GB2607108A

GB2607108A - SMA actuator assembly

Info

Publication number: GB2607108A
Application number: GB2107720.1A
Authority: GB
Inventors: Howarth James; Benjamin Simpson Brown Andrew; Eddington Robin; Farmer Geoffrey; matthew bunting Stephen
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2021-05-28
Filing date: 2021-05-28
Publication date: 2022-11-30
Anticipated expiration: 2041-05-28
Also published as: GB202107720D0; GB2607108B

Abstract

An SMA actuator assembly comprises a rotating part 40 with a rotation axis R and a movable part 20 arranged to move in a plane perpendicular to the rotation axis and along a boundary 45 of the rotating part. One of the rotating part 40 and the movable part 20 surrounds the other part in the plane perpendicular to the rotation axis. Three or more SMA wires 30 are arranged, on contraction, to move the movable part 20 along the boundary 45 of the rotating part 40, such that contact between the movable part 20 and the rotating part 40 drives continuous rotation of the rotating part about the rotation axis, thereby converting linear to rotary movement in a compact arrangement. The three or more SMA wires are capable of applying a torque to the movable part.

Description

SMA ACTUATOR ASSEMBLY

Field

The present invention generally relates to a shape memory alloy (SMA) actuator assembly, and in particular to rotary actuators or motors that are driven by SMA wires.

Background

Miniature rotary actuators find application in a wide variety of fields, for example as motors used to drive a pop-up camera in a smartphone, a panning camera in a drone, a miniature pump (e.g. for a medical device), a camera shutter or iris shutter, haptic devices/mechanisms, and medical devices. SMA wires may be particularly useful as actuators in miniature rotary actuators. Due to their high energy density, SMA wires may apply relatively high forces while taking up relatively little space in the rotary actuator. There remains a need for improved SMA rotary actuators that efficiently make use of the contraction forces of SMA wires to generate an output torque.

Summary

According to the present invention, there is provided an SMA actuator assembly comprising: a rotating part arranged to rotate about a rotation axis; a movable part arranged to move in a plane perpendicular to the rotation axis and along a boundary of the rotating part, wherein one of the rotating part and the movable part surrounds the other of the rotating part and the movable part in the plane perpendicular to the rotation axis; and three or more SMA wires arranged, on contraction, to move the movable part along the boundary of the rotating part, such that contact between the movable part and the rotating part drives continuous rotation of the rotating part about the rotation axis, wherein the arrangement of the three or more SMA wires is capable of applying a torque to the movable part.

Further aspects of the present invention are set out in the dependent claims.

Brief description of the drawings

Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schematic plan view of an SMA actuator assembly; Figs. 2a and 2b are schematic side views embodiments of the SMA actuator assembly; Figs. 3a and 3b are schematic plan and side views of another embodiment of the SMA actuator 35 assembly.

Detailed description

Broadly speaking, the present techniques provide continuous drive motors that are driven by shape memory alloy (SMA) wires. The present techniques use SMA wires to translationally move a movable member, which can in turn cause a rotating part to rotate. The present techniques may advantageously enable fine control of the velocity of the rotation the position of the rotating part, and may be enable the rotation to be driven continuously. The present techniques may, for example, be used to actuate a motor used to drive a pop-up camera in a smartphone, a panning camera in a drone, a miniature pump (e.g. for a medical device), a camera shutter or iris shutter, haptic devices/mechanisms, and medical devices. It will be understood that this is a non-exhaustive and non-limiting list of example applications/uses of the actuator assemblies described herein.

The SMA actuator assembly Figure 1 shows a schematic plan view of an SMA actuator assembly 1 according to an embodiment of the present invention. Figures 2a and 2b show schematic cross-sectional views of different embodiments of the SMA actuator assembly 1. The SMA actuator assembly 1 comprises a support structure 10, a movable part 20 and a rotating part 40. The SMA actuator assembly 1 further comprises SMA wires 30. The support structure 10 is used as a reference point to describe movement of the rotating part 40 and the movable part 20 herein. When the SMA actuator assembly 1 is included in a device, such as a smartphone, a drone, or any other device, the support structure 10 may be fixed relative to a main body of the device. However, in general the support structure 10 need not necessarily be stationary and may be movable relative to or within such a device.

The rotating part 40 is arranged to rotate about a rotation axis R. In figures land 2, the rotation axis R extends in the z direction, so movement along the rotation axis R is herein also referred to as movement along the z axis. The plane orthogonal to the rotation axis R is herein also referred to as the x-y plane. The rotation axis R may be fixed relative to the support structure 10. So, the rotating part 40 may be arranged to rotate relative to the support structure 10 about the rotation axis R. The rotating part 40 may be arranged to only rotate relative to the support structure 10 about the rotation axis R, so translational movement of the rotating part 40 relative to the support structure 10 may be constrained.

