Actuator assembly

Abstract

A blade 100 for a variable aperture assembly comprising a base and a rotatable part wherein the blade comprises: a body portion 101; a first flexure 102 extending from the body portion, and a coupling

Description

Field Certain embodiments of the present application relate to an actuator assembly providing rotary motion, for instance a variable aperture assembly. Certain embodiments relate to a blade for a variable aperture assembly and a variable aperture assembly incorporating a plurality of the blades. Background There are a variety of apparatuses in which it is desired to provide control of a movable element. SMA elements, for instance SMA wires, may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA actuator required to apply a given force to the movable element can be relatively small. One type of apparatus in which SMA wire is known for use as an actuator is in miniature cameras, for example those used in smartphones or other portable electronic devices. WO2011 / 104518 discloses examples of SMA actuation apparatuses which are suitable for use in miniature cameras. A variable aperture assembly can be used to provide an aperture of a controlled size. For example, in the context of a camera, a variable aperture assembly may be used to control the size of the aperture. For example, different aperture sizes may be used for different focal lengths. Variable aperture assemblies within a camera may use SMA elements (for instance, SMA wires) to move the blades so as to adjust the size of the variable aperture. An example of such a variable aperture assembly is disclosed in WO2024 / 057042: the variable aperture assembly comprises a base, a rotatable part, and an actuator assembly configured to drive rotation of the rotatable part relative to the base about a primary axis to any rotational position within a range of movement. The actuator assembly includes at least one SMA element coupled between the base and the rotatable parts. A plurality of blades is connected to the base and the rotatable part. Rotation of the rotatable part drives rotation of each blade of the plurality of blades to change the size of the variable aperture. In some conventional variable aperture assemblies, each blade is coupled to the base and the rotatable part to effect blade rotation with protrusions, for instance pins, and corresponding holes. However, assembling a plurality of blades, each coupled to the base and the rotatable part via a pair of protrusions can be difficult due to the need to accurately align the holes and protrusions. Typically, the sizes of the holes and protrusions are closely matched to minimise backlash in the blade drive mechanism, further exacerbating the complex assembly process. Furthermore, in an actuator assembly in which a rotatable part rotates relative to a base there may be a need to provide a bearing arrangement, for instance plain or rolling bearings or magnets, between the relatively rotating parts in order to provide control over the relative motion. This can present a limit on the extent to which an actuator assembly can be minimised. This may limit the ability to reduce the actuator assembly size in a direction normal to the plane of rotation. It is an aim of certain embodiments of the present invention to provide for a variable aperture assembly that is simpler to assemble. It is a further aim of certain embodiments of the present invention to provide an actuator assembly that can be reduced in size relative to convention actuator assemblies. Summary According to an aspect of the present invention, there is provided a blade for a variable aperture assembly comprising a base and a moveable part, wherein the blade comprises: a body portion; a first flexure extending from the body portion; and a coupling feature; wherein the body portion is connectable to one of the base and the moveable part via the first flexure and the body portion is connectable to the other of the base and the rotatable part via the coupling feature. Advantageously, the first flexure may be easier to connect to the base or the moveable part of a variable aperture assembly during manufacturing, than is the case for convention pin-hole blade coupling mechanisms. The flexure may define the rotation of the blade as the base and moveable part relative to one another. The moveable part may be a rotatable part. Suitably the base and the rotatable part may comprise nested inner and outer rings, where relative ring rotation drives blade rotation to control the size of the aperture. In some cases it is preferable for the flexure to connect to the base as this may ensure that the flexure remains under tension as the rotatable part rotates. In principle the flexure could be coupled to the base or the rotatable part and connected to the blade during assembly. The body portion may have a first end and a second end; and wherein the coupling feature may be closer to the first end than the point at which the first flexure extends from the body portion, or the point at which the first flexure extends from the body portion is closer to the first end than the coupling feature. The relative arrangement of the coupling feature and the flexure may define the direction of blade tip rotation as the rotatable part rotates relative to the base. The coupling feature may comprises a protrusion or a hole configured to engage a corresponding hole or protrusion defined by the other of the base and the moveable part. That is, it may comprise a conventional pin-hole coupling. The pin-hole coupling may be further from the blade tip than the point at which the flexure joins the blade body, which results in amplified blade tip rotation as the rotatable part rotates. Alternatively, the coupling feature may comprise a second flexure extending from the body portion; and the body portion may be connectable to the other of the base and the moveable part via the second flexure. Using two flexures further simplifies the assembly process as both may be connected