FR2778243A1 - Determining the granulometric composition of a mixture of particles, applicable to particles ranging from tenth of mm to centimeter and even to crushed rocks - Google Patents

Abstract

The particles to be measured circulate in a flat channel (5) within a transparent container (4) and are illuminated by a laser (2, 3). The light from the transparent container (4) is received by a video camera (7) with a charge coupled matricial detector (11) and the resulting signal is processed (8) using Fourier transformations to determine the granulometric composition

Description

The present invention relates to a device for determining in real time

  the particle size distribution of a mixture of particles,

  that is to say, to draw a curve representative of the varia-

  according to the diameter of the particles in the mixture, the proportion of particles having the same diameter. By the documents US-A-3 807 864, FR-A-2 300 337, FR-A-2 415 801 and US-A-4 274 741, for example, a device or granulometer 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 through which the fluid current carrying said particles passes, and a converging optical system (converging lens) to form a diffraction and / or diffusion pattern (depending on the size of said particles) particles illuminated by the

  laser beam. A set of photosensitive detectors, distributed radia-

  in the focal plane of said optical system, generates signals

  representative of said diffraction-diffusion figure and addresses

  sés to a computer, processing said electrical signals to determine

  the particle size distribution of said mixture of particles.

  Such a laser granulometer uses diffraction in a known manner

known as Fraunhofer's.

  For this purpose, we know that a particle of size much greater than the wavelength of the laser produces a diffraction pattern, whose intensity l (c) verifies the relation d2 J 1 (7 ._. Sin (ca )) (a) = 10 5.d. sin (() in which - 10 is a reference intensity in the axis - ca is a diffraction angle; - d is the diameter of the particle - X is the wavelength of the laser beam; and

  - J1 is the Bessel function of order 1.

  When said fluid current comprises a large number of parts

  cules, we obtain on average a linear combination of different intensities

  fractured. From a diffraction pattern analysis and treatment

  diffracted intensities matrix, we obtain an indication on the distribution

  particle size distribution sought.

  This method is all the more precise as the particles are

  small, since the width of the diffraction pattern is inversely pro-

  portional to the diameter d of the particle considered.

  To this end, the measurements made in the focal plane of the converging optical system (converging lens) 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 taken far of said axis, that is to say with high values of the angle a of diffraction

and / or broadcasting.

  Therefore, the measurement of large particles is very difficult,

  even impossible, for a granulometer of the aforementioned type, since the

  The signals to be measured are then embedded in the diffraction pattern of the laser beam. It will also be noted that the only known solution for measuring very large particles is to provide a longer optical bench, which

  then requires a larger and very expensive detector.

  The object of the present invention is to remedy these drawbacks

  nients. It relates to a simple and inexpensive device capable of

  determine, in real time and precisely, the particle size distribution

  than a mixture of particles comprising particles larger than those capable of being measured by a laser granulometer

usual, as mentioned above.

  To this end, according to the invention, said device for determining the re-

  particle size distribution of a mixture of particles is remarkable in

  that it includes at least one optical processing system which

  takes: - an image taking means for taking at least one image of at least part of said mixture of particles; and

  an image processing means for deducing information from the

  said image taken by said image taking means and to determine from this information, using at least one transformation of

  Fourier, said particle size distribution.

  Thus, as, thanks to the invention, the ta-

  ches (or shadows) generated by said particles, of which an image is taken, and not the resultant due to the transformation carried out by a converging lens, the optical information relating to the largest particles are not concentrated in the middle of said capture means. 'picture,

  which overcomes the aforementioned drawbacks.

  In addition, thanks to the Fourier transformation, we obtain in-

  training similar to that obtained with the laser granulometer

  aforementioned nude, since the intensity figure measured by said ganulometer

  known corresponds to the modulus of the Fourier transform of the image if-

  killed upstream of the converging lens, this upstream image being that measured by the device according to the invention, since the latter does not include a converging lens and takes the image of the

directly above.

  In addition, the treatments implemented by the present invention are much faster, as will be seen in more detail below, than the usual treatments (consisting in isolating the particles in the image, in measuring them, then in count), which makes it possible to take into account a larger number of images and thus to refine the measurements and

  to increase the precision of said device.

  In a first embodiment, advantageously, said

  image comprises a plurality of gray levels and said processing means

  image calculates the modulus of the bi-Fourier transform

  dimensional image formed by these gray levels, said module

  being representative of said particle size distribution.

