Actuator assembly

Abstract

An actuator assembly comprising: a support structure (2) defining a primary axis (P); a first part (10) that is movable relative to the support structure; a helical bearing arrangement (40) arranged to effect movement of the first part with respect to the support structure along the primary axis; and one or more actuating units (80) each configured to apply an actuating force capable of driving rotation of a rotatable part of the actuator assembly about the primary axis which the helical bearing arrangement converts into said movement of the first part along the primary axis. Each actuating unit comprises: a body portion (81); a shape memory alloy, SMA, element (84) connected between the body portion and the support structure, and configured, on actuation, to apply an input force to the body portion; and a force-modifying element (82) connected between the body portion and the support structure and configured to modify the input force so as to give rise to the actuating force.

Description

[0001] ACTUATOR ASSEMBLY

[0002] Field

[0003] The present application relates to an actuator assembly.

[0004] Background

[0005] It is known to use an actuator, for example comprising a shape memory alloy, SMA, element, to drive translational movement of a movable element with respect to a support structure. SMA has particular advantages in miniature devices and may be applied in a variety of devices including handheld devices, such as cameras and mobile phones. Such SMA elements may be used for example in an optical device such as a camera for driving translational movement of a camera lens element along its optical axis, for example to effect focussing (autofocus, AF), zoom or athermalisation (moving a lens to account for thermal variations in a device).

[0006] Some examples of SMA actuation apparatuses which are cameras of this type are disclosed in WO2019 / 243849 Al. In particular, SMA apparatuses comprising a helical bearing arrangement that converts rotation around a helical axis into helical movement are disclosed. Further examples of SMA actuation apparatuses comprising helical bearing arrangements are disclosed in WO2021 / 209767A1. In these further examples, the lens translates along the helical axis without rotation.

[0007] It is desirable to control the amount by which a movable part moves along an axis. In some cases it is desirable to move the movable part by a large distance along the axis. In other cases it may be desirable to very precisely control the position of the movable part along the axis and / or move the movable part by a very small distance. It may also be desirable to reduce or minimise the power consumption of an actuator assembly.

[0008] Summary

[0009] According to an aspect of the present invention, there is provided an actuator assembly comprising: a support structure defining a primary axis; a first part that is movable relative to the support structure; a helical bearing arrangement arranged to effect movement of the first part with respect to the support structure along the primary axis; and one or more actuating units each configured to apply an actuating force capable of driving rotation of a rotatable part of the actuator assembly about the primary axis which the helical bearing arrangement converts into said movement of the first part along the primary axis.

[0010] Each actuating unit comprises: a body portion;

[0011] - a shape memory alloy, SMA, element connected between the body portion and the support structure (or the rotatable part), and configured, on actuation, to apply an input force to the body portion; and

[0012] - a force-modifying element connected between the body portion and the support structure (or the rotatable part), and configured to modify the input force so as to give rise to the actuating force.

[0013] In this way, one or more actuating units are arranged to drive rotation of a rotatable part of the actuator assembly which a helical bearing arrangement converts into movement of a first part along a primary axis. The rotatable part may be the first part itself or may be another part of the actuator assembly, as will be explained further below.

[0014] The actuating units provide control over the force (magnitude and / or direction) applied to the rotatable part and / or the movement of the first part along the primary axis. In particular, the dimensions and relative positions of the various parts of each actuating unit may be selected to control the movement and / or force due to contraction of the SMA element(s). For example, the movement of the first part may be amplified, i.e. the first part may move along the primary axis by a distance which is greater than an amount by which the one or more SMA elements of the one or more actuating units contract. Alternatively, the movement of the first part may be de-amplified, i.e. the first part moves along the primary axis by a distance which is less than an amount by which the one or more SMA elements of the one or more actuating units contract. The input force may also be amplified or de-amplified to give rise to the actuating force. Additionally or alternatively the direction of the input force may be changed to give rise to the actuating force.

[0015] As mentioned above, the helical bearing arrangement is arranged to effect movement of the first part with respect to the support structure along the primary axis. The helical bearing arrangement guides relative helical motion between two parts of the actuator assembly, e.g. the first part and the support structure or the first part and the rotatable part (in embodiments in which the first part is different to the rotatable part). One part may rotate and the other may move along the primary axis (without rotating) relative to the support structure. Alternatively, one part may both rotate and translate along the primary (i.e. move helically) with respect to the support structure.

[0016] The helical bearing arrangement may comprise any suitable arrangement. In some embodiments, the helical bearing arrangement comprises one or more flexures arranged to guide movement of the first part along the primary axis on rotation of the rotatable part relative to the support structure.

[0017] In some embodiments, the helical bearing arrangement may comprise one or more bearing surfaces. The one or more bearing surfaces may extend helically around the primary axis, that is following a line that is helical. That said, in practical embodiments, the length of the one or more bearing surfaces may be short compared to the distance of the bearing surfaces from the primary axis, such that their shape is close to straight or even each being straight, provided that the one or more helical bearings of the helical bearing arrangement guide helical movement of one part relative to another. Plural helical bearings are typically present, located at different angular positions around the primary axis, in which case the helical bearings have different orientations so that they cooperate and maintain adequate constraints to guide the helical movement of one part relative to another, even if the one or more bearing surfaces of an individual helical bearing are straight.

[0018] In some embodiments, the one or more bearing surfaces may be at an angle to a plane perpendicular to the primary axis which is non-zero and less than 45 degrees. In some embodiments, the angle may be non-zero and less than 40 degrees. In some embodiments, the angle may be non-zero and less than 30 degrees. Such a shallow bearing angle may have a number of advantages. Firstly, such a shallow angle may reduce the risk of the rotatable part tilting, i.e. rotating about an axis perpendicular to the primary axis. It may be preferable to keep the rotatable part in a stable position and avoid movement of the rotatable part in degrees of freedom other than the intended helical movement. Secondly, a shallow bearing angle may have particular advantages when plain bearings are used, particularly when there is sufficient friction in the plain bearings to hold the first part still relative to the support structure when the SMA element(s) of the one or more actuating units are unpowered, as will be explained below.

[0019] A shallow-angle bearing may also be described in other terms. The helical bearing arrangement may comprise a surface which defines an amount by which a part moving along the surface moves along the primary axis for a given distance moved perpendicular to the primary axis. That surface may be at a nonzero angle of less than 45° to a plane perpendicular to the primary axis. In some embodiments the one or more bearing surfaces may be at an acute angle of greater than 45° to a plane perpendicular to the primary axis. Generally, the angle may also be measured with reference to the direction of the actuating force(s).

[0020] In some embodiments, the helical bearing arrangement comprises three helical bearings. By providing three helical bearings, the movement of the first part may be more reliably constrained to movement along the primary axis.

[0021] Taking a helical bearing arrangement between the support structure and the first part as an example, optionally the bearing surfaces of a first and a second helical bearing each comprises grooves on each of the support structure and the first part, and the bearing surfaces of a third helical bearing comprises a groove on one of the support structure and the first part and a planar surface on the other of the support structure and the first part. By providing grooves, the helical bearing may constrain movement of the first part in two degrees of freedom. This may reduce the number of helical bearings required. The three helical bearings may thus allow only a single degree of freedom of movement of the first part relative to the support structure, in particular only movement along a helical path. It will be appreciated that the helical bearings could instead by provided between two different parts of the actuator assembly (for example the first part and the third part) as opposed to between the first part and the support structure, and that the above features may be applied equally to a helical bearing arrangement between any two parts of the actuator assembly.

[0022] In some embodiments, the one or more actuating units are arranged to drive movement of a first surface across a second surface. In other words, application of the actuating force(s) to the rotatable part drives, either directly or indirectly, movement of a first surface across a second surface.

[0023] The actuator assembly may be arranged such that the first surface and the second surface are biased together with a normal force, thereby generating a frictional force therebetween to retain the first part in position relative to the support structure when the one or more SMA elements are unpowered. Accordingly, the first part can be held still relative to the support structure by friction when the one or more SMA elements are unpowered. The overall power consumption of the actuator assembly may be reduced as compared to a situation in which the SMA element(s) must be continuously powered to hold the first part in a particular position with respect to the support structure. Such an arrangement may be referred to as a zero-hold-power arrangement. Zero-hold-power arrangements may be particularly advantageous when the actuator assembly is embodied on a portable device, for example a wearable device. The first and second surfaces may be part of a plain bearing arrangement, e.g. as part of the helical bearing arrangement.

[0024] In some embodiments, the actuator assembly is arranged such that the coefficient of static friction between the first surface and the second surface is between 0.01 and 0.6, preferably between 0.05 and 0.4.

[0025] In some embodiments, the actuator assembly may be arranged such that the actuating force(s) applied by the one or more actuating units to the rotatable part reduce the normal force between the first surface and the second surface. In this way, on actuation of the first and second actuating units, the normal force and hence the friction between the first surface and the second surface is reduced. Therefore, the friction is reduced while rotation of the first part about the primary axis is driven by the one or more actuating units. In other embodiments, the normal force between the first part and the first surface remains substantially constant on actuation of the one or more actuating units, as will be explained below.

[0026] In some embodiments the one or more actuating units comprises: a first actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in a first sense about the primary axis relative to the support structure; and a second actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in a second sense about the primary axis relative to the support structure, wherein the first sense is opposite to the second sense.

[0027] The first and second actuating units may be arranged to apply a torque about an axis perpendicular to the primary axis so as to reduce the normal force between the first surface and the second surface. Such a torque may be referred to as an unloading torque. Further detail on such an arrangement may be found in WO2024 / 074841A1, which is incorporated herein by reference.

[0028] In some embodiments the one or more actuating units comprises: a third actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the first sense about the primary axis relative to the support structure; and a fourth actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the second sense about the primary axis relative to the support structure.

[0029] As mentioned above, in some embodiments, the normal force between the first part and the first surface remains substantially constant on actuation of the one or more actuating units. Such an arrangement may be simpler and / or may be easier to control as compared to an arrangement in which the one or more actuating units reduce the normal force on actuation.

[0030] The normal force between the first part and the second part may be provided by any suitable means. For example, gravity may be used to bias the first and second surfaces together.

[0031] In some embodiments, the actuator assembly comprises a biasing arrangement arranged to bias the first part and the first surface together. Such a biasing arrangement may otherwise be described as loading arrangement. The biasing arrangement may comprise one or more resilient elements, such as springs, and / or one or more magnets. This biasing may provide or contribute towards the normal force between the first and second surfaces.

[0032] In some embodiments the rotatable part may be the first part. In this case, the one or more actuating units are each configured to apply an actuating force capable of driving rotation of the first part around the primary axis which the helical bearing arrangement converts into said movement of the first part along the primary axis. Accordingly, the first part is driven to move along a helical path.

[0033] In some embodiments the actuator assembly may comprise a third part which is the rotatable part. In other words, a third part which is separate from the first part is the rotatable part.

[0034] The third part may be movable relative to the support structure and relative to the first part. The helical bearing arrangement may support the first part on the third part. The actuator assembly may further comprise a further bearing arrangement supporting the third part on the support structure. The one or more actuating units are thus each configured to apply their respective actuating force to the third part to drive rotation of the third part (relative to the support structure and the first part) which the helical bearing arrangement converts into the movement of the first part along the primary axis.

[0035] In other words, in some embodiments, the rotatable part is a third part of the actuator assembly (different to the first part) which is driven by the one or more actuating units to rotate (relative to the support structure and the first part) about the primary axis. The first part is supported (by the helical bearing arrangement) on the third part. Thus when the third part rotates, the first part is driven to move (without rotating) along the primary axis as a result of the helical bearing arrangement.

[0036] In some embodiments the actuator assembly is arranged such that the third part is allowed to rotate relative to the support structure about the primary axis but is constrained from translating along the primary axis and / or an axis perpendicular to the primary axis and / or from rotating about any axis perpendicular to the primary axis.

[0037] In some embodiments the actuator assembly may comprise an anti-rotation mechanism for preventing rotation of the first part about the primary axis. The anti-rotation mechanism may comprise a flexure, for example. Alternatively the anti-rotation mechanism may comprise a bearing arrangement supporting movement of the first part with respect to the support structure and preventing rotation of the first part about the primary axis (with respect to the support structure).

