ASYMMETRIC SMA ACTUATOR

Abstract

An SMA actuator (10) comprising SMA wires (31, 32) in which the wire arrangement is asymmetrical, allowing a greater range of motion from a rest position in a first direction than in a second direction, which may be opposite or orthogonal to the first direction. Where the directions are opposite, the angle between a principal axis and the wires providing motion in the first direction may be different from the angle between the principal axis and the wires providing motion in the second direction.

Claims

1. An SMA actuation apparatus comprising: a support structure; a movable element supported on the support structure in a manner allowing movement of the movable element relative to the support structure; and a plurality of SMA actuator wires connected between the support structure and the movable element and arranged such that, on contraction, a first group of one or more of the SMA actuator wires provides a force on the movable element in a first direction and, on contraction, a second group of one or more of the SMA actuator wires provides a force on the movable element in a second direction, wherein: the first and second directions are opposite and lie along a principal axis of the apparatus; and the first and second groups of SMA actuator wires are arranged such that the range of possible motion of the movable element from a rest position in the first direction is greater than the range of possible motion of the movable element from the rest position in the second direction.

2. (canceled)

3. An SMA actuation apparatus according to claim 1, wherein the angle between the principal axis and each wire in the first group of SMA actuator wires is smaller than the angle between the principal axis and each wire in the second group of SMA actuator wires.

4. An SMA actuation apparatus according to claim 1 wherein the first and second direction are perpendicular to each other.

5. An SMA actuation apparatus according to claim 1 wherein a third group of the SMA actuator wires are arranged such that, on contraction, they provide a force on the movable element in a third direction, wherein the first and second directions are opposite and the third direction is orthogonal to the first and second directions, and the groups of SMA actuator wires are arranged such that the ranges of possible motion of the movable element from the rest position in each of said directions are different.

6. An SMA actuation apparatus comprising: a support structure; a movable element supported on the support structure in a manner allowing movement of the movable element relative to the support structure at least along a principal axis of the apparatus; and a plurality of SMA actuator wires connected between the support structure and the movable element and arranged such that when the actuator is at a rest position, the arrangement of SMA actuator wires as viewed from any direction perpendicular to the principal axis is not symmetric about any plane perpendicular to the principal axis.

7. An SMA actuation apparatus according to claim 6 wherein the angle between the principal axis and each wire pulling in a fourth direction along the principal axis is smaller than the angle between the principal axis and each wire pulling in a fifth direction which is opposite to said fourth direction.

8. An SMA actuation apparatus comprising: a support structure; a movable element supported on the support structure in a manner allowing movement of the movable element relative to the support structure; and a plurality of SMA actuator wires connected between the support structure and the movable element and arranged such that, on contraction, a first group of one or more of the SMA actuator wires provides a force on the movable element in a first direction and, on contraction, a second group of one or more of the SMA actuator wires provides a force on the movable element in a second direction, wherein: the first and second groups of SMA actuator wires are arranged such that when all the wires have the same strain, the average of the angles between each of the wires in the first group and the first direction is smaller than the average of the angles between each of the wires in the second group and the second direction.

9. An SMA actuation apparatus according to claim 8 wherein the first and second directions are opposite.

10. An SMA actuation apparatus according to claim 9 wherein the first and second directions lie along a principal axis of the apparatus, and the angle between the principal axis and each wire in the first group of SMA actuator wires is smaller than the angle between the principal axis and each wire in the second group of SMA actuator wires.

11. An SMA actuation apparatus according to claim 1 wherein there are eight SMA actuator wires inclined with respect to a principal axis of the apparatus, with two SMA actuator wires on each of four sides around the principal axis, the SMA actuator wires being connected such that on contraction two groups of four SMA actuator wires provide a force on the movable element with a component in opposite directions along the principal axis, the SMA actuator wires of each group being arranged with two-fold rotational symmetry about the principal axis.

12. An SMA actuation apparatus according to claim 1 further including a bearing which is arranged to guide the motion of the movable element.

13. An SMA actuation apparatus according to claim 1 wherein the apparatus is a camera apparatus and further comprising an image sensor fixed to the support structure, wherein the movable element comprises a lens element arranged to focus an image on the image sensor.

14. An SMA actuation apparatus according to claim 13 further comprising a control circuit electrically connected to the SMA actuator wires and arranged to provide drive signals to the SMA actuator wires.