  Furthermore, in a second preferred embodiment, said

  image comprises a plurality of gray levels and said processing means

  image ment performs grayscale summations and calculates the mo-

  dule of the Fourier transform of said summations, said module being

  representative of said particle size distribution, which makes it possible to

reduce the calculation time.

  In this case, preferably, the gray levels of said image are arranged according to a matrix structure, and said image processing means:

  - performs gray level summations in one of the two directions

  vertical and horizontal tions of said matrix structure, for all the gray levels of the other direction;

  - calculates the modulus of the Fourier transform of all the summa-

tions thus carried out; and

  - deduce therefrom said particle size distribution.

  Furthermore, advantageously, said image taking means is a video camera which preferably comprises a matrix detector.

  of the charge transfer device type.

  Although, preferably, the mixture of particles is moved in front of the image-taking means, it is also possible in the context of the present invention: - either to produce a single image, for a single position of said means of taking an image in relation to said mixture of particles;

  - either to move said image taking means with respect to said metering

  lange of particles, which can then be stationary.

  The device according to the invention, as mentioned above, can therefore be used to measure all particle sizes, that is to say both particles of the order of a tenth of a millimeter and of the order of

  centimeter. It can even be used to take measurements of gra-

  Nullometry of large objects seen at relative distances-

mentally large.

  Also, an interesting application of the invention is to use the device according to the invention in a system intended to measure the particle size distribution of gravel or crushed rock (of dimensions equal to 500 mm for example), such a system being able to be used in particular in public works. In this case, a conveyor belt is preferably provided for moving the mixture of particles in front of said image-taking means, said mixture being observed by the latter - via a lens - either in the light of the day either by being

subject to auxiliary lighting.

  Furthermore, in another preferred application, the device

  form of the invention comprises at least a first particle size analyzer which

  takes: - at least one generator of light beams, preferably with parallel rays; - at least one transparent container, which is traversed by said mixture of particles and which is arranged in the path of a light beam emitted by said generator; and

  - at least said optical processing system, to analyze said beam

  light beam, downstream of the container in the direction of transmission of said

  light beam, and to deduce said particle size distribution therefrom

than.

  In this case, in a particularly advantageous embodiment

  tagous, said device also comprises at least one second granulomer

  likely to determine the particle size distribution of particles

  having a size at least partially different from the size of the

  the size distribution of which is determined by said pre-

  mier granulometer. This second granulometer can obviously be a laser granulometer of the known and aforementioned type, which is suitable for measuring small particles. It can optionally be incorporated into said first particle size analyzer. Thus, thanks to the invention, the range of measurement measurements is increased.

  particles, relative to their size.

  Furthermore, in another particularly advantageous embodiment, said device also comprises at least

  - a second granulometer capable of determining the gra-

  particle size zero having a size at least partially identical to the particle size, the particle size distribution of which is determined by said first particle size analyzer; and - a determining calculation unit, for particles of identical size,

  a final particle size distribution from the particle size distributions

  metrics determined respectively by said first and second particle size analyzers.

  Thus, as with the invention, the results are obtained from

  from measurements and treatments carried out by two granulometers of different types, we are able to increase the precision and refine these results. For this purpose, preferably, said final particle size distribution determined by said calculation unit is the average of said distributions

  particle sizes determined respectively by said first and se-

cond granulometers.

  Of course, the last two embodiments can be combined in a single device then having a measurement field

  important and high precision.

  Furthermore, advantageously, in order to determine said final particle size distribution, said calculation unit combines the results obtained respectively by said first and second particle size analyzers, using the relation

sinc = _.

  a in which: - is the wavelength of a light beam used by said second particle size analyzer; - A is a diffraction angle in a given direction of analysis, which is capable of being determined by said second particle size analyzer; - i is a serial number of the amplitude of the modulus of the transform of

  Fourier, which is likely to be determined by said first granulo-

  metre; and - a is the length of a measurement field of said image taking means

according to said direction of analysis.

