SMA actuator assembly

Abstract

An SMA actuator assembly comprises an SMA actuator to drive movement of a movable part relative to a support structure. A thermally activated stress-relief arrangement is activated by an increase in a

Description

The present invention relates to the protection of shape memory alloy (SMA) actuator assemblies against high temperatures. SMA actuator assemblies typically use one or more SMA actuator elements to drive movement of a movable part with respect to a support structure. An SMA actuator element contracts on heating and is arranged to drive movement on contraction. During actuation, the movement may be controlled by changing the temperature of the SMA actuator element by supplying a drive signal to the SMA actuator element causing electrical heating. SMA actuator assemblies have found use in a wide range of applications. Such an SMA actuator element can be damaged by excessive heating. Such damage impacts the phase transition of the SMA material in the SMA actuator element in a manner that the contraction is reduced, which is detrimental to performance of the SMA actuator assembly. During normal actuation, the drive signal may be controlled to limit the degree of electrical heating so as to reduce such use. However, in some applications, excessive heating creating a risk of damage to the SMA actuator element can be caused by increases in the ambient temperature of the SMA actuator element. For example, in many medical applications, such as in a camera in an endoscope, it is desired to apply an autoclave sterilising process that elevates the ambient temperature to levels, for example around 140 degrees in some cases, which risks serious damage to an SMA actuator assembly. It would be desirable to mitigate such risks, so as to allow an SMA actuator assembly to be used in applications where high ambient temperatures are expected. According to a first aspect of the present invention, there is provided an SMA actuator assembly comprising: a support structure; a movable part; at least one SMA actuator element arranged, on contraction, to drive movement of the movable part with respect to the support structure; and a stress-relief arrangement that is arranged to be thermally activated by an increase in ambient temperature and is arranged, on thermal activation, to relieve stress on the at least one SMA actuator element. Thus, the present invention employs a stress-relief arrangement that is thermally activated by an increase in ambient temperature. On thermal activation, the stress-relief arrangement relieves the stress on the at least one SMA actuator element. Damage to an SMA actuator element caused by high temperatures is reduced by lowering the stress applied to the SMA actuator element. Accordingly, it is possible to subject the SMA actuator assembly to ambient temperatures that would otherwise cause damage to the at least one SMA actuator element. This increases the range of applications of the SMA actuator assembly. As the thermal activation of the stress-relief arrangement is itself caused by an increase in ambient temperature, the SMA actuator element is intrinsically protected against damage without the need for external intervention to reduce temperatures or apply external forces to reduce the stress of the SMA actuator element. The stress-relief arrangement may be implemented in various ways. In a first implementation, the stress-relief arrangement may comprise at least one SMA relief element arranged, on thermal activation by an increase in ambient temperature, to relieve stress on the at least one SMA actuator element. In this example, the at least one SMA relief element acts as an actuator, albeit one activated by a change in ambient temperature, rather than a drive signal, which actively reduces stress on the SMA actuator element. As a result of being active, this allows a relatively high degree of stress-relief. Advantageously, the at least one SMA relief element may be arranged to provide a greater stress than the at least one SMA actuator element when at the same temperature. This may be a pre-determined temperature or pre-determined temperature range. For example, a cross-sectional area of the at least one SMA relief element may be greater than a cross-sectional area of the at least one SMA actuator element. For actuation, the SMA actuator assembly may further comprise a control circuit connected to the at least one SMA actuator element and arranged to supply drive signals thereto. In this case, the control circuit may not be connected to the at least one SMA relief element, because the at least one SMA relief element is activated by an increase in the ambient temperature, so does not need to be driven. In this sense, the SMA relief element is passive. This first implementation of the stress-relief arrangement may be applied in an SMA actuator assembly that uses resilient biasing of the at least one SMA actuator assembly. For example, the at least one SMA actuator element may be connected between the support structure and the movable part, and the SMA actuator assembly may further comprise a resilient bias assembly that is, for example, connected between the support structure and the movable part and is arranged to oppose contraction of the at least one SMA actuator element. In this case, the at least one SMA relief element may be connected between the support structure and the movable part in parallel with the at least one SMA actuator element. The resilient bias assembly may comprise one or more springs or flexures, for example. Alternatively, this first implementation of the stress-relief arrangement may be applied in an SMA actuator assembly that uses opposed SMA actuator elements. For example, the at least one SMA actuator element may comprise opposed SMA actuator elements arranged to oppose contraction of each other (e.g. a first SMA actuator element arranged to drive movement of the movable part in a first direction and a second SMA actuator element arranged to drive movement of the movable part in a second direction which is opposite to the first direction). In this case, the stress-relief arrangement may further comprise at least one resilient element, wherein the or each resilient element has a series connection with an SMA actuator element of the at least one SMA actuator element between the support structure and the movable part, and is arranged to deform in a manner that accommodates the relief of stress on the at least one SMA actuator element on thermal activation of the stress-relief arrangement. The or each resilient element