Actuator assembly

Abstract

An actuator assembly comprising a first part, a second part that is moveable relative to the first part, and one or more actuating units. Each actuating unit is configured to apply an actuating force to the second part capable of moving the second part relative to the first part, wherein at least a first actuating unit comprises at least a first SMA element configured, on actuation, to generate an input force so as to give rise to the actuating force. The first SMA element is slack when not actuated, and the actuator assembly is configured such that the first SMA element, when slack, is constrained from entering a restricted area.

Description

[0001] ACTUATOR ASSEMBLY

[0002] Field

[0003] The present application relates to an actuator assembly. According to some examples the actuator assembly comprises a variable aperture assembly.

[0004] Background

[0005] There are a variety of apparatuses in which it is desired to provide control of a movable element. SMA elements, for instance SMA wires, may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA actuator required to apply a given force to the movable element can be small.

[0006] One type of apparatus in which SMA wire is known for use as an actuator is in miniature cameras, for example those used in smartphones or other portable electronic devices. W02011 / 104518 discloses examples of SMA actuation apparatuses which are suitable for use in miniature cameras.

[0007] A variable aperture (VA) assembly can be used to provide an aperture of a controlled size. For example, in the context of a camera, a VA assembly may be used to control the size of the aperture. For example, different aperture sizes may be used for different focal lengths.

[0008] Variable aperture assemblies within a camera may use SMA elements (for instance, SMA wires) to move the blades to adjust the size of the variable aperture. An example of such a variable aperture assembly is disclosed in W02024 / 057042: the variable aperture assembly comprises a base, a rotatable part, and an actuator assembly configured to drive rotation of the rotatable part relative to the base about a primary axis to any rotational position within a range of movement. The actuator assembly includes at least one SMA element coupled between the base and the rotatable parts. A plurality of blades is connected to the base and the rotatable part. Rotation of the rotatable part drives rotation of each blade of the plurality of blades to change the size of the variable aperture.

[0009] SMA elements, for instance wires, may slack when not actuated. For a variable aperture assembly within a camera, if the slack SMA wire bows towards the optical axis it may move towards or enter the field of view of an image sensor forming part of a camera incorporating the variable aperture assembly. This can cause undesirable distortion. More generally, for any type of actuator assembly incorporating one or more SMA elements providing an actuating force, the SMA elements when not actuated may be slack and may move in an uncontrolled way, for instance if the actuator assembly is subject to external forces. There may be regions of the actuator assembly (termed herein restricted areas) where it would be problematic for a slack SMA element to enter. For instance, a restricted area may be an area where a slack SMA element is at risk of rubbing or snagging.

[0010] It is an aim of certain embodiments of the present invention to mitigate the problem of slack SMA elements entering restricted areas of an actuator assembly.

[0011] Summary

[0012] According to a first aspect of the present invention there is provide an actuator assembly comprising: a first part; a second part that is moveable relative to the first part; and one or more actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part, wherein at least a first actuating unit comprises at least a first SMA element configured, on actuation, to generate an input force so as to give rise to the actuating force; wherein the first SMA element is slack when not actuated, and wherein the actuator assembly is configured such that the first SMA element, when slack, is constrained from entering a restricted area.

[0013] An advantage of aspects of the invention is that slack wires within an actuator assembly may be controlled to reduce the risk of a wire entering a restricted area where there is a risk of damage to the wire (such as through rubbing or snagging) or other disadvantageous effects (for an optical system, this may include a wire entering a field of view).

[0014] The restricted area may comprise a part of a three-dimensional space of positions which would be reachable by portions of the first SMA element, when slack, without the first SMA element being constrained.

[0015] The first actuating unit may further comprise connection elements coupling to the first SMA element at its ends. The connection elements may be crimps.

[0016] At least a first connection element may be configured such that the first SMA element, when slack, extends from the first connection element at an angle relative to a wire axis extending from the first connection element along the path followed by the first SMA element when actuated. The first connection element may be configured by angling at least a portion of the connection element relative to the wire axis. Alternatively, the first connection element may be configured by angling the first SMA element relative to at least a portion of the connection element prior to coupling the first SMA element to the connection element. The first SMA element may be shaped proximal to at least a first connection element such that the first SMA element, when slack, extends from the first connection element at an angle relative to an axis extending from the first connection element along the path followed by the first SMA element when actuated.

[0017] The first actuating unit may further comprise at least one redirection element provided proximal to at least a first connection element such that each redirection element bends the first SMA element, when slack, away from a wire axis extending from the first connection element along the path followed by the first SMA element when actuated. The redirection element may comprise a damping gel or a compliant adhesive applied to the first SMA element to control the orientation of the first SMA element. The damping gel or compliant adhesive may couple the first SMA element to a further SMA element, the first part, the second part, or a further part of the actuator assembly.

[0018] The actuator assembly may further comprise at least a first deflection element configured to contact a middle portion of the first SMA element so as to deflect the middle portion of the first SMA element, when slack, away from the restricted area. The first SMA element may be arranged, on actuation, to slide across the first deflection element. The first deflection element may be deformable in a direction along the length of the first SMA element, so as to allow actuation of the SMA element. The first deflection element may comprise a resilient element configured to apply a biasing force to the first SMA element away from the restricted area, and wherein actuating the first SMA element applies a force to the first deflection element opposing the biasing force. The first deflection element may be configured such that when actuated the first SMA element passes the first deflection element in a straight line.

[0019] The actuator assembly may further comprise one or more spacers arranged between the first SMA element and the restricted area so as to limit the first SMA element, when slack, from entering the restricted area, and optionally the one or more spacers are coupled directly or indirectly to one or both of the first and second parts.

[0020] At least one connection element may incorporate or be coupled to a resilient portion configured to apply a tensioning force to the first SMA element to reduce an amount of slack within the first SMA element when not actuated. The first actuating unit may further comprise at least one end stop configured such that the resilient portion causes the connection element to bear against the end stop when the SMA element is actuated. According to another aspect of the present invention there is provided a variable aperture assembly comprising: an actuator assembly as described above, wherein the first part comprises a base and the second part comprise a rotatable part, and wherein the actuating force is capable of rotating the rotatable part relative to the base about a primary axis; and a plurality of blades arranged to define a variable aperture about the primary axis and configured such that rotation of the rotatable part relative to the base effects movement of the blades, thereby changing the size of the variable aperture; wherein the restricted area surrounds the primary axis.

