JPH0664075B2 - Object moving speed measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention uses a random interference pattern (speckle pattern) obtained by irradiating a moving object with coherent light such as laser light to obtain an object. The method of measuring the moving speed of the.

(B) Conventional Technology Generally, as shown in FIG. 3, when a moving object 3 is irradiated with coherent light 3 from a laser plateau 1 through a projecting optical system 2, the light transmitted or reflected by the moving object 4 is transmitted. When it is received at the light receiving point 5, the speckle pattern is accompanied by two movements of translation (translation) and boiling (the speckle does not move and the intensity of light vibrates on the spot) It is known that patterns move and deform (Magazine "Laser Research" Vol. 8, No. 2, Vol. 8, No. 3). In the figure, the normalized autocorrelation function Γ (τ, v) from which the direct current of the light fluctuation at the light receiving point 5 is removed is However, V: moving speed of coarse object, w: irradiation beam radius, Δx: speckle mean diameter, σ: speckle translation magnification, ρ: wavefront curvature of irradiation beam, R: distance from moving object to light receiving point, and Become.

The above equation (1) shows that the autocorrelation function attenuates as the object velocity V increases, and for example, the correlation is 1 / e
Let τc (autocorrelation length) be the time However, a: The radius of the receiving opening. The velocity V can be measured by this τc, and the inventors of the present application have already applied for a speckle velocity meter adopting this principle (Japanese Patent Laid-Open No. 59-54043).

Also, the number of zero crossings per unit time of light fluctuation No.
Calculated from equation (1), Becomes The velocity V can also be measured by counting the zero crossing number No. The inventors of the present application have already applied for a speckle velocity meter adopting this principle (Japanese Utility Model Publication No. 58-106376). , Japanese Patent Application Showa 59-
54041).

The above-mentioned already applied speckle velocimeters have all adopted the illumination method using the convergent beam or the expanded beam.

(C) Problems to be solved by the invention The conventional speckle velocimeter uses the convergent beam or the expanded beam for illumination and measures the velocity V based on the formula (3) or (4). Therefore, if the roughness correlation length of the rough object is relatively long compared to the illumination area, the speckle becomes non-Gaussian, and the speckle becomes small on paper and the like due to the bleeding phenomenon. There is a drawback that speckles with no size are created, and therefore the autocorrelation function of the light fluctuation changes, and the measurement cannot be performed correctly.
Further, when the measurement object changes, for example, from copper to paper, and then cloth, Δx changes depending on the measurement object, and therefore the signal processing unit must correct the object change each time. Furthermore, if the illumination beam is not a perfect spherical wave and contains wavefront aberrations, the translation of the speckle is strongly related to the wavefront curvature, so the translation becomes unstable, and the measurement accuracy also deteriorates. there were.

In view of the above, the present invention can measure the speed of an object with high accuracy even when the speckle is non-Gaussian, when the measurement object is changed, and when the illumination beam includes wavefront aberration. The purpose is to provide a measurement method.

(D) Means and Actions for Solving Problems In the present invention, speckles are boiled on the light receiving surface in order to solve the above problems. The optical arrangement in which the speckle is boiling places the measurement object in the waist area of coherent light having a convergence area, a waist area and a diffusion area, and places the light receiving surface in the diffusion area from the object, and the measurement object in the aberration area, The light receiving surface may be arranged in the waist region, but the present invention adopts the latter arrangement. That is, according to the object velocity measuring method of the present invention, the light receiving surface is arranged in the waist region of the coherent light emitted from the plateau, and the moving object is R = πw ₀ ² / λ.
(Λ: wavelength of laser light, w ₀ : waist radius) is arranged in the converging region on the light source side for a distance equal to or more than that, the moving object is irradiated with the coherent light, and the speckle pattern obtained by the light receiving element is subjected to signal processing It is processed by the department to measure the moving speed.

In this movement measurement method, R = −ρ in the convergence region, so R / ρ = −1, and from the above equation (2), the translation magnification σ
= 0. Therefore, the right term of the equations (1) and (4) becomes 0. Therefore, the autocorrelation length τc and the zero crossing number No are Therefore, in any case, even if Δx, ρ, etc. vary, the measurement result is not affected.

(E) Examples Hereinafter, the present invention will be described in more detail with reference to Examples.

