Assembly

Abstract

An assembly comprises a first part 2 defining a primary axis P, a second part 6 movable with respect to the first part, and a third part 4 movable with respect to the first and second parts and compri

Description

Field The present application relates to assemblies, in particular assemblies comprising three or more parts and an SMA element. Background There are a variety of apparatuses in which it is desirable to provide control of a movable part. Shape memory alloy (SMA) elements (e.g. 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 part can be relatively small. It may be desirable to move the movable part to a given position relative to a support structure and then retain the movable part in that position for a length of time. It may also be desirable to keep the power consumption of the actuator to an acceptably low level. This is particularly relevant when the actuator is embodied on a portable electronic device, such as a mobile phone or a wearable device. Summary According to a first aspect of the present invention there is provided an assembly comprising a first part defining a primary axis; a second part which is movable with respect to the first part; and a third part which is movable with respect to the first and second parts and which comprises a first friction surface which is biased against a second friction surface with a normal force. The second friction surface is disposed on one of the first and second parts. The assembly further comprises a first shape memory alloy (SMA) element arranged to drive movement of the third part along a first direction, at least a component of which is perpendicular to the primary axis. The third part is mechanically coupled to the first and second parts such that movement of the third part along the first direction causes movement of the second part relative to the first part along a second direction, different to the first direction. At least a component of the second direction is parallel to the primary axis. The first SMA element is arranged such that contraction of the first SMA element reduces or removes the normal force between the first and second friction surfaces. The assembly further comprises a force-application arrangement configured to: apply a force between the second part and the first part, thereby reducing or removing the normal force between the first and second friction surfaces, and cause movement of the second part along a third direction, at least a component of which is parallel to the primary axis and opposite to the at least a component of the second direction. In this way, an assembly is provided in which movement of a moving part (i.e. the second part) can be displaced along two opposite directions. In the case of each direction, friction between two friction surfaces is reduced during movement, thereby enabling a higher level of friction between two friction surfaces while the moving part is still and a lower level of friction while the moving part is moving. This difference in friction levels is advantageous because a higher level of friction is desirable in order to, for example, retain the moving part in a given position without needing to supply the SMA element with a high power or even power at all and a lower level of friction during movement of the second part is desirable so that a lower force is required to overcome that friction and drive movement of the second part. During the movement of the third part along the first direction, the first friction surface moves relative to the second friction surface. In some embodiments, a load (i.e. a loading force or a biasing force) may act on the second part relative to the first part. For example, the load may act to bias the moving part in a direction along the primary axis or in a direction having at least a component along the primary axis. This load may be provided by one or more of: a resilient element (such as a spring), a magnet or by other means, for example by pressure of a liquid inside a liquid lens which is engaged with the second part. The load may bias the first and second friction surfaces together. In particular, the biasing of the first and second friction surfaces together with a normal force as mentioned above may be provided in full or in part by the load. Alternatively or additionally, the first and second friction surfaces may be biased against each other by gravity and / or one or more resilient elements such as springs and / or one or more magnets. In some embodiments, the first direction is perpendicular to the primary axis or has at least a component which is perpendicular to the primary axis, the second direction is parallel to (or colinear with) the primary axis and the third direction is opposite to the second direction. The second part may be constrained to move along the primary axis only. The second part may be prevented from moving in any other degrees of freedom. To constrain the second part to move along the primary axis only, the assembly may comprise a bearing arrangement to guide movement of the second part relative to the first part. The bearing arrangement could be a rolling bearing arrangement or a plain bearing arrangement, for example. In some embodiments, the assembly is arranged such that the second part is retained in position (i.e. held still) with respect to the first part when the first SMA element is unpowered, at least in part due to friction between the first and second friction surfaces. This may reduce the overall power consumption of the assembly because the SMA element need not be continuously powered in order to hold the second (moving) part in a given position with respect to the first part. In some embodiments, the assembly is arranged such that the friction between the first and second friction surfaces is sufficient to retain the second part in position with respect to the first part when the first SMA element is unpowered. In some embodiments, multiple pairs of friction surfaces (e.g. multiple pairs within the assembly or one or more pairs in a further assembly) may, in total, be sufficient to retain the second part in position with respect to the first part when the first SMA element is unpowered. In some embodiments, one or both of the first and second friction surfaces are at a non-zero angle to the primary axis. In some embodiments, one or both of the first and second friction surfaces are at a non-zero, acute angle to the primary axis. Accordingly, the first and / or second friction surfaces may be used to provide friction and also to convert movement of the third part along a first direction into movement of the second part along a second, different direction, at least a component of the second direction being perpendicular to at least a component of the first direction. In some embodiments, the third part comprises a third friction surface which is biased against a fourth friction surface. The second friction surface may be on one of the first and second parts and the fourth friction surface may be on the other of the first and second parts. In this way, a pair of friction interfaces is used to provide friction in the assembly: a first interface comprising a first friction surface on the third part and a second friction surface on one of the first and second parts and a second interface comprising a third friction surface on the third part and a fourth friction surface on the other of the first and second parts. By using two interfaces, the amount of friction in the system can be varied across a greater range (i.e. the difference between the level of friction during movement as compared to when the second part is held still can be greater). Also, the total friction in the system may be higher (as compared to an embodiment with only one friction interface) and / or materials with a relatively lower coefficient of friction may be used. A level of friction in the first interface may be different to a level of friction in the second interface. In particular, a coefficient of friction may be associated with each of the first to fourth friction surfaces. Some or all of the coefficients of friction may be the same as each other. Some or all of the coefficients of friction may be different to each other. In some embodiments, one or both of the third and fourth friction surfaces are at a non-zero angle to the primary axis. In some embodiments, one or both of the third and fourth friction surfaces are at a non-zero, acute angle to the primary axis. As such, the third and fourth friction surfaces may also be used to drive a change in direction of movement, as between the third part and the second part. The acute, non-zero angle which one or both of the third and fourth friction surfaces make with the primary axis may be defined in an opposite way to the acute, non-zero angle which one or both of the first and second friction surfaces make with the primary axis. For example, the angled surface(s) in the interface between the third and first parts may slope upwards and the angled surface(s) in the interface between the third and second parts may slope downwards (or vice versa). Such an arrangement can increase the overall amount by which the second part is driven to move along the primary axis for a given amount of contraction of the first SMA element. In some embodiments, a combination of (a) friction between the first and second friction surfaces and (b) friction between the third and fourth friction surfaces is sufficient to retain the second part in position with respect to the first part when the first SMA element is unpowered. The third and fourth friction surfaces have been described above as providing friction in the system and optionally as being angled with respect to the primary axis. In some embodiments, the third and fourth friction surfaces may not necessarily provide friction (or may not provide more than a negligible amount of friction in the assembly). In this way, the third and fourth friction surfaces may simply be referred to as third and fourth surfaces. For example, a low-friction bearing arrangement may be implemented to guide relative movement between the third and fourth surfaces. This may be a rolling bearing or a low-friction plain bearing (e.g. involving surfaces with low coefficients of friction and / or a lubricant), for example. The third and / or fourth surfaces may be angled, as described above, but may have a low-friction interface between them. In other embodiments, a flexure bearing arrangement may used to guide movement of the third part relative to the first part, for example, and a friction interface may be provided between the third part and the second part in the way described above (or vice versa). In some embodiments, the force-application arrangement may be configured so that the force applied between the second part and the first part is controllable. In some embodiments, the force-application arrangement is configured to be supplied with a control signal in order to control the amount of force between the first and second parts. Such a control signal may comprise or correspond to a level of electrical power supplied to the force-application arrangement, for example. In some embodiments, the force-application arrangement comprises a second SMA element. The second SMA element may be arranged to apply a force between the second part and the first part either directly (e.g. by directly driving movement of the second part relative to the first part) or indirectly (e.g. by driving movement of an intermediate part which in turn drives movement of the second part relative to the first part. It will be appreciated that the SMA element may be arranged in any suitable way. In some embodiments, the force-application arrangement comprises two or more SMA elements. In some embodiments, the second SMA element may be of the same or of different length to the first SMA element. In some embodiments, the assembly is arranged such that the second part is retained in position (i.e. held still) with respect to the first part when both the first SMA element and the second SMA element are unpowered. In some embodiments, the force-application arrangement comprises a different (i.e. non-SMA) actuator, for example a voice coil motor or a piezoelectric actuator. In some embodiments, the force-application arrangement comprises a fourth part which is movable with respect to the first and second parts and which is mechanically coupled to the first and second parts such that movement of the fourth part relative to the first part drives movement of the second part relative to the first part. The fourth part may be described as an intermediate part. In some embodiments, the second SMA element is arranged to drive movement of the fourth part relative to the first part. In some embodiments, the assembly comprises a bearing arrangement configured to guide movement of the fourth part relative to the first and second parts. The bearing arrangement may have an associated level of mechanical resistance which is lower than a level of mechanical resistance provided by friction between the first and second friction surfaces. Optionally, where third and fourth friction surfaces are present, as described above, the bearing arrangement may have an associated level of mechanical resistance which is lower than a total level of mechanical resistance provided by friction between the first and second friction surfaces and friction between the third and fourth friction surfaces. Put differently, the fourth part may run on low-resistance (e.g. low-friction) bearings (e.g. rolling bearings, low-friction plain bearings, flexure bearings), whereas the third part runs on relatively high-friction bearings. Accordingly, friction in the system may be provided by the third part. The fourth part may be used to facilitate movement of the second part relative to the first part to at least partially relieve the friction provided by the third part. The associated level of mechanical resistance could be a level of friction. Put differently, in some embodiments the bearing arrangement has an associated level of friction which is lower than a level of friction between the first and second friction surfaces (optionally a total level of friction between the first and second friction surfaces and also between the third and fourth friction surfaces). In some embodiments, the bearing arrangement comprises a first bearing which guides movement of the fourth part relative to the first part and a second bearing which guides movement of the second part relative to the fourth part. In some embodiments, the first bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and / or second friction surfaces. In some embodiments, the second bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and / or second friction surfaces. In some embodiments, the first bearing comprises a surface which is at a non-zero angle to the primary axis. In some embodiments, the first bearing comprises a surface which is at a non-zero, acute angle to the primary axis. Specifically, the fourth part and / or the first part may comprise a surface which is at a non-zero (and optionally acute) angle to the primary axis. In some embodiments, the second bearing comprises a surface which is at a non-zero angle to the primary axis. In some embodiments, the second bearing comprises a surface which is at a nonzero, acute angle to the primary axis. Specifically, the fourth part and / or the second part may comprise a surface which is at a non-zero (and optionally acute) angle to the primary axis. In other words, one or both of the first and second bearings may comprise an angled surface in an analogous way to the angled surfaces described above with reference to the first, second and third parts. In some embodiments, the fourth part is mechanically coupled to the third part. In some embodiments, the assembly comprises a biasing arrangement which provides a biasing force between the fourth part and the third part. In some embodiments, the biasing arrangement is arranged to bias the fourth part and the third part away from