Haptic device

Abstract

A haptic device 10 has a plurality of haptic elements 11, each haptic element 11 belonging to one of a plurality of first groups 21 and one of a plurality of second groups 22. A plurality of first sha

Description

Field The present application relates to a haptic device, in particular a haptic array mechanically addressed by shape memory alloy (SMA) elements. Background A haptic device interacts with a person to make the person experience touch, i.e. to emulate the sensation of touching. The interaction may be the application of a motion such as a vibration or a force to the person. An example of a haptic device is a haptic button that can provide a stimulus to a finger of a person. This emulates, to an extent, the sensation of touching an object. The present invention is concerned with improving the extent to which a haptic device can emulate the sensation of touching an object and / or moving a finger over an object. Summary According to an aspect of the present invention, there is provided a haptic device comprising: a plurality of haptic elements arranged in a plurality of first groups and a plurality of second groups such that each haptic element belongs to a first group and a second group; a plurality of first shape memory alloy, SMA, elements associated with respective first groups of haptic elements; and a plurality of second SMA elements associated with respective second groups of haptic elements; wherein the first and second SMA elements are arranged, on activation of a first SMA element and a second SMA element, to actuate an addressed haptic element more than the other haptic elements, the addressed haptic element being a haptic element that belongs to the first group associated with the activated first SMA element and to the second group associated with the activated second SMA element. By providing a plurality of haptic elements that can each be actuated, it is possible to provide a spatially varying sensation. This improves the extent to which the haptic device can emulate the sensation of touching an object or moving a finger over an object. The haptic device may be able to simulate texture of a virtual object, for example. By providing that the haptic elements are actuated by activating a combination of first and second SMA elements which correspond to groups of the haptic elements, the haptic elements may be positioned closer to each other. The array of haptic elements may have a finer pitch. This reduces the size and coarseness of the haptic array. In addition it reduces the number of different elements that need to be activated in order to actuate the haptic elements. Optionally, the first and second SMA elements are arranged to actuate the addressed haptic element by adding forces applied by the activated first SMA element and the activated second SMA element. By adding forces in order to actuate haptic elements, the haptic elements are mechanically addressable. It is not necessary to provide each haptic element with a unique SMA element. Instead, one or more SMA elements in each array are activated and the haptic element associated with the combination of SMA elements is actuated such that the force induced by the activated SMA elements is added. Additionally, by actuating the haptic elements by adding forces, the construction of the haptic device may be kept relatively simple. The number of different components of the haptic device may be kept low. The haptic device may optionally be configured such that for haptic elements where forces are not added, there is little or no stimulus provided by that haptic element. This may improve the fineness by which the haptic elements of the haptic device may be controlled. The haptic elements may be controlled more independently of each other. Optionally, the first and second SMA elements are arranged to actuate the addressed haptic element by adding the movements induced by the activated first SMA element and the activated second SMA element. By adding movements in order to actuate haptic elements, the haptic elements are mechanically addressable. It is not necessary to provide each haptic element with a unique SMA element. Instead, one or more SMA elements in each array are activated and the haptic element associated with the combination of SMA elements is actuated such that the motion induced by the activated SMA elements is added. Optionally, the haptic device comprises: a support structure; wherein the first and second SMA elements are arranged, on activation, to actuate the addressed haptic element relative to the support structure. By providing the support structure, the haptic device may be provided with a static base. This helps to make the haptic device stable. The support structure may make it easier for the haptic device to be incorporated into a large apparatus, for example by being mounted or secured to another component of the apparatus. Optionally, the haptic device comprises: an intermediate component comprising portions associated with respective first groups of haptic elements, wherein the first SMA elements are arranged, on activation, to actuate the portions of the intermediate component associated with the respective associated first groups of haptic elements relative to the support structure. By providing an intermediate component, the effects of the first and second SMA elements may be separated such that the movements induced by them may be added together in the haptic elements. The intermediate component allows the activation of the first SMA elements to be faithfully translated into a motion. This motion can be combined with the effect of the activation of one or more second SMA elements. Optionally, the intermediate component is arranged such that movement of the portions induces movement of the respective associated first groups of haptic elements. The intermediate component allows the activation of the first SMA elements to be faithfully translated into actuation of the corresponding haptic elements. This may help the actuation of the haptic elements to be more precisely controlled. The movement of the haptic elements may be made sensitive to the activation of the first SMA elements. Optionally, the second SMA elements are arranged, on activation, to actuate the respective associated second groups of haptic elements relative to the intermediate component. The intermediate component allows the activation of the second SMA elements to be faithfully translated into actuation of the corresponding haptic elements. This may help the actuation of the haptic elements to be more precisely controlled. The movement of the haptic elements may be made sensitive to the activation of the second SMA elements. Optionally, the intermediate component comprises a plurality of thickness-varying elements corresponding to respective haptic elements, arranged such that movement of the haptic elements relative to the support structure along an actuation axis is dependent on a position of the corresponding thickness-varying element relative to the support structure in a plane nonparallel to the actuation axis. By providing thickness-varying portions, the extent to which the haptic elements are moved for a given activation of SMA elements may be controlled by controlling the topography of the thickness-varying elements. For example if desired, small movements of the haptic elements may be more accurately controlled. The SMA elements may indirectly induce movement of the haptic elements via the thickness-varying elements. The SMA elements may be located away from the haptic elements, if desired. This increases the design freedom affecting the shape of the volume taken up by the haptic device. Optionally, the first SMA elements are arranged, on activation, to actuate the thickness-varying elements corresponding to the respective associated first groups of haptic elements in a first direction within the plane; and / or the second SMA elements are arranged, on activation, to actuate the thicknessvarying elements corresponding to the respective associated second groups of haptic elements in a second direction within the plane and different from the first direction. The SMA elements may be disposed so as to act in a horizontal direction. This may simplify the arrangement of the SMA elements. Optionally, the haptic device comprises: a compliant structure; wherein the first and second SMA elements are arranged, on activation, to induce compliance of the compliant structure so as to actuate the addressed haptic element. By providing a compliant structure, the amplification of the haptic elements may be enhanced. Nonlinearities in the behaviour of the compliant structure may be taken advantage of so as to allow individual haptic elements to be actuated more independently from nearby haptic elements. Optionally, the compliant structure comprises: a support layer comprising support portions for respective first groups of haptic elements, the first SMA elements arranged, on activation, to actuate the respective support portions; and a deflection layer comprising deflection portions for respective first groups of haptic elements, the support portions arranged, on actuation, to deflect the respective deflection portions. By providing deflection portions, the haptic device may be configured such that deflection of a deflection portion acts as a trigger for allowing corresponding haptic elements to be actuated. This helps to allow activation of SMA elements without resulting in actuation of all associate haptic elements. The actuation of individual haptic elements may be controlled