may be configured so as to deform only on thermal activation of the stress-relief arrangement. Various configurations of the at least one SMA relief element and the at least one resilient element are possible, some examples of which are given below. In a second approach, the stress-relief arrangement may comprise at least one stressrelief assembly having a series connection with an SMA actuator element of the at least one SMA actuator element between the support structure and the movable part, the or each stressrelief assembly being thermally activated by an increase in ambient temperature and arranged, on thermal activation, to relieve stress on the at least one SMA actuator element. In a first example of the second approach, the or each stress-relief assembly may comprise a stress-relief actuator arranged, on thermal activation, to apply a force that reduces the stress on the SMA actuator element with which the stress-relief assembly has a series connection. In this example, stress-relief is provided by the stress-relief actuator, which actively reduces stress on the SMA actuator element. As a result of being active, this allows a relatively high degree of stress-relief. The or each stress-relief actuator may take any form, for example being a flexure. In one example, the stress-relief actuator may be a flexure that is a bi-metallic strip arranged, on thermal activation, to bend in a manner that drives the movement that reduces the strain of the SMA actuator element. Advantageously, the at least one stress-relief actuator may be arranged to provide a greater degree of movement than the contraction of the at least one SMA actuator element when at the same temperature. In a second example of the second approach to the stress-relief arrangement, the or each stress-relief component may be resilient and arranged, on thermal activation, to have an increased compliance so that it relieves stress on the one or more SMA actuator elements. In this example, stress-relief is provided by a stress-relief component having a compliance that changes, in particular increasing on thermal activation. In this manner, the stress-relief component may be provided with a compliance in an operating range of ambient temperatures that is chosen to limit the impact on the performance of the SMA actuator assembly in normal use. But at increased ambient temperatures, the increased compliance takes up some of the change in strain of the at least one SMA actuator element, resulting from its contraction, thereby providing stress-relief. The or each stress-relief component may take any form. In one example, the or each resilient stress-relief component may comprise: a resilient element; and a thermally activated material arranged to resist deformation of the resilient element, the thermally activated material being arranged, on thermal activation, to soften. In this case, the resilient element may take any form, for example a flexure. The thermally-activated material may have a softening temperature above the operating temperature of the SMA actuator assembly. The softening temperature may be below the temperature at which an autoclave operates (for example below 120°C, 150°C or 180°C). The thermally-activated material may comprise a resin, for example a thermoplastic or an elastomer, or similar material. The thermally-activated material may have a glass transition temperature between the predetermined operating range of ambient temperatures and the increased temperature at which thermal activation is desired. Alternatively, the thermally-activated material may comprise a gel or liquid (e.g. with a melting point between the highest expected operating temperature of the SMA actuator assembly and the lowest expected sterilisation temperature) encapsulated by a flexible outer shell. The thermally-activated material may comprise a self-supporting gel, such as a damping gel. This second implementation of the stress-relief arrangement may be applied in an SMA actuator assembly that uses resilient biasing of the at least one SMA actuator assembly. For example, the SMA actuator assembly may further comprise a resilient bias assembly arranged to oppose contraction of the at least one SMA actuator element. Alternatively, this first implementation of the stress-relief arrangement may be applied in an SMA actuator assembly that uses opposed SMA actuator assemblies. For example, the at least one SMA actuator element may comprise opposed SMA actuator elements arranged to oppose contraction of each other. In this case, the stress-relief arrangement may comprise a stress-relief assembly connected in series with each of the opposed SMA actuator elements. Any of the SMA actuator assemblies described herein may be part of a medical device, such as an endoscope. The movable part may comprise a lens. The one or more SMA actuator elements may be configured to drive movement of the movable part along the optical axis of the lens. The device may also comprise an image sensor and the lens may be arranged to focus an image on the image sensor. The movable part comprising a lens may be driven to move along the optical axis of the lens for the purpose of altering a focus of the image, for example as part of an auto-focussing process. Embodiments of the present invention will now be described by way of non-limitative example with reference to the accompanying drawings, in which: Fig. 1 is a side view of an SMA actuator assembly; Fig. 2 is a diagram of a control circuit of the SMA actuator assembly; Figs. 3 to 8 are side views of the SMA actuator assembly having various alternative constructions; Figs. 9 to 11 are side views of three alternative constructions of a stress-relief assembly in the SMA actuator assembly that may be applied in the alternative construction of Fig. 8; Figs. 12 to 14 are side views of the SMA actuator assembly having various alternative constructions; Fig. 15A is a side view of an SMA actuator assembly at a normal operating temperature; and Fig. 15B is a side view of the SMA actuator assembly of figure 15A at a high temperature. Fig. 1 illustrates an SMA actuator assembly 1 that is arranged as follows. The SMA actuator assembly 1 comprises a support structure 2 and a movable part 3. The movable part 3 is movable with respect to the support structure 2. Optionally, a bearing (not shown) of any type, for example a ball bearing, plain bearing or a flexure bearing, may be provided to guide the desired movement of the movable part 3 with respect to the support structure 2. The support structure 2 and the movable part 3 may take any