Field

The present application relates to an actuator assembly suitable for use in a portable electronic device. The actuator assembly may have a drive mechanism including a plurality of shape memory alloy (SMA) elements.

Background

SMA actuators have applications in cameras for smartphones and other portable electronic devices. For example, SMA actuators can be used to produce two-dimensional (2D) movement of a movable part which includes a lens or an image sensor so as to enable optical image stabilisation (OIS). Such SMA actuators are described in WO2013/175197 and WO2017/072525. This 2D movement of the movable part may be guided by: a) a set of bearings (e.g. ball bearings or plain bearings) limiting movement of the movable part towards a support structure, and b) a biasing arrangement (e.g. flexures or magnets) which biases the movable part towards the support structure.

Conventionally, the set of bearings consists of three or four symmetrically-arranged bearings, and the biasing arrangement produces a biasing force that is centralised relative to the bearings.

Support by three bearings is sometimes referred to herein as 'three-point support', and support by four bearings as 'four-point support'.

Summary

According to a first aspect of the present invention, there is provided an actuator assembly suitable for use in a portable electronic device. The actuator assembly comprises: a support structure and a movable part; a drive mechanism for moving the movable part relative to the support structure in a plane perpendicular to a primary axis; a set of four or more bearings, wherein each bearing is configured to limit movement of the movable part towards the support structure by producing a bearing force on the movable part which can be represented by a point force, wherein each point force acts on the movable part at a different bearing point; and a biasing arrangement configured to bias the movable part towards the support structure by producing a biasing force on the movable part which can be represented by a point force acting on the movable part at a biasing point. Throughout an operating range of movement of the movable part relative to the support structure, and when viewed along the primary axis, the biasing point is offset from each of the lines connecting the bearing points. Typically, the bearing points are not coplanar. For example, the bearing points may be nominally coplanar but may not be actually coplanar due to manufacturing tolerances.

Thus, the movable part is typically supported by only three of the bearings during normal operation. The identities of the three bearings will depend on the three-dimensional (3D) position of the bearing points associated with the set of bearings, and/or on the position of the biasing point.

The movable part may be supported by at least one bearing that is not one of the three bearings when the movable part is subject to an abnormal force - e.g. a force generated during manufacture or assembly or when the actuator assembly is subject to a mechanical shock or has a defective or degraded biasing arrangement.

Thus, the actuator assembly can have certain advantages associated with three-point support as well as certain (usually incompatible) advantages associated with four-point support.

With its three-point support during normal operation, the actuator assembly can avoid the disadvantage of conventional four-point support that, for example, relatively small deviations in the 3D position of the bearing points may lead to relatively large tilts of the movable part during normal operation - for example, as a result of changes in orientation of the actuator assembly or as a result of operation of the drive mechanism. Such tilting is described in more detail below with reference to Figure 3.

Furthermore, when the movable part is subject to an abnormal force, the actuator assembly can benefit from the additional support provided by the at least one bearing that is not one of the three bearings. This can have various advantages, for example: higher forces can be used during manufacture or assembly (e.g. when bonding components to the movable part); a reduced risk of damage to the bearings (particularly ball bearings) when the actuator assembly is subject to a mechanical shock, due to the greater contact area; a reduced risk of damage due to components coming into contact as a result of the movable tilting when the actuator assembly is subject to a mechanical shock; and/or the movable part tilting less if the biasing arrangement is defective or degraded (e.g. the biasing force is too low).

Three or more of the bearings may be non-compliant, and one or more of the bearings may be compliant and may be deformed or displaced by the biasing force. The one or more compliant bearings may be deformed such that the one or more bearing points associated therewith lie in the same plane as the bearing points associated with three of the non- compliant bearings.

The movable part may be additionally supported by at least one compliant bearing when the movable part is subject to an abnormal force. As will be appreciated, the at least one compliant bearing may exert a force on the movable part during normal operation but may exert a greater force on the movable part when the movable part is subject to an abnormal force.

Thus, this use of one or more compliant bearings provides similar advantages to those described above.

The offset may be such that, for each of the lines connecting the bearing points, the biasing force produces a torque on the movable part in a first sense about the line that is greater than the torques produced on the movable part in a second, opposite sense about the line during normal operation.

The torques produced during normal operation may comprise torques produced by the weight of the moving part in any orientation of the actuator assembly. The torques produced during normal operation may comprise torques produced by the drive mechanism.

