POSITION SENSOR

Abstract: Contact lenses can be made by machining an inner concave surface into a plastics material blank, then mounting the blank in a lathe and machining the outer convex surface thereon. Repeated measurement with a micrometer is needed during the latter machining to ensure that the lens vertex is correctly formed. To avoid this, the sensor described herein comprises a laser source (12) from which radiation passes over a knife edge member (13) and the blank (10) to a detector (15), the items being arranged so that, when the lens has been correctly machined, a surface diffraction effect occurs leading to a detectable signal from the detector (15). The disclosed sensor apparatus could be used during manufacture of items other than contact lenses.

Description: This invention relates to a sensor for detecting the position of a surface or interface and in particular, but not exclusively, to a sensor for determining the position of the vertex of a partially fabricated contact lens during the manufacture thereof. In one known method of manufacture of contact lenses for example that disclosed in U.S. Specification No. 4,455,901, a cylindrical blank of plastics material is machined on a lathe to form the inner concave surface of the lens to be produced. Manufacture of the lens is completed by attaching the machined concave surface to a domed metal arbour by means of wax or other suitable meltable material, mounting the metal arbour in the chuck of the lathe and machining the exposed material to form a convex surface defining the outer surface of the lens and then removing the lens from the arbour. It will be appreciated that, in order to ensure that the lens is neither too thick or too thin, it is necessary to be able to determine the vertex position - that is the point of intersection of the axis of symmetry of the lens with the inner surface - with a high degree of accuracy. If the lens is too thick or too thin it will need to be scrapped because it will not perform to its optical design specifications. At present, the position of the vertex of the lens is determined by using a micrometer to measure the distance between the vertex of the concave surface and the plain end of the blank, and then marking this dimension on the arbour. Knowing this dimension the blank is machined to form the concave surface defining the outer surface of the lens at a distance to provide a lens of the required thickness. This process is however time consuming and does not provide sufficient accuracy for production of thin lenses. In the particular manufacturing technique studied, it was necessary to be able to determine the position of the vertex with an accuracy in the region of ±3.0 microns. A need therefore exists for a sensor capable of determining the vertex position of the formed surface of a partially fabricated contact lens with sufficient accuracy to enable the other surface to be formed to produce a contact lens having an acceptable thickness. CH PS No. 238858 discloses a method and apparatus for setting the position of a shiny object so that its surface is at a predetermined position, in which an illumination means projects an optical image of a sight mark onto a screen beyond the predetermined position so that, as the object is adjusted towards the predetermined position, there can be seen on the screen two sight mark images, one received direct and one reflected from the shiny surface. When the two images are made to coincide, the reflecting surface is aligned with the required position. According to the invention there is provided apparatus for detecting the presence of surface or interface at a predetermined position within a sensing region with respect to the apparatus, the apparatus comprising: illuminating means for directing a collimated beam of optical radiation along a sensing axis through said sensing region; optical radiation detector means aligned with said sensing axis and located beyond said sensing region, and characterised by knife edge means located between said sensing region and said optical radiation detector means and arranged to protrude at least partly into said collimated beam in a given direction. By this arrangement, when the surface or interface is in the predetermined position within the sensing region, the beam of radiation is caused to be deflected by a surface diffraction effect thus altering the intensity of the radiation detected by the optical radiation detector means. The inventor has observed that as a surface is moved through the sensing region in a direction transverse to the sensing axis a curve representing intensity vs axial position reaches a sharply defined, symmetric peak at a unique position which is thought to be caused by surface diffraction effects akin to those which give rise to the sunburst phenomenon observed at dusk or dawn. It will be appreciated that the sharpness of the peak allows a datum position to be defined with a high degree of accuracy and moreover since the peak is symmetrical its exact location may be determined by taking the mean position of two equal intensity points one either side of the peak. Preferably, said illuminating means includes a laser source. Preferably, said optical radiation detects means comprises objective lens means adapted to form an image of the sensing region at an image plane and eyepiece means adapted to focus said image on to a detector element. In this arrangement a slit means may advantageously be provided at said image plane to select a predetermined portion of the sensing region for examination. By way of example only, two specific embodiments of this invention will now be described in detail, reference being made to the accompanying drawings, in which:- Figure 1 is a schematic view from above on part of a contact lens lathe, illustrating a first embodiment of position sensor; Figure 2 is a graph plotting intensity against axial position for the arrangement of Figure 1. Figure 3 is a view showing the view through the microscope of Figure 1, and Figure 4 is a schematic view of a second embodiment of position sensor. The position sensors illustrated in the Figures are designed to be attached to the tool slide of a computer controlled contact lens lathe to allow the vertex position of the concave inner surface of a contact lens to be determined with sufficient accuracy to enable a lens of the correct thickness to be produced. A lathe of this type is of conventional construction, including a chuck for holding a workpiece and a toolslide carrying a tool for machining a workpiece held in the chuck. In the manufacture of contact lenses a cylinder blank of plastics material is initially machined to provide the inner concave surface of the lens. The concave surface of the lens is then attached to the convex surface of a brass holder known as an arbour using a meltable wax adhesive, and the remainder of blank machined to provide the convex outer surface. The machined lens is then removed from the arbour. The sensors illustrated in the Figures are designed to provide to the computer controlling the lathe a datum signal representative of the vertex position of the concave inner surface so that the computer may determine the appropriate path for the tool to follow when machining the lens to ensure that the finished lens is of the correct