The movable part 20 is arranged to move in a plane (e.g. the x-y plane) perpendicular to the rotation axis R. The In some embodiments, the movable part 20 may be movable freely in the plane within a range of movement, i.e. translation along the x axis, translation along the y axis and rotation about the z axis may be relatively unconstrained within the range of movement. In alternative embodiments, the movable part 20 may be movable specifically along a constrained path in the plane, as will be explained in further detail below. In either case, the movable part 20 is movable along a boundary 45 of the rotating part 40 in the x-y plane. In the embodiment of figure 1, for example, the movable part 20 is movable along the circular boundary 45 of the rotating part 40.

The SMA wires 30 drive movement of the movable part 20. SMA material has the property that on heating it undergoes a solid-state phase change which causes the SMA material to contract. A range of contraction occurs as the temperature of the SMA increases over the range of temperature in which there occurs the transition of the SMA material from the Martensite phase to the Austenite phase.

Conversely, on cooling of the SMA wires 30 so that the stress therein decreases, the SMA wires 30 expand under tensile forces (e.g. from opposing ones of the SMA wires 30 or from springs or other resilient elements). This allows the movable part 20 to move in the opposite direction.

The position of the movable part 20 relative to the support structure 10 in the x-y plane may thus be controlled by selectively varying the temperature of the SMA wires 30. This may be achieved by passing through SMA wires 30 selective drive currents that provide resistive heating. Heating is provided directly by the drive current. Cooling is provided by reducing or ceasing the drive current to allow the SMA wires 30 to cool by conduction, convection and/or radiation to the surroundings.

The SMA wires 30 are arranged, on contraction, to move the movable part 20. For example, the SMA wires 30 may be connected at one end to the support structure 10 and at the other end to the movable part 20. The SMA wires 30 may move the movable part 20 translationally in the x-y plane, and in particular along the boundary 45 of the rotating part 40. In the depicted embodiment, the SMA wires move the movable part 20 along a circular path so as to move along the boundary 45 of the rotation part. In general, the movement path of the movable part 20 may be non-circular for other geometries of the rotating part 40 and/or movable part 20. The movable part 20 remains in constant contact with the rotating part 40 as it moves around the rotating part 40. Movement of the movable part 20 drives rotation of the rotating part 40. For example, as shown in figure 1, translational movement of the movable part 20 in direction 22 drives rotation 46 of the rotating part 40 around the rotation axis R. The SMA actuator assembly 1 is thus capable of generating continuous rotation. The rotating part 40 can be rotated indefinitely and without interruption. The SMA actuator assembly 1 may thus be used in any application requiring rotation or torque. Using SMA wires 30 to ultimately drive rotation has the added benefit of providing rotation with a relatively high torque using relatively little electrical power, due to the high energy density of SMA. The SMA actuator assembly 1 can further be made compact compared to motors making use of other actuators, such as induction coils.

Bearing arrangements The components of the SMA actuator assembly 1, in particular the support structure 10, the movable part 20 and the rotating part 40, as well as bearing arrangements defining movement of these components relative to one another, are described below in more detail.

The rotating part 40 is rotatable about the rotation axis R relative to the support structure 10. The SMA actuator assembly 1 may comprise a bearing arrangement 1040 between the rotating part 40 and the support structure 10. The bearing arrangement 1040 allows rotation about the rotation axis R of the rotating part 40 relative to the support structure 10. The bearing arrangement 1040 constrains translational movement, for example in the x-y plane and/or along the rotation axis R, of the rotating part 40 relative to the support structure 10. The bearing arrangement 1040 also constrains tilting about the x or y axes. In the depicted embodiment, the bearing arrangement 1040 comprises a plain bearing between the rotating part 40 and the support structure 10, i.e. each of the rotating part 40 and the support structure 10 comprise bearing surfaces that directly bear onto each other. In general, any other suitable type of bearing (such as a roller or ball bearing) may be used instead of the plain bearing.

As shown in Fig. 2a, the rotating part 40 may comprises a rotating shaft 41 and a driven part 42. The rotating shaft 41 and the driven part 42 are fixedly arranged relative to each other.

