WO2025040871A1

WO2025040871A1 - Sma actuator assembly

Info

Publication number: WO2025040871A1
Application number: PCT/GB2024/052073
Authority: WO
Inventors: Andrew Benjamin Simpson Brown
Original assignee: Cambridge Mechatronics Limited
Priority date: 2023-08-22
Filing date: 2024-08-05
Publication date: 2025-02-27

Abstract

An SMA actuator assembly (1) comprising a first part (10); a second part (20); an SMA element (30) arranged, on actuation, to move the second part relative to the first part; wherein the SMA actuator assembly is configured such that the second part is retained in position relative to the first part when the SMA element is not actuated; and a control circuit (100) configured to: obtain a target position of the second part relative to the first part, and in response to receiving the target position, apply a drive pulse to the SMA element capable of actuating the SMA element so as to move the second part from an initial position to the target position relative to the first part.

Description

SMA ACTUATOR ASSEMBLY

Field

The present application relates to an SMA actuator assembly, in particular comprising a controller configured to apply a drive pulse for actuating the SMA element of the SMA actuator assembly.

Background

Shape memory alloy (SMA) actuators are used in camera assemblies for effecting a range of motions of a lens carriage or an image sensor. For example, WO 2020/115260 Al describes an actuator wire arrangement in which SMA wires are configured to, on contraction, move a movable part in directions perpendicular to an optical axis to provide optical image stabilisation (OIS).

WO 2020/115260 Al further discloses a drive scheme for controlling the SMA wires of the actuator wire arrangement. In particular, a succession of pulse width modulation (PWM) voltage pulses is continuously applied to the SMA wires so as to maintain tension in the SMA wires and allow accurate positioning of the movable part relative to a support structure.

Summary

According to an aspect of the present invention, there is provided an SMA actuator assembly comprising a first part; a second part; an SMA element arranged, on actuation, to move the second part relative to the first part; wherein the SMA actuator assembly is configured such that the second part is retained in position relative to the first part when the SMA element is not actuated; and a control circuit configured to: obtain a target position of the second part relative to the first part, and in response to receiving the target position, apply a drive pulse to the SMA element capable of actuating the SMA element so as to move the second part from an initial position to the target position relative to the first part.

In some embodiments, the duration of the drive pulse is less than the time constant of heat dissipation from the SMA element. In particular, the ratio of the time constant of heat dissipation from the SMA element to the duration of the drive pulse is greater than 2, preferably greater than 5, further preferably greater than 10.

The drive pulse may be non-periodic. So, the drive pulse may not be a pulse-width modulation (PWM) pulse that is applied to the SMA element at a PWM frequency (which PWM frequency is usually in the range from 32kHz to 600kHz). The duration of the drive pulse may be more than 0.1 ms, preferably more than 0.5ms, further preferably more than 1ms. The duration of the drive pulse may be less than 5s, preferably less than 50ms, further preferably less than 5ms. The period between successive drive pulses may be greater than the duration of each of the successive drive pulses, preferably by a factor of at least 2, further preferably by a factor of at least 5 or at least 10.

After application of the drive pulse, the control circuit may be configured to cease powering the SMA element, thereby stopping actuation of the SMA element and allowing the second part to be retained in position relative to the first part.

In some embodiments, the SMA actuator assembly comprises at least two SMA elements, wherein one of the at least two SMA elements is arranged, on actuation, to move the second part relative to the first part at least partially in a first direction and another of the at least two SMA elements is arranged, on actuation, to move the second part relative to the first part at least partially in a second direction that is opposite to the first direction. The control circuit may be configured selectively to apply drive pulses to the at least two SMA elements such that the at least two SMA elements are not actuated concurrently.

In some embodiments, the SMA element is surrounded by a heat dissipating material, in particular immersed in a liquid or gel. Alternatively or additionally, the SMA element may be in direct contact with a heat dissipating material at at least one, at least two or at least three locations along the length of the SMA element between the end portions of the SMA element.

In some embodiments, after or during applying the drive pulse, the control circuit is further configured to obtain feedback regarding the position of the first part relative to the second part; and after obtaining the feedback, the control circuit is configured to: i) if the feedback indicates that the target position has been reached, cease applying further drive pulses to the SMA element such that the first part is retained in position relative to the second part; or ii) if the feedback indicates that the target position has not been reached, apply a further drive pulse so as to move the first part relative to the second part to the target position. The control circuit may obtain the feedback by determining a measure of the electrical characteristic of the SMA element. Alternatively, the SMA actuator assembly may comprise a position sensor for sensing the position of the second part relative to the first part, and the control circuit may be configured to obtain the feedback by receiving a sensed position of the second part relative to the first part.

The SMA elements may be slack, i.e. not in tension, when the SMA elements are not powered. The controller may obtain the measure of the electrical characteristic of the SMA elements by applying electrical power to the SMA elements that is sufficient to tension the SMA elements but insufficient to move the second part relative to the first part. The controller may then determine the measure of the electrical characteristic of the SMA elements while the SMA elements are tensioned, thereby obtaining the feedback regarding the position of the first part relative to the second part.

Prior to applying the drive pulse, the control circuit may be configured to determine characteristics of the drive pulse using a thermal model of the SMA element.

Some embodiments comprise one or more superelastic SMA elements configured to retain the second part in position relative to the first part when the SMA element is not actuated.

In some preferable embodiments, friction between surfaces coupled to the first and second parts retains the second part in position relative to the first part when the SMA element is not actuated.

Some other embodiment comprise a brake assembly configured to hold the second part in position relative to the first part when the SMA element is not actuated. The brake assembly may comprise a friction brake configured, when engaged, to frictionally hold the second part in position relative to the first part. The brake assembly may comprise an additional actuator component, such as an additional SMA element, configured to disengage the brake assembly on actuation.

