GB2588764A

GB2588764A - An electrical joint and method of forming

Info

Publication number: GB2588764A
Application number: GB1915923.5A
Authority: GB
Inventors: Langhorne Robert
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2019-11-01
Filing date: 2019-11-01
Publication date: 2021-05-12
Also published as: GB201915923D0; WO2021084257A1

Abstract

An electrical joint is formed by welding between a shape memory alloy (SMA) actuator wire 2 and an electrical connector 43. The SMA actuator wire is coated with an insulation coating (44, Fig 2). A heat source, such as a laser, is used to melt one or both of the connector and the wire to form a weld 46 and also to remove the insulation coating locally at the weld. Means, such as the connector itself acting as a heatsink, or a reflective or heat-insulating portion, are used to limit heat dissipation to the coated portion during weld formation. The connector may be crimped to the insulated wire before the weld is made. The end portion may absorb heat by being rough, having a black pigment added, or by being annealed to deposit carbon. The joint may be used in the lens mechanism of a camera.

Description

AN ELECTRICAL JOINT AND METHOD OF FORMING

Field

The present application relates to an electrical joint and method of forming the electrical joint, and in particular to an electrical joint having a shape memory alloy (SMA) actuator wire.

Background

Crimping is a technique used for mechanically connecting a plurality of cables, as well as connecting an end of a cable to an electrical terminal. The crimping process is made more complicated when connecting electrical cables, or wires. For example, electrical wires require an additional step of removing their insulation coatings, thereby exposing a conductive core, before they are crimped to effect electrical connection. The insulating coatings may be stripped from the electrical wires mechanically, for example with the use of a wire stripper or by abrasion.

However, such technique risks causing mechanical damage to the wire, as well as the need to accurately aligning the wire stripper with the wire in order to avoid overexposing the conductive core. Alternatively, polymeric insulation coatings may be removed by heat, for example heating the connecting end of the wires to melt or vaporise the insulation coatings thereat.

The quality and reliability of the electrical connection formed depends largely on the physical contact between the wire and the crimp. For example, intermittent electrical connection can be a serious problem for movable electrical connectors, for example those applied in actuators and motors. To compensate, crimping can be further improved by soldering where a solder is applied in between the wires and a connecting surface of the crimp. However, soldering can be difficult and imprecise for miniature devices, such as actuators for actuating miniature camera lenses. Furthermore, the residual heat energy from soldering can cause damage to heat sensitive materials, for example, insulation coatings as well as shape memory alloy (SMA) actuator wires. In the latter case, the heat energy from solder may cause the SMA wires to contract during soldering and therefore negatively affecting its tension or alignment.

Therefore, an improved crimping process that offers an electrical joint with improved performance and reliability, as well as simplicity, is highly desirable.

Summary

The present invention offers a process for welding an insulated SMA actuator wire to an electrical connector by melting a portion of the SMA actuator wire and/or the electrical connector to form the weld. Furthermore, the electrical connector comprises means for preventing heat dissipation to unwelded part of the wire. Advantageously, a desirable length of the insulation coating can be selectively 113 removed by the heat energy and therefore enabling exposure of just the correct amount of the electrical conductor in the SMA actuator wire. The process may not require the need for a separate soldering material for forming the electrical connection, nor does it require the removal of insulation coating prior to making the electrical connection. The process is particularly beneficial for crimping an SMA actuator wire in an actuator and as a result, a more reliable electrical joint may be formed.

According to a first aspect of the presently-claimed invention, there is provided a method of forming an electrical joint, comprising: contacting a smart memory alloy (SMA) actuator wire with an electrical connector, the SMA actuator wire being coated with an insulation coating; heating, with a heat source, to melt one or both of the electrical connector and the SMA actuator wire and thereby forming a weld for effecting electrical connection therebetween, wherein the heating removes the insulation coating locally at the weld to form a conductive portion, and thereby leaving a coated portion of the SMA actuator wire coated with an insulation coating; and limiting, with means, heat dissipation to coated portion during weld formation.

Optionally, the heating is effected by laser irradiation. More specifically, the laser irradiation is perform by a laser beam welding technique using any suitable laser light source, for example solid-state lasers such as ruby lasers and Nd:YAG lasers, or gas lasers that use mixtures of gases such as helium, nitrogen, and carbon dioxide, as a medium.

Optionally, the method further comprises crimping the SMA actuator wire to the electrical connector with a crimp of the electrical connector prior to heating.