[0021] The restricted area may be generally equal to or exceeds a maximum size of the variable aperture when viewed along the primary axis. The actuator assembly may be configured to prevent the first SMA element, when slack, from fully bowing towards the primary axis. The actuator assembly may be configured to cause the first SMA element, when slack, to bow away from the primary axis. The actuator assembly may be configured to cause the first SMA element to extend from at least one connection element at an angle away from the primary axis relative to an axis extending from where the first SMA element couples to the first connection element along the path followed by the first SMA element when actuated.

[0022] The variable aperture assembly may comprise at least first and second actuating units, each actuating unit comprising at least a first SMA element configured, on actuation, to generate an input force so as to give rise to an actuating force capable of rotating the rotatable part relative to the base about a primary axis, wherein the first and second actuating units are configured to rotate the rotatable part in opposite senses. The variable aperture assembly may comprise a bearing arrangement between the base and the rotatable part configured to constrain movement of the rotatable part to rotation about the primary axis.

[0023] According to yet another aspect of the present invention there is provided a camera comprising a variable aperture assembly as described above; a lens assembly; and an image capture device; wherein the optical axis of the lens assembly coincides with the primary axis, such that light passing through the variable aperture assembly passes is focused by the lens and is received by the image capture device.

[0024] According to yet another aspect of the present invention there is provided an electronic device incorporating the camera according to claim 22.

[0025] An advantage of certain examples of the present invention is that because the actuator assembly is configured to constrain the SMA element (for instance, a wire) from entering a restricted area, the risks associated with uncontrolled movement of a slack SMA element can be reduced. This may reduce the risk of SMA element rubbing or snagging, particularly if the actuator assembly is subject to external forces. For the specific example of a variable aperture assembly, the risk of optical distortion caused by uncontrolled movement of a slack SMA element is reduced.

[0026] In its most general sense, the present invention relates to actuator assemblies providing for relative movement between two parts. This may be translational or rotational movement. A variable aperture assembly is a particular example of such an actuator assembly, and much of the following description relates specifically to a variable aperture assembly. However, unless the context requires otherwise, features described in connection with embodiments of a variable aperture assembly should be considered to apply more generally to actuator assembly.

[0027] According to one embodiment of the present invention the actuator assembly comprises a variable aperture assembly comprising: a base; a rotatable part that is rotatable relative to the base about a primary axis; one or more blades arranged to define a variable aperture, wherein the one or more blades are coupled between the base and the rotatable part such that rotation of the rotatable part relative to the base changes the size of the variable aperture; one or more SMA wires configured, on actuation, to drive rotation of the rotatable part relative to the base about the primary axis to any rotational position within a range of movement, thereby adjusting the size of the variable aperture; wherein the one or more SMA wires are slack when unenergized, and wherein the variable aperture assembly is configured such that the one or more SMA wires, when slack, do not to fully bow towards the primary axis.

[0028] Brief description of the drawings

[0029] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0030] Figure 1 is a schematic plan view of a variable aperture assembly with a relatively closed variable aperture;

[0031] Figure 2 is a schematic plan view of a variable aperture assembly with a relatively open variable aperture;

[0032] Figure 3 is a schematic side view of a variable aperture assembly assembled on a lens assembly; Figures 4a and 4b are schematic plan views of an arrangement of SMA elements for adjusting the variable aperture and illustrate the problem of uncontrolled movement of slack SMA elements for the example of a variable aperture assembly;

[0033] Figures 5A-H and 6 to 8 illustrate alternative approaches for addressing the problem of uncontrolled movement of slack SMA elements for the example of a variable aperture assembly;

[0034] Figure 9A and 9B schematically illustrate another approach for addressing the problem of uncontrolled movement of slack SMA elements;

[0035] Figure 10 is a schematic plan view of an alternative arrangement of SMA elements for adjusting a variable aperture; and

[0036] Figure 11 schematically illustrates an alternative type of actuator assembly.

[0037] Detailed description

[0038] Certain example devices will now be described. Where similar or identical components are used in the different examples, they will be given the same reference numerals. For efficiency, description of similar or identical elements may not be repeated between the examples and characteristics and features of elements are to be understood as applying to those elements in all examples unless the description indicates otherwise.

[0039] Some of the following description, and the accompanying drawings, present different embodiments of a variable aperture assembly or portions of a variable aperture assembly. As previously noted, unless the context requires otherwise, these should be considered to be examples of an actuator assembly providing for relative movement between a first and second parts. Features presented in connection with a variable aperture assembly should be considered to be more generally applicable to any actuator assembly.

[0040] Furthermore, in the following description, the terms "actuator" and "actuating unit" are used broadly synonymously to refer to a part of the actuator assembly that provides a driving force to effect relative rotational movement. The embodiments presented particularly concern actuating units including one or more Shape Memory Alloy (SMA) element. This may be an SMA wire, and where the term SMA wire is used this should be considered to include other suitable forms of SMA elements. Actuation of the actuating unit is achieved by heating the SMA element (for instance, by passing an electrical current) causing it to contract.

[0041] Variable aperture assembly

[0042] Figures 1 to 3 schematically depict a variable aperture (VA) assembly 1. Figure 1 shows a plan view of the VA assembly 1 with a relatively small variable aperture, and Figure 2 shows a plan view of the VA assembly 1 with a relatively large variable aperture. Figure 3 shows a side view of the VA assembly 1 in combination with a lens assembly 50.

[0043] The VA assembly comprises a base 30 and a rotatable part 20. The rotatable part 20 is rotatable relative to the base 30, in particular about a primary axis O. At least one of the base 30 and the rotatable part 20 defines an aperture that allows light or fluid (for instance, gas or liquid) to pass. The aperture surrounds the primary axis O. The aperture may have rotational symmetry about the primary axis O. As illustrated, the rotatable part 20 at least partially surrounds the base. This arrangement may be reversed.

