Actuator assembly

Abstract

Intermittent control of an actuator 10 assembly e.g. a variable aperture (fig 2) or a lens 50 of a camera device to move a movable part 20 (e.g aperture blade) to a target location which may correspon

Description

Field The present application relates to an actuator assembly. Background An actuator assembly can be used to move a movable part relative to a support structure. As one example, an actuator assembly can be used to provide an aperture. For example, in the context of a camera, an actuator assembly may be used to control opening and closing of an aperture. Optionally, an actuator assembly may be used to control the size of the aperture. For example, different aperture sizes may be used for different focal lengths. During use of the actuator assembly, position of the movable part relative to the support structure may change undesirably due to forces experienced by the actuator assembly. In the context of an actuator assembly that defines an aperture, the size of the aperture may change undesirably due to forces experienced by the actuator assembly. For example, if the actuator assembly (or an apparatus comprising the actuator assembly) is dropped, then it may experience an acceleration that causes the size of the aperture to change. During use of the actuator assembly, the actuator assembly may experience forces that are less extreme than a drop but which nevertheless cause the size of the aperture to change undesirably. It is desirable to reduce the possibility of an actuator assembly having a movable part in a position that is different from its target position. Summary According to an aspect of the present invention, there is provided an actuator assembly, the actuator assembly comprising: a support structure; a movable part that is movable relative to the support structure; an actuator configured, on actuation, to move the movable part relative to the support structure, wherein the actuator assembly is arranged such that frictional forces between the movable part and the support structure constrain movement of the movable part relative to the support structure when the actuator is not actuating and when acceleration of the actuator assembly is less than or equal to a hold threshold; and a controller configured to control the actuator, wherein the controller is configured to, during a period in which a target position of the movable part is unchanged, intermittently control the actuator to move the movable part to the target position. By intermittently controlling the actuator to change the position of the movable part to the target position, the possibility of the movable part having a position that is different from the target position may be reduced. It is possible that the position of the movable part may be different from the target position for a shorter period of time. Optionally, the controller is configured to perform the intermittent control in response to receipt of a signal. By repositioning the actuator in response to receipt of a signal, power may be saved compared to continuously repositioning the actuator. Optionally, the signal is indicative of an acceleration of the actuator assembly greater than or equal to a threshold acceleration. By using the indication of the threshold acceleration being reached as the prompt for repositioning the actuator assembly, the amount of time for which the position of the movable part is different from the target position may be reduced Optionally, the threshold acceleration is dependent on an orientation of the actuator assembly and / or a direction of the acceleration. By providing different threshold accelerations dependent on the orientation of the actuator assembly and / or a direction of the acceleration, unnecessary actuation for repositioning the actuator assembly may be avoided. The power consumption of the actuator assembly may be reduced. Optionally, the threshold acceleration is dependent on the target position of the movable part. By providing different threshold accelerations dependent on the target position of the movable part, unnecessary actuation for repositioning the actuator assembly may be avoided. The power consumption of the actuator assembly may be reduced. Optionally, the signal is indicative of the movable part having a position outside of a tolerance range of movable part positions, the tolerance range comprising the target position. By repositioning only when the position of the movable part has changed, the possibility of the controller unnecessarily initiating repositioning of the actuator assembly may be avoided. Optionally, the controller is configured to issue a measurement request to the actuator, the signal being responsive to the measurement request. By measuring the position of the movable part the controller does not need to initiate repositioning of the actuator assembly in such a scenario. Power consumption may be reduced. Optionally, the controller is configured to issue the measurement request to the actuator periodically. By periodically issuing the measurement request, the maximum amount of time for which the position of the movable part may be different from the target position may be limited. Optionally, the measurement request is a signal of insufficient power to initiate actuation by the actuator. The power consumption may be reduced by providing that the measurement request is only a small measurement pulse. Optionally, the controller is configured to perform the intermittent control periodically. By repositioning the actuator at defined intervals, power may be saved compared to continuously repositioning the actuator. Optionally, the actuator assembly is arranged to define a variable aperture with a size that can be changed within a size range; the actuator is configured, on actuation, to change the size of the variable aperture within the size range; the target position of the movable part corresponds to a target size of the variable aperture; and the controller is configured to control the actuator to change the size of the variable aperture. By intermittently controlling the actuator to change the size of the aperture to the target size, the possibility of the aperture having a size that is different from the target size may be reduced. It is possible that the size of the aperture may be different from the target size for a shorter period of time. Optionally, the actuator assembly comprises: a plurality of blades arranged to define an aperture; wherein the actuator is configured to actuate the plurality of blades. Optionally, the actuator comprises: at least one shape memory alloy, SMA, element arranged, on contraction, to move the movable part relative to the support structure. Optionally, the actuator comprises: a voice coil motor. According to an aspect of the present invention, there is provided a camera assembly comprising: the actuator assembly described above. Optionally, the camera assembly comprises: an accelerometer configured to send signals to the controller indicative of an acceleration of the actuator assembly. Optionally, the camera assembly, comprises: a lens assembly configured to receive light; wherein the actuator assembly is mounted on the lens assembly. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an apparatus comprising an actuator assembly; Figure 2 is schematic plan view of an actuator assembly with a rotatable part in a first position relative to a base; Figure 3 is a schematic plan view of the variable aperture of Figure 2 with a rotatable part in a second position relative to a base; Figure 4 is a schematic cross-sectional side view of the actuator assembly of Figure 2 mounted on a lens assembly; and Figure 5 is a schematic plan view of an actuator comprising four shape memory alloy (SMA) wires. Detailed description Intermittent movable part position control Figure 1 is a schematic diagram of an apparatus 60 comprising an actuator assembly 1. The actuator assembly 1 comprises a support structure 30 and a movable part 20. The movable part 20 is movable relative to the support structure 30. The actuator assembly 1 comprises an actuator 10. The actuator 10 is configured, on actuation, to move the movable part 20 relative to the support structure 30. The movement of the movable part 20 relative to the support structure 30 causes the actuator assembly 1 to define an aperture. Optionally, the actuator assembly 1 is arranged to define an aperture. For example, the actuator assembly 1 may define the aperture of an optical system. Alternatively, the actuator assembly 1 may be arranged differently. Merely as an example, the actuator assembly 1 may be configured to move a lens system and / or an image sensor relative to each other or relative to another component. In the context of defining an aperture, optionally the actuator assembly 1 is arranged to define a variable aperture with a size that can be changed within a size range. In other words, the actuator assembly 1 may be a variable aperture (VA) assembly. Alternatively, the actuator assembly 1 may be configured to define an aperture with a fixed size. For convenience an embodiment is described primarily in the context of the actuator assembly 1 defining an aperture. However, it will be understood that the described features may be applied to alternative arrangements of actuator assembly. An example of an actuator assembly 1 is described in more detail below with reference particularly to Figures 2-4. The actuator assembly 1 shown in Figures 2-4 is a VA assembly in which the support structure 30 is a base 30 and the movable part is a rotatable part 20. In an alternative embodiment the movable part 20 is not rotatable. The actuator assembly 1 comprises an actuator 10. The actuator 10 is configured to change the size of the variable aperture within the size range. The actuator 10 moves one or more components of the actuator assembly 1 in order to make the size of the aperture substantially equal to a target size. The target position of the movable part 20 relative to the support structure 30 may vary over time. In the context of the actuator assembly 1 defining an aperture, the target position of the movable part 20 may correspond to a target size of the aperture. The target size for the aperture may vary over time. For example, different aperture sizes may be used for different focal lengths. Accordingly, the target size may vary depending on use of the actuator assembly 1. In one example, the apparatus 60 may be an electronic device comprising the actuator assembly 1. The apparatus 60 may comprise a camera assembly 63. The camera assembly 63 may comprise the actuator assembly 1 and a lens assembly 50. The apparatus 60 may comprise a processor configured to control the camera assembly 63. For example, the processor may run software for controlling the camera assembly 63. When the software is being run, the software may determine the target size of the aperture of the actuator assembly 1. The software may determine changes in the target size. As shown in Figure 1, optionally the actuator assembly 1 comprises a controller 61. The controller 61 is configured to control the actuator 10 to change the size of the variable aperture. For example, the controller 61 may receive a signal indicating the target size. The controller 61 may send a signal to the actuator 10 to cause the actuator 10 to change the size of the variable aperture to the target size. As mentioned above, the target size may be set by software controlling the camera assembly 63. Optionally, the apparatus 60 may be operated without the software for controlling the camera assembly 63 being run. For example, when the camera assembly 63 is not used to form images, the software may not be run so as to reduce power consumption. When the software is not being run, the target size may be set to a default target size. For example, the default target size may be substantially equal to the maximum size of the aperture. Alternatively, the default target size may be the minimum size for the aperture or a specific intermediate size for the aperture. During use of the actuator assembly 1, it is possible that the size of the aperture may undesirably change away from the target size. For example, the actuator assembly 1 may undergo an acceleration that causes the aperture size to increase or decrease away from the target size. Optionally, the controller 61 is configured to, during a period in which the target position of the movable part 20 relative to the support structure 30 is unchanged, intermittently control the actuator 10 to move the movable part 20 to the target position relative to the support structure 30. For example, the controller 61 may be configured to, during a period in which the target size of the aperture is unchanged )e.g. constant), intermittently control the actuator 10 to change the size of the variable aperture to the target size. The target size may be constant for a period of time. For example, when the software for controlling the camera assembly 63 is not running, the target size may remain constant at the default target size. As another example, when the camera assembly 63 is being used (e.g. when the software is running), the target size may remain constant for a period of time. For example, the desired focal length for the camera assembly 63 may be unchanged for a period of time. When the target size for the aperture is unchanged, it is desirable for the size of the aperture to remain at the target size. By intermittently controlling the actuator 10 to change the size of the variable aperture to the target size, the possibility of the variable aperture having a size that is different from the target size may be reduced. It is possible that the size of the variable aperture may be different from the target size for a short period of time. In particular, in between the intermittent performance of the control of the actuator 10, the size may be different from the target size. However, once the controller controls the actuator 10, the size of the variable aperture is changed to the target size. This may be a shorter period of time compared to if the controller were not configured to intermittently control the actuator 10 in this way. For example, the variable aperture may only then be changed to have the target size when the target size is changed such that the controller 61 controls the actuator 10 to change the size of the variable aperture to the target size. By controlling the actuator 10 to change the size of the variable aperture to the target size only intermittently, power consumption may be reduced. In particular, power consumption may be reduced compared to an alternative arrangement in which the controller 61 substantially continuously controls the actuator 10 to change or keep the size of the variable aperture at the target size. An embodiment of the invention is expected to reduce power consumption. An embodiment of the invention is expected to reduce the amount of time that the position of the movable part is different from the target position. Holding friction The actuator assembly 1 may be arranged such that frictional forces between the movable part 20 and the support structure 30 constrain movement of the movable part 20 relative to the support structure 30 when the actuator 10 is not actuating. For example, the actuator assembly 1 may be configured to generate frictional forces that constrain the change in size of the aperture at any position within the range of sizes when the actuator 10 is not actuated. The frictional forces may by generated by engagement between components of the actuator assembly 1. Examples of 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 30 may comprise a first friction surface and the movable part 20 may comprise a second friction surface that engages the first