each other. In some embodiments, the biasing arrangement is arranged to bias the fourth part and the third part towards each other. In some embodiments, the biasing arrangement comprises one or more of a resilient element and a magnet. For example, a spring could be disposed between the third and fourth parts either under tension (to bias the third and fourth parts towards each other) or under compression (to bias the third and fourth parts away from each other). Alternatively or additionally, one or more magnets could be disposed on one or both of the third and fourth parts to achieve the same effect. The first and second parts may be arranged relative to the third and fourth parts in any suitable way. A first side and a second side of the third and fourth parts are defined as being sides of the third and fourth parts which are spaced apart from each other along the primary axis. In some embodiments, the second part may be engaged with both the third and fourth parts on the same side of the third and fourth parts (e.g. the first side) and the support structure may be engaged with both the third and fourth parts on the same side of the third and fourth parts (e.g. the second side). Put differently, the second part may be above the third and fourth parts and the first part may be below the third and fourth parts when viewed along a direction perpendicular to the primary axis (or vice versa). In other embodiments, the second part may be engaged with the third part on the first side of the third part and engaged with the fourth part on the second side of the fourth part. In such embodiments, the first part may be engaged with the third part on the second side of the third part and engaged with the fourth part on the first side of the fourth part. Put differently, one portion of the second part may be above the third part and another portion of the second part may be below the fourth part (when viewed along a direction perpendicular to the primary axis) and a portion of the first part may be below the third part and another portion of the first part may be above the fourth part (when viewed along a direction perpendicular to the primary axis) or vice versa. In some embodiments, the first friction surface remains in contact with the second friction surface during normal operation of the assembly. The phrase 'normal operation of the assembly' is intended to refer to a state in which the assembly is functioning as intended and excludes e.g. a user dropping the device in which the assembly is embodied. In some embodiments, the fourth part is movable relative to the third part and relative movement between the third and fourth parts causes the first friction surface to disengage from the second friction surface. Accordingly, the assembly is configured so that the friction between the first and second friction surfaces is reduced to zero during movement of the second part relative to the first part, and then increased again when the first and second friction surfaces come back into contact when it is desired to hold the second part in a given position relative to the first part. In some embodiments, the assembly comprises a constraining arrangement which is configured to prevent relative movement between the third and fourth parts beyond a threshold amount. The constraining arrangement could comprise two or more endstop surfaces which are configured to engage each other once the third and fourth parts have moved relative to each other by the threshold amount. In another example, the constraining arrangement could comprise a tether arrangement, e.g. comprising a flexure or wire which is slack but then straightens and provides a tensional force to prevent the third and fourth parts moving relative to each other beyond the threshold amount. In another embodiment, the constraining arrangement may comprise a rigid bar comprising a first end arranged to contact the third part and a second end arranged to contact the fourth part when the third and fourth parts move apart from each other by more than the threshold amount. In some embodiments, the third part is constrained from moving relative to the fourth part. In this way, the third and fourth parts may move together as one. In some embodiments, the assembly is configured such that, on contraction of the first SMA element by a first distance, the second part moves along the primary axis by a second distance which is greater than the first distance. In other words, movement of the second part relative to the first part is amplified, relative to a distance by which the first SMA element contracts. This may be advantageous where it is desired to move the second part by a relatively long way, as compared to the small amount of contraction provided by SMA. This amplification may otherwise be described as gearing-up. In some embodiments, the assembly is configured such that, on contraction of the first SMA element by a first distance, the second part moves along the primary axis by a second distance which is smaller than the first distance. In other words, movement of the second part relative to the first part is de-amplified, relative to a distance by which the first SMA element contracts. This may be advantageous where it is desired to control the position of the second part relative to the first part very precisely. This de-amplification may otherwise be described as gearing-down. In some embodiments, the assembly comprises a counterbalance arrangement which is configured to cause movement of the third part relative to the first part when the forceapplication arrangement reduces or removes the normal force between the first and second friction surfaces. In some embodiments, the counterbalance arrangement comprises a resilient element arranged between the first part and the third part and / or a magnet acting between the first part and the third part. As mentioned above, in some embodiments a load (i.e. a loading force or a biasing force) may act on the second part. For example, the load may act to bias the moving part in a direction along the primary axis or in a direction having at least a component along the primary axis. The counterbalance arrangement may be arranged to partially counteract the action of the load on the third part. For example, the load may act (via the second part) to bias the third part along the first direction. The counterbalance arrangement may apply a force to counteract (at least partially) this force, e.g. by exerting a force on the third part in a direction opposite to the first direction. The counterbalance arrangement may be configured to drive movement of the third part back to or towards a central position (defined with reference to a range of movement of the third part relative to the first part) when the force-application arrangement reduces (or removes) the normal force between the first and second friction surfaces. The counterbalance arrangement may be configured so as to oppose contraction of the first SMA element. In a second aspect of the present invention there is provided a system comprising: a component; and a first assembly as described herein. The system is configured such that movement of the second part of the first assembly relative to the first part of the first assembly drives movement of at least part of the component. The component could be driven to move as a whole by the first assembly, for example when the component is rigid. Alternatively, the component may be flexible and so the assembly may be configured to drive movement of only part of the component. In some embodiments, the component is a deformable optical component. The first assembly may be configured such that movement of the second part of the first assembly relative to the first part of the first assembly drives deformation of the deformable optical element. For example, the deformable optical element may be a deformable lens (such as a liquid lens) or a deformable mirror. The deformation of the deformable optical element may alter an optical property of the deformable optical element, such as a focal length. The second part of the first assembly may be arranged to directly drive deformation of the deformable optical element. For example, the second part may be in contact with or integrally formed with a part of the deformable optical component. Alternatively, movement of the second part may indirectly drive deformation of the deformable optical element, e.g. via a further intermediate part. In some embodiments, the system comprises a second assembly as described herein. The first assembly may be arranged to drive movement of a first part of the component and the second assembly may be arranged to drive movement of a second part of the component, different to the first part of the component. In some embodiments, the third part of the first assembly is rigidly connected to the third part of the second assembly. Put differently, the third part of the first assembly may be the same part as the third part of the second assembly. In this way, a third part may be common to both the first and second assemblies. In embodiments in which the first and second assemblies each also comprise a respective fourth part, as described above, the fourth part of the first assembly may be rigidly connected to the fourth part of the second assembly. In this way, a fourth part may be common to both the first and second assemblies. The respective first parts of the first and second assemblies may also be rigidly connected to each other. In this way, a first part may be common to both the first and second assemblies. The assemblies and systems described herein are described with reference to a primary axis. This primary axis may be defined in any suitable way with reference to the first part. The first part may comprise a frame for supporting the parts of the assembly and the primary axis may be perpendicular to a plane defined by the frame. When viewed along the primary axis, the frame may form a closed loop around the primary axis. In embodiments where the assembly or assemblies are arranged to drive deformation of a deformable optical element such as a deformable lens, the primary axis may be an optical axis or a principal axis of the deformable lens, for example when the deformable lens is in a neutral position, i.e. when the lens is undeformed. The primary axis may be colinear with a line joining the centre of curvature of a deformable surface of the lens when the lens is minimally deformed and the centre of curvature of the deformable surface of the lens when the lens is maximally deformed. The terms minimum and maximum here refer to the minimum deformation (i.e. no deformation) and maximum deformation of the deformable optical element achievable by the assembly. In any case, the deformable optical element may be generally planar and / or define a plane and the primary axis may be perpendicular to that plane. In embodiments in which the deformable optical element is a deformable lens, a plane may be defined by the deformable lens when in an undeformed state and the primary axis may be perpendicular to that plane. The primary axis may be perpendicular to a plane defined by the average position of a number of points around the edge of the deformable optical element e.g. along a direction in which they are driven to move by the assembly. In other words, a plane may be defined by n points (where n>3) around the edge of the deformable optical component, wherein the plane is the best-fit plane of the n points. The best-fit plane may be defined such that the summed squared distance to all points is minimised and may be calculated using planar regression. The plane may be defined by the n points when the deformable component is in a deformed state or a 'neutral', undeformed state. The primary axis may be perpendicular to such a plane. The primary axis may pass through a centre of the deformable optical component (the centre being defined, for example, by an outer circumference of the component). At least some of the embodiments described herein rely on various angled surfaces. The following angles are defined: the angle between the primary axis and the angled surface present between the third part and the first part: 0i the angle between the primary axis and the angled surface present between the third part and the second part: 02 the angle between the primary axis and the angled surface present between the fourth part and the first part: 03 the angle between the primary axis and the angled surface present between the fourth part and the second part: 04. One or more (e.g. all) of these angles may be the same as each other. It should be noted that the numerical value of the angle may be the same as between two or more of the above angles but they may be defined with respect to the primary axis in opposite ways (e.g. some sloping upwards, some sloping downwards). One or more (or all) of the angles may be different to each other. In some embodiments, 0i may be the same as 02 (but defined in an opposite way) and 03 may be the same as 04 (but defined in an opposite way) but different to 0i and 02. Put differently, the angled surfaces on the third part may have the same slope as each other and the angled surfaces on the fourth part may have the same slope as each other (but a different slope to the angled surfaces on the third part). One or more of the angles may be acute and non-zero. One or more of the angles may be 90°. Assemblies have been described above which are arranged to provide a relatively high level of friction when it is desired to hold the second part in position with respect to the first part and a lower (or zero) level of friction during movement of the second part relative to the first part. One or more such assemblies may be present in a given system to provide control over the amount of friction in the system. Also present in the system may be one or more intermediate parts which are arranged to assist in driving movement of the second part relative to the first part. Such one or more intermediate parts may comprise one or more surfaces which are at an acute, non-zero angle with respect to the primary axis. A large number (e.g. 10 or more, 20 or more or 50 or more) of such surfaces may be present in a system and the system may also comprise one or more assemblies as described above in order to vary the level of friction in the system over time. Such angled surfaces may be engaged with other parts of the system and the interface between the angled surfaces and the other parts of the system with which they are engaged may be low-friction, i.e. they may have an associated level of friction which is lower than the level of friction between the first and second friction surfaces described above. A system having one or more intermediate parts with at least one angled surface is described in PCT / GB2024 / 052191, which is incorporated herein by reference in its entirety. 