[0044] The base 30 may be fixed within a larger device (such as a smartphone) within which the VA assembly 1 is incorporated. The base 30 may, for example, be fixed relative to a lens element of a lens assembly 50 that is provided in combination with the VA assembly 1. However, in general, the base 30 may also be movable within such a larger device. The base 30 is herein used as a reference structure relative to which movement of other components is described, unless explicitly stated otherwise.

[0045] The VA assembly 1 may comprise a bearing arrangement (not shown) between the rotatable part 20 and the base 30. The bearing arrangement may guide rotation of the rotatable part 20 relative to the base 30. The bearing arrangement may constrain one or more degrees of freedom of movement other than the rotation. For example, the bearing arrangement may constrain movement of the rotatable part 20 relative to the base 30 along the primary axis O. The bearing arrangement may comprise rolling bearings (such as roller or ball bearings), plain bearings (that is, sliding bearings) or flexure bearings (that is, arrangements of flexures constraining degrees of freedom of movement). The VA assembly 1 may also comprise a biasing arrangement (not shown), for instance an arrangement of flexures or other types of spring, for loading the bearing arrangement. The biasing arrangement may further bias the rotatable part 20 to a predetermined rotational position within the range of rotation.

[0046] The VA assembly 1 further comprises at least one blade 40, preferably a plurality of blades 40. The blades 40 may also be referred to as leaves 40. The at least one blade 40 defines a variable aperture. The variable aperture is preferably substantially circular, but in general may have other shapes, depending on the desired application of the VA assembly 1. Each blade 40 is coupled between the base 30 and the rotatable part 20 in a manner such that rotation of the rotatable part 20 relative to the base 30 changes the size of the variable aperture.

[0047] In the embodiment of Figures 1 and 2, each blade 40 is connected to the base 30 via a respective pin 31 and to the rotatable part 20 via a respective pin 21. Rotation of the rotatable part 20 relative to the base 30 causes relative movement of the pins 21, 31, thereby allowing the blades 40 to effectively pivot about the pins 21, 31 so as to change the variable aperture. The pins 21, 31 are spring loaded relative to each other, by flexure 22 in the particular embodiment of Figures 1 and 2, allowing each pin 21 to move along a circular path around respective pin 31 on rotation of the rotatable part 20. The skilled person will understand that this is only one mechanism for driving rotation of the blades 40 and that embodiments of the invention are not limited to the specific pin drive mechanism.

[0048] In Figures 1 and 2, the VA assembly 1 comprises a total of six blades 40. The plurality of blades 40 are stacked in two layers of three blades 40 on top of each other. The two layers overlap when viewed along the primary axis O. However, in general, the VA assembly 1 may comprise any number of blades 40, arranged in any number of layers. In some embodiments, the VA assembly 1 comprises only a single blade. Such a VA assembly 1 may act as a variable shutter, for example. The VA assembly 1 may comprise at least four blades 40, preferably at least 6 blades. In some embodiments, the VA assembly 1 comprises 8 blades 40. A higher number of blades 40 may allow for a more circular variable aperture, at the cost of additional moving parts and a more complex assembly of the VA assembly 1. The blades 40 need not be arranged in layers but may instead overlap sequentially around the variable aperture such that no blade 40 is finally on top.

[0049] In general, the coupling of the blades 40 to the base 30 and the rotatable part 20 may comprise any mechanism allowing movement of the blades 40 upon rotation of the rotatable part 20 so as to adjust the variable aperture. For example, the coupling may comprise a pin-slot arrangement in which one of the pins 21, 31 is allowed to slide within a slot of the blade. In further alternatives pins may not be used at all, and the blades 40 may be arranged so that rotation of the rotatable part 20 relative to the base 30 effects blade movement in other ways.

[0050] The VA assembly 1 further comprises an actuator assembly 10, schematically shown in Figure 3. The actuator assembly 10 is configured to drive rotation of the rotatable part 20 relative to the base 30 about a primary axis O. The actuator assembly 10 may rotate the rotatable part 20 relative to the base 30 to any rotational position within a range of movement. As such, the size of the variable aperture defined by the blades 40 may be adjusted to any size within a continuous range.

[0051] Figure 3 shows the VA assembly 1 in combination with a lens assembly 50. The base 30 of the VA assembly 1 may be mounted on the lens assembly 50, such that the lens assembly 50 is nested or provided within a through hole or opening of the base 30 which extends along a primary axis O of the VA assembly 1. The primary axis O may coincide with the optical axis of the lens assembly 50. The VA assembly 1 may thus adjust the amount of light entering the lens assembly 50. The light enters the lens assembly 50 along an optical path 2. The optical path 2 may be shaped, between the VA assembly 1 and the lens assembly 50, as a cone around the primary axis O.

[0052] Figures 4A and 4B schematically show an embodiment of the actuator assembly 10. The actuator assembly 10 comprises one or more SMA wires 11. The one or more SMA wires 11 are configured, on actuation, to drive rotation of the rotatable part 20 relative to the base 30 about the primary axis O. In some embodiments, the one or more SMA wires 11 drive the rotatable part 20 to any rotational position within a range of movement relative to the base. In some other embodiments, the one or more SMA wires 11 drive the rotatable part 20 to a set of predetermined positions within a range of movement relative to the base. The size of the variable aperture is thereby adjusted.

[0053] The SMA wires 11 are connected between the base 30 and rotatable part 20 by connection elements 42, 43. The connection elements 42, 43 may be crimps, for example. The SMA wires 11 may be directly connected between the base 30 and rotatable part 20, such that the connection elements 42, 43 are directly connected to the base 30 and rotatable part 20. Alternatively, intermediate elements (not shown) may be connected between the connection elements 42, 43 and the base 30 and / or rotating part 20, such that the SMA wires 11 are indirectly connected between the base and rotatable part 20. Such intermediate elements transfer the force in the SMA wires 11 to the rotatable part 20 so as to effect rotation of the rotatable part 20 relative to the base 10.

