Actuator assembly

Abstract

An actuator assembly comprises a first part and a relatively moveable second part, which is moved by an actuator component which may be an SMA element powered by a PWM signal from a controller. When the controller receives a request to move the second part, it sets a first actuation level 31 during a first period when the second part is stationary, and subsequently, during a second period when the second part is moving, sets a second, lower, actuation level 41. This technique provides a starting pulse with adequate actuation force to overcome static friction (slip-stick or stiction) but then provides a lower actuation force to give more accurately controllable movement of the second part.

Description

Field The present application relates to an actuator assembly. Background There are a variety of apparatuses in which it is desired to provide control of a movable element. Actuators may be used in such apparatuses. For example, shape memory alloy (SMA) elements (e.g. wires) may be advantageous as actuators in such apparatuses, for example due to their high energy density which means that the SMA actuator required to apply a given force to the movable element can be relatively small. One type of apparatus in which SMA wire is known for use as an actuator is in miniature cameras, for example those used in smartphones or other portable electronic devices. WO 2011 / 104518 discloses examples of SMA actuation apparatuses which are suitable for use in miniature cameras. Friction can make movement of the movable element undesirably jerky and / or inaccurate. It is desirable to allow movement of the movable element to be more smooth and / or more accurate. Summary According to an aspect of the present invention, there is provided an actuator assembly comprising: a first part; a second part that is movable relative to the first part; an actuator component arranged, on actuation, to drive movement of the second part relative to the first part; and a controller configured to control actuation of the at least one actuator component to drive the movement of the second part, wherein the controller is configured to: obtain a request to move the second part from a first position at which the second part is stationary to a second position, in response to the request: during a first control period during which the second part is stationary at the first position, set an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, set the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position. By setting the actuation level to the first value and subsequently to the second value lower than the first value, it may be possible to overcome the static friction during the first control period. After overcoming the static friction, the second part may be moved towards the second position more smoothly and / or more accurately. The stick-slip nature of motion may be reduced. Optionally, the controller is configured to carry out open-loop control during the first control period and to carry out closed-loop control during the second control period. By using open-loop control, the setting of the actuation level to the first value may be effected without requiring real time accurate measurements to be made. The system may be more reliable. By subsequently switching to closed-loop control, the possibility of the second part reverting to the static friction regime may be reduced. Optionally, the controller is configured to supply power to the at least one actuator component and wherein a time-averaged power which is supplied to the at least one actuator component during the first control period is higher than a time-averaged power which is supplied to the at least one actuator component during the second control period. Energy may be provided to the actuator component to overcome the static friction. The power may be increased in order to overcome the static friction so that when motion begins, it can be more smooth. Optionally, the controller is configured to increase and then subsequently decrease the actuation level of the at least one actuator component during the first control period. By increasing and then subsequently decrease the actuation level, a pulse of energy may be provided to the actuator component to overcome the static friction. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the at least one actuator component is not actuated (and / or not powered). The second part may be held in place by friction when it is not desired to move the second part. The power required to maintain the position of the second part may be reduced. Optionally, the at least one actuator component is arranged such that the normal force remains substantially constant on actuation of the at least one actuator component. By having a substantially constant normal force, the actuator assembly may be simplified. Optionally, the at least one actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. By reducing the normal force, the friction may be reduced such that movement of the second part may be less affected by the friction. Optionally, the at least one actuator component comprises a first actuator component and a second actuator component which are arranged, on actuation, to drive the second part relative to the first part in opposite directions. The position of the second part may be controlled accurately by selective actuation of the actuator components. Optionally, the controller is configured cause the supply of power to the second actuator component during the first control period and during the second control period, wherein a time-averaged power supplied to the second actuator component is lower during the first control period than during the second control period. By decreasing the actuation level of the second actuator component, the net driving force applied by the actuator components may be further increased. This may help to overcome the frictional force threshold, i.e. static friction. Optionally, the controller is configured to decrease and then subsequently increase the actuation level of second actuator component during the first control period. By temporarily decreasing the actuation level of the second actuator component, a pulse of energy urging the second part in one direction may help to overcome the frictional force threshold. Optionally, the controller is configured to, during the first control period, increase the actuation level of the first actuator component and decrease the actuation level of the second actuator component simultaneously. The difference in actuation level may be temporarily increased so as to overcome the static friction. Optionally, the second actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. By reducing the normal force, the friction may be reduced such that movement of the second part may be less affected by the friction. Optionally, the controller is configured to increase the actuation level of the first actuator component and increase the actuation level of the second actuator component simultaneously during the first control period. By actuating both actuator components to reduce the loading, the friction may be reduced more quickly and / or more effectively. Optionally, the controller is configured to increase the actuation level of the first actuator component and increase the actuation level of the second actuator component to substantially the same actuation level. By actuating both actuator components to the same level, the second part may be more stable as the friction is reduced. Optionally, the controller is configured to cause the supply of power to the second actuator component during the first control period and during the second control period, wherein a time-averaged power which is supplied to the second actuator component during the first control period is higher than a time-averaged power which is supplied to the second actuator component during the second control period. Energy may be provided to the actuator component to overcome the static friction. The power may be increased in order to overcome the static friction so that when motion begins, it can be more smooth. According to another aspect of the present invention, there is provided an actuator assembly comprising: a first part; a second part configured to move relative to the first part; two actuator components arranged, on actuation, to drive movement of the second part relative to the first part; and a controller configured to control actuation of the actuator components to drive movement of the second part, the controller configured to: obtain a request to move the second part from a first position at which the second part is stationary to a second position, in response to the request: determine whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuate the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance. By driving the second part to the intermediate position, the second part may be moved from the first position to the second position accurately, even when the distance between the first position and the second position is very small, for example smaller than the minimum achievable step size for the second part. Optionally, the controller is configured to actuate the actuator components to drive movement of the second part from the first position to the second position via the intermediate position by driving movement of the second part relative to the first part