FR2713773A1 - Laser particle size analyzer with prism effect. - Google Patents

Abstract

The present invention relates to a device for determining the particle size distribution of a mixture of particles in circulation, comprising:. a generator (1) of a monochromatic light beam (2); . a transparent container (3); . a converging optical system (5); . a set (6) of photosensitive detectors (7i); and. means (9) for processing electrical signals. According to the invention, said device further comprises a diopter (13), formed of an optical surface which separates a first and a second medium of different refractive indices and disposed, in an inclined manner, on the path of said light beam monochromatic (2) between said mixture of particles (P) in circulation and said optical system (5), the refractive index of the first medium, opposite said mixture of particles (P), being greater than the refractive index of second medium, facing said optical system (5).

Description

The present invention relates to a device for determining the particle size distribution of a mixture of particles, that is to say making it possible to draw a curve representative of the variations, as a function of the diameter of the particles having the same diameter.

From documents US-A-3 807 864, FR-A-2 300337, FR-A-2 415 801 and US-A-4 274 741, for example, a device is already known for determining the particle size distribution of a mixture of particles in circulation, for example suspended in a fluid stream (liquid or gas). Such a device comprises a laser source, the beam of which passes through a transparent container traversed by the fluid current transporting said particles, and a converging optical system to form a diffraction pattern and / or diffusion (depending on the size of said particles) of the illuminated particles. by the laser beam. A set of photosensitive detectors, distributed in the focal plane of said optical system, generates electrical signals representative of said diffraction-diffusion pattern and addressed to a computer, processing said electrical signals to determine the particle size distribution of said mixture of particles. This type of device depends on the theory of diffraction
Fraunhofer.

We also know that, with such a device, the measurements made in said focal plane are significant, for large particles, when they are taken in the vicinity of the axis of the laser beam, and for fine particles, when they are performed far from said axis, that is to say with high values of the diffraction angle and / or diffusion.

However, these measurements are often difficult to perform, especially for large particles. Indeed, the latter are difficult to identify, because of the low value of the diffraction angle and / or diffusion relative to the axis of the monochromatic light beam.

In order to facilitate detection, a known solution recommends using optical systems with larger focal lengths, which makes it possible to increase the angular dispersion of the rays to be detected. This solution poses, however, new problems, since it requires, in particular, to increase the length of the optical measurement bench carrying the various elements of the device.

The object of the invention is to remedy these drawbacks, by improving the device mentioned above, so as to allow an analysis of great finesse, whatever the size of the particles of the mixture.

To this end, according to the invention, the device for determining the particle size distribution of a mixture of particles in circulation, comprising: - a generator of a monochromatic beam of light; - a transparent container, placed on the path of said beam and traversed by said mixture of particles; - a converging optical system to form, in its focal plane, a diffraction-diffusion pattern of said particles illuminated by said beam; - a set of photosensitive detectors distributed in said focal plane of the optical system; and - means for processing the electrical signals emitted by said set of photosensitive detectors; is remarkable in that it comprises a diopter, formed of an optical surface which separates a first and a second medium of different refractive indices and disposed, in an inclined manner, on the path of said beam of monochromatic light between said mixture of particles in circulation and said optical system, The refractive index of the first medium, facing said mixture of particles, being greater than the refractive index of the second medium, facing said optical system.

Thus, the angular dispersion of the monochromatic rays from the transparent container increases with the passage of said diopter, that is to say while passing from said first medium to said second medium with a lower refractive index, which results in a greater spatial distribution of the rays to detect and thus allow an analysis of great finesse, particularly interesting for large particles.

Advantageously, said diopter is inclined relative to the axis of the monochromatic light beam by an angle O defined by the relation sin (7C / 24) = n2 / nl, with nl
The refractive index of said first medium and n2 The refractive index of said second medium.

Thus, a monochromatic ray which crosses the container without coming into contact with a particle and which therefore retains its initial direction corresponding to the axis of the beam of monochromatic light, suffers, due to the value as defined above of the angle inclination 0, a total reflection on said diopter. On the contrary, a monochromatic ray which is diffracted by a particle of the mixture, modifying its initial direction, is not reflected by said diopter and therefore crosses the latter. Consequently, by carrying out the optical measurements at the exit of the diopter, only the diffracted rays useful for the implementation of the invention are detected, while the non-diffracted incident rays, sources of noise, are deviated from the measurement field. .

This improvement therefore makes it possible to considerably increase the signal-to-noise ratio of the measurements carried out.

The device according to the invention can be produced, either by adding an additional element to the elements used by a device known from the state of the art, or by directly modifying one of these elements used.

Thus, according to a first embodiment, the device according to the invention comprises a dioptric element, made of a material with a refractive index greater than the refractive index of the ambient medium and placed on the path of said beam of light. monochromatic, the face of the dioptric element through which said beam of monochromatic light exits, consisting of said diopter.

