JPS59674A - Laser doppler speedometer - Google Patents

Abstract

PURPOSE:To improve measurement precision and to remove the evil influence of the surface condition of an object to be measured, by irradiating the object with a wide parallel light beam and converging and detecting scattered light from wide area, and measuring the speed. CONSTITUTION:Coherent light from a laser device 1 is collimated by a collimator lens system 2 into the wide parallel beam, which illuminates the object 13 of measurement. Reflected and scattered light 14 from the object 13 is transmitted through a half-mirror 3 and convered to a focal plane 15. Light beams incident to input terminals 5a and 5b are inputted to and multiplexed by a photocoupler 7, whose output is inputted to a photodetector 8 to perform heterodyne detection. The signal from the photodetector 9 is passed through a signal analyzing device 10 and multiplied by a specific constant at an output device 11 to generate a signal corresponding to the movement speed of the object 13. Thus, the measurement precision is improved and the evil influence of the surfac condition of the object to be measured is removed.

Description

Detailed Description of the Invention The present invention provides a laser dog (prior art and its problems) that detects the speed of an object by shining coherent light onto a moving object and detecting the Doppler shift of the scattered light. Conventional laser Doppler velocimeters include those that irradiate an object with a laser beam to form an interference pattern on the object and measure the speed by observing this interference pattern, and those that measure the speed by focusing a laser beam and irradiating it onto the object. There are known devices that detect the moving speed of an object by detecting the reflected light from the object.
ing.

However, in the conventional method, the accuracy of the optical system used must be extremely high, and the measurement accuracy decreases when the position of the object to be measured, especially in the optical axis direction, changes. There was an inconvenience. In addition, in the latter method, if there are irregularities on the measurement target that reflect the laser beam outside the specified range or shadow areas with low reflectance, it is necessary to There were disadvantages such as a period during which no signal could be obtained.

(Object of the Invention) The object of the present invention is to provide two optical waveguides on the focal plane of a lens that condenses the scattered light from the rug, and to mix the outputs of these optical waveguides. Based on the concept of heterodyne detection, the objective is to prevent measurement accuracy from decreasing due to variations in the distance between the speedometer and the object to be measured, and to eliminate the adverse effects of the surface condition of the object to be measured.

The present invention will be explained in detail below with reference to the drawings. The accompanying drawings schematically show the structure of a laser Doppler velocimeter according to an embodiment of the present invention. The speedometer shown in the figure includes a laser device 1 as a light source that generates coherent light, a collimator lens system 2 for converting the light from the laser device 1 into a wide parallel beam, a half mirror 3, a lens 4, an optical fiber, etc. optical waveguides 5 and 6, an optical coupler 7 that mixes the light from each optical waveguide 5 and 6 and sends it to another optical waveguide 8, and a light receiver 9 that photoelectrically converts the light from the optical waveguide 8. , for example, a signal analysis device 10 such as a spectrum analyzer, and an output device 11.

The input ends 5a and 6a of each optical waveguide 5 and 6 are lenses,
The optical axis 12 is located on the focal plane of the lens 4, that is, the Fourier transform plane 15.
installed in a symmetrical position. Further, the measurement object 13 is, for example, paper or an iron plate, and the lens 4
The optical axis 12 of the optical axis 12 is moved at a constant angle, for example, in a perpendicular direction.

In the above configuration, coherent light from the laser device 1 is converted into a wide parallel light beam by the collimator lens system 2, reflected by the half mirror 3, and irradiated onto the measurement object 13. Reflected and scattered light 14 from the measurement object 13 passes through the half mirror 3 and is focused onto the focal plane 15 by the lens 4. In this case, all of the scattered light from the measurement object 13 that is reflected at the same angle with respect to the optical axis 12, that is, parallel light rays, is focused on one point on the focal plane 15. Therefore, the scattered light reflected from the measurement object 13 at different fixed angles is focused on the input ends 5a and 6a of the optical waveguides 5 and 6, respectively. Also,
The light incident on each input end 5a and 6a is the measurement target 1.
The frequency changes due to the Doppler shift caused by the movement of 3. The frequency change due to this Doppler shift is proportional to the IJs value θ, where θ is the angle that the scattered light from the measurement object 13 makes with the optical axis 12, and τ is the moving speed of the measurement object 13. Therefore, by fixing the positions of the input ends 5a and 6a, the angle of the scattered light input from the measurement object 13 to the input ends 5a and 6a with respect to the optical axis 12 becomes constant. The amount of Doppler shift of the light input to each input terminal 5a and 6a is proportional to the speed υ of the measurement object 13. Each input end 5a and 6a
These lights incident on the optical waveguides 5 and 6 are input to the optical coupler 7 and mixed, and then input to the optical receiver 9 via the other optical waveguide 8. The light receiver 9 performs square law detection of the light inputted by, for example, a photodiode or the like and converts it into an electrical signal, and also performs heterodyne detection of coherent light having slightly different wavelengths, which constitutes the input light.

The signal from the light receiver 9 is input to the signal analyzer 10 and converted into a signal corresponding to the frequency difference of the light input to each input end 5a and 6a on the focal plane 15. By multiplying by the constant , a signal corresponding to the moving speed υ of the measurement object 13 is obtained.

Next, the operation of the above-described laser Doppler velocimeter will be explained using mathematical formulas. As shown in the accompanying drawings, the plane coordinates of the focal plane 15 are represented by coordinate axes X and y, and the plane of the measurement object 13 is represented by coordinate axes ξ and η. Further, it is assumed that the focal length of the lens 4 is 2, and that the measurement object 13 is at the focal position in front of the lens 4 to simplify the formula. At this time, if the amplitude of the scattered light on the surface of the measurement object 13 is A (ξ, η; t), then the amplitude of the focal plane U (x, y
:t) is represented by. Here, ωX and ωy are respectively in the X direction.

