WO2023099924A1

WO2023099924A1 - Sma actuator assembly

Info

Publication number: WO2023099924A1
Application number: PCT/GB2022/053090
Authority: WO
Inventors: James Howarth; Robin Eddington
Original assignee: Cambridge Mechatronics Limited
Priority date: 2021-12-03
Filing date: 2022-12-05
Publication date: 2023-06-08
Also published as: GB202117542D0

Abstract

An actuator assembly comprises: a static part (10); a movable part (20); a bearing arrangement (30) supporting the movable part on the static part, guiding relative movement of movable and static parts in a movement plane perpendicular to a primary axis, and resisting movement parallel to the primary axis; and four SMA wires (40). Each SMA wire connects the static and movable parts, and extends at an acute angle relative to the movement plane. Each SMA wire applies components of force to the movable part along each of orthogonal first and second axes in the movement plane such that different combinations of two of the SMA wires apply components of force to the movable part along each of the first and second axes. First and second pairs of the SMA wires are separated along the first axis and projections of the first and second pairs onto the second axis overlap the movable part.

Description

SMA ACTUATOR ASSEMBLY

Background

The present application relates to an actuator assembly wherein shape memory alloy wires are used to provide movement, for example translational movement, of a movable part with respect to a static part.

There are a variety of types of apparatus in which it is desired to provide positional control of a movable element. SMA wire is advantageous as an actuator in such an apparatus, in particular due to its high energy density which means that the SMA actuator required to apply a given force is of relatively small size.

One type of apparatus in which SMA wire is known for use as an actuator is a camera, particularly a miniature camera. Some examples are as follows. WO-2007/113478 discloses an SMA actuation apparatus in which SMA wire is used to drive movement of a camera lens element along the optical axis, for example for the purpose of focussing an image formed by the camera lens element on an image sensor. WO-2010/029316 and WO-2010/089529 each disclose an SMA actuation apparatus in which SMA wire is used to provide optical image stabilisation (OIS) in a camera by driving tilting of a camera unit including a camera lens element and an image sensor. The tilting is controlled to stabilise the image formed by the camera lens element on an image sensor against vibration, typically caused by user hand movement, that degrades the quality of the image captured by the image sensor. WO-2011/104518 discloses an SMA actuation apparatus in which SMA wire is used to provide OIS in a camera by driving tilting of a camera unit, but with additional degrees of freedom.

Miniature cameras are often used for applications such as mobile phones. It is desirable for handsets to minimise their thickness, and at the same time the demands on OIS systems continue to increase for driving heavier lens elements. Therefore, there is a need for arrangements that can provide OIS with relatively high applied forces in applications such as miniature cameras, while minimising the height of the assembly

Summary

According to the present embodiments, there is provided an actuator assembly comprising: a static part; a movable part; a bearing arrangement that supports the movable part on the static part and is arranged to guide movement of the movable part with respect to the static part in a movement plane perpendicular to a primary axis and to resist movement along the primary axis; and four SMA wires, each SMA wire being connected between the static part and the movable part, each SMA wire extending at a respective acute angle of greater than O⁹ with respect to the movement plane, and each SMA wire arranged to apply components of force to the movable part along each of orthogonal first and second axes lying in the movement plane such that different combinations of two of the SMA wires are arranged to apply components of force to the movable part in each of the opposed directions along each of the first and second axes, wherein first and second pairs of the SMA wires are separated along the first axis and projections of the first and second pairs onto the second axis overlap the movable part.

The use of SMA wire actuators is a feasible option to meet the need for reduced height OIS assemblies that can still provide sufficient applied force. The present embodiments solve this problem by using 4 shallow-angled SMA wires to drive the lens element in the X & Y directions. The acute angle with respect to the movement plane provides gearing of the SMA wires. The angled-V wires amplify the length change of the wire to give higher stroke. Separating the pairs of SMA wires along the first axis and overlapping their projections onto the second axis with the movable part allows the SMA wires to be positioned either side of the movable part to minimise z-height. As the wires are able to generate significant force (compared to VCM or other technology) they are able to move heavier lenses and be arranged to be more compact, specifically in the second axis, which is important for minimising device thickness.

In some embodiments, the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least half the overall extent of the first and second pairs projected onto the second axis. This provides sufficient space between the pairs of SMA wires to accommodate a lens.

In some embodiments, the overall extent of the first and second pairs of the SMA wires projected onto the second axis is less than the overall extent of the movable part projected onto the second axis. This allows the SMA wires to be kept within the height of the movable part, so that the SMA wires do not limit the minimum height of the assembly.

In some embodiments, the overall extent of the SMA wires projected onto the first axis is greater than the overall extent of the movable part projected onto the first axis. This allows the SMA wires to extend beyond the movable part to provide leverage and increased stroke through gearing.

In some embodiments each SMA wire extends at a respective acute angle of at least 30^g with respect to the movement plane. Extending at an acute angle to the movement plane creates gearing, so that the movement of the movable part produced by the SMA wire can be greater than the change in length of the SMA wire on contraction.

In some embodiments, the movable part is a lens assembly comprising at least one lens having an optical axis, the optical axis being collinear with or parallel to the primary axis. A lens assembly is particularly advantageous use for this type of compact assembly, as it can be used for miniature cameras, for example in mobile phones.

In some embodiments, the lens assembly has an output aperture, and the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least half the diameter of the output aperture. This allows the SMA wires to be arranged in a more compact fashion around the output aperture.

In some embodiments, the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least the diameter of the output aperture. This separation allows the SMA wires to be place either side of the output aperture, so that the SMA wires do not limit the height of the assembly.

In some embodiments, the lens assembly has an input aperture and the overall extent of each of the first and second pairs of the SMA wires projected onto the second axis is less than the diameter of the input aperture. This means that the SMA wires are within the height of the movable part, and so do not limit the height of the assembly.

In some embodiments, the lens assembly has an input end and an output end and is tapered inwardly from the input end to the output end, the SMA wires are connected to the lens assembly at the output end and are connected to the static part outwardly of the output end. Connecting the SMA wires at the narrower output end reduces the constraints on SMA wire placement without affecting the minimum height of the assembly.

In some embodiments, the SMA wires overlap the lens assembly in a direction along the optical axis. This allows the width of the assembly to be reduced as well by overlapping the SMA wires with the movable part.

In some embodiments, the actuator assembly further comprises an image sensor arranged to capture an image focussed by the lens assembly. By moving the lens assembly relative to the image sensor, the assembly can provide OIS functionality.

In some embodiments, the movable part comprises an image sensor arranged to capture an image focussed by a lens assembly. Placing the image sensor on the movable part, rather than the lens assembly, may be advantageous for providing OIS in some cases, for example if the lens assembly is very large or difficult to move.

