Patent Description:
Typical SMA actuator assemblies require power to be applied to the SMA actuator wire to retain given positions within a range of movement. This is a problem in some applications where only occasional actuation is required.

According to the present techniques, there is provided a shape memory alloy actuator assembly comprising: a first part, including a surface; a second part arranged to move relative to the first part across the surface; a resilient biasing element arranged to bias the second part into contact with the surface so as to generate frictional forces therebetween for retaining the second part in position on the surface; and at least one shape memory alloy actuator wire connected between the first part and the second part to the second part and arranged to, on contraction thereof, apply a force to the second part with a component orthogonal to the surface that reduces said frictional forces and with a component parallel to the surface so as to drive movement of the second part relative to the first part across the surface.

In this actuator assembly, the resilient biasing element biases the second part into contact with the surface of the first part, thereby generating frictional forces therebetween. The frictional forces are sufficient to retain the second part in position on the surface when no drive signal is applied to the at least one SMA actuator wire is not powered. However, when a drive signal is applied to the at least one SMA actuator wire causing contraction thereof, the applies a force to the second part with a component orthogonal to the surface that reduces said frictional forces and with a component parallel to the surface that drives movement of the second part relative to the first part across the surface. By applying a force with a component orthogonal to the surface, the at least one SMA actuator wire reduces the reaction between the second element and the surface which reduces the frictional forces. This reduction of the frictional forces assists with the overcoming of the frictional forces by the component of force applied by the at least one SMA actuator wire parallel to the surface. Thus, the actuator assembly tends to retain its position when no power is applied, but is capable of movement when power is applied. This is advantageous in applications where it is desired to minimise power consumption while allowing the second part to be retained in a predetermined position.

Advantageously, the at least one SMA actuator wire may comprise at least two opposed SMA actuator wires arranged to, on contraction thereof, apply forces to the second part with respective components orthogonal to the surface that reduce said frictional forces and with respective components parallel to the surface in opposite directions. In such an opposed arrangement, the two opposed SMA actuator wires bias each other, allowing the SMA actuator wires to expand when they cool. However, this is not essential and a single SMA actuator wire may be provided in which case biasing of the SMA actuator wire may be provided by the resilient element that biases the second part into contact with the surface or by an additional biasing element.

The at least one SMA actuator wire may be arranged on contraction thereof to apply a force to the second part with a component orthogonal to the surface that lifts the second part out of contact with the surface. This has the effect of reducing the frictional forces to zero on application of a drive signal to the SMA actuator.

Optionally, the actuator assembly may further comprise a bearing arrangement arranged to guide movement of the second part relative to the first part along a movement axis across the surface. For example, the bearing arrangement may comprise a pair of rolling or sliding bearings. Use of a bearing arrangement has the advantage of increasing the control over movement direction of the second part.

Where a bearing arrangement is provided, the at least one SMA may be inclined relative to the movement axis, as viewed orthogonally to the surface, at an acute angle of greater than <NUM>° so as to, on contraction thereof, apply a force to the second part with a component orthogonal to the surface that reduces said frictional forces and with a component parallel to the surface along the movement axis. Such inclination of the at least one SMA actuator wire advantageously increases the gearing between the travel of the second part and the strain of the SMA actuator wire.

In this case, at least two opposed shape memory alloy actuator wires may be arranged to, on contraction thereof, apply forces to the second part with respective components orthogonal to the surface that reduce said frictional forces and with respective components parallel to the surface and along the movement axis in opposite directions.

In this case, the two opposed shape memory alloy actuator wires may be arranged to, on contraction thereof, apply forces to the second part which apply a first couple to the second element around an axis normal to the surface, and the bearing arrangement may be arranged to apply a second couple to the second part around an axis normal to the surface in an opposite sense from the first couple. In this manner, the first couple applied by the SMA actuator wires may be used to reduce play which may otherwise be present in the bearing arrangement, thereby increasing the control over the movement direction of the second part.

As an alternative to providing a bearing arrangement, the second part may be arranged to move relative to the first part in two dimensions across the surface. In that case, there may be provided at least two SMA actuator wires arranged to, on selective contraction thereof, apply forces to the second part with respective components orthogonal to the surface that reduce said frictional forces and with components parallel to the surface in two dimensions. In that manner, the actuator assembly is capable of driving movement of the second part relative to the first part in two dimensions across the surface.

The SMA actuator assembly may be applied to any type of device that comprises a first part and a second part which is movable with respect to the first part. By way of non-limitative example, the actuator assembly may be, or may be provided in, any one of the following devices: a smartphone, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronics device, an image capture device, a 3D sensing device or system, a servomotor, a consumer electronic device, a mobile computing device, a mobile electronic device, a laptop, a tablet computing device, an e-reader (also known as an e-book reader or e-book device), a computing accessory or computing peripheral device (e.g. mouse, keyboard, headphones, earphones, earbuds, etc.), a security system, a gaming system, a gaming accessory (e.g. controller, headset, a wearable controller, etc.), an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device (e.g. a watch, a smartwatch, a fitness tracker, etc.), a drone (aerial, water, underwater, etc.), an aircraft, a spacecraft, a submersible vessel, a vehicle, and an autonomous vehicle. It will be understood that this is a non-exhaustive list of example devices.

Actuator assemblies as described herein may be used in devices/systems suitable for image capture, 3D sensing, depth mapping, aerial surveying, terrestrial surveying, surveying in or from space, hydrographic surveying, underwater surveying, scene detection, collision warning, security, facial recognition, augmented and/or virtual reality, advanced driver-assistance systems in vehicles, autonomous vehicles, gaming, gesture control/recognition, robotic devices, robotic device control, touchless technology, home automation, medical devices, and haptics.

