WO2025257525A1

WO2025257525A1 - Shape memory alloy actuator assembly

Info

Publication number: WO2025257525A1
Application number: PCT/GB2025/051188
Authority: WO
Inventors: Andrew Lam; Andrew Benjamin Simpson Brown
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2024-06-11
Filing date: 2025-06-02
Publication date: 2025-12-18
Anticipated expiration: 2026-12-11
Also published as: GB2643093A; GB202408330D0

Abstract

A shape memory alloy, SMA, actuator assembly (1) comprising a first part (4) comprising a first surface (4f); a second part (20) which is movable relative to the first part across the first surface; one or more SMA elements (80) arranged, on actuation, to drive movement of the second part relative to the first part, wherein the SMA actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a frictional force therebetween; and a controller configured to control actuation of the one or more SMA elements. The controller is configured to: cause the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

Description

SHAPE MEMORY ALLOY ACTUATOR ASSEMBLY

Field

The present application relates to a shape memory allow (SMA) actuator assembly and associated methods of control.

Background

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

It may be desirable to move the movable component to a given position relative to a support structure and then retain the movable component in that position for a length of time. It may also be desirable to keep the power consumption of the actuator to an acceptably low level. This is particularly relevant when the actuator is embodied on a portable electronic device, such as a mobile phone or a wearable device.

Summary

In a first aspect of the present invention, there is disclosed a shape memory alloy, SMA, actuator assembly comprising: a first part comprising a first surface; a second part which is movable relative to the first part across the first surface; and one or more SMA elements arranged, on actuation, to drive movement of the second part relative to the first part.

The SMA actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a frictional force therebetween. The SMA actuator assembly further comprises a controller configured to control actuation of the one or more SMA elements. The controller is configured to: cause the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

The SMA actuator assembly thus is configured to use a combination of friction between the second part (which may be referred to as a movable part) and a surface of the first part (which may be referred to as a support structure) and tension in an SMA element to hold the second part still relative to the first part.

Some previously known devices, such as those disclosed in W02020/120997A1, which is incorporated herein by reference in its entirety, employ SMA elements to drive movement of a movable part and are arranged so as to use friction alone to hold a movable part still when the SMA elements are unpowered. However, for some actuator designs a very high level of friction may be required to hold the movable part still. This is particularly true when additional forces are acting on the movable part. Such additional forces may be applied by one or more of: a spring or magnet, e.g. as part of a biasing arrangement to bias the movable part against the friction surface; an SMA wire, e.g. an opposing wire which may still have residual tension even when it is not powered; gravity (it may be desirable to hold the movable part still regardless of the orientation of the SMA actuator assembly and/or a device in which the actuator assembly is housed); inertial forces as a result of acceleration of a device in which the SMA actuator assembly is housed.

The high level of friction which would be required to counteract these forces may degrade actuator performance. For example, stick-slip behaviour may become an issue. High friction may also accelerate the wear of the surfaces involved, e.g. the first surface of the first part, and may increase the stress in the SMA element(s) required to overcome the friction to move the movable part. This may increase fatigue of the SMA element and reduce the life of the actuator.

The SMA actuator assemblies described herein overcome this issue by using a combination of friction and tension in an SMA element to hold a movable part still. The friction required is therefore less as compared to a situation in which the SMA element(s) are powered off completely. In addition to the above advantages of avoiding high friction, a lower level of friction may also provide more design freedom in that a total surface area of contact between the movable part and the friction surface that is required may be less.

The overall power consumption of the actuator assembly is still reduced (as compared to a typical SMA actuator in which power is supplied to drive movement of the movable part to a given position and a high power level is maintained to keep the movable part at that position) because a relatively low level of power is supplied to the SMA element.

As above, a combination of SMA element tension and friction may be used to counteract forces acting on the movable part. It will be appreciated that there may be various forces acting on the movable part which aid the tension and friction (i.e. which act in the same direction as the tension of the SMA element being supplied with power at the second power level). For example, the movable part may be being pulled back towards a central position by a biasing spring or magnet and this may be counteracted by a combination of friction, tension in an SMA wire and also a further force (e.g. gravity, a force applied by a different spring or magnet etc.).

As mentioned above, the controller is configured to: cause the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

The first power level and/or the second power level may be an average power level. The first power level and the second power levels are non-zero power levels.

It will be appreciated that the controller may be configured to supply one or more drive signals to a power supply, which in turn supplies power (e.g. in the form of electrical current) to the one or more SMA elements as a result. The SMA actuator assembly may comprise the power supply and/or the power supply may be part of the device in which the SMA actuator assembly is housed. The first power level may correspond to a first temperature of the one or more SMA elements and the second power level may correspond to a second temperature, lower than the first temperature, of the one or more SMA elements. The controller may therefore be configured to: drive the temperature of the one or more SMA elements to the first temperature to drive movement of the second part relative to the first part; and drive the temperature of the one or more SMA elements to the second temperature (or allow the one or more SMA elements to reach the second temperature), to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

In some embodiments, the second part is arranged to move relative to the first part over a range of motion comprising multiple possible positions of the second part relative to the first part. The actuator assembly may be configured such that: for a first subset of the multiple possible positions, the frictional force between the second part and the first surface is sufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered; and for a second subset of the multiple possible positions, different to the first subset, the frictional force alone is insufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered.

In other words, for a part of the possible range of motion of the second part, the SMA actuator assembly is arranged to operate in a zero-hold-power state in which the second part is held still by friction between the second part and the first surface when the one or more SMA elements are unpowered. In a different part of the possible range of motion of the second part, the SMA actuator assembly is arranged to operate in a low-hold-power state in which the second part is held still by a combination of (a) friction between the second part and the first surface and (b) tension in the one or more SMA elements.

In the second subset of the multiple possible positions, the controller is configured to cause the supply of power at the second power level.

The range of motion of the second part may comprise a continuum of possible positions of the second part relative to the first part or a set of discrete possible positions. In either case, the first subset of multiple possible positions may make up between 50% and 80% of the range of motion, preferably between 65% and 75%.

In some embodiments, the range of motion of the second part relative to the first part defines a central position (of the second part relative to the first part) in the range of motion. The positions of the second subset may be further from the central position than the positions of the first subset of positions are. In other embodiments, the positions of the first subset may be further from the central position than the positions of the second subset are.

In some embodiments, the one or more SMA elements comprises a first SMA element and the SMA actuator assembly comprises a second SMA element arranged, on actuation, to drive movement of the second part relative to the first part. The first and second SMA elements may be arranged such that contraction of the second SMA element opposes contraction of the first SMA element.

In other words, the SMA actuator assembly may comprise a pair of opposing SMA elements. The first SMA element may apply a force to the second part in a first direction and the second SM element may apply a force to the second part in a second direction. The first and second directions may comprise at least components in opposite directions. The first direction may be opposite to the second direction.