to the base and the rotatable part, for instance by gluing or welding, thereby avoiding pinhole couplings to the blade entirely. The first flexure or the second flexure may integrally formed with the body portion or coupled to the body portion. The blade may be formed from a sheet of material. The body portion may comprise an aperture-defining edge configured to partly define a variable aperture of the variable aperture assembly. In other respects the blade may formed as is known in the art of variable aperture construction. According to another aspect of the present invention there is provided a variable aperture assembly comprising: a base; a rotatable part; an actuating unit configured to drive rotation of the rotatable part relative to the base about a primary axis to any rotational position within a range of movement; and a plurality of the blade described above, wherein for each blade of the plurality of blades the body portion is connected to one of the base and the rotatable part via the first flexure and the body portion is connected to the other of the base and the rotatable part via the coupling feature, the plurality of blades being arranged around the base and the rotatable part to define a variable aperture about the primary axis; wherein rotation of the rotatable part relative to the base causes for each blade of the plurality of blades at least one flexure to flex such that the blade rotates about a blade rotation axis, wherein rotation of the plurality of blades changes the size of the variable aperture. There may be any number of blades, for instance 6, 8, or 10. The upper limit may be defined by space constraints and the degree to which the aperture is required to approximate a circle. 20 blades may be an upper limit. They may be provided in a single layer or in multiple stacked layers of groups of blades. The actuating unit may be of any conventional type, including the use of SMA wire. For instance, the actuator assembly may be configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, the actuating unit comprising at least one shape memory alloy, SMA, element coupled between the base and the rotatable part. Rotation of the rotatable part relative to the base may cause points at which each blade of the plurality of blades is connected to the base and rotatable part to move relative to one another to cause at least one flexure to flex. For a blade of the plurality of blades having a coupling feature comprising a hole or a protrusion, the blade rotation axis may pass through the hole or the protrusion. The base and the rotatable part may be arranged around the primary axis. The base may for instance be radially outward of at least part of the rotatable part. For each of the plurality of blades an end of the first flexure or the second flexure remote from the body portion of the blade may be fixed to the base or the rotatable part, for instance by welding or an adhesive. According to yet another aspect of the present invention there is provided an actuator assembly comprising: a base; a rotatable part; and an actuator assembly configured to drive rotation of the rotatable part relative to the base about a primary axis to any rotational position within a range of movement; wherein the base and the rotatable part are connected via at least one flexure configured to permit relative rotational movement about the primary axis while limiting relative translational movement along an axis perpendicular to the primary axis. Advantageously, the use of flexures to couple the base and the rotatable part may remove the need for other bearings and so the height of the actuator assembly along the primary axis may be reduced. In their most general sense, certain embodiments of the present invention relate to actuator assemblies providing for relative rotational motion between two parts (a rotary actuator assembly). A variable aperture assembly is a particular example of such a rotary actuator assembly, and much of the following description relates specifically to a variable aperture assembly. However, unless the context requires otherwise, features described in connection with embodiments of a variable aperture assembly should be considered to apply more generally to a rotary actuator assembly. The actuator assembly could be actuated by an actuating unit powered using SMA wire. Rotation of the rotatable part may further include motion along the primary axis: the motion may be helical, for instance as suited to driving an auto focus assembly. An actuating unit may be configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, the actuating unit comprising at least one shape memory alloy, SMA, element coupled between the base and the rotatable part. The at least one flexure may comprise a plurality of flexures spaced apart around the primary axis. They may be evenly spaced. The flexure arrangements may be rotationally symmetrical. There may be any number of flexures, for instance, at least 3, 4, 6, 8. An upper limit may be defined by space constraints. In some examples, 20 flexures may be the upper limit. Each flexure may define a first end connected to the base at a first location and a second end connected to the rotatable part at a second location, each flexure being configured to accommodate a change in distance between the first and second locations as the base and the rotatable part rotate relative to one another about the primary axis. A linear flexure may be suitable. Other known flexures may be used, for instance a coiled spring. Each flexure may be stiffer in a direction perpendicular to the primary axis than in a direction circumferential about the primary axis. That is, flexures may be more compliant in a circumferential direction about the primary axis. To achieve this compliance, in some examples each flexure includes a significant proportion extending radially or at a shallow angle to the radius. Each flexure may comprise a linear element incorporating at least one folded portion between the first and second ends, the