form, depending on the application. For example, in an application where the SMA actuator assembly 1 is part of a camera, the support structure 2 may support an image sensor and the movable part 3 may comprise a lens arrangement. In this case, the movable part 3 may be movable along the optical axis of the lens arrangement to provide focussing or perpendicular to the optical axis of the lens arrangement to provide optical image stabilisation. Many other applications of SMA actuator assemblies as described herein are envisaged, including medical applications. For example, the actuator assembly 1 may be part of a medical device. The movable part 3 may comprise a moving part of a medical device, such as a valve or part of a valve or a plunger. The SMA actuator assembly 1 comprises an SMA actuator element 5 connected between the support structure 2 and the movable part 3. Such mechanical connection may be provided by any suitable means, for example crimps that can be also be used to provide an electrical connection. The SMA actuator element 5 is arranged, on contraction, to drive movement of the movable part 3 with respect to the support structure 2. As mentioned above, a bearing may be provided to movement of the movable part 3 with respect to the support structure 2. Alternatively, the movable part 3 may be suspended on the support structure 2 by the SMA actuator element(s) and optionally a resilient bias assembly 6 (described below). In this example and all the examples below, the term 'SMA element', in the case of both the SMA actuator element 5 and the SMA relief element 10 that is described below, may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element 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 element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field. The SMA actuator assembly 1 includes control circuit 8 that is connected to the SMA actuator element 5, as shown in Fig. 2. The control circuit 8 supplies drive signals to the SMA actuator element 5 to control the movement of the movable part 3. In the SMA actuator assembly 1 shown in Fig. 1, the SMA actuator element 5 is resiliently biased. To achieve this, the SMA actuator assembly 1 comprises a resilient bias assembly 6 connected between the support structure 2 and the movable part 3 and arranged to oppose contraction of the SMA actuator element 5. The resilient bias assembly 6 may take any suitable form, for example being a component such as a spring or a flexure, or being a more complicated structure. The SMA actuator assembly 1 comprises an SMA relief element 10 connected between the support structure 2 and the movable part 3 in parallel with the SMA actuator element 5. The SMA relief element 10 is not driven, so the control circuit 8 is not connected thereto. Instead, the SMA relief element 10 is thermally activated by an increase in the ambient temperature. On such thermal activation, the SMA relief element 10 contracts and applies a force to the movable part 3 relative to the support structure 2 in the same direction as the SMA actuator element 5, thereby reducing the stress on the SMA actuator element 5. Thus, the SMA relief element 10 forms a thermally activated stress-relief arrangement that reduces the stress on the SMA actuator element 5 on thermal activation. As such stressrelief is provided by an actuator, albeit one activated by a change in ambient temperature rather than a drive signal, which actively reduces stress on the SMA actuator element 5, the active nature of the stress-relief arrangement permits a relatively high degree of stress-relief. Such reduction in the stress applied to the SMA actuator element 5 reduces damage to the SMA actuator element 5 that might otherwise be caused by the high ambient temperature that it experiences. Accordingly, it is possible to subject the SMA actuator assembly 1 to ambient temperatures than would otherwise be possible without causing damage to the SMA actuator element 5. In practical terms, this increases the range of applications of the SMA actuator assembly 1. For example, the SMA actuator assembly 1 may be part of a medical device which is to be sterilised by an autoclave. It is also noted that the SMA relief element 10 is thermally activated by the same increase in ambient temperature that might potentially damage the SMA actuator element 5, so in this manner provides intrinsic protection without the need for external intervention to reduce temperatures or apply external forces to reduce the stress of the SMA actuator element 5. The SMA relief element 10 may be arranged to provide a greater stress than the SMA actuator element 5 when at the same temperature. As such, the SMA relief element 10 provides a greater degree of movement of the movable part 3 relative to the support structure 2 than the SMA actuator element 5 when at the same temperature. This may be achieved by means of the cross-sectional area of the SMA relief element 10 being greater than the cross-sectional area of the SMA actuator element 5. That may be achieved by increasing the thickness of the SMA relief element 10 or applying plural SMA relief elements in parallel, so that their total cross-sectional area is greater than the cross-sectional area of the SMA actuator element 5. Performance of the SMA relief element 10 can also be improved by increasing its length. Some preferred design criteria for the SMA relief element 10 are as follows. Preferably, the SMA relief element 10, when fully in the austenitic phase, moves the movable part 3 relative to the support structure 2 to a position where the SMA actuator element 5 is slack. In this manner the stress relief and hence the protection against high temperature damage to the SMA actuator element 5 is maximised. However, lesser degrees of stress-relief still provide useful protection. Preferably, in a predetermined operating range of ambient temperature, the SMA relief element 10 is slack and so applies no force to the moving part 3 during normal use. To achieve this, the transition temperature of the SMA material in the SMA relief element 10 may be selected so that the martensite phase finish temperature is greater than the upper end of the predetermined operating range of ambient temperature. A work around for this point would be 8 to increase the force applied by the resilient bias assembly 6 in order to counter any residual stress in the SMA relief element 10. However, this is not preferred, as this would increase the stress in the SMA actuator element 5 during operation. Although a single SMA actuator element 5 and a single SMA relief