Actuator assembly

Abstract

An actuator assembly comprises a base 102, a rotatable part 105, and an actuating unit 106-109. The actuating unit can apply a force to the rotatable part 105 to rotate the rotatable part about a prim

Description

Field The present application relates to an actuator assembly. According to certain embodiments the actuator assembly comprises a rotary actuator assembly, for instance a variable aperture assembly. 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 VA assembly can be used to provide an aperture of a controlled size. For example, in the context of a camera, a VA 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. Where a variable aperture assembly having an actuator assembly is incorporated into a miniature camera, for example in a smartphone or other portable electronic device, it will be appreciated that space for the actuator assembly is constrained. This imposes limitations on the ability of the actuator assembly to control the relative rotation of the base and the rotatable part and to effect a change in size of the variable aperture. Additionally, actuation of the actuator assembly to effect a change in size of the variable aperture can result in heating of a camera lens. This can impact upon lens performance, particularly if that heating is not evenly distributed across the lens. It is an aim of certain embodiments of the present invention to provide for more space efficient actuator assemblies within a rotary actuator, for instance a variable aperture assembly. It is a particular aim of certain embodiments of the present invention to provide for improved actuator assemblies using SMA elements to actuator a rotary actuator. It is a further aim of certain embodiments of the present invention to reduce the effects of lens heating on lens performance in a variable aperture assembly. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: a base; a rotatable part; and an actuating unit configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, wherein the actuating unit comprises: at least two shape memory alloy, SMA, element segments coupled in series between the base and the rotatable part, the at least two SMA element segments being axially misaligned and configured, on actuation of the actuating unit, to apply the actuating force to the rotatable part; wherein the actuator assembly further comprises a pair of friction surfaces biased against each other by a biasing force extending transverse to the primary axis, thereby generating a frictional force between the pair of friction surfaces for maintaining the position of the rotatable part relative to the base when the actuating unit is not actuating; and wherein the actuating unit is arranged such that on actuation of the actuating unit the frictional force between the pair of friction surfaces is reduced. In its most general sense, the present invention relates 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. Additionally, (rotary) actuator assemblies according to various embodiments of the present invention have other applications, for instance focusing assembly such as an autofocus assembly, where one part is driven in rotation to adjust the focal length. Suitably, the rotational drive may be transferred to axial movement through a helical drive mechanism. However, the helical drive may be driven using actuator assemblies according to the present invention including at least two axially misaligned SMA element segments. In the following description where rotatory motion is mentioned, this should be considered to include helical motion. Greater SMA element length for a given package size An advantage of aspects of the present invention is that because the at least two SMA element segments of an actuating unit (also referred to herein as an actuator) are axially misaligned (that is, they are defined by axes that are noncoIlinear, for instance they are orientated at a non-zero angle relative to each other, or parallel but offset), a greater length of SMA element may be provided within an actuator assembly. That is, a greater length of SMA element can be provided within a given space constraint. This allows for the total contraction of the SMA element to be increased, thereby increasing the drive stroke deliverable to the rotatable part without an overall increase in the size of the actuator assembly. The result is that the extent of relative rotation between the base and the rotatable part can be increased. More even thermal heating A further advantage of aspects of the present invention is that because the at least two SMA element segments are axially misaligned relative to each other, where the actuator assembly forms part of a variable aperture of a camera, the SMA element or SMA elements can be distributed more evenly around the lens (for instance, approximating a rotationally symmetrical arrangement of SMA elements) such that any heating of the lens resulting from SMA actuation is more evenly applied. The result is that lens performance can be less affected by the actuator assembly. In some embodiments, the SMA element segments can be provided in an approximately rotationally symmetrical pattern around the actuator assembly such that any disadvantageous heating effects caused by operation of the SMA elements are more evenly distributed. In some examples, the SMA element segments may form an approximately diamond or square arrangement around the rotary actuator. ZHP A further advantage of aspects of the present invention is that the increased rotation of the rotatable part (and optionally reduced impact on lens performance for a variable aperture assembly) can be combined with the capacity to maintain the position of the rotatable part when the actuating unit is not actuating (that is, when the at least one SMA element is not powered). The rotatable part can be held stationary or its rotational movement significantly reduced. This may be referred to as Zero Hold Power (ZHP): the rotatable part is held in position against the effects of external forces applied to the actuator assembly when the actuating unit is unpowered. When the actuating unit is actuating, the static frictional force maintaining the position of the rotatable part can be reduced or entirely removed, permitting the actuating unit to drive rotation of the rotatable part. Advantageously, for embodiments in which the actuator assembly forms part of a variable aperture, the size of the variable aperture may thus be maintained even when the actuator is not powered. As such, the actuating unit does not need to be powered continuously, but only when the size of the variable aperture is to be changed. The energy efficiency of the variable aperture assembly may thus be improved. The SMA element may be an SMA wire but could instead be a flattened wire or an elongate SMA foil. The SMA element segments may be noncoIlinear. They could be parallel (but not collinear), or they could be orientated at a non-zero angle relative to each other (intersecting or non-intersecting). The requirement to be in series requires that each SMA element segment is connected in some way. In some examples the SMA element segments may be different portions, that is segments of a single SMA element, such as an SMA wire. In some examples the SMA element segments may be two separate SMA wires connected via a connecting element. Where they are separate lengths of wires they may be coupled also via a non-functional portion of wire as a result of a fabrication process, the non-functional portion being in parallel to the connecting element. In this context, non-functional means that if it contracts it does not contribute to an actuating force applied to the rotatable part. Where two wires are connected by a connecting element their connection points may not be close to one another. Connection in this series may suitable mean that contraction of one SMA segment acts upon the end of the next SMA segment in series so that stroke is increased (optionally, additively). In some examples, the actuator assembly may alternatively be referred to simply as an actuator or a rotary actuator. The biasing force could be (approximately) perpendicular to the primary axis, or angled relative to the primary axis, for instance at a shallow angle relative to a plan that is normal to the primary axis. The reduction of the static frictional force may be variable or may include the complete removal of the static frictional force. The at least two SMA element segments may comprise portions of a single SMA element coupled between the base and the rotatable part such that contraction of each portion of the single SMA element combines to generate a total contraction to apply the actuating force to the rotatable part. The contraction of each SMA element segment may additively combine to generate a total contraction. In some cases, the total contraction may be less than the additive sum of the contraction of each SMA element segment due to losses within the actuating unit, such as a compliance of one or more component effectively absorbing a portion of the contraction. The actuating unit may further comprise one or more redirection elements, each redirection element being coupled to the base, to the rotatable part, or an intermediate component moveable relative to the base or the rotatable part, such that between the base and the rotatable part the single SMA element passes each redirection element to define separate SMA element segments to either side of the redirection element. For instance, the single SMA element may pass over a redirection element. The redirection element may comprise a pulley, roller, post, rocker, flexure, or any further component known to the skilled person capable of changing the direction in which an SMA element, for instance a wire, extends. Contraction of the SMA element may cause the SMA element to slide over the redirection element and / or the redirection element may move (for instance, roll) to avoid or reduce sliding contact and hence avoid or reduce friction between the SMA element and the redirection element. The redirection element may be formed from an electrically insulative material, ceramic, plastic, polymer, electrically conductive material, low friction material, or a metal. Particularly where there is sliding contact, the redirection element may be covered in a low friction coating and / or an electrically insulative coating. Alternatively, the at least two SMA element segments may comprise at least two SMA elements mechanically coupled together in series such that contraction of the SMA elements combines to generate a total contraction to apply the actuating force to the rotatable part. The contractions of each SMA element may be additively combined, though the total contraction may be reduced as a result of losses particularly where the SMA element are coupled, for instance as a result of compliance in the system. The actuating unit may further comprise one or more connecting elements to couple together pairs of SMA elements, each connecting element being coupled to the base or the rotatable part. Alternatively, the actuating unit further comprises an intermediate component moveable relative to the base and the rotatable part. A connecting element may suitably be a rocker, flexure, or similarly structure to which ends of the pair of SMA elements are attached. Alternatively, the intermediate component may comprise a component such as a ring (full or partial) or slider coupled to the base and / or the rotatable base and configured to move such as in rotation or translation relative to the base or the rotatable part). Where there are two actuating units rotating the rotatable part in opposite directions, the connecting elements can be coupled to the same or different of the base and the rotatable part. Or they may be coupled to a single intermediate component, or one intermediate component may be provided for each actuating unit. In certain examples, each connecting element comprises a flexure connected to the base or the rotatable part, wherein contraction of a first of the pair of SMA element causes bending of the flexure and displacement of a second of the pair of SMA elements. In some examples, a flexure can couple to both the base and the rotatable part. The flexure may be configured to generate a rotational biasing force to a centre point or one or both rotational end points. Suitably, this can assist in biasing rotation of the rotatable part to a home position. The flexures may orient towards the centre of the actuator or away from it. Each connecting element may comprise one or more crimp components for mechanically coupling together a pair of SMA elements. Each of the at least two SMA elements may be a separate SMA element, though they can be formed from a single element that is crimped to leave a non-functional portion between the crimps on a single connecting element. Connecting elements may electrically couple together a pair of SMA elements. According to another aspect of the present invention, there is provided an actuator assembly comprising: a base; a rotatable part; and an actuating unit configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, wherein the actuating unit comprises: at least two SMA element segments, the SMA element segments being axially misaligned and configured, on actuation of the actuating unit, to apply the actuating force to the rotatable part; and at least one intermediate component moveable relative to the base and the rotatable part and configured to couple the two SMA elements in series between the base and the rotatable part. An embodiment incorporating an intermediate part may implement ZHP substantially as described for the embodiments above. In some examples, two SMA element segments (for instance, separate SMA wires) are interconnected by one intermediate component to form each actuating unit, however there may be