In this way, during normal operation, the movable part does not tilt about any of the lines connecting the bearing points.

The torque produced by the biasing force may be 25% or 50% or 100% greater than the torques produced during normal operation.

Each offset may be at least 0.25mm. Each offset may be at least 0.5mm, at least 1mm, at least 1.5mm, at least 2mm, at least 2.5mm or at least 3mm.

Each offset may be at least 10% of a dimension which corresponds to the distance between (a) the centroid of the bearing points and (b) the bearing point that is furthest from the centroid of the bearing points. Each offset may be at least 20%, at least 30%, at least 40% or at least 50% of the dimension.

When viewed along the primary axis, the biasing point may be located at, or near to, a centroid of three points, wherein the three points are two of bearing points and the crossing point of two of the lines connecting the bearing points. When viewed along the primary axis, at least one of the lines connecting the bearing points may pass through, or near to, a central point of the actuator assembly. This central point may correspond to the centre of mass of the movable part, for example. Such an arrangement of bearing points may be advantageous e.g. in relation to the layout of the actuator assembly. Alternatively, when viewed along the primary axis, the biasing point may be positioned at, or near to, the central point. Such a 'central' biasing point may be advantageous e.g. in relation to the layout of the actuator assembly. Alternatively, when viewed along the primary axis, none of the lines connecting the bearing points may pass through the central point, and the biasing point may be offset from the central point. Such an arrangement may enable the offset to be maximised.

When viewed along the primary axis, the bearing points may correspond to the vertices of a square (or another regular polygon with an even number of vertices), and the biasing point may be offset from the centroid of the bearing points. Alternatively, when viewed along the primary axis, the bearing points may correspond to the vertices of a regular polygon with an odd number of (>5) vertices and the biasing point may be positioned at, or near to, the centroid of the bearing points. The latter configuration can have the advantage of both a central biasing point and a symmetrical arrangement of bearing points.

Alternatively, the bearing points may correspond to the vertices of an irregular quadrilateral or other irregular polygon when viewed along the primary axis.

The set of bearings may comprise plain bearings. The set of bearings may comprise ball bearings. The biasing arrangement may comprise one or more resilient elements connected between the support structure and the movable part. The biasing arrangement may comprise one or more sets of magnetically interacting parts, wherein each set includes a part comprised in the support structure and a part comprised the movable part.

The biasing arrangement may comprise the one or more resilient elements and the one or more sets of magnetically interacting parts. One of these may produce a first biasing force with a first biasing point positioned at, or near to, the central point of the actuator assembly. The other of these may produce a second biasing force such that the biasing point is offset from the central point of the actuator assembly.

As will be appreciated, the bearing points and/or the biasing point may move relative to the movable part as the movable part moves relative to the support structure. The drive mechanism may comprise a plurality of SMA elements (e.g. wires) configured to move the movable part relative to the support structure.

In particular, the drive mechanism may have a total of four SMA elements each connected at one end to the movable part and at the other end to the support structure in an arrangement in which none of the SMA elements are collinear and the SMA elements are capable of being selectively driven to move the movable part relative to the support structure without applying any net torque to the movable part about the primary axis.

The movement of the movable part relative to the support structure may include rotation of the movable part about an axis parallel to the primary axis.

There may be provided a camera assembly comprising: a set of one or more lenses; an image sensor; and the actuator assembly. The set of lenses or the image sensor may be comprised in the movable part of the actuator assembly.

According to a second aspect of the present invention, there is provided an actuator assembly suitable for use in a portable electronic device. The actuator assembly comprises: a support structure and a movable part; a drive mechanism for moving the movable part relative to the support structure in a plane perpendicular to a primary axis; a set of four or more bearings, wherein each bearing is configured to limit movement of the movable part towards the support structure by producing a bearing force on the movable part which can be represented by a point force, wherein each point force acts on the movable part at a different bearing point; and a biasing arrangement configured to bias the movable part towards the support structure by producing a biasing force on the movable part which can be represented by a point force acting on the movable part at a biasing point Throughout an operating range of movement of the movable part relative to the support structure, and when viewed along the primary axis, the biasing point is offset from one of the diagonals of the bearing points, wherein the diagonals of the bearing points are those lines connecting the bearing points that cross each other.

Thus, the movable part is less likely to tilt than in actuator assemblies with conventional four-point support.