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 view of a known assembly; Figure 2 is a schematic view of an embodiment according to the present invention; Figure 3 is a schematic view of a further embodiment according to the present invention; Figure 4 is a schematic view of a further embodiment according to the present invention; Figure 5 is a schematic view of a further embodiment according to the present invention; Figure 6 is a schematic view of a further embodiment according to the present invention; Figure 7 is a schematic view of a further embodiment according to the present invention; Figure 8A is a schematic view of a further embodiment according to the present invention; Figure 8B is an enlarged view of part of Figure 8A; Figure 8C is a schematic view of a further embodiment according to the present invention; Figure 9A is a schematic view of a further embodiment according to the present invention; Figure 9B is a schematic view of a further embodiment according to the present invention; Figure 10 is a schematic view of a further embodiment according to the present invention; Figure 11A is a schematic, perspective view of a further embodiment according to the present invention; Figure 11B is a side view of the embodiment of Figure 11A; Figure 11C is a side view of the embodiment of Figure 11A, showing the other side of the assembly, as compared to Figure 11B; Figure 11D is an enlarged view of part of Figure 11C; Figure 12A is a schematic view of an implementation of the present invention; Figure 12B is a schematic, perspective view of an implementation of the present invention; Figure 13A is a schematic view of an implementation of the present invention; Figure 13B is a schematic, perspective view of the implementation of the present invention of Figure 13A;Figure 14A is a schematic view a liquid lens; Figure 14B is a cross-section of the lens of Figure 14A; Figure 15 is a further cross-section of the lens of Figure 14A; and Figures 16A and 16B are enlarged partial schematic views of liquid lenses. Detailed description With reference to Figure 1, a known assembly is described. The assembly 1 comprises a support structure 2, an intermediate part 4 and a moving part 6. The moving part 6 is movable with respect to the support structure 2. The intermediate part 4 is movable with respect to the support structure 2 and the moving part 6. A first SMA element 8 is connected between the support structure 2 and the intermediate part 4 and is arranged to apply a force along a first direction Fi. A second SMA element 10 is connected between the support structure 2 and the intermediate part 4 and is arranged to apply a force to the intermediate part 4 along a second direction Fj. The second SMA element 10 is arranged such that contraction of the second SMA element 10 opposes contraction of the first SMA element 8. A load, indicated by arrow L in Figure 1, acts on the moving part 6. This load could be applied by a spring, a magnet, gravity or by any other means. The intermediate part 4 is mechanically coupled to the support structure 2 and to the moving part 6. Specifically, a first surface 12 of the intermediate part 4 is in contact with a second surface 14 of the support structure 2. A third surface 16 of the intermediate part 4 is in contact with a fourth surface 18 of the moving part 6. Each of the first to fourth surfaces form an acute, non-zero angle with respect to a primary axis P. The moving part 6 is constrained to move along the primary axis P only (i.e. up and down in Figure 1). This constraint may be provided by a bearing arrangement between the moving part 6 and the support structure 2 (not shown), for example a rolling bearing or a plain bearing. Since each of the first to fourth surfaces are angled with respect to the primary axis P, it can be seen that as the first SMA element 8 is actuated (contracted), the intermediate part 4 will be driven to the left (as seen in Figure 1) and also downwards simultaneously. The moving part 6 will move downwards under the action of the load L. Similarly, when the second SMA element 10 is contracted, the intermediate part 4 will move to the right and also upwards simultaneously. This in turn drives the moving part 6 to move upwards, relative to the support structure 2. The first to fourth surfaces may be arranged to have sufficient friction to hold the intermediate part 4 still with respect to the support structure 2 when the first and second SMA elements 8,10 are unpowered. In this way, the assembly 1 may be described as a zero-hold power assembly. The level of friction in the interfaces must be chosen carefully to be high enough that the moving part 6 can be retained in position when the SMA elements are unpowered but low enough to allow relative motion between the various parts of the assembly. It can be seen that when the first SMA element 8 is actuated, the normal force between the support structure 2 and the intermediate part 4 and also the normal force between the intermediate part 4 and the moving part 6 are both reduced. This reduction in normal force also reduces the friction between the first and second surfaces 12,14 and the friction between the third and fourth surfaces 16, 18. However, when the second SMA element 10 is actuated, the normal force between the support structure 2 and the intermediate part 4 and also the normal force between the intermediate part 4 and the moving part 6 are both increased, thereby increasing the friction. This increase in friction during movement is undesirable as relative movement between the various parts is made more difficult and a higher force is required to drive movement of the intermediate part 4 and the moving part 6 relative to the support structure 2. Also, the asymmetry between the force required to move the moving part 6 upwards as compared to downwards is undesirable at least because it makes the whole assembly 1 difficult to control. Various assemblies are provided which overcome these issues and these are now described with reference to Figures 2 to 11D. With reference to Figure 2, an assembly 1 is described. Like reference numerals are used to refer to like parts as compared to the assembly of Figure 1 and the description of those parts will not be repeated. In the following description, the terms intermediate part, moving part and support structure will be used but those parts may otherwise be referred to as follows: the support structure may be referred to as a first part, the moving part may be referred to as a second part, and the intermediate part may be referred to as a third part. The assembly 1 of Figure 2 comprises a first SMA element 8 which is arranged to apply a force to the intermediate part 4 in the first direction Fi. The assembly 1 further comprises a force application arrangement 20 which is configured to apply a force between the support structure 2 and the moving part 6. The assembly 1 further comprises a spring 22 connected between the intermediate part 4 and the support structure 2. The spring 22 is under tension and is arranged to oppose contraction of the first SMA element 8. The spring 22 also partially counteracts the action of the load L on the intermediate part 4 (via the moving part 6), which acts to bias the intermediate part 4 to the left in Figure 2. The first surface 12 of the intermediate part 4 is biased against the second surface 14 of the support structure 2 under the action of the load L. The third surface 16 of the intermediate part 4 is biased against the fourth surface 18 of the moving part 6 also under the action of the load L. The first to fourth surfaces are arranged such that when the first SMA element 8 is unpowered the moving part 6 is retained in position with respect to the support structure 2, due to the friction between the first and second surfaces and between the third and fourth surfaces. The first surface 12 of the intermediate part 4 may be described as a first friction surface. The second surface 14 of the support structure 2 may be described as a second friction surface. The third surface 16 of the intermediate part 4 may be described as a third friction surface. The fourth surface 18 of the moving part 6 may be described as a fourth friction surface. However, the terms first to fourth surfaces will be used in the following description. The moving part 6 is constrained to move along the primary axis P (i.e. up and down in Figure 2). This constraint is provided by a bearing arrangement between the moving part 6 and the support structure 2 (not shown), for example a rolling bearing or a plain bearing. Operation of the assembly 1 will now be described. In order to drive the moving part 6 to move downwards (as seen in Figure 2), the first SMA element 8 is actuated and contracts. The first SMA element is driven to contract by supplying the first SMA element with electrical current which causes resistive heating in the first SMA element. The current is supplied by a power source (not shown). The supply of current may be controlled by a controller (not shown). Contraction of the first SMA element 8 drives the intermediate part 4 to move to the left in Figure 2. The intermediate part 4 also moves downwards (as seen in Figure 2) simultaneously due to the angle of the first and second surfaces 12, 14. The force applied by the first SMA element 8 to the intermediate part 4 reduces the normal force between the first and second surfaces 12, 14 and the normal force between the third and fourth surfaces 16,18 thus also reducing the friction between the intermediate part 4 and the support structure 2 and between the intermediate part 4 and the moving part 6. This allows movement of the intermediate part 4 relative to the support structure 2 and the moving part 6. As the intermediate part 4 moves to the left (and also downwards), the moving part 6 moves downwards under the action of the load L. In order to drive the moving part 6 upwards, the force application arrangement 20 is actuated and applies a force between the moving part 6 and the support structure 2. This causes the moving part 6 to move upwards, as desired, and also reduces the normal force between the first and second surfaces 12, 14 and the normal force between the third and fourth surfaces, 16, 18, thus also reducing the friction between the intermediate part 4 and the support structure 2 and the friction between the intermediate part 4 and the moving part 6. The reduction in friction allows movement of the intermediate part 4 relative to the support structure 2 and the moving part 6 and the intermediate part 4 moves to the right under the action of the spring 22. This movement of the intermediate part 4 to the right means that, as the moving part 6 is driven to move upwards, the first surface 12 and the third surface 16 of the intermediate part 4 are still in contact with the second surface 14 and the fourth surface 18 respectively. This is so that the moving part 6 can be held in position with respect to the support structure 2 by friction between the various surfaces once the SMA element 8 and the force application unit 20 are no longer acting on the various parts of the assembly 1. The force application arrangement 20 may take various forms. These will now be described with reference to Figures 3 to 11D. Like reference numerals are used to refer to like parts and only the differences as compared to the embodiment of Figure 2 will be described here. With reference to Figure 3, the force-application arrangement 20 comprises a second SMA element 10 which is connected between the support structure 2 and the moving part 6. The second SMA element 10 is configured to contract and drive the moving part 6 to move along the primary axis P (i.e. upwards in figure 3). As for the first SMA element 8, the second SMA element 10 is caused to contract by supplying current to the second SMA element 10, which causes resistive heating within the second SMA element 10. The current is supplied by a power source (not shown). The supply of current may be controlled by a controller (not shown). As an alternative, the second SMA element 10 could be replaced with any other suitable actuator, such as a voice coil motor (VCM), piezoelectric actuator or a motor. The assembly 1 comprises a counterbalance arrangement 100 which is configured to cause movement of the intermediate part 4 relative to the support structure 2 when the forceapplication arrangement 20 reduces the friction between the first and second surfaces 12, 14 and the friction between the third and fourth surfaces 16,18. The counterbalance arrangement 100 opposes contraction of the first SMA element 8. The counterbalance arrangement 100 may comprise one or more of: a spring, for example such as spring 22 described with reference to and illustrated in Figure 2, a magnet and an actuator (such as an SMA element, a voice coil motor or a piezoelectric actuator, for example). With reference to Figure 4, a further alternative embodiment of the force-application arrangement 20 is described. Like reference numerals are used to refer to like parts and only the differences as compared to the embodiment of Figure 2 will be described here. In the embodiment of Figure 4, the force-application arrangement 20 comprises a second intermediate part 24, which may otherwise be referred to as a fourth part. The second intermediate part 24 is arranged to move relative to the support structure 2 and the moving part 6. The force-application arrangement 20 also comprises a second SMA element 10 which is arranged to drive movement of the second intermediate part 24 relative to the support structure 2 and which is connected between the support structure 2 and the second intermediate part 24. The moving part 6 is constrained to move along the primary axis P (i.e. up and down in Figure 4). This constraint is provided by a bearing arrangement between the moving part 6 and the support structure 2 (not shown), for example a rolling bearing or a plain bearing. The assembly comprises a counterbalance arrangement in the form of a spring 22 connected between the intermediate part 4 and the support structure. The spring 22 is arranged to oppose contraction of the first SMA element 8. The assembly 1 comprises rolling bearing arrangement 26 which is arranged to guide movement of the second intermediate part 24 relative to the support structure 2. The assembly 1 further comprises a further rolling bearing arrangement 28 which is arranged to guide relative movement between the second intermediate part 24 and the moving part 6. Each rolling bearing arrangement comprises a plurality of rolling elements such as ball bearings between two surfaces which are each at a non-zero, acute angle to the primary axis P. Due to the angled surfaces, as the second intermediate part moves to the right (under the action of the second SMA element 10), the second intermediate part 24 also moves upwards. The moving part 6 is driven upwards relative to the support structure 2 as a result of the movement of the second intermediate part 24 driven by the second SMA element 10. The bearing arrangements 26 and 28 could instead be another type of low-friction bearing such as a flexure bearing or a plain bearing with a low coefficient of friction (and / or a lubricant). Operation of the assembly 1 of Figure 4 will now be described. In order to drive the moving part 6 downwards along the primary axis P, the first SMA element 8 is actuated and contracts. The first SMA element 8 thus applies a force to the intermediate part 4. This force drives the intermediate part 4 to move to the left and also downwards and also reduces the normal force between the first and second surfaces 12, 14 and the normal force between the third and fourth surfaces, 16, 18. This reduction in normal force causes a reduction in friction between the intermediate part 4 and the support structure 2 and a reduction in friction between the intermediate part 4 and the moving part 6. This reduction in friction allows movement of the intermediate part 4 relative to the support structure 2 and the moving part 6. As the intermediate part 4 moves to the left and downwards, the moving part 6 moves downwards, along the primary axis P, under the action of the load L. As the moving part 6 moves downwards, the second intermediate part 24 is driven to the left and downwards due to the angled surfaces which are part of the bearing arrangement 28 between the moving part 6 and the second intermediate part 24. In order to drive the moving part 6 upwards, the second SMA element 10 is actuated and contracts. This causes the second intermediate part 24 to move to the right and also upwards. The movement of the second intermediate part 24 causes the moving part 6 to move upwards relative to the support structure 2. Although contraction of the second SMA element increases a normal force in the rolling bearing arrangements, this does not adversely affect relative movement between the second intermediate part 24 and the support structure 2 and moving part 6 respectively because the rolling bearings 26, 28 are low-friction. As the moving part 6 moves upwards, the normal force (and hence the friction) between the first 12 and second 14 surfaces and also the normal force (and hence