID generally oppose each other. The force arrows are generally perpendicular to the plane of movement. The force arrows are in the direction of the SMA wires themselves. The SMA wires may be generally perpendicular to the plane of movement In general, however, the SMA wires may be oriented at an acute angle relative to an axis perpendicular to the plane of movement. When the movable part 104 moves along the plane of movement relative to the support structure 102, the angle of orientation of the SMA wires may vary. However, the forces and the SMA wires may remain generally approximately perpendicular to the plane of movement (or at least at an acute angle perpendicular to the plane of movement). Optionally, the forces applied by the SMA wires could be equal to each other in magnitude but applied in opposite directions. This would result in no overall force perpendicular to the plane of movement. However, the unloading torque could still be applied. This means that the loading of the bearing arrangement 106 can be controlled without adversely affecting the control of the position of the movable part 104 relative to the support structure 102. By providing an unloading torque so as to reduce loading of the bearing arrangement 106, the extent of loading of the bearing arrangement 106 may be varied in a controlled manner. For example, when it is desirable to move the movable element 104 in the plane of movement, then the loading of the bearing arrangement 106 may be reduced by applying the unloading torque. By reducing loading of the bearing arrangement 106, the friction in the bearing arrangement 106 (or generally the resistance to motion in the bearing arrangement) may be reduced. This allows the movable part 104 to move more freely relative to the support structure 102. Of course, it is desirable for the bearing arrangement 106 to remain loaded at least to some extent so that the bearing arrangement 106 can continue to reliably guide movement of the movable part 104 relative to the support 102 during use of the actuator assembly 100. It is desirable for the unloading torque to be less than a threshold amount which would result in the bearing arrangement 106 becoming unloaded. By providing that the unloading torque is applied by the actuator components 110 that drive rotation of the movable part 104 for unloading and cause the movable part 104 to move in the plane of movement, the loading of the bearing arrangement 106 can be controlled without requiring additional components for controlling the loading of the bearing arrangement 106. The actuator components 110 may be provided already in such an actuator assembly. The actuator components 110 are controlled in a new way so as to control loading of the bearing arrangement 106. By providing that the loading of the bearing arrangement 106 is reduced by an unloading torque about an axis parallel to the plane of movement, the possibility of the unloading torque itself directly resulting in movement of the movable part 104 is reduced. For example, if the reduction in loading of the bearing arrangement 106 were achieved by applying a force that acts primarily or purely in the plane of movement, then the unloading force itself may cause the movable part to move in the plane of movement. Hence the movement of the movable part may be affected in an undesirable way. By providing the unloading torque about the axis parallel to the plane of movement, undesirable effects on the movement may be reduced. However, in the present example, movement of the movable part 104 in the plane of movement and the control of said movement is wanted. As such, the actuator component 110 provides one component of force which provides the unloading torque and one component of force with provides the force for movement of the movable part 104. - Driving movement - Of course, it may be desirable to apply different forces by the different SMA wires. For example, it may be desirable to drive movement of the movable part 104 so as to move the movable part 104 relative to the support structure 102 in the plane of movement. Additionally, or alternatively, it may be desirable to control a difference in forces applied by the SMA wires in order to counteract other external forces such as gravity. In order to translate the movable component 104 in the plane of movement, the pair of SMA wires are contracted. Contracting the wires causes the movable component 104 to rotate anti-clockwise (in this example). Rotating the movable component 104 about an axis parallel to the plane of movementimparts an unloading torque about an axis parallel to the plane of movement. Contracting the first wire more than the second causes the movable component 104 to move upwards the plane of movement (and parallel to the primary axis in this example). Contracting the first wire less than the second causes the movable component 104 to move downwards in the plane of movement (and parallel to the primary axis in this example). The movement of the movable part 104 in the plane of movement requires less power because the friction between the movable part 104 and the support structure 102 has been reduced by imparting the unloading torque. Zero hold power - Figures 1A to ID When movement of the movable part 104 is not desired (for example when it is desired for the movable part 104 to maintain its position relative to the support structure), the loading of the bearing arrangement may be increased. For example, the unloading torque may be reduced so as to reduce any reduction in loading of the bearing arrangement 106 caused by the unloading torque. By increasing loading of the bearing arrangement 106, friction within the bearing arrangement 106 is increased. The friction helps to reduce the amount of power required by the actuator components 110 in order to keep the position of the moveable part 104 relative to the support structure 102. It is possible that the power required to maintain the position of the movable part in the plane of movement may be eliminated. In other words, when the actuator components 110 are not actuated, the friction within the bearing arrangement 106 is sufficient to keep the movable part 104 in position relative to the support structure 102. This may be referred to as zero hold power. As such, the bearing arrangement 106 has sufficient friction when loaded that the movable part 104 remains in position when the actuator components are not driving movement of the movable part 104. The bearing arrangement 106 is generally good at resisting linear forces caused by shocks, for example. Such a linear force may increase friction on one or more of the bearings of the bearing arrangement 106, thereby actually increasing the resistance to motion. Zero hold power actuators have a benefit of using no power when holding a position. This is particularly advantageous for devices that have limited power (e.g. a limited peak power) and / or energy (e.g. a limited average power). For example, wearables may have limited power and / or energy. Other battery powered devices may similarly have limited power and / or energy available. The bearing arrangement 106 is arranged to have sufficient friction when loaded that the movable part 104 remains in position when the actuator components 110 are not driving movement of the movable part 104. The bearing arrangement 106 is arranged to have sufficient friction when loaded that the movable part 104 remains in position when the actuator components 110 are not providing an un-loading torque. This allows the power and energy requirements of the actuator assembly 100 to be reduced while allowing the position of the movable part 104 to be controlled and maintained. The bearing arrangement 106 is arranged to have sufficient friction when loaded that the movable part 104, over a continuum of positions, remains in position when the actuator components 110 are not driving rotation of the movable part 104. This allows the movable part 104 to be controlled to maintain any arbitrary position relative to the support structure 102, at least within a range of movement of the movable part 104 relative to the support structure 102. This is an improvement over ratchet-type systems which may maintain the position of a component but only at a set of discrete intervals. The friction within the bearing arrangement 106 allows the movable part 104 to be held at any of a continuum of positions. The loading arrangement is arranged to load the bearing arrangement 106 so as to generate frictional forces therein that constrain the movement of the movable part 104 relative to the support structure 102 at any position within a range of movement when the actuator components 110 are not actuated. The constraining of the movable part 104 may be such that the position of the movable part 104 is maintained relative to the support structure 102. Once a desirable position of the movable part 104 has been found, it is not necessary to again control the movement of the movable part 104 in order to maintain that desirable position for a subsequent process (e.g. taking of a photograph with a camera). The pair of actuator components 110 is arranged, on actuation, to apply the unloading torque so as to reduce the frictional forces in the bearing arrangement 106. As shown in Figure ID, the unloading torque counteracts the loading torque. The loading torque and the unloading torque may be about the same axis (in this example, an axis parallel to the primary axis). The unloading torque acts to cancel out part of the loading torque. Of course, the loading torque may overall remain greater than the unloading torque such that the bearing arrangement 106 remains loaded, at least to an extent. By reducing the frictional forces in the bearing arrangement 106, the ease of movement of the movable part 104 relative to the support structure may be controlled. For example, when it is desirable to maintain the position of the movable part 104, the friction can be increased by reducing the unloading torque. When it is desirable to move the movable part 104 in the plane of movement, then the unloading torque may be increased so as to reduce the friction within the bearing arrangement 106. Figures 2A and 2B An embodiment of an actuator assembly 100 is illustrated in Figures 2A and 2B. Figures 2A and 2B are schematic perspective views of an actuator assembly 100. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. In Figures 1A-1D, the SMA wires are arranged to pull away from the bearing pins of the support structure 102. Conversely, in Figures 2A and 2B, the SMA wires are arranged to pull towards the bearing pins of the support structure 102. In this way, the support structure 102 does not require further support portions which the SMA elements of the actuator components are attached to. In Figures 2A and 2B, the SMA elements of the actuator components are attached to the bearing pins of the support structure 102. In Figures 1A-1D the distance between the pair of SMA wires is similar to the distance between the pair of bearings 106. In Figures 2A and 2B the pair of SMA wires are closer and the pair of bearings 106 are further apart. In this way, the force required to unload the torque loaded on the bearing arrangement 106 is reduced. Figures 3A and 3B An embodiment of an actuator assembly 100 is illustrated in Figures 3A and 3B. Figures 3A and 3B are schematic views of an actuator assembly 100. Figure 3A is a side view and Figure 3B is a plan view. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. The loading arrangement 108 for loading the bearing arrangement 106 comprises a magnetic loading arrangement. The magnetic loading arrangement comprises two magnets disposed on or in the movable component 104, each magnet opposite a respective bearing pin 112a, 112b, as shown in Figure 3B. The bearing pins 112a, 112b comprise magnetic material (e.g. magnetic steel). The magnetic force between the magnets and the bearing pins 112a, 112b keeps the movable component 104 in contact with the bearing pins 112a, 112b. In the arrangement shown in Figure 3B, there are two magnets and two respective portions of magnetic material. The magnetic loading arrangement is configured to provide the force for loading the bearing arrangement 106. By providing the magnets, the loading torque may be applied with little or even no lateral forces. The magnet 65 has a low lateral force over the stroke of the movable part 10. In Figures 1A-1D the actuator components 110 of the pair of actuator components are arranged on either side of the support structure 102. When the movable part 104 and / or the support structure 102 comprise a lens having an optical axis, the actuator components 110 of the pair of actuator components are arranged on either side of the lens. In the embodiment of Figures 3A and 3B, the actuator components 110 of the pair of actuator components are arranged on the same side of the support structure 102. When the movable part 104 and / or the support structure 102 comprise a lens having an optical axis, the actuator components 110 of the pair of actuator components are arranged on the same side of the lens. The sides of the lens may be defined as multiple sides extending in a loop around the primary axis, which may be the optical axis of the lens. The sides of the support structure may be defined as multiple sides extending in a loop around the primary axis, which may be the optical axis of the lens. Figure 4 An embodiment of an actuator assembly 100 is illustrated in Figure 4. Figure 4 is a cross sectional side view of an actuator assembly 100. The movable part is one single part, where the two portions of the movable part shown in Figure 4 are connected in an area not shown in this cross sectional side view of the actuator assembly. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. In Figures 1A-1D, Figures 2A-2B, Figures 3A-3B and Figure 4, the unloading torque is in a different degree of freedom to the movement of the movable part 104 relative to the support structure 102 in the plane of movement. In Figures 1A-1D, Figures 2A-2B, and Figures 3A-3B, the axis of the unloading torque is in the plane of movement, wherein the plane of movement is parallel to the primary axis. Conversely, in the embodiment of Figure 4 the axis of the unloading torque is perpendicular to the plane of movement, wherein the plane of movement is parallel to the primary axis. That is, in Figures 1A-1D, Figures 2A-2B, and Figures 3A-3B, the unloading torque is around the z axis, parallel to the plane of movement and the primary axis. Conversely, in the embodiment of Figure 4, the unloading torque is around the x axis, perpendicular to the plane of movement and the primary axis. Figure 5 An embodiment of an actuator assembly 100 is illustrated in Figure 5. Figure 5 is a schematic perspective view of an actuator assembly 100. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. In Figures 1A-1D, Figures 2A-2B, Figures 3A-3Band Figure 5 the axis of the unloading torque is in the plane of movement. In Figures 1A-1D, Figures 2A-2B, and Figures 3A-3B the plane of movement is parallel to the primary axis defined by the assembly. Conversely, in the embodiment of Figure 5, the plane of movement is perpendicular to the primary axis defined by the assembly. In Figures 1A-1D, Figures 2A-2B and Figures 3A-3B the bearing arrangement is arranged to guide one dimensional movement of the movable part in the plane of movement relative to the support structure. Conversely, in the embodiment of Figure 5, the bearing arrangement is arranged to guide two-dimensional movement of the movable part in the plane of movement relative to the support structure. That is, in Figure 5, the bearing arrangement 106 is arranged to guide movement of the movable part 104 relative to the support structure 102 in the XY plane. This embodiment has two pairs of actuator components are arranged, on actuation, to drive the movement of the movable part 104 relative to the support structure 102 in the XY plane. Two of the actuator components are raised with respect to two other actuator components, relative to the plane of movement. In this way, the unloading torque for reducing load on the bearing arrangement 106 can be applied when the actuator components are actuated. The actuator components are controlled (i.e. actuated) so as to control the position of the movable part in the XY plane. The actuator components may be controlled in the manner described in WO 2013 / 175197 Al. The loading arrangement is arranged to apply a loading torque for loading the bearing arrangement 106. The axis of the loading torque is perpendicular to the primary axis. That is, the axis of the loading torque is an axis which extends along the XY plane. The two pairs of actuator components are arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement. The unloading torque is applied around the XY axis. The bearing arrangement 106 supports the movable part 104 on the support structure 102. The bearing arrangement 106 comprises a pair of bearings. The pair of bearings are plain bearings comprising a bearing surface on the support structure 102 and a bearing surface on the movable part 104, arranged to slide against each other, as described above, with respect to the bearing surface of the support structure 102. The bearing surfaces of the support structure 102 are positioned on either side of the plane of movement and the movable part 104, such that one bearing surface is on top of the movable part 104 and the second bearing surface is underneath the movable part 104. Springs or magnets (not shown) are used to bias the moving part 104 against the two bearing friction surfaces. When power is applied to the SMA wires, the tension in the SMA wires creates a torque that reduces the normal force and hence the friction in the bearing friction surfaces. Figure 6 An embodiment of an actuator assembly 100 is illustrated in Figure 6. Figure 6 is an exploded schematic view of an actuator assembly 100. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. In Figures 1A-1D, Figures 2A-2B, Figures 3A-3Band Figure 5 the axis of the unloading torque is in the plane of movement. Conversely, in the embodiment of Figure 6, the axis of the unloading torque is perpendicular to the plane of movement. In Figures 1A-1D, Figures 2A-2B, and Figures 3A-3B the plane of movement is parallel to the primary axis defined by the assembly. Conversely, in the embodiment of Figure 6, the plane of movement is perpendicular to the primary axis defined by the assembly. In Figures 1A-1D, Figures 2A-2B and Figures 3A-3B the bearing arrangement is arranged to guide one dimensional movement of the movable part in the plane of movement relative to the support structure. Conversely, in the embodiment of Figure 6, the bearing arrangement is arranged to guide two-dimensional movement of the movable part in the plane of movement relative to the support structure. That is, in Figure 6, the bearing arrangement 106 is arranged to guide movement of the movable part 104 relative to the support structure 102 in the XY plane. This embodiment has at least one actuator component are arranged, on actuation, to drive the movement of the movable part 104 relative to the support structure 102 in the XY plane. The loading arrangement is arranged to apply a loading torque for loading the bearing arrangement 106. The axis of the loading torque is parallel to the primary axis. That is, the axis of the loading torque is the Z axis. The actuator components are arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement. The unloading torque is applied around the Z axis. The bearing arrangement 106 supports the movable part 104 on the support structure 102. The bearing arrangement 106 comprises an intermediate bearing component 130. The intermediate bearing