CN110007542B

CN110007542B - Shape memory alloy actuator

Info

Publication number: CN110007542B
Application number: CN201811543160.5A
Authority: CN
Inventors: 斯蒂芬·马修·邦廷; 安德鲁·本杰明·大卫·布朗; 马克-赛巴斯蒂安·肖尔茨
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2017-12-22
Filing date: 2018-12-17
Publication date: 2021-08-20
Anticipated expiration: 2038-12-17
Also published as: GB2569668C; GB201810201D0; GB2569668A; GB201721716D0; CN110007542A; GB2569668B

Abstract

The present application relates to shape memory alloy actuation devices. An SMA actuation apparatus comprises a support structure and a movable element supported on the support structure by a suspension system which guides movement driven by a length of SMA actuator wire. The length of SMA actuator wire is inclined at an angle in the range of 5 to 15 degrees relative to a plane orthogonal to the direction of movement.

Description

Shape memory alloy actuator

The present technology relates generally to Shape Memory Alloy (SMA) actuation apparatus in which at least one length of SMA actuator wire drives movement of a movable element relative to a support structure. In particular, the present technique relates to minimizing the overall height of the SMA actuation arrangement in the direction of movement.

The present technology provides a shape memory alloy actuation device comprising: a support structure; a movable element supported on a support structure by a suspension system (suspension system), the suspension system comprising a bearing arrangement (bearing arrangement) arranged to guide movement of the movable element relative to the support structure in a movement direction; at least one length of shape memory alloy actuator wire connected between the support structure and the movable element for driving movement of the movable element relative to the support structure, wherein the length of shape memory alloy actuator wire is inclined at an angle in the range of 5 to 15 degrees relative to a plane orthogonal to the direction of movement.

It is expected that such an angle will not be ideal as there is an increased mechanical gain (mechanical gain) as the angle is smaller, which in turn may be expected to make the actuator wire more sensitive and more difficult to control, making it difficult to maintain stability in the position of the movable element. Surprisingly, however, it has been found that in practice lens stability can in fact be maintained over such an angular range by using a suitable bearing arrangement to guide the movement of the movable element relative to the support structure in the direction of movement.

The present technique is applicable to an optical device in which the movable element is a lens element comprising at least one lens, wherein the direction of movement is along the optical axis of the lens element. For example, the SMA actuation apparatus may be a camera in which the support structure has an image sensor mounted thereon and the lens element is arranged to focus an image on the image sensor.

In the case of an optical device, the reduction of the height of the SMA actuation means in the direction of movement is particularly advantageous, since it is highly desirable in practical implementations to minimize this height.

Advantageously, the at least one lens may be made of glass. The use of glass lenses instead of plastic lenses is a recent trend, as glass lenses improve image quality and are generally thinner than their plastic analogues due to their higher refractive index. However, glass lenses are heavier than their plastic analogues and therefore require a greater actuation force. Further, since the glass lens is thinner than the plastic lens, the height of the lens element along the optical axis is reduced, however, the moving range (stroke) required to move the lens along the optical axis needs to be kept at least the same and must be realized without tilting the lens. The first aspect of the present invention reduces both of these problems. A smaller angle of the SMA actuator wires of at least one length provides a larger force suitable for a glass lens. Furthermore, the smaller angle of the at least one length of SMA actuator wire allows the range of movement to be maintained.

Preferred features of the present technique are set out in the appended dependent claims.

Drawings

Embodiments of the present technology will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a first SMA actuation apparatus;

FIG. 2 is an exploded view of a first SMA actuation apparatus;

FIG. 3 is a side view of a first SMA actuation apparatus;

figures 4 to 6 are plan views of different bearing arrangements for the first SMA actuation apparatus;

FIG. 7 is a cross-sectional view of an alternative bearing for the first SMA actuation apparatus, the cross-section being taken perpendicular to the direction of movement of the bearing;

FIG. 8 is a side view of the alternative bearing of FIG. 7;

FIG. 9 is a cross-sectional view of an alternative bearing for the first SMA actuation apparatus, the cross-section being taken perpendicular to the direction of movement of the bearing;

FIG. 10 is a side view of the alternative bearing of FIG. 9;

FIG. 11 is a perspective view of a second SMA actuation apparatus;

FIG. 12 is a perspective view of a step during manufacture of a second SMA actuation apparatus;

FIG. 13 is a perspective view of a third SMA actuation apparatus;

FIG. 14 is a side view of a strut element (strut element) used during manufacture of a third SMA actuation apparatus; and

fig. 15 and 16 are schematic side views of two alternative forms of a fourth SMA actuation arrangement.

In general, the present technology provides a shape memory alloy actuator device in which a length of shape memory alloy actuator wire is inclined at an angle in the range of 5 to 15 degrees relative to a plane orthogonal to the direction of movement.

It is known to use SMA actuator wires to drive translational movement of a movable element relative to a support structure. SMA actuator wires have particular advantages in miniature devices and can be applied in a wide range of consumer electronic devices including hand-held devices such as cameras and mobile phones.

For example, such SMA actuator wires may be used in an optical device (e.g. a camera) for driving translational movement of a camera lens element along its optical axis, e.g. to achieve focus (autofocus, AF) or zoom. Such cameras can be small and relatively low cost and provide a large number of functions not only for taking pictures, but also for video calls, scanning, object recognition, security, etc. Examples of SMA actuation arrangements for cameras of this type are disclosed in WO2007/113478, WO2009/056822 and WO 2017/134456.

