JPH0516548B2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field This invention irradiates a moving object with coherent light such as a laser beam, and utilizes the resulting random interference pattern (speckle pattern). This invention relates to a method for measuring the moving speed of an object.

(b) Prior Art Generally, as shown in FIG. 4, when a moving object 4 is irradiated with coherent light 3 from a laser light source 1 via a projection optical system 2, the light transmitted or reflected by the moving object 4 is When received at the light receiving point 5, the speckle pattern undergoes two movements: translation movement and boiling movement (the speckle does not move, and the intensity of light oscillates at that position). is known to move and displace (Magazine "Laser Research" Vol. 8, No. 2)
No. 8, No. 3). In the same figure, light receiving point 5
The normalized autocorrelation function ^{Γ (} τ, v) that ^{removes the direct} current of light fluctuation ^at τ ² )...(1) σ=1+R/ρ...(2) where, V: moving speed of coarse object, w: irradiation beam radius, Δx: speckle average value, σ: speckle translation magnification, ρ: wavefront of irradiation beam Curvature, R: Distance from the moving object to the light receiving point.

Equation (1) above shows that the autocorrelation function attenuates as the object speed V increases. For example, if the time when the correlation becomes 1/e is τc (autocorrelation length), then (1 ) From the equation, τc=1/V (1/w ² +σ ² /4a ² +Δx ² ) ^-1/2 ...(3) where a: radius of the socket opening. The velocity V can be measured using this τc, and the inventors of this application have already applied for a speckle velocimeter that adopts this principle (Japanese Patent Application No. 59-54043).
issue).

Also, the number of zero crossings per unit time of light fluctuation No.
is also calculated from the above equation (1), and becomes No=√2/πV(1/w ² +σ ² /4a ² +Δx ² ) ^1/2 (4). The velocity V can also be measured by counting this zero-crossing number No. The inventors of this application have also developed a speckle velocimeter that adopts this principle.
After all, it has already been applied (Utility Application No. 58-106376)
No., Special Application No. 59-54041).

The above-mentioned speckle velocimeter, which has already been applied for, is
All of them employed illumination methods using converging beams or expanding beams.

(c) Problems to be solved by the invention The conventional speckle velocimeter described above uses a convergent beam or an expanded beam for illumination,
Since the speed V is measured based on the formula,
If the roughness correlation length of the rough object is relatively long compared to the illumination area, the speckle will be non-Gaussian, and for paper etc., the speckle will be small due to the smearing phenomenon, and the speckle will not have a normal speckle size. As a result, the autocorrelation function of light fluctuations changed, resulting in inaccurate measurements. Furthermore, when the object to be measured changes, for example from copper to paper to cloth, Δx changes depending on the object to be measured, and therefore the signal processing section has to make corrections for each change in object. Furthermore, if the illumination beam is not a perfect spherical wave and contains wavefront aberration, the translation of the speckle is strongly related to the wavefront curvature, which causes the translation to become unstable, which also has the disadvantage of reducing measurement accuracy. Ta.

Therefore, in order to solve this drawback, the inventor of the present application placed a moving object in the waist region of a beam having a convergence region, a waist region, and a diffusion region, and received the speckle pattern in a boiling state in the diffusion region. He has created a method for measuring speed and has already filed a separate application. However, since this method involves placing an object in the waist region, power is concentrated and there is a risk of damaging the object to be measured.

In view of the above, the present invention makes it possible to measure the velocity of an object with high precision even when the speckle becomes non-Gaussian, when the measurement object is changed, or when the illumination beam includes wavefront aberration, and furthermore, even when the object to be measured is The purpose of this research is to provide a method for measuring the speed of an object that causes less damage to the object.

(d) Means and operation for solving the problem In order to solve the above problem, the present invention provides a system in which a moving object is placed in the waist region and light is received in a boiling state, and the cross section of the light beam is elliptical. , so that the object to be measured is irradiated. That is, the method of measuring the speed of an object according to the present invention uses an optical system to convert light from a coherent light source into a light beam having a convergence region, a waist region, and a diffusion region and an elliptical cross section. A moving object is placed in the area, a photoelectric conversion element is provided in the diffusion area, and a speckle pattern signal obtained from the photoelectric conversion element is processed by a signal processing unit.
I am trying to measure the moving speed of an object.

In this moving measurement method, the irradiation beam radius w is
w ₀ is made very small, and the speckle size Δx is large because R is large (Δx = λR / πw), and the (3)
In the formula, 1/w ² ≫σ ² /4a ² +Δx ² , and the right term can be almost ignored. Also, since the same holds true for equation (4), the cross-correlation length τc and zero crossover number No are τc = w/V, No = √2/πV, and both can be measured even if Δx, ρ, etc. vary. Does not affect results.

Furthermore, since the object is irradiated with an elliptical light beam, the optical power is dispersed and hits the object to be measured.

