RU2262660C1 - Method and device of contact-free optical measurement of object's sizes - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: according to method of contact-free optical measurement the object is placed between laser radiation source and photoreceiver. Power of laser radiation P is measured and compared with preset level of power P₀ . Laser radiation is optically scanned into beam of parallel rays at the area where object finds its place and size of object is found from size of shade of object onto photoreceiver while correcting time of exposure from value of difference (P₀-P). Device for realizing the method has laser, beam-splitting plate, short-focused cylindrical lens, output cylindrical lens, collimating lens, CCD, data processing unit, photoreceiving threshold unit.

EFFECT: improved precision of measurement.

5 cl, 1 dwg

Description

The invention relates to measuring equipment, in particular to non-contact optical means for measuring the geometric dimensions of various objects.

There is a method of non-contact optical measurement of the size of objects, also called shadow, which consists in placing the object under study between the laser and a multi-element photodetector, scanning the laser radiation in a beam of parallel rays in the area of the object and determining the size of the object by the magnitude of the shadow cast by it on the photodetector. Devices that implement the known method, laser shadow meters, consist of a laser radiation source, a lens system that forms a beam of parallel rays from the initial beam by optical scanning, and a multi-element photodetector connected to the information processing unit. The number of unlit pixels on the photodetector on the CCD line determines the size of the object (1, 2).

The use of optical scanning makes it possible to use a multi-element photodetector on a CCD line for continuous reading of information and to carry out information collection within one frame, the duration of which is adjustable over a wide range, up to 0.1 μs. This circumstance makes it possible to use laser shadow meters to measure the parameters of objects moving at high speed.

As a prototype of the claimed technical solution, the method of non-contact optical measurement of the size of objects is selected, which consists in placing the object under study between the laser and the photodetector, optical scanning of laser radiation in a beam of parallel rays in the area of the object and determining the size of the object by the magnitude of the shadow of the object on the photodetector. A device that implements the known method consists of a laser radiation source, a lens optical scanning system, a multi-element photodiode array, an information processing circuit and a computer (3).

The disadvantages of the known method and device with which the method is implemented are due to the following. The measurement accuracy when using the known method depends, first of all, on the accuracy of determining the boundaries of the contour of the investigated object. Diffraction effects lead to the fact that the transition from light to shadow on the surface of the photodetector is characterized by a certain extent, which for practical photodetectors on the CCD line is usually a few pixels. The blurring of the border between light and shadow reduces the accuracy of determining the size of an object, and the influence of this factor will be greater, the smaller the size of the object.

As shown above, the size of the object is determined by the number of unlit (darkened) pixels on the CCD line. A pixel is considered darkened, the video signal from which is less than a certain threshold.

It can be shown that the detail size will be determined by the number of pixels at which the voltage U _{t is} greater than the threshold U _pore

,

,

where E _max - the maximum power of laser radiation;

r is the current radius of the laser beam on the CCD line;

r _o is the radius of the laser beam at a point with a radiation power density e ² times less than the intensity in the center;

T _ex - exposure time;

RC is a parameter specific to a particular CCD line.

From the expression (1) it follows that the size of the object depends both on the power of the laser radiation and on the exposure time.

During the exposure, the number of pixels at which U _t ≥U _then will be determined by the power of the laser radiation, since the illumination of each pixel and, therefore, the rate of rise of the charge on it depends on the power of the laser radiation. As a result, the determined size of the object will depend on the magnitude of the laser radiation power. Therefore, in a known laser meter with power fluctuations, the accuracy of determining the size of an object is reduced.

The problem solved by the invention is to increase the accuracy of measurements.

This problem is solved by the fact that in the method of non-contact optical measurement of the size of objects, which consists in placing an object between a laser source and a photodetector, optical scanning of laser radiation in a beam of parallel rays in the area of the object and determining the size of the object by the magnitude of the shadow of the object on the photodetector, measure the laser radiation power P, compare it with a given level of P _about and in magnitude (P _about -P) adjust the exposure time of the photodetector. A device for implementing the method, comprising a laser beam source, means for optical scanning of the laser beam, a photodetector connected to the first input of the information processing unit, and an object located between the laser beam source and the photodetector, equipped with a beam splitter located between the laser beam source and optical scanning means, and a photodetector threshold device, the output of which is connected to the second input of the information processing unit. The means of optical scanning of the laser beam are made in the form of cylindrical lenses, and the beam splitter is in the form of a translucent plate.

