WO2019106340A1

WO2019106340A1 - Shape memory alloy actuator

Info

Publication number: WO2019106340A1
Application number: PCT/GB2018/053364
Authority: WO
Inventors: Eugene Yu Jen HO; Andrew Benjamin Simpson Brown; Jonathan Morgan
Original assignee: Cambridge Mechatronics Limited
Priority date: 2017-12-01
Filing date: 2018-11-21
Publication date: 2019-06-06
Also published as: GB2583294B; GB2583294A; GB2605924B; GB202210121D0; GB202009995D0; GB2605924A

Abstract

A shape memory alloy, SMA, actuator for use in a device comprising a static portion (2,3) and a moveable portion (1) which is moveable relative to the static portion. The shape memory alloy actuator comprises an SMA wire (5) which is connectable between the static portion (2) and the moveable portion (1) of a device and which is arranged, by contraction, to drive movement of the moveable portion from a first position to a second position. The actuator also comprises a restoring element (4) which is connectable to the moveable portion (1) to return the moveable portion to the first position from the second position; and a strain relief mechanism (6) which is connected to the SMA wire (5) to allow contraction of the SMA wire (5) when the moveable portion (1) is immobilised.

Description

Shape Memory Alloy Actuator

The present application generally relates to a shape memory alloy (SMA) actuator and devices that incorporate such actuators for moving a component within the device. Example devices include a latch or a haptic device that is incorporated in a consumer electronics device to provide a tactile or haptic sensation to a user by movement of a button when activated.

For example, the co-owned patent application GB2551657 (which claims priority from GB1709011.9) discloses a haptic button that seeks to improve the user experience by using an SMA actuator comprising SMA wire to generate a haptic or tactile sensation that is localised to the button in a compact package. The device works by shifting the button laterally when it detects the presence of a finger, which the user interprets as a vertical press. In one type of device, SMA wire is used to pull the button in a first direction, when the button is activated, and it is returned to its starting position by resilient means such as a spring, or a second SMA wire.

One problem that may arise when using such an SMA haptic device is that the SMA wire can become damaged if the button is immobilized while the SMA wire is activated, for example if the button is pressed with excessive force by a finger. In an extreme case the stress imposed on the wire by it being both held in position by a finger and simultaneously attempting to contract by virtue of its heating when powered may result in the wire breaking. Damage to the SMA wire may also arise during reliability testing when the wire is powered or unpowered and there is excessive movement of the moving portion It will be appreciated that similar problems may arise in other devices having a moveable component which is moved by an SMA actuator, when the moveable component is immobilised or during reliability testing.

The present applicant has identified the need for an improved SMA actuator.

In a first approach of the present techniques, there is disclosed a shape memory alloy actuator for use in a device comprising a static portion and a moveable portion which is moveable relative to the static portion, the shape memory alloy actuator comprising a shape memory alloy (SMA) wire which is connectable between a static portion and a moveable portion and which is arranged, by contraction, to drive movement of the moveable portion from a first position to a second position; a restoring element which is connectable to the moveable portion to return the moveable portion to the first position from the second position; and a strain relief mechanism which is connected to the SMA wire to reduce strain on the SMA wire. For example, the strain relief mechanism may allow contraction of the SMA wire when the moveable portion is immobilised. Alternatively, the strain relief mechanism may prevent over stretching of the SMA wire when the moveable portion moves an unexpectedly large distance.

In a second approach of the present techniques, there is disclosed a device comprising a static portion, a moveable portion and a shape memory alloy actuator as described above.

The devices may be any one of: a smartphone, a camera, a foldable smartphone, a foldable image capture device, a foldable smartphone camera, a foldable consumer electronics device, a foldable or flexible display screen/display device, an image capture device, a 3D sensing device or system, a servomotor, a consumer electronic device (including domestic appliances such as vacuum cleaners, washing machines and lawnmowers), a mobile or portable computing device, a mobile or portable electronic device, a laptop, a tablet computing device, an e-reader (also known as an e-book reader or e-book device), a computing accessory or computing peripheral device (e.g. mouse, keyboard, headphones, earphones, earbuds, etc.), a security system, a medical device (e.g. an endoscope), a gaming system, a gaming accessory (e.g. controller, headset, a wearable controller, joystick, etc.), a robot or robotics device, an audio device (e.g. headphones, headset, earphones, etc.), an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a haptics device, a wearable device (e.g. a watch, a smartwatch, a fitness tracker, etc.), a drone (aerial, water, underwater, etc.), an aircraft, a spacecraft, a submersible vessel, a vehicle, an autonomous vehicle (e.g. driverless car), a tool, a surgical tool, a remote controller (e.g. for a drone or consumer electronics device), clothing (e.g. a garment, shoes, etc.), a switch, dial or button (e.g. a light switch, a thermostat dial, etc.), a display screen, a touchscreen, and a near field communication (NFC) device. It will be understood that this is a non-exhaustive list of example devices.

For example, in a third approach of the present techniques, there is disclosed a haptic button device comprising a static portion in the form of a housing, a moveable portion in the form of a button and a shape memory alloy actuator as described above.

Preferred features are set out in the appended claims.

Implementations of the present techniques will now be described, by way of example only, with reference to the accompanying drawings, in which :

Figure 1A is a schematic cross-sectional view of a first haptic button device;

Figure IB is a schematic cross-sectional view of a second haptic button device;

Figure 1C is a schematic cross-sectional view of a third haptic button device;

Figures 2A and 2B are schematic cross-sectional views of variations of the device of Figure 1A and IB;

Figure 3A is an exploded view of another haptic button device;

Figure 3B is a schematic view of a circuit which can be used to control a haptic button device;

Figure 4 is a schematic cross-sectional view of a latch;

Figure 5 is a schematic cross-sectional view of another haptic button device;

Figure 6 is a schematic cross-sectional view of a haptic button device using a compression spring; Figures 7 and 8 are schematic cross-sectional views of two further haptic button devices;

Figures 9A and 9B are schematic cross-sectional views of another haptic button device in a rest and use position respectively;

Figures 10A and 10B are a schematic cross-sectional and plan views of a haptic button device using a flexible printed circuit (FPC); and

Figure 11 is a schematic plan view of another haptic button device using a flexible printed circuit (FPC).

