GB2124762A

GB2124762A - Position and/or dimensions of objects

Info

Publication number: GB2124762A
Application number: GB08321233A
Authority: GB
Inventors: Bernhard Schlapp; Bruno Richter
Original assignee: RICHTER BRUNO DIPL ING FA
Current assignee: RICHTER BRUNO DIPL ING FA
Priority date: 1982-08-05
Filing date: 1983-08-05
Publication date: 1984-02-22
Also published as: DE3229263C2; CH662879A5; GB2124762B; DE3229263A1; GB8321233D0

Abstract

A measuring apparatus comprises two light beams 3 and 4 directed on to a rotating mirror 9 and thence reflected to a collimating lens 10 to produce two beams in a measuring field 11 in which an object P to be measured is positioned. As mirror 9 rotates the beams execute sideways displacement scanning motions within the measuring field and are detected by a detector 15 except when obscured by the object P so that the time of obscuration is a measure of the dimension of the object in the scanning direction. To compensate for any oscillatory movement of object P while it is being measured the respective displacement motions of the beams are arranged to be in opposite directions and the output signal from detector 15 is averaged. Reversal of the scanning direction of one of the beams is achieved by inserting a reversing prism 12 in one of the beam paths. A parallel sided body 13 may be inserted in the other beam path to equalise the effective path lengths of the two beams. The scanning planes of the two beams in the measuring field may be parallel to each other or else may be inclined at an angle so that the planes intersect in the mid-field region of the measuring field. <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 instruments of this type are disclosed in US Patent 3,765,774 and German Offenlegungsschrift 2;849,252.

If the object to be measured moves while being measured then measurement errors result. If the object to be measured moves in the direction of the parallel-displacement motion an impression will be given of a larger dimension of this object in the scanning direction, while if the object moves in opposition to the parallel-displacement motion the result of the measurement will indicate a smaller dimension of the objects.

Falsification of the result of the measurement due to movement of the object also occurs when the object in question oscillates, since when mean values are formed from a large number of individual measurements the errors in these measurements do not average-out if the scanning frequency approximates to the oscillation frequency of the object to be measured, or if it is an uneven multiple of the oscillation frequency.

Attempts have been made in measuring instruments of the type initially described to avoid measurement errors resulting from movement of the object which is to be measured by means of a procedure in which the light beam executes first a parallel-displacement motion in one direction in the measurement field, and then executes a paralleldisplacement motion in the opposite direction, in an alternating manner. This may be achieved, for example, by use of an oscillating mirror instead of a rotating mirror. Measurement errors still occur when the oscillation frequency of the test objects coincides with that of the oscillating mirror, as well as with certain phase relationships. Moreover, even when these unfavourable conditions do not prevail it is impossible in the known instruments to eliminate the measurement errors completely, due to the large phase difference between successive scans.

An object of the invention is to provide measuring apparatus of the kind described above in which vibratory or other movements of the test object do not falsify the measurements.

According to the invention apparatus for measuring the position and/or dimensions of objects comprises means for generating two focussed beams of light and directing the beams onto rotating mirror means, collimating means for converting the rotational movements of the beams after reflection from the mirror means into parallel sideways displacement scanning motions within a measuring field in which an object to be detected can be positioned, optical means for providing that the respective sideways displacement motions of the two beams are in opposite directions to each other, detection means for detecting the beams after passage through the field except when they are obscured by an object so that the period of obscuration is a measure ofthe dimension of the object along a line parallel to the directions of displacement motion of the beams, and averaging means for averaging the output of the detection means.

Preferably the rotating mirror means comprises a common mirror onto which the light beams are directed, the collimating means comprises a single collimating lens and the detection means comprises a common photosensitive detector for both beams.

In carrying out the invention the said optical means may comprise a reversing prism inserted in the path of one of the beams after the collimating means. it may also be convenient to provide a plane parallel-sided optical body in the path of the other beam in order to equalise the effective path lengths of the two beams.

In one preferred embodiment the two light beams are both directed onto the same point of the rotating mirror which point is the focal point of the collimating lens, and the sideways scanning motions of the beams in the measuring field take place in planes which are parallel to and spaced apart from each other. With such an arrangement a test object will be scanned at cross-sections which are spaced a corresponding distance apart.

If however the dimension of a particular crosssection of the test object is of interest then the two light beams can be directed onto different points in the rotating mirror spaced apart from each other and the sideways scanning motions of the beams in the measuring field then take place in planes which are inclined to one another and intersect in a midfield region of the measuring field. The dimension of interest of the test object is then positioned to coincide with the line of intersection of the two planes.

By using a common rotating mirror and a single collimating lens the scanning movements of the two light beams can be rigorously symmetrical and simultaneous so that major expenditure for synchronising the scanning motions is not required.

In order that the invention may be more fully understood reference will now be made to the accompanying drawings in which: Figure 1 shows a diagrammatic perspective view of apparatus embodying the invention, and Figure 2 shows an alternative embodiment to the embodiment shown in Figure 1.

In Figure 1 two light sources 1 and 2 respectively direct sharply-focussed light beams 3 and 4 onto a lens 5. Beams 3 and 4 are parallel to one another.

The light sources 1 and 2 can comprise lasers. As an alternative to the arrangement shown in Figure 1 it is possible to derive the parallel spaced apart light beams 3 and 4from a single laser acting as a light source by means of a beam-splitter and mirrors.

