Bearning assembly

Abstract

Pivoting bearing having two bearing components 2, 5 and a constriction 20. The second bearing surface 6 rolls and rotates on first bearing surface 4 about a rotation axis 7, such that the rolling comp

Description

Field The present application relates to a bearing assembly. The bearing assembly comprises a first part and a second part that rotates relative to the first part such that it rolls along a bearing surface defined by the first part. An actuating unit may actuate the bearing assembly. Background A bearing assembly constrains relative motion between two parts to a desired motion. For example, a bearing assembly can constrain relative motion between first and second parts to rotation of the second part relative to the first part about a rotation axis while a portion of the second part bears against a portion of the first part. In some examples the second part may exert a normal force on the first part, for instance if a biasing arrangement applies a biasing force to the second part urging it towards the first part. Such an arrangement may be sketched or modelled as a sharp point formed by the second point bearing against a surface of the first part such that rotation of the second part comprises rotation about the point contact. However, in practice, point contact (particularly when subjected to a large biasing force urging the parts together) will give rise to excessive stress at the interface between the first and second parts. Accordingly, for any real device, it is preferable to arrange for a portion of the second part to roll or slide along a bearing surface defined by the first part as the second part rotates. However, uncontrolled sliding (for instance, if the bearing assembly is subjected to a large external force) can cause the position of the second part along a bearing surface of the first part to vary from a desired position for a given rotational position of the second part. It is an aim of certain embodiments of the present invention to provide a bearing assembly that allows a position of a second part along a bearing surface of a first part for a given rotational position of the second part to be controlled or periodically reset. Summary According to an aspect of the present invention, there is provided a bearing assembly comprising: a first part comprising a first bearing component; a second part comprising a second bearing component, the second part being configured to rotate relative to the first part about a rotation axis such that the second bearing component rolls along a first bearing surface defined by the first bearing component and exerts a normal force which can be represented by a point force acting on the first bearing surface at a contact point; and a constraining arrangement configured to constrain a relationship between a position of the contact point in at least one direction along the first bearing surface which is radial of the rotation axis and an orientation of the second part about the rotation axis for at least part of a range of rotation of the second part. In some examples the arrangement of bearing components and bearing surface may be reversed. The bearing assembly may alternatively be referred to as a rolling bearing assembly in that in normal operation the second part rolls along a bearing surface defined by the first part. However, the present invention is distinguished from a rolling bearing in which one or more ball bearings roll along one or more bearing races in that in such an example there is no constrained relationship between an orientation of a ball bearing and a contact point between the ball bearing and the bearing race: for a given position along the bearing race a ball bearing may adopt any orientation. An advantage of embodiments of the present invention is that the constraining arrangement may periodically, occasionally, or in some examples continuously reset a position of the contact point for a given orientation. The constraining arrangement may take effect in limited portions of the range of rotation, for instance towards ends of the range of rotation. In some examples, the constraining arrangement may take effect in a middle part of the range of rotation or over a significant proportion of the range of rotation (for instance, through the use of complementary guide portions). The constraint on the relationship between contact point position and second part orientation may comprise periodically resetting the contact point position to an expected position or limiting the difference between current contact point position and expected position from exceeding a threshold for a given orientation. Regardless of the shape of the respective bearing components and bearing surface, the normal force can be conceptualized as acting through a single contact point which can move along a bearing surface in a direction radial of the rotation axis. It will be understood that specifying movement of the contact point radial of the rotation axis excludes the constraining arrangement comprising a channel limiting movement along the rotation axis while the second part can freely rotate within the channel. The rotation axis may also move relative to the bearing surface as the contact point moves. In some examples, during relative motion of the first and second parts, the effective rotation axis may also move relative to the second part. Movement of the contact point defines a locus across the bearing surface as the second part rotates relative to the first part. The constraining arrangement may alternatively be considered to constrain a position of the contact point along the lotus to a range of permitted positions for a given orientation of the second part. In some examples the second part can rotate (and roll) in first and second opposite directions. The constraining arrangement may take effect in either or both directions. The normal force may cause frictional engagement between the first and second bearing components that limits or prevents the second bearing component from sliding along the first bearing surface unless the bearing assembly is subjected to an external force exceeding a threshold. The first and second bearing components may comprise a first friction surface and a second friction surface that are biased against each other by the normal force, thereby generating static frictional forces which limit or prevent the contact point sliding as the second bearing component rolls along the first bearing surface. The static frictional forces may be selected (through consideration of the surface materials and structure and the magnitude of the normal force) for preventing or limiting sliding while acceleration of the variable aperture assembly is below a threshold. The threshold may be greater than the gravitational acceleration of Earth, so greater than g (9.81 m / s2), preferably greater than 2g or greater than 5g, or greater than 10g. The bearing assembly may further comprise a biasing arrangement compressing the first and second bearing components together to generate the normal force. The biasing arrangement may comprise a spring or other resilient element interconnecting the first and second parts (directly or indirectly). Or the biasing arrangement may be provided also by one or more actuating units configured to apply an actuating force to the second part (directly, or indirectly) in order to cause relative rotation between the parts. Rotation of the second part relative to the first part in a first rotational direction may cause the contact point to move in a first direction along the first bearing surface for at least a part of the range of rotation of the second part. The at least a part of the range of rotation may comprise a portion of the range away from one or both extremes of the range. The first direction may be at least partly radial of the rotation axis and along or across the bearing surface. In some examples the first direction may be perpendicular to the rotation axis. The constraining arrangement may comprise at least one end-stop arrangement comprising first and second end-stop surfaces configured such that continued rotation of the second part relative to the first part about the rotation axis in the first rotational direction causes