Broadly speaking, embodiments of the present techniques provide a shape memory alloy actuator which is not damaged even when incorporated in a device in which the moveable portion is temporarily or permanently immobilised. Such actuators can be incorporated into many different devices.

For example, many consumer electronic devices contain a human machine interface (HMI). A commonly used HMI is a touchscreen, which detects and responds to the touch of a user's finger. Some functions, such as a master power control (on or off switch) are typically confined to a mechanical button or switch, and do not form part of the touch screen user interface. The use of mechanical buttons causes a number of problems. Firstly it restricts the size of the screen that can be placed in a device, as the screen has to be positioned around the mechanical button. Secondly the mechanical movement compromises the water resistance or water proofing of the device. Thirdly mechanical switches can take up a significant volume compared to the available volume in the device. Many of the embodiments below show haptic button devices or assemblies which comprise a button or other moveable portion which is moved to generate a haptic sensation for a user and attempt to mitigate these issues associated with mechanical buttons.

As shown below, a haptic sensation may be generated by moving the button in a lateral direction with respect to contact by the user - see also for example WO2018/046937 and GB255167. However, the haptic button may also be arranged to move in a direction that is normal to the surface of the button and the surface of the device in which the button is incorporated. Similarly, although many of the arrangements show a single SMA wire, it will be appreciated that multiple SMA wires may be used, e.g. if a greater force is required. It will be appreciated that these embodiments describing haptic button assemblies are merely illustrative and not intended to limit the applications of the present techniques for the SMA actuator.

Various devices are now described with respect to the Figures. It will be understood that elements or features described with respect to one particular Figure may equally apply to any of the other Figures. For example, the techniques for sensing movement of the moveable element, the endstops or the intermediate elements described with respect to particular Figures, may equally apply to any or all of the devices or assemblies described herein.

Figure 1A shows a schematic view of a shape memory alloy actuator incorporated in a simplified haptic button assembly 22 which comprises a moveable portion in the form of a button 1 and a static portion in the form of a housing 2,3. The button 1 has a pressable surface 24 and is connected to a sensing element 8 which could also be termed a sensing means. The sensing element is described in the co-owned patent application GB2551657 which claims priority from GB1709011.9 and which is incorporated by reference. Some details of the sensing element are described in more detail below.

The shape memory alloy actuator comprises an SMA wire 5 which is connected between the button 1 and the housing 2, 3. The SMA wire 5 is connected direct to the button and is connected to a portion of the housing 2 via a strain relief mechanism 6 which can also be termed a strain relief device. The SMA wire is connected via a crimp 7 to the strain relief element 6. The crimp forms an electrical and mechanical connection between the strain relief mechanism 6 and the SMA wire 5 and it will be appreciated that other similar forms of connection may also be used. The shape memory alloy actuator also comprises a restoring element 4 which is connected to the button 1 and to another portion of the housing 3 and which restores the position of the button 1 after contraction of the SMA wire. Each of the strain relief mechanism 6 and the restoring element 4 may be or may incorporate a resilient element such as a spring, elastic element or rubber element. Alternatively, the strain relief mechanism 6 and the restoring element 5 may be or may incorporate another SMA wire, for example as shown in Figure 1C below.

As explained in more detail below, when the sensing element 8 detects that the button 1 has been pressed, a signal is sent to the control unit to activate the SMA wire 5. When activated the wire 5 contracts, pulling the button 1 laterally towards the housing 2 to which the wire 5 is connected via the strain relief element 6. In other words, the moveable portion is moved from a first position (i.e. the normal rest position) to a second position (i.e. closer to the housing). When the SMA wire 5 contracts it extends the restoring element 4 and once the SMA wire begins to contract, the restoring element 4 returns the button to its first position from the second position. In normal operation the force produced by the wire 5 to move the button is less than the force used to alter the strain relief mechanism 6. For example, as shown in Figure 1A, when the strain relief mechanism 6 is a spring, in normal operation, the force produced by the wire 5 to move the button 1 is less than the force used to bias the spring. The crimp 7 therefore does not move and the button 1 is moved laterally towards the housing 2.

Sometimes, a user presses the button 1 so hard that it is effectively immobilised. If this occurs, all of the force generated by heating and hence contracting the SMA wire 5 pulls on the crimp 7. If this force exceeds the bias force of the strain relief mechanism 6, the contraction of the SMA wire 5 will extend the strain relief mechanism 6. In this example, the spring force required to extend the strain relief mechanism 6 is set low enough so that the spring of the strain relief mechanism 6 extends before the SMA wire 5 is damaged, but above the force typically imposed on the button by a normal finger press. The nature of finger presses means that this point may be determined heuristically.

Figure IB shows a design variant of the haptic button assembly shown in Figure 1A. The haptic button assembly 22' comprises a shape memory alloy actuator comprising a strain relief mechanism 6' (e.g. spring) connected between the button 1 and the housing 2, 3. As before, a crimp 7' or similar connector is used to connect the strain relief mechanism 6' to the SMA wire 5' and a restoring element 4 is used to restore the button 1 to its first position. In this arrangement, the strain relief mechanism 6' is connected to the button 1 and the SMA wire 5' rather than to the SMA wire and the housing as shown in Figure 1A. In Figure IB the SMA wire 5' is in turn connected to the button housing 2 and thus the SMA wire 5' is still connected between the button and the housing with the connection to the button being via the strain relief mechanism 6'.