The rotating shaft 41 may output the rotation generated by the SMA actuator assembly 1. When the SMA actuator assembly 1 is included in a device, the rotating shaft 41 may be connected to a part of the device that is to be rotated, such as to a pop-up camera in a smartphone or a panning camera of a drone. In the depicted embodiment, the rotating shaft 41 is inserted into an aperture in the support structure 10 to thereby constrain movement of the rotating part 40 in the x-y plane and tilting of the rotating part 40 about the x and y axes. The part of the rotating shaft 41 inserted into the aperture of the support structure 10 thus forms part of the bearing arrangement 1040 between rotating part 40 and support structure 10.

In the embodiment of Fig. 2a, the driven part 42 comprises a bearing surface that bears directly onto a corresponding bearing surface of the support structure 10, thus constraining movement of the rotating part 40 along the z-axis. These bearing surfaces thus form part of the bearing arrangement 1040 between the rotating part 40 and the support structure 10. The bearing arrangement 1040 optionally further comprises a resilient element (not shown), such as a flexure or other spring, to urge the bearing surfaces towards each other.

The movable part 20 is movable in the x-y plane relative to the rotation axis R (and so movable in the x-y plane relative to the rotating part 40 and relative to the support structure 10). In the embodiment of Fig. 2a, a bearing arrangement 1020 between the movable part 20 and the support structure 10 allows movement in the x-y plane. The bearing arrangement 1020 may constrain movement of the movable part 20 along the z-axis (so along the rotation axis R). The bearing arrangement 1020 of the depicted embodiment comprises a plain bearing 1020a that allows movement in the x-y plane (and constrains movement along the z-axis). The plain bearing 1020a comprises a bearing surface of the movable part 20 bearing directly upon a corresponding bearing surface of the support structure 10.

Optionally, the SMA actuator assembly 1 comprises a bearing arrangement that constrains movement of the movable part 20 to a path along the boundary of the rotating part 40. In the embodiment of Fig. 2a, for example, the bearing arrangement 1020 between the movable part 20 and the support structure 10 comprises a cam shaft 1020b that constrains movement of the movable part 20 to a circular path (which circular path runs along the boundary of the rotating part 40). The bearing arrangement 1020 may comprise multiple such cam shafts 1020b. Generally, any bearing arrangement that can constrain movement of the movable part 20 to a path along the boundary of the rotating part 40 may be used instead of the cam shaft 1020, such as a ball bearing comprising a ball guided in circular tracks in the movable part 20 and/or support structure 10.

Bearing plate Figure 2b depicts an alternative embodiment in which the rotating part 40 further comprises a bearing plate 43. The bearing plate 43 is fixed relative to the driven part 42 and the rotating shaft 41. In some embodiments, the bearing plate 43 is integrally formed with the rotating part 40. The bearing plate 43 comprises a bearing surface extending in the x-y plane that bears against the support structure 10, thereby providing part of the bearing arrangement 1040 between the rotating part 40 and the support structure 10. The bearing plate 43 further comprises a bearing surface extending in the x-y plane that bears against the movable part 20, thereby providing a bearing arrangement 2040 between the rotating part 40 and the movable part 20.

Compared to the embodiment of Fig. 2a, there is no bearing arrangement 1020 between the movable part 20 and the support structure 10. Instead, the bearing arrangement 2040 between the movable part 20 and the rotating part 40 allows movement of the movable part 20 in the x-y plane. The bearing arrangement 2040 may constrain movement of the movable part 20 along the z-axis (so along the rotation axis R).

A resilient element, such as a flexure or other spring, may be provided to apply a spring force urging the movable part 20 against the bearing plate 43. The resilient element may, for example, be connected between the movable part 20 and the support structure 10. The spring force may further urge the bearing plate 43 against the support structure 10, thereby ensuring that the bearing surfaces of the bearing arrangements 2040 and 1040 remain in contact with one another.

The resilient element may apply the spring force urging the movable part 40 against the bearing plate 32, and the bearing plate 43 against the support structure 10, to thereby constrain movement of the rotating part 40 relative to the movable part 20 when the SMA wires 30 are not powered. So, in the absence of contraction of the SMA wires 30, the bearing plate 43 may be constrained from rotating, and the movable part 20 may be constrained from moving, by virtue of the friction forces in the plain bearings. Optionally, the SMA wires 30 may be angled relative to the x-y plane so as to, on contraction, oppose the spring force by the resilient element. This may reduce the frictional forces at the plain bearings on contraction of the SMA wires 30.

As an alternative or addition to providing the resilient element, the SMA wires 30 may also be angled relative to the x-y axis so as to urge any of the above-described plain bearings towards one another.