Optionally, the method further comprises providing a heat sink to the electrical 5 connector to form the means. For example, the providing may comprise thermally connecting the heat sink with SMA actuator wire directly or via a heat conductor.

Optionally, the method further comprises selectively providing a pigment to the electrical connector for increasing local energy absorption at the weld during weld formation. Optionally, the pigment comprises a black pigment. For example, the pigment may be a paint and may be applicable using any one of painting, spraying, immersing and plating the selected surface with the paint.

Optionally, the step of selectively providing a pigment comprises annealing a part of the electrical connector for releasing the carbon thereat. For example, the released carbon may be deposited or coated on the surface of the electrical connector at the weld. Thus, the deposited carbon may increase local energy absorption thereat. Advantageously, such method allows the use of a laser light source for applying the pigment accurately, e.g. the same laser source for forming the weld may be used for annealing the electrical connector.

Optionally, the method further comprises applying a second surface finish onto a surface of electrical connector and/or SMA actuator wire adjacent to the weld for forming the means, the second surface finish is configured to reduce energy absorption by the respective electrical connector and/or SMA actuator wire through the surface during weld formation.

Optionally, applying a second surface finish comprises one or more of applying a heat insulating coating, applying a reflective coating, applying an ablative coating and polishing to effect a polished surface.

Optionally, the second surface finish is a liquid coating when it is applied, wherein the applying may comprises any one of painting, spraying, vapor deposition, immersing and plating the surface with the paint.

Optionally, the insulation coating having a melting point equal to or lower than a temperature required for forming the weld, thereby during weld formation the insulation coating at the weld melts to form electrical connection thereat.

According to a second aspect of the presently-claimed invention, there is provided an electrical joint comprising: a smart memory alloy (SMA) actuator wire having a coated portion coated with an insulation coating and a conductive portion free of the insulation coating; and an electrical connector electrically connected to the conductive portion of the SMA actuator wire by a weld formed from partially 113 melting at least one of the SMA actuator wire and the electrical connector, wherein the electrical connector comprises means for enabling localised heating at the weld during its formation, thereby reducing the amount of heat dissipation to the coated portion.

Optionally, the SMA actuator wire comprises one or more SMA actuator wires. Each of which may be independently insulated by an insulation coating or together all the electrically conductive wires many be insulated by a single insulation coating. The SMA actuator wires may comprise coaxial cables and/or heat activated actuator wires.

Advantageously, the means may reduce heat dissipation to the SMA actuator wire during the welding process, therefore it may avoid damaging or over-tensioning the SMA actuator wire during welding. In some cases, the means may allow heat energy to promptly transfer away from the SMA actuator wire during actuation, thus effecting a faster response time in the SMA actuator wire during normal use.

The insulation coating may be formed from heat sensitive material such as polymer, e.g. polyimide, or vanish by extrusion or by immersion coating. For example, the insulation coating at the weld may be configured to melt or vaporised during the welding process to expose the conductive portion of the conductor, whilst the insulation coating located adjacent or away from the weld may be preserved to insulate the coated portion.

The electrical connector may be a crimp at least partially formed by any electrically conductive material, such as carbon steel, stainless steel and copper alloy, e.g. phosphorus bronze. The conductive material may advantageously be ductile and capable of mechanically crimping the cable thereat. Alternatively, the connector may be a hook, a dowel pin, a clip or any other suitable connectors for effecting mechanical connection with the SMA actuator wire.

The SMA actuator wire and the electrical connector may be formed from the same or different material. For example, when the same material is used for forming the conductor and the connector, both of the conductor and the connector may melt to form the weld upon heating. Alternatively, when they are formed from 113 different materials, e.g. one of the conductor and the connector may possess a higher melting point than the other, thus only one of the conductor and connector may melt upon heating to form the weld.

The insulation coating may have the same or a lower melting point than a temperature required for forming the weld. Hence, the heat energy generated during formation of the weld may also melt the insulation coating sandwiched between the conductor and the connector. Therefore advantageously, the conventional step of wire stripping may be omitted. To prevent the heat energy from conducting through the rest of the SMA actuator wire, a means is provided to prevent a substantial rise in the temperature at locations away from, or adjacent to the weld. Advantageously, such arrangement may limit, or prevent, the insulation coating form melting at locations away from, or adjacent to the weld, as well as undesirable expansion or contraction in the SMA actuator wire that is associated with temperature fluctuation.

