GB2639201A

GB2639201A - Shape memory alloy actuator assembly

Info

Publication number: GB2639201A
Application number: GB2403450.6A
Authority: GB
Inventors: Samuel Johnson Benjamin; Lam Andrew
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2024-03-11
Filing date: 2024-03-11
Publication date: 2025-09-17
Also published as: WO2025191246A1; GB202403450D0

Abstract

A shape memory alloy (SMA) actuator assembly 1 comprises a second part 20 movable relative to a first part 4 across a first surface, and at least one SMA element 80 to drive movement of the second part. The SMA actuator assembly provides a bias between the second part and the first surface with a normal force to generate a static frictional force that constrains movement of the second part when the SMA element is not actuated. A controller controls actuation of the SMA element. The controller may, during a power-on sequence of the SMA element, increase power supplied to the SMA element gradually from zero to an operating power and/or, during a power-off sequence of the SMA element, decrease power supplied to the SMA element gradually from an operating power to zero. Alternatively, for at least one position of the second part, the actuator assembly may provide a residual net force along the movement direction when the SMA element is unpowered, and the controller obtains an indication of a target position of the second part, outputs one or more drive signals to the SMA element to move the second part to an intermediate position displaced from the target position along the movement direction, and power-off the SMA element such that second part moves towards the target position under the action of the residual net force.

Description

SHAPE MEMORY ALLOY ACTUATOR ASSEMBLY

Field

The present application relates to a shape memory allow (SMA) actuator assembly.

Background

There are a variety of apparatuses in which it is desired to provide control of a movable element. 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.

One type of apparatus in which SMA wire is known for use as an actuator is in miniature cameras, for example those used in smartphones or other portable electronic devices. WO 2011/104518 discloses examples of SMA actuation apparatuses which are suitable for use in miniature cameras.

It may be desirable to be able to power-on and/or power-off an actuator while allowing the position of the movable element to be consistent between the powered-on state and the powered-off state.

Summary

According to an aspect of the present invention, there is provided an SMA actuator assembly comprising: a first part; a second part which is movable relative to the first part across a first surface; at least one SMA element 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and a controller configured to control actuation of the at least one SMA element, wherein the controller is configured to: during a power-on sequence of the at least one SMA element, cause an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, cause a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.

By changing the power supplied to the at least one SMA element gradually, the possibility of the second part undesirably moving during a power-on sequence and/or during a power-off sequence may be reduced.

Causing an increase of power gradually or causing a decrease of power gradually may comprise causing one or more intermediate levels of power to be supplied to the at least one SMA wire during the power-on or power-off sequence. Each intermediate level of power may be greater than zero and less than an operating power.

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 static frictional force. The static frictional force may constrain the movement of the second part relative to the first part when the at least one SMA element is not powered.

Optionally, the controller is arranged to, during the power-on sequence and/or during the power-off sequence, control the power supplied to the at least one SMA element such that the second part is held relative to the first part by friction between the second part and the first surface. The SMA actuator assembly may be powered-off without changing the position of the second part between the powered state and the zero-hold power state.

Optionally, the SMA actuator assembly comprises two SMA elements arranged, on actuation, to drive movement of the second part relative to the first part in opposite directions. The position of the second part may be controlled accurately by controlling the SMA elements.

Optionally, during the power-on sequence and/or during the power-off sequence, the controller controls the power supplied to the SMA elements such that a net force applied to the second part by the SMA elements remains less than the static frictional force acting on the second part. By keeping the net tension below the frictional force threshold, the second part can remain in position relative to the first part during power-on/off.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the difference decreases until the power supplied to the SMA elements is zero. By decreasing the difference, the net tension may be gradually reduced so as to reduce or eliminate the risk of any jolt of the second part.

Optionally, the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the SMA elements stop being powered at substantially the same time. By stopping actuation at the same time, the overall time for the power-off sequence may be minimised.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the SMA element that had the greater power supplied stops being powered after the SMA element that had the lesser power supplied stops being powered. By keeping one SMA element actuated for longer, the SMA element may resist any undesirable slip of the second part, for example until the friction between the first part and the second part is sufficiently increased.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the difference is increased temporarily. By temporarily increasing the difference, the net tension may be increased so as to resist any undesirable slip of the second part, for example until the friction between the first part and the second part is sufficiently increased.

Optionally, the difference is increased such that a net driving force on the first part becomes substantially zero. By making the net driving force minimal, the second part may remain in place relative to the first part.

Optionally, the controller is configured to control the power supplied to the SMA elements during the power-on sequence such that the SMA elements start being powered at substantially the same time. By starting actuation at the same time, the overall time for the power-on sequence may be minimised.

Optionally, immediately after the power-on sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-on sequence such that the difference increases. By increasing the difference gradually, the risk of an undesirable jolt of the second part may be reduced.

Optionally, the controller is configured to control the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element changes linearly over time for at least part of the duration of the sequence. By changing the power linearly, the control algorithm may be simplified.

Optionally, the controller is configured to control the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element changes according to a polynomial shape over time for at least part of the duration of the sequence. By using a polynomial shape, the SMA elements may be optimised for reducing, or compensating for, any possible slip of the second part. The SMA actuator assembly can be calculated such that such a slip can be predicted and accounted for by using a polynomial shape for the actuation control.

Optionally, the controller is configured to control the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element comprises at least one step change. By providing a step change, the overall time required for the power-on or power-off sequence may be shortened.

According to another aspect of the present invention, there is provided an SMA actuator assembly comprising: a first part; a second part which is movable relative to the first part in a movement direction across a first surface; at least one SMA element 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; wherein, for at least one position of the second part relative to the first part in a range of movement of the second part, the actuator assembly is arranged such that the second part experiences a residual net force along the movement direction when the at least one SMA element is unpowered; and a controller configured to control actuation of the at least one SMA element, wherein the controller is configured to: obtain an indication of a target position of the second part relative to the first part; output one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and power-off the at least one SMA element such that second part moves towards the target position under the action of the residual net force.

By controlling actuation of the at least one SMA element so as to drive movement of the second part to the intermediate position, the second part may be moved so as to compensate for an expected slip of the second part. The accuracy with which the second part may be positioned relative to the first part may therefore be increased.

The second part is movable relative to the first part in a movement direction across a first surface. The second part may be movable relative to the first part in one or more degrees of freedom. For example, the second part may be movable in any direction in a plane relative to the first part. Alternatively, the second part may be constrained to move in a single degree of freedom, for example along a single axis.