In some embodiments, the controller is configured such that the second SMA element is unpowered whilst power is supplied to the first SMA element at the second power level. This may help to keep the overall power consumption of the actuator assembly particularly low.

In other embodiments, the controller is configured to supply power to the second SMA element at a further power level (which is non-zero) whilst power is supplied to the first SMA element at the second power level. The further power level may be less than the second power level.

In some embodiments, the one or more SMA elements comprises a first SMA element configured to apply a first force to the second part and wherein the SMA actuator assembly is arranged such that a second force is applied to the second part by gravity and/or by a force-application arrangement, wherein the second force opposes contraction of the first SMA element. The SMA actuator assembly may comprise the force-application arrangement. In other words, the contraction of the first SMA element may be opposed by something other than another SMA element, e.g. gravity or a force-application arrangement. The force-application arrangement may comprise one or more resilient elements and/or one or more magnets.

In some embodiments, the second power level is a pre-determined power level. The second power level may be a value stored in look-up table, for example. The second power level may depend on the position of the second part relative to the first part, i.e. the second power level may be different for each of a set of possible positions of the second part relative to the first part.

The pre-determined second power level for each possible position of the second part relative to the first part may have been determined in a calibration step, e.g. as part of a manufacture process.

To drive movement of the second part relative to the first part, the controller causes the supply of power to the one or more SMA elements at a first power level. Movement of the second part relative to the first part may be driven using open loop control or closed loop control, for example.

In some embodiments, the controller is configured to determine the second power level. For example, the second power level may be determined dynamically during operation of the SMA actuator assembly. Determining the second power level may comprise calculating the second power level, e.g. based on one or more inputs.

In some embodiments, the controller is configured to determine the second power level by causing the supply of power to the one or more SMA elements at a third power level and subsequently decreasing the level of power supplied to the one or more SMA elements until the controller detects that the second part has started to move.

The second power level may be taken to be the last power level supplied to the one or more SMA elements before the second part started to move. This second power level may be a minimum power level required to retain the second part in position relative to the first part.

The third power level may be the same as or different to the first power level. For example, the third power level may be lower than the first power level. The controller may be configured to decrease the level of power supplied to the one or more SMA elements in increments. Accordingly, the controller may supply power to the one or more SMA elements at a set of power levels. In some embodiments, the controller may be configured to determine a measure of electrical resistance of the one or more SMA elements for each of the power levels at which power is supplied. The controller may be configured to step through the set of power levels until the determined measure of electrical resistance changes, for example changes by a threshold amount. The last power level before the measure of electrical resistance changed (e.g. by a threshold amount) may be taken to be the second power level.

In an opposing wire actuator assembly, any opposing wires may be powered-off before the process of decreasing the power supplied to the one or more SMA elements in order to determine the second power level begins.

As an alternative to decreasing the power level in increments, the power level may be decreased continuously while a measure of electrical resistance of the one or more SMA elements is monitored. When a change in the measure of electrical resistance is detected (e.g. a change by more than a threshold amount), the power may be increased again (e.g. by a preset amount or until the measure of electrical resistance is back to its original value, before the change) to hold the second part still. This final power level may be taken to be the second power level.

As part of this method of determining a value for the second power level, if an inertial disturbance of the device in which the SMA actuator assembly is housed (e.g. via a gyroscope or some other acceleration detector), the power supplied to the one or more SMA elements might be increased again (e.g. to the first power level or to a power level less than the first power level) to restore the second part to the desired position relative to the first part. The process of decreasing the power supplied until movement of the second part is detected can then be resumed.

In some embodiments, the coefficient of static friction between the first surface and the second part is between 0.01 and 0.6, preferably between 0.05 and 0.4.

In some embodiments, the SMA actuator assembly is arranged such that a combination of:

(a) the frictional force between the first surface and the second part; and

(b) the force applied by the one or more SMA elements when supplied with power at the second power level; is equal to or greater than the weight of the second part.

The friction and tension may be great enough to support the weight of the second part when the SMA actuator assembly is in any orientation with respect to an external reference point.

In some embodiments, a combination of:

(a) the frictional force between the first surface and the second part; and

(b) the force applied by the one or more SMA elements when supplied with power at the second power level; is equal to or greater than a multiple the weight of the second part, for example 2, 3, 4, 5, 8 or 10 times the weight of the second part.

In some embodiments, a combination of:

(a) the frictional force between the first surface and the second part; and

(b) the force applied by the one or more SMA elements when supplied with power at the second power level; is equal to or greater than the combined weight of the second part and a component that the second part is arranged to hold, for example a lens, an image sensor, a display, an emitter of a part thereof. The combination may be equal to or greater than a multiple the combined weight, for example 2, 3, 4, 5, 8 or 10 times the combined weight.

In some embodiments, the one or more SMA elements are arranged, on contraction, to reduce the normal force between the first surface and the second part. The one or more SMA elements may be arranged to each apply a force to the second part with a component orthogonal to the first surface that reduces said frictional forces and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface.

In some embodiments, the one or more SMA elements are arranged on contraction thereof (when supplied with power at the first power level) to apply a force to the second part with a component orthogonal to the first surface that lifts the second part out of contact with the first surface.

In some embodiments, the one or more SMA elements are inclined relative to a direction of movement of the second part at an acute angle of greater than 0° so as to, on contraction thereof, apply a force to the second part with a component orthogonal to the first surface that reduces said frictional forces and with a component parallel to the first surface along the direction of movement.

In some embodiments the SMA actuator assembly comprises at least two opposed SMA elements arranged to, on contraction thereof, apply forces to the second part with respective components orthogonal to the first surface that reduce said frictional forces and with respective components parallel to the first surface and along the movement direction in opposite directions.

In some embodiments, the one or more SMA elements are arranged such that the normal force between the first surface and the second part remains substantially constant on contraction of the one or more SMA elements. This may facilitate simple control of the SMA actuator assembly.

In some embodiments, first power level is between 4 and 10 times greater than the second power level, preferably wherein the first power level is between 3 and 6 times greater than the second power level. The power level may be approximately five times the second power level.

In some embodiments, the one or more SMA elements comprises three or more SMA elements. The one or more SMA elements may comprise a total of three SMA elements.

In some embodiments, the one or more SMA elements comprises four or more SMA elements. The one or more SMA elements may comprise a total of four SMA elements. The four SMA elements may extend in a loop around a primary axis defined by the first part. Each of the four SMA elements may be disposed on a side of the SMA actuator assembly, the four respective sides extending in a loop around the primary axis.

