The present invention relates to the use of shape memory alloy (SMA) wires to provide positional control of a movable part supported on a support structure.

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

One type of actuator assembly in which SMA wire is known for use as an actuator is a camera, particularly a miniature camera. The actuator assembly may, for example, be used to provide optical image stabilization (OIS) in such a camera. WO 2013/175197 Al discloses a SMA actuator assembly in which a total of four SMA wires are used to provide OIS, by moving a movable lens element relative to an image sensor on a support structure to any position across a range of movement in two orthogonal directions, without applying any net torque to the movable part. WO 2017/072535 Al discloses an SMA actuator assembly in which SMA wires are used to provide OIS by moving a movable image sensor relative to a lens element.

The present invention is concerned with providing an alternative SMA actuator assembly for moving a movable part relative to a support structure in a plane, for example for the purpose of providing OIS.

According to the present invention, there is provided a shape memory alloy (SMA) actuator assembly comprising: a support structure; a movable part that is movable relative to the support structure across a range of movement in a plane; an anti-rotation mechanism configured to constrain rotation of the movable part relative to the support structure about any axis perpendicular to the plane; and a total of three SMA wires that are individually controllable so as to move the movable part relative to the support structure to any position in said range of movement.

Provision of the anti-rotation mechanism in combination with the three SMA wires allows for control of three degrees of freedom of the movable part: movement along the x-axis, movement along the y-axis, and tension in the SMA wires. The anti -rotation mechanism constrains rotation of the movable part, such that any torque applied to the movable part by the SMA wires is converted into translational movement and/or tension in the SMA wires. Controlling the tension allows for more accurate control of the SMA wires compared to an assembly in which such tension control is not provided. The SMA actuator assembly is thus capable of accurately and controllably moving the movable part within a plane of movement, for example for the purposes of providing OIS. According to the present invention, there is also provided a shape memory alloy (SMA) actuator assembly comprising: a support structure; a movable part that is movable relative to the support structure in a plane; an anti-rotation mechanism configured to constrain rotation of the movable part relative to the support structure about any axis perpendicular to the plane; a total of two SMA wires, wherein neither of the SMA wires are collinear and wherein the SMA wires are individually controllable so as to apply a force to the movable part in two directions in the plane; and a biasing arrangement configured to apply a force to the movable part that opposes the force applied by the two SMA wires.

Provision of the anti-rotation mechanism in combination with the two SMA wires allows for control of two degrees of freedom of the movable part: movement along the x-axis and movement along the y-axis. The anti -rotation mechanism constrains rotation of the movable part, such that any torque applied to the movable part by the SMA wires is converted into translational movement. The biasing arrangement opposes the force applied by the two SMA wires and so may move the movable part in a direction opposite to the direction in which the two SMA wires move the movable part. The SMA actuator assembly is thus capable controllably moving the movable part within a plane of movement, for example for the purposes of providing 01 S.

According to the present invention, there is also provided a camera apparatus comprising the SMA actuator assembly and an image sensor that is fixed relative to the support structure. The movable part comprises a lens assembly comprising one or more lenses configured to focus an image on the image sensor. The SMA actuator assembly may be used to provide OIS in the camera apparatus by moving the lens assembly laterally of the optical axis. The overall size of the camera apparatus may be reduced compared to a camera in which OIS is provided by tilting of the camera unit or of the image sensor, where the camera apparatus requires additional clearance in the z direction to allow for such tilting.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. la is a schematic side view of a camera apparatus incorporating an SMA actuator assembly;

Fig. lb is a schematic plan view of an SMA actuator assembly in accordance with an embodiment of the present invention;

Fig. 2a is a schematic plan view of a bearing arrangement and anti-rotation mechanism in an SMA actuator assembly in accordance with an embodiment of the invention;

Fig. 2b is a schematic plan view of another bearing arrangement and anti-rotation mechanism in an SMA actuator assembly in accordance with an embodiment of the invention;

Fig. 3a is a schematic plan view of an arrangement of SMA wires in an SMA actuator assembly in accordance with an embodiment of the invention; Fig. 3b is a schematic plan view of another arrangement of SMA wires in an SMA actuator assembly in accordance with an embodiment of the invention;

Fig. 3c-3e are schematic plan views of intermediary elements that may be included in the SMA actuator assembly of Fig. 3b;

Fig. 3f is a schematic plan view of another arrangement of SMA wires in an SMA actuator assembly in accordance with an embodiment of the invention;

Fig. 4a is a schematic plan view of another SMA actuator assembly in accordance with an embodiment of the invention; and

Fig. 4b is a schematic plan view of another SMA actuator assembly in accordance with an embodiment of the invention.