The lens 5 directs the beams 3 and 4 onto a point 6 which is located on the rotation axis 7 of a rotating mirror 9 this mirror being caused to rotate by means of a drive motor 8. Point 6 is situated at the focus of a collimating lens 10 which causes the swinging or rotational movements of the light beams 3 and 4 produced by rotating mirror 9 to be converted into parallel-displacement sideways scanning motions in a measurement field 11.

Immediately after the collimating lens 10 there follow a reversing prism 12 in the path of one beam and a glass body 13 in the path of the other beam.

Glass body 13 possesses plane-parallel entry and exit faces and is provided in order to equalise the effective path length of that light beam which is not led through the reversing prism 12.

Reversing prism 12 causes the light beams 3 and 4 to execute parallel-displacement motions within the measurement field 11 which are opposite to each other and are simultaneous to a high degree of precision. The respective directions of both beams are parallel within the measurement field 11 and the two opposite parallel-displacement sideways motions take place within planes which are at a defined distance from one another in a manner such that simultaneous measurements are made on cross sections of the test object P which are separated by a distance D corresponding to the distance between the above-mentioned planes.

The light rays are directed by means of a focussing device 14 onto a detecting device 15 which gener ates an output signal corresponding to the light beam 3 and an output signal corresponding to the light beam 4. These two signals are added and the result is halved, thereby representing a corrected measured value. In the evaluating unit 16 a mea sured value corresponding to a particular cross sectional dimension of test object P is obtained by correlating the movement of the rotating mirror with the variation as a function of time of the output signal from detecting device 15. For this purpose a signal line is led from the rotating mirror drive motor 8 to the evaluating unit 16. Details relating to this arrangement are familiar to a person skilled in the art and do not require further description.

The embodiment according to Figure 2 is of similar construction to the measuring apparatus shown in Figure 1 and identical reference numbers are used to designate like parts.

In the case of the embodiment of Figure 2 lens 5 is omitted and the light beams 3 and 4 are directed onto points 6a and 6b which are in spaced apart location on the rotation axis 7 of the rotating mirror 9. As rotating mirror 9 is driven the beams are caused to execute rotational or swinging movements in planes which are parallel with one another and are similarly spaced apart.Following the conversion of the rotational movement of the light beams 3 and 4 into a parallel-displacement sideways scanning mo tions within the measurement field 11 by means of collimating lens 10, light beam 4 passes through glass body 13 which has plane-parallel entry and exit faces while light beam 3 passes through the reversing prism 12 in a manner such that when, for example, the rotating mirror 9 is driven anticlockwise the parallel-displacement motion of the light beam 4takes place in an upward direction in measuring field 11 and the parallel-displacement motion of light beam 3 takes place simultaneously in the opposite downward direction, as is also indicated both in Figure 1 and in Figure 2 by the shaded arrows.

The distance separating the points 6a and 6b is chosen to be such that the planes in which the light beams 3 and 4 execute their parallel-displacement motions within the measurement field 11 intersect on a line 17 in the mid-field region of measuring field 11. Line 17 passes through the test object P and is, in particular, a centre line of a particular cross-section of the test object. While therefore cross-sections of the test object separated by the distance D, are scanned in in the case of the embodiment according to Figure 1, essentially one and the same crosssection of the test object is scanned by the twoscanning light beams in the case of the embodiment according to Figure 2, which are moved symmetrically and simultaneously in contrary directions.

In conclusion, it should be noted that both the glass body 13 and the reversing prism 12 can be provided with anti-reflection coatings on the faces at which the light enters and leaves in order to avoid interfering effects.

Claims

1. Apparatus for measuring the position and/or dimensions of objects comprising means for generating two focussed beams of light and directing the beam onto rotating mirror means, collimating means for converting the rotational movements of the beams after reflection from the mirror means into parallel sideways displacement scanning motions within a measuring field in which an object to be detected is positioned, optical means for providing that the respective sideways displacement motions of the two beams are in opposite directions to each other, detection means for detecting the beams after passage through the field except when they are obscured by an object so that the period of obscuration is a measure of the dimension of the object along a line parallel to the directions of displacement motion of the beams, and averaging means for averaging the output of the detection means.

2. Apparatus as claimed in Claim 1 in which the rotating means comprises a common mirror onto which the light beams are directed, the collimating means comprises a single collimating lens and the detection means comprises a common photosensitive detector for both beams.

3. Apparatus as claimed in either one of the preceding claims in which the said optical means comprises a reversing prism inserted in the path of one of the beams after the collimating means.

4. Apparatus as claimed in Claim 3 in which a plane parallel sided optical body is provided in the path of the other beam in order to equalise the effective path lengths of the two beams.

5. Apparatus as claimed in Claim 3 in which the entry and exit faces of the reversing prism and the plane parallel sided optical body carry anti-reflection coatings.

6. Apparatus as claimed in any one of Claims 2 to 5 in which the two light beams are both directed onto the same point on the rotating mirror which point is the focal point of the collimating lens, and the sideways scanning motions of the beams in the measuring field take place in planes which are parallel to and spaced apart from each other.

7. Apparatus as claimed in any one of Claims 2 to 5 in which the two light beams are directed onto different points in the rotating mirror which are spaced apart from each other, and the sideways scanning motions of the beams in the measuring field take place in planes which are inclined to one another and intersect in a midfield region of the measuring field.

8. Apparatus for measuring the position and/or dimensions of objects specifically as described herein with reference to Figure 1 or Figure 2 of the accompanying drawings.