friction surface. The actuator assembly 1 may comprise a biasing arrangement that is arranged to bias the first and second friction surfaces against each other. The biasing arrangement may be a resilient element, such as a spring, or a magnetic arrangement, for example. The biasing arrangement biases the first and second friction surfaces against each other, thereby generating a static frictional force that constraints the movement of the movable part 20 relative to the support structure 30 at any position within a range of movement when the actuator is not actuated (and when acceleration of the actuator assembly 1 is below a hold threshold). The magnitude of the frictional forces can be adjusted by adjusting the biasing force of the biasing arrangement, and / or by adjusting the coefficient of static friction between the first and second friction surfaces. The actuator (e.g. the SMA wire) may, on actuation, reduce the normal force and thus the frictional forces between the first and second friction surfaces, as disclosed in WO 2023 / 094813 Al. Alternatively, the normal force may remain substantially constant on actuation of the actuator (e.g. on contraction of an SMA wire), as disclosed in WO 2023 / 084251 Al. By providing that such frictional forces are generated, the extent by which it is necessary to use power to control the position of the movable part 20 may be reduced. For example, when the target position of the movable part 20 remains substantially constant, the target position may be maintained without requiring the actuator 10 to be powered. An embodiment of the invention is expected to reduce power consumption. The actuator assembly 1 may use friction in the mechanism to achieve zero hold power. Zero hold power refers to the position of the movable part 20 being held (i.e. maintained) without power to the actuator 10. Optionally, the actuator assembly 1 is arranged to achieve zero hold power during normal operating conditions. Normal operating conditions include the actuator assembly 1 undergoing small accelerations and forces. However, large accelerations may be outside of normal operating conditions. Optionally, the actuator assembly 1 is arranged such that frictional forces between the movable part 20 and the support structure 30 constrain movement of the movable part 20 relative to the support structure 30 when the actuator 10 is not actuated only when acceleration of the actuator assembly 1 is less than or equal to a hold threshold. The hold threshold is a magnitude of acceleration of the actuator assembly 1. When the acceleration is equal to or less than the hold threshold, the frictional forces generated may be sufficient to maintain the size of the variable aperture. When the acceleration is greater than the hold threshold, the frictional forces generated may be insufficient for maintaining the position of the movable part 20. The movable part 20 may change in position due to the acceleration higher than the hold threshold. By providing the hold threshold, it is not necessary for zero hold power to be achieved in all conditions. The design freedom for the actuator assembly 1 may be increased. Any disadvantages caused by the requirement to achieve zero hold power in all conditions may be avoided. For example, an embodiment of the invention is expected to increase transition times, increase positional accuracy, increase stroke capability over the lifetime of the actuator assembly 1, reduce particle generation due to wear of friction generating features and / or increase positional accuracy over lifetime of the actuator assembly 1 (by reducing part wear) compared to an actuator assembly 1 required to achieve zero hold power in all conditions. Optionally, the hold threshold is 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 power consumption required to maintain or revert the size of the aperture to the target size may be reduced. Optionally the hold threshold is at most 100g (980 m / s2), optionally at most 50g, optionally at most 20g, optionally at most 10g, and optionally at most 5g. By reducing the hold threshold, the design freedom for the actuator assembly 1 may be increased. By increasing the design freedom, one or more other properties of the actuator assembly 1 may be optimised. 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 actuator assembly 1 is being used. For example, the actuator assembly 1 may be used in an apparatus 60 such as a mobile phone. The actuator assembly 1 may undergo much larger accelerations. For example, the largest accelerations a mobile phone handset may be expected to withstand is drop testing with an expected maximum acceleration of about 10,000g (98,100 m / s2). Drop detection Optionally, the controller 61 is configured to perform the intermittent control in response to receipt of a signal. When the controller 61 does not receive a signal prompting the controller 61 to perform the intermittent control, the controller 61 does not control the actuator 10 to move the movable part 20 to the target position. Accordingly, the actuator 10 remains unpowered, thereby reducing power consumption. When the controller 61 receives a signal that prompts the controller 61 to control the actuator 10, the controller 61 performs the intermittent control. The position of the movable part 20 may be reverted to the target position before the target position is changed. The signal that prompts the controller 61 to perform the intermittent control is different from any signal indicating that the target position of the movable part 20 has changed. The signal is received during the period in which the target position of the movable part 20 is constant. Optionally, the signal is indicative of an acceleration of the actuator assembly 1 greater than or equal to a threshold acceleration. For example, as shown in Figure 1 the apparatus 60 comprising the actuator assembly 1 may further comprise an accelerometer 62. The accelerometer is configured to measure an acceleration undergone by the accelerometer. The accelerometer 62 and the actuator assembly 1 may be provided in the same apparatus 60. The acceleration of the accelerometer 62 therefore corresponds to the acceleration of the actuator assembly 1. As shown in Figure 1, the accelerometer 62 may be provided separately from the camera assembly 63. In an alternative arrangement, the camera assembly 63 may comprise the accelerometer 62. In such an arrangement, the accelerometer 62 may be provided separately from the actuator assembly 1. In an alternative arrangement, the actuator assembly 1 may comprise the accelerometer 62. By providing the threshold acceleration, the position of the movable part 20 may be reverted to the target position when the actuator assembly 1 undergoes high accelerations. Such high accelerations may be expected to cause the position of the movable part 20 to change away from the target position. By using the indication of the threshold acceleration being reached as the prompt for repositioning the actuator assembly 1, the amount of time for which the position of the movable part 20 is different from the target position may be reduced. By providing a system that detects accelerations that are likely to move an unpowered zero hold power actuator, friction can be optimised to achieve a balance between true zero hold power under normal operating conditions and maintaining position during more challenging impact conditions that are expected to happen less frequently. For example, repositioning may be performed when a drop is detected. A drop may be detected when the threshold acceleration is reached. Optionally, the accelerometer 62 is configured to send acceleration measurement signals to the controller 61 of the actuator assembly 1. The accelerometer 62 may send such signals when the apparatus 60 is powered on. When the apparatus 60 is powered off, the accelerometer 62 may not send signals to the controller 