In the following description, the present invention will be described with reference to a camera in which OIS is desired. However, this is one non-limiting example use of the present invention and it will be understood that the present invention may be used in any optical system or non-optical system and for any purpose. For example, the present invention may be used to improve the performance of a system used to perform 3D sensing (i.e. generate a 3D representation of a scene) or in haptics applications.

Fig. la schematically shows a camera apparatus 1 that incorporates an SMA actuator assembly 2 in accordance with the present invention. The camera apparatus 1 is to be incorporated in a portable electronic device such as a mobile telephone, or tablet computer. Thus, miniaturisation is an important design criterion.

The SMA actuator assembly 2 comprises a support structure 10 and a movable part 20. The movable part 20 is supported on the support structure 10. The movable part 20 is movable relative to the support structure 10 across a range of movement in a plane, in particular in the x-y plane. Movement in a direction perpendicular to the plane, i.e. along the z-axis, may be constrained or prevented.

The SMA actuator assembly 2 comprises plural SMA wires 30. The SMA wires 30 may be connected in tension between the support structure 10 and the movable part 20. The SMA wires 30 may be connected at their ends to the support structure 10 and/or to the movable part 20 using connection elements 33, for example crimp connections. The crimp connections may crimp the SMA wires to hold them mechanically, as well as providing electrical connections to the SMA wires 30. However, any other suitable connections may alternatively be used. The SMA wires 30 are capable, on selective contraction, of driving movement of the movable part 20 with respect to the support structure 10 in translational movement with two degrees of freedom (i.e. along the x and y axes).

The movable part 20 may be supported (so suspended) on the support structure 10 exclusively by the SMA wires 30. However, preferably, the SMA actuator assembly 2 comprises a bearing arrangement 40 that supports the movable part on the support structure 10. The bearing arrangement 40 may have any suitable form for allowing movement of the movable part 20 with respect to the support structure 10 in the x-y plane. For this purpose, the bearing arrangement 40 may, for example, comprise a rolling bearing, a flexure bearing or a plain bearing. The bearing arrangement 40 may constrain or prevent movement of the movable part 20 relative to the support structure 10 in the z- direction. The bearing arrangement 40 may constrain movement in the x-y plane within a particular range of movement.

The camera apparatus 1 further comprises a lens assembly 3 and an image sensor 4. The lens assembly 3 comprises one or more lenses configured to focus an image on the image sensor 4. The image sensor 4 captures an image and may be of any suitable type, for example a charge coupled device (CCD) or a CMOS device. The lens assembly 3 comprises a lens carrier, for example in the form of a cylindrical body, supporting the one or more lenses. The one or more lenses may be fixed in the lens carrier, or may be supported in the lens carrier in a manner in which at least one lens is movable along the optical axis O, for example to provide zoom or focus, such as auto-focus (AF). The camera apparatus 1 may be a miniature camera apparatus in which the or each lens of the lens assembly 3 has a diameter of 20mm or less, preferably of 12mm or less. For ease of reference, the z axis is taken to be the optical axis O of the lens assembly 3 and the x and y axes are perpendicular thereto. In the desired orientation of the lens assembly 3, the optical axis O is perpendicular to a light- sensitive region of the image sensor 4 and the x and y axes are parallel to the light-sensitive region of image sensor 20.

In the embodiment shown in Fig. la, the movable part 20 comprises the lens assembly 3. The image sensor 4 may be fixed relative to the support structure 10, i.e. mounted on the support structure 10. In other embodiments (not shown), the lens assembly 3 may be fixed (in the x-y plane) relative to the support structure 10 and the movable part 20 may comprise the image sensor 4. In either embodiment, in operation the lens assembly 3 is moved relative to the image sensor 4 orthogonally to the optical axis O in the x-y plane. This has the effect that the image on the image sensor 4 is moved. This is used to provide OIS, compensating for image movement of the camera apparatus 1, caused for example by a user’s handshake.