further intermediate components connecting a larger number of SMA element segments. The at least two SMA element segments may comprise at least two SMA elements mechanically coupled together in series such that contraction of the SMA elements combines to generate a total contraction to apply the actuating force to the rotatable part. At least one intermediate component may be configured to rotate about an axis substantially parallel to the primary axis or to translate along an axis extending substantially perpendicular to the primary axis (optionally the translation axis passes through the primary axis). Rotation of an intermediate may be about the primary axis or translation of an intermediate part may be radial to the primary axis. At least one intermediate component may extend at least partially around the primary axis, or the intermediate component is mounted upon the base or the rotatable part such that it can translate relative to the base or the rotatable part. The actuator assembly may further comprise a pair of friction surfaces biased against each other by a biasing force extending transverse to the primary axis, thereby generating a static frictional force between the pair of friction surfaces for maintaining the position of the rotatable part relative to the base when the actuating unit is not actuating; and the actuating unit may be arranged such that on actuation of the actuating unit the static frictional force between the pair of friction surfaces is reduced. Actuator assemblies as described above may include crimp components such that a first SMA element segment is mechanically coupled to the base via a base crimp component and a second SMA element segment is mechanically coupled to the rotatable part via a rotatable part crimp component. The base crimp component is couplable to a control circuit, or power supply. An actuating unit may comprise more than two SMA element segments coupled in series between the base and the rotatable part, each adjacent pair of SMA element segments being axially misaligned relative to each other. For instance, each actuating unit may include 3 or 4 coupled SMA element segments. An upper limit may be imposed by space constraints: for instance, in some examples the upper limit may be 10 SMA element segments. Where there are two actuating units, each may have the same or a differing number of coupled SMA element segments. In many examples there will be two actuating units generating actuating forces in different rotational directions about the primary axis. In some cases, there may be more actuating units, for instance pairs of actuating units operating in tandem in each rotational direction. Again, an upper limit may be set by space constraints, for instance 10 actuating units. Where there are two or more actuating units, they may be of the same types discussed above or they may be dissimilar. Two or more actuating units may be generally symmetrically arranged about a plane including the primary axis. Advantageously, this can reduce the impact of thermal gradients across a lens positioned closed to the actuator assembly. The SMA element segments of the actuating units may be generally arranged about the actuator assembly with rotational symmetry of at least order 2. The SMA element segments may be arranged in a square or diamond pattern about the primary axis or a higher order polygon. Rotational symmetry may be approximate in the sense that each SMA element segment may be roughly the same length and roughly evenly distributed about the primary axis, albeit the requirement to connect to base and rotatable part, plus the form of redirection elements, connecting elements, or intermediate components restricts true rotational symmetry. In some examples, the centres of each SMA element segment may be (approximately) evenly distributed about primary axis. In some examples the SMA element segments do not fully encircle the primary axis. Advantageously, greater mirror or rotational symmetry of the SMA element segments (at least relative to previous actuator assemblies) results in more even lens heating when forming part of a variable aperture assembly. The base may be provided within a hole that extends through the rotatable part along the primary axis, or the rotatable part may be provided within a hole that extends through the base along the primary axis. There may be a bearing arrangement between base and rotatable part to ease relative rotational movement. The base and rotatable part may form concentric rings, and in some examples the rotatable part is the inner ring. Further ZHP features In some embodiments, the variable aperture assembly comprises one or more pairs of friction surfaces, each pair of friction surfaces comprising a first friction surface and a second friction surface that are biased against each other by a normal force, thereby generating static frictional forces between first and second friction surfaces for maintaining the position of the blades when the actuator is not actuating. The static frictional forces are for maintaining the position of the blades, and so are deliberately chosen to be large enough to allow maintenance of the position. Unlike in conventional variable aperture assemblies, any frictional forces are thus not minimized. The static frictional forces may be for maintaining the position of the blades while acceleration of the variable aperture assembly is below a hold threshold. The hold threshold may be greater than the gravitational acceleration of Earth, so greater than g (9.81 m / s2), preferably greater than 2g or greater than 5g, or greater than 10g. In some embodiments, the actuator is arranged such that the normal force between at least one pair of friction surfaces is reduced on actuation of the actuating unit, thereby reducing the static frictional force between the pair of friction surfaces. Such a reduction in frictional force may ensure that the actuating unit can reliably move the blades and set the size of the variable aperture. The static frictional force between the friction surfaces can be set higher compared to a case in which the static frictional force remains constant. In some embodiments, the actuator is arranged such that at least one pair of friction surfaces disengages on actuation of the actuator. The frictional force between the friction surfaces may thus be reduced to zero, minimizing resistance to movement on actuation of the actuator. In some embodiments, the normal force biasing at least one pair of friction surfaces acts in a direction perpendicular to the primary axis. The friction surfaces may be generally parallel to the primary axis. Some embodiments comprise one or more biasing arrangements arranged to bias the pairs of friction surfaces with the biasing (normal) force, thereby giving rise to the frictional force. The at least one biasing arrangement may comprise a resilient element, such as a spring (coil spring, flexure, leaf spring) or elastic element (rubber band, etc). The at least one biasing arrangement may comprise a magnetic element. For example, the biasing arrangement may comprise a magnet on one component of the variable aperture assembly (such as the base or the rotatable part) and a magnet or ferromagnetic material on another component of the variable aperture assembly. The pair of friction surfaces may be provided respectively on the base and the rotatable part. The actuator assembly may comprise more than one pair of friction surfaces arranged such that the static frictional force between each pair of friction surfaces is reduced on actuation of at least one actuating unit. The actuator assembly may further comprise a biasing arrangement configured to bias the pair of friction surfaces with the biasing force, thereby giving rise to the static frictional force. The biasing arrangement may be a resilient element, such as a leaf or coil spring for flexure, or it could be magnetic. The biasing arrangement may be configured to bias the rotatable part against the base. The biasing force may be applied to the rotatable part in a first direction and at least one actuating unit may be configured to apply the actuating force to the rotatable part angled relative to the first direction such that a component of the actuating force opposes the biasing force. Each direction may be generally towards or away from 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 configured such that rotation of the rotatable part relative to the base effects movement of the blades, thereby changing the size of the variable aperture; wherein the static frictional force between the pair of friction surfaces maintains the position of the blades when the actuating unit is not actuating. Each of the plurality of blades may be coupled between the base and the rotatable part. Each blade may be coupled to the base via a first set of complementary coupling features and each blade is coupled to the rotatable part via a second set of complementary coupling features. Suitably, these may comprise pin and hole / slot couplings. 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. The variable aperture assembly may be mounted on the lens assembly such that the lens assembly is received within a hole in the base or the rotatable part aligned with the primary axis. In some examples, more than 50%, 60%, 70%, 80%, or 90% of the variable aperture assembly overlaps with the lens assembly along the primary axis. According to yet another aspect of the present invention, there is provided an electronic device incorporating the above camera. 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 and 6 are schematic plan views of other variable aperture assemblies with variable friction for achieving zero hold power functionality; Figures 7a to 7b are schematic plan views of alternative actuator assemblies; Figures 8a and 8b are schematic views of alternative connecting elements for coupling together pairs of SMA elements forming part of an actuating unit; Figures 9a and 9b are schematic plan views of further alternative actuator assemblies; Figures 10a to lOd are schematic plan views of alternative actuating units; Figure 11 is a schematic plan views of a further alternative actuating unit; Figures 12a and 12b are schematic plan views of further alternative actuating units; Figure 13 is a schematic plan view of a further alternative actuating assembly; and Figure 14 is a schematic plan view of a further alternative actuating 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, for instance and elongate, flattened SMA element, for instance a tape or foil. Actuation of the actuating unit is achieved by heating the SMA element (for instance, by passing an electrical current) causing it to contract. 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 30 (though they may partially overlap along a radial direction perpendicular to the primary axis). Alternatively, the rotatable part 20 may surround the base 30. For the alternative actuator assemblies shown starting with Figure 5 base is radially further from the primary axis O as for Figures 1 to 3. 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 a rotatable part 20 are formed generally as rings surrounding the primary axis O that 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) 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 roller 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. As the rotatable part 20 rotates relative to the base 30, for each blade 40 there is relative movement between its respective pins 23, 33. This drive rotation of the blade 40 to adjust the size of 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 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 flexure coupled to the base 30 or the rotatable part 20, 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 plurality of 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. In the depicted embodiment of Figures 1 to 3, the base 30 surrounds the rotatable part 20 when viewed along the primary axis O. In other embodiments, the rotatable part 20 surrounds the base 30, for instance as illustrated in Figure 4. In general, the base 30 and rotatable part 20 need not surround each other when viewed along the primary axis O. For example, the base 30 and rotatable part 20 may be stacked along 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 actuating unit 10 comprises one or more SMA wires 11. The one or more 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. The radial positions of the rotatable part 20 and the base 30 are reversed in Figure 4 compared to Figures 1 to 3. In some embodiments, the one or more 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 one or more 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, 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 four SMA wires 11. A first pair of SMA wires 11 (for instance, the top and bottom wires) is arranged, on contraction, to apply a torque to the rotatable part 20 in a first sense (for instance, clockwise). A second pair of SMA wires 11 (for instance, the left and right wires) 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. The four SMA wires 11 are arranged in a loop around the primary axis O. Each SMA wire 11 is arranged on one of four sides that are arranged in a loop around the primary axis 0. SMA wires 11 applying a torque in the same direction are arranged on opposite sides of the four sides. The SMA wires 11 may be arranged with two-fold rotational symmetry about the primary axis 0. This allows the torques applied by the first and / or second pairs of SMA wires 11 to be centred specifically about the primary axis 0, reducing and indeed avoiding off-axis forces on the rotatable part 20. Motion of the rotatable part 20 may thus be purely rotational upon actuation of the first and / or second pairs of SMA wires 11, avoiding the need for a bearing arrangement constraining movement to such rotation about the primary axis 0 or reducing adverse forces on such a bearing arrangement. Although the embodiment of Figure 4 shows four SMA wires 11, the number of SMA wires may be larger, for instance six or more SMA wires 11. There may be no theoretical upper limit to the number of SMA wires 11, however space for the wires and particularly space for attaching the wires to the rotatable part 20 and the base 30 may impose a practical upper limit, for instance 10 SMA wires 11. In some embodiments, the number of SMA wires may be reduced, such as those shown in Figures 5 and 6 in which, the variable aperture assembly 1 comprises two SMA wires 11. Where two SMA wires 11 (or more generally, two SMA elements) are used within an actuating unit, one SMA wire 11 may be arranged, on contraction, to rotate the rotatable part 20 in the first sense, and another SMA wire 11 is arranged, on contraction, to rotate the rotatable part 20 in the second sense. Zero hold power In conventional variable aperture assemblies, the actuator 10 is constantly powered to maintain 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 embodiments of the present invention, 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. So, the size of the variable aperture can be maintained without powering or energizing the actuator 10. 