the friction) between the third 16 and fourth 18 surfaces is reduced, thus allowing the intermediate part 4 to move to the right under the action of the tension in the spring 22. In the embodiment of Figure 4, the spring 22 could be connected between the intermediate part 4 and the second intermediate part 24 (instead of between the intermediate part 4 and the support structure 2). The spring 22 could also be replaced with another arrangement configured to apply a force to the intermediate part 4. For example, the spring 22 could be replaced with a magnet or another actuator, such as a further SMA element. In the embodiment of Figure 4, the intermediate part 4 and the second intermediate 24 may be arranged with respect to each other in any suitable way. In Figure 4, the second intermediate part 24 is illustrated as being next to and displaced from the intermediate part 4 along a direction perpendicular to the primary axis. However, the second intermediate part could instead be arranged behind or in front of the intermediate part 4 when viewed as in Figure 4, for example. Alternatively, the second intermediate part could be above or below the intermediate part 4 when viewed as in Figure 4 (i.e. stacked along the primary axis). With reference to Figure 5, a further assembly 1 is described. Only the differences between the embodiment of Figure 5 and the embodiment of Figure 4 will be described here. Like reference numerals are used to refer to like parts as compared to the assembly of Figure 4. The embodiment of Figure 5 is arranged and operates in a similar way to the embodiment of Figure 4, except that the direction of the angled surfaces of the intermediate part 4 have been reversed, which necessitates the reversal of the position of the support structure 2 and the moving part 6 with respect to the intermediate part 4. In particular, the support structure 2 is now in contact with the intermediate part 4 on an upper side of the intermediate part 4 (as seen in Figure 5) and the moving part 6 is now in contact with the intermediate part 4 on the lower side of the intermediate part 4. The two portions of the moving part 6 illustrated in figure 5 are rigidly connected together (they are part of the same component) and so move together as one. The two parts of the support structure 2 are also rigidly connected together (i.e. they are unable to move relative to each other). The assembly 1 comprises a counterbalance arrangement in the form of a spring 22 which is connected between the second intermediate part 24 and the support structure 2 and which will be referred to hereinafter as a first spring 22. The first spring 22, acting as a counterbalance arrangement, partially opposes the load L (which acts to drive the second intermediate part 24 to the left in Figure 5). A benefit of the counterbalance arrangement (the first spring 22) being connected between the second intermediate part 24 and the support structure 2, as opposed to being connected between the intermediate part 4 and the support structure 2 as in e.g. Figure 4, is that the bearings between the second intermediate part 24 and each of the support structure 2 and the moving part 6 are low friction. The first spring 22 is therefore driving movement on low-friction bearings and so can more effectively counterbalance the load L because less force is lost to friction as compared to the figure 4 embodiment. The assembly 1 also comprises a second spring 30 which is connected between the intermediate part 4 and the second intermediate part 24 and is under tension. The tension in the second spring 30 loads the bearing arrangements 26 and 28. The tension in the second spring 30 also loads the plain bearings, between the first and second surfaces 12, 14 of the intermediate part 4 and support structure 2 respectively and between the third and fourth surfaces 16, 18 of the intermediate part 4 and moving part 6 respectively. In this embodiment, the first and second surfaces 12,14 are biased together by the second spring 30 and not by the load L. The second spring 30 is configured in such a way that it provides no extra bias against the load L and so the characteristics of the second spring 30 (e.g. spring constant, pre-load distance etc.) can be selected independently and ideally. The assembly 1 further comprises a constraining arrangement 32 configured to limit relative movement between the intermediate part 4 and the second intermediate part 24. The constraining arrangement comprises a rigid bar which comprises a first end portion and a second end portion which extend into the page in Figure 4 and which are arranged so as to engage with the second intermediate part 24 and the intermediate part 4 respectively in the event that they move apart from each other by more than a pre-determined distance. The constraining arrangement 32 may prevent or reduce the risk of the spring 30 overextending and / or the SMA elements 8 and 10 overextending and / or breaking in the event that the assembly 1 is subjected to a large acceleration (e.g. if a device in which the assembly 1 is present is dropped). The constraining arrangement 32 is not necessary, however, and may be omitted. Operation of the assembly 1 will now be described. It is possible to drive the assembly 1 in two modes: a first mode in which the first 12 and second 14 surfaces and also the third 16 and fourth 18 surfaces remain in contact with each other during normal operation of the assembly 1 and a second mode in which at least one of the pair of surfaces disengage from each other. The first mode of operation works as follows. The second spring 30 is stiff enough so that it does not extend (or extends by only a negligible amount) during operation, i.e. on contraction of the first and second SMA elements 8,10. The force applied to the second intermediate part 24 and the intermediate part 4 by the second spring 30 is much greater than the load L which is applied to the moving part 6 (see the two arrows in figure 5). In order to move the moving part 6 upwards along the primary axis P, the first SMA element 8 is actuated and contracts. The force applied by the first SMA element 8 to the intermediate part 4 reduces a normal force and hence the friction between the first and second surfaces 12, 14 and between the third and fourth surfaces 16,18. Contraction of the first SMA element drives the intermediate part 4 to move to the right. At the same time, the second intermediate part 24 also moves to the right because the second intermediate part 24 is connected to the intermediate part 4 by the second spring 30 (which, in this first mode, is stiff enough so as not to be extended by the first SMA element). This movement of the second intermediate part 24 to the right causes the moving part 6 to move upwards along the primary axis P (by virtue of the bearing arrangements 26, 28). Since the bearing arrangement 28 comprises angled surfaces, it will be appreciated that the second intermediate part 24 is also itself displaced along the primary axis P. The moving part 6 is constrained to move along the primary axis P (i.e. up and down in Figure 5). This constraint is provided by a bearing arrangement between the moving part 6 and the support structure 2 (not shown), for example a rolling bearing or a plain bearing. To drive the moving part 6 to move downwards, along the primary axis P, the second SMA element 10 is actuated and contracts. This drives the second intermediate part 24 to move to the left and also downwards. As a result, the moving part 6 moves downwards under the action of the load L. This reduces the normal force between the third and fourth surfaces 16,18 which reduces the friction therebetween. As the second intermediate part 24 moves to the left, the intermediate part 4 also moves to the left due to the presence of the second spring 30. Again, due to the angled surface in bearing arrangement 28 the second intermediate part 24 is also displaced downwards, along the primary axis P. In the second mode of operation, the spring 30 provides a tensional force between the second intermediate part 24 and the intermediate part 4 but the spring is arranged to be weak enough so that the spring can be extended by actuating both the first and second SMA elements 8,10 simultaneously. Where the assembly 1 is to be driven in this second mode, the constraining arrangement 32 is configured to limit the relative movement between the intermediate part 4 and the second intermediate part 24. In a first step, both the first SMA element 8 and the second SMA element 10 are actuated and contract. This causes the intermediate part 4 to move to the right and the second intermediate part 24 to the left (relative to the support structure 2). The intermediate part 4 engages with a right-hand end of the rigid bar (specifically the second end portion) which acts as the constraining arrangement 32 and the second intermediate part 24 engages with a left-hand end of the bar (specifically the first end portion), thus preventing the intermediate part 4 and the second intermediate part 24 from moving any further apart. The movement of the second intermediate part 24 to the left (and downwards) causes the moving part 6 to move downwards under the action of the load L. This movement of the moving part 6 and also the movement of the intermediate part 4 to the right causes the fourth surface 18 of the moving part 6 to disengage from the third surface 16 of the intermediate part 4. Friction between the two surfaces is therefore reduced to zero. Once the third and fourth surfaces have disengaged, the relative amount of contraction of the first and second SMA elements 8, 10 is varied to control the position of the moving part 6 along the primary axis P. In the disengaged state, the intermediate part 4 and the second intermediate part 24 move left and right as one because the constraining arrangement 32 is limiting their relative positions (as long as enough tension in the first and second SMA elements 8,10 is maintained). To cause the moving part 6 to move downwards, the second SMA element 10 is actuated further, causing it to contact further. This drives the second intermediate part 24 and also the intermediate part 4 to the left. Due to the angled surfaces which are part of the bearing arrangement 26 between the second intermediate part 24 and the moving part 6, the moving part 6 moves downwards under the action of the load. To cause the moving part 6 to move upwards, the power to the second SMA element 10 is reduced, and actuation of the first SMA element 8 is increased, causing it to contract further. As a result, the first intermediate part and the second intermediate part are both driven to the right. This causes the blue part to be driven upwards (due to the angled surfaces which are part of the bearing arrangement 26 between the second intermediate part 24 and the moving part 6). To hold the moving part 6 still in a desired position with respect to the support structure 2, the first intermediate part 4 must be re-engaged with the moving part 6 when the moving part 6 is at the desired position. In order to accurately control the position of the moving part 6 during reengagement between the first intermediate part 4 and the moving part 6, the tension in the second SMA element 10 should be balanced with a combination of (a) the force being exerted on the moving part 6 by the load L, (b) the tension in the first spring 22, and (c) the tension the first SMA element 8, carried through the constraining arrangement 32. As the first SMA element cools, the tension in the first SMA element 8 reduces. Once the tension in the first SMA element 8 is less than the tension in the second spring 30, the first intermediate part 4 disengages from (i.e. comes out of contact with) the constraining arrangement 32 under the action of the second spring 30. Thereafter, power to the second SMA element 10 should be controlled so that the tension in the second SMA element 10 is be balanced with the combination of (a) the force being exerted on the moving part 6 by the load L, (b) the tension in the first spring 22, and (c) the tension in the second spring 30. This balance ensures that the moving part is held still with respect to the support structure 2. Once the tension in the first SMA element 8 has reduced enough to allow the first intermediate part 4 to move to the left (as seen in Figure 5), under the action of the second spring 30, and come into contact with the moving part 6 with a high enough normal force, movement of the moving part 6 with respect to the support structure is prevented by friction between the third and fourth surfaces 16,18. Then, the tension in first SMA element 10 may be released (by removing or reducing power to the second SMA element 10 and allowing it to cool). At least some of the embodiments described herein rely on various angled surfaces. The following angles are defined and illustrated in Figure 6: the angle between the primary axis and the angled surface present between the intermediate part 4 and the support structure 2: 0i the angle between the primary axis and the angled surface present between the intermediate part 4 and the moving part 6: 02 the angle between the primary axis and the angled surface present between the second intermediate part 24 and the support structure 2: 03 the angle between the primary axis and the angled surface present between the second intermediate part 24 and the moving part 6: 04. One or more (e.g. all) of these angles may be the same. One or more (or all) of the angles may be different. One or more of the angles may be non-zero, optionally acute and non-zero. One or more of the angles may be 90°. In some embodiments, 0i may be the same as 02 and 03 may be the same as 04 but 0i may be different to 02. Each of the four angles may be between 0 and 60° (e.g. 0 °< angle <60°), optionally between 20° and 50 ° (e.g. 20 °< angle <50°), optionally between 30° and 45° (e.g. 30 °< angle <45°). With reference to Figure 7, an embodiment in which angle 0i is 90° is shown. The embodiment of Figure 7 is equivalent to the embodiment of Figure 2 and operates in the same way, except that the intermediate part 4 moves only left to right and does not have a component of motion along the primary axis P. As compared to the embodiment of Figure 2, the embodiment of Figure 7 provides a smaller amount of movement of the moving part 6 for a given change in length of the first SMA element 8. This may or may not be desirable. It will be appreciated that two or more of angles 04-04 may be 90° but some mechanism which changes the direction of movement is required (for example at least one angled surface or one or more flexures, as will be explained with reference to Figure 9A). A further assembly 1 is described with reference to Figure 8A. The embodiment of Figure 8A is similar to and operates in fundamentally the same way as the embodiment of Figure 5. As compared to the embodiment of figure 5 the embodiment of Figure 8A has the following differences: the directions of the angled surfaces have been reversed, the first and second SMA elements 8,10 overlap along a direction perpendicular to the primary axis, and the second spring 30, arranged between the intermediate part 4 and the second intermediate part 24, is under compression as opposed to tension. The overlap between the first and second SMA elements 8, 10 may facilitate an overall smaller assembly and / or a longer length of SMA wire (as compared to an assembly with no overlap). The first spring 22 is present in the embodiment of figure 8A but is not shown for clarity. The first spring 22 is connected between the second intermediate part 24 and either the support structure 2 or the moving part 6. The first spring is configured to impart a force on the second intermediate part 24 to counteract the load L, i.e. acting to the left in Figure 8A. The moving part 6 is constrained to move along the primary axis P only. Operation of the embodiment of Figure 8A will now be described. To drive the moving part 6 upwards, along the primary axis