WO2018073585A1

WO2018073585A1 - Method of assembling an sma actuator assembly

Info

Publication number: WO2018073585A1
Application number: PCT/GB2017/053151
Authority: WO
Inventors: Stephen Matthew BUNTING; Andrew Benjamin David Brown; Robin Eddington
Original assignee: Cambridge Mechatronics Ltd
Current assignee: Cambridge Mechatronics Ltd
Priority date: 2016-10-20
Filing date: 2017-10-18
Publication date: 2018-04-26
Anticipated expiration: 2019-04-20
Also published as: CN109716226B; CN109716226A

Abstract

A device comprising a component (4) and an SMA actuator assembly (10) comprising a first part (12) and a second part (11) relatively movable and at least one SMA wire (10) connected between the two parts so as to drive relative movement of the first and second parts, wherein the component is placed on a bed of adhesive provided on the first part, while maintaining the at least one SMA wire at an elevated temperature at which the SMA wire is partially in an austenitic state and in tension, the position of the component is adjusted to a desired position with respect to the second part, and the adhesive is cured to fix the component to the first part in the desired position.

Description

Method Of Assembling An SMA Actuator Assembly

The present invention relates to assembly of a device including an SMA (shape memory alloy) actuator assembly.

An SMA actuator assembly may comprise one or more SMA wires connected in tension between two parts to act as actuators to drive relative movement of the two parts. Use of SMA wires has numerous advantages compared to other types of actuator, particularly for miniature devices. Such advantages include provision of high forces in compact arrangements.

In some types of SMA actuator assembly, plural SMA wires are connected in an arrangement in which the SMA wires are capable of driving relative movement of the two parts with plural degrees of freedom on selective contraction. This allows complex movements to be driven which is useful in many applications.

In one type of application, an SMA actuator assembly may be used in a device which is a miniature camera to effect focus, zoom or optical image stabilization (OIS).

By way of an example, WO-2011/104518, WO-2012/066285, WO-2014/076463 disclose SMA actuator assemblies employing eight SMA wires that provide translational movement with three degrees of freedom and also rotational movement with three degrees of freedom. In the case that the movable element is a camera lens element, these SMA actuator assemblies are capable of changing the focus and providing OIS.

By way of another example, WO-2013/175197 and WO-2014/083318 disclose

SMA actuator assemblies employing four SMA wires that provide translational movement with two degrees of freedom and also rotational movement with two degrees of freedom. In the case that the movable element is a camera lens element, these SMA arrangements are capable of providing OIS.

Such a device including an SMA actuator assembly comprises multiple components that need to be assembled together.

For example in the case of a device which is a camera, typically, the components are assembled from a base up, as follows. An SMA actuator assembly may be pre- assembled. Such an SMA actuator assembly may comprise a static part, a movable part, SMA wires, and in some cases a suspension system, for example balls and flexures.

Similarly, a lens assembly, which may have a fixed focus or a variable focus, may also be pre-assembled. The SMA actuator assembly may first be fixed to the base on which an image sensor is mounted. The lens assembly may then attached to the top of the SMA actuator assembly, specifically to the movable part of the SMA actuator assembly so that in use the SMA actuator assembly can move the lens assembly, for example to effect OIS. A screening can is then dropped over the lens assembly and fixed to the base.

During an assembly process of this type, difficulties in aligning components may arise. If a component is not correctly positioned or oriented, taking account of the relative motion between components driven by the SMA actuator assembly, then proper operation might not occur. Difficulties of this type become greater for miniaturised devices where tolerances between components are typically small in absolute terms. Such difficulties may reduce manufacturing yield.

For example, in the case of a device which is a camera, it is usually desirable to orient the lens assembly over the image sensor with the optical axis normal to the image sensor and to place the lens assembly at a particular distance from the image sensor. In a fixed focus camera without using an actuator assembly to provide OIS, this can be achieved by mounting the lens assembly in a high tolerance moulding that holds it in the correct position and orientation with respect to the image sensor. However, where an SMA actuator assembly is used, for example to provide auto focus or OIS, when a lens assembly is fixed to the SMA actuator assembly, there is the risk of error in the position and/or orientation of the movable part relative to the static part and so simple positioning of the lens assembly relative to the movable part may result in the lens assembly being at an undesirable position and orientation with respect to the image sensor.