The SMA actuator wire may form from any suitable shape memory alloy material, typically a nickel-titanium alloy (e.g. Nitinol), but may also contain tertiary components such as copper. The SMA actuator wire may have any cross-sectional profile and diameter suitable for the application. For example, the SMA wire may have a cross section diameter of 25,,Lm capable of generating a maximum force of between 120mN to 200mN whilst maintaining the strain in the SMA wire within safe limits (e.g. 2-3% reduction in length over original length). Increasing the diameter of each SMA wire from 254m to 35p.m approximately doubles the cross-sectional area of each SMA wire and thus approximately doubles the force provided by each SMA wire. Preferably, the SMA wire may be capable to deliver a high force, e.g. between 1.2 to 3N, more preferably between 1.2 to 10N, whilst maintaining the strain in the wire within safe limits (e.g. 2-3°/o reduction in length over original length). The force may be dependent on the target displacement required.

Optionally, the electrical connector comprises a crimp having first end and a second end opposite the first end, and wherein the weld is formed at the first end of the crimp. For example, the first end of the crimp may be a leading edge of the crimp. Alternatively, the electrical connector comprises a crimp having first end and a second end opposite the first end, and wherein the weld is formed between the first end and the second end of the crimp. That is, the weld may form anywhere along a length of the crimp away from the first and second ends.

Optionally, the SMA actuator wire is crimped between the first end and the second end of the crimp, and wherein the coated portion emerges from the second end of the crimp. Advantageously, such arrangement may allow the heat energy during the welding process to concentrate at weld, thus reducing or preventing excessive temperature rise at the coated portion.

The electrical connector may comprise a plurality of crimps where each of which crimps onto different portions of the conductor. The or each crimp may have a width of between 5001im to 754m. The crimps may be adjacent to each other and vertically aligned, i.e. may be in a stack. Alternatively, the adjacent crimps may be laterally offset from each other, or vertically offset from each other, or laterally and vertically offset from each other. When a plurality of crimps are used, weld may be only be formed on the first end of a leading crimp amongst the plurality of crimps. Advantageously, such arrangement may allow crimps with different physical properties to be used. For example, the leading crimp may be formed from a material with a lower melting point than the rest of the crimps in the electrical connector, therefore it may allow the weld to be formed at a lower temperature.

Optionally, the means comprising a heat sink and/or configured to be in thermal connection with an auxiliary heat sink, the heat sink is configured to absorb the heat energy during formation of the weld. For example, the heat sink may be a thermally conductive body of a crimp. The thermally conductive body may have a higher heat capacity than the SMA actuator wire. The heat sink and the auxiliary heat sink may comprise heat dissipating means such as fins. Alternatively, or in addition, the means may be a heat conductor that is in physical contact with an auxiliary heat sink. For example, the means may be a thermally conductive body of a crimp that is physically connected to an actuator body, wherein the actuator body comprises the auxiliary heat sink for absorbing the heat energy from the means. Advantageously, such arrangement allows at least some of the heat energy accumulated during the welding process to be dissipated through the means, rather than dissipating through the SMA actuator wire.

Optionally, the heat sink and/or the auxiliary heat sink are configured to absorb the heat energy during the actuation of the SMA actuator wire. For example, the means may also effect heat dissipation during the operation of SMA actuator wire, and thereby enables a faster response time in the actuator.

Alternatively, or in addition, the means may be a heat conductor that is configured to be in thermal connection with a cooling element, such as an external Peltier device or a fan. For example, the Peltier device or the fan may activate during the welding process, and subsequently be removed once the electrical joint is formed.

Advantageously, this may prevent damage to the coated portion during welding, yet prevents influencing the thermal profile of the SMA actuator wire during the operation of the SMA actuator.

Optionally, the means comprises a pigment selectively provided at the electrical connector for increasing local energy absorption at the weld during weld formation. Preferably, the pigment comprises a black pigment. For example, the pigment may reduce reflected non-ionising radiation such as laser irradiation, thereby the pigment may promote local energy absorption at selected sites, e.g. the weld. The pigment may be provided on selected surface or selected portion of the electrical connecter where the weld is formed. The pigment may be a surface coating. Optionally, the electrical connector is formed from phosphor bronze. Advantageously, such arrangement may significantly increase absorption of the laser irradiation at the surface. Hence, the weld may only form at location that is covered by the pigment during laser irradiation.

Optionally, the pigment comprises carbon diffused, or released, from the electrical connector during an annealing process prior to weld formation. For example, a selected part of the electrical connector where the weld is formed may be irradiated by defocused laser. That is, the defocussed laser may heat the carbon beneath the surface of the electrical connector, e.g., in the body of the crimp, thereby releasing the carbon and subsequently allowing it to diffuse to the surface of the electrical connector. This process is typically known as black annealed marking and is particular suitable for carbon-rich materials such as carbon steel or stainless steel.