[0054] As shown in Figures 4A and 4B, the VA assembly 1 may comprise four SMA wires 11. However, the following described techniques for controlling slack SMA wires are applicable to actuator assemblies with any number of SMA wires (or other forms of element), including a single SMA element. Where only a single SMA element is used, its contraction may be opposed by a resilient element to provide a return force. A first pair of SMA wires 11 (for instance the top and bottom wires) are arranged, on contraction, to apply a torque to the rotating part 20 in a first sense (for instance counterclockwise). A second pair of SMA wire 11 (for instance, the left and right wires) are arranged, on contraction, to rotate the rotating part 20 in a second sense (for instance clockwise). The second sense is opposite to the first sense. The four SMA wires 11 are arranged in a loop around the primary axis O. Each SMA wire 11 is arranged on one of four sides that are arranged in a loop around the primary axis O. SMA wires 11 applying a torque in the same direction.

[0055] The SMA wires 11 may be arranged with two-fold rotational symmetry about the primary axis O. This allows the torques applied by the first and / or second pairs of SMA wires 11 to be centered specifically about the primary axis O, reducing and indeed avoiding off-axis forces on the rotatable part 20. Motion of the rotatable part 20 may thus be purely rotational upon actuation of the first and / or second pairs of SMA wires 11, avoiding the need for a bearing arrangement constraining to such rotation about the primary axis O or reducing adverse forces on such a bearing arrangement.

[0056] In some embodiments, the SMA wires 11 of the first pair of SMA wires 11 may be electrically connected. So, the SMA wires 11 need not be independently controllable. A single drive channel may be used to control the first pair of SMA wires 11. Similarly, the SMA wires 11 of the second pair of SMA wires 11 may be electrically connected. So, the SMA wires 11 need not be independently controllable. A single channel is used to control the second pair of SMA wires 11. However, in general, more than 2 channels (for instance, four channels) may be used to drive the four SMA wires 11, and the SMA wires 11 of the pairs need not be electrically connected.

[0057] Although the embodiment of Figures 4A and 4B shows four SMA wires 11, in some other embodiments the VA assembly 1 comprises six or more SMA wires 11. In some embodiments, the VA assembly 1 comprises at least two SMA wires 11 (an example of which is presented below as Figure 10). One SMA wire 11 is arranged, on contraction, to rotate the rotatable part 20 in the first sense, and another SMA wire 11 is arranged, on contraction, to rotate the rotatable part 20 in the second sense. In further alternative embodiments, only a single SMA wire 11 is provided to drive rotation of the rotatable part 20 in the first sense. A biasing element, such as a flexure or other type of spring, may oppose the SMA wire 11 and drive rotation of the rotatable part in the second sense upon cooling of the SMA wire 11.

[0058] As shown in Figure 4A, the SMA wires 11 are held in tension between the connection elements 42, 43 upon actuation (that is, contraction) of the SMA wires 11. The SMA wires 11 may be arranged close to the optical path 2. The SMA wires 11 may be arranged to overlap the aperture in the base 30 and the aperture in the rotatable part 20 when viewed along the primary axis O, or they may be held outside of the aperture. The SMA wires 11 when actuated may be outside of (but close to) the optical path 2. As such, the footprint of the VA assembly 1 (viewed along the primary axis O) may be made compact compared to a situation in which the SMA wires 11 are further away from the primary axis O. Furthermore, by arranging the SMA wires 11 closer to the primary axis O, the amount of rotation achieved by the SMA wires 11 for a given amount of contraction is increased. However, the SMA wires 11 are arranged to be outside the optical path 2 when actuated. The SMA wires 11 thus do not enter the field of view of a camera of which the VA assembly 1 forms a part.

[0059] As shown in Figure 4B, the SMA wires 11 are arranged to be slack between the connection elements 42, 43 when the SMA wires 11 are not energized, that is, when the SMA wires 11 are not in the actuated or contracted state. When not energized, the length of the SMA wire 11 may be greater than the distance between the connection elements 42, 43 to which the SMA wire 11 is connected. The SMA wires 11 thus bow outward from a straight line between the connection elements 42, 43. The inventors have found that the SMA wires 11 may be arranged close enough to the optical path 2 such that, at least at the largest size of the variable aperture, the slack SMA wires 11 bowing towards the primary axis O may enter the optical path 2. This is schematically shown in Figure 4B, in which all four SMA wires 11 bow towards the primary axis O and enter the optical path 2. In practice, if no precautions are taken, the SMA wires 11 may bow randomly in any direction, such that there is a risk that one or more of the SMA wires 11 enter the optical path 2.

[0060] The inventors have realised that there is a risk that SMA wires 11 in the optical path degrade the quality of the image transmitted through the variable aperture 2. The present invention is thus concerned with avoiding or reducing the risk of SMA wires 11 entering the optical path 2.

[0061] According to embodiments of the present invention, the VA assembly 1 is configured such that the SMA wires 11, when slack, do not to fully bow towards the primary axis O. Put more generally, according to embodiments of the present invention an actuator assembly, such as a VA assembly, may be configured such that at least one SMA element, when slack, is constrained from entering a restricted area. The restricted area may comprise the optical path 2 shown in Figures 4a and 4b, or any larger or smaller region formed around the primary axis. For other types of actuator assemblies (but also for VA assemblies) a restricted area may be an area where wire rub / snagging would otherwise be possible. A restricted area comprises a part of a three-dimensional space of positions which would be reachable by portions of the first SMA element, when slack, without the first SMA element being constrained. That is, if the slack SMA element were not constrained, and if the actuator assembly were subject to external forces, the three-dimensional space is defined by those locations within which the SMA element may move. So, for the example of a VA assembly 1, the SMA wires 11 may not be allowed to bow towards the primary axis O fully and so may not be allowed to reach a restricted area around the primary axis O. In some embodiments, the SMA wires 11, when viewed along the primary axis O, do not extend beyond a line connecting the respective connection elements 42, 43. In some embodiments, the SMA wires 11, when viewed along the primary axis O, bow away from the primary axis O when viewed along the primary axis O.

