JP6191000B2 - Laser Doppler flow velocity measuring method and apparatus - Google Patents

Description

  The present invention relates to a laser Doppler flow velocity measuring method and apparatus for measuring a flow velocity by a laser Doppler method capable of simultaneously measuring flow velocity at multiple points.

Point measurement LDV (Laser Doppler) is a non-contact and pressure-free measurement method
The Velocimeter method is capable of measuring velocity distribution in unsteady flow in the KHz order with high response, high accuracy, and high resolution without being affected by density and clay. Is the method.

  In this point measurement LDV method, one laser beam is divided into two parallel beams of equal intensity by a beam splitter and a mirror, condensed by an achromatic lens, and the intersection of the two condensed beams is measured. As a method of measuring the velocity of seeding particles passing through this point.

  However, this point measurement LDV method has a problem that since the flow field is measured by a point, the amount of information is very small and it is difficult to measure with high accuracy. Therefore, the inventor of the present application has developed a line measurement LDV method capable of simultaneously measuring a flow field at multiple points as shown in Non-Patent Documents 1 and 2.

  The line measurement LDV method divides one laser beam into two parallel beams of equal intensity by a cube beam splitter and a mirror, condenses them by an achromatic lens, After the plane light of parallel light is made, each plane light is crossed. Each plane light is a direction orthogonal to the traveling direction, and crosses each plane light so that the width direction of each plane is parallel. Thereby, the intersecting line where the planes of the respective plane lights intersect becomes a measurement point sequence, and interference fringes are generated at the intersecting line, and the particles pass through the intersecting line, so that scattered light having a Doppler signal is obtained. The scattered light is guided to an avalanche photodiode (APD), converted from an optical signal to an electrical signal by APD, and subjected to FFT (Fast Fourier Transform) processing to calculate the flow velocity of the particles. The inventor of the present application developed and patented a laser Doppler blood flow measurement method and measurement disclosed in Patent Document 1 as an example of use of this measurement method.

Japanese Patent No. 5234470

Development of a Multi-point LDV by usingSemiconductor laser with FFT-based multi-channel Signal Processing; Experiences in Fluids 24 Springer-Verlag pp.70-76 Development of simultaneous measurement method of two-component flow velocity distribution using LDV and CCD area image sensor simultaneously; Proceedings of the Society of Instrument and Control Engineers Voi.39, No.3 (2003) 218-224

  The above-mentioned line measurement LDV method can measure the flow field from point to multi-point simultaneously, and the measurement accuracy is high. However, since the measurement area is a one-dimensional line measurement, the amount of information is still sufficient. However, the development of more accurate measurement technology has been demanded. In particular, a surface measurement technique with a two-dimensional measurement range has not yet been established.

  The present invention has been made in view of the above-described conventional background art, and is a laser that can measure a two-dimensionally wide range efficiently with a high measurement accuracy by a simple optical system and apparatus using a laser Doppler method. It is an object of the present invention to provide a Doppler flow velocity measuring method and apparatus.

  In the present invention, the laser light from the laser light source is branched, and each of the branched laser lights is made into a sheet-like thin sheet light, intersecting each other on the same surface so that the surfaces overlap each other, The scattered light from the particles in the fluid at the intersecting intersection is imaged on a planar light-receiving part, and incident on a plurality of light-receiving elements arranged two-dimensionally at the image-forming position, and incident on the light-receiving element. Each of the scattered light and the scattered light shifted by the Doppler frequency is converted into an electric signal by a photoelectric conversion element, and the flow rate of the fluid is changed for each photoelectric conversion element by the scattered light shifted by the Doppler frequency of the laser light. This is a laser Doppler flow velocity measuring method to be calculated.

  The intersecting region of the sheet-like laser light may be scanned in a direction intersecting the intersecting region, and the flow velocity of the intersecting region may be obtained three-dimensionally.

The present invention also includes a laser light source, a beam splitting element that splits the laser light from the laser light source, and a sheet light forming optical system that converts the laser light split by the beam splitting element into a planar thin sheet light, A condensing optical system that crosses each of the branched sheet light on the same surface so that the surfaces overlap each other, and light scattered by particles in the fluid in the intersecting region where the sheet light intersects are two-dimensionally connected. An imaging optical system for imaging, a plurality of light receiving elements arranged at an imaging position of the imaging optical system, a photoelectric conversion element for converting the scattered light incident on the light receiving element into an electrical signal, and the laser light Is a laser Doppler flow velocity measuring device including a calculating means for calculating the flow velocity of the fluid in the intersecting region shifted by the Doppler frequency for each photoelectric conversion element based on the electric signal.