Such SMA actuator wires may similarly be used in optical devices, such as cameras, for driving translational movement of a camera lens element transverse to the optical axis, for example to provide Optical Image Stabilization (OIS). Examples of SMA actuation arrangements for cameras of this type are disclosed in WO2013/175197 and WO 2014/083318.

In WO2007/113478, the movable element is a camera lens element supported on a support structure by a suspension system comprising flexures (flexures) that guide translational movement along the optical axis. In one example described in WO2007/113478, the SMA actuator wire is a piece of SMA wire (a piece of SMA wire) which is connected at its ends to the support structure and which hooks onto a hook on the camera lens element for driving the translational movement. Straight SMA actuator wires formed by portions of the piece of SMA wire on either side of the hook extend at an acute angle greater than 0 degrees relative to a direction of movement parallel to the optical axis. Angling the SMA actuator wires away from the direction of movement in this manner increases the amount of movement compared to SMA actuator wires extending in the direction of movement. To maximize the amount of travel, it is desirable to maximize the length of the SMA actuator wires. However, it is still desirable for the SMA actuator wires to be at an acute angle to the direction of movement, so that the component of contraction acts in the direction of movement. WO2007/113478 discloses an example in which the SMA actuator wires are inclined at an angle of 70 degrees to the optical axis, i.e. 20 degrees to a plane orthogonal to the direction of movement.

Thus, in contrast to WO2007/113478, in the present technique, the angle of the SMA actuator wires of at least one length with respect to a plane orthogonal to the direction of movement has been reduced such that the angle is in the range of 5 to 15 degrees, and preferably in the range of 8 to 12 degrees. A smaller angle allows the overall height of the SMA actuation arrangement in the direction of movement to be reduced, since the length of the SMA actuator wire itself necessarily has an extent (extended projected) in projection in the direction of movement, which sets a minimum dimension in the height of the SMA actuation arrangement.

In other words, embodiments of the present technology provide an actuator that combines a tilted/angled SMA actuator wire with a bearing arrangement to control and guide the movement of the movable element of the actuator. In general, a small tilt angle increases the gain of the actuator and may make it more difficult to maintain the stability of the movable element in a particular position. However, the combination of a small angle SMA actuator wire and bearing arrangement solves this problem because the bearing is able to resist any increased lateral force (linear force) caused by a small angle of inclination. One advantage of the present technique is that small actuators can be produced without any loss in stroke volume. Small actuators are desirable because actuators are used in consumer electronics devices (e.g., smart phones) and there is market pressure to make consumer electronics devices smaller and thinner.

An SMA actuation apparatus 1 is shown in fig. 1 and 2, the SMA actuation apparatus 1 being a camera. The SMA actuation apparatus 1 comprises a support structure 2, the support structure 2 having an image sensor 3 mounted thereon.

The support structure 2 comprises a base 4, the base 4 being a rigid plate. The image sensor 3 is fixed to the front side of the base 4. In the arrangement shown in fig. 1 for controlling the SMA actuation apparatus 1 using a Hall sensor 9, the support structure 2 supports an IC chip 5 fixed to the rear side of the base 4. Alternatively, the SMA actuation apparatus 1 may be controlled using resistance control, in which case the IC chip 5 may be external to the SMA actuation apparatus 1. The control circuit is implemented in the IC chip 5, as described further below.

The support structure 2 further comprises a base frame 6, the base frame 6 protruding from the base 4 and may be a molded part. As described below, the chassis 6 serves as a mounting platform for the various components and also defines any reference features required during assembly. The chassis 6 has a central hole 7 aligned with the image sensor 3. In the arrangement shown in fig. 1, the support structure 2 further comprises a flexible printed circuit 8 fixed to the outside of the chassis 6. Typically, the flexible printed circuit 8 provides VDD, GND, SCL and SDA connection pads (not shown). On the flexible printed circuit 8, the hall sensor 9 is fixed close to the magnet 26 supported by the support structure 6.

The SMA actuation apparatus 1 further comprises a shield (shield can)10, which shield 10 is fitted to the base 4 and covers all other components described below to prevent physical damage and ingress of dust.

The SMA actuation apparatus 1 further comprises a lens element 20, which lens element 20 is a movable element in this example. The lens element 20 comprises a lens carrier 25, the lens carrier 25 holding the lens 21, but alternatively there may be a plurality of lenses. The lens 21 may be made of glass or plastic. The lens element 20 has an optical axis O aligned with the image sensor 3 and is arranged to focus an image on the image sensor 3. The lens element 20 also has a projection 22, and the projection 22 is formed on a side projecting laterally of the optical axis O.

In the presence of the hall sensor 9, the lens carrier 25 of the lens element 20 also mounts a magnet 26, positioned relative to the hall sensor 9, on the support structure 2, such that the hall sensor 9 senses the position of the lens element 20 along the optical axis O.

The SMA actuation apparatus 1 is a miniature optical device. In some examples of micro-optical devices, the lens 21 (or lenses, when provided) may have a diameter of at most 20mm, preferably at most 15mm, more preferably at most 10 mm.

