KR100676629B1 - Adaptive signal processing apparatus and method in Laser Ultrasonic System - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an adaptive acquisition device for an interferometer signal and a method for acquiring the interferometer signal in a laser ultrasonic inspection device. A device for receiving a stable signal irrespective of the surface state of a measurement object by developing a device that receives a pulse at an optimized position at the moment of exciting the ultrasonic wave and corrects a sensitivity of an object of the received signal; The measurement method is provided. In the present invention, the device for generating a synchronization signal by grasping the magnitude of the laser ultrasound reception signal according to the surface state of the object in the process of constantly changing the interferometer signal, and adaptively determine the optimal point according to the present invention; The present invention provides a signal processing device and a method for correcting the magnitude and smallness of a signal according to a surface state.

Confocal Fabry-Perot Interferometer, Ultrasonic Excitation Pulse Laser, Continuous Oscillation Laser, PZT, Ultrasound

Description

Adaptive signal acquisition apparatus and method for acquiring the same in a laser ultrasonic inspection apparatus             

1 is an ultrasonic measurement principle using a confocal Fabry-Perot interferometer.

Fig. 2 shows the interferometer transmittance when the PZT voltage of the interferometer is changed into a triangular waveform,

3 is a pulse laser synchronization and signal correction method for adaptive signal acquisition;

4 is a schematic diagram of a laser ultrasonic apparatus using a confocal Fabry-Perot interferometer.

<Description of the symbols for the main parts of the drawings>

21: pulsed laser for ultrasonic excitation 22: shutter

(23): neutral concentration filter (24, 25, 33): mirror

(26, 37, 42, 44): Focusing lens 31: Continuous oscillation laser device

(32): polarizer 34: half-wave plate

(35, 39): polarizing beam filter (36, 41): 1/4 wave plate

(40, 47): high speed optical sensor (43): confocal Fabry-Perot interferometer

(45): PZT (46): relay lens

(48): PZT driver (49): signal acquisition controller

50: measurement object

The present invention relates to an adaptive signal acquisition device and a method for acquiring the same in a laser ultrasound inspection apparatus, and more particularly, to an apparatus and method for stably acquiring an interferometer signal among laser ultrasound inspection apparatuses using a confocal interferometer. The present invention relates to an apparatus and method for acquiring an interferometer signal by generating an ultrasonic wave at an optimum moment in a device that constantly changes a signal, and correcting an element for a surface state of an object to catch a stable signal irrespective of the surface state.

The laser ultrasonic inspection apparatus is a non-contact inspection apparatus that acquires an ultrasonic signal excited by a laser using a laser interferometer.

The laser excitation ultrasound signal generated by the laser ultrasound inspection apparatus is received using a laser interferometer device. The signal can be accurately captured only when the interferometer is in an optimal condition at the moment of generating the ultrasound.

To this end, most interferometers use a device that actively maintains an interferometer state, or a method of acquiring a laser ultrasonic signal at a specific point while constantly changing the signal of the interferometer.

However, even in this case, there remains a problem that the signal received from the interferometer is not the optimal point or the signal strength varies depending on the surface state of the object.

Looking at the prior art in this field in more detail as follows.

The laser ultrasonic device generates an ultrasonic signal by irradiating a strong pulsed laser beam to the surface of the object, and after measuring or transmitting the ultrasonic signal using a laser interferometer after measuring or propagating the ultrasonic signal from the object, the defect or characteristic of the object is measured. Device.

Laser ultrasonic devices have been studied in recent years because they have the advantage of generating / detecting ultrasonic waves in a non-contact manner as compared to conventional ultrasonic transducers using contact transducers. .

In general, laser ultrasonic devices use an interferometer to measure microdisplacements produced by ultrasound, the most common of which is the Confocal Fabry-Pero Interferometer (see Q. Shan, AS Bradford). , RJ Dewhurst, "New field formulas for the Fabry-Perot interferometer and their application to ultrasound detection", measurement Science and Technology, Vol. 9, p. 24-37, 1998).

There are many patents for laser interferometer devices using such confocal interferometers, the representative ones of which are as follows.

US Pat. No. 4,659,224 (Optical interferometric reception of ultrasonic energy) relates to an ultrasonic signal measuring apparatus using a confocal Fabry-Perot interferometer and is an early patent for a general confocal interferometer device.