element 10 are present here, in general any number of SMA actuator elements 5 could be provided to drive movement in one or more degrees of freedom and similarly in general any number of SMA relief elements 5 could be provided to provide the desired stress-relief of the SMA actuator element(s) 5. Some further non-limitative examples of the SMA actuator assembly 1 with modified constructions with respect to Fig. 1 are described below. In the following description, common elements of the SMA actuator assembly 1 are given the same reference numerals and the description above is equally applicable and so not repeated, except for the modifications that are described. Whereas the SMA actuator element 5 is resiliently biased in the SMA actuator assembly 1 of Fig. 1, as an alternative the SMA actuator assembly 1 may use opposed SMA actuator elements 5. Figs. 3 to 7 show five examples of this. In each of the SMA actuator assemblies 1 of Figs. 3 to 7, the SMA actuator assembly 1 is modified by omission of the resilient biasing element 6 omitted and provision of two SMA actuator elements 5 that oppose contraction of each other. As described in more detail below, each of the SMA actuator assemblies 1 of Figs. 3 to 7 also includes one or more SMA relief elements 10 that are connected in various different configurations such that the SMA relief elements 10 reduce the stress on the SMA actuator elements 5. However, as the SMA actuator elements 5 are opposed and there is no resilient biasing assembly 6, then in order to accommodate such stress relief, each of the SMA actuator assemblies 1 of Figs. 3 to 7 is also modified to include at least one resilient element 15 as part of the stress-relief arrangement. In particular, the or each resilient element 15 has a series connection with one of the SMA actuator elements 5 between the support structure 2 and the movable part 3. As such the resilient element deforms in a manner that accommodates the relief of stress on the corresponding SMA actuator element provided by the SMA relief elements 10 on thermal activation. However, the resilient element 15 is arranged to as not to deform (or only to deform minimally) during normal operation of the SMA actuator assembly 1. In other words, the resilient element 15 is arranged to only deform on contraction of the SMA relief element 10 and not on contraction of the SMA actuator element(s) 5. In each of the SMA actuator assemblies 1 of Figs. 3 to 7, the resilient element 15 is a shown as a flexure, but more generally the resilient element 15 can be a resilient element of any other form, for example a spring. The specific configurations of each of the SMA actuator assemblies 1 of Figs. 3 to 7 will now be described. In the SMA actuator assembly 1 of Fig. 3, the stress-relief arrangement comprises a resilient element 15 in respect of each of the opposed SMA actuator elements 5. The resilient elements 15 are opposed and each has a series connection with the corresponding one of the opposed SMA actuator elements 5. Each resilient element 15 is connected to the support structure 2. Each resilient element 15 has a series connection between the support structure 2 and the movable part 3 with a corresponding SMA actuator element 5, which is connected to the resilient element 15 and to the movable part 3. In addition, in the SMA actuator assembly 1 of Fig. 3, the stress-relief arrangement comprises an SMA relief element 10 in respect of each resilient element 15, which is connected to the resilient element 15 and to the movable part 3 in parallel with the SMA element 5. Each resilient element 15 is preloaded against an endstop 16 that forms part of the support structure 2, within a predetermined operating range of ambient temperatures. However, the SMA relief elements 10, on thermal activation by an increase in the ambient temperature, each contract and apply force between the movable part 3 and the corresponding resilient element 15 in the same direction as the corresponding SMA actuator element 5, thereby deforming the resilient element 15 in a manner that reduces the stress on the SMA actuator element 5. The resilient elements 15 accommodate this stress reduction, lifting off the endstops 16. The SMA actuator assembly 1 of Fig. 4 is the same as that of Fig. 3 except that the series connection of each resilient element 15 and SMA actuator element 5 is reversed. Thus, each resilient element 15 is connected to the movable part 3 and each resilient element 15 has a series connection between the support structure 2 and the movable part 3 with a corresponding SMA actuator element 5, which is connected to the resilient element 15 and to the support structure 2. In addition, in the SMA actuator assembly 1 of Fig. 4, the stress-relief arrangement comprises an SMA relief element 10 in respect of each resilient element 15, which is connected to the resilient element 15 and to the support structure 3 in parallel with the SMA element 5. The endstops 16 against which the resilient elements 15 are preloaded now form part of the movable part 3. The SMA relief elements 10, on thermal activation by an increase in the ambient temperature, each contract and apply force between the support structure 2 and the corresponding resilient element 15 in the same direction as the corresponding SMA actuator element 5, thereby deforming the resilient element 15 in a manner that reduces the stress on the SMA actuator element 5. The resilient elements 15 accommodate this stress reduction, lifting off the endstops 16. The SMA actuator assembly 1 of Fig. 5 is the same as that of Fig. 3 except that the SMA relief elements 10 in respect of each resilient element 15 are replaced by a single SMA relief element 10 that is connected between both resilient elements. Thus, the SMA relief element 10, on thermal activation by an increase in the ambient temperature, contracts and apply force between the resilient elements 15, thereby deforming the resilient elements 15 in a manner that reduces the stress on both of the SMA actuator elements 5 together. The resilient elements 15 accommodate this stress reduction, lifting off the endstops 16. The SMA actuator assembly 1 of Fig. 6 is the same as that of Fig. 3 except that the SMA relief elements 10 in respect of each resilient element 15 are connected between the resilient element 15 and the support structure 2. Thus, the SMA relief elements 10, on thermal activation by an increase in the ambient temperature, each contract and apply force between the support structure 2 and the resilient elements 15, thereby deforming the resilient