  Furthermore, advantageously, said second granulometer comprises

  door: - a generator of light beams

  - a transparent container, which is crossed by said mixture of particles

  and which is arranged in the path of a light beam emitted by said generator; - a converging optical system to form, in its focal plane, a diffraction-diffusion pattern of said particles illuminated by said light beam; - a set of photosensitive detectors distributed radially in

  said focal plane of the optical system converging and generating

  electrical signals representative of the detected light intensity; and

  - means for processing the electrical signals generated by said en-

  seems to be photosensitive detectors and deduce said particle size distribution. In this case, preferably - said transparent container of said second particle size analyzer corresponds to said transparent container of said first particle size analyzer; and or

  - said light beam generator of said second particle size analyzer

  corresponding to said light beam generator of said first beam

  nullometer and said device comprises means for forming, from

  of a light beam emitted by said single generator, two beams

  partial light beams treated respectively by said first and

second granulometers.

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

  denote similar elements.

  Figure 1 schematically shows a basic embodiment of a

device according to the invention.

  Figure 2 schematically shows a pre-

  of a device according to the invention.

  The device according to the invention and shown diagrammatically-

  ment in FIGS. 1 and 2 respectively in two embodiments

  tion 1A and lB different is intended to determine the particle size distribution

  stick of a mixture of particles in circulation.

  According to the invention, said device 1A comprises a particle size analyzer G1 which comprises a generator 2 generating a light beam 3 with parallel rays, for example a laser beam, of axis X-X. On the path of said light beam 3 is arranged a transparent container 4 in the form

  tank with parallel faces and provided with a flat duct 5, for example or-

  thogonal to the X-X axis, through which a mixture of particles circulates.

  These particles can be sprayed dry through said container 4 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 and exit one end of conduit 5 (arrow E)

be end (arrow S).

  It will be noted that, as a variant, said particles may not be in a tank, but simply be in the open air, by providing for

  means of circulation suitable and not shown.

  Downstream of the container 4 (relative to the direction of propagation of the light beam 3), the device 1A according to the invention comprises an optical processing system 6, for analyzing said light beam 3

  and to deduce therefrom said particle size distribution.

  To be able to easily and precisely measure particles

  having a wide range of diameters, for example between 200 and 2500 μm, said optical processing system 6 comprises, according to the invention: - an image taking means 7, for taking at least one image of said light beam 3 having passed through said transparent container 4; and - an image processing means 8, connected by a link 9 to said means 7, for deducing information from the images taken by said means 7 and for determining from this information, using at least one

  Fourier transformation, said particle size distribution.

  It will be noted that, in a variant embodiment not shown,

  said image taking means 7 directly takes an image of the particles

  illuminated in daylight, which allows you to do without a general

light beam 2 3.

  The results obtained by means 8 can be transmitted to a

  user device not shown via a link 10.

  The image generated by the means 7 comprises a plurality of ni-

gray calves.

  Also, according to the invention - in a first embodiment, said processing means 8

  image computes the modulus of the two-dimensional Fourier transform-

  of said gray levels and calculates from said module the distribution

  particle size sought and possibly the form factor; while - in a second embodiment, said means 8 for processing

  image performs previous summations of grayscale and cal-

  calculates the module of the Fourier transform of said summations, said

  module being representative of said particle size distribution.

  By way of example, this second mode of implementation can

  sister to perform in the vertical direction of the image the sum of the gray levels to obtain a "sum" line, then to calculate the modulus of the Fourier transform. This can also be done by doing the

sum in the horizontal direction.

  It will also be noted that a calibration of the image taking means 7 can be carried out, by measuring a signal in the absence of a sample (Fourier coefficient or Fourier module) and then subtracting it

  measurements made later.

  According to the invention, said image taking means 7 can be produced

in different ways.

  However, in a preferred embodiment shown, said means 7 is a video camera 7 comprising a matrix detector 11 of the type with charge transfer device. In this case, however,

  also provide an afocal optical means not shown, for agran-

  direct or reduce the image formed on said detector 1 1.

  Thus, the device 1A according to the invention measures directly

  the spots (or shadows) generated by the particles located on the par-

  course of the light beam 3, which makes it possible to remedy the problems associated with the measurement of large particles (for example of larger diameter

  at 400 pim) existing on conventional laser granulometers known.

  Said device 1A can thus measure particles having a

large diameter.

  In addition, the device according to the invention is not limited to the above values. It can indeed measure much more particles

  still large, such as gravel for example.

  Also, as an application, one can consider using the device

  tif according to the invention in a system, in particular of public works

  blics, to determine the particle size distribution of gravel or ro-

  crushed ches, which generally have dimensions up to

500 mm.