Alternatively, or in addition, the means comprises a first surface finish selectively provided at the electrical connector for increasing local energy absorption at the weld during weld formation, wherein the first surface finish having a higher surface roughness than the surface adjacent to the weld. A rough finish may reduce the amount of reflected irradiation and therefore such arrangement may advantageously promote local energy absorption. For example, the first surface finish at the weld may have a surface roughness an order of magnitude greater than the surface adjacent to the weld. The first surface finish may have a surface roughness of at least 2 microns, or at least 5 microns, or at least 10 microns, or at least 20 microns.

Optionally, the means further comprises a second surface finish provided on a surface of electrical connector and/or SMA actuator wire adjacent to the weld, wherein the second surface finish is configured to reduce energy absorption by the respective electrical connector and/or SMA actuator wire though the surface during weld formation. Optionally, the second surface finish comprises one or more of a heat insulating coating, a reflective coating, an ablative coating and a polished surface. For example, when the weld is formed by a radiative heat source, the second surface finish may shield the surface that is adjacent to the weld from the heat source, or it may reflect irradiation, and thereby reducing the temperature rise associated with heat energy absorption. In the case of an ablative coating, or a sacrificial coating, the ablative coating may configure to absorb heat energy before it is sublimed or vaporised, thus reducing energy absorption at the electrical connector. In other words, the second surface finish may reduce heat energy absorption at the surface by changing the physical properties thereat, for example changing the reflectivity and/or thermal conductivity of the surface. The coatings may be applied prior to welding. The second surface finish may be a liquid coating applicable by spraying, plating, painting, vapour deposition or any other suitable techniques. Alternatively, the second surface finish may be a tape or a decal that is applicable on the surface by adhesion. In the case of a polished or a mirror finish, the surface roughness of the polished or mirror surface may have a surface roughness less than the surface forming weld. For example, the first surface finish at position adjacent to the weld may have a surface roughness an order of magnitude less than the surface forming weld. The first surface finish may have a surface roughness of less than 2 microns, or less than 1 microns, or less than 500 nanometers.

Optionally, the weld is configured to be formed by laser irradiation, and wherein the second surface finish, or coating, is configured to cover the surface from the laser irradiation.

According to a third aspect of the presently-claimed invention, there is provided an actuation mechanism comprising: a static component; a moveable component that is moveable relative to the static component; the electrical joint of the first aspect, wherein the electrical connector is mechanically connected to one of the static component and the movable component, and wherein the SMA actuator wire is mechanically connected to the other of static component and the movable component.

The actuation mechanism may be used for any suitable applications. For example, the actuating mechanism may be configured to actuate various movable components in a mobile phone such as the lens or lens carrier carrying the lens, light emitter, optical sensors, haptic input/output devices. Advantageously, because of their robustness and compactness, the electrical joint in such actuator are particularly suitable for miniature consumer devices.

Optionally, the SMA actuator wire is connected to the other of static component and the movable component by a second electrical joint. For example, the electrical joint and the second electrical joint may each be electrically connected to a conductive portion at a respective end of the SMA actuator wire.

Optionally, the means is configured to dissipate heat energy to the static component or the moveable component it is connected to. For example, the means may be a heat conductor for transferring heat energy to an auxiliary heat sink that provided on the static component or the moveable component the electrical connector is connected to, wherein the auxiliary heat sink is configured to absorb heat energy from the electrical connector.

According to a further aspect of the presently-claimed invention, there is provided 113 an apparatus comprising any of the electrical joint, actuator mechanism and SMA actuators described herein. The apparatus may be a smartphone, a camera, a foldable smartphone, a foldable image capture device, a foldable smartphone camera, a foldable consumer electronics device, a camera with folded optics, an image capture device, an array camera, a 3D sensing device or system, a servomotor, a consumer electronic device, a mobile or portable computing device, a mobile or portable electronic device, a laptop, a tablet computing device, an e-reader, a computing accessory or computing peripheral device, an audio device, a security system, a gaming system, a gaming accessory, a robot or robotics device, a medical device, an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device, an autonomous vehicle, a vehicle, a tool, a surgical tool, a remote controller, clothing, a switch, dial or button, a display screen, a touchscreen, and a near-field communication (NFC) device. It will be understood that this is a non-exhaustive list of possible apparatus.