P, the first SMA element 8 is contracted. This reduces the friction in the plain bearings between the intermediate part 4 and the support structure 2 and between the intermediate part 4 and the moving part 6, thereby allowing movement of the intermediate part 4 relative to the support structure 2 and the moving part 6. Contraction of the first SMA element 8 also drives the intermediate part 4 to the left and upwards. Since the friction between the intermediate part 4 and the support structure 2 and between the intermediate part 4 and the moving part 6 has been reduced, the second intermediate part 24 also moves to the left and upwards under the action of the second spring 30 and the first spring (not illustrated in Figure 8A but described above). In this way, contraction of the first SMA element 8 reduces friction enough so that the first spring and the second spring 30 are able to overcome the load L and any tension in the second SMA element 10 to drive the second intermediate part to the left (and upwards) and hence drive the moving part 6 upwards along the primary axis P. It can be seen that as the intermediate part 4 and second intermediate part 24 move to the left they will also themselves also move upwards due to the angled surface present in the bearing arrangement 28 between the intermediate part 4 and the support structure 2. This movement of the second intermediate part 24 and the intermediate part 4 to the left (and upwards) drives movement of the moving part 6 upwards, due to the angled surfaces in the bearing arrangement 26 between the second intermediate part 24 and the moving part 6. To drive the moving part 6 downwards, the second SMA element 10 is actuated. As a result, the second intermediate part 24 and the intermediate part 4 move to the right and also downwards, due to the angle of the first surface 12 and the second surface 14. As the second intermediate part 24 (along with the intermediate part 4) moves to the right, the moving part 6 is allowed to move downwards under the action of the load L, due to the angle in the bearing arrangement 26 between the second intermediate part 24 and the moving part 6. Figure 8B is an enlarged version of part of Figure 8A. The assembly 1 comprises a constraining arrangement comprising a first endstop surface 34a and a second endstop surface 34b on the intermediate part 4 and a third endstop surface 34c and a fourth endstop surface 34d on the second intermediate part 24. The first endstop surface 32a is arranged to engage with the third endstop surface 34c after a certain amount of relative movement between the intermediate part 4 and the second intermediate part 24, specifically movement towards eachother. Similarly, the second endstop surface 34b is arranged to engage with the fourth endstop surface 34d after the relative movement. The endstop surfaces 34a-d are arranged to prevent further movement of the intermediate part 4 towards the second intermediate part 24. This may prevent damage to (e.g. overextension or even breaking of) the first and second SMA elements 8,10 when the assembly 1 undergoes a sudden acceleration. The endstop surfaces 34a-d also facilitate the driving of the assembly 1 in a second mode, different to the mode described above, which is analogous to the second mode in which the embodiment of Figure 5 may be driven. If the assembly is to be driven in the second mode, the spring 30 should be weak enough so that it can be compressed by actuating both the first 8 and second 10 SMA elements simultaneously. Where the assembly 1 is to be driven in this second mode, the endstop surfaces 34a-d are configured to limit the relative movement between the intermediate part 4 and the second intermediate part 24 as mentioned above. Operation of the assembly 1 in the second mode is as follows. In a first step, both the first SMA element 8 and the second SMA element 10 are actuated and contract simultaneously. This causes the intermediate part 4 to move to the left and the second intermediate part 24 to the right. The intermediate part 4 and the second intermediate part 24 are driven towards each other until the first and third endstop surfaces 34a, 34c engage each other and the second and fourth endstop surfaces 34b, 34d engage each other. The movement of the second intermediate part 24 to the right causes the moving part 6 to move downwards under the action of the load L, as described above. This movement of the moving part 6 downwards causes the fourth surface 18 of the moving part 6 to disengage from the third surface 16 of the intermediate part 4. Friction between the two surfaces is therefore reduced to zero. Once the third and fourth surfaces have disengaged, the amount of relative contraction between the first and second SMA elements 8,10 is varied to control the position of the moving part 6 along the primary axis. In the disengaged state, the intermediate part 4 and the second intermediate part 24 move left and right as one (as long as sufficient tension in the first and second SMA elements 8, 10 is maintained). To cause the moving part 6 to move downwards, the second SMA element 10 is actuated further, causing it to contact further. This drives the second intermediate part 24 and also the intermediate part 4 to the right. Due to the angled surfaces which are part of the bearing arrangement 26 between the second intermediate part 24 and the moving part 6, the moving part 6 moves downwards under the action of the load. To cause the moving part 6 to move upwards, the power to the second SMA element 10 is reduced, and / or the power to the first SMA element 8 is increased. As a result, the intermediate part 4 and the second intermediate part 24 move to the left which drives the moving part 6 upwards. To hold the moving part 6 still in a desired position with respect to the support structure 2, power to the first and second SMA elements 8,10 is controlled in an analogous way to that described above with reference to figure 5. With reference to Figure 8C, a further embodiment is described. The embodiment of Figure 8C has many features is common with the embodiment of Figures 8A and 8B and only the differences will be described here. The embodiment of Figure 8C is arranged and operates in a similar way to the embodiment of Figures 8A and 8B. A key difference is the shape of the first intermediate part 4, where the angle and angle direction of the first 12, second 14, third 16 and fourth 18 surfaces is the same, but the positions of the first 12 and second 14 surfaces have been swapped with the positions of the third 16 and fourth 18 surfaces. This difference in the shape of the first intermediate part means that the moving part 6 can be arranged on a single side of the intermediate parts (i.e. above the intermediate parts 4, 24) and the support structure 2 can also be arranged on a single side of the intermediate parts (i.e. below the intermediate parts 4, 24). This key difference relates to the arrangement of the embodiment Figure 8C, as the angles of all the surfaces are the same relative to each other, the operation of the embodiment of Figure 8C is the same as the operation of the embodiment of Figures 8A and 8B. The embodiments described above use surfaces which are at an acute, non-zero angle to the primary axis to convert movement of the intermediate part 4 along a direction having at least a component which is perpendicular to the primary axis into movement of the moving part 6 along the primary axis. An alternative way of changing the direction of movement is to use flexures arranged between an intermediate part and the moving part. Such an embodiment is illustrated in Figure 9A and will now be described. The embodiment of Figure 9A has many features in common with the embodiment of Figures 8A and 8B and only the differences will be described here. A key difference is that the bearing arrangements between (a) the second intermediate part 24 and the support structure 2 and (b) the second intermediate part 24 and the moving part 6 are flexure bearing arrangements (as opposed to rolling bearing arrangements in the embodiment of Figures 8A and 8B). This difference in bearing arrangement means that the moving part 6 can be arranged on a single side of the intermediate parts (i.e. below the intermediate parts 4, 24) and the support structure 2 can also be arranged on a single side of the intermediate parts (i.e. above the intermediate parts 4, 24). The bearing arrangement 26 between the second intermediate part 24 and the moving part 6 comprises a flexure 36 connected between the second intermediate part 24 and the moving part 6. The flexure 36 is integral with a body portion 40 of the second intermediate part 24 and is pivotally connected to the moving part 6. Similarly, the bearing arrangement 28 between the second intermediate part 24 and the support structure 2 comprises a flexure 38 connected between the second intermediate part 24 and the support structure 2. The flexure 38 is integral with the body portion 40 of the second intermediate part 24 and is pivotally connected to the support structure 2. Operation of the assembly 1 is as follows. To drive the moving part 6 downwards, the second SMA element 10 is actuated. This causes the second intermediate part 24 to move to the right. The intermediate part 4 also moves to the right because the second intermediate part 24 and the intermediate part 4 are coupled to each other via the second spring 30. Movement of the second intermediate part 24 to the right causes the flexures 36 and 38 to straighten, thereby pushing the second intermediate part 24 and also the moving part 6 downwards, away from the support structure 2. This is aided by the load L, which also acts to drive the moving part 6 downwards. This downward movement of the moving part 6 reduces the normal force and hence the friction between the third 16 and fourth 18 surfaces. As the intermediate part 4 is driven towards the right, it also moves downwards due to the angle of the first 12 and second 14 surfaces. To drive the moving part 6 upwards, the first SMA element 8 is actuated which pulls the intermediate part 4 and hence the second intermediate part 24 to the left. This movement reduces the normal forces and also the friction between (a) the first 12 and second 14 surfaces and (b) the third 16 and fourth 18 surfaces, thereby allowing relative movement between the intermediate part 4 and the support structure 2 and moving part 6. As the second intermediate part 24 is driven to the left, the flexures 36 and 38 fold inwards, towards each other, thereby pulling the moving part 6 upwards towards the support structure 2. With reference to Figure 9B, a further embodiment is described. The embodiment of Figure 9B has many features in common with the embodiments of Figure 9A and only the differences will be described here. Key differences include the bearing arrangement 26 between the second intermediate part 24 and the moving part 6 comprises a flexure 36 connected between the second intermediate part 24 and the moving part 6. The flexure 36 is pivotally connected to the moving part 6. Similarly, the bearing arrangement 28 between the second intermediate part 24 and the support structure 2 comprises a first flexure 38a and a second flexure 38b, both connected between the second intermediate part 24 and the support structure 2. The flexures 38a and 38b are pivotally connected to the support structure 2. The flexures 36, 38a, 38b are integral with each other and are formed from a stamped or etched sheet which is attached to the body portion 40 of the second intermediate part 24. The flexures 36, 38a, 38b are formed to maximise the length of each flexure within the footprint of the body portion 40 of the second intermediate part 24. Alternatively, the flexures 36, 38a, 38b may be integral with the body portion 40 of the second intermediate part 24. The connection points of the first flexure 38a and the second flexure 38b at the support structure 2 may be spaced apart along an axis perpendicular to the primary axis P. The connection points of the flexure 36, the first flexure 38a and the second flexure 38 at the moving part 6 and the support structure 2, respectively, may be spaced apart along an axis perpendicular to the primary axis P. The bearing arrangement 26 between the second intermediate part 24 and the moving part 6 and the bearing arrangement 28 between the second intermediate part 24 and the support structure 2 may together comprise three or more flexures, with each of the bearing arrangements comprising least one flexure. The three or more flexures may be arranged such that the second intermediate part 24 is rotationally constrained and does not rotate about an axis perpendicular to the primary axis P. With reference to Figure 10, a further embodiment is described. The embodiment of figure 10 is similar to the embodiment of Figure 5 and only the differences will be described here. Key differences include: both the first and second SMA elements 8,10 apply a force to the intermediate part 4 and second intermediate part 24 respectively in the same direction (to the left, as seen in Figure 10), the moving part 6 is on a single side of the second intermediate part 24 and the intermediate part 4 (above the intermediate parts 4, 24) and the support structure 2 is on a single side of the second intermediate part 24 and the intermediate part 4 (below the intermediate parts 4, 24), the second intermediate part 24 and intermediate part 4 are biased towards each other by a magnetic arrangement 44, specifically a magnet 46 on the intermediate part 4 and a magnet 48 on the second intermediate part 24. A first spring 22 is connected between the support structure 2 and the intermediate part 4. A second spring 42 is connected between the support structure 2 and the second intermediate part 24. The first and second springs 22, 42 act as counterbalance arrangements. Both of the springs counteract the load L, which acts via the moving part 6 to drive the intermediate part 4 towards the left and the second intermediate part towards the right. Operation of the embodiment of Figure 10 will now be described. To drive the moving part 6 downwards, the first SMA element 8 is actuated. This causes the intermediate part 4 to move to the left. This reduces the normal force and hence the friction between the first and second surfaces 12,14 and also between the third and fourth surfaces 16,18. As the intermediate part 4 moves to the left, the moving part 6 moves downwards under the action of the load L .The second intermediate part 24 is driven to the right as the moving part 6 moves downwards under the action of the load L due to the angled surface in the bearing arrangement 26 between the second intermediate part 24 and the moving part 6. The intermediate part 4 and the second intermediate part 24 therefore are driven apart from each other as the moving part 6 moves downwards. To drive the moving part 6 upwards, the second SMA element 10 is actuated, which drives the second intermediate part 24 to the left. This in turn drives movement of the moving part 6 upwards, due to the angled surface present in the bearing arrangement 26 between the moving part 6 and the second intermediate part 24. Movement of the moving part 6 upwards reduces the normal force and hence the friction between the first and second surfaces 12,14 and also between the third and fourth surfaces 16, 18. As the second intermediate part 24 moves to the left and the moving part 6 is driven upwards, the intermediate part 4 is driven to the right (towards the second intermediate part 24) by the magnetic force between the two magnets 46, 48 and also under the action of the first spring 22. A further embodiment will now be described with reference to Figures 11A-C. Figure 11A is a perspective view of the assembly 1. Figure 11B is a view of one side of the assembly, looking along a direction perpendicular to the primary axis. Figure 11C is a view of the other side of the assembly (as compared to that shown in Figure 11B). Figures 11A-C