The camera apparatus 1 comprises a vibration sensor 6 and a control circuit 8. The vibration sensor 6 may be a gyroscope sensor, for example, although in general other types of vibration sensor 6 could be used. The vibration sensor 6 detects vibrations that the camera apparatus 1 is experiencing and generates output signals representative of the vibration of the camera apparatus 1. The control circuit 8 may be implemented in an integrated circuit (IC) chip. The control circuit 8 generates drive signals for the SMA wires 30 in response to the output signals of the vibration sensor 6. SMA material has the property that on heating it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA wires 30, thereby heating the SMA wires 30 by allowing an electric current to flow, will cause the SMA wires 30 to contract and move the movable part 20. The drive signals are chosen to drive movement of the movable part 20 in a manner that stabilizes the image sensed by the image sensor 4. The control circuit 8 supplies the generated drive signals to the SMA wires 30, thereby providing OIS.

An actuator assembly with four SMA wires, as in WO 2013/175197 Al, may be used to provide OIS. Four SMA wires allow control of the movable part 20 with respect to the support structure 10 in four degrees of freedom, in particular control of: movement along the x-axis, movement along the y-axis, rotation about the z-axis, and tension in the SMA wires. In some situations, control of the rotation about the z-axis may not be required. For example, without limitation to the claimed invention, when the movable part 20 comprises the lens assembly 3 with only rotationally symmetric lenses, there may be no benefit in allowing the movable part 20 to rotate about the z-axis.

The inventors have found that providing an anti-rotation mechanism 7 that constrains rotation about the z-axis allows a total of three SMA wires 30 to control: movement along the x-axis, movement along the y-axis, and tension in the SMA wires 30. Use of a total of three SMA wires 30 may reduce the cost and power consumption of the SMA actuator assembly 2, compared to an actuator assembly with four or more SMA wires, while allowing OIS in the camera apparatus 1.

Fig. lb schematically depicts a plan view of an embodiment of the actuator assembly 2 with the anti -rotation mechanism 7 and a total of three SMA wires 30. The total of three SMA wires 30 consists of only three SMA wires 30 that are independently controllable. The anti -rotation mechanism 7 constrains rotation of the movable part 20 relative to the support structure 10 about any axis perpendicular to the plane. So, rotation of the movable part 20 about the z-axis is constrained, i.e. rotation about the z-axis is resisted or even prevented. The three SMA wires 30 are individually controllable so as to move the movable part 20 relative to the support structure 10 to any position in the range of movement. So, the three SMA wires may reversible and controllably move the movable part 20 in the x-y plane. This allows the movable part 20 to be moved to any x-y coordinates within the range of movement by controlling the SMA wires 30.

Each of the SMA wires 30 applies, upon contraction, a respective force to the movable part 20. The first SMA wire 30a applies a force in a first direction 3 la in the x-y plane, e.g. in the x direction. The second SMA wire 30b applies a force in a second direction 3 lb in the x-y plane, e.g. in the y direction. As shown in Fig. lb, the first and second directions 3 la, 3 lb may be orthogonal direction. However, in general, the first and second directions 3 la, 3 lb may be any two different directions in the x-y plane, and so may be two directions that are angled with respect to each other. The third SMA wire 30c applies, upon contraction, a force to the movable part 20 in a third direction 31c in the x-y plane. The force in the third direction 31c opposes the force applied by the first and/or second SMA wires 30a, 30b. So, the third direction 31c is opposite to the first direction 3 la, or opposite to the second direction 3 lb, or opposite to any direction within the acute angle between the first and second directions 3 la, 3 lb. Preferably, the third direction 31c opposes the force applied by both first and second SMA wires 30a, 30b, and so is opposite to any direction within the angle (i.e. within the right angle, acute angle, or obtuse angle) between first and second directions 3 la, 3 lb.

