Actuator assembly

Abstract

An actuator assembly (1) comprising first and second parts (10, 20) that are movable relative to each other, the first part comprising a first friction surface (lOf) and the second part comprising a second friction surface (20f); at least two SMA elements (49) configured, on actuation, to cause actuation forces in opposite directions to be applied to the second part for moving the second part in opposite directions relative to the first part; a biasing arrangement (30) configured to apply a normal force biasing the first and second friction surfaces against each other, thereby giving rise to at least part of a static frictional force that is capable of constraining relative movement of the first and second parts when the at least two SMA elements are not actuated; wherein the at least two SMA elements are configured, on actuation, to cause reduction of the normal force between the first and second friction surfaces, thereby reducing the static frictional force; and further comprising at least one additional actuator (45) configured, on actuation, to modulate the static frictional force.

Description

[0001] ACTUATOR ASSEMBLY

[0002] Field

[0003] The present application generally relates to an actuator assembly, for example an actuator assembly comprising shape memory alloy elements as actuators. The present invention also relates to a method for driving actuation of the actuator assembly.

[0004] Background

[0005] There are a variety of apparatuses in which it is desired to control movement of a movable component. Shape memory alloy (SMA) elements (such as SMA wires) may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA element required to apply a given force to the movable element can be relatively small.

[0006] The movable component may be retained in position by friction. The movable component is biased against a surface of a support structure by a biasing element to hold the movable part in any given position when SMA wires are not energised. Zero hold power functionality may thus be achieved. Upon actuation, the SMA wires act against the biasing element to reduce frictional forces, thereby enabling the movable component to be driven to a new position. W02020 / 120997 Al and WO2023 / 094813 Al, the disclosures of which are herein incorporated by reference, disclose various actuator assemblies with zero hold power functionality.

[0007] Summary

[0008] According to the present invention, there is provided an actuator assembly comprising first and second parts that are movable relative to each other, the first part comprising a first friction surface and the second part comprising a second friction surface; at least two SMA elements configured, on actuation, to cause actuation forces in opposite directions to be applied to the second part for moving the second part in opposite directions relative to the first part; a biasing arrangement configured to apply a normal force biasing the first and second friction surfaces against each other, thereby giving rise to at least part of a static frictional force that is capable of constraining relative movement of the first and second parts when the at least two SMA elements are not actuated; wherein the at least two SMA elements are configured, on actuation, to cause reduction of the normal force between the first and second friction surfaces, thereby reducing the static frictional force; and further comprising at least one additional actuator configured, on actuation, to modulate the static frictional force. The additional actuator may be used to selectively increase the static frictional force when required, for example to reduce the risk of slipping of the second part relative to the first part when relatively large loads act between the first and second parts. The performance of the actuator assembly may thus be improved and / or more reliable.

[0009] The first and second friction surfaces, when biased against each other with the normal force, may a give rise to a static frictional force therebetween that contributes to an overall static frictional force. The overall static frictional force is the sum of the frictional forces that act to constrain relative movement of the first and second parts, for example due to the provision of multiple pairs of first and second friction surfaces. The at least one additional actuator may be configured, on actuation, to modulate this overall static frictional force. In some embodiments, the static frictional force between the first and second friction surfaces (i.e. one pair of first and second friction surfaces) may correspond, or at least dominate (e.g. make up more than 80% or more than 90%) of the overall static frictional force. The static frictional force between the first and second friction surfaces may constrain relative movement of the first and second parts when the at least two SMA elements are not actuated.

[0010] The actuating force is the component of the force on the second part that is due to actuation of the SMA element and that acts in a direction parallel to instantaneous movement of the second part. The actuating force may be the component of the force on the second part that is due to actuation of the SMA element that does not cause a reduction of the normal force.

[0011] In some embodiments, the at least one additional actuator is configured, on actuation, to increase the static frictional force.

[0012] In some embodiments, the at least two SMA elements are configured to reduce and the at least one additional actuator is configured to modulate the static frictional force between the same first and second friction surfaces. The two SMA elements and the additional actuator may act on the same pair or set of friction surfaces. Alternatively, the two SMA elements and the additional actuator may act on different pairs or sets of friction surfaces.

[0013] Some embodiments further comprise a controller configured control actuation of the at least two SMA elements and of the additional actuator, wherein the controller is configured to: during a power-on period, cause an increase of power supplied to at least one of the at least two SMA elements so as to initiate actuation of the at least one of the at least two SMA elements and / or during a power-off period cause a decrease of power supplied to at least one of the at least two SMA elements so as to cease actuation of the at least one of the at least two SMA elements; and during the power-on period and / or during the power-off period, control actuation of the additional actuator so as to increase the static frictional force. On completion of the power-on period and / or power-off period, the actuation of the additional actuator may be controlled as to cease increasing the static frictional force.

[0014] In some embodiments, the controller is configured to control actuation of the additional actuator so as to increase the static frictional force during the power-on period and / or during the power-off period only when the second part is positioned at least a pre-determined distance from a reference position of the second part relative to the first part.

[0015] The position of the second part relative to the first part herein refers to both translational and rotational position of the second part, and so includes the orientation or pose of the second part. The reference position may correspond to a reference orientation or reference pose of the movable part. Displacement of the second part from the reference position may include rotation of the second part from the reference orientation or reference pose. The range of movement is the set of positions that can be reached by the second part during normal operation of the actuator assembly, i.e. due to actuation of the SMA elements.

[0016] In some embodiments, the controller is configured to cause power to be supplied to only one of the at least two SMA elements so as to cause movement of the second part relative to the first part. The normal force may be reduced due to actuation of only one of the at least two SMA elements.

[0017] In some embodiments, the at least two SMA elements are configured, on actuation, to disengage the first and second friction surfaces. The frictional force between the first and second friction surfaces may be reduced to zero.

[0018] In some embodiments, the at least one additional actuator is configured, on actuation, not to affect relative movement of the first and second parts. The additional actuator may apply an additional force that does not have a component in a direction parallel to the movement. Some embodiments comprise a decoupling bearing arrangement configured to decouple the actuation of the at least one additional actuator from the relative movement of the first and second parts.

[0019] Some embodiments comprise an additional part, wherein the additional actuator is arranged between the additional part and one of the first and second parts, and wherein the decoupling bearing arrangement is arranged between the additional part and the other of the first and second parts.

[0020] Some embodiments comprise a guide bearing arrangement configured to guide relative movement of the first and second parts.

[0021] In some embodiments, the first and second friction surfaces are separate from the guide bearing arrangement, and wherein the at least two SMA elements are arranged, on actuation, to disengage the first and second friction surfaces such that relative movement of the first and second parts is guided by the guide bearing arrangement.

[0022] In some embodiments, the same bearing arrangement acts as the decoupling bearing arrangement configured to decouple actuation of the at least one additional actuator from the relative movement of the first and second parts and as the guide bearing arrangement configured to guide relative movement of the first and second parts.