in a first direction from the first position to the intermediate position and subsequently in a second direction, opposite to the first direction, from the intermediate position to the second position. In other words, the controller drives movement of the second part in a first direction and then in a second direction, opposite to the first, in order to drive the second part to the second position. A minimum overall amount of movement of the second part may be implemented while moving the second part accurately to the second position. Optionally, the first position and second position are separated along a first direction and wherein the intermediate position is separated from the first position in a second direction, perpendicular to the first direction. In other words, the intermediate position is offset from a notional axis joining the first and second positions. The second part may be moved from the first position to the second position accurately, without requiring a greater range of movement along a straight line comprising the first position and the second position. For example, the second part may be moved from the first position to the second position accurately, even when the range of movement along such a straight line is limited. For example, the second part may be moved along a curved path in order to reach the second position. Optionally, the first position is between the intermediate position and the second position along one axis. A minimum overall amount of movement of the second part may be implemented while moving the second part accurately to the second position. Optionally, the second part may be constrained (e.g. by a bearing arrangement) to move along the axis. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the two actuator components are not actuated. The second part may be held in place by friction when it is not desired to move the second part. The power required to maintain the position of the second part may be reduced. Optionally, the first and second actuator components are arranged such that the normal force remains substantially constant on actuation of the first and second actuator components. By having a substantially constant normal force, the actuator assembly may be simplified. Optionally, the first and second actuator components are each arranged to apply a respective force to the second part, each respective force having a component orthogonal to the first surface that reduces the normal force and a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. By reducing the normal force, the friction may be reduced such that movement of the second part may be less affected by the friction. Optionally, each actuator component comprises a shape memory alloy, SMA, element. SMA elements are advantageous due to their high energy density which means that the SMA element required to apply a given force to the second part can be relatively small. Optionally, the second part is a lens element comprising at least one lens. Optionally, the first part has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor. Optionally, the first part has a display mounted thereon, the lens element being arranged to focus light emitted by the display. The actuator assembly may be used to improve a camera, projector or display, for example. Optionally, the second part has an image sensor mounted thereon. Optionally, the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor. The actuator assembly may be used to improve a camera, projector or display, for example. According to another aspect of the present invention, there is provided a method of controlling an actuator assembly comprising a first part, a second part and an actuator component, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: during a first control period during which the second part is stationary at the first position, setting an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, setting the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position. The method may be a computer-implemented method. Optionally, the method comprises carrying out open-loop control during the first control period and carrying out closed-loop control during the second control period. Optionally, the method comprises supplying power to the at least one actuator component such that a time-averaged power which is supplied to the at least one actuator component during the first control period is higher than a time-averaged power which is supplied to the at least one actuator component during the second control period. Optionally, the method comprises increasing and then subsequently decreasing the actuation level of the at least one actuator component during the first control period. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the at least one actuator component is not actuated. Optionally, the at least one actuator component is arranged such that the normal force remains substantially constant on actuation of the at least one actuator component. Optionally, the at least one actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, the method comprises actuating a first actuator component and a second actuator component to drive the second part relative to the first part in opposite directions. Optionally, the method comprises causing the supply of power to the second actuator component during the first control period and during the second control period, such that a time-averaged power supplied to the second actuator component is lower during the first control period than during the second control period. Optionally, the method comprises decreasing and then subsequently increasing the actuation level of second actuator component during the first control period. Optionally, the method comprises, during the first control period, increasing the actuation level of the first actuator component and decreasing the actuation level of the second actuator component simultaneously. Optionally, the second actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, the method comprises increasing the actuation level of the first actuator component and increasing the actuation level of the second actuator component simultaneously during the first control period. Optionally, the method comprises increasing the actuation level of the first actuator component and increasing the actuation level of the second actuator component to substantially the same actuation level. Optionally, the method comprises causing the supply of power to the second actuator component during the first control period and during the second control period, such that a time-averaged power which is supplied to the second actuator component during the first control period is higher than a time-averaged power which is supplied to the second actuator component during the second control period. According to another aspect of the present invention, there is provided a method of controlling an actuator assembly comprising a first part, a second part and two actuator components, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: determining whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuating the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance. The method may be a computer-implemented method. Optionally, the method comprises actuating the actuator components to drive movement of the second part from the first position to the second position via the intermediate position by driving movement of the second part relative to the first part in a first direction from the first position to the intermediate position and subsequently in a second direction, opposite to the first direction, from the intermediate position to the second position. Optionally, the first position and second position are separated along a first direction and wherein the intermediate position is separated from the first position in a second direction, perpendicular to the first direction. Optionally, the first position is between the intermediate position and the second position along one axis. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the two actuator components are not actuated. Optionally, the first and second actuator components are arranged such that the normal force remains substantially constant on actuation of the first and second actuator components. Optionally, the first and second actuator components are each arranged to apply a respective force to the second part, each respective force having a component orthogonal to the first surface that reduces the normal force and a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, each actuator component comprises a shape memory alloy, SMA, element. Optionally, the second part is a lens element comprising at least one lens. Optionally, the first part has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor. Optionally, the first part has a display mounted thereon, the lens element being arranged to focus light emitted by the display. Optionally, the second part has an image sensor mounted thereon. Optionally, the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor. According to another aspect of the present invention, there is provided a computer program product comprising instructions for instructing a controller to perform a method of controlling an actuator assembly comprising a first part, a second part and an actuator component, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: during a first control period during