illustrate a system 76 comprising two assemblies: a first assembly la and a second assembly lb. Each assembly corresponds to an assembly equivalent to that illustrated in the previous figures. Like reference numerals are used to refer to like parts where the part in question is common to the two assemblies. Reference numerals with an 'a' suffix refer to the first assembly la and reference numerals with a 'b' suffix refer to the second assembly lb. The first assembly la will now be described. The second assembly lb is structured and operates in the same way. The first assembly la comprises a support structure 2 and a first intermediate part 4. The intermediate part 4 comprises an elongate body portion 50, a first wedge portion 52a and a second wedge portion 54a. The elongate body portion 50, the first wedge portion 52a and the second wedge portion 54a are rigidly connected to each other and move together as one. Figures 11B and 11D are each enlarged views of part of the system 76 (of different sides of the system 76). As shown in Figure 11D, the first wedge portion 52a is in contact with the support structure 2 and comprises the first surface 12. The support structure 2 comprises the second surface 14. Turning to Figure 11B, the second wedge portion 54 a comprises the third surface 16 and the moving part 6a comprises the fourth surface 18. As compared to the embodiment in Figure 5, for example, the two angled surfaces of the intermediate part 4 in the embodiment of Figures 11A-D are now located differently, on their own respective wedge portions 52a, 54a. The interfaces between the first and second surfaces 12, 14 and the third and fourth surfaces 16,18 respectively provide friction for zero-hold-power functionality. The first assembly la also comprises a second intermediate portion 24. The second intermediate part 24 comprises an elongate body portion 56, a third wedge portion 58a and a fourth wedge portion 60a. As indicated in Figure 11B, the assembly 1 comprises a bearing arrangement 26 between the second intermediate part 24 (specifically the third wedge portion 58a) and the moving part 6a. As indicated in Figure 11D, the assembly la further comprises a bearing arrangement 28 between the second intermediate part 24 (specifically the fourth wedge portion 60a) and the support structure 2. The two bearing arrangements 26, 28 are low-friction plain bearings, i.e. plain bearings where the relevant surfaces of the intermediate part 4, second intermediate part 24, moving part 6a and the support structure 2 have a low coefficient of friction. In this way, a level of friction associated with the two bearing arrangements 26, 28 is lower than a level of friction between the first and second surfaces 12, 14 and between the third and fourth surfaces 16, 18. The assembly 1 comprises a first spring 22 which acts as a counterbalance arrangement (see figure 11B). The first spring 22 is connected between the second intermediate part 24 (in this case the elongate body portion 56) and the support structure 2. The first spring 22 acts to oppose contraction of the second SMA element 10 (i.e. the first spring 22 applies a force to the second intermediate part 24 to the right, as seen in in Figure 11B. Alternatively, the spring 22 could be connected to another component of the assembly which is unable to move left to right (in Figure 11B), for example the moving part 6a. The spring 22 is not shown in Figures 11A, 11C and 11D. The intermediate part 4 and the second intermediate part 24 are biased towards each other by flexures 62 which are connected between the intermediate part 4 and the second intermediate part 24. The flexures have effectively the same function as the second spring 30 in Figure 5. Only some of the flexures 62 are labelled in the figures for clarity. Specifically, the flexures 62 are integrally formed with both the intermediate part 4 and the second intermediate part 24. The flexures 62 are arranged so as not to extend along their respective direction of elongations but are arranged to bend in directions perpendicular to their respective directions of elongation so as to allow movement of the intermediate part 4 and second intermediate part 24 relative to the support structure in directions having at least a component which is perpendicular to the primary axis P. In an analogous way to the intermediate part 4, as compared to the embodiment in Figure 5, the two angled surfaces of the second intermediate part 24 in the embodiment of Figures 11A-D are now located differently, on their own respective wedge portions 58a, 60a. The assembly la comprises a first SMA element 8 which is connected between the intermediate part 4 and the support structure 2. The connection of the first SMA element 8 to the support structure 2 is not shown in Figures 11A-D. Specifically, the first SMA element 8 is connected between a first crimp 70 on the intermediate part 4 and the support structure (e.g. to a crimp on the support structure). However, the SMA element may be coupled to the intermediate part 4 and the support structure 2 in any other suitable way. The direction of force applied by the first SMA element 8 to the intermediate part 4 is indicated by arrow 66 in Figures 11A-C. The assembly la further comprises a second SMA element 10 which is connected between the second intermediate part 24 and the support structure 2. The connection of the second SMA element 10 to the support structure 2 is not shown in Figures 11A-D. Specifically, the second SMA element 10 is connected between a second crimp 72 on the second intermediate part 24 and the support structure (e.g. to a crimp on the support structure). However, the second SMA element 10 may be coupled to the second intermediate part 24 and the support structure 2 in any other suitable way. The direction of force applied by the second SMA element 10 to the second intermediate part 24 is indicated by arrow 68 in Figures 11A-C. The moving part 6a is constrained to move along the primary axis P (i.e. up and down in Figure 11B). This constraint is provided by a bearing arrangement between the moving part 6a and the support structure 2 (not shown), for example a rolling bearing or a plain bearing. As mentioned above, the second assembly lb is arranged in an analogous way to the first assembly la. It can be seen that the intermediate part 4 of the first assembly is the same as the intermediate part 4 of the second assembly. They are the same component. Put differently, the respective intermediate parts of the two assemblies are rigidly connected. Corresponding comments also apply to the respective second intermediate parts of the two assemblies. Operation of the embodiment of Figures 11A-D will now be described. The embodiment of Figures 11A-D functions in an analogous way to the embodiment of Figure 5. To drive the moving part 6a upwards, the first SMA element 8 is actuated. The normal force (and hence the friction) between the first and second surfaces 12, 14 (see figure 11D) is thereby reduced. The intermediate part 4 is driven to the left in Figure 11C by contraction of the first SMA element 8 and the second intermediate part is also driven to the left in figure 11C under the action of the flexures 62 and the counterbalance spring 22 (see Figure 11B - movement of the second intermediate part 24 to the left in Figure 11C corresponds to movement of the second intermediate part 24 to the right in Figure 11B). The force of the counterbalance spring 22 is able to drive movement of the second intermediate part 24 once the friction has been reduced between the first and second surfaces 12,14. Accordingly, both the intermediate part 4 and the second intermediate part 24 are driven to the left in Figure 11C (which corresponds to movement of the intermediate part 4 and the second intermediate part 24 to the right in Figure 11B). Due to the engagement of the second intermediate part 24 with the angled surface of the support structure 2 the second intermediate part 24 is also driven upwards as well as to the left in Figure 11C. This movement corresponds to movement upwards and to the right in Figure 11B and it can be seen that this drives movement of the moving part 6a upwards. The movement of the moving part 6a also drives the intermediate part 4 upwards and so the overall movement of the intermediate part 4 is also upwards and to the right in Figure 11B. To drive the moving part 6a downwards, the second SMA element 10 is actuated. The second intermediate part 24 is therefore driven to the left in Figure 11B which allows the moving part 6a to move downwards under the action of the load (indicated by arrow 74 in Figure 11B). This reduces the normal force and hence the friction between the third and fourth surfaces 16,18 (see figure 11B). The intermediate part 4 is also driven to the left (as seen in Figure 11B) by the tension in the flexures 62. This movement corresponds to movement of the intermediate part 4 to the right in Figure 11C and it can be seen that due to the engagement of the intermediate part 4 with the angled surface of the support structure (the second surface 14), the intermediate part 4 is also driven downwards. This causes the moving part 6a and also the second intermediate part 24 to move downwards too. Figures 12A-16B schematically show different variations of an apparatus incorporating one or more assemblies as described herein to drive movement of a moving part. The apparatus is, for example, a camera assembly, a lens assembly, or a display assembly. Generally, the apparatus is to be incorporated in a portable electronic device such as a smartphone or wearable device. Thus, miniaturization can be an important design criterion. The moving part being driven by the one or more assemblies could be any component or part of a component of such an apparatus. Multiple assemblies could be implemented in an apparatus to drive movement of multiple, separate moving parts in the same degree of freedom or multiple, different degrees of freedom. Multiple such assemblies could also be implemented in an apparatus to drive movement of a single moving part in multiple degrees of freedom. Figure 12A shows an apparatus with an assembly 1. The assembly 1 includes a support structure 2 and a movable part 6. The movable part 6 is movable relative to the support structure 2, as shown by the arrow M. When the assembly 1 is included e.g. in the apparatus, the support structure 2 may be fixed relative to the main body of the apparatus. However, in general, the support structure 2 need not be stationary and may be movable relative to or within the apparatus. The assembly of Figure 12A has many features in common with the assembly of Figure 10 and only the differences will be described here. Key differences include: the second intermediate part 24 and the intermediate part 4 are biased towards each other by a second spring 30, which is under tension, and the movable part 6 and the support structure 2 are biased towards each other by a third spring 23, which is under tension, and provides the load indicated by arrow L. Figure 12B shows an apparatus with two assemblies la and lb. The apparatus of Figure 12B has many features in common with the apparatus of Figure 12A and only the differences will be described here. The two assemblies may be arranged to provide movement of the movable part 6 relative to the support structure 2 in the same direction, or in directions perpendicular to each other. For example, the direction of movement Mi is perpendicular to the direction of movement M2. Figure 13A shows an apparatus with two assemblies la and lb. The apparatus of Figure 13A has many features in common with the apparatus of Figure 12A and only the differences will be described here. Key differences include: the first assembly la extending along a first side of the apparatus and the second assembly lb extending along a second, different side of the apparatus, where the first side and the second side are adjacent, the first assembly la is arranged to provide movement of a movable part 6 relative to the support structure 2 in direction Mi, the second assembly lb is arranged to provide movement of a movable part 6 relative to the support structure in direction M2. The direction of movement Mi may be perpendicular to the direction of movement M2, a first spring 45a extending along a third side of the apparatus, opposite the second side of the apparatus, and a second spring 45b extending along a fourth side of the apparatus, opposite the first side of the apparatus, wherein the first spring 45a and the second spring 45b are connected between the support structure 2 and the movable part 6, and The first assembly la, second assembly lb, first spring 45a, and second spring 45a are arranged around the periphery of the apparatus such that the center of the apparatus is unobstructed. For example, for a lens assembly. Figure 13B shows an apparatus with two assemblies la and lb arranged on adjacent sides of an apparatus. The apparatus of Figure 13B has many features in common with the apparatus of Figure 13A, and is arranged and operates in a similar way. The primary axis P can be defined with reference to the assembly 1 and / or the support structure 2. The primary axis P may extend through the assembly 1, e.g. through the centre of the assembly 1. In some examples, the assembly 1, support structure 2 and / or the movable part 6 extend predominantly in a direction perpendicular to the primary axis P. In other words, the extent of the assembly 1, the support structure 2 and / or the movable part 6 along the primary axis P is less than the extent thereof along any direction perpendicular to the primary axis. Alternatively or additionally, the support structure 2 and / or movable part 6 may include a planar component that extends perpendicularly to the primary axis P. Alternatively or additionally, in examples in which the apparatus includes an optical element (such as a lens assembly) with an optical axis, or an imaging element (such as an imager sensor) with an imaging axis, or a display element (such as a display) with a primary orientation axis, the primary axis P may be parallel to such an axis and / or may coincide with such an axis when the movable part is in a central position or orientation. In general, the movable part 6 may be movable relative to the support structure 2 with a single degree of freedom for each assembly 1. In the context of describing the degrees of freedom of movement, the primary axis P may also be referred to as the z axis, and two further axes that are perpendicular to the primary axis P and to each other may be referred to as the x and y axes. Figure 12A shows an apparatus with an assembly 1, Figure 12B shows an apparatus with two assemblies 1, Figure 13A shows an apparatus with two assemblies 1, Figure 13B shows an apparatus with two assemblies 1. The apparatus of any of these embodiments may be, for example, a camera assembly where the movable part 6 is an image sensor and / or a lens assembly or component of a lens assembly. In both of these variations, the assembly 1 may be configured to move the lens assembly relative to the image sensor in any direction in the plane perpendicular to the primary axis P and hence the optical axis O. Such movement has the effect of moving the image on the image sensor and enables optical image stabilisation (OIS) to be implemented in the camera module camera assembly. The camera assembly 1 may be a compact camera assembly in which each lens has a diameter of 20mm or less, for example of 12mm or less. The apparatus of any of these embodiments may alternatively be, for example, a lens assembly where the movable part 6 is a lens or component of a lens, or a display assembly where the movable part 6 is a display, a component of a display and / or a lens or component of a lens. In these variations, the assembly may be configured to move the movable part relative to the rest of the apparatus along the primary axis and hence the optical axis O. Such movement may have the effect of adjusting the focus of the image