[0023] In some embodiments, the decoupling bearing arrangement and / or the guiding bearing arrangement comprises a rolling bearing. Alternatively, the decoupling bearing arrangement and / or the guiding bearing arrangement may comprise a plain bearing or a flexure bearing, for example.

[0024] Some embodiments comprise an endstop that comprises two endstop surfaces spaced apart from one another when the at least two SMA elements are not actuated, wherein the at least two SMA elements are arranged, on actuation, to disengage the first and second friction surfaces and to engage the endstop.

[0025] In some embodiments, each of the at least one additional actuator comprises an additional SMA element that is arranged, on actuation, to modulate the static frictional force. In some embodiments, a ratio of i) a length of each one of the at least two SMA elements to ii) a length of each one of the at least one additional SMA element is greater than 2, preferably greater than 4.

[0026] In some embodiments, the at least one additional actuator comprises at least two additional actuators, wherein the actuation forces are applied to the second part by the at least two SMA elements at a location between additional forces applied to the second part by the at least two additional actuators.

[0027] In some embodiments, the first and second parts are translationally movable relative to each other in one degree of freedom.

[0028] According to the present invention, there is also provided a method for driving actuation of the actuator assembly, wherein the method comprises causing power to be supplied to at least one of the at least two SMA elements so as to modulate the static frictional force and to control movement of the second part relative to the first part, and wherein the method further comprises causing power to be supplied to the at least one additional actuator so as to modulate the frictional force.

[0029] Some embodiments comprise during a power-on period causing an increase of power supplied to at least one of the at least two SMA elements so as to initiate actuation of the at least one of the at least two SMA elements and / or during a power-off period causing a decrease of power supplied to at least one of the at least two SMA elements so as to cease actuation of the at least one of the at least two SMA elements; and during the power-on period and / or during the power-off period, controlling actuation of the additional actuator so as to increase the static frictional force.

[0030] Some embodiments comprise obtaining a measure indicative of the position of the second part compared to a reference position and controlling actuation of the additional actuator so as to increase the static frictional force during the power-on period and / or during the power-off period only when the measure is indicative of the second part being positioned at least a pre-determined distance from the reference position.

[0031] Some embodiments comprise causing power to be supplied to only one of the at least two SMA elements. According to the present invention, there is also provided a non-transitory computer-readable medium comprising instructions for carrying out the method. The non-transitory computer-readable medium may be any data storage carrier capable of storing computer-readable instructions.

[0032] Further aspects of the present invention are set out in the dependent claims and in the detailed description.

[0033] Brief description of the drawings

[0034] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0035] Figure 1 schematically shows an actuator assembly with zero hold power functionality;

[0036] Figure 2 schematically shows another actuator assembly with zero hold power functionality;

[0037] Figures 3A and 3B are graphs showing the forces acting on a movable part in an actuator assembly with zero hold power functionality, such as the actuator assembly of Figure 1 or Figure 2;

[0038] Figure 4 schematically shows an actuator assembly with zero hold power functionality as shown in Figure 1, comprising an additional actuator for modulating the frictional force;

[0039] Figures 5A and 5B schematically shows other actuator assemblies with zero hold power functionality as shown in Figure 1, comprising an additional actuator for modulating the frictional force;

[0040] Figures 6 schematically shows an actuator assembly with zero hold power functionality as shown in Figure 2, comprising an additional actuator for modulating the frictional force;

[0041] Figure 7 schematically shows another actuator assembly with zero hold power functionality as shown in Figure 1, comprising two additional actuators for modulating the frictional force; and

[0042] Figure 8 schematically shows another actuator assembly with zero hold power functionality as shown in Figure 1, comprising an additional actuator for modulating the frictional force.

[0043] Detailed description Actuator assembly

[0044] Figures 1 and 2 schematically show examples of an actuator assembly 1. The actuator assembly 1 includes a support structure 10 and a movable part 20. The movable part 20 is movable relative to the support structure 10. When the actuator assembly 1 is included in an apparatus, such as portable electronic device, the support structure 10 may be fixed relative to the main body of such an apparatus. However, in general, the support structure 10 need not be stationary and may be movable relative to or within such an apparatus. The support structure 10 herein serves as a reference structure relative to which movement of other components of the actuator assembly 1 is described, but it will be appreciated that the support structure 10 may equally move relative to the movable part 20, for example if the movable part 20 is fixed within a larger device. The support structure 10 and movable part 20 may also be referred to simply as a first part and a second part, or vice versa.

[0045] The actuator assembly 1 comprises two SMA elements 40, in particular two SMA wires 40. The SMA elements 40 are configured, on actuation, to apply actuating forces F to the movable part 20 capable of moving the movable part 20 relative to the support structure 10. More generally, the SMA elements 40 may apply an input force Fi (corresponding for example to tension in the SMA elements). The actuating force F is the force on the movable part 20 acting in a direction parallel to instantaneous movement of the movable part 20 on actuation of the SMA element 40. In Figures 1 and 2, the actuating force F is a component of the input force Fi. Purely for illustrative reasons, the input force Fi and the actuating force F are only shown for one SMA element 40 in the Figures, but in general any of the SMA elements 40 may, on actuation, give rise to such an input force Fi and actuating force F.

[0046] The actuator assembly 2 may include a bearing arrangement 50 and a biasing arrangement 30. The bearing arrangement 50 supports the movable part 20 on the support structure 10 and guides movement of the movable part 20. The bearing arrangement 50 is also referred to as a guide bearing arrangement 50 herein to allow distinction from other bearing arrangements that may be provided between other parts. The biasing arrangement 30 loads the bearing arrangement 50, thereby applying a force that helps guide movement of the movable part 20. The SMA elements 40 may support the movable part 20 on the support structure 10 together with the bearing arrangement 50 and the biasing arrangement 30.

[0047] The bearing arrangement 50 may have any suitable form for allowing movement of the movable part

[0048] 20 with respect to the support structure 10 with one or more degrees of freedom (DOFs). In general, the movable part 20 may be movable relative to the support structure 10 with up to six degrees of freedom (DOFs). The movable part 20 may be movable relative to the support structure 10 in all or in any subset (including only one) of three translational DOFs (so movement along three orthogonal axes, herein the x, y and z axes) and three rotational DOFs (so rotation about three orthogonal axes, in particular the x, y and z axes). Figures 1 and 2 show embodiments in which the movable part 20 generally moves in one DOF, in particular along a movement axis M.

[0049] The SMA elements 40 and / or the bearing arrangement 50 and biasing arrangement 30 may constrain, i.e. reduce or prevent, other DOFs of movement of the movable part 20 relative to the support structure 10. For this purpose, the bearing arrangement 50 may, for example, include one or more of the following bearings: a rolling bearing (such as a ball bearing), a flexure bearing (i.e. an arrangement of flexures or other resilient elements that guide movement), or a plain (i.e. sliding contact) bearing. The biasing arrangement 30 may include any element capable of applying a loading force to the bearing arrangement 50, for example a resilient element such as a spring or flexure, or a set of magnetic elements such as a magnet and a ferromagnetic material. The biasing arrangement 30 may load the bearing arrangement 50 by biasing bearing surfaces of the bearing arrangement 50 against each other (typically for a rolling bearing or a plain bearing) or by placing the bearing arrangement 50 in tension (typically for a flexure bearing).

