GB2620800A

GB2620800A - Actuator assembly

Info

Publication number: GB2620800A
Application number: GB2210798.1A
Authority: GB
Inventors: Langhorne Robert; Eddington Robin
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2022-07-22
Filing date: 2022-07-22
Publication date: 2024-01-24
Anticipated expiration: 2042-07-22
Also published as: GB2620800B; GB202210798D0

Abstract

An actuator assembly comprises relatively movable first and second parts 10, (20, figure 1A) and one or more actuating units (30, figure 1A), each of which comprises a body portion 31, a force-modifying mechanism 32 connected between the body portion and the first part, a coupling link (33, figure 2B) connected between the body portion and the second part, and a shape memory alloy (SMA) wire 34 connected between the first part and the body portion for applying an input force on the body portion thereby causing the body portion to apply an output force on the coupling link and causing the coupling link to apply an actuating force on the second part. The actuating force has at least a major component in a plane perpendicular to a primary axis, and the coupling link is compliant in directions perpendicular to the actuating force. One or more limiting arrangements 51 are configured to limit movement along the primary axis of at least a part of at least one of the actuating units. The actuator assembly may be part of a camera assembly which also has an image sensor having an imaging axis parallel to the primary axis, and the actuator assembly is for providing sensor-shift optical image stabilisation.

Description

ACTUATOR ASSEMBLY

Field

The present application relates to a shape memory alloy (SMA) actuator assembly.

Background

SMA actuator assemblies have various applications and, for example, can be used to provide optical image stabilisation (015) in compact cameras for smartphones and other electronic devices. For instance, W02013/175197 describes an actuator assembly with four SMA wires for moving the movable part in any direction in a plane perpendicular to a primary axis. W02022/084699 describes a high-stroke version of such an actuator assembly.

Summary

According to an aspect of the present invention, there is provided an actuator assembly comprising: first and second parts which are movable relative to each other; and one or more actuating units, each of which comprises: a body portion; a force-modifying mechanism connected between the body portion and the first part; a coupling link connected between the body portion and the second part; and an SMA wire connected between the first part and the body portion for applying an input force on the body portion thereby causing the body portion to apply an output force on the coupling link and causing the coupling link to apply an actuating force on the second part; wherein the actuating force has at least a major component in a plane perpendicular to a primary axis, and the coupling link is compliant in directions perpendicular to the actuating force; and wherein the actuator assembly further comprises one or more limiting arrangements, each of which is configured to limit movement along the primary axis of at least a part of at least one of the actuating units.

Thus, the limiting arrangement can limit (e.g. prevent or reduce) movement of the actuating unit along the primary axis which may occur due to unstable/unbalanced forces in the actuating unit and which may adversely affect performance or reliability of the actuator assembly.

The actuating force having at least a major component in a plane perpendicular to the primary axis means, for example, that the actuating force is directed at an angle of less than 45° to the plane. The angle is typically substantially less than this (e.g. less than 5°) and the angle may be approximately zero, that is to say that the actuating force is in the plane.

The coupling link may correspond to a coupling flexure and the force-modifying mechanism may correspond to a force-modifying flexure.

Alternatively, the coupling link and/or the force-modifying mechanism may correspond to any of the non-flexure examples provided in W02022/084699, which is incorporated by reference At least one of the limiting arrangements may comprise an endstop configured to engage a part of the actuating unit when the part of the actuating unit moves in one direction along the primary axis.

The at least one of the limiting arrangements may comprises a further endstop configured to engage a part of the actuating unit when the part of the actuating unit moves in the other direction along the primary axis.

The endstop may be comprised in the first part or the second part. The further endstop may be comprised in the first part or the second part.

At least one of the limiting arrangements may comprise a bearing between a part of the actuating unit and another part of the actuator assembly.

The other part may be comprised in the first part or the second part.

The actuator assembly may be configured to generate a biasing force to bias the part of the actuating unit against the other part of the actuator assembly.

The biasing force may be generated by way of a pre-stress in the force-modifying flexure.

The biasing force may be generated by the SMA wire when the SMA wire is in tension. To provide for this, the SMA wire and/or the force-modifying flexure may extend at a non-zero angle to the plane perpendicular to the primary axis.

The bearing may be a plain bearing or a rolling (e.g. ball) bearing.

The other part of the actuator assembly may comprise a planar surface that is perpendicular to the primary axis.

At least one of the limiting arrangements may comprise a first magnet on the actuating unit. The first magnet may repel a second magnet that is spaced from the first magnet in one direction along the primary axis, and the first magnet may repel a third magnet that is spaced from the first magnet in the other direction along the primary axis.

At least one of the limiting arrangements may comprise compliant material arranged between a part of the actuating unit and at least one of the first and second parts.

The compliant material may remain in contact with the part of the actuating unit during operation of the actuator assembly. The compliant material may be a gel.

At least one of the limiting arrangements may be configured to limit movement of at least part of the body portion of an actuating unit.

When viewed along the primary axis, the body portion may be wider than the coupling flexure and/or the force-modifying flexure.

