Actuator assembly

Abstract

An assembly for driving movement of at least part of a component which may be a deformable lens 6 comprises the component6, a first, support, part 10, a second, intermediate, part 12, and a third part

Description

Field The present application relates to assemblies for driving movement of at least part of a component, the component having an outer circumference which extends in a loop around a primary axis. Background Actuators are used for many different purposes and to move many different components. A given actuator may need to fit within a certain space envelope within a device and achieve movement of a component, or a part of a component, along a certain direction. An actuator may need to be designed with certain space restrictions in mind and make use of available space within a device in one dimension but minimise the space occupied by the actuator in another dimension. SMA actuators in particular can be made to be very compact. Space can be particularly constrained in devices such as smartphones, cameras and augmented reality (AR) and virtual reality (VR) devices such as glasses or headsets. It would be desirable to have an actuator capable of moving a component or part of a component along a particular direction which is capable of fitting within various space envelopes. Summary According to a first aspect of the present invention there is provided an assembly for driving movement of at least part of a component. The assembly comprises: the component; a first part; a second part which is movable relative to the first part; a first bearing arrangement supporting movement of the second part relative to the first part; a third part which is movable relative to the first part and to the second part and which is coupled to the component; a second bearing arrangement mechanically coupling the third part to the second part; and an actuator arrangement arranged to drive movement of the second part relative to the first part, wherein the actuator arrangement comprises one or more SMA elements. The component has an outer circumference which extends in a loop around a primary axis and the component defines a radial direction which is perpendicular to the primary axis and extends between the primary axis and the outer circumference of the component. The first and / or second bearing arrangements are configured such that in response to the actuator arrangement displacing the second part along a first direction which is parallel to the radial direction, the third part is displaced, relative to the first part, along a second direction which is parallel to the primary axis, which in turn drives movement of at least part of the component relative to the first part. The at least part of the component may be displaced, relative to the first part, along a direction parallel to the primary axis. There is therefore provided an assembly in which actuation of an intermediate part (the second part referred to above) along a generally radial direction, defined by the component itself, is used to drive movement of the component (or only a part of the component) along a direction parallel to the primary axis. Such movement along a direction parallel to the primary axis will be referred to in the following explanation as vertical movement. Such radial actuation of an intermediate part has a number of benefits compared to other systems which drive vertical movement of a component or part thereof. Firstly, radial-actuation assemblies may be more compact. Secondly, such assemblies may afford more design freedom. By introducing an intermediate part that moves radially in order to drive vertical movement of the component, a possible source of amplification or de-amplification of the movement of the component is introduced. Additional benefits also arise when the component is a deformable optical element such as a liquid lens, as will be explained below. The first part may be described as a support structure. The component may be any type of component. The component may be an optical component, for example a deformable optical component. The component may be generally planar. The radial direction may lie within a plane defined by the component. In some embodiments, the component may move as a whole. For example, the component may be rigid and / or may be driven to move without changing shape. In other embodiments, the component may be deformable and part of the component may be driven to move relative to another part of the component. In some embodiments, the third part, which is coupled to the component, may be attached to or integral with the component. In other embodiments, the third part may not be attached to or integral with the component and may simply be in contact with the component. As mentioned above, the actuator arrangement is arranged to drive movement of the second part relative to the first part and comprises one or more SMA elements. Such SMA elements may be arranged to drive movement of the second part relative to the first part directly. For example, the one or more SMA elements may be attached to or hooked onto the second part. Alternatively, the actuator arrangement may be arranged to drive movement of the second part relative to the first part indirectly, for example via another part (e.g. a second intermediate part) which is movable relative to the first part. Two or more of the SMA elements may be arranged to drive relative movement between the same two parts of the assembly. Put differently, two or more SMA elements may be arranged in mechanical parallel. Such an arrangement may enable a high force to be imparted on the part to be moved. Generally, as mentioned above, the presence of the second part in the assembly provides a possible source of amplification or de-amplification of the movement of the at least part of the component. The first bearing arrangement and / or the second bearing arrangement may be configured such that for a given contraction of the one or more SMA elements, the at least part of the component may be driven to move along the second direction by a distance greater than the amount by which the SMA contracts (amplification) or by a distance less than the amount by which the SMA contracts (de-amplification). Amplification may be useful where it is desired to move the at least part of the component by a large amount (as compared to the contraction of the SMA), whereas de-amplification may be useful where small but precise movements are required. The first and / or second bearing arrangement may be any type of bearing arrangement, for example a flexure bearing arrangement, a rolling bearing arrangement (e.g. comprising one or more rolling elements such as ball bearings) or a plain bearing arrangement. In the case of a plain bearing arrangement, the relevant bearing arrangement may simply be formed of two surfaces of other elements of the assembly (e.g. the first and second parts in the case of the first bearing arrangement) which are in contact with each other. As explained above, the component has an outer circumference which extends in a loop around a primary axis. The loop may be a closed loop. The primary axis may be defined in a number of different ways. For example, the primary axis may be defined with reference to the first part (which may be referred to as a support structure). The first part may comprise a frame located around the periphery of the component 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 first part may form a closed loop around the primary axis. In the case that the component is a deformable optical element and specifically a deformable lens, the movement of the at least part of the component may drive deformation of a surface the 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 defined as being 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 component may be generally planar and / or define a plane and the primary axis may be perpendicular to that plane. In embodiments in which the component 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. Where the component is deformable, the primary axis may be defined as being perpendicular to a plane defined by the average position of a number of points around the edge of the component 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 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 component (the centre being defined, for example, by an outer circumference of the component). It should be noted that any reference herein to a part being displaced along a particular direction should be interpreted to mean that at least a component of the movement of the part is along that direction. In other words, reference to a part being displaced along a direction should not be taken to mean that the part moves along that direction only. Instead, reference to a part being displaced along a direction should be taken to merely require a component of movement along that direction and should not be taken to exclude a component of the movement being along a perpendicular direction. The assembly described above can thus be described using the following alternative wording. Disclosed is an assembly for driving movement of at least part of a component. The assembly comprises: the component; a first part; a second