WO2021123780A1

WO2021123780A1 - Sma haptic assembly and assembly of an sma haptic assembly

Info

Publication number: WO2021123780A1
Application number: PCT/GB2020/053252
Authority: WO
Inventors: Andrew Benjamin Simpson Brown; Andreas FLOURIS; Nicolas HEIJNE; James Howarth; Marc-Sebastian SCHOLZ; Stephen Matthew BUNTING; Joshua CARR; Geoffrey Farmer
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2019-12-16
Filing date: 2020-12-16
Publication date: 2021-06-24
Anticipated expiration: 2022-06-16
Also published as: CN114867938B; CN114867938A

Abstract

An SMA actuator assembly comprises two parts relatively movable along a movement axis, and a length of SMA wire, each end of the SMA wire being connected to one of the parts. One part comprises a contact portion and the other part comprises plural contact portions, the contact portions making contact with the SMA wire on opposite sides along the movement axis. The contact portions alternate in a direction normal to the movement axis and are positioned to guide the SMA wire along a tortuous path such that the parts are driven in opposite directions on contraction of the SMA wire. Considering intermediate sections of the SMA wire between contacts with adjacent contact portions, on contraction of the SMA wire, the fractional change in the wire length of each intermediate section is equal to the overall strain of the SMA wire and there is no slippage of the SMA wire.

Description

SMA HAPTIC ASSEMBLY AND ASSEMBLY OF AN SMA HAPTIC ASSEMBLY

The present invention relates to actuators which use shape memory alloy (SMA) wires to provide relative movement between two components. The present invention also relates to methods for assembling actuators that use SMA wires to provide relative movement between two components. In particular, it relates to such actuators used in haptic assemblies.

SMA actuators are known for use in handheld electronic devices, such as cameras and mobile phones. In particular, they can be used to provide haptic functionality for tactile feedback, for example in response to a user selecting a particular area of a screen or pressing a button. Such actuators typically function by using the contraction of an SMA wire to cause relative motion of two components. The SMA wire is in contact with parts (e.g. teeth) of two opposing bodies which are forced apart due to the change in length of the SMA wire as it contracts.

However, in order to provide such functionality, the SMA wire is required to be in contact with the two opposing bodies. .Also, the SMA wire is attached at its ends to the bodies that are to be forced apart. In many prior art devices, the SMA wire makes contact with the body to which it is attached as it extends from its attachment point to the first tooth. When the SMA wire contracts, it may slip relative to the part of the bodies that it is in contact with, due to its changing length. Such slippage may abrade the SMA wire, and may cause fatigue as the SMA wire varyingly flexes around this part of the body. This will increase the risk of wire failure over repeated actuations of the assembly. This leads to reduced device lifetime and/or loss of functionality.

It may be possible to reduce the wear caused by such slippage by, for example, using low friction coatings on the parts of the bodies in contact with the SMA wire. However, this may increase manufacturing complexity, and therefore the cost of the device and the manufacturing process. Moreover, it may be possible to reduce the fatigue caused by the flexing by, for example, ensuring that the surfaces in contact with the SMA wire have a large radius of curvature. However, this may increase the size of the device, which is undesirable in many cases.

Therefore, it is desirable to provide an SMA actuator assembly with a configuration which reduces slippage of the SMA wire, and thereby reduces the chance of wire failure.

Furthermore, in order to make an SMA actuator assembly accurately, it is necessary to control the SMA wire length and the dimensions of features on the opposing bodies very carefully. This means very high manufacturing tolerances are required to produce satisfactory performance in the finished device. Equipment and processes to allow for sufficiently high tolerances are very expensive, and can lead to high wastage in rejected components. Such equipment and processes are also more time-consuming and difficult to control.

Therefore it is also desirable to provide a method of assembling a haptic assembly using shape memory alloy wires that allows for the accurate production of assemblies without requiring high manufacturing tolerances.

According to an aspect of the present invention, there is provided an SMA actuator assembly comprising first and second parts that are movable relative to each other along a movement axis, and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises plural contact portions making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the plural contact portions of the second part alternating in a direction normal to the movement axis and being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, and wherein, considering intermediate SMA sections of the length of SMA wire that extend between centre points of the contacts with adjacent contact portions of the first and second parts, the SMA actuator assembly is configured so that, on contraction of the length of SMA wire, the fractional change in the wire length of each intermediate SMA section is at least substantially equal to the overall strain of the length of SMA wire and there is at least substantially no slippage of the length of SMA wire over the centre points of the contacts.

As the strain in the length of SMA wire is the fractional change in wire length of the material of SMA wire on contraction, by configuring the SMA actuator assembly so as to match the fractional change in the wire length in each intermediate section of the length of SMA wire to the strain of the length of SMA wire overall, the length of SMA wire is prevented from slipping relative to the points at which it is in contact with the contact portions of the first and second parts. This reduces wear and/or flexing of the length of SMA wire, and reduces the likelihood of wire failure.

In some embodiments, the length of SMA wire is connected at each end to either one of the first and second parts by a respective connection element that holds the SMA wire. Connection elements hold the ends of the length of SMA wire in a fixed position relative to the relevant parts, such that the behaviour of the length of SMA wire and the SMA actuator assembly are well-defined.

In some embodiments, further considering end SMA sections of the length of SMA wire that extend from an exit point, where the length of SMA wire exits the connection element, to the centre point of the contact with the adjacent contact portion, the SMA actuator assembly is configured so that, on contraction of the length of SMA wire, in respect of each SMA section of the intermediate SMA sections and the end SMA sections, the fractional change in the wire length of each SMA section is at least substantially equal to the overall strain of the length of SMA wire, and there is no slippage of the length of SMA wire over the centre point of the contact with the contact portion adjacent to the connection element.

It is advantageous that the fractional change in the wire length is matched properly for all sections of the length of SMA wire, including those at the edges of the assembly where the wire is connected to the parts. This is particularly true if the end sections have a different geometry to sections in the centre of the assembly. If the end sections are not also designed in the correct manner, then slippage and increased wear on the length of SMA wire can still occur.

In some embodiments, the connection element is a crimp portion.

Crimp portions provide a convenient connection method that is straightforward to manufacture and assemble, and can also provide an electrical connection to the length of SMA wire.

In some embodiments, the connection element further comprises adhesive between the crimp portion and the adjacent contact portion.

Including adhesive in addition to the crimp portion means that the exit point from the connection element is closer to the adjacent contact portion, and can thereby reduce the likelihood of contact between the length of SMA wire and either of the parts between the exit point and the adjacent contact portion, which may increase wear on the SMA wire.

In some embodiments, the wire length of the SMA section and the angle that the SMA section makes to a plane normal to the movement axis vary between SMA sections.

These variations may be advantageous in some applications.

In some embodiments, for each SMA section, the ratio ( d/e ) of the relative movement along the movement axis developed between the first and second parts at each end of the SMA section ( d ) to the fractional change in the wire length of the SMA section e at least substantially equals the ratio ((L/sin(0)) of the wire length of the SMA section (L) to the sine of the angle that the SMA section makes to a plane normal to the movement axis (sin(0)).

In some embodiments, the separation along a direction normal to the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts varies between the intermediate SMA sections.

Allowing the separation between contacts of the wire with adjacent contact portions to vary can allow for a greater range of functionality in the haptic assembly, and greater flexibility of design.

In some embodiments, the height of each intermediate SMA section along the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts is the same.

Matching the heights of the SMA sections, in particular the heights in the resting position when the SMA wire is not contracted, means that all of the contact portions of the first and second parts can be made the same shapes, thereby simplifying manufacture.

In some embodiments, each of the first and second parts is rigid so that relative movement along the movement axis developed between the first and second parts at each end of the intermediate SMA section is the same for each of the intermediate SMA sections.

Where the first and second parts are rigid, they can be used to provide uniform haptic response across their entire surface.

In some embodiments, at least one of the first and second parts is flexible, flexing thereof at least partly providing relative movement along the movement axis between the first and second parts at each end of the intermediate SMA section that differs between the intermediate SMA sections.

Flexible parts permit greater flexibility of design and a greater variety of possible haptic responses, for example being able to provide a variation in haptic signal amplitude across the surface of the SMA actuator assembly.

In some embodiments, one of the first and second parts is a beam which extends between two fixed ends that have no movement with respect to the other one of the first and second parts and which is flexible so that the relative movement along the movement axis of the first and second parts at each end of the intermediate SMA section increases with increasing distance from the fixed ends towards a centre portion of the beam.

Using a flexible beams with fixed ends has the advantage that the ends of the beam at the edges of the SMA actuator assembly can remain flush with surrounding material even when the assembly is actuated to provide relative movement of the two parts in the centre of the SMA actuator assembly. This may provide a more aesthetically pleasing appearance, or may reduce the likelihood of ingress of fluid or dirt into the assembly.

In some embodiments, the separation along a direction normal to the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts increases with increasing distance from the fixed ends towards a centre portion of the beam.

Varying the separation of the contacts with the contact portions allows different amplitudes of relative movement to be provided in different regions of the SMA actuator assembly by varying the angle of the length of SMA wire in different SMA sections, even if, for example, the heights of the SMA sections along the movement axis are the same.

In some embodiments, one of the first and second parts is a flexible part of a casing of an electronic device.

In some embodiments, the length of SMA wire is connected at each end to the first part.

Connecting both ends of the length of SMA wire to the same one of the parts has the advantage that no net force is applied between the parts perpendicular to the movement direction. This can reduce stress on a suspension mechanism that is used to suspend the two parts relative to one another.

In some embodiments, each of the parts has plural contact portions. This increases the total force applied by the length of SMA wire while minimising the height along the movement axis.

In some embodiments, the parts are configured to be urged together only by a force applied by a user. This removes the need to provide an urging member to move the parts together.

In some embodiments, the first part comprises a first body, the second part comprises a second body, the at least one contact portion of the first body comprises at least one tooth, the plural contact portions of the second body comprise plural teeth, the teeth of the first and second bodies alternate in a direction normal to the movement axis and overlap in a direction parallel to the movement axis, and the contacts of the length of SMA wire with the teeth of the first and second bodies alternate between the teeth of the first body and the teeth of the second body.

According to another aspect of the present invention, there is provided an SMA actuator assembly comprising first and second parts that are movable relative to each other along a movement axis, and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, wherein the length of SMA wire is connected at each end to the first or second part by a respective connection element that holds the SMA wire; and wherein at each end the length of SMA wire extends from an exit point where the length of SMA wire exits the connection element to an adjacent contact portion without contact with the part to which the end of the length of SMA wire is connected.

By ensuring that the length of SMA wire does not make contact with the part to which it is attached prior to contacting the contact portions of the device, the wear and/or flexing of the wire is reduced. Therefore, the lifetime of the SMA assembly can be increased.

In some embodiments, each connection element is shaped such that the length of SMA wire extends inside the connection element up to the exit point of the length of SMA wire from the connection element along a direction that is at an exit angle of at most 15° to a line from the exit point to a point where the length of SMA wire first contacts the adjacent contact portion.

Having a high angle of bend of the length of SMA wire at the exit point can increase wear and/or flexing of the wire at this point, and increase the likelihood of wire failure. Therefore, ensuring the angle is sufficiently small can further improve the lifetime of the assembly.

In some embodiments, said exit angle is at most 10° to said line. In further embodiments, said exit angle is at most 5° to said line.

A smaller exit angle can further reduce the wear and/or flexing at the exit point, providing improved longevity of the assembly.

In some embodiments, the connection element is a crimp portion that is fixed to the first part and includes a crimp tab that is closed around the length of SMA wire so as to hold the length of SMA wire.

Using crimp tabs can be a convenient way to attach the length of SMA wire to the parts that provides for simple and rapid assembly of the SMA actuator.

In some embodiments, the crimp portion is made from sheet material that is curved to provide a part extending normal to the movement axis and a part including the exit point extending at an acute angle of greater than 0° to a plane normal to the movement axis.

Using a curved crimp portion allows the exit angle of the wire to be kept small, while allowing the wire to be held at a different angle in another portion of the crimp. This provides for greater flexibility in the design of the crimps, and of the manner in which the length of SMA wire is attached to the first and second parts.

In some embodiments, the part of the crimp portion extending normal to the movement axis is fixed to the first part.