elements 15 in a manner that reduces the stress on the SMA actuator element 5. The resilient elements 15 accommodate this stress reduction, lifting off the endstops 16. The SMA actuator assembly 1 of Fig. 7 is the same as that of Fig. 6 except that there is only one resilient element 15 in respect of one of the SMA actuator elements 5, the other one of the SMA actuator elements 5 being connected to the support structure 2, and there is only one SMA relief element 10 connected between the resilient element 15 and the support structure 2. Thus, the SMA relief element 10, on thermal activation by an increase in the ambient temperature, contracts and applies a force between the support structure 2 and the resilient element 15, thereby deforming the resilient element 15 in a manner that reduces the stress on both SMA actuator elements 5. The resilient element 15 accommodates this stress reduction, lifting off the endstop 16. Whereas the examples above provide a thermally activated stress-relief arrangement in the form of the SMA relief element 10, there will now be described some modified constructions of the SMA actuator assembly 1 in which alternative forms of thermally activated stress-relief arrangement are provided. Fig. 8 illustrates an SMA actuator assembly 1 that is arranged as follows. The SMA actuator assembly 1 comprises a support structure 2 and a movable part 3. The movable part 3 is movable with respect to the support structure 2. Optionally, a bearing (not shown) of any type, for example a ball bearing, plain bearing or a flexure bearing, may be provided to guide the desired movement of the movable part 3 with respect to the support structure 2. The support structure 2 and the movable part 3 may take any form, depending on the application. For example, in an application where the SMA actuator assembly 1 is a camera, the support structure 2 may support an image sensor and the movable part 3 may comprise a lens arrangement. In this case, the movable part 3 may be movable along the optical axis of the lens arrangement to provide focussing or along the optical axis of the lens arrangement to provide optical image stabilisation. As explained above, the SMA actuator assembly 1 may be part of any device, for example a medical device. The SMA actuator assembly 1 comprises an SMA actuator element 5 connected between the support structure 2 and the movable part 3. Such mechanical connection may be provided by any suitable means, for example crimps that can be also be used to provide an electrical connection. The SMA actuator element 5 is arranged, on contraction, to drive movement of the movable part 3 with respect to the support structure 2. In the SMA actuator assembly 1 shown in Fig. 8 the SMA actuator element 5 is resiliently biased. To achieve this, the SMA actuator assembly 1 comprises a resilient bias assembly 6 connected between the support structure 2 and the movable part 3 and arranged to oppose contraction of the SMA actuator element 5. The resilient bias assembly 6 may take any suitable form, for example being a component such as a spring or a flexure, or being a more complicated structure. The SMA actuator assembly 1 comprises a stress-relief assembly 20 that has a series connection with the SMA actuator element 5 between the support structure 2 and the movable part 4. In particular, the SMA actuator element 5 is connected to the movable part 3 and the stress-relief assembly 20 is connected to the support structure 2 and the SMA actuator element 5. The stress-relief assembly 20 is thermally activated by an increase in ambient temperature, on thermal activation, to relieve stress on the SMA actuator element 5. Figs. 9 to 11 show three alternative constructions of the stress-relief assembly 20, as follows. Fig. 9 shows a construction of the stress-relief assembly 20 comprising a stress-relief actuator 21 in the form of a flexure that has a bi-metallic construction of materials having different thermal expansion (i.e. a bi-metallic strip). The stress-relief actuator 21 is preloaded against an endstop 25 that forms part of the support structure 2, within a predetermined operating range of ambient temperature. The stress-relief actuator 21 is thermally activated by an increase in the ambient temperature. On such thermal activation, the stress-relief actuator 21 deforms towards the movable part 3, thereby reducing the stress on the SMA actuator element 5. On thermal activation, the stress-relief actuator 21 preferably lifts off the endstop 25, although this is not essential as sufficient stress-relief may be achieved without that. Thus, the stress-relief assembly 20 shown in Fig. 9 forms a thermally activated stressrelief arrangement that reduces the stress on the SMA actuator element 5 on thermal activation. As such stress-relief is provided by an actuator, albeit one activated by a change in ambient temperature rather than a drive signal, which actively reduces stress on the SMA actuator element 5, the active nature of the stress-relief arrangement permits a relatively high degree of stress-relief. Preferably, the stress-relief actuator 21, when thermally activated, deforms to an extent where the SMA actuator element 5 is slack. In this manner the stress relief and hence the protection against high temperature damage to the SMA actuator element 5 is maximised. However, lesser degrees of stress-relief still provide useful protection. The stress-relief actuator 21 may be arranged to provide a greater degree of movement than the contraction of the SMA actuator element 5 when at the same temperature. Figs. 10 and 11 show constructions of the stress-relief assembly 20 that is resilient and arranged, on thermal activation, to have an increased compliance. In each of Figs. 10 and 11, the stress-relief assembly 20 comprises a flexure 22 as a resilient element. The flexure 22 is preloaded against an endstop 25 that forms part of the support structure 2, within a predetermined operating range of ambient temperature. In each of Figs. 10 and 11, the stress-relief assembly 20 also comprises a thermally activated material 23 arranged to resist deformation of the flexure 22 by means of being provided between the flexure 22 and the support structure 2. The stress-relief assembly 20 is designed to be sufficiently stiff, within a predetermined operating range of ambient temperatures, relative to the stress within the SMA actuator element 5, so that the performance of the SMA actuator assembly 1 is not degraded by the presence of the stress-relief assembly 20. However, the thermally activated material 23 