[0038] In some embodiments the helical bearing arrangement may comprise a plain bearing. This may be particularly advantageous in situations which it is desirable to move the first part to a given position along the primary axis under action of the one or more actuating units and hold the first part in that position without needing to continuously power the one or more SMA elements. As above, such an arrangement may be referred to as a zero-hold-power arrangement. In this case, the surfaces of the plain bearing may be arranged such that friction between the two surfaces is sufficient to retain the first part in a given position along the primary axis whilst the one or more SMA elements are unpowered. Further details of such arrangements are available in WO 2020 / 120997A1, WO2023 / 084251A1 and WO 2023 / 094813A1, each of which are incorporated herein by reference in their entirety.

[0039] In some embodiments in which the actuator assembly comprises both a first part and a third part, the third part being the rotatable part, the further bearing arrangement (which supports the third part on the support structure) may comprise a helical bearing. Accordingly, the third part is supported on the support structure by a further helical bearing. Therefore, when the third part is driven to rotate by the one or more actuating units, this rotation is converted into movement of the third part along the primary axis. In this case, the overall movement of the first part along the primary axis is a combination of: i. the movement of the third part along the primary axis (due to the further helical bearing guiding helical movement of the third part relative to the support structure) which in turn drives translation of the first part along the primary axis; and ii. the movement of the first part relative to the third part along the primary axis, caused by rotation of the third part, which due to the helical bearing arrangement causes the first part to translate along the primary axis.

[0040] The first part can therefore be driven along the primary axis by a greater distance, as compared to a situation in which the third part rotates about the primary axis but does not translate along it. Further details of such a dual-helical arrangement can be found in GB application GB2213594.1 and also W02024 / 057043A1, both of which are incorporated by reference herein in their entireties.

[0041] As mentioned above, in some embodiments the one or more actuating units may comprise:

[0042] - a first actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part of the actuator assembly in a first sense about the primary axis relative to the support structure; and

[0043] - a second actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in a second sense about the primary axis relative to the support structure, wherein the first sense is opposite to the second sense.

[0044] An angular extent of the first and second actuating units may overlap when viewed along the primary axis. The respective SMA elements of the first and second units may be on the same side of the actuator assembly (e.g. when viewed along the primary axis). Alternatively, the respective SMA elements of the first and second units may be on opposite sides of the actuator assembly (e.g. when viewed along the primary axis). Equally, the first and second actuating units may be on the same side or opposite sides of the actuator assembly (e.g. when viewed along the primary axis).

[0045] In some embodiments the force-modifying element is or comprises a force-modifying flexure. In some embodiments the force-modifying element is elongate and is stiff along its length and compliant in a direction perpendicular to its length.

[0046] In some embodiments, each actuating unit may further comprise a coupling link connected between the body portion and the rotatable part. The coupling link may be configured to transmit the actuating force from the body portion to the rotatable part, and the coupling link may be compliant in a direction perpendicular to the actuating force. The coupling link allows the rotatable part to move in directions other than the direction of the actuating force. This can be needed, for example, where different actuating units cause the rotatable part to rotate in opposite directions about the primary axis. In some embodiments the coupling link is or comprises a coupling flexure. In some embodiments the coupling link is elongate and is stiff along its length and compliant in a direction perpendicular to its length.

[0047] In some embodiments the body portion, the coupling link and / or the force-modifying element are integrally formed.

[0048] In some embodiments, in at least one actuating unit the coupling link may be perpendicular or generally perpendicular to the SMA element of that actuating unit. The SMA element of each or an actuating unit may be disposed on a side of the actuating unit adjacent to a side on which the respective coupling link is disposed. In other words, an or each actuating unit may bend around a corner.

[0049] In some embodiments, in each or an actuating unit the SMA element is parallel or generally parallel to the respective coupling link. Alternatively, the SMA element may extend at a non-zero, acute angle to the respective coupling link.

[0050] In some embodiments, the first and second actuating units may be integrally formed. In particular, the body portions, force-modifying flexures and coupling flexures of the first and second actuating units may be integrally formed, i.e. formed from the same material. The body portions, force-modifying flexures and coupling flexures of the first and second actuating units may be formed from a single metal sheet, for example by etching.

[0051] As mentioned above, in some embodiments the one or more actuating units may comprise:

[0052] - a third actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the first sense about the primary axis relative to the support structure; and

[0053] - a fourth actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the second sense about the primary axis relative to the support structure.

[0054] The third and fourth actuating units may be integrally formed in an analogous way to the first and second actuating units.

[0055] One of each of the four actuating units may be disposed on each of four sides of the actuator assembly when viewed along the primary axis. The four sides may be arranged in a loop around the primary axis. The first and third actuating units may be disposed on opposite sides of the actuator assembly and the second and fourth actuating units may be disposed on opposite sides of the actuator assembly. In some embodiments none of the actuating forces of the first, second, third and fourth actuating units are colinear. In some embodiments the actuator assembly may comprise a total of four actuating units.

[0056] In some embodiments the actuator assembly may comprise eight actuating units configured to apply respective actuating forces, wherein two actuating forces, when visualised as vectors in space, are arranged on each of four sides of the actuator assembly around the primary axis, wherein the two actuating forces on each side are inclined in opposite senses relative to a plane perpendicular to the primary axis when viewed along a direction perpendicular to the primary axis. The four sides may be arranged in a loop around the primary axis. The actuator assembly may comprise a total of eight actuating units. In embodiments in which the actuator assembly comprises multiple actuating units, each of the actuating units may lie in a single plane. For example, the plane may be perpendicular to the primary axis and may be separated from the movable part when viewed along a direction perpendicular to the primary axis. In some embodiments, one or more actuating units may be disposed on a first side of the movable part and one or more actuating units may be disposed on a second, opposite side of the movable part. The first and second sides may be separated along the primary axis.

[0057] In some embodiments the actuator assembly may be configured such that on contraction of the one or more SMA elements of the one or more actuating units, the first part moves along the primary axis by a distance which is greater than a change in length of the one or more SMA elements. In other words, the movement of the first part along the primary axis is amplified, i.e. there is an overall gearing-up of the motion along the primary axis.

[0058] In some embodiments the actuator assembly may be configured such that on contraction of the one or more SMA elements of the one or more actuating units, the first part moves along the primary axis by a distance which is less than a change in length of the one or more SMA elements.

[0059] The one or more actuating units may be arranged to amplify or de-amplify the rotation of the rotatable part.

[0060] In some embodiments, the one or more actuating units are arranged to drive rotation of the first part about the primary axis such that a point on the rotatable part moves along a circular path by a distance which is greater than a change in length of the SMA element on contraction of the SMA element. In other words, the one or more actuating units are arranged so as to amplify the rotation of the rotatable part. Accordingly, rotation by a relatively far distance may be achieved as a result of a relatively small contraction of the SMA element. As a result, a given displacement of the first part along the primary axis may be achieved with shallower bearings as compared to a situation in which the rotation is not amplified (e.g. in a situation where an SMA element is connected directly to the rotatable part).

[0061] Such shallow bearings are preferable for a number of reasons. Firstly, steep bearings are prone to allowing the rotatable part to tilt (e.g. about one or more axes perpendicular to the primary / helical axis). The use of shallow bearings therefore reduces the risk of tilt. Shallow bearings are also preferable in a zero-hold-power arrangement, in which friction is used to hold the first part still when the SMA element(s) are unpowered, as described above. With steep bearings, friction from the bearings (due to the large normal force) means that the motion of the actuator is less uniform. Therefore using shallowangle bearings can lead to uniform motion of the movable part because the normal force is reduced. In some embodiments, the one or more actuating units are arranged to drive rotation of the first part about the primary axis such that a point on the rotatable part moves along a circular path by a distance which is less than a change in length of the SMA element on contraction of the SMA element. In other words, rotation of the rotatable part is de-amplified.

[0062] It will be appreciated that other aspects of the actuator assembly will also affect the overall amplification (or de-amplification) of the motion of the first part along the primary axis. For example, the helical bearing arrangement will affect how the contraction of the one or more SMA elements is converted into translation of the first part along the primary axis.

[0063] These two means of control (helical bearing and actuating units) may be used in combination to control whether the motion of the first part is geared up (amplified) or geared down (de-amplified) and to what extent. The two means may be used in in an additive way or one may partially counteract the other. In this way, a particularly advantageous arrangement is provided which provides particularly flexibility in the design of the actuator assembly and control over the overall motion of the first part. The following options are disclosed:

[0064] (1) In some embodiments the one or more actuating units are arranged to amplify the rotation of the rotatable part and the helical bearing arrangement is arranged to amplify the motion of the first part along the primary axis.

[0065] (2) In some embodiments the one or more actuating units are arranged to de-amplify the rotation of the rotatable part and the helical bearing arrangement is arranged to de-amplify the motion of the first part along the primary axis.

[0066] (3) In some embodiments the one or more actuating units are arranged to amplify the rotation of the rotatable part and the helical bearing arrangement is arranged to de-amplify the motion of the first part along the primary axis.

[0067] (4) In some embodiments the one or more actuating units are arranged to de-amplify the rotation of the rotatable part and the helical bearing arrangement is arranged to amplify the motion of the first part along the primary axis.

[0068] In this case, the helical bearing arrangement and the one or more actuating units may work together to both amplify (option 1) or both de-amplify (option 2) their respective motions or one may partially counteract the other. For either option 1 or option 2, the overall motion of the first part along the primary axis may be amplified or de-amplified as compared to the contraction of the one or more SMA elements. Option (3) may be particularly advantageous. For example, the helical bearing may comprise a set of angled ramps (e.g. as in Figure 8, described below) and the angle of these ramps relative to a plane perpendicular to the primary axis may be chosen to be shallow (i.e. less than 45°). This shallow angle would result in a gearing down of the translation of the first part along the optical axis, in that for a given distance by which the SMA element contracts, the first part would move along the primary axis by a smaller distance. In other words, the helical bearing arrangement de-amplifies the motion of the first part along the primary axis. However, the one or more actuating units may be arranged to amplify the rotation of the rotatable part to overcompensate for the gearing-down effected by the helical bearing arrangement to result in an overall amplification of the motion of the first part along the primary axis. Accordingly, overall amplification is provided for (which may be advantageous in that the first part may be moved relatively far) whilst still using a shallow-angle helical bearing. This may be advantageous for the reasons given above. Plain bearings may also be used when the bearing angle is shallow which may have advantages related to power reduction (as described above).

[0069] Another advantage of amplifying the overall motion of the movable part is that by amplifying, a given movement can be achieved with a relatively shorter length of SMA wire. By using shorter SMA wires, the power consumption of the actuator assembly can be reduced. This may be used in combination with a zero-hold-power arrangement in order to provide an actuator assembly with a particularly low power consumption.

[0070] In some embodiments a component of the input force along the primary axis has a smaller magnitude than a component of the input force in any direction perpendicular to the primary axis. In other words, in at least one of the actuating units the SMA element may be at an acute angle relative to a plane perpendicular to the primary axis. In some embodiments, in at least one of the actuating units (and optionally in each of the actuating units) the SMA element lies in a plane perpendicular to the primary axis.

[0071] In some embodiments a component of the actuating force along the primary axis has a smaller magnitude than a component of the actuating force in any direction perpendicular to the primary axis. In other words, in embodiments in which a coupling link is present, the coupling link may be at an acute angle relative to a plane perpendicular to the primary axis. In some embodiments, in at least one of the actuating units (and optionally in each of the actuating units) the coupling link lies in a plane perpendicular to the primary axis.

[0072] In some embodiments the support structure or the first part may comprise an optical component having an optical axis. The primary axis may be aligned with the optical axis. The optical component may be a lens or mirror. In embodiments in which the first part comprises a lens, the lens may be driven to move along the optical axis of the lens. The actuator assembly may also comprise an image sensor and the lens may be configured to focus an image on the image sensor. Movement of the lens may therefore be used to effect a change in focus of the image (e.g. autofocus) or zoom. The lens may also be moved to account for thermal variations in the actuator assembly (athermalisation).

[0073] In some embodiments the first part may comprise a light-emitting component, for example an emitter, a display, a projector or a part thereof. In such embodiments, movement of the light-emitting component may be used in a virtual reality (VR) or augmented reality (AR) device. In embodiments in which the first part comprises a light-emitting component it may be desirable to avoid rotation of the first part about the primary axis and hence an arrangement in which the rotatable part is a third part, as described above, may be used. In some embodiments the support structure may comprise a light-emitting component, for example an emitter, a display, a projector or a part thereof. In some embodiments the primary axis is perpendicular to a plane defined by the display and / or is parallel to a general direction in which radiation is emitted from the light-emitting component.

