Patent Publication Number: US-11048098-B2

Title: Shape memory alloy actuator arrangement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/763,751, filed Mar. 27, 2018, now allowed, which is a national stage application under 35 U.S.C. § 371 of International Application PCT/GB2016/000174, filed Sep. 29, 2016, which claims priority of GB Patent Application GB 1517202.6, filed Sep. 29, 2015. The disclosures of which are hereby incorporated by reference herein in their entireties. 
    
    
     In one aspect, the present invention relates to a shape memory alloy (SMA) actuator arrangement for a camera lens element that provides movement orthogonal to the optical axis. More generally, the present invention relates to a shape memory alloy (SMA) actuator arrangement for a movable element that provides movement. 
     In the case of a camera lens element, movement orthogonal to the optical axis may be utilised to provide optical image stabilisation (OIS) of an image focused by the camera lens element on an image sensor. 
     The purpose of OIS is to compensate for camera shake, that is vibration of the camera apparatus, typically caused by user hand movement, that degrades the quality of the image captured by the image sensor. OIS typically involves detecting the vibration by a vibration sensor such as a gyroscope sensor, and controlling, on the basis of the detected vibration, an actuator arrangement that adjusts the camera apparatus to compensate for the vibration. Several techniques for adjusting the camera apparatus are known. OIS by processing of a captured image is possible in principle but requires significant processing power. Accordingly, there has been developed mechanical OIS in which the optical system of the camera is adjusted mechanically. 
     A number of actuator arrangements employing mechanical OIS techniques are known and applied successfully in relatively large camera apparatuses, such as digital still cameras, are difficult to miniaturise. Cameras are becoming very common in a wide range of portable electronic equipment, for example mobile telephones and tablet computers, and in many such applications miniaturisation of the camera is important. The very tight packaging of components in miniature camera apparatuses presents great difficulties in adding OIS actuators within the desired package. 
     In one type of mechanical OIS, a camera lens element is moved orthogonal to the optical axis of the at least one lens. Such a type of OIS will be referred to herein as “OIS-shift”. OIS-shift has the potential to reduce the size of the overall package for the camera as compared to a type of OIS in which a camera unit comprising an image sensor and a camera lens assembly for focussing an image on the image sensor is tilted relative to the support structure around two notional axes that are perpendicular to each other and to the optical axis of the lens assembly, for example as disclosed in WO-2010/029316 and WO-2010/089529. This is because only the camera lens element is moved and the lateral movement thereof requires less clearance than tilting the entire camera. 
     WO-2014/083318 discloses an SMA actuator arrangement that employs OIS-shift. In WO-2014/083318, there is employed a suspension system for suspending the camera lens element on the support structure that uses at least one ball to permit movement of the camera lens element relative to the support structure orthogonal to the optical axis but prevent movement along the optical axis. This provides good performance in operation. However, balls are difficult to handle in assembly, particularly in a miniature camera lens assembly. This has a negative impact of the manufacturing yield. Also, in practice the balls have a minimum size that limits miniaturisation of the arrangement. 
     It would therefore be desirable to provide an alternative method of suspension that does not suffer from these problems, but maintains the operation performance in permitting movement of the camera lens element relative to the support structure. Many types of suspension have high friction which makes them unsuitable for a camera lens element. 
     According to a first aspect of the present invention, there is provided a shape memory alloy actuator arrangement for a camera lens element, the shape memory alloy actuator arrangement comprising: a support structure; a movable element for supporting a camera lens assembly comprising at least one lens having an optical axis; plural shape memory alloy actuator wires connected between the support structure and the movable element in an arrangement wherein the shape memory alloy actuator wires are arranged, on selective driving, to move the movable element relative to the support structure in any direction orthogonal to the optical axis; and at least one plain bearing that bears the movable element on the support structure, allowing movement of the movable element relative to the support structure orthogonal to the optical axis. 
     Thus, at least one plain bearing is used to bear the camera lens element on the support structure, allowing movement of the camera lens element relative to the support structure orthogonal to the optical axis. A plain bearing is a bearing comprising two surfaces in contact with each other and permitting relative sliding motion. Of course, a plain bearing is a simple type of bearing which is known for use in other applications. Inevitably the contact provides friction which adversely affects the performance, particularly in a miniature arrangement. However, surprisingly, plain bearings can in fact be used to provide good performance in which the friction is sufficiently low to allow movement perpendicular to the optical axis. This is possible, in part, because the SMA actuator wires provide a high actuation force compared to other forms of actuator. 