Fixing the part of the crimp normal to the movement axis to the part allows for a robust and consistent join to the part, regardless of the particular angle chosen for the curved portion.

In some embodiments, a part of the crimp portion including said exit point of the length of SMA wire overhangs a part of the first part to which the crimp portion is fixed.

Having the exit point on an overhanging part of the crimp portion allows the assembly to easily provide both features of a sufficiently small exit angle of the length of SMA wire, and no contact between the length of SMA wire and the parts to which it is fixed prior to making contact with the adjacent contact portion. The overhang can ensure no contact is possible, and allow the overhanging portion to be angled without interfering with the contact to the first part. It can also be more space efficient in the direction of the movement axis.

In some embodiments, said line is at an acute angle of greater than 0° to a plane normal to the movement axis.

This feature means that the part of the length of SMA wire between the connection element and the adjacent contact portion can also contribute to providing a force to separate the two parts when the length of SMA wire contracts.

Connecting both ends of the length of SMA wire to the same one of the parts has the advantage that no net force is applied between the parts perpendicular to the movement direction. This can reduce strain on a suspension mechanism that is used to suspend the two parts relative to one another.

In some embodiments, the first part has at least one contact portion, preferably plural contact portions, the second part has plural contact portions, the contact portions of the two parts alternate in a direction normal to the movement axis, and the contacts of the length of SMA wire with the contact portions alternate between the contact portions of the first and second parts. This increases the total force applied by the length of SMA wire while minimising the height along the movement axis.

In some embodiment, the first part comprises a first body, the second part comprises a second body, the at least one contact portion of the first body comprises at least one tooth, the at least one contact portion of the second body comprises at least one tooth, the teeth of the first and second bodies overlap in a direction parallel to the movement axis, and the contacts of the length of SMA wire with the teeth of the first and second bodies alternate between the teeth of the first body and the teeth of the second body.

According to another aspect of the present invention, there is provided a method of manufacturing an SMA actuator assembly which comprises first and second parts that are movable relative to each other along a movement axis, and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, wherein the method comprises controlling the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends of the length of SMA wire while assembling the SMA actuator assembly.

Thereby a method is provided wherein the characteristics of the completed SMA actuator assembly can be determined very precisely without requiring very high manufacturing tolerances on the component parts. This is achieved by compensating for variations that have occurred in manufacture of the components of the SMA actuator assembly during the assembly of the SMA actuator assembly.

In some embodiments, the method comprises connecting the length of SMA wire at each end to either one of the first and second parts and assembling the first and second parts with the length of SMA wire extending therebetween, the first and second parts being provided with an adjustable end-stop arrangement that limits relative movement of the first and second parts towards each other, and while applying a predetermined force between the first and second parts, adjusting the adjustable end-stop arrangement to control the tension in the length of SMA wire.

Adjustment of the end stops provides a convenient way to adjust the length of the tortuous path between the parts to provide the correct tension in the length of SMA wire.

This can be convenient in situations where adjusting the length of the wire is difficult or undesirable.

In some embodiments, the adjustable end-stop arrangement comprises at least one mechanically adjustable end-stop.

A mechanically adjustable end-stop provides a straightforward mechanism for adjusting to the correct dimension.

In some embodiments, the adjustable end-stop comprises at least one end-stop mounted in a bed of adhesive, the step of adjusting the adjustable end-stop arrangement comprises adjusting the at least one end-stop while the adhesive is uncured, and the method further comprises curing the adhesive after adjusting the adjustable end-stop arrangement.

Using adhesive is a convenient method to adjust the end-stop position, because the deformation of the adhesive during assembly retains the correct position of the end-stop. Curing of the adhesive then fixes this position in the final product.

In some embodiments, the method comprises assembling the first and second parts with the length of SMA wire extending therebetween, and connecting the ends of the length of SMA wire to either one of the first and second parts, the tension of the SMA wire between the ends of the length of SMA wire being controlled while holding the parts in a fixed position relative to each other, before completing connection of the ends of the length of SMA wire to either one of the first and second parts.

This embodiment allows the tension in the wire to be controlled to match the length of tortuous path defined by the fixed relative position of the parts. This is useful if a particular relative position of the parts is required in the finished product.

In some embodiments, during the step of connecting the ends of the length of SMA wire to either one of the first and second parts, the wire length of the length of SMA wire between the ends of the length of SMA wire is fixed and the tension is controlled by selecting at least one of the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts.

Choosing the positions at which to connect the ends of the length of SMA wire is a convenient way to adjust the tension in the wire during assembly, as the positions can be chosen while applying the required tension. In some embodiments, the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises connecting one of the ends of the length of SMA wire to either one of the first and second parts, and while holding the parts in a fixed position relative to each other, controlling the tension by adjusting the position at which the other one of the ends of the length of SMA wire is connected to either one of the first and second parts.

Fixing one end of the wire, and adjusting the position at which the other is fixed means that the position can be set in a controlled manner to provide the correct tension, while only requiring the position of a single component to be adjusted.

In some embodiments, the length of SMA wire is connected to either one of the first and second parts by crimp portions that crimp the ends of the length of SMA wire and the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts are selected by selecting positions at which the crimp portions are fixed to either one of the first and second parts.

Crimp portions are particularly convenient choices of fixing, as they are easy to assemble and can be made compact in their assembled state. Crimp portions can also be used to provide electrical connection to the length of SMA wire.

In some embodiments, the step of assembling the first and second parts with the length of SMA wire extending therebetween is performed with the length of SMA wire held by a fret comprising a sacrificial body and two crimp portions crimping the ends of the length of SMA wire, the sacrificial body having a resilient portion arranged to apply a predetermined tension to the length of SMA wire, the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises fixing the crimp portions to either one of the first and second parts at positions selected while the tension of the length of SMA wire between the ends of the length of SMA wire is controlled by the resilient portion of the fret, and after connecting the crimp portions to either one of the first and second parts, removing the sacrificial body leaving the crimp portions fixed to either one of the first and second parts.

Using a fret structure allows a standard tension to be applied to the wire in a controlled manner during assembly, thereby simplifying the process of assembling the SMA actuator assembly. Removing the sacrificial portion allows the crimp portions to be made compact in the completed assembly while still allowing for the standard predetermined tension to be applied consistently during assembly.

In some embodiments, during the step of connecting the ends of the length of SMA wire to either one of the first and second parts, the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts are fixed and the tension is controlled by adjusting the wire length of the length of SMA wire between those positions.

Adjusting the wire length provides a way to match the wire tension to the tortuous path length in a case where the length of SMA wire is required to be connected at particular positions, and the relative positions of the parts is also predetermined.

In some embodiments, the step of assembling the first and second parts with the length of SMA wire extending therebetween is performed with the length of SMA wire located in crimp portions that partially crimp the ends of the length of SMA wire, and the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises connecting the crimp portions to either one of the first and second parts at the fixed positions, controlling the tension in the length of SMA wire by adjusting the wire length of the length of SMA wire between the crimp portions, and compressing the crimp portions to complete the crimping of the length of SMA wire by the crimp portions.

Partially crimping the crimp portions allows the length of SMA wire to be held securely and placed correctly in the assembly during assembly, while still allowing the wire length to be adjusted by applying tension to the length of SMA wire.

In some embodiments, the first and second parts are provided with an end-stop arrangement that limits relative movement of the first and second parts towards each other, and the first and second parts are held in a fixed position relative to each other by holding the first and second parts together against the end-stop arrangement.

Using end-stops allows the relative position of the first and second parts to be fixed during assembly in cases where a particular dimension of the assembly or position of the parts is required in the final assembly.

In some embodiments, the length of SMA wire is connected at each end to the same one of the first and second parts.

Connecting both ends of the length of SMA wire to the same part reduces force applied between the parts in a direction normal to the movement direction, thereby reducing unwanted movement and reducing strain on any suspension mechanism holding the parts.

In some embodiments, the method comprises assembling a first one of the first and second parts and the length of SMA wire together with a further part that shapes the length of SMA wire to follow a path of predetermined length, connecting the ends of the length of SMA wire to the first part, the tension of the length of SMA wire between the ends of the length of SMA wire being adjusted while holding the first part and the further part in a fixed position relative to each other, before completing connection of the ends of the length of SMA wire to the first part, removing the further part, and assembling the second part with the first part and with the length of SMA wire extending therebetween.

Using a further part to predetermine the wire length of the length of SMA wire may be convenient in some situations where setting the wire length or tension of the length of SMA wire in situ in the assembly is difficult.

In some embodiments, during the step of connecting the ends of the length of SMA wire to the first part, the connection points at which the ends of the length of SMA wire are connected to the first part are in fixed positions and the tension is adjusted by adjusting the wire length of the length of SMA wire between the connection points.

Adjusting the wire length may be a preferred method in a case where the connection points are required to be in a particular position on the first part.

In some embodiments, during the step of connecting the ends of the length of SMA wire to the first part, the wire length of the length of SMA wire between the ends of the length of SMA wire is fixed and the tension is adjusted by adjusting the position of at least one of the connection points at which the ends of the length of SMA wire are connected to the first part.

Adjusting the positions of the connection points may be a convenient way to set the tension in the length of SMA wire when the relative position of the parts is predetermined, and to allow the correct tension to be consistently applied.

In some embodiments, the length of SMA wire is connected to either one of the first and second parts by crimp portions that crimp the ends of the length of SMA wire and are fixed to either one of the first and second parts.

Crimp portions provide a convenient way to secure the length of SMA wire to the first and second parts using purely mechanical means and does not require adhesives or curing processes. If the crimps are conductive, they can also be used to make electrical connections to the length of SMA wire.

In some embodiments, at least one of the parts, preferably each of the parts, has plural contact portions, the contact portions of the first and second parts alternate in a direction normal to the movement axis, and the contacts of the length of SMA wire with the contact portions alternate between the contact portions of the first and second parts.

In some embodiments, the controlling of the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends of the length of SMA wire is carried out with the parts in a particular relative position. The particular position may correspond to a resting position of the SMA actuator assembly.

In some embodiments, the first part comprises a first body, the second part comprises a second body, the at least one contact portion of the first body comprises at least one tooth, the at least one contact portion of the second body comprises at least one tooth, and the teeth of the first and second bodies overlap in a direction parallel to the movement axis.

According to a further aspect of the invention, there is provided an SMA actuator assembly comprising first and second parts that are movable relative to each other along a movement axis, and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, , the SMA actuator assembly having an arrangement that provides control of the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends.

According to a further aspect of the invention, there is provided a group of (e.g. >100) SMA actuator assemblies manufactured using the method, each of which has substantially the same tension in the length of SMA wire and/or substantially the same path length of the length of SMA wire between the connected ends of the length of SMA wire (e.g. to within say 5%, 1%, 0.5% or 0.1%).

Embodiments of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which:

Fig. 1 shows an SMA actuator assembly;

Fig. 2 shows a section of the assembly of Fig. 1 before and during actuation;

Fig. 3 shows an assembly where the spacing perpendicular to the movement direction of the contacts with the length of SMA wire varies across the assembly;

Fig. 4 shows an assembly with a flexible body during actuation;

Fig. 5 shows an assembly with a connection element comprising a crimp portion and an adhesive; Fig. 6 shows another SMA actuator assembly;

Fig. 7 shows a section of an SMA actuator assembly;

Fig. 8 shows a section of an SMA actuator assembly according to an embodiment of the present invention;

Fig. 9 shows an assembly with an overhanging crimp portion;

Fig. 10 shows a crimp portion prior to being formed with a curved portion;

Fig. 11 shows a crimp portion formed with a curved portion;

Fig. 12 shows an SMA actuator assembly;

Fig. 13 shows assembly of an SMA actuator assembly by adjusting an end-stop;

Fig. 14 shows assembly of an SMA actuator assembly by adjusting the position of a connection point of the length of SMA wire;

Fig. 15 shows a fret that may be used in the assembly shown in Fig. 14;

Fig. 16 shows assembly of an SMA actuator assembly by adjusting the wire length of the length of SMA wire;

Fig. 17 shows assembly of an SMA actuator assembly using a further part;

Fig. 18 is a flowchart of a manufacturing method comprising adjusting an end stop;

Fig. 19 is a flowchart of a manufacturing method comprising adjusting the connection point of the length of SMA wire to the part;

Fig. 20 is a flowchart of a manufacturing method comprising adjusting the wire length of the length of SMA wire; and

Fig. 21 is a flowchart of a manufacturing method comprising use of a further part.