In some embodiments, the actuator assembly further comprises a reflective element aligned with the optical axis on the input side of the lens assembly and inclined with a normal at an acute angle greater than O⁹ with respect to a plane normal to the second axis. This allows the assembly to be placed at an angle with respect to the path of light into the assembly, thereby reducing the height of the assembly while maintaining an optimum optical path length.

In some embodiments, the SMA wires are arranged to apply components of force to the movable part in directions along the first axis that are opposed as between the first and second pairs of SMA wires. This allows precise control of the movement of the movable part along the first axis by balancing the forces applied by the two pairs.

In some embodiments, the SMA wires are arranged to apply components of force to the movable part in directions along the second axis that are opposed as between the SMA wires of each of the first and second pairs of SMA wires. This allows precise control of the movement of the movable part along the second axis by balancing the forces applied by the SMA wires within each pair.

In some embodiments, the SMA wires of each of the first and second pairs of SMA wires are arranged overlapping each other in a direction along the second axis. This can reduce the depth of the assembly by arranging the SMA wires on two sides of the movable part.

In some embodiments, the SMA wires of each of the first and second pairs of SMA wires are arranged to apply a torque to the movable part when applying the components of force to the movable part in each of the opposed directions; and the actuator assembly further comprises a bearing arrangement arranged to resist the torque applied by the SMA wires to the movable part. The torque may be applied about the first axis and/or the second axis. This may allow crossing of the SMA wires in a more compact space. This increases the length of the individual SMA wires, and thereby the force they are able to apply.

In some embodiments, each SMA wire is a length of SMA wire connected at one end to the static part and at the other end to the movable part. This minimises the overall length of SMA wire needed for the assembly.

In some embodiments, each SMA wire is a piece of SMA wire connected at both ends to the static part and connected to the movable part by being hooked onto the movable part. This embodiment is easier to assemble, because it is not necessary to make a join to of one end of the SMA wire to the movable part, for example with a crimp or weld.

In some embodiments, the bearing arrangement is a flexure bearing arrangement. Flexures provide a convenient mechanism for incorporating a restoring force in the bearing arrangement.

In some embodiments, the flexure bearing arrangement comprises plural elongate flexures each extending parallel to the primary axis. Having the flexures extend along the primary axis minimises the dimensions of the assembly in the movement plane, in particular the height.

In some embodiments, the flexure bearing arrangement comprises plural flexures held in compression or tension by the flexure bearing arrangement. This makes placement of movable part on flexures more secure.

In some embodiments, the flexure bearing arrangement is configured to apply a centring force to the movable part. Providing a centring force reduces the power requirements of the assembly, because forces from the SMA wires are not needed to hold the movable part in a neutral position. This allows the SMA wires to be de-energised when not in use for OIS.

In some embodiments, the bearing arrangement is a sliding or rolling bearing arrangement. This type of bearing arrangement can be more compact.

In some embodiments, the SMA wires are arranged to apply a component of force along the primary axis to provide a bearing normal force between the movable part and the bearing arrangement. In this way, tension in the SMA wires loads the movable part onto the bearing arrangement (e.g. ball bearings in the case of a roller bearing arrangement or a surface of the static part in the case of a sliding bearing arrangement). The bearing normal force is the normal force imparted by the movable part on the bearing arrangement.

In some embodiments the actuator assembly comprises a preloading arrangement configured to provide a bearing normal force between the movable part and the bearing arrangement. In other words, the assembly may further comprises a preloading arrangement which loads the movable part onto the bearing arrangement.

In some embodiments the preloading arrangement may comprise a magnet. For example, a magnet may be disposed on the movable part and at least part of the static part may comprise a magnetic material, e.g. magnetic steel (or vice versa - a magnet may be disposed on the static part). Additionally or alternatively, the preloading arrangement may comprise a resilient element such as a spring.

In some embodiments, the SMA wires are arranged to apply a component of force along the primary axis to reduce the bearing normal force between the movable part and the bearing arrangement. Therefore when the SMA wires contract, the bearing normal force (and hence the friction) between the movable part and the bearing arrangement is reduced. In cases of high load on the bearings, motion of the movable part in the movement plane may not be smooth due to high friction and imperfections in the bearing arrangement (e.g. in one of the bearing surfaces). Advantageously, the described reduction in load on the bearing arrangement (provided by the SMA wires) may result in smoother motion of the movable part in the movement plane. The assembly is arranged such that the bearing normal force provided by the preloading arrangement (e.g. the magnetic force) is sufficient to ensure that the movable part is in contact with the bearing arrangement throughout operation (i.e. the bearing normal force provided by the preloading arrangement is greater than the component of force along the primary axis provided by the SMA wires). In some embodiments in which the bearing arrangement is a sliding bearing, the SMA wires being arranged to apply a component of force along the primary axis to reduce the bearing normal force between the movable part and the bearing arrangement (which in this case may be a surface of the static part) may have a further advantage. In the case of a sliding bearing arrangement, a surface of the movable part may be arranged to slide over a surface of the static part. These surfaces may be configured such that friction between them is sufficient to retain the movable part in position on the static part when the SMA wires are unpowered (and hence not contracted). In this way, a zero-hold- power arrangement may be provided. When the SMA wires contract, friction between the two surfaces is reduced and movement is driven but when power to the SMA wires is removed, friction between the surfaces holds the movable part in position with respect to the static part. Such a zerohold power arrangement may also be provided with other bearing arrangements.

The principle of the SMA wires reducing the bearing normal force may be applied to actuator assemblies more widely. According to an embodiment there is provided an actuator assembly comprising: a static part; a movable part; a bearing arrangement that is arranged to guide movement of the movable part with respect to the static part in at least one movement direction; a preloading arrangement configured to apply a preloading force to the bearing arrangement, thereby producing a normal force in the bearing arrangement; and at least one SMA wire arranged to apply a driving force to the movable part to cause the movable part to move in the at least one movement direction. The driving force has a component that counteracts the preloading force, thereby reducing the normal force. Optionally, in the absence of any driving forces a frictional force in the bearing arrangement is less than a weight of the movable part.

In other words, the one or more SMA wire acts to reduce the normal force in the bearing arrangement (i.e. the normal force imparted onto the bearing arrangement by the movable part). A benefit of such an arrangement is that when the one or more SMA wires contract, the bearing normal force (and hence the friction) between the movable part and the bearing arrangement is reduced. In cases of high load on the bearings, motion of the movable part with respect to the static part may not be smooth due to high friction and imperfections in the bearing arrangement.

Advantageously, the described reduction in load on the bearing arrangement (provided by the one or more SMA wires) may result in smoother motion of the movable part in the movement plane.

As mentioned above, optionally in the absence of any driving forces a frictional force in the bearing arrangement is less than a weight of the movable part. Accordingly, the friction in the bearing arrangement may not be sufficient to retain the movable part in place with respect to the static part when the one or more SMA wires are unpowered (and hence not contracted).