which the second part is stationary at the first position, setting an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, setting the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position. According to another aspect of the present invention, there is provided a computer-readable storage medium comprising instructions for instructing a controller to perform a method of controlling an actuator assembly comprising a first part, a second part and an actuator component, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: during a first control period during which the second part is stationary at the first position, setting an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, setting the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position. The computer-readable storage medium may be non-transitory. Optionally, the method comprises carrying out open-loop control during the first control period and carrying out closed-loop control during the second control period. Optionally, the method comprises supplying power to the at least one actuator component such that a time-averaged power which is supplied to the at least one actuator component during the first control period is higher than a time-averaged power which is supplied to the at least one actuator component during the second control period. Optionally, the method comprises increasing and then subsequently decreasing the actuation level of the at least one actuator component during the first control period. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the at least one actuator component is not actuated. Optionally, the at least one actuator component is arranged such that the normal force remains substantially constant on actuation of the at least one actuator component. Optionally, the at least one actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, the method comprises actuating a first actuator component and a second actuator component to drive the second part relative to the first part in opposite directions. Optionally, the method comprises causing the supply of power to the second actuator component during the first control period and during the second control period, such that a time-averaged power supplied to the second actuator component is lower during the first control period than during the second control period. Optionally, the method comprises decreasing and then subsequently increasing the actuation level of second actuator component during the first control period. Optionally, the method comprises, during the first control period, increasing the actuation level of the first actuator component and decreasing the actuation level of the second actuator component simultaneously. Optionally, the second actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, the method comprises increasing the actuation level of the first actuator component and increasing the actuation level of the second actuator component simultaneously during the first control period. Optionally, the method comprises increasing the actuation level of the first actuator component and increasing the actuation level of the second actuator component to substantially the same actuation level. Optionally, the method comprises causing the supply of power to the second actuator component during the first control period and during the second control period, such that a time-averaged power which is supplied to the second actuator component during the first control period is higher than a time-averaged power which is supplied to the second actuator component during the second control period. According to another aspect of the present invention, there is provided a computer program product comprising instructions for instructing a controller to perform a method of controlling an actuator assembly comprising a first part, a second part and two actuator components, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: determining whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuating the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance. According to another aspect of the present invention, there is provided a computer-readable storage medium comprising instructions for instructing a controller to perform a method of controlling an actuator assembly comprising a first part, a second part and two actuator components, the method comprising: obtaining a request to move the second part from a first position at which the second part is stationary to a second position; in response to the request: determining whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuating the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance. The computer-readable storage medium may be non-transitory. Optionally, the method comprises actuating the actuator components to drive movement of the second part from the first position to the second position via the intermediate position by driving movement of the second part relative to the first part in a first direction from the first position to the intermediate position and subsequently in a second direction, opposite to the first direction, from the intermediate position to the second position. Optionally, the first position and second position are separated along a first direction and wherein the intermediate position is separated from the first position in a second direction, perpendicular to the first direction. Optionally, the first position is between the intermediate position and the second position along one axis. Optionally, the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the two actuator components are not actuated. Optionally, the first and second actuator components are arranged such that the normal force remains substantially constant on actuation of the first and second actuator components. Optionally, the first and second actuator components are each arranged to apply a respective force to the second part, each respective force having a component orthogonal to the first surface that reduces the normal force and a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface. Optionally, each actuator component comprises a shape memory alloy, SMA, element. Optionally, the second part is a lens element comprising at least one lens. Optionally, the first part has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor. Optionally, the first part has a display mounted thereon, the lens element being arranged to focus light emitted by the display. Optionally, the second part has an image sensor mounted thereon. Optionally, the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic view of an actuator assembly; Figure 2 is a schematic view of an alternative actuator assembly; Figure 3 is a schematic view of an alternative actuator assembly; Figure 4 is a schematic view of power profiles applied to actuator components of an actuator assembly; Figure 5 is a schematic diagram showing movement of the second part of an actuator assembly over time corresponding to the power profiles shown in Figure 4; Figure 6 is a schematic view of power profiles applied to actuator components of an actuator assembly; Figure 7 is a schematic view of the movement of the second part of the actuator assembly corresponding to the power profiles shown in Figure 6; Figure 8 is a schematic view of possible sequence of movements for moving the second part of an actuator assembly from a first position to a second position; and Figure 9 is a schematic view of an alternative sequence of movements for moving the second part of an actuator assembly from a first position to a second position. Detailed description Figure 1 is a schematic view of an actuator assembly. As shown in Figure 1, the actuator assembly comprises a first part 4 and second part 20. The second part 20 is movable relative to the first part 4. The first part 4 may be referred to as a support structure. The second part 20 may be referred to as a movable part. The actuator assembly comprises at least one actuator component. As shown in Figure 1, the actuator component may be an SMA element such as an SMA wire 80. When the actuator component is an SMA element, the actuator assembly may be referred to as an SMA actuator assembly 1. For convenience, embodiments are described in the context of using one or more SMA wires 80. Other types of SMA elements may be used instead of SMA wires 80. Other types of actuator components may be used instead of SMA elements. Accordingly, the actuator assembly is not required to be an SMA actuator assembly 1. The at least one SMA wire 80 is arranged, on actuation, to drive movement of the second part 20 relative to the first part 4. For example, the SMA wire 80 may, on actuation, contract to drive movement of the second part 20 relative to the first part 4. The contraction of the SMA wire 80 may be controlled by applying electrical power to the SMA wire 80. Optionally, the SMA actuator assembly 1 comprises a controller. The controller is configured to control actuation of the at least one SMA wire 80 to drive movement of the second part 80. For example, the controller may be configured to control application of power P to the SMA wire 80 so as to control the extent by which the SMA wire 80 contracts. The SMA actuator assembly 1 may be arranged such that friction resists relative movement between the first part 4 and the second part 20. As shown in Figure 1, optionally the second part 20 is in contact with the first part 4. The