the first and second end-stop surfaces to contact one another. Corresponding end-stop surfaces may, for instance, be provide on or coupled to the first and second parts. Continue rotation in one direction serves to cause the end-stop surfaces to come into contact with one another. The end-stop surfaces may be provided at an extreme of the range of second part rotation. In some examples, in normal operation the end-stop surfaces do not (quite) make contact when the end of the rotation range is reached unless the contact point is misaligned in a particular direction along the bearing surface. The amount of rotation required for the end-stop surfaces to make contact (for instance, from a central rotational position) may vary according to whether the contact point is misaligned. The end-stops ensure that the position of the contact point is reset for a given orientation of the second part (for instance, maximum rotation in one direction). When in contact with one another the first and second end-stop surfaces form a pivot arrangement defining a pivot point; and wherein further rotation of the second part relative to the first part in the first rotational direction may cause the second part to pivot about the pivot point such that the position of the contact point shifts along the first bearing surface in the first direction or a second direction opposite to the first direction. Shifting of the contact point may comprise sliding of the contact point along the bearing surface. Pivoting of the second part may reduce the normal force. This may make it easier for the bearing components to slip relative to one another, by reducing the static frictional force between them. In some examples the pivoting may cause the bearing components to separate and the contact point to skip across the bearing surface. In some examples, there may also be sliding contact between the endstops as well as pivoting, for instance if the biasing force exceeds the frictional force between the endstops. In a further alternative, instead of end-stops which physically contact one another, a magnetic end-stop arrangement may be provided in which, for instance, as opposite pole magnetic elements approach one another during second part rotation, further rotation is resisted, causing the contact point position to shift. The first part may define the first end-stop, and the second part may define the second end-stop. In other examples, the end-stops may be separate and connected to the first and second parts in a fixed or limited spatial arrangement (for instance, connected to one or other part by a spring so they yield before they cause pivoting). The constraining arrangement may further comprise at least one further end-stop arrangement configured to shift the position of the contact point along the first bearing surface in the first direction or the second direction when the second part rotates relative to the first part in the first rotational direction or in a second rotational direction opposite to the first rotational direction. The constraining arrangement may comprise or further comprise complementary guide portions, the guide portions being arranged relative to one another such that for at least a part of the range of rotation of the second part, contact between the guide portions limits a position of the contact point along the first bearing surface to a first range of positions. For complementary guide portions the "at least part of a range of rotation of the second part" may be a mid-portion of the range of rotation, or optionally the whole or substantially the whole of the range of rotation. The physical arrangement of the guide portions prevents or limits misalignment of the contact point. The guide portions may be defined by or connected to the first and second parts. Complementary guide portions for realignment of the contact point (or limiting a permitted degree of misalignment) during a portion of the range of rotation may be combined with one or more end-stop arrangement to correct the position of the contact point towards an end of the range of rotation. Constraining arrangements, including end-stop arrangements and complementary guide portions may be provided (at least partially) in a plane perpendicular to the rotation axis which includes the contact point or at least intersects rolling contact between the first and second parts. Or at least a portion (of the whole) of a constraining arrangement may be provided in a plane perpendicular to the rotation axis that is offset along the rotation axis from rolling contact between the first and second parts. The assembly may further comprise a third part moveable relative to the first part and configured such that at least a part of the second part is compressed between the first and third parts; wherein the second part may be further configured to rotate relative to the third part about the rotation axis or a further rotation axis. The further rotation axis may be offset from the rotation axis. The third part may comprise a third bearing component and the second part may further comprise a fourth bearing component configured to roll along a second bearing surface defined by the third bearing component and to exert a further normal force which can be represented by a point force acting on the second bearing surface at a further contact point; and wherein relative movement of the first part and the third part may cause rotation of the second part relative to the first part or the third part. The second part rotates relative to the third part about a further rotation axis which is offset from the rotation axis. The second part may comprise a beam or rod which is under compression between the first and third parts such that movement of the third part may be transmitted through the second part to the first part (or vice versa) with rotational movement at either end of the second part facilitating this transmission. The third part may comprise a third bearing component and the second bearing component may be configured such that relative movement of the first part and the third part causes the second bearing component to roll along the first bearing surface and a second bearing surface defined by the third bearing component and to exert a further normal force which can be represented by a point force acting on the second bearing surface at a further contact point. A portion of the second part may be spherical or cylindrical (or a similar curved shape) and compressed between the first and third parts such that it rotates as the first and third parts move relative to each other. The constraining arrangement may further comprise at least one further end-stop arrangement configured to shift the position of the further contact point along the second bearing surface when the second part rotates relative to the third part in at least one rotational direction. According to another aspect of the present invention, there is provided an actuator assembly comprising: a bearing assembly as described above; and at least one actuating unit configured to apply an actuating force to the second part capable of rotating the second part relative to the first part. According to yet another aspect of the present invention, there is provided an actuator assembly comprising: a bearing assembly as described above; and at least one actuating unit configured to apply an actuating force to the first part or the third part capable of rotating the second part relative to the first part or the third part. The actuating unit or at least one actuating unit may comprise at least one shape memory alloy, SMA, element coupled directly or indirectly to one of the first, second, or third parts. According to yet another aspect of the present invention, there is provided a method of controlling an actuator assembly comprising an actuating unit, a rolling element, one or more pivot features and a support structure comprising a bearing surface, wherein the method comprises: actuating the actuating unit to drive the rolling element to roll in a first rolling direction along the bearing surface such that a pivot feature of the one or more pivot features contacts a surface, thereby shifting a contact point between the rolling element and the bearing surface along