WO2022263833A1

WO2022263833A1 - Sma haptic assembly

Info

Publication number: WO2022263833A1
Application number: PCT/GB2022/051526
Authority: WO
Inventors: Andrew Benjamin Simpson Brown; Stephen Matthew BUNTING
Original assignee: Cambridge Mechatronics Limited
Priority date: 2021-06-16
Filing date: 2022-06-16
Publication date: 2022-12-22
Also published as: CN117460889A; GB2607932B; GB202108619D0; GB2607932A

Abstract

An SMA actuator assembly comprises first and second parts 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. At least one of the first and second parts comprises at least one contact portion arranged to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion. The at least one contact portion is shaped to provide the curved section of the length of SMA wire with a radius of curvature that is greater in a central region of the curved section than in outer regions of the curved section.

Description

SMA HAPTIC ASSEMBLY

The present invention relates to actuators which use shape memory alloy (SMA) wires to provide relative movement between two components. In particular, the present invention 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 teeth on two opposing bodies which are forced apart due to the change in length of the SMA wire as it contracts.

It is desirable to minimise the height of haptic actuators, to reduce their impact on the dimensions of the electronic device in which they are used. The height of the actuator depends on the shape of the SMA wire. A longer stretch of wire in a steeply curved path between teeth of the opposing bodies will lead to a larger height of the actuator. A shorter stretch of wire in a flatter path will lead to a smaller height of the actuator. Conventional approaches therefore seek to reduce the actuator height by reducing the length of the wire.

The path of the wire between the teeth of the opposing bodies is correspondingly flattened to maintain the desired stroke.

However, reducing the length of wire in this way reduces the force with which the wire moves the opposing bodies apart. A reduction of force is undesirable, and therefore a compromise must be made between actuator height and applied force.

According to a first 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, at least one of the first and second parts comprising at least one contact portion arranged in contact with the SMA wire so as to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion and intermediate sections of the length of SMA wire extending between the first and second parts 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 at least one contact portion is shaped to provide the curved section of the length of SMA wire which extends therearound with a radius of curvature that is greater in a central region of the curved section than in outer regions of the curved section adjacent to the intermediate sections of the length of SMA wire. In the conventional approaches to reducing wire height discussed above, the wire angle is reduced. The wire angle is the angle between the SMA wire and a plane orthogonal to the movement axis at a contact point with a contact portion. In other words, the wire angle is a measure of how steep or flat the path of the wire is. Reducing the wire angle reduces the force applied to the moving parts when the wire contracts, for a given tension.

In the present invention, at least one contact portion is shaped to alter the path of the wire from the conventional uniformly curved path. Outer regions of the contact section are more curved than a central region. The more curved outer regions provide a large wire angle, and hence larger force. The shallower central region reduces the height of the wire path. The present invention thus reduces the height of the actuator without overly reducing the force applied to move the first and second parts.

In some embodiments, at least one contact portion is in continuous contact with the curved section of the length of SMA wire which extends therearound. Alternatively or additionally, at least one contact portion may comprise plural contact sections in contact with the curved section of the length of SMA wire which extends therearound, the contact portions being separated by at least one gap across which there is no contact with the curved section of the length of SMA wire. In particular embodiments, the at least one contact portion comprises two contact sections separated by a single gap.

In some embodiments each of the first and second parts comprises at least one contact portion which make contact with the SMA wire on opposite sides thereof. In some embodiments, the second part has plural contact portions, the contact portions of the two parts alternating in a direction normal to the movement axis. In some embodiments, each of the first and second parts has plural contact portions.

In some embodiments the length of SMA wire is connected at each end to the first or second part by respective connection elements that hold the SMA wire. In some embodiments the connection elements are crimp elements fixed to the first or second part. Using crimp elements can be a convenient way to attach the length of SMA wire to the parts, providing for simple and rapid assembly of the SMA actuator.

In some embodiments at least one of the first and second parts comprises a moulded body of material, such as a body with a base layer and protrusions extending from the base layer acting as the contact portions. Alternatively or additionally, at least one the first and second parts may comprise a sheet of material that is shaped to define the contact portions. Using a sheet of material may reduce the overall weight and dimensions of the actuator compared to using full moulded bodies, which is desirable for use in handheld electronic devices.

According to a second aspect of the invention there is provided an SMA actuator assembly according to any embodiment of the first aspect, the SMA actuator assembly being arranged to provide a haptic effect.