61. The intermittent control of the actuator 10 to change the position of the movable part 20 to the target position may be triggered after measuring an acceleration equal to or greater than the threshold acceleration. Optionally, the threshold acceleration is at least 2g, optionally at least 5g, optionally at least 10g, optionally at least 20g, optionally at least 50g and optionally at least 100g. By increasing the threshold acceleration, the frequency of the intermittent control of the actuator 10 may be reduced. This may reduce the power consumption of the actuator assembly 1. Optionally, the threshold acceleration is at most 200g, optionally at most 100g, optionally at most 50g, optionally at most 20g, optionally at most 10g, and optionally at most 5g. By reducing the threshold acceleration, the possibility that the position of the movable part 20 remains different from the target position for an extended period of time may be reduced. Merely as one example, the threshold acceleration may be about 8g. Optionally, the threshold acceleration is less than or equal to the hold threshold. By providing that the threshold acceleration is less than or equal to the hold threshold, the possibility of the actuator assembly 1 undergoing an acceleration which overcomes the frictional forces generated but which does not trigger repositioning of the actuator assembly 1 may be reduced or avoided all together. For smaller accelerations, the frictional forces generated maintain the size of the aperture. For larger accelerations, the actuator assembly 1 is repositioned using the actuator 10. Different trigger thresholds Optionally, the threshold acceleration is dependent on an orientation of the actuator assembly 1. Additionally or alternatively, the threshold acceleration may be dependent on a direction of the acceleration. Different threshold accelerations may correspond to different orientations of the actuator assembly 1 and / or to different directions of acceleration. Alternatively, the threshold acceleration may be a function of the orientation of the actuator assembly 1 and / or a direction of the acceleration. The actuator assembly 1 may be less sensitive to accelerations in certain orientations. For example, the actuator assembly 1 may be arranged such that the size of the aperture tends to increase in size when one side of the apparatus 60 is tapped, and to reduce in size when the apparatus 60 is tapped on the other side of the screen. The repositioning of the actuator assembly 1 may be triggered at different threshold accelerations dependent on orientation. For example, the actuator assembly 1 may be less sensitive to accelerations that are along the direction of the axis defined by the aperture. The axis defined by the aperture may be the axis perpendicular to the plane of the aperture. The axis may correspond to the optical axis of the camera assembly 63 and / or the lens assembly 50. Optionally, the threshold acceleration is greater for acceleration directions more in line with the axis defined by the aperture. The threshold acceleration may be smaller for accelerations across the axis (i.e. more in line with the plane defined by the aperture). For example, the threshold acceleration may be about 3g (29.4 m / s2) for accelerations within the plane of the aperture. The threshold acceleration may be about 10g (98.1 m / s2) for accelerations parallel to the axis defined by the aperture. The threshold acceleration may be a continuous function dependent on the direction of the acceleration. Alternatively, the possible directions of acceleration may be bracketed into a plurality of different groups, with each group having a threshold acceleration associated with it. By providing different threshold accelerations dependent on the orientation of the actuator assembly 1 and / or a direction of the acceleration, unnecessary actuation for repositioning the actuator assembly 1 may be avoided. The power consumption of the actuator assembly 1 may be reduced. Optionally, the threshold acceleration is dependent on the target position of the movable part 20. In other words, different trigger thresholds may be applied depending on where the actuator 10 is positioned during an acceleration event. For example, the mechanism of the actuator assembly 1 may inherently have greater friction for some sizes of aperture compared to other sizes of aperture. As an example, the friction may be greater when the aperture is fully opened (i.e. at the maximum size of the variable aperture) compared to that intermediate or smaller positions of the variable aperture. As a result of the differing friction, different accelerations may be expected to change the position of the movable part 20. For example, the threshold acceleration may be greater for target sizes at which the actuator assembly 1 is expected to generate greater frictional forces. For example, the threshold acceleration may be greater when the target position is at an end of the stroke of the position of the movable part 20. The threshold acceleration may be a function of the target position of the movable part 20. A plurality of different threshold accelerations may be set corresponding to different target positions of the movable part 20. Optionally, the threshold acceleration is dependent on the target position of the movable part 20 as well as being dependent on the direction of the acceleration (or the orientation of the actuator assembly 1). By providing that the threshold acceleration is dependent on the target position of the movable part 20, unnecessary repositioning of the actuator assembly 1 may be reduced. This may help to reduce the power consumption of the actuator assembly 1. Optionally, the accelerometer 62 is configured to send a signal to the controller 61 when the accelerometer 62 measures an acceleration equal to or above the threshold acceleration. When the controller 61 receives the signal, the controller 61 controls the actuator 10 to revert the position of the movable part 20 to the target position. When the accelerometer 62 measure accelerations below the threshold acceleration, the accelerometer 62 does not send a signal to the controller 61. In an alternative arrangement, the accelerometer 62 substantially continuously or periodically sends acceleration measurement signals to the controller 61. The controller 61 is configured to determine whether or not the acceleration measurement signal indicates an acceleration greater than or equal to the threshold acceleration. When the controller 61 determines that the acceleration measurement signal indicates an acceleration greater than or equal to the threshold acceleration, the controller 61 controls the actuator 10 to change the size of the aperture to the target size. When the controller 61 determines that the acceleration measurement signal indicates that the acceleration is less than the threshold acceleration, then the controller 61 does not send any control signals to the actuator 10. Interval monitoring As mentioned above, the controller 61 is configured to perform the intermittent control in response to receipt of a signal. Optionally, the signal is indicative of the movable part 20 having a position outside of a tolerance range of positions. The tolerance range comprises the target position. For example, the tolerance range may be very small such that the signal is indicative of the movable part 20 having a position different from the target position. Alternatively, a larger tolerance range may be set such that small variations from the target position are tolerated whereas larger changes are not tolerated. When the controller 61 determines that the movable part 20 has a position outside of the tolerance range of aperture sizes, the controller 61 controls the actuator 10 to move the movable part 20 to the target position. Optionally, the controller 61 is configured to issue a measurement request