JPS60243583A - Laser doppler speedometer - Google Patents

Abstract

PURPOSE:To make it possible to obtain a high S/N value with high accuracy, by mounting a means for performing linear irradiation in almost parallel to a moving direction and a means for performing linear prescription of reflected light to the direction crossing the moving direction almost at right angles. CONSTITUTION:Beam emitted from a laser beam source 1 is converted to parallel beam 3 by a lens 2 and passed through a lens 9 and a beam splitter 4 to be divided into two beams which are, in turn, condensed to the direction at a right angle to a moving direction through mirrors 6, 6' to linearily irradiate the surface of an object 7 as is parallel to the moving direction. Reflected and scattered beam from the object 7 is received by a beam detector 8 to analyze the frequency thereof and the moving speed V of the object 7 is detected. The magnitude of a spectrum becomes large in the direction at a right angle to the moving direction of the object and the number of spectra on the light detection surface are reduced. Therefore, the AC component of the output signal of the detector 8 becomes large and an S/N ratio is enhanced to easily enable the processing for frequency analysis with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a laser Doppler velocimeter that non-contactly detects the moving speed of an object or fluid.

[Prior art]

BACKGROUND ART Laser Doppler velocimeters have conventionally been used as devices for detecting the moving speed of objects or fluids with high precision in a non-contact state. What is a laser Doppler velocimeter? As is well known, when a moving object is irradiated, the frequency of light scattered by the object shifts (Doppler shift) in proportion to the object's moving speed (Doppler effect). This is a device that detects the moving speed of. As such a device, there has conventionally been a device having a configuration as shown in FIG. In the figure, 1 is a laser light source (for example, a semiconductor laser), 2 is a collimator lens, 3 is a parallel light beam, 4 is a beam splitter,
5 and 5' are two divided parallel light beams, 6 and 6' are mirrors, 7 is an object moving in the direction of the arrow shown at velocity V, and 8 is the detection of scattered light reflected by this object. It is a photodetector that

Next, the operation will be explained. A beam emitted from a laser light source 1 becomes a parallel beam 3 with a finite size by a collimator lens 2, is split into two beams 5.5' by a beam splitter 4, and is reflected by a mirror 6.6'. Two beams of light are incident on an object 7 moving at a speed V at an incident angle θ. Scattered light reflected from the surface of the object 7 is detected by a photodetector 8. The frequency of the reflected and scattered light by these two light beams 5.5' is proportional to the moving speed of the object 7, and the frequencies of the reflected and scattered light are each 10Δf
.

undergoes a Doppler shift of −Δf. The above Δf is calculated by the following formula +
11.

Δf=vSLrIθ/λ ・・・・・・・・・・・・(
1) Here, λ is the wavelength of the laser beam.

Then, the output frequency F of the photodetector 8 is F=2Δf=2
vsin θ/λ . . . It is given by (2). From equation (2), if the frequency F is measured using a spectrum analyzer or the like, the moving speed V of the object 7 can be determined.

The conventional laser Doppler velocimeter constructed based on the above-mentioned principle has a drawback in that the 81N value of the output signal of the photodetector 8 is low for the reason described below.

That is, as is clear from equation (1), the moving speed V
In order to accurately measure K, it is necessary that there is no expansion in the incident angle θ. Therefore, it is necessary that the two incident lights 5.5' be parallel light beams.

When a parallel laser beam with a diameter d is irradiated onto the surface of the object 7, the reflected and scattered light forms a so-called spectral pattern in the form of irregular spots, and is reflected on the surface of the photodetector 8 at a distance from the surface of the object 7. It is known that the average diameter of the spectral pattern is expressed as DωλL/d...13). That is, due to the parallel beam of diameter d, which is a finite size, and the finite distance ゛L,
In order to obtain effective sensitivity without increasing speckles on the photodetection surface, a large number of speckles are formed on the photodetection surface, which has a finite size. As shown in FIG. 2, for example, the output of the photodetector 8 has a drawback that the AC component (Doppler modulation) is smaller than the DC component, that is, it becomes a signal with a low S/'H.

〔the purpose〕

The present invention has been made in view of the above-mentioned problems, and aims to provide a laser Doppler speedometer that is highly accurate and obtains a high 8/N value by eliminating the above-mentioned drawbacks. ,ru.