  The imaging optical system is arranged with an optical axis in a direction orthogonal to the plane of the intersecting region of the sheet light.

  The light receiving element may be composed of a solid-state imaging element in which photoelectric conversion elements are integrally arranged on a plane.

  An acoustooptic element may be provided in an optical system composed of the beam splitting element and the sheet light forming optical system.

  According to the laser Doppler flow velocity measuring method and apparatus of the present invention, when measuring the flow velocity of a fluid, the flow velocity can be measured in a two-dimensional intersection region, and a large amount of flow velocity information can be obtained at the same time. It is highly efficient and can effectively measure fluids with a large flow rate and a wide range.

It is the schematic which shows the laser Doppler flow velocity measuring device optical system of one Embodiment of this invention. It is a schematic diagram which shows the measurement surface which is an intersection area | region of the sheet light by the laser Doppler flow velocity measuring apparatus of one Embodiment of this invention. It is the whole figure (a) which shows arrangement | positioning of the optical fiber of the laser Doppler flow velocity measuring apparatus of one Embodiment of this invention, and the front view (b) which shows the light-receiving part of a fiber array. It is a block diagram which shows the function from the photoelectric conversion element of the laser Doppler flow velocity measuring apparatus of one Embodiment of this invention to a computer. It is a graph which shows the change for 5 second of the flow velocity in one point of the measurement point of the flow velocity measurement area | region measured by the laser Doppler flow velocity measuring method of this invention. It is a graph which shows the change for 5 second of the flow velocity in each measurement point in the flow velocity measurement area | region measured by the laser Doppler flow velocity measuring method of this invention.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, a laser Doppler flow velocity measuring apparatus 10 according to this embodiment includes a laser light source 12 such as a laser diode and a beam splitter that is a beam splitting element that splits the laser light L from the laser light source 12 in two directions. 14, a reflecting mirror 16 that directs one of the laser beams L 1 and L 2 branched into two by the beam splitter 14 in a direction parallel to the other, and an achromaticity that focuses the laser beams L 1 and L 2 at a predetermined position. A condensing lens 18 that is a condensing optical system such as a cleansing lens is provided.

  Further, rod lenses 21 and 22 that are sheet light forming optical systems are provided on the output light side of the condenser lens 18 so as to coincide with the passing positions of the laser beams L1 and L2 and to be parallel to each other. The rod lenses 21 and 22 diffuse the laser beams L1 and L2 in a planar shape, and the diffused laser beams L1 and L2 are located on the same plane. In the vicinity of the light emitting side of each of the rod lenses 21 and 22, the cylindrical lens 23 of the sheet light forming optical system that forms the incident planar laser beams L1 and L2 into the sheet beams Ls1 and Ls2 that are planar parallel beams. , 24 are located. Cylindrical lenses 23 and 24 are also positioned in parallel with each other, and flat portions 23a and 24a face the rod lenses 21 and 22 side. The sheet lights Ls1 and Ls2 intersect at a predetermined angle 2θ, with the sheet surfaces positioned on the same plane. Here, θ is an angle at which the laser beams L1 and L2 with respect to the optical axis of the condenser lens 18 are refracted and emitted.

  Since the intersecting region D of the sheet light Ls1 and Ls2 intersects on the same surface so that the surfaces of the sheet light Ls1 and Ls2 that are parallel lights coincide with each other and overlap as shown in FIGS. A two-dimensional flow velocity measurement region formed by an optical fiber array 30 to be described later is set in the range of the intersection region D.

  An imaging lens 26 that is an imaging optical system such as a pair of achromatic lenses is disposed at a position separated by a predetermined distance in a direction orthogonal to the surface of the intersecting region D of the sheet light Ls1 and Ls2. The light receiving portion 30a of the optical fiber array 30 that is a light receiving element arranged two-dimensionally is located at the image forming position of the image forming lens 26 facing the outgoing light side. The imaging lens 26 of the imaging optical system is arranged with an optical axis in a direction orthogonal to the plane of the intersecting region D of the sheet lights Ls1 and Ls2.