more independently. Optionally, the haptic elements are secured to the deflection portions. By providing that the haptic elements are secured to the deflection portions, the actuation of an addressed haptic element may be enhanced when the appropriate combination of SMA elements is activated. Optionally, the second SMA elements are arranged, on activation, to actuate a haptic element by an extent dependent on the extent to which a deflection portion to which the haptic element is secured is deflected. The deflection portion may be deflected so as to trigger the possibility of particular haptic elements being actuated. The independence of the haptic elements from each other may be increased. Optionally, the first and second SMA elements are arranged such that the second SMA elements, on activation, apply a greater force and / or movement to any haptic elements belonging to a first group associated with a activated first SMA element than to other haptic elements. Accordingly, the motion of the SMA elements that are activated may be more focused on one or more specific haptic elements. Optionally, the first and second SMA elements are arranged such that the extent to which the second SMA elements, on activation, apply a force and / or movement to a haptic element is dependent on the extent to which a first SMA element associated with a first group to which the haptic element belongs is activated. Accordingly, the motion of the haptic elements that do not correspond to an activated combination of SMA elements can be reduced relative to the motion of the haptic element that does correspond to the activated combination of SMA elements. Optionally, the haptic elements are arranged in a two-dimensional (2D) array, the first groups being rows and the second groups being columns. By providing a 2D array, the extent to which the haptic device can emulate the sensation of moving a finger over an object can be increased. Optionally, the first SMA elements are arranged parallel to each other and / or the second SMA elements are arranged parallel to each other. Optionally, the first SMA elements intersect the second SMA elements when viewed in a direction perpendicular to a plane in which the first SMA elements are arranged. Optionally, the haptic elements overlap where the first and second SMA elements intersect when viewed in a direction perpendicular to a plane in which the first SMA elements are arranged. By arranging the SMA elements in a parallel way, the haptic elements may be associated with the intersection of a first SMA element and a second SMA element. This allows a simple way of addressing each haptic element of the haptic device. Optionally, the haptic device comprises resilient elements arranged to bias respective haptic elements against forces applied by the first and second SMA elements. By providing resilient elements, the positions of the haptic elements within the haptic device may be maintained. The possibility of crosstalk between an addressed haptic element and other haptic elements in the same group may be reduced. Optionally, the resilient elements are arranged to bias the respective haptic elements by a force greater than a maximum force applicable by only the associated first SMA element and greater than a maximum force applicable by only the associated second SMA element and less than a maximum force applicable by a combination of the associated first SMA element and the associated second SMA element. Accordingly, the haptic elements may be controlled more independently of each other. This increases the accuracy of the haptic stimulus provided by the haptic device. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a haptic device; Figure 2 is a schematic plan view of the haptic device of Figure 1; Figure 3 is a schematic side view of another haptic device; Figure 4 is another schematic side view of the haptic device of Figure 3; Figure 5 is a schematic plan view of another haptic device; Figure 6 is a schematic plan view of the haptic device of Figure 5 when SMA elements are activated; Figure 7 is a schematic plan view of another haptic device; Figure 8 is a schematic plan view of the haptic device of Figure 7 when an SMA element is activated; Figure 9 is a schematic plan view of the haptic device of Figure 7 when two SMA elements are activated; Figure 10 is a diagram illustrating a feature of a haptic device; Figure 11 is a further diagram illustrating the feature illustrated in Figure 10; Figure 12 is a schematic side view of another haptic device; Figure 13 is a schematic side view of the haptic device of Figure 12 when an SMA element is activated; and Figure 14 is a schematic plan view of the haptic device of Figure 12. Detailed description Array of haptic elements Figure 1 is a schematic side view of a haptic device 10. As shown in Figure 1, the haptic device 10 comprises a plurality of haptic elements 11. A haptic element 11 is configured to be actuated. When a haptic element 11 is actuated, the haptic element 11 applies a movement and / or force to a subject, for example to a finger of a person. The haptic elements 11 may be arranged in an array. The array may be a 2D array. The haptic elements 11 may be referred to as pixels or movable elements. The array may be a miniature array, with lateral dimensions (in a direction orthogonal to an actuation axis 13) of less than 3cm by 3cm, preferably less than 2cm by 2cm, optionally less than 1.5cm by 1.5cm. Figure 1 shows an actuation axis 13. The haptic elements 11 are arranged such that their primary direction of actuation is along the actuation axis 13. The haptic elements 11 may move a small amount in directions perpendicular to the actuation axis 13. However, the intended movement of the haptic elements 11 upon actuation is along the actuation axis 13. Upon movement of select haptic elements 11 along the actuation axis 13, haptic feedback may be provided to a user. Haptic feedback may enhanced and more varied compared to an actuator in which only a single haptic button is provided. Haptic feedback may simulate different textures of a virtual object, for example. The haptic elements 11 are arranged adjacent to each another in directions perpendicular to the actuation axis 13. As shown in Figure 1, optionally the haptic device 10 comprises a support structure 12. The support structure 12 may be referred to as a static part or a static component. The support structure 12 is used herein as a reference structure, relative to which movement of the haptic elements 11 and other movable components is described. However, in general the support structure 12 may itself be movable relative to a larger device in which the haptic device 10 is integrated. Optionally, the support structure 12 forms the base of the haptic device 10. As shown in Figure 1, optionally the support structure 12 comprises a base 14. The haptic elements 11 may be located above the support structure 12, i.e. the haptic elements 11 may be located on one side of the support structure 12 along the actuation axis 13. In use of the haptic device 10, the orientation of the haptic device 10 may change such that the support structure 12 is above the haptic elements 11. However, for clarity, the haptic device 10 is described primarily in the orientation shown in Figure 1. The haptic device 10 comprises a plurality of first SMA elements 31 and a plurality of second SMA elements 32. The first and second SMA elements 31, 32 are described herein also as first and second SMA wires 31, 32. The first and second SMA wires 31, 32 are arranged, on activation, to actuate one or more addressed haptic elements 11, optionally relative to the support structure 12. This is described in more detail below. By providing the support structure 12, the haptic device 10 may be provided with a static base. This helps to make the haptic device 10 stable. The support structure 12 may make it easier for the haptic device 10 to be incorporated into a larger apparatus, for example by being mounted or secured to another component of the apparatus. As shown in Figure 1, optionally the haptic elements 11 are arranged in a layer. So, the haptic elements 11 may overlap when viewed orthogonally to the actuation axis 13. The haptic elements 11 may be arranged such that their upper surfaces are in a plane when the haptic elements 11 are not actuated. Optionally, the haptic elements 11 can be actuated individually. Optionally, the haptic device 10 is or comprises a haptic button. The haptic device 10 may be arranged (e.g. shaped and sized) such that a human finger can be moved over it. The haptic device is configured to emulate the sensation of touching an object or moving a finger over an object. By providing a plurality of haptic elements 11 that can each be actuated, it is possible to provide a spatially varying sensation. This improves the extent to which the haptic device 10 can emulate the sensation of touching an object or moving a finger over an object. For example, the texture of a virtual object may be simulated using the haptic device 10. The haptic elements 11 are arranged in a plurality of first groups 21 and a plurality of second groups 22. Each haptic element 11 belongs to a first group 21 and a second group 22, in particular to a single first group 21 and a single second group 22. Accordingly, each haptic element 11 is addressable by a combination of a first group 21 and a second group 22. Each haptic element 11 may correspond to a unique combination of a first group 21 and a second group 22. Figure 1 shows the haptic elements 11 belonging to one first