The second aspect may include any suitable feature(s) of the first aspect. Brief description of the drawings

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

Figure 1 is a schematic cross-sectional view of a camera assembly including a reference example of an actuator assembly;

Figure 2 is a schematic plan view of the camera assembly of Figure 1;

Figure 3 illustrates the arrangement of the bearing force and the biasing forces in the actuator assembly of Figure 1;

Figure 4 illustrates the arrangement of the bearing force and the biasing forces in a first example of an actuator assembly;

Figures 5A illustrates the arrangement of the bearing force and the biasing forces in a second example of an actuator assembly;

Figure 5B is an enlarged version of Figure 5A;

Figure 6 illustrates the arrangement of the bearing force and the biasing forces in a third example of an actuator assembly;

Figure 7A is a schematic plan view of a first version of a camera assembly including the third example of the actuator assembly of Figure 6;

Figure 7B is a schematic plan view of a second version of a camera assembly including the third example of the actuator assembly of Figure 6;

Figure 8 illustrates the arrangement of the bearing force and the biasing forces in a fourth example of an actuator assembly;

Figure 9A is a schematic plan view of a camera assembly including a fifth example of an actuator assembly;

Figure 9B is a schematic cross-sectional view of a compliant bearing included in the camera assembly of Figure 9A; and

Figure 10 illustrates the arrangement of the bearing force and the biasing forces in a sixth example of an actuator assembly.

Detailed description

Camera assembly including a reference example of an actuator assembly Referring to Figures 1, 2 and 3, a camera assembly 100 will now be described.

Referring in particular to Figures 1 and 2, the camera assembly 100 includes a reference example of an actuator assembly 10, a lens assembly 11, and an image sensor assembly 12. The camera assembly 100 has a primary axis z. The actuator assembly 10 includes a support structure 1, a movable part 2, and a drive arrangement 3 configured to move the movable part 2 relative to the support structure 1. This movement of the movable part 2 relative to the support structure 1 is two-dimensional (2D) in that it includes translational movement in any direction in a plane perpendicular to the primary axis z (e.g. the x-y plane) and/or includes rotation about the primary axis z or an axis parallel thereto.

In this example, the drive arrangement 3 includes SMA wires 30 connected between the support structure 1 and the movable part 2. In particular, the drive mechanism 3 includes four SMA wires 30, each of which is connected at one end to the support structure 1 (via a 'static' crimp 31) and at the other end to the movable part 2 (via a 'moving' crimp 32). In this example, each of the SMA wires 30 is arranged along a different side of the generally-square movable part 2. More generally, the SMA wires 30 may be arranged such that none of the SMA wires 30 are collinear and the SMA wires 30 are capable of being selectively driven to translationally move the movable part 2 relative to the support structure 1 without applying any net torque to the movable part 2 about the primary axis z. The SMA wires 30 are also capable of being selectively driven to apply such a torque and so rotate the movable part 2 about the primary axis z or an axis parallel thereto. Such a drive arrangement 3 is described in WO2013/175197 and WO2017/072525, which are incorporated by reference.

In other examples, the drive arrangement 3 may include a different configuration of SMA wires 30, or may involve a different type of actuator, e.g. a voice coil motor (VCM), etc.

The actuator assembly 10 also includes a bearing arrangement 4 and a biasing arrangement 5. The bearing arrangement 4 and the biasing arrangement 5 are configured to guide the above-described 2D movement of the movable part 2 relative to the support structure 1.

The bearing arrangement 4 includes a set of bearings 40. In this example, there are four such bearings 40. Each bearing 40 limits movement of the movable part 2 towards the support structure 1 by producing a 'bearing' force FA (see Figure 3) on the movable part 2. Each bearing force FA can be represented by a point force acting on the movable part 2 at a 'bearing' point A (see Figure 3). The bearing forces FA are each directed in substantially the same (e.g. upward) direction along the primary axis z. In this example, the bearings 40 are ball bearings. However, at least some of the bearings 40 may be of a different type, e.g. plain bearings.