Said dioptric element can, for example, consist of a prism.

Advantageously, this dioptric element is mounted on a rotary support.

Thus, it is possible, by rotating said support, to tilt said diopter so as to obtain the desired reflection conditions. This solution makes it possible to adapt the device according to the invention to monochromatic light beams of variable directions.

According to a second embodiment, said diopter constitutes the face of said container through which emerges said beam of monochromatic light, said diopter being inclined relative to the opposite face of said container through which enters said beam of monochromatic light.

Thus, the advantages of the invention are obtained by directly modifying the transparent container in which the particles to be analyzed circulate.

The figures of the appended drawing will make it clear how the invention can be implemented. In these figures, identical references designate similar elements.

Figure 1 shows, schematically, a known device for determining the particle size distribution of a mixture of particles.

FIG. 2 schematically shows a device according to the invention, improving the device known from FIG. 1.

Figure 3 shows, schematically, an alternative embodiment of the device according to the invention.

The present invention is intended to improve the device of the known type, represented in FIG. 1 and making it possible to determine the particle size distribution of a mixture of particles P.

Said known device comprises a laser generator 1 (not shown) generating a laser beam 2 with parallel rays, of axes X-X. On the path of the laser beam 2, is arranged a transparent container 3 in the form of a tank with parallel faces and provided with a flat conduit 4, for example orthogonal to the axis XX, through which a mixture of particles P circulates. P particles can be sprayed dry through said container or else be suspended in a stream of liquid. The mixture of said particles is put into circulation by known means, not shown, to enter at one end of the conduit 4 (arrow E) and exit therefrom at its other end (arrow S).

Downstream of the container 3 (relative to the direction of propagation of the laser beam 2), the device of FIG. 1 comprises a converging optical system 5, coaxial with the axis X-X, so that its focus F is located on the latter.

In the focal plane of the optical system 5, a set of photosensitive detectors 6 is provided, the sensitive zones 7i of which are distributed radially from the axis X-X. The plurality of sensitive zones 7i are not necessarily centered on the same radius, but they are located at different radial distances from each other. The interval between the sensitive zones 7i and the surface of said sensitive zones 7i increases as a function of their distance f from the axis X-X.

By a link 8.1, the set of photosensitive detectors 6 is connected to a computer 9 capable of processing the signals received from said detectors and of emitting at its output 10 signals representative of the particle size distribution of the mixture of particles conveyed by the current fluid passing through the duct 4.

These particles 9, illuminated by the beam 2, diffract and / or diffuse (depending on their size) of light from different angles a1, as illustrated in FIG. 1. as a result, in the focal plane of the optical system 5 , a diffraction-diffusion pattern of revolution centered on the XX axis.

Depending on their size, the particles participate in the illumination of regions different from the focal plane of the optical system 5: the large particles send light in the vicinity of the axis XX (that is to say on sensitive surfaces 7i, the distance r from the axis XX is small), while the fine particles send light from the axis XX to regions distant from the axis XX (i.e. to surfaces sensitive 7i whose distance r to the axis XX is large). Thus, for example, for a diffracted radius of angle cxl as shown, the corresponding sensitive surface 7i is at a distance rl from the axis XX
Depending on what has been said previously, it will be readily understood that the precision of the measurements is limited for large particles, since these measurements must be carried out at very short distance from the axis XX. This known device therefore does not allow a very fine analysis for such particles.

The device, according to the invention and shown in Figure 2, is intended to remedy this drawback. For this purpose, it comprises a dioptric element, in this case a prism 11, made of a material, for example glass, with a refractive index n1 greater than the refractive index n2 of the ambient medium and placed on the path of said laser beam 2 between said transparent container 3 and said optical system 5.

The different faces 12, 13, 14 of said prism 11 each form a diopter, that is to say an optical surface separating two media with different refractive indices.

The diopter 12 opposite the container 3 is substantially orthogonal to the axis XX, while the diopter 13 located on the side opposite to said container 3 forms an angle A with the diopter 12. The value of this angle A will be specified below. .

A laser beam 15 from said laser beam 2, diffracted by a particle P from the mixture 4 and making an angle a1 with the axis XX, downstream of said container 3 and upstream of said diopter 12 (relative to the direction of propagation of the laser beam 2 ), undergoes refraction on the passage of said diopter 12, so as to present downstream of this diopter 12 an angle a2 relative to the axis XX, the angle a2 verifying the relation n2 sin a1 = nl sin a2. This laser beam 15 then crosses the diopter 13, at the passage of which it undergoes a second refraction, so that with an angle ss2 relative to the normal N of the diopter 13 upstream thereof, it leaves said diopter 13 with a angle ss1 with respect to this normal
N. The angles 131 and 132 also verify the relation n2 sin 131 = nl sin 132.