group 21. Only one first group 21 is shown in Figure 1. The haptic device 10 comprises a plurality of first groups 21. Optionally each first group 21 has a plurality of haptic elements 11 belonging to it. Alternatively, one or more of the first groups 21 may have only one haptic element 11 belonging to it. The haptic elements 11 shown in Figure 1 belong to different second groups 22. The haptic device 10 comprises a plurality of second groups 22. Optionally each second group 22 has a plurality of haptic elements 11 belonging to it. Alternatively, one or more of the second groups 22 may have only one haptic element 11 belonging to it. Figure 2 is a schematic plan view of the haptic device 10 shown in Figure 1. As shown in Figure 2, the haptic device 10 comprises a plurality of first SMA elements 31 associated with respective first groups 21 of haptic elements 11. In Figure 2, the first SMA elements 31 are depicted horizontally, and the first groups 21 of haptic elements 11 correspond to the rows of SMA elements. The second SMA elements 32 are depicted vertically, and the second groups 22 of haptic elements 11 correspond to the columns of SMA elements. The first SMA elements 31 may be referred to as first SMA wires 31, and the second SMA elements 32 as second SMA wires 32. Figure 1 shows one first SMA wire 31 corresponding to the first group 21 shown in Figure 1. Figure 2 shows a plurality of first SMA wires 31. As shown in Figure 1, optionally the support structure 12 comprises a plurality of supporting elements 15. The supporting elements are configured to support the SMA wires 31, 32. The supporting elements 15 may be referred to as protrusions or posts. However, it is not essential for the supporting elements 15 to be arranged as protrusions as shown in Figure 1. Figure 1 shows the supporting elements 15 supporting a first SMA wire 31. The supporting elements may also support the second SMA wires 32. As shown in Figure 1, optionally the supporting elements 15 are configured to support the SMA wire 31 to be distanced from the base 14. The supporting elements 15 may be configured to support the SMA wire 31 to have an undulating shape. The SMA wire 31 may take the form of a wave. Portions of the SMA wire 31 that are directly supported by the supporting elements 15 may be distanced further from the base 14. Intermediate portions of the SMA wire 31 between the supporting elements 15 may be closer to the base 14, for example due to gravity or due to a biasing force urging the haptic elements 11 towards the base 14. So, in general terms, the support structure may comprise a plurality of contact portions making contact with the length of SMA wires 31, 32 on a first side of the length of SMA wire 31, 32 along the actuation axis 13. The haptic elements 11 may together comprise plural contact portions (e.g. one such contact portion per haptic element) making contact with the length of SMA wire 31, 32 on a second side of the length of SMA wire 31, 32 along the actuation axis 13. The second side is opposite to the first side. The contact portions of the support structure 12 and the contact portions of the haptic elements 11 may alternate in a direction normal to the actuation axis 13 and be relatively positioned so as to guide the length of SMA wire 31, 32 along a tortuous path. So, the haptic elements 11 and the support structure 12 may be driven in opposite directions (i.e. forces urging the haptic elements 11 and support structure 12 apart may be applied) along the actuation axis 13 on actuation of the of the length of SMA wire 31, 32. As shown in Figure 2, the haptic device 10 comprises a plurality of second SMA wires 32 associated with respective second groups 22 of haptic elements 11. Figure 1 does not show any second SMA wire 32. Figure 2 shows a plurality of second SMA wires 32. Optionally, an SMA element such as an SMA wire 31 is activated by being contracted. For example, optionally the haptic device 10 comprises a controller. The controller is configured to control activation of the SMA wires 31, 32. Optionally the controller is arranged to control an electric current applied to the SMA wires 31, 32, for example by applying pulse width modulated (PWM) control signals. SMA material has the property that on heating it undergoes a phase change that leads to actuation of the SMA element. When embodied by a wire, the SMA element may contract. So, when an electric current is applied to an SMA wire 31, 32, the SMA wire 31, 32 contracts. As can be appreciated from Figure 1, when the first SMA wire 31 is activated (e.g. contracted), the length of wire suspended between the supporting elements 15 reduces. This causes the first SMA wire 31 to apply a force on the haptic elements 11 supported by the first SMA wire 31. The magnitude of the force may depend on the extent to which the first SMA wire 31 contracts. Optionally the controller is configured to control the force applied to the haptic elements 11 by controlling a control signal applied to the SMA wires 31, 32. Optionally, the force applied to the haptic elements 11 causes the haptic elements 11 supported by the first SMA wire 31 to be actuated, i.e. moved relative to the support structure 12. Alternatively, the force may not be sufficient to move the haptic elements 11. It may be necessary for a second SMA wire 32 to additionally be activated in order to move the addressed haptic element(s) 11. As shown in Figure 1, optionally the SMA wire 31 extends from the support structure 12, under the haptic element 11 (i.e. between the haptic element 11 and the support structure 12) and then returns to the support structure 12. The section of SMA wire 31 between supporting elements 15 may have a "V" shape. When the SMA wire 31 is heated it contracts and drives the haptic element 11 along the actuation axis 13 to deliver haptic feedback. Optionally, the first and second SMA wires 31, 32 are arranged, on activation of a first SMA wire 31 and a second SMA wire 32, to actuate an addressed haptic element 11 more than the other haptic elements 11. In particular, the first and second SMA wires 31, 32 may be arranged to apply a greater force to the addressed haptic element 11 than to the other haptic elements 11, or may be arranged to move the addressed haptic element 11 by a greater amount than the other haptic elements 11, or both. The addressed haptic element is a haptic element 11 that belongs to the first group 21 associated with the activated first SMA wire 31 and to the second group 22 associated with the activated second SMA wire 32. When the SMA wires 31, 32 cease being activated, the haptic element 11 that was actuated may return along the actuation axis 13 to its original unactuated state. By providing that the haptic elements 11 are actuated by activating a combination of first and second SMA wires 31, 32 which correspond to groups of the haptic elements 11, the haptic elements 11 may be positioned closer to each other. This is because there may be no or less need for electrical traces between adjacent haptic elements 11. The array of haptic elements 11 may have a finer pitch. This reduces the size and coarseness of the haptic array. In addition, the number of different SMA elements that need to be activated in order to actuate the haptic elements 11 is reduced compared to a situation in which each haptic element 11 is associated with a dedicated SMA element. Each SMA wire 31, 32 has two electrical connections, so as to control the current applied to it. By providing the first SMA wires 31 and the second SMA wires 32 to address the haptic elements 11, it is not necessary to provide a unique drive connection for each haptic element 11 to enable it to be driven. Instead, each haptic element 11 may be driven by activating the appropriate combination of SMA wires 31, 32. This reduces the number of connecting tracks required in the haptic device 10. So, instead of electrically addressing the haptic elements 11, the haptic array according to the present invention is mechanically addressed. As shown in Figure 2, optionally the SMA wires overlap (when viewed in plan view, i.e. along the actuation axis 13) with the haptic elements 11 of the group to which the SMA wire corresponds. However, this is not necessarily the case. The SMA wire corresponds to the group of haptic elements 11 in the sense that the activation of the SMA wire can contribute to actuation of the haptic elements 11 of the group to which the SMA wire corresponds. Optionally, activation of an SMA wire is not sufficient for actuation of a haptic element 11 belonging to a group corresponding to the SMA wire. Optionally, activation of an SMA wire may be necessary for actuation of a haptic element 11 belonging to a group corresponding to the SMA wire. Optionally, activation of two SMA wires, one first SMA wire 31 and one second SMA wire 32 is necessary for actuation of the haptic element 11 belonging to the first group 21 that corresponds to the activated first SMA wire 31 and to the second group 22 corresponding to the activated second SMA wire 32. Alternatively, activation of an SMA wire is sufficient for actuation of a haptic element 11 belonging to a group corresponding to the SMA wire. The haptic elements 11 may be actuated to a greater extent when the SMA wires corresponding to both groups to which the haptic element 11 belongs are activated. The haptic elements 11 may be arranged in an array having a relatively small pitch. Optionally, the haptic elements 11 are arranged at regular intervals. The pitch may be maintained substantially constant across