According to a first aspect of the present invention, there is provided a method of assembling a device comprising a component and an SMA actuator assembly, which SMA actuator assembly comprises a first part and a second part that are relatively movable and at least one SMA wire connected between the two parts so as to drive relative movement of the first and second parts, wherein the method comprises: placing the component on a bed of adhesive provided on the first part; and while maintaining the at least one SMA wire at an elevated temperature at which the SMA wire is partially in an austenitic state and in tension, adjusting the position of the component to a desired position with respect to the second part, and curing said adhesive to fix the component to the first part in the desired position.

This method therefore involves adjusting the position of a component to be in a desired position on the first part of the SMA actuation assembly while on a bed of adhesive, and then fixing the component in that desired position by curing the adhesive. Importantly, this process is carried out while the at least one SMA wire is maintained at an elevated temperature at which the SMA wire is partially in an austenitic state and in tension. In that state, the SMA wires are caused to contract and drive relative motion of the first and second parts to be in relative positions typical of normal use. As such, it is easier to fix the component in the correct alignment taking into account the relative motion driven by the SMA actuator assembly. This reduces difficulties in aligning the component being fixed and so increases manufacturing yield, particularly in a miniature device.

In general, misalignment of components can result in reduction of the available stroke of relative movement provided by the SMA actuator assembly. This may occur if components are misaligned such that they come into contact and physically limit relative motion. This may also occur if an attempt is made to accommodate misalignment of a fixed component by offsetting the control of the relative movement, where the misalignment is within a degree of freedom, for example position or orientation, of the SMA actuator assembly. For example, in the case of a device which is a camera, where an SMA actuator assembly drives relative translational and/or rotational movement of the lens assembly with respect to the image sensor, then misalignment of the optical axis of the lens assembly may be corrected by offsetting the control. However, in such a case, the centre of the controlled movement is offset, such that the available stroke within the limits of contraction and expansion of the SMA wires is reduced on one side of the centre. Put another way, the maximum stroke in both directions is available when the centre of the controlled motion is central position within the motion limits of the SMA actuator assembly, and so is reduced if the centre of the controlled motion is offset from that central position.

The method may also be used to increase the available stroke of relative movement provided by the SMA actuator assembly, by maintaining the at least one SMA wire at an elevated temperature corresponding to the centre of the available stroke, that is the relative motion driven by the SMA actuator assembly.

The method may be applied to various components within various types of device.

In one example, the method may be applied where the first part to which the component is fixed is a movable part and the second part is a static part. An example of this applies to manufacture of a camera where the SMA actuator assembly drives motion of a lens assembly relative to an image sensor. In that case, the component may be the lens assembly and the second part is a part for fixing to an image sensor, for example the second part being a plate which is itself subsequently fixed to a base mounting the image sensor or the second part itself comprising the base mounting the image sensor. In this case, the method provides the advantage that the lens assembly may be better aligned with the image sensor, both in terms of position and orientation. Similarly, the lens assembly may be better aligned with other components such as a screening can that is subsequently fixed to the device. This may improve performance and manufacturing yield.

In another example, the method may be applied where the first part to which the component is fixed is a static part and the second part is a movable part. An example of this applies to manufacture of a camera where the SMA actuator assembly drives motion of a lens assembly mounted on the second part relative to an image sensor mounted on the first part. In that case, the component may be a screening can. In this case, the method provides the advantage that the screening can may be better aligned with the image sensor avoiding risk of collision and improving manufacturing yield.

In other examples, the device may be of a type other than a camera.

In order to maintain the at least one SMA wire at an elevated temperature, the entire SMA actuator assembly may be heated, for example by carrying out the method with the SMA actuator assembly in an oven or on a hot plate. Such a type of heating of the SMA wire has the advantage of being easy to implement, simplifying manufacture and reducing costs. However, as the heat is applied externally of the device, care needs to be taken to minimise the creation of thermal gradients which may cause the SMA wires to contract unequally or in an unpredictable manner.

Alternatively, in order to maintain the at least one SMA wire at an elevated temperature, a drive signal may be applied to the at least one SMA wire. That is a convenient way to heat the SMA wires, because it is how the SMA wires are driven in normal use, and reduces the issues of thermal gradients that may affect a method employing heating of the entire SMA actuator assembly. Accordingly, this alternative provides reliable and effective control over the position of the first part while the method is performed. Generally speaking, it may further improve performance and manufacturing yield.

Furthermore, application of a drive signal may make it easier to maintain the at least one SMA wire at an elevated temperature corresponding to the centre of the available stroke.

