WO2020065332A1

WO2020065332A1 - Detecting degree of slack of a length of shape memory alloy wire

Info

Publication number: WO2020065332A1
Application number: PCT/GB2019/052728
Authority: WO
Inventors: Geoffrey Farmer
Original assignee: Cambridge Mechatronics Limited
Priority date: 2018-09-26
Filing date: 2019-09-26
Publication date: 2020-04-02
Also published as: CN112771271A; CN112771271B; GB2577515A; GB201815716D0; GB2577515B

Abstract

The degree of slack of a length of SMA wire is detected using images of the length of SMA wire captured from different orientations. The images are analysed to determine a three-dimensional path of the length of SMA wire and to derive a measure of the degree of slack therefrom. The method provides an accurate measure which may be used during manufacture of an SMA actuator assembly.

Description

Detecting Degree of Slack of a Length of Shape Memory Alloy Wire

The present application generally relates to manufacture of shape memory alloy (SMA) actuator assemblies.

In a first approach of the present techniques, there is provided a method of detecting the degree of slack of a length of shape memory alloy wire which is held slack, wherein the method uses images of the shape memory alloy wire captured from different orientations and comprises performing an analysis of the images which derives a measure of the degree of slack of the wire.

Generally speaking, the term "slack wire" may mean a wire which has zero tension. Alternatively, the term "slack wire" may mean a wire which has zero tension when the wire is unpowered. Alternatively, the term "slack wire" may mean a wire which has zero tension when it is unpowered and at ambient temperature (which may, in some cases be, 25°C). In other words, the term "slack wire" may mean a wire which is held between two crimps/crimp portions (i.e. is mechanically coupled at two points along its length to some other element), does not lie in a straight line between those two crimp portions when the wire is unpowered and at ambient temperature. Thus, in some cases, the slackness is present when the length of SMA wire is at a temperature of 25°C (which is a typical ambient temperature). When a drive signal is applied in use to the length of SMA wire to cause contraction, its temperature rises significantly above 25°C. Typically, when SMA wire is pulled from a spool (in which the wire is under tension), in some cases the wire may retain some tension due to hysteresis. Thus, alternatively, the term "slack wire" may mean that the wire is slack after any residual tension has been removed. The tension may be removed by, for example stretching the wire at ambient temperature. Once the tension is removed, if the length of the wire between the two crimps is greater than the distance between the crimps, the wire may be considered to be slack. A further definition of the term "slack wire" is a wire which is slack when the SMA wire is substantially martensitic. It will be understood that regardless of which of these definitions are used to provide slack wire, the present techniques can be used to measure/detect the degree of slack.

This method provides an accurate and reliable method detecting the degree of slack of the length of SMA wire. The degree of slack corresponds to the length of the three-dimensional path of the length of SMA wire. For example, if the ends of a slack length of SMA wire with zero tension are fixed, then the length of SMA wire will follow a non-linear path. It has been appreciated that the configuration of the length of SMA wire in three dimensions can be observed images captured from different orientation to a sufficient degree to allow a measure of the degree of slack to be derived from image analysis of such images.

By way of example, the image analysis may involve determination of the three-dimensional path of the length of SMA wire from the images. This may be achieved by detecting the length of SMA wire in the images, determining the two-dimensional path of the detected length of SMA wire in each image, and deriving the three-dimensional paths therefrom.

The measure of the degree of slack of the wire may be determined from the determined three-dimensional path. For example, the measure of the degree of slack may be the difference between the length of the three-dimensional path of the length of SMA wire and the distance between the ends of the length of SMA wire, although related measures may also be used.

This method is advantageous because it provides an accurate measure of the degree of slack without the need to apply a complex electrical or mechanical measurement process. The method is capable of being performed very quickly, making it suitable for application at any stage in the manufacturing process in an mass production environment, including as an end-of-line inspection to verify correct assembly manufactured of SMA actuator assemblies.

The method does not rely on independent variable properties of the length of SMA wire, such as the resistance of the length of SMA wire (which varies with resistivity and wire diameter along a length of SMA wire) or of associated components, such as their parasitic resistance.

The method also does not rely on the length of SMA wire being subject to tensions of the order experienced in normal use, which may be impractical at some stages of manufacture. For example, the length of SMA wire may be crimped but without being immediately ready to withstand such tensions that will be applied when it is powered, e.g. on an external crimp that is not yet attached to an SMA actuator assembly

The method is also advantageous over alternative methods that rely on the application of tension to relatively move the ends of the length of SMA wire, for example to pull it taut and measure the displacement or change in angle. Such alternative methods cannot be applied at stages of manufacture where the ends of the length of SMA wire are fixed. When the ends of the wire are movable, a tool applying the tension may affect the performance of the length of SMA wire performance and potentially cause damage either through the contact with the length of SMA wire or the applied tension itself. Moreover, such alternative methods are complex and unreliable because of difficulties in achieve accurate alignment of the tool applying the tension, variability in the tension that is being applied which will affect the measurement, and variability in the subsequent slack of the length of SMA wire when the tool is released.

