GB2129932A

GB2129932A - Position and/or dimensions of objects

Info

Publication number: GB2129932A
Application number: GB08325923A
Authority: GB
Inventors: Bruno Richter; Bernhard Schlapp
Original assignee: RICHTER BRUNO DIPL ING FA
Current assignee: RICHTER BRUNO DIPL ING FA
Priority date: 1982-11-11
Filing date: 1983-09-28
Publication date: 1984-05-23
Also published as: GB8325923D0; DE3241770C2; GB2129932B; DE3241770A1; CH662650A5

Abstract

A measuring apparatus comprises a light source 1 which, by means of beam splitters 3 and mirrors 4, produces a plurality of parallel focussed beams 5 all lying in a common plane. Beams 5 are directed onto a rotating mirror 7 the axis of rotation 6 of which lies in the plane of beams 5. The beams are reflected from mirror 7 onto a collimating lens 9 so that they execute a parallel sideways displacement scanning motion a measuring field 11 in which an object 10 is placed. The beams after passing through measuring field 11 are deflected by a focussing device 13 onto individual detectors 14, 15, 16 and 17. The period of obscuration of the beams as detected by the detectors is a measure of the dimension of object 10 in the direction of scanning of the beams in field 11. The outputs of the detectors are fed to a combining device 18 for evaluation. Device 18 may select that beam for providing the measurement signal which has the least time period of obscuration. <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 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 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, and a signal measuring circuit to which the output of the detection means is fed.

Measuring apparatus of this type has been disclosed in US Patent No 3,756,774 and in German Patent Publication 2,849,252.

When apparatus of the above-mentioned kind is being used it is occasionally found that particles of dirt settle on the collimating element or on other optical parts, and the detection means mistakenly recognizes these particles as objects to be measured, thus leading to errors.

Attempts have been made to eliminate this problem by a procedure wherein, before being incident on the rotating mirror, the light beam is widened in cross-section in a direction parallel to the axis about which the mirror rotates, for example by means of a cylindrical lens, in a manner such that the rotating mirror does not produce rotational movement of a narrow light beam, but produces rotational movement of a broad band of light. This band is then caused by the collimating element, to execute a parallel sideways displacement motion within the measuring field.

With regard to the improvement of reliability this known precaution does not however achieve its object when fibre-like or rod-like dirt particles or dirt particles which are elongated in any other way, settle onto the optical components of the apparatus. Such dirt particles occupy a substantial proportion of the cross-section of the scanning light-band and are still capable of giving rise to errors.

Incorrect measurements can result even when interfering particles in the beam paths do not occupy the entire beam cross-section, since the irregular shape of the intensity distribution in the originally sharply-focussed beam produces an inhomogenous intensity distribution in the widened beam cross-section and this inhomogeneity can result in a situation in which the obscuration of a small proportion of the beam cross-section can cause an incorrect measurement.

The invention is intended to achieve the object of configuring an optoelectric measuring apparatus of the kind described above in a manner such that errors caused by interfering particles in the path of the scanning beam can be avoided more reliably than in the case of known apparatus.

According to the invention apparatus for measuring the position and/or dimensions of an object comprises means for generating a plurality of parallel focussed beams of light lying in a common plane and directing the said beams onto a rotatable mirror having an axis of rotation lying in said plane, collimating means for converting the rotational movement of the beams after reflection from the mirror into respective parallel sideways displacement scanning motions within a measuring field in which an object to be detected is positioned, detection means for detecting the beams 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 beams, and a signal measuring circuit to which the output of the detection means is fed.

It will be recognised that, as a result of the use of a plurality of parallel light beams directed onto the rotating mirror the distribution of luminous intensity within a comparatively large beam crosssection is unimportant. It is possible to use a plurality of light beams, for example four light beams possessing essentially identical intensities.

Moreover the distances between the individual light beams in the direction of the axis about which the mirror rotates can be chosen to be comparatively large without involving major expense for the optical devices which are used for collimating and focussing purposes, so that the probability of the effective beam cross-section being obscured, even by very large dirt particles, is low.

According to one preferred embodiment focussing devices on the far side of the measuring field deflect each of the scanning light beams onto an individual detector associated with that beam and the respective detector output signals are further processed in order to provide the measurement result.

In the test which follows, an illustrative embodiment is described in more detail by reference to the attached diagrammatic drawing, which reproduces in simplified form a plan view of an optoelectric measuring apparatus embodying the invention.