At least one of the limiting arrangements may be configured to limit movement of a plurality of parts of an actuating unit, and the plurality of parts are spaced from each other when viewed along the primary axis.

In at least one of the actuating units, the force-modifying flexure may comprise a first portion and a second portion which is offset and/or spaced from the first portion in a direction along the primary axis, thereby forming a limiting arrangement.

In the at least one of the actuating units, the SMA wire and the first and second portions may each extend perpendicularly to the primary axis, and the position of the wire along the primary axis may be substantially midway between the positions of the first and second portions along the primary axis.

The at least one of the actuating units may comprises a foot portion via which the force-modifying flexure is connected to the first part.

The body portion, the force-modifying flexure, and the foot portion may be configured such that the force-modifying flexure is in tension when the SMA wire is in tension.

In the at least one of the actuating units, the foot portion, the body portion and the force-modifying flexure may each comprise a first portion and a second portion which is spaced from the first portion in a direction along the primary axis. The first portions may be integrally formed with each other, and the second portions may be integrally formed with each other. The body portion and the foot portion may each further comprise a third portion which is located between, and attached to each of, the first and second portions.

The first, second and third portions may be formed from metal sheet.

In the at least one of the actuating units, the SMA wire may be connected to the body portion via a crimp. The crimp may be integrally formed with the third portion of the body portion.

The crimp may be integrally formed with the first portion or the second portion of the body portion. The coupling flexure may be integrally formed with the first portion, second portion or third portion of the body portion.

The actuator assembly may comprise at least two actuating units arranged to apply actuating forces on the second part in perpendicular directions such that the coupling link of each of the two actuating units is compliant in the direction of the actuating force of the other of the two actuating units.

The first and second parts may be movable relative to each other in any direction in a plane perpendicular to the primary axis.

The actuator assembly may comprise four actuating units arranged so as to be capable of moving the second part relative to the first part in any direction in a plane perpendicular to the primary axis without applying any net torque to the second part about the primary axis.

The actuating units may be arranged so as to be capable of rotating the second part relative to the first part about any axis perpendicular to and intersecting the primary axis.

The actuator assembly may include a suitable bearing arrangement to allow such movements.

There may be provided a camera assembly comprising the actuator assembly, wherein the second part comprises an image sensor having an imaging axis parallel to the primary axis, and wherein the actuator assembly is for providing sensor-shift optical image stabilisation.

However, the actuator assembly may be used in any system or device, e.g. a head-mounted display.

Brief description of the drawings

Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1A illustrates a camera apparatus; Figure 1B illustrates an actuator assembly which may be used in the camera apparatus of Figure 1A; Figures 2A and 2B are plan views of an example of the actuator assembly of Figure 1B; Figures 3A and 3B illustrates a comparative example of an actuator assembly with an actuating unit in a central position (3A) and in two possible non-central positions (3B); Figure 4 illustrates an actuator assembly with a first example of a limiting arrangement; Figure 5 illustrates an actuator assembly with a second example of a limiting arrangement; Figure 6 illustrates an actuator assembly with a third example of a limiting arrangement; Figure 7 illustrates an actuator assembly with a fourth example of a limiting arrangement; Figure 8 illustrates an actuator assembly with a fifth example of a limiting arrangement; Figure 9 illustrates a possible location of the limiting arrangements illustrated in Figures 4 to 8; Figure 10 illustrates an actuator assembly with a sixth example of a limiting arrangement; Figure 11 is a perspective view of an example of the actuator assembly of Figure 10; Figure 12 is a perspective view of another example of an actuator assembly; Figure 13 illustrates a method of forming the actuator assembly of Figure 12; and Figure 14 illustrates another example of an actuator assembly.

Detailed description

Camera apparatus Figure 1A schematically shows an apparatus 1 incorporating an actuator assembly 2 in accordance with an embodiment of the present invention. Figure 1B schematically shows a plan view of the actuator assembly 2. The apparatus 1 is, for example, a camera apparatus 1. The apparatus 1 is to be incorporated in a portable electronic device such as a mobile telephone, or tablet computer. Thus, miniaturisation is an important design criterion.

The apparatus 1 comprises an actuator assembly 2 or may itself be considered an example of an actuator assembly 2. The actuator assembly 2 comprises a support structure 10 (an example of a first part 10) and a movable part 20 (an example of a second part 20). The movable part 20 is supported on the support structure 10. The movable part 20 is movable relative to the support structure 10. For example, the movable part 20 may be supported in a manner allowing movement of the movable part 20 relative to the support structure 10 in a plane orthogonal to a primary axis P. Movement along the primary axis P may be constrained or prevented. Alternatively, the movable part 20 may be supported in a manner allowing tilting of the movable part 20 relative to the support structure 10 about any axes orthogonal to the primary axis P. Movement other than such tilting may be constrained or prevented.

The support structure 10 is used as a reference point to describe movement of the movable part 20.