first part 4 may have a first surface. The first surface may face towards the second part 20. The second part 20 may have a second surface. The second surface may face towards the first part 4. Optionally, when the second part 20 moves, the second surface (of the second part 20) moves across the first surface (of the first part 4). In use of the SMA actuator assembly 1, friction between the first surface and the second surface may resist relative movement between the second part 20 and the first part 4. For example, the friction may prevent the second part 20 from moving relative to the first part 4 when a net driving force on the second part 20 is too low. The net driving force is the net force on the second part 20 for driving the second part 20 relative to the first part 4. The friction opposes the net driving force. The at least one SMA wire 80 is configured to apply a driving force to the second part 20. Optionally, SMA actuator assembly 1 may comprise one or more further driving components that apply driving forces to the second part 20. For example, although not shown in Figure 1, optionally the SMA actuator assembly 1 comprises one or more resilient members, for example one or more springs. A resilient member may be arranged to oppose the driving force applied by the SMA wire 80 shown in Figure 1. The net driving force on the second part 20 may be comprised of the driving force supplied by the SMA wire 80 and the force supplied by the resilient member. The SMA actuator assembly 1 may be arranged such that when a net driving force on the second part 20 is below a frictional force threshold, then the second part 20 remains in position relative to the first part 4. The frictional force threshold is a threshold that must be overcome in order for the second part 20 to move relative to the first part 4. When the second part 20 is stationary relative to the first part 4, then the frictional force threshold prevents movement of the second part 20 relative to the first part 4 until the frictional force threshold is overcome by the net driving force on the second part 20. Optionally, the second part 20 may undergo stick-slip motion relative to the first part 4. The friction of the SMA actuator assembly 1 may reduce smoothness of movement between the first part 4 and the second part 20. Optionally, the second part 20 is movable relative to the first part 4 across the first surface. The SMA actuator assembly 1 may be arranged such that the second part 20 and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part 20 relative to the first part 4 when the at least one SMA wire 80 is not actuated. For example, the SMA actuator assembly 1 is arranged to have sufficient friction that the second part 20 remains in position relative to the first part 4 when the at least one SMA wire 80 is not actuated. This may allow the second part 20 to be held in position relative to the first part 4 when zero power is applied to the at least one SMA wire 80. This may be referred to as zero hold power. The biasing together of the second part and the first surface may be provided by, for example, one or more of gravity, one or more resilient elements such as springs, and / or one magnets. Figure 2 is a schematic view of an alternative SMA actuator assembly 1. As shown in Figure 2, optionally the at least one SMA wire 80 comprises a first SMA wire 80a and a second SMA wire 80b which are arranged, on actuation, to drive the second part 20 relative to the first part 4 in opposite directions. The SMA actuator assembly 1 may comprise two SMA wires 80a, 80b. For example, as shown in Figure 2 the SMA wires 80a, 80b may be configured to drive movement of the second part 20 in opposite directions. The SMA wires 80 may have actuation levels. The actuation level of an SMA wire 80 may equivalent to the power supplied to the SMA wire 80. The value of the actuation level of an SMA wire 80 may be controlled, for example adjusted. The net driving force on the second part 20 may comprise the driving force applied by a first SMA wire 80a and the driving force applied by a second SMA wire 80b. When the SMA wires 80a, 80b are actuated to the same actuation level, then the net driving force may be substantially zero such that the second part 20 does not move relative to the first part 4. When the actuation level of one SMA wire is greater than the other, then the net driving force on the second part 20 may be non-zero. When the net driving force is greater than the frictional force threshold, then the second part 20 may move relative to the first part 4. When each SMA wire 80 is actuated, the SMA wire 80 has a tension that corresponds to the driving force that the SMA wire 80 applies to the second part 20. When there are two (or more) SMA wires 80, then the SMA wires 80 have a net tension. The net tension is the overall tension applied by the SMA wires 80 on the second part 20. The net tension may correspond to the net driving force when there are no components other than the SMA wires 80 that drive movement of the second part 20. When the SMA wires 80 are actuated to the same actuation level, then the net tension is substantially zero. Optionally, the at least one SMA wire 80 is arranged to apply a force to the second part 20 with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part 20 relative to the first part 4 across the first surface. An example is shown in Figure 3. Figure 3 schematically depicts an alternative SMA actuated assembly 1. Features of the SMA actuator assembly 1 may be the same as described above in relation to the SMA actuator assembly 1 shown in Figure 2, except where differences are described below. As shown in Figure 3, optionally the SMA actuator assembly 1 comprises a loader 41. In Figure 3, the loader 41 takes the form of a spring. The loader 41 is configured to increase the friction that opposes movement of the second part 20 relative to the first part 4. For example, in the arrangement shown in Figure 3, the loader 41 is configured to urge the second part 20 towards the first part 4, so as to increase the friction between the first surface of the first part 4 and the second surface of the second part 20. Although the loader 41 is shown as a spring in Figure 3, other types of component may be used as the loader 41. For example, one or more magnets may be used. Alternatively, gravity may be used to provide the loading. Optionally, the second SMA wire 80b is arranged to apply a force to the second part 20 with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part 20 relative to the first part 4 across the first surface. As shown in Figure 3, optionally the SMA wires 80a, 80b are arranged, on actuation, to apply an unloading force. The unloading force is for reducing the friction that opposes movement of the second part 20 relative to the first part 4. For example, as shown in Figure 3, optionally the SMA wires 80 are arranged, on actuation, to apply a force on the second part 20 away from the first part 4. By applying a force away from the first part 4, the SMA wires 80 act to reduce the friction between the first part 4 and the second part 20. The loader 41 applies a force that opposes the unloading force supplied by the SMA wires 80 when they are actuated. In general, the force supplied by the loader 41 is greater than the unloading force supplied by the SMA wires 80. The second part 20 remains engaged with the first part 4. During use of the SMA actuator assembly 1, the friction level varies. When the SMA wires 80 are not actuated, the friction level is generally higher. When the SMA wires 80 are at a greater actuation level, the friction that opposes movement of the second part 20 relative to the first part 4 reduces. It is not essential for the friction to be variable. In an alternative arrangement, the first and second SMA wires 80 are arranged such that the normal force remains substantially constant on actuation of the first and second SMA wires 80. Figure 4 is a schematic view of power profiles applied to SMA wires 80 of an SMA actuator assembly 1. Embodiments are described in the context of the actuator assembly being an SMA actuator assembly, and the actuator components of the actuator assembly being SMA wires. However, the embodiments may alternatively be realised using different types of actuator components. For example, SMA elements other than wires may be used as the actuator components. As a further alternative, non-SMA actuator components may be used. For example, one or more voice coil motors may be used as the actuator components. Accordingly, the actuator assembly is not necessarily required to be an SMA actuator assembly. Figure 4 shows two traces corresponding to the power profiles applied to two SMA wires 80. As noted above, it is not essential for the SMA actuator assembly 1 to comprise two SMA wires 80. It is possible for the SMA actuator assembly 1 to have only one SMA wire 80, for example as shown in Figure 1. Accordingly, although two power profiles for two SMA wires 80 are shown in Figure 4, it is understood that where only one SMA wire 80 is provided then the power profile shown in solid lines may be used without using the power profile shown in dashed lines. Figure 4 shows a power profile used when the second part 20 is to be driven from a first position xl to a second position x2. The second part 20 is stationary at the first position xl. There may be friction that opposes