Optionally, the at least one SMA element is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the surface so as to drive movement of the second part relative to the first part across the first surface. The friction may be reduced by contraction of the at least one SMA element when it is desired to move the second part. The friction may be increased again (by powering-off the SMA elements) when it is desired to keep the second part stationary.

Optionally, the second part is a lens element comprising at least one lens. Optionally, the first part has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor. Optionally, the first part has a display mounted thereon, the lens element being arranged to focus light emitted by the display. The SMA actuator assembly may be used to improve a camera, projector or display, for example.

Optionally, the second part has an image sensor mounted thereon. Optionally, the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor. The SMA actuator assembly may be used to improve a camera, projector or display, for

example.

It will be appreciated that the actuator assemblies and methods described herein may be used to move any element or component.

According to another aspect of the present invention, there is provided a method of controlling an SMA actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and during a power-on sequence of the at least one SMA element, causing an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, causing a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.

Optionally, the method comprises, during the power-on sequence and/or during the power-off sequence, controlling the power supplied to the at least one SMA element such that the second part is held relative to the first part by friction between the second part and the first surface.

Optionally, the method comprises, during the power-on sequence and/or during the power-off sequence, controlling the power supplied to opposing SMA elements such that a net force applied to the second part by the SMA elements remains less than the static frictional force acting on the second part.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the method comprises controlling the power supplied to opposing SMA elements during the power-off sequence such that the difference decreases until the power supplied to the SMA elements is zero.

Optionally, the method comprises controlling the power supplied to opposing SMA elements during the power-off sequence such that the SMA elements stop being powered at substantially the same time.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the method comprises controlling the power supplied to opposing SMA elements during the power-off sequence such that the SMA element that had the greater power supplied stops being powered after the SMA element that had the lesser power supplied stops being powered.

Optionally, immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the method comprises controlling the power supplied to opposing SMA elements during the power-off sequence such that the difference is increased temporarily.

Optionally, the method comprises increasing the difference such that a net driving force on the first part becomes substantially zero.

Optionally, the method comprises controlling the power supplied to opposing SMA elements during the power-on sequence such that the SMA elements start being powered at substantially the same time.

Optionally, immediately after the power-on sequence, there is a difference in the power supplied to each of the SMA elements, and the method comprises controlling the power supplied to opposing SMA elements during the power-on sequence such that the difference increases.

Optionally, the method comprises controlling the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element changes linearly over time for at least part of the duration of the sequence.

Optionally, the method comprises controlling the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element changes according to a polynomial shape over time for at least part of the duration of the sequence.

Optionally, the method comprises controlling the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element comprises at least one step change.

According to another aspect of the present invention, there is provided a method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; obtaining an indication of a target position of the second part relative to the first part; outputting one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and powering-off the at least one SMA element such that second part moves towards the target position under the action of a residual net force along the movement direction experienced by the second part for at least one position of the second part relative to the first part in a range of movement of the second part when the at least one SMA element is unpowered.

The method may be a computer-implemented method.

According to another aspect of the present invention, there is provided a computer program product comprising instructions for instructing a controller to perform a method of controlling an SMA actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and during a power-on sequence of the at least one SMA element, causing an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, causing a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.

According to another aspect of the present invention, there is provided a computer-readable storage medium comprising instructions for instructing a controller to perform a method of controlling an SMA actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and during a power-on sequence of the at least one SMA element, causing an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, causing a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero. The computer-readable storage medium may be non-transitory.

According to another aspect of the present invention, there is provided a computer program product comprising instructions for instructing a controller to perform a method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; obtaining an indication of a target position of the second part relative to the first part; outputting one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and powering-off the at least one SMA element such that second part moves towards the target position under the action of a residual net force along the movement direction experienced by the second part for at least one position of the second part relative to the first part in a range of movement of the second part when the at least one SMA element is unpowered.

According to another aspect of the present invention, there is provided a computer-readable storage medium comprising instructions for instructing a controller to perform a method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; obtaining an indication of a target position of the second part relative to the first part; outputting one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and powering-off the at least one SMA element such that second part moves towards the target position under the action of a residual net force along the movement direction experienced by the second part for at least one position of the second part relative to the first part in a range of movement of the second part when the at least one SMA element is unpowered. The computer-readable storage medium may be non-transitory.

Any of the optional features described herein may be applied to any of the aspects 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 an alternative SMA actuator assembly; Figures 4-9 are schematic diagrams showing power supplied to SMA elements during alternative power-off sequences; and Figures 10-13 are schematic diagrams showing power supplied to SMA elements during alternative power-on sequences.

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. The second part 20 is configured to move relative to the first part 4. 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 may be movable relative to the first part 4 in a movement direction across a first surface. The second part 20 may be movable in at least two dimensions relative to the first part 4. Alternatively, the second part 20 may be constrained to one dimensional movement relative to the first part 4.

The SMA actuator assembly 1 comprises at least one SMA element. As shown in Figure 1, the SMA element(s) may be SMA wire(s) 80. Embodiments are described in the context of using one or more SMA wires 80. Other types of SMA elements may be used instead of SMA wires 80.

The at least one 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.

Optionally, 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 80. 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 may be arranged such that friction resists relative movement between the first part 4 and the second part 20. The SMA actuator assembly 1 may be arranged such that the second part 20 and the first surface (across which the second part 20 is movable relative to the first part 4) are biased against each other with a normal force. The normal force thereby generates a static frictional force that constrains the movement of the second part 20 relative to the first part 4. For example, the static frictional force may constrain the movement of the second part 20 relative to the first part 4 when the at least one SMA wire 80 is not actuated. As shown in Figure 1, optionally the second part 20 is in contact with the first part 4. The first part 4 has the first surface. The first surface may face towards the second part 20. The second part 20 may have a second surface. The second surface may face towards the first part 4.

The biasing together of the second part and the first surface may be provided by, for example, one or more of gravity, one or more resilient elements such as springs, and/or one magnets.

Optionally, when the second part 20 moves, the second surface (of the second part 20) moves across the first surface (of the first part 4). In use of the SMA actuator assembly 1, friction between the first surface and the second surface may resist relative movement between the second part 20 and the first part 4. For example, the friction may prevent the second part 20 from moving relative to the first part 4 when a net driving force on the second part 20 is too low.

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. The at least one SMA wire 80 is configured to apply a driving force to the second part 20. Optionally, SMA actuator assembly 1 may comprise one or more further driving components that apply driving forces to the second part 20.