Brief Description of the Drawings

Certain embodiments of the presently-claimed invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an exploded view of SMA actuator wire arrangements in a camera; Figure 2 is a sectional view of an SMA actuator wire; Figure 3 is a perspective view of an electrical joint according to a first embodiment of the present invention; Figures 4A to 4C are sectional views of different stages of forming the electrical joint according to a first embodiment of the present invention; Figures 5A to 5C are sectional views of different stages of forming the electrical joint according to a second embodiment of the present invention; Figures 6A and 65 are sectional views of different stages of providing surface coating for forming the electrical joint according to a third embodiment of the 113 present invention; and Figure 7 is a sectional view of an electrical joint according to a fourth embodiment of the present invention.

Detailed Description

Figure 1 shows an exploded view of a shape memory alloy (SMA) actuator wire arrangement 10 in a miniature camera. The SMA actuator arrangement 10 includes a static part 5 that comprises a base 11 that is an integrated chassis and sensor bracket for mounting an image sensor, and a screening can 12 attached to the base 11. The SMA actuator arrangement 10 includes a moving part 6 that is a camera lens assembly comprising a lens carriage 13 carrying at least one lens (not shown).

In this example, the actuator 10 includes eight SMA wires 2 each attached between the static part 5 and the moving part 6. A pair of SMA wires 2 that cross each other are provided on each of four sides of the SMA actuator arrangement 10 as viewed along an optical axis. The SMA wires 2 are attached to the static part 5 and the moving part 6 in such a configuration that they are capable of providing relative movement of the moving part 5 with multiple degrees of freedom for providing both autofocus (AF) and optical image stabilisation (OIS). The electrical joint according to present invention is applicable to any SMA actuator arrangement.

Thus, in respect of each pair of SMA wires 2, the SMA wires 2 are attached at one end to two static mount portions 15, which are themselves mounted to the static part 5 for attaching the SMA wires 2 to the static part 5. The static mount portions are adjacent one another but are separated to allow them to be at different electrical potentials.

Similarly, in respect of each pair of SMA wires 2, the SMA wires 2 are attached at one end to a moving mount portion 16 which is itself mounted to the moving part 6 for attaching the SMA wires 2 to the moving part 6. The moving part 6 further 113 comprises a conductive ring 17 connected to each of the moving mount portions 16 for electrically connecting the SMA wires 2 together at the moving part 6.

The static mount portions 15 and the moving mount portions 16 comprise crimp tabs 23 which may be formed into crimps and used to hold the SMA wires 2. The moving mount portions 16 may comprise electrical connection tabs 31 for providing electrical connection to the conductive ring 17. Thus, in the example shown in Figure 1, the crimp tabs 23 that are formed into crimps are integral parts of the static and moving portions of the actuator arrangement 10. Methods for forming the crimps and trapping the SMA wires within the crimp tabs 23 are described in International Patent Publication No. W02016/189314.

Figure 2 shows a sectional view of an example SMA actuator wire 2. The SMA actuator wire 2 comprises a length of electrical conductor 42 coated with an insulation coating 44. The electrical conductor 42 may be formed from a single or multiple strands of SMA wires. International Patent Publication No. W02015/036761 describes having SMA wires coated with insulation coating along part of their length, but not at the crimps. In the actuator, the length of the part of the SMA actuator wire 2 that is not coated with the electrically insulating layer, i.e. a conductive portion, is greater than the length of contact between the SMA actuator wire 2 and the crimp 43, such that good contact is made within the crimp 43 while providing easy placement of the crimp. Accordingly, the conductive portion is formed prior to the crimping.

Figure 3 shows a perspective view of an electrical joint 40 according to a first embodiment of the present invention. The electrical joint 40 comprises an SMA actuator wire 2 crimped in a crimp 43. It is particularly important to achieve good mechanical and electrical contact between the SMA actuator wire 2 and the crimp 43, so that the SMA actuator wire 2 does not become loose or detach from the crimp 43. Hence, the wire 2 can be powered/driven during actuation. If the SMA actuator wire 2 is coated with an electrically insulating coating 44, the insulation coating may hinder this requirement especially if it is relatively thick (e.g. of the order of 1pm or more), as the mechanical action of closing the jaws of the crimp 43 during manufacture is not sufficient to break down the electrically insulating coating 40 and to effect good contact with the SMA material beneath.