movement of the second part 20 away from the first position xl. This friction may be referred to as static friction. The static friction may provide a friction force threshold that must be overcome in order for the second part 20 to be moved away from the first position xl. As shown in Figure 4, optionally the actuation level of the SMA wires 80 is zero at time to. In other words, the SMA wires are not actuated at time to. Figure 4 shows power profiles that may be applied when powering on the SMA actuator assembly 1. When the SMA actuator assembly 1 is powered off (e.g. before time to in Figure 4), then the friction that opposes movement of the second part 20 may be particularly high. For example, in the arrangement shown in Figure 3 the loading force loading the second part 20 onto the first part 4 may be at a maximum. As the SMA wires 80 are actuated, the SMA wires 80 provide an unloading force which causes the friction to be reduced. As a result of the high friction stick-slip movement is particularly likely when attempting to make a first movement of the second part 20 following power on of the SMA actuator assembly 1. However, in an alternative arrangement, the SMA wires 80 may already be powered on before the power profiles described below are applied to the SMA wires 80. Optionally, the controller is configured to obtain a request to move the second part 20 from the first position xl at which the second part is stationary to the second position x2. Optionally, the controller is configured to, in response to the request, during a first control period during which the second part 20 is stationary at the first position xl, set an actuation level of the first SMA wire 80a to a first value so as to start movement of the second part 20 from the first position xl. For example, as shown in Figure 4, optionally the power profile applied to the first SMA wire 80a comprises a pulse 31. The pulse 31 is a temporary increase in power applied to the first SMA wire 80a. Optionally, the controller is configured to, in response to the request, subsequently, during a second control period that begins after the second part 20 has started moving from the first position xl, set the actuation level to a second value lower than the first value so as to drive movement of the second part 20 towards the second position x2. The controller may be configured to increase and then subsequently decrease the actuation level of the first SMA wire 80a during the first control period. For example, optionally, the controller is configured to increase the actuation level of the first SMA wire 80a and subsequently decrease the actuation level so as to drive movement of the second part 20 towards the second position x2. Figure 4 shows the actuation level of the first SMA wire 80a being increased at the up-ramp of the pulse 31 (i.e. immediately to the left of the peak of the pulse 31 shown in Figure 4). Figure 4 shows a subsequent decrease of the actuation level by the down-ramp immediately to the right of the peak of the pulse 31 shown in Figure 4. As shown in Figure 4, in order for the second part 20 to be moved to the second position x2, the controller may apply a first target actuation level 41 to the first SMA wire 80a. the controller may apply a second target actuation level 42 to the second SMA wire 80b. The target actuation levels 41, 42 may correspond to the second position x2. As shown in Figure 4, the first target actuation level 41 of the first SMA wire 80a may be higher than the second target actuation level 42 of the second SMA wire 80b. This indicates that at the second position x2, the first SMA wire 80 is more contracted than the second SMA wire 80b. The second position x2 of the second part 20 may correspond to a location that is slightly to the right of the neutral position shown in Figure 2 or Figure 3, for example. Previously, it may have been expected that the actuation levels of the SMA wires 80 would be increased to the target actuation levels 41, 42. However, as shown in Figure 4 according to an embodiment of the invention the actuation level of the first SMA wire 80a is initially increased and then subsequently decreased so as to drive movement of the second part 20 towards the second position x2. The actuation level of the first SMA wire 80a may be decreased to the first target actuation level 41, rather than being increased to the first target actuation level 41. By providing the pulse 31 to the first SMA wire 80a, it may be possible to overcome the static friction. After overcoming the static friction, the second part 20 may be moved towards the second position x2 more smoothly and / or more accurately. The stick-slip nature of motion may be reduced. Figure 5 schematically shows the position of the second part 20 over time t when the power profiles shown in Figure 4 are applied to the SMA wires 80a, 80b. As shown in Figure 4, the pulse 31 may start at time tl. The controller may be configured to increase the actuation level of the first SMA wire 80a at time tl. The actuation level of the first SMA wire 80a may reach a peak at time t2. As shown in Figure 5, the second part may be expected to remain stationary at the first position xl until the start of the pulse 31 at time tl. This is shown by the flat line 51 in Figure 5. From the start of the pulse tl to the peak of the pulse at time t2, the second part 20 may move a small amount in the direction of the second position x2. This is shown by the shallow slope 52 in Figure 5. From the peak of the pulse 31 at time t2, the second part 20 moves more rapidly towards the second position x2. This is shown by the steep slope 53 in Figure 5. Until the pulse 31 begins at time tl, the second part 20 may remain stationary due to the friction. The second part 20 may be in a static friction regime relative to the first part 4. Once the static friction has been overcome by applying the pulse 31 at time tl, the second part 20 may enter a dynamic friction regime relative to the first part 4. The dynamic friction regime allows the second part 20 to move more smoothly towards the second position x2. During the first control period the second part 20 is stationary at the first position xl. The second control period begins after the second part 20 has started moving from the first position xl. The second part 20 starts moving from the first position xl at time tl. As shown in Figure 5, optionally the controller is configured to start the increase of the actuation level while the second part 20 is stationary at the first position xl. This is shown by the start of the pulse 31 at time tl being while the second part 20 is at the first position xl. The right amount of power may be pulsed into the SMA wires 80 to overcome the static friction. It is not essential for a sharp pulse to be provided. Optionally, the controller is configured to supply power to the SMA wires 80. A time-averaged power which is supplied to the first SMA wire 80a during the first control period is higher than a time-averaged power which is supplied to the first SMA wire 80a during the second control period. As shown in Figure 4, optionally the SMA actuator assembly 1 comprises two opposing SMA wires 80a, 80b. Different power profiles may be applied to the different SMA wires 80a, 80b. Optionally, the controller is configured cause the supply of power to the second SMA wire 80b during the first control period and during the second control period, wherein a time-averaged power supplied to the second SMA wire 80b is lower during the first control period than during the second control period. Optionally, the controller is configured to decrease and then subsequently increase the actuation level of second SMA wire 80b during the first control period. As shown in Figure 4, optionally the controller is configured to decrease an actuation level of the second SMA wire 80b. This is shown by the steep downward slope from time tl until time t2 in the dashed line. As shown in Figure 4, optionally the controller is configured to subsequently increase the actuation level of the second SMA wire 80b as the second part 20 is driven towards the second position x2. The 21 increase in the actuation level is shown in Figure 4 by the steep increase in actuation level from time t2. The time t2 may correspond to the trough of the power profile applied to the second SMA wire 80b. Optionally, the controller is configured to, during the first control period, increase the actuation level of the first SMA wire 80a and decrease the actuation level of the second SMA wire 80b simultaneously. For example, as shown in Figure 4, the controller may be configured to increase the actuation level of the first SMA wire 80a and to decrease the actuation level of the second SMA wire 80b simultaneously. This is shown in Figure 4, where the steep upward slope from tl to t2 for the power profile of the first SMA wire 80a is over the same time period as the steep downward slope for the actuation level of the second wire 80b from time tl to time t2. It is not essential for the increase of the first SMA wire 80a and the decrease of the second wire 80b to fully overlap each other in time. They may overlap in time only partially. For example, the peak of the pulse 31 applied to the first SMA wire 80a may be before or after the trough of the anti-pulse 32 applied to the second SMA wire 80b. By applying the anti-pulse 32 to the second SMA wire 80b, the