15. An SMA actuation apparatus according to claim 14, wherein the control circuit is arranged to generate the drive signals in order to focus the image on the image sensor and wherein the SMA actuator wires are arranged such that the range of possible motion of the movable element along an optical axis of the lens element away from the image sensor is greater than the range of possible motion of the movable element along the optical axis towards the image sensor.

16. An SMA actuation apparatus according to claim 14 further comprising a vibration sensor arranged to generate output signals representative of the vibration of the apparatus, the control circuit being arranged to generate the drive signals in response to the output signals in order to stablise the image sensed by the image sensor.

17. An SMA actuation apparatus according to claim 16 wherein the SMA actuator wires are arranged such that the range of possible motion of the movable element in a direction perpendicular to an optical axis of the lens element is greater than the range of possible motion of the movable element along the optical axis.

18. An SMA actuation apparatus according to claim 13 wherein the apparatus is any one of: a smartphone, a camera, a foldable smartphone, a foldable smartphone camera, a foldable consumer electronics device, an image capture device, a foldable image capture device, an array camera, a 3D sensing device or system, a servomotor, a consumer electronic device, a mobile or portable computing device, a mobile or portable electronic device, a laptop, a tablet computing device, an e-reader, a computing accessory or computing peripheral device, a security system, a gaming system, a gaming accessory, a robot or robotics device, a medical device, an augmented reality system, an augmented reality device, a virtual reality system, a virtual reality device, a wearable device, a drone, an aircraft, a spacecraft, a submersible vessel, a vehicle, and an autonomous vehicle.

Description

[0040] Embodiments of the present techniques will now be described by way of example with reference to the accompanying figures in which:

[0041] FIG. 1 shows a wire arrangement of a regular 8-wire SMA actuator;

[0042] FIG. 2 is a schematic illustration of the range of motion of the actuator shown in FIG. 1;

[0043] FIG. 3 shows an SMA actuator according to an embodiment of the present techniques; and

[0044] FIG. 4 is a schematic illustration of the range of motion of the actuator shown in FIG. 3.

[0045] FIG. 1 shows the wire arrangement in an 8-wire actuator as described in WO2011/104518at the centre of motion. The actuator 1 comprises a moving element 2 and eight SMA wires 3 (of which six are visible in the figure). Each SMA wire 3 is attached at one end 4, denoted by a circle, to the static part (the support, not shown for clarity). At the other end 5 of each wire 3, it is attached to the moving element 2. The principal direction 6 is also shown. The moving element 2 may for example be, or carry, a lens element in which case the principal axis 6 is the optical axis and movement along the optical axis provides autofocus (AF) while movement in the plane perpendicular to the optical axis provides optical image stabilization (OIS).

[0046] FIG. 2 shows a cross section of the octahedron of motion 7 for the arrangement shown in FIG. 1, with the centre of motion 8 marked. Two regions 9 are marked where the difference in tension between the wires that pull the moving portion up and the wires that pull the moving portion down is sufficiently high that good performance is not possible.

[0047] FIG. 3 shows an 8-wire actuator 10 of an embodiment of the present techniques at its centre of motion. In this figure the wires which move the movable element 2 upwards are labelled 31 and those which move the movable element 2 down are labelled 32 (there are four of each, not all visible). The principal plane is the plane perpendicular to the principal axis 6, and in FIG. 3 the upper and lower surfaces of the movable element 2 are shown parallel to this plane. In the actuator of this embodiment, the angle 11 between the principal plane and the wires 31 that pull the moving portion upwards is larger than the angle 12 between the principal plane and the wires 32 that pull the moving portion downwards.

[0048] FIG. 4 shows a cross section 71 of the octahedron of motion for the arrangement shown in FIG. 2, with the centre of motion 81 marked. Two regions 91 are marked where the difference in tension between the wires that pull the moving portion up and the tension that pull the moving portion down is sufficiently high that good performance is not possible.

[0049] In the arrangement according to an embodiment of the present techniques, as shown in FIGS. 3 and 4, it can be seen that the maximum motion in a direction normal to the principal axis, denoted by line 41, is closer to the bottom 42 of the range of motion along the principal axis 6. It can also be seen that the maximum motion in a direction normal to the principal axis is less than the maximum motion in a direction along the principal axis.

[0050] Such an asymmetric 3-D actuator is of particular use in applications where it is desired to have more motion in a first dimension than in second and third dimensions, and also where the motion in that first dimension is desired to be greater in one direction than in its opposite direction relative to the point of maximum stroke in the second and third dimensions.