When the actuator assembly 2 is included in an apparatus or device, such as a camera, smartphone, a drone, the support structure 10 may be fixed relative to a main body of the apparatus or device. However, in general the support structure 10 need not necessarily be stationary and may be movable relative to or within such a device. In some embodiments, the movable part 20 may be fixed relative to a main body of the device. Furthermore, although the support structure 10 is schematically depicted as one part in Figure 1A, in practice the support structure 10 may be formed from a plurality of layers, parts and components that are fixed relative to one another. Similarly, the movable part 20 may be formed from a plurality of layers, parts and components that are fixed relative to one another.

The actuator assembly 2 comprises actuating units 30. The actuating units 30 are connected between the support structure 10 and the movable part 20. The actuating units 30 are arranged to apply actuating forces F between the movable part 20 and the support structure 10. Selectively applying and varying the actuating forces F may move the movable part 20 relative to the support structure 10. The actuating units 30 are thus capable, on selective actuation, of driving movement of the movable part 20 relative to the support structure 10.

The actuator assembly 2 also includes limiting arrangements 50 which will be described in more detail below.

The actuator assembly 2 extends primarily in a direction orthogonal to a primary axis P. The extent of the actuator assembly 2 along the primary axis is less than the extent of the actuator assembly 2 along axes orthogonal to the primary axis P. The movable part 20 may be supported (so suspended) on the support structure 10 exclusively by the actuating units 30. However, preferably, the actuator assembly 2 comprises a bearing arrangement 40 that supports the movable part 20 on the support structure 10. The bearing arrangement 40 may have any suitable form for allowing movement of the movable part 20 with respect to the support structure 10. According to embodiments, the bearing arrangement 40 may guide movement of the movable part 20 relative to the support structure 10 in a plane, also referred to as a movement plane. In alternative embodiments, the bearing arrangement 40 guides movement of the movable part 20 relative to the support structure 10 such that the movable part 20 tilts about axes orthogonal to the primary axis P. The bearing arrangement 40 may constrain movement of the movable part relative to the support structure in other degrees of freedom. For this purpose, the bearing arrangement 40 may, for example, comprise a rolling bearing (such as a roller bearing or ball bearing), a flexure bearing (i.e. an arrangement of flexures or other resilient elements guiding movement), or a plain bearing or sliding bearing.

The camera apparatus 1 further comprises a lens assembly 3 and an image sensor 4. The lens assembly 3 comprises one or more lenses configured to focus an image on the image sensor 4. The lens assembly 3 defines an optical axis 0, which is aligned with the primary axis Pin Figure 1A. The image sensor 4 captures an image and may be of any suitable type, for example a charge coupled device (CCD) or a CMOS device. The lens assembly 3 comprises a lens carrier, for example in the form of a cylindrical body, supporting the one or more lenses. The one or more lenses may be fixed in the lens carrier, or may be supported in the lens carrier in a manner in which at least one lens is movable along the optical axis 0, for example to provide zoom or focus, such as auto-focus (AF). The lens carrier itself may be movable along the optical axis 0. The lenses or the lens carrier may be moved by a voice coil motor (VCM) or an arrangement of SMA wires (not shown), for example. The apparatus 1 may be a miniature camera apparatus in which the or each lens of the lens assembly 3 has a diameter of 20mm or less, for example of 12mm or less.

In the embodiment shown in Figure 1, the movable part 20 may be considered to comprise the image sensor 4. The lens assembly 3 may be fixed relative to the support structure 10, i.e. mounted on the support structure 10. In other embodiments (not shown), the image sensor 4 may be fixed relative to the support structure 10 and the movable part 20 may comprise the lens assembly 3. In either embodiment, in operation the lens assembly 3 is moved relative to the image sensor 4. This has the effect that the image on the image sensor 4 is moved. So, optical image stabilization (ON may be implemented in the apparatus 1.

The camera apparatus 1 further comprises a controllers. The controller 8 may be implemented in an integrated circuit (IC) chip. The controllers generates drive signals for the actuating units 30, in particular for SMA wires 34 forming part of the actuating units 30. 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 34, thereby heating the SMA wires 34 by allowing an electric current to flow, will cause the SMA wires 34 to contract and thus actuate the actuating unit 30, so as to move the movable part 20. The drive signals are chosen to drive movement of the movable part 20 in a desired manner, for example so as to achieve OIS by stabilizing the image sensed by the image sensor 4. The controller 8 supplies the generated drive signals to the SMA wires 34.

Optionally, the camera apparatus comprises an inertial measurement unit 6. The inertial measurement unit 6 may comprise one or more vibration sensors, such as gyroscopes, accelerometers or magnetometers, although in general other types of sensors could be used. The inertial measurement unit 6 detects changes in the orientation of and/or the forces on the camera apparatus 1 and generates sensor signals representative of the orientation of and/or forces on the camera apparatus 1. The controllers receives the sensor signals and generates the drive signals for the SMA wires 34 in response to the sensor signals, for example so as to counteract the changes in orientation and/or forces represented by the output signals. The controllers may thus control the SMA wires 34 to achieve 015.