at least one folded portion being configured to unfold to accommodate a change in distance between the first and second locations. Each flexure may define a flexure axis between the first and second ends; and each flexure may be more compliant under a load transverse to the flexure axis compared with a load along the flexure axis. The base, the rotatable part, and each flexure may be integrally formed (for instance, by injection molding); or the flexure or a plurality of flexures may be formed from a sheet material and attached to surfaces of the base and the rotatable part (for instance, by glueing or welding). In some cases, part or all of the base and / or the rotatable part may be integrally formed with the flexures, for instance by being formed from an etched or stamped metal layer. An actuator assembly may be built up in which the major components including the flexures are formed from stacked etched or stamped metal layers. Metal flexures may suitably electrically interconnect the base and the rotatable part. The base and the rotatable part may be arranged around the primary axis. According to yet another aspect of the present invention there is provided a variable aperture assembly comprising: an actuator assembly as described above; and a plurality of blades connected to the rotary actuator assembly and arranged to define a variable aperture about the primary axis; wherein rotation of the rotatable part relative to the base about the primary axis drives rotation of each blade about a blade rotation axis to change the size of the variable aperture. Each blade of the plurality of blades may be connected to the base and the rotatable part via respective first and second coupling features; and rotation of the rotatable part relative to the base may cause relative movement between the first and second coupling features to drive rotation of each blade about the blade rotation axis. Each blade of the plurality of blades may be connected to the base via a first protrusion and connected to the rotatable part via a second protrusion. Alternatively, the plurality of blades may be formed as described above. Each blade may comprise an aperture-defining edge configured to partly define a variable aperture of the variable aperture assembly. According to yet another aspect of the present invention there is provided a camera comprising: a variable aperture assembly as described above; a lens assembly; and an image capture device; wherein the optical axis of the lens assembly coincides with the primary axis, such that light passing through the variable aperture assembly passes is focused by the lens and is received by the image capture device. According to yet another aspect of the present invention there is provided an electronic device incorporating a camera as described above. 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 plan view of a variable aperture assembly with a relatively closed variable aperture; Figure 2 is a schematic plan view of a variable aperture assembly with a relatively open variable aperture; Figure 3 is a schematic side view of a variable aperture assembly assembled on a lens assembly; Figure 4 is a schematic plan view of an arrangement of SMA elements for adjusting the variable aperture; Figures 5 to 7 are schematic plan views of alternative blades suitable for use in a variable aperture assembly; Figure 8 is a schematic plan view of a flexure arrangement for coupling together a base and a rotatable part to form an actuator assembly; and Figures 9i to 9v are schematic plan views of alternative flexures for coupling together a base and a rotatable part to form an actuator assembly. Detailed description Certain example devices will now be described. Where similar or identical components are used in the different examples, they will be given the same reference numerals. For efficiency, description of similar or identical elements may not be repeated between the examples and characteristics and features of elements are to be understood as applying to those elements in all examples unless the description indicates otherwise. The following description, and the accompanying drawings, present different embodiments of a variable aperture assembly or portions of a variable aperture assembly. As previously noted, unless the context requires otherwise, these should be considered to be examples of an actuator assembly providing for relative rotational movement between a base and a rotatable part (a rotary actuator assembly). Features presented in connection with a variable aperture assembly should be considered to be more generally applicable to any rotational actuator assembly. Furthermore, in the following description, the terms "actuator" and "actuating unit" are used broadly synonymously to refer to a part of the actuator assembly that provides a driving force to effect relative rotational movement. The embodiments presented particularly concern actuating units including one or more Shape Memory Alloy (SMA) element. This may be an SMA wire, and where the term SMA wire is used this should be considered to include other suitable forms of SMA elements. Actuation of the actuating unit is achieved by heating the SMA element (for instance, by passing an electrical current) causing it to contract. Variable aperture assemblies Figures 1 to 3 schematically depict a variable aperture assembly 1. Figure 1 shows a plan view of the variable aperture assembly 1 with a relatively small variable aperture, and Figure 2 shows a plan view of the variable aperture assembly 1 with a relatively large variable aperture. Figure 3 shows a side view of the variable aperture assembly 1 in combination with a lens assembly 50. The variable aperture assembly comprises a base 30 and a rotatable part 20. The rotatable part 20 is rotatable relative to the base 30, in particular about a primary axis O. The base 30 is shown generally disposed around the rotatable part 20 (though they may partially overlap along a radial direction perpendicular to