In the illustrated embodiment, the SMA actuator wire 2 extends between a first end 43a and a second end 43b of the crimp 43. The crimped SMA actuator wire 2 exits the crimp 43 through the second end 43b. More specifically, the crimp 43 is crimped onto the insulation coating 44 of the SMA actuator wire 2 towards its second end 43b to effect mechanical connection therebetween. A weld 46 is formed at the first end 43a, or the leading edge of the crimp 43 to effect electrical connection with the electrical conductor 42 of the SMA actuator wire 2. In other embodiments, the weld may be formed at any position along the length of the crimp 43 between the first 43a and second ends 43b of the crimp 43.

Figures 4A to 4C illustrate different stages of forming the electrical joint 40. Figure 4A shows a first stage of the formation process wherein a length of the SMA actuator wire 2 is crimped in the crimp 43, with an end of the SMA actuator wire 2 protruding out of the first end 43a of the crimp 43. In other embodiments, the end of SMA actuator wire 2 may flush with an edge of the first end 43a, or it may recede into the first end 43a of the crimp 43. As shown in Figure 4A, the insulation coating 44 extends between the electrical conductor 42 core and the crimp 43 along its full length, thus there is no electrical connection between the electrical conductor 42 and the crimp 43 at this stage of the process.

Figure 4B illustrate a second stage of the forming process where welding is performed. In Figure 4B, a laser beam 102 is configured to focus on the first end 43a of the crimp 43, thereby heating the first end 43a to a temperature above the melting point of both the crimp 43 and the SMA actuator wire 2 to fuse or weld the two together. In the illustrated embodiment, the end portions of both the crimp 43 and the SMA actuator wire 2 are melted to form the weld. In other embodiments, one of the crimp 43 and the SMA actuator wire 2 has a higher melting point than the other, and the first end 43a is heated to a temperature such that only the first end 43a of the crimp 43 or the SMA actuator wire 2 is melted to form the weld 46. For example, the weld 46 may be formed by melting the electrical conductor 42 of the SMA actuator wire 2, or it may be formed by melting the first end 43a of the crimp 43.

In the illustrated embodiment, the heat energy for forming the weld 46 is imparted 113 by laser irradiation. More specially, a lens 100 is used for focusing the laser beam from a laser emitter (not shown) onto the first end 43a of the crimp 43. In other embodiments, the weld 46 may be formed by one or more of arc welding, electrical resistive heating, gas welding, inductive heating, electron beam welding, microwave or any other form of irradiation.

Regardless of the type of heat source applied, the heat energy generated during welding process is sufficient to melt or to vaporise/sublime the insulation coating 44 locally at the weld 46. The insulation coating 44 typically has the same or a lower melting point than at least one of the crimp 43 and the electrical conductor 42. This allows electrical connection to establish between the electrical conductor 42 and the crimp 43 at a conductive portion 52 of the SMA actuator wire 2, whilst the crimp 43 remains in mechanical connection with a coated portion of the SMA actuator wire 2 towards its second end 43b.

Heat energy may be conducted, at least along the electrical conductor 42, towards and beyond the second end 43b of the crimp 43. This may be undesirable as an excessive amount of insulation coating 44 may be melted or vaporised, and thereby weakening the mechanical connection between the SMA actuator wire 2 and the crimp 43. Furthermore, excessive heating may cause the SMA actuator wire 2 to contract during welding, thus inadvertently affecting the tension in the SMA actuator wire 2. Therefore, in the illustrated embodiment, the crimp 43 functions as a heat sink for absorbing the heat from the weld 46, and thereby reducing the amount of heat energy being conduct, or dissipated, along the electrical conductor 42. Furthermore, the crimp 43 also functions as a heat conductor for conducting heat energy to the respective static part 5 or the moving part 6 it is connected to, whereby the static part 5 or the moving part 6 may further comprises an auxiliary heat sink (not shown). As a result, temperature may significantly reduce along the length of the electrical conductor 42.

The heat sink provided herein may be the crimp, the static part 5 or the moving part 6, or it may be a crimp support (not shown) with a high heat capacity or a phase change material attached thereon. The heat sink may comprise a fin or other heat dissipation means for dissipating heat energy to the surroundings. In some embodiment where the crimp 43 is a heat conductor, it may be cooled by an external cooling means, such as a fan or a piezoelectric element.

Figure 4C shows a third stage of the process for forming the electrical joint 40. The first end 43a of the crimp 43 is shown to have an irregular edge that results from the weld 46. The SMA actuator wire 2 and the crimp 43 are in electrical connection at the conductive portion 52 and in mechanical connection at the coated portion 50. The weld 46 also provide mechanical connection between the two components.

