JPS5922907B2 - Laser speed measuring device - Google Patents

Description

[Detailed Description of the Invention] This invention is a non-ferrous material that is required in manufacturing lines for various steel plates and pipes, stainless steel plates and pipes, aluminum plates and pipes, copper plates and copper pipes, etc. in the steel and non-ferrous industries. This invention relates to a contact, high-precision laser speed measuring device.

Conventionally, the speed of the object to be measured in each of the above production lines has been measured using a contact type measuring roll.

In principle, this method can provide a measurement accuracy of 0.1% or higher and is highly accurate, but because it is a contact type, there are errors due to slipping at high speeds, errors due to roller wear, and it cannot be used in high-temperature environments. There are other problems. As a means to solve these problems, we have developed a speckle pattern that is generated on the surface of the object to be measured due to the diffraction phenomenon of laser light.
A non-contact laser speed measurement device that measures the frequency proportional to the speed of the object to be measured by processing it with a spatial filter (grid slits with a constant pitch, etc.). is being developed. With this method, errors due to slipping and roller wear could be resolved; however, due to the random nature of the speckle pattern, the signal-to-noise ratio (S/N) and Q value of the signal may not necessarily be good. First, the measurement accuracy (resolution) decreases, especially in the low speed region of the object to be measured, and it is considered extremely difficult to achieve accuracy equivalent to that of a measuring roll. On the other hand, a method has been developed that uses the Doppler effect, which utilizes the coherency of laser light, to measure the movement of particles in gases and liquids with high precision in a non-contact manner, and is used by various research institutions. This method is (a) non-contact;
(b) Good linearity in absolute measurement, (c) Fast response, (d) Measurement target is local and has high spatial resolution, (e) Speed measurement range is large, especially advantageous in low speed range. However, when trying to apply this method to the speed measurement of steel plates, etc., as described above, line fluctuations (
The positional fluctuation of the steel plate, etc.) is large, ranging from +5 mm to ±50 mm, so the high spatial resolution, which was one of the major features of this method, has the disadvantage of causing large measurement errors or making measurements impossible. However, its use was limited to measuring the movement of particles in gases and liquids. Therefore, the present invention provides a highly accurate laser velocity measuring device using the Doppler effect, which practically eliminates the influence of line fluctuations mentioned in 1-2 above.

This invention will be explained below with reference to the drawings. Figure 1 shows the fringe mode (Fringe mode) which is the most excellent method among the conventional laser velocity measurement devices using the Doppler effect.
geMOdO). In the figure, 1 is a laser oscillator, 2 is a semi-transparent mirror, 3 is a total reflection mirror,
4 is a focusing lens, 5 is an object to be measured, 6 is a light receiving lens,
7 is a photodetector, 8 is a signal processing section, R is a reference light, and S is a sample light. Now, the laser beam from the laser oscillator 1 is converted into a reference beam (hereinafter referred to as R light) and a sample beam (hereinafter referred to as S light) by a semi-transparent mirror 2.
When the beams are divided into two beams (referred to as "particles, steel plates, etc.") and the two beams are crossed by the focusing lens 4, a fringe is generated at the intersection, and the object to be measured 5 (particles, steel plate, etc.)
When passing through this fringe, if the object to be measured 5 is in the bright part of the fringe, the intensity of the scattered light is strong, and if it is in the dark part of the fringe, the intensity of the scattered light is weak. The light receiving lens 6 and the photodetector 7 collect the above scattered light and photoelectrically convert it. The signal processing unit 8 measures the Doppler frequency according to a relational expression described later, and calculates the velocity of the object to be measured 5. Now, for the sake of simplicity, suppose that the object to be measured 5 is moving at a speed v in the direction perpendicular to the above fringe, then the change Fd in the intensity of the scattered light per unit time is 7, where 7 is the spacing between the fringes. , is given.

On the other hand, if the intersection angle of the two laser beams is θ and the wavelength of the laser beam is λ, then from the interference of the two beams at equal inclination angles, 1
is given by the following equation. From equations (1) and (2), the output signal Fd of the photodetector 7 is expressed as follows. Here,
λ=0.6328μm (He-+Je gas laser), θ
Substituting the values of -106 and V-6 m/min into equation (3) yields the frequency, which allows frequency processing with sufficient accuracy and high-precision measurement even at low speeds of several m/min. It turns out that it is possible.

FIG. 2 is a diagram for enlarging the intersection of the beams in FIG. 1 and calculating the geometric dimensions of the intersection. In this diagram, holds true. Here, Dm: Minimum diameter of the beam (
Beam diameter at the lens focal point) f: Focal length of the lens Δθ: Beam convergence angle Db: Diameter of the laser beam.