part which is movable relative to the first part; a first bearing arrangement supporting movement of the second part relative to the first part; a third part which is movable relative to the first part and to the second part and which is coupled to the component; a second bearing arrangement mechanically coupling the third part to the second part; and an actuator arrangement arranged to drive movement of the second part relative to the first part, wherein the actuator arrangement comprises one or more SMA elements. The component has an outer circumference which extends in a loop around a primary axis and the component defines a radial direction which is perpendicular to the primary axis and extends between the primary axis and the outer circumference of the component. The first and / or second bearing arrangements are configured such that in response to the actuator arrangement moving the second part along a direction having at least a component which is parallel to the radial direction, the third part is driven to move, relative to the first part, along a direction having at least a component which is parallel to the primary axis, which in turn drives movement of at least part of the component relative to the first part. In particular, the at least part of the component may be driven to move, relative to the first part, along a direction having at least a component which is parallel to the primary axis. In some embodiments, the component is a deformable optical element. The assembly may be configured such that the movement of the at least part of the component alters an optical property of the deformable optical element. The optical property may be an optical magnification or a focal length, for example. A change in optical magnification may be referred to as a change in dioptre of the optical element. Movement of the at least part of the component may alter more than one optical property of the deformable optical element. In particular, movement of the at least part of the component along a direction parallel to the primary axis may alter the optical property or properties of the deformable optical element. Such movement may alter a shape or profile of a deformable surface of the deformable optical element. Where the component is a deformable optical element and in particular a deformable lens, the third part may be a lens stiffener, i.e. a stiff ring that keeps the deformable membrane of the lens under tension. In some embodiments, the deformable optical element is a deformable lens, e.g. a liquid lens. In this case, the primary axis may be aligned with (i.e. colinear with) an optical axis of the deformable lens. Movement of the at least part of the component may cause deformation of a deformable surface (e.g. a deformable membrane) of the deformable lens. This deformation may be caused by pressure of a fluid (e.g. a liquid) within the deformable lens and / or by virtue of the stiffness of the deformable surface. The assemblies described herein are particularly advantageous when the component to be moved is a liquid lens or a component of a liquid lens. For example, actuation of an intermediate part (the second part) along a generally radial direction (as opposed to another direction) may increase the deformation of a deformable surface of the liquid lens. In some embodiments, the deformable lens defines a chamber containing a liquid or gel and the assembly is configured such that the displacement of the second part along the first direction changes a footprint of the chamber when viewed along the primary axis. In other words, movement of the second part along a generally radial direction (defined by the deformable lens) changes a diameter of the liquid (or gel)-containing portion of the deformable lens (when viewed along the optical axis of the lens). This may be advantageous because liquid (or gel) is displaced along the primary axis as a result of the reduction in footprint of the liquid chamber and this may deform or further deform a deformable surface of the lens. In some embodiments, the deformable lens comprises a deformable surface and a further surface opposite the deformable surface and spaced from the deformable surface along the primary axis. The second part may be disposed between the deformable surface and the further surface when viewed along a direction perpendicular to the primary axis. In other words, the second part may be within the deformable lens itself, e.g. between the top and bottom surfaces of the lens. In other embodiments, the second part may be outside of the deformable lens. The further surface may be rigid (i.e. not deformable). For example, the further surface may be made of glass. Alternatively, the further surface may also be deformable. In some embodiments, the deformable optical element may be a deformable mirror. In some embodiments, the assembly may be configured such that in response to the actuator arrangement displacing the second part in the first direction, multiple parts of the component, the multiple parts being at different positions around a periphery of the component, are displaced by different amounts along respective directions which are each parallel to the primary axis. In other words, different parts of the component around the periphery of the component may be displaced by different amounts. This may be useful when the component is non-circular and a greater displacement is required for parts of the component which are further from the centre of the component (as compared to parts which are closer to the centre). This variation in displacement may be achieved by displacement of a single second part and the variation of displacement may be provided by the first and / or second bearing arrangement for example. Alternatively, the assembly may comprise multiple, separate second parts which are each actuated separately (e.g. by separate actuator arrangements) and which each drive displacement of a different part of the component. In this way, the different parts may be driven by separate amounts. The first bearing arrangement, which supports movement of the second part relative to the first part, may take various forms. In some embodiments, the first bearing arrangement comprises one or more lever elements which are each coupled to the first and second parts and which are arranged to displace the second part relative to the first part along a direction parallel to the primary axis when the second part is displaced by the actuator arrangement relative to the first part along the first direction. The one or more lever elements may be arranged to be in tension or compression during use. In some embodiments, the one or more lever elements may be arranged to be in compression when the second part is displaced relative to the first part along the first direction and in tension when the second part is displaced relative to the first part along a direction opposite to the first direction (or vice versa). The one or more lever elements may be coupled to the first and second parts in any suitable way. For example, the one or more lever elements may be attached to or integral with the first and / or second parts. Alternatively, the one or more lever elements may simply be in contact with the first and / or second parts without being attached to or integral with the first and / or second parts. The assembly may comprise multiple lever elements, each arranged between the first and second parts. Such multiple lever elements may be arranged in parallel with each other. In some embodiments, the multiple lever elements may each be attached to a flexible material which is arranged to flex as the second part moves along the first direction. This flexible material may be an outer surface of a liquid lens, for example. In this way, the lever elements may be attached to an outer surface of the liquid lens. Where multiple lever elements are present, the lever elements may be positioned at different points around the circumference of the component. The lever elements at different points may have different lengths. Advantageously, this may facilitate different amounts of movement of different parts of the component. This may be useful where the component is not circular (when viewed along the primary axis) and a greater amount of movement is required for parts of the component which are further from the centre of the component than other parts. If a large number of lever elements are present, the variation in displacement around the periphery of the lens may be near-continuous. In some embodiments, the first bearing arrangement may comprise 10 or more, optionally 20 or more, optionally 50 or more, optionally 100 or more lever elements, for example. The length of a lever element at a given point around the circumference of the component may be inversely proportional to a height of the second part at that point (i.e. a dimension of the second part along a direction parallel to the primary axis). Accordingly, in some embodiments, a height of the second part may vary around the circumference of the component. Generally, the one or more lever elements may also be referred to as flexures or jointed elements. In some embodiments, the first bearing arrangement comprises at least one first angled surface on one