the haptic device 10. Alternatively, the haptic elements 11 may be arranged irregularly. As shown in Figure 2, optionally the haptic elements 11 are arranged in a 2D array such as a grid. Such an arrangement may allow for a wide range of patterns of actuated haptic elements 11. This may allow the haptic device 10 to flexibly generate a wide range of sensations on a user. The 2D array may be at least 3 by 3 array, for example, optionally at least a 4 by 4 array or a 5 by 5 array. By providing that the haptic elements 11 are addressable by a combination of first and second SMA wires 31, 32, the haptic elements 11 are actuatable by fewer connections and drive circuits, e.g. when compared to a device in which each haptic element 11 is provided with a unique electrical connection. As shown in Figure 2, optionally the SMA wires 31, 32 are arranged in a 2D array. This allows the haptic elements 11 to be simply addressed and individually actuated by activation of a combination of a first SMA wire 31 and a second SMA wire 32. As shown in Figure 2, optionally the haptic device 10 comprises a haptic element 11 associated with each intersection of two SMA wires 31, 32. However, as described in more detail below, it is not essential for the haptic elements 11 to be physically located at the intersections. The haptic elements 11 are addressable by activation of a combination of two SMA wires 31, 32. As shown in Figure 2, optionally the first SMA wires 31 are arranged to be substantially orthogonal to the second SMA wires 32. However, this is not essential. For example the angle between the first SMA wires 31 and the second SMA wires 32 when viewed in plan view (i.e. along the actuation axis 13) may be other than a right angle. Optionally, when a first SMA wire 31 and a second SMA wire 32 are activated (e.g. driven), the haptic sensation is greatest at the haptic element 11 associated with the intersection of the two driven SMA wires 31, 32. The haptic sensation is dependent on the actuation of the haptic element 11. As shown in Figure 2, optionally the haptic device 10 comprises resilient elements such as flexures 33. In Figure 2, flexures 33 to connect the haptic elements 11 to the surround are shown in some of the elements in the top left of the figure. Such flexures 33 may be provided for all of the haptic elements 11 across the haptic device 10. The flexures 33 are not shown across the whole haptic device 10 in Figure 2 so as not to obscure other features shown in Figure 2. As shown in Figure 2, optionally the flexures 33 are configured to connect the haptic elements 11 to the support structure 12, for example to the supporting elements 15. Optionally, the flexures 15 are compliant in a direction parallel to the actuation axis 13. This allows the haptic elements 11 to be actuated without significant interference from the flexures 33. The flexures 33 may improve the stability of the array of haptic elements 11 without unduly reducing the amplitude of actuation of the haptic elements 11. Optionally, the flexures 33 are configured to resist excess motion in the directions perpendicular to the actuation axis 13, e.g. in the plane across which the haptic elements 11 are arranged. The flexures 33 may help to maintain the position of the haptic element 11 in the directions perpendicular to the actuation axis 13. For example, the haptic elements 11 may be maintained at or near the centre of the recess in the support structure 12. The recess may be defined by the supporting elements 15. The flexures 33 may be metallic. The flexures 33 may function as springs. As shown in Figure 2, optionally the flexures 33 are secured to the support structure 12 (e.g. to the supporting elements 15) at connection points 34. Optionally, the flexures 33 (or other resilient element) may provide a biasing force opposing the force of the first and / or second SMA elements 31, 32. As such, the flexures 33 (or other resilient element) may return the haptic elements 11 to a non-actuated position upon ceasing activation of the SMA elements 31, 32. Adding forces As shown in Figure 2, each haptic element 11 does not have a unique SMA wire assigned to it. Instead, the SMA wires 31, 32 extend across the whole array. As shown in Figure 2, optionally the SMA wires 31, 32 are in two orthogonal (ID) arrays. Optionally, the first and second SMA wires 31, 32 are arranged to actuate the addressed haptic element 11 by adding forces applied by the activated first SMA wire 31 and the activated second SMA wire 32. One or more SMA wires 31, 32 in each array are driven and the haptic elements(s) 11 associated with where the SMA wires 31, 32 cross is driven such that the force from the driven wires in the two arrays is added. Figure 2 schematically depicts just one embodiment of a haptic device in which forces may be added to mechanically address haptic elements 11 in a haptic array. In alternative embodiments, the SMA wires need not be arranged in a wave or V-shape, but could apply forces otherwise directly or via an intermediate element. In general, the first SMA wire 31 and the second SMA wire 32 may be mechanically arranged in parallel for the addressed haptic element 11 so as to add the actuation forces. The actuation forces of the first and second SMA wires 31, 32 may additively combine at the addressed haptic element 11. The force applied on the haptic element 11 by the SMA wires 31, 32 may be the sum of the force due to the first SMA wire 31 and the force due to the second SMA wire 32. By adding forces in order to actuate haptic elements 11, the haptic elements 11 are mechanically addressable. It is not necessary to provide each haptic element 11 with a unique SMA wire. Instead, one or more SMA wires 31, 32 in each array are activated and the haptic element 11 associated with the combination of SMA wires 11 is actuated such that the force induced by the activated SMA elements is added. As shown in Figure 1, optionally the support structure 12 is at the bottom of the haptic device 10 and the haptic elements 11 are placed above the support structure 12. Optionally, the SMA wires 31, 32 are located between the haptic elements 11 and the support structure 12. The first SMA wires 31 may extend along a first direction and the second SMA wires 32 extend in a second direction as shown in Figure 2. Optionally, the SMA wires 31, 32 are continuous across the array of haptic elements 11. When a first SMA wire 31 is heated it delivers a vertical force to a row of haptic elements 11. When a second SMA wire 32 is additionally activated then the haptic element 11 where the wires cross receives a combination of the forces, i.e. more than the other haptic elements 11. For example, when the first and second SMA wires 31, 32 are arranged to apply the same amplitude of force as each other, then the addressed haptic element 11 receives double the force applied to other haptic elements 11 in the corresponding row or column. This allows the force to be addressed. By actuating the haptic elements 11 by adding forces, the construction of the haptic device 10 may be kept relatively simple. The number of different components of the haptic device 10 may be kept low. The haptic device 10 may optionally be configured such that for haptic elements 11 where forces are not added, there is little or no stimulus provided by that haptic element 11. This may improve the fineness by which the haptic elements 11 of the haptic device 10 may be controlled. The haptic elements 11 may be controlled more independently of each other. For example, the haptic device 10 may comprise resilient elements (e.g. the flexures 33) arranged to bias respective haptic elements 11 against forces applied by the first and second SMA elements 31, 32. The resilient elements may be arranged to bias the respective haptic elements 11 by a force greater than a maximum force applicable by only the associated first SMA element 31, and greater than a maximum force applicable by only the associated second SMA element 32. As such, when only one of the SMA wire 31, 32 is actuated, the biasing force (or pre-load force) of the resilient element may not be overcome. The bias force of the resilient element may be less than a maximum force applicable by a combination of the associated first SMA element 31 and the associated second SMA element 32. As such, the bias force may be overcome by the added force of both SMA elements 31, 32, so as to be preferentially actuated and move. Adding motions Figure 3 is a schematic side view of another haptic device 10. As shown in Figure 3, the haptic elements 11 do not have a unique SMA wire 31 assigned to it. The first SMA wires 31 (and the second SMA wires 32 as shown in Figure 4) are arranged to extend across the whole haptic device 10. Optionally, the first SMA wires 31 and the second SMA wires are arranged in two respective arrays, e.g. ID arrays. Optionally the arrays are arranged to be orthogonal to each other, for example when viewed along the actuation axis 13. One or more SMA wires 31, 32 in each array are driven and the haptic element(s) 11 associated with the combination(s) of activated SMA wires 31, 32 is actuated such that the motion induced by the actuated SMA wires 31, 32 in the two arrays is added. Optionally, the first and second SMA wires 31, 32 are arranged to actuate the addressed haptic element 11 by adding the movements induced by the activated first SMA wire 31 and the activated second SMA wire 32. By adding movements in order to actuate haptic elements 11, the haptic elements 11 are mechanically addressable. It is not