the primary axis). Alternatively, the rotatable part 20 may surround the base 30. At least one of the base 30 and rotatable part 20 defines an aperture that allows light or fluid (for instance, gas or liquid) to pass. The aperture surrounds the primary axis O. The aperture may have rotational symmetry about the primary axis O. The base 30 and rotatable part 20 are formed generally as rings surrounding the primary axis O that light can pass through, in particular to a lens assembly 50 as shown in Figure 3. The base 30 may be fixed within a larger device (such as a smartphone or other portable electronic device) within which the variable aperture assembly 1 is incorporated. The base 30 may, for example, be fixed relative to a lens element of a lens assembly 50 that is provided in combination with the variable aperture assembly 1. However, in general, the base 30 may also be movable within such a larger device. The base 30 is herein used as a reference structure relative to which movement of other components is described, unless explicitly stated otherwise. The base 30 may comprise multiple parts that are fixed relative to each other to form the base 30. The variable aperture assembly 1 may comprise a bearing arrangement (not shown in Figures 1 to 3) between the rotatable part 20 and the base 30. The bearing arrangement may guide rotation of the rotatable part 20 relative to the base 30. The bearing arrangement may constrain one or more degrees of freedom of movement other than the rotation. For example, the bearing arrangement may constrain movement of the rotatable part 20 relative to the base 30 along the primary axis O. The bearing arrangement may comprise rolling bearings (such as rollers or ball bearings), plain bearings (for instance, sliding bearings) or flexure bearings (for instance, arrangements of flexures constraining degrees of freedom of movement). The variable aperture assembly 1 may also comprise a biasing arrangement (not shown), for instance, an arrangement of flexures or other types of spring, for loading the bearing arrangement. The variable aperture assembly 1 further comprises a plurality of blades 40. The blades 40 may also be referred to as leaves 40. The plurality of blades 40 defines a variable aperture. The variable aperture is preferably substantially circular, but in general may have other shapes, depending on the desired application of the variable aperture assembly 1. Each blade 40 is coupled between the base 30 and the rotatable part 20 in a manner such that rotation of the rotatable part 20 relative to the base 30 changes the size of the variable aperture. In the embodiment of Figures 1 and 2, each blade 40 is coupled to the base 30 via a respective pin 33 and to the rotatable part 20 via a respective pin 23. Rotation of the rotatable part 20 relative to the base 30 causes relative movement of the pins 23, 33, thereby allowing the blades 40 to effectively pivot about the pins 23, 33 so as to change the variable aperture. The distance between the pins 23, 33 will change during rotation of the rotatable part 20 and so each blade 40 is coupled to its respective pins 23, 33 by at least one pin being received in an elongate hole or slot formed in the blade 40 to accommodate this change. As illustrated, the pin 33 formed on the base 30 engages a slot extending to an edge of the blade 40. In an alternative embodiment, not illustrated, the pins 23, 33 are spring loaded relative to each other, by a flexure coupled to the base 30 or the rotatable part 20 (or both can be flexure coupled), allowing each pin 23 to move along a circular path around respective pin 32 on rotation of the rotatable part 20. In a further alternative, the pins may be provided upon the blades 40 and engage holes or slots formed within the base 30 and the rotatable part 20. In general, the coupling of the blades 40 to the base 30 and the rotatable part 20 may comprise any mechanism allowing movement of the blades 40 upon rotation of the rotatable part 20 so as to adjust the variable aperture. In Figures 1 and 2 the variable aperture assembly 1 comprises a total of six blades 40. The blades 40 are stacked in two layers of three blades 40 on top of each other. The two layers overlap when viewed along the primary axis O. However, in general, the variable aperture assembly 1 may comprise any number of blades 40, arranged in any number of layers. The variable aperture assembly 1 further comprises an actuating unit 10, schematically shown in Figure 3. The actuating unit 10 is configured to drive rotation of the rotatable part 20 relative to the base 30 about the primary axis O. The actuating unit 10 may rotate the rotatable part 20 relative to the base 30 to any rotational position within a range of movement. As such, the size of the variable aperture defined by the blades 40 may be adjusted to any size within a continuous range. Figure 3 shows the variable aperture assembly 1 in combination with a lens assembly 50. The base 30 of the variable aperture assembly 1 may be mounted on the lens assembly 50, such that the lens assembly 50 is nested or provided within a through hole or opening of the base 30 and the rotatable part 20 which extends along a primary axis O of the variable aperture assembly 1. The primary axis O may coincide with the optical axis of the lens assembly 50. The variable aperture assembly 1 may thus adjust the amount of light entering the lens assembly 50. The light enters the lens assembly 50 along an optical path 2. The optical path 2 may be shaped, between the variable aperture assembly 1 and the lens assembly 50, as a cone around the primary axis O. Although the variable aperture assembly 1 has been described as comprising a rotatable part 20 to which the blades 40 are coupled, in general the variable aperture assembly 1 may comprise any other mechanism capable of effecting movement of the blades 40. For example, the variable aperture assembly 1 may generally comprise a movable part instead of the rotatable part 20. The movable