Various techniques may be applied to adjust the position of the component to the desired position with respect to the second part. Some non-limitative examples are as follows.

The adjustment may be performed by the component and the second part contacting respective reference surfaces of a jig. This is simple to perform during manufacture.

Alternatively, where the component comprises a lens assembly, the adjustment may be performed in response to optical measurements taken through the lens assembly. One possibility is to take the optical measurements using an external optical system. Another possibility when the second part mounts an image sensor is for the optical measurements to be derived from output of the image sensor. By using optical measurements, the lens assembly may be directly put in a position in which it is properly optically aligned with an image sensor.

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

Fig. 1 is a schematic side view of a device which is a camera unit including an

SMA actuator assembly;

Fig. 2 is a flowchart of a first assembly method for fixing a lens assembly in the device;

Fig. 3 is a schematic side view of the device during the first assembly method; Fig. 4 is a flowchart of a second assembly method for fixing a lens assembly in the device;

Fig. 5 is a schematic side view of the device on a jig during the second assembly method;

Fig. 6 is a schematic side view of the device at the end of the second assembly method;

Fig. 7 is a flowchart of a third assembly method for fixing a lens assembly in the device;

Fig. 8 is a schematic side view of the device on a jig during the third assembly method;

Fig. 9 is a flowchart assembly method for fixing a screening can in the device;

Fig. 10 is a schematic side view of the device during the first assembly method; and Fig. 11 is a side view of a further device which is a camera including an SMA actuator assembly.

A device 1 including an SMA actuator assembly 10 is shown in an assembled state in Fig. 1 and will be described first before describing various methods for assembling the device 1.

The device 1 shown in Fig. 1 is a camera including an image sensor 3 and a lens assembly 4 arranged to focus an image on the image sensor 3.

As described below, the SMA actuator assembly 10 drives relative movement of the image sensor 3 and the lens assembly 4, with various degrees of freedom depending on the form of the SMA actuator assembly 10, for example movement along the optical axis O which changes the focus of the image, or movement orthogonal to the optical axis which may provide OIS. The lens assembly 4 comprises one or more lenses 5, a single lens 5 being illustrated in Fig. 1 for clarity. The device 1 is a miniature camera in which the one or more lenses 5 have a diameter of no more than 10mm.

Although in this example the device 1 is a miniature camera in which the lens assembly 4 is the movable element, that is not limitative, and in general the SMA actuator assembly 10 may be applied to any type of device to move any type of movable element.

The SMA actuator assembly 10 is arranged as follows. The SMA actuator assembly

10 comprises a static part 11, a movable part 12 and plural SMA wires 13 connected in tension between the static part 11 and the movable part 12, for example by crimping using crimp portions (not shown) fixed on the static part 11 and the movable part 12 providing mechanical and electrical connection to the SMA wires 13.

The words "movable" and "static" are used because the movable part 12 moves relative to the static part 11 in use, but of course the static part may in fact also be moved around in use for example as a user holds the device 1.

As an alternative, the SMA actuator assembly 10 may comprise a single SMA wire 13 and a resilient biasing element (not shown) providing resilient biasing acting against the contraction of the SMA wire 13.

The static part 11 and the movable part 12 may have various constructions. In a simple construction each of the static part 11 and the movable part 12 may comprise a single plate, for example of metal. Alternatively, either or both of the static part 11 and the movable part 12 may have a more complex construction of multiple parts, for example a laminated construction of plural layers which may include one or more metal plate and one or more insulator layers, or by moulded components.

In this example, the movable part 12 is suspended on the static part 11 by a suspension system 14. The suspension system 14 is shown schematically in Fig. 1 and may have any suitable form for allowing movement of the lens assembly 4 with respect to the support structure 10 with the desired degrees of freedom. For example, the suspension system 14 comprise flexures to allow movement in three dimensions, or may comprise ball bearings or sliding bearings to allow movement in two dimensions orthogonal to the optical axis O while constraining movement in a third dimension along the optical axis O. Some specific examples are described in the documents referenced below. However, the suspension system 14 is not essential. In some designs, the movable part 12 may be suspended on the static part 11 exclusively by the SMA wires 13.