In alternative embodiments (not shown), the movable part 20 may comprise (and optionally be integrally formed with) the bearing plate 43. A surface of the bearing plate 43 may bear against the rotating part 40, thereby providing the bearing arrangement 2040 between the movable part 20 and the rotating part 40. Gears

The driven part 42 is, in use, in contact with the movable part 20. As will be explained in more detail below, movement of the movable part 20 may, through contact with the rotating part 40 (and in particular through contact with the driven part 42), drive rotation of the rotating part 40. The rotating part 40 may comprise a contact surface (e.g. on the driven part 42) that is in contact with a complementary contact surface of the movable part 20. The contact surfaces may be configured to constrain, i.e. to limit or even to prevent, sliding between the rotating part 40 and the movable part 20 along the boundary 45 of the rotating part 40. In the depicted embodiments, sliding is thus prevented in a tangential direction around the rotation axis R. In the depicted embodiment, the driven part 42 is an inner gear 42a and the movable part 20 comprises an outer gear 22a. The gears 22a, 42a comprise complementary teeth, i.e. the teeth of the rotating part 40 and the teeth of the movable part 20 intermesh. The outer gear 22a comprises a greater number of teeth than the inner gear 42a. In particular, the outer gear 22a comprises at least one more tooth than the inner gear 42a. The outer gear 22a may, for example, comprise a number of teeth that is greater than 10, preferably greater than 25, further preferably greater than 50. The inner gear 42a may comprise, for example at least 1 fewer tooth than the outer gear 22a, or 2 or more fewer teeth than the outer gear.

In alternative embodiments, the movable part 20 and the rotating part 40 do not comprise complimentary teeth. Instead, the movable part 20 and the rotating part 40 may comprise surfaces that constrain sliding between the movable part 20 and the rotating part 40 along the boundary 45 of the rotating part 40. The surfaces may, for example, be relatively rough to constrain sliding therebetween.

Multiple movable parts As schematically depicted in Figs. 3a and 3b, the SMA actuator assembly 1 may optionally comprise a second movable part 20'. The second movable part is arranged to move in the x-y plane and along the boundary 45 of the rotating part 40. In the depicted embodiment, the second movable part 40' surrounds the rotating part 40 in the x-y plane. The SMA actuator assembly 1 further comprises three or more further SMA wires 30'. The arrangement of the three of more further SMA wires 30 relative to the second movable part 20' may be as described in relation to the three or more SMA wires 30 relative to the movable part 20. The three or more further SMA wires 30' are arranged, on contraction, to move the second movable part 20' along the boundary 45 of the rotating part 40, such that contact between the second movable part 20' and the rotating part 20 drives continuous rotation of the rotating part about the rotation axis R. The use of multiple movable parts (with multiple sets of SMA wires 30) may increase the torque of which the rotating part 40.

Preferably, the contact region between the second movable part 20' and the rotating part 40 is diametrically opposite about the rotation axis R compared to the contact region between the movable part 20 and the rotating part 40. The forces Fl, F1' applied by the movable parts 20, 20' to the rotating part 40 in a radial direction relative to the rotation axis R may be equal and opposite. So, the net force on the rotating part 40 in the radial direction relative to the rotation axis R may be zero. This may reduces frictional forces in the bearing arrangement 1040 between the support structure 10 and the rotating part 40. In some embodiments, a bearing arrangement constraining translational movement of the rotating part 40 in the x-y plane may not be required.

As shown in Fig. 3h, the second movable part 20' may be offset from the movable part 20 along the rotation axis R, in particular in embodiments in which the movable parts 20, 20' surround the rotating part 20 in the x-y plane.

Arrangement of SMA wires The arrangement of SMA wires 30 is capable of applying a torque to the movable part 20. For example, the arrangement of SMA wires 30 comprises at least two SMA wires capable of applying a force couple to the movable part 20. The SMA wires 30 may apply forces to the movable part 20 with force components that are parallel but not colinear.

This allows the SMA wires 30 to control the torque applied to the movable part 20, thereby adjusting the rotation of the movable part 20 relative to the support structure 10. The SMA wires 30 may, in particular, be capable of moving the movable part 20 along the boundary 45 of the rotating part 40 without any net rotation of the movable part 20 relative to the support structure. This means that over the course of one or more rotations of the rotating part 40, the movable part 20 does not rotate about the rotation axis R. This may be achieved without any bearing arrangements that constrain rotation about the rotation axis R of the movable part 20, because the SMA wires 30 are capable of applying a torque. The arrangement of SMA wires 30 thus allows the SMA actuator assembly 1 to be used without any bearing arrangement that constrains rotation of the movable part 20 about the rotation axis R, reducing the manufacturing cost/complexity of the SMA actuator assembly 1. If such a bearing arrangement that constrains rotation of the movable part 20 about the rotation axis R (such as the cam shaft 1020b in Fig. 2b) is optionally used, then the arrangement of SMA wires 30 at least reduces the forces acting on such a bearing arrangement, thereby reducing the risk of fatigue of or damage to such a bearing arrangement.