     Furthermore, plain bearings may be formed with inherently small size along the height of the bearing, that is along the optical axis, especially compared to a suspension system employing balls. This allows the size of the arrangement to be reduced along the optical axis compared to that disclosed in WO-2014/083318. 
     As the present motion orthogonal to the optical axis of the at least one lens, the plain bearing may comprise conforming surfaces in contact with each other which are planar. By using a plain bearing comprising planar conforming surfaces, it is possible to avoid contact at a point or along a line. Such contact at a point or along a line may be disadvantageous, as wear would be concentrated in a small area, which could over time cause changes in the area of contact and hence the properties of the bearing. In contrast, the advantage of a plain bearing comprising planar conforming surfaces is that wear is distributed across a larger area, which area remains stable over time. 
     Similar problems to those described above for a camera lens element occur also in the more general case of an SMA actuator arrangement for a movable element of any other type. 
     Thus, according to a second aspect of the present invention, there is provided a shape memory alloy actuator arrangement, the shape memory alloy actuator arrangement comprising: a support structure; a movable element; at least one shape memory alloy actuator wire connected between the support structure and the movable element in an arrangement wherein the shape memory alloy actuator wire is arranged, on driving thereof, to move the movable element relative to the support structure; and at least one plain bearing that bears the movable element on the support structure, allowing movement of the movable element relative to the support structure. 
     In this more general case, the movement may be in a plane in which case the at least one plain bearing may comprise conforming surfaces that are planar, or may be rotational in which case the at least one plain bearing may comprise conforming surfaces that are cylindrical sections. 
    
    
     
       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 cross-sectional view of a camera apparatus including an SMA actuator arrangement; 
         FIG. 2  is an exploded perspective view of the SMA actuator arrangement; 
         FIG. 3  is a side view of the SMA actuator arrangement; 
         FIG. 4  is a cross-sectional view of a plain bearing of the SMA actuator arrangement; 
         FIGS. 5 and 6  are side views of flexures of the SMA actuator arrangement in their relaxed and assembled states; 
         FIG. 7  is a cross-sectional view of an alternative arrangement of plain bearing; 
         FIGS. 8 and 9  are plan views of alternative arrangements of plain bearings; and 
         FIGS. 10 to 14  are cross-sectional views of further alternative arrangements of plain bearings. 
     
    
    
     A camera apparatus  1  that incorporates an SMA actuator arrangement  40  in accordance with the present invention is shown in  FIG. 1 , which is a cross-sectional view taken along the optical axis O. The camera apparatus  1  is to be incorporated in a portable electronic device such as a mobile telephone, or tablet computer. Thus, miniaturisation is an important design criterion. 
     The camera apparatus  1  comprises a camera lens assembly  20  suspended on a support structure  4  by an SMA actuator arrangement  40  that supports the lens assembly  20  in a manner allowing movement of the lens assembly  20  relative to the support structure  4  in two orthogonal directions each perpendicular to the optical axis O. 
     The support structure  4  is a camera support supporting an image sensor  6 . The support structure  4  comprises a base  5 , the image sensor  6  being mounted on the front side of the base  5 . On the rear side of the base  5 , there is mounted an IC (integrated circuit) chip  30  in which a control circuit is implemented, and also a gyroscope sensor  31 . The support structure  4  also comprises a can  7  protruding forwardly from the base  5  to encase and protect the other components of the camera apparatus  1 . 
     The camera lens assembly  20  comprises a lens carriage  21  in the form of a cylindrical body supporting two lenses  22  arranged along the optical axis O, although in general any number of one or more lenses  22  may be provided. The camera apparatus  1  is a miniature camera in which the lenses  22  (or lens  22  if a single lens is provided) have a diameter of at most 10 mm. 
     The lens assembly  20  is arranged to focus an image onto the image sensor  6 . The image sensor  6  captures the image and may be of any suitable type, for example a CCD (charge-coupled device) or a CMOS (complimentary metal-oxide-semiconductor) device. 
     In this example, the lenses  22  are supported on the lens carriage  21  in a manner in which the lenses  22  are movable along the optical axis O relative to the lens carriage  21 , for example to provide focussing or zoom. In particular, the lenses  22  are fixed to a lens holder  23  which is movable along the optical axis O relative to the lens carriage  21 . Although all the lenses  22  are fixed to the lens holder  23  in this example, in general one or more of the lenses  22  may be fixed to the lens carriage  21  and so not movable along the optical axis O relative to the lens carriage  21 , leaving at least one of the lenses  22  fixed to the lens holder  23 . 