To allow better understanding, an embodiment of the present techniques will now be described by way of non-limitative example with reference to the accompanying drawings, in which:.

Except where the context requires otherwise, the term "bearing" is used herein as follows. The term "bearing" is used herein to encompass the terms "sliding bearing", "plain bearing", "rolling bearing", "ball bearing", "roller bearing" and "flexure". The term "bearing" is used herein to generally mean any element or combination of elements that functions to constrain motion to only the desired motion and reduce friction between moving parts. The term "sliding bearing" is used to mean a bearing in which a bearing element slides on a bearing surface, and includes a "plain bearing". The term "rolling bearing" is used to mean a bearing in which a rolling bearing element, for example a ball or roller, rolls on a bearing surface. In embodiments, the bearing may be provided on, or may comprise, non-linear bearing surfaces.

In some embodiments of the present techniques, more than one type of bearing element may be used in combination to provide the bearing functionality. Accordingly, the term "bearing" used herein includes any combination of, for example, plain bearings, ball bearings, roller bearings and flexures.

<FIG> illustrate a first actuator assembly <NUM> which is an actuator assembly that is arranged as follows.

The first actuator assembly <NUM> includes a static part <NUM> and a movable part <NUM> which are the first part and the second part, respectively, in this example. The movable part <NUM> is movable with respect to the static part <NUM> and the terms "static" and "movable" are used for clarity of description, but are somewhat arbitrarily applied to the two parts as the movement to which they refer is a relative movement between the static part <NUM> and the movable part <NUM>.

The static part <NUM> includes a body <NUM> and a surface <NUM> held in a fixed position with respect to the body <NUM>, for example by a connecting portion (not shown). A gap is provided between the body <NUM> and the surface <NUM>.

The movable part <NUM> is located in the gap between the body <NUM> and the surface <NUM>. The movable part <NUM> is capable of movement relative to the static part <NUM> across the surface <NUM>. In this example, the movable part <NUM> is capable of movement across the surface <NUM> in any direction in two dimensions.

The movable part <NUM> may be any type of element, including any of the examples listed above.

The first actuator assembly <NUM> also includes a spring <NUM>, which is a coil spring, connected between the static part <NUM> and the movable part <NUM> by being connected at one end to the body <NUM> and at the other end to the movable part <NUM>. The spring <NUM> extends orthogonally to the surface <NUM> in this example, although that is not essential. The spring <NUM> is held in compression, and is therefore a resilient biasing element that acts as a biasing arrangement biasing the movable part <NUM> into contact with the surface <NUM>. This generates a reaction between the movable part <NUM> and the surface <NUM>, as well as generating frictional forces between the movable part <NUM> and the surface <NUM>.

The first actuator assembly <NUM> also includes two SMA actuator wires <NUM> arranged as follows. Each SMA actuator wire <NUM> is connected at one end to the body <NUM> and at the other end to the movable part <NUM>. Each SMA actuator wire <NUM> is inclined at an acute angle α of greater than <NUM>° with respect to the surface <NUM> so as to apply a force, on contraction of the SMA actuator wire <NUM>, with a component normal to the surface <NUM> that biases the movable part <NUM> away from the surface <NUM> and with a component parallel to the surface <NUM>.

As shown in <FIG>, which illustrates the first actuator assembly <NUM> with the body <NUM> omitted for clarity, the SMA actuator wires <NUM> are arranged in a common plane normal to the surface <NUM>, each SMA actuator wire <NUM> being inclined in opposite senses so that they are opposed so that the components of force applied thereby parallel to the surface <NUM> are in opposite directions.

The SMA actuator wires <NUM> are each connected to a control circuit <NUM> which may be implemented in an integrated circuit chip. The control circuit <NUM> in use applies drive signals to the SMA actuator wires <NUM> which resistively heat the SMA actuator wires <NUM> causing them to contract. The plural SMA actuator wires <NUM> may be driven independently or otherwise. The control circuit may also measure the resistance of the SMA actuator wires <NUM>, and use the measured resistance to calculate/determine the position of the movable part <NUM>.

In the absence of drive signals being applied, the SMA actuator wires <NUM> do not contract, and so the spring <NUM> biases the movable part <NUM> onto the surface <NUM> generating frictional forces that are sufficient to retain the movable part <NUM> on the surface <NUM>. In this state, the movable element <NUM> is retained in position with zero power consumption by the first actuator assembly <NUM>, so the first actuator assembly <NUM> may be referred to as a zero power hold actuator assembly, as may the other actuator assemblies disclosed herein.

When drive signals are applied, the SMA actuator wires <NUM> are heated, causing them to contract and thereby applying force to the movable part <NUM>.

In a direction orthogonal to the surface <NUM>, the spring <NUM> acts as a resilient biasing element against contraction of the SMA actuator wires <NUM> that occurs in common. Thus, the component of force applied by the SMA actuator wires <NUM> to the movable part <NUM> orthogonal to the surface <NUM> that biases the movable part <NUM> away from the surface <NUM> reduces the reaction between the movable part <NUM> and the surface <NUM>, thereby also reducing the frictional forces between the movable part <NUM> and the surface <NUM>. Optionally, the SMA actuator wires <NUM> may lift the movable part <NUM> out of contact with the surface <NUM>, thereby reducing the frictional forces between the movable part <NUM> and the surface <NUM> to zero.