of the first part and the second part. The at least one first angled surface is configured to engage with the other of the first part and the second part. The first angled surface may be at a non-zero, acute angle to the first direction. The first angled surface may be at a non-zero, acute angle to the first direction when viewed along a direction which is perpendicular to the first direction and to the primary axis. The angle between the at least one first angled surface and the first direction may be any angle greater than zero and less than 90 degrees. For example, the angle may be 45 degrees or less, optionally 30 degrees or less. Particular examples of angles include 30 degrees, 45 degrees and 60 degrees. One or both of the first and second parts may comprise at least one first angled surfaces. Where both the first and second parts comprise an angled surface, the respective angled surfaces may be parallel to each other. The at least one first angled surface may be arranged such that as the second part moves along the first direction, the second part is displaced along a direction parallel to the primary axis. The first bearing arrangement may comprise multiple angled surfaces at different positions around the circumference of the component. The angled surfaces at different positions may form different angles with the respective radial direction for that particular position. As such, different amounts of movement of different parts of the component around the edge of the component may be achieved. This may be particularly useful if the component is not circular (when viewed along the primary axis). For example, a steeper angle (i.e. a larger angle with respect to the radial direction) may be used for parts of the component which are further from the centre of the component than other parts. The various options for the first bearing arrangement are also applicable to the second bearing arrangement, which mechanically couples the third part to the second part. In some embodiments, the second bearing arrangement comprises one or more lever elements which are each coupled to the second and third parts and which are arranged to displace the third part along the second direction when the second part is displaced by the actuator arrangement along the first direction. The one or more lever elements may be arranged to be in tension or compression during use. In some embodiments, the one or more lever elements may be arranged to be in compression when the second part is displaced relative to the first part along the first direction and in tension when the second part is displaced relative to the first part along a direction opposite to the first direction (or vice versa). The one or more lever elements may be coupled to the second and third parts in any suitable way. For example, the one or more lever elements may be attached to or integral with the second and / or third parts. Alternatively, the one or more lever elements may simply be in contact with the second and / or third parts without being attached to or integral with the second and / or third parts. The assembly may comprise multiple lever elements, each arranged between the second and third parts. Such multiple lever elements may be arranged in parallel with each other. The multiple lever elements may be connected to a flexible material which is arranged to flex as the second part moves along the first direction. This flexible material may be an outer surface of a liquid lens, for example. In this way, the lever elements may be attached to an outer surface of the liquid lens. Where multiple lever elements are present, the lever elements may be positioned at different points around the circumference of the component. The lever elements at different points may have different lengths. Advantageously, this may facilitate different amounts of movement of different parts of the component around the edge of the component. This may be useful where the component is not circular (when viewed along the primary axis) and a greater amount of movement is required for parts of the component which are further from the centre of the component than other parts. If a large number of lever elements are present, the variation in displacement around the periphery of the lens may be near-continuous. In some embodiments, the second bearing arrangement may comprise 10 or more, optionally 20 or more, optionally 50 or more lever elements, for example. The length of the lever element at a given point around the circumference of the component may be inversely proportional to a height of the second part at that point (i.e. a dimension of the second part along a direction parallel to the primary axis). Accordingly, in some embodiments, a height of the second part may vary around the circumference of the component. Where both the first and second bearing arrangements each comprise one or more lever elements, the one or more lever elements of the first bearing arrangement may be arranged to cross over the one or more lever elements of the second bearing arrangement when viewed along a direction perpendicular to the primary axis and to the radial direction. This cross-over may provide a particularly compact arrangement in which longer levers can be used (as compared to a situation in which the lever elements do not cross over each other). Generally, the one or more lever elements may also be referred to as flexures or jointed elements. In some embodiments, the second bearing arrangement comprises at least one second angled surface on one of the second part and the third part which is configured to engage with the other of the second part and the third part. The second angled surface may be at a non-zero, acute angle to the first direction. The second angled surface may be at a non-zero, acute angle to the first direction when viewed along a direction which is perpendicular to the first direction and to the primary axis. One or both of the second and third parts may comprise at least one second angled surfaces. Where both the second and third parts comprise an angled surface, the respective angled surfaces may be parallel to each other. This at least one second angled surface may be arranged such that as the second part moves along the first direction, the third part is displaced along the second direction which is parallel to the primary axis. The angle between the second angled surface and the first direction may be any angle greater than zero and less than 90 degrees. For example, the angle may be 45 degrees or less, optionally 30 degrees or less. Particular examples of angles include 30 degrees, 45 degrees and 60 degrees. The second bearing arrangement may comprise multiple angled surfaces at different positions around the circumference of the component. The angled surfaces at different positions may form different angles with the respective radial direction at that particular position. As such, different amounts of movement of different parts of the component around the edge of the component may be achieved. This may be particularly useful if the component is not circular (when viewed along the primary axis). For example, a steeper angle (i.e. a larger angle with respect to the radial direction axis) may be used for parts of the component which are further from the centre of the component than other parts. The at least one first angled surface may be defined with reference to the first direction in an opposite way to the at least one second angled surface. In other words, one of the first and second angled surface(s) may slope upwards and the other of the first and second angled surface(s) may slope downwards. In some embodiments, the first and / or second bearing arrangements are configured such that the third part is displaced along the second direction by an amount greater than an amount by which the one or more SMA elements contract when powered. In this context, reference to the one or more SMA elements being powered should be taken to mean being supplied with a power sufficient to cause the SMA element(s) to contract. Put differently, there may be an overall amplification effect, whereby the third part is displaced by an amount which is greater than the distance by which the one or more SMA element contracts when actuated. This is advantageous because the amount of contraction of shape memory alloy materials generally is small and so by amplifying (i.e. gearing up) the movement, the third part can be displaced by a relatively large amount. In some embodiments, the first and / or second bearing arrangements are configured such that the third part is displaced along the second direction by an amount which is less than an amount by which the one or more SMA elements contract when powered. Again, reference to the one or more SMA elements being powered here should be taken to mean being supplied with a power sufficient to cause the SMA element(s) to contract. Put differently, there may be an overall de-amplification effect, whereby the third part is displaced by an amount which is less than the distance by which the one or more SMA element contracts when actuated. Such a de-amplification effect may be advantageous where particular precise movements are required since for a given contraction of the one or