[0062] Controlling a wire angle

[0063] Figures 5 and 6 show embodiments ensuring that the SMA wires 11 are configured, when slack, to bow away from the primary axis O. The inherent properties of the shape memory alloy material may be used to achieve this. Shape memory alloy is a material that may be configured to return to an initial shape.

[0064] As such, the SMA wires 11 may initially (for instance, during manufacturing of the VA assembly 1) be shaped or deformed to bow away from the primary axis O. In particular, the middle portion of the SMA wires 11 may be urged away from the primary axis O. A series of steps for doing so are depicted in order in Figures 5A-H. These steps are shown being performed on a plurality of SMA wires, but they may be applied to a single SMA wire or a subset of SMA wires.

[0065] Figure 5A depicts the SMA wires 11 in an initial configuration, before any steps have been taken to ensure that the SMA wires 11 do not fully bow towards the primary axis O. As shown, at least some of the SMA wires 11 enter the optical path 2.

[0066] Figure 5B shows a set of wire deforming elements 60 that are introduced into the apertures of the rotating part 20 and base 30, between the primary axis O and the SMA wires 11 when viewed along the primary axis O. The deforming elements 60 may be relatively soft so as not to damage the SMA wires 11.

[0067] As shown in Figure 5C, the deforming elements 60 are then moved radially outward from the primary axis O towards a middle portion of the SMA wires 11. The middle portion 11 is a portion between the connection elements 42, 43, preferably cantered between the connection elements 42, 43. The deforming elements 60 slowly move radially outward so as to deform the SMA wires 11 to bow away from the primary axis O.

[0068] A shown in Figure 5D, the deforming elements 60 may be held in this position for several seconds. As shown in Figure 5E, the deforming elements 60 are then moved radially inward towards the primary axis O. The deforming elements 60 may return to the position shown in Figure 5B, when first introduced into the apertures of the base 30 and rotatable part 20. The SMA wires 11 remain in their deformed state, bowing away from the primary axis O. That is, the SMA wires 11 have been deformed such that when on later occasions they are not actuated, and therefore slack, they bow away from the restricted area about the primary axis. The deformation may be proximal to the point at which an SMA wire 11 exits a connection element, for instance a crimp.

[0069] As shown in Figure 5F, the deforming elements 60 are then removed from the apertures of the base 30 and rotatable part 20. The SMA wires 11 remain in their deformed state, bowing away from the primary axis O. The SMA wires 11 are now configured, when slack, to bow away from the primary axis O.

[0070] Figure 5G shows the SMA wires 11 being contracted, for example on actuation of the VA assembly 1. The SMA wires 11 are arranged in tension between the connection elements 52, 53, and so are straight between the connection elements 52, 53.

[0071] Figure 5H shows the SMA wires 11 once cooled again, in the unenergized and uncontracted state. The SMA wires 11 return to their set configuration of bowing away from the primary axis O by virtue of the inherent properties of shape memory alloy.

[0072] The SMA wires 11 may thus be configured (for instance, by deformation) to bow away from the primary axis O, reducing the risk of SMA wires 11 entering the optical path. In some examples this deformation is such that at least one SMA wire 11, when slack bends away from an axis defined by the path that the SMA wire 11 takes extending from a connection element. This bend away from that axis may be away from the primary axis and thus away from the restricted area.

[0073] As a further option, Figure 5D shows that a damping gel or compliant adhesive 61 is applied to the SMA wires 11 close to the connection elements 42, 43 while the SMA wires 11 are bent away from the primary axis. In particular, the damping gel or compliant adhesive is shown applied to SMA wires 11 where a pair of wires cross close to their respective crimps, such that in effect the wires are resiliently bound together. However, for other actuator arrangements including where SMA wires do not cross, the damping gel or compliant adhesive may couple an SMA wire to another part of the actuating unit, or another portion of the actuator assembly such as a static first part or a moving second part. The damping gel or compliant adhesive 61 serves to ensure that the wires exit the connection elements when slack in a direction away from a restricted area, and so this may be referred to as a redirection element. When the SMA wires 11 are actuated, they may pull straight despite the presence of a redirection element, yet when not actuated and hence slack the redirection element may constrain the position of the SMA wire 11 so that it does not enter a restricted area. This use of a damping gel or compliant adhesive 61 may be in addition to or in place of the deformation of the wire described above.

[0074] Figure 6 schematically shows the VA assembly 1 with all SMA wires 11, when slack, configured to bow away from the primary axis O. In the example of Figure 6, the SMA wires 11 may be urged to bend away from the primary axis by arranging the SMA wires 11 such that each SMA wire 11 extends, when slack, from the respective connection elements 42, 43 at an angle away from the primary axis O. In particular, in one example, the crimps may be configured for instance by twisting away from the primary axis O when being arranged to the base 30 and rotatable part 20. So, the crimps may direct the SMA wires 11 away from the primary axis O. It may only be part of the crimp, such as a crimp head, which is twisted to adjust the angle at which a slack wire is directed. In another example a crimp may be configured to control a wire exit angle by angling the direction a wire extends prior to forming or closing the crimp around the wire. This configuration of the crimp to select a wire exit angle for a slack SMA wire may be in addition to or in place of the wire deflection techniques of Figure 5. In some examples the twisting of the crimp or the twisting of the wire exiting the crimp may be limited to a small angle relative to the direction the SMA wire pulls when actuated in order to minimise wire fatigue. For instance, the angle may be limited to less than 10°, such as less than 7.5°.

[0075] Deflection elements

[0076] Figure 7 schematically depicts another embodiment of the present invention. The concept of Figure 7 may be provided in addition or as an alternative to the concept of Figures 5 and 6. As shown, the VA assembly 1 comprises one or more deflection elements 62. Each deflection element 62 is arranged to contact a middle portion of a respective SMA wire 11. The deflection element 62 deflect the SMA wire 11 (in particular the middle portion thereof), when slack, away from the primary axis O.