According to the present invention there is also provided a deformable lens comprising the SMA actuator assembly, wherein the first part is a center portion of the deformable lens and the second part is an edge portion of the deformable lens, and wherein the SMA element is configured, on actuation, to move the edge portion relative to the center portion so as to deform the deformable lens, thereby adjusting the amount of focus of the deformable lens.

Further aspects of the present invention are set out in the detailed description.

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:

Figures 1A and IB are schematic views of SMA actuator assemblies in accordance with the present invention;

Figure 2 is a schematic view of a control circuit for implementing the present invention; Figures 3A to 3C are schematic views of drive pulses that may be applied to the SMA elements of the SMA actuator assemblies according to the present invention; and

Figure 4 is a flow chart illustrating the steps carried out to implement the present invention.

Detailed description

Actuator assembly

Figures 1A-B schematically depict embodiments of an actuator assembly 1, also referred to as SMA actuator assembly 1, according to the present invention.

The actuator assembly 1 comprises a support structure 10 and a movable part 20. The movable part 20 is movable relative to the support structure 10. In general, the support structure 10 and the movable part 20 may be referred to as a first part and a second part respectively, and the terms support structure 10 and movable part 20 are used herein for purely illustrative purposes. In this regard, the support structure 10 is used herein as a reference structure. Movement of any components of the actuator assembly 1 is described relative to the support structure 10, unless otherwise indicated. However, in general the support structure 10 may itself be movable, for example within a larger device into which the actuator assembly 1 is incorporated. In some embodiments, the support structure 10 may be made up of components that are movable relative to each other. The support structure 10 and the movable part 20 may be integrally formed as a single deformable component (such as a liquid lens or deformable lens), in which one part of the deformable component is movable relative to another part of the deformable component.

The movable part 20 is movable relative to the support structure 10 within a range of movement. The range of movement may define movement in any number of degrees of freedom (DOF). Preferably, the range of movement defines movement in up to three DOFs, for example one, two or three DOFs. The movable part 20 may be movable relative to the support structure 10 in a movement plane within the range of movement, or along a movement axis within the range of movement, for example.

The actuator assembly 1 comprises one or more SMA elements 30, such as SMA wires 30. Preferably, the actuator assembly 1 comprises at least two SMA elements 30. The SMA elements 30 are arranged, on actuation, to move the movable part 20 relative to the support structure 10. The at least two SMA elements 30 may oppose one another, so the at least two SMA elements 30 may, on actuation, apply forces to the movable part 20 relative to the support structure 10 that have components in opposite directions. The at least two SMA elements 30 may move or tilt the movable part 20 in opposite directions or senses relative to the support structure. The SMA elements 30 move the movable part 20 to any position within the range of movement. For example, the SMA elements 30 may move the movable part 20 in one DOF, in two DOFs or in three DOFs.

Each of the SMA elements 30 may be connected at one end to the support structure 10 by a corresponding coupling element (not shown) and at the other end to the movable part 20 by a corresponding coupling element (not shown). The coupling elements may be crimps, for example, although in general any intermediate mechanism may be provided between SMA elements 30 and corresponding part to transfer a force from the SMA element 30 to the part. The coupling elements may provide both mechanical and electrical connection to the SMA elements 30. The SMA elements 30 may each be electrically connected (via the coupling elements) to a control circuit 100. The control circuit in use applies drive signals to the SMA elements 30 which resistively heat the SMA elements 30, causing them to actuate. The plural SMA elements 30 may be driven independently or otherwise. The control circuit 100 may also measure the resistance of the SMA elements 30, and use the measured resistance to calculate/determine the position of the movable part 20. In general, however, the SMA elements 30 may be heated so as to contract by any other suitable means, such as via an external heat source, radiative heating or inductive heating.

In this regard, the range of movement comprises any movement of the movable part 20 relative to the support structure 10 that can be achieved by selective contraction of the arrangement of SMA elements 30. The range of movement may be defined as the movement achievable by selective contraction of the SMA elements 30. Optionally, the range of movement may be limited by endstops between the support structure 10 and the movable part 20, in particular when contraction of the SMA elements 30 causes an endstop between the support structure 10 and the movable part to engage. The range of movement may also be affected, at least in part, by a bearing arrangement 50 defining the DOFs in which the movable part 20 may be moved. Figure IB, for example, depicts a bearing arrangement 50 comprising a rolling bearing (such as a ball bearing) for guiding movement of the movable part 20 relative to the support structure 10. In Figure 1A, the friction surfaces lOf, 20f effectively act as a bearing arrangement 50 in the form of a plain bearing or sliding bearing.

The range of movement may thus be defined as the collection of locations and orientations to which the movable part 20 may be moved relative to the support structure 10 by the SMA elements 30. The range of movement may be affected by one or more of i) the arrangement of SMA elements 30 as well as control for driving the SMA elements 30, ii) the provision of endstops between movable part 20 and support structure 10 that limit the range of movement, iii) the provision of bearing arrangements 50 that define the DOFs of movement of the movable part 20 relative to the support structure 10. In some embodiments, the range of movement may define movement of the movable part 20 relative to the support structure 10 in a movement plane (in 2 or 3 DOFs) or along a movement path (in 1 DOF).

Zero hold power

The actuator assembly 1 is configured such that the movable part 20 is retained in position relative to the support structure 10 when the SMA element 30 is not actuated. So, when the control circuit does not provide a drive signal to the SMA element 30 so as not to actuate the SMA element 30, the movable part 20 remains stationary relative to the support structure 10. The power consumption of the actuator assembly 1 may thus be reduced compared to an actuator assembly 1 in which the SMA elements 30 need to be continuously powered so as to retain the movable part 20 in position. The movable part 20 may be retained in position at any point within the continuous range of movement within which the movable part 20 is moved upon actuation of the SMA elements 30. The movable part 20 may be retained in position as long as the acceleration (or deceleration) of the actuator assembly 1 is below a hold threshold. So, the movable part 20 may only move relative to the support structure 10 when the actuator assembly 1 undergoes a relatively high acceleration (or deceleration), such as during drops and other impact events. The hold threshold may be greater than g (9.81 m/s²), greater than 2g, greater than 5g, or greater than 10g, for example.