thickness. Referring to Figure 1, a partially formed blank contact lens 10 is shown attached to the convex end surface of a brass arbour 11. The contact lens blank 10 is of transparent plastics material. The other end of the arbour 11 is gripped by the chuck (not shown) of a lathe (not shown) for rotation about axis 'A'. The position sensor comprises a Helium Neon Laser 12, a knife edge 13, a microscope 14 and a photodiode 15 all secured to the tool slide 23 of the lathe. The tool slide is movable backwards and forwards parallel to axis 'A'. The laser 12 emits a collimated beam 16, typically 2mm wide along a sensing axis B which lies perpendicular to axis A and intersects it within a sensing region 17. The microscope 14 is aligned with the sensing axis B and includes a (X5) objective lens system 18 for producing an image of the intersection of axes 'A' and 'B' at an image plane 19. A slit 20 is also located at the image plane to select only a slot region extending in a direction orthogonal to axes A and B. The image at slit 20 is then passed on to the photodiode 15 by means of a (X10) eyepiece system 21 of the microscope. Adjacent the outer lens of the objective lens system 18 is located the knife edge 13 which extends orthogonal to axes 'A' and 'B' and which is positioned so that it protrudes into the beam 16 just sufficient to prevent it reaching the microscope when there is nothing in sensing region 17 to cause the beam to deviate. In use, the tool slide is moved towards the arbour in the direction of axis 'A' whilst the output of the photodiode 15 is monitored to record the intensity of this incident radiation. When there is no material in the sensing region, the incident radiation is at a low background level because it is almost wholly obscured by the knife edge 13. When however the outer end region of the blank 10 is adjacent or coincident with the beam 16, a surface diffraction effect causes the beam to deviate sufficiently to cause an increase in intensity of the radiation incident upon photodiode 15, and consequently its output rises to indicate the presence of the surface. Continued movement of the tool slide towards the chuck causes the laser beam 16 to impinge on the blank 10 intermediate it ends so that no surface effects are apparent and thus there is no deviation of the beam 16 and the intensity of this radiation incident upon the photodetector is therefore relatively low. When however the beam 16 approaches the convex interface between the concave surface of the lens blank 10 and the wax adhering the blank to the arbour 11 approaches the beam 16, the variation of intensity incident on photodiode 15 with axial position is as shown in figure 2. Referring to this figure, it will be noted that the intensity reaches a sharply-defined symmetric peak. By monitoring the output of the photodiode 15, the presence of the interface, and thus the position of the vertex of the concave surface of the blank can be determined precisely. In tests carried out, it was found that the position of the vertex could be determined to an accuracy of within ±3 microns using a laser beam of width 2mm. The output from the photodiode 15 may be monitored by supplying it to a meter (not shown) and then adjusting the position of the arbour to obtain maximum intensity and then reading off or marking the longitudinal position of the tool slide with respect to the lathe bed. Alternatively, the position may be determined by recording the longitudinal position of the tool slide in relation to the intensity detected by the photodiode as the tool slide is moved towards the arbour. In this case the datum position may be determined as before by sensing the position of maximum sensed intensity or by noting the position of two equal intensity points either side of the maximum and then calculating the mid-point which will correspond to the position of the maximum. The technique described above allows a sharply defined datum point to be fixed with respect to the lathe bed which corresponds to the axial position of the vertex of the concave surface of the partially formed lens blank 10. The lathe can then be calibrated to enable lenses of a particular thickness to be manufactured without the need to manually measure the axial length of the partially formed blank. Calibration may be effected by cutting the outer concave surface of a lens with the tool slide in approximately the correct longitudinal position, measuring the actual thickness of the lens produced and calculating the offset (if any) required to produce a lens of the required thickness. Referring to Figure 3, this shows the view seen by the photodiode with the slit removed; the position of the edges of the slit are shown in dotted lines. The Figure illustrates the position of the blank 10 and arbour 11 view of the microscope when the tool slide is at the datum position. A surface diffraction effect causes a large part of the concave interface to glisten. The slit 20 selects only that part of the image which is influenced by the presence of the interface. This enables the signal to noise ratio of the device to be relatively high. Referring now to Figure 4, the second embodiment of position sensor is of similar form to the first embodiment and similar items will be given similar references. The position sensor comprises a mounting bracket 30 attaching to the tool slide of the lathe. The mounting bracket supports a Helium-Neon laser 12, first and second turning mirrors 31,32 each for deflecting the beam 16 emitted from the laser 12 through 90°, and a microscope 14. The microscope 14 mounts at its front end a knife edge 13 arranged, as in the previous embodiment, to block the beam 16 when it is undeviated. The microscope includes a (X5) objective lens system 18, a (X10) eyepiece lens system comprising a field lens 33 and an eye lens 34, a slit 20 located between the eye lens and the field lens, and a photodiode 15 to measure the output. As before, the mounting bracket is moved towards the arbour 11 whilst the output from photodiode 15 is monitored and the datum position is set at the longitudinal position corresponding to the sharply defined symmetrical peak shown in Figure 3.

Claims: Apparatus for detecting the presence of surface or interface at a predetermined position within a sensing region (17) with respect to the apparatus, the apparatus comprising: illuminating means (12) for directing a collimated beam (16) of optical radiation along a sensing axis through said sensing region (17); optical radiation detector means (14, 15) aligned with said sensing axis (16) and located beyond said sensing region (17), and characterised by knife edge means (13) located between said sensing region (17) and said optical radiation detector means (15) and arranged to protrude at least partly into said collimated beam (16) in a given direction. Apparatus according to claim 1 in which the illuminating means (12) comprises a laser. Apparatus according to either preceding claim in which the optical radiation detection means includes a photodetector (15). Apparatus according to any preceding claim in which the surface whose presence is to be detected is a surface of a partially-machined contact lens (10).

Inventors: HARRISON, ANDREW B.
Applicants: BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: G01B11
IPC codes: G01B11