The biasing arrangement 5 biases the movable part 2 towards the support structure 1 by producing a 'biasing' force F _B (see Figure 3) on the movable part 2. The biasing force F _B can be represented by a point force acting on the movable part at a 'biasing' point B (see Figure 3). The biasing force F _B is directed in substantially the opposite (e.g. downward) direction along the primary axis z to the bearing forces FA. The biasing arrangement 5 includes a set of biasing elements 50. In this example, there are four biasing elements 50. The biasing force F _B is the sum of the forces produced by the biasing elements 50. In this example, each biasing element 50 is a mechanical biasing element in the form of a resilient member connected between the support structure 1 and the movable part 2. Each resilient member 50 may be, for example, a strip of metal. Such resilient members 50 are sometimes referred to as flexures or a spring arms. Typically, the resilient members 50 are elastically deformed during assembly of the actuator assembly 10 such that they produce suitable forces on the movable part 2 in the assembled actuator assembly 10. The resilient members 50 may be configured as illustrated in Fig. 2 or they may take a different form, e.g. as described in WO2017/072525, which is incorporated by reference. At least some of the biasing elements 50 may be of a different type, e.g. involving magnetically interacting parts included in (or forming part of the) the support structure 1 and the movable part 2.

The lens assembly 11 includes one or more lenses 110 configured to focus an image on the image sensor 12. The optical axis of each of the lenses 110 may correspond to the primary axis z. Each of the lenses 110 may have a diameter of 20mm or less, preferably 12mm or less, which is characteristic of a miniature camera. The camera assembly 100 may include a further actuator assembly (not shown) configured to move the lens assembly 11 relative to the image sensor 12 along the primary axis z so as to enable e.g. autofocus (AF) functionality.

The image sensor 12 is configured to capture an image and may be of any suitable type, e.g. a charge- coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) sensor.

In this ('sensor shift') example, the image sensor 12 is included in the movable part 2. In other ('lens shift') examples, the lens assembly 11 may be included in the movable part 2. In either case, the image sensor 12 can be moved relative to the lens assembly 11 with the effect that the image on the image sensor 4 is moved. This enables optical image stabilization (OIS) functionality, i.e. compensating for movement of the camera assembly 1 caused for example by a user's handshake.

In this example, the camera assembly 100 also includes a set of (one or more) movement sensors 13 and also includes control circuitry 14. The movement sensors 13 are configured to sense movement of the camera assembly 1 and may include, for example, a gyroscope and an accelerometer. The control circuitry 14, which may be implemented in an integrated circuit (IC), is configured to generate drive signals for the SMA wires 30. SMA has the property that, on heating, it undergoes a solid-state phase change that causes the SMA to contract. The drive signals cause electrical currents to flow through the SMA wires 30, heating the SMA wires 30, and causing the SMA wires 30 to contract, thereby applying a force on the movable part 2. The drive signals can be generated based on signals from the movement sensors 13 so as to move the movable part 2 in a way that stabilises the image sensed by the image sensor 12.

Arrangement of the bearing force and the biasing forces in the actuator assembly

Referring in particular to Figure 3, the arrangement of the bearing force FA and the biasing forces F _B in the actuator assembly 10 will now be described in more detail.

The actuator assembly 10 uses conventional four-point support.

As described above, each of the four bearing forces FA can be represented by a point force acting on the movable part 2 at a bearing point A, with each of the bearing forces FA directed in substantially the same (e.g. upward) direction along the primary axis z. The biasing force F _B can be represented by a point force acting on the movable part 2 at a biasing point B and directed in substantially the opposite (e.g. downward) direction along the primary axis z to the bearing forces FA.

As will be appreciated, the bearing points A may be at different 'heights', i.e. have different z coordinates, from the biasing point B. The description below of the relative positions of the bearing points A and the biasing point B will refer to these points when projected onto a plane perpendicular to the primary axis z (or, in other words, when viewed along the primary axis z).

In this example, when viewed along the primary axis z, the bearing points A1-A4 correspond to the four corners of square whose centre (or centroid) is at a central point C of the actuator assembly 10. In this example, the central point C corresponds to the centre of mass of the movable part 2. The biasing point B is also positioned at C. Accordingly, the biasing point B is positioned at the crossing point of the two lines L13, L24 connecting the bearing points A1-A4 that cross each other (these lines L13, L24 are generally referred to herein as diagonals).

The diagonals L13, L24 are potential tilt axes about which the movable part 2 may tilt if the bearing points A1-A4 are not completely coplanar. Moreover, because the biasing point B is nominally positioned on the diagonals L13, L24, this tilting may occur in the event of relatively small changes in any additional forces acting on the movable part 2, e.g. as a result of changes in the orientation of the actuator assembly 10 or as a result of operation of the drive mechanism 3. This can be disadvantageous.