This diffracted ray 15, which characterizes a particle size, thus leaves the prism according to a deviation D = al + ss1 - A. Considering that the center of the diffraction image is no longer located on the axis XX , but is located on an axis LL corresponding to the direction of the diopter 13 in the plane of the figure (as will be seen below),
The analysis will be made by placing the optical system 5 and the set of photosensitive detectors 6 relative to the axis LL in the same way as they are arranged relative to the axis XX in FIG. 1.

The diffracted ray 15 is thus no longer analyzed at an angle al, but at an angle R = 90 "- 131, much greater than al.

For example, with an angle A = 41.81 and with nl = 1.5 (glass) and n2 = 1 (air), we obtain R = 9.3 for a1 = 1 "and R = 1.2 "for cxl = 0.1". In these examples, the corresponding angles are multiplied by a factor close to ten.

Consequently, the corresponding sensitive surface 7i is located at a distance r2 from the axis LL greater than the distance rl in FIG. 1. Thanks to the invention, an increase in the angular resolution of the diffracted laser rays is therefore obtained, this which obviously allows greater finesse of measurement.

These remarks are not valid for a ray 16 of the laser beam 2, which is not diffracted by a particle P of the mixture and which therefore retains, when the container 3 passes, its initial direction parallel to the axis X-X. This ray 16 does not undergo any refraction from the passage of the diopter 12, since this diopter 12 is orthogonal to the axis X-X.

Said radius 16 makes an angle O with the diopter 13, with O = x / 2 - A. Said angle A (or angle 0) is determined from the relation sin A = n2 / nl. Because of this value of A (or 0) and from a fundamental characteristic of the diopters, said radius 16 is entirely reflected by the diopter 13 and is directed in a direction ZZ, also of angle O with respect to l 'axis LL.

Thus, thanks to this value O of the inclination of the diopter 13 relative to the axis XX, any non-diffracted ray of the laser beam (like said ray 16) is reflected by said diopter 13 and is therefore not addressed to the detectors photosensitive 7i. This makes it possible to considerably increase the signal-to-noise ratio of the measurements, since the main noise generators, namely the non-diffracted rays, are thus not taken into account in the measurements.

In the embodiment of Figure 3, the container 17, containing the mixture of particles P shown locally in this figure, is no longer parallel faces, but has a triangular shape in section, transverse to the direction of circulation represented by the arrow H of said particles P, said particles being carried by a fluid or a gas with a refractive index nl greater than the refractive index n2 of the ambient medium.

The face 12 of the container, through which between the laser beam (not shown) of axis XX, is always arranged orthogonally to this axis XX, while the outlet face 13 is inclined by an angle O defined above, so as to reflecting a non-diffracted ray 18 of said laser beam.

On the contrary, with nl = 1.3 (water) and n2 = 1 (air) and, therefore, with an angle A = 48.75, we obtain for a2 = 10, R = 33.65 and for a2 = 1 " , R = 10.10.

Claims (5)

 the refractive index of the second medium, opposite said optical system (5).  CLAIMS 1. Device for determining the particle size distribution of a mixture of particles in circulation, comprising: - a generator (1) of a monochromatic light beam (2); - a transparent container (3,17), arranged on the path of said beam (2) and traversed by said mixture of particles (P); - a converging optical system (5) to form, in its focal plane, a diffraction-diffusion pattern of said particles illuminated by said beam (2); - a set (6) of photosensitive detectors (7i) distributed in said focal plane of the optical system (5); - means (9) for processing the electrical signals emitted by said assembly (6) of photosensitive detectors; and characterized in that it comprises a diopter (13), formed of an optical surface which separates a first and a second medium of different refractive indices and arranged, in an inclined manner, in the path of said beam of monochromatic light ( 2) between said mixture of particles (P) in circulation and said optical system (5), The refractive index of the first medium, opposite said mixture of particles (P), being greater 2. Device according to claim 1, characterized in that said diopter (13) is inclined relative to the axis (XX) of the monochromatic light beam (2) of an angle O defined by the relation sin (X / 2 4 ) = n2 / nl, with nl The refractive index of said first medium and n2 The refractive index of said second medium. 3. Device according to one of claims 1 or 2, characterized in that it comprises a dioptric element (11), made of a material with a refractive index greater than the refractive index of the ambient medium and disposed on the path of said monochromatic light beam (2), the face of the dioptric element through which said monochromatic light beam exits consisting of said diopter (13). 4. Device according to claim 3, characterized in that said dioptric element (11) consists of a prism. 5. Device according to one of claims 1 or 2, characterized in that said diopter (13) constitutes the face of said container (17) through which exits said beam of monochromatic light (2), said diopter being inclined relative to the face opposite (12) of said container (17) through which enters said monochromatic light beam (2).