It represents the spatial frequency in the X direction. Further, λ represents the wavelength of light, and 2 represents the focal length of the lens. The l-th optical waveguide is placed on the X-axis at a point separated by X6 from the optical axis. Since the diameter of the fiber is considered to be sufficiently small compared to the range of scattered light, if this aperture function is set as δ(x x6 +y) by Dirac's δ function, the amplitude of the light guided into the waveguide ■ ) is V, (XO, t
) = // U(x,y:t)δ(x-xo,y)d
xdy=: U(XOIO:t)'・
...(3). Similarly, the amplitude of light guided into the first waveguide and the second waveguide placed symmetrically with respect to the optical axis v2 (xQ + L) is: 2(XOIt) −N U(x, y: t) δ(x+x
(1+y)dxdy=U(-x@, 0: t
) ...(4). vI+
When V2 is combined with an optical coupler, the combined amplitude 5 (xO, t) is 5 (x (1, t) = U (xo, O: t) + U (x
o, 0at) ・+...+ (5) Substituting equation (1) into equation (5) to find S, we get ωx
(Set as 1=2πx(1/λ2) to obtain.Since the light intensity that can be detected by the photodetector is I=lS12, if we calculate ■, we can write: Here, as A(ξ, η; t) Consider the following shape.

A(ξ, η:t) = A(ξ−υt, η)
...(8) This shows that the mother turn A(ξ, η) is moving at a speed ゛τ in the ξ direction without changing its shape. Therefore, by substituting equation (8) into equation (7), we obtain the following. Here * represents convolution integral. Further, A is the integral of A in the η direction, and can be generally considered to be a finite function. Let's perform a Fourier transform on (2) in terms of time and examine its frequency characteristics. Using the convolution theorem, 1, i'CI) = individuals X-ωXoυt) *A1 (v
month 2]. Here, A1 represents the complex conjugate of A1.

Also, 1 [...] represents Fourier transform. Also, Ri [c
lls(ωx(Ivt):)=2 (δ(fo−πυ
)+δage0+-iV month ・(1υde'), 7 CA
+(?7t)l)=4(fo), then equation (1t) becomes 1 ω
Ding) + δ (f + Pa ν)] * [δ (f - DiV) + δ (f Tomihi V) Katana 2π 2π l ωXQ 20,000 p14 (779 months・(2δ(f) + δCfo-,
+7)+δ(/+,,j). (In formula 12, the term before the brackets (...) is a constant, and the term before the brackets (...) is a constant.
The first term in ) means 0 frequency, that is, the DC component. It can be seen from the second and third terms that the optical signal (2) has a sharp peak at the frequency ωX6τ/π. And this frequency f
If it is detected, the speed τ can be calculated as .

The above analysis was performed when the object was on the focal plane of the lens, but it is thought that the intensity does not change even at other positions, only a phase term is added at the amplitude stage. Therefore, speed can be detected with this optical system.

In addition, in the above description, a spread tram analyzer is used as the signal analysis device 10, but the signal analysis device 10 is not limited to this, and it is also possible to detect frequency peaks using, for example, a dragging filter. It is also possible to use a device that measures the period, etc. of an input signal.

As described above, according to the present invention, since the speed is measured by irradiating the object to be measured with a wide parallel beam of light and converging and detecting the scattered light from a wide area, the distance between the speedometer and the object to be measured is The speed can be accurately measured without decreasing the measurement accuracy even if the speed changes, and the measurement signal will not be interrupted even if the surface of the object to be measured is uneven or the reflectance changes. .

[Brief explanation of the drawing]

The accompanying drawings are schematic diagrams showing the configuration of a laser Doppler velocimeter according to one embodiment of the present invention. l... Laser device, 2... Collimator lens system, 3...
・・Half mirror 14・・Lens, 5, 6.8・・・
Optical waveguide, 7... Optical coupler, 9... Light receiver, 10... Signal analyzer, 11... Output device, 12... Optical axis, 13...
...Measurement object, 14. Scattered light, 15. Focal plane. Patent Applicant Tateishi Electric Co., Ltd. Agent Patent Attorney Tatsuo Ito Patent Attorney Tetsuya Ito Procedural Amendment October 29, 1980 Commissioner of the Japan Patent Office Kazuo Wakasugi Indication of Case 1 Patent Application No. 109781 of 1982 2. Name of the invention Relationship to the Laser Doppler speedometer 3 correction case Patent applicant residence 10 Hanazono Tsuchido-cho, Ukyo-ku, Kyoto-shi, Kyoto Name (
294) Tateishi Electric Co., Ltd. Representative Takao Tateishi 4 Agent 105 Address 2-8-1 Toranomon, Minato-ku, Tokyo 5 Date of amendment order Voluntary amendment 6 In the specification subject to amendment, "Detailed description of the invention" Column 7. Correct each amendment. (2) In the same book, p. 9, 11fo" in the formula and line 12 of the same page, "f" and "
9! Corrected to "L(f)". (3) Correct the formula 0a on the same page in the same book as shown below. "

Claims (1)

[Claims] :+HiL/int light source, an optical system that spreads the light emitted from this coherent light source and makes it incident parallel to the object to be measured, a lens that condenses the scattered light from the object to be measured, and the manual end of the Two optical waveguides are attached to the focal plane of the lens symmetrically with respect to the optical axis, a light receiving device performs heterodyne detection by mixing the outputs of these two optical waveguides, and frequency analysis is performed on the optical signal from this light receiving device. A laser Doppler speedometer characterized by being equipped with a signal analysis device for measuring the frequency ridge or period.