According to another 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, at least one of the first and second parts comprising at least one contact portion arranged in contact with the SMA wire so as to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion and intermediate sections of the length of SMA wire extending between the first and second parts 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 at least one contact portion is shaped to provide the curved section of the length of SMA wire which extends therearound with a radius of curvature that is equal or greater in a central region of the curved section than in outer regions of the curved section adjacent to the intermediate sections of the length of SMA wire, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the outer regions of the curved section is 50 or less.

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 an SMA actuator assembly;

Fig. 3 shows an alternative SMA actuator assembly;

Figs. 4a and 4b show an alternative SMA actuator assembly; and

Fig. 5 shows another alternative SMA actuator assembly.

Fig. 1 shows an SMA actuator assembly 100 of the type in which the present disclosure may be implemented. The SMA actuator assembly 100 comprises a first part 101 and a second part 102 that are movable relative to each other along a movement axis M.

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 parts 101, 102 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 illustrated parts.

Although not shown in Fig. 1, the SMA actuator assembly 100 may comprise a suspension system which holds the two parts 101, 102 relative to one another and allows them to move along the movement axis M. The suspension system may permit movement of the two bodies 101, 102 relative to one another along the movement axis M, while restricting or preventing relative movement of the two bodies 101, 102 in the plane perpendicular to the movement axis M and/or restricting or preventing relative rotation of the two bodies 101,

102

In some embodiments, the SMA actuator assembly 100 is integrated into a larger device. In such embodiments, the first part 101 may be a static part, which does not move relative to the device during actuation of the SMA actuator assembly 100, and the second part 102 may be a moving part which does move relative to the device during actuation of the SMA actuator assembly 100. Alternatively, both parts may move during actuation.

The SMA actuator assembly 100 further comprises a length of SMA wire 103 connected at each end to either one of the first and second parts 101, 102. In some embodiments, the ends of the length of SMA wire 103 are connected to different ones of the two parts. Preferably, the length of SMA wire 103 is connected at each end to the same one of the parts, i.e. both ends of the length of SMA wire 103 are connected to the first part 3 or both ends are connected to the second part 4. This reduces the force between the first and second parts 101, 102 in a direction perpendicular to the movement axis M during actuation of the SMA actuator assembly 100. In some embodiments, the length of SMA wire 103 is connected at each end to the first part 101. This may be preferable in embodiments where the first part 101 is a static part. In the embodiment shown in Fig. 1, both ends of the length of SMA wire 10 are connected to the first part 101.

In some embodiments the length of SMA wire 103 is connected at each end to either one of the first and second parts 4, 6 by a respective connection element that holds the SMA wire 103. Any suitable means or wire attachment device may be used as the connection element to hold the length of SMA wire 103. For example, the connection element may comprise an adhesive, where the length of SMA wire 103 is set into the adhesive before curing the adhesive. In some embodiments, one or both of the connection elements is a crimp element. The crimp element crimps the end of the length of SMA wire 10. The crimp element may be fixed to the first part 101 or second part 102. In some embodiments, the crimp element 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 element 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 element may be desirable, in particular where the crimp is used to make electrical connection to the length of SMA wire 103 as well as fixing the length of SMA wire 103 to the first part 101 or second part 102.

At least one of the first and second parts 101, 102 comprises at least one contact portion 104, 105 arranged in contact with the SMA wire 103. In the embodiment illustrated in Fig. 1, both the first part 101 and the second part 102 each have at least one contact portion 104, 105. In the embodiment illustrated in Fig. 1, the contact portions 104 can be considered as teeth of the first part 101, and the contact portions 105 as teeth of the second part 102. The illustrated contact portions 104, 105 are substantially solid and moulded integrally with their respective part 101, 102. 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, as discussed further below in relation to Fig. 4.

In the embodiment shown in Fig. 1, the first part 101 has four contact portions 104, and the second part 102 has three contact portions 105. The contact portions 104, 105 of the first and second parts 101, 102 overlap in a direction parallel to the movement axis M. The overlapping of the contact portions 104, 105 means that the uppermost portion of a contact portion 104 on the first body 101 is above the lowermost portion of the adjacent contact portion 105 of the second part 102 (where ‘up’ for this purpose is defined as being in the direction of movement of the second part 102 relative to the first part 101 on contraction of the SMA wire 103). In some embodiments, the first part 101 has at least one contact portion 104, the second body 102 has plural contact portions 105, the contact portions 104, 105 of the two bodies alternating in a direction normal to the movement axis M, the length of SMA wire 103 contacts with alternating contact portions 104, 105 of the first and second parts 101, 102. In an embodiment such as that shown in Fig. 1, each of the parts 101, 102 has plural contact portions 104, 105.