The static part 11 is fixed to a base 6 of the device 1 on which the image sensor 3 is mounted. The lens assembly 4 is fixed to the movable part 12. The SMA wires 13 are capable of driving relative movement of the static part 11 and movable part 12, and hence of driving relative movement of the lens assembly 4 and the image sensor 3, on contraction thereof. The SMA wires 13 are in an arrangement in which they are capable of driving relative movement of the fixed part 11 and static part 12 with plural degrees of freedom on selective contraction of different SMA wires 13. The SMA actuator assembly 10 may have various constructions with various arrangements of SMA wires 13 to drive relative movement with different degrees of freedom. Some non-limitative examples are as follows.

In a first type of configuration, the SMA wires 13 are in an arrangement to drive, on selective contraction, relative movement of the movable part 12 relative to the static part 11 with three translational degrees of freedom (i.e. along three axes being the optical axis O and two axes orthogonal to the optical axis O) and also with rotational degrees of freedom (i.e. around the same three axes). The suspension system 14 permits relative motion with any of these degrees of freedom.

Relative motion along the optical axis O moves the lens assembly 4 to change the focus. Relative motion along the axes perpendicular to the optical axis O shifts the lens assembly 4 laterally to provide OIS. Relative motion around the axes perpendicular to the optical axis O therefore tilts the lens assembly 4 and is desired to be maintained at a constant value where the optical axis O is aligned perpendicular to the image sensor.

By way of example in the first type of configuration, the SMA actuator assembly 10 may have the arrangement of eight SMA wires described in further detail in any of WO- 2011/104518, WO-2012/066285 or WO-2014/076463.

Movement in each of the degrees of freedom is driven by contraction of different combinations of SMA wires 13. As the movements add linearly, movement to any translational and/or rotational position within the six degrees of freedom is driven by a linear combination of contractions of the SMA wires 13. Thus, the translational and rotational position of the lens assembly 4 is controlled by controlling the drive signals applied to each SMA wire 13.

In a second type of configuration, the SMA actuator assembly 10 has a suspension system 14 that mechanically constrains relative movement of the movable part 12 relative to the static part 11 along the optical axis O. In that case, the SMA wires 13 are in an arrangement to drive, on selective contraction, relative movement of the movable part 12 relative to the static part 11 with two degrees of freedom (i.e. along the axes orthogonal to the optical axis O) and also rotational movement with one degrees of freedom (i.e. around the optical axis O).

Relative motion along the axes perpendicular to the optical axis O shifts the lens assembly 4 laterally to provide OIS.

By way of example in the second type of configuration, the SMA actuator assembly 10 may have the arrangement of four SMA wires described in further detail in any of WO- 2013/175197 wherein the suspension system 14 comprises beams, WO-2014/083318 wherein the suspension system 14 comprises ball bearings, or WO-2017/055788 wherein the suspension system 14 comprises a sliding bearing.

Movement in each of the degrees of freedom is driven by contraction of different combinations of SMA wires 13. As the movements add linearly, movement to any trans lational and/or rotational position within the three degrees of freedom is driven by a linear combination of contractions of the SMA wires 13. Thus, the translational and rotational position of the lens assembly 4 is controlled by controlling the drive signals applied to each SMA wire 13.

The device 1 further comprises a screening can 7, typically made of metal, that is fixed to the base 6 and extends over the other components to protect them from physical damage and ingress of debris.

The device 1 further comprises a control circuit 20 implemented in an integrated circuit chip and which is arranged to supply drive signals to the SMA wires 13. The control circuit 20 varies the power of the drive signals under closed loop control on the basis of target signals and feedback signals obtained from the derived measures of resistance of the SMA wires 13, which provides accurate control within certain physical limits. The control circuit 20 may be configured to supply drive signals as described in greater detail in WO- 2017/098249, for example.

Assembly methods for fixing various components in the device 1 during

manufacture will now be described. In each of these methods, the SMA actuator assembly 10 is pre-assembled, and the lens assembly 4 is also pre-assembled.

In the first to third assembly methods, the component that is fixed comprises the lens assembly 4, the first part to which the lens assembly 4 is fixed comprises the movable part 12, and so the second part that is relatively movable to the first part may comprise the static part 11 prior to fixing the static part 11 to the base 6 on which the image sensor 5 is mounted, or may comprises both the static part 11 and the base 6 on which the image sensor 5 is mounted.

The first assembly method is shown in Fig. 2 and performed as follows.

In step Sl-1, the SMA actuator assembly 10 is fixed to the base 6, specifically by fixing the static part 11 to the base 6.