In a direction parallel to the surface <NUM>, the respective components of force applied by the SMA actuator wires <NUM> to the movable part <NUM> parallel to the surface <NUM> are in opposite directions. The net component of force applied to the movable part <NUM> parallel to the surface <NUM> drives movement of the movable part <NUM> relative to the static part <NUM> across the surface <NUM> when that net component of force is greater than the frictional forces. In use, the drive signals are selected to move the movable part <NUM> to a desired position relative to the static part <NUM> that is controlled by the power of the drive signals.

In use, the SMA actuator wires <NUM> are driven differentially by drive signals that generate differential contraction, as between the SMA actuator wires <NUM>, to provide such net component of force that is overcomes the frictional forces. The drive signals are selected to move the movable part <NUM> to a desired and controlled position relative to the static part <NUM>. In this example, there are no constraints on the movement of the movable part <NUM> across the surface <NUM> and so the movement axis M along which movement occurs is in the common plane in which the SMA actuator wires <NUM> are arranged.

When the drive signals cease to be applied, the frictional forces generated as a result of the biasing by the spring <NUM> again retain the movable part <NUM> on the surface <NUM>, which may be a position that is different compared to that before application of the drive signals. In this state, the movable element <NUM> is once again retained in position with zero power consumption by the first actuator assembly <NUM>.

Achievement of a net component of force that overcomes the frictional forces is assisted by the frictional forces also being reduced by the contraction of the SMA actuator wires <NUM> as described above. However, that is not essential. The SMA actuator wires <NUM> could alternatively apply a force the movable part <NUM> with no component normal to the surface <NUM>. In that case, a greater force would be required to overcome the frictional forces, which is practical but less desirable.

While the example described above includes two SMA actuator wires <NUM> which are opposed, as an alternative a single SMA actuator wire <NUM> could be provided. In that case, the single SMA actuator wire <NUM> may be resiliently biased against the component of contraction of the SMA actuator wires <NUM> parallel to the surface <NUM> either by the spring <NUM> or by an additional resilient biasing element connected between the static part <NUM> and the movable part <NUM>.

Further actuator assemblies will now be described. The further actuator assemblies are modified as compared to the first actuator assembly <NUM>. The modifications will be described. Apart from the modifications the further actuator assemblies, and in particular the commonly named elements, have the same construction as described above, which is not repeated, for brevity. It is also noted that the different modifications of the further actuator assemblies may be combined in any combination.

<FIG> illustrate a first modified form of the actuator assembly <NUM> which is an actuator assembly that is modified by the inclusion of a bearing arrangement <NUM> that guides movement of the movable part <NUM> relative to the static part <NUM> along a movement axis M across the surface <NUM>. The bearing arrangement comprises a pair of bearings <NUM> arranged on opposite sides of the movable part <NUM>. The bearings <NUM> are rolling bearings or sliding bearings.

<FIG> illustrates an example in which the bearing <NUM> is a rolling bearing. In this case, the bearing <NUM> comprises a pair of bearing surfaces <NUM> and <NUM>, provided on the static part <NUM> and the movable part <NUM>, respectively, and a rolling bearing element <NUM> disposed therebetween. The rolling bearing element <NUM> rolls on the bearing surfaces <NUM> and <NUM> so as to permit relative movement of the movable part <NUM> with respect to the static part <NUM> across the surface <NUM> along the movement axis M, while constraining such relative movement along a constraint axis C orthogonal to the movement axis M. The rolling bearing element <NUM> may be formed by any suitable element, for example a ball or roller.

<FIG> illustrates an example in which the bearing <NUM> is a sliding bearing. In this case, the bearing <NUM> is a plain bearing that comprises an elongate bearing surface <NUM> provided on either one of static part <NUM> and the movable part <NUM> and a protrusion <NUM> formed on the other of the static part <NUM> and the movable part <NUM>. The protrusion <NUM> has a bearing surface <NUM> on its end, which bears on the elongate bearing surface <NUM>. Although one protrusion <NUM> is shown in the example of <FIG>, in general any number of one or more protrusions <NUM> may be provided. The elongate bearing surface <NUM> and the bearing surface <NUM> are conformal, so as to permit relative movement of the movable part <NUM> with respect to the static part <NUM> across the surface <NUM> along the movement axis M, while constraining such relative movement along constraint axis C orthogonal to the movement axis M. The elongate bearing surface <NUM> and the bearing surface <NUM> desirably have a coefficient of friction of <NUM> or less.

<FIG> illustrates another illustrates an example in which the bearing <NUM> is a sliding bearing. In this case, the bearing <NUM> comprises an elongate slot <NUM> provided on either one of static part <NUM> and the movable part <NUM>, the slot <NUM> having opposed, internal bearing surfaces <NUM> formed on its internal surfaces. The bearing <NUM> further comprises a flange <NUM> provided on the other of the static part <NUM> and the movable part <NUM>. The flange <NUM> fits within the slot <NUM> and has opposed, external bearing surfaces <NUM> which bear on the internal bearing surfaces <NUM> of the slot <NUM>. The internal bearing surfaces <NUM> of the slot <NUM> and the external bearing surfaces <NUM> of the flange <NUM> are conformal, so as to permit relative movement of the movable part <NUM> with respect to the static part <NUM> across the surface <NUM> along the movement axis M, while constraining such relative movement along constraint axis C orthogonal to the movement axis M. The elongate bearing surface <NUM> and the bearing surface <NUM> desirably have a coefficient of friction of <NUM> or less.