Advantageously, the images may be captured from at least from two orientations that are orthogonal. This simplifies the image analysis, although in general any different orientations may be used.

The method may be applied to a length of SMA wire at any stage in a manufacturing process. The method is particularly suitable for a length of SMA wire that is held slack between crimp portions crimped around the length of SMA wire, but is more generally applicable to a length of SMA wire held in any other way.

In one example, the method may be applied to a length of SMA wire that is held slack between crimp portions which are crimped around the length of SMA wire and are respectively mounted on a static part and a movable part that is movable with respect to the static part in a SMA actuator assembly, for example an optical element which may be a lens element comprising at least one lens.

In another example, the method may be applied to a length of SMA wire that is held slack between crimp portions crimped around the length of SMA wire in an SMA sub-assembly comprising at least one body portion formed integrally with the pair of crimp portions from a sheet of material. Such an SMA sub- assembly may be used in the manufacture of an SMA actuator assembly.

The measure of the degree of slack may be used in various ways in the manufacture. In one example, the measure of the degree of slack may be used for quality control. In another example, the method may further comprise adjusting the degree of slack of the length of SMA wire in response to the derived measure. In that case, the length of SMA wire may initially be held slack between crimp portions that are partly crimped around the length of SMA wire, in which case the method may further comprise fully crimping the crimp portions around (portions or segments of) the length of SMA wire after said step of adjusting the degree of slack of the length of SMA wire.

The present techniques may be used to measure the degree of slack in a length of SMA wire which is held between crimp portions, either before attaching the crimp portions to an SMA actuator assembly or afterwards. Thus, the slack measurement may be performed after manufacture of an SMA sub-assembly but prior to manufacture of an SMA actuator assembly, or during or after manufacture of the SMA actuator assembly. The SMA actuator assembly be any type of device that comprises a static part and a moveable part which is moveable with respect to the static part. The SMA actuator assembly may be, or may be provided in, any one of the following devices: a smartphone, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronics device, an image capture device, a 3D sensing device or system, a servomotor, a consumer electronic device, a mobile computing device, a mobile electronic device, a laptop, a tablet computing device, an e-reader (also known as an e-book reader or e-book device), a computing accessory or computing peripheral device (e.g. mouse, keyboard, headphones, earphones, earbuds, etc.), a security system, a gaming system, a gaming accessory (e.g. controller, headset, a wearable controller, etc.), an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device (e.g. a watch, a smartwatch, a fitness tracker, etc.), a drone (aerial, water, underwater, etc.), an aircraft, a spacecraft, a submersible vessel, a vehicle, and an autonomous vehicle. It will be understood that this is a non- exhaustive list of example devices. Thus, the techniques described herein may be used to manufacture or calibrate an SMA actuator assembly that may be used for or in devices/systems suitable for image capture, 3D sensing, depth mapping, aerial surveying, terrestrial surveying, surveying in or from space, hydrographic surveying, underwater surveying, scene detection, collision warning, securing, facial recognition, augmented and/or virtual reality, advanced driver-assistance systems in vehicles, autonomous vehicles, gaming, gesture control/recognition, robotic devices, robotic device control, touchless technology, home automation, medical devices, and haptics.

In a second approach of the present techniques, there is provided apparatus for detecting the degree of slack of a length of shape memory alloy wire which is held slack, the apparatus comprising: at least one camera to capture images of the shape memory alloy wire from at least two different orientations; and at least one processor to: analyse the captures images; and derive, from the analysis, a measure of the degree of slack of the wire.

In a related approach of the present techniques, there is provided a non- transitory data carrier carrying processor control code to implement any of the methods described herein.

Preferred features are set out in the appended dependent claims.

As will be appreciated by one skilled in the art, the present techniques may be embodied as a system, method or computer program product. Accordingly, present techniques may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects.

Furthermore, the present techniques may take the form of a computer program product embodied in a computer readable medium having computer readable program code embodied thereon. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing.

Computer program code for carrying out operations of the present techniques may be written in any combination of one or more programming languages, including object oriented programming languages and conventional procedural programming languages. Code components may be embodied as procedures, methods or the like, and may comprise sub-components which may take the form of instructions or sequences of instructions at any of the levels of abstraction, from the direct machine instructions of a native instruction set to high-level compiled or interpreted language constructs.

Embodiments of the present techniques also provide a non-transitory data carrier carrying code which, when implemented on a processor, causes the processor to carry out any of the methods described herein.