necessary to provide each haptic element 11 with a unique SMA wire. Instead, one or more SMA wire 31, 32 in each array are activated and the haptic element 11 associated with the combination of SMA wires 31, 32 is actuated such that the motion induced by the activated SMA wires 31, 32 is added. As shown in Figure 3, the haptic device 11 comprises a support structure 12 (shown at the bottom of Figure 3). The support structure 12 may have the same features as described above with reference to Figure 1 and Figure 2. As shown in Figure 3, optionally the haptic device 10 comprises an intermediate component 40 (shown above the support structure 12 in Figure 3). Optionally the intermediate component 40 is flexible. The intermediate component may be arranged to be actuated by the first SMA wires 31. The view shown in Figure 3 is along the direction of the second SMA wires 32. As a result the second SMA wires 32 are not shown in Figure 3 (the second SMA wires 32 would extend into and out of the paper). One first SMA wire 31 is shown in Figure 3. When the first SMA wire 31 is activated then a portion 41 of the intermediate component 40 is actuated upwards. For example, as shown in Figure 3, the intermediate component 40 may comprises a plurality of struts 42. The struts 42 shown in Figure 3 are arranged to transmit force applied by activation of the first SMA wire to a portion 41 of the intermediate component 40 and optionally to the haptic elements 11 of the corresponding first group 21. As shown in Figure 3, optionally the first SMA wires 31 are located physically between the support structure 12 and the intermediate component 40. When the first SMA wire 31 is activated, the first SMA wire 31 applies a force to actuate part of the intermediate component 40 immediately above the activated first SMA wire 31. The intermediate component 40 comprises portions 41 associated with respective first groups 21 of haptic elements 11. The first SMA wires 31 are arranged, on activation, to actuate the portions 41 of the intermediate component 40 associated with the respective associated first groups 21 of haptic elements 11 relative to the support structure 12. By providing an intermediate component 40, the effects of the first and second SMA wires 31, 32 may be separated such that the movements induced by them may be added together in the haptic elements 11. The intermediate component 40 allows the activation of the first SMA wires 31 to be faithfully translated into a motion. This motion can be combined with the effect of the activation of one or more second SMA wires 32. Optionally, the intermediate component 40 is arranged such that movement of the portions 41 induces movement of the respective associated first groups 21 of haptic elements 11. For example, as shown in Figure 3, the struts 42 may be configured to induce the movement of the first group 21. The intermediate component 40 allows the activation of the first SMA wires 31 to be faithfully translated into actuation of the corresponding haptic elements 11. This may help the actuation of the haptic elements 11 to be more precisely controlled. The movement of the haptic elements 11 may be made sensitive to the activation of the first SMA wires 31. Figure 4 is another schematic side view of the haptic device 10 shown in Figure 3. Figure 4 shows a perpendicular view of the haptic device 10 compared to Figure 3. The view shown in Figure 4 is along the first SMA wires 31. As a result the first SMA wires 31 are not shown in Figure 4 (the first SMA wires 31 would extend into and out of the paper). One second SMA wire 32 is shown in Figure 4. As shown in Figure 4, optionally the second SMA wires 32 are located above the intermediate component 40. The intermediate component 40 is optionally located between the second SMA wires 32 and the support structure 12. Further struts 42 of the intermediate component may be configured to support the second SMA wires 32. When a second SMA wire 32 is activated, then the activated second SMA wire 32 pushes a haptic element 11 upwards (i.e. along the actuation axis 13) relative to the intermediate component 40. The flexibility of the intermediate component 40 allows other parts of the intermediate component 40 above other, non-activated second SMA wires 32 to remain substantially unmoved. Optionally, the second SMA wires 32 are arranged, on activation, to actuate the respective associated second groups 22 of haptic elements 11 relative to the intermediate component 40. For example, the second SMA wire 32 may be supported by further struts 42 to form "V" shapes. Each "V" shape may support a corresponding haptic element 11. When the second SMA wire 32 is activated the haptic elements 11 are actuated. The intermediate component 40 allows the activation of the second SMA wires 32 to be faithfully translated into actuation of the corresponding haptic elements 11. This may help the actuation of the haptic elements 11 to be more precisely controlled. The movement of the haptic elements 11 may be made sensitive to the activation of the second SMA wires 32. The haptic elements 11 are actuated by a combination of the motion of the intermediate component 40 (which may be referred to as an intermediate layer) transmitted through the second SMA wires 32 and the activation of the second SMA wires 32. So, in general, the first and second SMA wires 31, 32 may be arranged mechanically in series. In particular, actuation of the first SMA wire 31 may move corresponding portions 41 of the intermediate component 40 relative to the support structure 12, and actuation of the second SMA wire 32 may move corresponding haptic elements 11 relative to corresponding portions 42 of the intermediate component. Thickness-varying elements Figure 5 is a schematic plan view of another haptic device 10 in which motions are added to actuate the haptic elements 11. Figure 5 shows a plan view of the intermediate component 40 which comprises a plurality of thickness-varying elements 50. The colour indicates the thickness of the thickness-varying element 50 (blue is minimum thickness 51, red is maximum thickness 52). The thickness-varying elements 50 may be wedges, for example. In general, the thickness-varying elements 50 may be parts that are movable relative to the support structure 12 and relative to the haptic elements 11 in a direction that is non-parallel (e.g. 90 degrees) to the actuation axis 13. The thickness-varying elements 50 may be parts with varying extent (or thickness) along the actuation axis 13. As shown in Figure 5, optionally the intermediate component 40 comprises connections 54 between the thickness-varying elements 50. Optionally the connections are flexible laterally, i.e. in the plane in which the thickness-varying elements 50 are arranged (perpendicular to the actuation axis). This allows rows or columns of the thickness-varying elements 50 to move relative to each other, for example as 18 shown in Figure 6. Optionally the connections 54 are not compressible or extendable. The connections 54 may be configured to maintain distances between adjacent thickness-varying elements 50 within each row and column. Optionally, the haptic elements 11 are arranged in a 2D array, the first groups 21 being rows and the second groups being columns. By providing a 2D array, the extent to which the haptic device 10 can emulate the sensation of moving a finger over an object can be increased. Optionally, the first SMA wires 31 are arranged parallel to each other and / or the second SMA wires 32 are arranged parallel to each other. Optionally, the first SMA wires 31 intersect the second SMA wires 32 when viewed in a direction perpendicular to a plane in which the first SMA wires 31 are arranged (i.e. along the actuation axis 13). Optionally, the haptic elements 11 overlap where the first and second SMA wires 31, 32 intersect when viewed in a direction perpendicular to a plane in which the first SMA wires 31 are arranged. By arranging the SMA wires 31, 32 in a parallel way, the haptic elements 11 may be associated with the intersection of a first SMA wire 31 and a second SMA wire 32. This allows a simple way of addressing each haptic element 11 of the haptic device 10. The thickness-varying elements 50 are configured to convert the activation of the SMA wires 31, 32 into actuation of the haptic elements 11. In particular, the thickness-varying elements 50 may be configured to convert the activation of the SMA wires 31, 32 based on a wedge approach. Each thickness-varying element 50 of the intermediate component 40 bears on the support structure 12 and a haptic element 11 bears on each of the thickness-varying elements 50 of the intermediate component 40. The bearing point 55 where the haptic element 11 and the support structure 12 touch the thickness-varying elements 50 is shown in Figure 6 but not in Figure 5. Figure 6 is a schematic plan view of the haptic device 10 shown in Figure 5 when SMA wires 31, 32 are activated. Optionally, each row of thickness-varying elements 50 is associated with a first group 21 of haptic elements 11 and a first SMA wire 31. Each column of thickness-varying elements 50 is associated with a second group 22 of haptic elements 11 and a second SMA wire 32. When an SMA wire 31, 32 is activated, the associated row or column is moved laterally. The lateral movement causes actuation of the haptic elements 11 depending on the thickness of the thickness-varying elements 50 where the bearing point 55 is located. The bearing points 55 may remain stationary relative to the support structure 12 and the haptic