In step SI -2, a bed of adhesive 30 is applied to the movable part 12 and the lens assembly 4 is placed on the bed of adhesive 30, as shown in Fig. 3. The adhesive 30 is curable, but is not cured at this time. Alternatively, step SI -2 may be performed by placing the lens assembly 4 onto the movable part 12 prior to the application of the adhesive 30 but leaving a small gap between which is filled with liquid adhesive 30 so that the lens assembly 4 ends up placed on a bed of the adhesive 30.

In step SI -3, the SMA wires 13 are heated to raise the temperature of the SMA wires 13 to an elevated temperature at which the SMA wire 13 are partially in an austenitic state, that is in a transition region in which it has partially transformed into an austenitic state, without having fully transformed, although the method may also work with a full transformation. This places the SMA wires in tension. SMA material when heated transforms from a low temperature martensitic state to a high temperature austenitic state, accompanied by a decrease in length. Through this heating the temperature around the assemblies is raised sufficiently for the SMA wires 13 to transform from their room temperature slack state to their high temperature active state, causing them to contract. As the wires become taut, they move the movable part 12 relative to the static part 11 into a position that is at least approximately central with respect to the range of movement.

Step SI -3 may be performed by heating the entire device 1 including the SMA actuator assembly 10, for example in an oven or on a hot plate. As the heat is applied externally of the device 1, care needs to be taken to minimise the creation of thermal gradients which may cause the SMA wires to contract unequally or in an unpredictable manner.

Alternatively, step SI -3 may be performed by applying drive signals to the SMA wires 13. The drive signals may be applied by an external circuit that is temporarily electrically connected to the SMA wires 13, rather than being applied by the control circuit 20. This may be achieved by the provision of electrical connections in an assembly jig. Nonetheless, the drive signals in step SI -3 take the same form as the drive signals that are applied by the control circuit 20 in use. As such, the SMA wires 13 are driven as in normal use, which reduces the issues of thermal gradients being created and provides reliable and effective control over the position of the movable part 12. There are several approaches that may be taken.

In one possible approach, drive signals of equal power may be applied to each of the SMA wires 13. This may approximate to the centre of the available stroke of the relative motion driven by the SMA actuator assembly 10. However, care still needs to be taken to minimise the creation of thermal gradients caused by the SMA wires 13 cooling to their surroundings at different rates.

In another possible approach, the drive signals are applied while measuring the resistance of the at least one SMA wire 13 and performing closed-loop control of the power of the drive signal on the basis of the measured resistance and a target resistance. The closed-loop control may be performed in the same manner as is performed by the control circuit 20 in normal use. The target resistance may be selected appropriately, for example to maintain the SMA wires at an elevated temperature corresponding to the centre of the available stroke.

One possibility for selecting the target resistance is to drive the SMA wires 13 so that the relative movement is driven against physical limits, to measure the resistance of the SMA wires 13 at those physical limits and to derive the target resistance from the resistances so measured.

Another possibility for selecting the target resistance is to determine a range of target resistances to be used for closed-loop control of the power of the drive signal in normal operation and to select the target resistance to be at the centre of that range.

The heating in step SI -3 is maintained during the subsequent two steps SI -4 and

Sl-5.

In step SI -4, the position of the lens assembly 4 is adjusted to a desired position with respect to the static part 11. This is performed in response to optical measurements taken through the lens assembly 4 derived from output of the image sensor 3. Specifically, images are collected from the image sensor 5 and the focus of the image at different locations on the image sensor 5 is recorded. Preferably, phase detect information is gathered. The current error in the position (laterally and rotationally, i.e. orientation) of the lens assembly 4 is calculated and the position of the lens assembly 4 is adjusted to the desired position in proper alignment with the image sensor 5.

In step Sl-5, while the lens assembly 4 is in the desired position, the adhesive 30 is cured to fix the lens assembly 4 in that desired position.

The second assembly method is shown in Fig. 4 and performed as follows. In step S2-1, the SMA actuator assembly 10, which is not yet fixed to the base 6, is placed on an assembly jig 31.

In step S2-2, a bed of adhesive 30 is applied to the movable part 12 and the lens assembly 4 is placed on the bed of adhesive 30, as shown in Fig. 5. This step is the same as step SI -2 described in more detail above.

In step S2-3, the SMA wires 13 are heated to raise the temperature of the SMA wires 13 to an elevated temperature at which the SMA wire 13 are partially in an austenitic state. This step is the same as step SI -3 described in more detail above.

The heating in step S2-3 is maintained during the subsequent two steps S2-4 and

S2-5.