The techniques further provide processor control code to implement the above-described methods, for example on a general purpose computer system or on a digital signal processor (DSP). The techniques also provide a carrier carrying processor control code to, when running, implement any of the above methods, in particular on a non-transitory data carrier. The code may be provided on a carrier such as a disk, a microprocessor, CD- or DVD-ROM, programmed memory such as non-volatile memory (e.g. Flash) or read-only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and/or data) to implement embodiments of the techniques described herein may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (RTM) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, such code and/or data may be distributed between a plurality of coupled components in communication with one another. The techniques may comprise a controller which includes a microprocessor, working memory and program memory coupled to one or more of the components of the system. It will also be clear to one of skill in the art that all or part of a logical method according to embodiments of the present techniques may suitably be embodied in a logic apparatus comprising logic elements to perform the steps of the above-described methods, and that such logic elements may comprise components such as logic gates in, for example a programmable logic array or application-specific integrated circuit. Such a logic arrangement may further be embodied in enabling elements for temporarily or permanently establishing logic structures in such an array or circuit using, for example, a virtual hardware descriptor language, which may be stored and transmitted using fixed or transmittable carrier media.

In an embodiment, the present techniques may be realised in the form of a data carrier having functional data thereon, said functional data comprising functional computer data structures to, when loaded into a computer system or network and operated upon thereby, enable said computer system to perform all the steps of the above-described method.

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

Figure 1 is a flow chart of a method of detecting the degree of slack of a length of SMA wire;

Figure 2 is a perspective view of an image capture arrangement used to capture images of the length of SMA wire;

Figure 3 is a flow chart of a step of image analysis shown in Figure 1;

Figure 4 is a pair of images with construction lines illustrating derivation of a three-dimensional path;

Figure 5 is a top view of an SMA sub-assembly comprising a single body portion;

Figures 6 and 7 are perspective views of the crimp portion in partly and fully crimped states; and Figures 8 and 9 are perspective views of SMA actuator assemblies which are cameras.

Broadly speaking, embodiments of the present techniques generally relate to manufacture of shape memory alloy (SMA) actuator assemblies. The degree of slack of a length of SMA wire is detected using images of the length of SMA wire captured from different orientations. The images are analysed to determine a three-dimensional path of the length of SMA wire and to derive a measure of the degree of slack therefrom. The method provides an accurate measure which may be used during manufacture of an SMA actuator assembly.

SMA actuators are known for use in handheld electronic devices, such as cameras and mobile phones. Such actuators may be used for example in miniature cameras to effect focus, zoom or optical image stabilization (OIS). By way of example, International Patent Publication No. WO2007/113478 discloses an SMA actuator arrangement for a camera providing autofocus using a single length of SMA wire and International Patent Publication No. WO2013/175197 discloses a compact an SMA actuator arrangement for a camera providing OIS using four length of SMA wires. Further, International Patent Publication No. W02011/104518 discloses an SMA actuator arrangement comprising eight lengths of SMA wires capable of effecting both autofocus and OIS. In each of these disclosures, each length of SMA wire is fixed at its ends to a static part and a moving part, and the preferred method of fixing is crimping in which a crimp portion is crimped around the length of SMA wire to form a crimp holding the length of SMA wire.

In the prior art examples mentioned above, it has been assumed that it is necessary during manufacture to attach the length of SMA wire under tension such that its length and tension can be accurately known in the unpowered state. In such an actuator, the length of SMA wire is under greater tension in the unpowered state. However, the present techniques are concerned with manufacturing the SMA actuator assembly using length of SMA wire that is slack, for example being temporarily slack during manufacture, and/or being slack in the absence of a drive signal in the manufactured SMA actuator assembly in order to increase the available stroke. In such cases, it is desirable to measure the degree of slack of the SMA actuator assembly in order to control the manufacturing process and/or assess newly manufactured actuator assemblies. However, the degree of slack is difficult to measure due to the small size and uncontrolled path of the length of SMA wire. One might consider applying tension to the length of SMA wire to pull it taught but that tension can itself affect the measurement.

Turning to the Figures, Figure 1 shows an example method of detecting the degree of slack of a length of SMA wire 20, and optionally for adjusting the degree of slack based on the detection. The method may be applied during a manufacturing process of an SMA actuator assembly 30, an example of which is described below. The method may be applied at any stage of that manufacturing process in which the length of SMA wire 20 is held slack. Examples of situations where the length of SMA wire 20 is held slack and the method may be applied are described below, but these are non-limitative and the method is generally application to the length of SMA wire 20 being held slack in any situation.

In step SI of the example method shown in Figure 1, images of the length of SMA wire 20 are captured from different orientations.

By way of example, Figure 2 illustrates an image capture arrangement 100 that may be used to perform step SI as follows.

