LENS MOUNTING

Abstract: The invention relates to a lens mounting suitable for use in a high performance optical instrument. The mounting (10) comprises a body or housing, a linear guide (16; 18; 36; 46; 64) mounted on the body, a lens carriage (14; 34; 44; 62) moveable along the linear guide, and a linear drive, which may be in the form of a ball-screw drive and comprising a ball-screw (20; 80) extending substantially parallel to the linear guide and a ball-screw follower (22; 68) linked to the lens carriage such that rotation of the ball-screw moves the carriage axially along the guide. One or more elements of the ball-screw drive are mounted relative to the linear guide to permit a degree of relative movement therebetween to accomodate misalignment between the linear guide and ball-screw.

Description: LENS MOUNTING FIELD OF THE INVENTION This invention relates to a lens mounting for a high performance optical instrument. BACKGROUND OF THE INVENTION High performance optics often imply very tight tolerances for positioning lenses both axially and radially. To satisfy such tolerances lens carriages are often mounted on rolling element linear guides, and ball-screws provided for axially locating the lens carriages. The precision required in high performance optical instruments including such components gives rise to tight mechanical alignment requirements; errors in alignment of just a few microns can cause increases in friction sufficient to seize the mechanism. Various methods are utilised to address this problem, generally utilising some means of careful alignment of the guides and ball-screw followed by fixation. Providing such accurate alignment adds considerably to the costs of the mechanism, and also the mechanism remains sensitive to small deformations in its structure, for example due to thermally induced stresses. It is among the objects of the present invention to obviate and mitigate these disadvantages. SUMMARY OF THE INVENTION According to the present invention there is provided a lens mounting for a high performance optical instrument, the mounting comprising: a body; a linear guide mounted on the body; a lens carriage moveable along the linear guide; and a linear drive mounted on the body with a drive member extending substantially parallel to the linear guide, and a drive member follower being linked to the lens carriage such that actuation of the drive member or drive member follower moves the carriage axially along the guide, one or more elements of the linear drive being mounted relative to the linear guide to permit a degree of relative movement therebetween to accommodate misalignment between the linear guide and the drive member. Preferably, the linear drive is in the form of a ball- screw drive comprising a ball-screw extending substantially parallel to the linear guide and a ball-screw follower linked to the lens carriage such that rotation of the ball-screw moves the carriage axially along the guide. Preferably also, the coupling between the ball-screw drive and the linear guide substantially precludes said relative movement in the axial direction to ensure accurate axial location of the lens carriage by the ball-screw drive. The ball-screw may be mounted relative the body to accommodate a degree of relative movement therebetween, and thus between the ball-screw and the linear guide; one or more of the bearings supporting the ball-screw may be mounted to the body to permit such movement, for example by mounting an end bearing in an elastomeric compound. This arrangement allows provision of a substantially rigid coupling between the ball-screw follower and the lens carriage. Alternatively, a coupling may be provided between the ball- screw follower and the lens carriage that allows for a limited degree of relative movement therebetween; such a coupling may be in the form of a flexure, a spherical bearing or an elastomeric element. BRIEF DESCRIPTION OF THE DRAWINGS This and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end view of a lens mounting in accordance with a first embodiment of the present invention; Figure 2 is a perspective view from above of a flexure of the mounting of Figure 1; Figure 3 is an end view of a lens mounting in accordance with a second embodiment of the present invention; Figure 4 is an end view of a lens mounting in accordance with a third embodiment of the present invention; Figure 5 is an end view of a lens mounting in accordance with a fourth embodiment of the present invention; and Figure 6 is a side view of parts of a lens mounting in accordance with a fifth embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is first made to Figures 1 and 2 of the drawings which illustrate, somewhat schematically, a lens mounting 10 in accordance with a first embodiment of the present invention. The lens mounting 10 is itself mounted on a body or within a housing (not shown) . The illustrated arrangement is typical of an application in which relatively small displacements of a lens are required, such as for focus mechanisms. A lens 12 is mounted in a carriage 14 itself mounted on two linear guides 16, 18 in the form of cylindrical rods. One of the rods 16 provides a reference, while the other rod 18 is utilised to prevent rotation. A ball-screw 20 extends parallel to the guides 16, 18, and a ball-screw follower or nut 22 is rotatably mounted within a housing 24. An angled flexure 26 of a flexible but stiff material connects the housing 24 to the lens carriage 14 such that rotation of the ball-screw 20, resulting in the nut 22 travelling along the ball-screw, moves the lens carriage 14 axially along the guides 16, 18. The flexure 26 comprises a pair of thin planar members 28, 29 at a right angle to one another and arranged to lie in planes perpendicular to the guides 16, 18 and ball-screw 20. Thus, the flexure 26 is relatively rigid in the axial or Z- direction, to ensure accurate axial location of the lens carriage 14 but will accommodate bending in other planes and will also accommodate bending as a result of torsion. Accordingly, any minor misalignment between the ball-screw 20 and the guides 16, 18 may be accommodated by bending of the flexure 26. For larger lens displacements, such as required in zoom telescopes, the relatively low stiffness of rods, and the inability to support them along their length, becomes a problem. Thus, where larger displacements are required the arrangement as illustrated in Figure 3 of the drawings may be utilised. In this lens mounting 30, in accordance with a second embodiment of the present invention, the lens 32 and the lens carriage 34 are mounted on a linear rail 36, which may be supported along its length by a part of the lens body 31. The ball-screw 20 and flexure 26 are similar to those described in the first embodiment. Reference is now made to Figure 4 of the drawings, which, like the embodiment illustrated in Figure 3, provides a lens 42 mounted in a lens carriage 44 movable along a linear rail 46. However, the linkage between the ball-screw nut housing 48 and the carriage 44 is in the form of a spherical (rose) bearing 50. In addition, the pin 52 extending from the housing 48 to engage the bearing 50 is slidable in the bearing. This embodiment offers the advantage that the stresses induced in the mounting 40 between the linear rail and ball- screw are very low, being due only to friction in the bearing. However, due to the necessary clearances in the bearing 50, which may introduce backlash, the positioning accuracy of the mounting 40 may be lower than the first and second embodiments described above. Reference is now made to Figure 5 of the drawings which illustrates a fourth embodiment of the present invention. Again, the mounting 60 comprises a lens carriage 62 mounted on a linear rail 64. In this particular embodiment the housing 66 for the ball-screw nut 68 is mounted directly to the lens carriage 62. However, the nut 68 is "potted" within the housing 66, that is provided with an elastomeric mounting 70. This arrangement relies on the fact that the forces tending to deviate the nut 68 are larger than those needed to drive the linear carriage 62; the mounting 70 will only be subject to deformation when there is misalignment of the ball-screw and rail 64, but will not deform during normal usage. Reference is now made to Figure 6 of the drawings which illustrates a ball-screw 80 in accordance with a fifth embodiment of the invention and which offers a somewhat different solution from the embodiments described above: rather than providing a flexible linkage between the ball- screw nut housing and the lens carriage it is possible to mount the ball-screw 80 in such a manner that misalignment between the ball-screw 80 and linear guide (not shown) will be accommodated by movement of the ball-screw itself, as described below. A ball-screw 80 is generally mounted on rotary bearings 82, 84. One end of the ball-screw may be provided with a duplex bearing 82 to provide good axial stiffness, while the other end has a simple locating radial bearing 84. If this radial bearing 84 is mounted in an elastomeric compound 86 a small amount of flexibility is provided which permits the mechanism to continue to move in the event of misalignment. Although this method does not obviate in requirement for initial accurate alignment, it does allow some flexibility to account for thermally induced deformation. It will be apparent to those of skill in the art that the above-described embodiments provide relatively simple means for avoiding difficulties associated with the tight mechanical requirements alignment of ball-screws and linear guides in high performance optical instruments. It will also be apparent to those of skill in the art that the above- described embodiments are merely exemplary of the present invention and that various modifications and improvements may be made thereto, without departing from the scope of the invention.