〔Example〕

The present invention will be explained below based on the drawings. Figures 3 and 5
6 and 7 are configuration diagrams of respective embodiments of the laser Doppler velocimeter of the present invention.

In both cases, components that are the same (equivalent) to the conventional example shown in FIG. In FIG. 3, reference numeral 9 denotes a cylindrical lens that has refractive power in a direction perpendicular to the deflection direction (arrow in the figure) of the moving object 7, but has almost no refractive power in the moving direction, and focuses on the surface of the moving object. It is placed in such a position that it

Next, the operation of this embodiment will be explained. The light beam emitted from the laser light source 1 is converted into a parallel light beam 3 by the collimator lens 2.
After that, it passes through a cylindrical lens 9, is split into two beams by a beam splitter 4, and is focused through a mirror 6.6' in a direction perpendicular to the direction of movement, while remaining parallel to the direction of movement. In this state, 7 surfaces of the object are irradiated. That is, the object 7 surface is irradiated linearly. Then, the reflected and scattered light from the object 7 is received by a photodetector 8, its frequency is analyzed, and the moving speed V of the object 7 is detected.

With the above configuration, as shown in the figure, the average size and width of the spectrum becomes larger in the direction perpendicular to the moving direction of the object 7, and the number of spectra on the photodetection surface decreases. . Therefore, the alternating current component (Doppler modulation component) of the output signal of the photodetector 8 becomes large as shown in FIG. 4, increasing the S/N ratio and making signal processing for frequency analysis easier and more accurate. . Furthermore, since the light is incident in the direction of movement of the object T as a parallel light beam, there is no wave force at the angle of incidence in this direction, and the accuracy of speed detection is not impaired.

[Other Examples]

FIG. 5 shows a second embodiment of the invention.

10 and 10' are cylindrical lenses having a smaller radius of curvature than the cylindrical lens 9 in the first embodiment shown in FIG.

By disposing the mirror 6.6' in the optical path, that is, just before the object 7 surface, and reducing the irradiation width in the direction orthogonal to the moving direction of the object 7 surface, the multi-photodetector 8 surface As mentioned above, the number of spectra in this direction is increased, and the S/N of the output signal of the photodetector 8 is further increased, so that a greater effect than in the previous embodiment can be obtained.

Further, FIG. 6 shows a third embodiment of the present invention. Reference numeral 11 denotes a mask having a linear aperture in a direction parallel to the moving direction of the object 7, a so-called slit, which is arranged at a corresponding position in place of the cylindrical lens 9 of the first embodiment, and similarly,
Since the object 7 can be linearly irradiated in a direction parallel to the direction of movement thereof, the same effects as in the first embodiment can be obtained. It should be noted that the cylindrical lenses 10 and 10' of the second embodiment may be replaced with slits in accordance with the present embodiment.

Furthermore, FIG. 7 shows a fourth embodiment of the present invention.

Reference numeral 12 denotes a slit provided on the detection surface of the photodetector 8 in a direction perpendicular to the moving direction of the object 7.

If the width of the slit 12 is limited to the average size of the spectral pattern formed on the photodetection surface in the direction of object movement, the S/N of the output signal of the photodetector 8 will increase, and a similar effect can be obtained. can get.

The slit 12 is the first slit. It can be replaced with a cylindrical lens as shown in the second embodiment.

Incidentally, the four embodiments shown above may be employed individually, or may be employed in appropriate combinations.

〔effect〕

As described above based on each embodiment, according to the present invention, the S/N of the photodetector output signal can be increased by a simple and inexpensive configuration in which one cylindrical lens or stripe slit is provided in the optical path. By increasing the accuracy of the laser Doppler speedometer, it is possible to achieve high accuracy.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of an example of a conventional laser Doppler velocimeter, Figure 2 is an example of its photodetector output signal, Figures 3, 5,
6 and 7 are respective configuration diagrams of the first to fourth embodiments of the laser Doppler velocimeter of the present invention, and FIG. 4 is an example of the photodetector output signal of the first embodiment of FIG. 3. Yes, 1...... Laser light source 3, 5, 5'...... Laser parallel beam 4
......Beam splitter 7...
・・Moving object 8・・・・・・・Photodetector

Claims (1)

[Claims] A means for linearly irradiating a moving object whose moving speed is to be detected in a direction substantially parallel to its moving direction and/or a means for linearly regulating light reflected from the object in a direction substantially perpendicular to its moving direction. Features a laser Doppler speedometer.