The image capture arrangement 100 is shown applied to a case in which the length of SMA wire 20 is held slack between crimp portions 10 that are crimped around the length of SMA wire 20, which is a typical way of connecting a length of SMA wire 20 to other components. However, although the image capture arrangement 100 is equally applicable to other methods of mounting and restraining the length of SMA wire 20.

The image capture arrangement 100 comprises two cameras 101 that are mounted with viewing axes that are orthogonal to each other with overlapping fields of view 102. The length of SMA wire 20 is arranged in the overlap of the fields of view, so that both cameras 101 are focussed on the length of SMA wire 20. Thus, the cameras 101 capture images from two orthogonal orientations. The fields of view 102 of the cameras 101 include the full extent of the length of SMA wire 20 including at least the inner edges of the crimp portions 10.

The cameras 101 have a resolution sufficient to allow clear discrimination of the edges of the length of SMA wire 20 and identification of the three- dimensional path of the length of SMA wire 20. Pixilation of the profile of the length of SMA wire 20 will result in mathematical rounding errors that will make the analysis and measurement described below less accurate. For example, the resolution may be selected so that the width of the length of SMA wire 20 corresponds to at least 3 pixels in the images, preferably at least 6 pixels, and most preferably at least 10 pixels. Higher resolutions can also be used, but with reducing advantage in the accuracy from increased ratio of pixels to width of the length of SMA wire 20.

The cameras have a depth of field sufficient to maintain focus on the length of SMA wire 20 within the expected range of displacement from the theoretical wire axis.

The cameras 101, background, and surrounding illumination are chosen to provide sufficient image contrast to enable optical discrimination between the length of SMA wire 20 and the background or any surrounding features of item being assembled or the mounting structure.

The image capture arrangement 100 may be adapted in various ways, for example as follows.

Although the cameras 101 capture images from two orthogonal orientations and this simplifies the image analysis described below, this is not essential and the cameras 100 may capture images from any different orientations.

More than two cameras 101 may be provided to capture more than two images. Instead of plural cameras 101, the image capture arrangement 100 may comprise a single camera 101 that is re-positioned (i.e. moved during the image capture process) to capture the images from different orientations. To achieve this, the single camera 101 may be mounted on a powered mount.

When measuring the slack of plural length of SMA wire 20 mounted on a single actuator assembly 30, then : (a) images of successive lengths of SMA wire 20 may be taken in turn by the same cameras 101 by the actuator assembly 30 being repositioned in front of the cameras 101, for example using a powered mount; (b) images of successive lengths of SMA wire 20 may be taken in turn by the same cameras 101 by the cameras 101 being repositioned in front of the actuator assembly 30, for example using a powered mount; or (c) separate sets of cameras 101 may be provided for each length of SMA wire.

When measuring the slack of plural lengths of SMA wire 20 mounted on a single actuator assembly 30 to drive movement of a movable part relative to a static part, then the slack of the lengths of SMA wire 20 will vary if the movable part is not in the same relative position to the static part as when the lengths of SMA wire 20 were crimped. In that case, a centring spring may be used to pull the movable part to a central position. Alternatively, the images may be captured before the two movable and static parts are unclamped from an assembly fixture. As another alternative, the slack of each wire in an actuator can be measured separately and the average slack calculated therefrom, but this is less desirable.

The image capture arrangement 100 may include fixed or movable mirrors to capture images from different orientations.

The field of view of the cameras 101 may be smaller than the full length of the length of SMA wire 20, in which case the cameras 101 and/or length of SMA wire 20 may be traversed to build up a full image of the length of SMA wire 20.

Although Figure 2 shows the plane of the crimp portions 10 as being perpendicular to the viewing axis of one of the cameras 101, this is not essential. Returning to Figure 1, step S2 is an automated computer image analysis step performed on the capture images. As such, step S2 may be performed in a computer apparatus 110. In this case, a computer program capable of execution by the computer apparatus 110 is provided. The computer program is configured so that, on execution, it causes the computer apparatus 110 to perform step S2. The computer apparatus 110 may be any type of computer system but is typically a computer of conventional construction.

The computer program may be written in any suitable programming language.

The computer program may be stored on a computer-readable storage medium, which may be of any type, for example: a recording medium which is insertable into a drive of the computing system and which may store information magnetically, optically or opto-magnetically; a fixed recording medium of the computer system such as a hard drive; or a computer memory.

Figure 3 is a flow chart of a step S2 which is performed as follows on the captured images (e.g. images 111 shown in Figure 4).

In step S2-1, the length of SMA wire 20 is detected in each of the images. This may be done by conventional techniques, for example a simple threshold detection technique or a more complex feature detection technique.

In step S2-2, the two-dimensional path of the detected length of SMA wire in each image is determined. The two-dimensional paths may be plotted as a series of two-dimensional coordinates. For example, referring to Figure 4, a two-dimensional path may be plotted as XY coordinates in image 111 taken along the Z axis and as XZ coordinates in the other image 111 taken along the Y axis.

