Variable aperture assembly

Abstract

A variable aperture assembly comprising: a base (30); one or more blades (40) arranged to define a variable aperture when viewed along a primary axis (O), wherein the one or more blades are movable relative to the base; an actuator (10) arranged, on actuation, to drive movement of the blades to any position within a range of movement so as to adjust the size of the variable aperture; wherein the variable aperture assembly is configured such that the position of the one or more blades is maintained at any position within the range of movement when the actuator is not actuating.

Description

[0001] VARIABLE APERTURE ASSEMBLY

[0002] Field

[0003] The present application relates to a variable aperture assembly.

[0004] Background

[0005] There are a variety of apparatuses in which it is desired to provide control of a movable element. SMA 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.

[0006] 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.

[0007] Variable aperture assemblies may use SMA wires to move the blades so as to adjust the size of the variable aperture.

[0008] Summary

[0009] According to an aspect of the present invention, there is provided a variable aperture assembly comprising: a base; one or more blades arranged to define a variable aperture when viewed along a primary axis, wherein the one or more blades are movable relative to the base; an actuator arranged, on actuation, to drive movement of the blades to any position within a range of movement so as to adjust the size of the variable aperture; wherein the variable aperture assembly is configured such that the position of the one or more blades is maintained at any position within the range of movement when the actuator is not actuating. The size of the variable aperture may thus be maintained even when the actuator is not powered. As such, the actuator does not need to be powered continuously, but only when the size of the variable aperture is to be changed. The energy efficiency of the variable aperture assembly may thus be improved.

[0010] In some embodiments, the variable aperture assembly comprises one or more pairs of friction surfaces, each pair of friction surfaces comprising a first friction surfaces and a second friction surface that are biased against each other by a normal force, thereby generating static frictional forces between first and second friction surfaces for maintaining the position of the blades when the actuator is not actuating. The static frictional forces are for maintaining the position of the blades, and so are deliberately chosen to be large enough to allow maintenance of the position. Unlike in conventional variable aperture assemblies, any frictional forces are thus not minimized. The static frictional forces may be for maintaining the position of the blades while acceleration of the variable aperture assembly is below a hold threshold. The hold threshold may be greater than the gravitational acceleration of Earth, so greater than g (9.81 m / s2), preferably greater than 2g or greater than 5g, or greater than 10g.

[0011] In some embodiments, the actuator is arranged such that the normal force between at least one pair of friction surfaces remains constant on actuation of the actuator. The frictional force between the friction surfaces may thus be fixed, allowing the frictional force to easily be taken into account by the control of the actuator. Control of the actuator may be relatively simple.

[0012] In some embodiments, the actuator is arranged such that the normal force between at least one pair of friction surfaces is reduced on actuation of the actuator, thereby reducing the static frictional force between the pair of friction surfaces. Such a reduction in frictional force may ensure that the actuator can reliably move the blades and set the size of the variable aperture. The static frictional force between the friction surfaces can be set higher compared to a case in which the static frictional force remains constant.

[0013] In some embodiments, the actuator is arranged such that at least one pair of friction surfaces disengages on actuation of the actuator. The frictional force between the friction surfaces may thus be reduced to zero, minimizing resistance to movement on actuation of the actuator.

[0014] Some embodiments comprise an additional actuator configured, on actuation, to reduce the normal force between, or disengage, at least one pair of friction surfaces, thereby reducing the static frictional force therebetween. The additional actuator may not be used to effect movement of the blades. Adjustment of the frictional force and movement of the blades (and so setting of the size of the variable aperture) may thus be performed independently.

[0015] Some embodiments comprise both at least one pair of friction surfaces for which the normal force remains constant on actuation of the actuator and at least one pair of friction surfaces for which the normal force is reduced on actuation of the actuator (either due to actuation of the actuator, or due to actuation of an additional actuator). The overall frictional force, being the sum of frictional forces between all pairs of friction surfaces, may be reduced. The overall frictional force may be reduced up to a minimum frictional force corresponding to the sum of the frictional forces between all pairs of friction surfaces for which the normal force remains constant on actuation of the actuator assembly. In some embodiments, the additional actuator comprises an SMA element. SMA has a relatively high energy density, and so can be made relatively compact. The additional actuator thus need not add significantly to the footprint or form factor of the variable aperture assembly.

[0016] In some embodiments, the normal force biasing at least one pair of friction surfaces acts in a direction parallel to the primary axis. The friction surfaces may be generally perpendicular to the primary axis.

[0017] In some embodiments, the normal force biasing at least one pair of friction surfaces acts in a direction perpendicular to the primary axis. The friction surfaces may be generally parallel to the primary axis.

[0018] In some embodiments, one of i) the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces and ii) the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to remain constant on actuation of the actuator, and the other of i) the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces and ii) the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to be reduced on actuation of the actuator. So, the normal force that remains constant is perpendicular to the normal force that may be reduced. The friction surfaces may be between the same two components of the variable aperture assembly, for example between the base and the rotatable part.

[0019] In some embodiments, the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces is configured to remain constant on actuation of the actuator, and the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to be reduced on actuation of the actuator.

[0020] Some embodiments comprise one or more biasing arrangements arranged to bias the pairs of friction surfaces with the normal force, thereby giving rise to the frictional force. The at least one biasing arrangement may comprise a resilient element, such as a spring (coil spring, flexure, leaf spring) or elastic element (rubber band, etc). The at least one biasing arrangement may comprise a magnetic element. For example, the biasing arrangement may comprise a magnet on one component of the variable aperture assembly (such as the base, the rotatable part or the blades) and a magnet or ferromagnetic material on another component of the variable aperture assembly. In some embodiments, at least one biasing arrangement is formed integrally with a blade. As such, an additional biasing element may not be required, simplifying assembly and reducing the complexity of the variable aperture assembly.

[0021] Some embodiments further comprise a movable part that is movable relative to the base, wherein the one or more blades are coupled between the base and the movable part such that movement of the movable part relative to the base effects movement of the one or more blades, thereby changing the size of the variable aperture. The movable part may move translationally, rotationally, or both translationally and rotationally. The movable part may be movable in a plane perpendicular to the primary axis. The movable part may be a rotatable part that is rotatable relative to the base about the primary axis, wherein the one or more blades are coupled between the base and the rotatable part such that rotation of the rotatable part relative to the base effects movement of the one or more blades, thereby changing the size of the variable aperture.

[0022] In some embodiments, at least one pair of friction surfaces is arranged between i) the movable part or the rotatable part and ii) one of the base and the blades. At least one pair of friction surfaces may be arranged between the rotatable part and the base. At least one pair of friction surfaces may be arranged between the rotatable part and the base when viewed in a direction perpendicular to the primary axis. At least one (additional) pair of friction surfaces may be arranged between the rotatable part and the base when viewed in a direction parallel to the primary axis.

[0023] In some embodiments, the actuator comprises at least two actuator components, wherein one actuator component is arranged, on actuation, to drive movement of the blades in a first direction so as to increase the size of the variable aperture and another actuator component is arranged, on actuation, to drive movement of the blades in a second direction, opposite to the first direction so as to decrease the size of the variable aperture. The actuator may comprise four actuator components, with two actuator components being arranged, on actuation, to drive movement of the blades in a first direction so as to increase the size of the variable aperture and another two actuator components being arranged, on actuation, to drive movement of the blades in a second direction, opposite to the first direction so as to decrease the size of the variable aperture.