Arrangement of actuating units Figure 1B schematically shows the arrangement of actuating units 30 in the actuator assembly 2. As shown, the actuator assembly 2 may comprise a total of four actuating units 30. The four actuating units 30 may apply actuating forces F between the movable part 20 and the support structure 10. The actuating forces F may be applied to the movable part 20 relative to the support structure 10.

In the depicted embodiment, 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 20 in degrees of freedom allowed by the bearing arrangement 40. In some embodiment 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 20 and the support structure 10.

In the depicted embodiment, the four actuating units 30 are in an arrangement capable of applying actuating forces F so as to move the movable part 20 relative to the support structure 10 to any positions within a range of movement. The range of movement may be within a movement plane that is perpendicular to the primary axis P. In particular, two actuating units 30 (e.g. the top and bottom actuating units in Figure 1B) are arranged to apply actuating forces F in opposite directions parallel to a first axis (e.g. the x axis in Figure 1B). The other two of actuating units (e.g. the left and right actuating units in Figure 1B) are arranged to apply actuating forces F opposite directions parallel to a second axis (e.g. the y axis in Figure 1B), orthogonal to the first axis. By appropriately varying the difference in actuation amount between the opposing actuating units 30, the movable part 20 may thus be moved independently along the first and second axes. The opposing actuating forces F are not colinear, but offset from each other in a direction perpendicular to the actuating forces. Providing opposing actuating units 30 allows the tension in the SMA wires 30 of the respective actuating units 30 to be controlled, allowing for more accurate and reliable positioning of the movable part 20 compared to a situation in which actuating units 30 do not oppose each other.

In embodiments, none of the actuating forces F are collinear. This allows the arrangement of actuating units 30 to translationally move the movable part 20 without applying any net torque to the movable part 20. So, the movable part 20 can be moved translationally in the movement plane without rotating the movable part 20 in the movement plane. In general, the arrangement of actuating units 30 is capable of accurately controlling a torque or moment of the movable part 20 about the primary axis P. So, the actuating units 30 are capable of rotating (or not rotating) the movable part 20 relative to the support structure about the primary axis P. In particular, two actuating units 30 (e.g. the top and bottom actuating units in Figure 1B) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 20 and the support structure 2 in a first sense (e.g. clockwise) around the primary axis P. The other two actuating units 30 (e.g. the left and right actuating units 30 in Figure 1B) are arranged to apply actuating forces F so as to generate a torque or moment between the movable part 20 and the support structure 2 in a second, opposite sense (e.g. anti-clockwise) around the primary axis P. This allows the movable part 20 to be rotated by simultaneously increasing or decreasing the tension of SMA wires in any of the two actuating units 30.

As shown, two actuating units 30 may be arranged to apply actuating forces in a corner of the actuator assembly 2. The other two actuating units 30 may be arranged to apply actuating forces in another, opposite corner of the actuator assembly 2. The actuator assembly 2, and in particular the movable part 20 and/or the support structure 10, may have a square or rectangular footprint. Each actuating unit 30 may be provided on one of the four sides of the actuator assembly 2.

The arrangement of forces F applied between movable part 20 and support structure 10 corresponds to the arrangement of SMA wires 30 described in W02013/175197, which is herein incorporated by reference.

Although, for illustrative purposes, the arrangement of actuating units 30 was described as moving the movable part 20 in the movement plane (e.g. translationally along the x and y axis, or rotationally about the primary axis P), in other embodiments the movable part 20 may be moved differently. For example, the same arrangement of actuating forces F may be used to tilt the movable part 20 relative to the support structure 10 about axes orthogonal to the primary axis, due to appropriate movement constraints provided by the bearing arrangement 40. For example, the bearing arrangement 40 may comprise a plurality of flexures for guiding tilting of the movable part 20 about the axes orthogonal to the primary axis P. Examples of such bearing arrangement 40 are described in W02022/029441, which is herein incorporated by reference.

Although the actuator assembly 2 is described herein in the context of four actuating units 30, in general the actuator assembly 2 may comprise fewer actuating units 30. For example, the actuator assembly 2 may comprise two actuating units 30, e.g. the two actuating units 30 depicted in the top left of Figure 1B. The forces applied to the movable part 20 by the two actuating units 30 may be opposed by a biasing force of one or more resilient elements, such as springs. With reference to Figure 1B, the two actuating units 30 in the bottom right corner may be replaced with springs applying biasing forces along the corresponding depicted arrows, for example.

Actuating units Figure 2A shows, in plan, an embodiment of the actuator assembly 2 in further detail, including the components of the actuating units 30. Figure 2B depicts an enlarged version cut-out A of Figure 2A.

Figure 2C shows a side view of Figure 2B.

One actuating unit 30 is provided with reference numerals in Figure 2B, but it will be appreciated that the other actuating units 30 may comprise the same components described with reference to that actuating unit 30. The actuating units 30 may be substantially identical, i.e. the structure and components of the actuating units 30 may be the same, but the actuating units' arrangement relative to the support structure 10 and/or movable part 20 may differ.