Although the SMA actuation apparatus 1 is a camera in this example, this is not generally necessary. In some examples, the SMA actuation apparatus 1 may be an optical device in which the movable element is a lens element, but without an image sensor. In other examples, the SMA actuation apparatus 1 may be of a type that is not an optical device and in which the movable element is not a lens element and is free of an image sensor.

The SMA actuation apparatus 1 further comprises a suspension system 30, the suspension system 30 supporting the lens element 20 on the support structure 2. The suspension system 30 is configured to guide movement of the lens element 20 relative to the support structure 2 along the optical axis O (which is therefore the direction of movement in this example), while limiting movement of the lens element 20 relative to the support structure 2 in other degrees of freedom. The suspension system 30 includes a bearing arrangement, which will be described in more detail below.

The SMA actuation apparatus 1 further comprises two lengths of SMA actuator wire 40, the two lengths of SMA actuator wire 40 being arranged to drive movement of the lens element 20 along the optical axis O as follows. The two lengths of SMA actuator wire 40 are part of a piece of SMA actuator wire 41, which piece of SMA actuator wire 41 is connected at each end to the support structure 2 by a crimped portion (crimp) 42 fixed at opposite corners at the top of the chassis 6. The crimping portion 42 crimps the piece of SMA actuator wire 41 to provide a mechanical and electrical connection. The piece of SMA actuator wire 41 is also connected to the lens element 20 by hooking around the protrusion 22. Thus, each of these lengths of SMA actuator wire 40 is connected at one end to the support structure 2 and at the other end to the lens element 20.

The lengths of SMA actuator wire 40 have an angled V-shaped arrangement of the type disclosed in WO2007/113478, as described below.

The lengths of SMA actuator wire 40 have an angle of 90 degrees between them when viewed along the optical axis O, which in this example is the direction of movement. More generally, the orientation of the lengths of SMA actuator wire 40 may be varied such that the angle between them, viewed along the optical axis O, has any magnitude less than 180 degrees, preferably in the range 70 to 110 degrees.

Furthermore, the SMA actuator wires 40 of each length are inclined in the same direction (in the same sense) and at the same acute angle θ relative to a plane orthogonal to the optical axis O, which in this example is the direction of movement, as shown in fig. 3. In this example, the angle θ is selected to be in a range of a lower limit of 5 degrees, or more preferably 8 degrees, to an upper limit of 15 degrees, or more preferably 12 degrees, with respect to a plane orthogonal to the moving direction. Thus, the angle θ is relatively small, for example compared to WO 2007/113478. This allows the overall height of the SMA actuation apparatus 1 along the optical axis O to be reduced as the extent to which the lengths of SMA actuator wire 40 project along the optical axis O is reduced.

This low value of the angle θ increases the gain, which is the ratio of the amount of movement to the change in length of the lengths of SMA actuator wire 40 (ratio), and the strain on the lengths of SMA actuator wire 40 that requires an increase in the power of the drive signal. These factors may make it more difficult to maintain stability in the position of the lens element 20. However, it has surprisingly been found that in practice, in the SMA actuation apparatus 1, the stability of the lens element 20 can actually be maintained in such an angular range. This is due in part to the formation of the suspension system 30 as a bearing arrangement (described in more detail below) because the bearings resist the increased lateral forces. This also benefits from making the lens 21 from glass, as the additional mass compared to using plastic means that additional lateral forces from the lengths of SMA actuator 40 are advantageous, allowing a viable device to be manufactured. However, making the lens heavier will lower the resonance frequency, which may cause environmental vibrations to be a problem. Since a reduction in line angle also lowers the resonant frequency, there is a lower limit of the acceptable line angle range.

While it is advantageous to select the angle θ in the range of 8 degrees to 12 degrees, this is not required and the angle θ may be selected to have other values, such as values in excess of 12 degrees or in excess of 15 degrees.

The lengths of SMA actuator wire 40 drive the lens element 20 to move along the optical axis O upon application of a drive signal which causes heating and cooling of the lengths of SMA actuator wire 40. These lengths of SMA actuator wire 40 are resistively heated by the drive signal and cooled by thermal conduction with the surrounding environment as the power of the drive signal is reduced. The lengths of SMA actuator wire 40 contract when heated, driving movement of the lens element 20 in a first direction (upwards in fig. 1 and 2) along the optical axis O.

The SMA actuation apparatus 1 further comprises a compression spring 11, which compression spring 11 is connected between the base 4 of the support structure 2 and the lens element 20 and acts as a resilient biasing element for the lengths of SMA actuator wire 40. Thus, when the lengths of SMA actuator wire 40 cool, the compression springs 11 drive movement in opposite directions (downwards in fig. 1 and 2) along the optical axis O. Thus, the temperature of the lengths of SMA actuator wire 40, and hence the position of the lens element 20 along the optical axis O, can be controlled by controlling the power of the drive signal.

A control circuit implemented in the IC chip 5 generates drive signals and supplies them to the lengths of SMA actuator wire 40 to which they are connected. The control circuit receives an input signal indicative of a desired position of the lens element 20 along the optical axis O and generates a drive signal having a power selected to drive the lens element 20 to the desired position. The power of the drive signal may be linear or may be varied using pulse width modulation.