[0024] In some embodiments, the actuator comprises one or more SMA elements. The SMA elements may apply a relatively high actuation force compared to other types of actuator, and so are able to overcome forces (such as frictional forces) resisting movement of the blades. In some embodiments, the one or more SMA elements are connected between the rotatable part and the base. The friction surfaces may be arranged between the rotatable part and the base. The biasing arrangement may be configured to apply a biasing force between the rotatable part and the base in a first direction. The SMA wires are angled relative to the first direction such that a component of the force applied by the SMA wires opposes the biasing force. The angle of the SMA wires relative to the first direction is set such that a component, along the biasing force, of the actuation force applied by a stress of 200MPa in the SMA wires is in the range from 0.5 to 2 times the biasing force. The SMA wires may be angled relative to the biasing force so as to balance the actuation force that can be applied by the SMA wires against the biasing force that is applied by the biasing arrangement.

[0025] In some embodiments, the actuator comprises one or more superelastic SMA elements. The hysteretic properties of the superelastic SMA elements may be configured to hold the blades in position when the superelastic SMA elements are not actuated. The superelastic SMA elements have a phase transition temperature of less than 70°C, preferably less than 40°C, further preferably less than 20°C.

[0026] Further aspects of the present invention are set out in the detailed description.

[0027] Brief description of the drawings

[0028] Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0029] Figure 1 is a schematic plan view of a variable aperture assembly with a relatively closed variable aperture;

[0030] Figure 2 is a schematic plan view of a variable aperture assembly with a relatively open variable aperture;

[0031] Figure 3 is a schematic side view of a variable aperture assembly assembled on a lens assembly;

[0032] Figure 4 is a schematic plan view of an arrangement of SMA elements for adjusting the variable aperture;

[0033] Figure 5 is a schematic side view of a variable aperture assembly with constant friction for achieving zero hold power functionality;

[0034] Figures 6A and 6B are perspective views of biasing arrangements for applying a biasing force; Figure 7A is a schematic perspective view of a blade incorporating a biasing arrangement;

[0035] Figures 7B and 7C are schematic side views of variable aperture assembly with constant friction for achieving zero hold power functionality;

[0036] Figure 8 is a schematic plan view of another variable aperture assembly with constant friction for achieving zero hold power functionality;

[0037] Figures 9A and 9B are schematic plan views of variable aperture assemblies with variable friction for achieving zero hold power functionality;

[0038] Figures 10A and 10B are schematic plan views of other variable aperture assemblies with variable friction for achieving zero hold power functionality;

[0039] Figure 11 is a schematic plan view of another variable aperture assemblies with variable friction for achieving zero hold power functionality;

[0040] Figures 12A and 12B are schematic side views of other variable aperture assemblies with variable friction for achieving zero hold power functionality;

[0041] Figure 13A is a schematic plan view of a variable aperture assembly with a friction brake for achieving zero hold power functionality; and

[0042] Figure 13B is a schematic side view of a variable aperture assembly with a friction brake for achieving zero hold power functionality.

[0043] Detailed description

[0044] Figures 1 to 3 schematically depict a variable aperture (VA) assembly 1. Figure 1 shows a plan view of the VA assembly 1 with a relatively small variable aperture, and Figure 2 shows a plan view of the VA assembly 1 with a relatively large variable aperture. Figure 3 shows a side view of the VA assembly 1 in combination with a lens assembly 50.

[0045] The VA assembly comprises a base 30 and a rotatable part 20. The rotatable part 20 is rotatable relative to the base 30, in particular about a primary axis O. At least one of the base 30 and rotatable part 20 defines an aperture that allows light or fluid (e.g. gas or liquid) to pass. The aperture surrounds the primary axis O. The aperture may have rotational symmetry about the primary axis O.

[0046] The base 30 may be fixed within a larger device (such as a smartphone) within which the VA assembly 1 is incorporated. The base 30 may, for example, be fixed relative to a lens element of a lens assembly 50 that is provided in combination with the VA assembly 1. However, in general, the base 30 may also be movable within such a larger device. The base 30 is herein used as a reference structure relative to which movement of other components is described, unless explicitly stated otherwise. The base 30 may comprise multiple parts that are fixed relative to each other so as to form the base 30.

[0047] The VA assembly 1 may comprise a bearing arrangement (not shown in Figures 1 to 3) between the rotatable part 20 and the base 30. The bearing arrangement may guide rotation of the rotatable part 20 relative to the base 30. The bearing arrangement may constrain one or more degrees of freedom of movement other than the rotation. For example, the bearing arrangement may constrain movement of the rotatable part 20 relative to the base 30 along the primary axis O. The bearing arrangement may comprise rolling bearings (such as roller or ball bearings), plain bearings (i.e. sliding bearings) or flexure bearings (i.e. arrangements of flexures constraining degrees of freedom of movement). The VA assembly 1 may also comprise a biasing arrangement (not shown), e.g. an arrangement of flexures or other types of spring, for loading the bearing arrangement.

[0048] The VA assembly 1 further comprises at least one blade 40, preferably a plurality of blades 40. The blades 40 may also be referred to as leafs 40. The at least one blade 40 defines a variable aperture. The variable aperture is preferably substantially circular, but in general may have other shapes, depending on the desired application of the VA assembly 1. Each blade 40 is coupled between the base 30 and the rotatable part 20 in a manner such that rotation of the rotatable part 20 relative to the base 30 changes the size of the variable aperture.

[0049] In the embodiment of Figures 1 and 2, each blade 40 is coupled to the base 30 via a respective pin 33 and to the rotatable part 20 via a respective pin 23. Rotation of the rotatable part 20 relative to the base 30 causes relative movement of the pins 23, 33, thereby allowing the blades 40 to effectively pivot about the pins 23, 33 so as to change the variable aperture. The pins 23, 33 are spring loaded relative to each other, by flexure 22 in the particular embodiment of Figures 1 and 2, allowing each pin 23 to move along a circular path around respective pin 32 on rotation of the rotatable part 20.

[0050] In Figures 1 and 2, the VA assembly 1 comprises a total of six blades 40. The plurality of blades 40 are stacked in two layers of three blades 40 on top of each other. The two layers overlap when viewed along the primary axis O. However, in general, the VA assembly 1 may comprise any number of blades 40, arranged in any number of layers. In some embodiments, the VA assembly 1 comprises only a single blade. Such a VA assembly 1 may act as a variable shutter, for example. The VA assembly 1 may comprise at least four blades 40, preferably at least 6 blades. In some embodiments, the VA assembly 1 comprises 8 blades 40. A higher number of blades 40 may allow for a more circular variable aperture, at the cost of additional moving parts and a more complex assembly of the VA assembly 1. The blades 40 need not be arranged in layers, but may instead overlap sequentially around the variable aperture such that no blade 40 is finally on top.

[0051] In general, the coupling of the blades 40 to the base 30 and the rotatable part 20 may comprise any mechanism allowing movement of the blades 40 upon rotation of the rotatable part 20 so as to adjust the variable aperture. For example, the coupling may comprise a pin-slot arrangement in which one of the pins 23, 33 is allowed to slide within a slot of the blade.

[0052] The VA assembly 1 further comprises an actuator 10, schematically shown in Figure 3. The actuator 10 is configured to drive rotation of the rotatable part 20 relative to the base 30 about a primary axis O. The actuator 10 may rotate the rotatable part 20 relative to the base 30 to any rotational position within a range of movement. As such, the size of the variable aperture defined by the blades 40 may be adjusted to any size within a continuous range.

