GB2283093A

GB2283093A - Optical measuring and sorting system

Info

Publication number: GB2283093A
Application number: GB9420256A
Authority: GB
Inventors: Kevin Ross Manning
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1993-10-22
Filing date: 1994-10-07
Publication date: 1995-04-26
Also published as: GB9321850D0; GB9420256D0

Abstract

In a production line 40 for manufacturing objects, eg lenses, the objects are sorted into categories of predetermined manufacturing tolerancies eg different radii of curvature. The objects are fed to an optical measuring device 42 which is arranged to measure the tolerance in the form of the radius of curvature of the object. In dependence upon the measured result the sorting device 44 causes the objects to be placed in predetermined receptacles 46. The measuring device may also be used to control the production line, when for example, it is detected that the objects are outside preset tolerance limits. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO OPTICAL MEASURING SYSTEMS The present invention is concerned with optical measuring systems, and in particular the invention relates to a problem on a production line where a simple, but accurate, statement of whether the surface curvature of the object being manufactured is within specification, together with an indication of how far it is from the required curvature.

According to the present invention there is provided apparatus for sorting from a production line a plurality of objects having a plurality of wholly of partly reflective surfaces into categories of predetermined manufacturing tolerancies, comprising means for receiving from the production line the plurality of objects, means for feeding the objects to an optical measuring device and means for sorting each object in accordance with their measured tolerance determined by said measuring means.

The sorting means may be arranged to control a production line when the measured tolerance departs from predetermined limits.

The checking means may include means for determining a normal to the surface to be measured which is used to reference subsequent calculations in determining the radius of curvature.

The checking means may use two or more parallel beams set at a known distance apart which are reflected back from the surface under test and are used to calculate the radius of curvature of the objects.

The checking means may include a probe beam which is scanned across the surface of the object to provide continuous or variable sample intervals for generating a profile of the surface being scanned.

Various embodiments of the present invention will now be described with reference to the accompanying drawings wherein; Figure 1 shows a block diagram of the apparatus used for testing a surface of an object having a plurality of wholly or partly reflective surfaces.

Figure 2 shows a block diagram of an alternative form of scanning used in preference to that shown in Figure 1; Figure 3 shows a diagram of variant equation parameters, and, Figure 4 shows a block diagram of a production line process utilising the apparatus as shown in Figures 1 or 2 for sorting/checking each object.

The following description may be read with patent application number 9213626.6, for background information, the contents of which are incorporated herein by reference thereto.

Referring to Figure 1, the apparatus consists of a diode laser source 2, which if emitting in the visible range of wavelengths will provide an indication to the operator of where the measurements are being made. The laser emits a collimated beam that is split into mro beams by a prism 4 and is directed to the surface 8 to be measured by a beamsplitter 6.

One method for determining the radius of curvature requires the apparatus to determine the normal to the surface, according to the method in patent application number 9213626.6, and is derived by the beamsplitter 18, detector 20, and an analogue to digital converter 22. This normal is used as the reference for subsequent calculations of the radius of curvature. When this normal has been found the radius of curvature is calculated. The beam reflected from the second position, which is a known distance from the first beam that is used to determine the surface normal, is focused onto a detector by a transform lens 10. The lens 10 converts angular displacement to a physical displacement at the detector 12.The reflected beam position from the surface is recorded in memory 14 via the analog to digital converter 22, immediately after the normal has been determined and is used to calculate the radius of curvature. This value is compared by a comparator system 16, with the design value and an output is produced that indicates how far from 'ideal' the surface is. The apparatus is pre-programmed with the design radius of curvature by a secondary unit connectable to the apparatus. After programming, a Go/No Go unit can be used independently of the programming unit, until a new radius of curvature is required.

Data logging can also be accomplished should this be required. The programming is accomplished by the unit 24 which is connected to a processor 26, which has an associated keyboard 28 and display 30.

A different method may be used which does not require the normal to be found, this is similar to a spherometer, which is a contact device, but is non-contact and therefore potentially more accurate. In this case two, or more, parallel beams 32 a known distance apart impinge on the surface under test 34, as shown in Figure 2. When the returns are detailed on the detector 36 the positions are stored in memory and used to calculate the radius of curvature.

