KR100643351B1 - A System and Method for On-line Measurement of Grain Size using Laser-Ultrasonics - Google Patents

Abstract

According to the present invention, ultrasonic waves are generated non-destructively in a steel sheet by using a pulsed laser and the ultrasonic waves propagated inside the steel sheet are measured using a laser interferometer, and then attenuation coefficients of the ultrasonic waves are calculated. The present invention relates to an on-line crystal grain size measuring system and method using a laser-ultrasound measuring particle size.

The present invention collects the laser scattered light generated by the frequency shift by the ultrasonic wave reflected from the lower surface of the object and returned to the upper surface by irradiating a laser beam for ultrasonic generation and measurement to the same point of the measurement object and the collection The generated scattered light is reciprocated inside the laser-interferometer to generate interference to detect an ultrasonic signal using the intensity of the transmitted interference light and the reflected interference light caused by the ultrasonic wave, and then the ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal is calculated. The grain size of the measurement object is calculated.

According to the present invention, it is easy to measure the on-line grain size, and it is possible to precisely measure by preventing the loss of ultrasonic energy, and the efficient measurement is possible by reducing the influence of the existing plasma radiation light on the ultrasonic detector for ultrasonic measurement. .

Laser, ultrasound, laser-interferometer, grain size, on-line, scattered light, plasma,

Description

System and Method for On-line Measurement of Grain Size using Laser-Ultrasonics

1 is an overall schematic diagram of an on-line grain size measurement system using laser-ultrasound according to an embodiment of the present invention.

Figure 2 is a schematic diagram showing the mode conversion generated when the reflection of the ultrasonic wave (longwave) generated by the laser pulse according to an embodiment of the present invention.

3 is a schematic diagram showing circular spots of an ultrasonic wave generating laser beam and circular spots of an ultrasonic wave measuring laser beam according to an example of the present invention.

4 is an internal configuration diagram of scattered light collecting means according to an embodiment of the present invention.

5 is an exemplary view showing an incident angle of the scattered light and the optical fiber cross section by the condensing lens applied to the present invention.

6 is a schematic diagram illustrating a laser-interferometer according to an embodiment of the present invention.

Figure 7 is a schematic diagram showing a laser-interferometer combined with a plasma radiation blocking means according to an embodiment of the present invention.

FIG. 8 is a view showing a change in intensity with time of plasma emission light involved in generating ultrasonic waves according to an embodiment of the present invention.

9 is an exemplary view of a switch configuration of the plasma radiation blocking means according to an embodiment of the present invention.

10 is a view showing polarization directions of two linear polarizers of a switch according to an embodiment of the present invention.

11 is a schematic view showing the configuration of a laser-interferometer stabilization means according to an embodiment of the present invention.

12 is a schematic view showing the configuration of a laser-interferometer stabilization means according to another embodiment of the present invention.

Figure 13 is a schematic diagram showing the configuration of a laser-interferometer stabilization means according to another embodiment of the present invention.

14 is a diagram illustrating a relationship between reflected interference light and transmitted interference light of a laser-interferometer according to an embodiment of the present invention.

15 is a schematic diagram of the ultrasonic signal detection means according to an embodiment of the present invention.

16 is a diagram illustrating an increase of an ultrasonic signal by differential amplification according to an embodiment of the present invention.

17 is a flowchart illustrating a method for measuring on-line grain size using laser-ultrasound according to an embodiment of the present invention.

FIG. 18 is a diagram illustrating an ultrasonic signal measured by being continuously reflected from upper and lower surfaces of a measurement object according to an exemplary embodiment of the present invention.

19 is a view illustrating a longitudinal wave propagated by multiple reflections inside a measurement object according to an embodiment of the present invention.

20 is a view showing a frequency spectrum of the continuously measured reflected ultrasound signal according to an embodiment of the present invention.

FIG. 21 is a flowchart illustrating a correlation between a crystal grain size and an ultrasonic attenuation coefficient of a measurement object applied to an embodiment of the present invention. FIG.

22 is a graph derived from the correlation between the crystal grain size and the ultrasonic attenuation coefficient according to an embodiment of the present invention.

23 is a flowchart illustrating a scattered light collecting method according to an embodiment of the present invention.

24 is a flowchart illustrating a laser interferometer stabilization method according to an embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

10: pulsed laser 11,38: condenser lens

12: total reflection mirror 13: laser beam for ultrasonic generation

20: laser for ultrasonic measurement 30: scattered light collecting means

27: laser beam for ultrasonic measurement 28: scattered light

50: plasma radiation blocking means 60: laser interferometer

70: ultrasonic signal detection means 80: signal processing means

90: measurement object 71,72: photodetector

74: frequency filter 110: switch

111: switch regulator 112: narrow line width filter

113, 115: linear polarizer 114: single crystal

The present invention relates to an on-line grain size measurement system and method, and more particularly, to generate ultrasonic waves by irradiating a pulsed laser beam onto a steel sheet and to irradiate a laser beam for measurement at the same point as the ultrasonic wave generation point to laser by the ultrasonic waves. On-line grain size online measurement system and method using laser-ultrasound to collect scattered light to calculate the attenuation coefficient of the ultrasonic wave, and to measure the grain size of the steel sheet using the attenuation coefficient will be.

Non-destructive testing of mechanical properties and microstructure of metals and composite materials is usually performed using ultrasonic testing. Ultrasonic examination is used to find mechanical properties and microstructures by using the propagation characteristics of ultrasonic waves within a measurement object, and is widely used in various fields because it is basically a non-destructive inspection method. Ultrasonic examinations have usually used piezoelectric transducers or electro-magnetic acoustic transducers. The dual piezoelectric transducer requires an ultrasonic transfer medium between the measuring object and the transducer, and has a disadvantage in that its function is deteriorated under high temperature. In addition, the EMAT has a disadvantage in that it is usually used in close proximity to the measurement object up to several mm. Due to these shortcomings, the conventional ultrasonic inspection using the apparatus as described above is impossible to apply on-line under harsh environment such as production line, especially hot rolling process.

In order to solve this disadvantage, a laser-ultrasound method has been developed. The laser-ultrasound method generates ultrasonic waves using a pulsed laser and measures the ultrasonic waves propagated inside the measurement object using a laser interferometer. Since it is basically a non-contact method, ultrasonic flaw detection of high temperature measurement objects is possible. On-line applications in the line also have the advantage of ease.

In general, the production line of the industrial site is very poor compared to the laboratory environment, so in the rolling process of the production line to generate ultrasonic waves of higher intensity than in the case of the experimental test for the ultrasonic measurement in the steel plate of various thickness. When ultrasonic waves are generated by using a laser pulse beam, ultrasonic waves are generated by a thermoelastic effect or ablation of the surface to be measured by the laser pulse beam (Ref. Scruby, CB et. al., "Laser-Ultrasonics: Techniques and Applications", Adam Hilger, Bristol, UK, 1990.). Among these, fusion of the surface to be measured occurs when the intensity of the laser pulse beam is large. In this case, the surface material is ionized and vaporized to propagate in a direction perpendicular to the surface of the specimen, so that a reaction force acts on the surface of the specimen and ultrasonic waves are generated by the reaction force. The ultrasonic waves generated by the fusion are generally larger in intensity than those generated by the thermoelastic effect, and efficiently generate ultrasonic waves (long wave) propagating in a direction perpendicular to the measurement target surface to which the laser pulse is incident.

Conventional ultrasound generation and measurement were each made on the opposite side of the measurement object. That is, the laser beam was irradiated to the first surface (upper surface) to generate ultrasonic waves, and the generated ultrasonic waves were measured on the opposite surface (lower surface) of the first surface. In the case of generating and measuring the ultrasonic waves, there is a problem in that the detection efficiency of the ultrasonic waves is lowered. In addition, the ultrasonic generation point and the measurement point were different even when the generation and measurement of the ultrasonic waves were performed in the same plane. As such, when using ultrasonic waves propagating at a predetermined angle with respect to the surface of the measurement object, there is a problem that loss of ultrasonic intensity due to mode conversion occurs when ultrasonic waves are reflected from the opposite side of the surface to be measured. Furthermore, when the generation point and the measurement point of the ultrasonic wave are different, there is a problem in that the calculation of the propagation path and propagation distance of the ultrasonic wave required for deriving the ultrasonic attenuation coefficient is very complicated.

In the on-line grain size measurement in the conventional production line, it is difficult to quickly calculate the grain size by real-time signal processing. That is, the ultrasonic waves propagated inside the object to be measured are attenuated by scattering due to crystal grains, absorption by the material to be measured, diffraction, and the like. In the case of measuring the grain size using the attenuation of the ultrasonic waves, the effects of the effects other than scattering due to grains should be removed from the measured attenuation coefficients of the ultrasonic waves. It is not easy to derive the ultrasonic attenuation coefficient due to scattering by crystal grains from the ultrasonic signals thus measured, which is a time consuming task. Therefore, in the art, it is necessary to apply a signal processing method that can calculate the crystal grain size of the measurement target from the measured ultrasonic signal in real time.

Typically, laser interferometers are used for laser-ultrasound measurements. An interferometer is a device for observing the interference of a beam, and divides the light from one light source into two or more to change the wavefront in an appropriate manner to interfere. Dividing Light Rays into Multiple Ray Interferometers include Fabry-Perot interferometers and Rummer-Gerke parallel plates. They are used for the measurement of microstructures of wavelengths and spectra, and for spectrophotometry. Among them, the Fabry-Perot interferometer is most widely used in the production line, and the Fabry-Perot interferometer has a structure in which two sheets of glass plated with silver on one side are fixed in parallel to face each other. The two silver plated glass plates act as mirrors having a specific reflectivity to partially reflect the incident light to each other at a corresponding reflectance to generate interference.

In a conventional production line, there is a problem in that the performance of the ultrasonic measuring interferometer is degraded. Laser interferometers, which are typically used for non-contact measurement of ultrasonic waves reaching the surface of a measurement object, generally have characteristics that are sensitive to external vibrations. That is, there is a problem that the performance of the laser interferometer is deteriorated when there is vibration in the position where the measuring device is installed. In this respect, the confocal Fabry-Perot interferometer, among other laser interferometers, is in principle suitable for on-line measurement in production lines where external vibrations are present because they are less affected by external vibrations in principle. This is because the confocal Fabry-Perot interferometer is insensitive to vibrations that are relatively low frequency compared to ultrasound, such as external vibrations. In addition, the confocal Fabry-Perot interferometer has high light gathering efficiency, and thus, ultrasonic waves can be easily measured even on a measurement object having a rough surface such as a steel sheet in a production line. However, such a confocal Fabry-Perot interferometer also has a disadvantage of requiring additional stabilization when applied to the production line on-line. In other words, the cavity that constitutes the confocal Fabry-Perot interferometer is composed of curved partial reflection mirrors at both ends, and the distance between the two curved mirrors should be kept constant at all times. The distance between the two curved mirrors is influenced by the change in the ambient temperature or the alignment of the two curved winters. The minute change in the distance between the two curved mirrors also affects the ultrasonic measurement efficiency. Therefore, in order for confocal Fabry-Perot interferometers to be used efficiently in production lines in harsh environments, it is necessary to stabilize the ultrasonic wave at high efficiency by maintaining a constant distance between two curved mirrors.