the bearing surface in a first shift direction. The actuator assembly may comprise an actuator assembly as described above. According to yet another aspect of the present invention, there is provided a computer-readable storage medium having computer-readable program code stored therein that, in response to execution by a processor, causes the processor to perform the above method. According to yet another aspect of the present invention, there is provided an apparatus comprising a processor and a memory storing executable instructions that, in response to execution by the processor, cause the apparatus to perform the above method. Brief description of the drawings Certain embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figures 1 and 2 illustrate different bearing assemblies; Figure 3 illustrates an actuator assembly comprising a bearing assembly and an actuating unit; Figure 4 illustrates a further bearing assembly; Figure 5 illustrates a further actuator assembly; and Figures 6, 7a to 7, and 8a and 8b illustrate further alternative bearing assemblies. Detailed description Certain example bearing assemblies and actuator assemblies will now be described. Where similar or identical components are used in the different examples, they will be given the same reference numerals. For efficiency, description of similar or identical elements may not be repeated between the examples, and characteristics and features of elements are to be understood as applying to those elements in all examples unless the description indicates otherwise. Bearing assembly of figure 1 Figure 1 illustrates a bearing assembly 1 shown in a side view. The bearing assembly 1 comprises a first part 2 (only part of which is shown) which in turn comprises a first bearing component 3 which defines a first bearing surface 4. The first bearing component 3 in the example of figure 1 forms a pocket and the first bearing surface 4 forms a base of the pocket. The first bearing surface 4 may be planar, as illustrated, or it may take other forms (for instance, as illustrated in figure 2, described below). The bearing assembly further comprises a second part 5 (only part of which is shown) comprising a second bearing component 6 (generally, an end portion of the second part 5) which is received within the pocket of the first bearing component 3. The second part 5 is shown in a first position (solid lines) and in a second position (dotted lines) indicated by 5'. The second bearing component 6 is shown in cross section as having a curved surface. In some examples, the second part 5 may be formed from a sheet of material (for instance, a metal sheet or a sheet formed from a plastics material) that is cut to form the curved surface. The curved surface may be a portion of a circle, or it may have a different curved shape. In other examples, the second part 5 may be formed from a rod (for instance, a metal rod or a rod formed from a plastics material) and the curved surface forming the second bearing component 6 may comprise a portion of a sphere, or a different three-dimensional curved surface. The second part 5 is configured to rotate relative to the first part 2 about a rotation axis 7 such that the second bearing component 6 rolls along the first bearing surface 4 as indicated by the difference between the first position (solid line 5) and the (dotted line 5'). It can be seen that the orientation of the second part 5 has changed between the first position and the second position by angle a. Where the second bearing component 6 is defined by a portion of a circle or a portion of the sphere, the rotation axis 7 is a fixed-point relative to the second part 5 (and will move relative to the first bearing surface 4 during rolling). For instance, it can be seen that the rotation axis 7 for the first position of the second part 5 is different to the rotation axis 7' for the second position of the second part 5'. Where the shape of the second bearing component 6 has a different profile, the effective rotation axis 7 (relative to the second part 5) may change during rolling (as well as moving relative to the first bearing surface 4). Furthermore, the second bearing component 6 exerts a normal force on the first bearing surface 4. which can be represented by a point force (arrow 8) acting on the first bearing surface 4 at a contact point 9. It will be understood that as the second part 5 rotates relative to the first part 2 the contact point 9 is defined by a different part of the second bearing component 6 and will also move along the first bearing surface 4. For instance, it can be seen that the contact point 9 for the first position of the second part 5 is different to the contact point 9' for the second position of the second part 5'. The normal force may result from a biasing arrangement (not illustrated in figure 1) compressing the first and second bearing components 3, 6 together to generate the normal force. The biasing arrangement may take any form, for instance a spring or other reliant element extending under tension between the first and second parts 2, 5. The normal force causes frictional engagement between the first and second bearing components 3, 6 that limits or prevents the second bearing component 6 from sliding along the first bearing surface 4 unless the bearing assembly 1 is subjected to an external force exceeding a threshold. The first and second bearing components 3, 6 comprise respective first and second friction surfaces that are biased against each other by the normal force, thereby generating static frictional forces which limit or prevent the contact point 9 from sliding as the second bearing component 6 rolls along the first bearing surface 4. The static frictional forces may be selected (through consideration of the surface materials and structure and the magnitude of the normal force) for preventing or limiting sliding while acceleration of the variable aperture assembly is below a threshold. Such an external force may result from the bearing assembly 1 (or a larger system of which it forms a part) being dropped. The threshold may be greater than the gravitational acceleration of Earth, so greater than g (9.81 m / s2), preferably greater than 2g or greater than 5g, or greater than 10g. In normal operation, at least for a central portion of the rotational range of the second part 5, rotation of the second part 5 relative to the first part 2 in a first rotational direction (for instance, clockwise in the example of figure 1 moving from the solid line 5 to the dotted line 5') causes the contact point 9 to move in a first direction along the first bearing surface 4. The first direction is downwards in the view of figure 1. The static frictional force between the first and second bearing components 3, 6 means that the relative motion between the second bearing component 6 and the first bearing surface 4 is only rolling motion: there is no sliding. Accordingly, in normal operation, a relationship between a position of the contact point 9 in at least the first direction along the first bearing surface 4 and an orientation of the second part 5 about the rotation axis 7 for at least part of a range of rotation of the second part 5 is fixed and predictable. However, as noted above, if subject to an external force exceeding a threshold, the second bearing component 6 can slide along the first bearing surface 4 such that the position of the contact point 9 differs from its expected position for a given orientation of the second part 5. To mitigate this loss of expected position, according to an embodiment of the present invention the bearing assembly of figure 1 further comprises a constraining arrangement configured to constrain the relationship between a position of the contact point 9 in at least one direction along the first bearing surface 4 which is radial of the rotation axis 7 and an orientation of the second part 5 about the rotation axis 7 for at least part of a range of rotation of the second part 5. In the example of figure 1, the constraining arrangement comprises a pair of end-stop arrangements 20a, 20b, each arranged towards opposite ends of the range of rotation of the second part 5. Each end-stop arrangement serves to apply a centring force to the