elements 11. However, the location of the bearing points 55 relative to the thickness-varying elements 50 changes depending on the lateral movement induced by activation of the SMA wires 31, 32. Optionally, the thickness-varying elements 50 are arranged such that movement of the haptic elements 11 relative to the support structure 12 along an actuation axis 13 is dependent on a position of the corresponding thickness-varying element 50 relative to the support structure 12 in a plane nonparallel (e.g. perpendicular) to the actuation axis 13. By providing thickness-varying elements 50, the extent to which the haptic elements 11 are moved for a given activation of SMA wires 31, 32 may be controlled by controlling the topography of the thickness-varying elements 50. For example if desired, small movements of the haptic elements 11 may be more accurately controlled. The SMA wires 31, 32 may indirectly induce movement of the haptic elements 11 via the thickness-varying elements 50. The SMA wires 31, 32 may be located away from the haptic elements 11, if desired. This increases the design freedom affecting the shape of the volume taken up by the haptic device 10. Optionally, the haptic elements 11 are constrained to only move parallel to the actuation axis 13. This helps to focus the energy into actuating the haptic elements 11 so as to generate the target haptic force to the user. In the view shown in Figure 6, one of the first SMA wires 31 (corresponding to the third row) is activated and one of the second SMA wires 32 (corresponding to the fourth column) is activated. Accordingly, the haptic element 11 that belongs to the third row and the fourth column is most actuated because the thickness of the thickness-varying element 50 at the bearing point 55 is the greatest as a result of the activations of the SMA wires 31, 32. As shown in Figure 6, optionally the other thickness-varying elements 50 in the third row or the fourth column may have their bearing points 55 at slightly thickened locations compared to other rows and columns. The corresponding haptic elements 11 may be slightly actuated. Optionally, each row of the intermediate component 40 is moved by a first SMA wire 31 and each column on the intermediate component 40 is moved by a second SMA wire 32. Optionally all the haptic elements 11 in the row or column move along the actuation axis 13 actuated by substantially the same amount. Alternatively, by designing the topography of the thickness-varying elements 50 appropriately, the haptic elements 11 in the same row or column as the addressed haptic element 11 may be substantially not actuated. Optionally, the first SMA wires 31 are arranged, on activation, to actuate the thickness-varying elements 50 corresponding to the respective associated first groups 21 of haptic elements 11 in a first direction (e.g. left and right in the view shown in Figure 5 and Figure 6) within the plane. Optionally, the second SMA wires 32 are arranged, on activation, to actuate the thickness-varying elements 50 corresponding to the respective associated second groups 22 of haptic elements 11 in a second direction (e.g. the up and down direction in Figure 5 and Figure 6) within the plane and different from the first direction. The SMA wires 31, 32 may be disposed so as to act in a horizontal direction. This may simplify the arrangement of the SMA wires 31, 32. The topography of the thickness-varying elements, i.e. the variation in extent along the actuation axis, may be such that there is no or only small movement of the haptic elements 11 when only one of the first and second SMA wires 31, 32 is actuated, but relatively large movement of the addressed haptic element 11 when both corresponding first and second SMA wires 31, 32 are actuated. In particular, the extent of the thickness-varying elements along the actuation axis 13 may be relatively constant in movement directions corresponding to movement of the thickness-varying element due to actuation of only one of the first and second SMA wires 31, 32. The extent may be relatively variable (e.g. vary by more than 50%, or more than 100% of the variation in extend along the movement due to actuation of only one of the first and second SMA wires 31, 32) in movement directions corresponding to movement of the thickness-varying element due to actuation of both the first and second SMA wires 31, 32. The thickness-varying elements 50 may also be referred to as intermediate elements 50. Each such intermediate element 50 may correspond to a respective haptic element 11. The intermediate elements 50 may be movable in two degrees of freedom (e.g. along two axes in a movement plane, optionally perpendicular to the actuation axis 13) upon actuation of the first and second SMA wires 31, 32. So, the intermediate elements 50 may move along a first axis upon actuation of a first SMA wire 31 and along a second axis upon movement of a second SMA wire 32. A bearing arrangement between each intermediate element 50 and respective haptic element 11 may convert the two degree of freedom movement of the intermediate element 50 into movement of the haptic element along the actuation axis 13. The topography of the intermediate element may vary in the movement plane. In particular, the variation of topography may be less (e.g. less than 50%, optionally less than 25%, further optionally less than 10%) along each of the first and second axes than along a movement axis due to combined actuation of respective first and second SMA wires 31, 32. Enhanced amplification Figure 7 is a schematic plan view of another haptic device 10. Figure 8 is a schematic plan view of the haptic device 10 of Figure 7 when a second SMA wire 32 is activated. Figure 9 is a schematic plan view of the haptic device 10 of Figure 7 when a first SMA wire 31 and a second SMA wire 32 are activated. As shown in Figure 7, the haptic device 10 may comprise an intermediate component comprising thicknessvarying elements 50. The thickness-varying elements 50 may be as described above with reference to Figure 5 and Figure 6. The thickness-varying elements 50 shown in Figures 7-9 have a different topography from those shown in Figure 5 and 6. The topography may be freely selected depending on the application of the invention. Optionally, the haptic elements 11 do not have unique SMA wires assigned to them. Optionally, the SMA wires 31, 32 extend across the whole haptic device 10 and are in two arrays, which may be orthogonal. The haptic device 10 may have features as described with reference to any of Figure 1-6, except as described below. Optionally, the haptic device 10 is constructed so that when an SMA wire 31, 32 is activated, then the SMA wire 31, 32 that is activated does not apply the same force and contraction to all haptic elements. Optionally, the activation of the SMA wire is focused into one or more haptic elements 11. This is described in more detail below. Optionally, the haptic elements 11 are constructed so that the activation of a first SMA wire 31 is enhanced by the activation of a second SMA wire 32. Similarly, the haptic elements 11 may be constructed so that the activation of a second SMA wire 32 is enhanced by the activation of a first SMA wire 31. The haptic element 11 associated with the combination of activated SMA wires 31, 32 is actuated with a significantly larger amplitude or force (e.g. by a factor of 1.5, or 2, or 5) than other haptic elements 11 of the haptic device 10, even the other haptic elements 11 that are associated with one (but not both) of the activated SMA wires 31, 32. As shown in Figures 7-9, optionally the intermediate component 40 comprises a support member 53. The support member 53 may be secured to the thickness-varying elements 50 by further connections 54. The exemplary haptic device shown in Figures 7-9 may be configured to convert activation of the SMA wires 31, 32 into actuation of the haptic elements 11 using the wedge approach as described with reference to Figures 5 and 6. Figure 8 is a schematic plan view of the haptic device 10 of Figure 7 when a second SMA wire 32 is activated. The force applied by the activated second SMA wire is shown with a force arrow 56. When one second SMA wire 32 is actuated then the shape of the intermediate component 40 comprising the thickness-varying elements 50 changes as shown in Figure 8. As shown in Figure 8, the movement may result in angled connections 54 between the thickness-varying elements 50 of the fourth column and the adjacent columns. The angled connections 54 are angled relative to their configuration when the intermediate component 40 is in a neutral state, i.e. when none of the SMA wires 31, 32 are activated. Figure 9 is a schematic plan view of the haptic device 10 of Figure 7 when a first SMA wire 31 and a second SMA wire 32 are activated. Optionally, the angled connections 54 shown in Figure 8 provide an amplification mechanism so that when a first SMA wire 31 is activated, then the row of connections come under compression and the row buckles at the column that has been actuated. This is shown in Figure 9. Non-linearities in the behaviour of the intermediate component 40 are taken advantage of so as to allow individual haptic elements 11 to be actuated more independently from nearby haptic elements 11. The haptic device 10 shown in Figures 7-9 may provide an improved way of actuating the haptic elements 11 such that substantially only one or more addressed haptic elements 11 are actuated, leaving other non-addressed haptic elements unmoved. The first motion shown in Figure 8 