The three SMA wires 30 may each extend parallel to the x-y plane, and so may apply a force to the movable part 20 only in the x-y plane. This is advantageous in minimizing the size of the SMA actuator assembly 2 in the z direction. In some embodiment, the SMA wires 30 may be angled (i.e. inclined at a non-zero angle, preferably a small angle) relative to the x-y plane, in which case the forces applied by the SMA wires 30 in the first, second and third directions refer to the component that act in the x-y plane of the overall forces applied by the SMA wires 30. This may be useful, for example, when the bearing arrangement 40 comprises ball bearings, in which case the tension in the SMA wires 30 could be used to urge the bearing surfaces of the ball bearings together.

Figs. 2a and 2b schematically depict, in plan, bearing arrangements 40 that comprise or incorporate the anti-rotation mechanism 7. The bearing arrangements 40 of Figs. 2a and 2b constrain rotation of the movable part 20 relative to the support structure 10 about the z-axis. Each bearing arrangement 40 comprises in essence two bearing portions that are connected in mechanical series, each bearing portion allowing movement in a respective one of two non-colinear directions in the x-y plane.

Fig. 2a shows a bearing arrangement 40 comprises an arrangement of flexure arms connected between the support structure 10 and the movable part 20. The bearing arrangement 40 comprises two pairs of flexure arms 41a, 42a that are connected in mechanical series. A first pair of flexure arms 41a allows movement in one of two orthogonal directions in the x-y plane, for example in the x- direction. The first pair of flexure arms 41a constrains movement in any other direction. A second pair of flexure arms 42a allows movement in the other of the two orthogonal directions in the x-y plane, for example in the y-direction. The second pair of flexure arms 42a constrains movement in any other direction. In combination, the two pairs of flexure arms 41a, 42a thus allow movement in the x-y plane and constrain rotation about the z-axis. The bearing arrangement 40 may be a single piece, so all portions of the bearing arrangement may be integrally formed, for example from a sheet material such as a sheet metal. The bearing arrangement 40 may be rigidly connected to the movable part 20 and support structure 10, or may be integrally formed with (part of) the support structure 10 and/or movable part 20.

Fig. 2b shows an alternative bearing arrangement 40 that comprises an arrangement of rolling bearings connected between the support structure 10 and the movable part 20. The rolling bearings may be ball bearings, roller bearings or rocker bearings, for example. The rolling bearings may comprise a rolling element (such as a ball, a roller or a rocker) that bears upon two bearing surfaces The bearing arrangement 40 comprises two rolling bearings 41b, 42b that are arranged in mechanical series between the movable part 20 and the support structure 10. A first rolling bearing 41b allows movement in one of two orthogonal directions in the x-y plane, for example in the x-direction. The first rolling bearing 41b constrains movement in any other direction. The first rolling bearing 41b may comprise rolling elements that bear upon a surface of the movable part 20 and a surface of an intermediary plate 43. The movable part 20 may move in the x-direction with respect to the intermediary plate 43. A second rolling bearing 42b allows movement in the other of the two orthogonal directions in the x-y plane, for example in the y-direction. The second rolling bearing 42b constrains movement in any other direction. The second rolling bearing 42b may comprise rolling elements that bear upon a surface of the support structure 10 and a surface of the intermediary plate 43. The intermediary plate 43 may move in the y-direction with respect to the support structure 10. In combination, the two rolling bearings 41b, 42b thus allow movement in the x-y plane and constrain rotation about the z-axis.

Although Figs. 2a and 2b show two examples of bearing arrangements 40 that incorporate the anti-rotation mechanism, it will be appreciated that the anti-rotation mechanism 7 may be separate from the bearing arrangement 40 or incorporated in a different bearing arrangement 40. In general, the anti-rotation mechanism 7 may comprise any mechanism that constrains rotation of the movable part 20 relative to the support structure 10. Further examples of such anti -rotation mechanisms 7 are disclosed in co-pending GB 2005570.3, which examples are incorporated by reference herein. Furthermore, as will be explained with reference to Figs. 3d and 3e, the anti-rotation mechanism 7 may be incorporated in the connection between movable part 20 and the third SMA wire 30c.

The SMA actuator assembly 2 may comprise a bearing arrangement 40 other than those described in relation to Figs. 2a and 2b, such as different types of flexure bearings or rolling bearings, or any types of plain bearings, that allow movement in the x-y plane. A plain bearing comprises two bearing surfaces that bear upon each other, so the movable part 20 may comprise a bearing surface that bears upon a complimentary bearing surface of the support structure 10. In general, the bearing arrangement 40 may have any construction that allows movement of the movable part 20 relative to the support structure 10 in the x-y plane.