component 130 is positioned between the movable part 104 and the support structure 102. A loading torque is applied to the movable part 104. The loading torque is counteracted by bearing surfaces between the intermediate bearing component 130 and the movable part and the support structure 102. This counteraction generates a friction load that resists motion in the XY plane. The at least one actuator component is arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement. The at least one actuator component is further arranged, on actuation, to move the movable part 104 in the XY plane. In this way, the unloading torque for reducing load on the bearing arrangement 106 can be applied when the actuator components are actuated. The actuator components are controlled (i.e. actuated) so as to control the position of the movable part in the XY plane. Figure 7 An embodiment of an actuator assembly 100 is illustrated in Figure 7. Figure 7 is a cross sectional side view of an actuator assembly 100. The actuator assembly 100 operates using the same general principles as the embodiments described with reference to Figures 1A-1D, and has a number of features in common with them. Accordingly, only the differences will be described here. The loading arrangement 108 for loading the bearing arrangement 106 comprises a spring arrangement. The spring arrangement comprises a leaf spring 140 and a ball bearing 150. The ball bearing is arranged to run on the leaf spring to transmit the spring force into the movable part 104 without providing a vertical return force. The ball bearing sits within a recess in the movable part 104, which constrains the movement of the ball bearing. The loading force of the leaf spring is transferred to the movable part 104 through the ball bearing. The use of a ball bearing enables unrestrained movement of the movable part 104 relative to the support structure 102 in the Z axis. Figure 7 shows only one spring arrangement A second spring arrangement is located on the other side of the actuator assembly of Figure 7. The loading arrangement 108 of Figure 7 may be applied to any of the embodiments described herein. Other embodiments Support structure and movable part - other embodiments The actuator assembly 100 comprises a movable part 104 (or movable element or movable component). Optionally the movable part 104 is or comprises a lens assembly 120 having one or more lenses. The moveable part 104 may have an axis (for example an optical axis O) aligned with the image sensor 114 and may be arranged to focus an image on the image sensor 114. The actuator assembly 100 may be a miniature device. In some examples of a miniature device, the lens (or plural lenses, when provided) of the lens assembly 120 may have a diameter of at most 20mm, preferably at most 15mm, preferably at most 10mm. Although the actuator assembly 100 in some examples is a camera, that is not in general essential. In some examples, the actuator assembly 100 may be an optical device in which the movable part 104 comprises a lens assembly 114 but there is no image sensor. In other examples, actuator assembly 100 may be a type of apparatus that is not an optical device, and in which the movable part 104 is not a lens element and there is no image sensor. Examples include apparatuses for depth mapping, face recognition, game consoles, projectors and security scanners. The movable part 104 may be a lens element arranged to focus light emitted from an emitter. The lens element may comprise at least one lens. The plane of movement may be parallel to the primary axis. The plane of movement may be parallel to the optical axis of the lens element. The lens element may be arranged to focus light emitted from the emitter into the eye of a user. The movable part 104 may be a lens element arranged to focus reflected light. The lens element may comprise at least one lens. The plane of movement may be parallel to the primary axis. The plane of movement may be parallel to the optical axis of the lens element. The support structure may have an image sensor mounted thereon. The lens element may be arranged to focus reflected light on the image sensor. The movable part 104 may comprise one or more of an electronic component, an optical component, an image sensor, and a light source. All references to movement of a lens may refer to movement of a lens assembly, movement of one or more lenses or movement of a part thereof. Support structure - other embodiments The actuator assembly 100 comprises a support structure 102. The support structure 102 may have one or more components fixed to it, for example mounted on to it. For example, when the actuator assembly 100 is a camera, the support structure 102 may have an image sensor 114 mounted thereon. The support structure 102 may take any suitable form, typically including a base 116 to which the image sensor 114 is fixed. The support structure 102 may also support an IC chip 118. The support structure 102 may comprise one or more bearing surfaces which bear the movement of the movable component 104. In some examples, the one or more bearing surfaces may be one or more bearing pins. The one or more bearing surfaces of the support structure 102 may engage with one or more bearing surfaces of the movable component 104. Accordingly, at least one surface of the movable component 104 contacts the support structure 102. The at least one contact surface ensures that the movable component 104 moves relative to the support structure 102 in a plane of movement. The movable part 104 moves along or slides over the one or more bearing surfaces of the support structure 102, which may be in the same manner described with respect to the bearing pins of Figures 1A to ID. The support structure may further comprise support portions which the at least one actuator component is attached to. The support portions may also constrain movement of the movable part 104 in degrees of freedom other than that in which movement is desired when the at least one actuator component 110 is actuated. The bearing arrangement may be arranged to guide movement of the movable part 104 relative to the support structure 102 in a plane of movement. The bearing arrangement may be arranged to guide one or more of one dimensional movement, two-dimensional movement, and rotational movement of the movable part 104 relative to the support structure in a plane of movement. The plane of movement may be parallel to or have a component parallel to a primary axis defined by the assembly. The movement of the movable part 104 in the plane of movement relative to the support structure 102 may be one dimensional movement when guided by the bearing arrangement. The plane of movement may alternatively or additionally be perpendicular to or have a component perpendicular to a primary axis defined by the assembly. The movement of the movable part 104 in the plane of movement relative to the support structure 102 may be two-dimensional movement when guided by the bearing arrangement. The movement of the movable part 104 in the plane of movement relative to the support structure 102 may be rotational movement when guided by the bearing arrangement. It will be appreciated that it is not necessary that both the support structure and the movable component comprise surfaces shaped and angled in the ways shown in Figures 1A-1D, 2A, 2B, 3A, and 3B. For example, the engagement feature of the movable component may comprise a different shape (for example a cylindrical or otherwise shaped notch or projection) which engages with a surface the support structure. Similarly, the engagement feature of the movable component may comprise one or more surfaces and support structure may comprise a different shaped surface or feature which engages with the surface(s) of the engagement feature of the movable component. For example, the movable component could comprise a projection which moves in an angled slot on the support component (or vice versa). The support structure may align the movable component with one or more further components of the assembly (or of a device on which the assembly is disposed). For example, the actuator assembly 100 may be part of a projector system and the movable component may comprise a lens which is moved along the optical axis of the lens to account for thermal variations in the projector (i.e. to carry out athermalisation). The lens may be stacked on top of multiple multi-pixel arrays (otherwise referred to as pixel arrays or LED arrays), each of which provide a different colour for the pixels of an RGB image. The lens requires precise alignment with the arrays and the arrays themselves require precise alignment with each other. To achieve this, the support structure, for example the bearing pins, extend beyond the extent (e.g. along the optical axis) of the movement range of the movable component. The multiple arrays are positioned so as to engage with the support structure. Accordingly, the support structure acts as both a bearing component for the movable component and also as a surface against which further components of the assembly are placed so as to align them with each other and with the movable component. The support structure is received in respective apertures in the support structure. It will be appreciated that the support structure could be used to align only one component (e.g. an array) with the movable component or to align two components (e.g. two arrays) with each other, without aligning them with the movable component. In addition to aligning components within an assembly, the bearing component may also align the assembly as a whole with respect to a further assembly. If the alignment component is a sheet, a step of active alignment (using a motor to move one or more components of the assembly) may be required to remove any tilt between the movable component 104 and the array (or between multiple arrays