In step S2-3, the three-dimensional path is derived from the determined two-dimensional paths. This may be done by a simple transformation of the determined two-dimensional paths. For example, where the two-dimensional paths are plotted as a series of two-dimensional coordinates, the two- dimensional coordinates of corresponding segments may be combined to provide a series of three-dimensional coordinates. By way of example, Figure 4 is a pair of images 111 with construction lines illustrating the two-dimensional coordinates which are derived in step S2-2 and combined to provide the three- dimensional path in step S2-3.

In an example where the length of SMA wire 20 is held slack between crimp portions 10, then the method considers the length of SMA wire 20 inside those crimp portions 10. In step S2-4, a measure of the degree of slack of the length of SMA wire

20 is derived from the determined three-dimensional path determined in step S2-3. In this example, the measure is the difference between the length of the three-dimensional path of the length of SMA wire 20 and the distance between the ends of the length of SMA wire 20. The length of the three-dimensional path of the length of SMA wire 20 may be calculated by summing the vector lengths of successive segments of the three-dimensional path. The distance between the ends of the length of SMA wire 20 may be calculated simply as the geometrical distance between those points. This difference increases with the slack of the length of SMA wire 20. However, other measures of the degree of slack may also be calculated.

To illustrate calculation of the length of the three-dimensional path, then if the three-dimensional path is defined by a series of coordinates {xOyOzO, xlylzl, x2y2z2, x3y3z3, x4y4z4, xnynzn,

and given that the vector length between two points in three dimensions is \/(Dc². Ay². Dz²), then the vector lengths of successive segments may be derived as shown in the following table:

To facilitate this calculation, two dimensional paths derived in step S2-2 should be a significant number of pixels apart to prevent the pixel steps becoming dominant to the wire path calculation. There will be an optimal balance in the minimum pixel, the number of pixels between each coordinate, and expected minimum bend radius of the length of SMA wire 20. If the bend radius can be trusted to be relatively large then a larger number of pixels can be used between coordinates, increasing the accuracy of path calculation for any given camera resolution.

Of course, other mathematical methodologies could be used for these steps, for example using radial coordinates instead of Cartesian coordinates, or laying spline curves over the imaged path

The method provides an accurate measure of the degree of slack that is capable of being performed very quickly, making it suitable for application at any stage in the manufacturing process in a mass production environment, including as an end-of-line inspection to verify correct assembly manufactured of SMA actuator assemblies.

In some applications of the method, for example for quality control of a manufactured product, the measure of the degree of slack of the length of SMA wire 20 is output as data, for example for display or storage. In that case, the following steps of the method may be omitted.

In other applications of the method, the measure of the degree of slack of the length of SMA wire 20 is used to control the manufacturing process, for example by performing steps S2 and S3 of Figure 1. In this case, the crimp portions 10 are partly crimped around the length of SMA wire 20 as shown in Figure 6 and described below. As such, the crimp portions 10 hold the length of SMA wire 20 with a compressive force that is sufficiently low to allow the length of SMA wire 20 to be moved along its length to vary the degree of slack.

In step S3, a force is applied along the length of the length of SMA wire 20 to vary the degree of slack into the length of SMA wire 20.

In step S4, the crimp portions 10 are completely crimped (or closed) around the length of SMA wire 20. As such, the crimp portions 10 hold the length of SMA wire 20 with a compressive force that is sufficiently high to resist the tension developed in the length of SMA wire 20 under application of drive signals in normal use. This may be done using a conventional crimp tool. Thus, the crimp portions 10 hold the length of SMA wire 20 in a state in which it is slack between the crimp portions 10. The body portion 3 of the SMA sub-assembly 1 holds the crimp portions 10 and maintains the length of the length of SMA wire 20 when it is subsequently tensioned, as described below.

There will now be described some products including a length of SMA wire 20 and to which the present method may be applied. These products are described by way of non-limitative example.

Figure 5 shows an SMA sub-assembly 1 comprising a fret 2 including a body portion 3. The fret 2 is formed from a sheet of material as a flat strip. The material of the fret 2 may be metal, for example phosphor bronze, steel or laminate containing conductive components. A number of techniques for forming an SMA sub-assembly are described in United Kingdom Patent Application No. GB1815673.7, which is incorporated herein in its entirety.

The fret 2 also comprises a pair of crimp portions 10 formed integrally with the body portion 3 from the same sheet of material. In particular, the body portion 3 comprises an elongate portion 4 and laterally protruding arms 5 at the extremes of the elongate portion 4, the crimp portions 10 being formed by tabs on the ends of the arms 5. Thus, the crimp portions 10 are held apart by the body portion 4.

The SMA sub-assembly 1 may have a similar construction and arrangement to the fret disclosed in International Patent Publication No. WO2016/189314.