In the embodiment of figure 1, the SMA actuator assembly comprises a total of four SMA wires 30. Each SMA wire 30 is connected between the support structure 10 and the movable part 20.

Each SMA wire 30 is held in tension, thereby applying a force between the movable part 20 and the support structure 10 in a direction perpendicular to the rotation axis R. In operation, the SMA wires 30 move the movable part 20 relative to the support structure 10 in two orthogonal directions perpendicular to the rotation axis R, i.e. in the x-y plane, as described further below.

The SMA wires 30 may each extend perpendicular to the rotation axis R. In some embodiments, the SMA wires 30 extend in a common plane which is advantageous in minimising the size of the SMA actuator assembly 1 along the rotation axis R. This arrangement also minimises the force on the movable part 20 (and any bearing arrangement supporting the movable part 20) in a direction parallel to the rotation axis R. Alternatively, the SMA wires 30 may be arranged inclined at a non-zero angle to the orthogonal directions perpendicular to the rotation axis R, which angle is preferably small. In this case, the SMA wires 30 in operation generate a component of force along the rotation axis R that may tend to tilt or to move the movable part 20 in a direction parallel to the rotation axis R. Such a component of force may be resisted by the bearing arrangement to provide movement in the orthogonal directions perpendicular to the rotation axis R. Conversely, the degree of inclination of the SMA wires 30 that provides acceptably small tilting or movement in a direction along the rotation axis R may be dependent on the stiffness of the bearing arrangement along the rotation axis R. Thus, relatively high inclinations are permissible in the case of the bearing arrangement having a high stiffness along the rotation axis R, for example when comprising a plain bearing or ball bearings.

In the case where the bearing arrangement comprises a plain bearing or ball bearings, it may even be desirable for the SMA wires 30 to be inclined with a significant component in a direction parallel to the rotation axis R such that the tension in the SMA wires 30 pushes the movable part 20 onto the plain bearing or onto the ball bearings.

Irrespective of whether the SMA wires 30 are perpendicular to the rotation axis R or inclined at a small angle to the plane perpendicular to the rotation axis R, the actuator assembly 1 can be made very compact, particularly in the direction along the rotation axis R. The SMA wires 30 are themselves very thin, typically of the order of 25p.nt in diameter, to ensure rapid heating and cooling. The arrangement of SMA wires 30 barely adds to the footprint of the actuator assembly 1 and may be made very thin in the direction along the rotation axis R, since the SMA wires 30 are laid essentially in a plane perpendicular to the rotation axis R in which they remain in operation. The height along the rotation axis R then depends on the thickness of the other components such as the connection elements 33 described below and the height necessary to allow manufacture.

The SMA wires 30 are connected at one end to the movable part 20 by respective connection elements 33a and at the other end to the support structure 10 by connection elements 33b. The connection elements 33a, 33b crimp the SMA wire 30 to hold it mechanically, optionally strengthened by the use of adhesive. The connection elements 33a, 33b also provide an electrical connection to the SMA wires 30.

The connection elements 33a, 33b may, for example, be crimping members However, any other suitable means for connecting the SMA wires 30 may alternatively be used.

As shown in figure 1, the SMA wires 30 may have an arrangement around the rotation axis R as follows.

Each of the SMA wires 30 is arranged along one side of the movable part 20. Thus, the SMA wires 30 are arranged in a loop at different angular positions around the rotation axis R. Thus, the four SMA actuator wires 30 consist of a first pair of SMA wires arranged on opposite sides of the rotation axis R and a second pair of SMA wires arranged on opposite sides of the rotation axis R. The first pair of SMA wires is capable on selective driving to move the movable part 20 relative to the support structure 10 in a first direction in said plane, and the second pair of SMA wires is capable on selective driving to move the movable part 20 relative to the support structure 10 in a second direction in said plane transverse to the first direction. Movement in directions other than parallel to the SMA wires 30 may be driven by a combination of actuation of these pairs of the SMA actuator wires 30 to provide a linear combination of movement in the transverse directions. Another way to view this movement is that simultaneous contraction of any pair of the SMA wires 30 that are adjacent each other in the loop will drive movement of the movable part 20 in a direction bisecting those two of the SMA wires 30 (diagonally in figure 1).