  The surface of an optical system component such as a lens used in this embodiment is coated with an antireflection film for this wavelength in order to suppress the attenuation of the wavelength of the used laser beam.

  As shown in FIG. 3, the optical fiber lay 30 includes a light guide bundle 34 in which the optical fibers 32 are bundled. The end faces of the optical fibers 32 are aligned vertically and horizontally so that the light receiving unit 30 a of the optical fiber lay 30 is arranged. Forming. The other end of each optical fiber 32 is connected to an optical plug 36 for each optical fiber 32. Each of the optical plugs 36 is provided so as to be connectable to an optical receptacle 38. Each optical receptacle 38 is provided with an avalanche photodiode (hereinafter referred to as APD) 40 which is a photoelectric conversion element.

  As shown in FIG. 4, the APDs 40 are provided by the number of the optical fibers 32, and are arranged in the same manner as the arrangement of the light receiving units 30 a of the optical fibers 32. The APD 40 converts each light guided by the optical fiber 32 into an electrical signal, and the output of each APD 40 is input to each frequency filter 44 through each amplifier 42 provided by the number of the optical fibers 32. Each signal filtered by the frequency filter 44 is A / D converted by the data recorder 46 and recorded. The digital signal recorded in each data recorder 46 is input to the computer 48 via a USB connector or the like, and the computer 48 performs a predetermined calculation analysis process for measuring a flow velocity, which will be described later.

  Next, the laser Doppler flow velocity measuring method of this embodiment will be described below. In this laser Doppler flow velocity measuring method, the laser light emitted from the laser light source 12 is divided by the beam splitter 14, and as shown in FIGS. 1 and 2, the rod lenses 21 and 22 and the cylindrical lenses 23 and 24, The sheet lights Ls1 and Ls2 are formed and crossed at measurement positions on a predetermined plane. In the intersecting region D that is the measurement position, the flow velocity of the fluid passing through the intersecting region D can be obtained with a spatial resolution determined by the magnification of the imaging lens 26 that is the imaging optical system, with the optical fiber 32 as a unit. For example, if an optical fiber 32 having a diameter of 250 μm is used and the magnification of the optical system is 1, the spatial resolution is 250 μm.

  The intersecting region D of the sheet light Ls1 and Ls2 is a region in which the sheet-like parallel light intersects with each other so as to overlap each other, and thus has a rhombus shape, and interference fringes are caused by the phase difference between the sheet light Ls1 and Ls2. Can do. When the minute particles p pass along with the fluid at the flow velocity in the fluid in the intersection region D, the sheet lights Ls1 and Ls2 are reflected by the particles p in the fluid, and scattered light is generated. The Doppler frequency of the scattered light changes due to the Doppler effect due to the velocity of the particles p. Then, each scattered light by the particles p in the intersection region D is imaged by the imaging lens 26 at a point corresponding to each on the light receiving portion 30a of the optical fiber array 30.

  Scattered light whose frequency is changed by the flow velocity of the fluid is received by the light receiving unit 30a of the optical fiber array 30, is input to each APD 40 via each optical fiber 32, and is detected as an electrical beat signal. The frequency change and intensity of the signal detected here are values corresponding to the speed and number of particles p in the fluid. This is calculated | required for every optical fiber 32 in the light-receiving part 30a, and the flow velocity in the point corresponding to the cross | intersection area | region D is calculated. The input signal obtained by each APD 40 is A / D converted and input to the computer 48, and the digital signal is fast Fourier transformed and further subjected to noise removal processing and the like to calculate the Doppler frequency. The Doppler frequency is determined by the appearance of a peak at the Doppler frequency in the frequency spectrum of the detected signal. When the Doppler frequency is obtained, the velocity of the particle p in the fluid is calculated from the frequency in a known calculation direction, and the velocity is equal to the flow velocity of the fluid.

  The measurement result obtained by the optical fiber 32 thus obtained is shown in the graph of FIG. Here, the velocity of the particles p in the fluid in 5 seconds is plotted with a predetermined sampling period. As shown in FIG. 6, the measurement result of each optical fiber 32 in the entire light receiving unit 30 a accurately detects the change in the flow velocity at the corresponding location for each optical fiber 32 in the intersection region D.