The drive signal may be generated using closed loop control based on the output of the hall sensor 9 sensing the position of the lens element 20 along the optical axis O.

Alternatively, the drive signal may be generated using a resistive feedback control technique, which may be, for example, as described in WO 2013/175197; WO 2014/076463; WO 2012/066285; WO 2012/020212; WO 2011/104518; WO 2012/038703; WO2010/089529 or WO2010029316, each of which is incorporated herein by reference in its entirety.

The suspension system 30 will now be described with reference to fig. 4 to 6, which show three different bearing arrangements of the rolling bearing 31.

The suspension system 30 comprises a bearing arrangement of a plurality of rolling bearings 31. Each of the rolling bearings 31 comprises a bearing surface 32 on the support structure 2, in particular on the chassis 6, and a bearing surface 33 on the lens element 20, in particular on the lens carrier 25. Each of the rolling bearings 31 further comprises one or more balls 34 arranged between the bearing surfaces. The balls 34 thus act as rolling bearing elements, although as an alternative other types of rolling bearing elements, such as rollers, may also be used.

Ideally, the bearing surface 32 on the support structure 2 has a low surface roughness in order to minimize friction and dynamic tilting. In some cases it may be sufficient that the bearing surface 32 is a plastic molded surface. More preferably, the bearing surface 32 is made of metal. When the chassis 6 is a molded element, this is achieved in that the bearing surface 32 is a surface of a metal insert 35 arranged in the chassis 6. Similarly, alternatively or additionally, the bearing surface 33 on the lens element 20 may be a metal surface formed as a surface of a metal insert arranged in the lens carrier, which is also a molded element.

Forming bearing surface 32 (and/or bearing surface 33) from metal gives a smooth surface on which balls 34 can run and allows the required dynamic tilt performance to be achieved. Defects in bearing

surfaces

32 and 33 may cause dynamic tilting. Such defects may take the form of surface curvature and surface roughness. If the number of balls 34 is reduced or, in the case of a plurality of balls 34, if the distance between the contact points of the upper and lower balls 34 is reduced, the influence of the defects of the bearing surfaces 32 and 33 on the dynamic tilt increases.

Three different bearing arrangements of the bearing 31 are arranged as follows.

The first bearing arrangement shown in fig. 4 comprises four bearings 31a-31d arranged as follows.

In each of the bearings 31a-31d, the bearing surface 32 on the support structure 2 is a planar surface and the bearing surface 33 on the lens element 20 is a groove (although alternatively the bearing surface 32 on the support structure 2 may be a groove and the bearing surface 33 on the lens element 20 may be a planar surface). Here, and where the bearing surface is a groove, the following applies. When the balls 34 roll, the grooves of the bearing surface 33 come into contact with the balls 34 along a pair of parallel lines. The planar surfaces on the bearing surface 33 are substantially parallel to the pair of parallel lines of contact in the recess of the bearing surface 33.

Each of the bearings 31a-31d, by virtue of the inclusion of a planar surface on the bearing surface 32, restricts movement of the bearing surface 32 and the bearing surface 33 towards each other, but does not restrict lateral movement of the bearing surface 32 and the bearing surface 33.

The two

bearings

31a, 31b are positioned against the protruding portion 22 on opposite sides of the protruding portion 22 when viewed along the optical axis O. These

bearings

31a, 31b include a plurality of balls 34, typically three balls 34, as shown in fig. 1 and 2. In the case where three or more balls 34 are provided, the inner ball 34 may be smaller in diameter than the outer two balls 33, and serve as a spacer.

With the

bearings

31a, 31b positioned against the projection 22, these

bearings

31a, 31b apply a force to the lens element 20 proximate to the location of the length of SMA actuator wire 40, which helps to restrict rotation about an axis normal to the plane containing the optical axis O (i.e. the axis into the page in fig. 3 and the axis in the lateral direction (sideways) in fig. 4). This is because the couple (couple) between the forces exerted by the lengths of SMA actuator wire 40 and the

bearings

31a, 31b is reduced when viewed perpendicular to the optical axis O compared to the

bearings

31a, 31b which are located further away. This effect is improved by providing a plurality of balls 34 in the

bearings

31a, 31b, which increases the length of the

bearings

31a, 31b along the optical axis O. The dynamic tilting behavior is therefore dependent on the spacing of the two outer balls 34 in the

bearings

31a, 31b and the flatness of the bearing surfaces 32 and 33.

The

other bearings

31c, 31d are located close to where the lengths of SMA actuator wire 40 are connected to the chassis 6. Thus, the

bearings

31c, 31d provide a restriction of rotation about the axis close to the compression spring 11 in a plane orthogonal to the optical axis O. Since the main function of the

other bearings

31c, 31d is to prevent such rotation, the

other bearings

31c, 31d may each include a single ball 33. Rotation of the lens element 20 about this axis is undesirable during normal operation, but is important for controlling unpowered drop events.

The other two

bearings

31c, 31d are required because, in general, the rotation allowed by the

bearings

31a, 31b located against a projection in a plane orthogonal to the optical axis O can occur in either direction. However, the

other bearings

31c, 31d must be arranged so that only one of the two balls 33 is in contact with the support structure 2 and the lens element 20, in order not to unduly constrain the system.