[0074] In some embodiments, the first part or the support structure may comprise a light-emitting component such as a display. Such a display may be part of a VR device. The other of the support structure and the first part may comprise one or more optical components such as lenses. The optical components may form a virtual image, using light provided by the display, which the eye of the user focuses onto the retina. Movement of the first part with respect to the support structure may be carried out to effect relative movement between the display and the optics. This relative movement may alter the virtual image distance and it may be desirable to do this for a number of reasons which are as follows.

[0075] The first reason is to correct for vergence accommodation conflict (VAC). The vergence of a user's eyes refers to the simultaneous movement of the eye to maintain binocular vision, whereas the accommodation of the eye refers to the ability of the eye to change its focus, for example to focus on distant or near objects. VAC arises when there is conflict between the vergence and accommodation of the eyes and can cause discomfort. Figure 14a illustrates two eyes 202 of a user and an object 200 in the real world. In this scenario, the vergence and the accommodation of the eye match. Figure 14b illustrates the same set-up, but the object is displayed on a display 204 and a virtual image is created by the display and the optics. In this case, there is a conflict between the vergence of the eyes (solid line) and the accommodation (dashed lines). A way to compensate for this VAC is to change the virtual image distance. A second reason for changing the virtual image distance is to take into account intrinsic characteristics of a user's eye, for example an optical prescription and / or to compensate for presbyopia (age-related far-sightedness).

[0076] In some embodiments the first part may comprise a part of an optical component, e.g. a part of a deformable optical component such as a deformable lens (e.g. liquid lens) or deformable mirror. Movement of the first part may thus be driven to effect deformation of a deformable optical element, for example for the purpose of changing a focal length or some other optical property of the deformable optical component.

[0077] In some embodiments the helical bearing arrangement comprises one or more rolling bearings. For example, one or more rolling elements such as ball bearings may be disposed between a surface of the support structure and a surface of the rotatable part. Such roller bearings may reduce the friction between two parts of the actuator assembly.

[0078] Brief description of the drawings

[0079] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0080] Figures 1A-E are schematic cross-sectional views of different variations of a camera module incorporating an actuator assembly;

[0081] Figure 2 is a schematic perspective view of the actuator assembly;

[0082] Figure 3 is a schematic view of an SMA actuation apparatus that is a camera;

[0083] Figures 4 and 5 are perspective views of two helical bearings;

[0084] Figures 6 and 7 are schematic cross-sectional views of the SMA actuation apparatus of Figure 3 with different possible helical bearing arrangements;

[0085] Figure 7A is a schematic view of a sliding bearing;

[0086] Figure 7B is a different schematic view of the sliding bearing shown in Fig. 7A;

[0087] Figure 7C is a schematic view of a sliding bearing;

[0088] Figure 7D is a different schematic view of the sliding bearing shown in Fig. 7C;

[0089] Figure 8 is an exploded projection view of an actuator assembly;

[0090] Figure 9 is a projection view of the actuator assembly of Figure 8;

[0091] Figure 10 is a projection view of a helical flexure bearing;

[0092] Figure 10A is a schematic view of an actuator assembly; Figures 11A and 11B are perspective and plan views of an actuating unit forming part of an actuator assembly, and Figure 11C is a plan view of another such actuating unit;

[0093] Figures 12A and 12B are schematic views of pairs of actuating units;

[0094] Figure 13 is a schematic plan view of an arrangement of four actuating units;

[0095] Figure 14a is a schematic view of a user viewing an object; and

[0096] Figure 14b is a schematic view of a user viewing an object displayed on a display.

[0097] Detailed description

[0098] Figures 1A-E schematically show different variations of an apparatus incorporating an actuator assembly 1. The apparatus is, for example, a camera module. Generally, the apparatus is to be incorporated in a portable electronic device such as a smartphone. Thus, miniaturisation can be an important design criterion.

[0099] Figure 2 schematically shows the actuator assembly 1. The actuator assembly 1 includes a support structure 2 and a movable part 10. The movable part 10 is movable relative to the support structure 2. When the actuator 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 actuator assembly 1 includes one or more actuating units 80. Each actuating unit 80 is configured to apply an actuating force to the movable part 10 capable of moving the movable part 10 relative to the support structure 2.

[0100] The movable part 10 may be supported (i.e. suspended) on the support structure 2 exclusively by the actuating units 80. Alternatively, the actuator assembly 1 may include a bearing arrangement 40 that supports the movable part 10 on the support structure 2. The actuating units 80 and the bearing arrangement 40 may together support the movable part 10 on the support structure 2. The bearing arrangement 40 may have any suitable form for allowing movement of the movable part 10 with respect to the support structure 2 with one or more degrees of freedom (DOFs). The actuating units 80 and / or the bearing arrangement 40 may constrain, i.e. reduce or prevent, other DOFs of movement of the movable part 10 relative to the support structure 2. For this purpose, the bearing arrangement 40 may, for example, include one or more of the following bearings: a rolling bearing (such as a ball bearing), a flexure bearing (i.e. an arrangement of flexures or other resilient elements that guide movement), or a plain (i.e. sliding contact) bearing.

[0101] A primary axis P can be defined with reference to the actuator assembly 1 and / or the support structure

[0102] 2. The primary axis P may extend through the actuator assembly 1, e.g. through the centre of the actuator assembly 1. In some examples, the actuator assembly 1, the support structure 2 and / or the movable part 10 extend predominantly in a direction perpendicular to the primary axis P. In other words, the extent of the actuator assembly 1, the support structure 2 and / or the movable part 10 along the primary axis P is less than the extent thereof along any direction perpendicular to the primary axis P. The primary axis P may be the longitudinal axis of the actuator assembly 1 and / or the support structure 2. Alternatively or additionally, the support structure 2 and / or movable part 10 may include a planar component that extends perpendicularly to the primary axis P. Alternatively or additionally, in examples in which the apparatus 1 includes an optical element (such as a lens assembly 3) with an optical axis, or an imaging element (such as an image sensor 4) with an imaging axis, the primary axis P may be parallel to such an axis and / or may coincide with such an axis when the movable part 10 is in a central position or orientation (for example, see Figure 1A).

[0103] In general, the movable part 10 may be movable relative to the support structure 2 with up to six degrees of freedom (DOFs). In the context of describing the DOFs 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. The movable part 10 may be movable relative to the support structure 2 in all or in any subset (including only one) of the following DOFs:

[0104] - Tx and Ty: Translational movement in the x-y plane. In other words, the movable part 10 may be independently movable along the x and y axes. The movable part 10 may be movable to any position in the x-y plane within a range of movement. Instead of such planar movement, the movable part 10 may be movable linearly, e.g. along the x or y axis.

[0105] - Rx and Ry: Rotational movement (or simply rotation or tilting) about the x and y axes. In other words, the movable part 10 may be rotated about any line perpendicular to the primary axis P. The movable part 10 may be rotatable to any rotational position (i.e. to any orientation) within a range of movement. Instead of such two-axis rotation, the movable part 10 may be rotatable about a single axis, e.g. about the x or y axis.

[0106] - Tz: Translational movement along the z axis. The movable part 10 may be movable to any translational position along the z axis within a range of movement.

[0107] - Rz: Rotational movement (or simply rotation) about the z axis. The movable part 10 may be rotatable to any rotational position (i.e. to any orientation) within a range of movement.

[0108] In some examples, the movable part 10 may be supported, e.g. by the bearing arrangement 40, so as to allow translational movement in the x-y plane (Tx, Ty) and / or rotational movement about the z axis (Rz). Translational movement along the z axis (Tz) and rotational movement about the x and y axes (Rx, Ry) may be constrained. Such support may be provided, for example, with a bearing arrangement 40 with a suitable arrangement of ball bearings or plain bearings which produce bearing forces in the +z direction and a biasing arrangement which produces a biasing force in the -z direction. Examples of actuator assemblies with such a bearing arrangement are disclosed in WO 2013 / 175197 Al and WO 2017 / 072525 Al, each of which is herein incorporated by reference.

[0109] In some examples, the movable part 10 may be supported so as to allow tilting about the x and y axes (Rx, Ry) and optionally rotation about the z axis (Rz). The other DOFs of movement (i.e. Tx, Ty, Tz, Rz, or Tx, Ty, Tz) may be constrained. Such support may be provided by the bearing arrangement 40, for example in the form of a gimbal. Examples of such a bearing arrangement 40 are disclosed in WO 2021 / 209770 Al, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 80, similarly to WO 2011 / 104518 Al which discloses an actuator assembly with 8 SMA wires connected between the support structure 2 and the movable part 10. WO 2011 / 104518 Al is herein incorporated by reference.

[0110] In some examples, the movable part 10 may be supported so as to allow three-dimensional translational movement (Tx, Ty, Tz), while rotational movement (Rx, Ry, Rz) may be constrained. Such support may be provided by the bearing arrangement 40, for example in the form of nested linear bearings. Examples of such a bearing arrangement 40 are disclosed in WO 2021 / 209769 Al, which is herein incorporated by reference. Alternatively, such support may be provided exclusively by the actuating units 80, similarly to WO 2011 / 104518 Al.

[0111] The movable part 10 may, alternatively or additionally, move in other DOFs. The movable part 10 may move in DOFs that are a combination of any two or more of Tx, Ty, Tx, Rx, Ry and Rz. For example, the movable part 10 may move along a helical path (i.e. move helically) about the z axis, and so concurrently move along the z axis and rotate about the z axis. In other words, Tz and Rz movement may be coupled. An example of such a helical actuator assembly is disclosed in WO 2019 / 243849 Al, which is herein incorporated by reference.

[0112] The actuating units 80 are connected between the support structure 2 and the movable part 10. The actuating units 80 are arranged to apply actuating forces F (see e.g. Figure 13) between the movable part 10 and the support structure 2. Selectively varying the actuating forces F may cause the movable part 10 to move relative to the support structure 2, for example within the DOFs allowed by the bearing arrangement 40. The actuating units 80 are thus capable of driving movement of the movable part 10 relative to the support structure 2. The bearing arrangement 40 may cause the movable part 10 to move in directions which differ from the directions of the actuating forces F. In simple examples of this, one component of each actuating force F causes the movement of the movable part 10, and another component of each actuating force F acts against the bearing forces produced by the bearing arrangement 40.

[0113] The camera module 1 also includes a lens assembly 3 and an image sensor 4. The lens assembly 3 includes one or more lenses configured to focus an image on the image sensor 4. The lens assembly 3 defines an optical axis O. The lens assembly 3 may include a lens carrier, for example in the form of a cylindrical body, supporting the one or more lenses. The image sensor 4 captures an image and may be of any suitable type, for example a charge coupled device (CCD) or a complementary metal-oxide- semiconductor (CMOS) device. The camera module 1 may be a compact camera module in which each lens has a diameter of 20mm or less, for example of 12mm or less.

[0114] In the ("sensor-shift") variation of the camera module 1 shown in Figure 1A, the movable part 10 includes the image sensor 4. The lens assembly 3 may be fixed relative to the support structure 2, or may be movable relative to the support structure 2 along the optical axis O, as described below.

[0115] In the ("lens-shift") variation shown in Figure IB, the image sensor 4 is fixed relative to the support structure 2 and the movable part 10 includes the lens assembly 3. The lens assembly 3 may be movable relative to the movable part 10 along the optical axis O, as described below.

[0116] In both of these variations, the actuator assembly 1 is configured to move the lens assembly 3 relative to the image sensor 4 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 4 and enables optical image stabilisation (OIS) to be implemented in the camera module 1. In the sensor-shift variation, the movable part 10 may also be rotatable about the primary axis P so as to also enable compensation for roll.

[0117] In the ("module-tilt") variation shown in Figure 1C, the movable part 10 includes both the lens assembly 3 and the image sensor 4. Again, the lens assembly 3 may be movable relative to the movable part 10 along the optical axis O, as described below. The actuator assembly 1 is configured to tilt the movable part 10 about two axes perpendicular to the primary axis P and to each other, and optionally rotate the movable part 10 about the primary axis P, enabling OIS to be implemented in the camera module 1.

[0118] In the ("autofocus") variation shown in Figure ID, the movable part 10 includes the lens assembly 3, and the actuator assembly 1 moves the movable part 10 relative to the support structure 2 along the primary axis P and hence the optical axis O. Such movement has the effect of adjusting the focus of the image on the image sensor 4. So, auto-focus (AF) or zoom functionality can be implemented in the camera module 1.

