KR102272163B1 - Apparatus and method for measuring thickness of construction - Google Patents

Abstract

According to the present invention, a device for measuring the thickness of a structure comprises: an interdigital transducer (IDT) sensor which generates a predetermined wave so that a surface wave propagates along a structure; a scanning unit which scans vibrations generated in the structure; a signal collection unit measuring a vibration signal of the structure scanned by the scanning unit; and a signal processing unit calculating the thickness of the structure based on the vibration signal measured by the signal collection unit. More specifically, the frequency and wavelength of the surface wave generated by the IDT sensor is calculated based on wavenumber sensitivity and mode separation determined according to materials and the thickness of the structure.

Description

Apparatus and method for measuring the thickness of structures {APPARATUS AND METHOD FOR MEASURING THICKNESS OF CONSTRUCTION}

The present invention relates to an apparatus, method, and system for measuring the thickness of a structure and whether or not wall-thinning occurs using an IDT (Interdigital Transducer) sensor.

As structures installed in plants, including nuclear power plants, age, a thinning phenomenon occurs in which the thickness of the structures becomes thinner. The thinned structure cannot withstand the design pressure and may break and cause personal injury. Therefore, in order to prevent this, a device for measuring the thickness and defects of a structure in advance is called a structure thickness measuring device.

In general, a structure thickness measuring apparatus measures the thickness of a structure using a technique using ultrasonic waves, vibration signals, and pulses. Specifically, the apparatus for measuring the thickness of the structure transmits an ultrasonic wave, a vibration signal, or a pulse signal of the structure to the structure, and receives a signal reflected from a portion of the structure with thinning or defects to inspect the condition of the structure.

With respect to the structure thickness measuring apparatus, Korean Patent Registration No. 10-1764706 (registration date, July 28, 2017) discloses "a system for measuring the thickness of a structure using a local spatial wave number filtering technique and a method for measuring the same" .

The system for measuring the thickness of a structure disclosed in Korean Patent Registration No. 10-1764706 includes a piezoelectric element (PZT) attached to a structure to be inspected in order to generate image information related to the thickness of the structure, and a mirror that implements scanning. And, a laser vibration meter (Laser Doppler Vibrometer, LDV) for collecting a signal related to the thickness, and a signal processing unit for processing the collected signal.

However, with this configuration alone, it is possible to check the thickness of the structure to be inspected when it is reduced by about 25% or more compared to the normal thickness, so detailed monitoring is difficult. In order to guarantee the safety of the structure, it is necessary to detect a thickness change more than 25% finer than 25% in the actual field, and the need to represent this thickness change as image information is emerging.

That is, since the vibrating unit disclosed in Korean Patent Registration No. 10-1764706 is only composed of a contact-type piezoelectric element or a non-contact-type vibration sensor, it is limited to performing more detailed monitoring.

Meanwhile, recently, an IDT sensor, which is a sensor for monitoring the health of a structure, has been developed and studied. Interdigital transducers (IDT) sensors generate a surface wave (SAW) or Rayleigh wave by applying a voltage to an electrode arranged in a comb pattern on a piezoelectric material. Refers to a method of evaluating the integrity of a structure (eg, measuring the thinning state of a structure) using surface waves. Here, the IDT sensor refers to a device for oscillating or receiving a surface wave by forming an electrode on a flat plate-shaped piezoelectric substrates using an etched technology, and applying a voltage to the electrode.

However, since ultrasound does not propagate in the air, in order to measure the thinning state of a structure using an IDT sensor, an ultrasound medium capable of transmitting ultrasound between the structure and the IDT sensor, that is, a couplant is required. .