difference in actuation levels of the SMA wires 80a, 80b is temporarily increased (compared to if the anti-pulse 32 were not applied). By increasing the difference in actuation level, the net tension of the SMA wires 80 may be temporarily increased. The net driving force on the second part 20 may be temporarily increased. By increasing the net tension or the net driving force on the second part 20, the static friction may be overcome more quickly. Similarly, the controller may optionally be configured to decrease the actuation level of the first SMA wire 80a and to increase the actuation level of the second SMA wire 80b simultaneously. Figure 6 is a schematic view of power profiles applied to SMA wires 80a, 80b of an SMA actuator assembly 1 according to an alternative power sequence. The SMA wires 80 may be arranged to adjust loading of a bearing arrangement that guides movement of the second part 20 relative to the first part 4. For example, the SMA wires 80 may, on actuation, reduced loading of the bearing arrangement. This is shown in, for example, Figure 3. Optionally, the controller is configured to increase the actuation level of the first SMA wire 80a and increase the actuation level of the second SMA wire 80b simultaneously during the first control period. Optionally, the controller is configured to increase the actuation level of the first SMA wire 80a and increase the actuation level of the second SMA wire 80b to substantially the same actuation level. Optionally, the controller is configured to cause the supply of power to the second SMA wire 80b during the first control period and during the second control period, wherein a time-averaged power which is supplied to the second SMA wire 80b during the first control period is higher than a time-averaged power which is supplied to the second SMA wire 80b during the second control period. Optionally, the controller is configured to control actuation of the SMA wires 80 to reduce the loading of the bearing arrangement while the second part 20 is at the first position xl. This is shown in the power profiles shown in Figure 6. As shown in Figure 6, optionally the power profile applied to the SMA wires 80 comprises an unloading pulse 33. The unloading pulse 33 may be applied to both the first SMA wire 80a and to the second SMA wire 80b. The unloading pulse 33 is a temporary increase of the actuation level of the SMA wires 80. The controller may be configured to increase the actuation level of the SMA wires 80. This is shown by the steep increase from time tl to time t2 in Figure 6. At time tl, the unloading pulse 33 is begun by the controller. The peak of the unloading pulse 33 is at time t2. Following time t2, the actuation level of the SMA wires 80 is decreased. Hence, the controller is configured to subsequently decrease the actuation level of the SMA wires 80 so as to drive movement of the second part 20 towards the second position x2. As shown in Figure 6, optionally the unloading pulse 33 is applied to both SMA wires 80 substantially simultaneously. In particular, the controller may be configured to increase the actuation level of the first SMA wire 80a and increase the actuation level of the second SMA wire 80b simultaneously so as to reduce the loading of the bearing arrangement. Figure 7 schematically shows the position of the second part 20 over time t when the power profiles shown in Figure 6 are applied. As shown in Figure 7, the second part 20 remains stationary at the first position xl until the peak of the unloading pulse 33 at time t2. The second part 20 is stationary while the SMA wires 80 start being actuated at time to and when the unloading pulse 33 begins at time tl. This is shown by the flat line 51 during which time to and time tl occur in Figure 7. As shown in Figure 7, the second part 20 begins to move to the second position x2 at time t2. This is shown by the upward slope 53 shown in Figure 7. By providing the unloading pulse 33, it is possible to pulse the right amount of power into the system to unload the bearings. Optionally, the controller is configured to apply the unloading pulse 33 to the SMA wires 80 such that the second part 20 is momentarily lifted off the bearing arrangement. During normal operation of the SMA actuator assembly 1, it may not be desirable to apply a pulse as strong as the unloading pulse 33 shown in Figure 6. This is because such a strong pulse may lead to a reduction of control of movement of the second part 20. However, as shown in Figure 7, optionally the unloading pulse 33 is applied while the second part 20 is stationary at the first position xl. This is shown by the times tl and t2 both occurring while the second part 20 remains at its original first position xl. As a result, the use of the strong unloading pulse 33 may not lead to uncontrolled movement of the second part 20. The second part 20 may be under the static friction regime relative to the first part 4 until the unloading pulse 33 begins at time tl. As shown in Figure 7, at time t2 the second part 20 may begin to move towards the second position x2. When the unloading pulse 33 begins at time tl, the second part 20 may enter into the dynamic friction regime relative to the first part 4. After the unloading pulse 33, normal operation of the second part 20 may be resumed so as to move the second part 20 to the target second position x2. As shown in Figure 6, optionally the controller is configured to increase the actuation level of both SMA wires 80 to substantially the same actuation level. By applying the same actuation level to both SMA wires 80, the unloading of the bearing arrangement may be more stable. The possibility of the second part 20 undergoing undesirable uncontrolled movement in either direction may be reduced. As shown in Figure 7, optionally the controller is configured to reduce the loading of the bearing arrangement such that the second part 20 remains at the first position xl during the reduction of the loading of the bearing arrangement. Once the unloading has been performed, the second part may subsequently be moved to the second position x2. Open and closed loop control As mentioned above, the controller may be configured to, during a first control period during which the second part 20 is stationary at the first position xl, set an actuation level of the first SMA wire 80a to a first value so as to start movement of the second part 20 from the first position xl, and to subsequently, during a second control period that begins after the second part 20 has started moving from the first position xl, set the actuation level to a second value lower than the first value so as to drive movement of the second part 20 towards the second position x2. Optionally, the controller is configured to carry out open-loop control during the first control period and to carry out closed-loop control during the second control period. Optionally, the pulse 31 shown in Figure 4 and / or the unloading pulse 33 shown in Figure 6 may be applied using open-loop control. In particular, the controller may be configured to increase the actuation level of the first SMA wire 80a based on an open-loop control. The pulse 31 may be predetermined. The form and amplitude of the pulse 31 may be predetermined. The SMA actuator assembly 1 may be calibrated such that the pulse 31 provides sufficient power to the SMA wires 80 so as to overcome the static friction. Optionally, following the pulse 31, the controller may be configured to drive movement of the second part 20 to the second position x2 using closed-loop control. The right amount of power may be pulsed into the SMA wires 80 to overcome static friction and then control may be immediately switched to closed-loop control to move the second part 20 in a smooth motion. Optionally, the second part 20 never stops moving until it reaches the second position x2 such that the second part does not revert back to the static friction regime relative to the first part 4. Optionally, the controller is configured to reduce the loading of the bearing arrangement based on open-loop control. The unloading pulse 33 may have a predetermined shape and / or amplitude. The SMA actuator assembly 1 may be calibrated such that the form and amplitude of the unloading pulse 33 is appropriate for unloading the bearing arrangement such that the second part 20 may be moved smoothly towards the second position x2 without undergoing uncontrolled motion. Optionally, the controller is configured to apply the anti-pulse 32 to the second SMA wire 80b using open-loop control. For example, the form and amplitude of the anti-pulse 32 may be predetermined. Move to intermediate position As mentioned above, it may be possible to reduce the stick-slip nature of the movement of the second part 20 relative to the first part 4. However, for some designs of SMA actuator assembly 1, the stick-slip phenomenon may occur. As a result of the stick-slip phenomenon, it may not be possible to move the second part 20 smoothly from the first position xl to the second position x2 when the distance xl2 from the first position xl to the second position x2 is too small. For example, when it is attempted to drive movement of the second part 20 from the first position xl to the second position x2, the jerky movement of the second part 20 may cause the second part 20 to undesirably overshoot the second position x2. There may be a minimum achievable step