[0119] In some examples (not shown), the camera module 1 may include a first actuator assembly for providing OIS as illustrated in Figures 1A-C, and a second actuator assembly for providing AF as illustrated in Figure ID. One or both of the first and second actuator assemblies may correspond to actuator assemblies 1 as described herein. One of the first and second actuator assemblies may be another type of SMA actuator assembly or may be a non-SMA actuator assembly, e.g. a voice-coil motor (VCM) actuator assembly. As will be appreciated, in the lens-shift and module-tilt variations, the support structure 2 of the second actuator assembly 1 is fixed to (or corresponds to) the movable part 10 of the first actuator assembly 1.

[0120] In the ("AF+OIS") variation shown in Figure IE, the movable part 10 includes the lens assembly 3, and the actuator assembly 1 produces three-dimensional translational movement of the movable part 10 relative to the support structure 2, enabling both AF and OIS to be implemented using one actuator assembly 1.

[0121] Other variations are also possible. For example, in the autofocus variation or the AF+OIS variation, the movable part 10 may include the image sensor 4 rather than the lens assembly 3. The camera module 1 may include combinations of the above-described features, e.g. (a) lens shift and sensor shift, (b) module tilt and lens shift or sensor shift and autofocus, or (c) module tilt and AF+OIS.

[0122] The camera module 1 also includes a controller 8. The controller 8 may be implemented in an integrated circuit (IC) chip. The controller 8 generates drive signals for the actuating units 80, in particular for SMA wires 84 forming part of the actuating units 80. SMA material has the property that, on heating, it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA wires 84, thereby heating the SMA wires 84 by causing an electric current to flow, will cause the SMA wires 84 to contract and thus actuate the actuating unit 80 so as to move the movable part 10. The drive signals are chosen to drive movement of the movable part 10 in a desired manner, for example so as to achieve OIS by stabilizing the image sensed by the image sensor 4 or to achieve AF / zoom by adjusting the focus of the image sensed by the image sensor 4. The controller 8 supplies the generated drive signals to the SMA wires 84.

[0123] Optionally, the camera module 1 also includes a motion sensor (not shown), which may include a 3-axis gyroscope and a 3-axis accelerometer. The motion sensor can generate signals representative of the motion (specifically vibrations or "shake") of the camera module 1, which can be processed so as to produce signals representative of the required movement of the movable part 10 to compensate for such shake. The controller 8 receives such signals and can generate the drive signals for the SMA wires 84 to achieve OIS.

[0124] Although the actuator assembly 1 is described in connection with a camera module 1, it will be appreciated that the actuator assembly 1 may be used in any device in which movement of a movable part 10 relative to a support structure 2 is desired, e.g. to provide haptic feedback in a haptic feedback device or to move a projector or display in an augmented reality (AR) or virtual reality (VR) device. As described above, this might be for the purpose of changing a virtual image distance to correct for vergence accommodation conflict and / or to correct for one or more characteristics of a user's eyes (e.g. an optical prescription).

[0125] The present application focusses on movement along an axis, e.g. an optical axis of a lens (Figure ID), in particular in which a helical bearing arrangement is used. Various helical bearing arrangements will now be described.

[0126] First bearing arrangement

[0127] With reference to Figure 3, a first bearing arrangement is described. Such a bearing arrangement is suitable for use as bearing arrangement 40 in Figure 2. Further details of the bearing arrangement can be found in WO2019 / 243849A1, which is incorporated herein by reference in its entirety. The first bearing arrangement guides movement of the first part (referred to below as movable part 10) along a helical path.

[0128] An SMA actuation apparatus 1 that is a camera is shown schematically in Fig. 3. The SMA actuation apparatus 1 comprises a support structure 2 that has an image sensor 4 mounted thereon. The support structure 2 may take any suitable form, typically including a base 74 to which the image sensor is fixed. The support structure 2 may also support an IC chip 5 described further below.

[0129] The SMA actuation apparatus 1 also comprises a movable part 10, which may also be referred to as a first part. The movable part 10 comprises a lens 11, although it may alternatively comprise plural lenses. The lens 11 has an optical axis O aligned with the image sensor 4 and is arranged to focus an image on the image sensor 4. The SMA actuation apparatus 1 is a miniature device. In some examples of a miniature device, the lens 11 (or plural lenses, when provided) may have a diameter of at most 20mm, preferably at most 15mm, preferably at most 10mm.

[0130] Although the SMA actuation apparatus 1 in this example is a camera, that is not in general essential. In some examples, the SMA actuation apparatus 1 may be an optical device in which the movable element is a lens element but there is no image sensor. In other examples, SMA actuation apparatus 1 may be a type of apparatus that is not an optical device, and in which the movable element is not a lens element and there is no image sensor. Examples include apparatuses for depth mapping, face recognition, game consoles, projectors, security scanners and augmented reality (AR) or virtual reality (VR) headsets or glasses. In some cases there is no lens present in the assembly or device.

[0131] The SMA actuation apparatus 1 comprises a helical bearing arrangement 40 (shown schematically in Fig. 3) that supports the movable part 10 on the support structure 2. The helical bearing arrangement 40 is arranged to guide helical movement of the movable part 10 with respect to the support structure 2 around a helical axis H. The helical axis H in this example is coincident with the optical axis O and the helical movement is shown in Fig. 3 by the arrow M. The helical / optical axis may also be referred to as the primary axis. Preferably, the helical motion is along a right helix, that is a helix with constant radius, but in general any helix is possible. The pitch of the helix may be constant or vary along the helical motion. Preferably, the helical movement is generally only a small portion (less than one quarter) of a full turn of the helix.

[0132] The helical motion of the movable part 10 guided by the helical bearing arrangement 40 includes a component of translational movement along the helical axis H and rotational movement around the helical axis H. The translational movement along the helical axis H is the desired movement of the movable part 10 (and hence the lens 11), for example to change the focus of the image on the image sensor 4 and / or to change the magnification (zoom) of the image on the image sensor 4 and / or to compensate for thermal variations in the environment which affect the lens. The rotational movement around the helical axis H is in this example not needed for optical purposes, but is in general acceptable as rotation of the movable part 10 (and hence the lens 11) does not change the focus of the image on the image sensor 4, for example.

[0133] The helical bearing arrangement 40 may take a variety of forms.

[0134] One possibility is that the helical bearing arrangement 40 comprises one or more helical bearings 30 that are rolling bearings, examples of which are shown in Figs. 4 and 5. In each of Figs. 4 and 5, the helical bearing 30 comprises a pair of bearing surfaces 31 and 32 and plural rolling bearing elements 33, for example balls, disposed between the bearing surfaces 31 and 32. One of the bearing surfaces 31 and 32 is provided on the support structure 2 and the other of the bearing surfaces 31 and 32 is provided on the movable part 10.

[0135] The helical bearing 30 guides the helical movement of the movable part 10 with respect to the support structure 2 as shown by the arrow M. This may be achieved by the bearing surfaces 31 and 32 extending helically around the helical axis H, that is following a line that is helical. That said, in practical embodiments, the length of the bearing surfaces 31 and 32 may be short compared to the distance of the bearing surfaces 31 and 32 from the helical axis H, such that their shape is close to straight or even each being straight, provided that the one or more helical bearings of the helical bearing arrangement 40 guide helical movement of the movable part 10 with respect to the support structure 2. Plural helical bearings 30 are typically present, located at different angular positions around the helical axis H, in which case the helical bearings 30 have different orientations so that they cooperate and maintain adequate constraints to guide the helical movement of the movable part 10 with respect to the support structure 2, even if the bearing surfaces 31 and 32 of an individual helical bearing 30 are straight.

[0136] In the example of Fig. 4, the bearing surfaces 31 and 32 each comprise respective grooves 34 and 35 in which the rolling bearing elements 33 are seated. In this example, the grooves 34 and 35 constrain transverse translational movement of the movable part 10 with respect to the support structure 2, that is transverse to the direction of movement shown by arrow M. The grooves shown in figure 4 are V- shaped in cross-section, but other cross-sections are possible, for example curved as in portions of a circle or an oval. In general, the grooves 34 and 35 provide two points of contact with the respective rolling bearing elements 33. The grooves 34 and 35 may extend helically. Alternatively, in practical embodiments, the length of the bearing surfaces 31 and 32 may be short compared to the distance of the bearing surfaces 31 and 32 from the helical axis H, in which case the grooves 34 and 35 may be straight or close to straight, provided that the one or more helical bearings 30 of the helical bearing arrangement 40 guide helical movement of the movable part 10 with respect to the support structure 2.

[0137] In the example of Fig. 5, a first bearing surface 31 comprises a groove 36 in which the rolling bearing elements 33 are seated and a second bearing surface 32 wherein the bearing surface is 'planar'. The first bearing surface 31 comprising a groove 36 may be provided on either one of the support structure 2 and the movable part 10, with the second bearing surface 32 being provided on the other one of the support structure 2 and the movable part 10. In the example of Fig. 5, the helical bearing 30 does not constrain transverse translational movement of the movable part 10 with respect to the support structure 2, that is transverse to the direction of movement shown by arrow M. The bearing surface 32 is 'planar' in the sense that it is a surface which is not a groove and one which provides only a single point of contact with the ball. In other words, the bearing surface 32 is effectively planar across a scale of the width of the rolling bearing element 33, although be helical at a larger scale. For example, as pictured, the 'planar' surface is helical, being a line in cross section which twists helically along the movement direction, maintaining a single point of contact with the ball at any time. Alternatively and as mentioned above, in practical embodiments the length of the bearing surfaces 31 and 32 may be short, in which case the bearing surface 32 may be planar or close to planar, provided that the one or more helical bearings 30 of the helical bearing arrangement 40 guide helical movement of the movable part 10 with respect to the support structure 2.

[0138] A single rolling bearing element 33 is shown in Figs. 4 and 5 by way of example, but in general may include any plural number of rolling bearing elements 33.

[0139] In some examples, the helical bearing 30 may include a single rolling bearing element 33. In that case, the helical bearing 30 by itself does not constrain the rotational movement of the movable part 10 with respect to the support structure 2 about the single rolling bearing element 33, that is around an axis transverse to the direction of movement shown by arrow M. However, this minimises the overall size of the helical bearing 30, and in particular the height of the helical bearing 30 projected along the helical axis H as it is only needed to accommodate the size of the rolling bearing element 33 and the relative travel of the bearing surfaces 31 and 32.

[0140] In other examples, the helical bearing 30 may include plural rolling bearing element 33. In that case, the helical bearing 30 constrains the rotational movement of the movable part 10 with respect to the support structure 2 about either one of the rolling bearing elements 33, that is around an axis transverse to the direction of movement shown by arrow M. However, compared to use of a single rolling bearing element 33, this increases the overall size of the helical bearing 30, and in particular the height of the helical bearing 30 projected along the helical axis H.

[0141] The helical bearing arrangement may in general comprise any number of helical bearings 30 with a configuration chosen to guide the helical movement of the movable part 10 with respect to the support structure 2 while constraining the movement of the movable part 10 with respect to the support structure 2 in other degrees of freedom. Many helical bearing arrangements may comprise plural helical bearings 30 and at least one which comprises plural rolling bearing elements 30. For example, Figure 6 shows an embodiment in which the helical bearing arrangement comprises two helical bearings 37 and 38. Figure 7 shows an embodiment in which the helical bearing arrangement comprises three helical bearings 39, 40 and 41. The helical bearing arrangement 40 is arranged to guide helical movement of the movable part 10 with respect to the support structure 2 around a helical axis H. Rotation of the movable part 10 is driven by one or more actuating units 80 (not shown in Figures 3-7) and this rotation is converted into helical movement by the helical bearing arrangement. In this way, the movable part 10 moves along the helical axis.

[0142] Another possibility is that the helical bearing arrangement 40 comprises one or more helical bearings 30 that are plain bearings (which may be referred to as sliding bearings). For example, instead of helical bearing surfaces 31 and 32 being separated by one or more rolling bearing elements 33 (e.g. balls), the helical bearing surfaces 31 and 32 may be in direct contact with each other. The helical bearing surfaces may be biased together with a normal force, thereby generating a frictional force therebetween. This frictional fore may be sufficient to retain the movable part 10 in position relative to the support structure 2 when the actuator is unpowered.