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. Optionally the hold threshold is at most 100g (980 m / s2), optionally at most 50g, optionally at most 20g, optionally at most 10g. By reducing the hold threshold, the design freedom for the variable aperture assembly 1 is increased. By increasing the design freedom, one or more other properties of the variable aperture assembly 1 may be optimised. For example, a lower hold threshold may lead to an improvement of transition times, increased positional accuracy, increased stroke capability over the lifetime of the variable aperture assembly 1, reduce particle generation due to wear of friction generating features and / or increased positional accuracy over lifetime of the variable aperture assembly 1 (by reducing part wear). Merely as one example, the hold threshold may be about 8g (78.4 m / s2). Such an acceleration is likely to represent most conditions where the variable aperture assembly 1 is being used. For example, the variable aperture assembly 1 may be used in an apparatus such as a mobile phone. 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. For example, a frictional force may act between the base 30 and the rotatable part 20 (or other movable part to which the blades 40 are coupled). A frictional force may act between the base 30 and the blades 40. A frictional force may act between the rotatable part 20 (or other movable part to which the blades 40 are coupled) and the blades 40. A frictional force may act between blades 40 that are in contact with each other. 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. Figure 5 schematically shows another embodiment of a variable aperture assembly 1 enabling a reduction in the frictional force. The variable aperture assembly 1 comprises the base 30, the rotatable part 20, and two SMA wires 11 arranged to rotate the rotatable part 20 to any rotational position within a range of movement. One SMA wire 11 (the left SMA wire 11 in Figure 5) rotates the rotatable part 20 in one sense (clockwise in Figure 5) and the other SMA wire 11 (the right SMA wire 11 in Figure 5) rotates the rotatable part 20 in the opposite sense (counterclockwise in Figure 5). More than two SMA wires 11 may be provided. The two SMA wires 11 are arranged to be parallel to each other. The variable aperture assembly 1 comprises a biasing arrangement 35. The biasing arrangement 35 may comprise a spring (such as a coil spring or flexure, for example) or a magnetic arrangement. The biasing arrangement 35 urges the rotatable part 20 in a first direction (downward in Figure 5), 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 two SMA wires 11 apply forces to the rotatable part 20 in a direction parallel to the biasing force applied by the biasing arrangement 35. The SMA wires 11 are arranged to apply a force to the rotatable part 20 with a component opposite to the first direction (upward in Figure 5). As such, the SMA wires 11, on actuation, reduce the frictional force between the first and second friction surfaces 31, 21. 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 SMA wires 11 are arranged, on contraction, to reduce the normal force between first and second friction surfaces 31, 21. Put another way, the composite force acting between the friction surfaces 21, 31 due to stresses in the SMA wires 11 has a component that is parallel to and opposite in direction to the normal force. The stresses in the SMA wires 11 affect (in particular reduce) the normal force. In some embodiments, equal stresses (or tensions or strains) in the SMA wires 11 may reduce the normal force without moving the rotatable part 20 and / or blades 40. 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). The biasing arrangement 35 applies a biasing force. The biasing force comprises a component that is perpendicular to the first and second friction surfaces 31, 21, and so the biasing arrangement 35 applies a normal force between the friction surface 31, 21. The normal force is perpendicular to the friction surfaces 21, 31. The biasing arrangement 35 may apply the biasing force in the direction perpendicular to the friction surfaces 31, 21, or in a direction angled relative to the friction surfaces 21, 31. The biasing force of the biasing arrangement may be equal to the normal force, or equal to the combination of normal forces of multiple pairs of friction surfaces 21, 31. So, the biasing force may not have a component parallel to the range of movement, and thus not affect rotation of the rotatable part 20 relative to the base 30. 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. This normal force generates or gives rise to a static frictional force between the first and second friction surfaces 31, 21. The static frictional force constrains movement of the components of the variable aperture assembly 1, so as ultimately to constrain movement of the blades 40, in particular when the SMA wires 11 are not contracted. Such movement is constrained at any position of the blades 40 relative to the base 30, that is, at any rotational position of the rotatable part 20 relative to the base 30. 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 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, as may the other assemblies disclosed herein. The static frictional force F constrains movement of components of the variable aperture assembly 1. The magnitude of the static frictional force F is thus large enough to constrain such movement. The magnitude of the static frictional force F is proportional to the normal force N and the coefficient of static friction p, such that F = p * N. The static frictional force F may be increased by increasing the normal force N, which is achieved by appropriate design of the biasing arrangement 35, and / or by increasing the coefficient of static friction, which is achieved by appropriate design of the friction surfaces 21, 31. The magnitude of the static frictional force is great enough to constrain movement of the blades 40, in particular before any reduction due to actuation of the actuator 10. The magnitude of the overall static frictional force is greater than the combined weight of the movable parts of the variable aperture assembly 1, for instance, greater than the combined weight of the rotatable part 20 and blades 40. This ensures that movement of the movable part is constrained by the frictional force even when the actuator assembly 1 is turned on its side, for example. The combined weight of the movable parts is equal to the mass of the movable parts times earth's average gravitational acceleration (9.81 m / s2). Preferably, the ratio of the static frictional force to the combined weight of the movable parts is greater than 3, further preferably greater than 5, further preferably greater than 10. This ensures that movement of the movable part 20 is constrained even when the variable aperture assembly 1 undergoes acceleration. The actuator 10 may be arranged to reduce the frictional force upon contraction. In some embodiments, the overall frictional force is reduced by at least 20%, more preferably by at least 40% or by at least 50%. The overall frictional force may be reduced by at least 90%. In some embodiments, the overall frictional force is reduced by 100%, that is, the friction surfaces 21, 31 disengage on actuation. The magnitude of the static frictional force is low enough to allow the actuator 10 to overcome the static frictional force so as to effect movement of the blades, in some embodiments after reduction of the overall frictional force due to actuation. So, the magnitude of the (optionally reduced) static frictional force is less than the force applied by the actuator 10 to effect movement of the blades 40. The (optionally reduced) static frictional force may be less than 50%, preferably less than 20%, further preferably less than 10% of the force generated by a stress of 200MPa in the SMA wires 11. The coefficient of static friction between the first and second friction surfaces 21, 31 directly affects the magnitude of the static frictional force. The coefficient of static friction may be modified by appropriately processing or selecting the material of the first and second friction surfaces 21, 31. The coefficient of static friction may be in the range between 0.05 and 0.6. Preferably, the coefficient of static friction is in the range between 0.1 and 0.4. In general, lower coefficients of static friction can be compensated for by higher normal forces imparted by the biasing arrangement 30. Unless stated otherwise, any reference to a frictional force herein relates to a static frictional force. The requirements for the static frictional forces between first and second friction surfaces 21, 31 have been described above. These requirements may ensure that blades 40 remain in place relative to the base 30 once in position. Preferably, the same requirements apply to the dynamic or kinetic frictional forces between first and second friction surfaces 21, 31, thus ensuring that the blades 40 rapidly come to rest after being moved. For this purpose, the ratio of the dynamic frictional force to combined weight of the movable parts, the relation between dynamic frictional force and forces due to the actuator 10, and the coefficient of dynamic friction between the first and second friction surfaces 21, 31 may be as described in relation to the static frictional force. Preferably, the static friction coefficient between the first and second friction surfaces 21, 31 is substantially equal (for instance, varying by less than 5%, preferably less than 1%) to the dynamic friction coefficient between the first and second friction surfaces 21, 31. This makes the forces acting on the movable part more predictable, reducing the complexity of movement control. Two-wire variable aperture assembly Figure 6 schematically shows another embodiment of a variable aperture assembly 1 enabling a reduction in the frictional force. The variable aperture assembly 1 of Figure 6 operates in a manner similar to the variable aperture assembly 1 described with reference to Figure 5. The SMA wires 11 are angled relative to one another at an angle a. The SMA wires 11 are angled relative to a biasing force applied by the biasing arrangement 35 at an angle a / 2. The biasing arrangement 35 comprises a biasing element, in particular a resilient element in the form of a leaf spring. The biasing arrangement 35 applies a biasing force to the rotatable part 20 in a first direction (downwards in Figure 10B) that is perpendicular to the primary axis O. The biasing arrangement 35 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 arranged to apply a force with a component that is opposite to the first direction (upwards in Figure 6). The SMA wires 11, on mutual actuation, oppose the biasing force, thereby reducing the normal force (and hence the frictional forces) acting on the friction surfaces 21, 31. The SMA wires 11 are thus arranged, on actuation, to reduce the frictional forces between the friction surfaces 21, 31. The SMA wires 11 are angled relative to the biasing force. 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 a relatively greater proportion of the stress of the SMA wires 11 is capable of effecting rotation of the rotatable part 20, compared to the embodiment of Figure 5 in which the SMA wires 11 apply forces that are parallel to the biasing force. 