[0050] The SMA elements 40 are, directly or indirectly, connected between the support structure 10 and the movable part 20. The SMA elements 40 may be connected, directly or indirectly, to the support structure 10 and the movable part 20 by connection elements, such as crimps (not shown). Optionally, one or more intermediate parts may be connected between the SMA elements 40 and the movable part 20 and / or support structure 10. The SMA elements 40 may thereby, either directly or indirectly via one or more intermediate parts, cause actuating forces F to be applied to the movable part 20. Selectively varying the actuating forces F applied by the SMA elements 40 may cause the movable part 20 to move relative to the support structure 10, for example within the DOFs allowed by the bearing arrangement 50. The SMA elements 40 are thus capable of driving movement of the movable part 20 relative to the support structure 10.

[0051] The bearing arrangement 50 may cause the movable part 20 to move in directions which differ from the directions of the input forces Fi applied by the SMA elements 40. In the actuator assemblies of Figure 1 and 2, for example, one component of each input force Fi causes the movement of the movable part 20, and another component of each input force Fi acts against or with the bearing forces produced by the bearing arrangement 50.

[0052] The actuator assembly 1 may also include a controller (not shown). The controller may be implemented in an integrated circuit (IC) chip. The controller generates drive signals for the SMA elements 40 so as to actuate the SMA elements 40, for example by generating drive signals for contracting the SMA elements 40. SMA material has the property that, on heating, it undergoes a solid-state phase change that causes the SMA material to contract. Thus, applying drive signals to the SMA elements 40, thereby heating the SMA elements 40by causing an electric current to flow, will cause the SMA elements 40 to contract and thus apply the actuating force so as to drive relative movement of the movable part 20. The drive signals are chosen to drive relative movement of the movable part 20 in the desired manner.

[0053] The movable part 20 may comprise or be coupled to a component to be moved, such as a lens or an image sensor. Such a component to be moved may generally give rise to a load force acting on the movable part 20. Moving a lens or an image sensor may effect optical image stabilization and / or autofocus or zoom functionality in a camera apparatus, for example. In general, the actuator assembly 1 may be used in any device in which movement of a movable part 20 relative to a support structure 10 is desired, e.g. to provide haptic feedback in a haptic feedback device or to move a projector or display in an augmented reality (AR) or virtual reality (VR) device.

[0054] Zero hold power

[0055] The actuator assembly 1 may be configured such that the movable part 20 does not move relative to the support structure 10 at any position within a range of movement of the movable part 20 when the SMA elements 40 are not actuated. The movable part 20 may remain in position without needing to power the SMA elements 40. This is also referred to as zero hold power. Power consumption of the actuator assembly 1 is reduced compared to an actuator assembly 1 in which the SMA elements 40 are continuously powered so as to keep a desired position of the movable part 20 relative to the support structure 10.

[0056] In the actuator assemblies 1 of Figures 1 and 2, the support structure 10 comprises a first friction surface lOf and the movable part 20 comprises a second friction surface 20f. The first and second friction surfaces lOf, 20f may form part of the bearing arrangement 50 or be separate from the bearing arrangement 50. The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force. A static frictional force is thus generated between the first and second friction surfaces lOf, 20f. The magnitude of the static frictional force may be sufficient to constrain movement of the movable part 20 relative to the support structure 10 at any position within the range of movement of the movable part 20 relative to the support structure 10 when the SMA elements 40 are not actuated.

[0057] In the actuator assemblies 1 of Figures 1 and 2, the SMA elements 40 may be arranged such that, on actuation, the normal force between the first and second friction surfaces lOf, 20f is reduced. For example, when two opposing SMA elements 40 are equally actuated, the movable part 20 may be urged in a direction perpendicular to the first and second friction surfaces lOf, 20f, such that the normal force between the first and second friction surfaces lOf, 20f is reduced. The input force Fi comprises a component that is normal to the actuating force F and thereby may reduce the normal force between the first and second friction surfaces lOf, 20f.

[0058] The first and second friction surfaces lOf, 20f may remain in contact or may even disengage on reduction of the normal force. As a result, the static frictional force between the first and second friction surfaces lOf, 20f is reduced. The resistance to movement of the movable part 20 on differential actuation of the SMA elements 40 may thus be reduced, such that the SMA elements 40 may more effectively move the movable part 20 and / or a larger normal force may be applied when the SMA elements 40 are not actuated.

[0059] The first and second friction surfaces lOf, 20f may generally be provided between any two components of the actuator assembly 1 that are in engagement and move relative to each other. For example, first and second friction surfaces lOf, 20f may be arranged between any intermediate part and the support structure 10, between any intermediate part and the movable part 20, or between any two intermediate parts that are coupled between the movable part 20 and the support structure 10.

[0060] In general, the normal force is reduced on actuation of the SMA elements 40. WO 2020 / 120997 Al and WO 2023 / 094813 Al, which are herein incorporated by reference, describe various ways of arranging the first and second friction surfaces lOf, 20f for achieving zero hold power.

[0061] Movement of the movable part 20 at any position within the range of movement may be constrained, in particular, when the SMA elements 40 are not actuated and when acceleration of the actuator assembly 1 is less than or equal to a hold threshold. The hold threshold may be at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2), where g is the acceleration due to Earth's gravity.

[0062] With particular reference to Figure 1, the bearing arrangement 50 may comprise a plain bearing and the biasing arrangement 30 may comprise a compression spring. Each SMA element 40 is connected between the support structure 10 and the movable part 20. So, one end of the SMA element 40 is connected to the support structure 10 and the other end of the SMA element 40 is connected to the movable part 20.

[0063] The plain bearing comprises the first surface lOf on the support structure 10 and the second surface 20f on the movable part 20. The biasing arrangement 30 is arranged to load the bearing arrangement 50 by biasing the first and second surfaces lOf, 20f against each other. The movable part 20 is arranged to be movable relative to the support structure 10 along a movement axis M (the horizontal axis in Figure 1). The biasing arrangement 30 applies a force to the movable part 20 in a direction that is generally perpendicular to the movement axis M (downward in Figure 1).

[0064] The SMA elements 40 apply input forces Fi, and thereby actuating forces F, to the movable part 20. In Figure 1, the SMA elements 40 are arranged at an angle a relative to the movement axis M. The input forces Fi typically act in a direction along the length of the SMA elements 40. In Figure 1, the input forces Fi thus have a component along the movement axis M (corresponding to the actuating force F) and a component perpendicular to the movement axis M that acts against the biasing force of the biasing arrangement 30. The SMA elements 40 are arranged in opposition, i.e. the SMA elements 40 apply input forces Fi to move the movable part 20 with force components in opposite directions along the movement axis M. On actuation of one SMA element 40 (the left SMA element 40 in Figure 1), the movable part 20 moves in one direction (leftwards) and on actuation of the other SMA element 40 (the right SMA element 40 in Figure 1), the movable part 20 moves in an opposite direction (rightwards).