The drawing shows a laser 1 which emits a sharply-focussed light beam 2, the cross-section of this beam being exceptionally small. This light beam is directed onto an array of beam-splitters 3 which have the property of splitting an incident light beam into two emerging light beams each of which contains approximately 50% of the energy which enters. Reflectors 4 operate in conjunction with beam-splitters 3 in the manner shown in the drawing to produce a total of four scanning light beams 5 each of small cross-section. Beams 5 are parallel to one another and all lie in a plane containing the axis 6 about which a mirror 7 rotates. Rotating mirror 7 is of considerable size in the direction of axis 6.Rotating mirror 7 is caused to rotate by means of a motor 8 in a manner such that the scanning light beams 5 are caused to execute rotational movements about the axis 6 about which the mirror rotates, these movements taking place in planes which are radial with respect to axis 6.

After reflection at the rotating mirror 7 the scanning light beams are incident on a collimating device, in the form of a cylindrical lens 9. The cylindrical lens 9 causes the rotational movements of the scanning light beams 5 to be converted, within a measuring field 11 containing a test specimen 10, into parallel sideways displacement motions in planes which are located parallel to one another and are a defined distance apart, and which are oriented approximately normal to the length of the test specimen 10. In the plan view shown in the drawing the above-mentioned planes appear as straight lines which are indicated by the broken lines 12.

On that side of the test specimen 10 which is remote from the collimating device the measuring field 11 is bounded by a further cylindrical lens 13 which serves as a focussing device and which focusses each scanning light beam onto an individual detector. The detectors may comprise photocells 14, 15, 16 and 17. A combined signal is formed from the output signals from the photocells 14 to 17, the combination being effected in a combining device 18, and the combined signal is supplied to an evaluating device 20 via the line 1 9. In the evaluating device 20 this combined signal is correlated with timing signals which are derived from the drive motor 8 of the rotating mirror 7, and are supplied via the line 21, in order to derive a measurement signal corresponding to the position and/or to the size of the test specimen 10.The derivation of this signal is based on the fact that each scanning light beam is obscured during its parallel-displacement movement within the measuring field 11 for a defined time which is a function of the size of the cross-section of the test specimen in the scanning direction, and which can be measured from the output signals from the photocell detectors 14 to 17.

It will be recognised that the units 1 8 and 20 can be combined to form one unit, and that the output signals from the photocells 14 to 1 7 can also be individually correlated with the timing signals supplied via the line 21. According to this procedure the separate measurement results obtained in this way are evaluated in order to form a resultant measurement signal possessing a high measurement-reliability.

In order to effect a particularly effective evaluation of the multiple signals obtained from the photocells 14 to 17, the procedure described below can be utilised.

From the four separately-measured times for which the output signals from the photocells 14 to 1 7 are interrupted due to the obscuration of the scanning light beam by the test specimen 10 in the measuring field 11 , that time which is the shortest, but which exceeds a defined minimum time, is assigned to the measurement of the test specimen. Influences arising from small interfering particles in the measuring field are excluded by fixing a minimum time. The minimum time can be fixed by automatic means in the manner indicated for instance in German Patent 2,951, 677, the knowledge gained from each preceding measuring operation being utilised in the course of a large number of successive measuring operations on one and the same test specimen. Alternatively the minimum time can be set manually on starting to operate the measuring apparatus.

It should be mentioned in addition that an arrangement of strip lenses, prisms, light guides and similar devices can be used in order to produce the parallel scanning light beams which are directed onto the rotating mirror 7.

Claims

1. Apparatus for measuring the position and/or dimensions of objects comprising means for generating a plurality of parallel focussed beams of light lying in a common plane and directing the said beams onto a rotatable mirror having an axis of rotation lying in said plane, collimating means for converting the rotational movement of the beams after reflection from the mirror into respective parallel sideways displacement scanning motions within a measuring field in which an object to be detected is positioned, detection means for detecting the beams after passage through the field except when obscured by an object so that the period of obscuration is a measure of the object in the direction of displacement of the beams, and a signal measuring circuit to which the output of the detection means is fed.

2. Apparatus as claimed in Claim 1 in which the means for generating a plurality of parallel focussed beams of light comprises a common light source together with a plurality of beam splitters and mirrors.

3. Apparatus as claimed in either one of the preceding claims in which the collimating means comprises a singie collimating element onto which all the beams are directed.

4. Apparatus as claimed in any one of the preceding claims in which the detection means comprises a single detector for all the beams.

5. Apparatus as claimed in any one of Claims 1 to 3 in which the detection means comprises a plurality of detectors, individual ones of which are associated with each beam.

6. Apparatus as claimed in Claim 5 in which the outputs of the detectors are connected to selection means which selects as the measurement signal that detector output which has the least time period of obscuration.

7. Apparatus as claimed in Claim 6 in which the selection means selects only those outputs having periods of obscuration greater than a minimum value.

8. Apparatus as claimed in Claim 7 in which automatic means are provided for setting the said period of minimum value.

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