The arrangement of the SMA wires 30 of Fig. lb may require a comparably large footprint to achieve a given amount of stroke or movement of the movable part 20 with respect to the support structure 10.

Figs. 3a-3f schematically depict, in plan, SMA actuator assemblies 2 with a reduced footprint to achieve a given amount of stroke. In each of these SMA actuator assemblies 2, each SMA wire 30 extends along an edge or side of the movable part 10. The SMA wires 30 are arranged in a loop at different angular positions around the z-axis. Alternatively (not shown), some of the SMA wires 30 could extend towards the centre of the movable part 10 to achieve a reduced footprint. In either case, the SMA wires 30 do not extend in a direction radially away from the movable part 20, and so the gap between the movable part 20 and support structure 10 in the x-y plane may be reduced, thereby reducing the overall footprint in the x-y plane of the SMA actuator assembly 2.

The SMA actuator assemblies 2 of Figs. 3a-3f may correspond to the SMA actuator assembly 2 described in relation to Figs, la and lb, except that the arrangement of SMA wires 30 is different. In Figs. 3a-3f, the first SMA wire 30a extends in the x direction. The first SMA wire 30a applies a force to the movable part 20 in the x direction, and so may move the movable part 20 in the x direction. The second SMA wire 30b extends in the y-direction. The second SMA wire 30b applies a force to the movable part 20 in the y direction, and so may move the movable part 20 in the y direction. The first and second SMA wires 30a, 30b thus apply forces to the movable part 20 in their direction of extent. The first and second SMA wires 30a, 30b may be connected to the movable part 20 so as to apply torques in alternate senses around the z-axis. For example, as shown in Fig. 3a, the first SMA wire 30a may apply an anti -clockwise torque to the movable part 20, whereas the second SMA wire 30b may apply a clockwise torque to the movable part 20. Due to provision of the antirotation mechanism 7, such torque will may be (at least partially) converted into translational movement in the x and y directions.

The third SMA wire 30c also extends along an edge of the movable part 20. So, the third SMA wire 30c extends in a direction other than the third direction 31c, in which third direction 31 the third SMA wire 30c applies a force to the movable part 20. For the purpose of redirecting the tensile force in the third SMA wire 30c along the third direction 31c, the SMA actuator assembly 2 comprises an intermediary element 35. The intermediary element 35 redirects the tensile force in the third SMA wire 30c to act in the third direction 31c.

Figs. 3a-3f show different examples of the intermediary element 35. In the SMA actuator assembly 2 of Fig. 3a, the intermediary element 35 comprises a flexure arm 35a. The flexure arm 35a is connected at one end to the support structure 10 and at the other end to the movable part 20. The flexure arm 35a allows movement of the other end relative to the support structure 10 in the third direction 31c. The third SMA wire 30c is connected to the flexure arm 35a, for example to the end of the flexure arm 35a that is connected to the movable part 10. Upon contraction, the third SMA wire 30c thus deflects the flexure arm. This urges the other end of the flexure arm 35a in the third direction 31c, and so applies a force to the movable part 30 in the third direction 31c.

In the SMA actuator assembly 2 of Figs. 3b-3f, the third SMA wire 30c bends around a contact region 36 with the intermediary element 35. The third SMA wire 30c thus comprises two SMA portions that extend on either side of the contact region 36. The two SMA portions are angled relative to each other. The two SMA portions are straight portions of SMA wire. In the example of Figs. 3b-3f, one of the two portions extends along the x-axis and the other of the two portions extends along the y-axis. The two portions thus extend in different directions along the edge of the movable part 10. The intermediary element 35 is arranged between the third SMA 30c wire and the movable part 20. The intermediary element 35 thus extends from the contact region 36 to the movable part 20. Each portion of the third SMA wire 30c applies, upon contraction, a force in a different direction on the contact region 36. In the depicted examples, one of the two SMA portions applies a force in the x- direction and the other of the two SMA portions applies a force in the y-direction. The combined force of the two portions, and so the force applied by the third SMA wire 30c, acts in the third direction 31c.