In addition, conventionally, in order to detect a laser beam for ultrasonic measurement, scattered light of a laser beam generated by a frequency shift due to ultrasonic waves generated on an object is collected using a conventional convex lens and incident the collected scattered light into an optical fiber or the like. The transmission method is used. However, in this case, the ultrasound generation laser beam and the ultrasound measurement laser beam are irradiated at different positions, and the irradiation point and the scattered light condensing position of the ultrasound measurement laser beam are different from each other, so that there is a problem in that the scattering light is collected.

On the other hand, conventional techniques that apply the laser-ultrasonic method to the measurement of mechanical properties or microstructure of the measurement object (US Patent 6,684,701; System and method of determining porosity in composite materials using ultrasound), Measurement of physical properties of specimens using diffusion (US Pat. No. 6,532,821; Apparatus and method for evaluating the physical properties of a sample using ultrasonics), Measurement of physical properties of sheet using ultrasonic resonance (US Pat. No. 6,057,927; Laser-ultrasound spectroscopy apparatus and method with detection of shear resonances for measuring anisotropy, thickness, and other properties. Measurement of the porosity of the double composite material (US Pat. No. 6,684,701) relates to a method for measuring the porosity of a specimen using ultrasonic attenuation, which is different from the present invention for calculating a crystal grain size using ultrasonic attenuation coefficient. Although it is emphasized that the generation and measurement of the ultrasonic wave by the laser beam can be performed on the same plane (the same position) of the measurement object, there are no problems and solutions for the on-line measurement in the steel sheet production line. Did. Another conventional technique is to measure the properties of a specimen using ultrasonic diffusion (US Pat. No. 6,532,821), which describes the temporal variation of the diffusion signal when the ultrasonic pulse generated by the pulsed laser beam is diffused by continuous scattering inside the specimen. Measure The diffusion signal measured as described above provides a method of measuring the physical properties of the specimen in comparison with the theoretical diffusion equation using various physical properties (ultrasound absorption coefficient, yield strength, grain size) as parameters. According to this prior art, in principle, it is possible to measure the crystal grain size of the specimen as in the present invention, but since the intensity of the diffused ultrasonic wave is generally very small, the production line is generally in a harsh environment (large sheet thickness, various noise elements). In S / N ratio, the signal-to-noise ratio is very low, and in order to derive the spreading expression that is closest to the measured spreading signal, it is necessary to derive the spreading formula by continuously changing the parameters and compare it with the measured signal. However, there is a disadvantage in that a criterion for establishing a theoretical diffusion equation most similar to the measured signal must be provided. Due to these drawbacks, this prior art is not suitable for on-line measurements in production lines, although applicable in laboratory environments. In addition, there are no problems and solutions to on-line measurements in production lines. Another conventional technique for measuring the physical properties of the plate using ultrasonic resonance (US Patent 6,057,927) provides a method for measuring the thickness or mechanical properties of the specimen using the resonance frequency of the longitudinal or / and transverse wave, but in the present invention In order to be applied, a plurality of continuous ultrasonic wave signals have to be acquired. In addition, the resonance frequency must be analyzed from a plurality of measured ultrasonic signals and the thickness or physical properties of the measurement target should be calculated. However, in the prior art (US Pat. No. 6,057,927), a method for automatically performing resonance frequency analysis and property calculation Since it is not presented, it is difficult to apply on-line in a production line requiring real time signal processing.

There is a US patent (4,659,224; Optical interferometric reception of ultrasonic energy) as a prior art for laser interferometer for ultrasonic measurement. This patent provides a method for measuring the ultrasonic waves reaching the specimen surface using a confocal Fabry-Perot interferometer, and also suggests a method for stabilizing the interferometer as described above. However, in order to stabilize the interferometer by the method proposed in this patent, a device having a large, expensive, and complicated circuit configuration such as a rock-in amplifier and an electro-optic cell should be used. In addition, the prior art (US Patent 4,659, 224) has a disadvantage in that a very complex and sensitive control circuit must be configured because the frequency of the ultrasonic measurement laser must be adjusted at all times in order to maintain the optimum conditions of the confocal Fabry-Perot interferometer. In recent years, when configuring a measuring device using a laser, a commercially available laser is used to facilitate the device configuration, and such a commercialized laser is not easy to change its internal configuration. In addition, according to the US Patent 4,659,224, it is very difficult to implement the contents of the patent because it is not possible to use a commercialized laser as it is, and to change the internal configuration of the commercialized laser or to make the laser itself according to the purpose of the measuring device.

The present invention is to solve the conventional problems that occur when the non-contact grain size measurement using a laser as described above in the rolling process of the production line, to provide a non-destructive ultrasonic wave to the steel sheet using a pulsed laser And a laser beam for measurement at the same point as the ultrasonic wave generation point to collect the laser scattered light by the ultrasonic wave to calculate the attenuation coefficient of the ultrasonic wave, and to measure the crystal grain size of the steel sheet using the attenuation coefficient. An object of the present invention is to provide an on-line grain size measuring system and method using ultrasonic waves.

Another object of the present invention, by detecting the scattered light by the ultrasonic wave only by blocking the influence of the existing plasma radiation in the generation of high-intensity ultrasonic wave of the scattered light by the ultrasonic wave to enable accurate and efficient ultrasonic measurement, the measured ultrasonic signal The present invention provides an on-line grain size measuring apparatus and method for real-time grain size calculation.                         

Still another object of the present invention is that the ultrasonic measurement is performed at the same point as the point of generation of the ultrasonic wave so that the present detection device is present only on the upper part of the steel sheet in production so that the on-line grain size measuring device can be applied to the actual production line. To provide.

Still another object of the present invention is to output an ultrasonic measuring laser beam to measure an ultrasonic wave generated by irradiating a laser beam for ultrasonic generation to an object, and scattered light of the ultrasonic measuring laser beam having a frequency shift caused by the ultrasonic wave. It is an object of the present invention to provide an on-line laser beam detection apparatus and method for ultrasonic measurement to efficiently detect the on-line.

Still another object of the present invention is to stabilize the laser-interferometer for ultrasonic measurement by interfering the scattered light of the laser beam by the ultrasonic wave generated in the object, and the laser-interferometer stabilization apparatus and method for ultrasonic measurement for measuring ultrasonic waves with optimum efficiency The purpose is to provide.

On-line crystal grain size measuring apparatus using the laser-ultrasound of the present invention for achieving the above object,

Ultrasonic wave generation laser for irradiating a non-contact type pulsed laser beam on the upper surface of the measurement object to generate an ultrasonic wave inside the measurement object; Ultrasonic measurement laser for outputting a laser beam for measuring the ultrasound generated in the measurement object; The laser beam output from the ultrasonic measuring laser is irradiated non-contactly to the same point where the ultrasonic wave is generated by the pulsed laser beam irradiated from the ultrasonic wave generating laser, and is reflected from the lower surface of the object to generate the ultrasonic wave on the upper surface. Scattered light collecting means for collecting the laser scattered light by the ultrasonic waves returned to the; A laser-interferometer which reciprocates the collected scattered light internally to generate interference and outputs transmission interference light and reflection interference light of opposite phases due to the ultrasonic waves; Ultrasonic signal detection means for converting the intensity of the two interference light output from the laser interferometer into an electrical signal and detecting an ultrasonic signal using the converted electrical signal; And signal processing means for calculating the crystal attenuation coefficient of the measurement object by calculating an ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal and applying the calculated attenuation coefficient to a predetermined crystal grain size-ultrasonic attenuation coefficient correlation. The diameter of the circular spot of the beam irradiated to the surface of the measurement object in the ultrasonic measuring laser is smaller than the diameter of the circular spot of the beam irradiated to the same point of the surface in the laser for ultrasound generation.

Crystal grain size measurement system according to an embodiment of the present invention, the condensing lens for condensing a laser beam output from the laser for generating ultrasound; And a total reflection mirror installed above the ultrasonic generation point on the surface of the measurement object to reflect the laser beam such that the focused laser beam is irradiated substantially perpendicular to the same point as the ultrasonic generation point on the surface of the measurement object. You may.

In addition, in one embodiment of the present invention, the crystal grain size measurement system according to the present invention is included in the scattered light transmitted from the scattered light collecting means to the laser interferometer, the plasma emission light generated when generating ultrasonic waves on the surface of the measurement object It may further include a plasma radiation blocking means for blocking the incident to the laser-interferometer.

In addition, the on-line crystal grain size measuring method using the laser-ultrasound according to the present invention for achieving the above object,

A first step of irradiating a non-contact type pulsed laser beam on an upper surface of the measurement object to generate ultrasonic waves inside the measurement object; Irradiating a laser beam for measuring ultrasonic waves generated on the measurement object to the same point where ultrasonic waves are generated by the irradiated pulsed laser beam; A third step of collecting the laser scattered light by the ultrasonic waves generated on the upper surface of the measurement object and reflected on the lower surface and returned to the ultrasonic emission point of the upper surface; A fourth step of generating interference by reciprocating the collected scattered light internally and outputting transmission interference light and reflection interference light of opposite phases due to ultrasonic waves generated in the measurement object; A fifth step of converting the intensities of the two interfering light outputs into electrical signals and detecting an ultrasonic signal by using the converted electrical signals; A sixth step of calculating an ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal; And a seventh step of calculating the crystal grain size of the measurement object by applying the calculated attenuation coefficient to a predetermined crystal grain size-ultrasonic attenuation coefficient correlation equation.
The diameter of the circular spot of the ultrasonic laser beam for irradiation on the surface of the measurement object is smaller than the diameter of the circular spot of the laser beam for ultrasound generation irradiated at the same point on the surface.

In one embodiment of the present invention, the crystal grain size measuring method according to the present invention, the plasma emission light generated during the generation of the ultrasonic wave on the surface of the measurement object, which is included in the collected scattered light block the incident of the laser-interferometer The eighth step may be further included.

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The present invention in order to provide in real time the crystal grain size information necessary for efficient rolling work in the hot or cold rolling process of various steel sheets produced in steel mills, by generating a ultrasonic wave on the surface of the steel sheet using a pulsed laser and On-line crystal grain size for irradiating the laser beam for measurement to the same point to collect the scattered light of the laser beam by the ultrasonic wave propagated inside the steel sheet to obtain the ultrasonic attenuation coefficient and to measure the grain size of the steel sheet in real time It provides a measuring device and method. More specifically, the present invention facilitates the on-line application to the steel sheet during hot or cold rolling by performing the oscillation and measurement of the ultrasonic wave (long wave) on the same surface of the steel sheet, and apply the optimum ultrasonic generating position and measurement position By increasing the longitudinal measurement efficiency and reducing the interference by the plasma, and stabilized the ultrasonic measurement signal by applying an efficient laser interferometer, by using the calibration curve corrected for the ultrasonic diffraction, the direction of the ultrasonic oscillation and the temperature effect It is possible to derive the crystal grain size from the measured ultrasonic attenuation coefficient, and select the optimum frequency among the frequency components of the ultrasonic wave and apply only this frequency to the attenuation coefficient analysis and the grain size measurement to enable quick and easy crystal grain measurement. ON- enables easy measurement of grain size in the rolling process An apparatus and method for measuring line grain size are provided.

