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.

According to the present invention, there is also provided 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 one 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, and a controller configured to: measure an electrical characteristic of the one or more SMA wires, and determine an angular position of the rotating part based on the electrical characteristic. According to the present invention, there is also 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 one or more SMA wires (optionally two 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. The SMA actuator assembly may further comprise a bearing arrangement that allows movement of the movable part in the plane perpendicular to the rotation axis and constrains rotation of the movable part about the rotation axis. The SMA actuator assembly may comprise any of the other features described herein and/or set out in the dependent claims.

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 assembly;

Fig 4 is a schematic plan view of another SMA actuator assembly; and Fig 5 is a schematic perspective view of a further SMA actuator assembly.

Detailed description

Broadly speaking, the present invention provides continuous drive motors that are driven by shape memory alloy (SMA) wires. The present invention uses SMA wires to translationally move a movable member, which can in turn cause a rotating part to rotate. The present invention may advantageously enable fine control of the velocity of the rotation the position of the rotating part, and may enable the rotation to be driven continuously. The present invention 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. Flowever, in general the support structure 10 need not necessarily be stationary and may be movable relative to or within such a device. In some embodiments, the rotating part 40 may be fixed relative to a main body of the device.

The rotating part 40 is arranged to rotate about a rotation axis R. In figures 1 and 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. 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 yet alternative embodiments, the movable part 20 may be movable specifically in the x and y directions without allowing rotation about the z axis. 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 or voltages, for example pulse-width modulaled (PWM) control signals, that provide resistive heating. Heating is provided directly by the drive signals. Cooling is provided by reducing or ceasing the drive signals 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 rotating part 40. 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 45 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 40 about the rotation axis R. The use of multiple movable parts (with multiple sets of SMA wires 30) may increase the torque at which the rotating part 40 is driven.

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 FI, FI' 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 reduce 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. 3b, 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 40 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 25pm 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 to 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 30 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 along the x and y axes, 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).

Embodiments of the present invention in which torque need not be controlled (such as those comprising a controller for determining the angular position of the SMA wire) may comprise a single SMA wire to move the movable part along the path along the boundary of the rotating part. A single SMA wire may be sufficient because the movable part may be moved in one degree of freedom along the path. Alternatively, two SMA wires may be provided in such embodiments. Optionally, the bearing arrangement in such embodiments may constrain rotation of the movable part about the rotation axis, thus avoiding the need to control the torque of the movable part about the rotation axis.

Measuring the angular position of the rotating part

Another advantage of using SMA wires 30 to ultimately drive rotation of the rotating part 40 is that the SMA wires 30 may be used to measure an angular position of the rotating part.

It is known that the length of an SMA wire 30 is related to an electrical characteristic of the SMA wire, in particular to a measure of the resistance of the SMA wire 30. So, the length of each SMA wires 30 is a function of the electrical characteristic of the respective SMA wire 30. The actuator assembly 1 may comprise a controller configured to measure the electrical characteristic. For example, the controller may apply a current pulse with known amplitude to each SMA wire 30 and measure the voltage across the respective SMA wire 30 to obtain a measure of the resistance of the respective SMA wire. Alternatively, a sense resistor may be connected in series with the SMA wires 30, a voltage pulse with known amplitude may be applied to both the SMA wire and sense resistor, and the voltage across the SMA wire may be measured to obtain a measure of the resistance of the respective SMA wire.

Generally, any method suitable for determining the electrical characteristic of the SMA wire may be used.

Measuring the electrical characteristics of the SMA wires 30 may allow the position of the movable part 20 to be determined. This is because the position of the movable part 20 depends directly on the length of the SMA wires 30. For example, with reference to Figure 1, the length (or extent of contraction/ elongation) of the top SMA wire 30 may be measured to determine the x coordinate of the movable part 20, and the length (or extent of contraction/ elongation) of the left SMA wire 30 may be measured to determine the y coordinate of the movable part 20. It is noted that parameters other than the length of the SMA wire 30, such as the ambient temperature, can affect the electrical characteristic of the SMA wire 30. So, other such parameters may also be taken into account to convert the measured electrical characteristic into a length of the SMA wire.