[0053] Figure 3 shows the VA assembly 1 in combination with a lens assembly 50. The base 30 of the VA assembly 1 may be mounted on the lens assembly 50, such that the lens assembly 50 is nested or provided within a through hole or opening of the base 30 which extends along a primary axis O of the VA assembly 1. The primary axis O may coincide with the optical axis of the lens assembly 50. The VA assembly 1 may thus adjust the amount of light entering the lens assembly 50. The light enters the lens assembly 50 along an optical path 2. The optical path 2 may be shaped, between the VA assembly 1 and the lens assembly 50, as a cone around the primary axis O.

[0054] In the depicted embodiment, the rotatable part 20 surrounds the base 30 when viewed along the primary axis O. In other embodiments, some of which are shown in Figures 5 to 13, the base 30 surrounds the rotatable part 20. In general, the base 30 and rotatable part 20 need not surround each other when viewed along the primary axis O. For example, the base 30 and rotatable part 20 may be stacked along the primary axis O.

[0055] Although the VA assembly 1 has been described as comprising a rotatable part 20 to which the blades

[0056] 40 are coupled, in general the VA assembly 1 may comprise any other mechanism capable of effecting movement of the blades 40. For example, the VA assembly 1 may generally comprise a movable part instead of the rotatable part 20. The movable part may be translationally or pivotally movable relative to the base 30. The blades 40 may be coupled between the base 30 and the movable part such that movement of the movable part relative to the base 30 effects movement of the blades 40, thereby adjusting the size of the variable aperture. In yet other embodiments, the rotatable part 20 and / or other movable part may be omitted altogether. The blades 40 may be directly movable relative to the base 30. The actuator 10 may apply a force directly to the blades 40 so as to effect movement of the blades 40, for example by providing each blade 40 with a dedicated actuator 10 (such as a dedicated SMA wire 11 connected between each blade 40 and the base 30).

[0057] SMA actuator

[0058] Figure 4 schematically shows an embodiment of the actuator 10. The actuator 10 comprises one or more SMA elements, in particular in the form of SMA wires 11. The one or more SMA wires 11 are configured, on actuation, to drive rotation of the rotatable part 20 relative to the base 30 about the primary axis O. In some embodiments, the one or more SMA wires 11 drive the rotatable part 20 to any rotational position within a range of movement relative to the base. In some other embodiments, the one or more SMA wires 11 drive the rotatable part 20 to a set of predetermined positions within a range of movement relative to the base. The size of the variable aperture is thereby adjusted.

[0059] The SMA wires 11 are connected between the base 30 and rotatable part 20 by connection elements 42, 43. The connection elements 42, 43 may be crimps, for example. The SMA wires 11 may be directly connected between the base 30 and rotatable part 20, such that the connection elements 42, 43 are directly connected to the base 30 and rotatable part 20. Alternatively, intermediate elements (not shown) may be connected between the connection elements 42, 43 and the base 30 and / or rotatable part 20, such that the SMA wires 11 are indirectly connected between the base and rotatable part 20. Such intermediate elements transfer the force in the SMA wires 11 to the rotatable part 20 so as to effect rotation of the rotatable part 20 relative to the base 10.

[0060] As shown in Figure 4, the VA assembly 1 may comprise four SMA wires 11. A first pair of SMA wires 11 (e.g. the top and bottom wires) is arranged, on contraction, to apply a torque to the rotatable part 20 in a first sense (e.g. counterclockwise). A second pair of SMA wires 11 (e.g. the left and right wires) is arranged, on contraction, to rotate the rotatable part 20 in a second sense (e.g. clockwise). The second sense is opposite to the first sense. The four SMA wires 11 are arranged in a loop around the primary axis O. Each SMA wire 11 is arranged on one of four sides that are arranged in a loop around the primary axis O. SMA wires 11 applying a torque in the same direction are arranged on opposite sides of the four sides.

[0061] The SMA wires 11 may be arranged with two-fold rotational symmetry about the primary axis O. This allows the torques applied by the first and / or second pairs of SMA wires 11 to be centered specifically about the primary axis O, reducing and indeed avoiding off-axis forces on the rotatable part 20. Motion of the rotatable part 20 may thus be purely rotational upon actuation of the first and / or second pairs of SMA wires 11, avoiding the need for a bearing arrangement constraining to such rotation about the primary axis O or reducing adverse forces on such a bearing arrangement.

[0062] In preferred embodiments, the SMA wires 11 of the first pair of SMA wires 11 may be electrically connected together. So, the SMA wires 11 need not be independently controllable. A single drive channel may be used to control the first pair of SMA wires 11. Similarly, the SMA wires 11 of the second pair of SMA wires 11 may be electrically connected together. So, the SMA wires 11 need not be not independently controllable. A single channel is used to control the second pair of SMA wires 11. The pairs of SMA wires 11 may be electrically connected in parallel or in series. However, in general, more than 2 channels (e.g. four channels) may be used to drive the four SMA wires 11, and the SMA wires 11 of the pairs need not be electrically connected together.

[0063] Although the embodiment of Figure 4 shows four SMA wires 11, in some other embodiments the VA assembly 1 comprises six or more SMA wires 11. In some embodiments, such as those shown in Figures 8 to 13, the VA assembly 1 comprises at least two SMA wires 11. One SMA wire 11 is arranged, on contraction, to rotate the rotatable part 20 in the first sense, and another SMA wire 11 is arranged, on contraction, to rotate the rotatable part 20 in the second sense.

[0064] Zero hold power

[0065] In conventional VA assemblies, the actuator 10 is constantly powered to maintain position of the blades and maintain the variable aperture at a desired size. The power consumption of such conventional VA assemblies is thus relatively high.

[0066] According to embodiments of the present invention, the VA assembly 1 is configured such that the one or more blades 40 maintain their position when the actuator 10 is not actuating. So, the size of the variable aperture can be maintained without powering or energizing the actuator 10. As such, the actuator 10 only needs to be powered when the size of the variable aperture is adjusted. The energy efficiency of the VA assembly 1 according to embodiments of the present invention is thus improved. The size of the variable aperture may be maintained when the actuator 10 is not actuating and when acceleration of the VA assembly 1 is less than or equal to a hold threshold. The hold threshold is a magnitude of acceleration of the VA assembly 1. When the acceleration is equal to or less than the hold threshold, hold forces (such as frictional forces) are sufficient to maintain the size of the variable aperture. When the acceleration is greater than the hold threshold, the hold forces (such as frictional forces) may be insufficient for maintaining the position of the blades 40. The hold threshold is set by the overall force resisting movement of the blades 40 when the actuator is not actuating, and so may be determined by the coefficients of friction of friction surfaces, the area of friction surfaces and the normal force with which the friction surfaces are biased against each other.

[0067] The hold threshold may be at least lg (9.8 m / s2) or at least 2g (19.6 m / s2), optionally at least 5g (49.0 m / s2), optionally at least 10g (98.1 m / s2), optionally at least 20g (196 m / s2), and optionally at least 50g (490 m / s2). By increasing the hold threshold, the risk of undesired movement of the blades 40 is reduced. Optionally the hold threshold is at most 100g (980 m / s2), optionally at most 50g, optionally at most 20g, optionally at most 10g. By reducing the hold threshold, the design freedom for the VA assembly 1 is increased. By increasing the design freedom, one or more other properties of the VA assembly 1 may be optimised. For example, a lower hold threshold may lead to an improvement of transition times, increased positional accuracy, increased stroke capability over the lifetime of the VA assembly 1, reduce particle generation due to wear of friction generating features and / or increased positional accuracy over lifetime of the VA assembly 1 (by reducing part wear).