In addition, the present invention facilitates the on-line application to the steel sheet during hot or cold rolling by performing the oscillation and measurement of the ultrasonic wave (long wave) on the same surface of the steel sheet, the longitudinal wave measurement by applying the optimum ultrasonic oscillation position and measurement position The present invention provides an on-line laser beam detection apparatus and method for ultrasonic measurement which increases the efficiency and enables the measurement of the ultrasonic signal easily in the hot rolling process as a whole.

Furthermore, the present invention generates ultrasonic waves on the surface of the steel sheet using a pulsed laser and propagates the inside of the steel sheet in order to provide crystal grain size information necessary for efficient rolling in a hot or cold rolling process of various steel sheets produced in a steel mill in real time. When the ultrasonic wave is measured using a laser interferometer, there is provided an apparatus and method for stabilizing a laser interferometer so that ultrasonic measurement efficiency of a laser interferometer is not degraded even in a harsh environment of a production line.

The steel sheets being rolled on the production line are generally parallel to each other at the top and bottom. Therefore, in the present invention, the crystal grain size is measured using the attenuation coefficient due to scattering generated at the boundary of the crystal grain while the ultrasonic wave propagates inside the measurement object.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, detailed descriptions of preferred embodiments of the present invention will be described with reference to the accompanying drawings. It should be noted that reference numerals and like elements among the drawings are denoted by the same reference numerals and symbols as much as possible even though they are shown in different drawings. In the following description of the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

1 is an overall schematic diagram of an on-line grain size measurement system using laser-ultrasound according to an embodiment of the present invention. Referring to FIG. 1, in the crystal grain size measurement system according to the present invention, the pulsed laser beam 13 is irradiated on the upper surface 71 of the measurement object 90 in a non-contact manner so that ultrasonic waves are generated inside the measurement object 90. The ultrasonic wave generating laser 10, the ultrasonic wave measuring laser 20 for outputting a laser beam for measuring the ultrasonic waves generated on the measuring object 90, and the same point as the ultrasonic generating point of the measuring object 90 The laser beam by the ultrasonic wave that is irradiated from the ultrasonic beam laser 20 for non-contact by reflecting from the lower surface 92 of the object 90 and returned to the ultrasonic generating point of the upper surface 91 again Scattered light collecting means 30 for collecting the scattered light 28, the collected scattered light 28 is reciprocated from the inside to generate interference, and the transmitted and reflected interference light of opposite phases due to the ultrasonic wave Ultrasonic signal detection means for converting the intensity of the two interference light output from the laser-interferometer 60, the laser-interferometer 60 to the electrical signal, respectively, and detects the ultrasonic signal by using the converted electrical signal ( 70) and signal processing means 80 for calculating the crystal attenuation coefficient of the measurement object by calculating the ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal and applying the calculated attenuation coefficient to a predetermined crystal grain size-ultrasonic attenuation coefficient correlation. ).

In the embodiment of the present invention shown in FIG. 1, the crystal grain size measuring system includes a third condensing lens 11 for condensing a laser beam output from an ultrasonic wave generating laser 10 and an ultrasonic generation point on a surface of a measurement object. It may further include a total reflection mirror 12 is installed on top of the reflecting the laser beam so that the focused laser beam is irradiated substantially perpendicular to the surface of the measurement object.

In addition, in one embodiment of the present invention, the crystal grain size measurement system is included in the scattered light transmitted from the scattered light collecting means 30 to the laser interferometer 60, on the surface 91 of the measurement object 90 It may further include a plasma radiation blocking means 50 for blocking the plasma radiation generated in the generation of the ultrasonic wave incident on the laser-interferometer 60.

Furthermore, as shown in FIG. 1, the crystal grain size measuring system may further include stabilization means 40 for stabilizing the laser interferometer 60.

Referring to FIG. 1, the pulsed laser beam 13 generated by the ultrasonic wave generation laser 10 is incident substantially perpendicularly to the surface of the measurement object 90 to generate various ultrasonic waves (long wave, transverse wave and surface wave) ( References: Scruby, CB). In FIG. 1, as a preferred embodiment of the present invention, the pulsed laser beam 13 generated by the ultrasonic wave generating laser 10 is collected by the third condenser lens 11 and then measured by the total reflection mirror 12. 90) is incident on the surface. These are optical elements for allowing the ultrasonic wave generation laser beam 13 output from the ultrasonic wave generation laser 10 located at a remote location to be incident perpendicularly to the measurement object 90.

As described above, the generation efficiency of various ultrasonic waves generated by the incident of the pulsed laser beam 13 is determined by the power density of the incident pulse beam, and the intensity of the pulsed laser beam 13 is measured. If the density is large (about 5x10 ⁸ W / cm ² or more at the surface to be measured), ultrasonic waves are mainly generated by the ablation of the surface material. As described above, the ultrasonic flaw detection in the production line requires high-intensity ultrasonic waves, and therefore, generation of ultrasonic waves by the fusion effect is required. It is preferable to use a Q-switched laser that generates a laser beam having a short pulse width of several nanoseconds to several tens of nanoseconds for the generation of the ultrasonic wave due to the surface of the measurement target. Here, in the preferred embodiment of the present invention, the ultrasonic wave generating laser 10 is a pulsed laser beam according to the thickness of the measuring object 90 so that ultrasonic waves are generated by the effect of the surface material of the measuring object 90. It is possible to adjust the intensity of the.

In addition, even when using the same laser pulse, the intensity density of the pulsed laser beam on the surface of the measurement object 90 depends on the size of the focused pulsed light. Since the cross-section of the pulsed laser beam is generally circular, the pulsed light condensed on the surface of the measurement object 90 is also circular, and the intensity density increases as the size of the pulsed light, ie, the diameter S ₁ , decreases. In general, when the conditions for generating the effect of the ignition effect are implemented, the size (S ₁ ) of the pulsed light is adjusted. The size S ₁ of the pulsed light focused on the measurement target surface is the focal length F _{1 of} the lens 11 and the propagation distance D of the pulsed light between the lens 11 and the surface of the measurement target 90. ₁ ). That is, when D ₁ and F ₁ coincide, the size (S ₁ ) of the focused pulsed light is the smallest and S ₁ becomes larger as D ₁ becomes smaller than F ₁ . In the present invention, it is preferable to adjust the D ₁ and F ₁ appropriately so that the ultrasonic wave is mainly generated by the effect of fusion.

As shown in FIG. 1, the linearly polarized laser beam generated by the ultrasonic measuring laser 20 is preferably a polarization maintaining fiber for laser beam transmission by the condenser lens 23. After entering the PMF (25) is transmitted to the scattered light collecting means (30). The laser beam transmitted by the polarization maintaining optical fiber 25 is focused on the upper surface 91 of the measurement object 90 by the scattered light collecting means 30, thereby being reflected from the opposite surface 92 to the measurement surface 91. Used to measure returned denominations.

Figure 2 is a schematic diagram showing the mode conversion generated when the reflection of the ultrasonic wave (longwave) generated by the laser pulse according to an embodiment of the present invention. As described above, various ultrasonic waves (long wave, transverse wave and surface wave) are generated by the sparking effect, and the generated longitudinal wave is mainly perpendicular to the surface 91 of the measurement object 90 as shown in FIG. It propagates around one direction. The steel sheets being rolled in the production line are usually parallel to each other on the upper and lower surfaces 91 and 92. Therefore, in the present invention, the generation and the measurement of the ultrasound is preferably performed at the same point on the surface 91 of the measurement object 90 to facilitate the on-line measurement, while the ultrasound is generated while propagating the inside of the measurement object. If the crystal grain size is to be determined using the ultrasonic attenuation coefficient due to the crystal grains, ultrasonic waves propagating perpendicularly to the surface of the measurement object are generated as shown in FIG. 2 (b), and are reflected from the opposite surface 92 of the surface of the measurement object. It is most efficient to measure the returned ultrasound at the same point where the ultrasound is generated. If the ultrasonic wave is different from the measurement point of the ultrasonic wave, as shown in FIG. 2 (c), the ultrasonic wave propagated at a constant angle with respect to the surface of the measurement object 90 should be used. When a portion of the longitudinal energy is converted to a transverse wave by mode conversion, there is a disadvantage in that the loss of the longitudinal wave strength used for measuring the grain size occurs. In addition, as shown in FIG. 2 (c), when the generation point and the measurement point of the ultrasonic wave are different, the calculation of the propagation path and the propagation distance of the ultrasonic wave required for the calculation of the ultrasonic attenuation coefficient is very complicated. For the above technical reasons, when measuring the grain size on-line in a production line, it is most efficient to generate ultrasonic waves (vertical waves) vertically by the effect of pulsation and to measure ultrasonic waves at the same point as the origin of the ultrasonic waves.

3 is a schematic diagram showing circular spots of an ultrasonic wave generating laser beam and circular spots of an ultrasonic wave measuring laser beam according to an example of the present invention. As described above, the ultrasonic laser beam 27 for ultrasonic measurement is used to generate the ultrasonic pulsed laser pulse light 13 as shown in FIG. 3 (a) to facilitate the measurement of the reflected longitudinal wave and the calculation of the attenuation coefficient in the production line. ) And the laser beam 27 for the ultrasonic measurement are incident on the same point on the surface 91 of the measurement object 90 and circular spots of the ultrasonic pulsed laser pulse light focused on the measurement object surface as shown in FIG. 3 (b). (spot) The laser beam 27 for ultrasonic measurement is condensed at the center of the diameter S ₂ in the center. As shown in FIG. 3B, the smaller the diameter S ₂ of the circular spot 37 by the laser beam 27 for ultrasonic measurement is, the light scattered from the surface of the measurement object using the scattered light collecting means 30 is shown. After collecting and transmitting to the laser-interferometer 60 through the multiple pod optical fiber 31 is high.

Figure 4 is a schematic diagram of the configuration of the scattered light collecting means according to an embodiment of the present invention. 4 (a), the scattered light collecting means 30 according to the present invention receives the laser beam 27 for ultrasonic measurement through the first piece light-retaining optical fiber 25 and measures the received ultrasonic wave. A beam collimator 38 for converting the laser beam 27 into parallel light, and the converted ultrasonic beam for laser measurement 27 is incident perpendicularly to the surface 91 of the object 90. Light that reflects (or transmits) the ultrasonic beam for laser measurement 27, and transmits (or reflects) the scattered light 28 of the ultrasonic beam for laser measurement 27 due to the ultrasonic wave generated in the object 90. A circular polarized laser beam 27 that is reflected (or transmitted) from the aperture 33 and the light shimmer 33 to be perpendicular to the surface 91 of the object 90 is circularly polarized ( converted to a circular polarized laser beam, and circularly polarized acid from the surface 91 of the object 90 The first wave that collects the scattered light 28 from the object surface 91 converts the scattered light 28 back to linearly polarized scattered light and consequently rotates the polarization direction by 90 degrees. The quarter wavelength is converted by converting the convex lens 36, the second convex lens 37 focusing the focused scattered light 28, and the focused scattered light 28 into parallel light whose diameter is reduced to a specific size. And a concave lens 35 for outputting to the plate 34 and the light shroud 33. Preferably, the light shimmer 33, the quarter wave plate 34, the concave lens 35, and the second convex. The lens 37 and the first convex lens 36 are sequentially arranged on the same line. 4, the scattered light collecting means 30 according to the exemplary embodiment of the present invention is output from the concave lens 35 and passes through the quarter-wave plate 34 and the light shinger 33. It may further comprise a condenser lens 38 for condensing one scattered light 28 to the incident surface of the multimode optical fiber 31. The condenser lens 38 allows the scattered light 28 to be incident on the optical fiber 31 efficiently. That is, as the incident angle θ and the spot size of the beam focused on the incident surface of the optical fiber 31 by the condenser lens 38 are smaller, the amount of light entering the optical fiber 31 increases. .