to the actuator 10. The signal, which indicates the position of the movable part 20, may be responsive to the measurement request. For example, when the controller 61 issues a measurement request to the actuator 10, the controller 61 may receive a signal that is a measurement of the position of the movable part 20. The controller 61 may determine from the signal whether the aperture size is within the tolerance range of positions. When the controller 61 determines that the position of the movable part 20 is within the tolerance range, the controller 61 does not send any actuation signal to the actuator 10. When the controller 61 determines that the position of the movable part 20 is outside of the tolerance range, then the controller 61 issues actuation signals to the actuator 10 to revert the position of the movable part 20 to the target position. By repositioning only when the position of the movable part 20 has changed, the possibility of the controller 61 unnecessarily initiating repositioning of the actuator assembly 1 may be avoided. For example, it is possible that even a large acceleration may not cause the position of the movable part 20 to vary. By measuring the position of the movable part 20, the controller 61 does not need to initiate repositioning of the actuator assembly 1 in such a scenario. Power consumption may be reduced. Optionally, the controller 61 is configured to issue the measurement request to the actuator 10 periodically. The measurement request may be a small measurement pulse delivered to the actuator 10. The measurement request may be issued at definable intervals. By periodically issuing the measurement request, the maximum amount of time for which the position of the movable part 20 may be different from the target position may be limited. For example, the period between measurement requests may be at most 5s, optionally at most 2s, optionally at most Is, optionally at most 0.5s, optionally at most 0.1s, and optionally at most 0.1s. By reducing the period, the maximum amount of time for which the position of the movable part 20 may be different from the target position may be reduced. Optionally, the period may be at least 0.1s, optionally at least 0.2s, optionally at least 0.5s and optionally at least Is. By increasing the period, power consumption may be reduced. Optionally, the measurement request is a signal of insufficient power to initiate actuation by the actuator 10. The measurement pulse may be of sufficiently low power so as not to fully actuate actuator 10 and move the actuator 10 whilst still indicating the position of the actuator 10. The power consumption may be reduced by providing that the measurement request is only a small measurement pulse. Meanwhile, when it is determined that the position of the movable part 20 is outside of the tolerance range, the actuator 10 may be driven using a more powerful signal so as to correct the position of the actuator 10 if the actuator 10 is found to be in the wrong position. Optionally, the measurement is of the position of the actuator 10, for example the position of wires of the actuator 10. The position of the actuator 10 may determine the size of the aperture. Accordingly, by measuring the position of the actuator 10, the size of the aperture of the actuator assembly 1 may be indicated. Optionally, the actuator 10 is driven / controlled using a plurality of driven channels (and a common). For example, a single eight-channel drive chip could be used to drive the actuator 10 in addition to other functions such as an Optical Image Stabilisation (OIS) actuator and a focus (e.g. Auto Focus (AF)) actuator. Alternatively, a single four-channel drive chip could be used to drive the actuator 10 and an AF actuator. Another single four-channel drive chip could be used to drive the OIS actuator. Optionally, a monitoring mode is provided within the drive chip. The monitoring mode is for monitoring the size of the aperture, for example by measuring the position of the actuator 10. The monitoring mode may have a reduced feature set compared to a drive mode for driving the actuator 10 to change the size of the aperture of the actuator assembly 1. By providing a monitoring mode with a reduced feature set, power consumption may be further reduced. Intermittent position conformation Optionally, the controller 61 is configured to perform the intermittent control periodically. That is, the controller 61 may be configured to perform the intermittent control without requiring receipt of a signal. The controller 61 may actuate the actuator 10 to change (or maintain) the position of the movable part 20 at the target position at definable intervals. The controller 61 may be configured to reposition the actuator 10 at defined intervals. By repositioning the actuator 10 at defined intervals, power may be saved compared to continuously repositioning the actuator 10. By performing the intermittent control periodically, rather than responsive to a signal, the intermittent control may be implemented more easily. The complexity of the controller 61 may be reduced. Variable aperture Figs. 2 to 4 show an actuator assembly 1 comprising: a base 30, a rotatable part 20, a plurality of blades 40 which are connected to the base 30 and the rotatable part 20 via pins 21, 31 and which define a variable aperture and, and an actuator 10 (only schematically illustrated in Fig. 4) configured to drive rotation of the rotatable part 20 relative to the base 30 about a primary axis O to any rotational position within a range of movement so as to change the size of the variable aperture defined by the blades 40. As shown in Figure 4, the base 30 may be mounted onto a lens assembly 50. The base 30 is mounted 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 actuator assembly 1. The primary axis O coincides with the optical axis of the lens assembly 50. The base 30 may be described as a hollow tube with a base plate, provided on the lower end of the base 30, which radially protrudes outwards from the main body of the base 30 away from the optical axis O. The base 30 comprises a plurality of pivot pins 31 (also referred to as pivot protrusions herein) protruding from an upper surface of the main body of the base 30 in an upward direction parallel to the primary axis O. The plurality of pivot pins 31 form a loop around the primary axis O and are equally distanced from each other and equally distanced from the primary axis O. The main body of the base 30 is nested or provided within a hole or opening that extends through the rotatable part 20 along the primary axis O. The rotatable part 20 is mounted onto the base 30 such that it is capable of rotating relative to the base 30 about the primary axis O. The rotatable part 20 comprises a plurality of moving pins 21 (also referred to as moving protrusions herein) protruding from an upper surface of the rotatable part 20 in an upward direction parallel to the primary axis 0. The plurality of moving pins 21 form a loop around the primary axis O and are equally distanced from each other and equally distanced from the primary axis O. The moving pins 21 are connected to the main body of the rotatable part 20 via connecting arms 22. The connecting arms 22 in the illustrated embodiments are flexure arms 22 configured to elastically deform so as to allow movement of the moving pins 21 relative to the main body of the rotatable part 20 in directions generally perpendicular to the extent / length of the flexure arms 22. In other words, given that the flexure arms 22 extend in a circular manner around the primary axis O (i.e. wrap around the primary axis O), the flexure arms 22 are configured to elastically deform to accommodate movement of the moving pins 21 relative to the main body of the rotatable part 20 generally towards and away from the primary axis O. In Figs. 2 and 3, the connecting arms 22 extend in the same sense (anti-clockwise) around the primary axis O. However, some (e.g. half) of the connecting arms 22 may extend in a first sense (e.g. clockwise) around the primary axis O and some (e.g. the remaining half) of the connecting arms 22 may extend in a second opposite sense (e.g. anticlockwise) around the primary axis 0. The plurality of blades 40 are arranged to define a VA with a central axis which coincides with the primary axis O. The central axis of the VA also coincides with the optical axis of the lens assembly 50. The plurality of blades 40 are connected to the base 30 via the plurality of pivot pins 31 and connected to the rotatable part 20 via the plurality of moving pins 21. The plurality of blades 40 are provided on the upper sides of the base 30 and the rotatable part 20. In other words, the plurality of blades 40 (at least partially) cover or are provided at sides of the base 30 and the movable part 20 that generally face in upwards. The plurality of blades 40 (at least partially) overlap with the base 30 and / or the rotatable part 20 as viewed along the primary axis O. The plurality of blades 40 (at least partially) overlap with each other as viewed along the primary axis O. The plurality of blades 40 generally lie in a plane perpendicular to the primary axis 0 which sits on top of the rotatable part 20 and the base 30. The plurality of blades 40 are provided at sides of the base 30 and the movable part 20 that generally face in the same direction (e.g. upwards). Each blade 40 is connected to the base 30 via a single pivot pin 31 and connected to the rotatable part 20 via a single moving pin 21. The pivot pins 31 and the moving pins 21 extend through holes provided in the blades 40. The plurality of blades 40 are arranged such that, throughout the range of movement, the (variable) aperture defined by the plurality of blades 40 is continuously generally circular as viewed along the primary axis O, and / or the shape of the (variable) aperture defined by the plurality of blades 40 does not comprise any acute angles, and / or the shape of the (variable) aperture defined by the plurality of blades 40 is continuously an equilateral shape. The plurality of blades 40 are distributed around the primary axis O. In Figs. 2 and 3, the plurality of blades 40 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, and the layers overlap when viewed along the primary axis O. It will be appreciated that, the plurality of blades 40 may comprise any number of blades and any number of layers. For example, the plurality of blades may comprise six blades, the blades stacked in layers of two blades on top of each other wherein the layers overlap when viewed along the primary axis. For example, the plurality of blades may comprise eight blades, the blades stacked in layers of four blades on top of each other wherein the layers overlap when viewed along the primary axis. The moving pins 21 are connected to the blades 40 in a manner that prevents or restricts (any significant amount of) relative translational movement between each connected moving pin 21 and blade 40 in directions perpendicular to the primary axis O. The pivot pins 31 are also connected to the blades 40 in a manner that prevents or restricts (any significant amount of) relative translational movement between each connected pivot pin 31 and blade 40 in directions perpendicular to the primary axis O. In the illustrated embodiments, the moving pins 21 are fixed (e.g. integrally formed with, attached, welded, glued or soldered) to the connecting arms 22, and the moving pins 21 are connected to the blades 40 in a manner that allows each connected moving pin 21 and blade 40 to slidably rotate relative to each other, i.e. the moving pins 21 are rotatably / slidably mounted within openings in the blades 40. However, it will be appreciated that in alternative embodiments this may not be the case. For example, the moving pins 21 may instead be fixed to the blades 40 and connected to the connecting arms 22 in a manner that allows each connected moving pin 21 and connecting arm 22 to slidably rotate relative to each other, i.e. the moving pins 21 may be rotatably / slidably mounted within openings in the connecting arms 22. In another example, the moving pins 21 may be rotatably / slidably mounted within openings in the connecting arms 22 and also rotatably / slidably mounted within openings in the blades 40. In yet another example, the moving pins 21 may be fixed to the connecting arms 22 and fixed to the blades 40, and the moving pins 21 may be arranged to elastically deform to allow the moving pins 21 (i.e. at least the portions of the moving pins 21 engaging the blades 40) to rotate about the pivot pins 31. In the illustrated embodiments, the pivot pins 31 are fixed (e.g. integrally formed with, attached, welded, glued or soldered) to the base 30, and the pivot pins 31 are connected to the blades 40 in a manner that allows each connected pivot pin 31 and blade 40 to slidably rotate relative to each other, i.e. the pivot pins 31 are rotatably / slidably mounted within openings in the blades 40. However, it will be appreciated that in alternative embodiments this may not be the case. For example, the pivot pins 31 may instead be fixed to the blades 40 and connected to the base 30 in a manner that allows the pivot pins 31 to slidably rotate (about their own axes) relative to the base 30, i.e. the pivot pins 31 may be rotatably / slidably mounted within openings in the base 30. In another example, the pivot pins 31 may be rotatably / slidable mounted within openings in the base 30 and also rotatably / slidable mounted within openings in the blade 40. In yet another example, the pivot pins 31 may be fixed to the base 30 and fixed to the blades 40, and the pivot pins 31 may be arranged to elastically deform to allow the moving pins 21 to rotate about the pivot pins 31. The actuator 10 is provided between the base 30 and the rotatable part 20. In Fig. 4, the actuator 10 is mounted onto the base plate of the base 30 but it will be appreciated that this may not necessarily be the case. The actuator 10 is configured to, on actuation, drive rotation of the rotatable part 20 relative to the base 30 about a primary axis 0 in both clockwise and anticlockwise directions. As shown in e.g. Fig. 5, the actuator 10 may be a shape memory alloy (SMA) actuator. However, it will be appreciated that the actuator 10 may be any suitable actuator, for example, be a voice coil motor (VCM) actuator or a piezoelectric actuator, instead of a SMA actuator. The actuator assembly 1 is configured such that rotation of the rotatable part 20 relative to the base 30 about the primary axis O, drives relative movement between the pivot pins 31 and the moving pins 21, more specifically drives rotation of the moving pins 21 around the pivot pins 31. In other words, rotation of the rotatable part 20 relative to the base 30 about the primary axis O drives the moving pins 21 to move along circular paths centred around the pivot pins 31 (i.e. drives each moving pin 21 to move along a circular path centred around a respective pivot pin 31). In other words, the relative rotation of the rotatable part 20 about the primary axis O drives each moving protrusion 21 to rotate around a respective pivot protrusion 31 (i.e. the pivot protrusion 31 the moving protrusion 21 is connected to via one of the plurality of blades 40). In other words, the moving pins 21 are configured to rotate (e.g. move in a circular arc) around the pivot pins 31 (when the rotatable part 20 is rotated relative to the base 30). This in turn drives rotation of the plurality of blades 40 about the pivot pins 31. The rotation of the plurality of blades 40 changes the size of the variable aperture. In other words, the rotation of the moving pins 21 around the pivot pins 31 drives rotation of the plurality of blades 40 about the pivot pins 31, and