     An axial actuator arrangement  24  provided between the lens carriage  21  and the lens holder  23  is arranged to drive movement of the lens holder  21  and lenses  22  along the optical axis O relative to the lens carriage  21 . The axial actuator arrangement  24  may be any suitable type, for example being a voice coil motor (VCM) or an arrangement of SMA actuator wires, such as is described in WO-2007/113478 which is incorporated herein by reference. 
     In operation, the camera lens assembly  20  is moved orthogonally to the optical axis O in two orthogonal directions, shown as X and Y, relative to the image sensor  6 , with the effect that the image on the image sensor  6  is moved. This is used to provide OIS, compensating for image movement of the camera apparatus  1 , caused by for example hand shake. 
     The SMA actuator arrangement  40  will now be described in more detail with reference to  FIGS. 2 and 3 ,  FIG. 2  being an exploded view of the SMA actuator arrangement  40  omitting the SMA actuator wires  80 , and  FIG. 3  being a side view of the SMA actuator arrangement  40  expanded along the optical axis O. 
     The SMA actuator arrangement  40  comprises a support plate  50  that forms part of the support structure  4  and is connected to the base  5 . The SMA actuator arrangement  40  further comprises a moving plate  60 . In this embodiment the moving plate  60  is the moving element, but it could equally form part of a movable element including other components. The moving plate  60  supports the camera lens assembly  20  and is connected to the lens carriage  21 . The support plate  50  and the moving plate  60  are integral sheets made of metal, for example steel such as stainless steel. 
     Each of the support plate  50  and the moving plate  60  is provided with a central aperture aligned with the optical axis O allowing the passage of light from the camera lens assembly  20  to the image sensor  6 . 
     Movement of the camera lens assembly  20  relative to the support structure  4  is driven by a lateral actuation arrangement comprising plural SMA actuator wires  80  connected between the support structure  4  and the movable element. Specifically, the support plate  50  is formed with crimps  51  and the moving plate  60  is formed with crimps  61 , the crimps  51  and  61  crimping the four SMA actuator wires  80  so as to connect them to the support plate  50  and the moving plate  60 . The SMA wires  80  may be perpendicular to the optical axis O or inclined at a small angle to the plane perpendicular to the optical axis O. Each of the SMA actuator wires  80  is held in tension, thereby applying a force between the support plate  50  and the moving plate  60  in a direction perpendicular to the optical axis O. In operation, the SMA actuator wires  80  are selectively driven to move the camera lens assembly  20  relative to the support structure  4  in any direction orthogonal to the optical axis O. The overall arrangement of the SMA wires  80  to achieve this is the same as described in WO-2014/083318, as follows. 
     SMA material has the property that on heating it undergoes a solid-state phase change which causes the SMA material to contract. At low temperatures the SMA material enters the Martensite phase. At high temperatures the SMA enters the Austenite phase which induces a deformation causing the SMA material to contract. The phase change occurs over a range of temperature due to the statistical spread of transition temperature in the SMA crystal structure. Thus heating of the SMA actuator wires  80  causes them to decrease in length. 
     The SMA actuator wires  80  may be made of any suitable SMA material, for example Nitinol or another Titanium-alloy SMA material. Advantageously, the material composition and pre-treatment of the SMA actuator wires  80  is chosen to provide phase change over a range of temperature that is above the expected ambient temperature during normal operation and as wide as possible to maximise the degree of positional control. 
     On heating of one of the SMA actuator wires  80 , the stress therein increases and it contracts, causing movement of the camera lens element  20 . A range of movement occurs as the temperature of the SMA increases over the range of temperature in which there occurs the transition of the SMA material from the Martensite phase to the Austenite phase. Conversely, on cooling of one of the SMA actuator wires  80  so that the stress therein decreases, it expands under the force from opposing ones of the SMA actuator wires  80 . This allows the camera lens element  20  to move in the opposite direction. 
     The SMA actuator wires  80  have an arrangement in a loop at different angular positions around the optical axis O to provide two pairs of opposed SMA actuator wires  80  that are perpendicular to each other. Thus each pair of opposed SMA actuator wires  80  is capable on selective driving to move the camera lens element  20  in one of two perpendicular directions X and Y orthogonal to the optical axis O. As a result, the SMA actuator wires  80  are capable of being selectively driven to move the camera lens element  20  relative to the support structure  4  to any position in a range of movement in two directions orthogonal to the optical axis O. The magnitude of the range of movement depends on the geometry and the range of contraction of the SMA actuator wires  80  within their normal operating parameters. 