[0068] Merely as one example, the hold threshold may be about 8g (78.4 m / s2). Such an acceleration is likely to represent most conditions where the VA assembly 1 is being used. For example, the VA assembly 1 may be used in an apparatus such as a mobile phone.

[0069] Friction surfaces for zero hold power

[0070] In some embodiments, the position of the blades 40 is maintained via an overall frictional force between components of the VA assembly 1. The overall frictional force may consist of frictional forces acting between any of the components of the VA assembly 1. For example, a frictional force may act between the base 30 and the rotatable part 20 (or other movable part to which the blades 40 are coupled). A frictional force may act between the base 30 and the blades 40. A frictional force may act between the rotatable part 20 (or other movable part to which the blades 40 are coupled) and the blades 40. A frictional force may act between blades 40 that are in contact with each other. These frictional forces may differ in magnitude, direction and whether or not the frictional forces are affected by actuation of the actuator 10. For example, some of these frictional forces may remain constant on actuation of the actuator 10 and some other of these frictional forces may be reduced or negated on actuation of the actuator 10. All of the frictional forces may contribute to the overall frictional force that resists movement of the blades 40 when the actuator 10 is not actuating.

[0071] In particular, the VA assembly 1 comprises one or more pairs of friction surfaces 21, 31. Each pair of friction surfaces comprises a first friction surface 31 and a second friction surface 21. The first and second friction surfaces 31, 21 may be arranged on any components that are in direct contact with each other. The second friction surface 21 engages the first friction surface 31.

[0072] At least some of the first and second friction surfaces 21, 31 may engage each other throughout the range of movement. So, in normal use (i.e. under actuation of the actuator 10 for moving the rotatable part 20), at least some of the first and second friction surfaces 21, 31 may remain in engagement with one another. Alternatively or additionally, at least some of the first and second friction surfaces 21, 31 may disengage on actuation of the actuator 10.

[0073] The VA assembly 1 may further comprise at least one biasing arrangement 35. The biasing arrangement 35 is arranged to bias the first and second friction surfaces 31, 21 against each other. The biasing arrangement 35 applies a biasing force. The biasing force comprises a component that is perpendicular to the first and second friction surfaces 31, 21, and so the biasing arrangement 35 applies a normal force between the friction surface 31, 21. The normal force is perpendicular to the friction surfaces 21, 31. The biasing arrangement 35 may apply the biasing force in the direction perpendicular to the friction surfaces 31, 21, or in a direction angled relative to the friction surfaces 21, 31. The biasing force of the biasing arrangement may be equal to the normal force, or equal to the combination of normal forces of multiple pairs of friction surfaces 21, 31. So, the biasing force may not have a component parallel to the range of movement, and thus not affect rotation of the rotatable part 20 relative to the base 30.

[0074] In general, the biasing arrangement 35 may comprise any element or combination of elements capable of applying a force between two or more components of the VA assembly 1. The biasing arrangement 35 may, for example, comprise a resilient or elastic element, such as a spring (e.g. coil spring, flexure, leaf spring), rubber band, or other resilient or elastic element. The biasing arrangement 35 may be a magnetic arrangement, comprising a magnet on one part and a magnet or ferromagentic material on the other part. The biasing arrangement 35 may be arranged between two parts, or be incorporated into one or more of the parts of the VA assembly 1. This normal force generates or gives rise to a static frictional force between the first and second friction surfaces 31, 21. The static frictional force constrains movement of the components of the VA assembly 1, so as ultimately to constrain movement of the blades 40, in particular when the SMA wires 11 are not contracted. Such movement is constrained at any position of the blades 40 relative to the base 30, i.e. at any rotational position of the rotatable part 20 relative to the base 30.

[0075] The actuator 10, for example the SMA wires 11, may be used to move the blades 40 to any position within the range of movement. Upon energising (i.e. when drive signals are applied to the SMA wires by the control circuit), the SMA wires 11 contract and apply an actuating force for moving the blades. The actuating force is sufficient to overcome the frictional forces at the friction surfaces 31, 32 (in some embodiments after reduction or elimination of the frictional forces due to SMA wire contraction), in order to drive movement of the blades. Upon ceasing power supply to the SMA wires 11, and so when stopping contraction of the SMA wires 11, the variable aperture components (e.g. the blades) remain at their position within the range of movement due to the frictional forces between the first and second friction surfaces 31, 21. In this state, the blades 40 are retained in position with zero power consumption by the VA assembly 1, so the VA assembly 1 may be referred to as a zero hold power actuator assembly, as may the other assemblies disclosed herein.

[0076] The static frictional force F constrains movement of components of the VA assembly 1. The magnitude of the static frictional force F is thus large enough to constrain such movement. The magnitude of the static frictional force F is proportional to the normal force N and the coefficient of static friction p, such that F = p * N. The static frictional force F may be increased by increasing the normal force N, which is achieved by appropriate design of the biasing arrangement 35, and / or by increasing the coefficient of static friction, which is achieved by appropriate design of the friction surfaces 21, 31.

[0077] The magnitude of the static frictional force is great enough to constrain movement of the blades 40, in particular before any reduction due to actuation of the actuator 10. The magnitude of the overall static frictional force is greater than the combined weight of the movable parts of the VA assembly 1, e.g. greater than the combined weight of the rotatable part 20 and blades 40. 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 combined weight of the movable parts is considered to be equal to the mass of the movable parts times earth's average gravitational acceleration (9.81 m / s2). Preferably, the ratio of the static frictional force to the combined weight of the movable parts is greater than 3, further preferably greater than 5, further preferably greater than 10. This ensures that movement of the movable part 20 is constrained even when the VA assembly 1 undergoes acceleratation. The actuator 10 may be arranged to reduce the frictional force upon contraction. In some embodiments, the overall frictional force is reduced by at least 20%, more preferably by at least 40% or by at least 50%. The overall frictional force may be reduced by at least 90%. In some embodiments, the overall frictional force is reduced by 100%, i.e. the friction surfaces 21, 31 disengage on actuation.

[0078] The magnitude of the static frictional force is low enough to allow the actuator 10 to overcome the static frictional force so as to effect movement of the blades, in some embodiments after reduction of the overall frictional force due to actuation. So, the magnitude of the (optionally reduced) static frictional force is less than the force applied by the actuator 10 to effect movement of the blades 40. The (optionally reduced) static frictional force may be less than 50%, preferably less than 20%, further preferably less than 10% of the force generated by a stress of 200MPa in the SMA wires 11.

[0079] The coefficient of static friction between the first and second friction surfaces 21, 31 directly affects the magnitude of the static frictional force. The coefficient of static friction may be modified by appropriately processing or selecting the material of the first and second friction surfaces 21, 31. The coefficient of static friction may be in the range between 0.05 and 0.6. Preferably, the coefficient of static friction is in the range between 0.1 and 0.4. In general, lower coefficients of static friction can be compensated for by higher normal forces imparted by the biasing arrangement 30.