On the other hand, in the case of such a coupling agent, water or gel may be used. However, in the case of an IDT sensor (hereinafter, referred to as a wet IDT sensor) using a couplant such as water or gel, there is a problem in that the couplant must be applied or sprayed on the structure. That is, in the case of a wet IDT sensor using a coluent such as water, it is difficult to use a couplant such as water in a structure sensitive to moisture, for example, a structure including a sensitive electronic circuit, etc., so a wet IDT sensor is used. Therefore, there is a problem in that it is difficult to measure the thickness. Accordingly, there is a need for a method capable of measuring the thinning state of the structure without using a couplant such as water or gel.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus and method for measuring the thickness of a structure capable of visualizing the thickness of the structure and the initial thickness change of the structure.

Another object of the present invention is to provide an apparatus and method for measuring the thickness of a structure capable of detecting the thickness or thinning of a structure using a surface wave and further imaging a change in the thickness of the structure.

Another object of the present invention is to provide an apparatus and method for measuring the thickness of a structure capable of detecting the thickness or thinning of a structure and imaging a change in the thickness of the structure without using a couplant such as water or gel.

As a means for solving the above problems, the structure thickness measuring apparatus according to the present invention is an IDT (Interdigital transducers) sensor that generates a predetermined wave so that the surface wave propagates along the structure, and between the structure and the IDT sensor. Based on the formed cover layer, a scanning unit for scanning vibrations generated in the structure, a signal collection unit for measuring the vibration signal of the structure scanned from the scanning unit, and the vibration signal measured by the signal collection unit Thus, including a signal processing unit for calculating the thickness of the structure, the frequency and wavelength of the surface wave generated by the IDT sensor is calculated based on the wavenumber sensitivity and mode separation determined according to the material and thickness of the structure characterized.

In one embodiment, the wavenumber sensitivity is defined by the following equation,

는 파수변화율,

는 두께변화율, f는 주파수인 것을 특징으로 한다. in the above formula

is the rate of change of wavenumber,

is the thickness change rate, and f is the frequency.

In one embodiment, the mode separation degree is defined by the following equation,

와

는 서로 다른 두 모드의 파수인 것을 특징으로 한다.

Wow

is the wavenumber of two different modes.

In an embodiment, the signal processing unit performs bandpass filtering to extract a single frequency component of the vibration signal measured by the signal collection unit, and converts the vibration signal in a three-dimensional matrix form into a two-dimensional matrix form In order to do this, a Fast Fourier Transform (FFT) is performed to obtain a steady state response signal, a preset mode is separated, a local spatial wave number filtering technique is performed, and a dispersion diagram for each thickness of the structure is calculated. After that, it is characterized in that the thickness of the structure is calculated by calculating the correlation between the wave number and the thickness.

In an embodiment, the signal processing unit calculates the wavenumber sensitivity and mode separation according to the frequency by using the calculated distribution diagram for each thickness of the structure.

In one embodiment, the cover layer is formed of a material according to the acoustic impedance calculated according to the following equation,

Z _O is an acoustic impedance required for the cover layer, Z _I is an acoustic impedance of a piezoelectric element (PZT), and Z _L is an acoustic impedance according to the material of the structure.

In one embodiment, the cover layer, when the material of the structure is a steel material, characterized in that formed of alumina material.

In an embodiment, the cover layer is characterized in that it has a thickness corresponding to 1/4 of the wavelength of the wave generated by the IDT sensor.

In an embodiment, the IDT sensor further includes a support layer for supporting the IDT sensor on which the cover layer is formed, wherein the support layer is formed of a material having a strength greater than or equal to that of the cover layer.

In addition, the method for measuring the thickness of a structure according to an embodiment of the present invention includes the steps of detecting information related to the material and thickness of the structure, and calculating the wavenumber sensitivity and mode separation degree corresponding to the structure by using the detected material and thickness Step, detecting thinning information related to the structure using an IDT sensor designed according to the calculated wavenumber sensitivity and mode separation degree, and generating image information related to the thickness of the structure based on the detected thinning information Including, between the IDT sensor and the structure, a cover layer formed of a material having an acoustic impedance calculated based on the acoustic impedance according to the material of the structure according to Equation 1 is formed,

[Equation 1]

Z _O is an acoustic impedance required for the cover layer, Z _I is an acoustic impedance of a piezoelectric element (PZT), and Z _L is an acoustic impedance according to the material of the structure.