more SMA elements, the third part will be displaced by an even smaller amount. In some embodiments, the actuator arrangement comprises: a fourth part which is movable relative to the first part and to the second part; a third bearing arrangement supporting movement of the fourth part relative to the first part; and a fourth bearing arrangement mechanically coupling the second part to the fourth part. The one or more SMA elements may be arranged to drive displacement of the fourth part relative to the first part along a third direction which is perpendicular to the radial direction and to the primary axis. Such a direction may be described as a tangential or circumferential direction. The third and / or fourth bearing arrangements may be configured such the displacement of the fourth part relative to the first part along the third direction drives the displacement of the second part relative to the first part along the first direction. In this way, the fourth part may be referred to as a second intermediate part. Accordingly, there is provided an assembly with two intermediate parts: the second part, which drives movement of the third part in a direction parallel to the primary axis, and the fourth part, which drives movement of the second part along the first direction (parallel to the radial direction). The third part is movable relative to fourth part. The fourth part may be driven in a generally circumferential direction (defined by the component), i.e. along the periphery of the component. This movement may equate to rotation of the fourth part about the primary axis. The third and / or fourth bearing arrangements may comprise one or more lever elements, as described above with reference to the first and second bearing arrangements. Alternatively or additionally, the third and / or fourth bearing arrangements may comprise at least one angled surface, as described above with reference to the first and second bearing arrangements. In particular, the fourth bearing arrangement may comprise one or more angled surfaces. The one or more angled surfaces may each be at an acute, non-zero angle with respect to the third direction. As for the first and second bearing arrangements, the third and fourth bearing arrangements may be of the same or different types of bearing arrangement. In some embodiments, the assembly is configured so as to retain the at least part of the component in position with respect to the first part when the one or more SMA elements are unpowered. This may be referred to as 'zero-hold-power' functionality, in which the at least part of the component is held still when the SMA elements are unpowered. Such an arrangement can significantly reduce the power consumption of the assembly because power need only be supplied to the one or more SMA elements to drive movement and not to hold the component in a particular configuration. Reference to the one or more SMA elements being unpowered should be taken to include states in which no power is supplied to the one or more SMA elements and also states in which a very low power (i.e. a power significantly less than an actuating power) is supplied to the one or more SMA elements. In some embodiments, the assembly comprises a first friction surface and a second friction surface. The assembly may be arranged such that the first and second friction surfaces are biased against each other with a normal force so as to generate a friction force therebetween that resists movement of the at least part of the component with respect to the first part. In this way, the at least part of the component can be held still with respect to the first part by friction between the first and second friction surfaces. The first and second friction surfaces may be on various parts of the assembly. Possible combinations are: the first friction surface is on the first part, the second friction surface is on the second part (or vice versa); the first friction surface is on the first part, the second friction surface is on the third part (or vice versa); the first friction surface is on the second part, the second friction surface is on the third part (or vice versa). Where the assembly also comprises a fourth part, as described above, the first friction surface may be a surface of the second part and the second friction surface may be a surface of the fourth part (or vice versa). Alternatively, the first friction surface may be a surface of the fourth part and the second friction surface may be a surface of the first part (or vice versa). In some embodiments, the actuator arrangement is arranged such that the normal force between the first and second friction surfaces remains substantially constant on actuation of the one or more SMA elements. Such an arrangement may be referred to as a constant-friction zero-hold-power arrangement and may be particularly simple to control. In some embodiments, the assembly can achieve zero hold power by relying on the mechanical hysteresis of the one or more SMA elements. That is, the energy dissipated due to internal friction within the system means that there is reduced, or negligible, expansion of the one or more SMA elements and movement of the third part, even after the one or more SMA elements are powered down. In some embodiments, at least one of the one or more SMA elements is arranged along a periphery of of the component when viewed along the primary axis. Where the assembly comprises multiple SMA elements, two or more of the SMA elements may be arranged along a periphery of the component, for example parallel or substantially parallel to one another. Arranging the SMA in this way may facilitate a particularly compact arrangement. Where the component is an optical element, arranging the SMA around the outside of the component may prevent the SMA obscuring the field of vision of a user and / or crossing the path of light impinging on the optical element. In some embodiments, the second part is compliant along a direction which is perpendicular to the primary axis and to the radial direction and rigid along a direction parallel to the primary axis. Put differently, the second part may be compliant in a circumferential direction or in a tangential direction (each being defined by the component) and rigid along a direction parallel to the primary axis. Accordingly, as the second part is driven to move in a generally radial direction, the second part is able to expand when driven radially outwards (to fill a larger arc along the periphery of the component, for example) and collapse when driven radially inwards (to fill a shorter arc). In some embodiments, the second part may have a generally zig-zag or serpentine shape (when viewed along a radial direction). Such a shape may give the second part the desired compliance. In other embodiments, the second part may comprise a series of rigid beams arranged parallel to the primary axis, the beams being joined by a flexible material or membrane. The flexible material may be configured to stretch or unfold when the second part is driven radially outwards, for example. In some embodiments, the component is, when viewed along the primary axis, substantially circular, substantially elliptical or substantially rectangular. In some embodiments, the one or more SMA elements comprises one or more first SMA elements configured to drive displacement of the second part along the first direction and one or more second SMA elements configured to drive displacement of the second part along a fourth direction which is opposite to the first direction. In this way, the assembly may comprise a set of opposing wires. This may afford particularly accurate control over the position of the second part. In other embodiments, the SMA may only be capable of driving displacement of the second part along the first direction and another mechanism may be used to drive displacement of the second part in the opposite direction. This mechanism may comprise a magnet or a resilient element such as a spring, for example. Where the component is a liquid lens, pressure of the liquid in the liquid lens may oppose contraction of the SMA. In some embodiments: the radial direction extends between the primary axis and a first point on the outer circumference of the component; the third part is coupled to a first portion of the component; the second part, the first bearing arrangement, the third part, the second bearing arrangement, and the actuator arrangement define a first actuation unit; and the assembly comprises a second actuation unit. The second actuation unit comprises; a fifth part which is movable relative to the first part; a fifth bearing arrangement supporting movement of the fifth part relative to the first part; a sixth part which is movable relative to the first part and to the fifth part and which is coupled to a second portion of the component, different to the first portion; a sixth bearing arrangement mechanically coupling the sixth part to the fifth part; and a further actuator arrangement arranged to drive movement of the fifth part relative to the first part. The actuator arrangement may comprise one or more further SMA elements. The component defines a further radial direction which is perpendicular to the primary axis and extends between