For example, although not shown in Figure 1, optionally the SMA actuator assembly 1 comprises one or more resilient members, for example one or more springs. A resilient member may be arranged to oppose the driving force applied by the SMA wire 80 shown in Figure 1. The net driving force on the second part 20 may be comprised of the driving force supplied by the SMA wire 80 and the force supplied by the resilient member.

The SMA actuator assembly 1 may be arranged such that when a net driving force on the second part 20 is below a frictional force threshold, then the second part 20 remains in position relative to the first part 4. The frictional force threshold is a threshold that must be overcome in order for the second part 20 to move relative to the first part 4. When the second part 20 is stationary relative to the first part 4, then the frictional force threshold prevents movement of the second part 20 relative to the first part 4 until the frictional force threshold is overcome by the net driving force on the second part 20.

Optionally, the second part 20 may undergo stick-slip motion relative to the first part 4. The friction of the SMA actuator assembly 1 may reduce smoothness of movement between the first part 4 and the second part 20.

Optionally, the SMA actuator assembly 1 is arranged to have sufficient friction that the second part 20 remains in position relative to the first part 4 when the at least one SMA wire 80 is not actuated. This may allow the second part 20 to be held in position relative to the first part 4 when zero power is applied to the at least one SMA wire 80. This may be referred to as zero hold power.

Figure 2 is a schematic view of an alternative SMA actuator assembly 1. As shown in Figure 2, optionally the SMA actuator assembly 1 comprises two SMA wires 80a, 80b. 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.

For example, as shown in Figure 2 the SMA wires 80a, 80b may be configured to drive movement of the second part 20 in opposite directions.

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.

When each SMA wire 80 is actuated, the SMA wire 80 has a tension that corresponds to the driving force that the SMA wire 80 applies to the second part 20. When there are two (or more) SMA wires 80, then the SMA wires 80 have a net tension. The net tension is the overall tension applied by the SMA wires 80 on the second part 20. The net tension may correspond to the net driving force when there are no components other than the SMA wires 80 that drive movement of the second part 20. When the SMA wires 80 are actuated to the same actuation level, then the net tension is substantially zero.

Variable friction Figure 3 schematically depicts an alternative SMA actuated assembly 1. 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.

Optionally, the at least one SMA wire 80 is arranged to apply a force to the second part 20 with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the surface so as to drive movement of the second part 20 relative to the first part 4 across the first surface.

As shown in Figure 3, optionally the SMA actuator assembly 1 comprises a loader 41. In Figure 3, the loader 41 takes the form of a spring. The loader 41 is configured to increase the friction that opposes movement of the second part 20 relative to the first part 4. For example, in the arrangement shown in Figure 3, the loader 41 is configured to urge the second part 20 towards the first part 4, so as to increase the friction between the first surface of the first part 4 and the second surface of the second part 20. Although the loader 41 is shown as a spring in Figure 3, other types of component may be used as the loader 41. For example, one or more magnets may be used. Alternatively, gravity may be used to provide the loading.

As shown in Figure 3, optionally 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 3, optionally the SMA wires 80 are arranged, on actuation, to apply a force on the second part 20 away from the first part 4. By applying a force away from the first part 4, the SMA wires 80 act to reduce the friction between the first part 4 and the second part 20. The loader 41 applies a force that opposes the unloading force supplied by the SMA wires 80 when they are actuated. In general, the force supplied by the loader 41 is greater than the unloading force supplied by the SMA wires 80. The second part 20 remains engaged with the first part 4.

During use of the SMA actuator assembly 1, the friction level varies. When the SMA wires 80 are not actuated, the friction level is generally higher. When the SMA wires 80 are at a greater actuation level, the friction that opposes movement of the second part 20 relative to the first part 4 reduces.

Ramping power down When the at least one SMA wire 80 is not actuated, the power supplied to the at least one SMA wire 80 is zero. During use of the SMA actuator assembly 1, the power supplied to the at least one SMA wire 80 is an operating power. The operating power is greater than zero. The operating power is sufficient to cause the at least one SMA wire 80 to apply a driving force to the second part 20. The SMA wire(s) 80 comprise an SMA that has a transition temperature. When the SMA wire 80 is heated such that its temperature is greater than the transition temperature, then the SMA wire 80 contracts. During use of the SMA actuator assembly 1, the SMA wires have a temperature that is greater than an ambient temperature. The ambient temperature may be the temperature of the surroundings of the SMA actuator assembly 1. The operating power may be sufficient to heat the at least one SMA wire 80 such that its tension increases or is at least maintained. The tension may cause the at least one SMA wire 80 to contract. The at least one SMA wire 80 may be prevented from contracting due to other forces, for example tension in an opposing SMA wire 80.

When the SMA wire 80 is powered-on, the power supplied to the SMA wire 80 may increase from zero to the operating power. When the SMA wire 80 is powered-off, the power supplied to the SMA wire 80 may decrease from an operating power to zero. Optionally, during a power-off sequence of the at least one SMA wire 80, the power supplied to the SMA wire 80 decreases from an operating power to zero. Optionally, the controller is configured to, during a power-off sequence of the at least one SMA element, cause a decrease of a power supplied to the at least one SMA wire 80 gradually from the operating power to zero.

The controller may be configured to, during the power-off sequence, cause one or more intermediate levels of power to be supplied to the at least one SMA wire 80. Each intermediate level of power may be greater than zero and less than the operating power. Optionally, the controller is configured to cause the intermediate levels of power to be supplied to the at least one SMA wire 80 sequentially in order of decreasing power. The controller may be configured to, during the power-off sequence, cause a decrease of a power to be supplied to the at least one SMA wire 80 over a predetermined period of time. The predetermines period of time may be, for example, 100ms or longer, 150ms or longer or 200ms or longer. The power supplied to the at least one SMA wire 80 may be at the operating power at the beginning of the predetermined period of time. The power supplied to the at least one SMA wire 80 may be less than the operating power after the beginning of the predetermined period of time. The power supplied to the at least one SMA wire 80 may reach the zero at the end of the predetermined period of time. The power supplied to the at least one SMA wire 80 may be greater than zero until the end of the predetermined period of time.

Optionally, the controller is configured to, during a power-off sequence of the at least one SMA element, cause a decrease of a power supplied to the at least one SMA wire 80 from the operating power to zero by a plurality of step decreases in power. A step decrease in power is an instantaneous decrease. By providing a plurality of step decreases, the power supplied to the at least one SMA wire 80 may be an intermediate level for part of the power-off sequence.