part may be translationally or pivotally movable relative to the base 30. The blades 40 may be coupled between the base 30 and the movable part such that movement of the movable part relative to the base 30 effects movement of the blades 40, thereby adjusting the size of the variable aperture. SMA actuating unit Figure 4 schematically shows an embodiment of the actuating unit 10. The blades 40 are not shown, but it will be apparent that they may be coupled to the base 30 and the rotatable part in the same way as described above in connection with Figures 1 and 2. The actuating unit 10 comprises two SMA wires 11. The SMA wires 11 are configured, on actuation, to drive rotation of the rotatable part 20 relative to the base 30 about the primary axis O. In some embodiments, the SMA wires 11 drive the rotatable part 20 to any rotational position within a range of movement relative to the base. In some other embodiments, the SMA wires 11 drive the rotatable part 20 to a set of predetermined positions within a range of movement relative to the base. The size of the variable aperture is thereby adjusted. The SMA wires 11 are connected between the base 30 and rotatable part 20 by connection elements 42, 43. The connection elements 42, 43 may be crimps, for example. The SMA wires 11 may be directly connected between the base 30 and rotatable part 20 as illustrated, such that the connection elements 42, 43 are directly connected to the base 30 and rotatable part 20. Alternatively, intermediate elements (not shown) may be connected between the connection elements 42, 43 and the base 30 and / or rotatable part 20, such that the SMA wires 11 are indirectly connected between the base and rotatable part 20. Such intermediate elements transfer the force in the SMA wires 11 to the rotatable part 20 so as to effect rotation of the rotatable part 20 relative to the base 10. As shown in Figure 4, the variable aperture assembly 1 may comprise two SMA wires 11. A first SMA wire 11 (for instance, the left-hand wire) is arranged, on contraction, to apply a torque to the rotatable part 20 in a first sense (for instance, clockwise). A second SMA wire 11 (for instance, the right-hand wire) is arranged, on contraction, to rotate the rotatable part 20 in a second sense (for instance, counterclockwise). The second sense is opposite to the first sense. In alternative embodiments there may be more or fewer SMA wires, for instance a larger number of wires operating in groups operating in opposite rotational directions. Where there is only a single SMA wire there may be a biasing element providing a return force acting in an opposite rotational direction. SMA wires providing torque in a certain sense may be considered to be a single actuating unit such that there may be considered to be two or more actuating units (Figure 4 thus showing two actuating units each comprising a single SMA wire). The SMA wires may be arranged in a loop around the primary axis O, or part way around the primary axis O. Zero hold power In conventional variable aperture assemblies, the actuator 10 is constantly powered to maintain the position of the blades and maintain the variable aperture at a desired size. The power consumption of such conventional variable aperture assemblies is thus relatively high. According to some embodiments, including Figure 4, the variable aperture assembly 1 is configured such that the one or more blades 40 maintain their position when the actuator 10 is not actuating. As such, the actuator 10 only needs to be powered when the size of the variable aperture is adjusted. The energy efficiency of the variable aperture assembly 1 according to embodiments of the present invention is thus improved. The size of the variable aperture may be maintained when the actuator 10 is not actuating and when acceleration of the variable aperture assembly 1 is less than or equal to a hold threshold. The hold threshold is a magnitude of acceleration of the variable aperture assembly 1. When the acceleration is equal to or less than the hold threshold, hold forces (such as frictional forces) are sufficient to maintain the size of the variable aperture. When the acceleration is greater than the hold threshold, the hold forces (such as frictional forces) may be insufficient for maintaining the position of the blades 40. The hold threshold is set by the overall force resisting movement of the blades 40 when the actuator is not actuating, and so may be determined by the coefficients of friction of friction surfaces, the area of friction surfaces and the normal force with which the friction surfaces are biased against each other. The hold threshold may be at least lg (9.8 m / s2) or at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2). By increasing the hold threshold, the risk of undesired movement of the blades 40 is reduced. Friction surfaces for zero hold power In some embodiments, the position of the blades 40 is maintained via an overall frictional force between components of the variable aperture assembly 1. The overall frictional force may consist of frictional forces acting between any of the components of the variable aperture assembly 1. These frictional forces may differ in magnitude, direction, and whether the frictional forces are affected by actuation of the actuator 10. For example, some of these frictional forces may remain constant on actuation of the actuator 10 and some other of these frictional forces may be reduced or negated on actuation of the actuator 10. All the frictional forces may contribute to the overall frictional force that resists movement of the blades 40 when the actuator 10 is not actuating. The actuator of Figure 4 is configured to enable a reduction in the frictional force. The variable aperture assembly 1 comprises a biasing arrangement 35. The biasing arrangement 35 may comprise a leaf spring as illustrated. In general, the biasing arrangement 35 may comprise any element or combination of elements capable of applying a force between two or