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. The angle a may be selected such that the component, in the direction along the biasing force, of the actuation force that can be applied by the SMA wires 11 (for instance, the actuation force applied by a stress of 200MPa in the SMA wires 11) is in the range from 0.2 to 4 times the biasing force, preferably 0.5 to 2 times the biasing force, further preferably 1 to 1.5 times the biasing force. The angle a may be selected such that the component, in the direction along the biasing force, of the actuation force that can be applied by the SMA wires 11 (for instance, the actuation force applied by a stress of 200MPa in the SMA wires 11) is greater than the biasing force. In Figure 6, 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. Increased SMA wire length Embodiments of the present invention will now be described relating to actuator assemblies, particularly actuator assemblies in which an actuating unit comprising an SMA element (for instance, an SMA wire) drives rotation of a rotatable part relative to a base about a primary axis. While in its broadest sense the present invention, and the embodiments presented in the following description of figures 7 to 12, are not restricted to variable aperture assemblies, it will be apparent from the following discussion that variable aperture assemblies are a particularly suited application of the present invention. As noted previously, where a variable aperture assembly is incorporated into a miniature camera, for example in a smartphone or other portable electronic device, space for the actuator assembly is constrained. This imposes limitations on the ability of the actuator assembly to control the relative rotation of the base and the rotatable part and to effect a change in size of the variable aperture. More generally, these limitations apply equally to any actuator assembly configured to provide relative rotation between a base and a rotatable part. For a variable aperture assembly driven and controlled with SMA wires, the required degree of blade motion to provide a practically useful range of aperture size is significantly large relative to the SMA strain that can be achieved with a straight piece of SMA wire. Particularly, the required blade rotation is large relative to the strain provided by a length of straight SMA wire that can be accommodated by a typical camera, particularly a miniature camera within a smartphone or other portable electronic device. For instance, with reference to figure 6, the length of each straight SMA wire 11 is limited by the requirement that the wire does not extend outside of a boundary defined by the outer edge of base 30. A further constraint is that the SMA wires must not encroach too close to the primary axis O as otherwise they would enter the field of view of a lens of the camera. Accordingly, the variable aperture assembly requires significant gearing between the actuating SMA wires and each blade. For the variable aperture assemblies described above in connection with Figures 1 to 3, the actuating unit comprises one or more SMA elements coupled between a base and a rotatable part (also referred to as a static ring and a moving ring), which drives blade rotation via pairs of pins or other protrusions connecting each blade to the base and the rotatable part. High gearing to provide the required degree of blade rotation and variable aperture range is achieved by positioning the pins close to one another so that limited relative rotation of the base and the rotatable part results in significant rotation of each blade about one or other of the pins. However, this requirement for high gearing of the blades means that manufacturing tolerances of the gearing mechanisms (for instance, the pins and holes or slots within the blades) results in a significant impact on blade position accuracy. Furthermore, the need to ensure clearance within each hole to receive a corresponding pin can result in a large backlash, leading in turn to inaccurate size and shape of the aperture. Furthermore, high gearing multiplies the limitations of the accuracy of the control system. That is, the accuracy with which the length of an SMA wire within an actuating unit can be controlled is then magnified by the high gearing to degrade the accuracy with which blade position can be controlled. Furthermore, high gearing leads to higher stresses on the wires to achieve a required degree of blade rotation, an increased risk of resonances within the system, and poor stick-slip behaviour of bearings, including the friction surfaces 21, 31 shown in Figure 6. It is expected that the performance limitations highlighted above will be exacerbated in the future as the requirements imposed by camera manufacturers for increased variable aperture performance require ever larger variable aperture ranges (to increase the camera f / stop range available) while also requiring more exacting sizing for the variable aperture assembly (in particular, how much larger the actuator assembly is permitted to be compared to the lens aperture). The present inventors have identified that these performance limitations for a variable aperture assembly may be at least partially addressed by identifying ways to increase the length of SMA wire that can be fitted into an actuating unit for a given size of variable aperture assembly. More generally, increasing the length of SMA wire into any rotary actuator assembly with a constrained overall size will address the same or similar performance limitations: at a minimum allowing for greater relative rotation or reduced SMA wire strain. Increasing the length of SMA wire within an actuating unit can allow for a larger relative rotation of the rotatable part relative to the base for a given SMA wire strain. This is because the displacement of a free end of an SMA wire (the other end being fixed) is proportional to the wire strain and the length of the wire. This in turn can allow for a reduction in the required blade gearing to achieve the same variable aperture range for a given wire strain. Reducing the required gearing can address some of the issues identified above, such as blade position accuracy and backlash. Furthermore, the particular methods for providing a longer SMA wire length for a given actuator assembly size according to embodiments of the present invention (providing at least two SMA wire segments that are not axially aligned, either by folding an SMA wire within the assembly or connecting together two SMA wires, as discussed below) can allow for greater freedom in selecting the angle at which the SMA wire connects to the rotatable part. Increasing the angle between the wire at the point of connection to the rotatable part and a radial axis perpendicular to the primary axis increases the relative rotation of the rotatable part for a given wire strain and wire length. In contrast, it will be appreciated from inspection of Figure 6 that increasing the length of an SMA wire 11 extending in a straight line between the base 30 and the rotatable part 20 by moving the SMA wire 11 closer to the primary axis O will at the same time reduce the angle between the wire and the radial axis: the resulting reduced gearing will partially counteract the benefit of increasing the length of the wire. An alternative advantage of certain embodiments is that by arranging the SMA wire so that it is folded or coupled about the primary axis, the required length of SMA wire for a given relative rotation of the rotatable part and the base can be reduced. Similarly, it will be appreciated from inspection of Figure 6 that relative rotation of the rotatable part and the base can be maximised for a given wire length by connecting the wire to the rotatable part as close to the primary axis as possible along a radial axis. However, for the straight wire embodiment of Figure 6, doing so will bring the middle part of the SMA wire closer to the primary axis, which would risk the wire impinging on the field of view of a camera lens aligned with the primary axis. Even if not directly visible within the lens field of view, an SMA wire provided at the edge of the variable aperture can form a diffraction grating causing undesirable optical effects, and so this increases the need to locate the SMA wire as far from the optical axis as possible. This connection to the rotatable part closer to the optical axis can be achieved for embodiments of the present invention by increasing the angle between the radial axis and the wire at the point of connection to the rotatable part. More generally, the embodiments of the present invention described below allow for an increase in the length of SMA wire within an actuating unit without requiring the SMA wire to approach closer to the primary axis. Additionally, as previously noted, actuation of an actuator assembly to effect a change in size of the variable aperture can result in heating of a camera lens. This lens heating can impact upon lens performance, particularly if that heating is not evenly distributed across the lens, resulting in a thermal gradient. For the variable aperture assembly of Figure 6, the SMA elements 11 are generally distributed only on one side of the primary axis O, and hence on only one side of a lens positioned within the rotatable part 20. This undesirable heating can result from substantially any form of actuating unit, including those incorporating an SMA element and those based upon Voice Coil Motors (VCM). For an SMA based actuating unit, contraction of the SMA element results from heating of the SMA element as an electrical current is passed, and so heating of components such as a lens surrounding the actuating unit can be unavoidable where space constraints dictate that those components must be placed in close proximity. The present inventors have identified that this undesirable uneven distribution of lens heating within a variable aperture system can be at least partially addressed by spreading the heat sources (that is, the SMA elements) more evenly around the circumference of the actuator assembly rather than concentrating them on one side of a lens. This more even distribution can include the SMA elements extending around a greater proportion of the lens, including substantially around the whole of the lens circumference. This more even distribution can include the SMA elements being arranged rotationally symmetrically or approximately rotationally symmetrically. Additionally, as discussed above, embodiments of the present invention allow for an increased length of SMA wire to be used within a given actuator assembly size while keeping the wire further from the primary axis, and so minimising heating of a lens aligned with the primary axis. Turning now to Figures 7a and 7b, embodiments of the present invention are now described by comparison to Figure 6. It will be appreciated that the simplified representations are presented for the purposes of explaining how SMA wires are arranged, but in other respects the actuator assemblies may be generally the same as Figure 6. In particular, the structure of the base 30 and rotatable part 20, the friction surfaces 21, 31, the biasing arrangement 35, 35a, the blades, and the drive mechanism for the blades may be generally the same for each of the following embodiments as for Figure 6, and so will not be described in detail again. Folded wire Figure 7a illustrates a variable aperture assembly 101 as an example of a rotary actuator assembly. The variable aperture assembly 101 is generally inverted relative to the variable aperture assembly of Figure 6, although functionally it is similar and comprises a base 102 and a rotatable part 103 that is configured to rotate relative to the base 102. A bearing arrangement (not illustrated) may be provided between the base 102 and the rotatable part 103. The variable aperture assembly 101 defines a lens aperture 104 within the rotatable part 103, with the base 102 being at least partially around the outside of the rotatable part about a primary axis O. The base 102 may be referred to as an outer ring and the rotatable part 103 may be referred to as an inner ring. In other embodiments the base 102 and the rotatable part 103 may be reversed. The variable aperture assembly 101 further comprises at least one actuating unit 105. In this embodiment first and second actuating units 105 are provided, each configured to apply an actuating force to the rotatable part 103 capable of rotating the rotatable part 103 relative to the base 102 in opposite directions about the primary axis O. In an alternative embodiment there may only be a single actuating unit 105 configured to apply an actuating force to the rotatable part 103 to rotate the rotatable part 103 in a single direction, with a return force being provided by a spring or other biasing mechanism. Each of the following actuator assemblies may similarly be provided with opposed actuating units or a single actuating unit acting against a spring. Each actuating unit comprises an SMA wire 106 (an SMA element) extending between connecting elements 107 to connect the SMA wire 106 between the base 102 and the rotatable part 103. Each connecting element 107 may comprise a crimp. Details of the connection of an SMA wire falls outside of the scope of the present invention. The left-hand actuating unit 105 will apply an actuating force on contraction of SMA wire 106 to the rotatable part 103 that will apply an anticlockwise torque to the rotatable part 103. Similarly, the right hand actuating unit 105 will apply an actuating force on contraction of SMA wire 106 to the rotatable part 103 that will apply a clockwise torque to the rotatable part 103. Actuation of the first and second actuating units 105 will cause the rotatable part 103 to rotate relative to the base 102 in either direction about the primary axis O as indicated by arrow 108. For each actuating unit, the SMA wire 106 passes over a redirection element 109 at an intermediate point between the ends coupled to the base 102 and the rotatable part 103. The redirection elements 109 may comprise a post, pin, rocker, pulley, roller, bearing, flexure or any other component that serves to change the direction of the SMA wire 106, and optionally allow the SMA wire 106 to slide over the redirection element as it contracts. The redirection elements 109 may be coupled to the rotatable part 103, as illustrated, or they may be attached to the base 102. The coupling may be direct or indirect via another component. The redirection elements 109 may move relative to the rotatable part 103 or base 102 to which they are connected, for instance moving in rotation or translation. There may be more than one redirection element 109 configured to change the direction of an SMA wire 106, for instance one attached to the base 102 and one attached to the rotatable part 103. In some cases, one or more redirection element 109 may be coupled to an intermediate component that is moveable relative to the base 102, the rotatable part 103 or both, such as an intermediate ring as shown in