Db=0.5mm, λ=0.6328μM, f-300
m' θ-100 (4), (6), (7), (8)
Calculating the formula yields ~0.12mm Dm-°Wm=0.085mm ~ Hm=0.085mm tm=0.980mm.

In other words, the width is 0.085m and the height is 0.085w! m length 0
．． It can be seen that fringes occur only in a very small intersection of 980 mm, and that only the scattered light from this part is an effective signal. Therefore, it is useful for fields that require increased spatial resolution, such as laminar flow measurement for gases and liquids, but it is useful for measuring speeds of steel plates etc. that have line fluctuations as described above. is not applicable. FIG. 3 shows a schematic diagram of an embodiment of the present invention, in which 9 is a beam expander.

The zero beam expander 9, in which parts not related to the present invention are omitted in the figure, is, for example, an inverted telescope with a magnification of n, and converts the laser beam diameter Db from the laser oscillator 1 into parallel light having a diameter NDb. The R light divided by the semi-transparent mirror 2 is directly irradiated onto the object to be measured 5, and the S light is irradiated by the total reflection mirror 3 at an angle of θ with the optical axis of the R light and on the surface of the object to be measured. It is irradiated so as to intersect with the R light. In addition, the enlarged laser beam diameter ND. Intersection angle θ of R light and S light, optical components for sending R light and S light to the outside (semi-transparent mirror 2, total reflection mirror 3)
distance a between the centers of A relationship is established between the two to optimize the S//N} and Q of the received signal.
In other words, equations (9), (10), and (self) are obtained.

An example of numerically calculating equations (12) and 03) is shown in FIG. Since the R light and the S light are parallel lights even at the intersection, for example, in Fig. 4, θ-19.50, nDb=7mm, a
=350mm, t=1.000mm then δ=0.0
A laser velocimeter has been realized that is completely unaffected by fluctuations of the object to be measured 5 within +20 degrees while maintaining the signal S/N ratio and Q above the required values. become. FIG. 5 shows a schematic configuration diagram of another embodiment of the present invention, in which 10 is a total reflection mirror. In the figure, the R light and the S light are respectively positioned at the object to be measured 5 with respect to the optical axis of the light receiving lens 6.
In general, the intensity distribution in the cross section of a laser beam takes a Gaussian distribution, so the fringes that occur when two beams intersect also have an intensity distribution, and from all the intersection parts When the scattered light of
/N is known to decrease. Therefore, in FIG. 5, the effective aperture of the light-receiving lens 6 is determined so that the S/N is greater than the required value, and the light-receiving solid angle α is determined.
As is clear from the above description, according to the present invention, it is possible to non-contact and highly accurately measure the speed of steel plates, etc., which have large line fluctuations, which has been considered difficult in the past, with an extremely simple configuration. At the same time, it can also be used as a length gauge for these lines, which is in high demand, and its effects are thought to be extremely large.

It goes without saying that the present invention can also be applied to a length meter that measures length by integrating velocity over time.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of a laser velocimeter using the conventional Doppler effect (fringe mode), Fig. 2 is an enlarged view of the beam intersection part in Fig. 1, and Fig. 3 is a diagram showing one example of the present invention. FIG. 4 is a schematic configuration diagram showing the embodiment; FIG. 4 is a calculation example showing conditions for selecting each parameter for optimizing the signal-to-noise ratio $/N and Q value of the signal; FIG. The figure is a schematic configuration diagram showing another embodiment of the present invention. In the figure, 1 is a laser oscillator, 2 is a semi-transparent mirror, 3 is a total reflection mirror, 4 is a condensing lens, 5 is an object to be measured, 6 is a light receiving lens, 7 is a photodetector, and 8 is a signal processing unit. , 9 is a beam expander, and 10 is a total reflection mirror.

Claims (1)

[Claims] 1 a laser device that emits coherent light, a means for expanding the laser beam from the laser device in parallel and at a required magnification, and a means for dividing the expanded parallel laser beam into a reference beam and a sample beam; means for irradiating the divided two parallel beams, the reference beam and the sample beam, onto the surface of the object to be measured so as to intersect with each other; means for setting the optical axes of the reference beam and the sample beam at a required angle; The light receiving means for receiving scattered light when the reference light and the sample light are scattered by the object to be measured with a required aperture diameter, and means for photoelectrically converting the signal received by the light receiving means, The angle between the enlarged laser beam diameter, the optical axis of the reference beam and the optical axis of the sample beam, the distance between the centers of the dividing means and the irradiation means, and the length of the part where the reference beam and sample beam intersect. 1. A laser speed measuring device characterized in that a signal-to-noise ratio S/N and a Q value of a signal received by the laser are set to have a unique relationship that is optimal.