more components of the variable aperture assembly 1. The biasing arrangement 35 may, for example, comprise a resilient or elastic element, such as a spring (for instance, coil spring, flexure, leaf spring), rubber band, or other resilient or elastic element. The biasing arrangement 35 may be a magnetic arrangement, comprising a magnet on one part and a magnet or ferromagnetic material on the other part. The biasing arrangement 35 may be arranged between two parts or be incorporated into one or more of the parts of the variable aperture assembly 1. The biasing arrangement 35 urges the rotatable part 20 in a first direction (downward in Figure 4), thereby urging friction surfaces 21, 31 (provided respectively on the rotatable part 20 and the base 30) against each other. Two pairs of friction surfaces 21, 31 are illustrated, each pair of friction surfaces comprising a first friction surface 31 and a second friction surface 21 that engage one another. The actuator 10, for example the SMA wires 11, may be used to move the blades 40 to any position within the range of movement. Upon energising (that is, when drive signals are applied to the SMA wires 11 by the control circuit), the SMA wires 11 contract and apply an actuating force for moving the blades. The actuating force is sufficient to overcome the frictional forces at the friction surfaces 31, 32 (in some embodiments after reduction or elimination of the frictional forces due to SMA wire contraction), to drive movement of the blades. Upon ceasing power supply to the SMA wires 11, and so when stopping contraction of the SMA wires 11, the zero-hold components (for instance, the blades) remain at their position within the range of movement due to the frictional forces between the first and second friction surfaces 31, 21. In this state, the blades 40 are retained in position with zero power consumption by the variable aperture assembly 1, so the variable aperture assembly 1 may be referred to as a zero hold power actuator assembly. The biasing arrangement 35 of Figure 4 also comprises a coupling element 35a arranged between the biasing element and the rotatable part 20. The coupling element 35a is a ball bearing. The coupling element 35a allows the rotatable part 20 to move relative to the biasing element. The biasing arrangement 35 applies the biasing force throughout a range of movement of the rotatable part 20 relative to the base 30. The SMA wires 11 are angled relative to one another at an angle a and relative to the biasing force applied by the biasing arrangement 35 at an angle a / 2. As such, a relatively smaller proportion of the stress in the SMA wires 11 is used to reduce the force applied by the biasing arrangement 35 (and so reduce the frictional force between the first and second friction surfaces 31, 21) and a relatively greater proportion of the stress of the SMA wires 11 is capable of effecting rotation of the rotatable part 20. Equal actuation of the SMA wires 11 reduces the frictional force without rotating the rotatable part 20. Unequal actuation of the SMA wires 11 rotates the rotatable part 20. The angle a (and so the angle a / 2) may be selected in dependence on the magnitude of the biasing force applied by the biasing arrangement, the coefficient of friction and area of the friction surfaces 21, 31, and the actuation force applied on actuation of the SMA wires 11. In Figure 4, the SMA wires 11 cross over when viewed along the primary axis O. The SMA wires 11 are thus longer compared to a situation in which the SMA wires 11 are not allowed to cross over. The SMA wires 11 are allowed to cross over because the depicted angle a is relatively large. For smaller angles a, the SMA wires 11 may not need to cross over while maintaining the same length of SMA wire 11. The first and second friction surfaces 21, 31 may engage each other throughout the range of movement. So, in normal use (that is, under actuation of the actuator 10 for moving the rotatable part 20), at least some of the first and second friction surfaces 21, 31 may remain in engagement with one another (even if the actuation of the SMA wires 11 reduces the frictional force). Alternatively, the first and second friction surfaces 21, 31 may disengage on actuation of the actuator 10 (if the actuation force provided by the SMA wires 11 exceeds the biasing force provided by the biasing arrangement). Blade flexure coupling As noted in the background section above, it can be difficult when assembling a variable aperture assembly to align the pin / hole arrangement coupling the blade to the base and the rotatable part. According to certain embodiments of the invention, blade rotation (to control the aperture size) is defined using one or more flexures in place of one or both pin / hole couplings. This can reduce the complexity of variable aperture assembly manufacturing, particularly by reducing or removing the need to align pins and holes when attaching the blades. Figure 5 shows a blade 100 for a variable aperture assembly comprising a base and a moveable part. The moveable part may be a rotatable part, and the base and the rotatable part may be substantially as described above in connection with Figures 1 to 4 (except for details regarding the connection to the blades). The blade 100 comprises a body portion 101 which defines the majority of the blade 100 and serves to block portions of the aperture according to its rotational position. The blade 100 further comprises a first flexure 102 extending from the body portion 101 and a coupling feature 103. The body portion 101 is connectable to one of a base and a moveable part of a variable aperture assembly via the first flexure 102 and the body portion 101 is connectable to the other of the base and the rotatable part via the coupling feature 103. Flexure 102 comprises an elongate portion 104 extending from the body portion 101 and a foot 105. Foot 105 is configured to be attached to the base or the rotatable part and may have any suitable shape. It may be attached