GB2124761A - Measuring position and/or dimensions of objects - Google Patents

Abstract

A measuring apparatus 1 comprises a laser 2 the output beam 3 of which is incident on a rotating mirror 4 and is thence reflected onto a lens 7 to produce a parallel sideways displacement motion of the beam in a measuring field 8 in which an object P to be measured is positioned. The time of obscuration of the beam in measuring field 8 as mirror 4 rotates is determined and is a measure of the size of object P in the direction of displacement of the beam. The signal from detector 10 is fed to a measuring circuit 12 which is enabled by an enabling signal obtained from a photosensitive detector 15 positioned to receive light reflected back through lens 7 from a partially reflecting plate 16 positioned in the measuring field near lens 7. Plate 16 is inclined at an angle relative to the normal to the beam direction to allow detector 15 to be spaced from mirror 4 in the focal plane of lens 17. An apertured diaphragm 17 can be provided between lens 7 and plate 16 to define the width of measuring field 8. <IMAGE>

Description

SPECIFICATION Apparatus for measuring the position and/or dimensions of objects This invention relates to apparatus for measuring the position and/or dimensions of objects.

One kind of such apparatus comprises means for generating a focussed beam of light and directing the beam onto a rotating mirror, a collimating element for converting the rotational movement of the beam after reflection from the mirror into a parallel sideways displace-scanning motion within a measuring field in which an object to be detected is position, detection means for detecting the beam after passage through the field except when obscured by an object so that the period of obscuration is a measure of the dimension of the object in the direction of displacement of the beam, a signal measuring circuit to which the output of the detection means is fed, and means for enabling said measuring circuit so that it is operational during the time interval that the beam scans the measuring field.

Measuring apparatus of this type is shown in US Patent 3,765,774 and German Patent 2,849,252. In the known measuring apparatus a photosensitive element is located at the edge of the measurement field and the scanning light beam is incident on the element at the commencement of the paralleldisplacement rotation. Energisation of the photosensitive element generates an enabling signal for the measuring circuit.

A disadvantage of the above-described method of enabling resides in the fact that the photosensitive elements which are customarily available possess edge zones which are not sensitive and, in consequence, it is necessary to adopt special measures in orderto prevent the measuring circuit, following switching-over to the operational condition, from erroneously treating the condition in which the edge zone of the photosensitive element is being scanned by the light beam as the scanning of an object or test sample which is to be measured. Furthermore, the known measuring apparatus in which photosensitive elements are situated in the measurement field have a disadvantage in that the usable aperture of the optical system is restricted, or cannot be fully utilised for the parallel-displacement motion of the scanning light beam.

An object of the invention is to provide apparatus of the kind described in which the full available aperture of the collimating element can be effectively utilised.

According to the invention in apparatus of the kind described above the enabling means comprises a partially reflecting element positioned in the measuring field in proximity to the collimating element and arranged to reflect a small proportion of the light incident thereon back through the collimating element, the partially reflecting element being inclined at an angle relative to the normal to the beam direction so that the reflected light is focussed to a point spaced from the rotating mirror and a photosensitive element is located at said point energisation of which generates an enabling signal for the measuring circuit.

Preferably a diaphragm is provided in the measuring field between the collimating element and the partially reflecting element. Preferably also the diaphragm has an aperture corresponding to the aperture of the collimating element and the diaphragm may be adjustable.

The partically reflecting element can be provided with an anti-reflection coating on its surface facing away from the collimating element.

It has been found that when the partially reflecting element is positioned at only a small angle relative to the normal to the beam direction the aperture of the diaphragm need be only insignificantly smaller than the available aperture of the collimating element. In will be realised that the degree of inclination of the partially reflecting element determines the distance of the photosensitive element from the rotating mirror.

Additionally an advantage of the diaphragm is that it defines the boundaries of the measurement field in which an object to be measured is positioned.

Furthermore the diaphragm can be adjusted or shifted as required without the need to reposition the photosensitive element. It is also possible to use the diaphragm to limit the width of the measurement field without the need to make a corresponding adjustment to any of the electronic circuits.

In order that the invention may be more fully understood reference will now be made to the drawing, the single Figure of which shows in diagrammatic form apparatus embodying the invention.

The optoelectric measuring apparatus 1 shown in the Figure contains a light source in the form of a laser 2 which generates a sharply-focussed light beam 3. This light beam is directed onto the surface of a rotating mirror 4 which is driven by an electric motor 5 in a manner such that light beam 3 executes a rotational or scanning movement in a plane which is normal to the rotation axis of mirror 4, as is shown by broken-line arrows 6.