In some embodiments the component of the driving force that counteracts the preloading force is less than the preloading force throughout normal operation of the actuator assembly.

Advantageously, this prevents the movable part from disengaging from the bearing arrangement.

In some embodiments the component of the driving force that counteracts the preloading force is greater than the component of the driving force that causes the movable part to move in the at least one movement direction.

In some embodiments the bearing arrangement is a sliding or rolling bearing arrangement.

In some embodiments the preloading arrangement comprises a magnet. For example, a magnet may be disposed on the movable part and at least part of the static part may comprise a magnetic material, e.g. magnetic steel (or vice versa - a magnet may be disposed on the static part).

Additionally or alternatively, the preloading arrangement may comprise a resilient element such as a spring.

In some embodiments, the actuator assembly may be configured for altering a focus of an image on an image sensor (e.g. for autofocus). The actuator assembly may comprise an image sensor and the movable part may comprise one or more lenses arranged to focus an image on the image sensor. The assembly may comprise a controller configured to control the one or more SMA wires to focus an image on the sensor (e.g. to implement an autofocus algorithm). The movement direction may be collinear with or parallel to an optical axis of the one or more lenses.

In some embodiments, the actuator assembly may be configured for optical image stabilisation. The assembly may comprise a controller configured to control the one or more SMA wires to stabilise an image on an image sensor. The movable part may comprise the image sensor and optionally one or more lenses. Alternatively, the image sensor may be fixed relative to the static part and the movable part may comprise one or more lenses arranged to focus an image onto the image sensor. Brief description of the drawings

Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is an isometric view of the main components of an exemplary actuator assembly;

Fig. 2 is an isometric view of an actuator assembly with a flexure bearing arrangement;

Fig. 3 is a top-down view of an actuator assembly with a flexure bearing arrangement;

Fig. 4 is a front view of an actuator assembly with a flexure bearing arrangement;

Fig. 5 is a side view of an actuator assembly with a flexure bearing arrangement;

Fig. 6 is a side view of an actuator assembly with a flexure bearing arrangement with the static part removed;

Fig. 7 is a top view of an actuator assembly with a flexure bearing arrangement where the flexures are in tension;

Fig. 8 is a top view of an actuator assembly with a flexure bearing arrangement where the flexures are in compression;

Fig. 9 shows various attachment methods for fixing flexures to the static part;

Fig. 10 is an isometric view of an actuator assembly with a ball bearing arrangement;

Fig. 11 is a top view of an actuator assembly with a ball bearing arrangement;

Fig. 12 is a top view of an actuator assembly with a ball bearing arrangement where the bearing arrangement is located closer to the output aperture of the lens assembly;

Fig. 13 is a top view of an actuator assembly with a ball bearing arrangement where the bearing arrangement is located closer to the input aperture of the lens assembly;

Fig. 14 is a front view of an actuator assembly with a ball bearing arrangement having four bearing points;

Fig. 15 is a front view of an actuator assembly with a ball bearing arrangement having three bearing points;

Fig. 16 is a front view of an actuator assembly having a first arrangement of the pairs of SMA wire;

Fig. 17 is a top view of an actuator assembly having a first arrangement of the pairs of SMA wire;

Fig. 18 is a side view of an actuator assembly having a first arrangement of the pairs of SMA wire;

Fig. 19 is a front view of an actuator assembly having a second arrangement of the pairs of SMA wire;

Fig. 20 is a side view of an actuator assembly having a second arrangement of the pairs of SMA wire;

Fig. 21 is a front view of an actuator assembly having a third arrangement of the pairs of SMA wire;

Fig. 22 is a side view of an actuator assembly having a third arrangement of the pairs of SMA wire; Fig. 23 is a front view of an actuator assembly having a fourth arrangement of the pairs of SMA wire; Fig. 24 is a side view of an actuator assembly having a fourth arrangement of the pairs of SMA wire; Fig. 25 is a side view of an actuator assembly having an alternative implementation of the fourth arrangement of the pairs of SMA wire;

Fig. 26 is a front view of an actuator assembly having a fifth arrangement of the pairs of SMA wire; Fig. 27 is a front view of an actuator assembly having a sixth arrangement of the pairs of SMA wire; Fig. 28 is a side view of an actuator assembly having a tapered movable part;

Fig. 29 is a side view of an actuator assembly having a seventh arrangement of the pairs of SMA wire;

Fig. 30 is a top view of an actuator assembly having a bearing arrangement arranged to resist a torque applied by the SMA wires;

Fig. 31 is a front view of an actuator assembly having SMA wires hooked onto the movable part;

Fig. 32 is a side view of an actuator assembly having SMA wires hooked onto the movable part;

Fig. 33 is a front view of an actuator assembly, illustrating operation of the actuator assembly; and Fig. 34 is a is a top view of an actuator assembly with a ball bearing arrangement having a further arrangement of the pairs of SMA wires.

Detailed description

Figs. 1 and 2 schematically shows an exemplary actuator assembly 1, where the assembly is a camera. The actuator assembly 1 provides OIS compensation for a movable part 20 comprising a lens assembly. However, the actuator assembly 1 could also be used for other applications, such as moving an image sensor or some other component of a camera or other device.

The actuator assembly 1 uses individually driven SMA wires 40 to translate the movable part 20 in a movement plane, which in Fig. 1 is the X-Y plane. The SMA wires 40 are angled such that they deliver geared translation compared to that of a straight wire of equivalent length. By using the SMA wires 40 in this configuration, an OIS system can be implemented having a lower profile in the Y (first) direction, because the wires can be moved to either side of the movable part 20, taking advantage of space in the X (second) direction.

Movable Part

The actuator assembly 1 comprises a movable part 20. The movable part 20 is a lens assembly comprising at least one lens having an optical axis. The optical axis is referred to as the primary axis. Particular advantage is achieved when the movable part is a lens assembly comprising at least one lens, for example where the primary axis is the optical axis of the lens element. There are many applications where it is desirable to minimise the height of such a lens assembly. The advantages of size reduction achieved by the present techniques are particularly valuable in a handheld device where space is at a premium and in a miniature device, for example wherein the at least one lens has a diameter of at most 20mm, preferably at most 15mm, preferably at most 10mm. The lens assembly may comprise multiple lens elements that move relative to each other for zooming and focussing.

However, the present techniques may in general be applied to any type of device that comprises a static part and a movable part which is movable with respect to the static part. For example, it is not essential that the movable part 20 comprise the lens assembly. In an alternative embodiment, the movable part 20 comprises an image sensor 5 arranged to capture an image focussed by a static lens assembly. In other examples, SMA actuation apparatus 1 may be a type of apparatus that is not an optical device, and in which the movable part is not a lens element and there is no image sensor.