In one embodiment, the cover layer is formed of a material having an acoustic impedance within a preset error range from the acoustic impedance calculated by Equation 1, and when the material of the structure is steel, it is formed of an alumina material. characterized by being

In an embodiment, the cover layer is characterized in that it has a thickness corresponding to a length of 1/4 of the wavelength of the frequency excited through the IDT sensor.

In an embodiment, the wavenumber sensitivity is defined by Equation 2 below,

[Equation 2]

는 파수변화율,

는 두께변화율, f는 주파수이며, 상기 모드분리도는 아래의 수학식 3에 의해 정의되고,in the above formula

is the rate of change of wavenumber,

is the thickness change rate, f is the frequency, and the mode separation degree is defined by Equation 3 below,

[Equation 3]

와

는 서로 다른 두 모드의 파수인 것을 특징으로 한다.

Wow

is the wavenumber of two different modes.

By the above means, the apparatus for measuring the thickness of a structure according to the present invention can measure the thickness of a structure without using a coupling agent such as water, so that the IDT sensor is used even for structures that cannot use the water or the like. It has the effect of making it possible to measure the thickness.

1 is a conceptual view showing a conventional structure thickness measuring apparatus.
Figure 2 is a graph related to the operation of the structure thickness measuring apparatus shown in Figure 1;
Figure 3 is a graph related to the operation of the structure thickness measuring device shown in Figure 1;
Figure 4a is a conceptual view showing the IDT sensor included in the structure thickness measuring device according to the present invention.
Figure 4b is a conceptual diagram showing the IDT sensor included in the structure thickness measuring device according to the present invention.
Figure 4c is a conceptual diagram showing a dry IDT sensor according to the present invention.
Figure 4d is an exemplary view showing an example in which the dry IDT sensor according to the present invention is pressed to the structure.
5 is a flowchart illustrating a structure monitoring method according to the present invention.
Figure 6 is a conceptual view showing an inspection target of the structure thickness measuring apparatus according to the present invention.
7a to 7c are graphs related to the operation of the structure thickness measuring apparatus according to the present invention.
8a to 8c are graphs related to the operation of the structure thickness measuring apparatus according to the present invention.
9 is a conceptual diagram illustrating an image generated by the apparatus for measuring the thickness of a structure according to the present invention.

Hereinafter, the structure thickness measuring apparatus and system according to the present invention will be described in more detail with reference to the drawings.

The singular expression includes the plural expression unless the context clearly dictates otherwise.

In describing the embodiments disclosed in the present specification, if it is determined that detailed descriptions of related known technologies may obscure the gist of the embodiments disclosed in the present specification, the detailed description thereof will be omitted.

In the present specification, the same and similar reference numerals are given to the same and similar components even in different embodiments and modified examples, and a redundant description thereof will be omitted.

The accompanying drawings are only for easy understanding of the embodiments disclosed in the present specification, and the technical spirit disclosed in the present specification is not limited by the accompanying drawings, and all changes and equivalents included in the spirit and scope of the present invention It should be understood to include water or substitutes.

Terms including an ordinal number, such as first, second, etc., may be used to describe various elements, but the elements are not limited by the terms. The above terms are used only for the purpose of distinguishing one component from another.

1 shows an embodiment of a conventional structure thickness measuring apparatus.

As shown in FIG. 1 , the apparatus for measuring the thickness of a structure includes a PZT excitation part 20 that is adhered to a structure 31 to be inspected.

In addition, the structure thickness measuring apparatus may include a scanning unit formed of a plurality of mirrors (horizontal mirror, vertical mirror), a signal collecting unit 33 , and a signal processing unit 34 .

As shown in FIG. 1 , the PZT excitation unit 20 , the scanning unit 200 , the signal collection unit 33 and the signal processing unit 34 are wired or wirelessly connected to transmit and receive signals and data between each other. and is preferably performed according to the instructions of the manager (operator).