Claims: CLAIMS 1. A lens mounting for a high performance optical instrument, the mounting comprising: a body; a linear guide mounted on the body; a lens carriages moveable along the linear guide; and a linear drive mounted on the body and including a drive member extending substantially parallel to the linear guide, and a drive member follower being linked to the lens carriage such that actuation of the drive member or drive member follower moves the carriage axially along the guide, one or more elements of the linear drive being mounted relative to the linear guide to permit a degree of relative movement therebetween to accommodate misalignment between the linear guide and the drive member. 2. The mounting of claim 1 wherein the linear drive is in the form of a ball-screw drive comprising a ball-screw extending substantially parallel to the linear guide and a ball-screw follower linked to the lens carriage such that the rotation of the ball-screw moves the carriage axially along the guide. 3. The mounting of claim 2 where the coupling between the ball-screw and the linear guide substantially precludes said relative movement in the axial direction, to ensure accurate axial location of the lens carriage by the ball-screw drive. 4. The mounting of claim 2 or 3 wherein the ball-screw is mounted relative to the body to accomodate a degree of relative movement therebetween, and thus between the ball- screw and the linear guide. 5. The mounting of claim 4 wherein the ball-screw is supported on bearings and one or more of the bearings is mounted to the body to permit a degree of relative movement therebetween. 6. The mounting of claim 5 wherein said one or more of the bearings supporting the ball-screw are mounted in an elastomeric material. 7. The mounting of claim 2 or claim 3 wherein a flexible coupling is provided between the ball-screw follower and the lens carriage and allows for a limited degree of relative movement therebetween. 8. The mounting of claim 7 wherein said coupling is in the form of a flexure. 9. The mounting of claim 8 wherein the flexure comprises a pair of planar members at an angle to one another and arrange to lie in planes perpendicular to the linear guide and ball- screw. 10. The mounting of claim 7 wherein the coupling is in the form of a spherical bearing. 11. The mounting of claim 10 wherein said bearing includes a pin extending from the ball-screw follower and which is slideable in the bearing. 12. The mounting of claim 7 wherein the coupling is in the form of an elastomeric element. 13. The mounting of claim 12 wherein the ball-screw follower is potted in an elastomeric material.

Inventors: EVANS GRAHAM, GARDAM ALLAN, PARR-BURMAN PHILIP MICHAEL, EVANS, GRAHAM, GARDAM, ALLAN, PARR-BURMAN, PHILIP, MICHAEL
Applicants: EVANS GRAHAM, GARDAM ALLAN, PARR BURMAN PHILIP MICHAEL, PILKINGTON PERKIN ELMER LTD, EVANS, GRAHAM, GARDAM, ALLAN, PARR-BURMAN, PHILIP, MICHAEL, PILKINGTON P.E. LIMITED
Application year: 1995
Main classification: G02B7
IPC codes: G02B7