Figure 5A to 5C illustrate different stages of forming an electrical joint 40a according to a second embodiment of the present invention. The electrical joint 40a, as well as its method of formation, are similar to the electrical joint 40 as shown in Figures 3 and 4A to 4C. However, the crimp 43 in electrical joint 40a further comprises a second surface finish, or a coating 50, deposited on a part of its external surface, e.g. towards the first end 43a of the crimp 43.

Such arrangement is configured to provide selective heat energy insulation or rejection at predetermined locations at the crimp 43, i.e. away from first end 43a of the crimp 43. As shown in Figure 5B, the coating 50 is deposited at locations corresponding to coated portion 50 of the SMA actuator wire, thereby reducing the amount of heat energy being absorbed during weld formation. On the other hand, the surface of the crimp 43 towards the first end 43a is free of coating 50, thus the uncoated surface the crimp 43 is fully exposed to laser irradiation. As a result, during laser irradiation the uncoated portion of the crimp 43 may absorb more heat energy than the coated portion. This leads to a shaper rise in temperature at the first end 43a of the crimp, thereby allowing the electrical conductor 42 and/or crimp 43 to melt locally to form the weld 46.

The coating 50 may reduce energy absorption by reflecting or insulating the coated crimp surface from laser irradiation. For example, the coating 50 may comprises a heat insulating coating for shielding the crimp 43 from laser irradiation. Therefore, the weld may form selectively at positions along the crimp 43 that are free of heat insulating coating, e.g. towards the first end 43a of the crimp. In some embodiments, the coating 50 may be an ablative coating which is configured to vaporise or sublime upon absorbing an amount of energy through laser irradiation, and thereby reducing the energy absorbed to the surface of the crimp 43. In other embodiments, the coating 50 may be a reflective coating, e.g. a layer of reflective paint or a reflective adhesive tape. In other embodiments, the second surface finish may comprise polishing the crimp at positions adjacent to the weld and thereby to promote laser reflection thereat.

In Figures 5A to 5C, the coating 50 is used in combination with the heat sink.

However, the coating 50 may be applied independently for producing the electrical joint 40a of Figure 5C.

Figures 6A and 6B show a process for providing an electrical joint 40b according to another embodiment. As shown in Figure 6A, the first end 43a of the crimp (in this case a stainless or carbon steel) is annealed by a defocussed laser source 200 in order to deposit a layer of diffused carbon 60 at the surface of the crimp. More specifically, the laser source 200 emits a defocussed laser to heat the first end 43a of the crimp, thereby releasing the carbon form the body of the crimp. The released carbon 60 may then diffuse and deposit on the surface of the crimp 43, thereby improving local heat absorption thereat. Advantageously, this enables a hot spot to form at or near the black pigment for forming the weld.

In some other embodiments, for example in the case where the crimp 43 is formed from phosphate bronze, the first surface finish 60 may contain a black pigment (e.g. a black paint applicable by a marker or a black tape) to increase localised absorption of laser irradiation. Alternatively, or in addition, the first surface finish 60 may be a rough surface for improving the local absorption of laser irradiation.

Figure 7 shows another electrical joint 40c comprising the first surface finish 60 for promoting local energy absorption at the weld, as well as the second surface finish 50 adjacent to the weld to limit local laser absorption thereat. Therefore advantageously, the temperature profile along the length of the crimp 43 may be selectively controlled to produce the weld, whilst the rest of the crimp 43 may be shielded from laser irradiation, thereby preserving the SMA wire.