Optionally, the controller is configured to, during a power-off sequence of the at least one SMA element, cause a power supplied to the at least one SMA wire 80 to decrease at a finite and non-zero rate. For example, the power may be decreased linearly over time or in accordance with a polynomial 10 shape.

Optionally, the controller is configured to cause power to be supplied to the at least one SMA wire 80 by pulse width modulation. However, it is not essential for pulse width modulation to be used.

Optionally, during a power-off sequence, the controller controls actuation of the at least one SMA wire such that the net driving force remains below the frictional force threshold. The power applied to the at least one SMA wire 80 may be ramped down while keeping the net driving force below the frictional force threshold. By keeping the net driving force below the frictional force threshold, the second part 20 remains in position relative to the first part 4 during a power-off sequence. The SMA actuator assembly 1 may be powered-off without changing the position of the second part 20 between the powered state and the zero-hold power state.

Figure 4 is a schematic diagram showing the power P applied to two SMA wires 80a, 80b during a power-off sequence. The box made of dash lines in Figure 4 shows the power-off sequence. During a power-off sequence, the power applied to the SMA wires 80 is reduced to zero. Figure 4 shows two traces. The trace formed of a solid line shows the power P applied to a first SMA wire 80a over time t. The trace made of a dash line shows the power P applied to a second SMA wire 80b over time t.

Figure 4 shows two traces for the two SMA wires 80a, 80b shown in Figure 2, for example. In an alternative arrangement comprising only one SMA wire 80, for example, as shown in Figure 1, the equivalent power profile would comprise only one trace. For example, the trace for the first SMA wire 80a may represent the trace of the SMA wire 80 of an arrangement comprising only one SMA wire 80.

As shown in Figure 4, immediately before the power-off, the SMA wires 80a, 80b may have different actuation levels. The first SMA wire 80a has a higher actuation level 71. The actuation level corresponds to the level of power P applies to the SMA wire 80. The second SMA wire 80b has a lower actuation level 72. The difference in actuation levels of the two wires 80 may correspond to a position of the second part 20 away from a neutral position. For example, the neutral position of the second part 20 may be a position where the two SMA wires 80 are contracted by the same amount. Immediately before the power-off sequence shown in Figure 4, the first SMA wire 80a has the higher actuation level 71. The first SMA wire 80a may have a higher temperature and may be more contracted compared to the second SMA wire 80b.

During a power-off sequence, the temperatures of the SMA wires 80 reduce. As the temperatures of the SMA wires 80 reduce, the SMA wires 80 may become less contracted. This affects the tension of the SMA wires 80. It is possible for the net tension of the SMA wires 80 to vary during a power-off sequence. For example, the first SMA wire 80a may cool more quickly than the second SMA wire 80b. This could cause the tension in the first wire 80a to reduce more quickly than the tension in the second wire 80b. This can potentially lead to the second part 20 slipping in the direction of the driving force applied by the second SMA wire 80b.

By keeping the net driving force below the frictional force threshold, the second part 20 is prevented from slipping relative to the first part 4.

When the first SMA wire 80a has a greater actuation level 71 compared to the lower actuation level 72 of the second SMA wire 80b, the second part 20 may be located slightly towards the right-hand side of the neutral position in the orientation shown in a Figure 2. The traces in Figure 4 show the power profiles for the SMA wires 80. The power profile is the power level (i.e. actuation level) applied to the SMA wire 80 over time. As shown in Figure 4, optionally the power profile for the first SMA 80a comprises a decrease 101. The decrease 101 may be in the power off sequence (i.e. within the dash line box of Figure 4). As shown in Figure 4, optionally the decrease 101 is linear. Alternatively, the decrease 101 may have a polynomial shape and/or may comprise one or more step changes.

As shown in Figure 4, optionally the power profile for the second SMA wire 80b comprises a decrease 201. However, the form of the decrease 201 for the second SMA wire 80b may be different from the decrease 101 for the first SMA wire 80a, as shown in Figure 4. Optionally, immediately before the power-off, there is a difference in actuation level of the SMA wires 80a, 80b. The controller may be configured to control actuation of the SMA wires 80a, 80b during a power-off sequence such that the difference decreases until the actuator components stop being actuated. This is shown in Figure 4, where the difference in actuation level of the two SMA wires 80a, 80b decreases over time. This is shown by the vertical distance between the two traces decreasing as t increases. As shown in Figure 4, the first SMA wire 80a may stop being actuated at a stop time 73. As shown in Figure 4, optionally the second SMA wire 80b stops being actuated at the same stop time 73.

After the power-off sequence, both of the SMA wires 80 are not actuated. The actuation level of both SMA wires 80 is zero. There is no difference in actuation level of the SMA wires 80 after the power-off.

By reducing the difference gradually, the possibility of the second part 20 undesirably slipping may be reduced. By providing that both SMA wires 80 stop being actuated at substantially the same stop time 73, the overall time required to perform power-off may be reduced.

Resisting slip during a power-off sequence Figure 5 shows the power profiles applied to two SMA wires 80a, 80b according to an alternative power-off sequence. The power-off sequence is indicated by the dash line box in Figure 5. As shown in Figure 5, immediately before the power-off there may be a difference in actuation level of the SMA wires 80. The first SMA wire 80a may have a higher actuation level 71. The second SMA wire 80b may have a lower actuation level 72. As shown in Figure 5, the power profile for the first SMA wire 80a may comprise a decrease 101. As shown in Figure 5, the power profile applied to the second SMA wire 80b may comprise a corresponding decrease 211.

As shown in Figure 5, optionally the difference in actuation level of the two SMA wires 80 may remain substantially constant during the power-off sequence until the second SMA wire 80b stops being actuated at the second wire stop time 74.

As shown in Figure 5, optionally the controller is configured to control actuation of the SMA wires 80 during a power-off sequence such that the SMA wire 80a that had the higher actuation level (immediately before the power-off) stops being actuated after the SMA wire 806 that had the lesser actuation level (immediately before the power-off) stops being actuated. This is shown in Figure 5, where the first wire stop time 73 is after the second wire stop time 74.

Immediately before the power-off, the second SMA wire 80b has the lower actuation level 72, the second wire 80b may be generally at a lower temperature than the first SMA wire 80a. The second SMA wire 80b may be less contracted (e.g. longer) than the first SMA wire 80a. Immediately before power-off, the second part 20 may be located slightly to the right-hand side of the neutral position in the orientation shown in Figure 2.