As described in more detail in relation to Figure 1A, when the button 1 is pressed with a standard strength press and the activation of the button is detected by the sensing element 8, the SMA wire is powered electrically so that it heats up and contracts, moving the button 1 towards the portion of the housing 2 to which the SMA wire 5' is connected. When the button is pressed with sufficient force that it is immobilised, when the SMA wire is heated, there is sufficient force to overcome the bias force in the strain relief element 6'. The strain relief mechanism 6' extends and the crimp 7' moves towards the portion of the housing 2 to which the SMA wire 5' is connected.

Figure 1C shows a design variant of the haptic button assembly shown in Figures 1A and IB. The haptic button assembly 22" comprises a shape memory alloy actuator comprising a strain relief mechanism 6" having a resilient element connected between the button 1 and the housing 2, 3. In this arrangement, the strain relief mechanism 6" also comprises a sprung crimp 7" which is used to connect the resilient element to the SMA wire 5". The sprung crimp 7" may prevent damage to the SMA wire when the SMA wire is unpowered. For example, during reliability (drop) testing or during environment testing (e.g. excessive external heating (> 100 degrees Celsius)), there is a risk to damage to the SMA wire, particularly if there are no constraints on the movement of the moving portion. During these reliability tests, the moving portion may be forced to move to a position by an external force or through inertia of the moving portion during an event with high acceleration such as an impact event. In such cases, the SMA wire may be damaged and the sprung crimp may alleviate or reduce this damage. The sprung crimp may be used alone or with the other elements of the strain relief mechanism which reduce strain when the button is immobilised. It will also be appreciated that the sprung crimp may be incorporated in any of the other embodiments. As in Figures 1A and IB, a restoring element 4" is used to restore the button 1 to its first position. In this arrangement, the restoring element 4" is another SMA wire(s) rather than a resilient element. It will be appreciated that the resilient element in the strain relief mechanism may be similarly replaced with one or more SMA wires.

One issue with the above implementations is that SMA wire undergoes a finite contraction when heated. The length of the wire is limed by the size of the actuator and hence the distance that the wire contracts is also limited. In the above implementations any compliance of the resilient element within the strain relief mechanism during normal operation of the haptic button will generally reduce the motion of the haptic button and so reduce the haptic sensation generated. This is generally to be avoided but may be used to control the maximum force which is applied to the button and thus control the haptic sensation. There is thus a balance between ensuring that the compliance of the resilient element is sufficiently low to allow the resilient element to contract/extend to prevent breakage of the wire and sufficiently high to prevent (or control) the contraction /extension when the moveable portion is free to move. The biasing force of the resilient element may thus be defined as a percentage of the breaking force required to break the wire, e.g. between 10 to 90%, between 20 and 80% or 20 and 60% depending on the circumstances. Merely as an example, the breaking force of a 36mhi wire is likely to be approximately 1.2N .

As described in relation to Figures 2A and 2B, this issue can be alleviated by providing an endstop that prevents the resilient element in the strain relief mechanism from complying until a critical force is reached, in other words prevents movement of the resilient element when the force applied to the endstop is below a threshold. It will be appreciated that the endstop described in relation to Figures 2A and 2B can be incorporated into other variations described below where appropriate. Furthermore, the endstop shown in Figures 2A and 2B effectively forms an intermediate component (or element or structure; the terms may be used interchangeably) which connects the SMA wire and the resilient element of the strain relief mechanism. Thus, it will be appreciated that when a different components are described as being connected, the connection may be direct, e.g. as shown in Figures 1A and IB by using a crimp, or indirect, e.g. as shown in Figures 2A and 2B via the endstop.

Figure 2A shows a schematic view of another haptic button assembly 111. This figure is the same as Figure 1A except that the strain relief mechanism 56 comprises an endstop as well as a strain relief spring 116 which is similar to the spring shown in Figure 1A. In this figure as in Figure 1A, an extension strain relief spring is illustrated, but a compression strain relief spring could be used to provide the same force. The endstop comprises an intermediate component 51 that is attached to both the wire and the spring 116. The static portion in the form of the portion of the housing 2 comprises protrusions 57. At rest, the strain relief spring presses this intermediate component 51 against the protrusions 57 of the static portion 2 with a given force.

Once the SMA wire is activated, the tension in the wire is increased. The intermediate component 51 does not move until the tension in the wire is greater than the force that the strain relief mechanism 116 is applying to the intermediate component 51. Thus, in this arrangement the strain relief mechanism 56 (the spring 116 and endstop combination) offers no compliance to the wire until this critical tension or force is achieved. As described above, the critical force may be defined in terms of the force required to break the wire. In other words, the critical force may be set to between 10 and 90% of the breaking force of an SMA wire. Alternatively, the critical force may be set to between 20 and 80% or between 60 and 80% of the breaking force of an SMA wire depending on the circumstances.

Figure 2B shows a schematic view of another haptic button assembly 311. This figure is the same as Figure IB except that the strain relief mechanism incorporates an intermediate component 351 in the form of an endstop as well as a strain relief spring 316 which is the same as the one in Figure IB. In this case, the strain relief spring 316 is attached to the button. The button is formed with a protrusion 313 so that the endstop abuts the protrusion 313 and an edge of the pressable surface of the button in the rest position. Thus, the strain relief mechanism is referenced to the button 1 instead of being referenced to the static portion 2 as in Figure 2A. Otherwise, the operation of the strain relief mechanism is the same as that described above.

Figure 3A shows a button assembly incorporating a sensing element 13 which could be incorporated in the devices described above or below. In Figure 3A, as in previous arrangements, the button assembly 10 includes a button 11 having a pressable surface 12 which in this example is circular. The button assembly also comprises an SMA wire 20 which is hooked at an intermediate position around a retaining portion 23 formed on the button on one side. A coil spring 25 is connected at one end of the button 11 on the opposite side from the SMA wire. The coil spring 25 acts as a resilient biasing element to resiliently bias the SMA wire 20 and act as the restoring element as described above.