is arranged, on thermal activation, to soften such that the compliance of the stress-relief assembly 20 increases. The increased compliance allows the stress-relief assembly 20 to deform to a greater extent, thereby relieving stress on the SMA actuator element 5. Thus, the stress-relief assembly 20 shown in Figs. 10 and 11 forms a thermally activated stress-relief arrangement that reduces the stress on the SMA actuator element 5 on thermal activation. In the example of Fig. 10, the thermally activated material 23 is employed in tension. Cooling of the ambient temperature would result in the stress-relief assembly 20 returning to the endstop 25 before the thermally activated material 23 cools sufficiently to re-solidify. The stress-relief assembly 20 is designed to balance the temperatures and forces, such that the SMA actuator element 5 cools and returns to the endstop 25, without solidifying in place. The stress-relief assembly 20 is designed so as not to be subject to hot and cold spots which could lead to the thermally activated material 23 reducing temperature before the SMA actuator element, which again would lead to solidification in the wrong position. There are design opportunities to add heat sinks or heat reservoirs to provide control over this cooling profile. An advantage is that for some materials these properties can have relatively narrow transition temperature bands, meaning that the load change will have more of an on-off behaviour than a steady transition with temperature change. In this case, the thermally activated material 23 may be, for example, a resin, which may be a thermoplastic or an elastomer that has a glass transition temperature between the predetermined operating range of ambient temperatures and the increased temperature at which thermal activation is desired. In the example of Fig. 11, the thermally activated material 23 is employed in compression. In this case, the thermally activated material 23 may be a self-supporting gel, for example a damping gel. Although a flexure 22 is used as a resilient element in this example, other forms of resilient element may be employed instead of the flexure 22. In the example of Figure 10 or the example of figure 11, the thermally activated material may comprise a gel or liquid (e.g. with a melting point between the highest expected operating temperature of the SMA actuator assembly and the lowest expected sterilisation temperature) encapsulated by a flexible outer shell. In the SMA actuator assembly 1 of Fig. 8, the reduction in the stress applied to the SMA actuator element 5 reduces damage to the SMA actuator element 5 that might otherwise be caused by the high ambient temperature that it experiences. Accordingly, it is possible to subject the SMA actuator assembly 1 to ambient temperatures than would otherwise be possible without causing damage to the SMA actuator element 5. In practical terms, this increases the range of applications of the SMA actuator assembly 1. It is also noted that the SMA relief element 10 is thermal activated by the same increase in ambient temperature that might potentially damage the SMA actuator element 5, so in this manner provides intrinsic protection without the need for external intervention to reduce temperatures or apply external forces to reduce the stress of the SMA actuator element 5. Fig. 12 shows an alternative construction for the SMA actuator assembly 1 which is the same as that of Fig. 8 except that the series connection of the stress-relief assembly 20 and SMA actuator element 5 is reversed. Thus, the SMA actuator element 5 is connected to the support structure 2 and the stress-relief assembly 20 is connected to the movable part 3 and the SMA actuator element 5. Thus, the endstop 25 is formed on the movable part 3. With this alternative construction, the SMA actuator assembly 1 operates in the same manner as described above. Whereas the SMA actuator element 5 is resiliently biased in the SMA actuator assembly 1 of Figs. 11 and 12, as an alternative the SMA actuator assembly 1 may use opposed SMA actuator elements 5. Figs. 13 and 14 show two examples of this. In each case, the SMA actuator assembly 1 is modified by omission of the resilient biasing element 6 omitted and provision of two SMA actuator elements 5 that oppose contraction of each other. A stress-relief assembly 20 having the same form and operation as described above has a series connection with each of the opposed SMA actuator elements 5. In the construction of Fig. 13, the SMA actuator element 5 is connected to the movable part 3 and the stress-relief assembly 20 is connected to the support structure 2 and the SMA actuator element 5. In the construction of Fig. 14, the series connection of the stress-relief assembly 20 and SMA actuator element 5 is reversed, such that the SMA actuator element 5 is connected to the support structure 2 and the stress-relief assembly 20 is connected to the movable part 3 and the SMA actuator element 5. Any of the stress-relief assemblies 20 described herein (e.g. with reference to Figure 9,10 and / or 11) may be employed in the arrangements shown in Figures 13 and 14. Figures 15A and 15B show an SMA actuator assembly 1 comprising two stress-relief assemblies 20 in the form of bi-metallic strips. Figure 15A shows the SMA actuator assembly 1 during normal operation, e.g. at room temperature, in which the movable part 3 is driven to move by the application of drive signals (by a control circuit, not shown) to the SMA actuator elements 5. Figure 15B shows the actuator assembly at a high ambient temperature, e.g. during sterilisation in an autoclave. The bi-metallic strips have deformed away from endstops 25, reducing the stress in the SMA actuator elements 5 and causing them to go slack. It will be appreciated that there may be many other variations of the above-described examples. For example, the embodiments described with reference to Figures 3, 4, 6 and 7 comprise two opposing SMA actuator elements 5. However, each of these embodiments may instead comprise an SMA actuator element 5 which is opposed by a resilient biasing element, e.g. as described with reference to Figure 1, instead of two opposing SMA actuator elements. Embodiments have been described which enable movement of a moveable part along an axis. However, movement of the movable part in two or three dimensions may be provided. For example, a pair of opposing SMA actuator elements (or an SMA actuator element opposed by a resilient bias assembly) may be provided for each dimension in which the movable part is arranged to move.