The contact portions 104 of the first part 101 make contact with the length of SMA wire 103 from below on a first side of the length of SMA wire 103 along the movement axis M, and the contact portions 105 of the second body 102 make contact with the length of SMA wire 103 from above on a second side of the length of SMA wire 103 along the movement axis, opposite to the first side. The length of SMA wire 103 extends between the first and second bodies 101, 102 and is guided along a tortuous path between the first and second bodies 101, 102 by the contact portions 104, 105, with a curved section 106 of the length of SMA wire 103 extending around the or each contact portion 104, 105 and intermediate sections 107 of the length of SMA wire 103 extending between the first and second parts 101, 102. 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 103 are connected to the first or second parts 101, 102. The tortuous path followed by the length of SMA wire 103 will therefore have a length which is greater than the shortest distance between connection points at which the ends of the length of SMA wire 103 is connected to the first and/or second parts 101, 102. 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 points.

The length of SMA wire 103 is arranged so that when the length of SMA wire 103 contracts, the first and second parts 101, 102 move away from each other. This is caused by the overlapping of the contact portions 104, 105 of the first and second parts 101, 102, such that a force is exerted on the contact portions 104, 105 by the length of SMA wire 103 as it contracts. In other embodiments, the first and second parts 104, 105 may move together, as long as the first and second parts 101, 102 move relatively in opposite directions.

In some embodiments, the two bodies are provided with end-stops (not illustrated) that limit relative movement of the two parts 101, 102 towards each other. The end-stops may be provided both on the same one of the two parts 101, 102, for instance the first part 101 as shown in Fig. 1. Alternatively, the end-stops may be provided on different ones of the two parts 101, 102 e.g. at different ends of the SMA actuator assembly 101, or end-stops may be provided on both parts 101, 102 e.g. at both ends of the SMA actuator assembly 100. The end-stops define a minimum separation of the first and second parts 101, 102. In some embodiments, the minimum separation will be that in a resting position when the SMA actuator assembly 100 is not actuated, i.e. when the length of SMA wire 103 is relaxed, not contracted. In the resting state, the two parts 101, 102 are in contact with the end-stops 12.

In some embodiments, the assembly 100 includes an arrangement (e.g. a resilient element such as a spring) to provide 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 103 is reduced or stopped, the length of SMA wire 103 expands as it cools and the two parts 101, 102 move back e.g. towards the resting position.

The first and second parts 101, 102 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. An example of an actuator assembly 100 formed from a sheet material is shown in Figs. 4a and 4b. Fig. 4a shows an isometric view of the actuator assembly 100, and Fig. 4b shows a side-on view of the assembly 100. In this illustrated embodiment the length of SMA wire 103 is connected at both ends to the first part 101 by connection elements 108. The first and second parts 101, 102 are formed from sheets of material shaped to define the contact portions 104, 105, creating the tortuous path for the SMA wire 103. The sheets of material may be or comprise a metal. In the illustrated embodiment both the first and second parts 101, 102 comprise a sheet material, but in other embodiments only one of the first or second parts 101, 102 is formed of a sheet material.

In the illustrated embodiment the first and second parts 101, 102 contact the SMA wire 103 substantially along their full length. The contact portions 104, 105, as used herein, are those portions of the first and second parts 101, 102 that cause the length of SMA wire to bend into the tortuous path, analogously to the contact portions 104, 105 in Fig. 1.

As can be seen in both Fig. 1 and Figs. 4a, 4b, the or each contact portion 104, 105 is shaped to provide the curved section 106 of the length of SMA wire 103 which extends therearound with a radius of curvature that is greater in a central region 106b of the curved section than in outer regions 106a of the curved section 106 adjacent to the intermediate sections 107 of the length of SMA wire 103. The degree of curvature is thus lower in the central region 106b than in the outer regions 106a. This can be seen more clearly in Fig. 2, which shows a closer view of the assembly 100 of Fig. 1. At the initial point of contact between the SMA wire 103 and a contact portion 104, 105 in an outer region 106b, the radius of curvature is comparatively steep. This mean that the wire angle of the wire is comparatively large, increasing the amount of force applied by the SMA wire on the first and second parts 101, 102 during actuation for a given length of wire. The radius of curvature of the contact portion 104, 105, and hence the SMA wire 103, then decreases in the central region 106b. This means that the height of the path of the SMA wire 103 along the movement axis M is smaller than it would be if the radius of curvature in the outer region 106b was maintained in the central region 106b. Thus the overall height of the actuator assembly 100 can be smaller than would otherwise be possible, without impacting on the force applied during actuation of the SMA wire 103.