using any suitable fabrication technique, such as welding or adhesive. The coupling feature 103 is illustrated as a hole which may receive a protrusion such as a pin extending from the base or the rotatable part. Alternatively, the coupling feature may comprise a protrusion such as a pin configured to engage a hole formed within the base or the rotatable part, substantially as described above in connection with Figures 1 to 4. Any known coupling feature for a variable aperture blade may be used instead. In one example the flexure 102, particularly the foot 105 is configured to be attached to a base of a variable aperture assembly and the coupling feature is configured to engage a corresponding coupling feature formed upon a rotatable part of a variable aperture assembly. Rotation of the rotatable part relative to the base causes the coupling feature 103 and foot 105 to move together or apart. The blade 100, particularly the body portion 101 will rotate about an axis generally parallel to the primary axis and the blade rotation may be amplified relative to the rotation of the rotatable part. The axis of blade rotation may extend through or close to foot 105. Coupling feature 103 may move towards or away from the foot 105 as indicated by arrows 106, 107 as the rotatable part and the base rotate relative to one another, causing blade tip 108 to move in corresponding first and second directions indicated by arrows 109,110. Figure 6 shows a second example of a blade for a variable aperture assembly that is closely similar to the blade of Figure 5 and so the same reference numbers are used. Figure 6 differs in that flexure 102 couples to the body portion 101 at a position 111 which is closer to the coupling feature 103, which changes the effective gearing of the blade drive mechanism such that the blade tip 108 moves further for a given relative movement between coupling feature 103 and flexure foot 105, as indicated by elongated arrows 109,110. It will be appreciated that the shape and size, particularly length, of flexure 102 and where it couples to the body portion 110, particularly relative to coupling feature 103, may be controlled to give the desired movement of the blade 100 and so to control the aperture size. In figures 5 and 6 the flexures 102 couple to the body portion 101 between the coupling feature 103 and the blade tip 108. This arrangement may be changed so that the coupling feature is between the blade tip 108 and the flexure 102. One change this would introduce is to reverse the relative motion of the blade tip 108 for a given relative movement between the flexure foot 105 and the coupling feature 103. A further alternative would be for the coupling feature 103 to be attached to the base and the flexure foot 105 to be attached to the rotatable part (as these terms are used in the description of variable aperture assemblies given above). However, in some examples this reversed coupling arrangement could place the flexure 102 in compression during part of the range of motion of the base and the rotatable part, rendering the blade position less stable. Figure 7 shows a further alternative blade 100 in which the coupling feature comprises a second flexure 120 comprising an elongate portion 121 and a foot 122. Foot 122 may be attached to a rotatable part of a variable aperture assembly while foot 105 is attached to a base of a variable aperture assembly (or the other way round). Advantageously, this further simplifies the blade assembly process as there is no longer a need to align pins and holes. A plurality of blades may be located above the base and the rotatable part and the flexure feet welded or glued in place. For the blades of Figures 5 to 7 the flexure or both flexures may suitably be integrally formed with the body portion, or they may be separately formed and coupled to the body portion. The material of the flexures (or the whole blade, for integral formation) may be selected to give the desired degree of compliance to control blade rotation. In principle, one of both flexures may be formed as part of, or attached to, the base or the rotatable part and attached to the blade during assembly. Suitably, the blades including the flexures may be formed from a sheet of material, for instance a polymer material or a metal. The blades may be assembled as a stack and attached to the rest of the variable aperture assembly in one piece. Flexure controlled rotation For current variable aperture designs, including those discussed above in connection with Figures 1 to 4, when implemented with moulded parts forming the base and the rotatable part, a limitation on reducing the Z height (height of the assembly along the primary axis) is the need for bearings. According to certain embodiments of the invention, the base and the rotatable part can be coupled instead using an arrangement of flexures (at least one flexure, optionally a plurality of flexures, for instance 8). The base and the rotatable part are held appropriately spaced apart by the flexures yet remain able to achieve relative rotation without the further need for bearings. This can in turn permit size reduction for the base and the rotatable part, for instance they may be implemented using thinner structures such as etched metal plates and assembled with a flexure layer also formed as an etched metal plate. More generally, the flexures coupling the base and the rotatable part may be part of the mouldings forming the base and the rotatable part, thereby allowing a single moulded component and so reducing the part count for a variable aperture assembly. Or the flexures may be attached to the mouldings as an etched component (which could form an electrical connection). Figure 8 shows a schematic plan view of parts of a variable aperture assembly 200 according to embodiments of the invention. The variable aperture assembly 200 comprises a base 201 and a rotatable part 202 which may be functionally similar to the base 30 and rotatable part 20 of Figures 1 to 4. One or more