The crimp portions 10 are partly or fully crimped around a length of SMA wire 20, so that they hold the length of SMA wire 20. The crimp portions 10 therefore crimp the length of SMA wire 20 to provide both mechanical and electrical connection. The length of SMA wire 20 may be made of any suitable SMA material, for example Nitinol or another Titanium-alloy SMA material. In contrast to WO2016/189314 wherein the length of SMA wire is under tension in the fret, the length of SMA wire 20 is slack, the term "slack" being used herein with its normal meaning that the length of SMA wire 20 (i.e. the length of its three-dimensional path) between the crimp portions 10 is greater than the distance between the crimp portions 10. Typically, the length of SMA wire 20 will have zero tension when it is slack, except that this may be affected by hysteresis effects. That is, when a length of SMA wire 20 is pulled under tension, e.g. from a spool, even if the wire is slack when between the crimped portions, it may retain some tension. Such tension may be removed by stretching the length of SMA wire 20 (before or after the crimp portions 10 are crimped), for example at a tension of 300MPa.

In general, the slackness or tension of the length of SMA wire 20 depends on its temperature and in use a drive signal is applied to the length of SMA wire 20 to cause contraction. However, references herein to the length of SMA wire 20 being slack refer to the length of SMA wire being slack when at ambient temperature, for example at a temperature of 25°C which is significantly below the temperature of the SMA wire with the drive signal is applied in use. The length of SMA wire 20 may be brought to a temperature of 25°C simply by placing it within an ambient temperature 25°C and waiting a sufficient time to reach thermal equilibrium of the length of SMA wire 20 and any surrounding components.

The body portion 3 is sacrificial and is removable from the crimp portions 10, for example by mechanical or laser cutting.

The crimp portions 10 may be partly crimped around the length of SMA wire 20 as shown in Figure 6. In that case, the crimp portions 10 hold the length of SMA wire 20 with a compressive force that is sufficiently low to allow the length of SMA wire 20 to be moved along its length to vary the degree of slack or introduce tension.

Alternatively, the crimp portions 10 may be fully crimped around the length of SMA wire 20 as shown in Figure 7. In that case, the crimp portions 10 hold the length of SMA wire 20 with a compressive force that is sufficiently high to resist the tension developed in the length of SMA wire 20 under application of drive signals in normal use.

Figure 8 shows an example of an SMA actuator assembly 30 which is a camera arranged as follows.

The SMA actuator assembly 30 comprises a support structure 32 that has an image sensor 33 mounted thereon. The support structure 32 includes a base 34 which is a rigid plate. The image sensor 33 is fixed to the front side of the base 34. The support structure 32 also includes a chassis 36 that protrudes from the base 4 and may be a moulded component. The chassis 36 has a central aperture 37 aligned with the image sensor 33.

The SMA actuator assembly 30 further comprises a lens element 40 positioned in the aperture 37 and comprising a lens carriage 42 which holds a lens 41, although alternatively plural lenses may be present. The lens 41 may be made of glass or plastic. The SMA actuator assembly 30 is a miniature optical device in which the lens 41 has a diameter of at most 20 mm, preferably at most 15 mm, more preferably at most 10 mm.

The lens element 40 has an optical axis O aligned with the image sensor 33 and is arranged to focus an image on the image sensor 33. The lens element 40 also has a protrusion 43 that is formed on one side protruding laterally of the optical axis O.

The SMA actuator assembly 30 also comprises a suspension system 50 that supports the lens element 40 on the support structure 32. The suspension system 30 is configured to guide movement of the lens element 40 with respect to the support structure 32 along the optical axis O, while constraining movement of the lens element 40 with respect to the support structure 32 in other degrees of freedom. Such relative movement of the lens element 40 changes the focus of the image on the image sensor 33, for example for providing autofocus or zoom. Accordingly, in this example the support structure 32 is a static part and the lens element 40 is a movable part that is movable with respect to the support structure 32 along the optical axis O to. The terms "static" and "movable" refer to that relative motion. In particular, the suspension system 50 comprises a bearing arrangement of plural rolling bearings 51. Each of the rolling bearings 51 comprises a bearing surface 52 on the support structure 32, in particular on the chassis 36, and a bearing surface 53 on the lens element 40, in particular on the lens carriage 42. Each of the rolling bearings 51 also comprises balls 54 disposed between the bearing surfaces 52 and 53. The balls 54 therefore act as rolling bearing elements, although as an alternative other types of rolling bearing element could be used, for example a roller.

As an alternative, the rolling bearings 51 may be replaced by plain bearings comprising bearing surfaces on each of the support structure 32 and the lens element 40 that conform and bear on each other to guide the relative movement.