The example of the bearing shown in <FIG> may restrict additional degrees of freedom of the motion of the movable part <NUM> with respect to the static part <NUM>, compared to the other examples,
In each of the examples of the bearing <NUM> shown in <FIG>, the materials of the various bearing surfaces <NUM>, <NUM>, <NUM> and <NUM> are chosen to provide smooth movement and a long life. The bearing surfaces <NUM>, <NUM>, <NUM> and <NUM> may be unitary with the underlying component or may be formed by a surface coating. Suitable materials include, for example PTFE or other polymeric bearing materials, or metal. A lubricant may be provided on the bearing surfaces <NUM>, <NUM>, <NUM> and <NUM>. Such a lubricant may be a powder or a fluid, for example. A suitable lubricant is a low viscosity oil.

As shown in <FIG>, the common plane normal to the surface <NUM> in which the SMA actuator wires <NUM> are arranged is at an acute angle β of greater than <NUM>° relative to the direction of motion. Thus, the SMA actuator wires <NUM> are inclined relative to the movement axis M, as viewed orthogonally to the surface <NUM>, at an acute angle β of greater than <NUM>°. As a result, the SMA actuator wires <NUM>, on contraction, each apply a force to the movable part <NUM> with a component along the movement axis M and a component along the constraint axis C. As in the first actuator assembly <NUM>, the components of force applied to the movable part <NUM> parallel to the surface <NUM> and along the movement axis M by each SMA actuator wire <NUM> are in opposite directions. Thus, the first modified form of the actuator assembly <NUM> operates in the same manner as the actuator assembly <NUM> shown in <FIG>, except that the inclination of the SMA actuator wires <NUM> increases the gearing between the travel of the movable part <NUM> and the strain of the SMA actuator wires <NUM> which is advantageous.

As the first modified form of the actuator assembly <NUM> employs two bearings <NUM>, the tolerances of such a bearing assembly <NUM> means that there might be play between the bearings <NUM> and the movable part <NUM>. This means that the movable part <NUM> might have some residual motion along the constraint axis C as well as in the movement axis M. Some ways of reducing this problem by arranging the SMA actuator wires <NUM> to load the bearings <NUM> are as follows.

A first way to reduce the play between the bearings <NUM> and the movable part <NUM> is to modify the actuator assembly so that the SMA actuator wires <NUM> and the bearings <NUM> apply couples in opposite senses to the movable part <NUM>.

<FIG> illustrates a second modified form of the actuator assembly <NUM> which is an actuator assembly that is modified compared to the first modified form of the actuator assembly <NUM> shown in <FIG> to implement this first way of reducing play between the bearings <NUM> and the movable part <NUM>, as follows.

In the second modified form of the actuator assembly <NUM> shown in <FIG>, the two SMA actuator wires <NUM> are connected to the movable part <NUM> at shifted positions so that the two SMA actuator wires <NUM> are no longer in a common plane, although they each remain within parallel planes that are normal to the surface <NUM>. As a result, the SMA actuator wires <NUM>, on contraction, apply forces to the movable part <NUM> which apply a first couple to the movable element around an axis normal to the surface <NUM>, being anti-clockwise in the example of <FIG>.

In addition, the bearings <NUM> are shifted in opposite directions along the movement axis, so that they apply a second couple to the movable part <NUM> around an axis normal to the surface <NUM>, but in an opposite sense from the first couple, being clockwise in the example of <FIG>. The second couple balances the first couple. Thus, the first couple generated by the two SMA actuator wires <NUM> biases the movable part <NUM> against each of the bearings <NUM> thereby loading the bearings <NUM> and reducing play.

In the second modified form of the actuator assembly <NUM> shown in <FIG>, the SMA actuator wires are connected to the movable part <NUM> at positions along the movement axis M inside the bearings <NUM>. Thus, while the couple applied by the two SMA actuator wires <NUM> when driven together is balanced by the bearings <NUM>, a single one of the SMA actuator wires <NUM> should not be driven alone or else it will apply a couple around the adjacent bearing <NUM> that tends to bias the movable part <NUM> off the other bearing <NUM>. There will now be described third and fourth modified forms of the actuator assembly <NUM> which prevent this while reducing play in a similar manner to the second modified form of the actuator assembly <NUM>.

<FIG> illustrates a third modified form of the actuator assembly <NUM> which is similar to the second modified form shown in <FIG>, except that the SMA actuator wires <NUM> are connected to the movable part <NUM> at positions along the movement axis M where the line of the force applied by each SMA actuator wire <NUM> to the movable part <NUM> is outside the bearings <NUM>. Thus, the torque applied by each SMA actuator wire <NUM> about the adjacent bearing <NUM> is resisted by the other bearing <NUM>. This provides balancing couples from the SMA actuator wires <NUM> and the bearings <NUM> causing the SMA actuator wires <NUM> to load the bearings <NUM> in a similar manner to the second modified form of the actuator assembly <NUM>, while also allowing each SMA actuator wire <NUM> to be driven in isolation.

<FIG> illustrates a fourth modified form of the actuator assembly <NUM> which is similar to the second modified form shown in <FIG>, except that two bearings <NUM> are provided on each side of the movable part <NUM> (although the two bearings <NUM> could be replaced by one long bearing). Thus, the line of the force applied by each SMA actuator wire <NUM> to the movable part <NUM> is between the extremities of the bearings <NUM> onto which that force is applied, so the SMA actuator wires <NUM> load the bearings <NUM> which resist rotation of the movable part <NUM>.

A second way to reduce play between the bearings <NUM> and the movable part <NUM> is modify the bearing arrangement <NUM> to be formed by one or more bearings <NUM> on the same side of the movable part <NUM> and angle the SMA actuator wires <NUM> so that they provide a component of force that biases the movable part <NUM> against the one or more bearings <NUM>, as well as providing the components of force discussed above.