Claims

1. An SMA actuator assembly comprising:a support structure;a movable part;at least one SMA actuator element arranged, on contraction, to drive movement of the movable part with respect to the support structure; anda stress-relief arrangement that is thermally activated by an increase in ambient temperature and is arranged, on thermal activation, to relieve stress on the at least one SMA actuator element.

2. An SMA actuator assembly according to claim 1, wherein the stress-relief arrangement comprises at least one SMA relief element arranged, on thermal activation by an increase in ambient temperature, to relieve stress on the at least one SMA actuator element.

3. An SMA actuator assembly according to claim 2, wherein:the at least one SMA actuator element is connected between the support structure and the movable part;the SMA actuator assembly further comprises a resilient bias assembly arranged between the support structure and the movable part and arranged to oppose contraction of the at least one SMA actuator element; andthe at least one SMA relief element is connected between the support structure and the movable part in parallel with the at least one SMA actuator element.

4. An SMA actuator assembly according to claim 2 or 3, wherein:the stress-relief arrangement further comprises at least one resilient element, the or each resilient element having a series connection with an SMA actuator element of the at least one SMA actuator element between the support structure and the movable part and being arranged to deform in a manner that accommodates the relief of stress on the at least one SMA actuator element on thermal activation of the stress-relief arrangement.

5. An SMA actuator assembly according to any preceding claim, wherein the at least one SMA actuator element comprises opposed SMA actuator elements arranged to oppose contraction of each other.