themselves). Specifically, the support structure 102 may be moved by a small angle to remove tilt. Bearing arrangement - other embodiments The actuator assembly 100 comprises a bearing arrangement 106 which may be arranged in the same manner described with respect to Figures 1A to ID. The bearing arrangement 106 may comprise at least one bearing. The at least one bearing may comprise at least two bearing surfaces. The bearing surfaces may comprise a groove on one of the support structure 102 and the movable part 104 and a planar or convex surface on the other of the support structure 102 and the movable part 104. Alternatively, the bearing surfaces may comprise grooves on each of the support structure 102 and the movable part 104. The bearing arrangement 106 may comprise at least one pair of bearings. A distance between the actuator components 110 of each pair of the at least one pair of actuator components 110 may be less than a distance between the bearings of each pair of the at least one pair of bearings, as shown in Figure 2. In this way, the force required to unload the torque loaded on the bearing arrangement 106 is reduced. The bearing arrangement 106 may comprise at least one bearing that is a plain bearing comprising bearing surfaces on the support structure 102 and the movable part 104 arranged to slide against each other. The bearing arrangement 106 may be arranged to have sufficient friction when loaded that the movable part 104 remains in position when the actuator components 110 are not driving movement of the movable part 104.The bearing arrangement 106 may be arranged to have sufficient friction when loaded that the movable part 104, over a continuum of positions, remains in position when the actuator components 110 are not driving movement of the movable part 104. The bearing arrangement 106 may comprise at least one bearing that is a rolling bearing. The rolling bearing may comprise bearing surfaces on the support structure 102 and the movable element 104 and at least one rolling bearing element disposed between the bearing surfaces. The movement of the movable part 104 guided by the bearing arrangement 106 is in the plane of movement or includes a component of movement in the plane of movement. The movement in the plane of movement is the desired movement of the movable part 104. Movement of the movable part 104 not needed for optical purposes may be acceptable if it does not substantially impact the purpose of the actuator assembly 100, for example, change the focus of the image on the image sensor 114. The plane of movement may be parallel to or have a component parallel to a primary axis defined by the assembly, as shown in Figures 1 to 4. The movement of the movable part in the plane of movement relative to the support structure may be one dimensional movement when guided by the bearing arrangement, as shown in Figures 1 to 4. The plane of movement may be perpendicular to or have a component perpendicular to a primary axis defined by the assembly, as shown in Figures 5 and 6. The movement of the movable part in the plane of movement relative to the support structure may be two dimensional movement when guided by the bearing arrangement, as shown in Figures 5 and 6. The movement of the movable part in the plane of movement relative to the support structure may be rotational movement when guided by the bearing arrangement, as shown in Figure 5. - Sliding bearing - The bearing arrangement 106 may take a variety of forms. One possibility is that the bearing arrangement comprises one or more bearings that are sliding bearings, examples of which are shown in Figures 1A-1D, 2A, 2B, 3A, 3B. The sliding bearings may be a plain bearing that may comprise an elongate bearing surface on one of the support structure 102 and the movable part 104. The plain bearing may comprises protrusions formed on the other of the support structure 102 and movable part 104, the ends of the protrusions forming bearing surfaces which bear on the elongate bearing surface. Although two protrusions are shown in example embodiments in the Figures, in general any number of one or more protrusions may be provided. The elongate bearing surface and the bearing surfaces may be conformal, both being planar in example embodiments in the Figures, so as to permit relative movement of the movable part 104 with respect to the support structure 102. The elongate bearing surface and the bearing surfaces desirably have a coefficient of friction of 0.2 or more. A higher coefficient of friction may reduce or eliminate the power and / or energy to keep the movable part 104 in position. In general, however, lower coefficients of friction may be used and offset by larger loading forces so as to provide zero hold power, and vice versa. The loading arrangement and friction surfaces of the bearing arrangement may thus be designed to work together to provide zero hold power. In the first examples show in Figures 1A-1D one of the sliding bearings is a plain bearing that comprises a channel on one of the support structure 102 and the movable part 104, the inner surface of the channel forming a bearing surface. In each of the plain bearings, the materials of the bearing surfaces are chosen to provide smooth movement and a long life. The bearing surfaces may be unitary with the underlying component or may be formed by a surface coating. Suitable materials include, for example PTFE or other polymeric bearing materials, or metal. In each of the plain bearings, a lubricant may be provided on the bearing surfaces. Such a lubricant may be a powder or a fluid, for example. Suitable lubricants include: graphite; silicon paste or a low viscosity oil. - Rolling bearing - As mentioned above, the bearing arrangement 106 may take a variety of forms. Another possibility is that the bearing arrangement may comprise one or more bearings that are rolling bearings, examples of which are shown in Figure 7. In Figure 7, the bearing comprises a pair of bearing surfaces and plural rolling bearing elements, for example balls, disposed between the bearing surfaces. One of the bearing surfaces is provided on the support structure 102 and the other of the bearing surfaces is provided on the movable part 104. The bearing guides the movement of the movable part 104 with respect to the support structure 102 as shown by the arrow M. This may be achieved by the bearing surfaces extending along an axis parallel to a plane of movement. That said, in practical embodiments, the length of the bearing surfaces may be short compared to the distance of the bearing surfaces from the plane of movement. Plural bearings are typically present, located at different angular positions around the primary axis. In the example of Figure 7, the bearing surfaces each comprise respective grooves in which the rolling bearing elements are seated. In this example, the grooves constrain transverse translational movement of the movable part 104 with respect to the support structure 102, that is transverse to the direction of movement shown by arrow M. The grooves may be V-shaped in cross-section, but other cross-sections are possible, for example curved as in portions of a circle or an oval. In general, the grooves provide two points of contact with the respective rolling bearing elements. The grooves may extend linearly. In a different example, a first bearing surface may comprise a groove in which the rolling bearing elements are seated and a second bearing surface wherein the bearing surface is 'planar'. The first bearing surface may comprise a groove that may be provided on either one of the support structure 102 and the movable part 104, with the second bearing surface being provided on the other one of the support structure 102 and the movable part 104. In this example, the bearing does not constrain transverse translational movement of the movable part 104 with respect to the support structure 102, that is transverse to the direction of movement shown by arrow M. The bearing surface is 'planar' in the sense that it is a surface which is not a groove and one which provides only a single point of contact with the ball. In other words, the bearing surface is effectively planar across a scale of the width of the rolling bearing element. A single rolling bearing element is shown in Figure 7 by way of example, but in general may include any plural number of rolling bearing elements. In some examples, the bearing may include a single rolling bearing element. In that case, the bearing by itself does not constrain the movement of the movable part 104 with respect to the support structure 102 about the single rolling bearing element. However, this minimises the overall size of the bearing, and in particular the height of the bearing projected along an axis as it is only needed to accommodate the size of the rolling bearing element and the relative travel of the bearing surfaces. The bearing arrangement may in general comprise any number of bearings with a configuration chosen to guide the movement of the movable part 104 with respect to the support structure 102 while constraining the movement of the movable part 104 with respect to the support structure 102 in other degrees of freedom. Many bearing arrangements may comprise plural bearings and at least one which comprises plural rolling bearing elements. Loading arrangement - other embodiments The actuator assembly 100 comprises a loading arrangement 108 which may be arranged in the same manner described with respect to Figures 1A to ID. The loading arrangement 108 may be arranged to apply a loading torque for loading the bearing arrangement 106. The axis of the loading torque may be parallel to or have a component parallel to plane of movement. The axis of the loading torque may alternatively or additionally be perpendicular to or have a component perpendicular to plane of movement. The loading arrangement 108 may comprise at least