the primary axis and a second point, different to the first point, on the outer circumference of the component. The fifth and / or sixth bearing arrangements are configured such that in response to the further actuator arrangement displacing the fifth part along a fifth direction which is parallel to the further radial direction, the sixth part is displaced, relative to the first part, along a sixth direction which is parallel to the primary axis, which in turn drives movement of the second portion of the component relative to the first part. In this way, there is provided a first actuation unit comprising an intermediate and moving part (the second part and third part respectively) which is driven to move a first part of the component and a second actuation unit comprising a further intermediate part and a further moving part (the fifth part and sixth part respectively) which is driven to move a second, different part of the component. Each actuation unit may be driven separately to achieve different displacements of the third and sixth parts. According to a second aspect of the present invention there is provided a pair of glasses comprising an assembly as described herein. The pair of glasses may otherwise be referred to as a pair of spectacles. According to a third aspect of the present invention there is provided augmented reality device comprising an assembly as described herein. The augmented reality device may be a pair of glasses or a headset, for example. According to a fourth aspect of the present invention there is provided a virtual reality device comprising an assembly as described herein. The augmented reality device may be a headset, for example. Although the actuator arrangements described above comprise one or more SMA elements, different actuator types may be used instead. Accordingly, there is disclosed an assembly for driving movement of at least part of a component. The assembly comprises: the component; a first part; a second part which is movable relative to the first part; a first bearing arrangement supporting movement of the second part relative to the first part; a third part which is movable relative to the first part and to the second part and which is coupled to the component; a second bearing arrangement mechanically coupling the third part to the second part; and an actuator arrangement arranged to drive movement of the second part relative to the first part. The component has an outer circumference which extends in a loop around a primary axis and the component defines a radial direction which is perpendicular to the primary axis and extends between the primary axis and the outer circumference of the component. The first and / or second bearing arrangements are configured such that in response to the actuator arrangement displacing the second part along a first direction which is parallel to the radial direction, the third part is displaced, relative to the first part, along a second direction which is parallel to the primary axis, which in turn drives movement of at least part of the component relative to the first part. Any of the optional features described herein may be applied to such an assembly. The actuator arrangement may comprise one or more voice coil motors (VCMs) and / or one or more piezoelectric actuators, for example. In some embodiments, the assembly may not comprise an actuator arrangement at all and a part of the device may be driven manually by a user. Generally, movement of the at least part of the component may be any type of movement. The movement may be translation or rotation, for example. The component or part of the component may be driven to translate, e.g. along a direction having at least a component parallel to the primary axis. Alternatively, part of the component may be driven to rotate, for example about an axis of rotation which is perpendicular to the primary axis and to the radial direction. Such rotation may be used in embodiments in which the component is a deformable optical element and part of the component is rotated about an axis of rotation which is perpendicular to the primary axis and to the radial direction in order to deform a membrane of the deformable optical element using a stiffness of the membrane. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic, cross-sectional view of an assembly according to the disclosure; Figure 2 is a schematic, cross-sectional view of a liquid lens; Figure 3 is a schematic, plan view of the liquid lens of Figure 2; Figure 4A is a view of part of the assembly of Figure 1; Figure 4B is view of part of a further assembly according to the disclosure; Figure 5 is a view of part of the assembly of Figure 4A; Figure 6A is a cross-sectional, schematic view of an assembly according to the disclosure; Figure 6B is a cross-sectional, schematic view of a further assembly according to the disclosure; Figures 7A, 7B, 7C are schematic views of possible actuator arrangements; Figure 8A is a perspective, cut-away view of a further assembly according to the present disclosure; Figure 8B is a cross-sectional view of part of the assembly of Figure 8A; Figure 8C is a view of a part of the assembly of Figures 8A and 8B projected onto a plane; Figure 8D is a cross-sectional view of part of a further assembly; and Figure 9 is a schematic view of a further assembly according to the present disclosure. Detailed description With reference to Figure 1, an assembly 2 for driving movement of at least part of a component 4 is described. The assembly 2 comprises the component 4 which is a deformable lens, specifically a liquid lens. However, it will be appreciated that the lens 4 could be any form of deformable lens or another component. Figure 1 shows only half of the liquid lens 4 and it will be appreciated that the liquid lens also includes a second half, which is a mirror image of the half shown in Figure 1. The general shape of the liquid lens 4 is illustrated in Figure 2. The liquid lens 4 is configured to be deformed to adjust its optical properties, such as its focal length. Typically, the profile of at least one of the top and bottom surfaces (as seen in Figure 2) of the lens will be made more concave or convex in order to adjust the focal length. In the embodiment shown in Figure 1, the liquid lens 4 comprises a deformable surface 6 made of a flexible material and a rigid surface 8 made of glass. A liquid 4a is contained within the lens 4. The assembly 2 defines a primary axis P which is colinear with the optical axis OA of the lens 4. The assembly 2 further comprises a first part, which will hereafter be referred to as a support structure 10, and a second part, which will hereafter be referred to as an intermediate part 12. The assembly 2 further comprises a first bearing arrangement which is a first lever 14. The lever 14 supports movement of the intermediate part 12 relative to the support structure 10. The assembly 2 further comprises a third part, which will hereafter be referred to as a moving part 16. The moving part 16 is movable relative to the support structure 10 and relative to the intermediate part 12. A second bearing arrangement which is a second lever 18 mechanically couples the moving part 16 to the intermediate part 12. The assembly 2 further comprises an actuator arrangement (not shown in Figure 1) comprising one or more SMA elements and which is arranged to drive movement of the intermediate part 12 relative to the support structure 10 along a first direction F (indicated by an arrow in Figure 1). The first bearing arrangement 14 and the second bearing arrangement 18 are configured such that in response to the actuator arrangement displacing the intermediate part 12 along the first direction F, the moving part 16 is displaced, relative to the support structure 10, along a second direction S which is parallel to the primary axis P, which in turn drives movement of at least part of the component. Figure 3 shows a plan view of the liquid lens 4, i.e. a view of the liquid lens 4 which is shown schematically in Figure 2 looking along the primary axis P. The liquid lens 4 has an outer circumference which extends in a loop around a primary axis. As shown in Figure 3, the outer circumference of the lens 4 is generally rectangular but it may instead take another shape, e.g. a circle, oval or square. The lens 4 defines a radial direction R, which is perpendicular to the primary axis P and which extends between the primary axis P and the outer circumference of the component. An example of a radial direction R is shown in Figure 3. It will be appreciated that the radial direction R may connect the primary axis P and any point on the outer circumference of the lens and the direction shown in Figure 3 is merely an example. The radial direction R is also indicated in Figure 1. The first direction F is parallel to the radial direction R. An enlarged view of part of the assembly 2 is shown in Figure 4A and additional details of the various parts will now be described. As mentioned above, the first bearing arrangement takes the form of a first lever 14. The first lever 14 is connected between the rigid surface 8 of the liquid lens (which is fixed relative to the support structure 10) and the intermediate part 12. In other embodiments