The actuating unit 30 comprises a body portion 31. The body portion 31 is a substantially rigid part and is designed not to deform on actuation of the actuating unit 30.

The actuating unit 30 further comprises a force-modifying flexure 32. The force-modifying flexure 32 is connected between the body portion 31 and the support structure 10. One end of the force-modifying flexure 32 is connected to the body portion 31. The other end of the force-modifying flexure 32 is connected to the support structure 10, in particular via a foot portion 36. The foot portion 36 is fixed relative to the support structure 10. In the depicted design, the force-modifying flexure is formed integrally with the foot portion 36 and with part of the body portion 31, for example from a single sheet of material (such as metal). The force-modifying flexure 32 may, on flexing, allow the body portion 31 to move relative to the support structure 10 in a direction that is substantially orthogonal to the force-modifying flexure 32. The force-modifying flexure 32 effectively allows the body portion 31 to pivot relative to the support structure 10, with the pivot point P provided in a region along the force-modifying flexure 32. The force-modifying flexure 32 thus effectively provides the pivot point P. Although the pivot point P is depicted in the middle of force-modifying flexure 32 in Figure 2B, in practice the pivot point P may be virtual and need not lie on the force-modifying flexure 32.

The actuating unit 30 further comprises an SMA wire 34. The SMA wire 34 is connected between the body portion 31 and the support structure 10. One end of the SMA wire 34 is connected to the support structure 10, in particular by a respective crimp 15. The other end of the SMA wire 34 is connected to the body portion 31, in particular by a respective crimp 35.

The actuating unit 30 further comprises a coupling flexure 33. The coupling flexure 33 is connected between the body portion 31 and the movable part 20. One end of the coupling flexure 33 is connected to the body portion 31. The other end of the coupling flexure 33 is connected to the movable part 20.

The SMA wire 34 is arranged, on contraction, to apply an input force Fi on the body portion 31. The input force Fi acts parallel to the length of the SMA wire 34. The force-modifying flexure 32 is arranged to modify the input force Fi so as to cause the coupling flexure 33 to apply the actuating force F to the movable part 20. In particular, in the depicted embodiment the force-modifying flexure 32 is placed in tension on contraction of the SMA wire 34. The force-modifying flexure 32 is arranged at an angle a relative to the SMA wire 34. As a result, the body portion 31 is arranged, on SMA wire contraction, to move at an angle (of about 90 degrees minus a) relative to the length of the SMA wire 34. The force-modifying flexure 32 thus converts the input force Fi, in particular the magnitude and direction thereof, into the actuating force F. In the depicted embodiment, the change in magnitude of the force is dependent on (and indeed proportional to) the ratio of i) the (shortest) distance Ds of the SMA wire 34 from the pivot point P and ii) the (shortest) distance Dc of the coupling flexure 33 from the pivot point P. So, F/Fi is proportional to Ds/Dc. It will be appreciated that the ratio Ds/Dc is dependent, at least in part, on the angle a between SMA wire 34 and force-modifying flexure 32.

So, if the SMA wire 34 is relatively closer to the (virtual) pivot point P than the coupling flexure 33, then the input force Fi applied on contraction of the SMA wire 32 is de-amplified. At the same time, the movement of the movable part 20 is amplified relative to a change in length of the SMA wire 32.

Alternatively, if the SMA wire 34 is relatively further away from the (virtual) pivot point P compared to the coupling flexure 33, then the input force Fi applied on contraction of the SMA wire 32 is amplified. At the same time, the movement of the movable part 20 is de-amplified relative to a change in length of the SMA wire 32. The actuating unit 30 can thus be configured to amplify movement or to amplify force due to contraction of the SMA wire 34.

In some embodiments, at least one actuating unit 30, preferably each actuating unit 30, is configured such that the force-modifying flexure 32 amplifies an amount of contraction of the SMA wire 34 to a relatively greater amount of movement of the movable part 20 relative to the support structure 10. 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 angle a between the force-modifying flexure 32 and the SMA wire 34. The angle a may be in the range from 0 to 45 degrees, preferably from 13 to 40 degrees.

The coupling flexure may be at an angle of substantially 90 degrees relative to the SMA wire 34. This allows the actuating unit 30 to fold around a corner of the movable part 20 in a compact manner.

However, in general, the angle between coupling flexure 33 and SMA wire 34 may be any angle other than 90 degrees.

The coupling flexure 33 is compliant in a direction perpendicular to the actuating force F. This allows the movable part 20 to move in a direction perpendicular to the actuating force F, and in a direction perpendicular to the coupling flexure 33, for example due to actuation of a different actuation unit 30.

In the above-described embodiments, the force-modifying flexure 32 is placed in tension on contraction of the SMA wire 34. This reduces the risk of buckling of the force-modifying flexure 32. However, in general, the force-modifying flexure 32 could also be arranged so as to be placed under compression on contraction of the SMA wire 34.

In the above-described embodiments, the force-modifying flexure 32 and the SMA wire 34 connect at one end to the support structure 10, and the coupling flexure 33 connects at one end to the movable part 20. In general, this arrangement may also be reversed, with the force-modifying flexure 32 and the SMA wire 34 connecting at one end to the movable part 20, and the coupling flexure 33 connecting at one end to the support structure 10.