In the embodiment illustrated in Fig. 2, the central regions 106b of the contact portions 104, 105 are substantially flat, i.e. have substantially infinite radius of curvature (or zero degree of curvature). In practice, central regions with a non-zero degree of curvature may be preferable to maintain a smooth contact with the length SMA wire 103, and so minimise wear on the SMA wire 103. In general, the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the central region may be infinite or less, preferably 1000 or less, further preferably 500 or less.

The ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the outer region may be 50 or less, preferably 20 or less, further preferably 10 or less. A smaller such ratio, i.e. a relatively smaller radius of curvature, may ensure that the height of the SMA actuator along the movement axis M remains relatively small.

The ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire at any point along the length of SMA wire may be 5 or more, preferably 10 or more, further preferably 20 or more. By limiting the minimum radius of curvature of the SMA wire, the risk of damage to the SMA wire due to tight bend radii is reduced.

The ratio of the radius of curvature of the length of SMA wire in the central region to the radius of curvature of the length of SMA wire in the outer regions may be 2 or more, preferably 4 or more, further preferably 8 or more. By ensuring that the SMA wire bends less in the central region than in the outer regions, the height of the SMA actuator along the movement axis M can be reduced.

In the illustrated embodiments, a single radius of curvature is used for the outer regions 106a, and a single radius of curvature for the central regions 106b. In other embodiments the outer regions 106a and/or the inner regions 106b comprise multiple sections, each with a different radius of curvature. For example, the radius of curvature may gradually change from the initial contact point of the SMA wire 103 with an outer region 106a to the centre point of a central region 106b, providing a smoother transition.

In the embodiments illustrated in Figs. 1, 2, 4a, and 4b, each contact portion 104, 105 is in continuous contact with the curved section 106 of the length of SMA wire 103 which extends therearound. In other embodiments, one or more contact portions 104, 105 comprise plural contact sections 104a, 105a in contact with the curved section 106 of the length of SMA wire 103 which extends therearound, the contact portions 104, 105 being separated by at least one gap 104b, 105b across which there is no contact with the curved section 106 of the length of SMA wire 103.

Such an embodiment of an actuator assembly 100 is shown in Fig. 3. In this embodiment, each contact portion 104, 105 comprises two contact sections 104a, 105a separated by a gap. Contact portions 104, 105 of other embodiments may have more than two contact sections 104a, 105a, and may have differing numbers of contact sections 104a, 105a on different contact portions 104a, 105a. In some embodiments the contact portions 104, 105 of only one of the first part 101 and second part 102 have plural contact sections 104a, 105a.

In Fig. 3, the central sections 106b of the length of SMA wire 103 extending across the gaps 104a, 105a appear substantially flat (i.e. zero degree of curvature). In practice, the length of SMA wire 106 may have a low, non-zero degree curvature across the gaps 104b, 105b when examined at close scale. The gaps 104, 105a thus provide the smaller radius of curvature to the central sections 106b discussed above in relation to Fig. 2. The contact sections 104a, 105a provide the larger radius of curvature to the outer portions 106a of the length of wire 103. A pair of contact sections 104a and a gap 104b thus form a single contact portion 104 of the first part 101. Similarly a pair of contact sections 105a and a gap 105b form a single contact portion 105 of the second part 101. The individual contact sections 104a, 105a may have a uniform radius of curvature, as shown in Fig. 3.

In the embodiment illustrated in Fig. 3, the contact sections 104a, 105a are formed by protrusions extending from a main body of their respective contact portion 104, 105. This may be particularly suited to embodiments where the first and/or second parts 101, 102 comprise a moulded body of material. In embodiments where the first and or second parts 101, 102 comprise a sheet of material, as in Figs. 4a and 4b, the gaps 104b, 10b may be formed by cut-outs, or preferably by recesses that extend partway through the sheet of material.