[0080] Unless stated otherwise, any reference to a frictional force herein relates to a static frictional force. The requirements for the static frictional forces between first and second friction surfaces 21, 31 have been described above. These requirements may ensure that blades 40 remain in place relative to the base 30 once in position. Preferably, the same requirements apply to the dynamic or kinetic frictional forces between first and second friction surfaces 21, 31, thus ensuring that the blades 40 rapidly come to rest after being moved. For this purpose, the ratio of the dynamic frictional force to combined weight of the movable parts, the relation between dynamic frictional force and forces due to the actuator 10, and the coefficient of dynamic friction between the first and second friction surfaces 21, 31 may be as described in relation to the static frictional force. Preferably, the static friction coefficient between the first and second friction surfaces 21, 31 is substantially equal (e.g. varying by less than 5%, preferably less than 1%) to the dynamic friction coefficient between the first and second friction surfaces 21, 31. This makes the forces acting on the movable part more predictable, reducing the complexity of movement control.

[0081] Figures 5 to 11 show various different configurations of first and second friction surfaces 21, 31. Embodiments of the present invention may comprise any one or any subset, including all, of these configurations. For example, an embodiment of the present invention may comprise both a first pair of friction surfaces 21, 31 that enables constant friction and a second pair of friction surfaces 21, 31 that enables variable friction. The normal force acting on the friction surfaces 21, 31 that enable constant friction may be perpendicular to the normal force acting on the friction surfaces 21, 31 that enable variable friction. For example, the friction surfaces 21, 31 described in relation to Figures 5, 6A, 6B, 7A, 7B or 7C may be combined in a VA assembly 1 with the friction surfaces 21, 31 described in relation to Figures 9A, 9B, 10A, 10B, 11 and / or 13A. Similarly, the friction surfaces 21, 31 described in relation to Figure 8 may be combined in a VA assembly 1 with the friction surfaces 21, 31 described in relation to Figures 12A, 12B or 13B.

[0082] Constant friction zero hold power

[0083] In some embodiments, the actuator 10 is arranged, on actuation, not to reduce the normal force between (at least some pairs of) first and second friction surfaces 31, 21. For example, the SMA wires 11 are arranged such that the normal force between first and second friction surfaces 31, 21 remains substantially constant on contraction of the one or more SMA wires. Stresses in the SMA wires 11 do not affect the normal force. The normal force remains substantially constant in that it varies by less than 5%, preferably less than 1%, due to forces arising from stresses in the SMA wires 11.

[0084] Such an arrangement in which the normal fore is substantially unaffected by the SMA wires 11 reduces variation in the frictional forces. This makes control of the movement of blades 40 by the SMA wires 11 simpler. The arrangement of SMA wires 11 may also be less complex compared to a situation in which stresses and / or strains in the SMA wires 11 affect the normal force between the friction surfaces 31, 21.

[0085] Figure 5 schematically shows a cross-sectional side view of an embodiment of a VA assembly 1 with zero hold power functionality. The frictional force remains constant in this embodiment. The normal force acts in a direction parallel to the primary axis O. The frictional forces act in directions perpendicular to the primary axis O.

[0086] As shown in Figure 5, the base 30 may surround the rotatable part 20. A cover plate 34 is fixed relative to the base and may thus be considered to form part of the base 30. The blades 40 are arranged, when viewed perpendicularly to the primary axis O, between the rotatable part 20 and the base 30, in particular between the rotatable part 20 and the cover plate 34. The blades 40 are sandwiched between the rotatable part 20 and the base 30.

[0087] The VA assembly 1 comprises a biasing arrangement including a biasing element 35, examples of which are depicted in Figures 6A and 6B. The biasing element 35 comprises spring plates 35 with pre-deflected parts configured to apply the biasing force. Portions 531 may be fixed to the base 30 and portions 532 may slidably engage the rotatable part 20 and bias the rotatable part 20 against the base 30, schematically shown by the depicted arrows. Alternatively, portions 532 may be fixed to the rotatable part 20 and portions 531 may slidably engage the base 30 and bias the rotatable part 20 against the base 30.

[0088] In particular, the biasing element 35 may urge the rotatable part 20 in a direction along the primary axis O. The biasing element 35 biases the rotatable part 20 against the blades 40, thereby biasing the blades 40 against the base 30 (in particular against the cover plate 34). The biasing element 35 thus effectively wedges the blades 40 between the rotatable part 20 and the base 30. Frictional forces may arise between all of the base 30 and the rotatable part 20 (e.g. between the portions 532 of the biasing element 35 and the rotatable part 20), between the blades 40 and the rotatable part 20, and between the blades 40 and the base 30 (between the blades 40 and the cover plate 34). The first and second friction surfaces 31, 21 may thus be arranged between multiple components of the VA assembly 1.

[0089] The SMA wires 11 may apply a torque around an axis parallel to the primary axis O, as explained with reference to Figure 4. The normal force applied by the biasing element 35 may be in a direction parallel to the primary axis O. As such, the forces in the SMA wires 11 may not affect the frictional forces. The normal force, and thus the frictional force between the first and second friction surfaces 21, 31, remains substantially constant on contraction of the SMA wires 11.

[0090] Figures 7A to C schematically show further embodiments of a VA assembly 1 with zero hold power functionality. The frictional force remains constant in these embodiments. The normal force acts in a direction parallel to the primary axis O. The frictional forces act in directions perpendicular to the primary axis O.

[0091] Figure 7A shows a blade 40 and the pins 23, 33 of the rotatable part 20 and the base 30 that couple to the blade 40. The biasing arrangement 35 is integrally formed with the blade 40. The blade 40 comprises a resilient element, in particular in the form of a spring arm or flexure arm. The blade 40 is generally planar, and the spring arm protrudes out from the plane of the blade 40. When included in the VA assembly 1, the spring arm protrudes from the blade 40 in a direction along the primary axis O.

[0092] Figure 7B shows a side view of a portion of a VA assembly 1, showing two blades 40 with integrally formed biasing arrangement 35. The blades 40 overlap when viewed along the primary axis O, and so are adjacent to each other in the side view of Figure 7B. In the depicted embodiment, the base 30 comprises an upper cover plate and a lower cover plate. The blades 40 are arranged between different portions of the base 30, in particular between an upper cover plate and a lower cover plate when viewed in a direction perpendicular to the primary axis O.

[0093] The biasing arrangement 35, here in the form of the spring arms formed integrally with the blades 40, applies a normal force between the blades 40 and the base 30. The biasing arrangement 35 biases the blades 40 against each other. The biasing arrangement 35 further biases a portion of the blades 40 against the base 30, in particular a portion of the blades 40 arranged on the spring arm. The spring arm of one blade 40 extends in a first direction (downwards in Figure 7B) along the primary axis O, and the spring arm of another blade 40 extends in a second, opposite direction (upwards in Figure 7B) along the primary axis O. On assembly, the spring arms are deflected from their unstressed state so as to give rise to the biasing force. The blades 40 and biasing arrangement 35 are effectively wedged between the different portions of the base 30. The first and second friction surfaces 21, 31 are arranged both between the blades 40 and the base 30 and between adjacent blades 40. The SMA wires 11 are connected between the base 30 and the rotatable part 20, and so do not affect the normal force applied by the biasing arrangement 35 between the blades 40 and the base 30.

[0094] Figure 7C shows a side view of another VA assembly 1, showing two blades 40 with integrally formed biasing arrangement 35. The biasing arrangement 35, in the form of the spring arm, biases the blade 40 against the rotatable part 20. The biasing arrangement 35 further biases the rotatable part 20 against the base 30, for example against a lower plate forming part of the base 30. The first and second friction surfaces 21, 31 are arranged both between the blades 40 and the rotatable part 20, and between the rotatable part 20 and the base 30. The friction surfaces 21, 31 extend generally in directions perpendicular to the primary axis O. The SMA wires 11 are arranged perpendicularly to the primary axis O. The SMA wires 11 apply a force to the rotatable part 20 that is perpendicular to the primary axis O. So, the SMA wires 11 do not affect the normal force (and hence not the frictional forces) between the friction surfaces 21, 31.