[0065] The first and second surfaces lOf, 20f of the bearing arrangement 50 correspond to first and second friction surfaces lOf, 20f. The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force, thereby giving rise to a static frictional force capable of holding the movable part 20 in position so as to achieve zero hold power. So, the normal force loading the bearing arrangement 50 is equal to the normal force loading the friction surfaces lOf, 20f. The component of the input force Fi that is perpendicular to the movement axis M acts against the biasing arrangement 30, and so reduces the normal force. On actuation of the SMA element 40, the static frictional force between the first and second friction surfaces lOf, 20f is thus reduced.

[0066] The actuator assembly 1 of Figure 2 is similar to the actuator assembly 1 of Figure 1, except that the bearing arrangement 50 is embodied by a rolling bearing and that the first and second friction surfaces lOf, 20f are separate from the bearing arrangement 50.

[0067] As shown in Figure 2, the support structure 10 comprises a first portion 10a and a second portion 10b. The first and second portions 10a, 10b are coupled by a compression spring forming the biasing arrangement 30.

[0068] The first and second portions 10a, 10b of the support structure are movable relative to each other in a direction perpendicular to the movement axis M. However, in practice, movement of the first and second portions 10a, 10b relative to each other need not take place or may be negligible during operation of the actuator assembly 1. The first and second portions 10a, 10b may remain static relative to each other on actuation of the SMA wires 40a, and so are considered as a unit forming the support structure 10. Alternatively, the first portion 10a may be considered to correspond to the support structure 10 and the second portion 10b may be considered to correspond to an additional part 10b (such as a sprung part 10b). In general, movement of the first and second portions 10a, 10b in a direction along the movement axis M is constrained.

[0069] The bearing arrangement 50 comprises a ball bearing arranged between the movable part 20 and the support structure 10, in particular the first portion 10a thereof. The ball bearing guides movement of the movable part 20 along the movement axis M. The biasing arrangement 30, in the form of the compression spring, is arranged between the first and second portions 10a, 10b of the support structure 10. The compression spring loads the ball bearing via the second portion 10b and the movable part 20, i.e. the biasing force is transmitted to the ball bearing by the second portion 10b and the movable part 20. The biasing force acts in a direction perpendicular to the movement axis M.

[0070] The biasing arrangement 30 is thus effectively decoupled from the movement of the movable part 20 along the movement axis M by the bearing arrangement 50. Compared to the actuator assembly 1 of Figure 1, for example, the biasing arrangement 30 of Figure 2 does not apply any lateral force components to the movable part 20 even when the movable part 20 is displaced from a central position relative to the support structure. The biasing arrangement 30 in Figure 2 does not deform in a direction parallel to the movement axis M on movement of the movable part 20. The biasing arrangement 30 does not apply any force opposing movement of the movable part 20, such that the effectiveness of actuation of the SMA elements 40 is improved.

[0071] The SMA elements 40 are connected between the movable part 20 and the second portion 10b of the support structure 10. The SMA elements 40 are connected in opposition, so as to move the movable part 20 in opposite directions along the movement axis M. On actuation, the SMA elements 40 are arranged to further load the bearing arrangement 50, i.e. the SMA elements 40 are arranged to apply the input force Fi with a force component urging the movable part 20 towards the bearing arrangement 50.

[0072] The first and second friction surfaces lOf, 20f are arranged on the movable part 20 and the second portion 10b of the support structure 10. The biasing arrangement 30 biases the first and second friction surfaces lOf, 20f against each other with a normal force. The biasing arrangement 30 thus loads both the bearing arrangement 50 and the friction surfaces lOf, 20f, and so the normal force loading the bearing arrangement 50 is equal to the normal force loading the friction surfaces lOf, 20f. The SMA elements 40 are angled relative to the movement axis M so as to reduce the normal force on actuation, thereby reducing the frictional force between the first and second friction surfaces lOf, 20f.

[0073] The SMA elements 40 in either of the actuator assemblies 1 of Figures 1 and 2 may be arranged, on actuation, to disengage the friction surfaces lOf, 20f. In Figure 1, the SMA elements 40 may lift the movable part 20 off the bearing arrangement 50 and so off the friction surface lOf on the support structure 10. The SMA elements 40 may thus suspend the movable part 20 from the support structure 10 when actuated. In Figure 2, the SMA elements 40 may similarly lift the movable part 20 off the friction surface lOf on the support structure 10.

[0074] The actuator assembly of Figure 2 comprises an endstop 55. The endstop 55 comprises a pair of endstop surfaces that are spaced apart when the SMA elements 40 are not actuated. A first endstop surface is provided on the first portion 10a of the support structure 10 and a second endstop surface is provided on the second portion 10b of the support structure 10. The SMA elements 40 may be configured, on actuation, to disengage the first and second friction surfaces lOf, 20f and to engage the endstop 55. With particular reference to Figure 2, the movable part 20 may be moved downward, i.e. in a direction towards the bearing arrangement 50, on actuation of the SMA elements 40, thereby moving the second portion 10b towards the endstop 55 so as ultimately to engage the endstop. Movement of the movable part 20 along the movement axis M may then be guided by the bearing arrangement 50.

[0075] Forces on movable part in actuator assembly with zero hold power functionality

[0076] Figures 3A and 3B are graphs showing exemplary forces acting on the movable part 20 of the actuator assembly 1 of Figures 1 or 2 at a particular position of the movable part 20, for example at an edge of the range of movement of the movable part 20 relative to the support structure 10.

[0077] In general, forces acting on the movable part 20 may include the actuating force F of an actuating SMA element 40 (indicated by line (i) in Figure 3A), the static frictional force FFbetween the friction surfaces lOf, 20f (indicated by line (ii) in Figure 3A), a load force L of an external load (indicated by line (iii) in Figure 3A) and the opposing force FEof an opposing SMA element 40 (indicated by line (iv) in Figure 3A). The opposing force FEacts in a direction opposite to the actuating force F.

[0078] The load force L is due to an external load, which typically acts on the movable part 20 due to coupling of a component to be moved to the movable part 20. Such an external load may apply various forces to the movable part 20, such as gravitational forces due to the mass of the movable part 20 including any components to be moved or biasing forces due to any resilient or elastic elements, such as spring components, or magnetic elements. The external load on the movable part 20 typically comprises a mixture of these forces. The external load may apply a load force L to the movable part 20 that resists movement of the movable part 20, and that needs to be overcome by actuation of the SMA elements 40 so as to effect movement of the movable part. The load force L may vary with displacement of the movable part 20 relative to the support structure 10, and is typically largest at the edges of the range of movement of the movable part 20.