the first lever 14 may be connected between the intermediate part 12 and the support structure 10. The second bearing arrangement takes the form of a second lever 18 connected between the intermediate part 12 and the moving part 16. The levers 14,18 may be part of a surface that seals the liquid 4a within the liquid lens. For example, the levers 14,18 may be rigid beams connected to a membrane or other surface that extends around the circumference of the liquid lens 4 and seals the liquid 4a within the lens (in combination with the flexible surface 6 and the rigid surface 8). Such an arrangement is shown in Figure 5. An outer surface 36 of the lens is flexible membrane that seals liquid within the lens. Attached to the surface 36 are a plurality of levers 14a-c which are each connected between the support structure 10 (not shown in Figure 5) and the intermediate part 12. A further plurality of levers 18a-c are attached to the surface and are each connected between the intermediate part 12 and the moving part 16. The first and second levers 14, 18 may be part of such a plurality and further plurality of levers. With reference to Figures 1 and 4A, operation of the assembly 2 will now be described. In use, the actuator arrangement (not shown in Figures 1 and 4A) drives movement of the intermediate part 12 along the first direction F, specifically towards the centre of the lens 4 (i.e. to the right in Figures 1 and 4). The intermediate part 12 thus exerts a tension force on the levers 14 and 18 and the levers pivot, which in turn drives the moving part 16 along the second direction S, specifically downwards in Figures 1 and 4. Referring to Figure 1, it can be seen that if the moving part 16 is moved downwards, the flexible surface 6 of the lens 4 will become more convex due to the pressure of the liquid inside 4a the lens 4. When the actuator arrangement ceases to exert a force on the intermediate part 12 (or reduces the force), the pressure of the liquid 4a will drive the moving part 16 away from the rigid surface 8, along a direction opposite to the second direction S (upwards in Figures 1 and 4), thus causing the flexible surface 6 to become less convex. In this way, contraction of the one or more SMA elements is controlled in order to control the degree of deformation of the flexible membrane 6 and hence the optical properties of the lens 4. In some embodiments, instead of relying on liquid pressure to drive the moving part 16 upwards again, the actuator arrangement may also be configured to drive the intermediate part 12 along a fourth direction which is opposite to the first direction F. In this case, movement of the intermediate part along the fourth direction (to the left in Figures 1 and 4) places the lever arms under compression, which drives the moving part 16 along a direction parallel to the primary axis P and opposite to the second direction S (i.e. upwards in Figures 1 and 4). In such an arrangement, zero-hold-power functionality may be provided by providing sufficient friction between two surfaces which move relative to each other during use. The two surfaces may be biased against each other with a normal force. For example, the two surfaces may be on the moving part 16 and the support structure 10 respectively, e.g. at interface 40 (see figure 1). The friction may be sufficient to hold the moving part 16 still relative to the support structure 10 when the SMA is unpowered. The moving part 16 may have an annular shape, extending along the circumference of the lens 4, e.g. as illustrated in Figure 3, and may be flexible. In some embodiments, multiple actuation units may be present around the outside of the lens. Each actuation unit may comprise its own actuator arrangement, intermediate part 12, and first and second levers 14 and 18. Each actuating unit may drive movement of a part of the moving part 16. For example, eight such actuation units may be present around the lens but other numbers of actuation units are also possible. As mentioned, the moving part 16 may be deformable so that different actuation units may actuate different parts of the moving part 16 by different amounts. Alternatively, in some embodiments, multiple separate moving parts 16 may be present around the outside of the lens, each with its own associated actuation unit. In other embodiments, an assembly 2 comprising a single intermediate part 12, the first and second bearing arrangements 14,18 and a single actuator arrangement may drive movement of a moving part or multiple moving parts around the whole circumference of the component. The intermediate part 12 and the first and second bearing arrangements may take the form illustrated in figure 5, for example. In such a case, the levers 14a-c may be of different lengths and / or the levers 18a-c may be of different lengths. The height H of the intermediate part 12 (see figure 4A and figure 5), i.e. the length of the intermediate part 12 along a direction parallel to the primary axis P, may vary accordingly, with the intermediate part 12 having a larger height at points where the levers are shorter, and a smaller height where the levers are longer. With reference to Figure 4B, an alternative arrangement of the intermediate part 12 and the first and second levers 14,18 is shown. The first and second levers 14, 18 cross over each other when viewed along a direction perpendicular to the primary axis P and perpendicular to the radial direction R. Figure 4B is a view along such a direction. This cross-over may provide a particularly compact arrangement in which longer levers can be used (thereby increasing the amount of movement of the moving part 16 which is possible), as compared to an embodiment in which the levers do not cross (e.g. as shown in Figure 4A). In the embodiment shown in Figure 1, the intermediate part 12 and the first and second levers 14, 18 are disposed within the liquid lens 4. This arrangement is shown schematically in Figure 6A, with the intermediate part 12 and the first and second levers being represented by box 20. With reference to Figure 6B, the intermediate part 12 and the first and second levers 14, 18 may be disposed outside of the liquid lens 4. Specifically, they may be disposed between part of the support structure 10 and the moving part 16. Such an arrangement may be less compact, as compared to the arrangement in Figure 6A, but may be simpler to manufacture given that the intermediate part 12 and levers 14, 18 are separated from the liquid in the liquid lens 4. As mentioned above, the actuator arrangement comprises one or more SMA elements and is arranged to drive movement of the intermediate part 12 relative to the support structure 10. With reference to Figures 7A-C, three example actuator arrangements are illustrated and the first direction F along which the intermediate part 12 is displaced is shown in each case. With reference to Figure 7A, a direct-drive arrangement is shown in which the intermediate part 12 is directly connected to an SMA wire 22. The SMA wire 22 is also connected to the support structure 10. In this embodiment, on contraction of the SMA wire 22, the intermediate part 12 is driven along the first direction F. No opposing SMA wire is present and so some other force is used to drive movement of the intermediate part in the direction opposite to the first direction F. This could be pressure of the liquid in the liquid lens, as described above, a resilient element such as a spring, or a magnet for example. With reference to Figure 7B, an opposing-wire arrangement is shown comprising a first SMA wire 22a, connected between the intermediate part 12 and the support structure 10, and a second SMA wire 22b, also connected between the intermediate part 12 and the support structure 10. The SMA wires 22a, 22b are arranged such that contraction of one wire opposes contraction of the other wire. In this way, the two wires 22a, 22b are arranged to drive the intermediate part 12 in opposite directions. Another example SMA arrangement is illustrated in Figure 7C. The arrangement comprises an SMA wire 22 which is attached at both ends to the support structure 10 and is hooked onto the intermediate part 12. Contraction of the wire 22 causes it to straighten, thus driving movement of the intermediate part 12 along the first direction F. Many other SMA arrangements are possible, including a wire wrapped around the circumference (wholly or partially) of the liquid lens. In this case, on contraction of the SMA wire the diameter of the arc along which the SMA wire lies would reduce, thus exerting a force on the intermediate part along a direction parallel to the radial direction R. With reference to Figure 8A, a further embodiment of an assembly 2 is described. Like reference numerals are used for like parts (as compared to the embodiment of Figure 1) and only the differences between the embodiment of Figure 8A and the embodiment of Figure 1 will be described. Figure 8a is a perspective, cut-away view of the assembly 2. Part of the rigid surface 8 of the lens 4 is shown but the remaining parts of the lens 4 are omitted for clarity. The radial direction R is shown by an arrow. As in the embodiment of Figure 1, the assembly 2 comprises a moving part 16 which in this case