FIG. 1 is a schematic configuration diagram of a laser speckle velocity meter in which the present invention is implemented. In the figure, 11 is a laser light source that emits a beam such as coherent light, and 12 is a projection optical system that converts the light beam from the laser light source 11 into a light beam having a converging region waist region and a diffusion region. Reference numeral 13 denotes a transparent moving object, which is arranged in the converging region and moves at a velocity V in a direction perpendicular to the light beam. Reference numeral 14 is a photoelectric conversion element, and for example, a photoelectric tube or a semiconductor light receiving element is used. The photoelectric conversion element 14 is arranged in the waist region of the light beam. An opening plate 16 having a pinhole opening 15 is provided on the entire surface of the photoelectric element 14. Further, a signal processing unit 17 for processing the converted speckle pattern signal is connected to the optical conversion element 14.

As shown in FIG. 2, the signal processing unit 17 includes a wide band amplifier 18 for amplifying the speckle pattern signal from the photoelectric conversion element 14, and a DC component removing circuit for cutting a DC component from the amplified speckle pattern signal. 19, a Schmitt trigger circuit 20, a one-shot multi circuit 21, a counter circuit 22, and a clock circuit 23.

In the laser speckle velocimeter of this embodiment, when a laser beam is emitted from the laser light source 11, it is converted into a beam beam (preferably a Gaussian beam) having an emission diameter D and a focal length f in the projection optical system 12. In this case, the beam waist radius w ₀ has a wavelength of λ, Becomes

The photoelectric conversion element 14 is provided on the beam waist, and the moving object 13 (from the photoelectric conversion element 14 (to the light source side) Since it is located in the position where
Since R = −ρ and σ = 0, assuming that the measured object 13 is moving at the speed V, the speckle becomes a boiling state on the surface of the opening slope 16 and fluctuates with time.

This fluctuation signal is converted into an electric signal by the photoelectric conversion element 14 and input to the signal processing unit 17. The speed of change at this time is proportional to the moving speed V of the object 13, and the speed of this change is detected by the signal processing unit 17.

In the signal processing unit 17, first, the speckle signal (fluctuation signal) converted into an electric signal is amplified by the wide band amplifier 18 to a predetermined level, and the amplified signal is further removed by the DC component removing circuit 19 for detecting the number of zero crossings. The DC component is cut by and the Schmitt trigger circuit 20 creates a pulse train in which high and low are inverted at the time of zero crossing. This pulse train is shaped into a predetermined pulse width by the one-shot multi-circuit 21, and the counter circuit 22 makes the zero crossing count number per unit time No /
2 is counted. This zero-crossing number No is as a general theoretical formula (4), but since the light is received in the boiling state in this embodiment, the right term of the formula (4) can be ignored, Becomes Therefore, the count value of the counting circuit 22 is not affected by the speckle size Δx, the opening radius a, the distance R between the object and the photoelectric conversion element, and the wavefront curvature ρ of the beam, and even if the material of the object 13 is changed, Even if Δx is converted or the wavefront curvature ρ is distorted, the influence does not affect the velocity measurement, and no correction is necessary.

In the above embodiment, the zero crossing detection circuit has been taken as an example of the signal processing unit 17, but the present invention is not limited to this, and a circuit for calculating the autocorrelation length τc can be used.

(F) Effect of the Invention According to the present invention, the speed measurement is performed by processing the signal received in the speckle boiling state.
Since the speckle size and the wavefront curvature do not affect the measurement result, highly accurate measurement can be performed. Further, even if the object to be measured is changed on the way, the measurement can be continued without any correction.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a laser speckle velocity meter in which the present invention is implemented, FIG. 2 is a circuit diagram showing a specific example of a signal processing unit of the laser speckle velocity meter, and FIG. It is a figure which shows the optical arrangement for demonstrating the speed measurement by a laser speckle. 11: Laser light source, 12: Projection optical system, 13: Moving object, 14: Photoelectric conversion element, 17: Signal processing unit.

Claims (1)

[Claims] 1. A coherent light from a light source is converted by an optical system into a light beam having a converging region, a waist region and a diffusing region, a photoelectric conversion element is provided in the waist region, and a moving object is arranged in the converging region. A method for measuring the moving speed of an object, wherein the moving speed is measured by irradiating the moving object with the light beam and processing a speckle pattern signal obtained by the photoelectric conversion element by a signal processing unit.