size due to the stick-slip phenomenon. The minimum achievable step size may be the smallest distance that the second part 20 may move in a controlled way from one position to another. Figure 8 schematically shows a possible sequence movement for moving the second part 20 from the first position xl to the second position x2. The distanced xl2 between the first position xl and the second position x2 may be small. The distance xl2 may be smaller than the minimum achievable step size for the second size 20 due to the stick-slip motion. As shown in Figure 8, optionally the controller is configured to actuate the SMA wires 80 to drive movement of the second part 20 to an intermediate position x3. The intermediate position x3 is a position to which the second part 20 is moved when the second part 20 is moved from the first position 25 xl to the second position x2. The second part 20 is at the intermediate position at a time intermediate between the time that the second part 20 is at the first position xl and the time that the second part 20 is at the second position x2. The intermediate position x3 is not necessarily intermediate in space between the first position xl and the second position x2. The controller may be configured to obtain a request to move the second part 20 from the first position xl at which the second part 20 is stationary to the second position x2. The controller may be configured to, in response to the request, determine whether the distance xl2 between the second position x2 and the first position xl is below a threshold distance. The threshold distance may be, for example, at least the minimum achievable step size for the second part 20. The controller may be configured to, upon determination that the distance xl2 between the second position x2 and the first position xl is below the threshold distance, actuate the SMA wires 80 to drive movement of the second part 20 from the first position xl to the second position x2 via the intermediate position x3 along a path that has a length greater than the threshold distance. The path is not necessarily a straight path. The path may comprise changes in direction of the second part 20. The intermediate position x3 is a greater distance from the second position x2 than the distance xl2 between the first position xl and the second position x2. This is shown in Figure 8, where the distance x23 between the second position x2 and the intermediate position x3 is greater than the distance xl2 between the first position xl and the second position x2. The distance x23 between the second position x2 and the intermediate position x3 may be greater than the minimum achievable step size for the second part 20. As shown in Figure 8, the controller may be configured to subsequently drive movement of the second part 20 to the second position x2. In particular, as shown in Figure 8, the controller may be configured to actuate the SMA wires 80 at a time t3 to drive movement of the second part 20 from the first position xl to the intermediate position x3. At a subsequent time t4, the controller may be configured to drive the SMA wires 80 so as to move the second part 20 from the intermediate position x3 to the second position x2. By driving the second part 20 to the intermediate position x3, the second part 20 may be moved from the first position xl to the second position x2 accurately, even when the distance between the first position xl and the second position x2 is very small, for example smaller than the minimum achievable step size for the second part 20. As shown in Figure 8, optionally the first position xl is between the second position x2 and the intermediate position x3. For example, the first position xl may be between the intermediate position x3 and the second position x2 along one axis. The controller may be configured to actuate the SMA wires 80 to drive movement of the second part 20 from the first position xl to the second position x2 via the intermediate position x3 by driving movement of the second part 20 relative to the first part 4 in a first direction from the first position xl to the intermediate position x3 and subsequently in a second direction, opposite to the first direction, from the intermediate position x3 to the second position x2. For example, the controller may be configured to drive movement of the second part 20 in the opposite direction away from the second position x2, before subsequently driving the second part 20 to the second position x2. By applying a reverse movement to the second part 20, a minimum overall amount of movement of the second part 20 may be implemented while moving the second part 20 accurately to the second position x2. For example, the reverse movement may substantially correspond to the minimum achievable step size for the second part 20 moving relative to the first part 4. The subsequent movement from the intermediate position x3 to the second position x2 is a distance greater than the minimum achievable step size such that the movement may be performed relatively accurately. Optionally, the second part 20 is configured to move relative to the first part 4 in one degree of freedom. For example, the second part 20 may be movable relative to the first part 4 along two different axes. The first position xl may be between the second position x2 and the intermediate position x3 along one axis of potential movement between the second part 20 and the first part 4. Alternatively, the first position xl and second position x2 may be separated along a first direction and the intermediate position x3 may be separated from the first position xl in a second direction, perpendicular to the first direction. For example, the controller may be configured to actuate the SMA wires 80 such that the second part 20 moves along a curved path from the first position xl to the second position x2. The intermediate position x3 may be located on the curved path. The curved path is longer than a straight line from the first position xl to the second position x2. It is not essential for the first position xl to be between the second position x2 and the intermediate position x3. Figure 9 schematically shows an alternative movement sequence in which the second position x2 is between the first position xl and the intermediate position x3. As shown in Figure 9, optionally the controller is configured to drive movement of the second part 20 from the first position xl beyond the second position x2. The controller is configured to intentionally overshoot the second position x2. The controller is configured to subsequently reverse the direction of movement of the second part 20 such that the second part 20 is driven from the intermediate position x3 to the desired second position x2. Paragraph relating to SMA wire The above-described SMA actuator assemblies comprise an SMA wire. The term 'shape memory alloy (SMA) wire' may refer to any element comprising SMA. The SMA wire may have any shape that is suitable for the purposes described herein. The SMA wire 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 wire. It is also possible that the length of the SMA wire (however defined) may be similar to one or more of its other dimensions. The SMA wire may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two elements, the SMA wire can apply only a tensile force which urges the two elements together. In other examples, the SMA wire may be bent around an element and can apply a force to the element as the SMA wire tends to straighten under tension. The SMA wire may be beam-like or rigid and may be able to apply different (e.g. non-tensile) forces to elements. The SMA wire may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA wire may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA wire' may refer to any configuration of SMA wire acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA wire may comprise two or more portions of SMA wire that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA wire may be part of a larger piece of SMA wire. Such a larger piece of SMA wire might comprise two or more parts that are individually controllable, thereby forming two or more SMA wires. Paragraph introducing other variations It will be appreciated that there may be many other variations of the above-described examples. For example, in the power profiles shown in the drawings, two traces are shown corresponding to the power profiles applied to two SMA wires 30. However, the SMA actuator assembly 1 may comprise more than two SMA wires 80. For example, there may be two pairs of SMA wires 80. In the power profiles shown, the actuation levels of the SMA wires 80 are adjusted substantially linearly over time. However, it is not essential for the actuation level to be controlled linearly over time. In an alternative arrangement, the increases and / or decreases may have a polynomial shape or may comprise one or more step changes. It will be appreciated that the above-described example may be implemented as methods and / or as computer programs. For example, each example may be embodied as a method of controlling an SMA actuator assembly 1. The method may be a method performed by the controller. Each example may be embodied as a computer program. The computer program may comprise instructions for instructing the controller to perform a method of controlling an SMA actuator assembly 1. The computer program may be implemented by the controller.