the rotation of the plurality of blades 40 about the pivot pins 31 changes the size of the variable aperture. By having the moving protrusions 21 arranged to rotate around the pivot protrusions 31, a high-gain variable aperture mechanism (with e.g. a mechanical advantage of about 10:1) is provided that is suitable for use with low stroke (e.g. low displacement) high force actuators. The moving pins 21 and the pivot pins 31 may be configured (e.g. are close enough to each other) to provide, per degree of rotation of the rotatable part 20 about the primary axis O (relative to the base), at least 5, 10, or 20 degrees of rotation of the blades 40 about the pivot pins 31. Optionally, the actuator assembly comprises a holding arrangement configured to releasably hold the rotatable part at one or more positions within the range of positions that the rotatable part is capable of being driven to relative to the base by the actuator. Optional biasing element for friction The actuator assembly 1 may comprise at least one biasing element configured to bias the rotatable part 20 in a direction parallel to the primary axis O so as to generate frictional forces that constrain the movement of the rotatable part 20 relative to the base 30 at any position within the range of movement (i.e. at any position within the range of positions that the rotatable part 20 is capable of being driven to relative to the base 30 by the actuator 10) when the actuator 10 is not actuated. The frictional forces may by generated by engagement between the components provided between the rotatable part 20 and the base 30. For example, engagement between surfaces of the rotatable part 20, surfaces of the blades 40, surfaces of the base 30, (if provided) surfaces of one or more washer / spacer plates provided between the blades 40 and the rotatable part 20, and / or (if provided) surfaces of one or more washer / spacer plates provided between the blades 40 and the base 30 (e.g. an upper plate fixed to the base 30). The biasing element may comprise a ring-shaped main body which may be pre-deformed so as to provide the biasing force for generating the frictional forces. The biasing element may also comprise radially extending portions for attaching the biasing element to e.g. the base 30 or the rotatable part 20. The actuator 10 may be configured such that the frictional forces remain substantially constant on actuation. The actuator 10 may be configured to be capable of reducing the frictional forces on actuation. For example, the actuator may be configured to be capable of reducing the frictional forces on actuation by applying a force to the rotatable part, on actuation, which acts against the bias (force) of the biasing element and is large enough to meaningfully reduce the bias (force). SMA actuator The actuator 10 may be an SMA actuator 10 comprising one or more SMA elements configured to, upon contraction (e.g. upon heating the SMA elements by passing a current through them), (directly or indirectly) drive the rotation of the rotatable part 20 relative to the base 30. An example of an SMA actuator 10 is shown in Fig. 5. Similar actuator assemblies are described in detail in WO2013175197 which is incorporated herein by reference. The actuator 10 of Fig. 5 comprises a total of four SMA elements 11,12,13,14; a support structure 4 fixed to the base 30; and a movable part 3 coupled to the rotatable part 20. The SMA elements 11, 12, 13,14 are configured to, upon contraction, drive relative movement between the movable part 3 and the support structure 4 so as to drive the rotation of the rotatable part 20 relative to the base 30. The movable part 3 is fixed to the rotatable part 20; and the one or more SMA elements 11,12,13,14 are configured to, upon contraction, drive rotation of the movable part 3 relative to the support structure 4 (e.g. around the primary axis O) so as to drive the rotation of the rotatable part 20 relative to the base 30. The actuator assembly 1 may comprise a holding arrangement (not shown) configured to releasably hold the movable part 3 at one or more positions within the range of positions that the movable part 3 is capable of being driven to relative to the support structure 4, e.g. by the one or more SMA elements 11, 12,13, 14. The holding arrangement may be any suitable latch or catch arrangement known in the art, such as a roller ball catch arrangement. The four SMA elements 11,12,13,14 are configured to, indirectly via the movable part 3, drive the rotation of the rotatable part 20 relative to the base 30. The four SMA elements 11,12,13,14 are arranged in a loop at different angular positions around the primary axis O. Successive SMA elements 11, 12,13, 14 around the primary axis O are configured, on contraction, to, indirectly via the movable part 3, apply a force to the rotatable part 20 in alternate senses around the primary axis O. In an alternative embodiment, the four SMA elements 11,12,13,14 may be configured to directly drive the rotation of the rotatable part 20 relative to the base 30. Successive SMA elements 11, 12,13,14 around the primary axis O may be configured, on contraction, to directly apply a force to the rotatable part 20 in alternate senses around the primary axis O. A first pair of SMA elements 11,13 may be electrically connected together (in series or parallel), and arranged to apply a torque to the rotatable part 20 (directly, or indirectly via the movable part 3) for rotating the rotatable part 20 about the primary axis O in a first sense (e.g. anticlockwise); and a second pair of SMA elements 12, 14 may be electrically connected together (in series or parallel) and arranged to apply a torque to the rotatable part 20 (directly, or indirectly via the movable part 3) for rotating the rotatable part 20 about the primary axis O in a second sense (e.g. clockwise), wherein the second sense is opposite to the first sense. 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 or 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. Other variations It will be appreciated that there may be many other variations of the above-described examples. For example, it is not essential for the actuator 10 to comprise SMA elements. In an alternative arrangement, the actuator 10 comprises a voice coil motor. Optionally, the actuator assembly 1 comprises a holding arrangement configured to releasably hold the size of the aperture. The holding arrangement may, for example, be any suitable latch or catch arrangement known in the art, such as a roller / ball catch arrangement. The options for repositioning the actuator 10 described in this document may be combined with each other. For example, the controller 61 may actuate the actuator 10 based on a signal from the accelerometer 62 and also in response to signals indicative of the size of the variable aperture being different from the target size (or outside of the tolerance range). Hence, the control of the actuator 10 5 to revert the position of the movable part 20 back to the target size may be performed when one or both of a high acceleration is measured and a position outside of the tolerance range is measured. Alternatively, the controller 61 may be prompted to measure the position of the movable part 20 in response to receipt of a signal indicating a high acceleration. When the controller 61 receives a signal 10 indicating an acceleration equal to or greater than the threshold acceleration, the controller 61 may issue a measurement request to the actuator 10. The measurement request may be of insufficient power to actuate the actuator 10. If the measurement request results in a signal indicating the position of the movable part 20 being outside of the tolerance range, then the controller 61 actuates the actuator 10 to revert to the target size. This may reduce the possibility of unnecessary actuation of the 15 actuator 10, thereby reducing power consumption.