second bearing component that resets the position of the contact point 9 relative to the first bearing surface 4 for a given orientation of the second part 5 (or at least reduces the error in position). In principle there may only be one end-stop arrangement 20 acting towards one end of the range of rotation, however figure 1 shows two end-stop arrangements 20a, 20b, one at each end of the range of rotation. For figure 1, only a single end-stop arrangement 20a is described. The other end-stop arrangement 20b will be understood to operate in a similar way (with respective features given the suffix "b"). End-stop arrangement 20a comprises a first end-stop surface 21a defined by a side of the pocket forming the first bearing component 3. A second end-stop surface 22a is defined by a side of the second part 5. As the second part 5 rotates in the clockwise direction (moving to the dotted second position 5') continued rotation of the second part 5 relative to the first part 2 about the rotation axis 7 causes the first and second end-stop surfaces 21a, 22a to contact one another. That is, the end-stop surfaces 21a, 22a come into contact after a certain amount of rotation in the first (clockwise) rotational direction. The certain amount of rotation will be dependent on a starting orientation and whether the contact point 9 has slid from its expected position for a given orientation of the second part 5. If there has been no sliding, then the second part 5 may reach the end of its range of clockwise rotation either coming to rest against the first end-stop surface 21 or stopping just short of this. However, if the contact point 9 has slid downwards in the view of figure 1 from its expected position, then the end-stop surfaces 21a, 22a will come into contact before the clockwise end of the range of rotational movement has been reached. In this case, the first and second end-stop surfaces 21a, 22a form a pivot arrangement defining a pivot point 23, which may for instance be at an outer edge of the pocket 3. Further clockwise rotation of the second part 5 causes the second part 5 to pivot about the pivot point 23 such that the position of the contact point 9 shifts along the first bearing surface 4 upwards in the view of figure 1. It will be appreciated that this upwards movement of the contact point 9 is in the opposite direction to the normal rolling movement of the contact point 9 for clockwise rotation of the second part 5. Shifting of the contact point 9 may comprise sliding of the second bearing component 6 along the first bearing surface 4. This sliding motion may be facilitated by the pivoting of the second part 5 causing an effective reduction of the normal force 8. That is, the end-stop arrangement 20b applies a force to the second part 5 acting upwards and to the right in the view of figure 1. If the normal force 8 is reduced to zero, then the contact point 9 may be shifted upwards in a skipping motion where contact between the second bearing component 6 and the first bearing surface 4 is interrupted. Accordingly, the end-stop arrangements 20a, 20b cause the second part 5 (and particularly the second bearing component 6) to be repositioned so as to reset (or at least reduce the error in) the relationship between position of the contact point 9 and the orientation of the second part 5. Each end-stop arrangement 20a, 20b applies a centring force to the second part 5 (acting upwards in the view of figure 1 for end-stop arrangement 20b and downwards for end-stop arrangement 20a). Centring in this context refers to a force which serves to reposition the contact point 9 to its correct position relative to the first bearing surface for a given rotational position of the second part 5. Centring does not refer to moving the contact point 9 to a central position along the first bearing surface 4. It will be understood that if in the event of a relatively small misalignment between the contact point 9 and its expected position, and for operation of the second part 5 within a middle portion of its range of rotation, then the misalignment may go uncorrected unless or until the rotation increases (or the misalignment increases) such that the end-stops of one or other end-stop arrangement 20a, 20b come into contact. In some examples a method of operating the bearing assembly may comprise the second part 5 being periodically rotated to an end of its range of rotation (or both ends) in order to correct the alignment of the contact point 9. As such, the relationship between contact point position and second part orientation is constrained at least for a portion of either end of the range of rotation of the second part. Bearing assembly of figure 2 Turning now to the bearing assembly 1 of figure 2, this differs from the bearing assembly 1 of figure 1 in two respects. Firstly, the first bearing surface 4 is shown as convex. In other examples, the first bearing surface may be concave or have a more complex form. Secondly, the end-stop arrangements 20a, 20b comprise shoulder portions 24 on outer edges of the pocket 3. The end-stop surfaces 21a, 21b formed on the first part 2 are defined by the shoulder portions 24. The shoulders 24 (or more precisely, the wider pocket 3 behind the shoulders) may allow for a larger misalignment of the contact point 9 from its expected position to be accommodated within a middle part of the range of rotation of the second part 5 before the end-stop surfaces 21a, 22a; 21b, 22b come into contact. This can result in a larger contact point correction towards the ends of the range of rotation of the second part 5. The shape of the second part 5 is also shown in a slightly different form from that of figure 1, and other profiles will be apparent to the skilled person. Actuator assembly of figure 3 Turning now to figure 3, this shows an actuator assembly 30 comprising a bearing assembly 1 generally the same as that of figure 1 and first and second actuating units 31a, 31b configured to apply actuating forces to the second part 5 capable of rotating the second part 5 relative to the first part 2. The second part 5 and the actuating units 31a, 31b are shown in a first position indicated by solid lines and a second, unlabelled position indicated by dotted lines. Each actuating unit 31a, 31b comprises a shape memory alloy, SMA, element 32, for instance an SMA wire, coupled between the first part 2 and the second part 5 (generally at position remote from the second bearing component 6). Actuation of the actuating unit is achieved by heating the SMA element (for instance, by passing an electrical current) causing it to contract. The actuating units 31a, 31b are coupled to the respective parts 2, 5 in any suitable fashion, for instance using crimps 33. The first actuating unit 31a is configured, when actuated, to apply a force acting to rotate the second part 5 in an anticlockwise direction about the rotation axis 7. The second actuating unit 31b is configured, when actuated to apply a force acting to rotate the second part 5 in a clockwise direction about the rotation axis 7. Rotation of the second part 5 is dependent upon the balance of actuating forces applied by the actuating units 31a, 31b. Furthermore, in combination, the actuating units 31a, 31b apply a biasing force to the second part 5 urging it towards the first part 2 to give rise to the normal force 8 such that no separate biasing arrangement is required. Bearing assembly of figure 4 Figure 4 illustrates a further bearing assembly 1 in which two end-stop arrangements 20a, 20b are provided which each take effect towards a clockwise end of the range of rotation of the second part 5 to provide centring forces that act to shift the contact point 9 either upwards or downwards along the second bearing surface 4 dependent upon the direction in which the contact point 9 is misaligned from its expected position. In the event that the contact point 9 is misaligned from its expected position downwards in the view of figure 4, then as the second part 5 rotates clockwise the end-stop surfaces 21a, 22a of end-stop arrangement 20a will contact one another resulting in an alignment force upon the second part 5 acting to shift the contact point 9 upwards. In the event that the contact point 9 is