<FIG> illustrates a fifth modified form of the actuator assembly <NUM> which is an actuator assembly that is modified compared to the first modified form of the actuator assembly <NUM> shown in <FIG> to implement this second way of reducing play between the bearings <NUM> and the movable part <NUM>, as follows.

In this case, two bearings <NUM> are provided on the same side of the movable part <NUM> (although the two bearings <NUM> could be replaced by one long bearing).

In addition, each SMA actuator wire <NUM> is inclined relative to the movement axis M, as viewed orthogonally to the surface <NUM>, at an acute angle β of greater than <NUM>° on the same side of the movable part <NUM> as the bearings <NUM>. As a result, the components of force applied by each by each SMA actuator wire <NUM>, on contraction, along the constraint axis load the bearings <NUM>.

A third way to reduce play between the bearings <NUM> and the movable part <NUM> is to modify the bearing arrangement <NUM> to be formed by a single bearing <NUM> and angle the spring <NUM> so that it is not coplanar with the SMA actuator wires <NUM>, so that it biases the movable part <NUM> against the single bearing <NUM>, as well as providing the force normal to the surface <NUM>.

<FIG> illustrates a sixth modified form of the actuator assembly <NUM> which is an actuator assembly that is modified compared to the actuator assembly <NUM> shown in <FIG> as follows. In the sixth modified form of the actuator assembly <NUM> shown in <FIG>, the two SMA actuator wires <NUM> are disposed at one end of the movable part <NUM>. As in the actuator assembly <NUM> shown in <FIG>, the SMA actuator wires <NUM> are inclined relative to the movement axis M, as viewed orthogonally to the surface <NUM>, at an acute angle β of greater than <NUM>°, so that the SMA actuator wires <NUM>, on contraction, each apply a force to the movable part <NUM> with a component along the movement axis M and a component along the constraint axis C, but with the components of force applied to the movable part <NUM> parallel to the surface <NUM> and along the movement axis by each SMA actuator wire <NUM> are in opposite directions. To achieve this while providing the SMA actuator wires <NUM> at the same end, the movable part <NUM> includes an extension <NUM> to which one of the SMA actuator wires <NUM> is connected.

<FIG> illustrates a seventh modified form of the actuator assembly <NUM> which is an actuator assembly that is modified compared to the actuator assembly <NUM> shown in <FIG> as follows. In the seventh modified form of the actuator assembly <NUM> shown in <FIG>, the two opposed SMA actuator wires <NUM> are crossed as viewed parallel to the surface <NUM>. In this manner, space is conserved and the overall size of the seventh modified form of the actuator assembly <NUM> is reduced. The SMA actuator wires <NUM>.

Whereas the above examples include two opposed SMA actuator wires <NUM>, alternatively, the number of SMA actuator wires <NUM> may be increased to increase the force applied thereby. In general, any number of SMA wires <NUM> may be used.

For example, <FIG> show eighth to tenth modified forms of the actuator assembly <NUM> which are each modified compared to <FIG>, by replacing each SMA actuator wire <NUM> by a pair of SMA actuator wires <NUM>.

In the eighth modified form of <FIG>, the SMA actuator wires <NUM> of each pair are attached to the same point on the movable part <NUM> and are each inclined with respect to the movement axis M.

In the ninth modified form of <FIG>, the SMA actuator wires <NUM> of each pair are attached to adjacent corners of the movable part <NUM>, and are each inclined with respect to the movement axis M so that they cross one another as viewed from above.

In the tenth modified form of <FIG>, the SMA actuator wires <NUM> of each pair extend parallel to each other and to the movement axis M.

In the examples of <FIG>, each pair of SMA actuator wires <NUM> may be driven in common by common drive signals, and the two pairs of SMA actuator wires <NUM> may be driven differentially by drive signals that generate differential contraction, as between the pairs of SMA actuator wires <NUM>, to provide the same function as the examples described above.

<FIG> illustrate an eleventh to fourteenth modified forms of the actuator assembly <NUM> which are modified compared to the actuator assembly <NUM> shown in <FIG> as follows.

In each of the eleventh to fourteenth modified forms of the actuator assembly <NUM>, no bearing assembly is provided so the movable part <NUM> is free to move relative to the static part <NUM> in two dimensions across the surface <NUM>. However, in contrast to the first actuator assembly <NUM> where the SMA actuator wires <NUM> are arranged in a common plane and so drive movement along a single movement axis, the SMA actuator wires <NUM> are modified to apply components of force parallel to the surface <NUM> in two dimensions, so as to be capable of driving movement of the movable part relative to the static part <NUM> in two dimensions across the surface, on application of drive signals which selectively contract the SMA actuator wires <NUM>. <FIG> illustrate two examples of configurations of SMA actuator wires <NUM> which achieve this, but in general the SMA actuator wires <NUM> may have any configuration that is capable of applying components of force parallel to the surface <NUM> in two dimensions.

In the eleventh modified form of the actuator assembly <NUM> shown in <FIG>, three SMA actuator wires <NUM> are provided which are equally angularly spaced around the normal to the surface <NUM>. As a result, the three SMA actuator wires <NUM> are opposed in two dimensions, and are driven by drive signals that generate differential contraction in two orthogonal directions. Thus, the three SMA actuator wires <NUM> are capable of applying a net component of force parallel to the surface to the movable part <NUM> in any direction in two dimensions across the surface <NUM>, on selective contraction.