Referring to an embodiment of the present invention shown in FIG. 4, the ultrasonic beam for measuring ultrasonic waves 27 transmitted through the optical fiber 25 becomes a polarized light after being parallel light by a beam collimator 32. The light beam 33 is reflected in the vertical direction to the object 90. The reflectance of the laser beam by the polarization type light shimmer 33 is changed depending on the polarization direction of the laser beam. In one embodiment of the present invention by rotating the crystal axis of a predetermined half-wave plate (not shown in the figure) by properly rotating the polarization direction of the laser beam transmitted through the first polarization maintaining optical fiber 25 to the polarization The reflectance by the type light shimmer 33 is maximized. The laser beam 27 for ultrasonic measurement reflected by the polarization type light filter 33 in this manner passes through the quarter waveplate 34 and then the concave lens 35 and the second and first portions. The light is collected on the surface 91 of the object 90 through the convex lenses 37 and 35. At this time, the ultrasonic measuring laser beam 27, which was initially linearly polarized, passes through the quarter wave plate 34 and becomes circular polarization. The scattered light 28 reflected from the surface 91 of the object 90 becomes parallel light while passing through the first and second convex lenses 36 and 37 and the concave lens 35, and then the quarter wave plate 34. Is again linearly polarized. The polarization direction of the light linearly polarized again by the quarter wave plate 34 is rotated by 90 degrees with respect to the initial polarization direction, and according to the above principle, the reflectance by the polarized light shimmer 33 is minimized. do. By this principle, the scattered light 28 reflected from the surface 91 of the object 90 most efficiently passes through the polarized light shimmer 33 to the incident surface of the optical fiber 31 by the condenser lens 38. Condensed

On the other hand, referring to Figure 4, as described above, the ultrasonic beam for measuring the laser beam 27 transmitted through the first polarization maintaining optical fiber 25 is polarized after being parallel light by the beam collimator (32) The scattered light 28 that is reflected by the type light shader 33 and is output from the concave lens 35 and passes through the quarter-wave plate 34 passes through the polarized light shader 33 and condenses the lens. The light is collected by 107 and flows into the optical fiber 31. Here, in FIG. 4A, as an example of the present invention, the beam parallelizer 32 and the condenser lens 38 are respectively positioned at the side portions and the upper surface portions of the scattered light collecting means 30 of the present invention. In another embodiment of the present invention (FIG. 4 (b)), the beam parallelizer 32 and the condenser lens 38 may be positioned on the upper surface side and the side surface, respectively. In this case, as shown in Figure 4 (b), the ultrasonic measuring laser beam 27 transmitted through the optical fiber 25 becomes a parallel light by the beam parallelizer 32 located in the upper surface polarization type It is transmitted by the light shimmer 33. In addition, the scattered light 28 that is output from the concave lens 35 and passes through the quarter-wave plate 34 is reflected by the polarized light shimmer 33 and is collected by the condenser lens 38 positioned on the right side. It flows into the optical fiber 31. As described above, according to the exemplary embodiment of the present invention, the polarization type light filter 33 reflects or transmits the laser beam 27 for ultrasonic measurement according to the positions of the beam parallelizer 32 and the condenser lens 38. The laser beam 27 is incident perpendicularly to the object 90, and the scattered light 28 is reflected or transmitted to be incident on the condenser lens 38. Furthermore, in another embodiment of the present invention, the beam parallelizer 32 and the condenser lens 38 may be located on different surfaces. In this case, the polarized light shimmer 33 reflects or transmits the laser beam 27 for ultrasonic measurement according to their position so that the laser beam 27 is incident perpendicularly to the object 90 and scattered light. Reflect or transmit 28 to enter the condenser lens 38.

The optical fiber 31 applied to the present invention is typically a multimode optical fiber, and the light introduced into the core of the optical fiber is transmitted. It is preferable that the diameter of an optical fiber core is about several tens-several hundred micrometers. Therefore, as the size of spots focused on the core cross-section of the multimode optical fiber 31 by the condenser lens 38 decreases, the amount of light entering the optical fiber increases. As such, the size of circular spots focused on the core cross section of the optical fiber 31 is proportional to the size of circular spots S ₂ focused on the object surface 91 by an optical principle. Due to such technical reasons, the smaller the diameter (S ₂ ) of the circular spot by the laser beam for ultrasonic measurement receives the scattered light from the surface of the measurement object using the scattered light collecting means 30 of the present invention and then the optical fiber ( 31) the efficiency of transmission to the laser interferometer 60 is high. Another factor affecting the scattered light transmission efficiency by the optical fiber 31 is the incident angle of light focused by the condenser lens 38 to the optical fiber incident cross section.

5 is an exemplary view showing an incident angle of the scattered light and the optical fiber cross section by the condensing lens applied to the present invention. As shown in FIG. 5, when the incident angle θ of the light by the condenser lens 55 is large, the light introduced into the optical fiber core 56 is not absorbed and transmitted by the cladding 57 of the optical fiber. . According to the scattered light collecting means 30 of FIG. 4, since the scattered light 28 reflected from the surface of the measurement object 90 is collected by the condenser lens 38 in the cross section of the optical fiber 31, as shown in FIG. 5. It is advantageous that the incident angle θ is small. As described above, according to the scattered light collecting means 30 of FIG. 4, the laser beam transmitted by the first polarization maintaining optical fiber 35 is focused on the surface of the measurement object 90 with maximum efficiency without loss, and the measurement object ( 90) the scattered light 28 reflected from the surface can also be transmitted to the laser interferometer 60 through the second polarization maintaining optical fiber 31 at maximum efficiency.

6 is a schematic diagram illustrating a laser-interferometer according to an embodiment of the present invention. FIG. 6 shows a Fabry-Perot interferometer as an example. First, Figure 2 (a) is an exemplary diagram of the basic configuration of the Fabry-Perot interferometer, the scattered light 28 transmitted from the scattered light collecting means 30 through the multi-mode optical fiber 31 is a beam collimator ( 61 is converted into parallel light and incident to the cavity 66. The resonator 66 is preferably composed of curved partially reflective resonator mirrors 62 and 63 at both ends, wherein each of the resonator mirrors 62 and 63 is a mirror having one surface with a predetermined reflectance and the other opposite surface is It is preferable to consist of glass. The two mirrors 62 and 63 are arranged at opposite intervals to face each other and have a predetermined reflectance. Accordingly, the scattered light 28 incident to the resonator 66 generates the interference light 69 while being reciprocated by the partial reflection of the two resonator mirrors 62 and 63. At this time, the two resonator mirrors 62 and 63 have a predetermined reflectance, so that some light is transmitted. Accordingly, the reflected interference light 64 reciprocated as described above and output through the first resonator mirror 62 is incident to the first photodetector 71 in the ultrasonic signal detector 70 (shown in FIG. 1). In addition, the transmission interference light 65 output through the second resonator mirror 63 while being reciprocated as described above is incident to the second photodetector 72 in the ultrasonic signal detector 70.

As such, the scattered light 28 transmitted through the multimode optical fiber 31 is analyzed by the laser interferometer 60 to output an optical signal due to the ultrasonic signal. The laser interferometer 60 converts the wavelength shift of the scattered light by ultrasonic waves into the intensity change of the reflected interference light 64 and the transmitted interference light 65. In other words, the scattered light 28 collected by the scattered light collecting means 30 reciprocates internally to generate interference, and outputs reflected interference light 64 and transmitted interference light 65 of opposite phases due to ultrasonic waves.

Figure 2 (b) is an illustration of another configuration of the Fabry-Perot interferometer, it shows a configuration in which a plurality of optical elements are added in Figure 2 (a). Referring to FIG. 2 (b), the scattered light 28 transmitted from the scattered light collecting means 30 through the multimode optical fiber 31 is converted into parallel light by the beam parallelizer 61 and then polarized light beam The polarization direction of the scattered light 28 is rotated by 45 degrees by passing through the aperture 67 and the quarter-wave plate 68. The rotated scattered light 28 is incident on the resonator 66 and reciprocated inside to generate interference as described in FIG. Subsequent processes are the same as those described with reference to FIG. However, the reflected interference light 64 passing through the first resonator mirror 62 passes through the quarter wave plate 68 again, and its polarization direction is rotated by 45 degrees, resulting in 90 degrees of rotation. The light beams 67 do not penetrate through the light shield 67 and are incident to the first photodetector 71.

Figure 2 (c) is an illustration of another configuration of the Fabry-Perot interferometer, another quarter wave plate 73 and the polarization type in front of the second photodetector 72 in the laser interferometer of Figure 2 (b) Lightgargar (74) is added. In FIG. 2 (c), the transmission interference light 65 output from the resonator 66 is received by the quarter wave plate 73, and the polarization direction thereof is rotated by 45 degrees. The polarization direction passes through the rotated transmitted interference light 65 to be input to the second photodetector 72. 2 illustrates various configurations of the laser interferometer to which the present invention is applied, but the laser interferometer to which the present invention is applied is not limited thereto.

Hereinafter, for convenience of description, the operation of the laser interferometer according to the present invention will be described with reference to FIG. 6 (b). Referring to FIG. 6B, the scattered light 28 transmitted to the multimode optical fiber 31 is analyzed by a laser interferometer 60 to output an optical signal due to an ultrasonic signal. The laser-interferometer 60 converts a wavelength shift of scattered light by ultrasonic waves into a change in intensity of transmitted and reflected interference light therein. In other words, the scattered light 28 collected by the scattered light collecting means 30 reciprocates internally to generate interference, and outputs transmitted interference light and reflected interference light of opposite phases due to the generated ultrasonic waves. Referring to FIG. 6, the scattered light transmitted to the multimode optical fiber 31 becomes parallel light by the collimator 61, and then the polarized light filter 67 and the quarter wave plate 68 are removed. By passing through, the polarization direction of the scattered light 28 is rotated by 45 degrees. The rotated scattered light is reciprocated by resonator mirrors 62 and 63 having a specific reflectance to generate interference. At this time, the transmitted interference light 65 transmitted through the second resonator mirror 63 is output to the first photodetector 72 of the ultrasonic signal detecting means 70, and the reflected interference reflected by the first resonator mirror 62. As the light passes through the quarter-wave plate 68 again, the polarization direction is further rotated by 45 degrees to reflect the light from the polarized light filter 67 and the second photodetector 71 of the ultrasonic signal detecting means 70. The ultrasonic signal is outputted as

Figure 7 is a schematic diagram showing a laser-interferometer combined with a plasma radiation blocking means according to an embodiment of the present invention. FIG. 7 illustrates a form in which the plasma radiation blocking means 50 is coupled to the laser interferometer 60 in FIG. 6 (b). However, the plasma radiation blocking means 50 according to the present invention can be coupled to other types of laser interferometers, including FIGS. 7 (a) and 7 (c). Referring to Figure 7, the plasma radiation blocking means 50 according to the present invention, the excitation signal applying means 116 for outputting an excitation signal at the moment of oscillating the laser beam in the ultrasonic wave generation laser 10, the excitation signal Is received from the scattered light collecting means to the laser-interferometer 38 during the set time according to the switch control means 111 and the control signal of the switch control means 111 to control the operation of the switch to be described later for a set time. It includes a switch 110 for blocking the optical signal to be.