The sensing element 13 is a laminated structure disposed beneath the button 11. The laminated structure comprises a contact layer 14 which is depressable and acts as a contact portion. The contact layer may be made from a spring metal such as phosphor bronze. In order beneath the contact layer, the laminated structure further includes a first insulating layer 15, a switch layer 16 and a second insulating layer 17. The first insulating layer 15 separates the contact layer 14 and the switch layer 16 and has an aperture 18 aligned with the button 11. Depression of the contact layer 14 into the aperture 18 brings the contact layer 14 into electrical contact with the switch layer 16. The, the contact layer 14 and the switch layer 16 act as first and second switch elements. The electrical contact may be sensed and so the switch layer 16 acts as a sensor element or arrangement to sense depression of the contact layer in the first button assembly 10.

Figure 3B shows a control circuit which can be used for the first button assembly shown in Figure 3A. It will be appreciated that the control circuit can also be used with the other devices described herein when they incorporate the sensing arrangement shown in Figure 3A. The control circuit 30 is connected to the contact layer 14 and the switch layer 16 and detects when an electrical contact between the contact layer 14 and the switch layer 16 occurs, due to depression of the contact layer 14 by pressing the button 11. The control circuit 30 is also connected to the SMA wire 20 and applies a drive signal thereto. In use, the control circuit 30 drives the SMA wire 20 to move the button 11 in response to detecting pressing of the button 11.

In Figures 1A to 2B, variations of haptic button assemblies are shown. Typically, in such assemblies, the diameter of the SMA wire is around 36mGh. Haptic button assemblies are miniature devices. In each variation, there is disclosed a button assembly comprising a button having a pressable surface; and a sensor arrangement underneath the button arranged to sense pressing of the button; a shape memory alloy actuator arranged to drive movement of the button laterally to the direction of the button press; resilient means arranged to return the button to its original position after the button has been driven laterally by SMA and a strain relief device. Each variation may also be described as a button assembly a button having a pressable surface; and a sensor arrangement underneath the button arranged to sense depression of the button; a shape memory alloy actuator arranged to drive relative lateral movement of the button with respect to the contact portion laterally to the direction of travel of the button when pressed; resilient means arranged to return the button to its original position after the button has been driven laterally by SMA and a strain relief device.

It will be appreciated that the shape memory alloy actuator may be incorporated in many different devices and Figure 4 is a cross-section showing a latch. In this device, the moveable portion is a mass 601 and the static portion is a chassis 602 which supports the moveable portion. As shown in Figure 4, in a first or rest position the mass 601 protrudes from the chassis 602 and through or into another static portion, e.g. a recess in a door 603. The mass 601 is thus acting as a latch.

The SMA actuator comprises a wire 605 which extends along the length of the elongate chassis 602. The use of an elongate chassis 602 means that the SMA wire 605 is sufficiently long to provide the necessary distance of motion of the mass 601. The SMA actuator also comprises a restoring element 606, 607 in the form of two compression springs which restore the mass 601 to its original position after movement by the SMA wire and which is indirectly connected to the mass 601. There is also a strain relief mechanism comprising a resilient element 608, e.g. a spring which is mounted within a recess in the mass 601 and an intermediate component 609 which acts as an endstop. In normal use, the endstop is held against the mass 601 by the biasing force of the resilient element 608. In a latch, the SMA wire typically has a larger cross-section than in the haptics devices described above and thus are likely to require a higher breaking force. Nevertheless, the biasing force may be in the same ranges as described above because these are defined relative to the breaking force of the wire. The SMA wire 605 is connected between the mass 601 and the chassis 602. The connection to the mass 601 is via the strain relief mechanism, in particular via the intermediate component.

In normal use, when the latch is activated, the SMA wire 605 contracts and pulls on the endstop 609. The force is not sufficient to overcome the biasing force in the resilient element 608 and thus the resilient element 608 does not extend but moves the mass 601 towards the chassis. The mass 601 is thus moved out of the recess in the other static portion 603 and hence the latch is released. However, when the mass 601 is immobilised, the force generated by the contraction of the SMA wire 605 is sufficient to extend the resilient element 608 which moves toward the chassis without damaging the SMA wire 605. In both cases, the movement of the mass 601 or endstop 609 compresses the springs in the restoring element 606, 607 and each of the mass 601 or endstop 609 is restored to its original position by decompression of the springs of the restoring element 606, 607 after the SMA wire is cooled.

Figure 5 shows a haptic button device, which has been designed for compactness such that the SMA activation mechanism is substantially contained within the body of the button 101. The SMA actuator comprises an SMA wire 105 which is connected between a moveable portion in the form of a button 101 and a static portion in the form of a housing 102, 103. In this arrangement, the SMA wire 105 is connected direct at a first end to the inner surface of one part of the housing 102 and indirectly to the button 101 via an intermediate structure 109 (the terms intermediate component or element could be used interchangeably). The SMA wire is connected to the intermediate structure 109 using a crimp 107 and a strain relief mechanism 106 which as illustrated is a spring but could be any suitable element. The strain relief mechanism 106 is also connected between the button 101 and the housing because it is connected direct to the button 101 and indirectly to the housing via the intermediate structure 109 and the SMA wire 105. In this arrangement the connection between the strain relief mechanism and the SMA wire 105 is indirect, e.g. via the intermediate structure. The SMA actuator also comprises a restoring element 108 in the form of a resilient element (as illustrated a spring) which is attached between the intermediate structure 109 and an inner surface of another portion of the button housing 103. In other words, the restoring element 108 is connected, albeit indirectly to the button.

