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The present invention relates to an electro-active device having an electro-active structure extending along a minor axis ( 36 ) which is curved, for example in a helix around a major axis ( 37 ). The electro-active structure comprises successive electro-active portions ( 35 ) extending around said

The present invention relates to an electro-active device having an electro-active structure extending along a minor axis ( 36 ) which is curved, for example in a helix around a major axis ( 37 ). The electro-active structure comprises successive electro-active portions ( 35 ) extending around said minor axis and arranged with electrodes to bend, when activated, around the minor axis ( 36 ) such that bending of the successive portions ( 35 ) is concomitant with rotation of the electro-active structure about the minor axis ( 36 ) adding incrementally along the minor axis ( 36 ). The electro-active structure generates displacement out of the local plane in which the minor axis is curved. The electra-active structure may be continuous electro-active member ( 35 ) extending along and curving around the minor axis ( 35 ), for example helically to form a helical helix. Alternatively, the electro-active structure may be a plurality of discrete elements ( 211 ) connected together. The electro-active portions preferably have a bender structure formed from a plurality of layers. The device may be used as a driver, a sensor or a generator.

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1. An electro-active device having an electro-active structure extending along a minor axis, the minor axis being curved with a total curvature of at least 30°, the electro-active structure comprising successive electro-active portions extending around said minor axis and arranged with electrod

1. An electro-active device having an electro-active structure extending along a minor axis, the minor axis being curved with a total curvature of at least 30°, the electro-active structure comprising successive electro-active portions extending around said minor axis and arranged with electrodes to bend, when activated, around the minor axis such that bending of the successive portions is concomitant with rotation of the electro-active structure about the minor axis adding incrementally along the minor axis. 2. An electro-active device as claimed in claim 1, wherein the electro-active structure comprises a continuous electro-active member extending along and curving around the minor axis, such that successive finite portions of the continuous member constitute said successive electro-active portions. 3. An electro-active device as claimed in claim 2, wherein the continuous electro-active member is a tape. 4. An electro-active device as claimed in claim 2, wherein:the continuous electro-active member extends along the minor axis in a helix around the minor axis. 5. An electro-active device as claimed in claim 4, wherein the continuous electro-active member has layers oriented across the width of the member extending parallel to the minor axis. 6. An electro-active device as claimed in claim 2, wherein the continuous electro-active member has a shape which is twisted along the minor axis. 7. An electro-active device as claimed in claim 2, wherein the continuous electro-active member has electrodes in separate sections along the length of the minor axis. 8. An electro-active device as claimed in claim 2, wherein the electro-active structure comprises a plurality of inter-twined continuous electro-active members each extending along and curving around the minor axis. 9. An electro-active device as claimed in claim 1, wherein electro-active structure comprises a plurality of discrete electro-active elements connected together to constitute said successive electro-active portions. 10. An electro-active device as claimed in claim 9, wherein successive electro-active elements along the minor axis extend in the same sense around the minor axis so that bending of the successive elements of electro-active material in the same sense around the minor axis is concomitant with said rotation. 11. An electro-active device as claimed in claim 9, wherein successive discrete electro-active elements along the minor axis extend in alternate senses around the minor axis so that bending of the successive elements of electro-active material in alternate senses around the minor axis is concomitant with said rotation. 12. An electro-active device as claimed in claim 9, wherein the plurality of discrete electro-active elements are separately formed and connected by separate connecting elements. 13. An electro-active device as claimed in claim 9, wherein the plurality of discrete electro-active elements are connected by connecting portions which, together with the plurality of discrete electro-active elements, are formed as a unitary member. 14. An electro-active device as claimed in claim 1, wherein the minor axis extends in a curve which is one of: a helix; a spiral; a series of at least two coaxial spirals; a series of at least two coaxial helices; a circle; or an arc of a circle. 15. An electro-active device as claimed in claim 1, wherein the minor axis curves in a curve consisting of straight portions between at least one bend. 16. An electro-active device as claimed in claim 1, wherein the minor axis is curved with a total curvature of at least 90°. 17. An electro-active device as claimed in claim 1, wherein the minor axis curves in an irregular curve. 18. An electro-active device as claimed in claim 1, wherein the minor axis curves in a curve which curves around a single major axis and subtends less than 360° at the major axis. 19. An electro-active device as claimed in claim 1, wherein:the electro-active portions include piezoelec tric material; andthe piezoelectric material is a piezoelectric ceramic or a piezoelectric polymer. 