In addition, it is preferable that the defect information including the final thickness of the structure calculated through the signal processing unit 34 is transmitted and displayed to a separate monitoring means (not shown) so that the manager (operator) can easily check it.

In detail about each configuration, the PZT excitation unit 20 may transmit continuous wave excitation to a structure to be measured. In detail, the PZT excitation unit 20 may include a contact-type piezoelectric element or a non-contact vibration sensor, and may transmit excitation of several kHz to several MHz according to an administrator's instruction.

The PZT excitation unit 20 sets the frequency band of the continuous wave excitation transmitted to the structure to be measured, and further includes a power amplifying means 31 and a waveform generating means 32 for amplifying power according to the set frequency band. It is preferable to configure.

The power amplifying unit 31 and the waveform generating unit 32 may control the PZT excitation unit 20 to set a frequency band of continuous wave excitation transmitted to a structure to be measured under the control of an administrator.

The scanning unit 200 may scan the vibration generated in the structure by the continuous wave excitation of the PZT excitation unit 20 . As shown in FIG. 1 , the scanning unit 200 may scan vibrations generated in a structure to be scanned, and a means for irradiating a laser to the vibrating structure, scanning the structure in a certain direction, and reflecting It preferably comprises means for scanning the laser.

The signal collection unit 33 may measure the vibration signal of the structure scanned from the scanning unit 200 and transmit it to the signal processing unit 34 .

The signal collecting unit 33 is composed of a Laser Doppler Vibrometer (LDV).

The signal processing unit 34 may analyze the vibration signal measured by the signal collection unit 33 using a local spatial wave number filtering technique to measure the thickness of the structure.

In detail, the signal processing unit 34 may perform bandpass filtering to extract a single frequency component of the vibration signal measured by the signal collection unit 33 , and the vibration signal in the form of a three-dimensional matrix In order to transform α into a two-dimensional matrix form, a fast Fourier transform (FFT) may be performed to obtain a steady state response signal.

Using the steady-state response signal, a preset mode is separated and a local spatial wavenumber filter technique is performed to calculate a distribution diagram for each thickness of the structure, and then the correlation between the wave number and the thickness of the structure , and the thickness of the structure can be finally calculated using this.

In addition, through the thickness of the structure, the location of the defect included in the structure, the depth of the defect, the size, etc. can be effectively predicted.

Here, since the wave number is inversely proportional to the wavelength, when the thickness is thin, the wave number is large, and when the thickness is thick, the wave number is small. Using this, the correlation between the wave number and the thickness of the structure is calculated, and the thickness of the structure can be finally calculated.

In other words, in the signal processing unit 34 , the vibration signal measured from the LDV of the signal collecting unit 33 through the scanning unit 200 may be expressed as V[x,y,t] in the form of a three-dimensional matrix. And by performing bandpass filtering, it is possible to extract a single frequency component included in the vibration signal.

In addition, in order to convert a 3D matrix form into a 2D matrix form, by performing FFT, it is possible to obtain a steady-state response signal having only a frequency component f0.

By performing 2D FFT on the obtained steady-state response signal, V[kx,ky,f0] can be calculated, and after separating a preset mode (for example, S0 mode), removing the S0 mode It is preferable to multiply by Wh[kx,ky,f0] for

By multiplying the mode-separated V[kx, ky] by W[kx, ky, kc] and then again performing 2D FFT, it is possible to calculate kc at which |z[k, y, kc]| becomes the maximum value. Through this, it is preferable to divide the calculated n region and perform smoothing by performing a local spatial wave number filtering technique. That is, the local spatial wave number filtering technique is to smooth the original signal by minimizing the occurrence of an error with the actual wave number without distorting the original signal.

Thereafter, by calculating the distribution diagram of the structure for each thickness, a correlation expression representing the correlation between the wave number and the thickness of the structure may be derived, and the thickness of the structure may be calculated using the derived correlation expression.