Claims

1. An actuator assembly comprising:a first part;a second part that is movable relative to the first part;an actuator component arranged, on actuation, to drive movement of the second part relative to the first part; anda controller configured to control actuation of the at least one actuator component to drive the movement of the second part, wherein the controller is configured to:obtain a request to move the second part from a first position at which the second part is stationary to a second position,in response to the request:during a first control period during which the second part is stationary at the first position, set an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; andsubsequently, during a second control period that begins after the second part has started moving from the first position, set the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position.

2. An actuator assembly of claim 1, wherein the controller is configured to carry out open-loop control during the first control period and to carry out closed-loop control during the second control period.

3. An actuator assembly of any preceding claim, wherein the controller is configured to supply power to the at least one actuator component and wherein a time-averaged power which is supplied to the at least one actuator component during the first control period is higher than a time-averaged power which is supplied to the at least one actuator component during the second control period.

4. An actuator assembly of any preceding claim, wherein the controller is configured to increase and then subsequently decrease the actuation level of the at least one actuator component during the first control period.

5. An actuator assembly of any preceding claim, wherein the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a staticfrictional force that constrains the movement of the second part relative to the first part when the at least one actuator component is not actuated.

6. An actuator assembly of claim 5, wherein the at least one actuator component is arranged such that the normal force remains substantially constant on actuation of the at least one actuator component.

7. An actuator assembly of claim 5 wherein the at least one actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface.

8. An actuator assembly of any preceding claim, wherein the at least one actuator component comprises a first actuator component and a second actuator component which are arranged, on actuation, to drive the second part relative to the first part in opposite directions.

9. An actuator assembly of claim 8, wherein the controller is configured cause the supply of power to the second actuator component during the first control period and during the second control period, wherein a time-averaged power supplied to the second actuator component is lower during the first control period than during the second control period.

10. An actuator assembly of claim 8 or claim 9, wherein the controller is configured to decrease and then subsequently increase the actuation level of second actuator component during the first control period.

11. An actuator assembly of claim 10 when dependent on claim 4, wherein the controller is configured to, during the first control period, increase the actuation level of the first actuator component and decrease the actuation level of the second actuator component simultaneously.

12. An actuator assembly according to claim 8 when dependent on claim 7, wherein the second actuator component is arranged to apply a force to the second part with a component orthogonal to the first surface that reduces the normal force and with a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface.

13. An actuator assembly of claim 12, wherein the controller is configured to increase the actuation level of the first actuator component and increase the actuation level of the second actuator componentsimultaneously during the first control period.

14. An actuator assembly of claim 13, wherein the controller is configured to increase the actuation level of the first actuator component and increase the actuation level of the second actuator component to substantially the same actuation level.

15. An actuator assembly of claim 8, wherein the controller is configured to cause the supply of power to the second actuator component during the first control period and during the second control period, wherein a time-averaged power which is supplied to the second actuator component during the first control period is higher than a time-averaged power which is supplied to the second actuator component during the second control period.

16. An actuator assembly comprising:a first part;a second part configured to move relative to the first part;two actuator components arranged, on actuation, to drive movement of the second part relative to the first part; anda controller configured to control actuation of the actuator components to drive movement of the second part, the controller configured to:obtain a request to move the second part from a first position at which the second part is stationary to a second position,in response to the request:determine whether the distance between the second position and the first position is below a threshold distance; andupon determination that the distance between the second position and the first position is below the threshold distance:actuate the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance.

17. An actuator assembly of claim 16, wherein the controller is configured to actuate the actuator components to drive movement of the second part from the first position to the second position via the intermediate position by driving movement of the second part relative to the first part in a first direction from the first position to the intermediate position and subsequently in a second direction, opposite to the first direction, from the intermediate position to the second position.

18. An actuator assembly of claim 17, wherein the first position and second position are separated along a first direction and wherein the intermediate position is separated from the first position in a second direction, perpendicular to the first direction.

19. An actuator assembly of claim 16 or 17, wherein the first position is between the intermediate position and the second position along one axis.

20. An actuator assembly of any of claims 16 to 19, wherein the second part is movable relative to the first part across a first surface and wherein the actuator assembly is arranged such that the second part and the first surface are biased against each other with a normal force, thereby generating a static frictional force that constrains the movement of the second part relative to the first part when the two actuator components are not actuated.

21. An actuator assembly of claim 20, wherein the first and second actuator components are arranged such that the normal force remains substantially constant on actuation of the first and second actuator components.

22. An actuator assembly of claim 20 wherein the first and second actuator components are each arranged to apply a respective force to the second part, each respective force having a component orthogonal to the first surface that reduces the normal force and a component parallel to the first surface so as to drive movement of the second part relative to the first part across the first surface.

23. An actuator assembly according to any preceding claim, wherein each actuator component comprises a shape memory alloy, SMA, element.

24. An actuator assembly according to any preceding claim, wherein the second part is a lens element comprising at least one lens.

25. An actuator assembly according to claim 24, wherein the first part has an image sensor mounted thereon, the lens element being arranged to focus an image on the image sensor.

26. An actuator assembly according to claim 24, wherein the first part has a display mounted thereon, the lens element being arranged to focus light emitted by the display.

27. An actuator assembly according to any preceding claim, wherein the second part has an image sensor mounted thereon.

28. An actuator assembly according to claim 27, wherein the first part has a lens element comprising at least one lens, the lens element being arranged to focus an image on the image sensor.

29. A method of controlling an actuator assembly comprising a first part, a second part and an actuator component, the method comprising:obtaining a request to move the second part from a first position at which the second part is stationary to a second position;in response to the request: during a first control period during which the second part is stationary at the first position, setting an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, setting the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position.

30. A computer program product comprising instructions for instructing a controller to perform a method of controlling an actuator assembly comprising a first part, a second part and an actuator component, the method comprising:obtaining a request to move the second part from a first position at which the second part is stationary to a second position;in response to the request: during a first control period during which the second part is stationary at the first position, setting an actuation level of the actuator component to a first value so as to start movement of the second part from the first position; and subsequently, during a second control period that begins after the second part has started moving from the first position, setting the actuation level to a second value lower than the first value so as to drive movement of the second part towards the second position.

31. A method of controlling an actuator assembly comprising a first part, a second part and two actuator components, the method comprising:obtaining a request to move the second part from a first position at which the second part is stationary to a second position;in response to the request: determining whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuating the actuatorcomponents to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance.

32. A computer program product comprising instructions for instructing a controller to perform method of controlling an actuator assembly comprising a first part, a second part and two actuator components, the method comprising:obtaining a request to move the second part from a first position at which the second part is stationary to a second position;in response to the request: determining whether the distance between the second position and the first position is below a threshold distance; and upon determination that the distance between the second position and the first position is below the threshold distance: actuating the actuator components to drive movement of the second part from the first position to the second position via an intermediate position along a path that has a length greater than the threshold distance.

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