Thus, in a first bearing arrangement, four bearings 31a-31d are used to prevent rotation, but the design requires several parts with tight tolerances in order to be able to be assembled without undue restriction. The second and third bearing arrangements provide a simplified design that provides for loose tolerances and ease of assembly due to the reduced number of components.

The second bearing arrangement shown in fig. 5 comprises only two

bearings

31a, 31 b. For the

respective bearings

31a, 31b in the first bearing arrangement, these

bearings

31a, 31b are positioned against the projection 22 on opposite sides of the projection 22 when viewed along the optical axis O, and comprise a plurality of balls 34, typically three balls 34. In the case where three or more balls 34 are provided, the inner ball 34 may be smaller in diameter than the outer two balls 33, and serve as a spacer. In this way, these

bearings

31a, 31b help to limit rotation about an axis orthogonal to the plane containing the optical axis O, and the above explanation of the effect of the

respective bearings

31a, 31b in the first bearing arrangement is equally applicable to the second bearing arrangement.

However, the bearing surfaces 32, 33 of the

bearings

31a, 31b are altered compared to the first bearing arrangement for restricting rotation in a plane orthogonal to the optical axis O. In particular, in one of the bearings 31a, the bearing surface 32 on the support structure 2 and the bearing surface 33 on the lens element 20 are each a groove. As the balls 34 roll, each of the grooves of the bearing surface 32 and the grooves of the bearing surface 33 come into contact with the balls 34 along a pair of parallel lines, the two pairs of parallel lines being parallel to each other.

In the other bearing 31b there is still the case where the bearing surface 32 on the support structure 2 is a planar surface and the bearing surface 33 on the lens element 20 is a groove (however alternatively the bearing surface 32 on the support structure 2 may be a groove and the bearing surface 33 on the lens element 20 may be a planar surface).

This asymmetric arrangement of

bearings

31a, 31b limits the movement of lens element 20 to rotation in a plane orthogonal to optical axis O. Conceptually, this limitation can be understood by considering the bearing surface 32 of the bearing 31a and the groove on the bearing surface 33, which limits the motion in all degrees of freedom except for rotation about an axis passing through the bearing 31a itself, which is itself limited by the other bearing 31 b. Since the further bearing 31b comprises a planar bearing surface 33, this constraint is imposed without unduly limiting the overall bearing arrangement.

The third bearing arrangement shown in fig. 6 comprises only three bearings 31a-31 c. As in the first bearing arrangement, two of these

bearings

31a, 31b help to restrict rotation about an axis orthogonal to the plane containing the optical axis O, and the above explanation of the effect of the

respective bearings

31a, 31b in the first bearing arrangement is equally applicable to the third bearing arrangement.

However, the two

bearings

31a, 31b are asymmetrically arranged compared to the first bearing arrangement, such that they apply a couple to the lens element 2 in a plane orthogonal to the optical axis O. Such a couple generated by the two

bearings

31a, 31b in the third bearing arrangement tends to generate rotation in a particular direction, compared to the first bearing arrangement in which rotation in a plane orthogonal to the optical axis O may be in either direction. Thus, only a single further bearing 31c may be provided to limit the rotation, the bearing 31c being positioned against the lens element 22 in a position in which it prevents the rotation. Since the main function of the further bearing 31c is to prevent such rotation, the further bearing 31c may comprise a single ball 33.

In this further bearing 31c, the bearing surface 32 on the support structure 2 is a planar surface and the bearing surface 33 on the lens element 20 is a groove (however alternatively the bearing surface 32 on the support structure 2 may be a groove and the bearing surface 33 on the lens element 20 may be a planar surface). In this way, it is possible to impose restrictions on rotation without over-restricting the entire bearing arrangement.

Although the rolling bearing 31 is described above, the rolling bearing 31 may be replaced by plain bearings (plain bearings) as an alternative.

In one example, a first type of plain bearing 41 illustrated in fig. 7 and 8 may be used. The slide bearing 41 comprises an elongated bearing surface 43 on one of the support structure 2 and the lens element 20. The slide bearing 41 further comprises a projection 45 formed on the other of the support structure 2 and the lens element 20, an end of the projection 45 forming a bearing surface 46 bearing on the elongated bearing surface 43. Although two tabs 45 are shown in the example of fig. 8, in general, any number of one or more tabs 45 may be provided. The elongate bearing surface 43 and the bearing surface 46 are conformal (planar) in this example, allowing relative movement of the lens element 20 with respect to the support structure 2. Desirably, the elongated bearing surface 43 and the bearing surface 46 have a coefficient of friction of 0.2 or less.

In another example, a second type of plain bearing 51 shown in fig. 9 and 10 may be used. The slide bearing 51 comprises a channel 52 on one of the support structure 2 and the lens element 20, an inner surface of the channel 52 forming a bearing surface 53. The slide bearing 51 comprises a projection 55 formed on the other of the support structure 2 and the lens element 20, the end of the projection 55 forming a bearing surface 56 bearing on the bearing surface 53. Although two tabs 55 are shown in this example, in general any number of one or more tabs 55 may be provided. The elongate bearing surface 53 and the bearing surface 56 are conformal, in this example both planar, thereby allowing relative movement of the lens element 20 with respect to the support structure 2. Desirably, the elongated bearing surface 53 and the bearing surface 56 have a coefficient of friction of 0.2 or less.