Friction for zero hold power

Figures 1A and IB depict embodiments of the actuator assembly 1 in which friction is used to retain the movable part 20 in position. Such actuator assemblies 1 are disclosed in WO 2023/084251 Al and WO 2023/094813 Al, which are herein incorporated by reference.

In particular, the support structure 10 comprises a first friction surface lOf. The movable part 20 comprises a second friction surface 20f. The second friction surface 20f of the movable part 20 engages the first friction surface lOf of the support structure 10. The first and second friction surfaces lOf, 20f may engage each other throughout the range of movement. So, in normal use (i.e. under actuation of the SMA elements 30 for moving the movable part 20), the first and second friction surfaces lOf, 20f remain in engagement with one another.

The actuator assembly 1 further comprises a biasing arrangement 40. The biasing arrangement 40 is arranged to bias the first and second friction surfaces lOf, 20f against each other. The biasing arrangement 40 applies a biasing force between the support structure 10 and the movable part 20. The biasing force comprises a component that is perpendicular to the first and second friction surfaces, and so the biasing arrangement 40 applies a normal force N between support structure 10 and movable part 20. The normal force N is perpendicular to the range of movement and perpendicular to the friction surfaces lOf, 20f. The biasing arrangement 40 may comprise a spring or other resilient element connected between the support structure 10 and the movable part 20 (as shown in Figure 1A), or a spring or other resilient element connected between two portions 20a, 20b of the movable part 20 (as shown in Figure IB). Preferably, the biasing arrangement 40 applies the biasing force in the direction perpendicular to the range of movement and perpendicular to the friction surfaces lOf, 20f. The biasing force of the biasing arrangement may be equal to the normal force N. So, the biasing force may not have a component parallel to the range of movement, and thus not affect movement of the movable part 20 relative to the support structure 10.

This normal force N generates or gives rise to a static frictional force F between the first and second friction surfaces lOf, 20f. The static frictional force F constrains movement of the movable part 20 relative to the support structure 20, in particular when the SMA elements 30 are not contracted. Such movement is constrained at any position and/or orientation within the range of movement of the movable part 20 relative to the support structure 10. The magnitude of the static frictional force is great enough to constrain movement of the movable part 20. The ratio of the static frictional force to weight of the movable part may be greater than 1. So, the magnitude of the static frictional force is greater than the weight of the movable part. This ensures that movement of the movable part is constrained by the frictional force even when the actuator assembly 1 is turned on its side, for example. The weight of the movable part is considered to be equal to the mass of the movable part times earth's average gravitational acceleration (9.81 m/s²). Preferably, the ratio of the static frictional force to the weight of the movable part is greater than 3, further preferably greater than 5. This ensures that movement of the movable part 20 is constrained even when the actuator assembly 1 accelerates.

The SMA elements 30 may be used to move the movable part 20 to any position within the range of movement of the movable part 20 relative to the support structure 10. Upon energising (i.e. when drive signals are applied to the SMA elements 30 by the control circuit 100), the SMA elements 30 actuate and apply an actuating force for moving the movable part 20 in respective directions. The actuating force is sufficient to overcome the frictional forces at the friction surfaces lOf, 20f, in order to drive relative movement between the movable part 20 and the support structure 10. Upon ceasing power supply to the SMA elements 30, and so when stopping contraction of the SMA elements 30, the movable part 20 remains at its position within the range of movement due to the frictional forces between the first and second friction surfaces lOf, 20f. In this state, the movable part 20 is retained in position with zero power consumption by the actuator assembly 1, so the actuator assembly 1 may be referred to as a zero power hold actuator assembly, as may the other actuator assemblies disclosed herein. The movable part 20 is thus held in place without requiring power supply to the SMA elements 30, reducing the power consumption of the actuator assembly compared to a situation in which the SMA elements 30 need to be powered to hold the movable part in place.

In the actuator assembly 1 of Figure 1A, the SMA elements 30 are arranged such that the normal force N between first and second friction surfaces lOf, 20f remains substantially constant on actuation of the one or more SMA elements 30. Stresses in the SMA elements 30 do not affect the normal force N. Put another way, the composite force acting on the movable part 20 due to stresses in the SMA elements 30 does not have a component that is parallel to the normal force N, or such a component is negligible. The stresses in the SMA elements 30 do not (or only to a negligible extent) contribute to the normal force N. The normal force N remains substantially constant in that it varies by less than 5%, preferably less than 1%, due to forces arising from stresses in the SMA elements 30.

Such an arrangement in which the normal fore N is substantially unaffected by the SMA elements 30 reduces variation in the frictional forces F between movable part 20 and support structure 10. This makes control of the movement of the movable part 20 by the SMA elements 30 simpler. The arrangement of SMA elements 30 may also be less complex compared to a situation in which stresses and/or strains in the SMA elements 30 affect the normal force N between the friction surfaces lOf, 20f.

In the actuator assembly 1 of Figure IB, the SMA elements 30 are arranged, on actuation, to reduce the normal force N between first and second friction surfaces lOf, 20f. Put another way, the composite force acting on the movable part 20 due to stresses in the SMA elements 30 has a component that is parallel to and opposite in direction to the normal force N. The stresses in the SMA elements 30 affect (in particular reduce) the normal force N. In some embodiments, equal stresses (or tensions or strains) in the SMA elements 30 may reduce the normal force N without moving the movable part 20. Unequal strains in the SMA elements 30 may result in movement of the movable part 20.