misaligned from its expected position upwards in the view of figure 4, then as the second part 5 rotates clockwise the endstop surfaces 21b, 22b of end-stop arrangement 20b will contact one another resulting in an alignment force upon the second part 5 acting to shift the contact point 9 downwards. Accordingly, the end-stop arrangements 20a, 20b collectively act to align the second part 5 relative to the first part 2 at one end of the range of rotation of the second part 5. In an alternative, another pair of end-stop arrangements may also or instead be provided at an anticlockwise end of the range of rotation of the second part 5. The pair of end-stop arrangements 20a, 20b may alternatively be considered to be complementary alignment features 40, 41 formed respectively upon the first part 2 and the second part 5. The complementary alignment features each comprise respective alignment surfaces 21a, 21b for alignment feature 40 and alignment surfaces 22a, 22b for alignment feature 41. Actuator assembly of figure 5 Figure 5 (and also figure 6, described below) illustrates further examples where there are two rolling interfaces. Each rolling interfaces may be substantially as described above in connection with figures 1 to 4, and so will be described more briefly. For both figures 5 and 6, the second part 5 rotates and transmits a force between the rolling interfaces. The rolling interfaces may hold the rotating second part 5 in position and in contact with respective bearing surfaces. Figure 5 illustrates an actuator assembly 30 comprising a bearing assembly 1 including a first part 2 and a second part 5 connected together at a first rolling interface 45 comprising bearing components 3 and 6. Bearing component 6 of the second part 5 rolls along a first bearing surface 4 defined by the bearing component 3 of the first part. This first rolling interface 45 may be substantially as described above for figures 1 and 2. However, departing from those earlier figures, the second part 5 is formed as an elongate component, for instance a bar or a rod. The bearing assembly 1 comprises a third part 50 connected to the second part 5 via a second rolling interface 46 at an opposite end to the first rolling interface. The third part 50 is moveable relative to the first part 2 and configured such that the second part 5 is compressed between the first and third parts 2, 50. In the example of figure 5 both the first part 2 and the third part 50 are mounted on respective roller bearings 51, 52 such that each can roll along respective bearing surfaces 53, 54 of a fourth part 55 (which may be considered to be a static component). However, in other examples, either the first or third parts 2, 50 may be static or fixed. The second rolling interface 46 may be generally the same as or similar to the first rolling interface 45. Specifically, the third part 50 may comprise a third bearing component 60 defining a second bearing surface 61 and the second part 5 further comprises a fourth bearing component 62 configured to roll along the second bearing surface 61 and to exert a further normal force which can be represented by a point force acting on the second bearing surface 61 at a further contact point 63. The second part 5, particularly the fourth bearing component 62 rotates about a further rotation axis 64 in order to roll along the second bearing surface 61. The further rotation axis 64 is generally at an opposite end of the second part 5 from the rotation axis 7 which defines the rotation for the first rolling interface. The further rotation axis 64 may in some examples be approximately or exactly parallel to rotation axis 7, or they may diverge. The actuator assembly 30 of figure 5 further comprises a first actuating unit 31 configured to apply an actuating force to the third part 50 capable of rotating the second part 5 relative to the first part 2 and the third part 50. Specifically, the first actuating unit 31 comprises an SMA wire 32 extending between and the fourth part 55 and the third part 50, connected at each end with crimps 33. Contraction of the SMA wire 32 causes the third part 50 to move to the left in the view of figure 5, relative to the fourth part 55. Movement of the third part is facilitated by the roller bearings 52. Movement of the third part 50 to the left applies a compressive load to the second part 5 through the second rolling interface, which in turn is transmitted to the first part 2 through the first rolling interface 45, causing the first part 2 to move downwards. Relative movement of the first part 2 and the third part 50 causes rotation of the second part 5 relative to the first part 2 and the third part 50 through the rolling interfaces 45, 46. In a further example, not illustrated, there may be a second actuating unit, similar to the first actuating unit 31 applying an actuating force to the first part 2 configured to move the first part 2 upwards relative to the fourth part 55. Actuation of the first and second actuating units places the beam under compression. If the actuating forces are not balanced, this causes rotation of the second part 2 relative to the first and second parts 2, 50 about rotation axes 7, 64. Bearing assembly of figure 6 Figure 6 illustrates a further example of a bearing assembly 1 comprising first, second, and third parts 2, 5, 50 in which the third part 50 is moveable relative to the first part 2 (upwards and downwards in the view of figure 6, though suitable actuating units are not illustrated) and configured such that a part of the second part 5 is compressed between the first and third parts 2, 50 through first and second rolling interfaces 45, 46. Differing from figure 5, the second part 5 rotates relative to the first and third parts 2, 50 about a single rotation axis 7. Compression of the second part 5 between the first and third parts 2, 50 may be achieved through a separate biasing arrangement, such as a spring or other resilient element under tension between the first and second parts 2, 50 (not illustrated). In this example the second part 5 is formed from or defines two portions: a spherical or cylindrical rolling portion 70 that sits between a first bearing surface 4 of the first part 2 and a second bearing surface 61 of the second part; and a wing portion 71 that overlaps the first and second parts 2, 50. For instance, the wing portion 71 may be formed from a sheet material and attached (such as by gluing or welding) to a sphere or roller forming the rolling portion 70. When the third part 50 is moved upwards or downwards relative to the first part 2 the rolling interfaces 45, 46 cause the rolling portion 70 to rotate about the rotation axis 7, which in turn causes the wing portion 71 to rotate about the same axis 7. For instance, if the third part 50 is moved upwards, the rolling portion 70 is caused to rotate anticlockwise such that a first contact point 9 moves upwards along the first bearing surface 4 and a second contact point 63 moves downwards along the second bearing surface 61. The bearing assembly 1 of figure 6 further comprises end-stop arrangements 20 provided between the first and second parts 2, 5 and also between the second and third parts 5, 50. Differing from each of the embodiments of figures 1 to 5, the bearing assembly 1 of figure 6 is an example of a bearing assembly in which one or more (in this case, all) of the end-stop arrangements 20 are provided offset along the rotation axis 7 from the contact between bearing components. Specifically, the end-stop arrangements are formed from pins 80a, 80b, 80c, 80d or other protrusions extending from the first part 2 or the third part 50 against which wings 81a, 81b, 81c of the wing portion 71 comes to bear towards first and second ends of the range of rotation of the second part 5. As an example, when the third part 50 moves downwards causing clockwise rotation of the second part 5 from a first position indicated by the solid lines to a second position 5' indicated by the dotted lines, if there is misalignment of the contact points 9, 63 then a first wing portion 81a of the second part 5 defining an end-stop surface may come into contact with an end-stop surface defined by pin 80a, with the result that contact point 9 and / or contact