Movement of actuating units along the primary axis Referring in particular to Figure 3A and 3B, the issue of movement of actuating units along the primary axis will now be explained with reference to a comparative example of an actuator assembly without any limiting arrangements 50.

When an actuating unit 30 is subject to a low force, as illustrated in Figure 3A, the actuating unit 30, including the body portion 31, the force-modifying flexure 33, and the coupling link (not shown in these figures) extend along a direction parallel to the movement plane and are spaced from the support structure 10 and a cover 11 (which is attached to the support structure).

However, when an actuating unit 30 is subject to a sufficiently large force (e.g. due to the tension in the SMA wire 34 of the actuating unit and/or due to the other actuating units 30), as illustrated in Figure 3B, the actuating unit 30 is susceptible to bending out of plane so that at least part of it is displaced along the primary axis P. As will be appreciated, this may be due to unstable/unbalanced input and output forces on the body portion 31. Such movement may continue until part of the actuating unit contacts the support structure 10 or cover 11. As a result, the performance or reliability of the actuator assembly 2 may be adversely affected.

Limiting arrangement Referring in particular to Figures 4 to 14, several examples of limiting arrangements 50 will be described. The limiting arrangement 50 may be included in the above-described actuator assembly 2. Generally, the actuator assembly 2 has one limiting arrangement 50 for each actuating unit 3. The limiting arrangement 50 is configured to limit movement along the primary axis P of at least a part of an actuating unit 30.

First example of a limiting arrangement Referring in particular to Figure 4, a first example of a limiting arrangement 51 will now be described.

The limiting arrangement 51 includes two endstops, i.e. a 'lower' endstop 51a and an 'upper' endstop 51b.

The lower endstop 51a is configured to engage a part of the actuating unit 30, e.g. a part of the body 31, when the body 31 moves in a 'downwards' direction along the primary axis P. The upper endstop 51b is configured to engage a part of the actuating unit 30, e.g. a part of the body 31, when the body 31 moves in an 'upwards' direction along the primary axis P. In this way, only limited movement of the actuating unit 30 along the primary axis P is allowed.

In this example, and the lower endstop 51a is a separate piece that is attached to the support structure 10, and the upper endstop Sib is a separate piece that is attached to the cover 11. The endstops 51a, 51b may be attached to other components of the actuator assembly 2. The endstops 51a, 51b may be integrally formed with the support structure 10, the cover 11 or other components of the actuator assembly 2.

The different endstops 51a, 51b may engage different parts of the actuating unit 30.

Second example of a limiting arrangement Referring in particular to Figure 5, a second example of a limiting arrangement 52 will now be described.

The limiting arrangement 52 includes a 'lower' endstop 52a and a bearing 52b.

As above, the lower endstop 52a is configured to engage a part of the actuating unit 30, e.g. a part of the body 31, when the body 31 moves in a 'downwards' direction along the primary axis P. The lower endstop 52a may not be used during normal operation. In some examples, the lower endstop 52a may be omitted.

The bearing 52b is between a part of the actuating unit 30, e.g. a part of the body 31, and another part 52c of the actuator assembly (this other part will be referred to as the 'bearing part').

The actuator assembly 2 is configured to generate a biasing force to bias the actuating unit 30 against the bearing part 52c.

In some examples, this biasing force is generated by way of a pre-stress in the force-modifying flexure 32 (e.g. by ensuring that the force-modifying flexure 32 is elastically deformed during assembly of that actuator assembly 2).

In some examples, the biasing force may be generated by the SMA wire 32 when the SMA wire 32 is in tension. To provide for this, the SMA wire 34 may extend at a non-zero angle to the movement plane (e.g. be angled 'upwards' away from the crimp 35). Alternatively or additionally, the force-modifying flexure 32 may extend at a non-zero angle to the movement plane (e.g. be angled 'downwards' between the crimp 35 and the foot portion 36).

A separate biasing arrangement (e.g. a magnetic biasing arrangement) could also be used.

The bearing part 52c has a planar surface which is perpendicular to the primary axis P and which acts as a known reference surface along which the actuating unit 30 moves.

The bearing 52b is a plain bearing. To avoid stick-slip friction, which may degrade performance of the actuator (hold stability) and even prevent certain features (super resolution), the friction in the bearing 52b can be reduced by using materials and/or coatings for the contact surfaces with a low friction coefficient and/or by adding lubricants between the contact surfaces.

In some examples, the bearing part 52c may be located 'below' rather than 'above' the actuating unit 25 30.

In this example, the bearing part 52c is a separate piece that is attached to the cover 11. The bearing part 52c may be attached to another component of the actuator assembly 2. The bearing part 52c may integrally formed with the cover 11 or another component of the actuator assembly 2.

The limiting arrangement may comprise a bearing between a part of the actuating unit and a further part of the actuator assembly, and the further endstop may correspond to the further part.