Figures 9a and 9b. Each redirection element 109 is configured so that it changes the direction of the SMA wire 106 and permits the SMA wire 106 to pass or slide over the redirection element 109. Alternatively, the redirection element 109 may move (for instance in rotation or translation, or flexing) to accommodate movement of the SMA wire 106 with sliding of the wire, which might cause undesirable friction. The redirection elements 109 serve to divide an SMA wire 106 (an SMA element) into at least at least two SMA wire segments 106a, 106b (SMA element segments) coupled in series between the base 102 and the rotatable part 103. The SMA wire segments 106a, 106b are orientated at a non-zero angle relative to each other. The SMA wire segments 106a, 106b collectively define the SMA wire 106, and are configured, on actuation of the actuating unit 105, to apply an actuating force to the rotatable part 103. That is, when an SMA wire 106 is powered, it will contract, with each SMA wire segment 106a, 106b contracting and contributing to an overall wire contraction. Optionally, the SMA wire 106 slides over the redirection element 109 (or the redirection element rotating or otherwise accommodating wire movement) such that the strain within each SMA wire segment 106a, 106b is substantially the same. Embodiments of the present invention, including Figure 7a, uses two lengths of SMA wire - the SMA wire segments - connected mechanically in series, while being axially misaligned. In the example of Figure 7a the SMA wire segments 106a, 106b are separate portions of a single SMA wire 106 (for each actuating unit 105 where there is more than one actuating unit 105). Where there is more than one T1 redirection element 109 for a single actuating unit 105 and a single SMA wire 106 then the wire will be divided into a larger number of portions (that is, a larger number of SMA wire segments). It will be appreciated that the effect of a redirection element 109 is to increase the length of SMA wire 106 connected between connecting elements 107 attached to the base 102 and the rotatable part 103 compared to a straight length of SMA wire extending between the connecting elements 107. This increases the wire stroke length available, without requiring an increase in the actuator assembly size to accommodate a longer straight length of wire. Alternatively, the same length of SMA wire can be provided within a smaller actuator assembly area by folding the wire around a redirection element. In general, contraction of each SMA wire segment 106a, 106b (that is, each portion of the SMA wire 106) combines to generate a total contraction of the SMA wire 106 to apply an actuating force to the rotatable part 103. This combination may be additive combination of each contraction of the SMA wire segments to form a total contraction. Furthermore, it is apparent from Figure 7a that the longer SMA wire length is achieved while moving the SMA wire 106 further away from the primary axis O, thereby reducing the effect of heating upon a lens located within the lens aperture 104 (not illustrated, but comparable to Figure 3). Furthermore, by having the SMA wires 106 of the pair of actuating units 105 each folded around a redirection element 109, the SMA wire for the whole variable aperture assembly is more evenly distributed around the primary axis O and a lens to which the variable aperture assembly is mounted. Accordingly, any heating of the lens will be more evenly distributed and so thermal gradients across the lens will be reduced. In the example of Figure 7a the SMA wire 106 of each wire approximates a square or diamond pattern around the primary axis. It will be appreciated that where each actuating unit 105 incorporates a larger number of redirection elements 109, or where there are more than two actuating units 105 then the disposition of SMA wire about the primary axis O may approximate a polygon with a larger number of sides. In the example of Figure 7a, the arrangement of actuating units 105 and particularly the arrangement of SMA wire segments 106a, 106b is generally symmetrical with a mirror symmetry about an axis vertically aligned within the figure and passing through the primary axis O. Furthermore, the disposition of SMA wire segments 106a, 106b is approximately rotationally symmetrical with fourfold symmetry. It will be appreciated that the rotational symmetry is only an approximation, given the different attachment points to the base 102 and the rotatable element 103 at each end of the SMA wire, and the different positions and shapes of the redirection elements 109. Figure 7a further illustrates the variable aperture assembly 101 further comprising at least one pair of friction surfaces 110, 111 biased against each other by a biasing force represented by arrow 112 extending transverse to the primary axis O. In certain of the following embodiments of Figure 8a onwards there may not be friction surfaces explicitly identified, however ZHP may be provided substantially as will now be described. In the example of Figure 7a there are two pairs of friction surfaces 110, 111, though there may be more or fewer. For each pair, a first friction surface 110 is provided on the base 102 and a second friction surface 111 is provided on the rotatable part 103. The biasing force 112 generates a static frictional force between the or each pair of friction surfaces 110, 111 for maintaining the position of the rotatable part 103 relative to the base 102 when each actuating unit 105 is unpowered. The biasing force 112 may be generated by a biasing arrangement the same as or similar to the flexure or spring 35 shown in Figure 6, and so will not be described again. The biasing force 112 is applied between the base 102 and the rotatable part 103 transverse to the primary axis O. In the example of Figure 7a the biasing force 112 is applied perpendicularly to the primary axis O - that is, radially. More generally, the biasing force 112 need not be exactly perpendicular to the primary axis O, so long as it is not parallel to the primary axis O. As previously described for figures 5 and 6, when the actuating units 105 are actuated (by contraction of one or both SMA wires 106), the static frictional force between the or each pair of friction surfaces 110, 111 is reduced. This reduction of the static frictional force arises from each actuating unit 105 applies an actuating force to the rotatable part 103 that includes at least a component that acts radially towards the primary axis O and at least a component that acts circumferentially about the primary axis O to effect relative rotational movement of the rotatable part 103. It will be appreciated that the arrangement of frictional surfaces 110, 111 and the biasing force 112 opposed by actuation of the actuating units 105 represents a form of zero hold power generally the same as described above in connection with Figures 5 and 6, and so the above discussion of zero hold power for those figures should be considered to apply equally to Figure 7a. However, one benefit of folding the SMA wire around the actuator assembly as shown in Figure 7a is that this allows increased flexibility when selecting the relative angles of the SMA wires at the connecting elements 107 attaching the wire 106 to the rotatable part 103. Accordingly, the proportion of the actuating force provided by the SMA wire 106 (or each SMA wire 106 when there is a pair of actuating units 105) that is required to counteract the biasing force can be more easily adjusted. For instance, a weaker biasing arrangement may be used to hold the rotatable part 103 in position. In an alternative embodiment the actuation of each actuating unit 105 applies a force to the base 102 or the rotatable part 103 at an intermediate location, for instance at a redirection element 109, either directly or indirectly (via an intermediary component). This applied force counteracts the ZHP bias force to reduce the static frictional forces between the friction surfaces upon actuation of the actuating units. The frictional surfaces and their biasing arrangement may be arranged substantially as shown in Figures 7a and 7b with the force applied at the intermediate location along the wire being transferred to the friction surfaces, for instance via a flexure or lever arrangement. Or, the frictional surfaces and the biasing arrangement may be located in a different location (for instance, proximal to a redirection element 109 instead of proximal to an end of the SMA wire 106) so that the force applied to the rotatable part 103 or the base 102 at the intermediate point serves to reduce the static frictional force. More generally, the present invention allows increased flexibility over the location of the frictional surfaces, the biasing arrangement and where a force generated by the actuating units to counteract the bias force is generated. This can effectively separate the points at which a SMA wire 106 disengages the zero-hold power friction mechanism and the where the SMA wire 106 applies rotational torque to the rotatable part 103. One option would be for rotatable part 103 (or a base 102) to be deformable and the intermediate point of the SMA wire 106 serve to deform the deformable part to partially or fully disengage friction surfaces. In an alternative, there may be a separate brake mechanism between the base 102 and the rotatable part 103 that is engaged or disengaged by an intermediate portion of the actuating unit, such as the redirection element. Further options will be apparent to the skilled person, having in common that the arrangement of SMA wire into two or more axially misaligned SMA wire segments facilitates greater flexibility in incorporating zero hold power into a rotary actuator assembly. Divided wire Referring now to Figure 7b, a variable aperture assembly 201 according to another embodiment is shown. Variable aperture assembly 120 of Figure 7b is closely similar to variable aperture assembly 101 of Figure 7a. Corresponding features are given the same reference numbers incremented by 100 and will not be described again except where they differ from Figure 7a. In particular, the options for providing zero hold power may be generally identical to Figure 7a. Each actuating unit 205 of variable aperture assembly 201 comprises two or more SMA wire segments 206a, 206b that are axially misaligned (that is, not collinear) and connected between the base 202 and the rotatable part 203 as for variable aperture assembly 101. Again, each actuating unit 205 applies an actuating force to the rotatable part 203 capable of rotating the rotatable part 103 relative to the base 202 about the primary axis O. Again, the actuating force further serves to reduce the static frictional force between friction surfaces 210, 111 in the or each pair of friction surfaces. That is, the variable aperture assembly 220 can provide for zero hold power in the same way as for variable aperture assembly 101 of Figure 7a (and as previously described in connection with Figures 5 and 6). However, the actuating units 205 of variable aperture assembly 201 differ in that the two or more SMA wire segments 206a, 206b are not different portions of a single SMA wire, rather they comprise different SMA wires that are mechanically coupled together in series such that contraction of each separate SMA wire segment 206a, 206b combines to generate a total contraction to apply the actuating force to the rotatable part. For simplicity, the following description refers to SMA wires 206a, 206b, but conceptually the two or more SMA wires 206a, 206b may be considered as segments of a larger length of SMA wire within an actuating unit. Each actuating unit 205 further comprises one or more connecting elements 220 to couple together pairs of SMA wires 206a, 206b (that is, separate SMA wires). Each actuating unit 205 may comprise more than one connecting element 220 connecting two or more SMA wires 206a, 206b. Each connecting element 220 is coupled to the base 202, the rotatable part 203 (as illustrated in Figure 7b), or an intermediate component moveable relative to the base 202 or the rotatable part 203 (as illustrated in Figures 9a and 9b). Where there is more than one connecting element 220 within an actuating unit 205 then they may be connected to different parts of the actuator assembly 201. Each connecting element 220 may comprise at least in part a flexure configured such that contraction of a first of a pair of connected SMA wires 206a, 206b causes bending of the flexure and displacement of a second of the pair of SMA wires 206a, 206b. In place of a flexure, the connecting elements 220 may comprises rockers. Further options for interconnecting SMA wires 206a, 206b while permitting for relative movement will be apparent to the skilled person. Typically, both SMA wires 206a, 206b are powered such that they contract to generate a total contraction and thus a total actuating force applied to the rotatable part 203. The flexures 220 are configured to flex (or more generally, the connecting elements 220 are configured to move or deform) to accommodate a change in length of the SMA wires 206a, 206 that are coupled on either side. Each connecting element 220 may comprise a pair of crimp components 221 for mechanically coupling together a pair of SMA wires 206a, 206b (or other suitable wire connection portions). It will be appreciated that the actuating units 205 of Figure 7b (which may be referred to as divided wire actuating units) provide the same advantages as the actuating units 105 of Figure 7a (which may be referred to as folded wire actuating units), in terms of allowing for a greater amount of SMA wire to be provided for actuating the rotatable part 203 for a given assembly size (or reduced assembly size for a given length of wire), greater flexibility to choose the wire connection angle to the base and the rotatable part, moving