[0143] Examples of plain bearings are shown in Figs. 7A-D. In the first example shown in Figs. 7A and 7B, the sliding bearing is a plain bearing 181 that comprises an elongate bearing surface 183 on one of the support structure 2 and the movable part 10. The plain bearing 181 also comprises protrusions 185 formed on the other of the support structure 2 and movable part 10, the ends of the protrusions 185 forming bearing surfaces 186 which bear on the elongate bearing surface 183. Although two protrusions 185 are shown in this example, in general any number of one or more protrusions 185 may be provided. The elongate bearing surface 183 and the bearing surfaces 186 are conformal, both being planar in this example, so as to permit relative movement of the movable part 10 with respect to the support structure 2. The elongate bearing surface 183 and the bearing surfaces 186 desirably have a coefficient of friction of 0.2 or more. A higher coefficient of friction may reduce or eliminate the power and / or energy to keep the movable part 10 in position.

[0144] In the second example shown in Figs. 7C and 7D, the sliding bearing is a plain bearing 91 that comprises a channel 92 on one of the support structure 2 and the movable part 10, the inner surface of the channel 92 forming a bearing surface 93. The plain bearing 91 comprises protrusions 95 formed on the other of the support structure 2 and movable part 10, the ends of the protrusions 95 forming bearing surfaces 96 which bear on the bearing surface 93. Although two protrusions 95 are shown in this example, in general any number of one or more protrusions 95 may be provided. The elongate bearing surface 93 and the bearing surfaces 96 are conformal, both being planar in this example, so as to permit relative movement of the movable part 10 with respect to the support structure 2. The elongate bearing surface 93 and the bearing surfaces 96 desirably have a coefficient of friction of 0.2 or more. A higher coefficient of friction may reduce or eliminate the power and / or energy to keep the movable part 10 in position. In general, however, lower coefficients of friction may be used and offset by larger loading forces so as to provide zero hold power, and vice versa. The loading arrangement and friction surfaces of the helical bearing arrangement may thus be designed to work together to provide zero hold power.

[0145] In each of the plain bearings 181 and 91, the materials of the bearing surfaces 183, 186, 93, 96 are chosen to provide smooth movement and a long life. The bearing surfaces 183, 186, 93, 96 may be unitary with the underlying component or may be formed by a surface coating. Suitable materials include, for example PTFE or other polymeric bearing materials, or metal.

[0146] In each of the plain bearings 181 and 91, a lubricant may be provided on the bearing surfaces 183, 186, 93, 96. Such a lubricant may be a powder or a fluid, for example. Suitable lubricants include: graphite; silicon paste or a low viscosity oil.

[0147] Second bearing arrangement

[0148] In some cases it may be desirable to achieve translation of a movable part (e.g. the first part) along an axis without the movable part rotating about that axis. A second bearing arrangement which provides for such translation without rotation is now described with reference to Figures 8 and 9. In this example, in contrast to the first bearing arrangement, the actuator assembly 1 comprises a third part and the rotatable part is the third part (not the first part).

[0149] The following description concerns relative movements Tz of a first part relative to a support structure along (or parallel to) a primary axis z, whilst constraining movements Tx, Ty along (or parallel to) x and y axes (which are perpendicular to each other and to the z axis) and all rotations Rx, Ry, Rz (about x, y and z axis respectively) of the first part relative to the support structure.

[0150] Referring to Figures 8 and 9, an actuator assembly 1 (hereinafter actuator assembly) is shown. Figure 8 shows an exploded perspective view of the actuator assembly 1 and Figure 9 shows a perspective view.

[0151] The actuator assembly 1 includes a first part 24 (which is otherwise referred to as the movable part), a support structure 25, a bearing arrangement mechanically coupling the first part 24 to the support structure 25, and a drive arrangement 11, 20. As before, the functions of the first actuator assembly 1 shall be described with reference to a set of axes fixed to the support structure 25. A primary axis z corresponds to a direction which, when used in a camera, would coincide with or be parallel to the optical axis O. First x and second axes y are perpendicular to the primary axis z, and to one another.

[0152] The drive arrangement 11, 20 and the bearing arrangement are configured such that the first part 24 is movable towards or away from the support structure 25 along the primary axis z. The bearing arrangement is also configured to constrain movement Tx, Ty of the first part 24 relative to the support structure 25 along the first axis x and / or the second axis y, and to constrain rotation Rx, Ry, Rz of the first part 24 relative to the support structure 25 about any of the first, second and primary axes x, y, z.

[0153] The bearing arrangement includes a first bearing 27 mechanically coupling the first part 24 to a third part 28. In this case the third part 28 is the rotatable part (which is driven to rotate by one or more actuating units). The drive arrangement 11 comprises four actuating units (not shown) connected between the support structure 25 and the third part 28. The first bearing 27 is configured to generate, in response to a torque applied about the primary axis z by the drive arrangement 11, 20, movement of the first part 24 towards or away from the third part 28 (and support structure 25) along the primary axis z. The first bearing 27 provides this function by guiding helical movement [Tz, Rz] about and along the primary axis z, by coupling a rotation Rz about the primary axis z to a translation Tz along the primary axis z.

[0154] A rotation of the first bearing 27 about the primary axis z will correspond to a rotation of the third part 28 relative to the first part 24 (and support structure 25) about the primary axis z. The first part 24 does not rotate about the primary axis z relative to the support structure 25.

[0155] The bearing arrangement also includes a second bearing 29 mechanically coupling the first part 24 to the support structure 25 and configured to guide movement of the first part 24 towards or away from the support structure 25 along the primary axis z whilst constraining movement of the first part 24 relative to the support structure 25 along the first axis x and / or the second axis y, and also constraining rotation Rz of the first part 24 relative to support structure 25 about the primary axis z. In this sense the second bearing acts as an anti-rotation mechanism.

[0156] The bearing arrangement also includes a third bearing mechanically coupling the third part 28 to the support structure 25 in parallel with the drive arrangement 11, 20. The third bearing may be configured to constrain movement Tz of the third part 28 relative to the support structure 25 along the primary axis z, and to constrain rotation Rx, Ry of the third part 28 relative to the support structure 25 about the first and / or second axes x, y. The third bearing should not constrain (i.e. should permit) rotation Rz of the third part 28 relative to the support structure 25 about the primary axis z. The third bearing is not shown in Figures 8 and 9 but may be a planar bearing, for example. The third part 28 may be supported on and slide over three cylindrical protrusions on the support structure 25.

[0157] A specific implementation of the actuator assembly 1 is now described with reference to Figures 8 and 9. The can 8 is omitted from Figure 9 for visual purposes.

[0158] The actuator assembly 1 includes a flat actuator assembly 15, of which an annular plate 16 provides the support structure 25 and an annular sheet 17 provides the third part 28. The annular sheet 17 is coupled to the annular plate 16 using the four actuating units and slides over the third bearing in the form of a planar bearing provided by three or more cylindrical protrusions (not shown in Figure 8).

[0159] A first bearing 27 in the form of a helical roller bearing 31a mechanically couples the third part 28 in the form of the annular sheet 17 to a first part 24 in the form of a lens carriage 32a which performs the function of supporting a lens or lenses of a lens assembly 3 in the same way as lens carriage. The helical roller bearing 31a includes an annulus 33a having a circular inner perimeter defining a central aperture 1009, and an outer perimeter which alternates between rectangular and circular outlines. The annulus 33a supports four ramps 34i, 342, 34a, 344 equi-spaced in a loop about the central aperture 1009. Each ramp 34i, 342, 34a, 34 takes the form of a rectangular frame having an elongated aperture 35i, 352, 35a, 354 extending along a length of the ramp 34i, 342, 34a, 344. The ramps 34i, 342, 34a, 344 all make substantially equal angles to the annulus 33a (which lies in a plane parallel to first and second axes x, y). When assembled, each elongated aperture 35i, 352, 35a, 354 receives a corresponding ball bearing 10301, 10302, 1030a, 10304.

[0160] The lens carriage 32a is generally cylindrical about a central aperture 1009 for mounting of one or more lenses. The lens carriage 32a also includes four protrusions 36i, 362, 36a, 864 extending radially outwards from the generally cylindrical lens carriage 32a. The first protrusion 36i defines a first bearing surface 37i in the form of a V-shaped channel. The first bearing surface 37i is oriented generally upwards (normals to the first bearing surface 37i have components generally in the positive +z direction along the primary axis z). The second protrusion 862 defines a second bearing surface 372 in the form of a V-shaped channel oriented generally downwards (normals to the second bearing surface 372 have components generally in the negative -z direction along the primary axis z). The third protrusion 863 defines a third bearing surface 37a in the form of an angled planar surface oriented generally upwards (normals to the third bearing surface 37a have components generally in the positive +z direction along the primary axis z). The fourth protrusion 864 defines a fourth bearing surface 374 in the form of an angled planar surface oriented generally downwards (normals to the fourth bearing surface 374 have components generally in the negative -z direction along the primary axis z).

[0161] When assembled, each bearing surface 37i, 372, 37a, 37 is in rolling contact with the corresponding ramp 34i, 342, 34a, 344 via the respective ball bearing 10301, 10302, 1030a, 10304. However, the first and third bearing surfaces 37i, 37a will lie below (relative to the primary axis z) the corresponding ramps 34i, 34a, whereas the second and fourth bearing surfaces 372, 374 will lie above the corresponding ramps 342, 344. This arrangement may be observed in Figure 9.

[0162] The annulus 33a is fixed to the annular sheet 17 (third part 28), for example by welding, adhesive or another suitable attachment method. An upper surface (relative to the primary axis z) of the lens carriage 32a (first part 24) is fixed to a central annular portion 38 of the second bearing 29. The second bearing 29 takes the form of two-bar link 1001, additionally including a central annular portion 38 rigidly attached to (or integrated with) the second rigid portion 10022. The central annular portion 38 takes the form of a circular annulus.

[0163] The first rigid portion 10021 is then rigidly connected to the annular plate 16 (support structure 25) via the can 8 and base 5a, to complete the coupling between the support structure 25 in the form of the annular plate 16 and the first part 24 in the form of the lens carriage 32a. For example, the first rigid portion 10021 is attached to the can 8, and the interior boundaries of the can 8 are dimensioned to abut (or nearly abut) the edges of the first and second beam portions 10031, 10032 in order to prevent movement Ty of the second rigid portion 10022 relative to the first rigid portion 10021 along the second axis y (as oriented in Figures 8 and 9). This configuration leaves the first and second beam portions 10031, 10032 free to deflect along the primary axis z along with the lens carriage 32a (first part 24).

[0164] The combination of the first and second bearings 27, 29 constrains any response to a lateral force (substantially perpendicular to the primary axis z) applied by the first drive arrangement 11. In use, the first drive arrangement 11 will not be caused to apply a lateral force (since this would have no useful effect given the bearing arrangement).

[0165] However, when the first drive arrangement 11 is caused to apply a torque about the primary axis z, the third part 28 in the form of the annular sheet 17, and the attached annulus 33a and ramps 34i, 342, 34a, 344 will rotate Rz about the primary axis z in response. This rotation will cause the ball bearings 10301, 10302, 10303, 10304 to roll between the ramps 34i, 342, 34a, 344 and bearing surfaces 37i, 372, 37a, 374, displacing the lens carriage 32a (first part 24) up or down (relative to the primary axis z) depending on the direction of the torque and corresponding rotation Rz. However, the lens carriage 32a (first part 24) does not rotate Rz about the primary axis z because of the constraint provided by the second bearing. Besides facilitating up or down movement of the lens carriage 57 (first part 24), the under-over-under- over configuration of the ramps 34i, 342, 34a, 344 and bearing surfaces 37i, 372, 37a, 37 means that, when the actuator assembly 1 is assembled the ramps 34i, 342, 34a, 344 are flexed, providing a loading force for the first bearing 27.

[0166] Although shown in Figures 8 and 9 using the (flat) drive arrangement 11, an angled drive arrangement may be used instead. The angled drive arrangement comprises actuating units which apply actuating forces which are inclined relative to a plane perpendicular to the primary axis and which apply a component of force along the primary axis z in combination with a torque about the primary axis z, which may help with smoother helical movement [Tz, Rz] of the first bearing 27.

[0167] In this way, an AF function (for example) may be provided using a single drive arrangement 11, 20 including a total of four actuating units whilst also avoiding rotation Rz of lenses about the primary axis z. Compared to a simple helical flexure or bearing which would also rotate Rz a lens about the primary axis z, this may improve the quality of images by reducing the possibility of aberrations resulting from imperfect circular symmetry of one or more lenses. Further, in embodiments in which the first part comprises some component other than a lens (e.g. an image sensor or a light emitting component such as a display or projector) it may be necessary to avoid rotation of the component.