Such an arrangement in which the normal force N is reduced by the SMA elements 30 allows selective reduction in the frictional forces by appropriate contraction of the SMA elements 30. This reduction of the frictional forces assists with the overcoming of the frictional forces to allow movement of the movable part 20 within the range of movement. So, the stress in the SMA elements 30 required to move the movable part 20 may be reduced compared to a situation in which the frictional forces are not reduced. Furthermore, the frictional forces in the absence of actuation of the SMA elements 30 may be designed to be higher compared to a situation in which the frictional forces cannot be reduced, thus reducing the risk of inadvertent movement of the movable part 20 in the absence of SMA element actuation. Hysteresis for zero hold power

According to further embodiments of the invention, hysteresis in superelastic SMA elements 30 is used to retain the movable part 20 in position relative to the support structure 10. The hysteretic properties of the superelastic SMA elements 30 are configured to hold the movable part 20 in position when the superelastic SMA elements 30 are not actuated, i.e. when the superelastic SMA elements are unpowered or unenergized. Such actuator assemblies 1 are disclosed in WO 2023/118880 Al, which is herein incorporated by reference.

In particular, the material composition and pre-treatment of the superelastic SMA element is chosen such that the superelastic SMA elements 30, when unenergized and in the normal operating environment, exhibit pseudo-elastic properties in which the superelastic SMA elements 30 are in a state between a full austinite phase and a full martensite phase. For example, the superelastic SMA elements 30 may remain in the pseudo-elastic range between the full Martensite and the full Austenite phase even when unpowered. In contrast, SMA elements 30 used in conventional actuator assemblies may revert to the full Martensite phase when unpowered. The superelastic SMA elements thus have a phase transition (from the full Martensite to the full Austenite phase) within a temperature range that is below the average operating temperature at which the superelastic SMA elements normally operate. The average operating temperature of the superelastic SMA elements may be the ambient temperature of an environment within which the superelastic SMA operate (e.g. the body temperature of about 37°C of a human for operation within the human body, or an elevated temperature due to heating effects of nearby electronic components), or may be greater than the ambient temperature due to heating effects of operating the superelastic SMA elements themselves.

The superelastic SMA elements may have a phase transition temperature of below 70°C, preferably below 50°C. In general, the phase transition temperature is a temperature at which the SMA material, upon heating from a cooled state, undergoes phase transition from the full Martensite phase to the full Austenite phase. So, a phase transition temperature may be understood to be a temperature in which the SMA material operates in the pseudo-elastic range. The phase transition temperature may correspond to the temperature at which the SMA material, upon heating from a cooled state, reaches the full Austenite phase, i.e. the temperature at which the SMA material stops the phase transition from Martensite to Austenite phase (i.e. the upper temperature limit of the pseudo-elastic range). In some embodiments, the phase transition temperature may correspond to the temperature at which the SMA material, upon heating from a cooled state, starts transitioning from the full Martensite phase to the partial Austenite phase (i.e. the lower temperature limit of the pseudo-elastic range). In contrast, non-superelastic SMA elements used in conventional actuator assemblies are deliberately chosen to have higher phase transition temperatures above 70°C. Conventionally, such higher phase transition temperatures were chosen to reduce the impact of ambient temperature fluctuations on the actuation control of the actuator assembly 1.

According to embodiments of the present invention, the phase transition temperature at which the SMA material, upon heating from a cooled state, starts transitioning from the full Martensite phase to the partial Austenite phase (i.e. the lower temperature limit of the pseudo-elastic range) may be less than 70°C, preferably less than 50°C. In some preferred embodiments, this phase transition temperature is less than 35°C, optionally less than 25°C or even less than 15°C or 0°C. Phase transition temperatures below 35°C may allow making use of the benefits of the superelastic SMA elements in environments having elevated temperatures, such as use in the human body or near heat-generating electronic components. Phase transition temperatures below 25°C may extend the use of the benefits of the superelastic SMA elements to environments having lower elevated temperatures, such as in handheld devices near electric circuitry or components. Phase transition temperatures below 15°C may extend the use of the benefits of the superelastic SMA elements to most ambient temperatures. Phase transition temperatures below 0°C may extend the use of the benefits of the superelastic SMA elements to almost all ambient temperatures.

In general, the phase transition temperature at which the SMA material, upon heating from a cooled state, starts transitioning from the full Martensite phase to the partial Austenite phase (i.e. the lower temperature limit of the pseudo-elastic range) may be less than 50°C, less than 45°C, less than 40°C, less than 38°C, less than 35°C, less than 30°C, less than 25°C, less than 23°C, less than 20°C, less than 15°C, less than 10°C, less than 5°C or less than 0°C.

The phase transition temperature of SMA material may be tailored by a variety of known techniques. For example and without limitation, the chemical composition and pre-treatment (e.g. heat treatment) of SMA material affects the phase transition temperature. One commonly used SMA material is Nitinol, the phase transition temperatures of which may be tailored within a range from about -100°C to 120°C by suitable metallurgy. Addition of impurities (elements other than nickel and titanium) can further extend that range.

Any of the SMA elements 30 in the arrangements described herein may be provided as superelastic SMA elements 30. For example, the SMA elements in the arrangements described with reference to Figures 1A and IB may be provided as superelastic SMA elements (and the friction surfaces may be omitted). In general, the present invention extends to incorporating superelastic SMA elements either to replace SMA actuator elements (which herein are considered to be formed from SMA material having a phase transition temperature at which the SMA material, upon heating from a cooled state, starts transitioning from the full Martensite phase to the partial Austenite phase (i.e. the lower temperature limit of the pseudo-elastic range) of 70°C or more) or in addition to SMA actuator elements.