In each of the slide bearing 41 and the slide bearing 51, the material of the bearing surfaces 43, 46, 53, 56 is selected to provide smooth movement and long life. The bearing surfaces 43, 46, 53, 56 may be integral with the underlying component or may be formed by a surface coating. Suitable materials include, for example, PTFE or other polymer bearing materials or metals.

It may be surprising that the slide bearing can be fully used for applications where the lens element 20 is guided along the optical axis O, provided that the lens element 20 needs to move smoothly over its lifetime without jamming and slipping. However, it has been found that plain bearings function adequately by controlling the force experienced by the bearing in the plane and perpendicular to the direction of movement to within a suitable range. Thus, the sliding bearing can provide a similar function of restricting the movement in degrees of freedom other than along the optical axis O, as compared to the rolling bearing 31, while reducing some or all of the above-described problems, as described below.

The manufacture of the SMA actuator arrangement is simplified because of the reduced number of parts, and assembly of the bearings is easier to perform because there is no rolling bearing to operate. Similarly, the risk of rolling bearing elements falling off the bearing due to insufficient mechanical constraints during use is eliminated.

Furthermore, the use of a sliding bearing may allow the height of the SMA actuation arrangement along the optical axis to be reduced, which is desirable for miniaturisation of cameras and other optical devices. In particular, the total height of the bearing surface can be reduced compared to a bearing comprising rolling bearing elements, while providing the required degree of dynamic tilting.

More generally, the rolling bearing 31 can also be replaced by any of the following types of bearings: a jewel bearing; a fluid bearing; a magnetic bearing; a flexible bearing; or a composite bearing. Furthermore, the rolling bearing 31 may also be replaced by a rocker bearing or a pivot bearing, wherein the bearing elements pivot or rock on the moving element and the stationary element.

The second to fourth SMA actuation arrangements will now be described. Each of the second to fourth SMA actuation arrangements is a modified version of the first SMA actuation arrangement 1. Therefore, only the modifications will be described, further the second to fourth SMA actuation arrangements have the structure of the first SMA actuation arrangement 1 as described above.

A second SMA actuation arrangement 60 is shown in fig. 11. In the second SMA actuation apparatus 60, the two lengths of SMA actuator wire 40 in the first SMA actuation apparatus 1 are replaced by a single straight SMA actuator wire 61, which straight SMA actuator wire 61 is connected to the support structure 2 by a crimped portion 62 fixed to the tab and to the lens element 20 by a crimped portion 63 fixed to the lens carrier 25. The

crimp portions

62 and 63 crimp the SMA actuator wire 61 to provide both a mechanical connection and an electrical connection.

The SMA actuator wire 61 has the same configuration as one of the lengths of SMA actuator wire 40 in the first SMA actuation arrangement 1 and is therefore inclined at an acute angle θ greater than 0 degrees to a plane orthogonal to the optical axis O.

The manufacture of the second SMA actuation apparatus 60 is simplified compared to both the first SMA actuation apparatus 1 and the angled V-shaped arrangement of WO2007/113478, as there is only a single length of SMA actuator wire 61 and there is no need to hook this SMA actuator wire 61 onto the projection 22. The tilting of the SMA actuator wires 61 retains the advantage of providing a gain in movement amount and is beneficial in minimising the height of the second SMA actuation arrangement 60 along the optical axis O.

The shape memory alloy actuator wires 61 generate a considerable lateral force when compared to known arrangements using 2, 4 or 8 SMA actuator wires, which typically provide some balance of the lateral forces generated by the different SMA actuator wires. However, it has been recognized through analysis and experimentation that surprisingly, these lateral forces can be adequately counteracted by the suspension system 30 due to the bearing arrangement using the bearing 31.

The assembly of the second SMA actuation apparatus 60 may be performed using the method disclosed in WO2016/189314, for example as shown in figure 12. The method uses a strut member 65, which strut member 65 is shaped to include a sacrificial strut body 66 and crimp tabs 67 held apart by the sacrificial strut body 66.

The SMA actuator wire 61 is placed at a known tension, for example starting from a spool, across the crimping tabs 67, the crimping tabs 67 being folded and pressed against the SMA actuator wire 61 by a closing tool (not shown) to form the crimped portion 62 and the crimped portion 63 which hold the SMA actuator wire 61. Then, the crimping

portions

62 and 63 are attached to the support structure 2 and the lens element 20, respectively, for example by mechanical means or adhesive means.

Thereafter, the sacrificial post body 66 is removed, leaving the

crimp portions

62 and 63 attached to the support structure 2 and the lens element 20, respectively.

In the second SMA actuation apparatus 60, it is necessary to electrically connect both end portions of the SMA actuator wire 61. This connection is direct at the support structure 2, since this is where the flexible circuit board 8 is located. The connection at the lens element 20 may be made in any suitable manner, for example by a flexible connector or by some other component of the SMA actuation arrangement 60, such as the compression spring 11, or flexure (if used as a suspension system 30).

The second actuation device 60 generates half the amount of force in the direction along the optical axis O compared to the first SMA actuation device 1. A third SMA actuation apparatus 70 producing the same force as the first SMA actuation apparatus 1 is shown in fig. 13.