In addition, US patent 6,356,846 (System and method of reducing motion-induced noise in the optical detection of an ultrasound signal in a moving body of material) relates to the reduction of noise signals caused by movement in a scanning ultrasound laser ultrasonic apparatus. It is about how to synchronize the motion of an object.

In this way, the pulse ultrasonic signal generated by the pulse laser is important to the synchronization in the receiving device. Another patent US Patent 6,633,384 (Method and apparatus for ultrasonic laser testing) uses a pulsed laser as the receiving laser to accurately receive this synchronization signal. The method was described.

However, there is no patent for a device that adjusts the signal acquisition timing in consideration of the state of the object in the voltage sweep type laser ultrasound device and simultaneously compensates for the large and small signal.

An object of the present invention to solve the above problems is to continuously monitor the confocal Fabry-Perot interferometer signal that sweeps the PZT voltage of the interferometer at a constant rate, which is optimized at the moment the pulsed laser excites the ultrasonic waves. The present invention provides a device and a method of measuring the same to obtain a stable signal by developing a device for receiving at a position and correcting a sensitivity of an object of a received signal.

The present invention, which achieves the above object and accomplishes the problem of eliminating conventional defects, continuously measures confocal Fabry-Perot interferometer signals that sweep the PZT voltage of the interferometer at a constant rate and then pulses. An apparatus for receiving at an optimized position at the moment of laser excitation and correcting the change of the received signal on the surface of the object to maintain a constant ultrasonic sensitivity, and a measuring method using the same.

Specifically, the present invention provides a laser ultrasound apparatus for generating an ultrasonic wave to a measurement object with a pulse laser, and then detecting the thickness or defect of the object by detecting a confocal interferometer using a continuous oscillation laser after propagating the measurement object. In

Ultrasonic excitation pulse laser 21 for irradiating a laser beam to one side S of the measurement object 50, a shutter 22 for blocking or passing the beam generated therefrom, and a laser beam passing through the shutter Neutral density filter 23 for adjusting the intensity of the light, a mirror 24 and 25 for converting the incident angle of the laser beam passed through the neutral density filter, and a focusing lens 26 for focusing the laser beam past the mirror. Ultrasonic excitation device using a pulsed laser consisting of;

It consists of an interferometer which is excited by the focused laser beam and receives the ultrasonic wave propagated at the other position (T) along the surface of the measurement object,

The interferometer includes a continuous oscillation laser device 31 for irradiating a laser, a polarizer 32 for linearly polarizing the laser beam, a mirror 33 for reflecting the linearly polarized beam, a half wave plate 34 for rotating polarization, A polarization beam splitter 35 for reflecting the beam passing through the half-wave plate, a quarter wave plate 36 for changing the polarization state of the beam reflected by the beam splitter, and a beam passing through the quarter wave plate The focusing lens 37 incident on the other side position T of the measurement object 50 and the beam reflected from the object pass through the focusing lens 37 and the quarter wave plate 36 again, and the polarization is rotated by 90 degrees. The polarizing beam filter 39 incident through the beam splitter 35, the quarter wave plate 41 for circularly polarizing the beam passing through the polarizing beam filter, and the quarter wavelength plate ( A focusing lens 42 for focusing the circularly polarized beam via 41, a confocal Fabry-Perot interferometer 43 for receiving the focused beam from the focusing lens 42, and this confocal interference PZT 45 for adjusting the distance between the mirrors at both ends of the mirror, PZT driver 48 for emitting a triangular PZT voltage signal at a specific period connected thereto, and a focusing lens 44 for focusing a beam passing through a confocal interferometer. A relay lens 46 for relaying the beam, a high speed optical sensor 47 for receiving a beam passing through the relay lens, a signal of the beam received by the high speed optical sensor 47, and a beam reflected by a confocal interferometer The polarization is rotated 90 degrees through the focusing lens 42 and the quarter wave plate 41 and reflected by the polarization beam filter 39 to obtain a signal of a beam received by another high speed optical sensor 40. Characterized in that the signal acquisition control unit 49 for measuring the defect, the thickness and the like.