In the twelfth and thirteenth modified forms of the actuator assembly <NUM> shown in <FIG>, four SMA actuator wires <NUM> are provided so that two of the SMA actuator wires <NUM> are opposed in a first dimension and the other two of the SMA actuator wires <NUM> are opposed in a second dimension. The SMA actuator wires <NUM> that are opposed in each dimension are differentially driven by drive signals that generate differential contraction in the two orthogonal directions. Thus, the four SMA actuator wires <NUM> are capable of applying a net component of force parallel to the surface to the movable part <NUM> in any direction in two dimensions across the surface <NUM>, on selective contraction.

In the fourteenth modified form of the actuator assembly <NUM> shown in <FIG>, two SMA actuator wires <NUM> are provided which are orthogonal to each other as viewed normal to the surface <NUM>. As a result, the two SMA actuator wire <NUM> on contraction, apply components of force parallel to the surface <NUM> in orthogonal directions, thus applying a net component of force parallel to the surface to the movable part <NUM> in any direction in two dimensions across the surface <NUM> on selective contraction. As the components of force parallel to the surface <NUM> are orthogonal, the SMA actuator wires <NUM> are not opposed, because are each SMA actuator wire <NUM> drives movement in an orthogonal direction. In respect of the contract causing that movement, each SMA actuator wire <NUM> is resiliently biased by the spring <NUM>. Optionally, each SMA actuator wire <NUM> could be resiliently biased by an additional resilient biasing element (not shown) connected between the static part <NUM> and the movable part <NUM>. While the example described above includes two SMA actuator wires <NUM> which are opposed, as an alternative a single SMA actuator wire <NUM> could be provided. In that case, the single SMA actuator wire <NUM> may be resiliently biased against the component of contraction of the SMA actuator wires <NUM> parallel to the surface <NUM> either by the spring <NUM> or by an additional resilient biasing element connected between the static part <NUM> and the movable part <NUM>.

In each of the eleventh to fourteenth modified forms of the actuator assembly <NUM>, in use, the drive signals are selected to move the movable part <NUM> to a desired position relative to the static part <NUM> that is controlled in two dimensions by the power of the drive signals.

There will now be described some modifications to the biasing arrangement formed by the spring <NUM> in the above examples. Any of the following modifications may be applied to any of the forms of the actuator assembly <NUM> described above.

In general, the spring <NUM> may be replaced by any other type of biasing arrangement, including at least one resilient element, including: springs of any type, including a coil spring or a leaf spring; types of resilient element other than a spring; and any number of springs <NUM> or other type of resilient element. Examples of types of resilient biasing element that may be used instead of the spring <NUM> include a flexure, a block of resilient material or a spring arranged in tension between the movable part <NUM> and the surface <NUM>. Some examples are as follows.

<FIG> show a first modification to the biasing arrangement in which plural springs <NUM> are used. In this example, the movable part <NUM> has wings <NUM> protruding laterally of the movement axis M and the plural springs <NUM> engage with each wing <NUM> at different positions along the movement axis M. As the moveable part <NUM> moves, at least one of the springs <NUM> applies a biasing force to the movable part <NUM>, providing the same function as the single spring <NUM> in the above examples. This arrangement assists in preventing the moveable part <NUM> from lifting off the surface <NUM> of the static part <NUM> when the movable part <NUM> is in motion.

<FIG> show a second modification to the biasing arrangement in which two springs <NUM> are provided and are not connected to the body <NUM>. Instead the static part <NUM> includes arms <NUM> that protrude from the surface <NUM> and overhang the movable part <NUM>. The springs <NUM> are connected at one end to the arms <NUM> and at the other end to the movable part <NUM>, to provide the same function as the single spring <NUM> in the above examples.

<FIG> show a third modification to the biasing arrangement in which the spring <NUM> is replaced by a pair of flexures <NUM> that each protrude from the surface <NUM> and engage the upper surface <NUM> of the movable part <NUM>. The flexures <NUM>, due to their resilience, bias the movable part <NUM> into contact with the surface <NUM>, and therefore act as resilient biasing elements providing the same function as the single spring <NUM> in the above examples.

In general, the spring <NUM> may be replaced by a type of biasing arrangement that applies biasing by means other than a resilient element, for example a magnetic biasing arrangement. <FIG> illustrates a fourth modification to the biasing arrangement in which the spring <NUM> is replaced by a magnetic biasing arrangement <NUM> formed by a static magnet <NUM> and a movable magnet <NUM>. The static magnet <NUM> is provided in the static part <NUM> below the surface <NUM>. The movable magnet <NUM> is provided in the movable part <NUM>. The static magnet <NUM> and the movable magnet <NUM> are attracted together magnetically to bias the movable part <NUM> into contact with the surface <NUM>, and thereby provide the same function as the spring <NUM>, as described above.

In the embodiments described above the static part <NUM> includes a single surface <NUM> which is larger than the movable part <NUM> and which contacts a single, facing surface of the movable part <NUM>. However, that is not essential. In other embodiments, the static part <NUM> may plural bearing surfaces corresponding to surface <NUM> which contact the movable part <NUM>. Similarly, the movable part <NUM> may include plural bearing elements which each comprise a facing surface that contacts the surface <NUM>, or the plural bearing surfaces, if present.

By way of example, <FIG> show a second actuator assembly <NUM> that is suitable for providing optical image stabilisation (OIS) when incorporated in a camera apparatus <NUM> as shown in <FIG>, or other optical apparatus. The second actuator assembly <NUM> is arranged as described below, but in general terms has a similar arrangement and function to the actuator arrangement described in <CIT>, except for some differences described below. Accordingly, reference is made to <CIT>, that discloses a shape memory alloy actuator assembly according to the preamble of claim <NUM>.