Fig. 1 shows an SMA actuator assembly 2 of the type in which the present disclosure may be implemented. The SMA actuator assembly 2 comprises a first body 4 and a second body 6 that are movable relative to each other along a movement axis M. The first and second bodies 4, 6 are examples of the first and second parts. The first and second bodies 4,

6 shown in Fig. 1 are solid bodies that may be formed by injection moulding or milling. However, it is not essential that the parts be formed in this way, and in some embodiments, the parts may take other forms, for example being hollow or formed from sheet material. The first and second parts provide two portions that can move relative to each other in order to cause a haptic signal to be provided to a user. Throughout the description, embodiments will be described generally with reference to the first and second bodies 4, 6 of the embodiments shown in the figures. However, any of the embodiments described herein may also be implemented using other types of first and second part other than the first and second bodies. Although not shown in Fig. 1, the SMA actuator assembly 2 may comprise a suspension system which holds the two bodies 4, 6 relative to one another and allows them to move along the movement axis M. The suspension system may permit movement of the two bodies 4, 6 relative to one another along the movement axis M, while restricting or preventing relative movement of the two bodies 4, 6 in the plane perpendicular to the movement axis M. The suspension system (or some other arrangement) may also restrict or prevent relative rotation of the two bodies 4, 6, for example due to an off-centre force applied to the SMA actuator assembly 2 by a user.

In some embodiments, the SMA actuator assembly 2 is integrated into a larger device. In some embodiments, the first body 4 may be a static body, which does not move relative to the device during actuation of the SMA actuator assembly 2, and the second body 6 may be a moving body which does move relative to the device during actuation of the SMA actuator assembly 2. Alternatively, both bodies 4, 6 may move during actuation.

The first body 4 has at least one tooth 8 and the second body 6 has plural teeth 8. The teeth 8 of the first and second bodies 4, 6 are examples of the contact portions of the first and second parts. In the embodiments shown in the figures, the teeth 8 of the first and second bodies 4, 6 are substantially solid and moulded integrally with the respective body. However, in general, this is not essential, and the contact portions of the parts may take other forms, for example being hollow, being formed separately from the bodies, or being formed from other materials such as metal.

In the embodiment shown in Fig. 1, the first body 4 has two teeth, and the second body 6 has three teeth. The teeth 8 of the first and second bodies 4 and 6 alternate in a direction normal to the movement axis M and overlap in a direction parallel to the movement axis M. This means that for any tooth 8 on either of the two bodies, the nearest adjacent teeth 8 will be teeth 8 of the other of the two bodies. The overlapping of the teeth means that the uppermost portion of a tooth 8 on the first body is above the lowermost portion of the adjacent teeth 8 of the second body 4 (where ‘up’ for this purpose is defined as being in the direction of movement of the second body 6 relative to the first body 4 on contraction of the SMA wire 10).

The shape of the teeth 8 can be any suitable shape to provide contact with the length of SMA wire 10 as described below. In the embodiment shown in the figure, the uppermost portion of the teeth 8 has a curved shape. However, other shapes could be used. For example, the uppermost portion of the teeth 8 may have a pointed shape, or the uppermost portion of the teeth may be flat. Furthermore, the lowermost portion of the teeth 8 may be triangular, as shown in the figures, or may be rectangular or any other shape. In embodiments such as that shown in Fig. 1, each of the bodies has plural teeth 8.

The SMA actuator assembly 2 further comprises a length of SMA wire 10 connected at each end to either one of the first and second bodies 4 and 6. In some embodiments, the ends of the length of SMA wire 10 are connected to different ones of the two bodies. Preferably, the length of SMA wire 10 is connected at each end to the same one of the bodies, i.e. both ends of the length of SMA wire 10 are connected to the first body 4 or both ends are connected to the second body 6. Connecting both ends to the same body reduces the force between the first and second bodies 4 and 6 in a direction perpendicular to the movement axis M during actuation of the SMA actuator assembly. In some embodiments, the length of SMA wire 10 is connected at each end to the first body 4. This may be preferable in embodiments where the first body 4 is a static body. In the embodiment shown in Fig. 1, both ends of the length of SMA wire 10 are connected to the first body 4.

In some embodiments, the length of SMA wire 10 is connected at each end to either one of the first and second bodies 4 and 6 by a respective connection element 18 that holds the length of SMA wire 10. Any suitable means or wire attachment device may be used as the connection element 18 to hold the length of SMA wire 10. In some embodiments, one or both of the connection elements 18 is a crimp portion. The crimp portion may be fixed to the first body 4 or second body 6. The crimp portion crimps the end of the length of SMA wire 10. This may be achieved by compressing the end of the wire 10 between two pieces of deformable material. Using a metal crimp portion may be desirable, in particular where the crimp is used to make electrical connection to the length of SMA wire 10 as well as fixing the length of SMA wire 10 to the first body 4 or second body 6.

In some embodiments, the connection element 18 comprises a crimp portion 7 and adhesive 19 between the crimp portion 7 and the adjacent tooth 8. Such a connection element

18 is illustrated in Fig. 5 and may be applied in any SMA actuator assembly 2 disclosed herein. Compared to use of the crimp portion 7 alone, as the SMA wire inside the adhesive

19 is fixed, the adhesive 19 can be used to effectively reduce the length of the end SMA section between the exit point of the connection element 18 which is now formed by the adhesive 19 and the adjacent tooth 8. This can be used in part to fulfil the condition (discussed further below) that the fractional change in the wire length of each SMA section matches the strain of the length of SMA wire 10.

The teeth 8 of the first body 4 make contact with the length of SMA wire 10 from below on a first side of the length of SMA wire 10 along the movement axis, and the teeth 8 of the second body 6 make contact with the length of SMA wire 10 from above on a second side of the length of SMA wire 10 along the movement axis, opposite to the first side. The length of SMA wire 10 extends between the first and second bodies 4 and 6 and is guided along a tortuous path between the first and second bodies 4 and 6 by the teeth 8, making contact with the teeth 8. The tortuous path is any path which is not a straight line between the points at which the ends of the length of SMA wire 10 are connected to the first or second bodies. The tortuous path followed by the length of SMA wire 10 will therefore have a wire length (that is the length following the path of the length of SMA wire 10) which is greater than the shortest distance between the connection elements 18. The tortuosity of the tortuous path may be measured using a ratio of the length of the tortuous path to the shortest distance between the connection elements 18.

The contacts of the length of SMA wire 10 alternate between the teeth 8 of the first and second bodies 4 and 6. In some embodiments, the length of SMA wire crosses from the first body 4 to the second body 6 (and back again) two or more times. In some embodiments, such as that shown in Fig. 1, the length of SMA wire 10 makes contact with all of the teeth 8 of the first body 4 and all of the teeth 8 of the second body 6 in the aforementioned alternating manner.

The teeth 8 of the first body 4 and the teeth 8 of the second body 6 are relatively positioned, and the alternating contacts of the length of SMA wire 10 with the teeth 8 of the first and second bodies 4 and 6 are such that the first and second bodies 4 and 6 are driven apart along the movement axis M on contraction of the length of SMA wire 10. The length of SMA wire 10 is arranged so that when the length of SMA wire 10 contracts, the first and second bodies move away from each other. This is caused by the overlapping of the teeth 8 of the first and second bodies 4 and 6, such that a force is exerted on the teeth 8 by the length of SMA wire as it contracts. In other embodiments, the first and second bodies 4, 6 may move together, as long as the first and second bodies 4, 6 move in opposite directions.

In some embodiments, the two bodies are provided with end-stops 12 that limit relative movement of the two bodies towards each other. The end-stops 12 may be provided both on the same one of the two bodies, for instance the first body 4 as shown in Fig. 1. Alternatively, the end-stops 12 may be provided on different ones of the two bodies e.g. at different ends of the SMA actuator assembly 2, or end-stops 12 may be provided on both bodies e.g. at both ends of the SMA actuator assembly 2. The end-stops 12 define a minimum separation of the first and second bodies 4 and 6. In some embodiments, the minimum separation will be that in a resting position when the SMA actuator assembly 2 is not actuated, i.e. when the length of SMA wire 10 is not contracted. In the resting position, the two bodies are in contact with the end-stops 12.

In some embodiments, the SMA actuator assembly 2 includes an arrangement (e.g. a resilient element such as a spring) to provides a force (“a return force”) urging the two bodies 4, 6 together along the movement axis M such that, when the power to the length of SMA wire 10 is reduced or stopped, the length of SMA wire 10 expands as it cools and the two bodies 4, 6 move back e.g. towards the resting position. In other embodiments, the SMA actuator assembly 2 does not include such an arrangement, in which case the return force can be provided by a user (e.g. by a finger press) on an area of the electronic device to which the SMA actuator assembly 2 is coupled.

As illustrated in Fig. 2, the length of SMA wire 10 can be considered to consist of a series of sections 44 where the boundaries between sections 44 are the points of contact 40,

42 with the adjacent teeth 8. In embodiments where the length of SMA wire 10 is in contact with each tooth 8 for some distance (for example if the uppermost point of the tooth 8 has a curved or flat surface) the boundary between sections 44 is the middle of the contact region where the length of SMA wire 10 is in contact with the adjacent teeth 8. For ease of understanding, reference will generally be made to embodiments as illustrated in Fig. 2 where the length of SMA wire 10 is in contact with each tooth 8 at a point or at least for a very short distance. However, the description also applies mutatis mutandis to embodiments where the length of SMA wire 10 is in contact with each tooth 8 for some distance, in which case parameters such as Q and L (see below) may be defined with reference to complete sections 44 or to only those portions of the sections 44 that are in space, i.e. not in contact with any teeth 8.

In some embodiments, the height h of each intermediate SMA section 44 along the movement axis M between the centre points of the contacts with adjacent teeth 8 of the first and second bodies 4 and 6 is the same. The height h along the movement axis M is the projection of the intermediate SMA section 44 onto the movement axis M. In particular, the height h of each intermediate SMA section 44 is the same in the resting position, before actuation of the SMA actuator assembly 2. In the case where the first and second bodies 4 and 6 are rigid, the height of each intermediate SMA section 44 is also the same in the actuated position.

Considering the intermediate SMA sections 44 of the length of SMA wire 10 that extend between centre points 40, 42 of the contacts with adjacent teeth 8 of the first and second bodies 4 and 6, the SMA actuator assembly 2 is configured so that, on contraction of the length of SMA wire 10, the fractional change in the wire length of each intermediate SMA section 44 is equal to the strain of the length of SMA wire 10. As the strain in the length of SMA wire 10 is the fractional change in wire length of the material of SMA wire 10, configuring the first and second bodies 4 and 6 so that the each intermediate SMA section 44 has the same fractional change in wire length means that when the length of SMA wire 10 contracts, causing the first and second bodies 4 and 6 to move relative to each other, the length of SMA wire 10 does not slide over the contact points. This can be achieved by configuring the first and second bodies 4 and 6, for example the length and angle of the length of SMA wire 10 between the points of contact 40, 42, so that for a given relative movement of the first and second bodies 4 and 6, the fractional change in the wire length in all the intermediate sections 44 of the length of SMA wire 10 is the same, and matches the overall strain of the length of SMA wire 10.

Avoiding the slipping of the length of SMA wire 10 prevents damage to the length of SMA wire 10 where the length of SMA wire 10 contacts the teeth 8 of the first and second bodies 4 and 6. This damage may occur due to rubbing and abrasion and/or flexing and fatigue if the length of SMA wire 10 slips over the teeth 8. Avoiding this type of damage can substantially increase the expected lifetime of the actuator assembly 2.

In some embodiments, further considering end SMA sections of the length of SMA wire 10 that extend from an exit point, where the length of SMA wire 10 exits the connection element 18, to the centre point of the contact with the adjacent tooth 8, the SMA actuator assembly 2 is configured so that, on contraction of the length of SMA wire 10, in respect of each SMA section of the intermediate SMA sections 44 and the end SMA sections, the fractional change in the wire length of each SMA section is equal to the strain of the length of SMA wire 10.