The light beam, after reflection from rotating mirror 4 is incident on a collimating lens 7 which converts the rotational movement of the light beam into a parallel sideways displacement scanning motion within a measurement field 8. Forthis purpose the point of incidence of light beam 3 on mirror4 lies on its rotational axis, which point is also the focal pointofcollimating lens 7.

A test object P is situated in the measurement field 8, the intention being to determine the crosssectional dimensions of this test object in the direction of the parallel sideways displacement motion, namely at right angles to the direction of the light beam in the measurement field. A collector lens 9 is positioned on that side of the test object facing away from the collimating lens 7. Collector lens 9 focusses the light beam onto a detecting device 10 in a manner such that a detection signal is generated on an output line 11 so long as no obscuration of the light beam by test object P occurs during its scanning movement in the measurement field 8 and the light rays can, via the collector lens 9, be incident the detecting device 10. The output signal from detector 10 disappears when the test object P interferes with the access of the scanning light beam to the detector.

A measuring circuit 12 takes account of changes in the level of the output signal from the detector only during the time that light beam 3 is incident on the aperture of the collimating lens 7. For this purpose, a gate-type switching element, for example, is provided in measuring circuit 12 to which signals indicating the position of the rotating mirror or of the drive motor 5 are supplied via a line 13. This gate-type switching element acts to pass on the detector signals supplied via the line 11 for measurement. The gate-type switching element is itself triggered by means of an enabling signal which is supplied via the line 14.

In the instrument described here the enabling signal is generated by a photosensitive element 15 which is situated at a defined distance Z in the transverse direction from the rotation axis of the rotating mirror 4.

A partially reflecting transparent plate 16 of glass or other material is located in the measurement field 8 on that side of collimating lens 7 which faces away from the rotating mirror 4 and at a comparatively short distance from collimating lens 7. Plate 16 is inclined at an angle relative to the optical axis which departs slightly from 90". The angle selected is such that a proportion of the light is reflected bythe surface of the transparent plate 16 which faces the collimating lens 7 and is focussed by lens 7 onto the photosensitive element 15.Thus as soon as the collimating lens 7 emits a light beam which is parallel to the optical axis in the direction of the collector lens 9 the photosensitive element 15 experiences excitation by a small proportion of light which is reflected as a result of the partial reflection at that surface of the plate 16 which faces rotating mirror 4.

Adiaphragm 17 is situated between collimating lens 7 and plate 16 in order to effect a precise definition of the size of measurement field 8 in the scanning direction. This diaphragm permits a defined measurement-field width to be selected if a particular test object is to be marked out for investigation from a plurality of mutually adjacent test objects. Diaphragm 17 can be designed to be adjustable. It is found that whether the diaphragm is set larger or smaller no re-adjustment of any kind need be made to the electrical switching devices in order to generate the enabling signal on line 14.

Claims (6)

1. Apparatus for measuring the position and/or dimensions of objects comprising means for generating a focussed beam of light and directing the beam onto a rotating mirror, a collimating element for converting the rotational movement of the beam after reflection from the mirror into a parallel sidways displacement scanning motion within a measuring field in which an object to be detected is positioned, detection means for detecting the beam after passage through the field except when obscured by the object so that the period of obscuration is a measure of the dimension of the object in the direction of displacement of the beam, a signal measuring circuit to which the output of the detection means is fed and means for enabling said measuring circuit so that it is ready to operate during the time interval that the beam scans the measuring field, said means comprising a partially reflecting element positioned in the measuring field in proximity to the collimating element and arrange to reflect a small proportion of the light incident thereon back through the collimating element, the partially reflecting element being inclined at an angle relative to the normal to the beam direction so that the reflected light is focussed to a point spaced from the rotating mirror, a photosensitive element located at said point energisation of which generates an enabling signal for the measuring circuit.

2. Apparatus as claimed in Claim 1 in which a diaphragm is provided in the measuring field between the collimating element and the partially reflecting element.

3. Apparatus as claimed in Claim 2 in which the diaphragm has an aperture corresponding to the aperture of the collimating element.

4. Apparatus as claimed in Claim 2 in which the diaphragm is adjustable.

5. Apparatus as laimed in any one of the preceding claims in which the partially reflecting element has an anti-reflection coating on its surface facing away from the collimating element.

6. Apparatus for measuring the position and/or dimensions of objects substantially as described herein with reference to the accompanying drawing.