In step S2-4, the position of the lens assembly 4 is adjusted to a desired position with respect to the static part 11. This is performed in response to optical measurements taken through the lens assembly 4 derived from output of an external sensor 33 mounted on the assembly jig 32. The output of the external sensor is used in the same manner as the output of the image sensor in step SI -4, as described in more detail above.

In step S2-5, while the lens assembly 4 is in the desired position, the adhesive 30 is cured to fix the lens assembly 4 in that desired position.

In step S2-6, the SMA actuator assembly 10 is removed from the assembly jig 31 and fixed to the base 6, leaving the device 1 in the state shown in Fig. 6.

The third assembly method is shown in Fig. 7 and performed as follows.

In step S3-1, the SMA actuator assembly 10, which is not yet fixed to the base 6, is placed on an assembly jig 34. The assembly jig has first and second reference surfaces 35 and 36 and the static part 11 is in contact with the first reference surface 35.

In step S3 -2, a bed of adhesive 30 is applied to the movable part 12 and the lens assembly 4 is placed on the bed of adhesive 30, as shown in Fig. 8, with part of the lens assembly in contact with the second reference surface 36. This step is the same as step SI -2 described in more detail above. However, in contrast to the first assembly method, in the third assembly method, the first and second reference surfaces 35 and 36 are arranged to put the position of the lens assembly 4 in a desired position with respect to the static part 11. This is the same as the desired position in the first assembly method in which the lens assembly 4 is in proper alignment with the image sensor 5, but is reached by mechanical position, not optically.

In step S3 -3, the SMA wires 13 are heated to raise the temperature of the SMA wires 13 to an elevated temperature at which the SMA wire 13 are partially in an austenitic state. This step is the same as step SI -3 described in more detail above. As the SMA wires 13 contract in step S3 -3, the position of the lens assembly 4 on the movable part 12 is physically adjusted by the assembly jig 34 to be in desired position with respect to the static part 11. As an alternative to placing the SMA actuator assembly 10 on the assembly jig 34 in step S3-1 and subsequently heating the SMA actuator wires 13 in step S3-3, the method may start with step S3-2 and the assembly jig 34 may be applied to adjust the position after the SMA wires 13 have been heated in step S3-3

The heating in step S3-3 is maintained during the subsequent step S3-4.

In step S3-4, while the lens assembly 4 is in the desired position, the adhesive 30 is cured to fix the lens assembly 4 in that desired position.

In step S3-5, the SMA actuator assembly 10 is removed from the assembly jig 31 and fixed to the base 6, leaving the device 1 in the state shown in Fig. 6.

In the fourth assembly method, the component that is fixed comprises the screening can 7, the first part to which the screening can 7 is fixed comprises both the base 6 on which the images sensor is mounted and the static part 12 to which the base 6 is fixed, and so the second part that is movable relative to the first part comprises both the movable part 12 and the lens assembly 4 which is fixed to the movable part 12, for example using any of the first to third assembly methods.

The fourth assembly method is shown in Fig. 9 and performed as follows.

The fourth assembly method starts with the device 1 having the lens assembly 4 and the base 6 fixed to the SMA actuator assembly 4, for example as shown in Fig. 6.

In step S4-1, a bed of adhesive 37 is applied to the base 6 and the screening can 7 is placed on the bed of adhesive 37, as shown in Fig. 10. The adhesive 37 is curable, but is not cured at this time. Alternatively, step S4-1 may be performed by placing the screening can 7 onto the base 6 prior to the application of the adhesive 37 but leaving a small gap between which is filled with liquid adhesive 37 so that the screening can 7 ends up placed on a bed of the adhesive 37.

In step S4-2, the SMA wires 13 are heated to raise the temperature of the SMA wires 13 to an elevated temperature at which the SMA wire 13 are partially in an austenitic state. This step is the same as step SI -2 described in more detail above.

The heating in step S4-2 is maintained during the subsequent two steps S4-3 and

S4-4.

In step S4-3, the position of the screening can 7 is adjusted to a desired position with respect to the static part 12 and the base 6. The desired position centralises the screening can 7 with respect to the remainder of the device 1. This provides the maximum possible clearance between the screening can 7 and the lens assembly 4 on all sides of the lens assembly 4

In step S4-4, while the screening can 7 is in the desired position, the adhesive 37 is cured to fix the screening can 7 in that desired position.