the wires further away from the primary axis, and more even distribution of wire around the primary axis. The actuator assemblies 101 and 201 of Figures 7a and 7b each include a pair of actuating units 105, 205 that are generally identical to one another but configured to rotate the rotatable part 103, 203 relative to the base 102, 202 about the primary axis O in opposite senses. That is, one actuating unit 105, 205 when powered will apply a torque to the rotatable part 103, 203 to drive the rotatable part 103, 203 clockwise and one actuating unit 105, 205 when powered will apply a torque to the rotatable part 103, 203 to drive the rotatable part 103, 203 anticlockwise. For each actuator assembly 101, 201 the torques deliverable by their respective actuating units 105, 205 may be approximately equal (resulting from their mirror symmetry). However, the present invention is not limited to this. There may be only a single actuating unit 105, 205, which may be configured to apply torque to the rotatable part 103, 203 that is resisted by a resilient element, such as a flexure or spring. There may two or more actuating units 105, 205 configured to drive the rotatable part 103, 203 in any direction. Where there are two or more actuating units 105, 205, each may include different lengths of SMA wire or otherwise be configured to provide differing torques to the rotatable part 103, 203. An actuator assembly may only include actuating units of the first type (folded wire) shown in Figure 7a or only include actuating units of the second type (divided wire) shown in Figure 7b, or there may be one or more of each type. Furthermore, the present invention is not limited to any particular configuration of base and rotatable part. These actuator components may be formed as rings with the base at least partially extending radially further from the primary axis O than the rotatable part (such as the forming a first ring encircling the rotatable part formed as second ring, as illustrated). Or, the radial arrangement of base and rotatable part may be reversed. The invention concerns primarily only the arrangement of SMA wire segments within an actuating unit forming part of a rotary actuator assembly. Similarly, where a rotary actuator assembly according to the present invention is embodied as a variable actuator assembly, it will be appreciated that the blades may be driven by relative rotational movement between the base and the rotatable part. However, the form of the blades and how they are driven is not germane to the present invention. The variable aperture assembly may itself form part of a camera, including a camera incorporated into a phone or other electronic device, the details of which are also not germane to the present invention. Flexure connecting elements Turning now to Figures 8a and 8b, these illustrate certain optional designs for a connecting element 220 forming part of a divided wire actuating unit according to Figure 7b. The connecting elements 220 of Figures 8a and 8b are formed as flexures. As noted previously, the connecting elements 220 can be attached to any part of the actuator assembly 201, particularly the base 202 or the rotatable part 203. Figure 8a illustrates a connecting element 220a suitable for connecting to an inner ring (the rotatable part 203 in the arrangement of Figure 7b). Figure 8b illustrates a connecting element 220b suitable for connecting to an outer ring (the base 202 in the arrangement of Figure 7b). Figure 8a shows connecting element 220a comprising a fixing plate 301a configured to be attached to the inner ring (rotatable part 203), for instance via welding, adhesive, or any other suitable technique. The connecting element 220a further comprises a flexure 302a (optionally, there may be more than one) connecting the fixing plate 301a to a flexure head 303a. SMA wires 206a, 206b are connected to the flexure head 303a, for instance by the use of crimps (such as the crimps 221 shown in Figure 7b), or by welding, adhesive, friction, mechanical encapsulation or any other suitable technique. The flexure 302a is configured to flex to allow the flexure head 303a to move laterally along arrow 304a to accommodate relative rotation between the base 202 and the rotatable part 203 and optionally different wire strain in SMA wires 206a, 206b (although they may suitably be driven to have the same wire strain). When incorporated into the actuator assembly of Figure 7b, the flexure 302a permits the flexure head 303a to move generally circumferentially about the primary axis O (or more precisely, to rotate about an axis extending generally parallel to the primary axis O and proximal to the flexure 302a). The flexure 302a and the flexure head 303a may extend from the fixing plate 301a either away from the primary axis O (as illustrated) or towards the primary axis O. Figure 8b shows connecting element 220b comprising a fixing plate 301b configured to be attached to the outer ring (base 202), for instance via welding, adhesive, or any other suitable technique. The connecting element 220b further comprises a pair of flexures 302b (optionally, there may be more or fewer) connecting the fixing plate 301b to a flexure head 303b. SMA wires 206a, 206b are connected to the flexure head 303a for instance by the use of crimps 221 (though again other techniques may be used). The flexures 302b are configured to flex to allow the flexure head 303b to move laterally along arrow 304b to accommodate relative rotation between the base 202 and the rotatable part 203 and optionally different wire strain in SMA wires 206a, 206b. Figure 8b shows the flexures 302b and the flexure head 303b extending towards the primary axis O, though again this may be reversed. The SMA wires 206a, 206b in Figure 8b may be separately connected to the flexure head 303b. An alternative (as illustrated) allows for reduced assembly and crimping time. The actuating unit 205 from a single SMA wire 206 crimped into two "active" sections (forming the pair of SMA wires 206a, 206b -the SMA wire segments). The active sections are joined by a "non-active" link of SMA wire 305. Once assembled, the SMA wire link 305 performs no function during operation of the actuator assembly 201, however it facilitates actuator assembly by allowing the two sections of the divided wire to be assembled as a set by hooking the single wire over a wire guide hook 306 and then closing crimps 221 over the wire. Advantageously, this may make it easier to provide uniform wire tension between SMA wires 206a, 206b. The non-active wire link 305 may be removed after the crimps 221 are formed, or it may be left. The two crimps 221 may be electrically connected, by virtue of being made from the same piece of metal or welded to a common metal baseplate. Accordingly, the wire link 305 will conduct relatively negligible current and have negligible heating upon operation of the actuating unit 205. It will be appreciated that the connecting elements 220 (such as elements 220a, 220b) can be coupled to any part of the actuator assembly. However, the use of a flexure results in forces and torques being transmitted through the flexure to the component to which they are mounted. Accordingly, the choice of mounting location can have an impact on the operating performances of the actuator assembly. For example, this may result in a bias force being applied to the rotatable part 203 biasing it towards a centre position or either or both end positions for the drive range of rotational movement. This can have advantages either to balance other forces in the system or to create a bias towards a chosen location, to achieve increased speed of motion or assist in biasing towards a target "home position" for the rotatable part. Intermediate components Turning now to Figures 9a and 9b, these illustrate two further actuator assemblies 401a, 401b that represent alternative options for a divided wire type actuator assembly according to Figure 7b. Features that are the same as or similar to the actuator assembly 201 of Figure 7b are given the same reference numbers, incremented by 200 and are not further described except to highlight differences. For Figure 9a, each actuator assembly comprises first and second SMA wires 406a, 406b coupled between the base 402 and the rotatable part 403 (the first actuator assembly comprising the left-hand pair of wires and the second actuator assembly comprising the right-hand pair of wires in Figure 9a). Differing from Figure 7b, in place of the SMA wires 406a, 406b being connected by a connecting element (for instance, a flexure connected to the base or the rotatable part), each pair of wires is connected to an intermediate component formed as a third ring 430. Ring 430 may be positioned radially intermediate of the base ring 402 and the rotatable part ring 403, as illustrated. However, more generally it may be any structure configured to rotate about the primary axis O (or an axis generally parallel to the primary axis O) or otherwise slide or move relative to one or both of the base 402 and the rotatable part 403. A sliding alternative intermediate component is shown in Figure 13. The intermediate ring 430 may extend all of the way around the primary axis O (as illustrated), or it may not form a complete loop. Intermediate ring 430 is shown mounted upon bearings 431 coupled to one or both of the base 402 and the rotatable part 403. The bearings 431 may take any form such as a ball race or plain bearings and have any number or arrangement. Alternatively, in place of bearings, the intermediate ring 431 may be coupled to the base 402 or the rotatable part 403 (or both) through a flexure system or a rocker / pivot arrangement. The intermediate component 430 may be coupled to or between both the base 402 and the rotatable part 403 or it may be coupled to only one of the two. The intermediate ring 430 connects the SMA wires 406a, 406b of each actuating unit through crimps 421 mounted upon the intermediate ring 431 (the other end of each wire 406a, 406b being connected to the base 402 and the rotatable part 403 via crimps 407). In an alternative, there may be separate intermediate rings provided for each actuating unit. Other methods for mechanically connecting SMA wires may be used. In other embodiments there may be more than one intermediate ring 431 such that a greater number of SMA wires 406a, 406b are connected for each actuating unit in an analogous fashion. The intermediate ring 431 is configured to rotate clockwise or anticlockwise relative to the base 402 or the rotatable part 403 as indicated by the arrows 432 according to the relative wire strain within each actuating unit. For instance, where the left-hand actuating unit is powered so that it contracts to rotate the rotatable part 403 anticlockwise as indicated by arrow 433 (while the right-hand actuating unit is unpowered or passing a lower current so that it is less contracted), the intermediate ring 431 will rotate anticlockwise also. The rotation of the intermediate ring 430 may be less than the rotation of the rotatable part 403. Similarly, when the right-hand actuating unit is powered so that the wires contract, the rotatable part 403 is driven clockwise according to arrow 434, with the intermediate ring 430 also being driven clockwise by a lesser amount. It will be appreciated that motion of the intermediate ring 430 and also the rotatable part 403 will be a function of the power delivered to the SMA wire within each actuating unit. Advantageously, using an intermediate ring 431 avoids the SMA wires 406a, 406b of each actuating unit having to overcome the spring back force arising from using flexures within the connecting elements 220 of Figure 7b, albeit at the expense of complexity. Although not specifically illustrated in Figures 9a, at least one pair of friction surfaces may be provided between the base 402 and the rotatable part 403 for the delivery of ZHP functionality. It will be appreciated that as for the embodiment of Figure 7b, the force applied to the rotatable part 403 by each actuating unit includes a component applying a torque for rotating the rotatable part 403 relative to the base 402 and a component acting radially towards the primary axis O for modulating the friction between the friction surfaces. Figure 9b illustrates an actuator assembly 401b according to an alternative intermediate component in which in place of an intermediate ring 431, the SMA wires of each actuating unit are connected between the base 402 and the rotatable part 403 via independent sliders 440 which can move independently of one another. The sliders 440 are mounted via a suitable bearing arrangement or similar to the base 402 or the rotatable part 403. Independent movement of sliders 440 and hence the actuating units may allow for greater flexibility in how the actuating units are driven and how they move circumferentially about the primary axis O as indicated by arrows 441. It will be appreciated that the operation of actuator assembly 401b is generally similar to that of actuator assembly 401a, with the exception that the sliders 440 are not connected. Alternative actuating units Turning to Figures 10a to lOd, some alternative arrangements for configuring an actuating unit to drive a rotatable part 503 relative to a base 502 are presented. Each of Figures 10a to lOd illustrates part or the whole of a single actuator unit, though typically there may be two or more actuating units as illustrated in Figures 7a and 7b to drive the rotatable part 503 in opposite directions. Figure 10a illustrates a folded wire actuating unit according to Figure 7a, while Figures 10b to lOd illustrate options for a divided wire actuating unit according to Figure 7b. Features the same as presented in Figures 7a and 7b are given the same reference number incremented by 300. Figure 10a shows a single length of SMA wire 506 extending between