The invention is illustrated in the drawing, which schematically shows a device with which the inventive method is implemented. It includes a laser 1, a beam-splitting translucent plate 2, means for optical scanning of a laser beam, consisting of a short-focus cylindrical lens 3 and an output cylindrical lens 4, a collimating lens 5, a photodetector on a CCD line 6 connected to the first input of the information processing unit 7, and a photodetector threshold device 8 connected to the second input of block 7 and representing a photodetector with a comparison circuit. The beam splitter plate 2 and the photodetector threshold device 8 form a channel for adjusting the exposure time. The beam splitter plate 2 is located at an angle to the path of the laser beam 1 in order to ensure the removal of part of the radiation power to the photodetector threshold device 8. The measured object 9 is placed between the lenses 4 and 5.

The inventive method is as follows. The radiation from the laser 1 hits the beam splitter plate 2. Part of the radiation is deflected by the plate 2 to the photodetector threshold device 8, and the rest passes into the optical system of lenses 3 and 4, which scan the radiation into a beam of parallel rays. As a result, the studied object 9 is illuminated by a flat beam and an image of the object is formed on the photodetector 6 corresponding to the shadow cast by the object 9 on the surface of the photodetector 6. In block 7, the image signal is processed and the size of the object 9 is determined. In the threshold device 8, a part of the laser radiation power is compared received by the device 8, with a threshold value corresponding to a given radiation power. If the power value is different from the set value, a differential signal will be generated at the output of the threshold device 8 and fed to the second input of block 7. In accordance with the value of the received signal, block 7 adjusts the exposure time of the photodetector 6. If the actual laser radiation power is greater than the set value, block 7 reduces exposure time, if less - increases.

As a result, adjusting the charge time of the pixels even in the presence of fluctuations in the laser radiation power ensures high measurement accuracy.

Thus, the inventive method and device by adjusting the exposure time depending on the power of the laser radiation provide - compared with the prototype device - improving the accuracy of measuring the size of objects.

LITERATURE

1. A.Z. Venediktov, V.N. Demkin, D.S. Dokov, A.V. Komarov. The use of laser methods to control the parameters of automatic couplers and springs. New technologies for rail transport. Collection of scientific articles with international participation, part 4. Omsk 2000, p.232-233.

2. V.N. Demrin, D.S. Dokov, V.N. Tereshkin, A.Z. Venediktov. Optical control of geometrical dimensions for railway cars automatic coupling. Third Internat. Workshop on New Approaches to High-Tech: Nondestructive Testing and Computer Simulations in Science and Engineering. Proceedings of SPAS, Vol. 3. 7-11 June 1999, St. Petersburg, p. A17.

3. V.V. Antsiferov, M.V. Muraviev. Non-contact laser measuring instrument of geometrical sizes of bearings rollers. New technologies for rail transport. Collection of scientific articles with international participation, part 4. Omsk 2000, p.210-213 (prototype).

Claims (4)

1. The method of non-contact measurement of the size of objects, which consists in placing the object between the laser radiation source and the photodetector, optical scanning of the laser radiation in a beam of parallel rays in the area of the object, and determining the size of the object by the magnitude of the shadow from the object on the photodetector, characterized in that the power is measured laser radiation P, compare it with a given level of P _about and the magnitude (P _about -P) carry out the adjustment of the exposure time of the photodetector. 2. A device for non-contact optical measurement of the size of objects, containing a laser beam source, means for optical scanning of the laser beam, a photodetector connected to the first input of the information processing unit, and an object located between the means of optical scanning of the laser beam and the photodetector, characterized in that it is provided a beam splitter located between the optical radiation source and optical scanning means and optically coupled to a photodetector threshold device, the output of which is below connected to the second input of the information processing unit. 3. The device according to claim 2, characterized in that the optical scan means of the laser beam are made in the form of cylindrical lenses. 4. The device according to claim 2, characterized in that the beam splitter is made in the form of a translucent plate.