Fig. 5 schematically depicts another embodiment of an SMA actuator assembly 100 according to the present invention. The SMA actuator assembly 100 may comprise any combination of the features described above in connection with the SMA actuator assembly of Figs 1 to 4, and a detailed description thereof will be omitted for reasons of conciseness. The SMA actuator assembly 100 comprises first and second parts 101, 102 that are movable relative to each other along a movement axis M. The SMA actuator assembly 100 further comprises a length of SMA wire 103, each of the ends of the length of SMA wire being connected to the first or second part 101, 102. At least one of the first and second parts 101, 102 comprising at least one contact portion arranged in contact with the SMA wire 103 so as to guide the length of SMA wire 103 along a tortuous path with a curved section 106 of the length of SMA wire 103 extending around the or each contact portion and intermediate sections 107 of the length of SMA wire 103 extending between the first and second parts 101, 102 such that the first and second parts are driven in opposite directions along the movement axis M on contraction of the length of SMA wire 103. Compared to the above-described embodiments, in the embodiment of Fig. 5 the radius of curvature of a central region 106b of the curved section 106 may be equal or greater than the radius in outer regions 106a of the curved section 106.

In the embodiment of Fig, 5, the ratio of the radius of curvature of the length of SMA wire 103 to the diameter of the length of SMA wire 103 in the outer regions 106a of the curved section 106 is 50 or less. Preferably, this ratio is 20 or less, and further preferably this ratio is 10 or less. A maximum limit on the radius of curvature of the SMA wire 103 in the outer regions 106a, and so a minimum limit on the degree of curvature, leads to an SMA actuator 100 with reduced height along the movement axis M. Such a compact SMA actuator assembly 100 may be particularly suitable for integration in devices with limited space, such as mobile phones.

The radius of curvature in the central region 106b may be equal to the radius of curvature in the outer regions 106b. This may reduce the extent of the SMA actuator assembly 100 in the direction lateral to the movement axis M, making the SMA actuator assembly 100 more compact. However, in general, the radius of curvature in the central region 106b may be greater than the radius of curvature in the outer region 106a, for example in the manner described above in relation to Figs. 1 to 4.

There may be a lower limit on the radius of curvature of the central region to thereby maintain reduced lateral dimensions. The ratio of the radius of curvature of the length of SMA wire 103 to the diameter of the length of SMA wire 103 in the central region of the curved section may be 50 or less, preferably 20 or less, further preferably 10 or less.

Preferably, the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire at any point along the length of SMA wire is 5 or more, preferably 10 or more, further preferably 20 or more. This may reduce the risk of damage to the SMA wire 103 due to tight bend radii.

In the above, the radius of curvature of the SMA wire 103 is defined in relation to the diameter of the SMA wire 103. This is appropriate because the diameter of the SMA wire 103 may be pre-selected independently from a consideration of the SMA actuator assembly dimensions, for the purposes of achieving a desired electrical characteristic, a desired haptic response or for reasons of ease of manufacturing. The embodiments described above may thus achieve a reduced height along the movement axis M for a given SMA wire diameter.

In any of the embodiments (of Figs. 1 to 5) described above, the SMA wire 103 may have a diameter in the range from 10pm to 200pm. Preferably, the SMA wire 103 has a diameter greater than 20pm, further preferably greater than 25pm, most preferably greater than 30pm. Using a larger diameter SMA wire 103 may increase the amount of force delivered along the movement axis M, or reduce the number of parallel wires (and so the complexity of manufacture) for a given force along the movement axis M. Preferably, the SMA wire 103 has a diameter less than 77pm, further preferably less than 50pm, most preferably less than 40pm. An upper limit on the SMA wire diameter may increase the rate of cooling of the SMA wire 103, and so the response time of the SMA actuator assembly 100.

There is thus also disclosed 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, at least one of the first and second parts comprising at least one contact portion arranged in contact with the SMA wire so as to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion and intermediate sections of the length of SMA wire extending between the first and second parts 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 SMA wire has a diameter in the range from 20 to 77pm, preferably from 25 to 50pm, further preferably from 30 to 40pm. The inventors have found that such an SMA actuator assembly may achieve the desired compromise between easy of manufacture and response time that is especially suitable for use in devices such as mobile phones.

The term ‘shape memory alloy (SMA) wire’ may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term ‘SMA wire’ may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. Those skilled in the art will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing the present disclosure, the present disclosure should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that the present invention has a broad range of applications, and that the embodiments may take a wide range of modifications without departing from scope of the claims.