Claims

1. An actuator assembly comprising:a support structure;a movable part that is movable relative to the support structure;an actuator configured, on actuation, to move the movable part relative to the support structure, wherein the actuator assembly is arranged such that frictional forces between the movable part and the support structure constrain movement of the movable part relative to the support structure when the actuator is not actuating and when acceleration of the actuator assembly is less than or equal to a hold threshold; anda controller configured to control the actuator, wherein the controller is configured to, during a period in which a target position of the movable part is unchanged, intermittently control the actuator to move the movable part to the target position.

2. An actuator assembly according to claim 1, wherein the controller is configured to perform the intermittent control in response to receipt of a signal.

3. An actuator assembly according to claim 2, wherein the signal is indicative of an acceleration of the actuator assembly greater than or equal to a threshold acceleration.

4. An actuator assembly according to claim 3, wherein the threshold acceleration is dependent on an orientation of the actuator assembly and / or a direction of the acceleration.

5. An actuator assembly according to claim 3 or 4, wherein the threshold acceleration is dependent on the target position of the movable part.

6. An actuator assembly according to any of claims 2-5, wherein the signal is indicative of the movable part having a position outside of a tolerance range of movable part positions, the tolerance range comprising the target position.

7. An actuator assembly according to any of claims 2-6, wherein the controller is configured to issue a measurement request to the actuator, the signal being responsive to the measurement request.

8. An actuator assembly according to claim 7, wherein the controller is configured to issue the measurement request to the actuator periodically.

9. An actuator assembly according to claim 7 or 8, wherein the measurement request is a signal of insufficient power to initiate actuation by the actuator.

10. An actuator assembly according to any preceding claim, wherein the controller is configured to perform the intermittent control periodically.

11. An actuator assembly according to any preceding claim, wherein:the actuator assembly is arranged to define a variable aperture with a size that can be changed within a size range;the actuator is configured, on actuation, to change the size of the variable aperture within the size range;the target position of the movable part corresponds to a target size of the variable aperture;andthe controller is configured to control the actuator to change the size of the variable aperture.

12. An actuator assembly according to any preceding claim, comprising:a plurality of blades arranged to define an aperture;wherein the actuator is configured to actuate the plurality of blades.

13. An actuator assembly according to any preceding claim, wherein the actuator comprises:at least one shape memory alloy, SMA, element arranged, on contraction, to move the movable part relative to the support structure.

14. An actuator assembly according to any preceding claim, wherein the actuator comprises: a voice coil motor.

15. A camera assembly comprising: the actuator assembly of any preceding claim.

16. A camera assembly according to claim 15, comprising:an accelerometer configured to send signals to the controller indicative of an acceleration of the actuator assembly.

17. A camera assembly according to claim 15 or 16, comprising:a lens assembly configured to receive light;wherein the actuator assembly is mounted on the lens assembly.

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