is a flexible ring which extends along the circumference of the lens 4. The assembly 2 also comprises an intermediate part, which will be referred to hereafter as a first intermediate part 12. The first bearing arrangement takes the form of a first angled surface 14 which engages with a corresponding angled surface on the support structure 10 (described below with reference to Figure 8B). The second bearing arrangement takes the form of a second angled surface 18 which engages with the moving part 16, in particular with a corresponding angled surface on the moving part. A cross-sectional view of the assembly 2, looking along a circumferential direction, is shown in Figure 8B. Turning back to Figure 8A, the assembly 2 further comprises a second intermediate part 24, which may also be referred to as a fourth part. The second intermediate part 24 is in contact with the first intermediate part 12. A third bearing arrangement (not shown in the figures) supports movement of the second intermediate part 24 relative to the support structure. This may be a plain bearing or rolling bearing, for example. The assembly 2 comprises a plurality of SMA wires 22 which extend in a circumferential direction. The plurality of SMA wires 22 are parallel to each other and are connected between the second intermediate part 24 and the support structure 10 (not shown in Figure 8A). In particular, each of the SMA wires 22 are connected at a first end to the second intermediate part 24 and at a second end to the support structure 10. Accordingly, the SMA wires 22 are arranged to drive movement of the second intermediate part 24 relative to the support structure 10 along a third direction T which is a generally tangential direction. The third direction T is indicated with an arrow in Figure 8A and is perpendicular to the radial direction and to the primary axis P. The third direction T could be a circumferential direction. The second intermediate part 24 comprises a third angled surface 30. The angled surface 30 is at a non-zero, acute angle to the third direction T. The first intermediate part 12 is engaged with the third angled surface 30. In this sense, the third angled surface 30 acts as a fourth bearing arrangement which mechanically couples the first intermediate part 12 to the second intermediate part 24. In use, the SMA wires 22 are actuated and drive movement of the second intermediate part 24 along the third direction T. Due to the engagement of the third angled surface 30 with the first intermediate part 12, movement of the second intermediate part 24 along the third direction drives movement of the first intermediate part 12 along the first direction F. This in turn drives movement of the moving part 16 along a direction parallel to the primary axis P (upwards, as seen in Figures 8A and 8B). The moving part 16 may be restricted to move along a direction parallel to the primary axis only and movement in other degrees of freedom may be restricted by a bearing arrangement, for example. The engagement between the various parts can be seen in Figure 8B. Movement of the second intermediate part 24 along the third direction (which is into the page in Figure 8B), drives movement of the first intermediate part 12 along the first direction F (due to the angled surface 30 - see Figure 8A). The angled surface 18 causes the first intermediate part itself to be driven along the second direction S. The movement of the first intermediate part 12 drives movement of the moving part 16 along the second direction S due to the angled surface 18 but also due to the movement of the first intermediate part along the second direction S. The angled surface 30 of the second intermediate part is biased against the first intermediate part 24. This biasing could be provided by a resilient element such as a spring, for example, or by pressure of the liquid in the liquid lens. This biasing causes friction between the second intermediate part 24 and the first intermediate part 12. The surfaces of the first and second intermediate parts may be arranged such that the friction between them is sufficient to hold the second intermediate part 24 still with respect to the first intermediate part 12 when the SMA wires 22 are unpowered. When there is no relative movement between the first and second intermediate parts, the other components of the assembly also do not move and hence the whole assembly 2 is held in a particular configuration. Zero-hold-power functionality is thereby provided. This may reduce the power consumption of the assembly 2 because it is not necessary to power the SMA wires 22 in order to hold the assembly 2 in a particular configuration. It will be appreciated that one of the first and second angled surfaces 14 and 18 need not be present. For example, the first angled surface 14 could be flat, i.e. perpendicular to the primary axis P. In this case, the first intermediate part would not itself move along the second direction but only along the first direction F. Such an arrangement is shown in Figure 8D. In another variation of the embodiment of Figure 8A, the SMA wires 22 may instead be connected between the support structure 10 and the first intermediate part 12. The second intermediate part may be fixed relative to the support structure 10. On actuation, the SMA wires 22 drive the first intermediate part 12 along the third direction T and the presence of the angled surface 30 causes the first intermediate part 12 to also move along the first direction F. The movement of the first intermediate part 12 along the first direction F would then drive the moving part 16 along the second direction S, as described above. As another alternative to the SMA wires 22 being connected between the support structure 10 and the second intermediate part 24, in some embodiments no SMA wires 22 are present and the second intermediate part 24 could instead be actuated manually by a user, e.g. by rotating the second intermediate part 24 about the primary axis P. Since the first intermediate part 12 moves along the first direction (parallel to the radial direction), the circumferential extent of the first intermediate part 12 changes, depending on its position along the first direction. In a first position, far from the centre of the lens, the first intermediate part 12 spans an arc of a first length. In a second position, closer to the centre of the lens than the first position, the first intermediate part spans an arc of a second length, shorter than the first length. Accordingly, the first intermediate part 12 is preferably compliant along a circumferential direction. This compliance is provided by a zig-zag shape of the first intermediate part 12. Figure 8C shows the shape of the first intermediate part 12 projected onto a plane extending along the circumference of the lens 4. The x-axis 32 in Figure 8C is the circumferential direction the y-axis 34 is a direction parallel to the primary axis P. The embodiment of figures 8A-C makes use of angled surfaces to drive movement of the various parts of the assembly 2. The embodiment of Figure 1 makes use of levers to drive movement of the moving part 16. These features may be combined into an embodiment which uses a combination of levers and angled surfaces to drive movement of the various parts. Such an embodiment is described with reference to Figure 9. The embodiment of Figure 9 is a modified version of the embodiment of Figure 1. Like reference numerals are used and only the differences between the embodiments of Figures 9 and 1 are described. The intermediate part 12 is a first intermediate part 12 configured as described with reference to Figure 5. The assembly 2 comprises a second intermediate part 24 which operates in the way described with reference to Figure 8A. The second intermediate part 24 comprises an angled surface which engages with the first intermediate part 12. The assembly 2 comprises one or more SMA wires (not shown in Figure 9) arranged to drive the second intermediate part 24 into the page. The engagement of the angled surface 30 of the second intermediate part 24 (which is illustrated in figure 8A) drives the first intermediate part 12 along the first direction F, which in turn causes the levers 14 and 18 to pull the moving part 16 downwards (see Figure 9). As described with reference to Figure 1, pressure of the liquid in the lens 4 is used to drive the moving part 16 upwards again when desired. 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. The liquid lens 4 described above in various embodiments comprises a flexible surface 6 and a rigid surface 8. It will be appreciated that the lens 4 may instead comprise two flexible surfaces which can each be deformed. In such a case, movement of the intermediate part 12 may change the separation between two moving parts (one coupled to a first flexible surface and another coupled to a second flexible surface), as opposed to only moving the moving part 16 relative to the support structure (and hence the rigid surface 8). Generally, various angled surfaces may also be disposed on different parts of the assembly. For example, in the embodiment of Figure 8A, the angled surface 30 may be present on the first intermediate part 12 instead of the second intermediate part 24.