[0168] Although the actuator assembly 1 has been explained with the support structure 25 corresponding to a support structure of a camera and the first part 24 corresponding to a lens carriage, 32a of a lens assembly 3, the roles may be reversed so that the support structure 25 corresponds to a lens carriage 9, 32a and the first part 24 provides a support structure. Equally, the actuator assembly 1 need not be restricted to use in a camera, and the actuator assembly may be arranged to drive movement of any parts requiring the relative motion Tz.

[0169] Although shown in Figures 8 and 9 using a two-bar link 1001 additionally constrained against lateral motion Ty by abutment of the can, the second bearing 29 may be implemented using any bearing which guides motion Tz along the primary axis z whilst constraining rotation Rz about the primary axis z. For example, the second bearing 29 may take the form of a z-flexure 1011, a z-translation bearing 1081, or any other type of bearing having the motions and constraints described hereinbefore.

[0170] Although shown in Figures 8 and 9 as an annular sheet 17, the third part 28 may in general be any structure suitable for mechanically coupling the first and second bearings 27, 29. Although shown in Figures 8 and 9 using a particular type of helical roller bearing 31a, the first bearing 27 may be any type of bearing or flexure which responds to an applied torque with helical motion [Tz, Rz], For example, a helical flexure bearing 1090 as shown in Figure 10 (and described below) may be used. Alternatively, any of the options described in WO 2019 / 243849 Al may be used. Instead of roller bearings, plain bearings may be used, for example a plain bearing such as that described with reference to Figures 7A and 7B or with reference to Figures 7C and 7D. This may facilitate the provision of a zero- hold-power actuator assembly.

[0171] With reference to Figure 10, a helical flexure bearing 1090 is described. In general, a helical flexure may take the form of a flat ring (or annulus) and at least three flexures extending from the flat ring. The flat ring and flexures may be a single-piece.

[0172] The helical flexure bearing 1090 includes a circular annulus 1091 having a central aperture 1009 and connected to three or more (preferably four or five) helical beam portions 1092. In the example shown in Figure 10, there are four helical beam portions 10921, 10922, 1092a, 10924. At the end not connected to the circular annulus, each helical beam portion 10921, 10922, 1092a, 1092 is connected to a pad 10931, 10932, 1093a, 10934, for example for connection to a layer or structure below (in relation to the primary axis z as drawn) the circular annulus 1091.

[0173] Each helical beam portion 10921, 10922, 1092a, 10924 is approximately tangential to the circular annulus 1091 (in the same sense) and its span includes both a first component parallel to the plane containing the first and second axes x, y and a second component parallel to the primary axis z. If the pads 10931, 10932, 1093a, 10934 are clamped and a force is exerted upwards (positive z direction) on the circular annulus 1091, then in response the helical beam portions 10921, 10922, 1092a, 10924 will deflect in the direction of that force. However, in doing so, the ends connected to the circular annulus 1091 are also deflected closer the respective pad 10931, 10932, 1093a, 10934, causing the circular annulus 1091 to rotate clockwise about an axis parallel to the primary axis z. Conversely, a force exerted downwards (negative z direction) on the circular annulus 1091 will result in both a downwards movement of the circular annulus 1091 and also an anti-clockwise (counter-clockwise) rotation of the circular annulus 1091.

[0174] In this way, the helical flexure bearing 1090 acts to convert a relative displacement parallel to the primary axis z into a rotation about the primary axis z and to convert a rotation about the primary axis z into a relative displacement parallel to the primary axis z. However, the movements are not independent of one another, and relative to clamped pads 10931, 10932, 1093a, 10934 the circular annulus 1091 is constrained to move along an approximately helical path. Since this does not reflect independent degrees-of-freedom, the motion will be denoted as [Tz, Rz] to highlight the relationship between translation Tz parallel to the primary axis z and rotation Rz about the primary axis z for this bearing type.

[0175] Although the helical beam portions 10921, 10922, 1092a, 10924 shown in Figure 10 are curving, in other examples of helical flexure bearings 1090 the helical beam portions 1091 may be straight.

[0176] With reference to Figure 10A, an actuator assembly comprising a helical bearing arrangement is described. The actuator assembly 1 comprises a support structure 2 and a first part 10 which is movable relative to the support structure 2 along a helical axis H. The helical axis may otherwise be referred to as the primary axis of the actuator assembly. The actuator assembly further comprises a third part 14 which is movable relative to the support structure 2 and relative to the first part 10.

[0177] The first part 10 is supported on the third part 14 by a helical bearing arrangement 40 comprising two helical bearings 42. The third part 14 is supported on the support structure 2 by a further bearing arrangement 50 which is also a helical bearing arrangement.

[0178] The further bearing arrangement 50 comprises two helical bearings 51. The further bearing arrangement 50 is arranged to guide helical movement of the third part 14 with respect to the third part 14 around the axis H. The helical motion may be along a circular helix, i.e. a helix with constant radius, but in general any helix is possible. The pitch of the helix may be constant or vary along the helical motion. The helical movement may be only a portion (e.g. equal to or less than one quarter, half, or three quarters) of a full turn of the helix.

[0179] The actuator assembly 1 comprises first and second actuating units 141 and 142, shown schematically in Figure 10A. The first actuating unit 141 is configured to, upon actuation, drive rotation of the third part 14 in a first sense around the axis H (e.g. drive rotation of the third part 14 in a clockwise direction when the actuator assembly 1 is viewed along the axis H). The first actuating unit 141 is coupled to the support structure 2 and the third part 14.

[0180] The second actuating unit 142 is configured to, upon actuation, drive rotation of the third part 14 in a second opposite sense around the axis H (e.g. drive rotation of the third part 14 in an anti-clockwise direction when the actuator assembly 1 is viewed along the axis H). The second actuating unit 142 is coupled to the support structure 2 and the third part 14. In other words, the first and second actuating units 141, 142 are configured to, upon contraction, drive relative rotation between the third part 14 and the support structure 2 around the axis H in opposite senses around the axis H. This rotation is converted into helical movement by the further bearing arrangement 50. The actuator assembly 1 comprises a rotation control arrangement. The rotation control arrangement is capable of limiting rotation of the first part 10 around the axis H (i.e. the primary axis) relative to the support structure 2. The rotation control arrangement comprises an anti-rotation arrangement 13 (i.e. a third bearing arrangement) configured to prevent rotation of the first part 10 around the axis H relative to the support structure 2. In other words, the anti-rotation arrangement 13 is configured to restrict movement of the first part 10 to be in a linear direction along the axis H. The anti-rotation arrangement 13 is a linear bearing arrangement comprising, for example, plain bearings and / or ball bearings. The anti-rotation arrangement 13 is configured to guide the first part 10 to move relative to the support structure 2 only along the direction of the axis H.

[0181] As mentioned above, the first part 10 is supported on the third part 14 by a helical bearing arrangement 40 comprising two helical bearings 42. The helical bearings 42 are inclined in a sense opposite to the sense in which the helical bearings 51 are inclined. The helical bearings 51 are positively angled relative to a plane normal to the axis H, and the helical bearings 42 are negatively angled relative to a plane normal to the axis H. In other words, the helical bearings 51 has a positive slope / gradient relative to a plane normal to the axis H, and the helical bearings 42 have a negative slope / gradient relative to a plane normal to the axis H. The helical bearing arrangement 40 and the further bearing arrangement 50 are both configured to guide helical movement around a common axis H.

[0182] When the actuating units 141,142 drive rotation of the third part 14 around the axis H relative to the support structure 2, the further bearing arrangement 50 converts the rotation of the third part 14 into helical movement of the third part 14 around the axis H relative to the support structure 2. The helical motion of the third part 14 includes a component of translational movement along the axis H and a component of rotational movement around the axis H.

[0183] When the third part 14 is driven to rotate in a first direction and thus move helically, due to the helical bearing arrangement 40 between the third part 14 and the first part 10 (which is inclined in an opposite sense to the further bearing arrangement 50) and the anti-rotation arrangement 13 preventing rotation of the first part 10 about the axis H, the first part 10 is driven to move translationally upwards (with no rotation) along the axis H relative to the third part 14 and the support structure 2. Rotation of the third part 14 in a second opposite direction around the axis H causes the third part 14 to translationally move downwards along the axis H relative to the support structure 2, and simultaneously causes the first part 10 to translationally move downwards (with no rotation) along the axis H relative to the third part 14. The first part, third part and the support structure may be biased together, e.g. by gravity or by a biasing arrangement. The distance moved by the first part 10 along the axis H may be greater than the distance moved by the third part 14 in this direction. For example, when the helical bearings 51, 42 are of the same design (except for being in opposite senses), the distance moved by the first part 10 along the axis H may be twice the distance moved by the third part 14 in the direction along the axis H.

[0184] The total linear movement of the first part 10 along the axis H with respect to the support structure 2 is equal to the sum of (i) the amount the third part 14 is moved along the axis H relative to the support structure 2 and (ii) the amount the first part 10 is moved along the axis H relative to the third part 14. The first part 10 may move a greater distance along the primary axis H for a given level of actuation of the actuator arrangement as compared to a situation in which the further bearing arrangement 50 is not a helical bearing arrangement.

[0185] The bearings 51, 42 are illustrated as being rolling bearings but it will be appreciated that plain bearings may be used instead. As described above, friction in the plain bearings may be used to provide zero- hold-power functionality to the system. Shallow-angle bearings may be used in such a system to provide the benefits described above (tilt prevention and smooth operation of the actuator assembly 1) in conjunction with the actuating units 141, 142 providing amplification of the rotation of the third part 14. In this way, a large amount of movement of the first part along the axis H may be provided whilst avoiding tilting of the third part 14 and providing for smooth operation (e.g. avoiding stick-slip behaviour).

[0186] Actuating unit

[0187] Various bearing arrangements have been described which convert rotation of a rotatable part (the movable part 10 in Figures 3-7 and the third part 28 in Figures 8 and 9) of an actuation apparatus into movement of a first part along an axis. Such rotation is driven by one or more actuating units.

[0188] An example of an actuating unit will now be described with reference to Figures 11A and 11B. Figure 11A shows a perspective view of an example of an actuating unit 80. Figure 11B shows part of the actuating unit 80 in plan view.

[0189] A single actuating unit 80 is shown in Figures 11A and 11B, but it will be appreciated that the actuator apparatus 1 generally has multiple actuating units 80, each of which may include the same components described with reference to Figures 11A and 11B. The actuating unit 80 includes a body portion 81 to which several other components of the actuating unit 80 are connected as described below. Typically, the body portion 81 is relatively rigid compared to the other components of the actuating unit, and does not deform significantly on actuation of the actuating unit 80. In some examples, the body portion 81 is not a distinct part of the actuating unit 80. For example, the body portion 81 may be defined as part of one of the other components of the actuating unit 80 or simply as a connection point between other components of the actuating unit 80. The actuating unit 80 also includes a force-modifying flexure 82. The force-modifying flexure 82 is connected between the body portion 81 and the support structure 2. One end of the force-modifying flexure 82 is connected to the body portion 81. The other end of the force-modifying flexure 82 is connected to the support structure 2, e.g. via a foot portion 86. The foot portion 86 is fixed relative to the support structure 2. In the depicted design, the force-modifying flexure 82 is formed integrally with the foot portion 86 and with the body portion 81, for example from a single sheet of material (such as metal). The force-modifying flexure 82 allows the body portion 81 to pivot relative to the support structure 2 about an effective pivot point P. Although the effective pivot point P is shown in Figure 11B as being positioned in the middle of force-modifying flexure 82, the effective pivot point P may have a different position and also need not lie on the force-modifying flexure 82. Such pivotal movement of the body portion 81 relative to the support structure 2 is initially in a direction that is substantially perpendicular to the force-modifying flexure 82.

[0190] The actuating unit 80 also includes an SMA element 84. In this example, the SMA element 84 is an SMA wire 84. The SMA wire 84 is connected between the body portion 81 and the support structure 2. One end of the SMA wire 84 is connected to the support structure 2, e.g. by a crimp 87. The other end of the SMA wire 84 is connected to the body portion 81, e.g. by a crimp 85.