The intermediate structure 109 comprises an elongate body which extends along the length of the wire and comprises two protrusions, one at either end of the body. A first protrusion extends into the inside of the button and the strain relief mechanism 106 is attached to the first protrusion. A second protrusion at the opposed end projects from the opposed surface of the body and the SMA wire 105 is attached to the second protrusion. The intermediate structure may thus be considered to have a "dog-leg" construction. The body is a similar length to the width of the button. As the amount of movement generated by an SMA wire is proportional to its length, the intermediate structure is used to give a longer length of the wire by using the whole width of the button even though the button is compact in size.

The functionality of the SMA actuator is similar to that described above. Thus, when the button is activated, electrical current is fed into the wire 105, which heats up due to the resistance of the wire. Upon heating the SMA wire contracts, pulling the intermediate structure 109 and hence the button 101 towards the housing 102 to which the SMA wire is connected (i.e. from a first position to a second position). In normal circumstances when the button is pressed with a standard pressure, the force required to slide the button under the finger to give the haptic effect is less than the force that can be easily generated by the SMA wire 105 and also less than the force used to bias the strain relief mechanism 106. The strain relief mechanism 106 therefore does not move, i.e. the spring does not extend, and all of the force generated by the SMA wire 105 is used to pull the button 101.

If a high force is used to press the button 101, the button may be temporarily or permanently immobilised. In these circumstances, the force exerted on the strain relief mechanism 106 exceeds the bias force causing it to move, i.e. for the spring to stretch in this arrangement. The intermediate piece 109 moves towards the surface of the portion of the housing 102 to which the SMA wire is attached, allowing the SMA wire to contract in response to being heated. Thus, even though the button 101 itself does not move, the SMA wire is prevented from snapping, or being over strained and damaged.

In both normal and high force use, the movement of the intermediate structure 109 extends the restoring element 108. Thus, as the SMA wire 105 cools, the restoring element 108 between the intermediate piece and the housing 103 returns the button 101 to its original position. It is possible that the strain relief mechanism 106 may also contribute to returning the button 101 to its original position. However, this will generally only occur if the wire has exceeded its normal limits and has engaged the strain relief mechanism.

Another example of a haptic button assembly incorporating an endstop as described in Figures 2A and 2B is shown in Figure 6. Like the arrangement of Figure 5, the haptic button assembly is compact but in the Figure 6 arrangement, the restoring element is in the form of a compression spring rather than an extension spring as used in Figure 5. The haptic button device comprises a moveable portion in the form of a button 61 and a static portion in the form of housing 63 for the button 61. The SMA actuator comprises an SMA wire 62 which is connected between the button 61 and the housing 63. In this arrangement, the SMA wire 62 is connected direct at a first end to the inner surface of one part of the housing 63 and indirectly to the button 61 via an intermediate structure 64 which is connected at the opposite end and a strain relief mechanism 66 which as illustrated is a spring but could be any suitable element. The strain relief mechanism 66 is also connected between the button 61 and the housing because it is connected direct to the button 61 and indirectly to the housing via the intermediate structure 64 and the SMA wire 62. The SMA actuator also comprises a restoring element 65 in the form of a resilient element (as illustrated a spring) which is attached between the intermediate structure 64 and an inner surface of another portion of the button housing 63. In other words, the restoring element 65 is connected, albeit indirectly to the button. The SMA wire 62 is connected directly at one end to the housing 63 and indirectly to the button 61 via an intermediate component 64 which is connected at the opposite end and a strain relief mechanism 66 which as illustrated comprises a spring but could be any suitable element. The SMA wire 62 is configured so that it moves the intermediate component 64 when it is heated and contracts. The SMA actuator also comprises a restoring element 65 in the form of a compression bias spring which acts on the intermediate component 64 to extend the SMA wire 62 when it is cold and to return the haptic button 61 to its initial position.

The intermediate component 64 may be considered to be part of the strain relief mechanism because the spring presses the intermediate component 64 against an inner surface of the button 61 and a protrusion from the housing 63. Thus, an endstop 67 is formed.

In normal operation when the wire is heated it contracts. This pulls the intermediate component 64 to the portion of the housing 63 to which the SMA wire is attached, compressing the restoring element 65 (i.e. compressing the bias spring). When the lateral force that the user's finger applies to the button is less than the force that the strain relief mechanism applies to the intermediate component 64, the button 61 and strain relief mechanism will also be pulled towards the portion of the housing 63 to which the SMA wire 62 is attached. No change to the strain relief mechanism 66 will thus occur.

In the case that the button 61 is constrained not to move (i.e. immobilised) or if the user applies a force to the button 61 in a direction away from the portion of the housing 63 to which the SMA wire is attached that is greater than the force that the strain relief mechanism applies to the intermediate component 64 then the button 61 will not move. Instead, the intermediate component 64 will move towards the portion of the housing 63 to which the SMA wire is attached and will compress the spring in the strain relief mechanism 66 (or otherwise bias the strain relief mechanism if another arrangement is used). This means that the wire 62 will still be able to contract even if the button 61 is not allowed to move, thus the tension in the wire 62 is limited by the strain relief mechanism 66 which will prevent wire damage. An issue with the above examples is that two springs (or similar resilient elements) are required in the actuator. One spring is part of the restoring element and provides the bias force to lengthen the SMA wire and restore the moveable portion to its first position. The second spring is within the strain relief mechanism and only plays a role if the wire tension exceeds a certain value, i.e. when the moveable portion is immobilised or otherwise not free to move. The following examples illustrate how the functions of the two springs may be combined to use a single spring with a pivoting element.

In these arrangements as described in more detail below, there is an SMA wire which extends between a static portion and an intermediate portion, a spring which also extends between the static portion and the intermediate portion, an actuated portion which is in contact with the intermediate portion and a pivot where the wire, the spring, the contact point between the actuated portion and the intermediate portion, and the pivot are arranged so that the intermediate portion rotates about the pivot in normal use, and if the force required to move the moving portion exceeds some value the intermediate portion stops rotation about the pivot and thus allows the wire to continue contracting. In this arrangement, the resilient means between the wire and the button is formed of the intermediate portion which is able to rotate. Furthermore, in this arrangement, if the force exceeds a certain value then the moving portion loses contact with the pivot and moves laterally instead of purely rotating about the pivot which thus allows the wire to continue contracting.