In some embodiments in which the one or more SMA elements comprises three or more SMA elements, the controller is configured to: cause the supply of power at the first power level to at least one of the three or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at the second power level, lower than the first power level, to a first subset of the three or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and second part, to hold the second part in position relative to the first part while second subset, different to the first subset, of the three or more SMA elements are unpowered. In some embodiments, the first subset may comprise two SMA elements, for example a total of two SMA elements.

In some embodiments, the one or more SMA elements comprises four SMA elements and the controller is configured to: cause the supply of power at the first power level to at least one of the four SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at the second power level, lower than the first power level, to a first pair of the four SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and second part, to hold the second part in position relative to the first part while a second pair of the four SMA elements, different to the first pair, are unpowered.

In some embodiments, the controller is configured to determine a measure of electrical resistance of the one or more SMA elements whilst the second power level is applied to the one or more SMA elements.

In some embodiments, the second power level is sufficient to straighten the one or more SMA elements. When the one or more SMA elements are unpowered or supplied with a power level lower than the second power level, the one or more SMA elements may be slack. Application of the second power level may cause the one or more SMA elements to straighten. It is beneficial to power the one or more SMA elements to straighten it as a measure of electrical resistance of the straight SMA element may be used to determine an indication of position of the second part. This is because the resistance of the SMA element when straight is dependent on the position of the second part.

In some embodiments, the second power level is a minimum power level required to straighten the one or more SMA elements.

In some embodiments, the second power level is a minimum power level required to retain the second part in position relative to the first part. This may minimise or at least reduce the power consumption of the actuator assembly. In some embodiments, the SMA actuator assembly is configured to constrain movement of the second part to one degree of freedom. For example, the second part may be allowed to move along a movement axis but may be constrained from moving in other degrees of freedom.

In some embodiments, the SMA actuator assembly is configured to constrain movement of the second part to two degrees of freedom. For example, the second part may be movable in a plane of movement but unable to move outside of the plane.

The SMA actuator assembly may comprise a bearing arrangement configured to guide movement of the second part relative to the first part in one or more degrees of freedom and optionally to constrain the second part from moving in other degrees of freedom.

In some embodiments, the one or more SMA elements are arranged to drive translational movement of the second part relative to the first part. In some embodiments, the one or more SMA elements are arranged to drive rotation of the second part relative to the first part about an axis of rotation. In some embodiments, the second part may be arranged to move helically, i.e. along a helical path.

In some embodiments, the SMA actuator assembly comprises a plurality of blades arranged to define a variable aperture, wherein movement of the second part relative to the first part drives a change in the size of the variable aperture. An example of a variable aperture assembly is provided in W02024057042A1, which is incorporated herein by reference in its entirety.

In some embodiments, the SMA actuator assembly comprises a biasing arrangement arranged to bias the second part and the first surface against each other. The biasing arrangement may comprise one or more resilient elements and/or one or more magnets. The biasing may alternatively or additionally be provided by some external force, e.g. gravity or a force applied by some other part of the SMA actuator assembly or a device which the SMA actuator assembly is part of. For example, the biasing force could be provided by a housing of the device.

In some embodiments, the second part comprises a display, an emitter or a part thereof. The first part may comprise one or more lenses which may be arranged to focus light emitted from the display or emitter, for example onto the retina of an eye of a user. In some embodiments, the second part comprises one or more lenses. The first part may comprise an image sensor. The lenses may be arranged to focus light onto a light-sensitive region of the image sensor. In some embodiments, the first part may comprise a display, an emitter or a part thereof. The lenses on the second part may be arranged to focus light emitted from the display or emitter, for example onto the retina of an eye of a user.

In any embodiment in which the SMA actuator assembly comprises one or more lenses, the second part may be arranged to move relative to the first part along an optical axis of the lens.

In some embodiments, the one or more SMA element may be parallel to the first the surface. In some embodiments, the one or more SMA element may be at an acute, non-zero angle to the first the surface.

In a second aspect of the present invention there is provided method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and one or more SMA elements. The method comprises: causing the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part across a first surface of the first part, the actuator assembly being arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a frictional force therebetween; and causing the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and second part, to hold the second part in position relative to the first part.

In some embodiments, the second power level is a pre-determined power level.

In some embodiments, the method comprises determining the second power level. For example, the second power level may be determined dynamically during operation of the SMA actuator assembly. Determining the second power level may comprise calculating the second power level, e.g. based on one or more inputs. In some embodiments, the method may comprise determining the second power level by causing the supply of power to the one or more SMA elements at a third power level and subsequently decreasing the level of power supplied to the one or more SMA elements until it is detected that the second part has started to move.

The third power level may be the same as or different to the first power level. For example, the third power level may be lower than the first power level.

The method may comprise decreasing the level of power supplied to the one or more SMA elements in increments. Accordingly, the method may comprise supplying power to the one or more SMA elements at a set of power levels. In some embodiments, the method may comprise determining a measure of electrical resistance of the one or more SMA elements for each of the power levels at which power is supplied. The method may comprise stepping through the set of power levels until the determined measure of electrical resistance changes, for example changes by a threshold amount. The last power level before the measure of electrical resistance changed (e.g. by a threshold amount) may be taken to be the second power level.

Alternatively, the power level may be decreased continuously while a measure of electrical resistance of the one or more SMA elements is monitored. When a change in the measure of electrical resistance is detected (e.g. a change by more than a threshold amount), the power may be increased again slightly (e.g. by a preset amount or until the measure of electrical resistance is back to its original value, before the change) to hold the second part still. This power level may be taken to be the second power level.

As part of this method of determining a value for the second power level, if an inertial disturbance of the device in which the SMA actuator assembly is housed (e.g. via a gyroscope or some other acceleration detector), the power supplied to the one or more SMA elements might be increased again (e.g. to the first power level or to a power level less than the first power level) to restore the second part to the desired position relative to the first part. The process of decreasing the power supplied until movement of the second part is detected can then be resumed. In some embodiments, the first power level is between 4 and 10 times greater than the second power level, preferably wherein the first power level is between 3 and 6 times greater than the second power level.

In some embodiments the method comprises determining a measure of electrical resistance of the one or more SMA elements whilst the second power level is applied to the one or more SMA elements.

In some embodiments, the second power level is sufficient to straighten the one or more SMA elements. When the one or more SMA elements are unpowered or supplied with a power level lower than the second power level, the one or more SMA elements may be slack. Application of the second power level may cause the one or more SMA elements to straighten.

In some embodiments, the second power level is a minimum power level required to retain the second part in position relative to the first part.

In some embodiments, the method may comprise determining the minimum power level required to retain the second part in position relative to the first part.

In a third aspect of the present invention there is provided a computer-readable storage medium comprising instructions for instructing a controller to perform a method as described herein. The computer-readable storage medium may be non-transitory.