When the SMA wires 80 are powered-off such that their actuation levels are zero, the second SMA wire 80b remains stretched in comparison to the first SMA wire 80a. As a result, the second SMA wire 80b may be expected to apply a greater tension to the second part compared to tension applied by the first SMA wire 80a. There may be a residual force when both SMA wires 80 are powered-off, the force urging the second part 20 towards the neutral position. By stopping actuation of the second SMA wire 80b (i.e. the SMA wire that has a lower temperature) before stopping actuation of the first SMA wire 80a, the force balance on the second part 20 may be better maintained until both SMA wires 80 are at a very low actuation level. This may help to reduce the possibility of an undesirable jilt or slip of the second part 20 during the power-off sequence.

By maintaining the force balance until the actuation level of both SMA wires 80 is low, the force balance is maintained until a time when the friction is sufficiently increased. In particular, when the SMA wires are arranged to apply an unloading force (for example, in the arrangement of Figure 3), the friction is greater when the actuation level of the SMA wires 80 has reached a low level.

In general, the difference in actuation level of the SMA wires 80a, 80b may correspond to a net tension on the second part. By maintaining the net tension until the friction has sufficiently increased, the possibility of the second part undesirably moving during the power-off sequence is reduced.

Alternative power-off sequences Figure 6 is a schematic diagram of power profiles applied to the SMA wires 80a, Bob according to an alternative power-off sequence. As shown in Figure 6, the power profile applied to the first SMA wire 80a may comprise a decrease 101. As shown in Figure 6, optionally the power profile applied to the second SMA wire 80b comprises a corresponding decrease 211. However, the decrease start point 76 for the first SMA wire 80a is before the decrease start point 77 for the second SMA wire Bob.

As shown in Figure 6, optionally during the power-off sequence the actuation level for the second SMA wire 80b becomes greater than the actuation level for the first SMA wire 80a. During the power-off sequence, the second SMA wire 80b may be maintained at a higher actuation level compared to the first SMA wire 80a until both SMA wires 80 are powered-off. As shown in Figure 6, the second SMA wire 80b stop being actuated at the second wire stop time 75 after the first wire stop time 73 at which the first SMA wire 80a stops being actuated.

Optionally, the SMA actuator assembly 1 may comprise a component that urges the second part 20 towards one or both extreme ends of its stroke. A stroke of a second part 20 is the range of positions that the second part 20 may take relative to the first part 4. For example, the SMA actuator assembly 1 may be a bistable assembly. The SMA actuator assembly 1 may have neutral positions at either extreme ends of the stroke of the second part 20. In the absence of a net tension by the SMA wires 80 or friction, the second part 20 may tend to take a position at either extreme ends of its stroke.

During a power-off sequence of such an SMA actuator assembly 1, there is a possibility that as the SMA wires 80 are powered down, the force that urges the second part 20 towards an extreme end of the stroke may dominate, causing the second part 20 to slip. By providing that the second SMA wire 80b has a higher actuation level during the power-off sequence, the possibility of such a slip occurring may be reduced. As shown in Figure 6, the difference in actuation level of the SMA wires 80a, 80b eventually decreases after the first wire stop time 73 at which the first SMA wire 80a stops being actuated.

However, at this point in the power-off sequence the actuation levels of the SMA wires 80 is sufficiently low that the friction may have sufficiently increased to prevent the second part 20 from moving.

Figure 7 shows a schematic view of power profiles applied to the SMA wires 80 according to an alternative power-off sequence. As shown in Figure 7, optionally the power profile applied to the second SMA wire 80b matches the power profile applied to the first SMA wire 80a for at least part of the power-off sequence. The difference in actuation level of the SMA wires 80 may be reduced to zero at an earlier time point.

Increasing net tension during a power-off sequence Figure 8 schematically shows power profiles applied to the SMA wires 80 according to an alternative power-off sequence. As shown in Figure 8, the power profile for the second SMA wire 80b comprises a decrease 221. The power profile applied to the first SMA wire 80a may comprise an initial decrease 111, followed by a shallower decrease 112 (or alternatively no decrease or an increase for a period of time), followed by a final decrease 113. As shown in Figure 7, optionally the shallow decrease 112 involves a lower decrease of power over time compared to the decrease 221 of the second SMA wire 80b. As a result, the difference in actuation level of the SMA wires 80 increases for a period during a power-off sequence.

As shown in Figure 8, optionally immediately before the power-off, there is a difference in actuation level of the SMA wires 80. The controller may be configured to control actuation of the SMA wires 80a, 80b during a power-off sequence such that the differences increased temporarily.

By matching the actuation levels of the SMA wires 80, the net forces may be reduced so as to a make the loading of the second part 20 against the first part 4 more stable.

As mentioned above, it is possible that during a power-off sequence of the SMA actuator assembly 1, the relatively stretched second SMA wire 80b has a tension that causes a force that undesirably moves the second part 20 during a power-off sequence. By temporarily increasing the difference in actuation level of the SMA wires 80 during a power-off sequence, the possibility of the second SMA wire 80b causing the second part 20 to undesirably slip it may be reduced.

Figure 9 schematically shows power profiles applied to SMA wires 80 according to an alternative power-off sequence. As shown in Figure 9, the difference in actuation level of the SMA wires 80 is temporarily increased during the power-off sequence compared to the difference immediately before power-off. In the power profiles shown in Figure 8, the temporarily increase in the differences is effected by a temporary slowing of the decrease in actuation level of the first SMA wire 80a. In the power profiles shown in Figure 9, the temporary increase in different actuation levels is effected by a temporary increase in the rate by which the actuation level of the second SMA wire 80b is reduced. As shown in Figure 9, optionally the power profile applied to the second SMA wire 80b comprises a step change 231 followed by a subsequent decrease 232. The step change 231 causes the difference in actuation levels of the SMA wires 80a, 80b to be increased. The difference is subsequently decreased over time. By temporarily increasing the difference in the actuation levels, the possibility of the second SMA wire 80b undesirably jerking the second part 20 may be reduced.

In an alternative power-off sequence, the rate of decrease of the actuation level of the first SMA wire 80a may be temporarily decreased (e.g. as in Figure 8) and the rates of decrease of the actuation level of the second SMA wire 80b may be temporarily increased (e.g. as in Figure 9). In other words, the features shown in Figure 8 and Figure 9 may be combined.

Overshoot during a power-off sequence As described with reference to Figure 8 and Figure 9, for example, the difference in actuation level between the SMA wires 80 may be temporarily increased during the power-off sequence. Optionally, this may be done first to prevent the second part 20 from moving relative to the first part 4.