  In this application, not shown, said device preferably comprises a means, for example a conveyor belt, for moving the gravel in the open air and said means 7 observes the gravel in circulation,

  either in daylight or under auxiliary lighting.

  Furthermore, thanks to the aforementioned Fourier transformation, said G1 particle size analyzer provides information of the same kind as that provided by conventional laser particle size analyzers, as will be seen in more detail below, which avoids any subsequent processing and facilitates

adaptation of the results.

  As can be seen in Figure 2, said device 1 B comprises

  the granulometer G1 of FIG. 2. Consequently, this device LB pre-

  already feels the aforementioned advantages and peculiarities of said G1i granulometer. In addition, said device 1B comprises a particle size analyzer G2, of the usual laser particle size analyzer type, which comprises:

  a laser generator 13 generating a laser beam 14 with pa-rays

  alignments, of axis Y-Y, passing through said container 4 for example in an or-

  thogonal so that the X-X and Y-Y axes are parallel

  a converging optical system 15, for example a converging lens,

  gente, to form, in its focal plane, a diffraction figure-

  scattering of said particles illuminated by said laser beam 14; - A set 16 of photosensitive detectors distributed radially in said focal plane of the converging optical system 15 and generating electrical signals representative of the detected light intensity; and - means 17 connected by a link 18 to said assembly 16, for processing the electrical signals generated by said assembly 16 of detectors

  photosensitive and deduce therefrom said particle size distribution.

  Therefore, as the usual G2 particle size analyzer allows

  to measure small particles having for example a mid-diameter

  nimal up to 0.1 pm, the IB device according to the invention has a very wide measurement field, for example from 0.1 pm to 2500 pm or more.

  In addition, as a variant or in addition to the above, when-

  as the particle size analyzers G1 and G2 measure particles of the same size, for example between 200 μm and 400 μIm, said device 1 B makes it possible to refine

  the results corresponding to this measurement range, from the results

  states provided by means 8 and 17.

  To this end, said device 1B comprises a calculation unit 19 representing

  linked to means 8 and 17 respectively by links 10 and 20, of which

  the results can be transmitted by a link 21 to a device used

sator not shown.

  The calculation unit 19 can, for example, carry out an average of the results provided by said particle size analyzer G1 and G2, to deduce therefrom

  final and refined particle size distribution of said mixture of particles.

  However, any possible respective weighting of the different re-

  results is of course conceivable within the framework of the present invention.

  tion. Furthermore, in a variant not shown, the device 1B comprises a single laser generator and a means for forming from a laser beam emitted by this single generator the two beams 3 and 14 above. In a particularly advantageous embodiment, the means 8 and 17 and the calculation unit 19 are arranged in a single unit

central 22.

  As previously indicated, the results provided by the granu-

  lometer G1 (after Fourier transformation) are of the same nature as

  those supplied by the G2 granulometer.

  Also, said calculation means 19 can easily link together said results using the following relation sinc = _.i a in which - X is the wavelength of beam 14; oa is a diffraction angle in a given direction of analysis, said angle a being able to be determined by means 17; - i is a serial number of the amplitude of the module of the Fourier transform, said number i can be determined by means 8; and

  - A is the length of the detector 1 1 according to said direction of analysis.

  Of course, in the context of the present invention, it is possible to

  associate any type of particle size analyzer with the G1 particle size analyzer, to obtain an increase in the measurement field and / or the accuracy of the measurements, and

  not just a laser particle sizer such as the G2 particle sizer.

  In addition, it will be noted that, thanks to the two-dimensional Fourier transform, it is possible to determine the average form factor

  measured particles, that is to say the average elongation thereof.