[0095] Figure 8 schematically shows another embodiment of a VA assembly 1 enabling a constant frictional force. The normal force here acts in a direction perpendicular to the primary axis O. The frictional forces act in directions perpendicular to the primary axis O.

[0096] The VA assembly 1 comprises a bearing arrangement 37, in particular comprising a rolling bearing (such as a ball bearing). The bearing arrangement 37 is provided between the base 30 and the rotatable part 20. The bearing arrangement 37 comprises a first bearing surface on the base 30, a second bearing surface on the rotatable part 20, and a rolling bearing element (such as a ball or roller) between the first and second bearing surfaces. The bearing arrangement 37 guides rotation of the rotatable part 20 relative to the base 30.

[0097] The SMA wires 11 are arranged, on contraction, to urge the bearing surfaces of the bearing arrangement 37 against each other. The SMA wires 11 are further arranged, on contraction, to rotate the rotatable part 20 to any angular position within a range of movement.

[0098] The biasing arrangement 35 applies the biasing force so as to bias the friction surfaces 31, 21 against each other. The biasing arrangement 35 is depicted in Figure 8 simply as a force arrow, and may be implemented by a resilient element or magnetic arrangement. The biasing arrangement 35 biases first and second friction surfaces 21, 31 against each other with the normal force, thereby giving rise to the frictional force. The normal force is further transmitted through the friction surfaces 21, 31 and the rotatable part 20 to the bearing arrangement 37, thereby loading the bearing arrangement 37 also in the absence of contraction of the SMA wires 11. The SMA wires 11 act in the same direction as the biasing arrangement 35. The frictional force between the friction surfaces 21, 31 is not affected by the SMA wires 11.

[0099] Variable friction zero hold power

[0100] In some embodiments, the actuator 10 is arranged, on actuation, to reduce the normal force between first and second friction surfaces 31, 21. For example, the SMA wires 11 are arranged, on contraction, to reduce the normal force between first and second friction surfaces 31, 21. Put another way, the composite force acting between the friction surfaces 21, 31 due to stresses in the SMA wires 11 has a component that is parallel to and opposite in direction to the normal force. The stresses in the SMA wires 11 affect (in particular reduce) the normal force. In some embodiments, equal stresses (or tensions or strains) in the SMA wires 11 may reduce the normal force without moving the rotatable part 20 and / or blades 40. Unequal strains in the SMA wires 11 may result in movement of the rotatable part 20 and / or blades 40.

[0101] Such an arrangement in which the normal force is reduced by the SMA wires 11 allows selective reduction of the frictional forces by appropriate contraction of the SMA wires 11. This reduction of the frictional forces assists with the overcoming of the frictional forces to allow movement of the rotatable part 20 and / or the blades 40 within the range of movement. So, the stress in the SMA wires 11 required to move the rotatable part 20 and / or blades 40 may be reduced compared to a situation in which the frictional forces are not reduced. Furthermore, the frictional forces in the absence of contraction of the SMA wires 11 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 rotatable part 20 in the absence of SMA wire contraction.

[0102] Figures 9A and 9B schematically show an embodiment of a VA assembly 1 enabling a reduction in the frictional force. The normal force acts in a direction perpendicular to the primary axis O. The frictional forces act in directions perpendicular to the primary axis O. The SMA wires 11 are arranged, in essence, as described with reference to Figure 4.

[0103] The friction surfaces 31, 21 generally extend in a direction parallel to the primary axis O. The friction surfaces 31, 21 are located between the base 30 and the rotatable part 20 when viewed along the primary axis O. The friction surfaces 31, 21 may be curved and form an arc of a circle centered about the primary axis O when viewed along the primary axis.

[0104] In the embodiment of Figures 9A and 9B, a portion of the base 30 is formed as a deformable ring. The deformable ring acts as a biasing arrangement 35 that biases the friction surfaces 31, 21 against each other, thus giving rise to the frictional forces.

[0105] Figure 9A shows the VA assembly 1 in a state in which the SMA wires 11 are not actuated. The first friction surface 31 is arranged on the base 30, in particular on the deformable ring forming part of the base 30. The second friction surface 21 is arranged on the rotatable part 20. When the SMA wires 11 are not actuated, the biasing element 35 (embodied within the deformable ring forming part of the base 30) biases the first and second friction surfaces 31, 21 against each other with a normal force. The normal force is large enough to prevent rotation of the rotatable part 20 relative to the base 30.

[0106] Figure 9B shows the VA assembly 1 in a state in which the SMA wires 11 are actuated. The deformable ring is deformed so as to reduce the normal force between the first and second friction surfaces 31, 21. As shown in Figure 9B, the first and second friction surfaces 21, 31 may disengage from each other so as to eliminate the frictional forces. Alternatively, the first and second friction surfaces 21, 31 may remain in constant contact, but the normal force may be reduced so as to reduce the frictional forces.

[0107] Figure 10A schematically shows another embodiment of a VA assembly 1 enabling a reduction in the frictional force. The VA assembly 1 comprises the base 30, the rotatable part 20, and two SMA wires 11 arranged to rotate the rotatable part 20 to any rotational position within a range of movement. One SMA wire 11 (the left SMA wire 11 in Figure 10A) rotates the rotatable part 20 in one sense (clockwise in Figure 10A) and the other SMA wire 11 (the right SMA wire 11 in Figure 10A) rotates the rotatable part 20 in the opposite sense (counter-clockwise in Figure 10A). More than two SMA wires 11 may be provided. The two SMA wires 11 are arranged to be parallel to each other.

[0108] The VA assembly 1 comprises a biasing arrangement 35. The biasing arrangement 35 may comprise a spring (such as a coil spring or flexure, for example) or a magnetic arrangement. The biasing arrangement 35 urges the rotatable part 20 in a first direction (downward in Figure 10), thereby urging the friction surfaces 21, 31 against each other. The two SMA wires 11 apply forces to the rotatable part 20 in a direction parallel to the biasing force applied by the biasing arrangement 35. The SMA wires 11 are arranged to apply a force to the rotatable part 20 with a component opposite to the first direction (upward in Figure 10). As such, the SMA wires 11, on actuation, reduce the frictional force between the first and second friction surfaces 31, 21. Equal actuation of the SMA wires 11 reduces the frictional force without rotating the rotatable part 20. Unequal actuation of the SMA wires 11 rotates the rotatable part 20.

[0109] Figure 10B schematically shows another embodiment of a VA assembly 1 enabling a reduction in the frictional force. The VA assembly 1 of Figure 10B operates in a manner similar to the VA assembly 1 described with reference to Figure 10A. The SMA wires 11 are angled relative to one another at an angle a. The SMA wires 11 are angled relative to a biasing force applied by the biasing arrangement 35 at an angle a / 2.

[0110] The biasing arrangement 35 comprises a biasing element, in particular a resilient element in the form of a leaf spring. The biasing arrangement 35 applies a biasing force to the rotatable part 20 in a first direction (downwards in Figure 10B) that is perpendicular to the primary axis O. The biasing arrangement 35 comprises a coupling element 35a arranged between the biasing element and the rotatable part 20. The coupling element 35a is a ball bearing. The coupling element 35a allows the rotatable part 20 to move relative to the biasing element. The biasing arrangement 35 applies the biasing force throughout a range of movement of the rotatable part 20 relative to the base 30.