In this embodiment, slipping of the length of SMA wire 10 over parts of the first body 4 and/or second body 6 is also avoided at the ends of the length of SMA wire 10 where the wires are connected to the bodies, and where they make contact with the first and/or last teeth during actuation. To avoid this slippage, the connection element 18 (and/or any other components at the ends of the SMA actuator assembly 2 which hold the length of SMA wire, for example where the connection element 18 comprises both a crimp portion and adhesive) is positioned in such a way that the length of SMA wire 10 contracts in an equivalent manner on either side of the first/last tooth. This is achieved by ensuring that the end SMA sections obey the same constraint as the intermediate SMA sections 44.

The condition to avoid slipping of the length of SMA wire 10 relative to the teeth 8 is illustrated by the parameters in Fig. 2. The solid lines show the configuration of the SMA actuator assembly 2 in the resting position when the length of SMA wire 10 is not contracted. The dashed lines show the configuration in the actuated state, when the length of SMA wire 10 is contracted. In the embodiment of Fig. 2, the first body 4 is a static body, and the relative motion of the two bodies 4, 6 is entirely due to movement of the second body 6. The condition to avoid slipping can be achieved when the distance d of the relative movement along the movement axis M of the two bodies 4, 6 during actuation has the same relationship to the fractional change in the wire length e for each SMA section of the length of SMA wire 10. This applies to both the intermediate SMA sections 44, and to the end SMA sections in embodiments where the end sections obey the same constraint as the intermediate SMA sections 44. Considering the case where the intermediate SMA section 44 (that spans between the centre points 40, 42 of contact of the length of SMA wire 10 with two adjacent teeth of the first body 4 and the second body 6, as described above) makes an angle theta Q to the plane that is normal to the movement axis M then the length of wire in a section L should be given by:

L = -_E sin(0) (Eq. 1)

As illustrated in Fig. 2, the angle theta Q and the length L are defined e.g. when the SMA actuator assembly 2 is in the resting position, where the length of SMA wire 10 is not contracted and the first body 4 and second body 6 are separated by the minimum separation. Eq. 1 reflects the gearing effect created by providing the SMA sections at an angle to the movement axis M, i.e. that the relative movement d of the two bodies is greater than the change in length of the SMA section (Le) by a factor (e.g. l/sin(0)) that increases with decreasing angle theta Q. Also from Eq. 1, the ratio (d/e) of the relative movement between the two bodies to the fractional change in the wire length for each SMA section is equal to the ratio (L/sin(0) and can be controlled by suitably controlling the angle theta Q together with the wire length L of the SMA section. Where the two bodies 4, 6 are rigid, these ratios should be the same for each SMA section. Broadly speaking, in such an assembly 2, an SMA section that is at a greater angle theta Q to a plane normal to the movement axis M than another SMA section must have a greater wire length L.

In the embodiment shown in Fig. 3, the separation along a direction normal to the movement axis M between the centre points 40, 42 of the contacts with adjacent teeth 8 of the first and second bodies 4 and 6 varies between the intermediate SMA sections 44. The separation may vary due to the spacing of the teeth 8 along a direction perpendicular to the movement axis M on the first body 4 not being the same as the spacing of the teeth 8 on the second body 6. Alternatively or additionally, the separation may vary because the spacing of the teeth 8 on one or both of the bodies is not the same along the entire length of the body. In the embodiment of Fig. 3, the spacing of the teeth 8 on the second body 6 varies along the second body 6, and is also different to the spacing of the teeth 8 on the first body 4. Therefore, the separation a differs from the separation b. Slipping of the length of SMA wire 10 relative to the teeth 8 is particularly likely in such embodiments if the parameters are not chosen carefully. Therefore, ensuring that the angles theta Q and lengths L of the sections of the length of SMA wire 10 obey the above condition is particularly advantageous in such embodiments.

In general terms, the angle theta Q and length L of the SMA sections is varied along the length of the SMA actuator assembly 2. If higher force is required on a particular tooth 8, then the angle theta Q is increased (i.e. the angle between the wire and the movement axis M is reduced) e.g. for the same length of the corresponding intermediate SMA section 44. If greater motion or force is required, then it may be preferable to increase the length L of the SMA sections.

In some embodiments, such as the one shown in Fig. 3, each of the first and second bodies 4 and 6 is at least substantially rigid so that relative movement along the movement axis M developed between the first and second bodies 4 and 6 at each end of the intermediate SMA section 44 is the same for each of the intermediate SMA sections 44. In such embodiments, in order to obey the above constraint on the lengths and angles of the SMA sections, the SMA actuator assembly 2 may be configured so that the angle between the centre points 40, 42 of the contacts with adjacent teeth 8 of the first and second bodies 4 and 6 varies between the intermediate SMA sections 44. This ensures that the angle can be matched properly where the separation along a direction normal to the movement axis M between the centre points 40, 42 of the contacts with adjacent teeth varies for different intermediate SMA sections. A variation such as that shown in Fig. 3 - i.e. SMA sections having larger angles theta Q (and hence higher forces on the teeth 8) towards the ends of the length of SMA wire 10 - may be advantageous in some applications, e.g. for increased rotational stability and/or to act against uneven forces e.g. associated with having to move a beam-like body (which may be a portion of the case of the device).

One or both of the first and second bodies 4 and 6 may be flexible. In embodiments where the first body 4 is a static body fixed to a device in which the SMA actuator assembly 2 is used, the second body 6 may be flexible. In particular, this may be true where the second body 6 is constrained not to move at both ends but is able to flex to produce movement in the centre. In this case different displacements and or different forces may be required at different positions along the SMA actuator assembly 2 and these can be achieved with a suitable variation of the angle theta Q and the length L of the SMA sections along the length of the SMA actuator assembly 2.

Fig. 4 illustrates an embodiment in which at least one of the first and second bodies 4 and 6 is flexible, flexing thereof at least partly providing the relative movement of the first and second bodies 4 and 6 along the movement axis M, whereby the relative movement along the movement axis M developed between the first and second bodies 4 and 6 at each end of the intermediate SMA section 44 differs as between the intermediate SMA sections 44. In embodiments where one of the bodies is flexible, it is advantageous to vary the separation along a direction normal to the movement axis M between the centre points 40, 42 of the contacts with adjacent teeth 8. As discussed above, this will change the angle Q between the SMA sections, so that a different force and relative displacement of the two bodies is provided at different points along the SMA actuator assembly 2. In the example of Fig. 4, the first body 4 is rigid and the second body 6 is flexible, although in general either or both of the bodies may be flexible.

In the embodiment of Fig. 4, the SMA actuator assembly 2 is designed so that the heights of the teeth 8 of the first body along the movement axis M are constant along the length of the SMA actuator assembly 2 to maintain a constant Z height. However, the angle between the wire and the direction of motion is smaller at the edges of the device (and therefore the angle theta Q defined above is larger) to follow the reduced displacement in the direction of the movement axis M that is possible at the edges of the SMA actuator assembly 2

Example parameters of the embodiment shown in Fig. 4 are given in Table 1 below. Although the SMA actuator assembly of Fig. 4 is shown in the actuated state (where the centre of the second body 6 is displaced in the movement axis M) the parameters of the angle theta Q and length L are measured in the resting position (i.e. non-actuated state) as described above.

Table 1

In some embodiments, one of the first and second bodies 4 and 6 is a beam which extends between two fixed ends that have no movement with respect to the other one of the first and second bodies 4 and 6 and which is flexible so that the relative movement along the movement axis M of the first and second bodies 4 and 6 at each end of the intermediate SMA section 44 increases with increasing distance from the fixed ends towards a centre portion of the beam. In such embodiments, the separation along a direction normal to the movement axis M between the centre points 40, 42 of the contacts with adjacent teeth 8 of the first and second bodies may be increased with increasing distance from the fixed ends towards a centre portion of the beam. This will reduce the angle Q near the centre compared to near the fixed ends, so that a larger relative movement of the two bodies is provided near the centre of the SMA actuator assembly 2. This further provides the effect that the centre of the SMA actuator assembly 2 bulges upward, providing a haptic effect even when the ends of the beam are fixed and do not move relative to the other of the bodies.

In some embodiments, one of the first and second bodies 4 and 6 is a flexible part of a casing of an electronic device. Use of flexible bodies is particularly advantageous in such embodiments of the SMA actuator assembly 2, because the flexibility means that the edges of the assembly 2 can be seamless and flush with the casing of the electronic device.

According to another aspect of the present invention, there is also provided an SMA actuator assembly, wherein the length of SMA wire is connected at each end to the first or second part by a respective connection element that holds the SMA wire; and wherein at each end the length of SMA wire extends from an exit point where the length of SMA wire exits the connection element to an adjacent contact portion without contact with the part to which the end of the length of SMA wire is connected. This aspect of the invention is described in the following with reference to Figs. 6 to 11.

Fig. 6 shows another SMA actuator assembly 2 of the type in which the present disclosure may be implemented. The SMA actuator assembly 2 comprises a first body 4 and a second body 6 that are movable relative to each other along a movement axis M. The first and second bodies 4, 6 are examples of the first and second parts. The first and second bodies 4, 6 shown in Fig. 6 are solid bodies that may be formed by injection moulding or milling. However, it is not essential that the parts be formed in this way, and in some embodiments, the parts may take other forms, for example being hollow or formed from sheet material. The first and second parts provide two portions that can move relative to each other in order to cause a haptic signal to be provided to a user. Throughout the description, embodiments will be described generally with reference to the first and second bodies 4, 6 of the embodiments shown in the figures. However, any of the embodiments described herein may also be implemented using other types of first and second part other than the first and second bodies.

Although not shown in Fig. 6, the SMA actuator assembly 2 may comprise a suspension system which holds the two bodies 4, 6 relative to one another and allows them to move along the movement axis M. The suspension system may permit movement of the two bodies 4, 6 relative to one another along the movement axis M, while restricting or preventing relative movement of the two bodies 4, 6 in the plane perpendicular to the movement axis M and/or restricting or preventing relative rotation of the two bodies 4, 6.

In some embodiments, the SMA actuator assembly 2 is integrated into a larger device. In such embodiments, the first body 4 may be a static body, which does not move relative to the device during actuation of the SMA actuator assembly 2, and the second body 6 may be a moving body which does move relative to the device during actuation of the SMA actuator assembly 2. Alternatively, both bodies may move during actuation. The first body 4 and the second body 6 bodies each have at least one tooth 8. The teeth 8 of the first and second bodies 4, 6 are examples of the contact portions of the first and second parts. In the embodiments shown in the figures, the teeth 8 of the first and second bodies 4, 6 are substantially solid and moulded integrally with the respective body. However, in general, this is not essential, and the contact portions of the parts may take other forms, for example being hollow, being formed separately from the bodies, or being formed from sheet material.

In the embodiment shown in Fig. 6, the first body 4 has two teeth, and the second body 6 has three teeth. The teeth 8 of the first and second bodies 4, 6 overlap in a direction parallel to the movement axis M. The overlapping of the teeth means that the uppermost portion of a tooth 8 on the first body 4 is above the lowermost portion of the adjacent teeth 8 of the second body 6 (where ‘up’ for this purpose is defined as being in the direction of movement of the second body 6 relative to the first body 4 on contraction of the SMA wire 10). In some embodiments, the first body 4 has at least one tooth 8, the second body 6 has plural teeth 8, the teeth 8 of the two bodies alternate in a direction normal to the movement axis M, and the contacts of the length of SMA wire 10 with the teeth 8 alternate between the teeth 8 of the first and second bodies 4, 6.

The shape of the teeth 8 can be any suitable shape to provide contact with the length of SMA wire 10 as described below. In the embodiment shown in the figures, the uppermost portion of the teeth 8 has a curved shape. However, other shapes could be used. For example, the uppermost portion of the teeth 8 may have a pointed shape, or the uppermost portion of the teeth may be flat. Furthermore, the lowermost portion of the teeth 8 may be triangular, as shown in the figures, or may be rectangular or any other shape. In an embodiment such as that shown in Fig. 6, each of the bodies has plural teeth 8.