The second actuator assembly <NUM> includes a movable plate <NUM> and a support plate <NUM> which are the first part and the second part, respectively, in this example. The movable plate <NUM> is movable with respect to the support plate <NUM> and the term "movable" is used for clarity of description because the support plate <NUM> is often held stationary by a user in normal use, but the movement is relative so if the movable plate <NUM> were held stationary then the support plate <NUM> would move.

The support plate <NUM> and the movable plate <NUM> are integral sheets made of metal, for example steel such as stainless steel. The support plate <NUM> is fixed to a support sheet <NUM>.

The second actuator assembly <NUM> may be incorporated in an optical apparatus such as a camera apparatus <NUM> as shown in <FIG> and as will now be described. The movable plate <NUM> supports a lens assembly <NUM>. The support sheet <NUM> is fixed to a base <NUM> on which an image sensor <NUM> is mounted, although in other types of optical apparatus the image sensor <NUM> may be omitted. Each of the support plate <NUM> and the movable plate <NUM> is provided with a central aperture aligned with an optical axis O allowing the passage of light from the lens assembly <NUM> to the image sensor <NUM> to allow the image sensor <NUM> to capture an image formed by the lens assembly <NUM>.

The second actuator assembly <NUM> includes four plain bearings <NUM> spaced around the optical axis O and each having a structure shown in more detail in <FIG>. Each plain bearing <NUM> comprises a bearing element <NUM> mounted on the support plate <NUM>, for example by adhesive, and a bearing surface <NUM> which is a surface of the movable plate <NUM> (shown in <FIG> and on the underside of the movable plate <NUM> as viewed in <FIG>). The bearing element <NUM> bears on the bearing surface <NUM>. In particular, an outer surface <NUM> of the bearing element <NUM> contacts the bearing surface <NUM>, the outer surface <NUM> of the bearing element <NUM> and the bearing surface <NUM> conforming with each other. The plain bearings <NUM> may be arranged as described in further detail in <CIT>.

Thus, the movable plate <NUM> is capable of movement relative to the static plate <NUM> across the bearing surfaces <NUM> of the plain bearings <NUM> in any direction in two dimensions orthogonal to the optical axis O.

As an alternative, the plain bearings <NUM> may be reversed to comprise a bearing element mounted on the movable plate <NUM> and a bearing surface which is a surface of the support plate <NUM>. In that case, the support plate <NUM> would form the first part and the movable plate <NUM> would form the second part. In that sense, the lens assembly <NUM> may be mounted on either one of the first and second parts.

The second actuator assembly <NUM> includes comprises two flexures <NUM> connected between the support plate <NUM> and the movable plate <NUM>. In this example, the flexures <NUM> are formed integrally with the movable plate <NUM> at one end thereof and are mounted to the support plate <NUM> at the other end thereof, although the flexures <NUM> could be formed integrally with the support plate <NUM> and mounted to the movable plate <NUM>, or else could be separate elements mounted to each of the support plate <NUM> and the movable plate <NUM>.

The flexures <NUM> are resilient and are therefore resilient biasing elements. The flexures <NUM> are arranged to act as a resilient biasing arrangement biasing the support plate <NUM> into contact with bearing surfaces <NUM> of the movable plate <NUM>. This may be achieved by configuring the flexures <NUM> so that they are deflected from their relaxed state to provide a pre-loading force that provides the biasing. This generates a reaction between the movable plate <NUM> and the bearing surfaces <NUM>, as well as generating frictional forces between the movable plate <NUM> and the bearing surfaces <NUM>.

Simultaneously, the flexures <NUM> permit movement of the movable plate <NUM> relative to the support plate <NUM> orthogonal to the optical axis O.

The flexures <NUM> are made of a suitable material that provides the desired mechanical properties and is electrically conductive so that the flexures <NUM> may electrically connect SMA actuator wires <NUM> that are connected thereto, for carrying the drive current supplied to the SMA actuator wires <NUM>. Typically the material is a metal having a relatively high yield, for example steel such as stainless steel.

The second actuator assembly <NUM> also includes four SMA actuator wires <NUM> connected between the support plate <NUM> and the movable plate <NUM>. Specifically, the support plate <NUM> is formed with crimps <NUM> and the movable plate <NUM> is formed with crimps <NUM>, wherein the crimps <NUM> and <NUM> crimp the four SMA actuator wires <NUM> so as to connect them to the support plate <NUM> and the moving plate <NUM>. In contrast to arrangement disclosed in <CIT> of the SMA actuator wires <NUM> extending perpendicular to the optical axis O, each SMA actuator wire <NUM> is inclined at an acute angle α of greater than <NUM>° with respect to the bearing surfaces <NUM> so as to apply a force ("upforce"), on contraction of the SMA actuator wire <NUM>, with a component normal to the bearing surfaces <NUM> that biases the support plate <NUM> away from the bearing surfaces <NUM> and with a component parallel to the bearing surfaces <NUM>.

The SMA actuator wires <NUM> have an arrangement around the optical axis O which is the same as that described in <CIT> so that each SMA actuator wires <NUM> applies a component of force parallel to the bearing surfaces <NUM> in different directions and the SMA actuator wires <NUM> are capable of driving movement of the movable plate <NUM> relative to the support plate <NUM> in two dimensions across the bearing surfaces <NUM>.

As the SMA actuator wires <NUM> are opposed, their average tension and hence the upforce can be controlled at least substantially independently of the movement.