[0079] As explained with reference to Figures 1 and 2, the actuator assembly 1 may comprise at least two SMA elements 40 applying actuating forces F in opposite directions, so as to effect movement of the movable part 20 in opposite directions. For illustrative purposes, the SMA element 40 that effects movement of the movable part 20 away from a reference position (such as a central position shown in Figures 1 and 2) is referred to herein as the actuating SMA element 40 and the other SMA element 40 is referred to herein as the opposing SMA element 40. When implemented as SMA wires 40, the actuating SMA element 40 may be a contracting SMA wire and the opposing SMA element 40 may be an extending SMA wire. With reference to Figure 1, for example, if the movable part 20 is moved towards the right due to actuation of the right SMA element 40, the right SMA element 40 is considered to be the actuating SMA element 40 and the left SMA element 40 is considered to be the opposing SMA element 40. In practice, however, any SMA element 40 may act as either the actuating SMA element 40 or the opposing SMA element 40 in dependence on the position of the movable part 20.

[0080] So as to effect movement of the movable part 20, the actuating SMA element 40 needs to apply an actuating force F of a magnitude that is greater than the magnitude of the combination of load force L, the opposing force FEand the static frictional force FF, SO F > L + FE+ FF.

[0081] The opposing force FEis desirably reduced or minimized, ideally to zero, so as to improve the efficiency of driving movement of the movable part 20 by the actuating SMA element 40. However, not actuating the opposing SMA element 40 may in some situations risk affecting the performance of the zero hold power functionality of the actuator assembly 2, for example in particular conditions in which in the SMA element 40 disengages the friction surfaces lOf, 20f. For example, if only the input force Fi applied by the actuating SMA element 40 disengage the friction surfaces lOf, 20f and the opposing SMA element 40 is not actuated, then on reduction of the input force Fi of the actuating SMA element 40 there is no increase in frictional force until the friction surfaces lOf, 20f re-engage.

[0082] The condition for holding the movable part 20 in position may be expressed as F + FF> L + FE. Figure 3A shows the magnitude of F + FFas dotted line (i)+(ii) and the magnitude of L + FEas dotted line (iii)+(iv) for a situation in which the movable part 20 is at an edge of the range of movement. It is noted that the magnitude of FEmay be greater than 0 even when the opposing SMA element 40 is not actuated, i.e. when no drive signal is applied to the opposing SMA element 40, because the opposing SMA element 40 may be stretched or generally not held slack even when unactuated when the movable part 20 is positioned near an edge of the range of movement by the actuating SMA element 40. As shown in Figure 3A, for a range of stresses in the actuating SMA element 40, the magnitude of L + FEexceeds the magnitude of F + FF, such that the movable part 20 may slip and not be held in position at the edge of the range of movement. In the particular example shown in Figure 3A, the movable part 20 may not be held in position when the stress in the actuating SMA element 40 is in the range from about 75 MPa to 375 MPa, but in practice this range may differ from the illustrative example given. The stress of the actuating SMA element 40 may relatively quickly bridge this range of stresses on being powered on or powered off, but there may be a short time period in which the stress is within such a range. The difference between the onset of the decrease of the actuating force F and the onset of the increase of the frictional force FFmay thus result in transient slipping of the movable part 20.

[0083] The graph of Figure 3B shows the magnitude of the frictional force in dependence on the stress of the actuating SMA element 40, illustrating the slip region (III) experienced by the movable part 20 on reduction of the input force Fi from a discrete friction operating region (IV) or from increase from a continuous friction operating region (II). When the input force Fi is relatively low (and so the stress in the actuating SMA element 40 is relatively low), the actuating force F is not sufficient to effect movement of the movable part 20 and the movable part 20 is held in position, as illustrated by region (I) of Figure 3B. The static frictional force decreases with increasing input force Fi. In region (II), the actuating force F is large enough to hold the movable part 20 or to controllably move the movable part 20 while the friction surfaces lOf, 20f remain in engagement. In region (III), the input force Fi is sufficient to disengage the friction surfaces lOf, 20f (such that the static frictional force is reduced to zero), but the actuating force F is insufficient to maintain the position of the movable part 20 due to the load force L and opposing force FEexperienced at the edges of the range of movement. The position of the movable part 20 may thus, in some situations, not be maintained. In region (IV) the actuating force F is large enough to maintain the position or controllably move the movable part 20.

[0084] Additional actuator for modulating frictional force

[0085] Figures 4 to 8 show embodiments of an actuator assembly 1 comprising an additional actuator 45, in particular an additional SMA element 45. The additional SMA element 45 is configured, on actuation, to modulate the static frictional force acting on the movable part 20. The additional SMA element 45 may thus selectively increase the static frictional force FF, for example.

[0086] Figure 3A shows a dashed line (i)+(ii)+add corresponding to a combination of the actuating force F (as per line (i)), the static friction force FFbefore increase by the additional actuator 45 (as per line (ii)), and the increase in the static frictional force FFdue to the additional actuator 45. The offset between the lines (i)+(ii) and (i)+(ii)+add corresponds to the increase in the static frictional force FF due to the additional actuator 45. As apparent from Figure 3A, the increase in the static frictional force FFdue to actuation of the additional actuator 45 is such that F + FF> L + FEfor the entire range of stresses in the actuating SMA element 40. The movable part 20 may thus reliably be held in position, even at the edges of the range of movement where the load force L may be large, during the transient period in which the actuating SMA element 40 undergoes the transition from a low input force Fi to a high input force Fi or vice versa, i.e. even then the SMA element 40 is powered on or off.

[0087] Figure 4 schematically shows an actuator assembly 1 corresponding to the actuator assembly 1 described in relation to Figure 1, except that the actuator assembly 1 comprises an additional actuator 45 in the form of an additional SMA element 45. The additional SMA element 45 is connected between the support structure 10 and the movable part 20, for example by connection elements such as crimps (not shown). The additional SMA element 45 is arranged to increase the static frictional force when actuated. In general, the additional SMA element 45 may directly or indirectly apply an additional force to the movable part 20 that urges the movable part 20 against the support structure 10, so as to urge the first and second friction surfaces lOf, 20f against each other. The additional force thus acts in the same direction as the normal force applied by the biasing element 30. The additional force acts in the opposite direction to the force component of the input forces Fi applied by the SMA elements 40 that are perpendicular to the direction of movement of the movable part 20.

[0088] The additional SMA element 45 may increase the static frictional force during a power-on period or during a power-off period of the SMA elements 40. As explained in relation to Figures 3A and 3B, slipping of the movable part 20 may occur specifically in situations in which the tension in the SMA wires 40, or generally the input force Fi of the SMA element 40, is transitioning between an operating value that is relatively high and an off-value that is relatively low. Such transitioning may happen during the power-on period or during the power-off period. During the power-on period, power (in the form of electrical power, such as electrical current) is supplied to at least one of the SMA elements 40 so as to initiate actuation of the SMA element 40. During the power-off period, the power supplied to at least one of the SMA elements 40 is decreased so as to cease actuation of the at SMA element 40. The additional actuator 40 may be controlled so as to increase the static frictional force specifically (e.g. only) during the power-on period or during a power-off period of the SMA elements 40. The additional actuator 40 may be controlled so as to cease increasing the static frictional force on completion of the power-on period or on completion of the power-off period of the SMA elements 40, for example when the SMA element 40 is at an operating temperature (or generally above an upper phase transition temperature of the SMA material) or when the SMA element 40 has cooled close to ambient temperature (or generally below a lower phase transition temperature of the SMA material). The additional actuator 40 may not be controlled to increase the static frictional force (e.g. not be controlled to be actuated) during normal operation of the actuator assembly 1, for example not during any periods other than the power-on period or power-off period.