The SMA actuator assembly 2 further comprises a length of SMA wire 10 connected at each end to either one of the first and second bodies 4, 6. In some embodiments, the ends of the length of SMA wire 10 are connected to different ones of the two bodies. Preferably, the length of SMA wire 10 is connected at each end to the same one of the bodies, i.e. both ends of the length of SMA wire 10 are connected to the first body 4 or both ends are connected to the second body 6. This reduces the force between the first and second bodies 4, 6 in a direction perpendicular to the movement axis M during actuation of the SMA actuator assembly. In some embodiments, the length of SMA wire 10 is connected at each end to the first body 6. This may be preferable in embodiments where the first body 4 is a static body. In the embodiment shown in Fig. 6, both ends of the length of SMA wire 10 are connected to the first body 6.

The length of SMA wire 10 is connected at each end to either one of the first and second bodies 4, 6 by a respective connection element 18 that holds the SMA wire 10. Any suitable means or wire attachment device may be used as the connection element 18 to hold the length of SMA wire 10. For example, the connection element 18 may comprise an adhesive, where the length of SMA wire 10 is set into the adhesive before curing the adhesive. In some embodiments, one or both of the connection elements 18 is a crimp portion. The crimp portion crimps the end of the length of SMA wire 10. The crimp portion may be fixed to the first body 4 or second body 6. In some embodiments, the crimp portion includes a crimp tab that is closed around the length of SMA wire 10 so as to hold the length of SMA wire 10. The crimp portion crimps the ends of the length of SMA wire 10. This may be achieved by compressing the end of the wire 10 between two pieces of deformable material. Using a metal crimp portion may be desirable, in particular where the crimp is used to make electrical connection to the length of SMA wire 10 as well as fixing the length of SMA wire 10 to the first body 4 or second body 6.

The teeth 8 of the first body 4 make contact with the length of SMA wire 10 from below on a first side of the length of SMA wire 10 along the movement axis, and the teeth 8 of the second body 6 make contact with the length of SMA wire 10 from above on a second side of the length of SMA wire 10 along the movement axis, opposite to the first side. The length of SMA wire 10 extends between the first and second bodies 4, 6 and is guided along a tortuous path between the first and second bodies 4, 6 by the teeth 8, making contact with the teeth 8. The tortuous path is any path which is not a straight line between the points at which the ends of the length of SMA wire 10 are connected to the first or second bodies 4, 6. The tortuous path followed by the length of SMA wire 10 will therefore have a length which is greater than the shortest distance between the connection elements 18. The tortuosity of the tortuous path may be measured using a ratio of the length of the tortuous path to the shortest distance between the connection elements 18.

The contacts of the length of SMA wire 10 alternate between the teeth 8 of the first and second bodies 4, 6. In some embodiments, the length of SMA wire crosses from the first body 4 to the second body 6 (and back again) two or more times. In some embodiments, such as that shown in Fig. 6, the length of SMA wire 10 makes contact with all of the teeth 8 of the first body 4 and all of the teeth 8 of the second body 6 in the aforementioned alternating manner.

The teeth 8 of the first body 4 and the teeth 8 of the second body 6 are relatively positioned, and the alternating contacts of the length of SMA wire 10 with the teeth 8 of the first and second bodies 4, 6 are such that the first and second bodies 4, 6 are driven apart along the movement axis M on contraction of the length of SMA wire 10. The length of SMA wire 10 is arranged so that when the length of SMA wire 10 contracts, the first and second bodies 4, 6 move away from each other. This is caused by the overlapping of the teeth 8 of the first and second bodies 4, 6, such that a force is exerted on the teeth 8 by the length of SMA wire 10 as it contracts. In other embodiments, the first and second bodies 4, 6 may move together, as long as the first and second bodies 4, 6 move in opposite directions.

In some embodiments, the two bodies are provided with end-stops 12 that limit relative movement of the two bodies towards each other. The end-stops 12 may be provided both on the same one of the two bodies, for instance the first body 4 as shown in Fig. 6. Alternatively, the end-stops 12 may be provided on different ones of the two bodies e.g. at different ends of the SMA actuator assembly 2, or end-stops 12 may be provided on both bodies e.g. at both ends of the SMA actuator assembly 2. The end-stops 12 define a minimum separation of the first and second bodies 4, 6. In some embodiments, the minimum separation will be that in a resting position when the SMA actuator assembly 2 is not actuated, i.e. when the length of SMA wire 10 is not contracted. In the resting state, the two bodies are in contact with the end-stops 12.

In some embodiments, the assembly 2 includes an arrangement (e.g. a resilient element such as a spring) to provides a force (“a return force”) urging the two bodies 4, 6 together along the movement axis M such that, when the power to the length of SMA wire 10 is reduced or stopped, the length of SMA wire 10 expands as it cools and the two bodies 4, 6 move back e.g. towards the resting position.

Fig. 7 shows a section of an SMA actuator assembly that is not an embodiment of the invention as a comparative example. In the SMA actuator assembly of Fig. 7, the end of the length of SMA wire 10 is attached to the first body 4 via the connection element 18. The length of SMA wire 10 exits the connection element 18 at an exit point 46 and then is in contact with another portion 48 of the first body 4 before extending across a gap to the first tooth 8 of the second body 6. When the length of SMA wire 10 contracts during actuation, the wire will rub on, and flex around, the first body 4 at the point 48, and this can cause wire damage.

As shown in the embodiments of Figs. 8 and 9, in the present invention, at each end the length of SMA wire 10 extends from an exit point 46 where the length of SMA wire 10 exits the connection element 18 to an adjacent tooth 8 without contact with the body to which the end of the length of SMA wire 10 is connected. Ensuring that the length of SMA wire 10 extends in this way prevents damage to the length of SMA wire 10 where it makes contact with the body to which it is attached.

The length of SMA wire 10 exits from the connection element 18 to the point where the length of SMA wire 10 contacts the first of the teeth 8 of the bodies. The angle between the direction 50 of the length of SMA wire 10 as it exits the connection element 18, and the line 52 between the point where the SMA wire 10 exits the connection element 18 and the point where the length of SMA wire 10 contacts the first tooth is preferably minimised. Minimising this angle reduces the bend in the length of SMA wire 10 at the exit point 46, thereby reducing wear on the length of SMA wire 10. Therefore, in embodiments such as the one shown in Fig. 8, the connection element 18 is shaped such that the length of SMA wire 10 extends inside the connection element 18 up to the exit point 46 of the length of SMA wire 10 from the connection element 18 along a direction 50 that is at an exit angle Q of at most 15° to a line 52 from the exit point 46 to a point 40 where the length of SMA wire 10 first contacts the adjacent tooth 8. In some embodiments, said line 52 is at an acute angle of greater than 0°, optionally greater than 5°, optionally greater than 10°, to a plane normal to the movement axis M. This means that the length of SMA wire 10 is able to exert force on the teeth of the bodies when it contracts. In some embodiments, said exit angle Q is at most 10° to said line 52, optionally at most 5° to said line 52.

The exit angle Q may be limited in a variety of ways. For example, as shown in Fig.

8, the connection element 18 may be shaped such that it holds the length of SMA wire 10 at an angle which is not aligned with the outer dimensions of the connection element 18, such that the length of SMA wire 10 is not perpendicular to the movement axis M inside the connection element 18 even where the connection element 18 is mounted on the body perpendicularly to the movement axis 18. Alternatively or additionally, the surface of the body to which the connection element 18 is fixed may not be perpendicular to the movement axis M. This ensures that the length of SMA wire 10 is not perpendicular to the movement axis M inside the connection element 18 even where the connection element 18 is mounted on the body perpendicularly to the movement axis 18 and holds the length of SMA wire 10 at an angle which is aligned with the outer dimensions of the connection element 18.

Another configuration which achieves the limitation on the exit angle Q is shown in Fig. 9. In the embodiment of Fig. 9, the connection element 18 is a crimp portion, where the crimp portion is made from sheet material that is curved to provide a part 54 extending normal to the movement axis M and a part 56 extending at an acute angle of greater than 0° to a plane normal to the movement axis M. In some embodiments, the part 54 of the crimp portion extending normal to the movement axis M is fixed to the first body 4. In embodiments such as that shown in Fig. 9, a part of the crimp portion including said exit point 46 of the length of SMA wire 10 overhangs a part of the first body 4 to which the crimp portion is fixed. Using an overhang in this way ensures that the curved connection element 18 can easily be mounted to the body, and that no contact occurs with the body to which the connection element 18 (and therefore also the end of the length of SMA wire 10) is fixed.

The crimp portions may be formed as shown in Figs. 10 and 11. The piece of sheet metal which is to form the crimp portion lies in the plane normal to the movement axis M and is placed between a punch 60 and anvil 62, as shown in Fig. 10. The sheet metal is then compressed between the punch 60 and the anvil 62 as shown in Fig. 11. Although the process is shown in Figs. 10 and 11 with the length of SMA wire 10 in the connection element 18, this is not necessary, and the forming of the crimp portion may be performed without the length of SMA wire 10 present, and the length of SMA wire 10 introduced later. The result is that the crimp portion holds the length of SMA wire 10 so that the length of SMA wire 10 exits along a line that makes an acute angle with the plane in which the sheet metal lies, which is a plane perpendicular to the movement axis M.

According to another aspect of the present invention, there is provided a method of manufacturing an SMA actuator assembly, wherein the method comprises controlling the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends of the length of SMA wire while assembling the SMA actuator assembly. This aspect of the present invention is described in the following with reference to Figs. 12 to 21

The present disclosure concerns methods of manufacturing an SMA actuator assembly. Fig. 12 shows an SMA actuator assembly 2 of the type which may be assembled using embodiments of the manufacturing method disclosed herein. The SMA actuator assembly 2 comprises two bodies 4, 6 that are movable relative to each other along a movement axis M. The two bodies 4, 6 are examples of the first and second parts. The two bodies 4, 6 shown in Fig. 12 are solid bodies that may be formed by methods such as injection moulding or milling. However, it is not essential that the parts be formed in this way, and in some embodiments, the parts may take other forms, for example being hollow or formed from sheet material. The first and second parts provide two portions that can move relative to each other in order to cause a haptic signal to be provided to a user. Throughout the description, embodiments will be described generally with reference to the two bodies 4,

6 of the embodiments shown in the figures. However, any of the embodiments described herein may also be implemented using types of first and second part other than the two bodies.

Although not shown in Fig. 12, the SMA actuator assembly 2 may comprise a suspension system which holds the two bodies 4, 6 relative to one another and allows them to move along the movement axis M. The suspension system may permit movement of the two bodies 4, 6 relative to one another along the movement axis M, while restricting or preventing relative movement of the two bodies 4, 6 in the plane perpendicular to the movement axis M and/or restricting or preventing relative rotation of the two bodies 4, 6.

In some embodiments, the SMA actuator assembly 2 is integrated into a larger device. In some embodiments, one of the two bodies 4, 6 may be a static body, which does not move relative to the device during actuation of the SMA actuator assembly 2, and the other of the two bodies 4, 6 may be a moving body which does move relative to the device during actuation of the SMA actuator assembly 2. Alternatively, both bodies may move during actuation.

The two bodies 4, 6 each have at least one tooth 8. The teeth 8 of the two bodies 4, 6 are examples of the contact portions of the first and second parts. In the embodiments shown in the figures, the teeth 8 of the two bodies 4, 6 are substantially solid and moulded integrally with the respective body. However, in general, this is not essential, and the contact portions of the parts may take other forms, for example being hollow, being formed separately from the bodies, or being formed from sheet material.

In the embodiment shown in Fig. 12, the two bodies 4, 6 of the SMA actuator assembly 2 comprise a lower body 4 having three teeth 8, and an upper body 6 having four teeth 8. The teeth 8 of the two bodies 4, 6 overlap in a direction parallel to the movement axis M. The overlapping of the teeth means that the uppermost portion of a tooth 8 on the lower body 4 is above the lowermost portion of the adjacent teeth 8 of the upper body 6 (where ‘up’ for this purpose is defined as being in the direction of movement of the body 6 relative to the body 4 on contraction of the SMA wire 10). In some embodiments where one of the two bodies 4, 6 is a static body and the other of the two bodies 4, 6 is a moving body, the lower body 4 is preferably the static body.