     The position of the camera lens element  20  relative to the support structure  4  perpendicular to the optical axis O is controlled by selectively varying the temperature of the SMA actuator wires  80 . This is achieved by passing through SMA actuator wires  80  selective drive signals that provide resistive heating. Heating is provided directly by the drive current. Cooling is provided by reducing or ceasing the drive current to allow the SMA wire  80  to cool by conduction, convection and radiation to its surroundings. 
     The drive signals for the SMA actuator wires  80  are generated and supplied by the control circuit implemented in the IC chip  30 . 
     The control circuit uses the output signals of the gyroscope sensor  31  which is a vibration sensor. The gyroscope sensor  31  detects the vibrations that the camera apparatus  1  is experiencing and its output signals represent the angular velocity of the camera lens element  20 . The gyroscope sensor  31  is typically a pair of miniature gyroscopes, for detecting vibration around two axes perpendicular to each other and the optical axis O, although in general larger numbers of gyroscopes or other types of vibration sensor could be used. 
     The drive signals are generated by the control circuit in response to the output signals of the gyroscope sensor  31  so as to drive movement of the camera lens element  20  to stabilise an image focused by the camera lens element  20  on the image sensor  6 , thereby providing OIS. The drive signals may be generated using a resistance feedback control technique for example as disclosed in any of International Patent Application No. PCT/GB2013/051325; International Patent Application No. PCT/GB2013/052959; WO-2012/066285; WO-2012/020212; WO-2011/104518; WO-2012/038703; WO-2010/089529 or WO-2010029316, each of which is incorporated herein by reference. 
     In addition, the SMA actuator arrangement  40  comprises three plain bearings  100  spaced around the optical axis O to bear the moving plate  60  on the support plate  50 . Each plain bearing  100  is arranged as follows and as illustrated in more detail in  FIG. 4 . 
     Each plain bearing  100  comprises a bearing member  101 . The bearing member  101  separates the moving plate  60  and the support plate  50 , and the thickness of the bearing members  101  is chosen accordingly. Due to the simplicity of the bearing members  101 , it possible to minimise the separation and hence the height of the SMA actuator arrangement  40  along the optical axis O. 
     In one alternative, the bearing member  101  is fixed to the support plate  50 , for example by adhesive. In this case, the bearing member  101  also bears on the moving plate  60 . That is, a surface  102  of the bearing member  101  on the opposite side from the support plate  50  and a surface  103  of the moving plate  60  are planar, conforming surfaces which contact one another. 
     In another alternative, the bearing member  101  is fixed to the moving plate  60 , for example by adhesive. In this case, the bearing member  101  also bears on the support plate  50 . That is, a surface  104  of the bearing member  101  on the opposite side from the moving plate  60  and a surface  105  of the support plate  50  are planar, conforming surfaces which contact one another. 
     Thus, the contact between the conforming surfaces  102  and  103  or between the conforming surfaces  104  and  105  supports and bears the movable plate  60  on the support plate  50 , allowing relative motion parallel to their extent, that is perpendicular to the optical axis O. 
     The material properties of the conforming surfaces  102  and  103  or the conforming surfaces  104  and  105  are chosen to provide a low friction and low wear plain bearing. One option is that the conforming surface  102  or  104  of the bearing member  101  is coated by a material having suitable properties. Another option is that the conforming surface  102  or  104  of the bearing member  101  is not coated, but instead the material of the bearing member  101  is chosen to have suitable properties. With either option, the other conforming surface  103  or  105  may also be coated with a material having suitable properties, but that is not essential. Where coatings are used, the coating has lower friction and/or lower wear than the material which is coated. The bearing member  101  or the coating, where used, may be made from a polymer, for example nylon, polytetrafluoroethylene (PTFE) (e.g. Teflon), an acetal (e.g. Delrin) or an Ultra High Molecular Weight Polyethylene (UHMWPE). 
     Although  FIG. 2  illustrates an example having three bearing members  101 , in general there may be any number of one or more bearing members  101 . Inclusion of at least three bearing members  101  spaced around the optical axis O assists in providing stable support and many practical embodiments include three or four bearing members  101 . 
     The total area of contact of the bearing member  101 , that is the area of the conforming surfaces  102  or  104  of the bearing member  101  summed over all bearing members present, is chosen to control the friction in the bearing. 