Alternatively, in an arrangement the difference is increased so as to drive movement of the second part 20. In other words, the second part 20 may be intentionally moved relative to the first part 4 during the power-off sequence. This may be referred to as a controlled overshoot of the position of the second part 20. As mentioned above, it is possible that the relatively uncontracted second SMA wire 80b causes the second part 20 to slip to the left in the orientation shown in Figure 2 or Figure 3. The distance by which the second part 20 slips may be predictable. The distance of the slip may depend on the location of the second part 20 relative to the first part 4 immediately before power-off. The SMA actuator assembly 1 may be calibrated by determining the slip expected during a power-off sequence, dependent on the location of the second part 20 immediately before the power-off.

As mentioned elsewhere, the second part 20 may be movable relative to the first part 4 in a movement direction across a first surface. Optionally, for at least one position of the second part 20 relative to the first part 4 in a range of movement of the second part 20, the SMA actuator assembly 1 is arranged such that the second part 20 experiences a residual net force along the movement direction when the at least one SMA wire 80 is unpowered.

The residual net force may be at least partly the result of a biasing force. For example, the SMA actuator assembly 1 may comprise a biasing element configured to apply a biasing force such that the second part 20 experiences the residual net force along the movement direction when the at least one SMA wire 80 is unpowered. The biasing element may comprise a resilient member such as a spring. The biasing element may comprise a magnetic element.

The residual net force may be at least partly the result of a centering force. The centering force may urge the second part 20 towards a centering position along the range of movement. The centering position is between the ends of the range of movement. For example, the centering position may be a centre of the range of movement. The SMA actuator assembly 1 may comprise a centering element configured to apply a centering force such that the second part 20 experiences the residual net force along the movement direction when the at least one SMA wire 80 is unpowered. The centering element may comprise a resilient member such as a spring. The centering element may comprise a magnetic element.

The residual net force may be at least partly the result of relaxation of a component, for example the first part 4, of the SMA actuator assembly 1 when the at least one SMA wire 80 is powered-off.

The residual net force may be at least partly the result of hysteresis of the at least one SMA wire 80.

Optionally, the power-off sequence is selected first to cause an intentional movement of the second part 20 that counters the expected slip. The second part 20 may be moved a distance (for example, towards the right in the orientation shown in Figure 2 or Figure 3), in the expectation that the second part 20 subsequently slips in the opposite directions (e.g. to the left in the orientation shown in Figure 2 or Figure 3). As a result, the position of the second part 20 immediately before may be substantially the same as the position of the second part 20 after the power-off sequence. The intentional movement of the second part 20 may compensate for the slip of the second part 20.

Optionally, the controller is configured to obtain an indication of a target position of the second part 20 relative to the first part 4. The target position is a position to which the second part 20 is to be moved. The controller may receive the indication of the target position. The indication of the target position may be comprised in a signal, such as a data signal or a control signal, received by the controller.

Optionally, the controller is configured to output one or more drive signals for application to the at least one SMA wire 80. The drive signals are to drive movement of the second part 20 to an intermediate position. The intermediate position is displaced from the target position along the movement direction. The intermediate position is a position to which the second part 20 is moved before the second part 20 is moved to the target position.

Optionally, the controller is configured to power-off the at least one SMA wire 80 such that second part 20 moves towards the target position under the action of the residual net force. The second part 20 may be moved to the target position under the action of the residual net force. It is possible that the second part 20 may be moved to a position displaced from the target position under the action of the residual net force. In other words, there may be some inaccuracy of movement of the second part 20.

However, the second part 20 is moved towards the target position under the action of the residual net force.

Power profiles such as shown in Figure 8 or Figure 9, for example, may allow for such an intentional movement to an intermediate position of the second part 20 relative to the first part 4. Alternatively, the second part may be driven to the intermediate position and then the SMA wire(s) powered-off instantaneously (as opposed to gradually).

By providing an overshoot that compensates for the slip, the design freedom for the SMA actuator assembly 1 may be increased. For example, with an arrangement having variable friction as shown in Figure 3, the friction that opposes movement of the second part 20 relative to the first part 4 may be relatively low when the second part 20 is at a more extreme position along its stroke (because there is greater actuation of one of the SMA wires 80). Such a low friction may be allowable when making use of the overshoot during the power-off sequence that compensates for the expected slip.

In overshoot mode, the controller may be configured to control actuation of the at least one SMA wire 80 so as to drive movement of the second part 20 a predetermined distance in a direction of driving force on the second part 20 applied by the SMA wire 80 that has the highest actuation level. For example, when the SMA actuator assembly 1 has only one SMA wire 80 (e.g. as shown in Figure 1), then the SMA wire 80 that has the highest actuation level is the only SMA wire 80. The controller may drive movement of the second part 20 so that the SMA wire 80 pulls the second part 20 in the expectation that the second part 20 will subsequently slip back during a power-off sequence of the SMA wire 80.

Alternatively, when two SMA wires 80a, 80b are provided, the first SMA wire 80a may have a higher actuation level (e.g. as shown in Figures 4-9). The controller may be configured to control actuation of the SMA wires 80 so as to drive movement of the second part 20 a predetermined distance in the direction of the driving force on the second part 20 applied by the first SMA wire 80a. This is because the second part 20 will be expected to slip due to the second SMA wire 80b being less contracted when the SMA wires 80 are powered-off.

Selection of power profile Optionally, the controller is configured to select a power profile for power-off dependent on the position of the second part 20 relative to the first part 4. For example, when the second part 20 is generally closer to neutral position (e.g., in the middle of the stroke), then a power profile similar to as shown in Figure 4 may be used. Such a power profile has an advantage of stopping actuation of both of the SMA wires 80 earlier, i.e. the power-off sequence is shorter. This is possible when the second part 20 is closer to the neutral position because the friction that opposes movement of the second part 20 may be expected to be generally greater.

Alternatively, when the location of the second part 20 is closer to an extreme end of the stroke, the friction may be lower. The controller may select an alternative profile such as shown in Figure 4, where the difference in actuation level of the SMA wires 80 is maintained until the friction is increased sufficiently. Similarly, a power profile as shown in Figure 8 or Figure 9 may be used when the second part has a starting position immediately before the power-off sequence that corresponds to a lower friction. Similarly, the controller may be configured to select a profile that causes an overshoot when the friction immediately before the power-off sequence is expected to be low.