Three arrangements are shown in Figures 7, 8, 9A and 9B. Although these are shown in the context of haptic devices, it will be appreciated that other contexts, e.g. latches or other devices, are also applicable. In each of the Figures below, the frame of reference is the static portion. However, it will be appreciated that alternative versions which are in the frame of reference of the moving portion may be constructed. If the frame of reference is adjusted, effectively the moving portion and the static portion are exchanged. The wire is thus between the moving portion and the intermediate portion and the pivot is between the intermediate portion and the moving portion. There is also a contact point between the static portion and the intermediate portion. Figure 7 shows a haptic button device comprising a haptic button 71 which acts as the moving portion and which moves relative to a static portion which is the housing 73 of the device. The SMA actuator comprises at least one SMA wire 72 which is connected between the button 71 and the housing 73. As illustrated in this example, the SMA wire 72 is connected at one end to the static portion 73 and at the other end to an intermediate portion 74 (the term intermediate portion may be used interchangeably with intermediate component) which connects it to the button 71 by contacting an outer surface of the button 71. As explained in more detail below, the intermediate portion 74 forms a strain relief device. A restoring element 75 in the form of a spring (as illustrated) is also connected to the button 71 indirectly via the intermediate portion 74 which is connected to one end of the restoring element 75. The restoring element 75 is connected at the opposed end to the static portion 73. As illustrated, the housing 73 is provided with a protrusion to which one end of the restoring element 75 is connected. The SMA wire 72 is connected direct to an inner surface of the housing 73 rather than to the protrusion. This allows the length of the SMA wire 72 to be maximised to provide the necessary movement. On an opposed inner surface of the housing adjacent the intermediate portion, there is another protrusion which acts as a pivot 76 for the intermediate portion.

The intermediate portion 74 is thus in contact with each of the pivot 76, the restoring element 75, the SMA wire 72 and the button 71. The pivot 76 is between the intermediate portion and the static portion. At each point of contact, there are forces acting on the intermediate portion 74 as follows: a point of force from the pivot 76, a point of force 77 from the contact with the restoring element 75, a point of force 78 from the contact with the SMA wire 72 and a point of force 79 from the contact with the button 71. In normal operation, when the wire 72 contracts the intermediate portion 74 rotates around the pivot 76, compressing the restoring element 75 by pushing against the point of force 77 and moving the button 71 from the first position to a second position by pushing against the point of force 179. As the wire 72 cools, the restoring element 75 expands and returns the button 71 and the intermediate portion 74 to the original positions. Pivot 76 may be considered a first pivot. If the force required to move the button 71 exceeds a certain value, i.e. if the button is immobilised, the intermediate portion 74 is still able to move. However, in this case, the intermediate portion 74 will rotate around the point of contact 79 with the button 71 in preference to moving the button and so the button will not move. The point of contact 79 may thus be considered to be acting as a second pivot. As the SMA wire 72 contracts, the movement of the intermediate portion applies a force at the point of force 77 which compresses the restoring element 75 and moves the intermediate portion 74 away from the pivot 76 as illustrated schematically by the dotted lines. In other words, the arrangement may comprise a first pivot on the static portion (pivot 76 on the housing) and a second pivot on the moveable portion (point 77 on the button) and the intermediate portion is rotatable about the first pivot to move the moveable portion from the first position to the second position and the intermediate portion is rotatable about the second pivot to activate the strain relief mechanism.

Figure 8 illustrates a variation of the arrangement shown in Figure 7. In Figure 8, the haptic button device comprises a haptic button 171 and a housing 173. The SMA actuator comprises at least one SMA wire 172 which is connected between the button 171 and the housing 173. As in Figure 7, the SMA wire 172 is connected at one end to the static portion 173 and at the other end to an intermediate portion 174 which connects it to the button 171. However, in the arrangement of Figure 8, the intermediate portion 174 contacts an inner surface of the button 171 rather than the outer surface as shown in Figure 7. A restoring element 175 in the form of a spring (as illustrated) is also connected to the button 171 indirectly via the intermediate portion 174. The restoring element 75 is connected at the opposed end to the static portion 73. In this arrangement, the SMA wire 172 is connected direct to an inner surface of the housing 173 below the protrusion to which the restoring element 175 is attached rather than above the protrusion as shown in Figure 7.

As in Figure 7, the intermediate portion 174 is thus in contact with each of the pivot 176, the restoring element 175, the SMA wire 172 and the button 171. The pivot 176 is between the intermediate portion and the static portion. At each point of contact, there are forces acting on the intermediate portion 174 as follows: a point of force from the pivot 176, a point of force 177 from the contact with the restoring element 175, a point of force 178 from the contact with the SMA wire 172 and a point of force 179 from the contact with the button 171. The order of the different points of force is different in Figure 8 to that shown in Figure 7. In Figure 8, the point of force from the pivot 176 is adjacent the point of force 179 from the contact with the button 171 rather than at opposed ends of the intermediate portion 174 as shown in Figure 7.

In normal operation, when the wire 172 contracts the intermediate portion 174 rotates around the pivot 176, compressing the restoring element 175 and moving the button 171. As the wire 172 cools, the restoring element 175 expands and returns the button 171 and the intermediate portion 174 to the original positions. If the force required to move the button 171 exceeds a certain value, i.e. the button is immobilised or otherwise not free to move, the intermediate portion 174 will rotate around the point of contact 179 with the button 171. As the SMA wire 172 contracts, the intermediate portion 174 away from the pivot 176 as illustrated schematically by the dotted lines. In other words, the arrangement may comprise a first pivot on the static portion (pivot 176 on the housing) and a second pivot on the moveable portion (point 179 on the button) and the intermediate portion is rotatable about the first pivot to move the moveable portion from the first position to the second position and the intermediate portion is rotatable about the second pivot to activate the strain relief mechanism.