     Surprisingly, the plain bearings  10  can in fact provide good performance with friction that is sufficiently low to allow movement perpendicular to the optical axis O, having regard to the force applied by the SMA actuator wires. Typically, in order to limit the friction, the total area of contact is at most 3 mm 2 , preferably at most 1.5 mm 2 . 
     Equally, the plain bearings maintain a relatively high total area of contact, due to the contact over the conforming surfaces, compared to a bearing having a point or line contact. This reduces the impact of wear occurring over time and changing the area of contact and hence the bearing properties. Typically, the total area of contact is at least 0.1 mm 2 , preferably at least 0.2 mm 2 . 
     In addition, the SMA actuator assembly  40  comprises two flexures  67  connected between the support structure and the movable element to act as a biasing arrangement that biases the support structure and the movable element against each other whilst permitting the movement of the movable element relative to the support structure orthogonal to the optical axis O. The flexures  67  are arranged as follows. 
     The flexures  67  each extend between the support plate  50  and the movable plate  60 . The flexures  67  have a dual purpose of providing a mechanical function as described below and providing electrical connections from the support structure  4  to the camera lens assembly  20 . 
     In this example, the flexures  67  are formed integrally with the movable plate  60  at one end thereof and are mounted to the support plate  50  at the other end thereof. Alternatively, the flexures  67  could be formed integrally with the support plate  50  and are mounted to the movable plate  60 , or else could be separate elements mounted to each of the support plate  50  and the movable plate  60 . The mounting of the flexures  67  may be achieved by soldering which provides both mechanical and electrical connection. 
     The flexures  67  are arranged as follows to provide their mechanical function. Each flexure  67  is an elongate beam connected between the support structure  4  and the camera lens assembly  20 . 
     The flexures  67 , due to their intrinsic resilience, bias the support structure  4  and the camera lens element  20  together, the biasing force being applied parallel to the optical axis O. This maintains the contact in the plain bearings  100 . At the same time, the flexures  67  may be laterally deflected to permit said movement of the camera lens assembly  20  relative to the support structure  4  orthogonal to the optical axis O, to permit an OIS function. 
     The flexures  67 , again due to their intrinsic resilience, provide a lateral biasing force that biases the camera lens assembly  20  towards a central position from any direction around the central position in which the optical axis O of the camera lens assembly is aligned with the centre of the light-sensitive region of the image sensor  6 . As a result, in the absence of driving of the lateral movement of the camera lens assembly  20 , the camera lens assembly  20  will tend towards the central position from any direction around the central position. This ensures that the camera apparatus  1  remains functional to capture images, even in the absence of driving of the SMA actuator wires  80 . 
     The flexures  67  are designed as follows to provide a suitable retaining force along the optical axis O for the plain bearings  101 , and also to permit lateral movement with a lateral biasing force. The magnitude of the lateral biasing force is kept low enough as not to hinder OIS, whilst being high enough to centre the camera lens assembly  20  in the absence of driving. 
     Each flexure  67  has a cross-section with an average width orthogonal to the optical axis O is that is greater than its average thickness parallel to the optical axis O. Each flexure  67  extends in an L-shape around the optical axis O, it in general being desirable that the angular extent is at least 90° as measured between the ends of the flexure  67 . 
     In the assembled state of the SMA actuator assembly  40 , the flexures  67  are deflected from their relaxed state to provide a pre-loading force that biases the support structure  4  and the movable element together. This is illustrated in  FIGS. 5 and 6 , wherein  FIG. 5  shows the flexures  67  in their relaxed state and  FIG. 6  shows the flexures  57  in their assembled state where the flexures are deflected from their relaxed state by a distance d. 
     The flexures  67  are made of a suitable material that provides the desired mechanical properties and is electrically conductive. Typically the material is a metal having a relatively high yield, for example steel such as stainless steel. 
     The SMA actuator arrangement  40  shown in  FIG. 2  employs a particular configuration of plain bearings  100  as an example, but plain bearings may be applied in a any other suitable configuration. By way of non-limitative example, some alternative configurations of plain bearings will now be described. Such alternative configurations of plain bearings may be applied as alternatives to the plain bearings  100 , but with the SMA actuator arrangement  40  being otherwise as described above. 