[0089] As also explained in relation to Figures 3A and 3B, slippage of the movable part 20 may occur specifically when the movable part 20 is at an edge of the range of movement of the movable part 20 relative to the support structure 10, where the load force L may be largest. As such, the additional actuator 45 may be controlled to increase the static frictional force during the power-on period and / or during the power-off period specifically when (e.g only when) the second part is positioned at least at a pre-determined distance from a reference position (such as a central position within a range of movement) of the movable part 20 relative to the support structure 10.

[0090] As also explained in relation to Figures 3A and 3B, it may be desirable to drive actuation of only the actuating SMA element 40 and not to actuate the opposing SMA element 40, for example at the edge of the range of movement. Providing the additional actuator 45, so as to temporarily increase the static frictional force, may thus allow power to be supplied to only one of the SMA elements 40 to cause movement of the movable part 20 in an actuator assembly 2 with zero hold power functionality.

[0091] The additional SMA element 45 may increase the static frictional force compared to a situation in which the additional SMA element 45 is not actuated. With reference to Figure 4, for example, when the additional SMA actuator 45 is actuated during a power-on period of one or both of the SMA elements 40, the static frictional force may be reduced due to actuation of the SMA elements 40. However, such a reduction in the static frictional force may be less or slower due to the actuation of the additional SMA element 45, because the additional SMA element 45 acts to increase the static frictional force.

[0092] The additional SMA element 45 modulates the frictional force but is typically not required to effect large displacement of the movable part 20 or any other part. As such, the additional SMA element 45 may be relatively short compared to the SMA elements 40 that drive movement of the movable part 20. The additional SMA element 45 may be less than half, or less than a quarter, of the length of the SMA elements 40. A ratio of the length of the SM A elements 40 to the length of the additional SMA element may be greater than 2, preferably greater than 4. The ratio may be greater than 8, for example.

[0093] Figures 5A and 5B show further actuator assemblies 1 comprising the additional SMA element 45. The actuator assemblies 1 generally correspond to the actuator assembly 1 described in relation to Figure 4, except that the additional actuator 45 is not connected directly between the support structure 10 and the movable part 20. The actuator assemblies 5A and 5B comprise an additional part 10b, 20b. The additional SMA element 45 is connected between the additional part 10b, 20b and one of the support structure 10 (as in Figure 5A) and the movable part 20 (as in Figure 5B).

[0094] In Figure 5A, the additional SMA element 45 is connected between the additional part 10b and the support structure 10, i.e. one end of the additional SMA element 45 is connected to the additional part 10b and the other end of the additional SMA element 45 is connected to the support structure 10. The additional part 10b may be considered to form a second portion 10b of the support structure 10. The support structure 10 of Figure 5A may thus comprise a first portion 10a and a second portion 10b as described in relation to Figure 2, for example. The additional part 10b is constrained from moving with the movable part 20, for example by a suitable bearing arrangement.

[0095] A decoupling bearing arrangement 52 is arranged between the additional part 10b and the movable part 20. The decoupling bearing arrangement 52 decouples the additional actuator 45 from the movable part 20, so as to allow movement of the movable part 20 relative to the additional SMA element 45. Compared to the actuator assembly 1 of Figure 4, the additional SMA element 45 does not move with the movable part 20, in particular none of the portions or ends of the additional SMA element 45 move with the movable part 20. The additional SMA element 45 thereby does not affect the movement of the movable part 20 relative to the support structure 10, compared to the actuator assembly 1 of Figure 4 in which the additional SMA element 45 may apply a lateral force component along the movement direction when the movable part 20 is displaced from the reference position shown in Figure 4.

[0096] As shown, the decoupling bearing arrangement 52 may comprise a rolling bearing. Such a rolling bearing may comprise bearing surfaces and a rolling bearing element, such as a ball or roller, configured to roll along the bearing surfaces. In general, the decoupling bearing arrangement 52 may comprise any mechanism allowing decoupling of the additional SMA element 45 from the movement of the movable part 20 relative to the support structure 10. The decoupling bearing arrangement 52 may, for example, comprise a plain bearing (e.g. a low-friction plain bearing) or a flexure bearing.

[0097] Figure 5A further shows an additional biasing arrangement 35 that loads the additional bearing arrangement 52. The additional biasing arrangement 35 is connected between the support structure 10 and the additional part 10b. The additional biasing arrangement 35 applies a biasing force, thereby sandwiching the movable part between the support structure 10 and the additional part 10b. The biasing force applied by the additional biasing arrangement 35 further loads the first and second friction surfaces lOf, 20f via the additional part 10b, the decoupling bearing arrangement 52 and the movable part 20. The additional biasing arrangement 35 thereby aids the biasing arrangement 30 in applying the normal force between the first and second friction surfaces lOf, 20f. Indeed, in some embodiments similar to Figure 2, the additional biasing arrangement 35 may act as the main biasing arrangement such that the biasing arrangement 30 depicted in Figure 5A can be omitted.

[0098] In Figure 5B, the additional SMA element 45 is connected between the additional part 20b and the movable part 20, i.e. one end of the additional SMA element 45 is connected to the additional part 20b and the other end of the additional SMA element 45 is connected to the movable part 20. The additional part 20b may be considered to form a second portion 20b of the movable part 20. The movable part 20 of Figure 5B may thus be considered to comprise a first portion and a second portion 20b, in analogous manner to the first and second portions 10a, 10b described in relation to Figure 2, for example. The additional part 20b is coupled to the movable part 20 so as to move with the movable part 20, for example by a suitable bearing arrangement (not shown).

[0099] A decoupling bearing arrangement 52 is arranged between the additional part 20b and the support structure 10. The decoupling bearing arrangement 52 decouples the additional actuator 45 from the support structure 10, so as to allow movement of the additional SMA element 45 relative to the support structure 10. Compared to the actuator assembly 1 of Figure 4, the entire additional SMA element 45 moves with the movable part 20, in particular both ends of the additional SMA element 45 move with the movable part 20. The additional SMA element 45 thereby does not affect the movement of the movable part 20 relative to the support structure 10, compared to the actuator assembly 1 of Figure 4. The decoupling bearing arrangement 52 of Figure 5B may generally be configured as described in relation to Figure 5A. Although not shown in Figure 5B, an additional biasing element may bias the additional part 20 against the support structure 10 so as to load the decoupling bearing arrangement 52. The additional biasing element may be connected between the movable part 20 and the additional part 20b, for example to sandwich a portion of the support structure 10 and the decoupling bearing arrangement 52 between the additional part 20b and the movable part 20.

[0100] Figure 6 schematically shows a further actuator assembly 1 comprising the additional SMA element 45. The actuator assembly 1 generally corresponds to the actuator assembly 1 described in relation to Figure 2, except that the actuator assembly 1 comprises an additional actuator 45 in the form of an additional SMA element 45.