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, at least one of the first and second parts comprising at least one contact portion arranged in contact with the SMA wire so as to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion and intermediate sections of the length of SMA wire extending between the first and second parts 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 at least one contact portion is shaped to provide the curved section of the length of SMA wire which extends therearound with a radius of curvature that is greater in a central region of the curved section than in outer regions of the curved section adjacent to the intermediate sections of the length of SMA wire.

2. An SMA actuator assembly according to claim 1, wherein each contact portion is in continuous contact with the curved section of the length of SMA wire which extends therearound.

3. An SMA actuator assembly according to claim 1, wherein each contact portion comprises plural contact sections in contact with the curved section of the length of SMA wire which extends therearound, the contact portions being separated by at least one gap across which there is no contact with the curved section of the length of SMA wire.

4. An SMA actuator assembly according to claim 3, wherein each contact portion comprises two contact sections separated by a single gap.

5. An SMA actuator assembly according to any one of the preceding claims, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire at any point along the length of SMA wire is 5 or more, preferably 10 or more, further preferably 20 or more.

6. An SMA actuator assembly according to any one of the preceding claims, wherein the ratio of the radius of curvature of the length of SMA wire in the central region to the radius of curvature of the length of SMA wire in the outer regions is 2 or more, preferably 4 or more, further preferably 8 or more.

7. An SMA actuator assembly according to any one of the preceding claims, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the central region is infinite or less, preferably 1000 or less, further preferably 500 or less.

8. An SMA actuator assembly according to any one of the preceding claims, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the outer region is 50 or less, preferably 20 or less, further preferably 10 or less.

9. An SMA actuator assembly according to any one of the preceding claims, wherein each of the first and second parts comprise at least one contact portion which make contact with the SMA wire on opposite sides thereof.

10. An SMA actuator assembly according to claim 9, wherein the second part has plural contact portions, the contact portions of the two parts alternating in a direction normal to the movement axis.

11. An SMA actuator assembly according to claim 10, wherein each of the first and second parts has plural contact portions.

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

13. 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 or second part by respective connection elements that hold the SMA wire.

14. An SMA actuator assembly according to claim 13, wherein the connection elements are crimp elements fixed to the first or second part.

15. An SMA actuator assembly according to any one of the preceding claims, wherein at least one of the first and second parts comprises a moulded body of material.

16. An SMA actuator assembly according to any one of the preceding claims, wherein at least one the first and second parts comprises a sheet of material that is shaped to define the contact portions.

17. An SMA actuator assembly according to claim 16 when dependent on any one of claims 3 to 6, wherein the gaps are formed by recesses that extend partway through the sheet of material.

18. An SMA actuator assembly according to any one of the preceding claims, the SMA actuator assembly being arranged to provide a haptic effect.

19. 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, at least one of the first and second parts comprising at least one contact portion arranged in contact with the SMA wire so as to guide the length of SMA wire along a tortuous path with a curved section of the length of SMA wire extending around the or each contact portion and intermediate sections of the length of SMA wire extending between the first and second parts 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 at least one contact portion is shaped to provide the curved section of the length of SMA wire which extends therearound with a radius of curvature that is equal or greater in a central region of the curved section than in outer regions of the curved section adjacent to the intermediate sections of the length of SMA wire, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the outer regions of the curved section is 50 or less.

20. An SMA actuator assembly according to claim 19, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the outer regions of the curved section is preferably 20 or less, further preferably 10 or less.

21. An SMA actuator assembly according to claim 19 or 20, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire in the central region of the curved section is 50 or less, preferably 20 or less, further preferably 10 or less.

22. An SMA actuator assembly according to one of claims 19 to 21, wherein the ratio of the radius of curvature of the length of SMA wire to the diameter of the length of SMA wire at any point along the length of SMA wire is 5 or more, preferably 10 or more, further preferably 20 or more.

23. An SMA actuator assembly according to any one of claims 19 to 22, wherein the ratio of the radius of curvature of the length of SMA wire in the central region to the radius of curvature of the length of SMA wire in the outer regions is 2 or more, preferably 4 or more, further preferably 8 or more.

24. An SMA actuator assembly according to any one of claims 19 to 23, wherein each of the first and second parts comprises plural contact portions which make contact with the SMA wire on opposite sides thereof, the contact portions of the two parts alternating in a direction normal to the movement axis.

25. An SMA actuator assembly according to any one of claims 19 to 24, wherein the length of SMA wire is connected at each end to the first or second part by respective connection elements that hold the SMA wire, wherein the connection elements are crimp elements fixed to the first or second part.