The plasma radiation blocking means 20 will be described in more detail with reference to FIG. 7. The switch 110 passes or blocks the scattered light 28 transmitted to the multimode optical fiber 31. The switch 110 is used for the purpose of reducing the effect of plasma radiation generated when the ultrasonic wave generated by the fusion effect. That is, when measuring the grain size on-line in the production line as described above, it is necessary to generate a high-intensity ultrasonic wave by the fusion effect, the reflection on the opposite side of the ultrasonic wave generation surface for easy calculation of the ultrasonic attenuation coefficient It is most efficient to measure the returned ultrasonic wave at the same point where the ultrasonic wave is generated. However, when the ultrasonic wave is generated by such a fusion effect, a plasma is generated that emits light of continuous wavelengths at the ultrasonic wave generating point, and when the ultrasonic wave is generated and measured at the same or in close proximity, the plasma is generated. The light emitted from the light enters the laser interferometer 60 for ultrasonic measurement and acts as an obstacle for ultrasonic measurement. As described above, the plasma emission light accompanying the ultrasonic generation is very strong initially (within several m seconds) from the time when the ultrasonic pulsed laser pulse beam is incident on the surface to be measured. On the other hand, since the minimum propagation distance L of the ultrasonic wave used for measuring the grain size corresponds to twice the thickness of the steel sheet to be measured, for example, in the case of the 10 mm thick measuring object, the arrival time of the ultrasonic wave (long wave) is approximately 3.3 ms. This is the point in time where the intensity of plasma radiation is reduced. Generation of high-intensity ultrasound due to the fusion effect is mainly necessary for measuring the grain size of a measurement object with a thick thickness. Accordingly, the arrival time of the ultrasound is usually 3-4 ms or more. Therefore, the intensity of plasma radiation typically reaches its peak before ultrasonic waves arrive. However, since a very sensitive photodetector such as an APD (avalanche photodiode) is used for the measurement of ultrasonic waves, the ultrasonic wave is saturated with plasma radiation. Thus, once saturated photodetector takes a certain time to restore to the normal operating state even if the intensity of the plasma radiation is reduced. Therefore, even when ultrasonic waves arrive at the time when the intensity of plasma radiation is decreased, normal ultrasonic measurement is difficult because the photodetector is not completely restored in the saturated state. In general, optical noise generated by the inflow of light other than the laser beam for measurement is generally solved by providing an optical filter at the input of the photodetector. However, since the plasma radiation has a continuous wavelength spectrum, it has the same wavelength component as the wavelength of the laser beam for measurement. Therefore, the conventional optical filter cannot solve the effect of the plasma radiation. The switch 110 shown in FIG. 7 is to solve such a conventional problem, and prevents the very strong plasma generation light from flowing into the photodetector for ultrasonic measurement at the initial stage of the ultrasonic generation by the pulsed laser beam. Make ultrasonic measurements possible. That is, the pulsed laser 10 for ultrasonic measurement applies a trigger signal from the excitation signal applying means 116 to the switch control means 111 while oscillating the pulsed laser beam. The switch regulator 111 opens a gate of the high-speed switch 110 after passing a scattered light after a predetermined time DT has elapsed from the moment of receiving the excitation signal, and the scattered light is incident on the interferometer and ultrasonic measurement starts. Be sure to That is, the switch 110 is operated immediately before the first reflected ultrasonic wave (longwave) reaches the measurement position by the switch regulator 111 as shown in Figure 8 to pass through the scattered light, The operating time DT of 110 is calculated using the thickness of the measurement target and the propagation speed of the ultrasonic wave (longwave). When the intensity of plasma radiation is lowered by this action, the scattered light for ultrasonic measurement begins to flow into the interferometer, and the ultrasonic wave is measured. Since no saturation occurs, normal ultrasonic measurements are possible.

FIG. 9 is an exemplary view illustrating a configuration of a switch of the plasma radiation blocking means 20 according to an embodiment of the present invention, and FIG. 10 is a view illustrating polarization directions of two linear polarizers of the switch according to an embodiment of the present invention. 9 and 10, a specific embodiment of the switch regulator 111 according to the present invention is shown in FIG. 9 for the purpose of enabling stable ultrasonic measurement by reducing the influence of plasma radiation as described above. . Referring to FIG. 9, the switch 110 transmits only the light having the same wavelength as the laser beam 27 for ultrasonic measurement among the optical signals transmitted from the scattered light collecting means 30 to the laser interferometer 60. (112), the linearly polarized light according to the control signal of the first linear polarizer 113, the switch control means 111 for converting a plurality of polarization light passing through the filter 112 into a linearly polarized light in a single direction It includes a single crystal 114 for rotating the polarization direction of the light beam and the second linear polarizer 115 disposed perpendicular to the polarization direction of the first linear polarizer 113, preferably the filter 112, the first The linear flat light 113, the single crystal 114 and the second linear polarizer 115 are sequentially arranged on the same line.

In FIG. 9, the light transmitted by the multimode optical fiber 31 includes scattered light 28 and plasma emission light reflected by the laser beam 27 for ultrasonic measurement at the surface 91 of the measurement object 90. Among them, the laser beam 27 for ultrasonic measurement is light of a single wavelength l, and the plasma emission light is light composed of various wavelengths. The narrow bandpass filter 112 of FIG. 9 is a filter for passing only light having a specific wavelength l. In the present invention, only the light having the same wavelength l as the laser beam 27 for ultrasonic measurement passes. Let's do it. Therefore, the light passing through the narrow line width wavelength filter 112 is composed of the light having the same wavelength as the laser beam 27 for ultrasound measurement and the laser beam 27 for ultrasound measurement among the plasma radiation. In general, the light transmitted to the multimode optical fiber 31 is not constant in the polarization direction, and light in various polarization directions is mixed. As such, the light in the random polarization direction transmitted to the multimode optical fiber 31 is linearly polarized by the first linear polarizer 113 through which only light polarized in a specific direction passes. As a result, the light transmitted by the multimode optical fiber 31 passes through the narrow line width wavelength filter 112 and the first linear polarizer 113 to become linearly polarized light having a single wavelength. As such, linearly polarized light having a single wavelength passes through a single crystal 114 and its polarization direction is rotated. The single crystal 114 of FIG. 9 is a material having no birefringence in a natural state but having a birefringence in a state in which a voltage is applied. Potassium dihydrogen phosphate (KDP) and ammonium dihydrogen Phosphate (ammonium dihydrogen phosphate (ADP), potassium dideuterium phosphate (KD * P), lithium niobate, barium sodium niobate, etc. Can be used). In FIG. 9, the polarization direction of the second linear polarizer 115 behind the single crystal 114 is perpendicular to the polarization direction of the first linear polarizer 113 in front of the single crystal 114. The polarization directions of the two linear polarizers 113 and 115 have been described with reference to FIG. 10. Referring to FIG. 10, if the polarization direction of the first linear polarizer 113 in front of the single crystal 114 has an angle of a, for example, the second linear polarizer 115 behind the single crystal 114 is (a + π). / 2) angle. Therefore, in the state where there is no rotation in the polarization direction by the single crystal 114, light passing through the first linear flatr 113 in front of the single crystal 114 may pass through the second linear flatr 115 behind the single crystal 114. It is not there. However, as shown in FIG. 10, when a voltage V _o having an appropriate magnitude is applied to the single crystal 114, light passing through the single crystal 114 may rotate by 90 ° (π / 2) in its polarization direction. . As such, the light having the polarization direction rotated by 90 ° (π / 2) passes through the second linear flattener 115. As described above, according to the exemplary embodiments of the present invention illustrated in FIGS. 9 and 10, the light transmitted to the multimode optical fiber 31 can be easily opened and closed in order to reduce the influence of the plasma radiation.

11 is a schematic diagram of a configuration of a laser-interferometer stabilization means according to an embodiment of the present invention, FIG. 12 is a schematic diagram of a configuration of a laser interferometer stabilization means according to another embodiment of the present invention, and FIG. Schematic diagram of the configuration of the laser interferometric stabilization means according to another embodiment of the. 11 to 13 show an example in which the laser interferometer stabilization means according to the present invention is applied to the Fabry-Perot interferometer shown in FIG. 7 as a preferred embodiment of the present invention. However, it should be noted that the present invention can be applied to other interferometers without departing from the technical spirit of the present invention.

First, Figure 11 illustrates a laser interferometer stabilization means applied to a laser interferometer according to an embodiment of the present invention. Referring to FIG. 11, the laser interferometer stabilization means 40 according to the present invention is positioned so as to face each other at regular intervals and reciprocates an incident laser beam therein to output two interference beams 150. In the laser interferometer for ultrasonic measurement comprising a resonator 66 having mirrors 62 and 63, a photodetector 41 for detecting the intensity of the interference light 150 signal by the laser beam incident on the resonator 66. The piezoelectric actuator 44 adjusts the intensity of the interference light 150 signal by adjusting the distance between the two resonator mirrors 62 and 63, and sets the intensity of the detected interference light 150 signal to It comprises a stabilization control circuit 42 for outputting a predetermined control signal compared to the allowable range, and a piezoelectric actuator controller 43 for controlling the driving of the piezoelectric driver 44 according to the control signal.

)가 최소가 되도록 소정의 제어신호(Vc)를 출력한다. 상기 두 신호의 세기의 차이와 제어신호의 관계는 하기 식 1과 같이 나타낼 수 있다.Referring to Figure 11 will be described in more detail the laser interferometer stabilization means 40 of the present invention. Referring to FIG. 11, as described above, some ultrasonic measuring laser beams output from the ultrasonic measuring laser 20 are transmitted to the multimode optical fiber 31 and incident on the resonator 66 of the laser interferometer. The incident laser beam is reciprocated by two resonator mirrors 62 and 63 in the resonator 66 to generate the interference light 150. At this time, the photodetector 41 detects the intensity of the interference light 150 signal output from the resonator 66. The stabilization control circuit 42 receives the intensity Is of the detected interference light 150 signal, compares it with a preset optical signal intensity Ir, and compares the strengths of the two signals (

Outputs a predetermined control signal Vc such that? The relationship between the difference between the strengths of the two signals and the control signal may be expressed by Equation 1 below.