Static Part

The actuator assembly 1 comprises a static part 10. The static part 10 is rigidly connected to an image sensor 5. The image sensor 5 is arranged to capture an image focussed by the movable part 20, i.e. the lens assembly. The lens assembly of the movable part 20 is arranged to focus an image on the image sensor 5. However, it is not essential that the static part 10 comprise an image sensor 5. In some examples, the actuator assembly 1 may be an optical device in which the movable part 20 is a lens assembly but there is no image sensor 5.

The static part 10 comprises two distinct bodies, one on either side of the movable part 20 along the first axis. The static part 10 may include a base 7 to which the image sensor is fixed, which may also cover the assembly.

However this is not essential, and in general the static part 2 may take any suitable form. For example, the static part 10 may be a single joined body. The static part 2 may also support an IC chip for controlling the SMA wires and the image sensor, and receiving data from the image sensor. The static part 10 may be manufactured using any suitable technique, for example moulding or etching. Folded Optics

The actuator assembly 1 comprises a reflective element 3 aligned with the optical axis on the input side of the lens assembly 1 and inclined with a normal at an acute angle greater than O⁹ with respect to a plane normal to the second axis. In the case of Figs. 1 and 2, the angle is approximately 90⁹. This arrangement may be referred to as a folded camera or folded optics. Since there is more space along the primary axis Z with a folded camera layout, it is possible to use longer SMA wires 40, because the increased length mostly takes up space in the Z direction. This is advantageous because a longer SMA wire 40 of the same angle will produce a larger absolute change in length on actuation, thereby increasing the achievable stroke for OIS.

The actuator assembly 1 system configuration could alternatively be implemented in a traditional camera format without the reflective element 3, and with the lens assembly aligned directly out of the camera. This may be preferred in some embodiments, but is generally less preferred for miniature camera applications as it would increase the effective height of the assembly. In a thin device, this would limit space for wiring and autofocussing arrangements.

Bearing arrangement

The actuator assembly 1 comprises a bearing arrangement 30 that supports the movable part 20 on the static part 10. The bearing arrangement 30 is arranged to guide movement of the movable part 20 with respect to the static part 10 in a movement plane perpendicular to a primary axis and to resist movement along the primary axis. As mentioned above, the primary axis may be the optical axis of the lens assembly of the movable part 20.

Except where the context requires otherwise, the term "bearing" is used to encompass the terms "sliding bearing", "plain bearing", "rolling bearing", "ball bearing", "flexure bearing", "roller bearing", and "air bearing" (a bearing in which pressurised air is used to float a load). The term "bearing" is used herein to generally mean any element or combination of elements that functions to constrain motion to only the desired motion and reduce friction between moving parts. The term "sliding bearing" is used to mean a bearing in which a bearing element slides on a bearing surface, and includes a "plain bearing". The term "rolling bearing" is used to mean a bearing in which a rolling bearing element, for example a ball or roller, rolls on a bearing surface. Such a rolling bearing element may be a compliant element, for example a sac filled with gas. In some embodiments, more than one type of bearing element may be used in combination to provide the bearing functionality. Accordingly, the term "bearing" used herein includes any combination of, for example, plain bearings, ball bearings, roller bearings and flexures.

In some embodiments, the bearing arrangement 30 is arranged to resist a torque applied to the movable part 20.

Flexure bearing arrangement

Figs. 3 to 6 show embodiments in which the bearing arrangement 30 is a flexure bearing arrangement. The flexure bearing arrangement comprises plural elongate flexures 34. The flexures 34 extend between the movable part 20 and the static part 10. The flexures 34 are mechanically connected to different corners of the movable part 20. An advantage of a flexure bearing arrangement is that it has relatively low static friction compared to ball bearing arrangements.

The flexures 34 may also be referred to as pins or pin flexures. Figs. 3 to 6 show four flexures 34, although in general any number of flexures 34 may be provided, for example three or more flexures 34. Each flexure 34 extends parallel to the primary axis and to the other flexures 34. In general this is not essential, and the flexures 34 could extend at a non-zero angle to the primary axis, provided that the flexures 34 are transverse to the movement plane. However, smaller angles are preferred so that the flexure bearing arrangement is able to resist movement along the primary axis.

The flexures 34 are connected to the static part 10 at one end, and connected to the movable part 20 at the opposite end. The flexures 34 are fixed to each of the static part 10 and the movable part 20 in a manner that the flexures 34 cannot rotate, for example by being soldered. The flexures 34 thereby support the movable part 20 relative to the static part 10 in such a manner as to allow movement of the movable part 20 relative in the movement plane by means of the flexures 34 bending, in particular in an S-shape.

During operation of the actuator assembly as the SMA wires contract and generate a resultant force to move the movable part 20, the flexures 34 deform in the X and Y axes (i.e. in the movement plane perpendicular to the primary axis.

Conversely, the flexures 34 resist movement parallel to the primary axis, and deform minimally in the Z-axis (i.e. perpendicular to the movement plane). This helps to ensure that the focus of the lens assembly on the movable part 20 is not disrupted by movement in the movement plane used for image stabilisation.

The flexures 34 could be used as an electrical connection from the movable part 20 to the static part 10, if required. The flexures 34 could be made from a conductive material, or support an electrical connection such as a wire.

The flexures 34 may be made from any suitable material and construction that provides the desired properties. For example, the flexures 34 may be round bars (pins), metal wires, or etched sheet material. The flexures 34 may also be manufactured by plastic moulding. However, this is generally less preferred due to creep of the plastic over time, which may change the positioning and properties of the bearing arrangement. Each flexure should ideally have similar stiffness in all directions in the X-Y (movement) plane, i.e. the restoring force against deformations should be the same for all deformations in the movement plane.

The flexure bearing arrangement is configured to apply a centring force to the movable part 20. This means that, in the absence of forces applied by the SMA wires 40, the flexure bearing arrangement is configured to return the movable part 20 to a centred or neutral position. The centred or neutral position may be substantially in a centre of a range of motion of the movable part 20 relative to the static part 10 achievable by the actuator assembly. An advantage of a flexure bearing arrangement is that it is relatively straightforward to arrange the bearing arrangement to apply such a centring force to the movable part 20. This means that the movable part 20 has a set position that it returns to by design in the absence of an applied force from the SMA wires 40. This also means that the SMA wires 40 do not need to be powered to suspend the movable part in the centred or neutral position, since this is done by the flexures 34. This in turn reduces the power requirements of the actuator assembly 1.

Figs. 7 and 8 show embodiments in which the flexure bearing arrangement comprises plural flexures 34 held in compression or tension by the flexure bearing arrangement. In general, the tension arrangement of Fig. 7 is preferred, because this reduces the risk of the flexures 34 buckling during use. However, the compression arrangement of Fig. 8 may be preferred in other situations, as it reduces constraints on the shape and size of the static part 10 relative to the movable part 20. This may have cost benefits, and/or layout benefits for other components.