In particularly advantageous embodiments, the SMA wires 30 comprise two pairs of opposing SMA wires. Each pair of SMA wires comprises two SMA wires 30 that exert forces on the movable part 30 in opposite directions. The SMA wires 30 in each pair of SMA wires may be parallel. With reference to the example of Figure 1, the top and bottom SMA wires 30 make up a first pair of SMA wires and the left and right SMA wires 30 make up a second pair of SMA wires 30. Generally, the two pairs of SMA wires 30 may exert forces on the movable part in non-parallel directions, for example in directions that are perpendicular to one another.

An advantage of making use of opposing SMA wires is that the position of the movable part 20, and thus ultimately the angular position of the rotating part 40, may be more accurately determined. For example, with reference to Figure 1, the difference in the measure of resistance between the SMA wires of the first pair of SMA wires (i.e. the top and bottom SMA wires 30) may provide a measure of the x coordinate of the movable part 20. The difference in the measure of resistance between the SMA wires of the second pair of SMA wires (i.e. the left and right SMA wires 30) may provide a measure of the y coordinate of the movable part 20. Environmental parameters (such as the ambient temperature) may affect both SMA wires in each pair of SMA wires equally and so the effect of such environmental parameters on the measurement of the position of the movable part 20 may cancel out by determining specifically the difference in electrical characteristic of opposing SMA wires. The translational position of the movable part 20 may thus more accurately be determined from the electrical characteristic of the SMA wires 30. It is noted that measuring the electrical characteristic of the SMA wires comprises measuring a difference in electrical characteristic, i.e. no absolute measurement of an electrical characteristic is required for the purposes of this invention. The measure of the resistance of the SMA wires may be a measure of the absolute resistance or a measure of the relative resistance (i.e. the resistance of one SMA wire relative to another, opposing, SMA wire).

The translational position of the movable part 20 may be used to determine the angular position of the rotating part. With reference to Figure 1, a full revolution of the movable part 20 around the rotating part 40 may result in rotation of the rotating part 40 by 1 or more teeth. In a particular example in which the rotating part 40 comprises 95 teeth and the movable part comprises 100 teeth, each revolution of the movable part 20 thus leads to an 18 degree rotation (5% of 360 degrees) of the rotating part 40. In this particular example, the absolute translational position of the movable part 20 may thus be used accurately to determine the angular position of the rotating part 40 within an 18 degree angle. The length of SMA wires can be measured relatively accurately using known techniques, which ultimately allows the angular position of the rotating part 40 to be determined with high accuracy.

In some embodiments, the translational position of the movable part 20 need not be determined at all, and the angular position of the rotating part 40 may be determined from the measures of the electrical characteristics of the SMA wires 30 directly, for example using suitable mathematical equations or look up tables. SMA actuator assembly with linear translation

Another embodiment of the SMA actuator assembly 1 is depicted in Figure 4. The SMA actuator assembly of Figure 4 may comprise any of the features described above in relation to Figures 1-3, although only the rotating part 40 is depicted for reasons of illustrative clarity.

In general, the SMA actuator assembly 1 comprises the rotating part 40 arranged to rotate about the rotation axis R and the movable part 20 arranged to move in a plane perpendicular to the rotation axis R and along a boundary of the rotating part 40, 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 R. The SMA actuator assembly 1 further comprises one 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 R.

The SMA actuator assembly 1 may comprise any of the additional features described in this application.

As shown in Figure 4, the SMA actuator assembly further comprises a linearly movable part 60, such as a rack 60. The linearly movable part 60 is movable along a movement axis 62 perpendicular to the rotation axis R, in particular along the x axis in Figure 4. The linearly movable part 60 is movable linearly. Rotation of the linearly movable part 60 or movement along axes other than the movement axis 62 may be constrained.

The linearly movable part 60 is in contact with the rotating part 40 such that rotation of the rotating part 40 drives linear movement of the linearly movable part 60. For example, the linearly movable part 60 may comprise teeth that are complementary to teeth in the rotating part 40. Alternatively, the linearly movable part 60 may comprise a contact surface, and friction between the contact surface and a corresponding contact surface of the rotating part 40 may drive movement of the linearly movable part 60 along the movement axis 62.