As a result, the SMA wires 30 are capable of being selectively driven to move movable part 20 relative to the support structure 10 to any position in a range of movement in two orthogonal directions perpendicular to the rotation axis R. The magnitude of the range of movement depends on the geometry and the range of contraction of the SMA wires 30 within their normal operating parameters.

The arrangement of the SMA wires 30 along respective sides of the movable part 20 assists in providing a compact arrangement since each of the SMA wires 30 fits largely or entirely within the profile of the movable part 20 as viewed from that side, unlike for example an arrangement in which wires extend radially of the rotation axis R, which would increase the footprint of the SMA actuator assembly. However as a result of not being radial, each SMA wire 30 individually applies a torque to the movable part 20 in the plane of the two orthogonal directions around the rotation axis R. Such torques may potentially increase the requirements any bearing arrangement which needs to resist any net torque, whilst permitting movement in that plane.

However, since none of the wires are collinear, they can be arranged to apply cancelling torques when operated together. Successive SMA wires 30 around the rotation axis R are connected to apply a force to the movable part 20 in alternate senses around the rotation axis R. That is, as viewed outwardly of the rotation axis R, one SMA wire 30 is connected at its left end to the support structure 10 and its right end to the movable part 20, but the next SMA wire 30 is connected at its left end to the movable part 20 and its right end to the support structure 10, and so on. As a result, successive SMA wires 30 around the rotation axis R also apply a torque in alternate senses around the rotation axis R. That is, one SMA wire applies a force to the movable part 20 in an anticlockwise sense, but the next SMA wire 30 applies a force to the movable part 20 in a clockwise sense, and so on.

This means that the first pair of SMA wires 30 generates a net torque to the movable part 20 in said plane around the rotation axis R in a first sense (e.g. anti-clockwise), and the second pair of SMA wires 30 generate a net torque to the movable part 20 in said plane around the rotation axis R that is in an opposite sense (e.g. clockwise). As a result, for an arbitrary degree of heating in each SMA wire 30, the torques tend to cancel.

Moreover, with this arrangement movement to any position in the range of movement may in principle be achieved without applying any net torque to the movable part 20 in the plane of the two orthogonal directions around the rotation axis R. To appreciate this, one can consider the first pair of SMA wires 30 separately from the second pair of SMA wires 30. For movement to any given position in two dimensions, the movement derived from the first pair of SMA wires 30 may be obtained with a range of stresses in the first pair of SMA wires 30, and hence with a range of torques in the first sense. Similarly the movement derived from the second pair of SMA wires 30 may be obtained with a range of stresses in the second pair of SMA wires 30, and hence with a range of torques in the second sense. This means the torques can be balanced by appropriate selection of the stresses in each SMA wire 30, based on a simply geometrical calculation relating the desired position and the arrangement of SMA wires 30.

In contrast, if all the SMA wires 30 were connected to apply a force to the movable part 20 in the same sense around the rotation axis R then they would always generate a net torque around the rotation axis R irrespective of how they were driven.

When moving the movable part 20 in other directions that are a linear combination of movement in directions X and Y, some degree of balancing is a natural effect of the arrangement, and indeed by appropriate selection of the forces generated in each one of the SMA wires 30, it is possible to cause the SMA wires 30 to generate no net torque around the rotation axis R. Furthermore, the SMA wires 30 may be controlled to cancel any torque on the movable part 20 that is due to contact of the movable part 20 with the rotating part 40. As such, the torque acting on the movable part 20 (due to forces from the SMA wires 30 and forces due to contact with the rotating part 40) during operation of the SMA actuator assembly 1 may be zero.

This reduction of torque around the rotation axis R reduces the tendency for the movable part 20 to rotate around the rotation axis R. The reduction or balancing of torques around the rotation axis R reduces the constraints on the bearing arrangement. In fact, in some embodiments, the constraints may be reduced to the extent that no bearing arrangement is needed, and the movable part 20 is instead supported by the SMA wires 30 themselves.

It is noted in particular that these benefits can be achieved in this actuator arrangement 10 employing just a single set of four SMA wires 30, which provides for a very simple and compact arrangement.

In this SMA actuator assembly 1, the SMA wires 30 extend in a common plane which is advantageous in minimising the size of the actuator assembly 1 along the rotation axis R. Alternatively, the SMA wires 30 could be offset from each other along the rotation axis R and still obtain the benefits described above, if they meet the more general requirement that projections of the four SMA wires 30 onto a notional plane perpendicular to the rotation axis R have the arrangement shown in figure 1 when viewed in that direction.