Claims (15)

  1. Device for determining the particle size distribution of a mixture of particles,   characterized in that it comprises at least one optical treatment system   that (6) which comprises: - image taking means (7) for taking at least one image of at least part of said mixture of particles; and - an image processing means (8) for deducing information from   said image taken by said image taking means (7) and for determining   ner from this information, using at least one transformation   of Fourier, said particle size distribution.   2. Device according to claim 1, characterized in that said image comprises a plurality of gray levels and said image processing means (8) calculates the module of the two-dimensional Fourier transform of the image formed of these levels   of gray, said module being representative of said particle size distribution   than. 3. Device according to claim 1, characterized in that said image comprises a plurality of levels of   gray and in that said image processing means (8) performs   grayscale and calculates the modulus of the Fou- transform   laugh at said summations, said module being representative of said distribution particle size distribution.   4. Device according to claim 3, characterized in that the gray levels of said image are arranged according to a matrix structure, and in that said means (8) for image processing   - performs gray level summations in one of the two directions   vertical and horizontal tions of said matrix structure, for all the gray levels of the other direction;   - calculates the modulus of the Fourier transform of all the summa- tions thus carried out; and   - deduce therefrom said particle size distribution.   5. Device according to any one of claims 1 to 4,   characterized in that said image taking means (7) is a video camera.   6. Device according to any one of the preceding claims. your,   characterized in that said image taking means (7) comprises a   matrix guard (11) of the charge transfer device type.   7. Device according to any one of the preceding claims.   tes, characterized in that said mixture of particles is displaced in front of said image capture means (7).   8. Device according to claim 7, characterized in that it comprises at least a first particle size analyzer (G1) which comprises - at least one generator (2) of light beams   - at least one transparent container (4), which is traversed by said medium   mixture of particles and which is arranged in the path of a light beam   neux (3) emitted by said generator (2); and - at least said optical processing system (6), for analyzing said light beam (3), downstream of said transparent container (4) in the direction of transmission of said light beam (3), and for deducing therefrom   said particle size distribution.   9. Device according to claim 7,   characterized in that it comprises a conveyor belt for moving said mete range of particles.   10. Device according to claim 8, characterized in that it further comprises at least one second granulomè-   tre (G2) capable of determining the particle size distribution of   cells having a size at least partially different from the size of the particles, the particle size distribution of which is determined by said first particle size analyzer (G1).   11. Device according to claim 8, characterized in that it further comprises at least - a second particle size analyzer (G2) capable of determining the distribution   particle size distribution with at least a partial size-   identical to the size of the particles, the size distribution of which   trique is determined by said first particle size analyzer (G1); and   - a calculating unit (19) determining, for particles of identical size   tick, a final particle size distribution from the distributions   particle sizes determined respectively by said first and se- cond granulometers (G1, G2).   12. Device according to claim 1 1, characterized in that, to determine said particle size distribution   final, said calculation unit (19) combines the results obtained respectively   by said first and second granulometers (G1, G2), using the relation sinca = _.ia in which - X is the wavelength of a light beam (14) used by said second granulometer (G2 ); - (a is a diffraction angle in a given direction of analysis, which is capable of being determined by said second particle size analyzer (G2); - i is a sequence number of the amplitude of the modulus of the transform of   Fourier, which is likely to be determined by said first granulo-   meter (G1); and - a is the length of a measurement field of said gripping means (7)   image according to said direction of analysis.   13. Device according to one of claims 1 1 and 12,   characterized in that said final particle size distribution determined   by said calculation unit (19) is the average of said granular distributions   lometric determined respectively by said first and second granulometers (G1, G2).   14. Device according to any one of claims 10 to 13,   characterized in that said second particle size analyzer (G2) comprises - a generator (13) of light beams;   - a transparent container (4), which is traversed by said mixture of par-   which is arranged in the path of a light beam (14) emitted by said generator (13); - a converging optical system (15) to form, in its focal plane, a diffraction-diffusion pattern of said particles illuminated by said light beam (14); - a set (16) of photosensitive detectors distributed radially in said focal plane of the converging optical system (15) and generating   electrical signals representative of the light intensity detected   tee; and - means (17) for processing the electrical signals generated by said   set (16) of photosensitive detectors and deducing said repair therefrom particle size distribution.   15. Device according to claim 14, characterized in that said transparent container (4) of said second granule   lometer (G2) corresponds to said transparent container (4) of said first graph nullometer (Gi).   1 6. Device according to one of claims 14 and 1 5,   characterized in that said generator (13) of light beams of said second particle size analyzer (G2) corresponds to said generator (2) of light beams of said first particle size analyzer (G1) and in that said device (1 B) comprises means for forming, at from a light beam emitted by said single generator, two partial light beams treated   respectively by said first and second granulomers (G1, G2).   17. System for determining the particle size distribution of gravel, characterized in that it comprises a device such as that specified under   any one of claims 1 to 7 and 9.