[0191] The actuating unit 80 also includes a coupling link 83. In this example, the coupling link 83 is a coupling flexure 83. The coupling flexure 83 is connected between the body portion 81 and the movable part 10 (which may be the first part or the third part in the examples above). One end of the coupling flexure 83 is connected to the body portion 81. The other end of the coupling flexure 83 is connected to the movable part 10. The coupling link 83 transfers or transmits an actuating force F from the body portion 81 to the movable part 10. The coupling link 83 is compliant (i.e. deformable) in a direction (or in multiple directions) perpendicular to the actuating force F. This allows the movable part 10 to move in directions other than the direction of the coupling flexure 83 and actuating force F. This can be needed, for example, where different actuating units 80 cause the movable part 10 to move in different directions. In this example, the body portion 81, the force-modifying flexure 82, the coupling flexure 83 and the foot portion 86 are integrally formed, for example from a single sheet of material (such as metal). In other examples, one or more or these features, if present, may be formed from different parts or materials.

[0192] The SMA wire 84 is arranged, on contraction, to apply an input force Fi on the body portion 81. The input force Fi acts parallel to the length of the SMA wire 84. The force-modifying flexure 82 and the body portion 81 are arranged to modify the input force Fi so as to give rise to the actuating force F, which is transmitted from the body portion 81 to the movable part 10 by the coupling flexure 83. In particular, the input force Fi deforms the force-modifying flexure 82, thereby causing the body portion 81 to pivot about the effective pivot point P. In simple terms, the force-modifying flexure 82 and the body portion 81 act like a lever. The force-modifying flexure 82 and the body portion 81 may modify the direction and / or the magnitude of the input force Fi so as to give rise to the actuating force F.

[0193] In the example illustrated in Figures 11A and 11B, the coupling flexure 83 is at an angle of ~90° relative to the SMA wire 84. Also, in this example, the force-modifying flexure 82 is arranged at an angle a of ~30° relative to the SMA wire 84, and the force-modifying flexure 82 is placed in tension on contraction of the SMA wire 84. Hence, on contraction of the SMA wire 84 and on resulting deformation of the force-modifying flexure 82, the body portion 81 initially moves at an angle of ~60° (90°-a) relative to the length of the SMA wire 84. Thus, it will be appreciated that, in this example, the force is de-amplified and the stroke is amplified, while the direction of the forces / movements is changed by an angle of ~90°. More generally, the change in direction of the force depends on the angle between the SMA wire 84 and the coupling flexure 83. Also more generally, the change in magnitude of the force is dependent on the ratio of i) the distance Ds from the effective pivot point P to the line on which the SMA wire 84 lies and ii) the distance De from the effective pivot point P to the line on which the coupling flexure 83 lies. In particular, F / Fi is proportional to Ds / Dc. If the SMA wire 84 lies on a line that is closer to the effective pivot point P than the line on which the coupling flexure 83 lies, then the input force Fi is de-amplified. At the same time, the movement of the movable part 10 is amplified, i.e. increased relative to a change in length of the SMA wire 84. Alternatively, if the SMA wire 84 lies on a line that is further away from the effective pivot point P than the line on which the coupling flexure 83 lies, then the input force Fi is amplified. At the same time, the movement of the movable part 10 is de-amplified, i.e. decreased relative to a change in length of the SMA wire 84. The actuating unit 80 can thus be configured to amplify movement or to amplify force due to contraction of the SMA wire 84. The actuating unit 80 can also be configured to change the direction of the input force Fi. In some examples, the actuating unit 80 is configured to change the direction of the input force Fi without changing the magnitude of the force or movement. The ratio Ds / Dc is dependent on the location of the end of the SMA wire 84 that is connected to the body portion 81, and on the location of the end of the coupling flexure 83 that is connected to the body portion 81. By way of example, the distance De could be increased by connecting the coupling flexure 83 further to the left of body portion 81 shown in Figure 11B, thereby decreasing Ds / Dc and so increasing the amount of stroke amplification. The ratio Ds / Dc is also dependent on the orientation of the SMA wire 84, and on the orientation of the coupling flexure 83. Such orientations can be defined with reference to the force-modifying flexure 82 (as above) or any suitable reference line. By way of example, the distance Ds could be decreased by angling the SMA wire 84 shown in Figure 11B so that it passes closer to the effective pivot point P, thereby decreasing Ds / Dc and so increasing the amount of stroke amplification. In summary, the amount by which the force-modifying flexure 82 amplifies or deamplifies the force / stroke of the SMA wire 84 may be tailored by:

[0194] - adjusting the orientation of the SMA wire 84 (and thus of the input force Fi);

[0195] - adjusting the location of the connection point between the SMA wire 84 and the body portion 81 (and thus the location at which the input force Fi acts on the body portion 81);

[0196] - adjusting the orientation of the coupling flexure 83 (and thus of the actuating force F); and / or

[0197] - adjusting the location of the connection point between the coupling flexure 83 and the body portion 81 (and thus the location from which the body portion 81 applies the actuating force F).

[0198] In some examples, at least one actuating unit 80 (preferably each actuating unit 80) is configured such that the force-modifying flexure 82 and the body portion 81 amplifies an amount of contraction of the SMA wire 84. Such amplification, for example, may be by a factor greater than 1.5, preferably greater than 2, further preferably greater than 3. For this purpose, in the example illustrated in Figures 11A and 11B, the angle a between the SMA wire 84 and the force-modifying flexure 82 may be in the range from 0 to 45 degrees, preferably from 13 to 40 degrees. However, in general, the angle a may have other values and the connection points of the SMA wire 84 and / or coupling flexure 83 to the body portion 81 may be adjusted to achieve a desired amount of amplification.

[0199] As described above, in the example illustrated in Figures 11A and 11B, the coupling flexure 83 is at an angle of about 90 degrees relative to the SMA wire 84. This allows the actuating unit 80 to fold around a corner of the movable part 10 in a compact manner. The angle between the coupling flexure 83 and the SMA wire 84 may be in the range from 70 to 12 degrees, preferably from 80 to 20 degrees. However, in general, the angle between coupling flexure 83 and SMA wire 84 may be outside these ranges.

[0200] For instance, in the actuating unit 80 illustrated in Figure 11C, the force-modifying flexure 82, the coupling flexure 83 and the SMA wire 84 are substantially parallel to one another. In the above-described examples, the actuating unit 80 is arranged in a plane. In particular, the SMA wire 84, the coupling flexure 83 and the force-modifying flexure 82 are arranged so as to substantially extend in a common plane, at least when the actuator assembly 1 is in an initial configuration. This allows for a compact configuration of the actuating unit 80. The body portion 81, when embodied by a plate, may further be arranged to extend in the plane. However, in general, the components of the actuating unit 80 need not be arranged in a common plane. The SMA wire 84 and / or the coupling flexure 83 may be angled relative to the plane, for example.

[0201] In the above-described examples, the force-modifying flexure 82 is placed in tension on contraction of the SMA wire 84. This reduces the risk of buckling of the force-modifying flexure 82, reducing the risk of damage to the actuator assembly 1 and making the actuator assembly 1 more reliable. However, the force-modifying flexure 82 could instead be arranged so as to be placed under compression on contraction of the SMA wire 84. With reference to Figure 11B, for example, the force-modifying flexure 82 could extend to the bottom-right from the connection point between the body portion 81 and the force-modifying flexure 82, and so be placed under compression on contraction of the SMA wire 84. An arrangement in which the force-modifying flexure 82 is placed under compression is disclosed in WO 2022 / 084699 Al, which is herein incorporated by reference in its entirety.

[0202] In the above-described examples, the force-modifying flexure 82 and the SMA wire 84 connect at one end to the support structure 2, and the coupling flexure 83 connects at one end to the movable part 10. In general, this arrangement may also be reversed, with the force-modifying flexure 82 and the SMA wire 84 connecting at one end to the movable part 10, and the coupling flexure 83 connecting at one end to the support structure 2.

[0203] In the above-described examples, the actuating unit 80 includes a coupling link 83 in the form of a coupling flexure 83. The purpose of the coupling link 83 is to allow movement of the movable part 10 in directions perpendicular to the actuating force F. In general, however, the actuating unit 80 need not include a coupling link 83, e.g. in examples in which there is no movement of the movable part 10 in directions perpendicular to the actuating force F. Furthermore, the coupling link 83 may be embodied by components other than the coupling flexure 83, for example by a ball bearing or plain bearing configured to transmit the actuating force F to the movable part 10 while allowing movement of the movable part 10 in directions perpendicular to the actuating force F. Such alternative examples of the coupling link 83 are disclosed in WO 2022 / 084699 Al, which is incorporated herein in its entirety. The coupling link 83 may (or may not) be formed by an SMA element such as an SMA wire, which may (or may not) be integral with the SMA wire 84 and may (or may not) be driven together with the SMA wire

[0204] 84. Furthermore, instead of the force-modifying flexure 82, the actuator assembly may include a different type of force-modifying element configured to enable the above-described movement of the body portion 81 relative to the support structure 2. Such a force-modifying element may include, for instance, a rigid member with one end connected to the support structure 2 via a suitable pivoting connection (e.g. a pin joint) and the other end connected to the body portion 81. Further examples of forcemodifying elements are disclosed in WO2022084699A1, which is incorporated herein by reference.

[0205] With reference to Figure 12A and 12B, a further actuating unit is described.

[0206] Figure 12A shows two actuating units 80, side by side. Figure 12B also shows two actuating units 80, side by side.

[0207] Each actuating unit 80 comprises a body portion 81 to which several components are connected. The body portion 81 is configured not to deform during use (i.e. during contraction or actuation of the SMA wires). The body portion 81 is thus relatively rigid.

[0208] Each actuating unit 80 further comprises a force-modifying flexure 82. The force-modifying flexure 82 is connected between the body portion 81 and the support structure 2. One end 82m of the forcemodifying flexure 82 is connected to the body portion 81. The other end 82s of the force-modifying flexure 82 is connected to the support structure 2. The force-modifying flexure 82 may, on flexing, allow the body portion 81 to move relative to the support structure 2 in a direction that is substantially orthogonal to the force-modifying flexure 82. The force-modifying flexure 82 effectively allows the body portion 81 to pivot relative to the support structure 2. The force-modifying flexure 82 is configured to provide an effective pivot point P about which the body portion is allowed to pivot relative to the support structure 2.

[0209] The actuating unit 80 further comprises an SMA wire 84. The SMA wire 84 is connected between the body portion 81 and the support structure 2. One end of the SMA wire 84 is connected to the support structure 2, in particular by a crimp 35s. The other end of the SMA wire 70 is connected to the body portion 81, in particular by a crimp 35m.

[0210] The actuating unit 80 further comprises a coupling flexure 83. The coupling flexure 83 is connected between the body portion 81 and the movable part 10. One end of the coupling flexure 83 is connected to the body portion 80a. The other end of the coupling flexure 83 is connected to the movable part 10. The SMA wire 84 is arranged, on contraction, to apply an input force Fi on the body portion 81. The input force acts parallel to the length of the SMA wire 84. The force-modifying flexure 82 is arranged to modify the input force Fi so as to cause the coupling flexure 83 to apply an actuating force Fa to the movable part 10. In particular, in the depicted embodiment the force-modifying flexure 82 is placed in tension on contraction of the SMA wire 84. The force-modifying flexure 82 is arranged at an angle and / or offset relative to the SMA wire 84. As a result, the body portion 81 is arranged, on SMA wire contraction, to deform on contraction of the SMA wire 84. The body portion 81 pivots about the effective pivot point provided by the force-modifying flexure 82. The force-modifying flexure 82 thus converts the input force Fi, in particular the magnitude and direction thereof, into the actuating force Fa.

[0211] The actuating unit 80 can be configured to amplify movement or to amplify force due to contraction of the SMA wire 84. In the depicted embodiment, the distance between the input force Fi applied by the SMA wire and the effective pivot point P is smaller than the distance between the actuating force Fa and the effective pivot point P. The actuating unit 80 effectively acts as a lever to amplify the movement of contraction of the SMA wire 84.

[0212] In some embodiments, at least one actuating unit 80, preferably each actuating unit 80, is configured such that the force-modifying flexure 82 amplifies an amount of contraction of the SMA wire 84 to a relatively greater amount of movement of the movable part 10 relative to the support structure 2. Such amplification, for example, may be by a factor greater than 1.5, preferably greater than 2, further preferably greater than 3. This may be achieved, for example, by appropriate selection of the distance between the SMA wire 84 and the effective pivot point P, for example by modifying the angle between SMA wire 84 and force-modifying flexure 82 or by altering the extent of the body portion 81. The angle between SMA wire 84 and force-modifying flexure 82 may be in the range from 0 to 45 degrees, preferably from 13 to 40 degrees.