Power-on Figure 10 is a schematic diagram of a power-on sequence. Optionally, during a power-on sequence of the at least one SMA wire 80, the power supplied to the SMA wire 80 increases from zero to an operating power. Optionally, the controller is configured to, during a power-on sequence of the at least one SMA element, cause an increase of a power supplied to the at least one SMA wire 80 gradually from zero to the operating power. By changing the power supplied to the at least one SMA element gradually, the possibility of the second part undesirably moving during a power-on sequence may be reduced.

The controller may be configured to, during the power-on sequence, cause one or more intermediate levels of power to be supplied to the at least one SMA wire 80. Each intermediate level of power may be greater than zero and less than the operating power. Optionally, the controller is configured to cause the intermediate levels of power to be supplied to the at least one SMA wire 80 sequentially in order of increasing power. The controller may be configured to, during the power-on sequence, cause an increase of a power to be supplied to the at least one SMA wire 80 over a predetermined period of time. The predetermines period of time may be, for example, 10ms or longer, 15ms or longer or 20ms or longer. The power supplied to the at least one SMA wire 80 may reach the operating power at the end of the predetermined period of time. The power supplied to the at least one SMA wire 80 may be lower than the operating power until the end of the predetermined period of time.

Optionally, the controller is configured to, during a power-on sequence of the at least one SMA element, cause an increase of a power supplied to the at least one SMA wire 80 from zero to the operating power by a plurality of step increases in power. A step increase in power is an instantaneous increase. By providing a plurality of step increases, the power supplied to the at least one SMA wire 80 may be an intermediate level for part of the power-on sequence.

Optionally, the controller is configured to, during a power-on sequence of the at least one SMA element, cause a power supplied to the at least one SMA wire 80 to increase at a finite and non-zero rate. For example, the power may be increased linearly over time or in accordance with a polynomial shape.

Optionally, the controller controls actuation of the at least one SMA wire 80 such that the net driving force remains below the frictional force threshold during a power-on sequence.

For example, as shown in Figure 10, optionally the controller is configured to control actuation of the SMA wires 80 during a power-on sequence such that the SMA wires 80 start being actuated at substantially the same time. For example, Figure 10 shows a first wire start time 83 that coincides with the second start wire time 83. The power profile applied to the first SMA wire 80a may comprise an increase 121. The power profile applied to the second SMA wire 80b may comprise a different increase 241. The rate of increase in power applied to the two SMA wires 80a, 80b may be different from each other. As shown in Figure 10, immediately after the power-on sequence, the SMA wires 80 may have a different actuation level. For example, the first SMA wire 80a may have a higher actuation level 81. The second SMA wire 80b may have a lesser actuation level 82. The power applied to the SMA wires 80 may be increased first to reach the target actuation levels 81, 82. The target actuation levels 81, 82 may correspond to the operating powers to be supplied to the SMA wires 80.

As shown in Figure 10, optionally the controller is configured to control actuation of the SMA wires during a power-on sequence such that the difference in actuation level increases. The increase 241 in power for the second SMA wire 80b may be at a lower rate than the increase 121 for the first SMA wire 80a. By starting actuation of the SMA wires 80 at the same time, the time required to complete the power-on sequence may be reduced.

Figure 11 shows a schematic diagram of the power profiles applied to the SMA wires 80a, 80b according to an alternative power-on sequence. As shown in Figure 11, optionally, the first wire start time 83 at which the first SMA wire 80a starts being actuated is before the second wire start time 84 at which the second SMA wire 80b starts being actuated. As shown in Figure 11, optionally the difference in actuation levels of the SMA wires 80 is substantially maintained for most of the power-on sequence. The power profile shown in Figure 10 may be substantially the reverse of the power profile shown in Figure 5, for example. As shown in Figure 11, optionally the power profile applied to the first SMA wire 80a comprises an increase 121. The power profile applied to the second SMA wire 80b may comprise a corresponding increase 251. The corresponding increase 251 may have substantially the same rate of increase as the increase 121 for the first SMA wire 80a.

Figure 12 schematically shows the power profiles applied to SMA wires 80 according to an alternative power-on sequence. As shown in Figure 12, optionally the second wire start time 85 at which the second SMA wire 80b starts being actuated is before the first wire start time 83 at which the first SMA wire 80a starts being actuated. The power profiles for the two SMA wires 80, may compromises increases 121, 251 that are at substantially the same rate. As shown in Figure 12, optionally the first wire target time 86 at which the first wire 80a reaches its target actuation level 81 is after the second wire target time 87 at which the second wire 80b reaches its target actuation level 82. The power profiles shown in Figure 12 may be substantially the reverse of the power profiles shown in Figure 6 for

example.

Figure 13 schematically shows power profiles applied to SMA wires 80 according to an alternative power-on sequence. As shown in Figure 13, optionally the SMA wires 80a, 80b start being actuated at substantially the same start time 83. As shown in Figure 13, optionally the first wire target time 86 is after the second wire target time 87, similar to as shown in Figure 12. As shown in Figure 13, optionally the power profile applied to the second wire 80b substantially matches the power profile applied to the first wire 80a for at least part of the power-on sequence, and optionally for most of the power-on sequence. By matching the actuation levels for the power-on sequence between the two SMA wires 80 the loading of the second part 20 against the first part 4 may be more stable.

Camera Optionally the SMA actuator assembly 1 is used within a camera, for example a miniature camera. Such a camera may be comprised in an electronic device such as a mobile phone.

The first part and/or the second part may comprise components of a camera. For example, optionally the second part 20 is, or comprises a lens element comprising at least one lens. Optionally the first part 4 is, or comprises an image sensor. For example, an image sensor may be mounted on the first part 4. The lens elements may be arranged to focus an image on the image sensor.

Alternatively, the first part 4 may have a display mounted on it. The lens element may be arranged to focus light emitted by the display.

In an alternative arrangement, the second part 20 may have an image sensor mounted on it. The first part 4 may have a lens element comprising at least one lens. The lens element may be arranged to focus an image on the image sensor.

Paragraph relating to SMA wire 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.

Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, the power-off sequence and/or the power-on sequence may involve the controller controlling actuation of the SMA wires 80 in open loop control. The power profiles applied to the SMA wires 80 may be predetermined. Alternatively, the controller may be configured to control the actuation levels of the SMA wires 80 during a power-on sequence and/or during a power-off sequence in closed loop control.

The second part 20 may be in direct contact with the first part 4. Alternatively, the SMA actuator assembly 1 may comprise a bearing arrangement. The bearing arrangement may be, configured to guide movement of the second part 20 relative to the first part 4. The bearing arrangements may be loaded, for example by a loader 41 as shown in Figure 3. The bearing arrangement may be partially unloaded by actuation of the SMA wires 80.