[0077] The SMA wires 11 may be arranged, on actuation, to slide across the deflection elements 62. Contraction of the SMA wire 11 is thus allowed. Alternatively, the deflection elements 62 may be deformable so as to allow the SMA wires 11 to be tensioned and / or to contract, thereby rotating the rotatable part 20 relative to the base 10. The deflection elements 62 may be deformable in a direction along the length of the respective SMA wire 11, so as to allow actuation of the SMA wire 11. The deflection elements 62 in some examples may comprise crimps that are attached to the wire, or they may comprise hooks that the wire passes over. The deflection elements 62 may comprise a resilient element configured to bias the SMA wire 11 away from the primary axis. The deflection element 62 may deform in a direction perpendicular to the length of the SMA wire, such that on actuation the SMA wire 11 may extend straight between the connection elements 42, 43.

[0078] Spacers

[0079] Figure 8 schematically depicts another embodiment of the present invention. The concept of Figure 8 may be provided in addition or as an alternative to the concept of Figures 5 to 7. As shown, the VA assembly 1 may comprise one or more spacers 64. The spacers 64 are arranged between a respective SMA wire 11 and the variable aperture so as to limit the SMA wire, when slack, from fully bowing towards the variable aperture. The spacers 64 may be arranged on a spacer plate that is fixed relative to the base 30 or relative to the rotatable part 20, or they may be formed directly on portions of the base or the rotatable part. The spacers 64 may be protrusions extending along the primary axis O between the SMA wires 11 and the primary axis O. The spacers 64 may be arranged in close proximity of the SMA wires 11 and are configured to directly contact the SMA wires 11 to prevent the SMA wires 11 from fully bowing towards the primary axis O. In a further example, the spacers 64 may be formed on another component of a camera, for instance on a portion of a lens or a lens support structure.

[0080] Alternative wire deflection of Figure 9

[0081] Figures 9A and 9B schematically depict another embodiment of the present invention. The concept of Figures 9A and 9B may be provided in addition or as an alternative to the concept of Figures 5 to 8. An SMA wire 11 may be connected at respective ends to crimps 42, 43. One or both crimps 42, 43 are sprung away from end stops 71, 72. The crimps 42, 43 may be sprung against end stops 71, 72 using flexures 73, 74 extending from feet 75, 76 which are attached to the base 30 or the rotatable part 20 (not shown). When the SMA wire 11 is actuated, as shown in Figure 9A, crimps 42, 43 will be pulled onto the end stop 71, 72, for precise actuation of the rotatable part 20. When the SMA wire 11 is not actuated, and so slack as shown in Figure 9B, the sprung crimps 42, 43 will pull away from the end stops 71, 72 when the tension is removed. This could remove some of the slack, or all of the slack in the SMA wire 11 depending on the spring parameters, and if the wire is designed to reach its relaxed length before or after the spring normalises.

[0082] The spring force and displacement could be tuned to the application to ensure performance, including the correct amount of slack take-up, and could be used to initially apply slack if appropriate. Different ways of providing the spring force could be investigated. It will be appreciated that different resilient structures for biasing crimps 42, 43 away from end stops can be envisaged. Different types of flexures or different types of resilient structures, for instance a coiled spring, can be used.

[0083] Alternative SMA arrangement for VA

[0084] Figure 10 schematically shows an alternative actuating unit 10. The blades 40 are not shown, but it will be apparent that they may be coupled to the base 30 and the rotatable part in the same way as described above. The actuating unit 10 comprises two SMA wires 11. The SMA wires 11 may be slack when not actuated, and the slack SMA wires may be constrained from entering a restricted area such as the aperture of a field of view of an image sensor using any of the techniques for figures 5 to 9. The whole of the description of Figures 5 to 9 should be considered to apply to Figure 10, adapted as appropriate, particularly for the differing number of wires. The SMA wires 11 are configured, on actuation, to drive rotation of the rotatable part 20 relative to the base 30 about the primary axis O. In some embodiments, the SMA wires 11 drive the rotatable part 20 to any rotational position within a range of movement relative to the base. In some other embodiments, the SMA wires 11 drive the rotatable part 20 to a set of predetermined positions within a range of movement relative to the base. The size of the variable aperture is thereby adjusted.

[0085] The SMA wires 11 are connected between the base 30 and rotatable part 20 by connection elements 42, 43. The connection elements 42, 43 may be crimps. The SMA wires 11 may be directly connected between the base 30 and rotatable part 20 as illustrated, such that the connection elements 42, 43 are directly connected to the base 30 and rotatable part 20. Alternatively, intermediate elements (not shown) may be connected between the connection elements 42, 43 and the base 30 and / or rotatable part 20, such that the SMA wires 11 are indirectly connected between the base and rotatable part 20. Such intermediate elements transfer the force in the SMA wires 11 to the rotatable part 20 to effect rotation of the rotatable part 20 relative to the base 10.

[0086] A first SMA wire 11 (for instance, the left-hand wire) is arranged, on contraction, to apply a torque to the rotatable part 20 in a first sense (for instance, clockwise). A second SMA wire 11 (for instance, the right-hand wire) is arranged, on contraction, to rotate the rotatable part 20 in a second sense (for instance, counterclockwise). The second sense is opposite to the first sense.

[0087] Zero hold power In conventional variable aperture assemblies, the actuator 10 is constantly powered to maintain the position of the blades and maintain the variable aperture at a desired size. The power consumption of such conventional variable aperture assemblies is thus relatively high.

[0088] According to some embodiments, including Figure 10, the variable aperture assembly 1 is configured such that the one or more blades 40 maintain their position when the actuator 10 is not actuating. As such, the actuator 10 only needs to be powered when the size of the variable aperture is adjusted. The energy efficiency of the variable aperture assembly 1 according to embodiments of the present invention is thus improved.

[0089] The position of the blades 40 is maintained via an overall frictional force between components of the variable aperture assembly 1.