The superelastic SMA elements 30 may be arranged to be in tension at the operating temperature of the actuator assembly. The superelastic SMA elements may be stretched to a strain that places them halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region. Alternatively, the superelastic SMA elements 30 may be arranged to be in compression at the operating temperature of the actuator assembly. The superelastic SMA elements may be compressed to a strain that places them halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region. The superelastic SMA elements 30 may be arranged to be in tension or in compression at a temperature of less than 50°C, less than 45°C, less than 40°C, less than 38°C, less than 35°C, less than 30°C, less than 25°C, less than 23°C, less than 20°C, less than 15°C, less than 10°C, less than 5°C or less than 0°C.

There is thus provided an actuator assembly 1 comprising a support structure 10; a movable part 20 that is movable relative to the support structure 10; an arrangement of superelastic SMA elements 30 arranged, on actuation, to move the movable part 20 relative to the support structure 10, wherein the hysteretic properties of the superelastic SMA elements 30 are configured to hold the movable part in position when the superelastic SMA elements 30 are not actuated. The movable part may be movable relative to the support structure 10 in at least two degrees of freedom, and the superelastic SMA elements may, upon actuation, move the movable part 20 in the at least two degrees of freedom relative to the support structure 10.

Brake assembly for zero hold power

In some other embodiments, a dedicated brake assembly (not shown) may be provided to hold the second part in position relative to the first part. The brake assembly may comprise a friction brake configured, when engaged, to apply a frictional force to the movable part 20, thereby holding the movable part 20 in position relative to the support structure 10. The brake assembly may comprise an additional actuator component, such as an additional SMA element, configured to disengage the brake assembly on actuation.

Control circuit Figure 2 schematically depicts a control circuit 100 for controlling the actuator assembly 1. The control circuit 100 applies drive signals to the SMA elements 30 of the actuator assembly 1. The control circuit 100 may be implemented by a single component, such as an integrated circuit (IC) chip, or may be implemented by multiple interconnected components (such as multiple IC chips and/or separate electrical circuits). Part of the control circuit 100 may be provided by a processor of a device in which the actuator assembly 1 is integrated.

The control circuit 100 comprises a controller 110 and one or more SMA drivers 120. The controller 110 may calculate or otherwise determine a measure of wire power P that is to be provided to the SMA elements 30 in response to a given input. The input may, for example, be a request for positioning the movable part 20 at a target position relative to the support structure 10. The controller 110 may determine wire powers P so as to actuate the SMA elements 30 to drive the movable part 20 to the target position. This determination may be made, for example, based on look-up tables that link a given target position to a set of wire powers P. Alternatively, the controller 110 may use a model of the SMA elements 30 to calculate the required wire powers P based on a given target position. Alternatively or additionally, the controller 110 may also take into account feedback of the actual position of the movable part 20 relative to the support structure 10, i.e. the controller 110 may implement closed loop control (such as a PID controller).

The controller 110 communicates the determined wire powers P to the SMA drivers 120. A dedicated SMA driver 120 may be provided for each SMA element 30, or an SMA driver 120 may provide drive signals to multiple SMA elements 30. The SMA drivers 120 generate and apply drive signals to the SMA elements 30 based on the wire powers 120. The SMA drivers 120 may comprise a drive circuit 122. The drive circuit 122 may comprise, for example, a constant current source or a constant voltage source that is selectively turned on or off so as to apply the drive signal to the respective SMA element 30. The drive circuit 122 may conventionally apply pulse width modulated (PWM) drive signals to the SMA elements 30. Such PWM drive signals are periodic and applied at a PWM frequency, usually in the range from 32 kHz to 300 kHz. The drive signals may lead to current flowing through the SMA elements 30, resulting in the SMA elements 30 heating up due to resistive heating. SMA has the property that on heating the SMA undergoes a phase change, which may lead to actuation (e.g. contraction) of the SMA element 30. So, the SMA elements 30 actuate when suitable drive signals are applied.

The SMA drivers 120 may further comprise a measurement circuit 124. The measurement circuit 124 comprises circuitry for measuring an electrical characteristic of the SMA element 30. The measurement circuit 124 may, for example, measure the resistance of the SMA element 30. The resistance of the SMA element 30 is a measure of the length (and so a measure of the actuation amount) of the SMA element 30. The SMA drivers 120 may feed back the measured electrical characteristic R to the controller 110. The controller 110 may use the electrical characteristic to determine whether the target position has been reached and/or use the measured electrical characteristic in closed loop control for determining the wire powers P. Instead of a measured electrical characteristic, other feedback signals may be used by the controller 110. For example, a dedicated position sensor, such as a Hall sensor, may be used to measure the position of the movable part 20 directly.

Drive pulse

Conventionally, the SMA elements 30 may be driven by a PWM drive signal that powers the SMA elements 30 for extended periods of time. All SMA elements 30 may be powered concurrently and held in tension so as to accurately position the movable part 20 relative to the support structure 10. Measures of the actuation amount of the SMA elements 30 may be taken repeatedly while the SMA elements 30 are powered so as to provide feedback as to the position of the movable element and enable closed loop control of the SMA elements 30 to achieve a target position of the movable part 20. Such a conventional drive scheme is disclosed, for example, in WO 2020/115260 Al.

One drawback of the relatively long time period over which drive signals are applied to SMA elements 30 according to such a conventional drive scheme is that heat from the SMA element 30 may be lost to the environment. This drawback is particularly relevant in situations in which the SMA element 30 is in good thermal contact with a surrounding material, for example when the SMA element 30 bends around a corner or when the SMA element 30 is immersed in a liquid or gel, or surrounded by other material. Such heat loss may reduce the energy efficiency of the actuator assembly. It may further be undesirable in some situations to heat the environment of the SMA element 30, for example in surgical applications. An environment with elevated temperature will also increase the time required for the SMA element 30 to cool down, increasing response times of the SMA element 30.