In addition, the method of the present invention in the ultrasonic wave generated by excitation with a pulsed laser propagates the object and then detected by a laser interferometer to check the state of the thickness, defects, etc. of the object,

Irradiating a continuous oscillation laser on the surface of the object after having the device described above and then injecting the reflected laser beam into a confocal interferometer;

When the ultrasonic signal generated by the pulse laser causes a small displacement on the surface of the object, the Doppler frequency shift occurs in the continuous oscillation laser beam incident on the interferometer, changing the transmittance of the confocal interferometer, and the intensity of the beam passing through the interferometer. An interferometer measuring step of measuring an ultrasonic signal by measuring that the modulated signal is substantially proportional to the frequency shift;

In the interferometer measurement step, the PZT driver finely adjusts the distance between the mirrors of the confocal interferometer to change the intensity of the continuous oscillation laser beam passing through the interferometer at a constant cycle by applying a triangular wave signal to the interferometer at a constant cycle, and at a specific position Generating a signal to obtain an ultrasonic signal.

In addition, the specific position at the moment when the optical sensor signal is half of the maximum value, including the step of generating a pulse laser at this time characterized in that it is possible to obtain an ultrasonic signal at a position having the maximum sensitivity at all times do.

In addition, after recording the maximum value of the signal of the optical sensor characterized in that it comprises the step of dividing the measured ultrasonic signal by the maximum value signal and measuring and correcting the magnitude of the signal according to the surface state of the object.

Hereinafter, the configuration and the operation of the embodiment of the present invention will be described in detail with reference to the accompanying drawings.

1 is a view illustrating a principle of obtaining an ultrasonic signal in a confocal Fabry-Perot interferometer, where the horizontal axis represents the frequency of the laser beam and the vertical axis represents the transmittance of the interferometer. The point showing the maximum transmittance is A, and the point showing half the transmittance is B.

When the distance between two mirrors of the interferometer is finely adjusted with PZT to set the transmittance to B, if the frequency of the incident laser beam changes slightly (indicated by input in the figure), the laser beam in the interferometer is shown in the figure. The transmittance of V varies greatly with the output.

The frequency of the laser beam reflected from the object is a Doppler movement according to the surface displacement. The frequency shift is made by changing the transmittance as shown in the figure to detect the ultrasonic signal.

If the PZT is installed on the mirrors at both ends of the confocal Fabry-Perot interferometer to change the distance between the mirrors in the form of a triangular wave, the transmittance of the interferometer will also change as shown in FIG. As illustrated in FIG. 1, the measurement sensitivity is the best when the frequency change of the laser beam is measured at the position C or D where the slope of transmittance is largest. Therefore, most ultrasonic measuring methods measure ultrasonic waves by synchronizing C or D points, which have a maximum transmittance of about half, with a pulsed laser.

The problem of the ultrasonic measuring device of this method is that the ultrasonic measuring device continuously measures a constant signal as shown in FIG. 2 according to the surface state of the object when measuring while moving the object, that is, scanning while scanning in one or two dimensions. It can't be done.

That is, as shown in FIG. 3, the ultrasonic signal is large in a place where the surface of the object is good (signal 1), and when the surface state of the object is bad, the intensity of the signal decreases (coral 2).

Most ultrasonic scan inspection apparatuses inspect the defects or characteristics of an object with the magnitude of the signal, and thus the reliability of the inspection is deteriorated when the magnitude of the signal is in and out.

The present invention aims to solve the irregularity of the signal according to the surface state (or alignment state) of such an object, and the specific method is as follows.

As shown in Fig. 3, while sweeping the voltage of the PZT that finely controls the length of the interferometer in a triangular waveform, the intensity of the optical signal is continuously measured and the maximum value (A in Fig. 3) is recorded and half of this maximum value is recorded. When the ultrasonic wave is measured at this time by giving the pulse laser generation signal at the place (F or I in FIG. 3), the ultrasonic signal can be measured at the place where it is always half of the maximum value.

That is, even when the signal strength is small as in position 2 of FIG. 3, the pulse laser signal may be synchronized to a point at half of the maximum value to receive the signal at the optimal position (I in FIG. 3).

In addition, when the same ultrasonic signal is received, a strong ultrasonic signal is received at the position signal 1 having a good surface state, and a small signal is received at the position signal 2 having a bad surface state. However, by using the method of continuously measuring and recording the maximum signal, the signal can be normalized by dividing the measured signal by the magnitude (max1 and max2) of these maximum received signals after the measurement. In other words, it is possible to correct the size difference of the ultrasonic signal caused by the different surface conditions.