The intermediary element 35 preferably allows movement of the movable part 20 in the x-y plane relative to the contact region 36. This reduces the effect of the third SMA wire 30c on movement of the movable part 20 in the x-y plane due to forces applied by the first and second SMA wires 30a, 30b. For example, as shown in Fig. 3b, the intermediary element 35 may comprise a flexure arm 37a. The flexure arm 37a extends substantially in the third direction 31c. One end of the flexure arm 37a is connected to the movable part 20 and another end of the flexure arm is connected to the contact region 36. The flexure arm 37a resists buckling, and so transmits a force from the third SMA wire 30c to the movable part 20 in the third direction 31c. The flexure arm 37a allows movement of the movable part 20 relative to the contact region 36 in directions orthogonal to the third direction 31c. The movable part 20 may thus move in the x-y plane due to forces applied by the first and second SMA wires 30a, 30b.

Fig. 3c shows another example of the intermediary element 35 that may replace the intermediary element 35 depicted in Fig. 3b. The intermediary element 35 comprises a rolling bearing 37b, for example a ball bearing, roller bearing or rocker bearing. The rolling bearing 37b comprises a rolling element (such as a ball, a roller or a rocker) between two bearing surfaces. A first of the bearing surfaces is connected to the movable part 20. A second of the bearing surfaces is connected to the contact region 36. The two bearing surfaces extend substantially perpendicularly to the third direction 31c. The rolling bearing 37b allows movement of the movable part 20 relative to the contact region 36 in directions orthogonal to the third direction 31c. The movable part 20 may thus move in the x-y plane due to forces applied by the first and second SMA wires 30a, 30b.

Figs. 3d and 3e show further examples of the intermediary element 35 that may replace the intermediary element 35 depicted in Fig. 3b. In Figs. 3d and 3e, the intermediary element 35 comprises or incorporates the anti -rotation mechanism 7. The intermediary element 35 thus constrains rotation of the movable part 20 relative to the support structure 10 about the z-axis. In particular, as shown in Fig. 3d, the intermediary element 35 may comprise two flexure arms 37c. Each flexure arm extends between the contact region 36 and the movable part 20. The two flexure arms 37c extend substantially in the third direction 31c. The two flexure arms 37c are spaced apart from each other in the x-y plane, in particular in a direction orthogonal to the third direction 31c. The two flexure arms 37c thus allow movement of the movable part 20 with respect to the contact region 36 in any direction orthogonal to the third direction 31c. Rotation of the movable part 20 relative to the contact region 36 about the z-axis is constrained.

Alternatively, as shown in Fig. 3e, the intermediary element 35 may comprise two rolling elements 37d. The two rolling elements 37d are arranged between a bearing surface connected to the contact region 36 and a bearing surface connected to the movable part 20. The two rolling elements 37d are in contact and bear on the bearing surfaces. The two rolling elements 37d are spaces apart from each other in the x-y plane, in particular in a direction orthogonal to the third direction 31c. The two rolling elements 37d thus prevent rotation of the two bearing surface with respect to each other about the z-axis. This prevents rotation of the movable part 20 relative to the support structure 10 about the z-axis.

In the embodiments of Figs. 3a-3e, the third SMA wire 30c is arranged on a side of the movable part 20 opposite to the first and second SMA wires 30a, 30b. Alternatively, the third SMA wire 30c may be arranged on the same side, so in general on the same half of the perimeter, of the movable part as the first and second SMA wires 30c. An example of this is schematically depicted, in plan, in Fig. 3f. The SMA actuator assembly 2 of Fig. 3f corresponds in essence to the SMA actuator assembly 2 of Fig. 3b, except that the three SMA wires 30 are arranged on the same half of the perimeter of the movable part 20. This may reduce the space required in the other half of the perimeter of the movable part 20, thus allowing the movable part 20 to be located closer to a comer of any device in which the SMA actuator assembly 2 is to be incorporated, such as the camera apparatus 1 or a mobile phone or other device. Although Fig. 3f only depicts an example of the SMA actuator assembly 2 of Fig. 3b with the three SMA wires arranged on the same side, it will be appreciated that the SMA actuator assemblies of any one of Figs. 3a-3e may be modified in a similar manner to arrive at a configuration in which the three SMA wires 30 are arranged on the same side.