The shape of the teeth 8 can be any suitable shape to provide contact with the length of SMA wire 10 as described below. In the embodiment shown in the figures, the uppermost portion of the teeth 8 has a curved shape. However, other shapes could be used. For example, the uppermost portion of the teeth 8 may have a pointed shape, or the uppermost portion of the teeth 8 may be flat. Furthermore, the lowermost portion of the teeth 8 may be triangular, as shown in the figures, or may be rectangular or any other shape. In some embodiments, at least one of the bodies 4, 6 has plural teeth 8, and the teeth 8 of the two bodies 4, 6 alternate in a direction normal to the movement axis M. This means that for any tooth 8 on either of the two bodies 4, 6, the nearest adjacent teeth 8 will be teeth 8 of the other of the two bodies 4, 6. In an embodiment such as that shown in Fig. 12, each of the bodies has plural teeth 8.

The SMA actuator assembly 2 further comprises a length of SMA wire 10 connected at each end to either one of the first and second bodies 4, 6. In some embodiments, the ends of the length of SMA wire 10 are connected to different ones of the two bodies 4, 6. Preferably, the length of SMA wire 10 is connected at each end to the same one of the two bodies 4, 6. Connecting both ends to the same body reduces the force between the two bodies 4, 6 in a direction perpendicular to the movement axis M during actuation of the SMA actuator assembly. In some embodiments where one of the bodies 4, 6 is a static body the length of SMA wire 10 may be connected at each end to the static body. In the embodiment shown in Fig. 12, both ends of the length of SMA wire 10 are connected to the lower body 4. In some embodiments, the length of SMA wire 10 is connected at each end to either one of the bodies 4, 6 by a respective connection element 18 that holds the SMA wire. Any suitable means or wire attachment device may be used as the connection element 18 to hold the length of SMA wire 10. For example, the connection element 18 may comprise an adhesive, where the length of SMA wire 10 is set into the adhesive before curing the adhesive. Preferably, the connection element 18 is a crimp portion. The crimp portion may be fixed to either of the bodies 4, 6. In some embodiments, the length of SMA wire 10 is connected to either one of the bodies 4, 6 by crimp portions, wherein the crimp portions are fixed to either one of the bodies 4, 6. The crimp portion crimps the end of the length of SMA wire 10. Crimping may be achieved by compressing the end of the wire 10 between two pieces of deformable material. Using a metal crimp portion may be desirable, in particular where the crimp portion is used to make electrical connection to the length of SMA wire 10 as well as fixing the length of SMA wire 10 to either one of the bodies 4, 6.

The teeth 8 of the first body 4 make contact with the length of SMA wire 10 from below on a first side of the length of SMA wire 10 along the movement axis, and the teeth 8 of the second body 6 make contact with the length of SMA wire 10 from above on a second side of the length of SMA wire 10 along the movement axis, opposite to the first side. The length of SMA wire 10 extends between the two bodies 4, 6 and is guided along a tortuous path between the two bodies 4, 6 by the teeth 8, making contact with the teeth 8. The tortuous path is any path which is not a straight line between the points at which the ends of the length of SMA wire 10 are connected to the two bodies 4, 6. The tortuous path followed by the length of SMA wire 10 will therefore have a length which is greater than the shortest distance between the connection element 18. The tortuosity of the tortuous path may be measured using a ratio of the length of the tortuous path to the shortest distance between the connection elements 18.

The teeth 8 of the first body 4 and the teeth 8 of the second body 6 are relatively positioned, and the length of SMA wire 10 makes contact with the teeth 8, so that the two bodies 4, 6 are driven apart along the movement axis on contraction of the length of SMA wire 10. The length of SMA wire 10 is arranged so that when the length of SMA wire 10 contracts, the two bodies 4, 6 move away from each other. This is caused by the overlapping of the teeth 8 of the two bodies 4, 6, such that a force is exerted on the teeth 8 by the length of SMA wire as it contracts. In other embodiments, the first and second bodies 4, 6 may move together, as long as the first and second bodies 4, 6 move in opposite directions.

In some embodiments where at least one of the bodies has plural teeth 8, and the teeth 8 of the two bodies 4, 6 alternate in a direction normal to the movement axis M, the contacts of the length of SMA wire 10 with the teeth 8 alternate between the teeth 8 of the two bodies 4, 6. In some embodiments, the length of SMA wire 10 crosses from one of the two bodies 4, 6 to the other of the two bodies 4, 6 two or more times. In some embodiments, such as that shown in Fig. 12, the length of SMA wire 10 makes contact with all of the teeth 8 of both of the two bodies 4, 6 in the aforementioned alternating manner. The alternating contacts of the length of SMA wire with the teeth 8 of the two bodies 4, 6 contribute to the driving apart of the two bodies 4, 6 along the movement axis M on contraction of the length of SMA wire 10.

In some embodiments, the two bodies 4, 6 are provided with an end-stop arrangement 12 that limit relative movement of the two bodies 4, 6 towards each other. The end-stop arrangement 12 may be provided both on the same one of the two bodies 4, 6, for instance the lower body 4 as shown in Fig. 12. Alternatively, the end-stop arrangement 12 may be provided on different ones of the two bodies 4, 6 e.g. at different ends of the SMA actuator assembly 2, or end-stop arrangements 12 may be provided on both bodies e.g. at both ends of the SMA actuator assembly 2. The end-stop arrangement 12 defines a minimum separation of the two bodies 4, 6. In some embodiments, the minimum separation will be that in a resting position when the SMA actuator assembly 2 is not actuated, i.e. when the length of SMA wire 10 is not contracted. In the resting state, the two bodies 4, 6 are in contact with the end-stop arrangement 12.

In some embodiments, the end-stop arrangement 12 is an adjustable end-stop arrangement. For example, the adjustable end-stop arrangement may comprise at least one mechanically adjustable end-stop, such as a grub screw. Alternatively or additionally, the adjustable end-stop arrangement may comprise at least one end-stop 14 mounted in a bed of adhesive 16, for example a component sitting on a bed of glue.

In some embodiments, the assembly includes an arrangement (e.g. a resilient element such as a spring) that urges the two bodies 4, 6 together along the movement axis M such that, when the power to the length of SMA wire 10 is reduced or stopped, the length of SMA wire 10 expands as it cools and the two bodies 4, 6 move back towards the resting position.

Methods of manufacturing SMA actuator assemblies such as those shown in Fig. 12 will now be described. In order to make an SMA actuator assembly 2 accurately, it is necessary to control the difference between the path length (where the path length is the length of the tortuous path between the two bodies 4, 6 along which the length of SMA wire 10 extends) and the length of the SMA wire 10 very accurately. In an SMA actuator assembly 2 where the length of SMA wire 10 is in contact with parts of the assembly such as the teeth 8 of the two bodies 4, 6 (i.e. parts other than the points at which the length of SMA wire 10 is connected to the bodies 4, 6) the path length is dependent on the tolerance of the parts of the assembly that the length of SMA wire 10 is in contact with.

Hence it is desirable to provide a manufacturing method that minimises the error in the difference between the path length and the wire length of the length of the SMA wire 10. Such error can arise due to variations in the path length from the tolerance of the components, and variations in the wire length of the length of SMA wire 10. One method of achieving this is to use high tolerance parts and a high tolerance assembly process to ensure that both the wire length of the length of SMA wire 10 and the path length are controlled accurately, thus ensuring accurate control of the difference between the path length and the wire length. However, such high accuracy assembly and parts are expensive and difficult to control. Therefore, embodiments of the manufacturing method disclosed herein provide an adjustment method to directly control the difference between the wire length of the length of SMA wire 10 and the path length, by adjusting one of either the path length or the wire length of the length of SMA wire 10 depending on the length of the other.

The manufacturing method comprises controlling the tension in the length of SMA wire 10 and/or the path length of the length of SMA wire 10 between the connected ends of the length of SMA wire 10 while assembling the SMA actuator assembly 2.

In some embodiments shown in Fig. 18, the method comprises a step S10 of connecting the length of SMA wire 10 at each end to either one of the bodies 4, 6. This may be achieved in any suitable manner, as discussed above. Preferably both ends of the length of SMA wire 10 are connected to the same one of the two bodies 4, 6. In some embodiments where one of the bodies is a static body, the length of SMA wire 10 may be connected at both ends to the static body. The method further comprises a step S12 of assembling the two bodies 4, 6 with the length of SMA wire 10 extending therebetween. The gap between the bodies for a given wire tension will be dependent on the tolerances of the bodies.

As additionally illustrated in Fig. 13, the method further comprises a step S14 of, while applying a predetermined force F between the two bodies 4, 6, adjusting the adjustable end-stop arrangement 12 to control the tension in the length of SMA wire 10. The adjustable end-stop arrangement 12 is set to exactly cross the gap L between the bodies when the correct force (and therefore wire tension) is being applied. Adjusting the adjustable end-stop arrangement 12 adjusts the path length, that is the length of the tortuous path between the two bodies 4, 6 along which the length of SMA wire 10 extends. Adjusting the path length while applying the correct force between the bodies ensures that the path length is correctly matched to the tension in the length of SMA wire 10 created by the applied force regardless of the tolerances on the bodies and the wire length of the length of SMA wire 10.

The force F may be applied to one of the two bodies 4, 6 while holding the other of the two bodies 4, 6 in a fixed position, as shown in Fig. 13. This may be preferable where the other of the two bodies 4, 6 is a static body and the one of the two bodies 4, 6 is a moving body of the SMA actuator assembly 2. Where the adjustable end-stop arrangement 12 of the SMA actuator assembly 2 comprises at least one end-stop 14 mounted in a bed of adhesive 16, the step S14 of adjusting the adjustable end-stop arrangement 12 comprises adjusting the at least one end-stop 14 while the adhesive 16 is uncured, and the method further comprises a step S16 of curing the adhesive 16 after adjusting the adjustable end-stop arrangement 12.

In the embodiments shown in Figs. 19 and 20, the method comprises a step S20 of assembling the two bodies 4, 6 with the length of SMA wire 10 extending therebetween, and a step S22 of connecting the ends of the length of SMA wire 10 to either one of the bodies. Connecting the ends of the length of SMA wire 10 to either of the bodies may be performed by connecting a connection element 18 (which can be any type of wire attachment component, such as a crimp portion or an adhesive) to both ends of the length of SMA wire 10, and connecting the connection elements 18 to either of the bodies. Alternatively, the connection elements 18 may first be connected to either of the bodies, and the connection elements 18 subsequently connected to the ends of the length of SMA wire 10. As discussed above, the connection elements 18 may be connected to the same one of the two bodies 4, 6, or different ones of the two bodies 4, 6. In some embodiments where one of the two bodies 4, 6 is a static body, the connection elements 18 are preferably both connected to the static body.

During the step S22 of connecting the ends of the length of SMA wire 10 to either one of the bodies, the tension of the SMA wire 10 between the ends of the length of SMA wire 10 is controlled while holding the bodies 4, 6 in a fixed position relative to each other before completing connection of the ends of the length of SMA wire 10 to either one of the bodies 4, 6. In some embodiments, such as those illustrated in Figs. 14 and 16, the two bodies 4, 6 are provided with an end-stop arrangement 12 that limits relative movement of the two bodies 4,

6 towards each other, and the two bodies 4, 6 are held in a fixed position relative to each other by holding the two bodies 4, 6 together against the end-stop arrangement 12. For example, in some embodiments where one of the two bodies 4, 64 is a static body, the end- stop arrangement 12 is provided on the static body, and the moving body provided by the other of the two bodies 4, 6 is placed in contact with the end-stop arrangement 12 on the static body.

In these embodiments, the fixed position of the bodies 4, 6 relative to each other fixes the path length of the tortuous path between the two bodies 4, 6 along which the length of SMA wire 10 extends. Controlling the wire tension while holding the path length fixed ensures the wire tension is correctly matched to the path length regardless of the tolerances on the bodies and the length of the SMA wire 10.

The tension in the length of SMA wire may be controlled in different ways during the step of connecting the ends of the length of SMA wire 10 to the bodies. One method of controlling the tension is to adjust the position of where the length of SMA wire 10 is attached to the bodies according to the length of the SMA wire 10. This method is shown in Fig. 19 and also illustrated in Fig. 14.