[0213] The coupling flexure 83 is compliant in a direction perpendicular to the actuating force Fa. This allows the movable part 10 to move in a direction perpendicular to the actuating force Fa, and in a direction perpendicular to the coupling flexure 83, for example due to actuation of a different actuating unit 80.

[0214] In the above-described embodiments, the force-modifying flexure 82 is placed in tension on contraction of the SMA wire 84. This can reduce the risk of buckling of the force-modifying flexure 82. However, in general, the force-modifying flexure 82 could also be arranged so as to be placed under compression on contraction of the SMA wire 84. Further details and alternative examples of actuating units are described in WO 2022 / 084699 Al, which is incorporated by reference.

[0215] In the above-described embodiments, the force-modifying flexure 82 and the SMA wire 84 connect at one end to the support structure 2, and the coupling flexure 83 connects at one end to the movable part 10. In general, this arrangement may also be reversed, with the force-modifying flexure 82 and the SMA wire 84 connecting at one end to the movable part 10, and the coupling flexure 83 connecting at one end to the support structure 2.

[0216] Figures 12A and 12B show pairs of actuating units 80 that are integrally formed. In particular, the body portions 81, force-modifying flexures 82 and coupling flexures 83 of the pair of actuating units 80 may be integrally formed, i.e. formed from the same material. The body portions 81, force-modifying flexures 82 and coupling flexures 83 of the pair of actuating units 80 may be formed from a single metal sheet, for example by etching. The pair of actuating units 80 may connect at the movable part 10 when assembled in an actuator. The pair of actuating units 80 may extend substantially in a common plane.

[0217] As shown in Figure 12A, the SMA wires 84 of the pair of actuating units 80 may cross over when viewed in a direction perpendicular to the SMA wires 84 and / or coupling flexures 83. Alternatively, as shown in Figure 12B, the SMA wires 84 of the pair of actuating units may be arranged to be parallel.

[0218] The body portion 81 of the actuating units 80 of Figures 12A and 12B comprises two arms extending from the connection point 82m to the force-modifying flexure 82 back towards the effective pivot point P. The two arms extend towards the force-modifying flexure 82. The two arms extend away from the SMA wire 84 and / or coupling flexure 83. This allows the SMA wire 84 and / or coupling flexure 83 to have an increased length compared to a situation in which the arms do not extend back. A longer SMA wire 84 may extend the stroke capabilities of the actuating units 80. A longer coupling flexure 83 may reduce the lateral stiffness of the coupling flexure 83.

[0219] An arrangement as described in figure 12A or 12B may be used to drive rotation of a rotatable part as described above, to drive movement of a first part along a primary axis. One actuating unit of the pair shown in Figure 12A or 12B may be used to drive rotation of the rotatable part (movable part 10 in Figures 12A and 12B) in a first sense about the primary axis and the other actuating unit of the pair may be used to drive rotation of the rotatable part in a second, opposite sense.

[0220] Arrangement of four actuating units Figure 13 schematically shows a plan view of an example of the actuator assembly 1, showing an arrangement of actuating units 80. In this example, the actuator assembly 1 includes a total of four actuating units 80. The four actuating units 80 may apply actuating forces F between the movable part 10 and the support structure 2. The actuating forces F are applied to the movable part 10 relative to the support structure 2.

[0221] The arrangement of actuating units 80 of Figure 13 may be used to drive rotation of the movable part 10.

[0222] In particular, two actuating units 80 (e.g. the top and bottom actuating units in Figure 13) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 10 and the support structure 2 in a first sense (e.g. clockwise) around the primary axis P (the primary axis P is directed into the page). The other two actuating units 80 (e.g. the left and right actuating units 80 in Figure 13) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 10 and the support structure 2 in a second, opposite sense (e.g. anti-clockwise) around the primary axis P. This allows the movable part 10 to be rotated by simultaneously increasing or decreasing the tension of SMA wires in any of the two actuating units 80.

[0223] As shown, two actuating units 80 may be arranged to apply actuating forces F in a corner of the actuator assembly 1. The other two actuating units 80 may be arranged to apply actuating forces F in another, opposite corner of the actuator assembly 1. The actuator assembly 1, and in particular the movable part 10 and / or the support structure 2, may have a square or rectangular footprint. Each actuating unit 80 may be provided on one of the four sides of the actuator assembly 1. In particular, each actuating unit 80 may bend around a corner of the movable part 10 such that the SMA wire 84 and the coupling flexure 83 of each actuating unit 80 extend along adjacent edges of the movable part 10. So, the actuating unit 80 may be as configured in Figures 11A and 11B, for example. The four SMA wires 82 of the four actuating units 80 may extend along the four different edges of the movable part 10.

[0224] The arrangement of actuating forces F applied between movable part 10 and support structure 2 corresponds to the arrangement of SMA wires 80 described in WO2013 / 175197 Al, which is herein incorporated by reference in its entirety.

[0225] In this example, the actuating forces F are perpendicular to the primary axis P, and may be parallel to the movement plane. However, in general the actuating forces F may be angled relative to the movement plane. The actuating forces F may thus have a component along the primary axis P. This component along the primary axis P may be resisted by the bearing arrangement 40, for example, to provide movement of the movable part 10 in degrees of freedom allowed by the bearing arrangement 40. In some examples, it may even be desirable for actuating forces F to have a component in parallel to the primary axis P, for example so as to load plain or rolling bearings arranged between the movable part 10 and the support structure 2. Alternatively, the component parallel to the primary axis P may reduce a normal force in the bearing arrangement 40. This may be advantageous in a zero-hold-power arrangement, as described above.

[0226] Although the actuator assembly 1 is described herein in the context of four actuating units 80, in general the actuator assembly 1 may include fewer actuating units 80. For example, the actuator assembly 1 may include two actuating units 80, e.g. the two actuating units 80 depicted in the top left of Figure 13. The forces applied to the movable part 10 by the two actuating units 80 may be opposed by a biasing force of one or more resilient elements, such as springs. With reference to Figure 13, the two actuating units 80 in the bottom right corner may be replaced with springs applying biasing forces along the corresponding depicted arrows, for example.

[0227] As mentioned above, Figure 14a shows an object 200 in the real world, viewed by eyes 202 of a user. The vergence 206 of the eyes 202 matches the accommodation 208 of the eyes 202.

[0228] Figure 14b shows a scenario in which the object 200 is shown on a display 204, appearing to be displayed in the same position as the real-world scenario. The user's eyes focuses on the virtual image created by the display and optics, whereas the vergence of the eyes is based on the stereoscopic distance at which the object appears to be displayed. The accommodation 208 therefore does not match the vergence 206. Vergence accommodation conflict (VAC) therefore arises and the user may feel some discomfort, for example nausea.

[0229] As explained above, a way of correcting for VAC is to alter the virtual image distance. This may be achieved by changing a distance between a display and the associated optics (e.g. one or more lenses). This change in virtual image distance may also be used to account for a user's eye prescription.

[0230] Other variations

[0231] It will be appreciated that there may be many other variations of the above-described examples.

[0232] For example, instead of a force-modifying flexure, the actuator assembly may include another type of force-modifying element configured to enable the above-described movement of the body portion 81 relative to the support structure 2. Such a force-modifying element may include, for instance, a rigid member with one end connected to the support structure 2 via a suitable pivoting connection (e.g. a pin joint) and the other end connected to the body portion 81. SMA

[0233] 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.

[0234] It will be appreciated that a reference to a component being "connected between" two other components means, for example, that the component is directly or indirectly connected to each of the other components. Such an indirect connection may involve a connection via further component(s) (e.g. a connector) with fixed position(s) relative to one of the other components. Such an indirect connection may involve a connection via further component(s) which is / are movable relative to the other components. For example, an SMA element may be connected to the one of the first and second parts via a further flexure, e.g. as described in WO 2022 / 144541 (which is herein incorporated by reference).

[0235] The third part may be referred to as a second part.

Claims

Claims1. An actuator assembly comprising: a support structure defining a primary axis; a first part that is movable relative to the support structure; a helical bearing arrangement arranged to effect movement of the first part with respect to the support structure along the primary axis; and one or more actuating units each configured to apply an actuating force capable of driving rotation of a rotatable part of the actuator assembly about the primary axis which the helical bearing arrangement converts into said movement of the first part along the primary axis, wherein each actuating unit comprises: a body portion; a shape memory alloy, SMA, element connected between the body portion and the support structure, and configured, on actuation, to apply an input force to the body portion; and a force-modifying element connected between the body portion and the support structure and configured to modify the input force so as to give rise to the actuating force.

2. An actuator assembly according to claim 1, wherein each actuating unit further comprises a coupling link connected between the body portion and the rotatable part, wherein the coupling link is configured to transmit the actuating force from the body portion to the rotatable part, and wherein the coupling link is compliant in a direction perpendicular to the actuating force.

3. An actuator assembly according to claim 1 or 2 wherein the one or more actuating units are arranged to drive movement of a first surface across a second surface, wherein the actuator assembly is arranged such that the first surface and the second surface are biased together with a normal force, thereby generating a frictional force therebetween to retain the first part in position relative to the support structure when the one or more SMA elements are unpowered.

4. An actuator assembly according to claim 3 arranged such that the normal force between the first surface and the second surface remains substantially constant on actuation of the one or more actuating units.

5. An actuator assembly according to claim 3 arranged such that the normal force between the first surface and the second surface is reduced on actuation of the one or more actuating units.

6. An actuator assembly according to any preceding claim wherein the one or more actuating units comprises: a first actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in a first sense about the primary axis relative to the support structure; and a second actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in a second sense about the primary axis relative to the support structure, wherein the first sense is opposite to the second sense.

7. An actuator assembly according to claim 6 when dependent on claim 5, wherein the first and second actuating units are arranged to apply a torque about an axis perpendicular to the primary axis so as to reduce the normal force between the first surface and the second surface.

8. An actuator assembly according to claim 6 or 7, wherein the one or more actuating units comprises: a third actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the first sense about the primary axis relative to the support structure; and a fourth actuating unit arranged to apply an actuating force capable of driving rotation of the rotatable part in the second sense about the primary axis relative to the support structure.

9. An actuator assembly according to any preceding claim wherein the rotatable part is the first part.

10. An actuator assembly according to any of claims 1 to 8, further comprising: a third part, wherein the third part is movable relative to the support structure and the first part and wherein the helical bearing arrangement supports the first part on the third part; and a further bearing arrangement supporting the third part on the support structure, wherein the rotatable part is the third part.

11. An actuator assembly according to claim 10 arranged such that the third part is allowed to rotate relative to the support structure about the primary axis but is constrained from translating along the primary axis or an axis perpendicular to the primary axis and from rotating about any axis perpendicular to the primary axis.

12. An actuator assembly according to claim 10, wherein the further bearing arrangement comprises a helical bearing.

13. An actuator assembly according to any preceding claim, wherein the helical bearing arrangement comprises one or more bearing surfaces, wherein the one or more bearing surfaces are at a non-zero angle to a plane perpendicular to the primary axis, wherein the angle is less than 45 degrees.

14. An actuator assembly according to any preceding claim, wherein the one or more actuating units are arranged to drive rotation of the rotatable part about the primary axis such that a point on the rotatable part moves along a circular path by a distance which is greater than a change in length of the SMA element on contraction of the SMA element.

15. An actuator assembly according to any preceding claim, wherein the support structure or the first part comprises an optical component having an optical axis and the primary axis is aligned with the optical axis.

16. An actuator assembly according to claim 15, wherein the optical component is a lens.

17. An actuator assembly according to any preceding claim, wherein the helical bearing arrangement comprises a plain bearing.

18. An actuator assembly according to any preceding claim configured such that on contraction of the one or more SMA elements of the one or more actuating units, the first part moves along the primary axis by a distance which is greater than a change in length of the one or more SMA elements.

19. An actuator assembly according to any of claims 1 to 17 configured such that on contraction of the one or more SMA elements of the one or more actuating units, the first part moves along the primary axis by a distance which is less than a change in length of the one or more SMA elements.

20. An actuator assembly according to any preceding claim, wherein a component of the input force along the primary axis has a smaller magnitude than a component of the input force in any direction perpendicular to the primary axis.

21. An actuator assembly according to any preceding claim, wherein the helical bearing arrangement comprises one or more flexures arranged to guide movement of the first part along the primary axis on rotation of the rotatable part relative to the support structure.

22. An actuator assembly according to any preceding claim, wherein the helical bearing arrangement comprises one or more rolling bearings.

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