The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. The computer-readable storage medium may be, for example, a solid state memory, a microprocessor, programmed memory such as non-volatile memory (such as Flash), or read-only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. Reference has been made to actuator assemblies which are configured to hold a second part in position relative to a first part by a combination of (a) friction and (b) tension in one or more SMA elements. It will be appreciated that the actuator assembly may not be configured to hold the second part still relative to the first part under the action of any possible external force. For example, the actuator assembly may be capable of holding the second part still in a given range of accelerations of the actuator assembly (or a device on which the assembly is embodied). The controller of any SMA actuator assembly described herein may be configured to cause the supply of power at the second power level, lower than the first power level (as described herein), to one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part when acceleration of the actuator assembly is less than or equal to a hold threshold. The hold threshold may be a multiple of gravitational acceleration G, e.g. 2G, 3G, 5G, 8G or 10G. The same comments apply to the methods described herein.

Brief description of the drawings

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

Figure 1 is a schematic view of an SMA actuator assembly;

Figure 2 is a schematic view of an alternative SMA actuator assembly;

Figure 3 is a schematic view of the actuator assembly of Figure 2 during use;

Figure 4 is a schematic diagram showing a method of control of an SMA actuator assembly;

Figure 5 is a schematic diagram showing the power supplied to an SMA actuator assembly over time; Figure 6 is a schematic view of an alternative SMA actuator assembly; and Figure 7 is a schematic view of a further alternative SMA actuator assembly.

Detailed description

SMA actuator assembly

Figure 1 is a schematic view of an SMA actuator assembly 1. As shown in Figure 1, the SMA actuator assembly comprises a first part 4 and second part 20. The second part 20 is movable relative to the first part 4 across a first surface 4f. The first part 4 may be referred to as a support structure. The second part 20 may be referred to as a movable part. The second part 20 is movable along a movement direction relative to the first part 4.

The SMA actuator assembly 1 comprises an SMA element. As shown in Figure 1, the SMA element is an SMA wire 80. The SMA wire 80 is arranged, on actuation, to drive movement of the second part 20 relative to the first part 4. For example, the SMA wire 80 may, on actuation, contract to drive movement of the second part 20 relative to the first part 4. The contraction of the SMA wire 80 may be controlled by applying electrical power to the SMA wire 80.

The SMA actuator assembly 1 comprises a controller (not shown). The controller is configured to control actuation of the at least one SMA wire 80 to drive movement of the second part 20. For example, the controller may be configured to control application of power P to the SMA wire 80 so as to control the extent by which the SMA wire 80 contracts.

The SMA actuator assembly 1 is arranged such that the second part 20 is biased against the first surface 4f with a normal force, thereby generating a frictional force between the second part 20 and the first surface 4f. This biasing is indicated schematically in figure 1 by an arrow 6. The biasing may be provided by any suitable means. For example, the biasing may be provided by gravity. The actuator assembly may comprise a biasing arrangement to provide the biasing. The biasing arrangement may comprise one or more resilient elements, such as springs. The one or more resilient elements may be disposed (and optionally connected) between the first part 4 and the second part 20. Additionally or alternatively the biasing arrangement may comprise one or more magnets.

When the second part 20 moves on actuation of the SMA wire 80, the second part 20 moves across the first surface 4f and friction between the first surface 4f and the second part 20 may resist relative movement between the second part 20 and the first part 4.

On contraction, the SMA wire 80 drives movement of the second part 20 in a first direction relative to the first part 4. In use, a force is applied to the second part 20 to drive movement of the second part 20 in a second direction, opposite to the first direction, relative to the first part 4. The actuator assembly 1 may comprise a force-application arrangement 12 to apply this force. The forceapplication arrangement 12 may oppose contraction of the SMA wire 80. Alternatively, the force may be provided by gravity. The force applied by the force-application arrangement 12 is indicated schematically by arrow 10. The force-application arrangement 12 may comprise any suitable arrangement for applying a force to the second part 20. For example, the force application arrangement may comprise one or more of the following: one or more resilient elements, such as springs; one or more magnets; one or more actuators, for example one or more voice coil motors (VCMs).

In a simple example, the force application arrangement 12 may comprise a spring connected between the first part 4 and the second part 20.

The net driving force is the net force on the second part 20 for driving the second part 20 relative to the first part 4. The friction opposes the net driving force.

Figure 2 is a schematic view of an alternative SMA actuator assembly 1. The embodiment of figure 2 is the same as that illustrated in figure 1 except that instead of an SMA wire and a force-application arrangement 12, the SMA actuator assembly 1 composes a pair of opposing SMA wires 80a, 80b.

Each of the SMA wires is connected between the second part 20 and the first part 4. The SMA wires 80 are arranged, on actuation, to drive movement of the second part 20 relative to the first part 4 in opposite directions. Contraction of one of the SMA wires 80a, 80b opposes contraction of the other of the SMA wires 80a, 80b. In this sense, the two SMA wires 80a, 80b may be referred to as opposing wires.

In the case of opposing wires, the net driving force on the second part 20 may comprise the driving force applied by a first SMA wire 80a and the driving force applied by a second SMA wire 80b. When the SMA wires 80a, 80b are actuated to the same actuation level, then the net driving force may be substantially zero such that the second part 20 does not move relative to the first part 4. The actuation level of an SMA wire 80 is equivalent to the power supplied to that SMA wire 80. When the actuation level of one SMA wire is greater than the other, then the net driving force on the second part 20 may be non-zero. When the net driving force is greater than the frictional force threshold, then the second part 20 may move relative to the first part 4.

With reference to the actuator assemblies shown in Figures 1 and 2, in previous designs of such an SMA actuator assembly, the assembly may have been arranged such that the friction between the second part 20 and the first surface 4f is high enough to hold the second part 20 in position relative to the first part 4 when the SMA wires(s) are unpowered. Such an arrangement may be referred to as a zero-hold-power arrangement.

However, for some designs the friction may not be sufficient to hold the second part 20 in position with respect to the first part when the SMA element(s) are unpowered. This may be the case for some or all possible positions of the second part 20 relative to the first part 4. This may be because additional forces are acting on the second part 20, as will now be explained.

In addition to driving forces imparted on the second part 20, e.g. by the SMA wire(s), and friction, there may be other forces acting on the second part 20. Such forces may have at least a component of force along a direction of movement of the second part 20. For example, the SMA actuator assembly 1 may comprise a biasing arrangement which comprises a spring. The force of the spring may act downwards, as seen in Figures 1 and 2, when the second part 20 is at a central position in the range of motion of the second part. The force of the spring is represented by the arrow 6 in Figures 1 and 2.