point 63 may be shifted along bearing surfaces 4, 61, for instance by sliding. In addition, end-stop surfaces defined by pin 80b and wing 81b may similarly come into contact and cause contact point shifting. Anticlockwise rotation may similarly cause pairs of wings and pins 80d, 81a; 80c 81c to come into contact and cause contact point shifting such that the relationship between the positions of the contact points 9, 63 along the bearing surfaces 4, 61 and the orientation of the second part 5 about the rotation axis 7 is constrained for at least portions of the range of rotation of the second part close to the ends of the range of rotation. Bearing assembly of figures 7a to 7c Figures 7a to 7c illustrate a yet further example of a bearing assembly 1, in which the second part 5 is illustrated in three different rotational positions. The second part includes a bearing component 6 shown in this example as being part circular about a rotation axis 7 and configured to roll along a bearing surface 4 defined by a first part 2. The bearing component 6 exerts a normal force on the bearing surface 4. The normal force can be represented by a point force acting on the bearing surface 4 at a contact point 9 that moves along the bearing surface 4 (upwards and downwards in the view of figures 7a to 7c). Differing from preceding embodiments, the constraining arrangement comprises complementary guide portions 90, 91 formed respectively on the first part 2 and the second part 5. The guide portions 90, 91 are arranged relative to one another such that for at least a part of the range of rotation of the second part, contact between the guide portions 90, 91 limits a position of the contact point 9 along the bearing surface 4 to a first range of positions. In this example, a first guide portion 90 comprises a tooth protruding from the first part 2 towards the second part 5 and a second guide potion 91 comprises a corresponding recess shaped to receive the tooth. As for the embodiment of figure 6, the constraining arrangement - here the complementary guide portions 90, 91 - is offset along the rotation axis 7 from contact between the bearing component 6 and the bearing surface 4. Accordingly, the rolling contact between the bearing component 6 and the bearing surface 4 is uninterrupted (as indicated by the circular profile of the bearing component 6 disappearing behind the tooth 90 in figure 7a). However, in an alternative embodiment, the guide portions 90, 91 may be in the same plane as rolling contact between the bearing component 6 and the bearing surface 4, such that when the guide portions 90, 91 are engaged, the normal force is transmitted through the guide potions 90, 91 or the contact point moves from one side of the guide portions to the other. Guide tooth 90 is shown with a curved profile, though it will be appreciated that the shape may vary. As the second part 5 rolls such that the tooth 90 begins to enter the recess 91 (figure 7b) if the contact point 9 is incorrectly positioned along the bearing surface for the orientation of the second part 5 about the rotation axis 7 then portions of the tooth 90 and the recess 91 will come into contact. The tooth 90 and the recess 91 being formed with curved surfaces may facilitate sliding contact between the two in the event of misalignment of the contact point 9. Further rotation will generate a force acting parallel to the bearing surface causing the contact point to be shifted either upwards or downwards in the view of figure 7b. It will be appreciated that the direction of contact point shift 9 may be in the same direction that the contact point 9 is moving as the second part 5 rotates, or it may be in the opposite direction dependent on which side of the recess 91 engages the tooth 90 first. Figure 7c shows the second part 5 at an orientation in which the tooth 90 is fully received in the recess 91. The location of the contact point 9 (not visible in figure 7c as it is behind the tooth 90 in the illustrated view) is reset to its correct position for the current orientation (or within a narrow position range, dependent upon manufacturing tolerances for the guide portions 90, 91). The guide portions 90, 91 constrain the position of the contact point 9 within a portion of the rotational range of the second part 5. In the illustrated example, the portion of the rotational range is generally a middle portion of the rotational range, however it will be appreciated that it may be provided at any part of the range, by suitably locating the guide portions. Furthermore, there may be more than one arrangement of corresponding guide portions provided at different parts of the rotational range. For instance, there may be a plurality of teeth 90 and corresponding recesses 91. The plurality of guide portions may be arranged such that the contact point position is effectively continuously constrained throughout a rotation range. Or the plurality of guide portions may be separated such that between successive teeth being engaged the contact point position is not constrained. In a further variation the arrangement of tooth and recess may be reversed, such that the tooth protrudes from the second part 5. The shape of the recess may conform closely to the shape of the tooth, as illustrated, such that the position of the contact point 9 is narrowly constrained. However, in other examples the recess 91 may be enlarged such that contact between tooth and recess is only made if the contact point position has deviated from its expected position by more than a threshold amount. The constraining arrangement may in some examples include one or more pairs of complementary guide portions and one or more end-stop arrangement. It will be appreciated that the form of the guide portions may vary widely from that shown in figures 7a to 7c. At a minimum at least one guide portion or guide surface is defined by or coupled to the rotating second part such that it moves during rotation of the second part, and at least one guide portion or guide surface is defined by or coupled to the first part (both either fixedly or with a limited degree of movement) such that a complementary pair of guide portions or surfaces come into contact with one another during at least part of the range of rotation of the second part, if the contact point where the normal force is exerted by the second part on the first part is misaligned. Contact between the guide portions or surfaces results in a force acting upon the second part such that the misalignment of the contact point is reduced or removed. Bearing assembly of figures 8a and 8b Figures 7a to 7c illustrate an embodiment in which the complementary guide portions may come into contact throughout a relatively small proportion of the range of rotation of the second part. Figures 8a and 8b illustrates an alternative in which the guide portions are arranged such that the relationship between the position of the contact point and the orientation of the second part about the rotation axis is constrained for substantially the whole of the range of rotation. As for figures 7a to 7c, the constraining arrangement may further include one or more end-stop arrangements and / or one or more further pair of guide portions. Figures 8a and 8b show the second part 5 in two different rotational positions. The second part includes a bearing component 6 shown in this example as being part circular about a rotation axis 7 and configured to roll along a bearing surface 4 defined by a first part 2. The bearing component 6 exerts a normal force on the bearing surface 4. The normal force can be represented by a point force acting on the bearing surface 4 at a contact point 9 that moves along the bearing surface 4 (upwards and downwards in the view of figures 8a and 8b). The constraining arrangement of figures 8a and 8b comprises two pairs of complementary guide portions 90, 91. For each pair, a first guide portion 90 comprises a bearing surface and a second guide portion 92 comprises a wing extending from the second part 5. The bearing surfaces forming the first guide portions are defined by surfaces of further part 92, which may be either integrally formed with the first part 2 or connected to the first part 2. In the example of