[0090] The actuator of Figure 10 is configured to enable a reduction in the frictional force. The variable aperture assembly 1 comprises a biasing arrangement 35. The biasing arrangement 35 may comprise a leaf spring as illustrated. In general, the biasing arrangement 35 may comprise any element or combination of elements capable of applying a force between two or more components of the variable aperture assembly 1. The biasing arrangement 35 may, for example, comprise a resilient or elastic element, such as a spring (for instance, coil spring, flexure, leaf spring), rubber band, or other resilient or elastic element. The biasing arrangement 35 may be a magnetic arrangement, comprising a magnet on one part and a magnet or ferromagnetic material on the other part. The biasing arrangement 35 may be arranged between two parts or be incorporated into one or more of the parts of the variable aperture assembly 1. The biasing arrangement 35 urges the rotatable part 20 in a first direction (downward in Figure 10), thereby urging friction surfaces 21, 31 (provided respectively on the rotatable part 20 and the base 30) against each other. Two pairs of friction surfaces 21, 31 are illustrated, each pair of friction surfaces comprising a first friction surface 31 and a second friction surface 21 that engage one another.

[0091] The actuator 10, for example the SMA wires 11, may be used to move the blades 40 to any position within the range of movement. Upon energising (that is, when drive signals are applied to the SMA wires 11 by the control circuit), the SMA wires 11 contract and apply an actuating force for moving the blades. The actuating force is sufficient to overcome the frictional forces at the friction surfaces 31, 32 (in some embodiments after reduction or elimination of the frictional forces due to SMA wire contraction), to drive movement of the blades. Upon ceasing power supply to the SMA wires 11, and so when stopping contraction of the SMA wires 11, the zero-hold components (for instance, the blades) remain at their position within the range of movement due to the frictional forces between the first and second friction surfaces 31, 21. In this state, the blades 40 are retained in position with zero power consumption by the variable aperture assembly 1, so the variable aperture assembly 1 may be referred to as a zero hold power actuator assembly.

[0092] The biasing arrangement 35 of Figure 4 also comprises a coupling element 35a arranged between the biasing element and the rotatable part 20. The coupling element 35a is a ball bearing. The coupling element 35a allows the rotatable part 20 to move relative to the biasing element. The biasing arrangement 35 applies the biasing force throughout a range of movement of the rotatable part 20 relative to the base 30. The SMA wires 11 are angled relative to one another at an angle a and relative to the biasing force applied by the biasing arrangement 35 at an angle a / 2. The first and second friction surfaces 21, 31 may engage each other throughout the range of movement. So, in normal use (that is, under actuation of the actuator 10 for moving the rotatable part 20), at least some of the first and second friction surfaces 21, 31 may remain in engagement with one another (even if the actuation of the SMA wires 11 reduces the frictional force). Alternatively, the first and second friction surfaces 21, 31 may disengage on actuation of the actuator 10 (if the actuation force provided by the SMA wires 11 exceeds the biasing force provided by the biasing arrangement).

[0093] Although the VA assembly 1 has been described in the context of an optical system, for adjusting the amount of light entering a lens assembly 50, it will be appreciated that the VA assembly 1 may be used in other applications. The VA assembly 1 may be used to adjust passage of any material though an aperture. For example, the VA assembly 1 may be used as a variable valve for adjusting the flow of a fluid (for instance a liquid or a gas, such as air) though a conduit. The VA assembly 1 could also be used to control the passage of particles other than photons, for example to adjust a charged particle beam, such as in an electron microscope or the like.

[0094] Alternative actuator assemblies

[0095] As noted previously, the above disclosed techniques for controlling slack SMA elements are not limited to variable aperture assemblies. SMA actuator assemblies may be used in a variety of applications for moving a movable part relative to a support structure.

[0096] For example, WO 2013 / 175197 Al describes a camera in which four SMA wires are arranged to move a lens element relative to an image sensor in a plane that is perpendicular to the optical axis of the lens element, thereby effecting optical image stabilization (OIS). WO 2010 / 029316 Al describes SMA actuator wires used to provide OIS in a camera by tilting a camera module. WO 2011 / 104518 Al describes an actuator assembly having eight SMA wires capable of effecting positional control of a movable element with multiple degrees of freedom. Any or all the techniques described above in connection with Figures 5 to 9 may be applied to any of the actuator assemblies disclosed in these documents.

[0097] Figure 11 corresponds to Figure 3 of WO 2013 / 175197 Al marked up to show restricted areas 120 (bounded by dashed boxes) for which it is desired to constrain SMA wires 111, 112, 113 and 114 from entering to minimize the risk of wire rubbing on portions of the actuator.

[0098] The actuator arrangement 110 of Figure 11 comprises a total of four SMA wires 111 to 114 connected between a support block 116 and a movable platform 115. Each of the SMA actuator wires 111 to 114 when actuated is held in tension, thereby applying a force between the movable platform 115 and the support block 116 in. In operation, the SMA actuator wires 111 to 114 move the moveable platform 115 relative to the support block 116 in two orthogonal directions. Each wire extends between a crimp 117 coupled to the moveable platform 115 and a crimp 118 coupled to the support block 116. The SMA actuator wires 111 to 114 extend in a common plane.

[0099] When not actuated each SMA wire 111 to 114 may have a degree of slack. It can be seen that the SMA wires 111 to 114 run close to portions of the moveable platform 115. If left uncontrolled, slack SMA wires 111 to 114 may touch the moveable platform or other components of the actuator assembly and be susceptible to damage. Accordingly, restricted areas 120 are defined and the slack SMA wires constrained from entering the restricted areas using the above-described techniques.

[0100] Other variations

[0101] It will be appreciated that there may be many other variations of the above-described examples.

[0102] SMA

[0103] The above-described SMA actuator assemblies comprise at least one SMA element. Each SMA element may be divided into one or more SMA element segments. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (for instance, non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion.

[0104] The SMA element may exhibit any shape memory effect, for instance, a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, for instance, by Joule heating, another heating technique or by applying a magnetic field.

[0105] Alternative ways of heating SMA

[0106] The heating of the heat-activated actuator(s), such as SMA material, in order to cause the moving portion to move, could be achieved in a number of ways.

[0107] In one arrangement, the material could be heated by passing a current through it. This current might come from a local or external power supply. Alternatively, the current might be induced in the wire by inductive coupling with an external alternating field. Where there are two actuators, the two actuators might be designed so that they couple to two different frequencies of the inductive power source, thus allowing the two actuators to be heated differentially.