The spring may be connected to both the first part 4 and the second part 20. Figure 3 illustrates the SMA actuator assembly 1 as shown in Figure 2 in a state in which the second part 20 has been driven to move by the second SMA wire 80b from a central position towards the right (as seen in Figure 3). In the embodiment of figure 3, the biasing force is provided by a spring, 6a, which is illustrated schematically. In the state shown in Figure 3, the biasing arrangement is no longer applying a force to the second part 20 which is perpendicular to the direction of movement of the second part. Instead, there is a component of force (indicated by arrow 6m) along the direction of movement of the second part 20, as well as a component of force (indicated by arrow 6n) perpendicular to the direction of movement.

This is an example of a situation in which the friction between the second part 20 and the first surface 4f may be insufficient to hold the second part 20 still with respect to the first part 4 when the SMA element(s) are unpowered. In other words, friction alone may not be enough to counteract the component of force 6m imparted by the spring. The friction may be sufficient to hold the second part still at some positions of the second part (e.g. a central position and a subset of positions around the central position) but not at other positions (e.g. nearer the ends of stroke). It will be appreciated that other forces from other sources may additionally or alternatively act on the second part 20 to lead to a situation in which the friction is insufficient to enable a zero-hold- power state for all positions of the second part relative to the first part. As described above, such forces may arise due to one or more of: a spring or magnet, e.g. as part of a biasing arrangement to bias the movable part against the friction surface; an SMA wire, e.g. an opposing wire which may still have residual tension even when it is not powered; gravity (it may be desirable to hold the movable part still regardless of the orientation of the SMA actuator assembly); inertial forces as a result of acceleration of a device in which the SMA actuator assembly is housed.

In any of the embodiments described herein (e.g. those shown in Figures 1, 2, 3, 4 and 6), the actuator assembly may be configured such that it is capable of zero hold power for some positions of the second part 20 relative to the first part 4 but not for other positions. In these other positions, power must be supplied to at least some of the SMA elements in order to hold the second part still.

In other words, the second part 20 is arranged to move relative to the first part 4 over a range of motion comprising multiple possible positions of the second part relative to the first part. The actuator assembly may be configured such that: for a first subset of the multiple possible positions, the frictional force between the second part 20 and the first surface 4f is sufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered; and for a second subset of the multiple possible positions, different to the first subset, the frictional force alone is insufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered.

In the second subset of the multiple possible positions, the controller causes the supply of power at a second power level, as will be explained below.

The range of motion of the second part may comprise a continuum of possible positions of the second part relative to the first part or a set of discrete possible positions. In either case, the first subset of multiple possible positions may make up between 50% and 80% of the range of motion, preferably between 65% and 75%. The range of motion of the second part 20 relative to the first part 4 may define a central position in the range of motion. Such a central position may be the position of the second part shown in Figure 1 or 2, for example. The positions of the second subset may be further from the central position than the positions of the first subset of positions are. With reference to Figure 3, it can be seen that if the biasing force is provided by a spring connected between the first and second parts, the magnitude of the component of force indicated by arrow 6m may increase as the displacement of the second part 20 from the central position (shown in figure 2) is increased.

In other embodiments, the positions of the second subset may be further from the central position than the positions of the first subset are.

With reference to Figure 4, a method of control is disclosed. As an overview, the method comprises supplying power to an SMA element (e.g. an SMA wire 80, 80a or 80b) at a low power (relative to a power supplied to an SMA element in order to actuate it) to hold the second part 20 still with respect to the first part 4. This low power provides enough tension in the SMA element such that a combination of: the tension and the friction between the second part 20 and the first surface 4f is sufficient to counteract any forces which are acting along a direction of movement of the second part and hold the second part 20 still relative to the first part 4.

As mentioned above, the SMA actuator assembly 1 comprises a controller. The controller is configured to control actuation of the at least one SMA wire 80 to drive movement of the second part 20. For example, the controller may be configured to control application of power P to the at least one SMA wire 80 so as to control the extent by which the SMA wire 80 contracts. The method of control described with reference to Figure 4 may be carried out by the controller and may be applied to any of the SMA actuator assemblies described herein. The embodiment of Figure 1 will be used as an example for the purposes of describing the method but it will be appreciated that the method of control may be applied to different actuator assemblies.

With reference to Figure 4, at step 30 power at a first power level is supplied to the SMA wire 80 to drive movement of the second part 20 relative to the first part 4 across the first surface 4f. The first power level is sufficient to cause the SMA wire 80 to contract and drive movement of the second part 20 relative to the first part 4. The second part 20 is driven to move until a desired position of the second part 20 relative to the first part 4 is reached.

At step 32, power at a second power level, which is lower than the first power level, is supplied to the SMA wire 80 to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface 4f and second part 20, to hold the second part 20 in position relative to the first part 4. As described above, the combination of tension in the SMA wire 80 and the friction counteracts a resultant force (provided by e.g. a spring, a magnet etc.) which would otherwise cause the second part 20 to move relative to the first part 4.

Advantageously, a combination of friction and tension in an SMA element is used to hold a moving part still. Less power is therefore needed to hold the moving part still, as compared to a situation in which friction is minimised (as may be the case in a typical SMA actuator assembly in which the SMA wire(s) must be continuously powered at a high level to hold a moving part still). By using wire tension in addition to friction, the friction in the system can be set at an acceptable level for good actuator performance. In some cases, if the friction was particularly high (e.g. high enough to hold the moving part still for all positions of the moving part in a range of movement of the moving part) the actuator performance may be degraded. For example, stick-slip behaviour may be an issue.

It will be appreciated that after the SMA element 80 is supplied with power at the second power level for a period of time to hold the second part still during that period, the method may be repeated in order to: drive the second part 20 to a new position relative to the first part 4 (by supplying power to the SMA element 80 at a subsequent power level which is higher than the second power level); and hold the second part 20 at that new position (by supplying power to the SMA element 80 at a power level which is lower than the subsequent power level).

With reference to Figure 5, the power supplied to each of two SMA wires in an SMA actuator assembly in accordance with the method described above is shown. The actuator assembly of Figures 2 and 3 will be used as an example. As explained above, the SMA actuator assembly 1 comprises a pair of opposing SMA wires: a first SMA wire 80a and a second SMA wire 80b. Figure 5 shows the power levels which are supplied to each wire over time in order to drive the second part 20 to the position shown in Figure 3. The upper line on the plot indicates the power supplied to the second SMA wire 80b and the lower line on the plot indicates the power supplied to the first SMA wire 80a.

In a first control period 40a power is supplied to each of the first and second SMA wires in order to drive the second part 20 to a desired position relative to the first part 4. This position may be that shown in Figure 3, for example. At this position, the biasing arrangement is applying a force to the second part 20 which has a component in a direction parallel to the first surface 4f back towards the central position, as indicated by arrow 6m in Figure 3. A first power 50a is supplied to the second SMA wire 80b during the first control period 40a. As can be seen from figure 5, a non-zero power is also supplied to the first SMA wire 80a during the first control period 40a.