one pair of loading components. The loading components of each pair of the at least one pair of loading components may be arranged to apply force components to the movable part 104 relative to the support structure 102 in opposite directions for loading the bearing arrangement 106. The loading arrangement 108 may be arranged to load the bearing arrangement 106 so as to generate frictional force components therein that constrain the movement of the movable part 104 relative to the support structure 102 at any position within a range of movement when the actuator components 110 are not actuated. - Magnetic loading arrangement - The loading arrangement 108 for loading the bearing arrangement 106 may comprise a magnetic loading arrangement. The magnetic loading arrangement may be configured to provide the force for loading the bearing arrangement 106. The magnetic loading arrangement may comprise one or more magnets disposed on or in the movable component 104, each magnet opposite a portion of the support structure 102, for example, as shown in Figure 3B. The portion of the support structure 102 may comprise magnetic material (e.g. magnetic steel). The magnetic force between the magnets and support structure 102 keeps the movable component 104 in contact with the bearing surface of the support structure 102. The magnetic loading arrangement may comprise a pair of magnets and a pair of magnetic materials. The number of magnets and magnetic materials is not particularly limited. In order to provide the loading torque to the bearing arrangement 106, it is desirable to have at least two magnets and two magnetic materials. However, the number of magnets may be four and the number of magnetic materials may be four, for example. By providing the magnets, the loading torque may be applied with little or even no lateral forces. The magnet has a low lateral force over the stroke of the movable part 104. In particular, if the magnetic material is provided such that it is wider than the magnet in the direction perpendicular to the plane of movement, then the magnetic field shift may be expected to be not particularly significant over the stroke of movement of the movable part 104in the plane of movement. The magnetic material may be provided as a metal shim, for example. - Resilient loading arrangement - The loading arrangement may comprise a resilient loading arrangement. The resilient loading arrangement may comprise a pair of resilient elements (e.g. springs). The resilient elements may exert a force that urges the bearing arrangement 106 together. The loading arrangement 108 may be provided in a variety of different forms, as explained in further detail below. The resilient loading arrangement may comprise at least one resilient element between the support structure 102 and the movable part 104. A resilient loading arrangement has the advantage that it does not need to be actuated in order to apply the load to the bearing arrangement 106. For example, the resilient element may be preloaded such that when it is mounted within the actuator assembly 100 it acts to urge the movable part 104 relative to the support structure 102 so as to provide the loading torque to the bearing arrangement 106. A resilient loading arrangement may comprise a pair of resilient elements 151. By providing a pair of resilient elements 151, a loading torque may be provided on the bearing arrangement 106 by combining the forces applied on the movable part 104 by the two resilient elements 151. The resilient elements 151 are between the support structure 102 and the movable part 104. For example, the resilient elements 151 may be connected between the support structure 102 and the movable part 104. For example, the resilient element 151 may comprise a static part 152 configured to be fixed to the support structure 102 and a moving part 153 configured to be fixed to the movable part 104. The resilient loading arrangement may comprise at least one resilient element between the support structure 102 and the movable part 104. The resilient element may be stressed in its mounted position connected between the support structure 102 and the movable part 104 so as to load the bearing arrangement 106, whereby parts of the resilient element that engage with the support structure 102 and the movable part 104 are less distanced in a direction along the plane of movement than if the resilient element were not stressed. The difference in how distanced along the plane of movement the parts of the resilient element that engage with the support structure 102 and the movable part 104 are may be greater than a possible range of movement of the movable part along the plane of movement. The resilient loading arrangement may comprise at least one resilient element that engages with at least one of the support structure 102 and the movable part 104 via a bearing arrangement 108. As the movable part 104 moves in the plane of movement, the forces applied on the movable part 104 by the resilient elements 151 varies. This is because the shape and / or orientation of the resilient element 151 changes. In particular, the distance in the plane of movement between the parts 152, 153 of the resilient element 151 that engage with the support structure to and the movable part 104 varies as the movable part 104 moves. By providing a relatively thick (in the direction of the plane of movement) resilient element 151, the change in the desired preload force over the stroke may be reduced. As mentioned above, the resilient element 51 may be preloaded with stress so that it applies a loading force on the movable part 104 when it is mounted in the actuator assembly 100. Optionally, the resilient loading arrangement comprises at least one resilient element 151 between the support structure 102 and the movable part 104. The resilient element 151 is stressed in its mounted position connected between the support structure 102 and the movable part 104 so as to load the bearing arrangement 106. By this, parts 152, 153 of the resilient element 151 that engage with the support structure 102 and the movable part 104 are less distanced in a direction in the plane of movement than if the resilient element 151 were not stressed. During manufacture of the resilient element 151, the resilient element 151 may be bent, for example a jog may be included in the resilient element 151. When the resilient element 151 is incorporated into the actuator assembly 100, it is mounted in a position so as to be deformed (e.g. to a more flat shape) compared to the bent shape during manufacture. Hence, there is a difference in the distance or extent in the plane of movement between the parts 152, 153 of the resilient element 151 when the resilient element 151 is mounted in the actuator assembly 100 compared to before it is mounted. So, the resilient element 151 may be pre-loaded during manufacture. Optionally, each resilient element is connected between the support structure 102 and the movable part 104. The resilient element may be fixedly connected at one end to the support structure 102 and at another end to the movable part 104. The resilient element may comprise a static part that engages with the support structure 102. The resilient element may comprise a moving part that engages with the movable part 104. For example, the moving part may be fixed to the movable part 104. Optionally, the moving part of the resilient element and the moving crimp may be provide as an integral component. However, this is not essential. In an alternative arrangement the moving part of the resilient element and the moving crimp may be provided as separate components. The moving part of the resilient element and the moving crimp may both be fixed relative to the movable part. Actuator component - other embodiments The actuator assembly 100 comprises at least one actuator component. The at least one actuator component may beat least one pair of actuator components 110. Optionally, at least one pair of actuator components are SMA elements, for example SMA wires. For example, as shown in Figure 1A, the actuator assembly 100 may comprises two SMA wires as the actuator components. However, other types of actuator components may be used. The at least one actuator component may be connected between the support structure 102 and the movable part 104. The at least one actuator component may be connected to the support structure 102 via a connection element such as a static crimp 122. The at least one actuator component may be connected to the movable part 104 via a connection element such as a moving crimp 124. In general, any connection element capable of fixing the at least one actuator component to the support structure 102 and / or movable part 104 may be used. The at least one pair of actuator components 110 may be arranged, on actuation, to apply an unloading torque for reducing a load on the bearing arrangement 106 in the same manner described with respect to Figures lAto ID. Preferably, the movable part 104 does not actually rotate, the rotation of the movable part for applying the unloading torque merely reduces the load on the bearing arrangement such that the normal force is reduced, without any actual rotational movement of the movable part 104. The at least one actuator component 110 may be arranged, on actuation, to drive the movement of the movable part 104 relative to the support structure 102 in a plane of movement. When the least one actuator component comprises a pair of actuator components 110, the driving force may be applied in in the same manner described with respect to Figures 1A to ID. Optionally, the at least one actuator component 110 may be driven by a control circuit or controller implemented in the IC chip 118. In particular, the control circuit may generate drive signals (e.g. PWM drive signals) for each actuator component 110 and supply the drive signals to the actuator component 