First example of an actuator assembly

Referring to Fig. 4, a first example of an actuator assembly 10' will now be described. The actuator assembly 10' is the same as the reference example except for the positions of the bearings 40 and hence the bearing points A1-A4. In particular, the bearing point Ai is at a different position than in the reference example. When viewed along the primary axis z, the bearing points A1-A4 correspond to the vertices of an irregular quadrilateral with one diagonal L24 passing through the central point C of the actuator assembly 10' and one diagonal L13 not passing through C. Accordingly, the biasing point B (which, like in the reference example, is at C) is positioned on only one diagonal L24.

The biasing point B is offset from the other diagonal L13 such that, if the bearing points A1-A4 are not completely coplanar (which is generally the case), the movable part 2 will prefer to be supported by three of the bearing points during normal operation, i.e. bearing points A1-A3 in this example in which the biasing point B is offset from L13 in a direction towards bearing point A2. Hence this arrangement is more stable than the reference example. However, the diagonal L24 (on which the biasing point B is positioned) is still a potential tilt axis as described above.

Alternatively or additionally, the biasing force F _B may be re-positioned compared to the reference example so as to be positioned on only one diagonal.

Second example of an actuator assembly

Referring to Figures 5A and 5B, a second example of an actuator assembly 10" will now be described.

This actuator assembly 10" is the same as the reference example except for the positions of the bearings 40 and hence the bearing points A1-A4. In particular, two of the bearing points Ai, A4 are at different positions than in the reference example. When viewed along the primary axis z, the bearing points A1-A4 correspond to the vertices of an irregular quadrilateral with neither diagonal L13, L24 passing through the central point C of the actuator assembly 10". Accordingly, the biasing point B (which, like in the reference example, is at C) is offset from both diagonals L13, L24. In particular, the biasing point B is offset from diagonal L13 by distance Di and is offset from diagonal L24 by distance D2.

The biasing point B is preferably sufficiently offset from each of the diagonals L13, L24 that neither of the diagonals L13, L24 is a potential tilt axis as described above. In particular, for each diagonal L13, L24, the offset is preferably large enough that the biasing force F _B produces a torque on the movable part 2 about the diagonal L13, L24 that is greater (e.g. 25% greater) than any opposite torques produced on the movable part 2 about the diagonal L13, L24 during normal operation, e.g. as a result of changes in the orientation of the actuator assembly 10" or as a result of operation of the drive mechanism 3. In some examples, this corresponds to each of the distances Di, D2 being at least 0.25mm. In some examples, this corresponds to each of the distances Di, D2 being at least 10% of a characteristic dimension of the bearing arrangement 4. This characteristic dimension may correspond to the distance between (a) the centroid of the bearing points A1-A4 and (b) the bearing point A that is furthest from this centroid.

If the bearing points A1-A4 are not completely coplanar (which is generally the case), the movable part 2 will be supported by three of the bearing points during normal operation, i.e. bearing points Ai, A2 ,A ₃ or bearing points A2, A3 ,A4 depending on the 3D position of the bearing points A1-A4. As explained above, such three-point support is more precise than e.g. the conventional four-point support in the reference example.

Moreover, as explained above, the fourth bearing point, i.e. bearing point A4 or Ai, can support the movable part 2 when the movable part 2 is subject to abnormal forces.

In this example, the biasing point B is positioned at the central point C of the actuator assembly 10". This can be produced by a biasing arrangement 5 with similar biasing elements 50 symmetrically placed about C (see e.g. Fig. 2).

The biasing point B is positioned within the quadrilateral whose vertices correspond to the bearing points A1-A4. The biasing point B is also preferably sufficiently offset from each of the sides L12, L23, L34, L41 of this quadrilateral such that none of these sides L12, L23, L34, L41 is a potential tilt axis. Accordingly, the biasing point B may be located at, or near to, a centroid of three points, wherein the three points are two of bearing points and the crossing point of two of the lines connecting the bearing points.

Third example of an actuator assembly

Referring to Figures 6 and 7A, a third example of an actuator assembly 10"' will now be described.

The actuator assembly 10"' is the same as the reference example except for the biasing arrangement 5 and the position of the biasing point B. In particular, the biasing arrangement 5 includes an additional magnetic biasing element 51. The magnetic biasing element 51 may have any suitable form and, for example, may include a permanent magnet provided on the movable part 2 which interacts with a ferromagnetic material on the support structure 1. The magnetic biasing element 51 is positioned away from the central point C of the actuator assembly 10'" and so the sum of the forces produced by the magnetic biasing element 51 and the mechanical biasing elements 50 (i.e. the biasing force F _B ) acts at a point (i.e. the biasing point B) which is offset from C. Accordingly, the biasing point B is offset from each of the diagonals L13, L24 associated with the bearing points A1-A4 which (like in the reference example) cross at C. Such an offset can provide the advantages described above in relation to the second example.