Figures 9A and 9B illustrate another variation of the arrangement shown in Figure 7. In Figures 9A and 9B, as before, the haptic button device comprises a haptic button 271 and a housing 273. Similarly, the SMA actuator comprises at least one SMA wire 272 which is connected between the button 271 and the housing 273. As in Figure 7, the SMA wire 272 is connected at one end to the static portion 273 and at the other end to an intermediate portion 174 which connects it to the button 271. However, in the arrangement of Figures 9A and 9B, the intermediate portion 274 contacts an inner surface of the button 271 rather than the outer surface as shown in Figure 7. A restoring element 275 in the form of a spring (as illustrated) is also connected to the button 271 indirectly via the intermediate portion 274. The restoring element 275 is connected at the opposed end to the static portion 273. In this arrangement, the SMA wire 272 is connected direct to the opposed inner surface of the housing 273 than that of the SMA wire 272. Accordingly, a protrusion extends from the inner surface of the housing 273 to which the SMA wire 272 is attached toward the protrusion to which the restoring element 275. The pivot 276 is formed on the protrusion extending from the inner surface of the housing 273 to which the SMA wire 272 is attached.

As in Figures 7 and 8, the intermediate portion 274 is thus in contact with each of the pivot 276, the restoring element 275, the SMA wire 272 and the button 271. The pivot 276 is between the intermediate portion and the static portion. At each point of contact, there are forces acting on the intermediate portion 274 as follows: a point of force from the pivot 276, a point of force 277 from the contact with the restoring element 275, a point of force 278 from the contact with the SMA wire 272 and a point of force 279 from the contact with the button 271. The order of the different points of force is different again to those shown in Figures 7 and 8. In Figure 9A and 9B, the point of force from the SMA wire is at the opposed end to the point of force from the restoring element.

Figure 9A illustrates the first or rest position and in normal operation, when the wire 272 contracts the intermediate portion 274 rotates around the pivot 276, compressing the restoring element 275 and moving the button 271. As the wire 272 cools, the restoring element 275 expands and returns the button 271 and the intermediate portion 274 to the original positions. If the force required to move the button 271 exceeds a certain value, as shown in Figure 9B the intermediate portion 274 will rotate around the point of contact 279 with the button 271. As the SMA wire 272 contracts, the intermediate portion 274 is pulled away from the pivot 276 and the restoring element 275 contracts. As the wire 272 cools, the restoring element 275 expands and returns the button 271 and the intermediate portion 274 to the original positions shown in Figure 9A. In each arrangement the maximum force transmitted to the button and the force that is applied to the wire by the restoring force when it extends can be controlled by adjusting the relative positions of the points of force. Furthermore, in each arrangement a single resilient element in conjunction with the intermediate portion is effectively providing both a bias force to restore the button to its original element and allows the SMA wire to contract when the button is immobilised.

Another example structure is shown in Figures 10A and 10B which show a button structure that uses a flexible printed circuit (FPC) 204 as an intermediate structure. The button structure comprises a moveable portion in the form of a button 201 and a cover and a static portion in the form of a housing 202,203. The shape memory alloy actuator comprises a SMA wire 209 placed in a U shape around the button 201 and attached at either end of the SMA wire 209 to the FPC 204 via two crimps 212 and 213. The SMA actuator also comprises a strain relief element 206, 208 in the form of two bias springs which are attached to the FPC via two crimps 210, 211. The strain relief mechanism is thus indirectly connected to the SMA wire via the FPC. The button is also attached to the button housing 202 via a restoring element 205, 207 in the form of a pair of resilient means which return the button 201 to its original position if it has been moved by activating the SMA wire 209.

The FPC 204 contains two slots that arranged around two posts 214, 215 that guide the FPC if it is moved. The posts project from the housing. The SMA wire is thus connected between the housing and the button; the connection to the button is direct and the connection to the housing is via the FPC and posts. When the button 201 is pressed with normal force, current is fed through the SMA wire 209, and the button is drawn towards a second part of the housing 203. The button is returned to its original position by the restoring element 205 and 207 after the SMA wire 209 cools. If the button has been pressed with sufficient force that it is immobilised, then the bias force in the two bias springs 206, 208 is overcome and the springs stretch allowing the FPC 204 to move thus protecting the SMA wire 209 from being over strained.

Another example structure is shown in Figure 11, again using an FPC 416 as an intermediate structure within the device. As in Figure 10A, the button structure comprises a moveable button 401 and a static housing 402, 403. In this arrangement, the FPC 416 extends beyond the housing at one side. The shape memory alloy actuator comprises a SMA wire 405 placed in a U shape around the button 401. The strain relief element comprises two bias springs 408, 406 which are attached via two crimps 407, 409 to the FPC 416 and which are fixed to the housing 403, 404. As above, the restoring element 414, 415 comprises a pair of springs.

As in the previous example, the FPC 416 contains two slots that arranged around two posts 412, 413. When the button 401 is pressed with normal force, the SMA wire 405 contracts and the button 401 is drawn towards a second part of the housing 403 404. If the button has been pressed with sufficient force that it is immobilised, then the bias force in the two bias springs 406, 408 is overcome and the springs stretch allowing the FPC 416 to move. Having the FPC 416 extend beyond the housing means that movement of the FPC 416 compresses the springs 406, 408, rather than extending them as in previous examples.

It will be appreciated that a similar arrangement can be incorporated in the previous examples to use a compression spring rather than an extension spring to provide the strain relief mechanism. Similarly, it will be appreciated that in the examples above, the resilient means can operate between the wire and the static portion instead of between the button and the wire where it is illustrated in this position.