Third example of a limiting arrangement Referring in particular to Figure 6, a third example of a limiting arrangement 53 will now be described.

The limiting arrangement 53 is the same as the second example except that there is no lower endstop. Furthermore, the bearing 53 that constitutes the limiting arrangement 53 is a ball bearing rather than a plain bearing. The use of such a rolling bearing will reduce or avoid slip stick friction behaviour.

Fourth example of a limiting arrangement Referring in particular to Figure 7, a fourth example of a limiting arrangement 54 will now be described.

The limiting arrangement 54 consists of a gel 54 which is arranged between a part of the actuating unit 30, e.g. part of the body portion 31, and the support structure 10. In this example, the gel 54 is attached to and/or remains in contact with both the actuating unit 30 and the support structure 30 during operation of the actuator assembly 2.

The gel 54 is applied such that the body portion 31 is suspended on or within the gel and so as to allow a full range of motion in the movement plane, while providing enough stiffness in directions along the primary axis P. The gel 54 may also have the additional benefit of damping oscillations and resonant behaviour of the actuator assembly 2.

The gel may be attached to a part of the actuator assembly 2 other than the support structure 10.

Instead of a gel, another type of compliant material may be used.

Fifth example of a limiting arrangement Referring in particular to Figure 8, a fifth example of a limiting arrangement 55 will now be described.

The limiting arrangement 55 includes a first magnet 55a on the actuating unit 30, e.g. on the body portion 31. The first magnet 55a repels a second magnet 55b that is spaced from the first magnet 55a in one direction ('downwards') along the primary axis P. The first magnet 55a also repels a third magnet 55c that is spaced from the first magnet 55a in the other direction ('upwards') along the primary axis P. Such an arrangement has the advantages of being zero-friction and also not involving the assembly complexity of using ball bearings.

Location and number of limiting arrangements In the above-described examples, the limiting arrangement 50 is located on or near (and directly limits the movement of) the body portion 31 of the actuating unit 30, as illustrated in Figure 9.

In other examples, the limiting arrangement 50 may be differently located.

In some examples, the limiting arrangement 50 may be located on or near (and directly limit the movement of) a plurality of parts of the actuating unit 30 (which are spaced from each other when viewed along the primary axis P). This is because, in practice, the actuating unit 30 could be damaged by excessive deflection at various points and as such may require multiple points of protection.

Sixth example of a limiting arrangement Referring in particular to Figures 10 to 14, a sixth example of a limiting arrangement 56 will now be described.

Referring in particular to Figure 10, the limiting arrangement 56 involves the force-modifying flexure 32' comprise a first portion 32a and a second portion 32b which is offset and/or spaced from the first portion in a direction along the primary axis P. This increases the stiffness of the force-modifying flexure 32 and reduce the likelihood of it bending out of plane.

In the examples of Figures 10 and 11, the first and second portions 32a, 32b are spaced from each other, thereby further increasing the relevant stiffness of the force-modifying flexure 32, without increasing the stiffness of the amplification mechanism (beyond the initial increase associated with the two portions 32a, 32b).

Furthermore, in the examples of Figures 10 and 11, the SMA wire 34 is positioned midway between the two portions 32a, 32b of the force-modifying flexure 32'. This is preferable as it makes the system more

stable.

In other examples, e.g. the example of Figure 12, the two portions 32a, 32b of the force-modifying flexure 32 may be in contact with each other.

In the example of Figure 11, the foot portion 36, the body portion 31 and the force-modifying flexure 32 each comprise a first portion (labelled a) and a second portion (labelled b) which is spaced from the first portion in a direction along the primary axis P. The first portions may be integrally formed with each other, and the second portions may be integrally formed with each other. The body portion 31 and the foot portion 36 may each further comprise a third portion (labelled c) which is located between, and attached to each of, the first and second portions.

The first, second and third portions may be formed from metal sheet, e.g. by etching.

The moving crimps 35 may be integrally formed with the third portion 31c of the body portion 31c and so have a 'midway' location as described above.

Alternatively, the moving crimp 34 may be integrally formed with the first portion 31a or the second 32b portion of the body portion, e.g. as in Figure 12. Similarly, the coupling flexure 33 may be integrally formed with the first portion 31a, second portion 31b or third portion 31c of the body portion.

Such actuating units 30 may be formed by folding a patterned metal sheet, as illustrated in Figure 13.

Instead of using a third portion of the body portion 31 and/or the foot portion 36 as 'spacers as described above, the actuating unit may include a 'jog' 37 as illustrated in Figure 14 to provide the space between the different portions 32a, 32b of the force-modifying flexure 32'.

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

For example, the connections of the elements of the actuating units to the moving and static parts may be reversed.

Multiple actuating units 30 may share limiting arrangements 50.

Different types of limiting arrangements 50 may be used in combination.