The inventors have realized that actuator assemblies 1 that are capable of retaining the movable part 20 in position relative to the support structure 10 when the SMA elements 30 are unpowered, such as those described in relation to Figures 1A and IB, provide the opportunity for a new drive scheme that overcomes some of the drawbacks of conventional drive schemes.

According to the present invention, the control circuit 100 is configured to apply a drive pulse to the SMA element 30. Applying the drive pulse to the SMA element 30 actuates the SMA element 30 and moves the movable part 20 relative to the support structure 10. In particular, the drive pulse is capable of actuating the SMA element 30 so as to move the movable part 20 from an initial position to the target position relative to the support structure 10. After application of the drive pulse, the control circuit 100 ceases powering the SMA element 30. Actuation of the SMA element 30 is thereby stopped. The movable part 30 is retained in position relative to the support structure 10.

The drive pulse has a defined start and end time, and so in contrast to the conventional drive scheme is not a continuously applied drive signal. The drive pulse is configured to actuate the SMA element 30 so as to move the movable part 20 from an initial position to the target position, and so is not equivalent to one PWM pulse of the conventional drive scheme. The drive pulse is non-periodic (i.e. non-cyclic), so the drive pulse is not applied at a fixed or variable frequency. The drive pulse may be considered to be a single drive pulse.

Figures 3A to 3C show examples of the drive pulse (left side) along with examples of the change in position of the movable part (right side). In Figure 3A, the drive pulse is a square pulse starting at to and ending at tl. Before to, the movable part 20 is retained in an initial position Pi. Upon application of the drive pulse, the movable part 20 moves from the initial position Pi to the target position Pt. After tl, the movable part 20 is retained at the target position Pt. It is noted that Figures 3A-3C purely illustrative, but in practice the movable part 20 may start moving shortly after the start of the drive pulse.

The square pulse of Figure 3A is just one example of a possible pulse shape. In general, the drive pulse may have any suitable shape for applying electrical power to the SMA element 30 so as to actuate the SMA element 30. Without limitation, the drive pulse may be a square pulse, a triangular pulse, a trapezoidal pulse, a sine pulse or a rounded pulse. The drive pulse may be a continuous drive pulse, i.e. power may continuously be applied to the SMA element 30 for the duration of the drive pulse. This allows the drive pulse to be relatively short to apply a given amount of electrical energy to the SMA element 30.

The drive pulse may also comprise a plurality of shorter pulses or spikes, as depicted in Figure 3B. The drive pulse may comprise a plurality of PWM pulses, for example, such that the power of the drive pulse may be varied by the duty of the PWM pulses. The drive pulse may still be considered to be a single drive pulse in this embodiment. As in Figure 3A, the drive pulse starts at to and ends at tl. The SMA element 30 may be continuously heated for the duration of the drive pulse.

The duration of the drive pulse is preferably less than the time constant of heat dissipation from the SMA element. The time constant of heat dissipation from the SMA element is the time it takes for a temperature difference between the SMA element and the environment of the SMA element to reduce to 1/e ("'36.8%) of an initial temperature difference, when subject to an initial step function change in difference in temperature. The ratio of the time constant of heat dissipation from the SMA element 30 to the duration of the drive pulse may be greater than 2, preferably greater than 5, further preferably greater than 10.

The duration of the drive pulse may be greater than 0.1ms, preferably greater than 1ms. The duration of the drive pulse may be sufficient to allow time for the SMA element 30 to heat up and actuate. The drive pulse may not be a single PWM drive pulse of a PWM drive signal. The duration of the drive pulse may be less than 10s, preferably less than 5s or less than 2s. The duration of the drive pulse may be less than 50ms, preferably less than 10ms, further preferably less than 5ms. The drive pulse may be less than the time during which the actuator assembly operates or is active. The drive pulse may be relatively short so as to avoid or reduce heating of the environment of the SMA element 30.

Figure 3C schematically depicts two drive pulses that are applied to two different SMA elements 30. The two SMA elements 30 may be opposing SMA elements 30, e.g. correspond to the two SMA elements 30 depicted in the actuator assemblies 1 of Figures 1A and IB. The control circuit 100 may selectively apply the drive pulses to the at least two SMA elements such that the at least two SMA elements are not actuated concurrently. So, the drive pulse applied to the first SMA element 30 does not overlap with the drive pulse applied to the second SMA element 30 in Figure 3C.

Figure 3C further schematically illustrates a situation in which the drive pulse applied to an SMA element 30 does not achieve the objective of positioning the movable part 20 at the target position Pt. In particular, the control circuit 100 may receive a target position for the movable part and determine characteristics of the drive pulse so as to move the movable part 20 to the target position Pt. As shown in Figure 3C, following application of the drive pulse, the movable part 20 may (undesirably) overshoot the target position Pt. After application of the drive pulse, the position of the movable part 20 may be determined. Following a determination that the movable part 20 has not reached the target position Pt, a second drive pulse may be applied to the second SMA wire to correct the position of the movable part 20 to the target position.

The SMA elements 30 may be slack, i.e. not in tension, when the SMA elements 30 are not powered. The control circuit 100 may obtain a measure of the electrical characteristic of the SMA elements 30 by applying electrical power to the SMA elements 30 that is sufficient to tension the SMA elements 30 but insufficient to move the movable part 20 relative to the support structure 10. The control circuit 100 may then determine the measure of the electrical characteristic of the SMA elements 30 while the SMA elements 30 are tensioned. The control circuit 100 may determine the position of the movable part 20 relative to the support structure 10 based on the measured electrical characteristics of the tensioned SMA elements, for example based on the difference of the measured electrical characteristics.

Figure 4 is a flow chart showing steps carried out by the control circuit 100 so as to apply the drive pulse to the SMA element 30.