20. An electro-active device as claimed in claim 1, wherein:the electro-active portions include piezoelectric material; andthe piezoelectric material is lead zirconate titanate (PZT) or polyvinylidenefluoride (PVDF). 21. An electro-active device as claimed in claim 1, wherein:the successive electro-active portions have at least two layers of electro-active material to act as a bimorph or multimorph. 22. An electro-active device as claimed in claim 1, wherein:the successive electro-active portions have a plurality of layers including at least one layer of electro-active material; anda sensor layer with associated electrodes. 23. An electro-active device as claimed in claim 1, wherein:along the length of the minor axis the electro-active structure has a variation to vary the degree of bending or the stiffness;the variation being in at least one of: the width of the electro-active portions; the cross-section of the electro-active portions; the cross-section of one or more layers of the electro-active portions; the number of layers; or the composition of at least one layer. 24. An electro-active device as claimed in claim 1, wherein the electro-active structure is provided with external polymeric or elastomeric material. 25. An electro-active device as claimed in claim 2, wherein the continuous electro-active member extends along the minor axis in a curve having at least two turns around the minor axis. 26. An electro-active device as claimed in claim 2, wherein the continuous electro-active member has been formed by being wound around the minor axis. 27. An electro-active device as claimed claim 2, wherein the continuous electro-active member is formed with conductive electrodes extending continuously along the electro-active member. 28. An electro-active device having a continuous electro-active member extending along a minor axis, the minor axis being curved with a total curvature of at least 30°, the continuous electro-active member curving around the minor axis and arranged with electrodes to bend, when activated, around the minor axis such that bending of successive finite portions of the continuous electro-active member is concomitant with rotation of the electro-active member about the minor axis adding incrementally along the minor axis. 29. An electro-active device having an electro-active structure extending along a minor axis, the minor axis being curved with a total curvature of at least 30°, the electro-active structure comprising a plurality of discrete electro-active elements coupled together to extend around said minor axis and arranged with electrodes to bend, when activated, around the minor axis such that bending of the successive electro-active elements is concomitant with rotation of the electro-active structure about the minor axis adding incrementally along the minor axis. 30. An electro-active device having an electro-active structure extending along a minor axis, the minor axis extending in a curve with a total curvature of at least 30°, the electro-active structure comprising successive electro-active portions extending around said minor axis and arranged with electrodes to bend, when activated, around the minor axis, the successive electro-active portions being coupled together such that bending of the successive portions is concomitant with rotation of the electro-active structure about the minor axis adding incrementally along the minor axis, which rotation is concomitant with displacement of the structure out of the local plane of the curve of the minor axis. 31. An electro-active device as claimed in claim 30, wherein the electro-active structure comprises a continuous electro-active, member extending along and curving around the minor axis, such that successive finite portions of the continuous member constitute said successive electro-active portions. 32. A method of making an electro -active device comprising winding a deformable, continuous electro-active member into a structure as claimed in claim 28. 33. A method as claimed in claim 32, further comprising initially making the continuous electro-active member by extrusion, co-extrusion or calendering. 34. A method as claimed in claim 33, wherein said step of winding the continuous electro-active member comprises:winding the continuous electro-active member into a curve around a minor axis; andcurving the wound continuous electro-active member. 35. A method as claimed in claim 33, wherein said step of winding the continuous electro-active member comprises flat-winding the continuous electro-active member around a minor former. 36. A method as claimed in claim 35, wherein:said minor former is straight and deformable; andsaid step of curving the minor curve of the continuous electro-active member comprises curving the minor former and the continuous electro-active member. 37. A method as claimed in claim 33, further comprising setting the electro-active member. 38. A method as claimed in claim 37, wherein:the deformable, continuous electro-active member is a sinterable plasticised ceramic mixture; andsaid step of setting the electro-active member comprises burning out the plasticising materials and densifying the resultant material through sintering. 39. A method of making an electro-active device comprising connecting together a plurality of separate electro-active elements by connecting elements to form a structure as claimed in claim 29. 40. A method of making an electro-active device comprising:cutting an elongate electro-active member to form discrete electro-active elements positioned longitudinally along the elongate member and connected by connecting portions;curving the elongate electro-active member extending longitudinally along the elongate member; andcurving the curved elongate electro-active member around the major axis to form a structure as defined in claim 13. 