There will now be described an alternative type of assembly method to the assembly methods described above. In this alternative, instead of adjusting the position of a component on a bed of adhesive while the SMA wires are at an elevated temperature, the device is modified to include a region that is compliant allowing the device 1 to be mechanically deformed to put a component in a desired position while the SMA wires 13 are at an elevated temperature.

An example of a device 41 including an SMA actuator assembly 50 having such a compliant part is shown in Fig. 11 and will now be described.

The device 41 shown in Fig. 11 is a camera including an image sensor 43 and a lens assembly 44 comprising one or more lenses and being arranged to focus an image on the image sensor 43. The device 41 is a miniature camera in which the one or more lenses have a diameter of no more than 10mm.

The SMA actuator assembly 50 is arranged as follows. The SMA actuator assembly 50 comprises a static part 51, a movable part 52 and plural SMA wires 53 connected in tension between the static part 51 and the movable part 52. The connection of the SMA wires 53 is made by crimping using crimp portions 54 fixed on the static part 11 and crimp portions 55 fixed on the movable part 12, the crimp portions 54 and 55 providing mechanical and electrical connection to the SMA wires 13. The static part 51 is fixed to a base 46 of the device 1 on which the image sensor 43 is mounted and the lens assembly 44 is fixed to the movable part 52.

The words "moving" and "static" are used because the movable part 52 moves relative to the static part 51 in use, but of course the static part may in fact also be moved around in use for example as a user holds the device 1.

As an alternative, the SMA actuator assembly 50 may comprise a single SMA wire 53 and a resilient biasing element (not shown) providing resilient biasing acting against the contraction of the SMA wire 53.

The static part 51 and the movable part 52 may have various constructions, as described above with reference to the device 1 shown in Fig. 1. The movable part 52 may be suspended on the static part 51 by a suspension system (not shown) or exclusively by the SMA wires 53.

The SMA wires 53 are capable, on contraction thereof, of driving relative movement of the fixed part 51 and static part 52, and hence of driving relative movement of the lens assembly 44 and the image sensor 43 with three translational degrees of freedom (i.e. along three axes being the optical axis O and two axes orthogonal to the optical axis O) and also with rotational degrees of freedom (i.e. around the same three axes). Relative motion along the optical axis O moves the lens assembly 4 to change the focus. Relative motion along the axes perpendicular to the optical axis O shifts the lens assembly 4 laterally to provide OIS. Relative motion around the axes perpendicular to the optical axis O therefore tilts the lens assembly 4 and is desired to be maintained at a constant value where the optical axis O is aligned perpendicular to the image sensor. By way of example, the SMA actuator assembly 10 may have the arrangement of SMA wires described in detail in any of WO-2011/104518, WO-2012/066285 or WO-2014/076463.

Some of the crimp portions 54 fixed on the static part 11 comprise a compliant region 56 formed by a line of thinned material, so that the crimp portions are more flexible in the compliant region than elsewhere. Thus the crimp portions 54 can be mechanically deformed in the compliant regions 56. The compliant regions 56 are shaped so that such mechanical deformation changes the tilt of the lens assembly 44 with respect to the image sensor 43. As an alternative to the compliant region 56 in the crimp portions 54 any other part of the device 41 may have a compliant region that can be deformed to achieve the same effect of changing the tilt of the lens assembly 44 with respect to the image sensor 43.

During assembly the compliant regions 56 are deformed to adjust the tilt of the lens assembly 44 with respect to the image sensor 43 to the desired tilt. Then the compliant regions 56 are made non-compliant, for example by adding additional material across the compliant regions 56, for example adhesive which is then cured or an additional component that braces the compliant regions 56. This may be achieved by modifying any of the first to third assembly methods described above instead of adjusting the position of a component on a bed of adhesive while the SMA wires 53 are at an elevated temperature, the compliant regions 56 are mechanically deformed to put the lens assembly 44 in a desired position while the SMA wires 13 are at an elevated temperature. Such deformation may be achieved by driving the SMA wires 13 to drive relative movement in the desired direction.

Although in this example the device 41 is a miniature camera in which the lens assembly 4 is the movable element and employs an SMA actuator assembly 50, that is not limitative. In general, the device 41 may be of any type to move any type of movable element. Similarly, the SMA actuator assembly 50 may be replaced by any type of actuator assembly employing an actuator other than SMA wire.

Also, although this method is applied when assembling the lens assembly 44, more generally a similar method could be applied when assembling any other component.