actuating units drive relative rotational movement between the base and the rotatable part but are not shown in Figure 8 for clarity (these may be as previously described or take any other form). A plurality of flexures 203 (for instance, eight, but there may be more or fewer) are positioned around the circumference of the base 201 and the rotatable part 202. Each flexure 203 connects the base 201 and the rotatable part 202. As previously discussed, the base 201 may be static and the rotatable part 202 free to rotate about the primary axis O. The arrangement of the base and the rotatable part may be reversed such that the rotatable part surrounds the base. The flexures 203 are compliant in rotation (or perpendicular to their length) to permit rotational motion (circumferential about the primary axis O), but comparatively stiff along their length (radially about the primary axis O) to restrict translational (radial) motion between the base and the rotatable part. In some examples a small amount of compliance may be necessary along the length of each flexure 203 to allow the rotational motion because as the rotatable part 202 rotates relative to the base 201, the distance between the two connecting points for each flexure 203 gets larger. Therefore, longitudinal compliance is required to allow each flexure 203 to extend its length slightly. Each flexure 203 is shown extending between first and second ends coupled to the base 201 and rotatable part 202 respectively. The first and second ends are spaced apart from one another along a radial axis extending from the primary axis O, however they are not radially aligned except perhaps at one point with the range of rotational movement between the base and the rotatable part. Each flexure 203 is stiffer in a radial direction about the primary axis O than in a direction circumferential about the primary axis O. Alternatively expressed, each flexure is more compliant transverse to the radial direction. It can seen that each flexure 203 includes a significant proportion extending generally radially or at a shallow angle to the radius (in at least one position of the rotatable part), which contributes to the stiffness. Each flexure 203 may comprise a linear element incorporating at least one folded portion between the first and second ends, the at least one folded portion being configured to unfold to accommodate a change in distance between the first and second locations. Figures 9i to 9v show various alternative flexure geometries. More geometries will be apparent to the skilled person. Figures 9i, ii and ill in some examples may limit radial movement between the base and the rotatable part more than Figures iv and v. The flexures 203 may in some examples be formed collectively as an etched or stamped sheet metal component which is either welded or glued to the base and the rotatable part. One or more flexures, or the whole flexure layer, may be electrically conductive to carry electrical current to the rotatable part. Other portions of the variable aperture assembly, including the base, rotatable part, crimps, spacers may be similarly formed as sheets and stacked together (and connected by welding or adhesive) to form a shim stack. Figure 8 shows a plurality of flexures 203, specifically eight, spaced apart around the primary axis O. There may be more or fewer. An upper limit may be determined by space requirements and by the expected rotational and translation loads placed on the rotatable part 202. There may for instance be 20 or more flexures. The flexures 203 may be evenly distributed about the circumference of the assembly, for instance having rotational symmetry equal to the number of flexures, though other patterns are possible. In some examples the flexures may take other forms, for instance coiled springs. The base 201, the rotatable part 202, and each flexure 203 may be integrally formed or the flexures 203 may be attached to one or both of the base 201 and the rotatable part 202. As previously noted, for a variable aperture assembly, blades may be coupled to the base 201 and the rotatable part 202 using any conventional technique, including the arrangement described above in connection with Figures 1 to 4. Alternatively, at least one blade may be as described in Figures 5 to 7. Other variations It will be appreciated that there may be many other variations of the above-described examples. SMA The above-described SMA actuator assemblies comprise at least one SMA element. Each SMA element may be divided into one or more SMA element segments. 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 (for instance, 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, for instance, a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, for instance, by Joule heating, another heating technique or by applying a magnetic field. Alternative ways of heating SMA The heating of the heat-activated actuator(s), such as SMA material, in order to cause the moving portion to move, could be achieved in a number of ways. In one arrangement, the material could be heated by passing a current through it. This current might come from a local or external power supply. Alternatively, the current might be induced in the wire by inductive coupling with an external alternating field. Where there are two actuators, the two actuators might be designed so that they couple to two different frequencies of the inductive power source, thus allowing the two actuators to be heated differentially. In another arrangement, the material could be heated by external radiation such as a visible or infra-red laser. The external radiation could be focussed so that one actuator is heated preferentially over another actuator, thus allowing differential actuation. Alternatively, or additionally, different actuators, or portions of the actuators, could be treated (for example with a surface coating) so that the different actuators heat at different rates depending on the nature (for instance, the frequency) of the incident radiation.