(E) Examples The present invention will be explained in more detail below with reference to Examples.

FIG. 1 is a schematic diagram of a laser speckle velocimeter in which the present invention is implemented. In the same figure, 11
is a laser light source that emits a coherent beam of light,
12 is a light projecting optical system for illuminating the moving object 14 at the waist position of the beam light. Further, the cylindrical lens 13 is provided to make the cross-sectional shape of the light beam elliptical, and a prism may be used instead. In other words, the moving object 14
is placed at the waist position of the beam light.
Moreover, as shown in FIG. 3, a light beam having an elliptical cross section is irradiated. Further, the moving object 14 is a transparent moving object. 15 is a photoelectric conversion element, for example, a phototube or a semiconductor light receiving element is used. An aperture plate 17 having pinhole apertures 16 is provided on the front surface of the photoelectric conversion element 15 .
The photoelectric conversion element 15 also includes a signal processing section 18 for processing the converted speckle pattern signal.
is connected.

This signal processing section 18, as shown in FIG.
A broadband amplifier 19 that amplifies the speckle pattern signal from the photoelectric conversion element 15, a DC component removal circuit 20 that removes the DC component from the amplified speckle pattern signal, and a Schmitt trigger circuit 2.
1, a one-shot multi-circuit 22, a counter circuit 23, and a clock circuit 24.

In this example laser speckle velocimeter,
When a laser beam is emitted from the laser light source 11, the projection optical system 12 converts the laser beam into a light beam having a convergence region, a waist region, and a diffusion region, and the cross section of the light beam is made elliptical by the cylindrical lens 13. Ru. Assuming that this measured object 14 is moving perpendicular to the optical axis at a speed V, the object 1
Since the light-receiving face plate 16, which is a distance R away from the light-receiving surface plate 16, is provided in the diffusion region (R≧πw ₀ ² /λ), the speckle is in a boiling state and fluctuates over time. This fluctuation signal is converted into an electrical signal by the photoelectric conversion element 15 and input to the signal processing section 18 . The speed of the fluctuation at this time is proportional to the moving speed V of the object 14, and the speed of this fluctuation is detected by the signal processing unit 18.

In the signal processing unit 18, first, the speckle signal (fluctuation signal) converted into an electrical signal is sent to the broadband amplifier 1.
9, the amplified signal is further amplified to a predetermined level by a DC component removal circuit 20 for zero-crossing detection.
The DC component is cut off, and the Schmitt trigger circuit 21 creates a pulse train whose high and low levels are inverted at the time of zero crossing. This pulse train is shaped into a predetermined pulse width by a one-shot multi-circuit 22, and a counter circuit 23 calculates the number of zero-crossing counts per unit time.
No/2 is counted. This zero crossing number No is as shown in equation (4) as a general theoretical equation, but in this example, since the light is received in a boiling state, the right term of equation (4) can be ignored, and No = √ It becomes 2V/πw. Therefore, the count value of the counting circuit 23 is not affected by the speckle size Δx, the aperture area a, the distance R between the object and the photoelectric conversion element, and the wavefront curvature ρ of the beam, even if the material of the object 14 is changed. , Δx change, or even if there is distortion in the wavefront curvature ρ, these effects do not affect the speed measurement, and there is no need for any correction.

In the above embodiment, the signal processing section 18 includes:
Although the zero-crossing number detection circuit is taken as an example, the present invention is not limited to this, and it is also possible to use a circuit that calculates the autocorrelation length τc.

(F) Effects of the Invention According to the present invention, since the speed measurement is performed by processing the signal received in the boiling state of the speckle, the speckle size and wavefront curvature do not affect the measurement results. , it is possible to perform highly accurate measurements. Further, even if the object to be measured is changed during the process, the measurement can be continued without any correction.

In addition, since the moving object is irradiated with an elliptical light beam, the optical power is not concentrated but dispersed.
No damage to the object being measured.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a laser speckle velocimeter in which the present invention is implemented, and FIG. 2 is a circuit diagram showing a specific example of a signal processing section of the laser speckle velocimeter.
FIG. 3 is a cross-sectional view of a light beam irradiated onto a moving object in the same laser speckle velocimeter, and FIG. 4 is a diagram showing an optical arrangement for explaining velocity measurement using a conventional laser speckle. 11: Laser light source, 12: Projection optical system, 13:
Cylindrical lens, 14: Moving object, 15:
Photoelectric conversion element, 18: signal processing section.

Claims (1)

[Claims] 1. Using an optical system, light from a coherent light source is converted into a light beam that has a convergence region, a waist region, and a diffusion region and has an elliptical cross section. A moving object is placed in the waist region of this light beam, and photoelectric conversion is performed. A method for measuring the speed of an object, in which an element is provided in a diffusion region, and a speckle pattern signal obtained from the photoelectric conversion element is processed by a signal processing section to measure the moving speed of the object.