[0051] This is useful for example in a miniature camera such as used in mobile phones, where the SMA actuator serves to provide Autofocus (AF) and Optical Image Stabilization (OIS). In this case the movable element is or carries the lens element of the camera, the principal direction is the optical axis of the lens element and the principal plane is perpendicular to this. If the optical axis is labelled the z direction, then the x-y plane is perpendicular to it and movement in this plane provides OIS. The amplitude of motion needed for OIS is considerably less than the amplitude of motion required for AF along the optical axis. The x-y excursion may by about 80 microns while the range of z motion is required to be 300 microns.

[0052] In the camera application, at rest the lens element is close to the image sensor, providing infinity focussing. In use, the lens is moved along the optical axis in the direction away from the image sensor in the positive z direction, in order to bring near scenes into focus. The majority of images are taken when the optical element is close to the image sensor and, so it is desirable that the maximum x-y excursion when the optical element is in this position. It is not required for the lens to move from this position in the negative z direction. It is therefore advantageous to have an actuator with asymmetric stroke in the z direction, since by doing so it increases the available +z stroke.

[0053] A first example of the present techniques is as follows. An 8-wire SMA actuator is provided in order to move the lens in a mobile phone camera to facilitate AF by moving the lens along the optical axis of the lens and to facilitate OIS by moving the lens in the plane perpendicular to the optical axis. The actuator is 8.5 mm8.5 mm and has a height of 2.5 mm. The lens element is the movable element and the optical axis of the lens element is the principal axis.

[0054] In a symmetrical design of an 8-wire SMA actuator, at the centre of motion the wires make an angle of 12 degrees to the plane of the OIS motion (the principal plane). The length of the wires between the points where the wires are attached to the moving portion and the static portion is 6.12 mm.

[0055] When the lens is moved 200 m from the centre of motion along the optical axis, then the wires that pull the lens in that direction (pulling wires), make an angle of 10.16 degrees to the plane of OIS motion while the wires that pull the actuator back towards the centre of motion (return wires) make an angle of 13.82 degrees to the plane of OIS motion. If we ignore the effect of gravity and any springs in the system on the wire tensions, then the pulling wires will have a tension that is 35% higher than the return wires. This difference in tension means that if the tension in the pulling wires is not allowed to increase to prevent damage to the wires, then the return wires will have a tension that is 35% lower. This means that the transition temperature of these wires will be reduced which will significantly reduce the rate at which these wires are able to cool and consequently the speed at which the lens can be moved.

[0056] According to the present techniques the symmetry of the actuator is changed at the centre of motion or conversely the centre of motion is moved away from the position where all the wires have the same angle to the plain of the OIS motion.

[0057] One example of a non-symmetrical design according to the present techniques, based on the above example is as follows. At the centre of motion, the pulling wires make an angle of 13 degrees to the plane of OIS motion and the return wires make an angle of 11 degrees to the plane of OIS motion. The length of both sets of wires as projected onto the plane of OIS motion is the same, but due to the difference in wire angle the length of the pulling wires is 6.140 mm while the length of the return wires is 6.095 mm.

[0058] When the lens is moved 109 m from the centre of motion along the optical axis in the direction pulled by the pulling wires, then the angle that the pulling wires make to the plane of OIS motion is reduced to 12 degrees and the angle that the return wires make to the plane of OIS motion is increased to 12 degrees.

[0059] When the lens is moved 200 m from the centre of motion, along the optical axis in the direction pulled by the pulling wires, then the angle that the pulling wires make to the plane of OIS motion is 11.2 degrees and the angle that the return wires make to the plane of OIS motion is 12.8 degrees. If we ignore the effect of gravity and any springs in the system on the wire tensions, then the pulling wires will have a tension that is only 15% higher than the return wires. This is a much smaller difference in tension than the 35% observed in the symmetrical design and allows more controllable motion.

[0060] Whilst embodiments and examples of the present techniques have been explained with reference to an actuation apparatus having eight SMA actuator wires, the skilled person will appreciate that the principles are equally applicable to actuation apparatuses having different numbers of actuator wires and/or different arrangements of actuator wires. In particular, the principles are applicable to arrangements having four actuator wires and arrangements in which the movable element is supported by a bearing which guides the motion of the movable element, for example to guide the motion of the movable element along the principal axis.

[0061] Those skilled in the art will appreciate that while the foregoing has described what is considered to be the best mode and where appropriate other modes of performing present techniques, the present techniques should not be limited to the specific configurations and methods disclosed in this description of the preferred embodiment. Those skilled in the art will recognise that present techniques have a broad range of applications, and that the embodiments may take a wide range of modifications without departing from any inventive concept as defined in the appended claims.