The SMA actuator assembly 30 also comprises two lengths of SMA wire 20 (one of which is visible in Figure 8 that are arranged as follows to drive movement of the lens element 40 along the optical axis O. Each length of SMA wire 20 is connected to the support structure 32 at one end and to the lens element 40 at the other end by crimp portions 10 (which are the crimp portions 10 of an SMA sub-assembly as described in more detail below).

The lengths of SMA wire 20 have an angled-V arrangement of a similar type to that disclosed in International Patent Publication No. WO2007/113478. That is, each length of SMA wire 20 is inclined in the same sense and at the same acute angle Q with respect to a plane normal to the optical axis O which is the movement direction in this example. The angle Q is selected to provide gain between the change in length of the length of SMA wire 20 and the movement along the optical axis O, while also reducing the height projected along the optical axis, typically being in a range from a lower limit of 5 degrees, or more preferably 8 degrees, to an upper limit of 20 degrees, preferably 15 degrees, or more preferably 12 degrees, with respect to a plane normal to the optical axis O. The lengths of SMA wire 20 also have an angle therebetween of 90 degrees as viewed along the optical axis O which is the movement direction in this example. In an alternative, simpler arrangement, one of the lengths of SMA wire 20 may be omitted.

The SMA actuator assembly 30 further comprises a compression spring 45 that is connected between the base 34 of the support structure 32 and the lens element 40 and acts a resilient biasing element for the lengths of SMA wire 20. Thus, when the lengths of SMA wire 20 cool, the compression spring 45 drives movement along the optical axis O in the opposite direction (downwards in Figures 1 and 2). As a result, the temperature of the length of SMA wire 20 and hence the position of the lens element 40 along the optical axis O can be controlled by control of the power of the drive signals.

The length of SMA wire 20 are arranged to be slack in the absence of a drive signal being applied thereto. It has been appreciated by experiment and analysis that it is not necessary to hold the lengths of SMA wire 20 in tension in the unpowered state. The length of SMA wire 20 is however configured so that a tension suitable for driving the SMA actuator assembly 30 may be applied to the length of SMA wire by application of suitable drive signals to heat the wire and cause it to contract. This may be achieved by controlling the degree of slack in the length of SMA wire 20.

In addition, such a case of slack length of SMA wire in the unpowered state provides significant advantages. If a length of SMA wire is under tension in the unpowered state, then the SMA actuator assembly typically loses a large amount of its theoretical stroke, for example of the order of 50pm to lOOpm in typical optical device. This is significant because the achievable stroke is often be a limiting factor in miniaturisation of the SMA actuator assembly. On the other hand, by providing length of SMA wire 20 that are slack in the unpowered state, the length of the length of SMA wires 20 is increased, improving the stroke capability of the SMA actuator assembly 30, possibly up to its theoretical maximum.

A control circuit implemented in an IC chip (not shown) generates the drive signals and supplies them to the length of SMA wire 20, to which the control circuit is connected. The control circuit receives an input signal representing a desired position for the lens element 40 along the optical axis O and generates drive signals having powers selected to drive the lens element 40 to the desired position. The power of the drive signals may be either linear or varied using pulse width modulation.

Figure 9 shows an example of an SMA actuator assembly 30 which is a camera similar to that shown in Figure 8 but with the following modification to provide an angled-V arrangement of the type disclosed in International Patent Publication No. WO2007/113478. Instead of providing two lengths of SMA wire 20 that are separate pieces of SMA wire each connected by crimp portions 10 at each end as in Figure 8, a single length of SMA wire 20 is connected at each end to the support structure 32 by crimp portions 10 and is connected to the lens element 40 by being hooked over a hook feature 44 formed on the protrusion 43. As a result, the two parts of the length of SMA wire 20 on either side of the protrusion 43 form respective length of SMA wire 22 which have the same configuration, and hence the same function and operation, as the two length of SMA wire 20 in Figure 8.

Although particular SMA actuator assemblies 30 are shown in Figures 8 and 9 as an example, the SMA sub-assembly 1 could be used to manufacture other types of SMA actuator assembly 30. In one alternative, the SMA actuator assembly 30 may be a camera providing OIS of the type disclosed in International Patent Publication No. WO2013/175197, or a camera providing both multiple functions of the type disclosed in International Patent Publication No. W02011/104518. In other alternatives, the SMA actuator assembly 30 may be an optical device in which the movable element is a lens element but there is no image sensor. In other alternatives, the SMA actuator assembly 30 may be an optical device wherein the movable part is an optical element other than a lens element, for example a diffractive optical element, a filter, a prism, a mirror, a reflective optical element, a polarising optical element, a dielectric mirror, a metallic mirror, a beam splitter, a grid, a patterned plate, or a grating, which may be a diffraction grating.