During a second control period, 40b, the second part 20 is held in the position shown in Figure 3. A second power 50b, is supplied to the second SMA element 80b. The second power 50b is lower than the first power 50a. When the second power level 50b is supplied to the second SMA wire 80b, the first SMA wire 80a is unpowered (i.e. no power is supplied to the first SMA wire 80b). However, in some embodiments, some power may be supplied to the first SMA wire 80b.

In order to move between the first power level 50a and the second power level 50b supplied to the second SMA wire 80b, any suitable transition may be used (i.e. any suitable profile of power over time). In Figure 5, it can be seen that a gradual reduction from the first power level 50a to the second power level 50b may be used. Alternatively, an instantaneous (or near-instantaneous) reduction, i.e. a step change, from the first to second power levels may be used. In another example, two or more step-changes may be used to transition from the first power level 50a to the second power level 50b.

As described above, the second power level may be a pre-determined power level. The second power level may be a value stored in look-up table, for example. The second power level may depend on the position of the second part relative to the first part, i.e. the second power level may be different for each of a set of possible positions of the second part relative to the first part. The lookup table may therefore comprise a set of possible positions of the second part 20 relative to the first part 4 and an associated value of the second power level. The pre-determined second power level for each possible position of the second part relative to the first part may have been determined in a calibration step, e.g. as part of a manufacture process. Alternatively, the second power level may be determined dynamically, i.e. during control of the SMA actuator assembly. For example, the second power level may be determined by causing the supply of power to the second SMA wire 80b at a third power level and subsequently decreasing the level of power supplied to the second SMA wire 80b until it is detected that the second part has started to move. This detection of movement of the second part 20 relative to the first part 4 may be achieved by monitoring a measure of electrical resistance of the second SMA wire 80b.

In one example, the level of power supplied to the second SMA wire 80b is decreased from the third power level in increments. Accordingly, power is supplied to the second SMA wire 80b at a set of power levels and a measure of electrical resistance of the second SMA wire 80b for each of the power levels determined. The set of power levels is stepped through until the determined measure of electrical resistance changes, for example changes by a threshold amount. The last power level before the measure of electrical resistance changed (e.g. by a threshold amount) is then taken to be the second power level.

In another example, the level of power supplied to the second SMA wire 80b is decreased continuously while a measure of electrical resistance of the second SMA wire 80b is monitored. When a change in the measure of electrical resistance is detected (e.g. a change by more than a threshold amount), the power is increased again (e.g. by a preset amount or until the measure of electrical resistance is back to its original value, before the change) to hold the second part 20 still. This final power level is taken to be the second power level and is maintained to hold the second part still.

As part of a method of determining a value for the second power level, if an inertial disturbance of the device in which the SMA actuator assembly is housed (e.g. via a gyroscope or some other acceleration detector), the power supplied to the one or more SMA elements may be increased again (e.g. to the first power level or to a power level less than the first power level) to drive movement of the second part 20 relative to the first part 4 and restore the second part to the desired position relative to the first part. The process of decreasing the power supplied until movement of the second part is detected can then be resumed As described above the second power level may be taken to be the last power level supplied to the one or more SMA elements before it is detected that the second part has started to move. This second power level may be a minimum power level required to retain the second part in position relative to the first part.

The third power level may be the same as the first power level or may be different to the first power level. For example, the third power level may be lower than the first power level.

As described above, in order to drive movement of the second part relative to the first part, the controller causes the supply of power to the first SMA wire 80a at a first power level 50a. Movement of the second part relative to the first part may be driven using open loop control or closed loop control, for example.

Generally, the options for the second power level also apply the first power level. The first power level may be pre-determined, i.e. there may be a pre-determined value of the first power level for each of a set of possible positions of the second part 20 relative to the first part 4. Alternatively, the value of the first power level may be determined dynamically during the control process.

Any reference to a power level made herein may refer to an average power level. Any reference to a power level may be to a level of electrical power.

Variable friction

Figure 6 schematically depicts an alternative SMA actuated assembly 1 with which the control method of Figure 4 may be used. Features of the SMA actuator assembly 1 may be the same as described above in relation to the SMA actuator assembly 1 shown in Figure 2, except where differences are described below.

With reference to Figure 6, the first and second SMA wires 80a, 80b are arranged to apply a force to the second part 20 with: a component orthogonal to the first surface 4f that reduces the normal force between the second part and the first surface; and a component parallel to the surface 4f so as to drive movement of the second part 20 relative to the first part 4 across the first surface 4f. The first and second SMA wires 80a, 80b are each inclined relative to the direction of movement of the second part 20 (and also relative to the first surface 4f) at an acute angle of greater than 0°.

In this way, the SMA wires 80a, 80b are arranged, on actuation, to apply an unloading force. The unloading force is for reducing the friction that opposes movement of the second part 20 relative to the first part 4. For example, as shown in Figure 6, optionally the SMA wires 80 are arranged, on actuation, to apply a force on the second part 20 away from the first surface 4f. By applying a force away from the first surface 4f, the SMA wires 80 act to reduce the friction between the first part 4 and the second part 20. As indicated by arrow 6, a force that opposes the unloading force supplied by the SMA wires 80 when they are actuated is provided, e.g. by gravity or a biasing arrangement such as a spring. In general, the force that biases the second part 20 against the first surface 4f is greater than the unloading force supplied by the SMA wires 80a, 80b. The second part 20 therefore remains engaged with the first part 4.

This arrangement facilitates a method of control in which friction is reduced during movement of the second part 20 parallel to the first surface 4f. To achieve this, both SMA wires 80a and 80b are actuated but one wire is contracted more than the other to move the second part 20 along the first surface 4f. When power to the wires is reduced, the normal force and hence the friction increases again and the second part 20 can be held still by a combination of friction and at least one of the wires in a low power mode as described above.

In some embodiments, the SMA wires 80a and 80b may be arranged to lift the second part 20 off of the first surface 4f completely during movement of the second part parallel to the first surface 4f. When power to the SMA wires 80a and 80b is reduced, the second part 20 then comes into contact with the first surface 4f again and the second part 20 can be held still as described above.

In the examples described with reference to Figure 1, 2, 3 and 6, the second part 20 is driven to move in one dimension, i.e. back and force along a movement direction. It will be appreciated that the second part 20 may move in more than one dimension (or degrees of freedom), for example in two dimensions in a plane of movement.

Figure 7 schematically shows a plan view of an example of the actuator assembly 1 in which the second part is movable in two dimensions, in a plane of movement. The actuator assembly 1 comprises a first part 4 and a second part 20, which is movable relative to the first part 4. Movement of the second part 20 relative to the first part 4 is supported by a plain bearing in that one or more surfaces of the second part 20 are in contact with a first surface 4f of the first part 4 and move across the first surface 4f when the second part 20 is driven to move. The second part 20 and the first surface 4f are biased together so as to generate frictional forces therebetween, in the same way as described for the embodiments shown in Figures 1-3 and 6.

