GB2590518A

GB2590518A - SMA haptic assembly

Info

Publication number: GB2590518A
Application number: GB2001864.4A
Authority: GB
Inventors: Benjamin Simpson Brown Andrew; Flouris Andreas; O Heijne Nicholas; Howarth James; Scholz Marc-Sebastian
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2019-12-16
Filing date: 2020-02-11
Publication date: 2021-06-30
Also published as: GB201918548D0; GB202001864D0

Abstract

An SMA actuator assembly 2 comprises two parts 4, 6 relatively movable along a movement axis M, and a length of SMA wire 10, each end of the SMA wire 10 being connected to one of the parts 4, 6. One part comprises a contact portion 8 and the other part comprises plural contact portions 8, 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. The contact portions 8 are arranged so that for intermediate sections 44 of the SMA wire between contacts with adjacent contact portions, the fractional change in the wire length on contraction of the SMA wire is equal to the overall strain of the SMA wire and there is no slippage of the SMA wire.

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, 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 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. 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.

According to 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 30 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 5 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.

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. 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 NI. 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 SNIA 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 SNIA 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 unions mutant/is 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 0 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 0 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 = -d s n ( 0) (Eq. 1) £ As illustrated in Fig. 2, the angle theta 9 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 NI, 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. 1/sin(6)) that increases with decreasing angle theta 0. 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 (9) and can be controlled by suitably controlling the angle theta 0 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 0 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 1 I first and second bodies 4 arid 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 B 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 8 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 U 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 NI 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 0 (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 U 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 NI, 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 NI between the centre points 40, 42 of the contacts with adjacent teeth 8. As discussed above, this will change the angle U 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 U 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 NI) the parameters of the angle theta 0 and length L are measured in the resting position (i.e. non-actuated state) as described above.

Amplitude of centre displacement 0.1 mm Total actuator assembly length 45 mm Dead region (n) 5 mm a 3.08 mm b 5.67 mm Height of SMA section (h, height along M) 1.5 mm Theta 0 (a) 25.9 deg Theta 0 (b) 14.8 deg Surface motion at (a) 0.0286 mm Surface motion at (b) 0.0852 mm Resting wire length L (a) 3.43 mm Resting wire length L (b) 5.86 mm Actuated wire length (a) 3.42 mm Actuated wire length (b) 5.84 mm Fractional change in wire length E (a) 0.00362 Fractional change in wire length e (b) 0.00362

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 NI 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 0 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

Claims

CLAIMSAn 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 1 6 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 SNIA 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(8)).
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 SIM 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.