[0111] The SMA wires 11 are arranged to apply a force with a component that is opposite to the first direction (upwards in Figure 10B). The SMA wires 11, on mutual actuation, oppose the biasing force, thereby reducing the normal force (and hence the frictional forces) acting on the friction surfaces 21, 31. The SMA wires 11 are thus arranged, on actuation, to reduce the frictional forces between the friction surfaces 21, 31.

[0112] The SMA wires 11 are angled relative to the biasing force. As such, a relatively smaller proportion of the stress in the SMA wires 11 is used to reduce the force applied by the biasing arrangement 35 and a relatively greater proportion of the stress of the SMA wires 11 is capable of effecting rotation of the rotatable part 20, compared to the embodiment of Figure 10A in which the SMA wires 11 apply forces that are parallel to the biasing force. The angle a (and so the angle a / 2) may be selected in dependence on the magnitude of the biasing force applied by the biasing arrangement, the coefficient of friction and area of the friction surfaces 21, 31, and the actuation force applied on actuation of the SMA wires 11. The angle a may be selected such that the component, in the direction along the biasing force, of the actuation force that can be applied by the SMA wires 11 (e.g. the actuation force applied by a stress of 200MPa in the SMA wires 11) is in the range from 0.2 to 4 times the biasing force, preferably 0.5 to 2 times the biasing force, further preferably 1 to 1.5 times the biasing force. The angle a may be selected such that the component, in the direction along the biasing force, of the actuation force that can be applied by the SMA wires 11 (e.g. the actuation force applied by a stress of 200MPa in the SMA wires 11) is greater than the biasing force.

[0113] In Figure 10B, the SMA wires 11 cross over when viewed along the primary axis O. The SMA wires 11 are thus longer compared to a situation in which the SMA wires 11 are not allowed to cross over. The SMA wires 11 are allowed to cross over because the depicted angle a is relatively large. For smaller angles a, the SMA wires 11 may not need to cross over while maintaining the same length of SMA wire 11.

[0114] Figure 11 schematically shows another embodiment of a VA assembly 1 enabling a reduction in the frictional force. The VA assembly 1 comprises a bearing arrangement 37, in particular comprising a rolling bearing (such as a ball bearing). The bearing arrangement 37 guides rotation of the rotatable part 20 relative to the base 30. The biasing arrangement 35 is arranged between the base and the bearing arrangement 37. The biasing arrangement 35 applies the biasing force so as to load the bearing arrangement 37. The biasing force is transferred through the bearing arrangement and the rotatable part 20 to the friction surfaces 21, 31, thereby biasing the friction surfaces 31, 21 against each other. The SMA wires 11 are arranged to apply a force with component opposing the biasing force. The SMA wires 11, on contraction, may thus unload the friction surfaces 31, 21. The SMA wires 11, on contraction, may disengage the friction surfaces 31, 21 and lift the rotatable part 20 off the base 30 until an endstop arranged on the bearing arrangement 37 stops further deformation of the biasing element 35.

[0115] Figures 12A and 12B schematically show further embodiments of a VA assembly 1 enabling a reduction in the frictional force. The VA assemblies 1 of Figures 12A and 12B function in a similar manner. The upper schematic in Figures 12A and 12B shows the VA assembly 1 when the SMA wires 11 are not actuated, and the lower schematic in Figures 12A and 12B shows the VA assembly when the SMA wires 11 are actuated.

[0116] In Figure 12A the biasing arrangement 35 is a magnetic arrangement. The magnetic arrangement comprises a magnet arranged on the rotatable part 20 and a ferromagnetic material (such as a metal plate) fixed relative to the base 30. The magnet magnetically attracts the ferromagnetic material so as to give rise to the biasing force.

[0117] In Figure 12B, the biasing arrangement comprises a resilient element, such as a spring. The resilient element is connected between the base 30 and the rotatable part 20. So, one end of the resilient element is connected to the base 30 and the other end of the resilient element is connected to the rotatable part 20. The resilient element need not be connected to both the base 30 and the rotatable part 20, and instead a portion of the resilient element may slide relative to the base 30 or rotatable part, as described with reference to the biasing elements 35 of Figures 6A and 6B.

[0118] The biasing element 35 urges the rotatable part 20 in a direction along the primary axis O. The biasing element 35 may wedge the blades 40 between the rotatable part 20 and the base 30, or else urge the rotatable part 20 against the base 30 in a direction along the primary axis O. The first and second friction surfaces 31, 21 are thus urged into engagement so as to give rise to the frictional forces. The SMA wires 11 are angled relative to the plane perpendicular to the primary axis O, so as to apply a force with a component that opposes the biasing force of the biasing arrangement. As such, on contraction of the SMA wires 11, the normal force between the friction surfaces 31, 21 is reduced.

[0119] Additional actuator for releasing friction brake for zero hold power

[0120] Figures 13A and 13B schematically show further embodiments of a VA assembly 1 with zero hold power functionality. The VA assembly 1 comprises a friction brake with an additional actuator 10 for releasing the friction brake.

[0121] The embodiment of Figure 13A is in essence the same as the embodiment described with reference to Figure 8, except for the provision of an additional actuator 10b. The normal force urging the friction surfaces 21, 31 against each other acts in a direction perpendicular to the primary axis O. In Figure 13A, the additional actuator 10 is embodied by an SMA wire lib, but in general any other type of actuator 10b may be used. The additional actuator 10b is arranged to disengage or urge apart the friction surfaces 21, 31. So, the additional actuator 10b is arranged to reduce the normal force, and optionally eliminate the normal force, thereby reducing or eliminating the frictional force between the friction surfaces 21, 31. The additional actuator 10b may not affect movement of the rotatable part 20 and / or of the blades 40 relative to the blades 40. So, the sole purpose of the additional actuator 10b may be to reduce the frictional force, thereby making it easier for the actuator 10 to effect movement of the rotatable part 20 and / or of the blades 40. The actuator 10, in the form of SMA wires 11, drives movement of the rotatable part 20 and / or the blades 40.

[0122] The embodiment of Figure 13B is in essence the same as the embodiment described with reference to Figure 7, except for the provision of an additional actuator 10b, depicted as an additional SMA wire lib. The normal force urging the friction surfaces 21, 31 against each other acts in a direction parallel to the primary axis O. The additional actuator 10b is arranged to disengage or urge apart the friction surfaces 21, 31. So, the additional actuator 10b is arranged to reduce the normal force, and optionally eliminate the normal force, thereby reducing or eliminating the frictional force between the friction surfaces 21, 31. The additional actuator 10b may not affect movement of the rotatable part 20 and / or of the blades 40 relative to the blades 40. The actuator 10, in the form of SMA wires 11, drives movement of the rotatable part 20 and / or the blades 40.

[0123] Hysteresis for zero hold power

[0124] The VA assembly 1 may rely on mechanisms other than friction to achieve zero hold power functionality. For example, hysteresis in the SMA wires 11 may be used to hold the rotatable part 20 and / or the blades 40 in position at any point in the range of movement. The VA assembly 1 may be as described with reference to Figure 4, with superelastic SMA elements 11 (such as superelastic SMA wires 11) replacing the SMA wires 11. The hysteretic properties of the superelastic SMA elements are configured to hold the blades 40 (and optionally the rotatable part 20 or other movable part) in position when the superelastic SMA elements are not actuated (i.e. not powered or energized). WO2023 / 118880 Al, the contents of which are herein incorporated by reference, explains the mechanisms behind superelastic SMA wire holding a movable part in position.