Looking at the preferred embodiment according to the present invention.

The following description is of an example of a laser ultrasonic surface defect inspection apparatus using a very common confocal Fabry-Perot interferometer, but also for other uses such as longitudinal wave characteristic inspection or thickness measurement, and the same for other types of laser ultrasonic inspection apparatuses. The principle can be applied.

The configuration of the laser ultrasonic apparatus of the adaptive signal processing method according to the present invention is as shown in FIG. 4.

The beam irradiated from the ultrasonic excitation pulse laser 21 includes a shutter 22 for blocking or passing the beam and a neutral density filter 23 for adjusting the intensity of the laser beam passing through the shutter. After roughing, the light is focused through the mirrors 24 and 25 to the focusing lens 26 to be incident to the S position of the measurement object 50.

In the measurement object, ultrasonic waves are excited by the laser beam, and the ultrasonic waves propagate along the surface. Ultrasound is received by the confocal interferometer device at the location of T.

The interferometer device for laser ultrasound reception operates as follows.

The laser beam emitted from the continuous oscillation laser device 31 is linearly polarized through the polarizer 32, then passes through the mirror 33 and the half wavelength plate 34, and is reflected by the polarization beam splitter 35, thereby reflecting the quarter wave plate ( After passing through 36, the light is focused at the focusing lens 37 to be incident on the T position of the measurement object 50.

The beam reflected from the measurement object passes through the focusing lens 37 and the quarter wave plate 36 again, and the polarization is rotated by 90 degrees to pass through the beam splitter 35 to pass another polarized beam splitter 39. After passing through the quarter wave plate 41 and circularly polarized, the light is focused by the focusing lens 42 to enter the confocal Fabry-Perot interferometer 43.

The distance between both mirrors of this confocal interferometer is precisely adjusted using PZT 45 and PZT driver 48.

The beam transmitted through the confocal interferometer is focused by the focusing lens 44 and received by the high speed optical sensor 47 via the relay lens 46.

The beam reflected from the confocal interferometer is again rotated by 90 degrees through the focusing lens 42 and the quarter wave plate 41, and thus is reflected by the polarizing beam splitter 39 so that another high speed optical sensor 40 is provided. Is received.

The signals of the two high speed optical sensors 47 and 40 are processed by the signal acquisition control unit 49 to measure defects or thicknesses.

The intensity of the laser beam from the continuous oscillation laser device 31 is monitored by another optical sensor 38.

At this time, the PZT driver 48 emits a triangular PZT voltage signal as shown in Figs. The signal detected by the optical sensor 47 then continues to change as shown in FIGS. 2 and 3.

If the surface state of the object is uniform, a signal having a constant size appears continuously as shown in FIG. 2, but there is no problem in measurement. In general, a signal having a different size continues to appear as shown in FIG.

In this case, if the pulse laser is generated at the moment that is half of the maximum value of the optical sensor signal, the ultrasonic signal can always be obtained at the optimum position.

In addition, since the maximum value of the signal of each optical sensor can be recorded, measuring the large and small of the signal according to the surface state of the object, and then correcting it using the maximum value at this time, the frequency shift signal (ultrasound signal) of the same sensitivity is obtained. You can get it.

The present invention exemplifies a general laser excited surface ultrasonic inspection apparatus, but the same principle can be applied to laser ultrasonic inspection apparatuses for other uses such as property inspection and thickness measurement. That is, the ultrasonic detection apparatus using almost all laser interferometers can be applied as a method of compensating the instability of the signal according to the surface state.

The present invention is not limited to the above-described specific preferred embodiments, and various modifications can be made by any person having ordinary skill in the art without departing from the gist of the present invention claimed in the claims. Of course, such changes will fall within the scope of the claims.

In the present invention as described above, in the process of constantly changing the interferometer signal, the characteristics of the laser ultrasound received signal according to the surface state of the object, etc. are determined and the optimal point is adaptively determined accordingly to generate a synchronization signal. By providing a signal processing device that corrects the magnitude and the smallness of the signal according to the surface condition, and the laser ultrasound inspection device can identify the characteristics of the laser reception signal according to the surface condition of the object in real time and specify the optimum reception time for each pulse. And it is a useful invention that has the advantage of being able to correct the size of the ultrasonic signal according to the surface state is an invention that is expected to use greatly in the industry.