Fig. 9 shows various ways in which the flexures 34 may be connected to the movable part 20 and the static part 10. In general, it is preferred if the ends of each flexure 34 are connected to the movable part 20 and static part 20 using the same technique for manufacturing simplicity. However, in other embodiments, it may be advantageous to join the ends of each flexure 34 with different techniques.

In some situations, a flexure bearing arrangement may not be robust to sudden, large shocks, for example during drop testing. The rigid connection (1) is the simplest to manufacture and assemble but it may not provide sufficient resistance to buckling under drop testing. This may require the introduction of an intermediate joint to ensure that the movable part 20 can translate in the Z-axis (parallel to the primary axis) under high shock loading conditions only, but not during normal use. This will allow the movable part 20 to move to an end stop position in the Z-axis. End-stops may also be required in the X and Y-axes to prevent the flexures 34 from being over stressed.

For example, a flexible joint could be used. Example (2) of Fig. 9 comprises a compliant potted joint that may comprise an elastic material such as silicone or another flexible polymer. This joint will provide enough movement to allow the movable part 20 to move to its end stops, and will absorb energy from the impact. Examples (3) and (4) of Fig. 9 provide flexible end couplings that provide sufficient stiffness parallel to the primary axis for normal operation, but are flexible under high shock loads to allow for movement of the movable part 20 to the end stop position. The end couplings could be a separate flexure element or formed as part of the rigid interface to the static part 10 or movable part 20. Example (5) of Fig. 9 is a ball joint option that reduces the bending load in the flexure 34. All of the example of Fig. 9 could be used at either or both ends of the flexures 34 to join to the static part 10 and/or the movable part 20.

Ball bearing arrangement

Figs. 10 to 15 show embodiments of the actuator assembly 1 in which the bearing arrangement 30 is a rolling bearing arrangement. The rolling bearing arrangement comprises a rolling element. In Figs. 10-15 the rolling element is provided by ball bearings 36, but may alternatively be provided by a roller bearing, or a rocker bearing. In general, any rolling or rotary element can be used that is spherical or with curved surfaces that bear against the movable part 20 and the static part, and is able to roll back and forth and around in operation. In further alternative embodiments, the bearing arrangement may comprise a sliding bearing arrangement.

The materials of the surfaces on which the ball bearings 36 bear are chosen to provide smooth movement and a long life. The surfaces may be unitary with the underlying component (i.e. the movable part 20 or the static part 10) or may be formed by a surface coating. Suitable materials include, for example, PTFE or other polymeric bearing materials, or metal. In some embodiments, a combination of different materials may be used.

The ball bearings 36 are disposed between the movable part 20 and the static part 10. The movable part 20 is thus supported on the static part 10 by the ball bearings 36. The ball bearing arrangement comprises plural ball bearings 36, for example three ball bearings 36.

In the case where the bearing arrangement 30 comprises ball bearings 36, it is advantageous for the SMA wires 40 to be inclined with a significant acute angle with respect to the movement plane. This means that tension in the SMA wires 40 pushes the movable part 20 onto the ball bearings 36.

A ball bearing arrangement can remove the problem of flexure buckling by providing a bearing that provides a constraint in the Z-axis (parallel to the primary axis) but is able to move freely or with minimal friction in the X-Y (movement) plane. The bearing normal force in the Z-axis is provided by the tension in the SMA wires 40 to ensure that the movable part 20 does not rotate about the X and/or Y axis. In some ball bearing arrangements, the bearing contact force may be relatively high. In Fig. 11, the four ball bearings 36 are secured in pockets in the static part.

The ball bearing arrangement could be changed to suit layout requirements, or to vary the number of bearing interfaces provided by ball bearings 36. In general, any number of ball bearings 36 could be provided. However, it is preferable to provide at least three ball bearings 36 to prevent relative tilting of the movable part 20 and the static part 10.

Fig. 14 shows an embodiment using a ball bearing arrangement that includes four ball bearings 36. Using four ball bearings 36 is likely to be more stable than using a smaller number of ball bearings 36. Using four ball bearings 36 helps to ensure the motion of the bearing arrangement 30 is purely in the X-Y plane, with no rotation about the X or Y axes. Fig. 15 shows a tripod bearing arrangement with three ball bearings 36. Three ball bearings 36 are sufficient to support the movable part 20 without tilting out of the movement plane, and the provision of three ball bearings 36 has the advantage of easing the tolerances required to maintain point contact with each ball bearings 36 in a common plane. This arrangement may be used to avoid over constraining the bearing arrangement 30, and requiring a 'floating' fourth bearing interface. However, this arrangement may have reduced stability compared to a bearing arrangement with four bearing interfaces. Stability in this context may refer to always having sufficient positive reaction forces at the bearing interfaces.

As mentioned above, in some ball bearing arrangements, the bearing contact force may be relatively high. This is also the case for sliding bearing arrangements. Such high loading forces may result in 'sticky' motion (i.e. motion that is not smooth) due to high friction and imperfections in the bearing arrangement. An embodiment in which such high loading forces are reduced is described with reference to Figure 34. Figure 34 shows a roller bearing arrangement but it will be appreciated that the wire configuration shown in Figure 34 may also be applied to other bearing arrangements, for example sliding bearing arrangements.

With reference to Figure 34, the actuator assembly 1 comprises a roller bearing arrangement comprising ball bearings 36 disposed between the static part 10 and the movable part 20. The actuator assembly further comprises a preloading arrangement which comprises magnets 50 and 52 disposed on the movable part 20. At least a portion of the static part 10, e.g. two portions each opposite a respective magnet, comprises or consists of a magnetic material, e.g. magnetic steel, such that there is a magnetic force F between the movable part 20 and the static part 10.

The SMA wires 40 are connected between the movable part 20 and the static part 10 and are arranged to, on contraction, apply a component of force along the Z direction and hence reduce the bearing normal force (F). Accordingly, friction is reduced and smoother motion of the movable part 20 is facilitated. The SMA wires 40 also apply components of force perpendicular to the primary axis (in the x-y plane) to move the movable part 20.

This principle may be applied to other wire arrangements, for example as described herein or otherwise. For example, the principle of the SMA wire(s) reducing the bearing load may be applied to wire arrangements having fewer than four or greater than four wires. Such wire arrangements could be used for optical image stabilisation and/or changing a focus of an image (autofocus, for example). The principle may also be applied to other bearing arrangements such as a sliding bearing arrangement.

In an embodiment comprising a sliding bearing arrangement, the ball bearings 36 shown in Figure 34 would be dispensed with and a surface of the movable part 20 would be in contact with a surface of the static part 10 directly. These surfaces and/or the preloading arrangement may be configured such that friction between the surfaces is sufficient to retain the movable part in position on the static part when the SMA wires are unpowered (and hence not contracted). In this way, a zero-hold- power arrangement may be provided. In operation, when the SMA wires 40 contract, friction between the two surfaces is reduced and movement of the movable part 20 in the movement plane is driven. When power to the SMA wires is then removed, the bearing normal force F increases again (as the opposition to the magnetic force has been removed) and the increased friction between the surfaces holds the movable part in position with respect to the static part. An advantage of this is that the power consumption of the assembly may be reduced (as compared to arrangements in which the SMA wires must be powered in order to hold the movable part 20 still).