The actuator assembly 1 comprises four SMA elements 80a, 80b, 80c and 80d which are shown schematically in Figure 7. Each SMA element 80a-d is connected between the first part and the second part and is arranged, on contraction, to drive movement of the second part 20 relative to the first part 4. Each SMA element 80a-d applies a respective force F to the second part 20 on contraction. The directions of the respective forces F are indicated with arrows in Figure 7.

The SMA actuator assembly 1 comprises a controller (not shown). The controller is configured to control actuation of the SMA wires 80a-d. For example, the controller may be configured to control application of power P to the SMA wires 80a-d so as to control the extent by which each SMA wire 80 contracts.

The arrangement of SMA elements 80a-d of Figure 7 may be used, for example, to drive movement of the second part 20 relative to the first part 4 in a movement plane, specifically the XY plane as seen in Figure 7. Accordingly, movement in one or more directions (or any direction) within the plane may be possible. This may include one or more of translation along the X axis, translation along the Y axis and rotation about the Z axis. The X and Y axes are labelled in figure 7 and the Z axis is directed into and out of the page.

The four SMA elements 80a-d are in an arrangement capable of applying actuating forces F so as to move the second part 20 relative to the first part 4 to any position within a range of movement. The range of movement may be within a movement plane that is perpendicular to a primary axis P (which is parallel to the Z axis).

In particular, two SMA elements 80a and 80b (i.e. the top and bottom SMA elements in Figure 7) are arranged to apply actuating forces F in opposite directions parallel to a first axis (e.g. the x axis). The other two SMA elements 80c and 80d (i.e. the left and right SMA elements in Figure 7) are arranged to apply actuating forces F in opposite directions parallel to a second axis (e.g. the y axis), perpendicular to the first axis. By appropriately varying the difference in actuation amount between the opposing SMA elements, the second part 20 may thus be moved independently along the first and second axes. The opposing actuating forces F are not colinear, but offset from each other in a direction perpendicular to the actuating forces F. Providing opposing SMA elements allows the tension in the SMA elements 80 to be controlled, allowing for more accurate and reliable positioning of the second part 20 compared to a situation in which the SMA elements do not oppose each other.

In some examples, none of the actuating forces F are collinear. This allows the arrangement of SMA elements 80a-d to translationally move the second part 20 without applying any net torque to the second part 20. So, the second part 20 can be moved translationally in the movement plane without rotating the second part 20 in the movement plane. In general, the arrangement of SMA elements 80a-d is capable of accurately controlling a torque or moment of the second part 20 about the primary axis P. So, the SMA elements 80a-d are capable of rotating (or not rotating) the second part 20 relative to the first part 4 about the primary axis P.

In particular, two SMA elements 80a and 80b (i.e. the top and bottom SMA elements in Figure 7) are arranged to apply actuating forces F so as to generate a torque or moment between the second part 20 and the first part 4 in a first sense (e.g. clockwise) around the primary axis P. The other two SMA elements 80c and 80d (i.e. the left and right SMA elements in Figure 7) are arranged to apply actuating forces F so as to generate a torque or moment between the second part 20 and the first part 4 in a second, opposite sense (e.g. anti-clockwise) around the primary axis P. This allows the second part 20 to be rotated by simultaneously increasing or decreasing the tension of the SMA wires.

As shown, two SMA elements 80 may be arranged to apply actuating forces F in a corner of the actuator assembly 1. The other two SMA elements may be arranged to apply actuating forces F in another, opposite corner of the actuator assembly 1. The actuator assembly 1, and in particular the second part 20 and/or the first part 4, may have a square or rectangular footprint. Each SMA element 80a-d may be provided on one of the four sides of the actuator assembly 1. The four SMA wires may extend in a loop around the primary axis (which may be defined by the first part 4). Each of the four SMA elements may be disposed on a side of the SMA actuator assembly 1, the four respective sides extending in a loop around the primary axis.

The arrangement of actuating forces F applied between the second part 20 and the first part 4 corresponds to the arrangement of SMA wires described in WO2013/175197 Al, which is herein incorporated by reference. l ' l A control method for use with the actuator assembly 1 will now be described. The control method is in line with the control method described with references to Figure 4, but applied to the four SMA elements 80a-d.

The controller causes the supply of power to at least one of the SMA elements 80a-d at a first power level. For example, power may be supplied to the SMA wires 80a and 80b to drive rotation of the second part 20 about the primary axis P. Once the desired position of the second part 20 relative to the first part 4 is reached, the controller causes the supply of power at a second power level, lower than the first power level, to one or more of the SMA elements 80a-d (i.e. a first subset of them) to apply a force to the second part 20 which is sufficient, in combination with the frictional force between the first surface 4f and second part 20, to hold the second part 20 in position relative to the first part 4 while a different one or more of the SMA elements 80a-d, are unpowered.

In one example, the controller causes a supply of power at the second power level to a first pair of the SMA elements (e.g. SMA elements 80a and 80d) while a second, different pair of the SMA elements (e.g. SMA elements 80b and 80c) are unpowered. The second part 20 is held still by a combination of: friction; and tension in the first pair of wires.

In the example shown in Figure 7, the actuating forces F are perpendicular to the primary axis P, and may be parallel to the movement plane. However, in general the actuating forces F may be angled relative to the movement plane. The actuating forces F may thus have a component along the primary axis P. This component along the primary axis P may be resisted by the plain bearing arrangement, for example, to provide movement of the second part 20 in degrees of freedom allowed by the bearing arrangement. In some examples, it may even be desirable for actuating forces F to have a component in parallel to the primary axis P, for example so as to load the plain bearing. In other examples, the actuating forces may have a component along the primary axis so as to, on contraction, reduce a normal force between the second part 20 and the first surface 4f and hence to reduce the friction therebetween. In this way, the variable friction aspects of the embodiment shown in Figure 6 may be applied to the embodiment shown in Figure 7.

Although the actuator assembly 1 of Figure 7 is described in the context of four SMA elements 80a- d, in general the actuator assembly 1 may include fewer SMA elements 80a-d. For example, the actuator assembly 1 may include two SMA elements, e.g. the two SMA elements 80a, 80d depicted in the top left of Figure 7. The forces applied to the second part 20 by the two SMA elements 80a, 80d may be opposed by a biasing force of one or more resilient elements, such as springs. With reference to Figure 7, the two SMA elements 80b, 80c in the bottom right corner may be replaced with springs applying biasing forces along the corresponding depicted arrows, for example.