41. A method of converting an electrical signal into relative displacement comprising using an electro-active device as claimed in claim 1 as a driver to convert a signal applied to the electrodes of the electro-active device into relative displacement. 42. A method of converting displacement into an electrical signal comprising using of an electro-active device as claimed in claim 1 as a sensor to convert relative displacement into a signal on the electrodes of the electro-active device. 43. A lens positioning device comprising an electro-active device as claimed in claim 1 mounting a lens for movement of the lens by activation of the electro-active device. 44. A positioning device comprising:a support;an object to be positioned;a plurality of electro-active devices each as claimed in claim 1 and being connected to the support and the object to be positioned. 45. An electro-active device according to claim 1, wherein the successive electro-active portions have a plurality of layers including at least one layer of electro-active material. 46. An electro-active device as claimed in claim 28, wherein the continuous electro-active member is a tape. 47. An electro-active device as claimed in claim 28, wherein the continuous electro-active member extends along the minor axis in a helix around the minor axis. 48. An electro-active device as claimed in claim 28, wherein the continuous electro-active member has layers oriented across the width of the member extending parallel to the minor axis. 49. An electro-active device as claimed in claim 28, wherein the continuous electro-active member has a shape which is twisted along the minor axis. 50. An electro-active device as claimed in claim 28, wherein the continuous electro-active member has electrodes in separate sections along the length of the minor axis. 51. An electro-active device as claimed in claim 28, wherein the electro-active structure comprises a plurality of inter-twined continuous electro-active members each extending along and c urving around the minor axis. 52. An electro-active device as claimed in claim 28, wherein the minor axis curves in a curve consisting of straight portions between at least one bend. 53. An electro-active device as claimed in claim 28, wherein the minor axis is curved with a total curvature of at least 90°. 54. An electro-active device as claimed in claim 28, wherein the minor axis curves in an irregular curve. 55. A electro-active device as claimed in claim 28, wherein the minor axis curves in a curve which curves around a single major axis and subtends less than 360° at the major axis. 56. An electro-active device as claimed in claim 28, wherein the minor axis extends in a curve which is one of: a helix; a spiral; a series of at least two coaxial spirals; a series of at least two coaxial helices; a circle; or an arc of a circle. 57. An electro-active device as claimed in claim 28, wherein the electro-active member includes piezoelectric material, the piezoelectric material being a piezoelectric ceramic or a piezoelectric polymer. 58. An electro-active device as claimed in claim 28, wherein the electro-active member includes piezoelectric material, the piezoelectric material being lead zirconate titanate (PZT) or polyvinylidenefluoride (PVDF). 59. An electro-active device as claimed claim 28, wherein the electro-active member has a plurality of layers including at least one layer of electro-active material. 60. An electro-active device as claimed in claim 28, wherein the electro-active member has at least two layers of electro-active material to act as a bimorph or multimorph. 61. An electro-active device as claimed in claim 28, wherein the electro-active member has a plurality of layers including at least one layer of electro-active material and a sensor layer with associated electrodes. 62. An electro-active device as claimed in claim 28, wherein along the length of the minor axis the electro-active member has a variation to vary the degree of bending or the stiffness, the variation being in at least one of: the width of the electro-active member; the cross-section of the electro-active member; the cross-section of one or more layers of the electro-active member; the number of layers of the electro-active member; or the composition of at least one layer of the electro-active member. 63. An electro-active device as claimed in claim 28, wherein the electro-active structure is provided, with external polymeric or elastomeric material. 64. An electro-active device as claimed in claim 28, wherein the continuous electro-active member extends along the minor axis in a curve having at least two turns around the minor axis. 65. An electro-active device as claimed in claim 28, wherein the continuous electro-active member has been formed by being wound around the minor axis. 66. An electro-active device as claimed claim 28, wherein the continuous electro-active member is formed with conductive electrodes extending continuously along the electro-active member. 67. An electro-active device as claimed in claim 29, wherein successive electro-active elements along the minor axis extend in the same sense around the minor axis so that bending of the successive elements of electro-active material in the same sense around the minor axis is concomitant with said rotation. 68. An electro-active device as claimed in claim 29, wherein successive discrete electro-active elements along the minor axis extend in alternate senses around the minor axis so that bending of the successive elements of electro-active material in alternate senses around the minor axis is concomitant with said rotation. 69. An electro-active device as claimed in claim 29, wherein the plurality of discrete electro-active elements are separately formed and connected by separate connecting elements. 70. An electro-active device as claimed in claim 29, wherein the plurality of discrete electro-active elements are connected by connecting portions which, together with the plurality of discrete electro-active elements, are formed as a unitary member.