Wire Angle

The actuator assembly 1 comprises four SMA wires 40. Each SMA wire 40 is connected between the static part 10 and the movable part 20.

As shown in Figs. 3 to 5, each SMA wire 40 extends at a respective acute angle of greater than 0⁹ with respect to the movement plane. The acute angle of each SMA wire 40 may be at least 20⁹, optionally at least 30⁹, optionally at least 45⁹, optionally at least 60⁹, optionally at least 70⁹, optionally at least 80⁹.

As shown in Fig. 3, an angle between each SMA wire 40 and the primary axis Z in the X-Z plane formed by the first axis X and the primary axis is referred to as theta 0. The respective acute angle is therefore given by (9O⁹-0).

As shown in Fig. 4, an angle between each SMA wire 40 and the second axis Y in the X-Y movement plane is referred to as lowercase phi cp. As shown in Fig. 5, an angle between each SMA wire 40 and the primary axis Z in the Y-Z plane formed by the second axis Y and the primary axis is referred to as uppercase phi <t>. Uppercase phi <t> may be at most 30⁹, optionally at most 20⁹, optionally at most 10⁹.

In some embodiments, either or both of uppercase phi <t> and lowercase phi cp may be equal to or greater than theta 0. Lowercase phi cp may be equal to or greater than either or both of theta 0 and uppercase phi <t>. Lowercase phi cp may be at most 45⁹.

Uppercase phi <t> and theta 0 may be larger in embodiments where more force is required for stability and/or where less absolute movement of the movable part 20 is required. Reducing the angles uppercase phi <t> and theta 0 produces hearing gearing, or stroke amplification, whereby a change in length of the SMA wire 40 on actuation produces a larger change in the position of the movable part 20. However, reducing the angles uppercase phi <t> and theta 0 also reduces the force applied by the SMA wire 40 to the movable part 20 in the direction of motion.

SMA Wire Arrangement and Degree of Separation

Each SMA wire 40 is arranged to apply components of force to the movable part 20 along each of orthogonal first X and second Y axes lying in the movement plane such that different combinations of two of the SMA wires 40 are arranged to apply components of force to the movable part 20 in each of the opposed directions along each of the first X and second Y axes. This type of arrangement ensures that the SMA wires 40 can move the movable part 20 in any direction in the movement plane to provide optical image stabilisation.

First and second pairs of the SMA wires 40 are separated along the first axis X and projections of the first and second pairs onto the second axis Y overlap the movable part 20. By separating the pairs of SMA wires 40 in the first axis X, and overlapping the SMA wires 40 with the movable part 20 in the second axis Y, the overall extent of the actuator assembly 1 along the first axis X can be reduced. This is advantageous in applications where the height of the actuator assembly is highly constrained.

The first and second pairs of the SMA wires 40 may be separated from each other by a separation projected onto the first axis X that is at least half the overall extent of the first and second pairs projected onto the second axis Y. The four SMA wires 40 may be arranged in multiple different ways such that the SMA wires 40 are separated as above, and are at an acute angle to the movement plane as discussed above.

Further, the SMA wires 40 of each of the first and second pairs of SMA wires 40 may be arranged overlapping each other in a direction along the second axis Y.

Figs. 16 to 32 show a variety of wire arrangements that do not produce a net torque on the movable part 20 around the primary axis Z. These wire arrangements can easily be used with any of the bearing arrangements discussed previously.

The SMA wires 40 are arranged to apply components of force to the movable part 20 in directions along the first axis X that are opposed as between the first and second pairs of SMA wires 40. This allows the force applied by each pair to be balanced to move the movable part 20 to a particular position along the first axis X. The SMA wires 40 are arranged to apply components of force to the movable part 20 in directions along the second axis Y that are opposed as between the SMA wires 40 of each of the first and second pairs of SMA wires 40. This allows the force applied by the pairs to be balanced to move the movable part 20 to a particular position along the second axis Y.

In the figures, the movable part 20 comprises a lens assembly, and the lens assembly has an output aperture. Preferably, the first and second pairs of the SMA wires 40 are separated from each other by a separation projected onto the first axis X that is at least half the diameter of the output aperture. This allows the pairs of SMA wires 40 to be positioned along the second axis Y to overlap with the extent of the output aperture. This reduces the height of the actuator assembly 1. Optionally, the separation projected onto the first axis X may be at least 60% of the diameter of the output aperture, preferably 80% of the diameter of the output aperture. In the examples shown in the figures, the first and second pairs of the SMA wires 40 are separated from each other by a separation projected onto the first axis X that is at least the diameter of the output aperture.

The lens assembly also has an input aperture. The overall extent of each of the first and second pairs of the SMA wires 40 projected onto the second axis Y is preferably less than the diameter of the input aperture. This is the case in the embodiments of the figures, and allows the height of the actuator assembly 1 along the second axis 1 to be limited only by the input aperture of the lens assembly, and not by the extent of the SMA wires 40. The movable part 20 may have an overall extent in the first and second axes X, Y that is different from that of the input or output apertures of the lens assembly. The overall extent of the projections of the first and second pairs of the SMA wires 40 onto the second axis Y is preferably less than 1.5 times, preferably less than 1.2 times, more preferably less than 1.1 times, most preferably less than the extent of the movable part 20 along the second axis Y.

The separation of the first and second pairs of SMA wires 40 projected onto the first axis X is preferably at least 50% of, preferably at least 60% of, more preferably at least 80% of, most preferably greater than the extent of the movable part 20 along the first axis X.

Figs. 16 to 18 show an embodiment where the four SMA wires 40 are arranged in a "V" configuration. In this embodiment, the arrangement of the SMA wires 40 in the movement plane has reflection symmetry about a line parallel to the first axis, and about a line parallel to the second axis Y.

Figs. 19 and 20 show an embodiment in which the arrangement of the SMA wires 40 in the movement plane has reflection symmetry about a line parallel to the first axis, and but does not have reflection symmetry about a line parallel to the second axis Y. Instead, the first and second pairs of SMA wires 40 have translation symmetry with respect to one another along the first axis X.

Figs. 21 and 22 show an embodiment similar to Figs. 16 to 18 in which the arrangement of the SMA wires 40 in the movement plane has reflection symmetry about a line parallel to the first axis, and about a line parallel to the second axis Y. However, in Figs. 21 and 22, the first connections 42 are further apart along the second axis Y than the second connections 44. This is in contrast to the embodiment of Figs. 16 to 18, in which the second connections 44 are further apart along the second axis Y than the first connections 42.