[0101] The additional SMA element 45 is connected between the first portion 10a and the second portion 10b of the support structure 10, or more generally between the additional part 10b and the support structure 10 corresponding to the first part 10a depicted in Figure 6. The additional SMA element 45 may thereby apply the additional force, via the bearing arrangement 50 and the movable part 20, to modulate the frictional force between the first and second friction surfaces lOf, 20f.

[0102] In Figure 6, the additional SMA element 45 is decoupled from the movement of the movable part 20 by the bearing arrangement 50 that guides movement of the movable part 20 relative to the support structure 10. The same bearing arrangement 50 may thus be used to guide movement of the movable part 20 and to decouple the additional SMA element 45 from such movement. The same bearing arrangement 50 may also decouple the biasing arrangement 30 from movement of the movable part, as explained in relation to Figure 2.

[0103] Figure 7 shows a further actuator assembly 1. The actuator assemblies 1 generally correspond to the actuator assembly 1 described in relation to Figure 5B, except that two additional actuators 45 in the form of two additional SMA elements 45 are provided. Each of the two additional SMA elements 45 is configured as described in relation to Figure 5B. The two additional SMA elements 45 are spaced apart from each other in a direction perpendicular to the length of the two additional SMA elements 45, i.e. spaced apart along an axis that is perpendicular to the normal force. The two additional SMA elements 45 may thus apply additional forces to the movable part at locations that are on either side of the locations at which the input forces Fi by the SMA elements 40 are applied. Compared to the actuator assembly of Figure 5B, for example, movement of the movable part 20 may be more stable by providing the two additional SMA elements 40. In Figure 5B, the SMA elements 40 and the additional SMA element 45 act on a common point on the movable part 20, such that the movable part 20 may rotate when the friction surfaces lOf, 20f are disengaged if such rotation is not otherwise constrained (for example by a suitable bearing arrangement). Providing the two additional SMA actuators 45 may allow control or constraining of any such rotation, thereby allowing movement of the movable part 20 to be more reliable.

[0104] In general, the two SMA elements 40 need not act on the same location of the movable part 20, unlike shown in Figures 4, 5, 7 and 8. The two SMA elements 40 may act at locations on the movable part 20 that are spaced apart, such as shown in Figures 1 and 2.

[0105] Although the two additional SMA elements 45 are shown as parallel to each other in Figure 7, in general the two additional SMA elements 45 may be angled relative to each other, for example in equal and opposite ways relative to an axis along the normal force urging the first and second friction surfaces lOf, 20f against each other.

[0106] The two additional SMA elements 45 of Figure 7, or indeed more than the two additional SMA elements 45, may be provided in any of the other actuator assemblies 1 described herein.

[0107] Figure 8 shows a further actuator assembly 1. The actuator assemblies 1 generally correspond to the actuator assembly 1 described in relation to Figure 4, except that the additional actuator 45 reduces the static frictional force on actuation.

[0108] The additional SMA element 45 of the actuator assemblies 1 of Figures 4 to 7 is configured to increase the static frictional force on actuation. However, in general, the additional SMA element 45 may otherwise modulate the static friction force on actuation, for example reduce the static frictional force on modulation. With reference to Figure 8, for example, the additional SMA element 45 is arranged between the movable part 20 and an additional part 20b. The additional part 20b may move with the movable part 20 as described in relation to Figure 5B, for example. In Figure 8, the decoupling bearing arrangement 52 is embodied by a plain bearing, although any other suitable bearing arrangement 52 may be used. An additional biasing element 35 is connected between the additional part 20b and the movable part 20. The additional biasing element 35 urges the additional part 20b and the movable part 20 apart and against different portions of the support structure 10. The additional biasing element 35 thereby increases the normal force, and so the static frictional force, between the friction surfaces lOf, 20f. The plain bearing provided by the decoupling bearing arrangement 52 may provide another pair of friction surfaces (not labelled in Figure 8) so as to increase the static frictional force.

[0109] The additional SMA element 45 urges the additional part 20b and the movable part 20 towards each other on actuation. The additional SMA element 45 thereby reduces the normal force acting between the first and second friction surfaces lOf, 20f, and optionally between the other pair of friction surfaces provided by the decoupling bearing arrangement 52. The additional SMA element 45 may be driven in opposite manner to that described in relation to Figure 4 of the other embodiments. In particular, the additional SMA element 45 may be actuated during operation of the SMA elements 40, and may cease to actuate or may not actuate during the power-off period an / or the power-on period, so as to increase the static frictional force during the power-off period and / or power-on period.

[0110] Configuring the additional SMA element 45 such that the frictional force is decreased on actuation, as described in relation to Figure 8, may be less preferable compared to an additional SMA element 45 that increases the frictional force on actuation, as described in relation to Figures 4 and 7. The power-on and power-off periods are typically short compared to the operation period of the SMA elements 40, such that actuating the additional SMA element 45 during the operation period of the SMA elements 40 may be less energy efficient than actuating the additional SMA element 45 specifically during the power-on and power-off periods.

[0111] The additional SMA element 45 effecting increase of the static frictional force on actuation is connected between the movable part 20 and the additional part 20b in Figure 8. Alternatively, the additional SMA element 45 may be connected between an additional part 10b and the support structure, or between the support structure 10 and the movable part 20 in a manner similar to that described in relation to Figures 2, 5A, B, 6 or 7 and be arranged to effect increase of the static frictional force on actuation. Modifications or Alternatives

[0112] In the embodiments of Figures 4 to 8, the additional actuator 45 is depicted as modulating the static frictional force between the same friction surfaces lOf, 20f that are urged apart on actuation of the SMA elements 40. However, in general, the additional actuator 45 may act on a separate pair of friction surfaces lOf, 20f. Such a separate pair of friction surfaces lOf, 20f may, for example, be arranged in the plane of Figures 4 to 8, and the additional actuator 45 may apply a force into or out of the plane of Figures 4 to 8 so as to modulate the static frictional force. Such a separate pair of friction surfaces lOf, 20f could also be arranged between any intermediate part (not shown in the Figures) that is coupled between the movable part 20 and the support structure 10 and is configured to move upon movement with the movable part 20. The static frictional force modulated by the additional actuator 45 may thus in general be considered to be an overall static frictional force constraining movement of the movable part 20.

[0113] Modulation of the frictional force by the additional actuator 45 may be particularly advantageous in embodiments in which the SMA elements 40 disengage the friction surfaces lOf, 20f, as described in relation to Figures 3A and 3B. However, in general, provision of the additional actuator 45 also has benefits in other zero hold power actuator assemblies 1, for example when the SMA elements 40 are configured to reduce the normal force without disengaging the friction surfaces lOf, 20f. Especially when the normal force is reduced close to zero, there is a risk of slippage of the movable part 20 at the edges of the range of movement, which risk may be reduced by the provision of the additional actuator 45.