[Equation 1]

)가 존재할 경우 압전구동기(44)에 전압(Vc)을 인가하여 두 공진기거울(62,63)의 간격을 조정함으로써 두 신호의 세기의 차이(

)를 저감하는 것이다. 이와 같은 안정화제어회로(42)에 의해 두 신호의 세기 차가 미리 설정된 허용범위(△Ir) 보다 크지 않으면 레이저 간섭계는 안정화된 것이다. 여기서, 상기 미리 설정된 광신호 세기는 상기 초음파 측정용 레이저(20)로부터 전송된 레이저 빔에 의해 결정된다. 즉, 상기 레이저 빔의 세기가 커지면 간섭광(150) 신호의 세기(Is)도 커지므로, 미리 설정된 광신호 세기(Ir)도 이에 비례하여 증대시킨다.In the above formula, C is a conversion constant, and Vc corresponds to a voltage applied to the piezoelectric actuator 44. That is, the strength difference between two signals (

), If there is a voltage (Vc) to the piezoelectric actuator 44 to adjust the distance between the two resonator mirrors (62, 63),

). The laser interferometer is stabilized by the stabilization control circuit 42 when the difference in intensity between the two signals is not larger than the preset allowable range DELTA Ir. Here, the predetermined optical signal intensity is determined by the laser beam transmitted from the ultrasonic laser 20 for measurement. That is, since the intensity Is of the interference light 150 signal increases as the intensity of the laser beam increases, the preset optical signal intensity Ir also increases proportionally.

The piezoelectric actuator controller 43 receiving the control signal adjusts the driving of the piezoelectric driver 44 according to the control signal. The piezoelectric actuator 44 is fixed to one resonator mirror and adjusts the distance between the two resonator mirrors 62 and 63 by adjusting the position of the one resonator mirror according to the control signal of the piezoelectric actuator controller 43. As a result, the intensity of the interference light signal 150 is adjusted.

As described above, the laser interferometer stabilization means 40 of the present invention, when the minute change in the distance between the two resonator mirrors 62,63 of the laser interferometer in a poor external environment due to ambient temperature or vibration, the two resonator mirrors 62 The two resonator mirrors 62 and 63 are detected by detecting the intensity of the interference light 150 signal according to the change of the distance between the two and resonator mirrors 63 and adjusting the distance between the two resonator mirrors 62 and 63 according to the detection result. The distance between them is always kept constant so that the laser interferometer is always in a stable state.

FIG. 12 illustrates a laser interferometer stabilization means applied to a laser interferometer according to another embodiment of the present invention, wherein the interfering light 150 output from the resonator 66 is supplied to the laser interferometer stabilization apparatus shown in FIG. 11 as an example. A first polarization type light filter for reflecting interference light passing through the first quarter wave plate 46 and the first quarter wave plate 46 to rotate the polarization direction by 45 degrees and outputting it to the photodetector 41 ( 45) is additionally included.

In addition, Figure 13 illustrates a laser interferometer stabilization means applied to the laser interferometer according to another embodiment of the present invention, for ultrasonic measurement incident to the resonator 66 to the laser interferometer stabilization means shown in FIG. A beam parallelizer 47 for converting a laser beam into parallel light, a second polarized light beam 48 for reflecting the laser beam converted into parallel light, and rotating the polarization direction of the reflected laser beam by 45 degrees The second quarter-wave plate 49 for outputting to the resonator 66 is further included. The same components in FIGS. 11 to 13 perform the same operation, and the laser beam transmitted through the polarization maintaining optical fiber 26 is received by the beam parallelizer 47 to be the resonant paper 66 or the second polarization type. It enters into the light beam 48.

On the other hand, the intensity of the interference light 150 interfered by the multi-reflection in the resonator 66 is determined by the interference conditions. At this time, the interference condition for determining the intensity of the interference light 150 is determined by the frequency d of the light and the distance d between the two resonator mirrors 62 and 63, as shown in FIG. 14 is a diagram illustrating a relationship between reflected interference light and transmitted interference light in a Fabry-Perot interferometer applied to an embodiment of the present invention. Referring to FIG. 14, when the frequency υ of the scattered light 28 is changed by the ultrasonic waves reaching the surface of the measurement object or the distance d between the two resonator mirrors 62 and 63 of the resonator 66 is changed, the laser interferometer The intensity A of reflected light and the intensity B of transmitted light change. Therefore, when the distance d between the two resonator mirrors 62 and 63 is constant, the change of the intensity A of the light reflected by the interferometer and the intensity B of the transmitted light may be used for the measurement of the ultrasonic wave. The laser interferometer stabilization region shown in FIG. 14 represents a region where the intensity (A) of reflected light and the intensity (B) of transmitted light are greatest with respect to a change in frequency (υ) of a constant scattered light 28, and a laser Ultrasonic measurement efficiency is highest when the interferometer is in this area. Therefore, the interference condition of the laser interferometer is always required to be in this region. Since the center frequency of the scattered light is always constant, the change in the interference condition of the laser interferometer is mainly due to the change in the distance d between the two resonator mirrors 62 and 63. In the present invention, when the laser interferometer is stabilized, it is shown in FIGS. 11 to 13. One piezoelectric actuator (PZT actuator) 44 is used. That is, the piezoelectric actuator 44 adjusts the distance between the two resonator mirrors 62 and 63 so that the laser interferometer is always in the interferometer stabilization region shown in FIG.

Referring back to FIG. 13, as shown in FIG. 1, a portion of the laser beam output from the ultrasonic wave measuring laser 20 is reflected by the polarization type light filter 22, and then the polarization maintaining optical fiber is formed using the condenser lens 24. Send to 26. As shown in FIG. 13, the laser beam transmitted to the optical fiber 26 is converted into parallel light by the beam parallelizer 47 and reflected by the second polarized light splitter 48 to reflect the second quarter wave plate 49. The polarization direction is rotated by 45 degrees while being converted into circularly polarized light, and then enters the resonator 66 of the laser interferometer. The laser interferometer forms the interference light 150, and the interference light 150 passing through the laser interferometer passes through the first quarter wave plate 46 and is further rotated by 45 degrees to be circularly polarized and linearly polarized. At the same time, the light is rotated by 90 degrees in the first polarization direction, and is then reflected by the first polarized light shimmer 45 to be incident on the photodetector 41. The photodetector 41 measures the intensity of the interfering light signal. At this time, the stabilization control circuit 42 and the piezoelectric actuator regulator 43 shown in FIG. 13 are adjusted so that the output signal of the photodetector 41 is always in the interferometer stabilization region shown in FIG. In this manner, in addition to the interference light 69 for ultrasonic measurement, another interference light 150 for stabilizing a separate laser interferometer is generated and used for stabilization of the laser interferometer.

The stabilization method of such a laser interferometer has the following advantages. Firstly, it does not cause any loss in the interference light 69 used for ultrasonic measurement, and secondly, it is possible to stabilize the laser interferometer stably at all times irrespective of the change in the intensity of the scattered light incident on the laser interferometer.

15 is a schematic diagram of the ultrasonic signal detection means according to an embodiment of the present invention. As shown in FIG. 15, the intensity change of the reflected interference light and the transmitted interference light of the interferometer represents a signal of ultrasonic waves reaching the surface of the measurement object 90. At this time, the ultrasonic signals measured by the two photodetectors (71, 72) are opposite in phase. Therefore, by using the differential amplifier 73 as shown in FIG. 15, the ultrasonic signal can be doubled. This differential amplification also has the effect of removing the electrical noise of the same phase applied to the two photodetectors (71, 72). An example of differentially amplifying two photodetector signals is shown in FIG. 16. 16 is a diagram illustrating an increase of an ultrasonic signal by differential amplification according to an embodiment of the present invention. In FIG. 15, the signal of the differential amplifier 73 is applied to the signal processing means 80 shown in FIG. 1 through a bandpass filter 74 to calculate a grain size of the measurement object.

17 is a flowchart illustrating a method for measuring on-line grain size using laser-ultrasound according to an embodiment of the present invention. Referring to FIG. 17, a pulsed laser beam for generating ultrasound is irradiated to the measurement object 90 to measure the crystal grain size (S10). Here, preferably, the pulsed laser beam is irradiated to the measurement object to generate ultrasonic waves remotely. In addition, although not shown in the drawing, in the step S10, it is preferable to adjust the intensity of the pulsed laser beam according to the thickness of the measurement object so that ultrasonic waves are generated by the effect of the surface material of the measurement object 90, The laser beam preferably has a pulse width of several nanoseconds to several tens of nanoseconds. Subsequently, when the ultrasonic waves are generated, the laser beam for measuring the generated ultrasonic waves is irradiated to the same point as the ultrasonic generation point of the measurement object 90 (S12), and the laser beam due to the ultrasonic waves generated on the measurement object 90. Scattered light is collected in a substantially vertical position (S14). The collected scattered light is preferably generated using a confocal Fabry-Perot interferometer (S16), and outputs the transmitted and reflected interference light from the confocal Fabry-Perot interferometer (S18). The ultrasound signal generated in the measurement object is detected using the transmitted and reflected interference light (S20). The ultrasonic attenuation coefficient for each frequency is calculated from the detected ultrasonic signal (S22), and the determined ultrasonic attenuation coefficient is substituted into the ultrasonic attenuation coefficient-crystal grain correlation (S24) to calculate the crystal grain size of the measurement object ( S26).

Here, although not shown in the drawing, the crystal grain size measuring method according to the present invention is preferably included in the collected scattered light, the plasma radiation generated when generating ultrasound on the surface of the measurement object is incident to the laser-interferometer Block it. In more detail, the ultrasonic generation laser outputs an excitation signal at the moment of oscillating the laser beam, and when the excitation signal is received, controls the operation of the switch to be described later for a set time. The transmission of the captured optical signal is interrupted for the predetermined time according to the control signal.

The step S20 will be described in more detail by converting the intensity of the reflected interference light and the transmitted interference light into an electrical signal, respectively, and first and second ultrasonic signals reaching the surface of the measurement object from the change of the electrical signal, respectively. Detect. Subsequently, the first and second ultrasonic signals are differentially amplified and filtered using the differentially amplified ultrasonic signals into ultrasonic signals of a specific frequency band.

FIG. 18 is a view illustrating ultrasonic signals that are continuously reflected from upper and lower surfaces of a measurement object according to an embodiment of the present invention, and FIG. 19 is propagated by multiple reflections inside the measurement object according to an embodiment of the present invention. 20 is a view illustrating a longitudinal wave, and FIG. 20 is a view illustrating a frequency spectrum of a continuously measured reflected ultrasound signal according to an embodiment of the present invention, and FIG. 21 is a grain size of a measurement object applied to an embodiment of the present invention. FIG. 22 is a flowchart illustrating a correlation between the ultrasonic attenuation coefficients and FIG. 22 is a graph showing the correlation between the crystal grain size and the ultrasonic attenuation coefficient according to an embodiment of the present invention.