Alternatively, the actuator assembly 1 may comprise a total of three SMA elements. Such SMA elements may be arranged as described in WO2022106856A1, which is incorporated herein by reference in its entirety.

Different types of movement

In the embodiments described with reference to Figures 1, 2 and 3, the second part 20 moves relative to the first part 3 along a movement direction. In the embodiment of Figure 7, the second part 20 is movable relative to the first part 4 in two degrees of freedom, in a plane.

Generally, for example as described with reference to Figure 7, the second part 20 may be rotatable relative to the first part 4, for example rotatable about an axis of rotation.

In some embodiments, multiple degrees of freedom may be combined. For example, the second part 20 may be movable relative to the first part 4 helically, e.g. along a helical path. Rotation of the second part 20 may be driven as described above with reference to Figure 7 and this rotation may be converted into helical movement by a helical bearing arrangement. An example of a helical bearing arrangement is provided in WO2019243849A1 which is incorporated herein by reference in its entirety.

SMA

The above-described SMA actuator assemblies comprise at least one SMA element. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA element may or may not include material(s) and/or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and/or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, e.g. a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, e.g. by Joule heating, another heating technique or by applying a magnetic field.

Claims

1. A shape memory alloy, SMA, actuator assembly comprising: a first part comprising a first surface; a second part which is movable relative to the first part across the first surface; one or more SMA elements arranged, on actuation, to drive movement of the second part relative to the first part, wherein the SMA actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a frictional force therebetween; and a controller configured to control actuation of the one or more SMA elements, wherein the controller is configured to: cause the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

2. An SMA actuator assembly according to claim 1 wherein: the second part is arranged to move relative to the first part over a range of motion comprising multiple possible positions of the second part relative to the first part; and the actuator assembly is configured such that: for a first subset of the multiple possible positions, the frictional force between the second part and the first surface is sufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered; and for a second subset of the multiple possible positions, different to the first subset, the frictional force alone is insufficient to retain the second part in position relative to the first part when the one or more SMA elements are unpowered.

3. An SMA actuator assembly according to claim 2, wherein the range of motion of the second part relative to the first part defines a central position of the second part in the range of motion and wherein the positions of the second subset are further from the central position than the positions of the first subset of positions are.

4. An SMA actuator assembly according to any preceding claim, wherein the one or more SMA elements comprises a first SMA element and wherein the SMA actuator assembly comprises a second SMA element arranged, on actuation, to drive movement of the second part relative to the first part, wherein the first and second SMA elements are arranged such that contraction of the second SMA element opposes contraction of the first SMA element.

5. An SMA actuator assembly according to claim 4, wherein the controller is configured such that the second SMA element is unpowered whilst power is supplied to the first SMA element at the second power level.

6. An SMA actuator assembly according to any of claims 1 to 3 wherein the one or more SMA elements comprises a first SMA element configured to apply a first force to the second part and wherein the SMA actuator assembly is arranged such that a second force is applied to the second part by gravity and/or by a force-application arrangement, wherein the second force opposes contraction of the first SMA element.

7. An SMA actuator assembly according to any preceding claim, wherein the second power level is a pre-determined power level.

8. An SMA actuator assembly according to any of claims 1 to 6, wherein the controller is configured to determine the second power level.

9. An SMA actuator assembly according to claim 8, wherein the controller is configured to determine the second power level by causing the supply of power at a third power level to the one or more SMA elements and subsequently decreasing the level of power supplied to the one or more SMA elements until the controller detects that the second part has started to move relative to the first part.

10. An SMA actuator assembly according to any preceding claim arranged such that the coefficient of static friction between the first surface and the second part is between 0.01 and 0.6, preferably between 0.05 and 0.4.

11. An SMA actuator assembly according to any preceding claim arranged such that a combination of: (a) the frictional force between the first surface and the second part; and

12. An SMA actuator assembly according to any preceding claim, wherein the one or more SMA elements are arranged, on contraction, to reduce the normal force between the first surface and the second part.

13. An SMA actuator assembly according to any of claims 1 to 11, wherein the one or more SMA elements are arranged such that the normal force between the first surface and the second part remains substantially constant on contraction of the one or more SMA elements.

14. An SMA actuator assembly according to any preceding claim, wherein the first power level is between 4 and 10 times greater than the second power level, preferably wherein the first power level is between 3 and 6 times greater than the second power level.

15. An SMA actuator assembly according to any previous claim, wherein the one or more SMA elements comprises three or more SMA elements.

16. An SMA actuator assembly according to claim 15, wherein the controller is configured to: cause the supply of power at the first power level to at least one of the three or more SMA elements to drive movement of the second part relative to the first part; and cause the supply of power at the second power level, lower than the first power level, to a first subset of the three or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part while a second subset, different to the first subset, of the three or more SMA elements are unpowered.

17. An SMA actuator assembly according to any preceding claim, wherein the controller is configured to determine a measure of electrical resistance of the one or more SMA elements whilst the second power level is supplied to the one or more SMA elements.

18. An SMA actuator assembly according to claim 16, wherein the second power level is a minimum power level required to straighten the one or more SMA elements.

19. An SMA actuator assembly according to any preceding claim, wherein the second power level is a minimum power level required to retain the second part in position relative to the first part.

20. A method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and one or more SMA elements, the method comprising: causing the supply of power at a first power level to the one or more SMA elements to drive movement of the second part relative to the first part across a first surface of the first part, wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a frictional force therebetween; and causing the supply of power at a second power level, lower than the first power level, to the one or more SMA elements to apply a force to the second part which is sufficient, in combination with the frictional force between the first surface and the second part, to hold the second part in position relative to the first part.

21. A method according to claim 20, wherein the second power level is a pre-determined power level.

22. A method according to claim 20 comprising determining the second power level.

23. A method according to claim 22 comprising determining the second power level by causing the supply of power at a third power level to the one or more SMA elements and subsequently decreasing the level of power supplied to the one or more SMA elements until it is detected that the second part has started to move relative to the first part.

24. A method according to any of claims 20 to 23, wherein the first power level is between 4 and 10 times greater than the second power level, preferably wherein the first power level is between 3 and 6 times greater than the second power level.

25. A method according to any of claims 20 to 24 comprising determining a measure of electrical resistance of the one or more SMA elements whilst the second power level is supplied to the one or more SMA elements.

26. A method according to any of claims 20 to 25 wherein the second power level is sufficient to straighten the one or more SMA elements.

27. A method according to any of claims 20 to 26 wherein the second power level is a minimum power level required to straighten the one or more SMA elements.

28. A method according to any of claims 20 to 27 wherein the second power level is a minimum power level required to retain the second part in position relative to the first part.

29. A computer-readable storage medium comprising instructions for instructing a controller to perform a method according to any one of claims 20 to 28.