In the third SMA actuation apparatus 70, the single piece of SMA actuator wire 41 hooked over the tab in the first SMA actuation apparatus 1 is replaced by two lengths of SMA actuator wire 71, the two lengths of SMA actuator wire 71 each being connected to the support structure 2 by a crimped portion 72 fixed to the tab 22 and each being connected to the lens element 20 by a crimped portion 73 fixed to the lens carrier 25. The

crimp portions

72 and 73 crimp the SMA actuator wires 71 to provide both a mechanical connection and an electrical connection.

The two lengths of SMA actuator wire 71 have the same configuration as the lengths of SMA actuator wire 40 in the first SMA actuation apparatus 1, except for the manner of connection at the projections 22. Thus, the SMA actuator wires 71 have an angled V-shaped arrangement of the type disclosed in WO2007/113478, as described below.

The two lengths of SMA actuator wire 71 have an angle of 90 degrees between them when viewed along the optical axis O, which in this example is the direction of movement. More generally, the orientation of the lengths of SMA actuator wire 71 may be varied such that the angle between them, viewed along the optical axis O, has any magnitude less than 180 degrees, preferably in the range 70 to 110 degrees.

Furthermore, the SMA actuator wires 71 of each length are inclined in the same direction and at the same acute angle θ relative to a plane orthogonal to the optical axis O, which in this example is the direction of movement. The lengths of SMA actuator wire 71 are also positioned at the same height along the optical axis O. In this example, the angle θ is selected to be in the range of 8 to 12 degrees with respect to a plane orthogonal to the moving direction. This provides the same advantages as using such an angle in the first SMA actuation apparatus 1, and the explanations with respect to this aspect of the first SMA actuation apparatus 1 apply equally to the third SMA actuation apparatus 70.

However, while it is advantageous to select the angle θ in the range of 8 degrees to 12 degrees, this is not required and the angle θ may be selected to have other values, such as values in excess of 12 degrees or in excess of 15 degrees.

The manufacture of the second SMA actuation apparatus 70 is simplified compared to both the first SMA actuation apparatus 1 and the angled V-shaped arrangement of WO2007/113478, as there is no need to hook lengths of SMA actuator wire 71 over the projection 22, and the crimped portions 73 may each be attached to the projection 22. The tilting of the lengths of SMA actuator wire 71 retains the advantage of providing a gain in the amount of movement and is beneficial in minimising the height of the third SMA actuation apparatus 70 along the optical axis O.

In the third SMA actuation apparatus 70, to provide electrical connections to two lengths of SMA actuator wire 71, electrical connectors 78 are provided on the lens element 2, forming electrical connections between the lengths of SMA actuator wire 71. In one type of embodiment, the two lengths of SMA actuator wire 71 and the electrical connector 78 may each be part of a single piece of SMA actuator wire. In another type of embodiment, the two lengths of SMA actuator wire 71 may be a single piece of SMA actuator wire and the electrical connector 78 may be a separate component, for example an electrically conductive track or metal element moulded into the lens carrier 24 connecting the two crimping portions 73, or alternatively a body integral with the crimping portions 73. Thus, an electrical connection to the two lengths of SMA actuator wire 71 can be made at the support structure by the crimp portion 72, which is direct as this is where the flexible circuit board 8 is located. This allows the drive signal to be provided by two lengths of SMA actuator wire 71 in series without the need for any electrical connection at the lens element 20.

The assembly of the third SMA actuation apparatus 70 may be performed using the method disclosed in WO2016/189314, for example as shown in figure 12.

The method may use a separate strut element 75 for each length of SMA actuator wire 71, and then assemble each length of SMA actuator wire 71 by performing the single wire 61 assembly method twice in the second SMA actuation assembly 60 as described above.

Alternatively, the method may use a single strut element 75, the single strut element 75 being shaped to include a sacrificial strut body 76 and crimp tabs 77 for two lengths of SMA actuator wire 71 held apart by the sacrificial strut body 76. The strut element 75 has a 90 degree bend at the bend line 78 so that two lengths of SMA actuator wire 71 may be attached in their respective orientations.

The lengths of SMA actuator wire 71 are placed at a known tension, for example starting from a spool, across crimping tabs 77, the crimping tabs 77 being folded over and pressed over the lengths of SMA actuator wire 71 by a closing tool (not shown) to form crimped

portions

72 and 73 holding the lengths of SMA actuator wire 71. The two lengths of SMA actuator wire 71 and the electrical connector 78 may be part of a single piece of SMA actuator wire, or alternatively the two lengths of SMA actuator wire 71 may be a single piece of SMA actuator wire 71 and the electrical connector 78 may be part of the strut element 75.

Then, the crimping

portions

72 and 73 are attached to the support structure 2 and the lens element 20, respectively, for example by mechanical means or adhesive means.

Thereafter, the sacrificial post body 76 is removed, leaving the

crimp portions

72 and 73 attached to the support structure 2 and the lens element 20, respectively.

The third SMA actuation apparatus 70 may be modified to vary the configuration or number of lengths of SMA actuator wire 71, including by way of non-limiting example arrangements, in which: parallel inclined lengths of SMA actuator wire 71 on the same side of the SMA actuation apparatus 70; SMA actuator wires 71 providing crossing lengths of opposite movement on one side of the SMA actuation arrangement 70; or two pairs of parallel length lengths of SMA actuator wire 71 on adjacent sides of the SMA actuation apparatus 70.