In the embodiment of Fig. 19, during the step S22 of connecting the ends of the length of SMA wire 10 to either one of the bodies, the wire length of the length of SMA wire 10 between the ends of the length of SMA wire 10 is fixed and the tension is controlled by selecting the positions at which the ends of the length of SMA wire 10 are connected to either one of the bodies. The fixed position of the bodies relative to each other fixes the path length of the tortuous section of the path length between the teeth of the two bodies 4, 6, and selecting the connection position adjusts the path length beyond the tortuous part of the path, i.e. the length of the part of the path between the connection point of the length of SMA wire 10 to the bodies and the first of the teeth of the bodies. This ensures the wire tension and the total path length are matched regardless of the tolerances on the bodies and the length of the SMA wire 10.

In some embodiments, the step S22 of connecting the ends of the length of SMA wire 10 to either one of the bodies 4, 6 comprises connecting one of the ends of the length of SMA wire 10 to either one of the bodies 4, 6, and while holding the bodies 4, 6 in a fixed position relative to each other, controlling the tension by adjusting in step S24 the position at which the other one of the ends of the length of SMA wire 10 is connected to either one of the bodies 4, 6. As described above, the bodies 4, 6 may be held in a fixed position relative to each other using an end-stop arrangement 12. The other one of the ends of the length of SMA wire 10 is then fixed to either of the bodies in step S26.

In the embodiment shown in Fig. 14, the length of SMA wire 10 is connected to the bodies using connection elements 18. Where the length of SMA wire 10 is connected to the bodies using connection elements 18, the ends of the length of SMA wire 10 are the points at which the length of SMA wire 10 exits the connection elements 18 along the tortuous path.

In the embodiment of Fig. 14, the connection elements 18 are crimp portions that crimp the ends of the length of SMA wire 10. The positions at which the ends of the length of SMA wire 10 are connected to either one of the bodies are selected by selecting positions at which the crimp portions are fixed to either one of the bodies.

The positions at which the ends of the length of SMA wire 10 are connected may be selected by applying a force perpendicular to the movement axis M to adjust the position at which the ends of the length of SMA wire 10 are connected to the body. The force may be applied to both ends of the length of SMA wire 10, or it may be applied to one end of the length of SMA wire 10 after having fixed the position of the other end of the length of SMA wire 10. In the embodiment of Fig. 14, the length of SMA wire 10 is connected by a connection element 18, the force is applied to the connection element 18, and the application of the force adjusts the position of the connection element 18, which in this embodiment is on the lower body 4. Applying the force creates a known tension in the length of SMA wire 10, and the position of the connection of the length of SMA wire 10 to the body under that applied force will be dependent on the tolerances on the dimensions of the two bodies 4, 6, and the wire length of the length of SMA wire 10. Once the position is selected under application of the force, the length of SMA wire 10 is fixed to the body, for example by fixing the connection element 18 to the body at that position.

One way to apply the force to the length of SMA wire 10 would be to crimp the length of SMA wire 10 onto a fret 20 as shown in Fig. 15. In such some embodiments, the step of assembling the two bodies 4, 6 with the length of SMA wire 10 extending therebetween is performed with the length of SMA wire 10 held by a fret 20 comprising a sacrificial body 22 and two crimp portions 18 crimping the ends of the length of SMA wire 10, the sacrificial body 22 having a resilient portion 24 arranged to apply a predetermined tension to the length of SMA wire 10. The resilient portion 24 may comprise a spring. When using a fret 20, the step of connecting the ends of the length of SMA wire 10 to either one of the bodies 4, 6 comprises fixing the crimp portions 18 to either one of the bodies 4, 6 at positions selected while the tension of the length of SMA wire 10 between the ends of the length of SMA wire 10 is controlled by the resilient portion 24 of the fret 20. Connecting the ends of the length of SMA wire 10 further comprises, after connecting the crimp portions 18 to either one of the bodies 4, 6, removing the sacrificial body 22 leaving the crimp portions 18 fixed to either one of the bodies 4, 6. This provides a reliable and reproducible way to apply the required force to the length of SMA wire 10.

An alternative method to control the tension in the length of SMA wire 10 during the step of connecting the ends of the length of SMA wire 10 to the bodies 4, 6 is to adjust the wire length of the length of SMA wire 10 according to the path length. An embodiment using this approach is shown in Fig. 20, and illustrated in Fig. 16.

In the embodiment of Fig. 20, during the step S32 of connecting the ends of the length of SMA wire 10 to either one of the bodies 4, 6, the positions at which the ends of the length of SMA wire 10 are connected to either one of the bodies 4, 6 are fixed and the tension is controlled by adjusting in step S34 the wire length of the length of SMA wire 10 between those positions. The ends of the length of SMA wire 10 can then be fixed in step S36. The wire length of the length of SMA wire 10 will be dependent on the tolerances. In this embodiment, the fixed position of the bodies 4, 6 relative to each other, and the fixed positions where the ends of the length of SMA wire 10 are connected to the bodies, fixes the path length of the tortuous path between the two bodies 4, 6 along which the length of SMA wire 10 extends. Adjusting the wire length of the length of SMA wire 10 controls the wire tension while holding the path length fixed to ensure the wire tension is correctly matched to the path length regardless of the tolerances on the two bodies 4, 6.

In the embodiment shown in Fig. 16, the step S30 of assembling the two bodies 4, 6 with the length of SMA wire 10 extending therebetween is performed with the length of SMA wire 10 located in crimp portions that partially crimp the ends of the length of SMA wire 10. Then, the step S22 of connecting the ends of the length of SMA wire 10 to either one of the bodies comprises connecting the crimp portions to either one of the bodies 4, 6 at the fixed positions. In some embodiments, this comprises attaching open crimp portions (preferably to the static body if one of the bodies 4, 6 is a static body) at either end of the SMA actuator assembly 2. In the embodiment of Fig. 16, no sacrificial or disposable element between the crimp portions (e.g. a crimp coupon) is used when connecting the crimp portions to either one of the bodies 4, 6, although in other embodiments a fret 20 similar to that shown in Fig. 15 may be used. The step S22 of connecting the ends of the length of SMA wire 10 further comprises controlling the tension in the length of SMA wire 10 by adjusting the wire length of the length of SMA wire 10 between the crimp portions, and compressing the crimp portions to complete the crimping of the length of SMA wire 10 by the crimp portions. This process can also be called on-board crimping.

The wire length of the length of SMA wire 10 between the crimp portions may be adjusted as shown in Fig. 16 by applying a tension to the length of SMA wire 10, and then compressing the crimp portions to attach the length of SMA wire 10 to the crimp portions. The wire length may be adjusted by feeding the length of SMA wire into the crimp portions, compressing the crimp portion holding one end of the length of SMA wire 10 to form a crimp at one end, applying a force to the other end of the length of SMA wire 10 (i.e. the end with the open crimp) to achieve the correct wire length in the SMA actuator assembly 2, and finally compressing the crimp portion holding the other of the ends of the length of SMA wire 10, i.e. closing the second crimp. Alternatively, the wire length may be adjusted by applying a force to both ends of the length of SMA wire 10 simultaneously, and then compressing both of the crimp portions at the same time.

During the controlling of the tension in the length of SMA wire 10, the two bodies 4,

6 are held in a fixed position relative to one another, for example using an end stop arrangement 12. Where one of the two bodies 4, 6 is a static body and the other of the two bodies 4, 6 is a moving body, the end stop arrangement 12 may be provided on the static body, and the moving body is brought into contact with the end stop arrangement 12 on the static body.

In any of the embodiments discussed above, the step S12, S20, S30 of assembling the two bodies 4, 6 may comprise assembling one of the two bodies 4, 6 together with a dummy body or further part. In embodiments where the one of the two bodies 4, 6 is a static body and the other of the two bodies 4, 6 is a moving body, the dummy body or further part is preferably assembled with the static body 4. The profile of the dummy body or further part is designed for the purpose of setting the correct wire length of the length of SMA wire 10, and is configured so that the path length of the tortuous path extending between the dummy body and the one of the two bodies 4, 6 matches the path length desired in the final assembled SMA actuator assembly 2. The profile of the dummy body may be different to that of either of the two bodies 4, 6 (and hence the shape of the tortuous path may be different). Figure 17 shows such a body 30. In some embodiments where a dummy body or further part is used, the method further comprises steps of removing the dummy body or further part and assembling the other of the two bodies 4, 6 with the one of the two bodies 4, 6 and with the length of SMA wire 10 extending therebetween, as shown in Fig. 21. As will be appreciated, where a dummy body or further part is used, tolerances in relation to the other one of the two bodies 4, 6 (e.g. the moving body 6) will not necessarily be compensated for.

In embodiments where crimp portions are used to connect the ends of the length of SMA wire 10 to e.g. the first body, the two crimp portions may be connected by a disposable connecting portion similar to that shown in Fig. 15 but without a resilient portion. During the step of connecting the ends of the length of SMA wire 10 to the crimp portions, the length of SMA wire 10 is laid between the two crimp portions and then deflected using e.g. a jig to increase the wire length of the length of SMA wire 10 between the two crimp portions to correspond to a desired wire length. The crimp portions are then closed, the crimp portions are connected to the first body, and the disposable connecting portion is removed.

Embodiments of the method described above may be implemented using an SMA actuator assembly 2 such as that shown in Fig. 12. The SMA actuator assembly 2, comprises two bodies 4, 6 that are movable relative to each other along a movement axis M, the two bodies 4, 6 each having at least one tooth 8 that overlap in a direction along the movement axis M, and a length of SMA wire 10 connected at each end to either one of the bodies and extending along a tortuous path between the two bodies 4, 6 in contact with the teeth 8 so that the two bodies 4, 6 are driven apart along the movement axis M on contraction of the length of SMA wire 10, the SMA actuator assembly 2 having an arrangement that provides control of the tension in the length of SMA wire 10 and/or the path length of the length of SMA wire 10 between the connected ends. The arrangement may correspond to any of the embodiments of the assembly method discussed above, for example an adjustable end stop arrangement 12.

Therefore, as described above, the tension in the length of SMA wire 10 and/or the path length of the length of SMA wire 10 between the connected ends of the length of SMA wire 10 can be controlled and, in particular, can be controlled for a particular relative position of the two bodies 4, 6. This position may correspond to the resting position of the SMA actuator assembly or to some other position (e.g. a midpoint of the operating range of relative movement of the two bodies 4, 6). Thus, for example, the tension throughout the operating range of movement is suitably controlled.

Claims

1. An SMA actuator assembly comprising: first and second parts that are movable relative to each other along a movement axis; and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises plural contact portions making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the plural contact portions of the second part alternating in a direction normal to the movement axis and being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, wherein, considering intermediate SMA sections of the length of SMA wire that extend between centre points of the contacts with adjacent contact portions of the first and second parts, the SMA actuator assembly is configured so that, on contraction of the length of SMA wire, the fractional change in the wire length of each intermediate SMA section is equal to the overall strain of the length of SMA wire and there is no slippage of the length of SMA wire over the centre points of the contacts.

2. An SMA actuator assembly according to claim 1, wherein the length of SMA wire is connected at each end to either one of the first and second parts by a respective connection element that holds the SMA wire.

3. An SMA actuator assembly according to claim 2, wherein, further considering end SMA sections of the length of SMA wire that extend from an exit point, where the length of SMA wire exits the connection element, to the centre point of the contact with the adjacent contact portion, the SMA actuator assembly is configured so that, on contraction of the length of SMA wire, in respect of each SMA section of the intermediate SMA sections and the end SMA sections, the fractional change in the wire length of each SMA section is equal to the overall strain of the length of SMA wire and there is no slippage of the length of SMA wire over the centre point of the contact with the contact portion adjacent to the connection element.

4. An SMA actuator assembly according to claim 2 or 3, wherein the connection element is a crimp portion.

5. An SMA actuator assembly according to claim 4, wherein the connection element further comprises adhesive between the crimp portion and the adjacent contact portion.

6. An SMA actuator assembly according to any one of the preceding claims, wherein the wire length of the SMA section and the angle that the SMA section makes to a plane normal to the movement axis vary between SMA sections.