110. The control circuit receives an input signal representing a desired position of the movable part 104 around an axis and generates drive signals selected to drive the movable part 104 to the desired position. The drive signals may be generated using a resistance feedback control technique, in which case the control circuit measures the resistance of the lengths of the actuator components, for example SMA wires, and uses the measured resistance as a feedback signal to control the power of the drive signals. As an alternative, the control circuit may include a sensor which senses the position of the movable part 102, for example a Hall sensor which senses the position of a magnet fixed to the movable part 102. In this case, the drive signals use the sensed position as a feedback signal to control the power of the drive signals. The at least one actuator component 110 may be any suitable actuator. For example, the actuator may comprise one or more SMA elements. Alternatively, any other suitable actuator could be used, such as voice coil motors (VCM), a piezo actuator, a MEMS (microelectromechanical system) drive system and / or shape memory polymer. - Unloading torque - In order to reduce the loading the bearing arrangement by providing the unloading torque around an axis, the at least one actuator component 110 is actuated. The unloading torque or a component of the unloading torque may be in a different degree of freedom to the movement of the movable part 104 or a component of the movement of the movable part 104 relative to the support structure 102 in the plane of movement. The unloading torque may be in a different degree of freedom to the movement of the movable part relative to the support structure in the plane of movement. For example, the movement of the movable part may be movement in the plane of movement and the unloading torque may be rotation about an axis in the plane of movement. As another example, the movement of the movable part may be movement in the plane of movement and the unloading torque may be rotation about an axis perpendicular to the plane of movement. As another example, the movement of the movable part may be rotation in the plane of movement and the unloading torque may be rotation in a plane perpendicular to the plane of movement. As another example, the movement of the movable part may be rotation in the plane of movement and the unloading torque may be rotation in a plane perpendicular to the plane of movement. There are six degrees of freedom in this instance. In the context of describing the degrees of freedom, the primary axis may also be referred to as the z axis, and two further axes that are perpendicular to the primary axis and to each other may be referred to as the x and y axes. The degrees of freedom are the following: movement along the x axis (Tx), movement along the y axis (Ty), movement along the z axis (Tz), rotation around the x axis (Rx), rotation around the y axis (Ry), rotation around the z axis (Rz). When at least one actuator component 110 is actuated, the force it applies on the movable component 104 may have components of force in the plane of movement and / or perpendicular to the plane of movement. A component of force produced by the at least one actuator component 110 reduces the normal force acting between the movable component 104 and the bearing surface of the support structure 102. This means that when the at least one actuator component 110 is actuated, the normal force (and hence the frictional forces) between the movable component 104 and the bearing surface of the support structure may be is reduced. Friction may therefore be lower during motion of the movable component 104 and relatively higher when the movable component is stationary. The assembly may be configured such that when the movable component 104 is stationary the friction is high enough to hold it in position with respect to the support structure 102. Power therefore does not need to be supplied to the SMA wires to hold the movable component still. The power consumption of the device is therefore reduced. The at least one pair of actuator components (e.g. SMA wires) 110 may be arranged to apply an unloading torque about an axis in the plane of movement or with a component in the plane of movement so as to reduce loading of the bearing arrangement. Alternatively or additionally, the at least one pair of actuator components (e.g. SMA wires) 110 may be arranged to apply an unloading torque about an axis perpendicular to or with a component perpendicular to the plane of movement so as to reduce loading of the bearing arrangement. The at least one pair of actuator components (e.g. SMA wires) 110 may be arranged to apply forces to the movable part 104 relative to the support structure 102 that are offset from each other along an axis in the plane of movement or which has a component in the plane of movement. Alternatively or additionally, the at least one pair of actuator components (e.g. SMA wires) 110 may be arranged to apply forces to the movable part 104 relative to the support structure 102 that are offset from each other along an axis perpendicular to or which has a component perpendicular to the plane of movement. This offset along an axis allows the forces to combine to form the unloading torque. The at least one pair of actuator components 110 may be arranged to apply forces in opposite directions perpendicular to the plane of movement such that the unloading torque can be applied without applying an overall force perpendicular to the plane of movement. When the at least one actuator component comprises at least one SMA element comprising at least one SMA wire, the SMA wires may be generally perpendicular to the plane of movement. Alternatively, or additionally, the SMA wires may be generally parallel to the plane of movement. In general, however, the SMA wires may be oriented at an acute angle relative to an axis perpendicular to the plane of movement. When the movable part 104 moves in the plane of movement relative to the support structure 102, the angle of orientation of the SMA wires may vary. However, the forces and the SMA wires may remain generally approximately perpendicular to the plane of movement (or at least at an acute angle perpendicular to the plane of movement). Optionally, the forces applied by the SMA wires may be equal to each other in magnitude but applied in opposite directions. This would result in no overall force perpendicular to the plane of movement. However, the unloading torque could still be applied. This means that the loading of the bearing arrangement 106 can be controlled without adversely affecting the control of the position of the movable part 104 relative to the support structure 102. By providing an unloading torque so as to reduce loading of the bearing arrangement 106, the extent of loading of the bearing arrangement 106 may be varied in a controlled manner. For example, when it is desirable to move the movable element 104 in the plane of movement, then the loading of the bearing arrangement 106 may be reduced by applying the unloading torque. By reducing loading of the bearing arrangement 106, the friction in the bearing arrangement 106 (or generally the resistance to motion in the bearing arrangement) may be reduced. This allows the movable part 104 to move more freely relative to the support structure 102. Of course, it is desirable for the bearing arrangement 106 to remain loaded at least to some extent so that the bearing arrangement 106 can continue to reliably guide movement of the movable part 104 relative to the support 102 during use of the actuator assembly 100. It is desirable for the unloading torque to be less than a threshold amount which would result in the bearing arrangement 106 becoming unloaded. By providing that the unloading torque is applied by the actuator components 110 that drive rotation of the movable part 104 for unloading and cause the movable part 104 to move in the plane of movement, the loading of the bearing arrangement 106 can be controlled without requiring additional components for controlling the loading of the bearing arrangement 106. The actuator components 110 may be provided already in such an actuator assembly. The actuator components 110 are controlled in a new way so as to control loading of the bearing arrangement 106. By providing that the loading of the bearing arrangement 106 is reduced by an unloading torque about an axis parallel to plane of movement, the possibility of the unloading torque itself directly resulting in movement of the movable part 104 is reduced. For example, if the reduction in loading of the bearing arrangement 106 were achieved by applying a force that acts primarily or purely in the plane of movement, then the unloading force itself may cause the movable part to move in the plane of movement. Hence the movement of the movable part may be affected in an undesirable way. By providing the unloading torque about the axis parallel to the plane of movement, undesirable effects on the movement may be reduced. However, in the present example, movement of the movable part 104 in the plane of movement and the control of said movement is wanted. As such, the actuator component 110 provides one component of force which provides the unloading torque and one component of force with provides the force for movement of the movable part 104. - Driving movement - Of course, it may be desirable to apply different forces by the different actuator components. For example, it may be desirable to drive movement of the movable part 104 so as to move the movable part 104 relative to the support structure 102 in a plane of movement. Additionally, or alternatively, it may be desirable to control a difference in forces applied by the actuator components in order to counteract other external forces such as gravity. In order to translate or rotate the movable component 