[0125] The material composition and pre-treatment of the superelastic SMA element is chosen such that the superelastic SMA elements, when unenergized and in the normal operating environment, exhibit pseudo-elastic properties in which the superelastic SMA elements are in a state between a full austinite phase and a full martensite phase. For example, the superelastic SMA elements may remain in the pseudo-elastic range (ii) between the full Martensite (i) and the full Austenite (iii) phase even when unpowered. In contrast, SMA wires used in conventional actuator assemblies may revert to the full Martensite phase (i) 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, or may be greater than the ambient temperature due to heating effects of operating the superelastic SMA elements themselves.

[0126] 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 (i) to the full Austenite phase (ii). 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 (iii), 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 (i) to the partial Austenite phase (ii) (i.e. the lower temperature limit of the pseudo-elastic range).

[0127] In contrast, non-superelastic SMA wires used in conventional actuator assemblies are deliberately chosen to have higher phase transition temperatures above 70°C. Conventionally, such higher phase transition temperatures are chosen to reduce the impact of ambient temperature fluctuations on the actuation control of the actuator. The inventors of the present invention, however, have realized the benefits of operating with superelastic SMA elements (in particular of making use of hysteretic characteristics to maintain the position of the movable part), overcoming technical prejudice to incorporate superelastic SMA wires with relatively lower phase transition temperatures in a VA assembly.

[0128] 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 (i) to the partial Austenite phase (ii) (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.

[0129] In general, the phase transition temperature at which the SMA material, upon heating from a cooled state, starts transitioning from the full Martensite phase (i) to the partial Austenite phase (ii) (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 large number 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.

[0130] The superelastic SMA elements 11 may be arranged to be in tension at the operating temperature of the VA assembly 1. The superelastic SMA elements llmay be stretched to a strain that places them halfway across the pseudo-elastic region or more than halfway across the pseudo-elastic region depicted in Figure 5. Alternatively, the superelastic SMA elements 11 may be arranged to be in compression at the operating temperature of the VA assembly 1. 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 11 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.

[0131] SMA

[0132] 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 and / or other forming process(es). 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.

[0133] Although the invention has been described primarily with reference to SMA wires 11 acting as the actuator, in general any other type of actuator may be used. In any of the embodiments, the actuator may apply forces in the direction of the length of the depicted SMA wire 11. Other types of actuators that may be used include, without limitation, voice coil motors and piezo actuators.

Claims

Claims1. A variable aperture assembly comprising: a base; one or more blades arranged to define a variable aperture when viewed along a primary axis, wherein the one or more blades are movable relative to the base; an actuator arranged, on actuation, to drive movement of the blades to any position within a range of movement so as to adjust the size of the variable aperture; wherein the variable aperture assembly is configured such that the position of the one or more blades is maintained at any position within the range of movement when the actuator is not actuating.

2. A variable aperture assembly according to claim 1, wherein the variable aperture assembly comprises one or more pairs of friction surfaces, each pair of friction surfaces comprising a first friction surfaces and a second friction surface that are biased against each other by a normal force, thereby generating static frictional forces between first and second friction surfaces for maintaining the position of the blades when the actuator is not actuating.

3. A variable aperture assembly according to claim 2, wherein the actuator is arranged such that the normal force between at least one pair of friction surfaces remains constant on actuation of the actuator.

4. A variable aperture assembly according to claim 2 or 3, wherein the actuator is arranged such that the normal force between at least one pair of friction surfaces is reduced on actuation of the actuator, thereby reducing the static frictional force between the pair of friction surfaces.

5. A variable aperture assembly according to any one of claims 2 to 4, wherein the actuator is arranged such that at least one pair of friction surfaces disengages on actuation of the actuator.

6. A variable aperture assembly according to any one of claims 2 to 5, comprising an additional actuator configured, on actuation, to reduce the normal force between, or disengage, at least one pair of friction surfaces, thereby reducing the static frictional force therebetween.

7. A variable aperture assembly according to claim 6, wherein the additional actuator comprises an SMA element.

8. A variable aperture assembly according to any one of claims 2 to 7, wherein the normal force biasing at least one pair of friction surfaces acts in a direction parallel to the primary axis.

9. A variable aperture assembly according to any one of claims 2 to 8, wherein the normal force biasing at least one pair of friction surfaces acts in a direction perpendicular to the primary axis.

10. A variable aperture assembly according to claims 8 and 9, wherein one of i) the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces and ii) the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to remain constant on actuation of the actuator, and the other of i) the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces and ii) the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to be reduced on actuation of the actuator.

11. A variable aperture assembly according to claim 10, wherein the normal force that acts in a direction parallel to the primary axis for biasing the respective pair of friction surfaces is configured to remain constant on actuation of the actuator, and the normal force that acts in a direction perpendicular to the primary axis for biasing the respective pair of friction surfaces is configured to be reduced on actuation of the actuator.

12. A variable aperture assembly according to any one of claims 2 to 11, further comprising one or more biasing arrangements arranged to bias the pairs of friction surfaces with the normal force, thereby giving rise to the frictional force.

13. A variable aperture assembly according to claim 12, wherein at least one biasing arrangement comprises a resilient element.

14. A variable aperture assembly according claim 12 or 13, wherein at least one biasing arrangement comprises a magnetic element.

15. A variable aperture assembly according to any one of claims 12 to 14, wherein at least one biasing arrangement is formed integrally with a blade.

16. A variable aperture assembly according to any preceding claim, further comprising a movable part that is movable relative to the base, wherein the one or more blades are coupled between the baseand the movable part such that movement of the movable part relative to the base effects movement of the one or more blades, thereby changing the size of the variable aperture.

17. A variable aperture assembly according to claim 16, wherein the movable part is a rotatable part that is rotatable relative to the base about the primary axis, wherein the one or more blades are coupled between the base and the rotatable part such that rotation of the rotatable part relative to the base effects movement of the one or more blades, thereby changing the size of the variable aperture.

18. A variable aperture assembly according to claim 16 or 17 when dependent on claims 2 to 13, wherein at least one pair of friction surfaces is arranged between i) the movable part or the rotatable part and ii) one of the base and the blades.

19. A variable aperture assembly according to any preceding claim, wherein the actuator comprises at least two actuator components, wherein one actuator component is arranged, on actuation, to drive movement of the blades in a first direction so as to increase the size of the variable aperture and another actuator component is arranged, on actuation, to drive movement of the blades in a second direction, opposite to the first direction so as to decrease the size of the variable aperture.

20. A variable aperture assembly according to any preceding claim, wherein the actuator comprises one or more SMA elements.

21. A variable aperture assembly according to claim 12, 17 and 20, wherein the one or more SMA elements are connected between the rotatable part and the base, and wherein the friction surfaces are arranged between the rotatable part and the base, and wherein the biasing arrangement is configured to apply a biasing force between the rotatable part and the base in a first direction, wherein the SMA wires are angled relative to the first direction such that a component of the force applied by the SMA wires opposes the biasing force.

22. A variable aperture assembly according to claim 21, wherein the angle of the SMA wires relative to the first direction is set such that a component, along the biasing force, of the actuation force applied by a stress of 200MPa in the SMA wires is in the range from 0.5 to 2 times the biasing force.

23. A variable aperture assembly according to any preceding claim, wherein the actuator comprises one or more superelastic SMA elements.

24. A variable aperture assembly according to claim 21, wherein the superelastic SMA elements have a phase transition temperature of less than 70°C, preferably less than 40°C, further preferably less than 20°C.

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