functions as the trigger that directs the motion of the compression shown in Figure 9. One column (or row) is primed for motion by the activation of the additional SMA wire. Optionally, the first and second SMA wires 31, 32 are arranged such that the second SMA wires 32, on activation, apply a greater force and / or movement to any haptic elements 11 belonging to a first group 21 associated with an activated first SMA wire 31 than to other haptic elements 11. Accordingly, the motion of the SMA wires 31, 32 that are activated may be more focused on one or more specific haptic elements 11. Optionally, the first and second SMA wires 31, 32 are arranged such that the extent to which the second SMA wires 32, on activation, apply a force and / or movement to a haptic element 11 is dependent on the extent to which a first SMA wire 31 associated with a first group 21 to which the haptic element 11 belongs is activated. Accordingly, the motion of the haptic elements 11 that do not correspond to an activated combination of SMA wires 31, 32 can be reduced relative to the motion of the haptic element 11 that does correspond to the activated combination of SMA wires 31, 32. Compliant structure Another haptic device 10 that achieves enhanced amplification (as described with reference to Figures 7-9) is described below with reference to Figures 12-14. A principal of the way that the haptic device 10 functions is described with reference to Figures 10 and 11. Figure 10 is a diagram illustrating a feature of a haptic device 10. Figure 11 is a further diagram illustrating the feature illustrated in Figure 10. As shown in Figure 10, optionally an SMA wire 60 is arranged in a wave, namely with undulating portions that form "V" shape. The SMA wire 60 is supported by five shaping elements 61-65. Between adjacent shaping elements 61-65, the SMA wire 60 forms a "V" shape. The shaping elements 61-65 may be laterally compliant. Figure 10 shows the device when the SMA wire 60 is not contracted. In Figure 11 the SMA wire 60 is contracted along its length. The SMA wire 60 does not slide over the contact points with the shaping element 61-65. One shaping element 62 is actuated and the others comply laterally to allow the motion induced by the shrinkage of the SMA wire 60 to be transmitted to the second shaping element 62. As shown in Figure 11, when the SMA wire 60 contracts, the shaping elements 61-65 that form the SMA wire 60 into a wave are laterally deflected so that the motion of the SMA wire 60 can be focussed in to a single point In particular the motion is focused onto the point corresponding to the location of the second shaping element 62. It may be desirable to focus contraction of the SMA wire 60 on one region of the SMA wire 60. The shaping elements 61-65 are laterally compliant so that when they comply laterally they cause one to move vertically (i.e. substantially orthogonally to the SMA wire 60). Figure 12 is a schematic side view of another haptic device 10. As shown in Figure 12, optionally the haptic device 10 comprises a compliant structure 70. The first and second SMA wires 31, 32 are arranged, on activation, to induce compliance of the compliant structure 70 so as to actuate the addressed haptic element 11. The mechanism is described in further detail below. The mechanism makes use of the principal described with reference to Figures 10 and 11 for focusing actuation. By providing a compliant structure 70, the amplification of the haptic elements 11 may be enhanced. Nonlinearities in the behaviour of the compliant structure 70 may be taken advantage of so as to allow individual haptic elements 11 to be actuated more independently from nearby haptic elements 11. Features of the haptic device 10 shown in Figure 12 may be as described above with reference to Figures 3-4, for example. Description of such features is not repeated so as to avoid redundant description. The description below focuses on features that have not been described with reference to other Figures. Figure 12 shows a first SMA wire 31 shaped by the support structure 12 into a wave. The second SMA wires 32 are arranged as another layer above the first SMA wires 31. Two second SMA wires 32 are shown as dots because the second SMA wires 32 extend into and out from the paper. There are two arrays of SMA wires 31, 32. The first SMA wires 31 may function as triggers. As shown in Figure 12, optionally the first SMA wires 31 extend in the X direction and deviate in the Z direction to form a wave. Figure 13 is a schematic side view of the haptic device 10 of Figure 12 when a first SMA wire 31 is activated. Optionally, the compliant structure 70 comprises a support layer comprising support portions 71 (e.g. struts) for respective first groups 21 of haptic elements 11. The first SMA wires 31 are arranged, on activation, to actuate the respective support portions 71. Optionally, the compliant structure 70 comprises a deflection layer comprising deflection portions 72 for respective first groups 21 of haptic elements 11. The support portions 71 are arranged, on actuation, to deflect the respective deflection portions 72. As shown in Figure 13, when a first SMA wire is activated, features in a line in the X direction are actuated in the Z direction to deflect a deflection portion 72 in the Z direction. The force from the first SMA wire 31 may be transmitted to the deflection portion 72 via the support portions 71. When the first SMA wire 31 contracts, the middles of the "V" shapes moves upwards, forcing the support portions 71 to move so as to buckle the deflection portion 72. By providing deflection portions 72, the haptic device 10 may be configured such that deflection of a deflection portion 72 acts as a trigger for allowing corresponding haptic elements 11 to be actuated. This helps to allow activation of SMA wires without resulting in actuation of all associated haptic elements 11. The actuation of individual haptic elements 11 may be controlled more independently. The deflection portion 72 may be secured to the support structure 12, for example, at fixed points 73 such that contraction of the first SMA wire 31 does not cause the whole of the deflection portion 72 to be raised. Instead, the deflection portion 72 buckles as shown in Figure 13. Optionally, the haptic elements 11 are secured to the deflection portions 72. By providing that the haptic elements 11 are secured to the deflection portions, the actuation of an addressed haptic element 11 may be enhanced when the appropriate combination of SMA wires 31, 32 is activated. Figure 14 is a schematic plan view of the haptic device 10 of Figure 12. Figure 14 shows the second SMA wires 32. The second SMA wires 32 may function as drive wires to drive the haptic elements 11 that are associated with a triggered first SMA wire 31. As shown in Figure 14, optionally the second SMA wires 32 extend primarily in the Y direction and deviate in the X direction to form a wave. The deflection portion 72 and other shaping elements 73 that form the wave are compliant in the Y direction, but are not generally compliant in the X direction. However, the deflection portion 72 that has been deflected by the activation of the first SMA wire 31 is compliant in the X direction. Thus when the second SMA wire 32 is activated, then the deflection portion 72 that has been deflected buckles further, causing motion in the Z direction at the middle of the deflection portion 72. The other parts of the wave deflect in the Y direction transmitting the motion due to the contraction of the second SMA wire 32 to the addressed haptic element 11. Optionally, the second SMA wires 32 are arranged, on activation, to actuate a haptic element 11 by an extent dependent on the extent to which a deflection portion 72 to which the haptic element 11 is secured is deflected. The deflection portion 72 may be deflected so as to trigger the possibility of particular haptic elements 11 being actuated. The independence of the haptic elements 11 from each other may be increased. The motion of the haptic elements 11 that are not at the intersection of the activated SMA wires 31, 32 can be less than half of the motion of the haptic element that is at the intersection of the activated SMA wires 31, 32. SMA wire The above-described haptic device comprise SMA elements such as SMA wires. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, optionally the haptic device 10 comprises resilient elements arranged to bias respective haptic elements 11 against forces applied by the first and second SMA wires 31, 32. By providing resilient elements, the positions of the haptic elements 11 within the haptic device 10 may be maintained. The possibility of crosstalk between an addressed haptic element 11 and other haptic elements 11 in the same group 21, 22 may be reduced. Optionally, the resilient elements are arranged to bias the respective haptic elements 11 by a force greater than a maximum force applicable by only the associated first SMA wire 31 and greater than a maximum force applicable by only the associated second SMA wire 32 and less than a maximum force applicable by a combination of the associated first SMA wire 31 and the associated second SMA wire 32. Accordingly, the haptic elements 11 may be controlled more independently of each other. This increases the accuracy of the haptic stimulus provided by the haptic device 10. Optionally, the haptic device 10 is provided with end stops to encourage actuation only of haptic elements 11 corresponding to an activated first SMA wire 31 and an activated second SMA wire 32.