)를 구한다. Hereinafter, referring to FIGS. 18 to 22, the process of calculating the crystal grain size using the ultrasonic signal efficiently measured by the operation of the present invention as described above will be described in more detail. Usually, the attenuation coefficient of the ultrasonic wave propagating inside the material depends on the grain size of the material. In addition, the correlation between the attenuation coefficient and the crystal grain size of the ultrasonic wave depends on the frequency of the ultrasonic wave. Therefore, the grain size (D) of the measurement target is usually measured as follows. In a first step, a continuous reflected ultrasound signal as shown in FIG. 19 is measured. In the ultrasonic signal waveform shown in FIG. 19, a series of consecutive ultrasonic signals, denoted by R _i (i = 1, 2, 3,...), Is shown in FIG. 18 as shown in FIG. 18. These signals are reflected at 91 and reach the measurement point continuously. In a second step, a Fourier transform (FFT) is performed on each of the consecutive reflected ultrasonic signals as shown in FIG. 19, thereby performing a frequency spectrum of the continuous reflected ultrasonic signals as shown in FIG. Get) In the third step, the frequency spectrum of the continuous reflected ultrasonic signal of FIG.

)

[Equation 2]

는 초음파 주파수

에 대한 i번째 반사파(Ri)의 초음파 세기를 나타낸다. 마지막 단계로 상기 절차에 의해 구한 주파수별 초음파 감쇠계수(

)를 분석하여 측정 대상체의 결정입경을 산출한다. Where l is the thickness of the specimen,

Ultrasonic frequency

It represents the ultrasonic intensity of the i-th reflected wave Ri with respect to. As a final step, the ultrasonic attenuation coefficient for each frequency

) To calculate the grain size of the measurement object.

However, as described above, it is very time consuming to calculate the information on the crystal grain size by analyzing the FFT of the continuous reflection ultrasonic signals and analyzing the ultrasonic spectrum by frequency obtained through the FFT, and usually requires a complicated comparative analysis algorithm. It may not be suitable for on-line measurements in production lines where real-time grain size is required.

)를 미리 선정한 후 이 주파수(

)의 감쇠계수(

)만을 이용하여 빠르게 결정입경을 산출하는 방법을 제시한다. 즉, 도 21에 나타낸 바와 같이, 생산라인에서 생산 중에 있는 강판 또는 강재 중 결정입경이 품질에 중요하여 결정입경의 제어가 필요한 한 개 또는 다수의 측정대상을 선정하고, 측정실험을 통해 이들 측정대상 각각에 대해 결정입경과 특정 주파수(

)의 초음파 감쇠계수(

) 사이의 상관식을 미리 도출한 후 이를 온-라인 측정 시 결정입경의 산출에 이용하는 방법을 제시한다. 이와 같이 온-라인 측정의 수행 전에 실험적으로 미리 결정입경과 특정 주파수(

)의 초음파 감쇠계수(

) 사이의 상관식을 도출할 때 사용되는 측정장치의 구성은 도 1에 나타낸 바와 같은 측정장치와 동일하게 한다. 이는 펄스형 레이저 빔에 의해 발생된 초음파는 펄스 광의 반점크기(S₁)에 따라 달라지는 회절(diffraction)되는 정도가 다르기 때문이다. 도 16에 제시된 방법을 통해 선정된 최적 주파수(F = 7.8 MHz)와 결정입경 사이의 상관성을 실험적으로 얻은 실시예를 도 22에 도시하였다. 도 21에 제시된 절차 중 마지막 단계에서는 도 22에 나타낸 바와 같은 데이터 들의 상관성 분석을 통해 다음과 같은 식 3의 상관식을 산출한다.In order to solve such a problem, in the present invention, one specific frequency having the highest correlation with the grain size for a specific measurement object (

), Then select this frequency (

Damping coefficient of

We present a method to quickly calculate grain size using only). That is, as shown in FIG. 21, one or more measurement targets for which control of the crystal grain size is necessary because the crystal grain size is important for quality among the steel sheets or steels being produced in the production line, and these measurement targets For each, the grain size and the specific frequency (

Ultrasonic Attenuation Coefficient of

We propose a method of deriving the correlation between and before using it to calculate the grain size for on-line measurement. Thus, before the on-line measurement is performed, the experimental grain size and the specific frequency (

Ultrasonic Attenuation Coefficient of

The configuration of the measuring device used when deriving a correlation between the same) is the same as that of the measuring device shown in FIG. This is because the ultrasonic waves generated by the pulsed laser beam have different diffraction degrees depending on the spot size S ₁ of the pulsed light. FIG. 22 shows an example in which the correlation between the optimal frequency selected by the method shown in FIG. 16 (F = 7.8 MHz) and the crystal grains was experimentally obtained. In the final step of the procedure shown in Figure 21 through the correlation analysis of the data as shown in Figure 22 to calculate the correlation of the following equation (3).

[Equation 3]

는 측정대상 M_i의 결정입경이다. 특정 측정 대상체에 대해 이와 같이 얻은 상관식을 이용하면, 온-라인 상에서 특정 측정 대상체에 대해 특정 초음파 주파수의 감쇠계수를 측정하면 이 측정 대상체의 결정입경이 자동적으로 산출될 수 있다. In expression 3

Is the grain size of the measurement object M _i . Using the correlation obtained as described above for a specific measurement object, the crystal grain size of the measurement object may be automatically calculated by measuring the attenuation coefficient of the specific ultrasound frequency for the specific measurement object on-line.

) 스펙트럼을 산출하고(S36), 상기 식1을 이용하여 상기 초음파 주파수(

)별로 감쇠계수(

)를 산출 한다(S38). 이어 상기 측정 시편의 주파수(

)를 감쇠계수(

)와 결정입경 간의 상관성을 분석한다(S40). 한편, 상기 측정된 반사 초음파 신호들은 신호처리수단(80)으로 입력되고 주파수 필터(bandpass filter)(74)는 특정(최적) 주파수(

)를 통과시키도록 조절되어 결정입경 도출을 위한 최적 초음파 주파수가 선정된다(S42). 이어서 측정대상에 대하여 초음파 주파수의 초음파 감쇠계수와 결정입경 사이의 상관식을 도출한다(S44). 이로써 상기 신호처리수단(80)의 프로세서(processor)에 미리 입력되어 있는 식 3와 같은 상관식에 감쇠계수를 대입하여 결정입경을 산출하는 것이다. 이와 같이 본 발명에 의해 고안된 결정입경 산출방법을 적용하면 생산라인에서 측정대상의 결정입경을 매우 간단한 신호처리 절차에 의해 빠르게 측정할 수 있는 것이다.Hereinafter, a method of calculating the crystal grain size of the measurement object 90 in real time during on-line measurement will be described using a correlation between the crystal grain diameter and the ultrasonic attenuation coefficient of a specific (optimal) frequency. First, the measurement object 90 being produced in the production line is selected (S30). Prepare a measurement object specimen having a variety of grain size (S32). The reflected ultrasonic signals (R ₁ , R ₂ , R _3, etc. of FIG. 19) are continuously measured for the plurality of specimens by the above-described technical procedure (S34). By performing FFT on the measured reflected ultrasonic signals, ultrasonic frequencies of the reflected ultrasonic signals (

) A spectrum (S36), and the ultrasonic frequency (

Attenuation coefficient ()

) Is calculated (S38). Then the frequency of the test specimen (

) Is the damping coefficient (

) And the correlation between the grain size (S40). On the other hand, the measured reflected ultrasonic signals are input to the signal processing means 80 and the frequency filter (bandpass filter) 74 is a specific (optimal) frequency (

(S42) is adjusted so as to pass through) is optimized for the determination of the grain size. Subsequently, a correlation between the ultrasonic attenuation coefficient of the ultrasonic frequency and the crystal grain size is derived for the measurement target (S44). As a result, the crystal grain size is calculated by substituting the attenuation coefficient into the correlation equation (3) previously inputted into the processor of the signal processing means 80. In this way, if the method for calculating the grain size devised by the present invention is applied, the grain size of the object to be measured in the production line can be quickly measured by a very simple signal processing procedure.

23 is a flowchart showing an on-line laser beam collecting method in scattered light collecting means according to an embodiment of the present invention. 4 and 23, the beam parallelizer 32 receives the laser beam 27 for ultrasonic measurement transmitted through the optical fiber 25 (S50), and receives the received laser beam 27 for ultrasonic measurement. Is converted into parallel light (S52). Subsequently, the ultrasonic measuring laser beam 27 in which the polarization type light shimmer 33 is converted into the parallel light is incident on the upper surface 91 of the object 90 substantially perpendicularly. ) Is reflected (or transmitted) (S54). In the step (S54), the reflectance (or transmittance) of the polarization type light filter 33 according to the exemplary embodiment of the present invention is preferably 100% (or transmittance is 0%), but such reflectance (or transmittance) is applied. It can be changed optimally according to the conditions of the space, characteristics and other parts of the beam to be. In addition, the reflection or transmission of the laser beam for ultrasonic measurement in the step (S14) is determined according to the position of the beam parallelizer 32 and the condenser lens 38 to be described later. The reflection or transmission process of the polarization type light shimmer 33 according to the position of the beam parallelizer 32 and the condenser lens 38 has been described in the scattered light collecting means 30 shown in FIG. Omit.

Subsequently, in step S54, the laser beam 27 for ultrasonic measurement, which is reflected (or transmitted) by the polarization type light filter 33 and is incident perpendicularly to the surface 91 of the object 90, may be used. The polarization direction is rotated 45 degrees (S56). The scattered light 28 of the ultrasonic measuring laser beam 27 due to the ultrasonic wave generated by the ultrasonic generating laser beam 13 incident perpendicularly to the surface 91 of the object 90 is received (S58). ). The received scattered light 28 is focused (S60), and the focused scattered light 28 is converted into parallel light whose diameter is reduced to a specific size (S62). The polarization direction of the scattered light converted into the parallel light is further rotated by 45 degrees and output to the polarized light shimmer 33 (S64). The scattered light 28 transmitted (or reflected) through the polarization type light shimmer 33 is collected on the incident surface of the optical fiber 31 (S66). As a result, the scattered light 28 is transmitted to the laser interferometer 60 through the optical fiber 31 and used for ultrasonic measurement.

24 is a flowchart illustrating a method of stabilizing a laser interferometer for ultrasonic measurement according to an embodiment of the present invention. First, as shown in FIGS. 11 to 13, two resonator mirrors 62 and 63 are positioned to face each other at regular intervals and reciprocate the incident laser beam therein to output the interference light 150. In the laser interferometer for ultrasonic measurement including a resonator 66 having a detected intensity of the interference light signal 150 by the laser beam incident on the resonator 66 (S70). The intensity of the detected interference light signal is compared with the allowable range of the set optical signal intensity (S72), and when the intensity of the detected interference light signal is out of the allowable range of the set optical signal intensity (S74), the two resonator mirrors A control signal for adjusting the interval of (62, 63) is output (S76). Subsequently, the distance between the two resonator mirrors 62 and 63 is adjusted according to the control signal (S78) to adjust the intensity of the interference light signal (S80). If the result of the comparison in step S74 is that the detected intensity of the interference light signal does not exceed the allowable range of the set optical signal intensity, the process returns to step S70 to continuously detect the intensity of the interference light 150 signal. (S70).