In step S100, the control circuit 100 may obtain a target position of the movable part 20 relative to the support structure 10. The target position may be in the form of an absolute position of the movable part 20, a relative position compared to an initial position, or an actuation amount of one or more SMA elements. In general, the target position may be any measure indicative of a particular position at which the movable part 20 should be positioned relative to the support structure 10.

In step S200, the control circuit 100 (e.g. the controller 110) determines drive pulse characteristics for positioning the movable part 20 at the target position. The drive pulse characteristics may, for example, be the duration of the drive pulse, the amplitude of the drive pulse, the shape of the drive pulse, the total energy of the drive pulse, or the duty of PWM pulses that together form the drive pulse. The drive pulse characteristics may be any measure that affects the total energy provided to the SMA element 30.

Determining the drive pulse characteristics may be based on the target position. A relatively large movement to the target position may require a relatively greater actuation amount, and so a relatively greater amount of total energy of the pulse drive. Conversely, a relatively small movement to the target position may require a relatively smaller actuation amount, and so a relatively smaller amount of total energy of the pulse drive.

The determination of the drive pulse characteristics may be done using a look-up table, for example. A set of target positions may be associated with a set of pulse characteristics. The look-up table may be established by experiments or during an initial calibration phase.

Preferably, the determination of the drive pulse characteristics may be made using a thermal model of the SMA element 30 and/or the environment of the SMA element 30. The control circuit 100 may calculate or otherwise model the characteristics of the drive pulse required to move the movable part to the target position. The model may be established based on initial experiments or during a calibration phase, for example, and take into account other variables that may affect SMA element performance, such as environmental data (e.g. temperature, humidity), actuation history, age of the SMA element, etc. As an example of a very simple model, the model could be a curve fitted to experimental data matching SMA element actuation amount to pulse characteristics. In step S300, the controller 100 (e.g. the SMA driver 120) applies the drive pulse to the SMA element 30. The drive pulse provides electrical energy to the SMA element 30, thereby resistively heating the SMA element 30 so as to actuate the SMA element 30. As a result, the movable part 20 may be driven towards the target position.

Optionally, in step S400, the controller 100 may determine feedback regarding the position of the movable part 20 relative to the support structure 10. The step S400 of determining feedback regarding the position of the movable part 20 relative to the support structure 10 may alternatively or additionally be taken prior to step S200 of determining the drive pulse characteristics. Determination of the drive pulse characteristics may take into account the feedback.

Determining the feedback may comprise measuring an electrical characteristic, such as the resistance, of the SMA element 30. The resistance of the SMA element may have a predetermined relation with (e.g. be inversely proportional to) the length of the SMA element, and so may provide a measure of the actuation amount of each SMA element, and so a measure of the position of the movable part. The electrical characteristic may be measured during the drive pulse, for example during time periods within the drive pulse during which the current is reduced, shown for example in Figure 3B. Alternatively, the electrical characteristic could be determined after the drive pulse has been applied, preferable directly after application of the drive pulse. The electrical characteristic may be determined as the SMA element 30 cools. Furthermore, in embodiments comprising opposing SMA elements 30 (i.e. SMA elements 30 pulling in opposite directions), the electrical characteristic of an SMA element 30 opposing the SMA element 30 to which the drive pulse is applied may be measured so as to determine a measure of the position of the movable part 20. The SMA elements 30 may be tensioned (without causing the movable part 20 to move) prior to measuring the electrical characteristic of the SMA elements 30.

Alternatively, a position sensor may be provided to sense the position of the movable part 20, and provide direct feedback to the control circuit 100 as to whether the target position has been reached. The position sensor may be a Hall sensor, for example, although in general any sensor capable of measuring the position of the movable part 20 relative to the support structure 10 may be used. In some embodiments in which movement of the movable part 20 relative to the support structure 10 is done to achieve a further effect, such as in a focus actuator, the further effect (e.g. the amount of focus) may be measured and used as feedback indicative of the position of the movable part 20 relative to the support structure 10. The controller 100 may make a determination D401 whether the target position has been achieved. The target position may be considered to be achieved if the determined feedback indicates that the movable part is exactly at or within a predetermined threshold distance of the target position. If the target position is reached, the control circuit 100 ceases powering the SMA element 30 (i.e. does not apply a further drive pulse) such that the movable part 20 is held at the target position relative to the support structure 10. If the target position is not reached, the control circuit 100 may determine and apply a further drive pulse to the same or to another SMA element 30 so as to drive movement of the movable part 20 to the target position. Multiple drive pulses may be determined and applied until the movable part 20 has reached the target position.

Application of SMA actuator assembly

The present invention provides particular advantage when applied to actuator assemblies in which the SMA element is surrounded or in direct contact with a material that provides heat dissipation. In such applications, prolonged heating of the SMA wire as per conventional drive schemes may lead to undesirable energy loss and heating of surroundings.

In particular, the SMA element 30 may be surrounded by a heat dissipating material. For example, the SMA element 30 may be immersed in a liquid or gel. This may be the case in a liquid lens or deformable lens, for example. In such a liquid lens or deformable lens, the SMA element 30 may be used to deform the lens so as to effect a change in focus of the lens. The SMA element 30 may be provided within the lens to allow deformation thereof.

There is thus provided a deformable lens that comprises the SMA actuator assembly 1 described herein. The support structure 10 or first part described herein may, for example, be a center portion of the deformable lens and the movable part 20 or second part may be an edge portion of the deformable lens. The SMA element 30 may thus move the edge portion relative to the center portion so as to deform the deformable lens, thereby adjusting the amount of focus of the deformable lens. The controller 100 may apply a drive pulse to the SMA element according to the drive scheme of the present invention.