[0114] In general, the additional actuator 45 may be used to modulate the frictional force for a variety of reasons. For example, the provision of the additional actuator 45 and temporary increase during power-on or power-off may be used to allow the SMA elements 40 to be powered on or off more quickly, while reducing the risk of slippage or stick-slip behaviour due to a mismatch in dynamic and static friction coefficients of the friction surfaces. The power to the SMA elements 40 may be ramped up or down gradually to avoid such slippage or stick-slip behaviour in actuator assemblies 1 without the additional actuator 45. Powering the SMA elements 40 on or off more quickly than allowed by such a gradual ramp may improve the response time of the actuator assembly 1.

[0115] The additional actuators 45 of the embodiments of Figures 4 to 8 are depicted and described additional SMA elements 45. However, in general, the additional actuators 45 may be any actuator capable of applying a force for modulating the static frictional force. The additional actuator 45 may, for example, be a voice coil motor or a piezoelectric actuator. Such other types of additional actuator 45 may be configured to apply the additional force in a direction that is along the length of the SMA element 45 shown in Figures 4 to 8.

[0116] The present invention has been described in connection with SMA elements 40, for example in the form of SMA wires 40. The term '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.

[0117] The foregoing has described some embodiments of the present invention, but the present invention is not limited to these embodiments. The scope of the invention is defined in the appended claims.

Claims

Claims1. An actuator assembly comprising first and second parts that are movable relative to each other, the first part comprising a first friction surface and the second part comprising a second friction surface; at least two SMA elements configured, on actuation, to cause actuation forces in opposite directions to be applied to the second part for moving the second part in opposite directions relative to the first part; a biasing arrangement configured to apply a normal force biasing the first and second friction surfaces against each other, thereby giving rise to at least part of a static frictional force that is capable of constraining relative movement of the first and second parts when the at least two SMA elements are not actuated; wherein the at least two SMA elements are configured, on actuation, to cause reduction of the normal force between the first and second friction surfaces, thereby reducing the static frictional force; and further comprising at least one additional actuator configured, on actuation, to modulate the static frictional force.

2. An actuator assembly according to claim 1, wherein the at least one additional actuator is configured, on actuation, to increase the static frictional force.

3. An actuator assembly according to claim 1 or 2, wherein the at least two SMA elements are configured to reduce and the at least one additional actuator is configured to modulate the static frictional force between the same first and second friction surfaces.

4. An actuator assembly according to any one of the preceding claims, further comprising a controller configured control actuation of the at least two SMA elements and of the additional actuator, wherein the controller is configured to: during a power-on period, cause an increase of power supplied to at least one of the at least two SMA elements so as to initiate actuation of the at least one of the at least two SMA elements and / or during a power-off period cause a decrease of power supplied to at least one of at least two SMA elements so as to cease actuation of the at least one of the at least two SMA elements; and during the power-on period and / or during the power-off period, control actuation of the additional actuator so as to increase the static frictional force.

5. An actuator assembly according to claim 4, wherein the controller is configured to control actuation of the additional actuator so as to increase the static frictional force during the power-on period and / or during the power-off period only when the second part is positioned at least a predetermined distance from a reference position of the second part relative to the first part.

6. An actuator assembly according to claim 4 or 5, wherein the controller is configured to cause power to be supplied to only one of the at least two SMA elements so as to cause movement of the second part relative to the first part.

7. An actuator assembly according to any one of the preceding claims, wherein the at least two SMA elements are configured, on actuation, to disengage the first and second friction surfaces.

8. An actuator assembly according to any one of the preceding claims, wherein the at least one additional actuator is configured, on actuation, not to affect relative movement of the first and second parts.

9. An actuator assembly according to any one of the preceding claims, comprising a decoupling bearing arrangement configured to decouple actuation of the at least one additional actuator from the relative movement of the first and second parts.

10. An actuator assembly according to claim 9, comprising an additional part, wherein the additional actuator is arranged between the additional part and one of the first and second parts, and wherein the decoupling bearing arrangement is arranged between the additional part and the other of the first and second parts.

11. An actuator assembly according to any one of the preceding claims, comprising a guide bearing arrangement configured to guide relative movement of the first and second parts.

12. An actuator assembly according to claim 11, wherein the first and second friction surfaces are separate from the guide bearing arrangement, and wherein the at least two SMA elements are arranged, on actuation, to disengage the first and second friction surfaces such that relative movement of the first and second parts is guided by the guide bearing arrangement.

13. An actuator assembly according to claim 9 or 10 and according to claim 11 or 12, wherein the same bearing arrangement acts as the decoupling bearing arrangement configured to decouple the actuation of the at least one additional actuator from the relative movement of the first and second parts and as the guide bearing arrangement configured to guide relative movement of the first and second parts.

14. An actuator assembly according to any one of claims 9 to 13, wherein the decoupling bearing arrangement and / or the guiding bearing arrangement comprises a rolling bearing.

15. An actuator assembly according to any one of the preceding claims, further comprising an endstop that comprises two endstop surfaces spaced apart from one another when the at least two SMA elements are not actuated, wherein the at least two SMA elements are arranged, on actuation, to disengage the first and second friction surfaces and to engage the endstop.

16. An actuator assembly according to any one of the preceding claims, wherein each of the at least one additional actuator comprises an additional SMA element that is arranged, on actuation, to modulate the static frictional force.

17. An actuator assembly according to claim 16, wherein a ratio of i) a length of each one of the at least two SMA elements to ii) a length of each one of the at least one additional SMA element is greater than 2, preferably greater than 4.

18. An actuator assembly according to any one of the preceding claims, wherein the at least one additional actuator comprises at least two additional actuators, wherein the actuation forces are applied to the second part by the at least two SMA elements at a location between additional forces applied to the second part by the at least two additional actuators.

19. An actuator assembly according to any one of the preceding claims, wherein the first and second parts are translationally movable relative to each other in one degree of freedom.

20. A method for driving actuation of the actuator assembly of any one of the preceding claims, wherein the method comprises causing power to be supplied to at least one of the at least two SMA elements so as to modulate the static frictional force and to control movement of the second partrelative to the first part, and wherein the method further comprises causing power to be supplied to the at least one additional actuator so as to modulate the static frictional force.

21. A method according to claim 20, comprising during a power-on period causing an increase of power supplied to at least one of the at least two SMA elements so as to initiate actuation of the at least one of the at least two SMA elements and / or during a power-off period causing a decrease of power supplied to at least one of the at least two SMA elements so as to cease actuation of the at least one of the at least two SMA elements; and during the power-on period and / or during the power-off period, controlling actuation of the additional actuator so as to increase the static frictional force.

22. A method according to claim 20 or 21, comprising obtaining a measure indicative of the position of the second part compared to a reference position, and controlling actuation of the additional actuator so as to increase the static frictional force during the power-on period and / or during the power-off period only when the measure is indicative of the second part being positioned at least a pre-determined distance from the reference position.

23. A method according to any one of claims 20 to 22, comprising causing power to be supplied to only one of the at least two SMA elements.

24. A non-transitory computer-readable medium comprising instructions for carrying out the method of any one of claims 20 to 23.

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