104 in a plane of movement, the actuator components may be actuated, for example, contracting a pair of SMA wires. Actuatign the actuator components may cause the movable component 104 to rotate anti-clockwise or anti-clockwise about an axis. Rotating the movable component 104 about an axis imparts an unloading torque about said axis. The axis of the unloading torque may be in the plane of movement or have a component in the plane of movement. Alternatively or additionally, the axis of the unloading torque may be perpendicular to or have a component perpendicular to the plane of movement. Actuating a first component more than a second may cause the movable component 104 to move in a first direction in a plane of movement. Contracting the first component less than the second may cause the movable component 104 to move in a second, opposite, direction in the plane of movement. The movement of the movable part 104 in the plane of movement requires less power because the friction between the movable part 104 and the support structure 102 has been reduced by imparting the unloading torque. Zero hold power-other embodiments When movement of the movable part 104 is not desired (for example when it is desired for the movable part 104 to maintain its position relative to the support structure), the loading of the bearing arrangement 108 may be increased. For example, as described with respect to Figures 1A to ID. Zero hold power actuators may be arranged in the same manner described with respect to Figures 1A to ID. For example, the actuator assembly may be used in the context of an autofocus function of a camera. It may be desirable to maintain a focussed position of the movable part relative to the support structure between shots taken by the camera. In another example, the actuator assembly may be used in the context of providing athermalisation in an optical system. It may be desirable to maintain a position of the movable part relative to the support structure while the ambient temperature remains constant. 'Smart' materials Example devices described herein use heat-activated material as actuator(s) to control movement of components of the device. Examples of heat-activated material that may be used in these devices are: SMA (Shape Memory Alloy); this is typically a nickel-titanium alloy (e.g. Nitinol), but may also contain tertiary components such as copper. Physically crosslinked SMP (Shape Memory Polymer); representative shape memory polymers include polyurethanes, polyurethanes with ionic or mesogenic components made by a prepolymer method. Other block copolymers also show the shape-memory effect, including: a block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(l,4-butadiene), and an ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran. Chemically crosslinked SMPs; examples include crosslinked polyurethane or PEO-based crosslinked SMPs. The network polymer can be synthesized by either polymerization with multifunctional (3 or more) crosslinker or by subsequent crosslinking of a linear or branched polymer. In devices having two or more actuators, different actuators may be made from different ones of the above materials (or from two different materials of the same type). This may be useful to achieve an arrangement in which the actuators have different properties, either in terms of their mechanical properties or how they are actuated. SMA The above-described SMA actuator assemblies comprise actuator components. Optionally the actuator components comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. Similar / identical components in different examples Certain example devices are described above. Where similar or identical components are used in the different examples, they are given the same reference numerals. For efficiency, description of similar or identical elements may not be repeated between the examples and characteristics and features of elements are to be understood as applying to those elements in all examples unless the description indicates otherwise. Alternative ways of heating SMA The heating of the heat-activated actuator(s), such as SMA material, in order to cause the moving portion to move, could be achieved in a number of ways. In one arrangement, the material could be heated by passing a current through it. This current might come from a local or external power supply. Alternatively, the current might be induced in the wire by inductive coupling with an external alternating field. Where there are two actuators, the two actuators might be designed so that they couple to two different frequencies of the inductive power source, thus allowing the two actuators to be heated differentially. In another arrangement, the material could be heated by external radiation such as a visible or infra-red laser. The external radiation could be focussed so that one actuator is heated preferentially over another actuator, thus allowing differential actuation. Alternatively or additionally, different actuators, or portions of the actuators, could be treated (for example with a surface coating) so that the different actuators heat at different rates depending on the nature (e.g. the frequency) of the incident radiation." Applications The assembly may correspond to (part of) an illumination source which may be for use in a 3D sensing system such as described in WO2020 / 030916 or in an augmented reality (AR) display system. Where the movable part comprises an emitter or a display (or a part thereof), the movable part may be moved to achieve wobulation, for example for the display of a super-resolution image (i.e. an image having a resolution higher than that of the intrinsic resolution of the emitter or display). In this case, a high-resolution image is displayed (or projected) by displaying a number of lower-resolution images at different positions in rapid succession. The image displayed at each position is a lower-resolution image formed of a subset of pixels of the high-resolution image. The movable part may be moved between the positions in a repeated pattern at a high frequency, for example greater than 30 Hz, preferably greater than 60 Hz, further preferably greater than 120 Hz. The succession of lower-resolution images is thus perceived by the human eye as one high-resolution image. Plane of movement and primary axis The primary axis is defined based on the structure or orientation of components of the actuator assembly and the plane of movement is defined based on the movement of components of the actuator assembly. The plane of movement may be defined as the plane in which the movable part 104 moves relative to the support structure 102 when guided by the bearing arrangement. The plane of movement is not defined by movement of the movable part when unloading torque for reducing load on the bearing arrangement is applied. The primary axis may be defined by the assembly. For example, the primary axis may be defined by the optical axis of one or more lens elements of the movable part and may be colinear with or parallel to the optical axis. As another example, the primary axis may be defined by a plurality of sides of the movable part extending in a loop around the primary axis. As another example, the primary axis may be defined by the support structure, where the primary axis is the central axis of the support structure about which the support structure has rotational symmetry. The primary axis may alternatively be defined as the axis around which the at least one actuator components are arranged. For example, such that there is at least one actuator component arranged on each of two sides around the primary axis. There may be no actuator components on two other sides around the primary axis. As another example, there is at least one actuator component arranged on each of four sides around the primary axis. The primary axis may alternatively be defined as the axis around which one or more of the support structure 102, movable part 104, bearing arrangement 106, loading arrangement 108 and the at least one actuator component may have rotational symmetry. Said rotational symmetry may be 2-fold rotational symmetry. In the case that the movable part comprises a display, the display may define the plane and the primary axis may be perpendicular to the plane defined by the display. The plane of movement may be substantially perpendicular or parallel to the plane defined by the display. In any case, the primary axis may be aligned with a general direction in which light is emitted from the display. In the case that the movable assembly comprises an emitter, the emitter may define a plane and the primary axis may be perpendicular to the plane defined by the emitter. The plane of movement may be substantially perpendicular or parallel to the plane defined by the emitter. For example, the emitter may comprise a VCSEL array and the primary axis may be perpendicular to the plane of the VCSEL array. In any case, the primary axis may be aligned with a general direction in which radiation is emitted by the emitter. Display and emitter The display may be a display panel, for example a LCOS (liquid crystal on silicon) display, a MicroLED display, a digital micromirror device (DMD) or a laser beam scanning (LBS) system. The emitter is configured to emit radiation (visible light or non-visible radiation, e.g. near infrared (NIR) light, short-wave infrared (SWIR) light). The emitter may comprise one or more LEDs or lasers, for example VCSELs (vertical-cavity surface-emitting lasers) or edge-emitting lasers. The emitter may comprise a VCSEL array. The emitter may otherwise be referred to as an illumination source and / or may comprise an image projector. Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, the actuator assembly 1 may comprise a mixture of sliding bearing and rolling bearings. As a further alternative the bearing arrangement may comprise a flexure arrangement.