Alternative SMA wire arrangements The above-described embodiments of the SMA actuator assembly 1 comprise a total of four SMA wires 30. However, fewer SMA wires 30 may be used in some embodiments of SMA actuator assembly 1. For example, one of the SMA wires 30 of the above-described arrangement may be replaced by a spring or other resilient element. Alternatively, the SMA actuator assembly 1 may comprise a total of three SMA wires 30, wherein first and second SMA wires are arranged to, upon contraction, respectively apply a force to the movable part in first and second (orthogonal) directions in the x-y plane, and a third SMA wire is arranged to, upon contraction, apply a force to the movable part in a third direction in the x-y plane so as to oppose the force applied by the first and/or second SAM wires. So, the third SMA wire may for example apply a force in a diagonal direction (bisecting the x and y axes). The third SMA wire may extend in a direction other than the third direction, and the SMA actuator assembly may further comprises an intermediary element that is configured to redirect a tensile force in the third SMA wire along the third direction.

For example, the intermediary element may comprise a flexure arm connected at one end to the support structure and at the other end to the movable part, wherein the flexure arm is arranged so as to allow movement of the other end relative to the support structure in the third direction. The third SMA wire may be connected to the flexure arm and configured to, upon contraction, deflect the flexure arm, thereby urging the other end of the flexure arm in the third direction.

Alternatively, the intermediary element may be arranged between the third SMA wire and the movable part, wherein the third SMA wire bends around a contact region with the intermediary element, thereby forming two SMA portions on either side of the contact region, the two SMA portions being angled relative to each other.

In principle, embodiments of the present invention may require only two SMA wires 30 to control two degrees of freedom, in particular movement along the path along the boundary 45 of the rotating part (the first degree of freedom) and torque about the rotation axis R (the second degree of freedom).

Alternative embodiments In the embodiment of figure 1, the movable part 20 surrounds the rotating part 40. In alternative embodiments (not shown), the rotating part 40 may surround the movable part 20 in the x-y plane. For example, the SMA wires 30 may be connected to the part numbered "40" in figure 1 so as to move that part in the x-y plane. The part numbered "20" may be rotatable about a rotation axis. The SMA wires 30 may move the part numbered "40" along the inner boundary of the part numbered "20", to thereby drive rotation of the part numbered "20" about the rotation axis. In embodiments with the second movable part 20', the rotating part 40 may also surround that second movable part 20' in the x-y plane.

The above-described bearing arrangements 1020, 1040 and 2040 comprise plain bearings. A lubricant or other friction reducer may be provided on the bearing surfaces of the plain bearings so as to reduce friction between the bearing surfaces. In alternative embodiments (not shown), some or all of the bearing arrangements 1020, 1040 and 2040 may comprise other types of bearings, such as ball bearings or roller bearings. The bearing arrangement 1020 between the support structure 10 and the movable part 20 may also comprise an arrangement of flexures to guide movement of the movable part 20 relative to the support structure 10. The use of bearings other than plain bearings may further reduce friction between the support structure 10, the movable part 20 and the rotating part 40. Generally, the use of bearing arrangements is not required. The movable part 20 may be supported entirely by the SMA wires 30, for example. The rotating part 40 may be supported entirely by two movable parts 20, 20' in the embodiment of Fig. 3, for example.

The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible.

In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material.

Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires.

Those skilled in the art will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing the present disclosure, the present disclosure should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that the present invention has a broad range of applications, and that the embodiments may take a wide range of modifications without departing from any inventive concept as defined in the appended claims.