the base 502 and the rotatable part 503. The end of the wire 506 coupled to the base 502 is attached via a mount 550 such that the wire 506 couples to the base 503 radially far from the primary axis O, which helps to keep the wire 506 relatively far from a lens located within lens aperture 504, to minimise any lens heating effect. The end the wire 506 coupled to the rotatable part 503 is attached via a mount 551, which again is configured to keep the attachment part radially remote from the primary axis O: indeed the mount 551 extends so that it overlaps the base 502 to couple to the wire at a more radially remote point. At an intermediate point the SMA wire 506 passes over a dowel pin 552 or another rolling element that is mounted upon the base 502 so that it can roll or otherwise move circumferentially. Circumferential motion of the dowel pin 552 may be constrained within a recess 553 upon the base 502. In an alternative, the dowel pin 552 may be mounted in a similar recess upon the rotatable part 503. Figure 10b shows wire segments 506a, 506b being connected via a flexure arrangement 520 mounted upon the rotatable part 503 (alternatively, it may be mounted upon the base 502). The flexure arrangement 520 incorporates first and second flexures 554 interconnecting a fixing plate 555 (mounted on the rotatable part 503 and shown formed in two separate parts, but it could be unitary) and a flexure head 556 that is configured to move as the wires 506a, 506b contract and the rotatable part 503 rotates relative to the base 502. The flexure arrangement 520 constrains the movement of the flexure head 556 to be primarily translational in a direction orthogonal to the length of the flexures 554. The flexure arrangement 520 may be configured to provide a resistance against a contracting wire in one or both directions. Figure 10c shows wire segments 506a, 506b being connected via a rocker 557 mounted upon a rocker bearing 558 formed on the base 502 (but could instead be formed on the rotatable part 503. Figure lOd shows wire segments 506a, 506b being connected via a flexure arrangement 560 mounted upon both the rotatable part 503 via a first, central flexure 561 and the base 502 via a pair of outer flexures 562. In Figure 10b the lateral stiffness of the flexures 554 will increase the tension in SMA wire 506a as the SMA wire 506a contracts due to SMA wire 506a being connected (indirectly) to the rotatable part at both ends. The connection of flexure arrangement 560 in Figure lOd to both the base 502 and the rotatable part 503 in Figure lOd serves to cause the wire connection points 521 to move laterally with a translation that is between that of the rotatable part 503 and that of the base 502 in the absence of any force from the wires. That is, relative rotation of the points at which the flexure arrangement is connected to the rotatable part 503 and the base 502 will cause the flexure arrangement 560 to adopt an intermediate position. This means that an increase in the tension of SMA wire 506a (connected at its other end to the rotatable part 503, as for Figure 10b) due to the lateral stiffness of the flexures 561, 562 will be reduced relative to a corresponding increase in tension for SMA wire 506a in the arrangement of Figure lOd for an equivalent relative rotation of the base 502 and the rotatable part 503. Therefore, both sections of SMA wire 506a, 506b will have a more similar tension (relative to the wire tensions of Figure lOd). This may be achieved by generating a bias force that is centring. Alternatively, the flexures 561, 562 may be offset to provide a return force in one direction of motion. Certain of the previously described actuator assemblies (both for folded wire and for divided wire actuating units) show the wire connection points to the base or the rotatable part (for a pair of actuating units) arranged such that the wires generally point to a common intersect. However, this is not required. As shown in Figure 11, for a pair of actuating units 605, one or both sets of wire connection points may be configured such that the wires cross each other with the crimps outside the adjacent wires. Figure 11 illustrates a pair of actuating units 605 based upon the divided wire actuator assembly of Figure 7b. Corresponding features are given the same reference number incremented by 500. Crimps 707a connected to the rotatable part (not illustrated) are arranged so that they ensure SMA wires 706a cross. Crimps 707b connected to the base (not illustrated) may similarly be arranged so that they ensure SMA wires 706b cross, though this is not illustrated in Figure 11. Crossing the wires at one or both ends of the actuating units in this way can provide for a further increase in wire length for a given size of actuator assembly. Further adaptations of the wire arrangement for actuating units are presented in Figures 12a and 12b. While both are presented as folded wire actuating unit embodiments, with reference numbers incremented by 600 from Figure 7a, the same adaptations are applicable also to divided wire actuating unit embodiments. Figure 12a shows that the wires forming the actuating units need not extend all (or substantially all) of the way around the primary axis O. The base and the rotatable part are not separately shown, rather the package size of the actuator assembly (in a plane normal to the primary axis O) is represented by circle 870. First and second actuating units 805a are formed from wires 806 passing over a redirection element 809 between crimps 807 connecting the wire to the base and the rotatable part. Figure 12a shows the wires 806 being generally arranged along four consecutive sides of a hexagon. While the actuating units 805a retain approximate mirror symmetry, they are no longer approximately rotationally symmetrical. The present invention is not limited to any particular proportion of a circle about the primary axis O provided with SMA wire. Figure 12b shows that each actuating unit 805b includes a pair of redirection elements 809 dividing wires 806 between their end crimps 807 such that the arrangement of wires approximates an octagon. Any regular or irregular polygon (wholly, or only certain sides) may be formed following the techniques of Figures 12a and 12b. It will be appreciated that increasing the number of SMA wire segments within an actuating unit provides for a further reduction in thermal gradients across a lens by providing heat sources more evenly about the circumference of a rotary actuator assembly. In some examples, each actuating unit may provide a different number of SMA wire segments and may extend round a different proportion of the circumference of the assembly. Alternative intermediate components Turning now to Figure 13, this illustrates a further actuator assembly 901 that represents an alternative option for a divided wire type actuator assembly. Actuator assembly 901 incorporates an intermediate component 930 that provides for a similar mode of actuation to that of actuator assemblies 401a and 401b of Figures 9a and 9b. Features that are the same as or similar to the actuator assemblies 401a and 401b of Figures 9a and 9b are given the same reference numbers, incremented by 500 and are not further described except to highlight differences. For Figure 13, two actuator assemblies are shown for driving rotation of the rotatable part 903 relative to the base 902 in opposite directions. A first actuator assembly comprises first and second SMA wires 906a, 906b coupled between the base 902 and the rotatable part 903. A second actuator assembly comprises first and second SMA wires 906c, 906d coupled between the base 902 and the rotatable part 903. As for Figure 9a, the pairs of SMA wires for each actuating unit are connected by an intermediate component formed as a third ring 930. Differing from Figure 9a however, the SMA wires 906a, 906b, 906c, 906d for the first and second actuating units are generally on opposite sides of the primary axis O, yet remain connected in series between the base 902 and the rotatable part 903 via the intermediate ring 930. Ring 930 may be positioned radially intermediate of the base ring 902 and the rotatable part ring 903, as illustrated. However, more generally it may be any structure configured to move relative to the base 902. In particular, the ring 930 is configured to slide along an axis perpendicular to the primary axis (optionally intersecting the primary axis O) as indicated by arrows 980 (up and down as illustrated). Ring 930 is constrained from rotating about the primary axis O relative to the base 902 by bearings 931, such that contraction of SMA wires 906a, 906c causes movement of the ring 930 along arrows 980 (upwards and downwards as illustrated). Ring 930 is coupled to the rotatable part 903 via further bearings 932 such that the rotatable part 903 can rotate relative to intermediate ring 930 (and the base 902). When the wires 906a, 906b of the first actuator assembly contract they apply a torque to the rotatable part 903 causing the rotatable part to rotate clockwise about the primary axis O as indicated by arrow 981. When the wires 906c, 906d of the second actuator assembly contract they apply a torque to the rotatable part 903 causing the rotatable part to rotate anticlockwise about the primary axis O as indicated by arrow 982. The effect of this sliding movement of ring 930 along arrows 980 is to magnify the rotation delivered to rotatable part 903 for a given wire contraction (relative to the actuator assembly 201 of Figure 7b). In some cases, further magnification of the effect of the contraction of the wires extending from base 902 is provided compared to the rotating intermediate ring 430 of Figure 9a. The intermediate ring 930 may extend all of the way around the primary axis O (as illustrated), or it may not form a complete loop. It may take any suitable shape for enabling movement along an axis perpendicular to the primary axis O. Intermediate ring 930 is shown mounted upon bearings 931, 932. The bearings 931, 932 may take any form such as a ball race or plain bearings and have any number or arrangement. Alternatively, in place of bearings, the intermediate ring 930 may be coupled to the base 902 or the rotatable part 903 (or both) through a flexure system. The intermediate ring 930 connects the SMA wires 906a, 906b and 906c, 906d of each actuating unit through crimps 921 mounted upon the intermediate ring 931 (the other end of each wire being connected to the base 902 and the rotatable part 903 via crimps 907). In an alternative, there may be separate intermediate components provided for each of the first and second actuating units. Other methods for mechanically connecting SMA wires may be used. In other embodiments there may be more than one intermediate ring 931 such that a greater number of SMA wires are connected for each actuating unit in an analogous fashion. Although not specifically illustrated in Figure 13, at least one pair of friction surfaces may be provided between the base 902 and the rotatable part 903 for the delivery of ZHP functionality. It will be appreciated that as for the embodiments of Figure 7b and 9a, the force applied to the rotatable part 903 by each actuating unit includes a component applying a torque for rotating the rotatable part 903 relative to the base 902 and a component acting radially towards the primary axis O for modulating the friction between the friction surfaces. Elongated connecting element Turning now to Figure 14, a yet further actuator assembly 1001 is shown. This comprises a divided wire type actuator similar in certain respects to actuator assembly 201 of Figure 7b. Accordingly, features that are the same as or similar to actuator assembly 201 of Figure 7b are given the same reference numbers, incremented by 800 and are not further described except to highlight differences. Rotatable part 1003 is driven to rotate relative to base 1002 in first and second senses about primary axis O by first and second actuating units 1005a, 1005b. Actuating unit 1005a comprises a pair of SMA wires 1006a, 1006b connected in series between the base 1002 and the rotatable part 1003 by a connecting element 1020a. Similarly, actuating unit 1005b comprises a pair of SMA wires 1006c, 1006d connected in series between the base 1002 and the rotatable part 1003 by a connecting element 1020b. Differing from the connecting elements 220 of Figure 7b, the connecting elements 1020a, 1020b are elongated such that wires 1006a-d may be provided generally parallel to one another (they need not be exactly parallel), while remaining outside of lens aperture 1004. Particularly, the wires within each actuating unit are generally parallel, as are the respective wires of the pair of actuating units. This has the benefit that no wires need to cross. It can be seen by visual inspection of Figures 7b and 14 that this arrangement (enabled by elongate connecting elements 1020a, 1020b) provides for an increase in the length of SMA wire that can be provided within the actuator assembly 1001. It will be appreciated that if the connecting elements 1020a, 1020b were both coupled to the base 1002 or both coupled to the rotatable part 1003, then there would be no resultant force on the rotatable part 1003 from tension in all the wires that can be used to modulate ZHP friction at friction surfaces 1010, 1011. Instead, this design has the connecting element 1020a of the first actuating unit 1005a mounted on the rotatable part 1003 at point 1090 and the connecting element 1020b of the second actuating unit 1005b mounted on the base 1002 at point 1091, for instance with any form of flexible, pivoting or rotating coupling of the form described above to create a net force on the rotatable part 1002 that can be used to reduce the friction from the plain bearings 1010, 1011. 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.