Claims

1. An actuator assembly comprising:a support structure;a movable part;a bearing arrangement arranged to guide movement of the movable part relative to the support structure in a plane of movement;a loading arrangement for loading the bearing arrangement; andat least one actuator component arranged, on actuation, to apply an unloading torque for reducing load on the bearing arrangement.

2. An actuator assembly according to any preceding claim, wherein a component of the unloading torque is in a different degree of freedom to a component of the movement of the movable part relative to the support structure in the plane of movement.

3. An actuator assembly according to any preceding claim, wherein the axis of the unloading torque has a component in the plane of movement.

4. An actuator assembly according to any preceding claim, wherein the axis of the unloading torque has a component perpendicular to the plane of movement.

5. An actuator assembly according to any preceding claim, wherein the movable part comprises a plurality of sides extending in a loop around a primary axis and the plane of movement has a component parallel to the primary axis.

6. An actuator assembly according to claim 5, wherein the bearing arrangement is arranged to guide one dimensional movement of the movable part in the plane of movement relative to the support structure.

7. An actuator assembly according to any one of claims 1 to 4, wherein the movable part comprises a plurality of sides extending in a loop around a primary axis and the plane of movement has a component perpendicular to the primary axis.

8. An actuator assembly according to claim 7, wherein the bearing arrangement is arranged to guide two dimensional movement of the movable part in the plane of movement relative to the support structure.

9. An actuator assembly according to any one of claims 5 and 7, wherein the bearing arrangement is arranged to guide rotational movement of the movable part in the plane of movement relative to the support structure.

10. An actuator assembly according to any preceding claim, the at least one actuator component further arranged, on actuation, to drive the movement of the movable part relative to the support structure in the plane of movement.

11. An actuator assembly according to any preceding claim, wherein the at least one actuator component comprises at least one pair of actuator components, and the actuator components of each pair of the at least one pair of actuator components are arranged, on actuation, to drive the movement of the movable part relative to the support structure in opposite directions in the plane of movement.

12. An actuator assembly according to any preceding claim, wherein the at least one actuator component is arranged to apply force components to the movable part relative to the support structure, wherein a first component of the force components applies the unloading torque for reducing load on the bearing arrangement and a second component of the force components drives the movement of the movable part relative to the support structure in the plane of movement.

13. An actuator assembly according to any preceding claim, wherein the at least one actuator component is arranged to reduce the loading of the bearing arrangement by less than the loading applied by the loading arrangement.

14. An actuator assembly according to any preceding claim, wherein the loading arrangement is arranged to load the bearing arrangement so as to generate frictional force components therein that constrain the movement of the movable part relative to the support structure at any position within a range of movement when the actuator components are not actuated.

15. An actuator assembly according to claim 14, wherein the at least one actuator component is arranged, on actuation, to apply the unloading torque so as to reduce the frictional force components in the bearing arrangement.

16. An actuator assembly according to any preceding claim, wherein the loading arrangement is arranged to apply a loading torque for loading the bearing arrangement.

17. An actuator assembly according to any preceding claim, wherein the at least one actuator component comprises at least one pair of actuator components and the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure for applying an unloading torque for reducing load on the bearing arrangement, wherein the force components are offset from each other along an axis perpendicular to the axis of the unloading torque.

18. An actuator assembly according to any preceding claim, wherein the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged on either side of the plane of movement.

19. An actuator assembly according to any one of claims 1 to 17, wherein the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged on the same side of the plane of movement.

20. An actuator assembly according to any preceding claim, wherein the at least one actuator component comprises an actuator unit comprising a shape memory alloy, SMA, element.

21. An actuator assembly according to claim 20, wherein the resultant force applied by the actuator unit is applied at an acute, non-zero angle to the plane of movement.

22. An actuator assembly according to any preceding claim, wherein:the loading arrangement comprises at least one pair of loading components; and the loading components of each pair of the at least one pair of loading components are arranged to apply force components to the movable part relative to the support structure in opposite directions for loading the bearing arrangement.

23. An actuator assembly according to any preceding claim, wherein:the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure in opposite directions for applying an unloading torque for reducing load on the bearing arrangement; anda force component applied by the loading components for loading the bearing arrangement and a force component applied by the actuator components for applying an unloading torque are parallel with each other.

24. An actuator assembly according to any preceding claim , wherein:the at least one actuator component comprises at least one pair of actuator components and the actuator components of each pair of the at least one pair of actuator components are arranged to apply force components to the movable part relative to the support structure in opposite directions for applying an unloading torque for reducing load on the bearing arrangement; anda force component applied by the loading components for loading the bearing arrangement and a force component applied by the actuator components for applying an unloading torque are perpendicular with each other.

25. An actuator assembly according to any preceding claim, wherein:the at least one actuator component comprises at least one pair of actuator components;the bearing arrangement comprises at least one pair of bearings; anda distance between the actuator components of each pair of the at least one pair of actuator components is less than a distance between the bearings of each pair of the at least one pair of bearings.

26. An actuator assembly according to any preceding claim, wherein the bearing arrangement comprises at least one bearing that is a plain bearing comprising bearing surfaces on the support structure and the movable part arranged to slide against each other.

27. An actuator assembly according to any preceding claim, wherein the loading arrangement comprises a magnetic loading arrangement or a resilient loading arrangement for resiliently loading the bearing arrangement.

28. An actuator assembly according to any preceding claim, wherein the movable part is a lens element comprising at least one lens, wherein the primary axis is the optical axis of the lens element and wherein the lens element is arranged to focus light emitted from an emitter.

29. An actuator assembly according to any of claims 1 to 27, wherein the movable part is a lens element comprising at least one lens, wherein the primary axis is the optical axis of the lens element and wherein the lens element is arranged to focus reflected light on an image sensor mounted on the support structure.44

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