[0108] In another arrangement, the material could be heated by external radiation such as a visible or infra-red laser. The external radiation could be focussed so that one actuator is heated preferentially over another actuator, thus allowing differential actuation. Alternatively, or additionally, different actuators, or portions of the actuators, could be treated (for example with a surface coating) so that the different actuators heat at different rates depending on the nature (for instance, the frequency) of the incident radiation.

Claims

Claims1. An actuator assembly comprising: a first part; a second part that is moveable relative to the first part; and one or more actuating units each configured to apply an actuating force to the second part capable of moving the second part relative to the first part, wherein at least a first actuating unit comprises at least a first SMA element configured, on actuation, to generate an input force so as to give rise to the actuating force; wherein the first SMA element is slack when not actuated, and wherein the actuator assembly is configured such that the first SMA element, when slack, is constrained from entering a restricted area.

2. An actuator assembly according to claim 1, wherein the restricted area comprises a part of a three-dimensional space of positions which would be reachable by portions of the first SMA element, when slack, without the first SMA element being constrained.

3. An actuator assembly according to claim 1 or 2, wherein the first actuating unit further comprises connection elements coupling to the first SMA element at its ends, and optionally wherein the connection elements are crimps.

4. An actuator assembly according to claim 3, wherein at least a first connection element is configured such that the first SMA element, when slack, extends from the first connection element at an angle relative to a wire axis extending from the first connection element along the path followed by the first SMA element when actuated.

5. An actuator assembly according to claim 4, wherein the first connection element is configured by angling at least a portion of the connection element relative to the wire axis; or wherein the first connection element is configured by angling the first SMA element relative to at least a portion of the connection element prior to coupling the first SMA element to the connection element.

6. An actuator assembly according to any one of claims 3 to 5, wherein the first SMA element is shaped proximal to at least a first connection element such that the first SMA element, when slack, extends from the first connection element at an angle relative to an axis extending from the first connection element along the path followed by the first SMA element when actuated.

7. An actuator assembly according to any one of claims 3 to 5, wherein the first actuating unit further comprises at least one redirection element provided proximal to at least a first connection element such that each redirection element bends the first SMA element, when slack, away from a wire axis extending from the first connection element along the path followed by the first SMA element when actuated.

8. An actuator assembly according to claim 7, wherein the redirection element comprises a damping gel or a compliant adhesive applied to the first SMA element to control the orientation of the first SMA element, and optionally wherein the damping gel or compliant adhesive couples the first SMA element to a further SMA element, the first part, the second part, or a further part of the actuator assembly..

9. An actuator assembly according to any one of the preceding claims, further comprising at least a first deflection element configured to contact a middle portion of the first SMA element so as to deflect the middle portion of the first SMA element, when slack, away from the restricted area.

10. An actuator assembly according to claim 9, wherein the first SMA element is arranged, on actuation, to slide across the first deflection element.

11. An actuator assembly according to claim 9 or 10, wherein the first deflection element is deformable in a direction along the length of the first SMA element, so as to allow actuation of the SMA element.

12. An actuator assembly according to any one of claims 9 to 11, wherein the first deflection element comprises a resilient element configured to apply a biasing force to the first SMA element away from the restricted area, and wherein actuating the first SMA element applies a force to the first deflection element opposing the biasing force.

13. An actuator assembly according to claim 12, wherein the first deflection element is configured such that when actuated the first SMA element passes the first deflection element in a straight line.

14. An actuator assembly according to any one of the preceding claims, further comprising one or more spacers arranged between the first SMA element and the restricted area so as to limit the firstSMA element, when slack, from entering the restricted area, and optionally the one or more spacers are coupled directly or indirectly to one or both of the first and second parts.

15. An actuator assembly according to claim 3 or any claim dependent thereto, wherein at least one connection element incorporates or is coupled to a resilient portion configured to apply a tensioning force to the first SMA element to reduce an amount of slack within the first SMA element when not actuated; and wherein the first actuating unit further comprises at least one end stop configured such that the resilient portion causes the connection element to bear against the end stop when the SMA element is actuated.

16. A variable aperture assembly comprising: an actuator assembly according to any one of the preceding claims, wherein the first part comprises a base and the second part comprise a rotatable part, and wherein the actuating force is capable of rotating the rotatable part relative to the base about a primary axis; and a plurality of blades arranged to define a variable aperture about the primary axis and configured such that rotation of the rotatable part relative to the base effects movement of the blades, thereby changing the size of the variable aperture; wherein the restricted area surrounds the primary axis.

17. A variable aperture assembly according to claim 16, wherein the restricted area is generally equal to or exceeds a maximum size of the variable aperture when viewed along the primary axis.

18. A variable aperture assembly according to claim 16 or claim 17, wherein the actuator assembly is configured to prevent the first SMA element, when slack, from fully bowing towards the primary axis.

19. A variable aperture assembly according to any one of claims 16 to 18, wherein the actuator assembly is configured to cause the first SMA element, when slack, to bow away from the primary axis.

20. A variable aperture assembly according to any one of claims 16 to 19 when dependent on claim 3, wherein the actuator assembly is configured to cause the first SMA element to extend from at least one connection element at an angle away from the primary axis relative to an axis extending from where the first SMA element couples to the first connection element along the path followed by the first SMA element when actuated.

21. A variable aperture assembly according to any one of claims 16 to 20, comprising at least first and second actuating units, each actuating unit comprising at least a first SMA element configured, on actuation, to generate an input force so as to give rise to an actuating force capable of rotating the rotatable part relative to the base about a primary axis, wherein the first and second actuating units are configured to rotate the rotatable part in opposite senses.

22. A variable aperture assembly according to any one of claims 16 to 21, comprising a bearing arrangement between the base and the rotatable part configured to constrain movement of the rotatable part to rotation about the primary axis.

23. A camera comprising: the variable aperture assembly according to any one of claims 16 to 22; a lens assembly; and an image capture device; wherein the optical axis of the lens assembly coincides with the primary axis, such that light passing through the variable aperture assembly passes is focused by the lens and is received by the image capture device.

24. An electronic device incorporating the camera according to claim 22.

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