Claims

1. An assembly for driving movement of at least part of a component, the assembly comprising:the component;a first part;a second part which is movable relative to the first part;a first bearing arrangement supporting movement of the second part relative to the first part;a third part which is movable relative to the first part and to the second part and which is coupled to the component;a second bearing arrangement mechanically coupling the third part to the second part; andan actuator arrangement arranged to drive movement of the second part relative to the first part, wherein the actuator arrangement comprises one or more SMA elements;wherein the component has an outer circumference which extends in a loop around a primary axis and wherein the component defines a radial direction which is perpendicular to the primary axis and extends between the primary axis and the outer circumference of the component; andwherein the first and / or second bearing arrangements are configured such that in response to the actuator arrangement displacing the second part along a first direction which is parallel to the radial direction, the third part is displaced, relative to the first part, along a second direction which is parallel to the primary axis, which in turn drives movement of at least part of the component relative to the first part.

2. An assembly according to claim 1 wherein the component is a deformable optical element and wherein the assembly is configured such that the movement of the at least part of the component alters an optical property of the deformable optical element.

3. An assembly according to claim 2, wherein the deformable optical element is a deformable lens.

4. An assembly according to claim 3, wherein the deformable lens defines a chamber containing a liquid or gel and wherein the assembly is configured such that the displacement of the second part along the first direction changes a footprint of the chamber when viewed along the primary axis.

5. An assembly according to claim 3 or claim 4, wherein the deformable lens comprises a deformable surface and a further surface opposite the deformable surface and spaced from the deformable surface along the primary axis and wherein the second part is disposed between the deformable surface and the further surface when viewed along a direction perpendicular to the primary axis.

6. An assembly according to any preceding claim configured such that in response to the actuator arrangement displacing the second part in the first direction, multiple parts of the component, the multiple parts being at different positions around a periphery of the component, are displaced by different amounts along respective directions which are each parallel to the primary axis.

7. An assembly according to any preceding claim wherein the first bearing arrangement comprises one or more lever elements which are each coupled to the first and second parts and which are arranged to displace the second part relative to the first part along the second direction when the second part is displaced by the actuator arrangement relative to the first part along the first direction.

8. An assembly according to any of claims 1 to 6, wherein the first bearing arrangement comprises at least one first angled surface on one of the first part and the second part which is configured to engage with the other of the first part and the second part, wherein the first angled surface is at a non-zero, acute angle to the first direction when viewed along a direction which is perpendicular to the first direction and to the primary axis.

9. An assembly according to any preceding claim wherein the second bearing arrangement comprises one or more lever elements which are each coupled to the second and third parts and which are arranged to displace the third part along the second direction when the second part is displaced by the actuator arrangement along the first direction.

10. An assembly according to any of claims 1 to 8 wherein the second bearing arrangement comprises at least one second angled surface on one of the second part and the third part which is configured to engage with the other of the second part and the third part, wherein the second angled surface is at a non-zero, acute angle to the first direction.

11. An assembly according to any preceding claim, wherein the first and / or second bearing arrangements are configured such that the third part is displaced along the second direction by an amount greater than an amount by which the one or more SMA elements contract when powered.

12. An assembly according to any preceding claim, wherein the actuator arrangement comprises:a fourth part which is movable relative to the first part and to the second part;a third bearing arrangement supporting movement of the fourth part relative to the first part; anda fourth bearing arrangement mechanically coupling the second part to the fourth part;wherein the one or more SMA elements are arranged to drive displacement of the fourth part relative to the first part along a third direction which is perpendicular to the radial direction and to the primary axis, andwherein the third and / or fourth bearing arrangements are configured such the displacement of the fourth part relative to the first part along the third direction drives the displacement of the second part relative to the first part along the first direction.

13. An assembly according to any preceding claim configured so as to retain the at least part of the component in position with respect to the first part when the one or more SMA elements are unpowered.

14. An assembly according to claim 13 comprising a first friction surface and a second friction surface, wherein the assembly is arranged such that the first and second friction surfaces are biased against each other with a normal force so as to generate a friction force therebetween that resists movement of the at least part of the component with respect to the first part.

15. An assembly according to claim 13 or claim 14 wherein the actuator arrangement is arranged such that the normal force between the first and second friction surfaces remains substantially constant on actuation of the one or more SMA elements.

16. An assembly according to any preceding claim wherein at least one of the one or more SMA elements is arranged along a periphery of of the component when viewed along the primary axis.

17. An assembly according to any preceding claim wherein the second part is compliant along a circumferential direction, defined by the component, and rigid along a direction parallel to the primary axis.

18. An assembly according to any preceding claim, wherein the component is, when viewed along the primary axis, substantially circular, substantially elliptical or substantially rectangular.

19. An assembly according to any preceding claim, wherein the one or more SMA elements comprises one or more first SMA elements configured to drive displacement of the second part along the first direction and one or more second SMA elements configured to drive displacement of the second part along a fourth direction which is opposite to the first direction.

20. An assembly according to any preceding claim, wherein:the radial direction extends between the primary axis and a first point on the outer circumference of the component;the third part is coupled to a first portion of the component;the second part, the first bearing arrangement, the third part, the second bearing arrangement, and the actuator arrangement define a first actuation unit; andthe assembly comprises a second actuation unit which comprises;a fifth part which is movable relative to the first part;a fifth bearing arrangement supporting movement of the fifth part relative to the first part;a sixth part which is movable relative to the first part and to the fifth part and which is coupled to a second portion of the component, different to the first portion;a sixth bearing arrangement mechanically coupling the sixth part to the fifth part; anda further actuator arrangement arranged to drive movement of the fifth part relative to the first part, wherein the actuator arrangement comprises one or more further SMA elements;wherein the component defines a further radial direction which is perpendicular to the primary axis and extends between the primary axis and a second point, different to the first point, on the outer circumference of the component; andwherein the fifth and / or sixth bearing arrangements are configured such that in response to the further actuator arrangement displacing the fifth part along a fifth direction which is parallel to the further radial direction, the sixth part is displaced, relative to the first part, along a sixth direction which is parallel to the primary axis, which in turn drives movement of the second portion of the component relative to the first part.

21. A pair of glasses comprising an assembly according to any preceding claim.

22. An augmented reality device comprising an assembly according to any of claims 1 to 20.

23. A virtual reality device comprising an assembly according to any of claims 1 to 20.

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