Claims

1. An actuator assembly comprising:a base;a rotatable part; andan actuating unit configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, wherein the actuating unit comprises:at least two shape memory alloy, SMA, element segments coupled in series between the base and the rotatable part, the at least two SMA element segments being axially misaligned and configured, on actuation of the actuating unit, to apply the actuating force to the rotatable part;wherein the actuator assembly further comprises a pair of friction surfaces biased against each other by a biasing force extending transverse to the primary axis, thereby generating a static frictional force between the pair of friction surfaces for maintaining the position of the rotatable part relative to the base when the actuating unit is not actuating; andwherein the actuating unit is arranged such that on actuation of the actuating unit the static frictional force between the pair of friction surfaces is reduced.

2. An actuator assembly according to claim 1, wherein the at least two SMA element segments comprise portions of a single SMA element coupled between the base and the rotatable part such that contraction of each portion of the single SMA element combines to generate a total contraction to apply the actuating force to the rotatable part.

3. An actuator assembly according to claim 2, wherein the actuating unit further comprises one or more redirection elements, each redirection element being coupled to the base, to the rotatable part, or an intermediate component moveable relative to the base or the rotatable part, such that between the base and the rotatable part the single SMA element passes each redirection element to define separate SMA element segments to either side of the redirection element.

4. An actuator assembly according to claim 3, wherein the redirection element is a pulley, roller, post, rocker, or flexure.

5. An actuator assembly according to claim 1, wherein the at least two SMA element segments comprise at least two SMA elements mechanically coupled together in series such that contraction of the SMA elements combines to generate a total contraction to apply the actuating force to the rotatable part.

6. An actuator assembly according to claim 5, wherein the actuating unit further comprises one or more connecting elements to couple together pairs of SMA elements, each connecting element being coupled to the base or the rotatable part; orwherein the actuating unit further comprises an intermediate component moveable relative to the base and the rotatable part.

7. An actuator assembly according to claim 6, wherein each connecting element comprises a flexure connected to the base or the rotatable part, wherein contraction of a first of the pair of SMA element causes bending of the flexure and displacement of a second of the pair of SMA elements.

8. An actuator assembly according to claim 6 or claim 7, wherein each connecting element comprises one or more crimp components for mechanically coupling together a pair of SMA elements.

9. An actuator assembly comprising:a base;a rotatable part; andan actuating unit configured to apply an actuating force to the rotatable part capable of rotating the rotatable part relative to the base about a primary axis, wherein the actuating unit comprises:at least two SMA element segments, the SMA element segments being axially misaligned and configured, on actuation of the actuating unit, to apply the actuating force to the rotatable part; andat least one intermediate component moveable relative to the base and the rotatable part and configured to couple the two SMA elements in series between the base and the rotatable part.

10. An actuator assembly according to claim 9, wherein the at least two SMA element segments comprise at least two SMA elements mechanically coupled together in series such that contraction of the SMA elements combines to generate a total contraction to apply the actuating force to the rotatable part.

11. An actuator assembly according to claim 9 or claim 10, wherein at least one intermediate component is configured to rotate about an axis substantially parallel to the primary axis or to translate along an axis extending substantially perpendicular to the primary axis.

12. An actuator assembly according to any one of claims 9 to 11, wherein at least one intermediate component extends at least partially around the primary axis, or the intermediate component is mounted upon the base or the rotatable part such that it can translate relative to the base or the rotatable part.

13. An actuator assembly according to any one of claims 9 to 12, wherein the actuator assembly further comprises a pair of friction surfaces biased against each other by a biasing force extending transverse to the primary axis, thereby generating a static frictional force between the pair of friction surfaces for maintaining the position of the rotatable part relative to the base when the actuating unit is not actuating; andwherein the actuating unit is arranged such that on actuation of the actuating unit the static frictional force between the pair of friction surfaces is reduced.

14. An actuator assembly according to any one of the preceding claims, wherein a first SMA element segment is mechanically coupled to the base via a base crimp component and a second SMA element segment is mechanically coupled to the rotatable part via a rotatable part crimp component.

15. An actuator assembly according to any one of the preceding claims, wherein the actuating unit comprises more than two SMA element segments coupled in series between the base and the rotatable part, each adjacent pair of SMA element segments being axially misaligned relative to each other.

16. An actuator assembly according to any one of the preceding claims, wherein the actuating unit is configured to rotate the rotatable part relative to the base about the primary axis in a first sense, and the actuator assembly further comprises a further actuating unit configured to rotate the rotatable part relative to the base about the primary axis in a second sense opposite to the first sense.

17. An actuator assembly according to claim 16, wherein the actuating unit and the further actuating unit are generally symmetrically arranged about a plane including the primary axis.

18. An actuator assembly according to claim 16 or claim 17, wherein the SMA element segments of the actuating unit and the further actuating unit are generally arranged about the actuator assembly with rotational symmetry of at least order 2.

19. An actuator assembly according to any one of the preceding claims, wherein the pair of friction surfaces are provided respectively on the base and the rotatable part.

20. An actuator assembly according to any one of the preceding claims, further comprising a biasing arrangement configured to bias the pair of friction surfaces with the biasing force, thereby giving rise to the static frictional force.

21. An actuator assembly according to any one of the preceding claims, wherein the biasing force is applied to the rotatable part in a first direction and at least one actuating unit is configured to apply the actuating force to the rotatable part angled relative to the first direction such that a component of the actuating force opposes the biasing force.

22. An actuator assembly according to any one of the preceding claims, wherein the biasing force is applied to the rotatable part in a first direction and an intermediate point of at least one actuating unit between a pair of SMA element segments is configured on actuation of the actuating unit to apply a force to the base or to the rotatable part to oppose the biasing force.

23. A variable aperture assembly comprising:the actuator assembly of any one of claims 1 to 22; anda plurality of blades configured such that rotation of the rotatable part relative to the base effects movement of the blades, thereby changing the size of the variable aperture;wherein the static frictional force between the pair of friction surfaces maintains the position of the blades when the actuating unit is not actuating.

24. A camera comprising:the variable aperture assembly of claim 23;a lens assembly; andan 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.

25. An electronic device incorporating the camera of claim 24.

Images