The SMA actuator assembly 2 of Figs, lb and 3a-3f comprises a total of three SMA wires 30, thus allowing independent control of: movement along the x-axis, movement along the y-axis, and tension in the SMA wires 30. In some applications, for example in devices requiring less accurate and reliable positioning, it may not be required to control the tension in the SMA wires 30. In such applications, one of the three SMA wires 30 may be redundant, and a total of two SMA wires 30 may be used to independently control movement along the x-axis and movement along the y-axis. Such an SMA actuator assembly 2 may correspond to the SMA actuator assembly 2 as described above, with the exception that the third SMA wire 30c is replaced with a biasing arrangement 50.

Such an SMA actuator assembly 2 is schematically depicted in Figs. 4a and 4b. The SMA actuator assembly 2 comprises the support structure 10 and the movable part 20 that is movable relative to the support structure 10 in the x-y plane. The SMA actuator assembly 2 further comprises the anti-rotation mechanism that constrains rotation of the movable part 20 relative to the support structure 10 about the z-axis. However, unlike described in relation to Figs, lb and 3a-3f, the SMA actuator assembly 2 comprises a total of two SMA wires 30a, 30b. Neither of the two SMA wires 30a, 30b are collinear. The SMA wires 30a, 30b are individually controllable so as to apply a force to the movable part 20 in two directions (e.g. two orthogonal directions) in the x-y plane. For example, the first SMA wire 30a may apply a force in the x-direction, and the second SMA wire 30b may apply a force in the y-direction. The first and second SMA wires 30a, 30b may extend along an edge or towards a centre of the movable part 20. Instead of the third SMA wire 30c, the SMA actuator assembly 2 comprises the biasing arrangement 50. The biasing arrangement 50 applies a force (biasing force) to the movable part 20 that opposes the force applied by the two SMA wires 30a, 30b. The biasing arrangement 50 may be a spring or other resilient element. As for example shown in Fig. 4a, the spring or other resilient element is connected at one end to the support structure 10 and at the other end to the movable part 20. The spring or other resilient element may be arranged in the same manner as the third SMA wire 30c in Figs, lb and 3a-3f and be biased to contract, thereby imparting a force in the third direction 31c in the manner described above. Alternatively, the spring or other resilient element may be biased to expand and arranged in a manner substantially opposite to the third SMA wire 30c in Figs, lb and 3a- 3f.

As shown in Fig. 4b, the biasing arrangement 50 may also be incorporated into the bearing arrangement 40. So, the bearing arrangement 40 may both support the movable part 20 on the support structure 10 in a manner allowing movement in the x-y plane, and provide a biasing force that opposes the force applied by the first and second SMA wires 30a, 30b. For example, the bearing arrangement 40 may comprise one or more flexures connected between the support structure 10 and the movable part 20. The one or more flexures may be pre-biased (pre-loaded) so as to impart the biasing force on the movable part 20.

The SMA actuator assembly 2 of Fig. 4b comprises the bearing arrangement 40 described in relation to Fig. 2a. The bearing arrangement 40 may thus also function as the anti-rotation mechanism 7. So, in the SMA actuator assembly 2 of Fig. 4b, the purpose of the bearing arrangement 40 is to i) support the movable part 20 on the support structure 10 in a manner allowing movement in the x-y plane, ii) apply a biasing force to the movable part 20 that opposes the force applied by the two SMA wires 30a, 30b, and iii) constrain rotation of the movable part 20 relative to the support structure 10 about the z axis. Providing a bearing arrangement 40 for all of these purposes may make the SMA actuator assembly more compact compared to an SMA actuator assembly in which these functions are implemented in separate components. However, in line with the disclosure above, the anti -rotation mechanism 7 may be implemented in any other manner, e.g. using the bearing arrangement 40 of Fig. 2b, or by appropriate connection between biasing arrangement 50 and movable part 20 in the manner discussed in relation to Figs. 3d and 3e. Similarly, the biasing arrangement 50 may be implemented using a component separate from the bearing arrangement 40.