In each of the haptic devices, the above examples have described the movement of the button in a lateral direction and return movement under the action of a return spring, together with the strain relief device. The button device is used to transmit a haptic effect to the user, that is, to provide a tactile sensation to the finger pressing the button. In use, the haptic effect may be enhanced by moving the button back and forth multiple times for a single button press. If such an effect is desired, the control system moves the button the required multiple times after the single button press has been detected.

In the examples above, the strain relief device may be attached between the SMA wire and the button via a connecting piece OR between the SMA wire and the housing via a connecting piece. The strain relief device may consist of resilient means, e.g. a bias spring. The bias force in the spring may be set to between 10 and 90% of the breaking force of an SMA wire, more particularly, between 20 and 80% of the breaking force of an SMA wire and optionally between 60 and 80% of the breaking force of an SMA wire.

As an example, the force required to break a 25 micron wire made from a typical material such as nitinol (i.e. nickel-titanium alloy) is approximately 0.6N. It will be appreciated that the force will depend on the thickness and type of material and that these values are merely indicative. The force may also be expressed in terms of stress and may be between 0.9 and 1.9 Gigapascals.

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 present techniques, the present techniques 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 present techniques have a broad range of applications, and that the embodiments may take a wide range of modifications without departing from any inventive concept as defined in the appended claims.

Claims

1. A shape memory alloy, SMA, actuator for use in a device comprising a static portion and a moveable portion which is moveable relative to the static portion, the shape memory alloy actuator comprising

an SMA wire which is connectable between a static portion and a moveable portion of a device and which is arranged, by contraction, to drive movement of the moveable portion from a first position to a second position;

a restoring element which is connectable to the moveable portion to return the moveable portion to the first position from the second position; and

a strain relief mechanism which is connected to the SMA wire to reduce strain on the SMA wire.

2. The SMA actuator of claim 1, wherein the strain relief mechanism comprises a resilient element.

3. The SMA actuator of claim 2, wherein the resilient element has a biasing force which is between 10% to 90% of a breaking force which breaks the SMA wire.

4. The SMA actuator of claim 2 or claim 3, wherein the resilient element is connected direct to the SMA wire.

5. The SMA actuator of claim 2 or claim 3, wherein the resilient element is connected to the SMA wire via an intermediate component.

6. The SMA actuator of claim 5, wherein the intermediate component is an endstop which prevents movement of the resilient element when the force applied to the endstop by the SMA wire is below a threshold.

7. The SMA actuator of claim 5, wherein the intermediate component is a flexible printed circuit.

8. The SMA actuator of claim 5, wherein the intermediate component is an elongate structure which extends along at least part of the length of the SMA wire.

9. The SMA actuator of claim 1, wherein the strain relief mechanism comprises a rotatable intermediate component which is connected to the SMA wire whereby contraction of the SMA wire rotates the intermediate component from a rest position.

10. The SMA actuator of claim 9, wherein the restoring element is connected to the rotatable intermediate component to restore the rotatable intermediate component to the rest position.

11. The SMA actuator of any one of claims 1 to 10, wherein the strain relief mechanism comprises a sprung crimp connected to the SMA wire.

12. The SMA actuator of any one of claims 1 to 11, wherein the restoring element comprises a resilient element or at least one SMA wire.

13. A device comprising a static portion, a moveable portion and an SMA actuator as set out in any one of claims 1 to 12.

14. The device of claim 13, wherein the strain relief mechanism is attached to both the SMA wire and the moveable portion.

15. The device of claim 13, wherein the strain relief mechanism is attached to both the SMA wire and the static portion.

16. The device of claim 13, when dependent on claim 9, comprising a first pivot on the static portion and a second pivot on the moveable portion, wherein the intermediate component is rotatable about the first pivot to move the moveable portion from the first position to the second position and the intermediate component is rotatable about the second pivot to reduce strain on the SMA wire.

17. The device of claim 16, wherein the first and second pivots contact opposed ends of the intermediate component.

18. The device of claim 16 or claim 17, wherein the SMA wire and the restoring element are connected to the intermediate component between the contacts for the first and second pivots.

19. The device of claim 18, wherein the SMA wire is connected to the intermediate component adjacent the first pivot.

20. The device of claim 16, wherein the first and second pivots contact the intermediate component at adjacent points on the intermediate component.

21. The device of claim 20, wherein the SMA wire is connected to one end of the intermediate component and the first pivot contacts the intermediate component at the opposed end.

22. The device of claim 20, wherein the SMA wire and the restoring element are connected to the intermediate component either side of the first and second pivots.

23. The device of any one of claims 13 to 22, wherein the static portion is a housing and the moveable portion is a button having a pressable surface whereby the device is a button assembly which is used to produce a haptic effect.

24. The device of claim 23, wherein multiple movements of the button relative to the housing are produced in response to a single button press.

25. The device of any one of claims 13 to 22, wherein the static portion is a chassis and the moveable portion is a mass whereby the device is a latch.

26. The device as claimed in any one of claims 13 to 22, where the device is any one of: a smartphone, a camera, a foldable smartphone, a foldable image capture device, a foldable smartphone camera, a foldable consumer electronics device, a foldable or flexible display screen/display device, an image capture device, a 3D sensing device or system, a servomotor, a consumer electronic device, a domestic appliance, a mobile or portable computing device, a mobile or portable electronic device, a laptop, a tablet computing device, an e-reader, a computing accessory or computing peripheral device, a security system, a medical device, a gaming system, a gaming accessory, a robot or robotics device, an audio device, an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a haptics device, a wearable device, a drone, an aircraft, a spacecraft, a submersible vessel, a vehicle, an autonomous vehicle, a tool, a surgical tool, a remote controller, clothing, a switch, dial or button, a display screen, a touchscreen, and a near field communication (NFC) device.