figures 8a and 8b the first guide portions 90 form a continuous bearing surface defined by a pair of arcs that are swept by the tips of the wings 91 as the second part 5 rotates and rolls along bearing surface 4. The pairs of guide portions 90, 91 are arranged relative to one another such that for at least a part of the range of rotation of the second part, contact between the guide portions 90, 91 limits a position of the contact point 9 along the bearing surface 4 to a first range of positions. As for the embodiments of figures 6 and 7a to 7c, the constraining arrangement - the pairs of complementary guide portions 90, 91 - is offset along the rotation axis 7 from contact between the bearing component 6 and the bearing surface 4. Accordingly, the rolling contact between the bearing component 6 and the bearing surface 4 is uninterrupted as the second part rotates, as will be understood from comparison of figures 8a and 8b. The wings 91 overlap at least a portion of the first part 2 defining the bearing surface 4. In the embodiment of figures 8a and 8b as the second part 5 rotates and rolls the tips of wings 91 sweep across the bearing surfaces 90. If the contact point 9 is correctly aligned the pairs of guide portions 90, 91 may be separated. If the contact point shifts from its expected position for a given orientation of second part 5 about its rotation axis 7, one or other wing 91 will engage its corresponding bearing surface 90 limiting the degree of contact point misalignment. A range of permitted positions for the contact point 9 along bearing surface 4 for a given orientation of second part 5 will be defined by a set spacing between the guide portions 90, 91. In the embodiment of figures 8a and 8b the pairs of guide portions 90, 91 effectively constrain the relationship between contact point position and second part orientation for the whole or substantially the whole of the range of rotation of the second part 5. However, it will be appreciated that this may be changed, for instance by changing the shape of further part 92 such that the wings and bearing surface only approach one another for a portion of the rotational range of second part 5. Other variations It will be appreciated that there may be many other variations of the above-described examples. The first, second, third, and fourth parts of the various embodiments described above may be formed from any suitable materials, including metals or polymer materials. Factors in material selection may include the required amount of static friction between the parts in order to minimise or avoid sliding movement of the bearing components during normal operation, but to permit sliding during operation of a constraining arrangement such as an end-stop arrangement or when the apparatus is subject to a large external force. The various embodiments described above show second parts in which bearing components are formed with spherical or curved two dimensional or three-dimensional profiles such that contact with a bearing surface of a first or third part is a point contact or a line contact that can roll along the bearing surface. However, the profile may comprise a polygonal shape such that rolling comprises planar contact and then pivoting about a vertex of the polygonal shape. The bearing assemblies and actuator assemblies described above may be suitable for use in portable electronic devices. As noted previously, a constraint on the relationship between a position of the contact point in at least one direction along the first bearing surface which is radial of the rotation axis and an orientation of the second part about the rotation axis for at least part of a range of rotation of the second part may comprise an end-stop arrangement at or close to an end of the range of rotation. In some examples a method of controlling an actuator assembly may comprise occasionally or periodically actuating the actuating unit to drive the rotation of the second part towards an end of the range of rotation in order to provide a centring force resetting the relationship. This may be done even if normal operation of the actuator assembly comprises operation in a centre part of the range of rotation in which the end-stop arrangements are not operational. Such a method of control may be implemented in computer-readable program code. SMA The above-described SMA actuator assemblies comprise at least one SMA element. Each SMA element may be divided into one or more SMA element segments. The term 'shape memory alloy (SMA) element' may refer to any element comprising SMA. The SMA element may be described as an SMA wire. The SMA element may have any shape that is suitable for the purposes described herein. The SMA element may be elongate and may have a round cross section or any other shape cross section. The cross section may vary along the length of the SMA element. The SMA element might have a relatively complex shape such as a helical spring. It is also possible that the length of the SMA element (however defined) may be similar to one or more of its other dimensions. The SMA element may be sheet-like, and such a sheet may be planar or non-planar. The SMA element may be pliant or, in other words, flexible. In some examples, when connected in a straight line between two components, the SMA element can apply only a tensile force which urges the two components together. In other examples, the SMA element may be bent around a component and can apply a force to the component as the SMA element tends to straighten under tension. The SMA element may be beam-like or rigid and may be able to apply different (for instance, non-tensile) forces to elements. The SMA element may or may not include material(s) and / or component(s) that are not SMA. For example, the SMA element may comprise a core of SMA and a coating of non-SMA material. Unless the context requires otherwise, the term 'SMA element' may refer to any configuration of SMA material acting as a single actuating element which, for example, can be individually controlled to produce a force on an element. For example, the SMA element may comprise two or more portions of SMA material that are arranged mechanically in parallel and / or in series. In some arrangements, the SMA element may be part of a larger SMA element. Such a larger SMA element might comprise two or more parts that are individually controllable, thereby forming two or more SMA elements. The SMA element may comprise an SMA wire, SMA foil, SMA film or any other configuration of SMA material. The SMA element may be manufactured using any suitable method, for example by a method involving drawing, rolling, deposition, sintering or powder fusion. The SMA element may exhibit any shape memory effect, for instance, a thermal shape memory effect or a magnetic shape memory effect, and may be controlled in any suitable way, for instance, by Joule heating, another heating technique or by applying a magnetic field. The SMA element may typically comprise nitinol (nickel titanium), although generally other types of SMA material may be used. Alternative ways of heating SMA The heating of the heat-activated actuator(s), such as SMA material, in order to cause the moving portion to move, could be achieved in a number of ways. In one arrangement, the material could be heated by passing a current through it. This current might come from a local or external power supply. Alternatively, the current might be induced in the wire by inductive coupling with an external alternating field. Where there are two actuators, the two actuators might be designed so that they couple to two different frequencies of the inductive power source, thus allowing the two actuators to be heated differentially. In another arrangement, the material could be heated by external radiation such as a visible or infra-red laser. The external radiation could be focussed so that one actuator is heated preferentially over another actuator, thus allowing differential actuation. Alternatively, or additionally, different actuators, or portions of the actuators, could be treated (for example with a surface coating) so that the different actuators heat at different rates depending on the nature (for instance, the frequency) of the incident radiation.