In other examples, SMA actuator assembly 30 may be a type of device that is not an optical device and in which the movable element is not an optical element. As mentioned above, the SMA actuator assembly be any type of device that comprises a static part and a moveable part which is moveable with respect to the static part. The SMA actuator assembly may be, or may be provided in, any one of the following devices: a smartphone, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronics device, an image capture device, a 3D sensing device or system, a servomotor, a consumer electronic device, a mobile computing device, a mobile electronic device, a laptop, a tablet computing device, an e-reader (also known as an e- book reader or e-book device), a computing accessory or computing peripheral device (e.g. mouse, keyboard, etc.), a security system, a gaming system, a gaming accessory (e.g. controller, headset, a wearable controller, etc.), an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device (e.g. a watch, a smartwatch, a fitness tracker, etc.), a drone (aerial, water, underwater, etc.), an aircraft, a spacecraft, a submersible vessel, a vehicle, and an autonomous vehicle. It will be understood that this is a non-exhaustive list of example devices.

Thus, the techniques described herein may be used to manufacture or calibrate an SMA actuator assembly that may be used for or in devices/systems suitable for image capture, 3D sensing, depth mapping, aerial surveying, terrestrial surveying, surveying in or from space, hydrographic surveying, underwater surveying, scene detection, collision warning, securing, facial recognition, augmented and/or virtual reality, advanced driver-assistance systems in vehicles, autonomous vehicles, gaming, gesture control/recognition, robotic devices, robotic device control, touchless technology, home automation, medical devices, and haptics.

Those skilled in the art will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing present techniques, the present techniques should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that present techniques have a broad range of applications, and that the embodiments may take a wide range of modifications without departing from any inventive concept as defined in the appended claims.

Claims

1. A method of detecting the degree of slack of a length of shape memory alloy wire which is held slack, wherein the method uses images of the shape memory alloy wire captured from different orientations and comprises performing an analysis of the images which derives a measure of the degree of slack of the wire.

2. The method according to claim 1, wherein the step of performing an analysis of the images comprises:

determining a three-dimensional path of the shape memory alloy wire from the images; and

deriving a measure of the degree of slack of the wire from the determined three-dimensional path.

3. The method according to claim 2, wherein the step of determining a three- dimensional path of the shape memory alloy wire comprises:

detecting the shape memory alloy wire in the images;

determining the two-dimensional path of the detected shape memory alloy wire in each image; and

deriving the three-dimensional paths from the determined two- dimensional paths.

4. The method according to claim 2 or 3, wherein the measure of the degree of slack of the shape memory alloy wire is the difference between the length of the three-dimensional path of the shape memory alloy wire and the distance between the ends of the shape memory alloy wire.

5. The method according to any preceding claim, wherein images of the shape memory alloy wire are images captured from at least from two orthogonal orientations.

6. The method according to any preceding claim, wherein the resolution of the images is selected so that such the width of the shape memory alloy wire corresponds to at least 3 pixels in the images.

7. The method according to any one of the preceding claims, wherein the length of shape memory alloy wire is held slack between crimp portions crimped around the length of shape memory alloy wire.

8. The method according to claim 7, wherein the length of shape memory alloy wire is held slack between crimp portions crimped around the length of shape memory alloy wire in a shape memory alloy sub-assembly comprising at least one body portion formed integrally with the pair of crimp portions from a sheet of material.

9. The method according to claim 7, wherein the length of shape memory alloy wire is held slack between crimp portions which are crimped around the length of shape memory alloy wire and are respectively mounted on a static part and a movable part that is movable with respect to the static part in a shape memory alloy actuator assembly.

10. The method according to claim 8, wherein the movable part is an optical element.

11. The method according to claim 9, wherein the movable part is a lens element comprising at least one lens.

12. The method according to any preceding claim further comprising adjusting the degree of slack of the length of shape memory alloy wire in response to the derived measure.

13. The method according to claim 12, wherein the length of shape memory alloy wire is initially held slack between crimp portions that are partly crimped around the length of shape memory alloy wire, and the method further comprises fully crimping the crimp portions around the length of shape memory alloy wire after said step of adjusting the degree of slack of the length of shape memory alloy wire.

14. A non-transitory data carrier carrying processor control code to implement the method of any of claims 1 to 13.

15. Apparatus for detecting the degree of slack of a length of shape memory alloy wire which is held slack, the apparatus comprising:

at least one camera to capture images of the shape memory alloy wire from at least two different orientations; and

at least one processor to:

analyse the captures images; and

derive, from the analysis, a measure of the degree of slack of the wire.

16. The apparatus as claimed in claim 15 wherein the apparatus comprises a single camera provided on a moveable mount, and the at least one processor is configured to:

generate control signals to move the moveable mount to enable the camera to capture images from the at least two different orientations.

17. The apparatus as claimed in claim 15 or 16 wherein the at least one processor:

adjusts, responsive to the derived measure, the degree of slack of the length of shape memory alloy wire.