     In  FIG. 2 , the bearing member  101  is fixed to one of the support plate  50  and the moving plate  60 , and the other of the support plate  50  and the moving plate  60  provide a conforming surface which contacts the conforming surface of the bearing member  101 .  FIG. 7  illustrates an alternative in which the plain bearing  100  comprises bearing members  101  fixed to both of the support plate  50  and the moving plate  60 . In this case, the bearing members  101  provide conforming surfaces  106  and  107  which contact one another. 
     In  FIG. 2 , the bearing members  101  are separate elements.  FIG. 8  illustrates in plan view an alternative in which the bearing members  101  are integrally formed with an annular ring  108  of the same material. In this example, the bearing members  101  and the annular ring  108  are fixed to the support plate  50  (but the arrangement could be reversed so that the annular ring  108  is fixed to the movable plate  60 ). The annular ring  108  extends around the optical axis O outside the aperture  52  in the support plate  50  but inside the aperture  62  in the moving plate  60 , so that the annular ring  108  does not contact the moving plate  60 . The bearing members  101  protrude outwardly of the annular ring  108  beyond the aperture  61  so that they overlap and bear on the moving plate  60 . This arrangement provides an advantage in manufacture that the annular ring  108  and bearing members  101  may be formed as an integral element at the same time, which facilitates manufacture and improves yield. 
     As an alternative to providing plural plain bearings  100  spaced around the optical axis O, it is possible to use a single plain bearing that is an annular bearing extending around the optical axis O. By way of example,  FIG. 9  illustrates in plan view a single plain bearing  100  that is modified to be annular. The plain bearing  100  comprises an annular bearing member  109  fixed to one of the support plate  50  and the movable plate  60  and bears on the other of the support plate  50  and the movable plate  60  (although for clarity the other of the support plate  50  and the movable plate  60  is not shown in  FIG. 9 ). Such an arrangement provides a stable support. The annular plain bearing  100  is formed as a single annular member, which facilitates manufacture and improves yield. 
     A potential risk with a plain bearing is that wear particles may be formed by wear of the plain bearing and that such wear particles may obscure the image capture or create mechanical or electrical problems within the SMA actuator assembly  40 . It is therefore desirable to provide means for containing wear particles formed at the plain bearing. A variety of such means are possible. There will now be described some non-limitative examples of means for containing wear particles that may be applied to any of the plain bearings  100  described above. 
       FIG. 10  illustrates in cross-section a plain bearing  100  comprising a labyrinth path  111  as a means for containing wear particles. The labyrinth path  111  is formed between the support plate  50  and the movable plate  60 , by shaping those components. 
       FIG. 11  illustrates in cross-section a plain bearing  100  comprising a trap  112  comprising an adherent surface  113  as a means for containing wear particles. The trap  112  is formed in a recess  114  in the support plate  50 , but could be formed additionally or instead on the movable plate  60 . The recess  113  contains a material  115  providing the adherent surface  113 . The material  115  is chosen to be adherent to the wear particles. For example, the material may be silicone or an adhesive. 
       FIG. 12  illustrates in cross-section another plain bearing  100  comprising a trap  116  comprising an adherent surface  117  as a means for containing wear particles. However, in this case the trap  116  is formed in a recess  118  in the bearing member  101  of the plain bearing  100 . Again, the recess  118  contains a material  119  providing the adherent surface  117 . The material  119  is chosen to be adherent to the wear particles. For example, the material may be silicone or an adhesive. 
       FIG. 13  illustrates in cross-section a plain bearing  100  comprising a wiper portion  120  as a means for containing wear particles. The wiper portion  120  is formed in the member  101  of the plain bearing  100 . In this example, the bearing member  101  is fixed to the support plate  50  and the wiper portion  120  extends to the surface of the movable element  60 , but this could be reversed. The wiper portion  120  wipes the movable element and collects wear particles  121 . 
     The arrangements of  FIGS. 12 and 13  are effective in containing wear particles, although the structures do require sufficient height and so may increase the overall height of the SMA actuator assembly along the optical axis O, albeit to a lesser extent than a suspension system employing balls. 
       FIG. 14  illustrates in cross-section a plain bearing  100  having a diaphragm  122  that contains the plain bearing  100  as a means for containing wear particles. In this case, the plain bearing  100  comprises bearing members  101  fixed to both of the support plate  50  and the moving plate  60 , so that the bearing members  101  provide conforming surfaces  106  and  107  which contact one another, similar to the arrangement shown in  FIG. 7 . However, in addition, the diaphragm  122  extends around the bearing members  101 , thereby containing the plain bearing  100 . The diaphragm  122  may be made of any suitable material, for example polyimide.