GB2590517A

GB2590517A - SMA Haptic assembly

Info

Publication number: GB2590517A
Application number: GB2001861.0A
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
Anticipated expiration: 2040-02-11
Also published as: GB201918461D0; GB2590517B; GB202001861D0

Abstract

An SMA actuator assembly 2 comprises first and second parts 4, 6 movable relative to each other along a movement axis M, and a length of SMA wire 10. The parts 4, 6 each comprise a contact portion 8 making contact with the length of SMA wire 10 on opposite sides of the length of SMA wire along the movement axis M, the contact portions 8 being relatively positioned to guide the length of SMA wire 10 along a tortuous path such that the parts 4, 6 are driven in opposite directions on contraction of the length of SMA wire 10. The SMA wire 10 is connected to the parts 4 by a connection element 18; and at each end the SMA wire 10 extends from the connection element 18 at an exit point 46, to an adjacent contact portion 8 without contact with the part 4 to which the end of the SMA 10 wire is connected, thus avoiding a potential source of damage to 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 teeth on two opposing bodies which are forced apart due to the change in length of the SMA wire as it contracts.

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 slips relative to this part of the body it is attached to, due to its changing length. Such slippage 15 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 andJor 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 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 20 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 00 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 00 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 20 the two parts when the length of SMA wire contracts.

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 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.

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 a section of an SMA actuator assembly according to an embodiment of the present invention; Fig 4 shows an assembly with an overhanging crimp portion; Fig 5 shows a crimp portion prior to being formed with a curved portion; Fig 6 shows a crimp portion formed with a curved portion.

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 Nil, 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. 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, 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 NI, 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. 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, 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. 1, 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. 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, 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. 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, 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. 2 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. 2, 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. 3 and 4, 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. 3, 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 0 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 00, optionally greater than 5°, optionally greater than 10°, to a plane normal to the movement axis Ni. 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 B is at most 100 to said line 52, optionally at most 50 to said line 52.

The exit angle 0 may be limited in a variety of ways. For example, as shown in Fig. 3, 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 NI. This ensures that the length of SMA wire 10 is not perpendicular to the movement axis NI 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 B is shown in Fig. 4. In the embodiment of Fig. 4, 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 00 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 Ni is fixed to the first body 4. In embodiments such as that shown in Fig. 4, 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. 5 and 6. The piece of sheet metal which is to form the crimp portion lies in the plane normal to the movement axis NI and is placed between a punch 60 and anvil 62, as shown in Fig. 5. The sheet metal is then compressed between the punch 60 and the anvil 62 as shown in Fig. 6. Although the process is shown in Figs. 5 and 6 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 NI.

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 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.
2. An SMA actuator assembly according to claim 1, 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 150 to a line from the exit point to a point where the length of SMA wire first contacts the adjacent contact portion
3 An SMA actuator assembly according to claim 2, wherein said exit angle is at most to said line.
4. An SMA actuator assembly according to claim 3, wherein said exit angle is at most 5° to said line.
5. An SMA actuator assembly according to any one of claims 1 to 4, 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.
6. An SMA actuator assembly according to claim 5, 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 00 to a plane normal to the movement axis.
7. An SMA actuator assembly according to claim 6, wherein the part of the crimp portion extending normal to the movement axis is fixed to the first part.
8. An SMA actuator assembly according to any one of claims 5 to 7, 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.
9. An SMA actuator assembly according to any one of claims 2 to 8, wherein said line is at an acute angle of greater than 00 to a plane normal to the movement axis.
10. An SMA actuator assembly according to any one of claims 1 to 9, wherein the length of SMA wire is connected at each end to the first part.
11. An SMA actuator assembly according to any one of claims 1 to 10, 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.
12. An SMA actuator assembly according to claim 11, wherein each of the first and second parts has plural contact portions.
13. 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; 1 1 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.