Referring to Figure 3, and the equations, they show that there is a simple relationship between the two measured angles, the distance between the beams and the required radius of curvature. In order to distinguish between the returns from the various input beams, one way would be to activate each beam separately such that the return, or returns for back surface reflections, are uniquely identified. This could be accomplished, for example, by having separate lasers for each beam position and switching them on in sequence. Another way to accomplish this could be to use polarised light and separate detectors for each polarisation. This method removes the requirement to determine the normal before the measurement is made. It should be noted that for widely spaced beams this method will only be accurate for spherical surfaces.However, if the beams are close to each other, e.g. sufficient sampling frequency, then the method will work for any smooth surface profile.

An alternate method to using multiple beams would be to scan the probe beam across the surface by having, for example, a scanning mirror/prism at the focus of the transform lens, or an LCD light shutter, thus providing continuous, or variable, sample intervals for providing a profile of the surface.

The measurements could be accomplished in real time, such that as the surface is machined, the output from the apparatus may be used to control the cutting tool. This will provide an automatic system that will modify the machining as required or stop when the required radius of curvature, or profile, is accomplished.

When the measurements are made the apparatus could, for example, be hand-held, affixed to the lathe stock or temporarily affixed near the surface to be tested.

The following is the equation for determining the radius of curvature, r, for two parallel beams a linown distance, x, apart, as shown in Figure 3

where: r = required radius of curvature y = distance from intersect of perpendicular from upper radius normal with lower radius normal x = beam separation ## = extra distance between x and intersect with lower radius normal e = upper measured angle of surface normal o = lower measured angle of surface normal In addition, it is possible to derive the height of the surface from a plane surface by the following: ah = ycos

# r = sin # - sin # therefore:

where:: = = height between impact points of the incident beams.

By using wavelength diversity for each of the lasers it is possible to isolate the lasers by wax elength filters and use linear detectors for each wavelength. Using this technique it is possible to have the beams very close together, i.e. small x, and still have resolution between the returns. This can be of benefit when the mechanical scanning is used where improved resolution of the radius of curvature is obtained.

Referring to Figure 4, a Go/NoGo unit is used to control a lens, e.g. on a spectacle production line. As the lenses are released from the manufacturing process line 40, they are passed to various conveyor belts that flow into one. If, according to the present invention, the unit 42 is positioned at this point it is possible to sort the lenses via sorting apparatus 44 into the various storage bins 46 for the 'standard' range of outer surfaces.

The unit would be programmed with the various 'standard' radii of curvature and having measured the lens as it passed would send control signals to the sorting apparatus for placing the lenses into their correct storage bins or machining bins. In addition, the results obtained from the unit can be used to control the manufacturing process. This could be of significant importance as lenses can be manufactured at approximately one per second and with the feedback it is possible to determine when the surfaces are going out of specification. The earlier the errors are detected the less scrap lenses will be manufactured and the down time for refurbishment of the manufacturing process is minimised.

It will be readily by those skilled in the art that various modifications are possible which fall within the scope of the invention.

Claims

1. Apparatus for sorting from a production line a plurality of objects having a plurality of wholly or partly reflective surfaces into categories of predetermined manufacturing tolerances, comprising means for receiving from the production line the plurality of objects, means for feeding the objects to an optical measuring device and means for sorting each object in accordance with a measured tolerance determined by said measuring means.

2. Apparatus as claimed in claim 1, wherein said sorting means is provided with means for controlling said production line when the measured tolerance departs from predefined limits.

3. Apparatus as claimed in claim 1 or claim 2, wherein said checking means includes means for determining a normal to the surface which is used to reference subsequent calculations of the radius of curvature of the objects.

4. Apparatus as claimed in claim 1 or claim 2, wherein said checking means utilises t'vo or more parallel beams set at a known distance apart which are reflected back from the surface under test and are used to calculate the radius of curvature of the objects.

5. Apparatus as claimed in claim 1 or claim 2, wherein the checking means includes a probe beam arranged to be scanned across the surface providing continuous or variable sample intervals for generating a profile of the surface being scanned.

6. Apparatus as claimed in any preceding claim arranged to control a machining process in the manufacture of the objects.

7. Apparatus as hereinbefore described with reference to the accompanying drawings.