SMA wire The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire 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 wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. For instance, the SMA 'wire' may actually be a foil (and may be manufactured accordingly). The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

Claims

Claims 1. An actuator assembly comprising: first and second parts which are movable relative to each other; and one or more actuating units, each of which comprises: a body portion; a force-modifying mechanism connected between the body portion and the first part; a coupling link connected between the body portion and the second part; and an SMA wire connected between the first part and the body portion for applying an input force on the body portion thereby causing the body portion to apply an output force on the coupling link and causing the coupling link to apply an actuating force on the second part; wherein the actuating force has at least a major component in a plane perpendicular to a primary axis, and the coupling link is compliant in directions perpendicular to the actuating force; and wherein the actuator assembly further comprises one or more limiting arrangements, each of which is configured to limit movement along the primary axis of at least a part of at least one of the actuating units.
2. An actuator assembly according to claim lwherein the coupling link corresponds to a coupling flexure and the force-modifying mechanism corresponds to a force-modifying flexure.
3. An actuator assembly according to claim 1 or 2 wherein at least one of the limiting arrangements comprises an endstop configured to engage a part of the actuating unit when the part of the actuating unit moves in one direction along the primary axis.
4. An actuator assembly according to claim 3 wherein the at least one of the limiting arrangements comprises a further endstop configured to engage a part of the actuating unit when the part of the actuating unit moves in the other direction along the primary axis.
5. An actuator assembly according to any preceding claim wherein at least one of the limiting arrangements comprises a bearing between a part of the actuating unit and another part of the actuator assembly.
6. An actuator assembly according to claims when dependent on claim 3 wherein the endstop corresponds to the other part of the actuator assembly.
7. An actuator assembly according to claim 5 configured to generate a biasing force to bias the part of the actuating unit against the other part of the actuator assembly.
8. An actuator assembly according to claim 7 when dependent on claim 2 wherein the biasing force is generated by way of a pre-stress in the force-modifying flexure.
9. An actuator assembly according to claim 7 or 8 wherein the biasing force is generated by the SMA wire when the SMA wire is in tension.
10. An actuator assembly according to any one of claims 5 to 9 wherein the bearing is a plain bearing or a rolling bearing.
11. An actuator assembly according to any one of claims 5 to 10 wherein the other part of the actuator assembly comprises a planar surface that is perpendicular to the primary axis.
12. An actuator assembly according to any preceding claim wherein at least one of the limiting arrangements comprises a first magnet on the actuating unit, wherein the first magnet repels a second magnet that is spaced from the first magnet in one direction along the primary axis, and the first magnet repels a third magnet that is spaced from the first magnet in the other direction along the primary axis.
13. An actuator assembly according to any preceding claim wherein at least one of the limiting arrangements comprises compliant material arranged between a part of the actuating unit and at least one of the first and second parts.
14. An actuator assembly according to any preceding claim wherein at least one of the limiting arrangements is configured to limit movement of at least part of the body portion of an actuating unit.
15. An actuator assembly according to any preceding claim wherein at least one of the limiting arrangements is configured to limit movement of a plurality of parts of an actuating unit, wherein the plurality of parts are spaced from each other when viewed along the primary axis.
16. An actuator assembly according to claim 2 or any one of claims 3 to 15 when dependent on claim 2 wherein, in at least one of the actuating units, the force-modifying flexure comprises a first portion and a second portion which is offset and/or spaced from the first portion in a direction along the primary axis, thereby forming a limiting arrangement.
17. An actuator assembly according to claim 16 wherein, in the at least one of the actuating units, the SMA wire and the first and second portions each extend perpendicularly to the primary axis, and the position of the wire along the primary axis is substantially midway between the positions of the first and second portions along the primary axis.
18. An actuator assembly according to claim 16 or 17 wherein the at least one of the actuating units comprises a foot portion via which the force-modifying flexure is connected to the first part.
19. An actuator assembly according to claim 18 wherein, in the at least one of the actuating units, the foot portion, the body portion and the force-modifying flexure each comprise a first portion and a second portion which is spaced from the first portion in a direction along the primary axis, wherein the first portions are integrally formed with each other and the second portions are integrally formed with each other, and wherein the body portion and the foot portion each further comprise a third portion which is located between, and attached to each of, the first and second portions.
20. An actuator assembly according to claim 19 wherein, in the at least one of the actuating units, the SMA wire is connected to the body portion via a crimp, wherein the crimp is integrally formed with the third portion of the body portion.
21. An actuator assembly according to any preceding claim, comprising at least two actuating units arranged to apply actuating forces on the second part in perpendicular directions such that the coupling link of each of the two actuating units is compliant in the direction of the actuating force of the other of the two actuating units.
22. An actuator assembly according to any preceding claim, wherein the first and second parts are movable relative to each other in any direction in a plane perpendicular to the primary axis.
23. An actuator assembly according to claim 22, comprising four actuating units arranged so as to be capable of moving the second part relative to the first part in any direction in a plane perpendicular to the primary axis without applying any net torque to the second part about the primary axis.
24. A camera assembly comprising an actuator assembly according to any preceding claim wherein the second part comprises an image sensor having an imaging axis parallel to the primary axis, and wherein the actuator assembly is for providing sensor-shift optical image stabilisation.