Claims

Claims 1. A method of forming an electrical joint, comprising: contacting a smart memory alloy (SMA) actuator wire with an electrical connector, the SMA actuator wire being coated with an insulation coating; heating, with a heat source, to melt one or both of the electrical connector and the SMA actuator wire and thereby forming a weld for effecting electrical connection therebetween, wherein the heating removes the insulation coating locally at the weld to form a conductive portion, and thereby leaving a coated portion of the SMA actuator wire coated with an insulation coating; and limiting, with means, heat dissipation to coated portion during weld formation.
2. A method according to claim 1, further comprises crimping the SMA actuator wire to the electrical connector with a crimp of the electrical connector prior to heating.
3. A method according to claim 1 or 2, further comprises providing a heat sink to the electrical connection to form the means.
4. A method according to claims 1 to 3, further comprises selectively providing a pigment to the electrical connector for increasing local energy absorption at the weld during weld formation.
5. A method according to claim 4, wherein selectively providing a pigment comprises annealing a part of the electrical connector for releasing the carbon thereat.
6. A method according to claim 4 or 5, wherein the pigment comprises a black pigment.
7. A method according to any one of the preceding claims, further comprises selectively providing a first surface finish at the electrical connector for increasing local energy absorption at the weld during weld formation, wherein the first surface finish having a higher surface roughness than the surface adjacent to the weld.
8. A method according to any one of the preceding claims, further comprises applying a second surface finish onto a surface of electrical connector and/or SMA actuator wire adjacent to the weld for forming the means, the second surface finish configured to reduce energy absorption by the respective electrical connector and/or SMA actuator wire through the surface during weld formation.
113 9. A method according to claim 8, wherein the applying a second surface finish comprises one or more of applying a heat insulating coating, applying a reflective coating, applying an ablative coating and polishing to effect a polished surface.
10. An electrical joint according to any one of the preceding claims, wherein the insulation coating having a melting point equal to or lower than a temperature required for forming the weld, thereby during weld formation the insulation coating at the weld melts to form electrical connection thereat.
11. A method according to any one of the preceding claims, wherein the heating is effected by laser irradiation.
12. An electrical joint comprising: a shape memory alloy (SMA) actuator wire having a coated portion coated with an insulation coating and a conductive portion free of the insulation coating; and an electrical connector electrically connected to the conductive portion of the SMA actuator wire by a weld formed from partially melting at least one of the SMA actuator wire and the electrical connector, wherein the electrical connector comprises means for enabling localised heating at the weld during its formation, thereby reducing the amount of heat dissipation to the coated portion.
13. An electrical joint according to claim 12, wherein the electrical connector comprises a crimp having first end and a second end opposite the first end, and wherein the weld is formed at the first end of the crimp.
14. An electrical joint according to claim 12, wherein the electrical connector comprises a crimp having first end and a second end opposite the first end, and wherein the weld is formed between the first end and the second end of the crimp.
15. An electrical joint according to claim 13 or 14, wherein the SMA actuator wire is crimped between the first end and the second end of the crimp, and wherein the coated portion emerges from the second end of the crimp.
16. An electrical joint according to any one of claims 12 to 15, wherein the 113 means comprising a heat sink and/or configured to be in thermal connection with an auxiliary heat sink, the heat sink and/or the auxiliary heat sink are configured to absorb heat energy during formation of the weld.
17. An electrical joint according to claim 16, wherein the heat sink and/or the auxiliary heat sink are configured to absorb heat energy during the actuation of the SMA actuator wire.
18. An electrical joint according to any one of claims 12 to 17, wherein the means comprises a pigment selectively provided at the electrical connector for increasing local energy absorption at the weld during weld formation.
19. An electrical joint according to claim 18, wherein the pigment comprises carbon diffused from the electrical connector during an annealing process prior to weld formation.
20. An electrical joint according to claim 18 or 19, wherein pigment comprises a black pigment.
21. An electrical joint according to any one of claims 12 to 20, wherein the means comprises a first surface finish selectively provided at the electrical connector for increasing local energy absorption at the weld during weld formation, wherein the first surface finish having a higher surface roughness than the surface adjacent to the weld.
22. An electrical joint according to any one of claims 12 to 21, wherein the means further comprises a second surface finish provided on a surface of electrical connector and/or SMA actuator wire adjacent to the weld, wherein the second surface finish is configured to reduce energy absorption by the respective electrical connector and/or SMA actuator wire though the surface during weld formation.
23. An electrical joint according to claim 22, wherein the second surface finish comprises one or more of a heat insulating coating, a reflective coating, an ablative coating and a polished surface.
24. As electrical joint according to claim 22 or claim 23, wherein the second surface finish having a lower surface roughness less than the surface of electrical connector that forms the weld.
25. An electrical joint according to any one of claims 12 to 24, wherein the SMA actuator wire comprises a plurality of SMA actuator wires.
26. An actuation mechanism comprising: a static component; a moveable component that is moveable relative to the static component; the electrical joint of any one of claims 12 to 25, wherein the electrical connector is mechanically connected to one of the static component and the movable component, and wherein the SMA actuator wire is mechanically connected to the other of static component and the movable component.
27. An actuation mechanism according to claim 26, wherein the SMA actuator wire is connected to the other of static component and the movable component by a second electrical joint.
28. An actuation mechanism according to claim 26 or 27, wherein the means is configured to dissipate heat energy to the static component or the moveable component it is connected to.