Claims

Claims 1. An SMA actuator assembly comprising: a rotating part arranged to rotate about a rotation axis; a movable part arranged to move in a plane perpendicular to the rotation axis and along a boundary of the rotating part, wherein one of the rotating part and the movable part surrounds the other of the rotating part and the movable part in the plane perpendicular to the rotation axis; and three or more SMA wires arranged, on contraction, to move the movable part along the boundary of the rotating part, such that contact between the movable part and the rotating part drives continuous rotation of the rotating part about the rotation axis, wherein the arrangement of the three or more SMA wires is capable of applying a torque to the movable part.
2. The SMA actuator assembly of claim 1, wherein the SMA wires are arranged to move the movable part along the boundary of the rotating part without net rotation of the movable part.
3. The SMA actuator assembly of any preceding claim, wherein the SMA wires arranged to translationally move the movable part along the boundary of the rotating part.
4. The SMA actuator assembly of any preceding claim, wherein the SMA wires comprise a total of four SMA wires in an arrangement wherein none of the forces applied by the SMA wires are collinear, and wherein the SMA wires are capable of being selectively driven to move the movable part relative to the support structure to any position along the boundary of the rotating part without rotating the movable part.
5. The SMA actuator assembly of claim 4, wherein two of the SMA wires are arranged to apply a torque to the movable part in said plane in a first sense around the rotation axis and the other two SMA wires are arranged to apply a torque to the movable part in said plane in a second, opposite sense around the rotation axis.
6. An apparatus according to claim 4 or 5, wherein the four SMA actuator wires are arranged in a loop at different angular positions around the rotation axis, successive SMA actuator wires around the primary axis being connected to apply a force to the movable element in alternate senses around the primary axis.
7. The SMA actuator assembly of any preceding claim, further comprising a bearing arrangement that allows movement of the movable part in the plane perpendicular to the rotation axis and rotation of the movable part about the rotation axis.
8. The SMA actuator assembly of any preceding claim, wherein the movable part surrounds the rotating part in the plane perpendicular to the rotation axis.
9. The SMA actuator assembly of any preceding claim, wherein the SMA wires are arranged, on contraction, to move the contact region between the movable part and the rotating part along the boundary of the rotating part in a first sense around the rotation axis, thereby driving continuous rotation of the rotating part in a second sense about the rotation axis, the second sense being opposite to the first sense.
10. The SMA actuator assembly of any preceding claim, further comprising a support structure, wherein the rotating part is configured to rotate relative to the support structure, and wherein the SMA wires are arranged, on contraction, to move the movable part relative to the support structure.
11. The SMA actuator assembly of claim 10, wherein the SMA wires are connected between the movable part and the support structure.
12. The SMA actuator assembly of any preceding claim, wherein the SMA wires are connected to the movable part by connection elements, wherein the connection elements are integrally formed with the movable part.
13. The SMA actuator assembly of any preceding claim, further comprising a bearing arrangement configured to constrain movement of the movable part to a path along the boundary of the rotating part.
14. The SMA actuator assembly of any preceding claim, further comprising a bearing plate with a bearing surface extending in the plane perpendicular to the rotation axis, wherein the bearing plate is fixed relative to one of the rotating part and the movable part and the bearing surface bears against the other one of the rotating part and the movable part.
15. The SMA actuator assembly of claim 14, wherein the bearing plate is integrally formed with the one of the rotating part and the movable part.
16. The SMA actuator assembly of claim 14 or 15, further comprising a resilient element configured to apply a spring force urging the other one of the rotating part and the movable part against the bearing surface.
17. The SMA actuator assembly of claim 16, wherein the SMA wires are angled relative to the plane perpendicular to the rotation axis so as to, on contraction, oppose the spring force by the resilient element.
18. The SMA actuator assembly of claim 16 or 17, wherein, in the absence of contraction of the SMA wires, the resilient element applying the spring force urging the other one of the rotating part and the movable part against the bearing surface is configured to constrain movement of the rotating part relative to the movable part.
19. The SMA actuator assembly of any preceding claim, wherein the rotating part and the movable part comprise respective contact surfaces that are in contact with one another, wherein the contact surfaces are configured to constrain sliding between the contact surfaces along the boundary of the rotating part.
20. The SMA actuator assembly of any preceding claim, wherein the rotating part and the movable part comprise gears with complimentary teeth.
21. The SMA actuator assembly of any preceding claim, wherein the one of the rotating part and the movable part that surrounds the other of the rotating part and the movable part comprises a greater number of teeth than the other of the rotating part and the movable part.
22. The SMA actuator assembly of any preceding claim, further comprising a second movable part arranged to move in a plane perpendicular to the rotation axis and along a boundary of the rotating part, wherein one of the rotating part and the second movable part surrounds the other of the rotating part and the second movable part in the plane perpendicular to the rotation axis; and three or more further SMA wires arranged, on contraction, to move the second movable part along the boundary of the rotating part, such that contact between the second movable part and the rotating part drives continuous rotation of the rotating part about the rotation axis.
23. The SMA actuator assembly of claim 22, wherein the contact region between the second movable part and the rotating part is diametrically opposite about the rotation axis compared to the contact region between the movable part and the rotating part.
24. The SMA actuator assembly of claim 22 or 23, wherein the movable part and the second movable part surround the rotating part, and wherein the second movable part is offset from the movable part along the rotation axis
25. The SMA actuator assembly of any preceding claim, wherein the rotating part and/or the movable part are formed from sheet material.