on the image sensor or display or providing zoom functionality. So, auto-focus (AF) or zoom functionality can be implemented in the apparatus. In some examples (e.g. Figure 12B), the apparatus may include a first assembly for providing OIS and a second assembly for providing AF or zoom. One or both of the first and second assemblies may correspond to the assemblies 1 as described herein. One of the first and second assemblies may be another type of SMA assembly or may be a non-SMA assembly, e.g. a voice-coil motor assembly. Is this AF and OIS variation, three-dimensional translational movement of the movable part 6 relative to the support structure 2 may be enabled by the combination of the assemblies. The assemblies described herein drive movement of a moving part 6. This moving part could be any component or part of a component. Multiple such assemblies could be implemented in a device to drive movement of multiple, separate moving parts. Alternatively, each assembly could drive part of a single component. For example, such a component may be flexible and each assembly may drive movement of a part of that flexible component. Accordingly, a system is disclosed comprising multiple assemblies as described herein. Such a system is illustrated in Figures 11A-D. The system 76 comprises a first assembly la and a second assembly lb. The two moving parts 6a, 6b of the respective assemblies may be separate moving parts of the system 76. Alternatively, the two moving parts 6a, 6b may be integral with (or attached to or merely in contact with) a common component to be driven to move and / or deform. One example of an implementation of such a system is in driving deformation of a deformable optical component. An example of such a deformable optical component is illustrated in Figure 14A. Figure 14A shows a plan view of the deformable optical element 80 which in this case is a liquid lens, however the deformable optical component could be a different type of deformable optical component such as a deformable mirror. Figure 14B shows a cross-sectional view of the deformable optical element 80 in the plane 82'. The liquid lens 80 is suitable for use in glasses and electronic headsets - such as virtual reality or augmented reality headsets. The liquid lens defines an optical axis OA (see Figure 14B). The liquid lens 80 is configured to be deformed to adjust its optical properties, such as its focal length. Typically, the profile of at least one of the front or rear surface of the lens will be made more concave or convex in order to adjust the focal length. A system 72 as described with reference to Figures 11A-D may be used to drive deformation of the liquid lens 80 to adjust the focal length of the liquid lens 80. Equally, any of the assemblies described herein may be used to deform the liquid lens 80. It will be appreciated that three or more assemblies may be part of the system 76. Although the present disclosure primarily discusses adjusting the focal length, it will be understood that the deformation of the deformable lens may instead, or additionally, be for adjusting other properties of the lens. In the example of Figure 14A, the assemblies configured to drive deformation of the liquid lens 80 are located around the circumference of the liquid lens 80 and are in contact with a deformable ring 84. However this disclosure is not limited as such and in other examples the assembly or assemblies may be located elsewhere. Here, the assemblies are located substantially within the outer envelope of the liquid lens 80, however in other examples the assemblies may be located outside of the envelope of the liquid lens 80. The liquid lens 80 of Figure 14A is substantially rectangular when viewed along the optical axis. The assemblies of the present disclosure may be used with a deformable optical component of any shape. Figures 14B and 15 are cross-sections of the liquid lens 80 in the plane 82' shown in Figure 14A. Figure 14B shows the liquid lens 80 in a neutral state. Figure 15 illustrates the deformation of the liquid lens 8. A lower membrane 88 of the liquid lens 8 is schematically shown as moving between a concave and convex arrangement. As the membrane 88 deflects and the liquid lens 8 deforms, the optical properties of the liquid lens 80 are adjusted - for example by adjusting the focal length. In the present arrangement, the upper portion of the liquid lens 80 (specifically surface 86) is glass and thus cannot be deformed, with a lower portion of the liquid lens 80 including the membrane 88 and being deformable. In other arrangements, the opposing (upper) surface 86 of the liquid lens 80 may be deformed or, alternatively, both portions / surfaces of the deformable lens may be deformed. It is understood that the nature and extent of the deformation of Figure 15 is schematic only. In embodiments of the disclosure, deformation of the liquid lens 80 may be less pronounced and the deflection of the membrane 88 may be between two different concave, or convex, profiles. Turning now to Figures 16A and 16B, enlarged schematic views of the liquid lens 80 of Figure 15 are provided. Figures 16A and 16B schematically show two different deformation mechanisms, both of which change the shape of the membrane 88 of the liquid lens 80 and hence deform the liquid lens 80. In Figure 16A, the assemblies are configured to rotate the edge of the membrane 88 about its edge 90. The membrane 88 is rotated about an axis perpendicular to the optical axis OA of the liquid lens 80. In this configuration, the stiffness of the membrane 88 causes deformation of the membrane 88 and liquid lens 80. In Figure 16B, the assemblies are configured to move the membrane 88 parallel to the optical axis OA, thus deforming the membrane 88 and the liquid lens 8 and adjusting the optical properties of the liquid lens 8. In order to drive deformation of the liquid lens 80, multiple assemblies may be positioned around the circumference of the liquid lens 80. Two assemblies are illustrated in Figures 11A-C but it will be appreciated that a greater number of assemblies may be positioned around the circumference of the liquid lens 80 and arranged in contact with a portion of the liquid lens 80 (for example deformable ring 84) to drive deformation of the liquid lens. For example, 10, 20, 50 or 100 assemblies could be positioned around the circumference of the liquid lens. Referring back to Figures 11A-D, it can be seen that the first and second assemblies are different. In particular, the angle of the angled surfaces (i.e. the angle between the angled surface and the primary axis P) present on the moving part 6a and the first and second wedge portions 54a, 56a are different to the angle of the corresponding surfaces in the second assembly lb, i.e. the surfaces between the moving part 6b and the wedge portions of the second assembly lb. This difference in angle means that for a given movement of the intermediate part 4, for example, (which is common to both assemblies la, lb), the moving part 6a will move by a distance along the primary axis which is different to a distance which the moving part 6b will move along the primary axis. In this way, the amount of displacement of the moving part in each assembly can be varied around the circumference of the liquid lens. This may be particularly advantageous when the liquid lens is not circular (when viewed along the optical axis OA). In particular, it may be desirable to drive a greater amount of movement of parts of the liquid lens which are relatively far from the centre of the liquid lens (when viewed along the optical axis OA) and a smaller amount of movement of parts of the liquid lens which are relatively close to the centre of the liquid lens. The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. It will be appreciated that there may be many other variations of the above-described examples. For example, the various options for various bearings (flexure, rolling, plain bearings) can be combined in various different combinations. Also disclosed is the following: 1. An assembly comprising: a first part defining a primary axis; a second part which is movable with respect to the first part; a third part which is movable with respect to the first and second parts and comprises a first friction surface which is biased against a second friction surface with a normal force, wherein the second friction surface is disposed on one of the first and second parts; a first actuator arrangement arranged to drive movement of the third part along a first direction, at least a component of which is perpendicular to the primary axis, wherein the third part is mechanically coupled to the first and second parts such that movement of the third part along the first direction causes movement of the second part relative to the first part along a second direction, different to the first direction, wherein at least a component of the second direction is parallel to the primary axis, wherein the first actuator arrangement is arranged such that actuation of the first actuator arrangement reduces or removes the normal force between the first and second friction surfaces; and a force-application arrangement configured to: apply a force between the second part and the first part, thereby reducing or removing the normal force between the first and second friction surfaces, and cause movement of the second part along a third direction, at least a component of which is parallel to the primary axis and opposite to the at least a component of the second direction. 2. An assembly according to item 1, wherein the assembly is arranged such that the second part is held still with respect to the first part when the first actuator arrangement is unpowered, at least in part due to friction between the first and second friction surfaces. 3. An assembly according to item 1 or 2, wherein one or both of the first and second friction surfaces are at a non-zero, acute angle to the primary axis. 4. An assembly according to any preceding item, wherein the third part comprises a third friction surface which is biased against a fourth friction surface, wherein the second friction surface is on one of the first and second parts and the fourth friction surface is on the other of the first and second parts. 5. An assembly according to item 4, wherein one or both of the third and fourth friction surfaces are at a non-zero, acute angle to the primary axis. 6. An assembly according to item 4 or 5, wherein a combination of (a) friction between the first and second friction surfaces and (b) friction force between the third and fourth friction surfaces is sufficient to retain the second part in position with respect to the first part when the first actuator arrangement is unpowered. 7. An assembly according to any preceding item wherein the force-application arrangement comprises a second actuator arrangement. 8. An assembly according to any preceding item, wherein the force-application arrangement comprises a fourth part which is movable with respect to the first and second parts and which is mechanically coupled to the first and second parts such that movement of the fourth part relative to the first part drives movement of the second part relative to the first part. 9. An assembly according to item 8 when dependent on item 7, wherein the second actuator arrangement is arranged to drive movement of the fourth part relative to the first part. 10. An assembly according to item 8 or 9 comprising a bearing arrangement configured to guide movement of the fourth part relative to the first and second parts, wherein the bearing arrangement has an associated level of mechanical resistance which is lower than a level of mechanical resistance provided by friction between the first and second friction surfaces. 11. An assembly according to item 10, wherein the bearing arrangement comprises a first bearing which guides movement of the fourth part relative to the first part and a second bearing which guides movement of the second part relative to the fourth part. 12. An assembly according to item 11, wherein the first bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and second friction surfaces. 13. An assembly according to item 11 or item 12, wherein the second bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and second friction surfaces. 14. An assembly according to any of items 11 to 13, wherein the first bearing comprises a surface which is at a non-zero, acute angle to the primary axis. 15. An assembly according to any of items 11 to 14, wherein the second bearing comprises a surface which is at a non-zero, acute angle to the primary axis. 16. An assembly according to any of items 8 to 15, wherein the fourth part is mechanically coupled to the third part. 17. An assembly according to any of items 8 to 16 comprising a biasing arrangement which provides a biasing force between the fourth part and the third part. 18. An assembly according to item 17, wherein the biasing arrangement is arranged to bias the fourth part and the third part away from each other. 19. An assembly according to item 17, wherein the biasing arrangement is arranged to bias the fourth part and the third part towards each other. 20. An assembly according to any of items 17 to 19, wherein the biasing arrangement comprises one or more of a resilient element and a magnet. 21. An assembly according to any preceding item, wherein the first friction surface remains in contact with the second friction surface during normal operation of the assembly. 22. An assembly according to any of items 1 to 20, wherein the fourth part is movable relative to the third part and wherein relative movement between the third and fourth parts causes the first friction surface to disengage from the second friction surface. 23. An assembly according to any of items 8 to 22 comprising a constraining arrangement which is configured to prevent relative movement between the third and fourth parts beyond a threshold amount. 24. An assembly according to any of items 1 to 21, wherein the third part is constrained from moving relative to the fourth part. 25. An assembly according to any preceding item comprising a counterbalance arrangement which is configured to cause movement of the third part relative to the first part when the force-application arrangement reduces or removes the normal force between the first and second friction surfaces. 26. An assembly according to item 25, wherein the counterbalance arrangement comprises a resilient element arranged between the first part and the third part and / or a magnet acting between the first part and the third part. 27. A system comprising: a component; and a first assembly according to any preceding item; wherein the system is configured such that movement of the second part of the first assembly relative to the first part of the first assembly drives movement of at least part of the component. 28. A system according to item 27, wherein the component is a deformable optical component, wherein the first assembly is configured such that movement of the second part of the first assembly relative to the first part of the first assembly drives deformation of the deformable optical element. 29. A system according to item 27 or 28 comprising a second assembly according to any of items 1 to 26, wherein the first assembly is arranged to drive movement of a first part of the component and the second assembly is arranged to drive movement of a second part of the component. 30. A system according to item 29, wherein the third part of the first assembly is rigidly connected to the third part of the second assembly. In the items set out above, the first actuator arrangement may comprise one or more of the following: one or more SMA elements, one or more voice coil motors (VCMs), one or more piezoelectric actuators, one or more motors. In the items set out above, the second actuator arrangement may comprise one or more of the following: one or more SMA elements, one or more voice coil motors (VCMs), one or more piezoelectric actuators, one or more motors.