In the embodiments of Figs. 16 to 22, the overall extent of the SMA wires 40 projected onto the first axis X is greater than the overall extent of the movable part 20 projected onto the first axis X.

Fig 23 shows an embodiment in which the projection of the overall extent of the SMA wires 40 onto the first axis is entirely within the projection of the movable part 20 onto the first axis. In some embodiments, the projection of the movable part 20 onto the first axis X overlaps at least 50%, preferably at least 60%, more preferably at least 80%, most preferably 100% of the projection of the overall extent of the SMA wires 40 onto the first axis X. This can be achieved using an in which the SMA wires 40 overlap the lens assembly of the movable part 20 in a direction along the optical axis.

Fig. 24 shows an embodiment in which the SMA wires 40 extend outwards from one end of the movable part 20, such that the projection of the overall extent of the SMA wires 40 onto the primary axis Z does not overlap the projection of the movable part 20 onto the primary axis Z. This could also be implemented for any of the arrangements of the SMA wires 40 in the movement plane shown in Figs. 16 to 22. This can also be used as a relatively straightforward way to implement an embodiment such as shown in Fig. 23 in which the SMA wires 40 overlap the lens assembly of the movable part 20 in a direction along the optical axis. The SMA wires 40 can be connected at either end of the movable part 20, as shown in Fig. 25.

Figs. 26 and 27 show embodiments in which the projections onto the second axis Y of the SMA wires 40 in the first pair of SMA wires overlap one another. Similarly, the projections onto the second axis Y of the SMA wires 40 in the second pair of SMA wires also overlap one another. This can be achieved by crossing the SMA wires 40, i.e. so that the SMA wires 40 in each of the first and second pairs of SMA wires 40 overlap one another along the first axis. Crossing the wires in this way allows each individual SMA wire 40 to be made longer. This allows the actuator assembly 1 to apply greater force to the movable part 20, and also allows for greater OIS resolution and shorter response times.

Fig. 28 shows an embodiment in which the movable part 20 comprises a lens assembly having an input end and an output end. The lens assembly is tapered inwardly from the input end to the output end. This means that the output end is smaller in extent in one or both of the first and second axes X, Y compared to the input end. The SMA wires 40 are connected to the lens assembly at the output end and are connected to the static part 10 outwardly of the output end. That is, the SMA wires 40 are connected to the static part 10 at positions projected onto the movement plane that are outside the area of the projection of the output end of the lens assembly onto the movement plane.

It is common for folded cameras to have a larger front optical element at the input end and a smaller rear optical element at the output end. The arrangement of Fig. 28 makes use of the tapering of the lens assembly to offer some space saving advantage, because the SMA wires 40 do not need to extend as far to either side of the movable part 20 in the first direction. This arrangement can also be used to implement embodiments similar to that of Fig. 23, in which the SMA wires 40 overlap the lens assembly of the movable part 20 in a direction along the optical axis.

Preferably, the SMA wires 40 overlap the lens assembly in a direction along the optical axis along at least 50%, preferably at least 60%, more preferably at least 80% of the projection of the overall extent of the SMA wires 40 onto the optical axis.

Fig. 29 shows a further embodiment, in which the SMA wires are arranged to apply a torque to the movable part 20 when applying the components of force to the movable part 20 in each of the opposed directions. The torque in Fig. 29 is applied about the first axis X and the second axis Y, but in other embodiments may be applied about only one of the first and second axes. The actuator assembly 1 comprises a bearing arrangement arranged to resist the torque applied by the SMA wires to the movable part.

Both flexure and ball bearing arrangements such as described above are able to resist torques applied about the first and second axes X and Y. However, in some embodiments, additional constraints may need to be applied to the bearing arrangement 30 to fully counteract the torque applied to the movable part 20. This is particularly true for any embodiments that apply a torque about the primary axis Z (although none are illustrated here).

Fig. 30 shows an actuator assembly 1 using a flexure bearing arrangement that further comprises a bearing constraint 38. The bearing constraint 38 mechanically couples the movable part 20 to the static part 10, and guides movement of the movable part 20 relative to the static part 10 in the movement plane. The bearing constraint 38 substantially prevents rotation of the movable part 20 around the primary axis relative to the static part 10.

This arrangement is particularly useful when the movable part 20 comprises a lens assembly. In practice, any lens will have non-uniformities across its surface. It is therefore desirable to maintain the rotational position of the lens assembly with respect to the image sensor 5. The bearing constraint 38 maintains this rotational position. The bearing arrangement comprising the bearing constraint 38 could be, without limitation, any one of the non-rotating general bearings described in relation to Figs. 10A, 10B, 11, and 12 of GB2005573.6, which is incorporated herein by reference. Single lengths and hooked pieces ofSMA wire

In the embodiments of Figs. 3 to 30, each SMA wire 40 is a length of SMA wire connected at one end to the static part 10 and at the other end to the movable part 20. Each SMA wire 40 is connected to the movable part 20 with a first connection 42, and connected to the static part 10 with a second connection 44. The first and second connections 42, 44 may use any suitable connection method, for example a crimp, welding, soldering, or an adhesive. The first and second connections 42, 44 may use the same connection method, or the first connection 42 may use a different connection method from the second connection 44.

Figs. 31 and 32 show an alternative embodiment, in which each SMA wire 40 is a piece of SMA wire connected at both ends to the static part 10 and connected to the movable part 20 by being hooked onto the movable part 10. The SMA wires 40 are connected to the static part 10 by second connections 44 such as discussed above. The SMA wires 40 are hooked onto the movable part at a hook 46. The SMA wires 40 therefore form a loop around the respective hooks 46. This connection option removes the need to have a moving electrical connection on the movable part 20 to electrically connect to the end of the SMA wire 40 connected to the movable part 20. It also has the potential to provide more force that will improve the hold stability and reduce response time of the actuator assembly 1.

The hooks 46 may allow the respective SMA wires 40 to slide around the hooks 46. However, this may increase wear on the SMA wires 40. Alternatively, the hooks 46 may be flexible to provide some flexural compliance. The flexibility of the hooks 46 should be such as to ensure that the tension in each segment of the loop of the SMA wires 40 is similar.

Operation

The actuator assembly 1 may be operated as follows. Fig. 33 shows the four SMA wires 40 labelled A-D.

To provide motion along the first axis X, SMA wires A and C may be powered. The components of force applied along the second axis Y cancel, but the components of force applied along the first axis X are added together. Therefore, the movable part 20 translates in the -X direction. Alternatively, when SMA wires B and D are powered, the movable part translates in the +X direction. To provide motion along the second axis Y, A and B may be powered. The components of force applied along the first axis X cancel, but the components of force applied along the second axis Y are added together. Therefore, the movable part 20 translates in the -Y direction. Alternatively, when C and D are powered, the movable part 20 translates in the +Y direction.