Claims (4)

In the laser ultrasonic apparatus for generating an ultrasonic wave to the measurement object with a pulse laser, the ultrasonic wave propagates the measurement object and then detected by a confocal interferometer using a continuous oscillation laser to inspect the defect or state of the object, Ultrasonic excitation pulse laser 21 for irradiating a laser beam to one side S of the measurement object 50, a shutter 22 for blocking or passing the beam generated therefrom, and a laser beam passing through the shutter Neutral density filter 23 for adjusting the intensity of the light, a mirror 24 and 25 for converting the incident angle of the laser beam passed through the neutral density filter, and a focusing lens 26 for focusing the laser beam past the mirror. Ultrasonic excitation device using a pulse laser consisting of; It consists of an interferometer which is excited by the focused laser beam and receives the ultrasonic wave propagated at the other position (T) along the surface of the measurement object, The interferometer includes a continuous oscillation laser device 31 for irradiating a laser, a polarizer 32 for linearly reflecting the laser beam, a mirror 33 for reflecting the linearly polarized beam, and a half-wave plate 34 for rotating polarization; A polarization beam splitter 35 for reflecting the beam passing through the half-wave plate, a quarter wave plate 36 for changing the polarization state of the beam reflected by the beam splitter, and a beam passing through the quarter wave plate Is focused on the other side position T of the measurement object 50 and the beam reflected from the object passes through the focusing lens 37 and the quarter wave plate 36 again and the polarization is 90 degrees. A polarized beam splitter 39 that rotates and enters through the beam splitter 35, a quarter wave plate 41 for circularly polarizing the beam that has passed through the polarized beam splitter, and the quarter wavelength plate A focusing lens 42 for focusing the circularly polarized beam via (41), a confocal Fabry-Perot interferometer 43 for receiving the focused beam from the focusing lens 42, and this confocal interference PZT 45 for adjusting the distance between the mirrors at both ends of the mirror, PZT driver 48 for emitting a triangular PZT voltage signal at a specific period connected thereto, and a focusing lens 44 for focusing a beam passing through a confocal interferometer. A relay lens 46 for relaying the beam, a high speed optical sensor 47 for receiving a beam passing through the relay lens, a signal of the beam received by the high speed optical sensor 47, and a beam reflected by a confocal interferometer The polarization is rotated 90 degrees through the focusing lens 42 and the quarter wave plate 41 and reflected by the polarization beam filter 39 to obtain a signal of a beam received by another high speed optical sensor 40. A signal acquisition device for an adaptive interferometer in a laser ultrasonic inspection device, characterized in that the signal acquisition control device (49) for measuring and processing defects and thickness. In the ultrasonic wave generated by excitation with a pulsed laser propagates the object and then detected by a laser interferometer to check the thickness of the object, such as defects, Irradiating a surface of an object with a continuous oscillation laser after the apparatus of claim 1 is incident and then injecting a reflected laser beam into a confocal interferometer; When the ultrasonic signal generated by the pulse laser causes a small displacement on the surface of the object, the Doppler frequency shift occurs in the continuous oscillation laser beam incident on the interferometer, changing the transmittance of the confocal interferometer, and the intensity of the beam passing through the interferometer is frequency. An interferometer measuring step of measuring an ultrasonic signal by measuring the modulation in proportion to the movement; In the interferometer measurement step, the PZT driver finely adjusts the distance between the mirrors of the confocal interferometer to change the intensity of the continuous oscillation laser beam passing through the interferometer at a constant cycle by applying a triangular wave signal to the interferometer at a constant cycle, and at a specific position A signal acquisition method of an adaptive interferometer in a laser ultrasound inspection apparatus, characterized in that the step of generating a signal to obtain an ultrasonic signal. The method of claim 2, Making the specific position at a moment when the optical sensor signal is half of the maximum value, and generating a pulse laser at this time so that an ultrasonic signal can be obtained at a position having the maximum sensitivity at all times. Signal Acquisition Method of Adaptive Interferometer in Laser Ultrasound Inspection System. The method of claim 2, And recording and measuring the maximum value of the signal of the optical sensor and dividing the measured ultrasonic signal into the maximum value signal and measuring and correcting the magnitude of the signal according to the surface state of the object. Signal Acquisition Method for Adaptive Interferometer

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