It will be appreciated that the above-described example may be implemented as methods and/or as computer programs. For example, each example may be embodied as a method of controlling an SMA actuator assembly 1. The method may be a method performed by the controller. Each example may be embodied as a computer program. The computer program may comprise instructions for instructing the controller to perform a method of controlling an SMA actuator assembly 1. The computer program may be implemented by the controller.

Any computer program product disclosed herein may be embodied in a computer readable medium having computer readable program code thereon. 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.

Claims

Claims 1. A shape memory alloy, SMA, actuator assembly comprising: a first part; a second part which is movable relative to the first part across a first surface; at least one SMA element 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and a controller configured to control actuation of the at least one SMA element, wherein the controller is configured to: during a power-on sequence of the at least one SMA element, cause an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, cause a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.
2. An SMA actuator assembly of claim 1, wherein the controller is arranged to, during the power-on sequence and/or during the power-off sequence, control the power supplied to the at least one SMA element such that the second part is held relative to the first part by friction between the second part and the first surface.
3. An SMA actuator assembly of claim 1 or 2, comprising: two SMA elements arranged, on actuation, to drive movement of the second part relative to the first part in opposite directions.
4. An SMA actuator assembly of claim 3, wherein during the power-on sequence and/or during the power-off sequence, the controller controls the power supplied to the SMA elements such that a net force applied to the second part by the SMA elements remains less than the static frictional force acting on the second part.
5. An SMA actuator assembly of claim 3 or 4, wherein immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the difference decreases until the power supplied to the SMA elements is zero.
6. An SMA actuator assembly of any of claims 3 to 5, wherein the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the SMA elements stop being powered at substantially the same time.
7. An SMA actuator assembly of claim 3 or 4, wherein immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the SMA element that had the greater power supplied stops being powered after the SMA element that had the lesser power supplied stops being powered.
8. An SMA actuator assembly of any of claims 3 to 7, wherein immediately before the power-off sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-off sequence such that the difference is increased temporarily.
9. An SMA actuator assembly of claim 8, wherein the difference is increased such that a net driving force on the first part becomes substantially zero.
10. An SMA actuator assembly of any of claims 3 to 9, wherein the controller is configured to control the power supplied to the SMA elements during the power-on sequence such that the SMA elements start being powered at substantially the same time.
11. An SMA actuator assembly of any of claims 3 to 10, wherein immediately after the power-on sequence, there is a difference in the power supplied to each of the SMA elements, and the controller is configured to control the power supplied to the SMA elements during the power-on sequence such that the difference increases.
12. An SMA actuator assembly according to any preceding claim, wherein the controller is configured to control the power supplied to the at least one SMA element such that during the power-on sequence and/or during the power-off sequence the power supplied to at least one SMA element changes linearly over time for at least part of the duration of the sequence.
13. An SMA actuator assembly according to any preceding claim, wherein the controller is configured to control the power supplied to the at least one SMA element such that during a power-on sequence and/or during a power-off sequence the power supplied to at least one SMA element changes according to a polynomial shape over time for at least part of the duration of the sequence.
14. An SMA actuator assembly according to any preceding claim, wherein the controller is configured to control the power supplied to the at least one SMA element such that during the power-on sequence and/or during the power-off sequence the power supplied to at least one SMA element comprises at least one step change.
15. A shape memory alloy, SMA, actuator assembly comprising: a first part; a second part which is movable relative to the first part in a movement direction across a first surface; at least one SMA element 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; wherein, for at least one position of the second part relative to the first part in a range of movement of the second part, the actuator assembly is arranged such that the second part experiences a residual net force along the movement direction when the at least one SMA element is unpowered; and a controller configured to control actuation of the at least one SMA element, wherein the controller is configured to: obtain an indication of a target position of the second part relative to the first part; output one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and power-off the at least one SMA element such that second part moves towards the target position under the action of the residual net force.
16. An SMA actuator assembly of claim 15, comprising: two SMA elements arranged, on actuation, to drive movement of the second part relative to the first part in opposite directions.
17. An SMA actuator assembly according to any preceding claim, wherein the at least one SMA element is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the surface so as to drive movement of the second part relative to the first part across the first surface.
18. An SMA actuator assembly according to any preceding claim, wherein the second part is a lens element comprising at least one lens.
19. An SMA actuator assembly according to claim 18, wherein the first part comprises an image sensor, the lens element being arranged to focus an image on the image sensor.
20. An SMA actuator assembly according to claim 18, wherein the first part comprises a display, the lens element being arranged to focus light emitted by the display.
21. An SMA actuator assembly according to any preceding claim, wherein the second part comprises an image sensor.
22. An SMA actuator assembly according to claim 21, wherein the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor.
23. An SMA actuator assembly according to any preceding claim, wherein the at least one SMA element is arranged such that the normal force remains substantially constant on contraction of the at least one SMA element.
24. A method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and during a power-on sequence of the at least one SMA element, causing an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, causing a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.
25. A computer program product comprising instructions for instructing a controller to perform a method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; and during a power-on sequence of the at least one SMA element, causing an increase of a power supplied to the at least one SMA element gradually from zero to an operating power; and/or during a power-off sequence of the at least one SMA element, causing a decrease of a power supplied to the at least one SMA element gradually from an operating power to zero.
26. A method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; obtaining an indication of a target position of the second part relative to the first part; outputting one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and powering-off the at least one SMA element such that second part moves towards the target position under the action of a residual net force along the movement direction experienced by the second part for at least one position of the second part relative to the first part in a range of movement of the second part when the at least one SMA element is unpowered.
27. A computer program product comprising instructions for instructing a controller to perform a method of controlling a shape memory alloy, SMA, actuator assembly comprising a first part, a second part and at least one SMA element, the method comprising: controlling actuation of the at least one SMA element to drive movement of the second part relative to the first part in a movement direction across a first surface, 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 static frictional force that constrains the movement of the second part relative to the first part when the at least one SMA element is not actuated; obtaining an indication of a target position of the second part relative to the first part; outputting one or more drive signals for application to the at least one SMA element to drive movement of the second part to an intermediate position which is displaced from the target position along the movement direction; and powering-off the at least one SMA element such that second part moves towards the target position under the action of a residual net force along the movement direction experienced by the second part for at least one position of the second part relative to the first part in a range of movement of the second part when the at least one SMA element is unpowered.