6. An SMA actuator assembly according to claim 5, whereinthe at least one resilient element comprises a resilient element in respect of each of the opposed SMA actuator elements,in respect of each resilient element:the resilient element is connected to the support structure; andthe SMA actuator element having a series connection with the resilient element is connected to the resilient element and the movable part, andthe at least one SMA relief element comprises an SMA relief element connected between the resilient elements.

7. An SMA actuator assembly according to claim 5, whereinthe at least one SMA relief element comprises an SMA relief element in respect of the or each resilient element, andin respect of the or each resilient element:the resilient element is connected to the support structure;the SMA actuator having a series connection with the resilient element is connected to the resilient element and to the movable part, andthe SMA relief element in respect of the resilient element is connected between the resilient element and the support structure.

8. An SMA actuator assembly according to claim 7, wherein the at least one resilientelement comprises opposed resilient elements, each having a series connection with one of the opposed SMA actuator elements.

9. An SMA actuator assembly according to claim 5, whereinthe stress-relief arrangement comprises a resilient element in respect of each of the opposed SMA actuator elements,the at least one SMA relief element comprises an SMA relief element in respect of the or each resilient element, andin respect of each resilient element:the resilient element is connected to the support structure;the SMA actuator element having a series connection with the resilient element is connected to the resilient element and to the movable part; andthe SMA relief element in respect of the resilient element is connected to the resilient element and to the movable part in parallel with the SMA element.

10. An SMA actuator assembly according to claim 5, whereinthe stress-relief arrangement comprises a resilient element in respect of each of the opposed SMA elements, andthe at least one SMA relief element comprises an SMA relief element in respect of the or each resilient element, andin respect of each resilient element:the resilient element is connected to the movable part;the SMA actuator element having a series connection with the resilient element is connected to the resilient element and to the support structure; andthe SMA relief element in respect of the resilient element is connected to the resilient element and to the support structure in parallel with the SMA actuator element.

11. An SMA actuator assembly according to any one of claims 4 to 10, wherein the at least one resilient element is a flexure.

12. An SMA actuator assembly according to any one of claims 4 to 11, wherein the at least one SMA relief element is arranged to provide a greater stress than the at least one SMA actuator element when at the same temperature.

13. An SMA actuator assembly according to any one of claims 2 to 11, wherein a cross-sectional area of the at least one SMA relief element is greater than a cross-sectional area of the at least one SMA actuator element.

14. An SMA actuator assembly according to any one of claims 2 to 13, further comprising a control circuit connected to the at least one SMA actuator element and arranged to supplydrive signals thereto, wherein the control circuit is not connected to the at least one SMA relief element.

15. An SMA actuator assembly according to claim 1, wherein the stress-relief arrangement comprises at least one stress-relief assembly having a series connection with an SMA actuator element of the at least one SMA actuator element between the support structure and the movable part, the or each stress-relief assembly being thermally activated by an increase in ambient temperature and arranged, on thermal activation, to relieve stress on the at least one SMA actuator element.

16. An SMA actuator assembly according to claim 15, wherein the or each stress-relief assembly comprises a stress-relief actuator arranged, on thermal activation, to apply a force that reduces the stress on the SMA actuator element with which the stress-relief assembly has a series connection.

17. An SMA actuator assembly according to claim 16, wherein the stress-relief actuator comprises a flexure.

18. An SMA actuator assembly according to claim 17, wherein the flexure is a bi-metallic strip that is arranged, on thermal activation, to bend in a manner that drives the movement that reduces the strain of the SMA actuator element.

19. An SMA actuator assembly according to any one of claims 16 to 18, wherein the at least one stress-relief actuator is arranged to provide a greater degree of movement than the contraction of the at least one SMA actuator element when at the same temperature.

20. An SMA actuator assembly according to claim 15, wherein the or each stress-relief assembly is resilient and arranged, on thermal activation, to have an increased compliance so that it relieves stress on the one or more SMA actuator elements.

21. An SMA actuator assembly according to claim 20, wherein the or each resilient stressrelief assembly comprises:a resilient element; anda thermally activated material arranged to resist deformation of the resilient element, the thermally activated material being arranged, on thermal activation, to soften.

22. An SMA actuator assembly according to claim 21, wherein the resilient element is a5 flexure.

23. An SMA actuator assembly according to any one of claims 15 to 22, wherein the SMA actuator assembly further comprises a resilient bias assembly arranged to oppose contraction of the at least one SMA actuator element.1024. An SMA actuator assembly according to any one of claims 15 to 22, wherein the at least one SMA actuator element comprises opposed SMA actuator elements arranged to oppose contraction of each other.15 25. An SMA actuator assembly according to claim 24, wherein the stress-relief arrangementcomprises a stress-relief assembly connected in series with each of the opposed SMA actuator elements.

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