Figure 5 schematically depict, in plan, an SMA actuator assembly 2 with an alternative SMA wire 30 arrangement, in accordance with an aspect of the present invention. The SMA actuator assembly 2 comprises a support structure 10, a movable part 20, an anti -rotation mechanism 7 and three SMA wires 30, as already described in relation to the embodiments of Figures lb and 3a-f, for example.

As shown in Figure 3, at least one of the SMA wires 30, preferably two SMA wires 30, may be provided underneath the movable part 20. So, at least one of the SMA wires 30, preferably two SMA wires 30, are provided between the support structure 10 and the movable part 20 along the z- axis. This may be particularly applicable in situations in which an image sensor, rather than a lens, is provided on the movable part 20. At least one of the SMA wires 30, preferably two SMA wires 30, may extend diagonally underneath the movable part 20. At least one of the SMA wires 30, preferably two SMA wires 30, may extend between two opposing comers of the movable part 20, in particular when the movable part 20 has a rectangular or square cross-section. Arranging at least one of the SMA wires 30 underneath the substrate may reduce the footprint, in the x-y plane, of the actuator assembly 2. Arranging at least one of the SMA wires 30 diagonally may increase the length of the SMA wire 30, without increasing the footprint of the actuator assembly, compared to a situation in which the SMA wire is not diagonally arranged.

In Figure 5 there are thus provided three SMA wires 30a, 30b, 30c. Two SMA wires 30a, 30b are arranged diagonally underneath the movable part 20. The two SMA wires 30a, 30b are arranged perpendicularly to each other. The two SMA wires 30a, 30b are arranged to apply a force in the same direction along a first axis (the x-axis) and a force in opposing directions along a second axis (the y- axis) that is perpendicular to the first axis. The third SMA wire 30c is arranged to apply a force along the first axis in a direction opposing the forces of the two SMA wires 30a, 30b.

The embodiment of Figure 5 is just one example in which SMA wires 30 are arranged underneath the movable part 20. Although not shown, many other arrangements of SMA wires 30 are possible. In some embodiments and with reference to Figure 3a, for example, the two SMA wires 30a, 30b may be arranged adjacent to the movable part 20 (as described in relation to Figure 3a), and the third SMA wire 3c may extend diagonally underneath the movable part 20 (in particular in the direction along arrow 31c in Figure 3a). Furthermore, in the embodiment of Figure 4a, one or both of the two SMA wires 30a, 30b may be arranged underneath the movable part 20, for example as described in relation to SMA wires 30a, 30b of Figure 5.

Although the schematic plan views of the figures described above show an example in which the movable part 20 has a square footprint in the x-y plane, it will be appreciated that the movable part 20 may generally have any other shape. For example, the movable part 20 may be substantially rounded and follow, for example, the outline of a cylindrical lens carrier. When reference is made to the SMA wires 30 extending along an edge or side of the movable part 20, it is thus not required that the SMA wires are parallel to such an edge or side. Rather, the intention is that the SMA wires 30 are arranged in a manner allowing the footprint of the SMA actuator assembly 2 in the x-y plane to be reduced compared to a situation in which the SMA wires 30 extend radially away from the optical axis O, while achieving a given amount of stroke (i.e. movement of the movable part 20). The SMA wires 30 may, in particular, extend in a manner similar to that depicted in the figures, regardless of the shape of the movable part 20 or support structure 10.

It will be appreciated that the direction in which any one of the above-described SMA wires 30 applies a force to the movable part 20 may change, to some degree, as the movable part 20 moves relative to the support structure 10. The above description of the forces and the direction of forces generally applies to a situation in which the movable part 20 is in a central position relative to the support structure 10, so in a position that is substantially central in a movement envelope defined by the possible movement of the movable part 20 relative to the support structure 10. The above description of the forces and the direction of forces may or may not remain applicable at or towards the extreme ends of the movement envelope, i.e. at the borders of the movement envelope beyond which the movable part 20 may not move during normal operation. Normal operation refers to a situation in which the movable part 20 moves due to forces applied by the SMA wires 30 and/or any optional biasing arrangements, for example for the purpose of OIS.

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.

Those skilled in the art will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing the present disclosure, the present disclosure should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that the present invention has a broad range of applications, and that the embodiments may take a wide range of modifications without departing from scope of the claims.