Claims

1. A haptic device comprising:a plurality of haptic elements arranged in a plurality of first groups and a plurality of second groups such that each haptic element belongs to a first group and a second group;a plurality of first shape memory alloy, SMA, elements associated with respective first groups of haptic elements; anda plurality of second SMA elements associated with respective second groups of haptic elements;wherein the first and second SMA elements are arranged, on activation of a first SMA element and a second SMA element, to actuate an addressed haptic element more than the other haptic elements, the addressed haptic element being a haptic element that belongs to the first group associated with the activated first SMA element and to the second group associated with the activated second SMA element.

2. The haptic device of claim 1, wherein the first and second SMA elements are arranged to actuate the addressed haptic element by adding forces applied by the activated first SMA element and the activated second SMA element.

3. The haptic device of claim 1 or 2, wherein the first and second SMA elements are arranged to actuate the addressed haptic element by adding the movements induced by the activated first SMA element and the activated second SMA element4. The haptic device of any preceding claim, wherein the first and second SMA elements are arranged such that the second SMA elements, on activation, apply a greater force and / or movement to any haptic elements belonging to a first group associated with an activated first SMA element than to other haptic elements.

5. The haptic device of any preceding claim, wherein the first and second SMA elements are arranged such that the extent to which the second SMA elements, on activation, apply a force and / or movement to a haptic element is dependent on the extent to which a first SMA element associated with a first group to which the haptic element belongs is activated.

6. The haptic device of claim 5, wherein the first and second SMA elements are arranged such that the extent to which the second SMA elements, on activation, apply a force and / or movement to a hapticelement is greater for any haptic elements associated with an activated first SMA element than for any haptic element associated with a non-activated first SMA element.

7. The haptic device of any preceding claim, wherein the amount by which a haptic element is actuated due to activation of only one of the associated first and second SMA elements is less than 50%, preferably less than 20%, further preferably less than 10% of the amount by which a haptic element is actuation due to activation of both of the associated first and second SMA elements.

8. The haptic device of any preceding claim, comprising:a support structure;wherein the first and second SMA elements are arranged, on activation, to actuate the addressed haptic element relative to the support structure.

9. The haptic device of claim 8, comprising:an intermediate component comprising portions associated with respective first groups of haptic elements, wherein the first SMA elements are arranged, on activation, to actuate the portions of the intermediate component associated with the respective associated first groups of haptic elements relative to the support structure.

10. The haptic device of claim 9, wherein the intermediate component is arranged such that movement of the portions induces movement of the respective associated first groups of haptic elements.

11. The haptic device of claim 9 or 10, wherein the second SMA elements are arranged, on activation, to actuate the respective associated second groups of haptic elements relative to the intermediate component.

12. The haptic device of any of claims 9-11, wherein the intermediate component comprises a plurality of thickness-varying elements corresponding to respective haptic elements, arranged such that movement of the haptic elements relative to the support structure along an actuation axis is dependent on a position of the corresponding thickness-varying element relative to the support structure in a plane nonparallel to the actuation axis.

13. The haptic device of claim 12, wherein:the first SMA elements are arranged, on activation, to actuate the thickness-varying elements corresponding to the respective associated first groups of haptic elements in a first direction within theplane; and / orthe second SMA elements are arranged, on activation, to actuate the thickness-varying elements corresponding to the respective associated second groups of haptic elements in a second direction within the plane and different from the first direction.

14. The haptic device of any preceding claim, comprising:a compliant structure;wherein the first and second SMA elements are arranged, on activation, to induce compliance of the compliant structure so as to actuate the addressed haptic element.

15. The haptic device of claim 14, wherein the compliant structure comprises:a support layer comprising support portions for respective first groups of haptic elements, the first SMA elements arranged, on activation, to actuate the respective support portions; anda deflection layer comprising deflection portions for respective first groups of haptic elements, the support portions arranged, on actuation, to deflect the respective deflection portions.

16. The haptic device of claim 15, wherein the haptic elements are secured to the deflection portions.

17. The haptic device of claim 15 or 16, wherein the second SMA elements are arranged, on activation, to actuate a haptic element by an extent dependent on the extent to which a deflection portion to which the haptic element is secured is deflected.

18. The haptic device of any preceding claim, wherein the haptic elements are arranged in a two-dimensional array, the first groups being rows and the second groups being columns.

19. The haptic device of any preceding claim, wherein the first SMA elements are arranged parallel to each other and / or the second SMA elements are arranged parallel to each other.

20. The haptic device of any preceding claim, wherein the first SMA elements intersect the second SMA elements when viewed in a direction perpendicular to a plane in which the first SMA elements are arranged.

21. The haptic device of claim 20, wherein the haptic elements overlap where the first and second SMA elements intersect when viewed in a direction perpendicular to a plane in which the first SMAelements are arranged.

22. The haptic device of any preceding claim, comprising resilient elements arranged to bias respective haptic elements against forces applied by the first and second SMA elements.

523. The haptic device of claim 22, wherein the resilient elements are arranged to bias the respective haptic elements by a force greater than a maximum force applicable by only the associated first SMA element and greater than a maximum force applicable by only the associated second SMA element and less than a maximum force applicable by a combination of the associated first SMA element and the10 associated second SMA element.

Images