Claims

1. A bearing assembly comprising:a first part comprising a first bearing component;a second part comprising a second bearing component, the second part being configured to rotate relative to the first part about a rotation axis such that the second bearing component rolls along a first bearing surface defined by the first bearing component and exerts a normal force which can be represented by a point force acting on the first bearing surface at a contact point; anda constraining arrangement configured to constrain a relationship between a position of the contact point in at least one direction along the first bearing surface which is radial of the rotation axis and an orientation of the second part about the rotation axis for at least part of a range of rotation of the second part.

2. A bearing assembly according to claim 1, wherein the normal force causes frictional engagement between the first and second bearing components that limits or prevents the second bearing component from sliding along the first bearing surface unless the bearing assembly is subjected to an external force exceeding a threshold.

3. A bearing assembly according to claim 1 or claim 2, further comprising a biasing arrangement compressing the first and second bearing components together to generate the normal force.

4. A bearing assembly according to any one of the preceding claims, wherein rotation of the second part relative to the first part in a first rotational direction causes the contact point to move in a first direction along the first bearing surface for at least a part of the range of rotation of the second part.

5. A bearing assembly according to claim 4, wherein the constraining arrangement comprises at least one end-stop arrangement comprising first and second end-stop surfaces configured such that continued rotation of the second part relative to the first part about the rotation axis in the first rotational direction causes the first and second end-stop surfaces to contact one another.

6. A bearing assembly according to claim 5, wherein when in contact with one another the first and second end-stop surfaces form a pivot arrangement defining a pivot point; andwherein further rotation of the second part relative to the first part in the first rotational direction causes the second part to pivot about the pivot point such that the position of the contactpoint shifts along the first bearing surface in the first direction or a second direction opposite to the first direction.

7. A bearing assembly according to claim 6, wherein pivoting of the second part reduces the normal force.

8. A bearing assembly according to claim 6 or claim 7, wherein shifting of the position of the contact point in the first direction or the second direction comprises the second bearing component sliding along the first bearing surface.

9. A bearing assembly according to any one of claims 5 to 8, wherein the first end-stop is defined by the first part and the second end-stop is defined by the second part.

10. A bearing assembly according to any one of claims 6 to 9, wherein the constraining arrangement further comprises at least one further end-stop arrangement configured to shift the position of the contact point along the first bearing surface in the first direction or the second direction when the second part rotates relative to the first part in the first rotational direction or in a second rotational direction opposite to the first rotational direction.

11. A bearing assembly according to any one of the preceding claims, wherein the constraining arrangement comprises or further comprises complementary guide portions, the guide portions being arranged relative to one another such that for at least a part of the range of rotation of the second part, contact between the guide portions limits a position of the contact point along the first bearing surface to a first range of positions.

12. A bearing assembly according to any one of the preceding claims, wherein the assembly further comprises a third part moveable relative to the first part and configured such that at least a part of the second part is compressed between the first and third parts;wherein the second part is further configured to rotate relative to the third part about the rotation axis or a further rotation axis.

13. A bearing assembly according to claim 12, wherein the third part comprises a third bearing component and the second part further comprises a fourth bearing component configured to roll along a second bearing surface defined by the third bearing component and to exert a further normal force which can be represented by a point force acting on the second bearing surface at a further contact point; andwherein relative movement of the first part and the third part causes rotation of the second part relative to the first part or the third part.

14. A bearing assembly according to claim 12, wherein the third part comprises a third bearing component and the second bearing component is configured such that relative movement of the first part and the third part causes the second bearing component to roll along the first bearing surface and a second bearing surface defined by the third bearing component and to exert a further normal force which can be represented by a point force acting on the second bearing surface at a further contact point.

15. A bearing assembly according to any one of claims 12 to 14, wherein the constraining arrangement further comprises at least one further end-stop arrangement configured to shift the position of the further contact point along the second bearing surface when the second part rotates relative to the third part in at least one rotational direction.

16. An actuator assembly comprising:a bearing assembly according to any one of claims 1 to 11; andat least one actuating unit configured to apply an actuating force to the second part capable of rotating the second part relative to the first part.

17. An actuator assembly comprising:a bearing assembly according to any one of claims 12 to 15; andat least one actuating unit configured to apply an actuating force to the first part or the third part capable of rotating the second part relative to the first part or the third part.

18. An actuator assembly according to claim 16 or claim 17, wherein the actuating unit comprises at least one shape memory alloy, SMA, element coupled directly or indirectly to one of the first, second, or third parts.

19. A method of controlling an actuator assembly comprising an actuating unit, a rolling element, one or more pivot features and a support structure comprising a bearing surface, wherein the method comprises:actuating the actuating unit to drive the rolling element to roll in a first rolling direction along the bearing surface such that a pivot feature of the one or more pivot features contacts a surface, thereby shifting a contact point between the rolling element and the bearing surface along the bearing surface in a first shift direction.

20. A method according to claim 18, wherein the actuator assembly comprises an actuator assembly according to any one of claims 16 to 18.

21. A computer-readable storage medium having computer-readable program code stored therein that, in response to execution by a processor, causes the processor to perform the method of claim 19 or claim 20.

22. An apparatus comprising a processor and a memory storing executable instructions that, in response to execution by the processor, cause the apparatus to perform the method of claim 19 or claim 20.s