Two alternative forms of the fourth SMA actuation apparatus 80 are shown in fig. 15 and 16.

In the fourth SMA actuation apparatus 80, the bearing arrangement forming the suspension system 30 is changed such that the bearing 31 guides movement of the lens element 20 relative to the support structure 2 in a movement direction M which is inclined at a first acute angle α greater than 0 degrees relative to the optical axis O of the lens element 20. Thus, the lens element 20 is allowed to have a component of lateral movement perpendicular to the optical axis O. However, such lateral movement may be acceptable in many applications. Furthermore, this has the advantage of reducing the forces transmitted through the suspension system 30 and also the advantage of reducing the gain.

The lengths of SMA actuator wire 40 are inclined at a second acute angle β, greater than the first acute angle α, with respect to the optical axis O. This means that the lengths of SMA actuator wire 40 are still inclined with respect to the direction of movement M, but the angle θ of the lengths of SMA actuator wire 40 with respect to the normal to the optical axis O is reduced. This allows the overall height of the fourth SMA actuation apparatus 80 to be reduced whilst maintaining the degree of travel and maintaining good control. In the fourth SMA actuation apparatus 80, the key angle determining the performance of the lengths of SMA actuator wire 40 is no longer the angle θ between the lengths of SMA actuator wire 40 and a plane orthogonal to the optical axis O, but the angle γ between the lengths of SMA actuator wire 40 and a plane orthogonal to the direction of movement M of the lens element 20 relative to the support structure 20. Therefore, the dimensions of the fourth SMA actuation apparatus 80 along the optical axis O are no longer limited by the lengths of SMA actuator wire 40.

In the first form of the fourth SMA actuation apparatus 80 shown in fig. 15, the lengths of SMA actuator wire 40 extend in a plane orthogonal to the optical axis O. In this case, the lengths of SMA actuator wire 40 have a minimum extent projected along the optical axis O.

In a second form of the fourth SMA actuation apparatus 80 shown in fig. 16, the lengths of SMA actuator wire 40 extend at an angle θ greater than 0 degrees relative to a plane orthogonal to the optical axis O. In this case, the lengths of SMA actuator wire 40 have a range projected along the optical axis O, but this can be controlled by adjusting the angle θ to fit within the dimensional limitations of some other component of the SMA actuation apparatus. For example, the range of SMA actuator wires 40 of these lengths projected in the direction of the optical axis O may be arranged to be the same as or smaller than the range of bearing arrangement of the bearing 31 projected in the direction of the optical axis O.

It should be appreciated by those of skill in the art that while the foregoing has described what is considered to be the best mode and other modes of carrying out the present technology where appropriate, the present technology should not be limited to the specific constructions and methods of the preferred embodiments disclosed in this specification. Those skilled in the art will recognize that the present technology has a wide range of applications, and that the embodiments can be modified in a wide range without departing from any inventive concept defined by the appended claims.

Claims

1. A shape memory alloy actuation apparatus comprising:

a support structure;

a movable element comprising a lens element having at least one lens, the lens element having an optical axis, the movable element being supported on the support structure by a suspension system, the suspension system comprising a bearing arrangement arranged to guide movement of the movable element relative to the support structure in a movement direction;

at least one length of shape memory alloy actuator wire connected between the support structure and the movable element for driving controlled movement of the movable element relative to the support structure, wherein the length of shape memory alloy actuator wire is inclined at an angle in the range of 8 to 15 degrees relative to a plane orthogonal to the direction of movement throughout an operational range of movement, and a projection of the length of shape memory alloy actuator wire is offset from the optical axis when viewed along the optical axis, and wherein the bearing arrangement is configured to counteract a force generated by the length of shape memory alloy actuator wire in a direction orthogonal to the direction of movement.

2. A shape memory alloy actuation apparatus according to claim 1, wherein the bearing arrangement comprises at least one rolling bearing comprising bearing surfaces on the support structure and the movable element and at least one rolling bearing element disposed between the bearing surfaces.

3. A shape memory alloy actuation apparatus according to claim 2, wherein at least one of the bearing surfaces is made of metal.

4. A shape memory alloy actuation apparatus according to claim 2, wherein at least one of the support structure and the movable element is a moulded element and the bearing surface on at least one of the support structure and the movable element is a surface of a metal insert provided in the moulded element.

5. A shape memory alloy actuation apparatus according to claim 1, wherein the bearing arrangement comprises at least one plain bearing.

6. A shape memory alloy actuation apparatus according to any one of the preceding claims, wherein the at least one lens is made of glass.

7. A shape memory alloy actuation apparatus according to any one of the preceding claims, wherein the direction of movement is along an optical axis of the lens element.

8. A shape memory alloy actuation apparatus according to claim 7, wherein the support structure has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor.

9. A shape memory alloy actuation apparatus according to any one of the preceding claims, wherein the at least one lens has a diameter of at most 20 mm.

10. A shape memory alloy actuation apparatus according to any one of the preceding claims, wherein the length of shape memory alloy actuator wire is inclined at an angle in the range 8 to 12 degrees to a plane orthogonal to the direction of movement.