Alternatively, the SMA element 30 may be in direct contact with a heat dissipating material between the end portions of the SMA element 30. For example, the SMA element 30 may form part of an elongate instrument, such as a steerable endoscope or guidewire of Figure 4 of WO 2023/118880 Al, which is herein incorporated by reference. The elongate instrument comprises a number of SMA elements (in the form of SMA wires) that are close contact. The elongate instrument may be selectively bent by heating one SMA wire in preference to the others. A conventional drive scheme may lead to heating of all the other SMA wires, which may prevent actuation. Applying a drive pulse in accordance with the present invention may heat a selected SMA wire before the heat can dissipate significantly into the neighbouring SMA wires, thus achieving more reliable differential heating and actuation of the actuator.

In alternative arrangements, the ends of the SMA element 30 may be held by crimps, for example, and the SMA element 30 may bend around or otherwise contact the heat dissipating material along the length of the SMA element 30. In some embodiments, a major portion (i.e. at least 50%) of the length of the SMA element 30 may be in direct contact with the heat dissipating material. The SMA element 30 may be in direct contact at at least one, at least two or at least three locations along the length of the SMA element.

The SMA element 30 may follow a curved path along at least part of the length of the SMA element 30, for example to fit the SMA element 30 within space constraints. One example of a device in which an SMA element 30 is arranged in such a way is a variable aperture. Two SMA elements 30 may act in opposition to rotate a component that in turn opens and closes an aperture. The variable aperture may be cylindrical, and for efficient space usage SMA elements 30 may lie around a component and slide over the component upon actuation.

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. An SMA actuator assembly comprising a first part; a second part; an SMA element arranged, on actuation, to move the second part relative to the first part; wherein the SMA actuator assembly is configured such that the second part is retained in position relative to the first part when the SMA element is not actuated; and a control circuit configured to: obtain a target position of the second part relative to the first part, and in response to receiving the target position, apply a drive pulse to the SMA element capable of actuating the SMA element so as to move the second part from an initial position to the target position relative to the first part.

2. An SMA actuator assembly according to claim 1, wherein the duration of the drive pulse is less than the time constant of heat dissipation from the SMA element.

3. An SMA actuator assembly according to any one of the preceding claims, wherein the ratio of the time constant of heat dissipation from the SMA element to the duration of the drive pulse is greater than 2, preferably greater than 5, further preferably greater than 10.

4. An SMA actuator assembly according to any one of the preceding claims, wherein the drive pulse is non-periodic.

5. An SMA actuator assembly according to any one of the preceding claims, wherein the duration of the drive pulse is more than 0.1 ms, preferably more than 1ms.

6. An SMA actuator assembly according to any one of the preceding claims, wherein the duration of the drive pulse is less than 50ms, preferably less than 5ms.

7. An SMA actuator assembly according to any one of the preceding claims, wherein after application of the drive pulse, the control circuit is configured to cease powering the SMA element, thereby stopping actuation of the SMA element and allowing the second part to be retained in position relative to the first part.

8. An SMA actuator assembly according to any one of the preceding claims, wherein the SMA actuator assembly comprises at least two SMA elements, wherein one of the at least two SMA elements is arranged, on actuation, to move the second part relative to the first part at least partially in a first direction and another of the at least two SMA elements is arranged, on actuation, to move the second part relative to the first part at least partially in a second direction that is opposite to the first direction; and wherein the control circuit is configured selectively to apply drive pulses to the at least two SMA elements such that the at least two SMA elements are not actuated concurrently.

9. An SMA actuator assembly according to any one of the preceding claims, wherein the SMA element is surrounded by a heat dissipating material, in particular immersed in a liquid or gel.

10. An SMA actuator assembly according to any one of the preceding claims, wherein the SMA element is in direct contact with a heat dissipating material at at least one, at least two or at least three locations along the length of the SMA element between the end portions of the SMA element.

11. An SMA actuator assembly according to any one of the preceding claims, wherein the control circuit is further configured to obtain feedback regarding the position of the first part relative to the second part, optional after or during applying the drive pulse.

12. An SMA actuator assembly according to claim 11, wherein after obtaining the feedback, the control circuit is configured to: i) if the feedback indicates that the target position has been reached, cease applying further drive pulses to the SMA element such that the first part is retained in position relative to the second part; or ii) if the feedback indicates that the target position has not been reached, apply a further drive pulse so as to move the first part relative to the second part to the target position.

13. An SMA actuator assembly according to claim 11 or 12, wherein the control circuit is configured to obtain the feedback by determining a measure of the electrical characteristic of the SMA element.

14. An SMA actuator assembly according to claim 13, wherein SMA element is slack when unpowered, and wherein the control circuit is configured, prior to obtaining the feedback, to apply electrical power to the SMA element sufficient to tension the SMA element but insufficient to move the second part relative to the first part.

15. An SMA actuator assembly according to any one of claims 11 to 14, wherein the SMA actuator assembly comprises a position sensor for sensing the position of the second part relative to the first part, and wherein the control circuit is configured to obtain the feedback by receiving a sensed position of the second part relative to the first part.

16. An SMA actuator assembly according to any one of the preceding claims, wherein, prior to applying the drive pulse, the control circuit is configured to determine characteristics of the drive pulse using a thermal model of the SMA element.

17. An SMA actuator assembly according to any one of the preceding claims, comprising one or more superelastic SMA elements configured to retain the second part in position relative to the first part when the SMA element is not actuated.

18. An SMA actuator assembly according to any one of the preceding claims, wherein friction between surfaces coupled to the first and second parts retains the second part in position relative to the first part when the SMA element is not actuated.

19. An SMA actuator assembly according to any one of the preceding claims, comprising a brake assembly configured to hold the second part in position relative to the first part when the SMA element is not actuated.

20. A deformable lens comprising the SMA actuator assembly of any preceding claim, wherein the first part is a center portion of the deformable lens and the second part is an edge portion of the deformable lens, and wherein the SMA element is configured, on actuation, to move the edge portion relative to the center portion so as to deform the deformable lens, thereby adjusting the amount of focus of the deformable lens.