7. An SMA actuator assembly according to any one of the preceding claims, wherein, for each SMA section, the ratio ( d/e ) of the relative movement along the movement axis developed between the first and second parts at each end of the SMA section ( d ) to the fractional change in the wire length of the SMA section (e) equals the ratio ((L/sin(0)) of the wire length of the SMA section (L) to the sine of the angle that the SMA section makes to a plane normal to the movement axis (sin(0)).

8. An SMA actuator assembly according to any one of the preceding claims, wherein the separation along a direction normal to the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts varies between the intermediate SMA sections.

9. An SMA actuator assembly according to any one of the preceding claims, wherein the height of each intermediate SMA section along the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts is the same in a resting position of the SMA actuator assembly.

10. An SMA actuator assembly according to any one of the preceding claims, wherein each of the first and second parts is rigid so that relative movement along the movement axis developed between the first and second parts at each end of the intermediate SMA section is the same for each of the intermediate SMA sections.

11. An SMA actuator assembly according to any one of claims 1 to 9, wherein at least one of the first and second parts is flexible, flexing thereof at least partly providing relative movement along the movement axis between the first and second parts at each end of the intermediate SMA section that differs between the intermediate SMA sections.

12. An SMA actuator assembly according to claim 10, wherein one of the first and second parts is a beam which extends between two fixed ends that have no movement with respect to the other one of the first and second parts and which is flexible so that the relative movement along the movement axis of the first and second parts at each end of the intermediate SMA section increases with increasing distance from the fixed ends towards a centre portion of the beam, wherein the separation along a direction normal to the movement axis between the centre points of the contacts with adjacent contact portions of the first and second parts increases with increasing distance from the fixed ends towards a centre portion of the beam.

13. An SMA actuator assembly according to any one of claims 10 to 12, wherein one of the first and second parts is a flexible part of a casing of an electronic device.

14. An SMA actuator assembly according to any one of the preceding claims, wherein the length of SMA wire is connected at each end to the first part.

15. An SMA actuator assembly according to any one of the preceding claims, wherein each of the parts has plural contact portions.

16. An SMA actuator assembly according to any one of the preceding claims, wherein the parts are configured to be urged together only by a force applied by a user.

17. An SMA actuator assembly according to any one of the preceding claims, wherein: the first part comprises a first body; the second part comprises a second body; the at least one contact portion of the first body comprises at least one tooth; the plural contact portions of the second body comprise plural teeth; the teeth of the first and second bodies alternate in a direction normal to the movement axis and overlap in a direction parallel to the movement axis; and the contacts of the length of SMA wire with the teeth of the first and second bodies alternate between the teeth of the first body and the teeth of the second body.

18. An SMA actuator assembly comprising: first and second parts that are movable relative to each other along a movement axis; and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, wherein the length of SMA wire is connected at each end to the first or second part by a respective connection element that holds the SMA wire; and wherein at each end the length of SMA wire extends from an exit point, where the length of SMA wire exits the connection element, to an adjacent contact portion without contact with the part to which the end of the length of SMA wire is connected.

19. An SMA actuator assembly according to claim 18, wherein each connection element is shaped such that the length of SMA wire extends inside the connection element up to the exit point of the length of SMA wire from the connection element along a direction that is at an exit angle of at most 15° to a line from the exit point to a point where the length of SMA wire first contacts the adjacent contact portion.

20. An SMA actuator assembly according to claim 19, wherein said exit angle is at most 10° to said line.

21. An SMA actuator assembly according to claim 20, wherein said exit angle is at most 5° to said line.

22. An SMA actuator assembly according to any one of claims 18 to 21, wherein the connection element is a crimp portion that is fixed to the first part and includes a crimp tab that is closed around the length of SMA wire so as to hold the length of SMA wire.

23. An SMA actuator assembly according to claim 22, wherein the crimp portion is made from sheet material that is curved to provide a part extending normal to the movement axis and a part including the exit point extending at an acute angle of greater than 0° to a plane normal to the movement axis.

24. An SMA actuator assembly according to claim 23, wherein the part of the crimp portion extending normal to the movement axis is fixed to the first part.

25. An SMA actuator assembly according to any one of claims 22 to 24, wherein a part of the crimp portion including said exit point of the length of SMA wire overhangs a part of the first part to which the crimp portion is fixed.

26. An SMA actuator assembly according to any one of claims 19 to 25, wherein said line is at an acute angle of greater than 0° to a plane normal to the movement axis.

27. An SMA actuator assembly according to any one of claims 18 to 26, wherein the length of SMA wire is connected at each end to the first part.

28. An SMA actuator assembly according to any one of claims 18 to 27, wherein the first part has at least one contact portion, the second part has plural contact portions, the contact portions of the two parts alternate in a direction normal to the movement axis, and the contacts of the length of SMA wire with the contact portions alternate between the contact portions of the first and second parts.

29. An SMA actuator assembly according to claim 28, wherein each of the first and second parts has plural contact portions.

30. An SMA actuator assembly according to any one of claims 18 to 29, wherein: the first part comprises a first body; the second part comprises a second body; the at least one contact portion of the first body comprises at least one tooth; the at least one contact portion of the second body comprises at least one tooth; the teeth of the first and second bodies overlap in a direction parallel to the movement axis; and the contacts of the length of SMA wire with the teeth of the first and second bodies alternate between the teeth of the first body and the teeth of the second body.

31. A method of manufacturing an SMA actuator assembly which comprises: first and second parts that are movable relative to each other along a movement axis; and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, wherein the method comprises controlling the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends of the length of SMA wire while assembling the SMA actuator assembly.

32. A method according to claim 31, wherein the method comprises: connecting the length of SMA wire at each end to either one of the first and second parts and assembling the first and second parts with the length of SMA wire extending therebetween, the first and second parts being provided with an adjustable end-stop arrangement that limits relative movement of the first and second parts towards each other; and while applying a predetermined force between the first and second parts, adjusting the adjustable end-stop arrangement to control the tension in the length of SMA wire.

33. A method according to claim 32, wherein the adjustable end-stop arrangement comprises at least one mechanically adjustable end-stop.

34. A method according to claim 32, wherein the adjustable end-stop arrangement comprises at least one end-stop mounted in a bed of adhesive, and the step of adjusting the adjustable end-stop arrangement comprises adjusting the at least one end-stop while the adhesive is uncured, and the method further comprises curing the adhesive after adjusting the adjustable end-stop arrangement.

35. A method according to claim 31, wherein the method comprises: assembling the first and second parts with the length of SMA wire extending therebetween; and connecting the ends of the length of SMA wire to either one of the first and second parts, the tension of the SMA wire between the ends of the length of SMA wire being controlled while holding the first and second parts in a fixed position relative to each other, before completing connection of the ends of the length of SMA wire to either one of the first and second parts.

36. A method according to claim 35, wherein, during the step of connecting the ends of the length of SMA wire to either one of the first and second parts, the wire length of the length of SMA wire between the ends of the length of SMA wire at which connection is fixed and the tension is controlled by selecting at least one of the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts.

37. A method according to claim 36, wherein the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises: connecting one of the ends of the length of SMA wire to either one of the first and second parts, and while holding the first and second parts in a fixed position relative to each other, controlling the tension by adjusting the position at which the other one of the ends of the length of SMA wire is connected to either one of the first and second parts.

38. A method according to claim 36 or 37, wherein the length of SMA wire is connected to either one of the first and second parts by crimp portions that crimp the ends of the length of SMA wire and the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts are selected by selecting positions at which the crimp portions are fixed to either one of the first and second parts.

39. A method according to claim 38, wherein the step of assembling the first and second parts with the length of SMA wire extending therebetween is performed with the length of SMA wire held by a fret comprising a sacrificial body and two crimp portions crimping the ends of the length of SMA wire, the sacrificial body having a resilient portion arranged to apply a predetermined tension to the length of SMA wire, the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises: fixing the crimp portions to either one of the first and second parts at positions selected while the tension of the length of SMA wire between the ends of the length of SMA wire is controlled by the resilient portion of the fret, and after connecting the crimp portions to either one of the first and second parts, removing the sacrificial body leaving the crimp portions fixed to either one of the first and second parts.

40. A method according to claim 35, wherein, during the step of connecting the ends of the length of SMA wire to either one of the first and second parts, the positions at which the ends of the length of SMA wire are connected to either one of the first and second parts are fixed and the tension is controlled by adjusting the wire length of the length of SMA wire between those positions.

41. A method according to claim 40, wherein the step of assembling the first and second parts with the length of SMA wire extending therebetween is performed with the length of SMA wire located in crimp portions that partially crimp the ends of the length of SMA wire, and the step of connecting the ends of the length of SMA wire to either one of the first and second parts comprises: connecting the crimp portions to either one of the first and second parts at the fixed positions; controlling the tension in the length of SMA wire by adjusting the wire length of the length of SMA wire between the crimp portions; and compressing the crimp portions to complete the crimping of the length of SMA wire by the crimp portions.

42. A method according to any one of claims 35 to 41, wherein the first and second parts are provided with an end-stop arrangement that limits relative movement of the first and second parts towards each other, and the first and second parts are held in a fixed position relative to each other by holding the first and second parts together against the end-stop arrangement.

43. A method according to any one of the claims 31 to 42, wherein the length of SMA wire connected is at each end to the same one of the first and second parts.

44. A method according to claim 31, wherein the method comprises: assembling a first one of the first and second parts and the length of SMA wire together with a further part that shapes the length of SMA wire to follow a path of predetermined length; connecting the ends of the length of SMA wire to the first part, the tension of the length of SMA wire between the ends of the length of SMA wire being adjusted while holding the first part and the further part in a fixed position relative to each other, before completing connection of the ends of the length of SMA wire to the first part; removing the further part; and assembling the second part with the first part and with the length of SMA wire extending therebetween.

45. A method according to claim 44, wherein, during the step of connecting the ends of the length of SMA wire to the first part, the connection points at which the ends of the length of SMA wire are connected to the first part are in fixed positions and the tension is adjusted by adjusting the wire length of the length of SMA wire between the connection points.

46. A method according to claim 44, wherein, during the step of connecting the ends of the length of SMA wire to the first part, the wire length of the length of SMA wire between the ends of the length of SMA wire at which connection is fixed and the tension is adjusted by adjusting the position of at least one of the connection points at which the ends of the length of SMA wire are connected to the first part.

47. A method according to any one of claims 31 to 46, wherein the length of SMA wire is connected to either one of the first and second parts by crimp portions that crimp the ends of the length of SMA wire and are fixed to either one of the first and second parts.

48. A method according to any one of claims 31 to 47, wherein at least one of the parts has plural contact portions, the contact portions of the two parts alternate in a direction normal to the movement axis, and the contacts of the length of SMA wire with the contact portions alternate between the contact portions of the two parts.

49. A method according to any one claims 31 to 48, wherein each of the parts has plural contact portions.

50. A method according to any one claims 31 to 29, wherein the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends of the length of SMA wire is controlled with the parts in a particular relative position.

51. A method according to claim 50, wherein the particular position corresponds to a resting position of the SMA actuator assembly.

52. A method according to any one of claims 31 to 51, wherein: the first part comprises a first body; the second part comprises a second body; the at least one contact portion of the first body comprises at least one tooth; the at least one contact portion of the second body comprises at least one tooth; and the teeth of the first and second bodies overlap in a direction parallel to the movement axis.

53. An SMA actuator assembly comprising: first and second parts that are movable relative to each other along a movement axis; and a length of SMA wire, each of the ends of the length of SMA wire being connected to the first or second part, wherein the first part comprises at least one contact portion making contact with the length of

SMA wire on a first side of the length of SMA wire along the movement axis, the second part comprises at least one contact portion making contact with the length of SMA wire on a second side of the length of SMA wire along the movement axis, opposite to the first side, the at least one contact portion of the first part and the at least one contact portion of the second part being relatively positioned so as to guide the length of SMA wire along a tortuous path such that the first and second parts are driven in opposite directions along the movement axis on contraction of the length of SMA wire, the SMA actuator assembly having an arrangement that provides control of the tension in the length of SMA wire and/or the path length of the length of SMA wire between the connected ends.

54. A group of SMA actuator assemblies manufactured using a method according to any one of claims 31 to 52, each having substantially the same tension in the length of SMA wire and/or substantially the same path length of the length of SMA wire between the connected ends of the length of SMA wire.