When the distance between the two resonator mirrors 62 and 63 in the resonator 66 of the laser interferometer is changed, the intensity of the interference light 150 signal output from the laser interferometer is changed. In the stabilization method, a change in the intensity of the signal of the interference light 150 according to a change in the distance between the two resonator mirrors 62 and 63 is detected, and the distance between the two resonator mirrors 62 and 63 is driven by driving the piezoelectric actuator 44. By adjusting, the distance between the two resonator mirrors 62, 63 is always kept constant. As a result, stabilization of the laser interferometer is realized by using another interfering light 110 for stabilizing a separate laser interferometer in addition to the interfering light 69 for ultrasonic measurement.

The above description and the contents of the drawings are limited to one embodiment of the present invention, and thus the present invention is not limited thereto. It will be possible to substitute, change or delete the component according to the present invention within the scope of the technical idea of the present invention.

Accordingly, the scope of the present invention should be determined by the appended claims rather than by the foregoing description and drawings.

According to the present invention, the generation and measurement of the ultrasonic waves can be made remotely so that the on-line ultrasonic flaw detection of the measurement object being transferred in the production line is possible.

Further, according to the present invention, the generation and measurement of ultrasonic waves are performed on the same surface and the same point of the surface to be measured so that the measuring device is present only on the upper part of the steel sheet being produced, thereby easily installing the measuring system and method of the present invention on the production line. And it can be applied, by generating a high-intensity ultrasonic wave by the effect of the ablation (ablation) has the effect of enabling the determination of the crystal grain size even for a thick measurement target.

Further, according to the present invention, the ultrasonic energy loss due to the mode conversion generated when the ultrasonic wave (long wave) is reflected from the surface of the measurement object by performing the generation and measurement of the ultrasonic wave at the same plane and the same point of the measurement object. It is effective in preventing and facilitating the calculation of the ultrasonic propagation distance, which in turn facilitates the calculation of the ultrasonic attenuation coefficient.

In addition, according to the present invention, by reducing the effect of the existing plasma in the ultrasonic detector for measuring the high-intensity ultrasound generated by the fusion effect, there is an advantage to enable more accurate measurement of the attenuation coefficient through the efficient ultrasonic measurement .

In addition, according to the present invention, in real-time on-line grain size measurement is performed by preselecting one specific frequency having the highest correlation with the grain size for a specific measurement object and quickly calculating grain size using only the attenuation coefficient of the frequency. There is an advantage to facilitate.

Further, according to the present invention, through the configuration of a measuring device using an efficient confocal Fabry-Perot interferometer, firstly, it does not cause a loss in the output light of the interferometer used for ultrasonic measurement, and secondly, It is possible to stabilize the interferometer stably at all times regardless of the intensity change, and thirdly, the two kinds of output light of the interferometer, that is, the reflected interference light and the transmitted interference light, are measured by the two photodetectors. There is an advantage to increase.

Claims (26)

In the on-line grain size measurement system using laser-ultrasound, Ultrasonic wave generation laser for irradiating a non-contact type pulsed laser beam on the upper surface of the measurement object to generate an ultrasonic wave inside the measurement object; Ultrasonic measurement laser for outputting a laser beam for measuring the ultrasound generated in the measurement object; The laser beam output from the ultrasonic measuring laser is irradiated non-contactly to the same point where the ultrasonic wave is generated by the pulsed laser beam irradiated from the ultrasonic wave generating laser, and is reflected from the lower surface of the object to generate the ultrasonic wave on the upper surface. Scattered light collecting means for collecting the laser scattered light by the ultrasonic waves returned to the; A laser-interferometer which reciprocates the collected scattered light internally to generate interference and outputs transmission interference light and reflection interference light of opposite phases due to the ultrasonic waves; Ultrasonic signal detection means for converting the intensity of the two interference light output from the laser interferometer into an electrical signal and detecting an ultrasonic signal using the converted electrical signal; And Signal processing means for calculating a crystal grain size of the measurement object by calculating an ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal and applying the calculated attenuation coefficient to a predetermined crystal grain size-ultrasonic attenuation coefficient correlation; Including, A laser beam, characterized in that the diameter of the circular spot of the beam irradiated to the surface of the measurement object in the ultrasonic measuring laser is smaller than the diameter of the circular spot of the beam irradiated to the same point of the surface in the laser for ultrasound generation -On-line grain size measurement system using ultrasonic waves. delete The method of claim 1, The laser beam transmitted from the ultrasonic measuring laser to the scattered light collecting means and the laser scattered light transmitted from the scattered light collecting means to the laser interferometer are respectively transmitted through the multimode optical fibers 33 and 41. -On-line grain size measurement system using ultrasonic waves. The method of claim 1, And the laser interferometer is a confocal Fabry-Perot interferometer. The method according to claim 1 or 4, wherein the laser interferometer, And a wavelength shift of the scattered light due to the generated ultrasonic waves to a change in intensity of transmitted and reflected interference light therein. The method of claim 1, wherein the ultrasonic signal detection means, A first photodetector (99) for converting the intensity of reflected interference light output from the laser interferometer into an electrical signal and detecting a first ultrasonic signal reaching the surface of the measurement object from the change of the electrical signal; A second photodetector (100) for converting the intensity of transmitted interference light output from the laser-interferometer into an electrical signal and detecting a second ultrasonic signal reaching the surface of the measurement object from the change of the electrical signal; A differential amplifier 108 for differentially amplifying the first and second ultrasonic signals; And A band pass filter 109 which filters the ultrasonic signals of a specific frequency band using the differentially amplified ultrasonic signals and outputs the ultrasonic signals to the signal processing means; On-line grain size measurement system using a laser-ultrasonic, comprising a. The method of claim 1, wherein the laser interferometer, A collimator 91 which receives the laser scattered light transmitted from the scattered light collecting means and converts the scattered light into parallel light; The polarized laser beams are installed to face each other on an optical path through which the polarized laser beams travel, thereby reflecting the advanced laser beams back and forth with a specific reflectance to each other in a reciprocating manner to generate interference, and transmitting the remaining light to reflect reflected and transmitted interference light, respectively. Outputting first and second resonator mirrors 94 and 95; The polarized laser beam is transmitted to the first and second resonator mirrors 94 and 95 on the optical path through which the polarized laser beam travels, and the reflected interference light reflected from the resonator mirrors 94 and 95 is ultrasonic. A polarized light filter 92 for reflecting the signal detection means; And Converts the polarized laser beam transmitted through the polarization type light filter 92 into a circularly polarized laser beam, and converts the circularly polarized reflected interference light reflected from the resonator mirrors 94 and 95 into linearly polarized interference light. A first quarter wave plate 93 which changes and consequently rotates the polarization direction by 90 degrees; Additionally contains The polarization type light filter 92, the first quarter wave plate 93, the first resonator mirror 94, the second resonator mirror 95 are laser-ultrasound, characterized in that arranged in the same line sequentially On-line grain size measurement system. The method of claim 1, Further comprising a plasma radiation blocking means for blocking the plasma radiation generated in the generation of ultrasonic waves on the surface of the measurement object, which is included in the scattered light transmitted from the scattered light collecting means to the laser-interferometer On-line grain size measurement system using a laser-ultrasonic. The method of claim 8, wherein the plasma radiation blocking means, Excitation signal applying means (116) for outputting an excitation signal at the moment of oscillating the laser beam in the ultrasonic wave generating laser; Switch control means (111) for controlling the operation of the switch to be described later for a set time when the excitation signal is received; And A switch 110 which blocks an optical signal transmitted from the scattered light collecting means to the laser interferometer during the set time according to the control signal of the switch control means 111; On-line grain size measurement system using a laser-ultrasonic, comprising a. The method of claim 9, wherein the switchgear, A filter (112) for passing only light having the same wavelength as the laser beam for ultrasonic measurement among the optical signals transmitted from the scattered light collecting means to the laser interferometer; A first linear polarizer 113 for converting light in a plurality of polarization directions passing through the filter 112 into linearly polarized light in a single direction; A single crystal 114 for rotating the polarization direction of the linearly polarized light according to the control signal of the switch control means 111; And A second linear flattener 115 disposed perpendicular to the polarization direction of the first linear polarizer; Including; On-line crystal grain size measurement system using laser-ultrasonic wave, characterized in that the filter 112, the first linear flatr 113, the single crystal 114 and the second linear polarizer 115 are sequentially arranged on the same line. . In the on-line grain size measuring method using a laser-ultrasound, A first step of irradiating a non-contact type pulsed laser beam on an upper surface of the measurement object to generate ultrasonic waves inside the measurement object; Irradiating a laser beam for measuring ultrasonic waves generated on the measurement object to the same point where ultrasonic waves are generated by the irradiated pulsed laser beam; A third step of collecting the laser scattered light by the ultrasonic waves generated on the upper surface of the measurement object and reflected on the lower surface and returned to an ultrasonic wave generation point of the upper surface; A fourth step of generating interference by reciprocating the collected scattered light internally and outputting transmission interference light and reflection interference light of opposite phases due to ultrasonic waves generated in the measurement object; A fifth step of converting the intensities of the two interfering light outputs into electrical signals and detecting an ultrasonic signal by using the converted electrical signals; A sixth step of calculating an ultrasonic attenuation coefficient for each frequency of the detected ultrasonic signal; And A seventh step of calculating the crystal grain size of the measurement object by applying the calculated attenuation coefficient to a predetermined crystal grain size-ultrasonic attenuation coefficient correlation equation; Including, Using a laser-ultrasonic wave, characterized in that the diameter of the circular spot of the laser beam for ultrasonic measurement irradiated on the surface of the measurement object is smaller than the diameter of the circular spot of the laser beam for ultrasound generation irradiated at the same point of the surface On-line grain size measurement method. The method of claim 11, wherein the fifth step, Converting the intensity of the reflected interference light into an electrical signal and detecting a first ultrasonic signal reaching the surface of the measurement object from the change of the electrical signal; Converting the intensity of the transmitted interference light into an electrical signal and detecting a second ultrasonic signal reaching the surface of the measurement object from the change of the electrical signal; And Differentially amplifying the first and second ultrasonic signals and filtering the ultrasonic signals of a specific frequency band using the differentially amplified ultrasonic signals; On-line crystal grain size measuring method using a laser-ultrasonic wave comprising a. The method of claim 11, And an eighth step included in the collected scattered light, to block the incident plasma radiation generated on the surface of the measurement object from being incident on the laser-interferometer. On-line grain size measurement method. The method of claim 13, wherein the eighth step is Outputting an excitation signal at the moment of oscillating a laser beam in the ultrasonic wave generating laser; Controlling an operation of a switch to be described later for a set time when the excitation signal is received; And Blocking transmission of the captured optical signal during the predetermined time according to the control signal; On-line crystal grain size measuring method using a laser-ultrasonic wave comprising a. delete delete delete delete delete delete delete delete delete delete delete delete

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