Powering different combinations of the four SMA wires 40 to different levels can produce motion to any position in the movement plane.

Modifications

It will be appreciated that there may be many other variations of the above-described embodiments.

The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non- tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

Claims

1. An actuator assembly comprising: a static part; a movable part; a bearing arrangement that supports the movable part on the static part and is arranged to guide movement of the movable part with respect to the static part in a movement plane perpendicular to a primary axis and to resist movement along the primary axis; and four SMA wires, each SMA wire being connected between the static part and the movable part, each SMA wire extending at a respective acute angle of greater than O⁹ with respect to the movement plane, and each SMA wire arranged to apply components of force to the movable part along each of orthogonal first and second axes lying in the movement plane such that different combinations of two of the SMA wires are arranged to apply components of force to the movable part in each of the opposed directions along each of the first and second axes, wherein first and second pairs of the SMA wires are separated along the first axis and projections of the first and second pairs onto the second axis overlap the movable part.

2. An actuator assembly according to claim 1, wherein the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least half the overall extent of the first and second pairs projected onto the second axis.

3. An actuator assembly according to claim 1 or 2, wherein the overall extent of the projections of the first and second pairs of the SMA wires onto the second axis is less than the overall extent of the movable part projected onto the second axis.

4. An actuator assembly according to any one of the preceding claims, wherein the overall extent of the SMA wires projected onto the first axis is greater than the overall extent of the movable part projected onto the first axis.

5. An actuator assembly according to any one of the preceding claims, wherein each SMA wire extends at a respective acute angle of at least 30⁹ with respect to the movement plane.

6. An actuator assembly according to any one of the preceding claims, wherein the movable

26 part is a lens assembly comprising at least one lens having an optical axis, the optical axis being the primary axis.

7. An actuator assembly according to claim 6, wherein the lens assembly has an output aperture, and the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least half the diameter of the output aperture.

8. An actuator assembly according to claim 7, wherein the first and second pairs of the SMA wires are separated from each other by a separation projected onto the first axis that is at least the diameter of the output aperture.

9. An actuator assembly according to any one of claims 6 to 8, wherein the lens assembly has an input aperture and the overall extent of each of the first and second pairs of the SMA wires projected onto the second axis is less than the diameter of the input aperture.

10. An actuator assembly according to any one of claims 6 to 9, wherein the lens assembly has an input end and an output end and is tapered inwardly from the input end to the output end, the SMA wires are connected to the lens assembly at the output end and are connected to the static part outwardly of the output end.

11. An actuator assembly according to any one of claims 6 to 10, wherein the SMA wires overlap the lens assembly in a direction along the optical axis.

12. An actuator assembly according to any one of claims 6 to 11, further comprising an image sensor arranged to capture an image focussed by the lens assembly.

13. An actuator assembly according to any one of claims 1 to 6, wherein the movable part comprises an image sensor arranged to capture an image focussed by a lens assembly.

14. An actuator assembly according to any one of claims 6 to 13, wherein the actuator assembly further comprises a reflective element aligned with the optical axis on the input side of the lens assembly and inclined with a normal at an acute angle greater than O⁹ with respect to a plane normal to the second axis.

15. An actuator assembly according to any one of the preceding claims, wherein the SMA wires are arranged to apply components of force to the movable part in directions along the first axis that are opposed as between the first and second pairs of SMA wires.

16. An actuator assembly according to any one of the preceding claims, wherein the SMA wires are arranged to apply components of force to the movable part in directions along the second axis that are opposed as between the SMA wires of each of the first and second pairs of SMA wires.

17. An actuator assembly according to any one of the preceding claims, wherein the SMA wires of each of the first and second pairs of SMA wires are arranged overlapping each other in a direction along the second axis.

18. An actuator assembly according to any one of the preceding claims, wherein: the SMA wires of each of the first and second pairs of SMA wires are arranged to apply a torque to the movable part when applying the components of force to the movable part in each of the opposed directions; and the actuator assembly further comprises a bearing arrangement arranged to resist the torque applied by the SMA wires to the movable part.

19. An actuator assembly according to claim 18, wherein the torque is applied about the first axis and/or the second axis.

20. An actuator assembly according to any one of the preceding claims, wherein each SMA wire is a length of SMA wire connected at one end to the static part and at the other end to the movable part.

21. An actuator assembly according to any one of claims 1 to 19, wherein each SMA wire is a piece of SMA wire connected at both ends to the static part and connected to the movable part by wire being hooked onto the movable part.

22. An actuator assembly according to any one of the preceding claims, wherein the bearing arrangement is a flexure bearing arrangement.

23. An actuator assembly according to claim 22, wherein the flexure bearing arrangement comprises plural elongate flexures each extending parallel to the primary axis.

24. An actuator assembly according to claim 22 or 23, wherein the flexure bearing arrangement comprises plural flexures held in compression or tension by the flexure bearing arrangement.

25. An actuator assembly according to any one of claims 22 to 24, wherein the flexure bearing arrangement is configured to apply a centring force to the movable part.

26. An actuator assembly according to any one of claims 1 to 21, wherein the bearing arrangement is a sliding or rolling bearing arrangement.

27. An actuator assembly according to claim 26, wherein the SMA wires are arranged to apply a component of force along the primary axis to provide a bearing normal force between the movable part and the bearing arrangement.

28. An actuator assembly according to claim 26 comprising a preloading arrangement configured to provide a bearing normal force between the movable part and the bearing arrangement.

29. An actuator assembly according to claim 28, wherein the SMA wires are arranged to apply a component of force along the primary axis to reduce the bearing normal force between the movable part and the bearing arrangement.

30. An actuator assembly according to claim 28 or 29, wherein the preloading arrangement comprises a magnet.

31. An actuator assembly comprising: a static part; a movable part; a bearing arrangement that is arranged to guide movement of the movable part with respect to the static part in at least one movement direction; a preloading arrangement configured to apply a preloading force to the bearing

29 arrangement, thereby producing a normal force in the bearing arrangement; and at least one SMA wire arranged to apply a driving force to the movable part to cause the movable part to move in the at least one movement direction, wherein the driving force has a component that counteracts the preloading force, thereby reducing the normal force; wherein, in the absence of any driving forces, a frictional force in the bearing arrangement is less than a weight of the movable part.

32. An actuator assembly according to claim 31 wherein the component of the driving force that counteracts the preloading force is less than the preloading force throughout normal operation of the actuator assembly.

33. An actuator assembly according to claim 31 or 32 wherein the component of the driving force that counteracts the preloading force is greater than the component of the driving force that causes the movable part to move in the at least one movement direction.

34. An actuator assembly according to any one of claims 31 to 33, wherein the bearing arrangement is a sliding or rolling bearing arrangement.

35. An actuator assembly according to any one of claims 31 to 34, wherein the preloading arrangement comprises a magnet.

30