Although not shown in Figure 4, the linearly movable part 60 may be offset from the movable part 20 along the rotation axis R. So, the linearly movable part 60 may effectively be stacked on top of the movable part 20. The SMA actuator assembly 1 of Figure 4 provides a linear output actuator with increased stroke and bi directional drive compared to using a single SMA wire as a linear output actuator. Another advantage is that the linear output actuator may provide for continuous (i.e. uninterrupted) linear actuation. The position of the linear output actuator may be accurately controlled by controlling contraction of the SMA wires 30.

Another SMA actuator assembly with linear translation

Another embodiment of the SMA actuator assembly 100 is depicted in Figure 5. This SMA actuator assembly 100 comprises an actuator assembly 1 as described above in relation to Figures 1-3 and several further features.

In particular, the actuator assembly 100 comprises a linearly movable part 102 which is linearly movable along the rotation axis R of the rotating part 40, in particular along the z axis in Figure 5. The actuator assembly 100 comprises a helical bearing arrangement 104 for converting the rotation of the rotating part 40 into the linear movement of the linearly movable part 102.

In this example the helical bearing arrangement 104 is formed as follows. A collar 106 extends from the rotating part 40. The collar 106 and the rotating part 40 may be formed as a single piece. The collar 106 has a hollow cylindrical shape and has an axis that is collinear with the axis R. The linearly movable part 102 has a cylindrical shape, also with an axis that it collinear with the axis R. The linearly movable part 102 fits inside the collar 106 along at least some of its length. The collar 106 includes a helical slot 106a which receives a protrusion (i.e. a 'pin') 102a provided on the linearly movable part 102. In general, there may be multiple sets of slots and pins.

As will be appreciated, other types of helical bearing arrangement 104 may be used instead. For instance, the helical bearing arrangement 103 may comprise ball bearings or may comprise flexures arranged to constrain small amplitudes of motion to an approximately helical motion.

The actuator assembly 100 comprises a constraining arrangement 108 configured to guide the linearly movable part to move along the axis R, i.e. to constrain movement in x and/or y directions and to constrain rotation about the x, y and z (R) axes. In this example, the constraining arrangement 108 comprises a pair of cross-shaped bearings 108a, 108b that are offset from each other in the z direction as depicted in Figure 5. Again, other types of constraining arrangement 108 may be used instead. The actuator assembly 100 also comprises a further constraining arrangement (not depicted in Figure 5) which prevents the rotating part 40 from moving in the z direction (relative to the support structure 10). This further constraining arrangement may correspond to the bearing surfaces and resilient element as described above with reference to Figure 2a. Alternatively, the further constraining arrangement may be formed by a pair of opposed bearing surfaces, e.g. that interact with the rotating part 40 and/or the collar 102.

Therefore, in operation, the rotating part 40 rotates (as described above) and the collar 102 also rotates and causes the linearly movable part 102 to move in one direction or another (depending on the sense of the rotation) along the axis R.

In other examples, a mechanism (e.g. gears) may be used to offset the axis of movement of the linearly movable part 102 from the axis of rotation R of the rotating part 40.

Applications of the SMA actuator assemblies with linear movement

The SMA actuator assemblies 1, 100 described above with reference to Figures 4 and 5 may be particularly useful for moving a camera (or a part of a camera) in a portable electronic device such as a smartphone.

For example, the linearly movable part 102 may correspond to a (miniature) camera module and the SMA actuator assembly 100 may be arranged so as to move the camera module 102 from under the screen to a side of the screen (e.g. the top of the device). Flence camera functionality can be provided without requiring a notch in the screen or requiring the camera to capture images through the screen.

Alternatively, the movable part 102 may correspond to one or more optical elements (e.g. lenses) that are spaced in the z direction from the image sensor. By linearly moving these optical elements 102, the camera can have a small z dimension (i.e. 'height') when not in use and have a longer TTL (total track length) when in use. Such a camera may be referred to as a 'pop-up camera'.

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 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.

In the above-described embodiments, the arrangement of SMA wires is capable of applying a torque to the movable part. In alternative embodiments, the arrangement of SMA wires is not required to be capable of applying a torque. The SMA wires may be arranged, on contraction, to move the movable part along the boundary of the rotating part in the plane that is perpendicular to the rotation axis. Two or more SMA wires may be used to effect movement in two degrees of freedom in the plane. The bearing arrangement may allow movement of the movable part in the plane perpendicular to the rotation axis and constrain rotation of the movable part about the rotation axis.

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 the present disclosure should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. 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.