  Further, by moving the position of the intersecting region D in the direction perpendicular to the surface and measuring the flow velocity in the same manner, and repeating this scanning, the flow velocity of the intersecting region D can be measured three-dimensionally.

  According to the laser Doppler flow velocity measuring method and apparatus of this embodiment, when measuring the flow velocity of the fluid, the flow rate can be measured in the two-dimensional intersection region D, and a large amount of flow velocity information can be obtained at the same time. . In addition, the spatial resolution in this case can be appropriately set by the optical system.

  The laser Doppler flow velocity measuring method and apparatus according to the present invention is not limited to the above-described embodiment, and the optical system between the laser light source and the condenser lens is replaced with an acousto-optic element and a reflecting mirror. Alternatively, the following effects can be obtained by using an acoustooptic device. It is possible to distinguish between positive and negative flow rates, separate pedestal and Doppler signals, and measure turbulence when the average flow rate is low. Furthermore, even when the average flow velocity is low and the fluctuation frequency is high, it is effective for measurement of unsteady flow. With a single signal processing device, it is possible to measure a wide band, and even if the frequency characteristics of a filter or the like are determined, the frequency of the signal can be changed. It becomes possible. In addition, since a known signal is given to the measurement volume, it is easy to adjust the optical system, and a signal corresponding to the frequency shift can be obtained even at zero flow velocity. Normally, it is confirmed that a signal can be obtained by flowing a mist such as water vapor to the measurement volume, but a signal corresponding to the frequency shift can be obtained by simply placing a transparent ruler or the like on the measurement volume.

  Further, the light receiving unit and the photoelectric conversion element may be a solid-state imaging element of an image sensor such as a CCD or a C-MOS. The optical fiber can also be selected as appropriate, such as glass fiber or plastic fiber.

DESCRIPTION OF SYMBOLS 10 Laser Doppler flow velocity measuring apparatus 12 Laser light source 14 Beam splitter 16 Reflector 18 Condensing lens 21, 22 Rod lens 23, 24 Cylindrical lens 26 Imaging lens 30 Fiber array 30a Light receiving part 32 Optical fiber 40 Avalanche photodiode (APD)
D crossing region Ls1, Ls2 sheet light p particle

Claims (6)

Branch the laser light from the laser light source, make each branched laser light into a sheet-like thin sheet light, intersect each other on the same surface so that the surfaces overlap,
The scattered light from the particles in the fluid in the intersecting region where the sheet light intersects is imaged on a planar light receiving unit, and is incident on a plurality of light receiving elements arranged two-dimensionally at the imaging position,
Each of the scattered light incident on the light receiving element, and the scattered light shifted by the Doppler frequency is converted into an electric signal by a photoelectric conversion element,
A laser Doppler flow velocity measuring method, wherein the flow velocity of the fluid is calculated for each photoelectric conversion element by using the scattered light shifted by the Doppler frequency of the laser light.   The laser Doppler flow velocity measuring method according to claim 1, wherein the intersecting region of the sheet-like laser light is scanned in a direction intersecting the intersecting region, and the flow velocity of the intersecting region is obtained three-dimensionally. A laser light source; a beam splitting element for splitting the laser light from the laser light source; a sheet light forming optical system for converting the laser light split by the beam splitting element into a planar thin sheet light; A condensing optical system that intersects light on the same surface so that the surfaces overlap each other, and imaging optics that forms a two-dimensional image of light scattered by particles in the fluid in the intersecting region where the sheet light intersects System, a plurality of light receiving elements arranged at the imaging position of the imaging optical system, a photoelectric conversion element that converts the scattered light incident on the light receiving element into an electrical signal, and the laser beam shifted in Doppler frequency A laser Doppler flow velocity measuring apparatus, comprising: a calculating means for calculating the flow velocity of the fluid in the intersecting region for each photoelectric conversion element based on the electrical signal.
  4. The laser Doppler flow velocity measuring device according to claim 3, wherein the imaging optical system is arranged with an optical axis in a direction orthogonal to the plane of the intersecting region of the sheet light.   5. The laser Doppler flow velocity measuring device according to claim 4, wherein the light receiving element is a solid-state imaging element in which photoelectric conversion elements are integrally arranged on a plane. 4. The laser Doppler flow velocity measuring apparatus according to claim 3, wherein an acousto-optic element is provided in an optical system comprising the beam splitting element and the sheet light forming optical system.

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