Claims

1. An assembly comprising:a first part defining a primary axis;a second part which is movable with respect to the first part;a third part which is movable with respect to the first and second parts and comprises a first friction surface which is biased against a second friction surface with a normal force, wherein the second friction surface is disposed on one of the first and second parts;a first shape memory alloy, SMA, element arranged to drive movement of the third part along a first direction, at least a component of which is perpendicular to the primary axis, wherein the third part is mechanically coupled to the first and second parts such that movement of the third part along the first direction causes movement of the second part relative to the first part along a second direction, different to the first direction, wherein at least a component of the second direction is parallel to the primary axis, wherein the first SMA element is arranged such that contraction of the first SMA element reduces or removes the normal force between the first and second friction surfaces; anda force-application arrangement configured to:apply a force between the second part and the first part, thereby reducing or removing the normal force between the first and second friction surfaces, andcause movement of the second part along a third direction, at least a component of which is parallel to the primary axis and opposite to the at least a component of the second direction.

2. An assembly according to claim 1, wherein the assembly is arranged such that the second part is retained in position with respect to the first part when the first SMA element is unpowered, at least in part due to friction between the first and second friction surfaces.

3. An assembly according to claim 1 or 2, wherein one or both of the first and second friction surfaces are at a non-zero, acute angle to the primary axis.

4. An assembly according to any preceding claim, wherein the third part comprises a third friction surface which is biased against a fourth friction surface, wherein the secondfriction surface is on one of the first and second parts and the fourth friction surface is on the other of the first and second parts.

5. An assembly according to claim 4, wherein one or both of the third and fourth friction surfaces are at a non-zero, acute angle to the primary axis.

6. An assembly according to claim 4 or 5, wherein a combination of (a) friction between the first and second friction surfaces and (b) friction between the third and fourth friction surfaces is sufficient to retain the second part in position with respect to the first part when the first SMA element is unpowered.

7. An assembly according to any preceding claim wherein the force-application arrangement comprises a second SMA element.

8. An assembly according to any preceding claim, wherein the force-application arrangement comprises a fourth part which is movable with respect to the first and second parts and which is mechanically coupled to the first and second parts such that movement of the fourth part relative to the first part drives movement of the second part relative to the first part.

9. An assembly according to claim 8 when dependent on claim 7, wherein the second SMA element is arranged to drive movement of the fourth part relative to the first part.

10. An assembly according to claim 8 or 9 comprising a bearing arrangement configured to guide movement of the fourth part relative to the first and second parts, wherein the bearing arrangement has an associated level of mechanical resistance which is lower than a level of mechanical resistance provided by friction between the first and second friction surfaces.

11. An assembly according to claim 10, wherein the bearing arrangement comprises a first bearing which guides movement of the fourth part relative to the first part and a second bearing which guides movement of the second part relative to the fourth part.

12. An assembly according to claim 11, wherein the first bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and second friction surfaces.

13. An assembly according to claim 11 or claim 12, wherein the second bearing is one of a flexure bearing, a rolling bearing, a plain bearing comprising a lubricant or a plain bearing having a lower coefficient of friction than a coefficient of friction associated with the first and second friction surfaces.

14. An assembly according to any of claims 11 to 13, wherein the first bearing comprises a surface which is at a non-zero, acute angle to the primary axis.

15. An assembly according to any of claims 11 to 14, wherein the second bearing comprises a surface which is at a non-zero, acute angle to the primary axis.

16. An assembly according to any of claims 8 to 15, wherein the fourth part is mechanically coupled to the third part.

17. An assembly according to any of claims 8 to 16 comprising a biasing arrangement which provides a biasing force between the fourth part and the third part.

18. An assembly according to claim 17, wherein the biasing arrangement is arranged to bias the fourth part and the third part away from each other.

19. An assembly according to claim 17, wherein the biasing arrangement is arranged to bias the fourth part and the third part towards each other.

20. An assembly according to any of claims 17 to 19, wherein the biasing arrangement comprises one or more of a resilient element and a magnet.

21. An assembly according to any preceding claim, wherein the first friction surface remains in contact with the second friction surface during normal operation of the assembly.

22. An assembly according to any of claims 1 to 20, wherein the fourth part is movable relative to the third part and wherein relative movement between the third and fourth parts causes the first friction surface to disengage from the second friction surface.

23. An assembly according to any of claims 8 to 22 comprising a constraining arrangement which is configured to prevent relative movement between the third and fourth parts beyond a threshold amount.

24. An assembly according to any of claims 1 to 21, wherein the third part is constrained from moving relative to the fourth part.

25. An assembly according to any preceding claim configured such that, on contraction of the first SMA element by a first distance, the second part moves along the primary axis by a second distance which is greater than the first distance.

26. An assembly according to any preceding claim comprising a counterbalance arrangement which is configured to cause movement of the third part relative to the first part when the force-application arrangement reduces or removes the normal force between the first and second friction surfaces.

27. An assembly according to claim 26, wherein the counterbalance arrangement comprises a resilient element arranged between the first part and the third part and / or a magnet acting between the first part and the third part.

28. A system comprising:a component; anda first assembly as claimed in any preceding claim;wherein the system is configured such that movement of the second part of the assembly relative to the first part of the assembly drives movement of at least part of the component.

29. A system according to claim 28, wherein the component is a deformable optical component, wherein the first assembly is configured such that movement of the second part of the first assembly relative to the first part of the first assembly drives deformation of the deformable optical element.

30. A system according to claim 28 or 29 comprising a second assembly as claimed in any of claims 1 to 27, wherein the first assembly is arranged to drive movement of a first part of the component and the second assembly is arranged to drive movement of a second part of the component.

31. A system according to claim 30, wherein the third part of the first assembly is rigidly connected to the third part of the second assembly.

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