Unlike in the second example, in this example, the bearings 50 are symmetrically placed about the central point C of the actuator assembly 10"'.

Such an off-centre biasing point B may be achieved in other ways. For example, the biasing arrangement 5 may include one or more (mechanical and/or magnetic) biasing elements 50, 51 which are dissimilar and/or not symmetrically placed around the central point C of the actuator assembly 10"'.

Referring to Figure 7B, in one such example, the biasing arrangement 5 include three similar magnetic biasing elements 51 which are non-symmetrically placed around the central point C.

In another such example, the biasing arrangement 5 includes mechanical biasing elements 50 which are symmetrically placed around the central point C (e.g. as illustrated in Figure 2), but which are dissimilar. Such dissimilarities may be achieved by introducing different amounts of elastic deformation in the elements 50 during assembly of the actuator assembly 10'" and hence different forces on the movable part 2 in the assembled actuator assembly 10'". This may be achieved by subjecting initially similar flexures to different amounts of pre-forming, i.e. plastic deformation.

Fourth example of an actuator assembly

Referring to Figure 8, a fourth example of an actuator assembly 10"" will now be described.

The actuator assembly 10"" is the same as the reference example except that it has the bearing arrangement 4 of the second example and the biasing arrangement 5 of the third example. Accordingly, the diagonals L , L24 do not pass through the central point C of the actuator assembly 10'", and the biasing point B is positioned away from C. In particular, this is done in such a way as to increase the offsets Di, D2 between the biasing point B and the diagonals L13, L24 (compared to the second example or the third example). For example, the crossing point of the diagonals L13, L24 may be in one direction (e.g. -Y) from C, and the biasing point B may be in the opposite direction (e.g. +Y).

Fifth example of an actuator assembly

Referring to Figures 9A and 9B, a fifth example of an actuator assembly 10* will now be described.

The actuator assembly 10 is broadly the same as the reference example except that one of the bearings

41 is compliant. In this example, the bearing 41 includes a resilient element 41a. The bearing 41 may be as described in GB2557006, which is incorporated by reference. The bearing 41 may be configured such that its bearing surface (and hence bearing point A) can be urged in a direction along the primary axis z (e.g. downwards). The bearing 41 could be extended (e.g. raised) slightly to ensure that it preferentially contacts the movable part 2 (or the support structure 1), but since it is compliant, it would then deform, in response to the biasing force F _B , to allow the other three bearings 40 to come into contact. In this situation, the four bearing points A lie in the same plane. The benefit of such a configuration is that it allows the position of the bearings 40, 41 to remain symmetric or approximately symmetric. The disadvantage is that the normal force from the compliant bearing 41 will reduce the biasing force F _B on the other bearings 40. As will be appreciated, the compliance may be provided in any suitable way, and the actuator assembly 10* may include more than one compliant bearing 41.

Sixth example of an actuator assembly

Referring to Figure 10, a sixth example of an actuator assembly 10** will now be described.

The actuator assembly 10** is the same as the reference example except for the bearing arrangement 4. In particular, the actuator assembly 10** has five bearings 40 and hence five bearing points AI-A ₅ . When viewed along the primary axis z, the bearing points AI-A ₅ correspond to the five corners of a regular pentagon whose centre is at a central point C of the actuator assembly 10**. None of the diagonals L , L14, L24, 25, 35 of the pentagon pass through its centre (which is the result of the pentagon being a regular polygon with an odd rather than an even number of sides). Hence none of the diagonals L13, L14, L2 , 25, 35 pass through the central point C of the actuator assembly 10**. Accordingly, the biasing point B (which, like in the reference example, is at C) is offset from each of the diagonals L13, L14, L2 , 25, 35. Moreover, the biasing point B is preferably sufficiently offset from each of the diagonals L13, L14, L2 , 25, 35 that none of the diagonals L13, L14, L2 , 25, 35 is a potential tilt axis.

SMA elements

The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA)element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field.

Other variations

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

For example, the drive mechanism need not include SMA wire and may involve a different type of actuator such as a voice coil motor (VCM) actuator.