Accordingly, according to a further aspect of the present invention, there is provided an assembly that includes an actuator, for example SMA wires, that can control the tilt of a movable part relative to a static part, the assembly having a compliant region which can be made noncompliant, for example by adding an additional material or component around the compliant region, the compliant region being configured so that if the movable part and the static part are held at a constant relative orientation the compliant region will deform when the actuator is driven. The deformation of the compliant region may be adjusted to place a component attached to the moving portion of the actuator at a desired orientation with respect to a component attached to the static portion of the actuator.

As an alternative to mechanically deforming a compliant region, a similar adjustment of the tilt of the lens assembly 44 with respect to the image sensor 43 to the desired tilt may be performed by applying a cycle of drive signals that causes the SMA wires 43 to stretch to different respective amounts and to work harden making them progressively less compliant to further changes.

Claims

1. A method of assembling a device comprising a component and an SMA actuator assembly, which SMA actuator assembly comprises a first part and a second part that are relatively movable and at least one SMA wire connected between the first and second parts so as to drive relative movement of the first and second parts, wherein the method comprises:

placing the component on a bed of adhesive provided on the first part; and while maintaining the at least one SMA wire at an elevated temperature at which the SMA wire is partially in an austenitic state and in tension, adjusting the position of the component to a desired position with respect to the second part, and curing said adhesive to fix the component to the first part in the desired position.

2. A method according to claim 1, wherein said step of adjusting the position of the component to a desired position with respect to the second part is performed by the component and the second part contacting respective reference surfaces of a jig.

3. A method according to claim 1 or 2, wherein the component comprises a lens assembly, and said step of adjusting the position of the component to a desired position with respect to the second part is performed by adjusting the position of the component in response to optical measurements taken through the lens assembly.

4. A method according to claim 3, wherein the second part mounts an image sensor, and the optical measurements are derived from output of the image sensor.

5 . A method according to any one of the preceding claims, wherein said elevated temperature corresponds to the centre of the relative motion driven by the SMA actuator assembly.

6. A method according to any one of the preceding claims, wherein said step of maintaining the at least one SMA wire is at said elevated temperature is performed by heating the entire SMA actuator assembly.

7. A method according to any one of claims 1 to 5, wherein said step of maintaining the at least one SMA wire at said elevated temperature is performed by applying a drive signal to the at least one SMA wire.

8. A method according to claim 7, wherein the at least one SMA wire comprises plural SMA wires, and drive signals of equal power are applied to each of the SMA wires.

9. A method according to claim 7, wherein the drive signal is applied while measuring the resistance of the at least one SMA wire and performing closed-loop control of the power of the drive signal on the basis of the measured resistance and a target resistance.

10. A method according to claim 9, further comprising selecting the target resistance.

11. A method according to claim 10, wherein said step of selecting the target resistance comprises measuring the resistance of the at least one SMA wire when the relative movement is driven against physical limits and deriving the target resistance from the resistances so measured.

12. A method according to claim 10, wherein said step of selecting the selected target resistance comprises determining a range of target resistances to be used for closed-loop control of the power of the drive signal in normal operation and selecting the selected target resistance to be at the centre of said range.

13. A method according to any one of the preceding claims, wherein the at least one SMA wire comprises plural SMA wires.

14. A method according to any one of the preceding claims, wherein the second part is a static part and the first part is a movable part.

15. A method according to any one of claims 1 to 13, wherein the component comprises a lens assembly.

16. A method according to claim 15, wherein the second part comprises a base to which the image sensor is fixed.

17. A method according to any one of claims 1 to 13, wherein the first part is a static part and the second part is a movable part.

18. A method according to any one of claims 1 to 13, wherein an image sensor is mounted on the first part, a lens assembly is fixed to the second part and the component comprises a screening can.

19. A method according to any one of the preceding claims, wherein the at least one SMA actuator wire is connected between the two parts so as to drive relative movement of the first and second parts that tilts the lens assembly.

20. A method according to claim 19, wherein the SMA actuator assembly further comprises a suspension system that suspends one of the first and second parts on the other in a manner allowing movement of the first and second parts that tilts the lens assembly.

21. A method according to any one of claims 1 to 19, wherein the at least one SMA wire is connected between the two parts so as to drive relative movement of the first and second parts that shifts the lens assembly laterally of the optical axis of the lens assembly.

22. A method according to claim 21, wherein the SMA actuator assembly further comprises a suspension system that suspends one of the first and second parts on the other in a manner allowing movement of the first and second parts that shifts the lens assembly laterally of the optical axis of the lens assembly.