The SMA actuator wires <NUM> are each connected to a control circuit which may be implemented in an integrated circuit chip. The control circuit in use applies drive signals to the SMA actuator wires <NUM> which resistively heat the SMA actuator wires <NUM> causing them to contract. In operation, the SMA actuator wires <NUM> are selectively driven to move the movable plate <NUM> relative to the support plate <NUM> along a movement axis in any direction orthogonal to the optical axis O. Such control may be used to move the lens assembly relative to image sensor orthogonally to the optical axis O so as to provide OIS as described in <CIT>.

In the absence of drive signals being applied, the SMA actuator wires <NUM> do not contract, and so the flexures <NUM> bias the movable plate <NUM> onto the bearing surfaces <NUM> generating frictional forces that are sufficient to retain the movable plate <NUM> in position on the bearing surfaces <NUM>. In this state, the movable plate <NUM> is retained in position with zero power consumption by the second actuator assembly <NUM>.

The flexures <NUM> may be designed to provide sufficient frictional forces to reduce motion and thereby improve stability of the second actuator assembly <NUM> and/or reduce the risk of audible noise when the SMA actuator wires <NUM> are in an unpowered state. This is important as being able to turn off OIS in situations where it is not effective (e.g. very high light levels) will reduce power consumption. In such a state, the frictional forces should retain the movable plate <NUM> in position on the bearing surfaces <NUM> in the event of typical forces acting on the second actuator assembly <NUM>, including gravitational forces which can lead to movement that is dependent on the orientation (posture dependence) and inertial impact forces. Otherwise, there is a risk that the second actuator assembly <NUM> is insufficiently stable and/or that audible noise is generated (e.g. between the movable plate <NUM> and the bearing surfaces <NUM> or between the lens assembly <NUM> and an enclosure of the camera apparatus <NUM>) when the second actuator assembly <NUM> vibrates, for example due to a haptic effect of a device such as a mobile telephone in which the second actuator assembly <NUM> is incorporated. When the second actuator assembly <NUM> is unpowered the SMA actuator wires <NUM> will slacken off and not exert much force. The position of the lens assembly <NUM> will therefore be determined by the interaction of the following forces:.

For example, when the camera apparatus <NUM> is held with the optical axis O horizontal the lens position will "sag" until the restoring force of the flexures <NUM> and frictional forces counterbalance the weight.

Generally, the frictional forces and hence the strength of the biasing force from the flexures <NUM> need to be increased with increasing mass of the cameral lens assembly that is to be mounted on the movable plate <NUM>.

Additionally, when the camera is accelerated hard inertia may move the lens assembly <NUM> relative the image sensor <NUM>. Both effects are undesirable, leading to blur from the motion and potential interference with OIS. A rigid stable system is desired for optimal OIS performance. The frictional forces generated between the movable plate <NUM> and the bearing surfaces <NUM> in the absence of contraction of the SMA actuator wires <NUM> may be less than the combined weight of the lens assembly <NUM> and the movable plate <NUM>. In that case, the movable plate <NUM> is maintained in position on the bearing surfaces <NUM> under the effect of gravitational forces when the camera apparatus <NUM> is held with the optical axis horizontal and ignoring the other forces in the system.

If frictional forces of a suitable level to achieve these effects were encountered when the SMA actuator wires <NUM> were driven, then this may hinder OIS performance. However, due to the inclination of the SMA actuator wires <NUM>, the force applied by the SMA actuator wires <NUM> on the support plate <NUM> has a component normal to the bearing surfaces <NUM> that biases the support plate <NUM> away from the bearing surfaces <NUM>, thereby reducing the frictional forces therebetween so as to reduce the impact on OIS performance.

In order to provide an appropriate degree of reduction, the ratio between (i) the frictional forces generated when the SMA actuator wires <NUM> drive the maximum degree of relative movement of the movable part <NUM>, and (ii) the frictional forces generated in the absence of contraction of the SMA actuator wires <NUM> may be less than <NUM> and more preferably less than <NUM>. The inventors have found that this can be achieved with practical sets of design parameters, which includes, amongst other things, an angle α of greater than <NUM>°. In smaller actuators, angles of <NUM>° or less are generally associated with unpractically small height differences between the ends of the SMA actuator wires <NUM> whereas, in larger actuators, such small angles generally do not provide sufficient upforce. Larger angles may be used but generally lead to taller actuators.

An alternative approach of increasing the stiffness of the flexures <NUM> (in the movement plane) would reduce sag as the restoring force will be greater, but is undesirable as the stiffness interferes with the performance of the SMA actuator wires <NUM>, for example by reducing stroke and slew rate.

An alternative approach of increasing the coefficient of friction of the plain bearings <NUM>, e.g. by surface roughness, is undesirable because it can lead to instabilities in OIS and reduce performance.

In principle, the problems would be lessened by reducing lens mass, but relatively heavy lens assemblies are preferred for improvement of camera performance generally.

Claim 1:
A shape memory alloy actuator assembly (<NUM>) comprising:
A first part (<NUM>), including a surface (<NUM>);
a second part (<NUM>) arranged to move relative to the first part across the surface;
a biasing arrangement (<NUM>) arranged to bias the second part into contact with the surface so as to generate frictional forces therebetween for retaining the second part in position on the surface; and
at least one shape memory alloy actuator wire (<NUM>) connected between the first part and the second part and arranged to, on contraction thereof, apply a force to the second part with a component orthogonal to the surface and with a component parallel to the surface so as to drive movement of the second part relative to the first part across the surface, characterized in that the component orthogonal to the surface reduces said frictional forces.