That is, the wavenumber sensitivity and mode separation according to the frequency can be calculated using the calculated wavenumber sensitivity and mode separation diagram for each thickness of the structure, and the thickness of the structure can be finally calculated using the calculated wavenumber sensitivity and mode separation diagram.

In more detail, as shown in the table below, it can be represented.

2 and 3 are graphs related to the operation of the structure thickness measuring apparatus of FIG. 1 described above.

According to the graphs shown in FIGS. 2 and 3 , the apparatus for measuring the thickness of the structure of FIG. 1 has a problem in that the thickness of the structure to be inspected can only be detected when the thickness of the structure to be inspected is reduced to about 70% or less of the initial thickness.

This problem is due to the fact that the structure thickness measuring apparatus described in FIG. 1 uses an inspection technique using a difference in wavenumber for each thickness of the inspected object. That is, since it is impossible to inspect the structure thickness measuring apparatus of FIG. 1 if there is no difference in wavenumber according to thickness, it is mainly inspected using a low frequency (0 to 250 kHz).

Referring to FIGS. 2 and 3 , the sensitivity of thickness change monitoring sharply decreases in the case of 250 kHz or higher.

In order to solve this problem, the structure thickness measurement apparatus proposed in the present invention replaces the PZT excitation unit 32 with an IDT sensor in the structure thickness measurement apparatus shown in FIG. 1 , so that a more detailed thickness change can be detected. is derived

Two approaches are considered to increase the sensitivity of structure monitoring.

For the structure monitoring method using the IDT sensor, a mode with high wavenumber sensitivity is selected or a mode with a high degree of separation between modes is selected.

First, the IDT sensor 400 included in the apparatus for measuring the thickness of the structure proposed by the present invention is shown in FIGS. 4a to 4d.

The IDT sensor 400 is manufactured by laser precision processing of a piezoelectric material, has a lower unit price than a general sensor, can be manufactured according to the size, shape, and frequency desired by the user, and can easily generate induced ultrasound (Lamb wave). have. In addition, it has the advantage that it can be applied to various materials such as steel structure composites, plastics, and concrete.

4A and 4B show one embodiment of an IDT sensor 400 .

The IDT sensor 400 shown in FIG. 4A generates a surface wave having a predetermined wavelength. The IDT sensor 400 provided in the apparatus for measuring the thickness of a structure according to the present invention is designed and manufactured by processing PZT composed of an upper electrode and a lower electrode on a PZT substrate into a shape desired by a user by processing the upper electrode with a laser.

Referring to FIG. 4B , the standard of the electrode formed by the IDT sensor 400 is shown, and the wavelength of the surface wave can be changed by adjusting the distance λ between the branch electrodes of one electrode.

Also, by adjusting the length L of one branch electrode, the width We of the branch electrodes, and the distance Ws between different electrodes, the intensity of the surface wave may be changed.

Meanwhile, FIGS. 4c and 4d show an example of a dry IDT sensor capable of measuring the thickness of the structure 470 without a couplant according to an embodiment of the present invention.

First, referring to FIG. 4C, the dry IDT sensor according to an embodiment of the present invention includes a cover layer 410 covering the IDT sensor 400 shown in FIGS. 4A and 4B and the IDT sensor 400 supporting the The support layer 420 may be formed by bonding. Here, the cover layer 410 may be a layer disposed between the IDT sensor 400 and the structure 470 .

Meanwhile, the cover layer 410 may be formed of a material having an acoustic impedance according to the material of the structure and an acoustic impedance calculated according to the acoustic impedance of the PZT. For example, when the acoustic impedance of the PZT is Z _I and the acoustic impedance according to the material of the structure is Z _{L , the acoustic impedance Z O} required for the cover layer 410 may be calculated as in Equation 1 below.

where Z _O is the acoustic impedance required for the cover layer, Z _I is the acoustic impedance of PZT, and Z _L is the acoustic impedance according to the material of the structure.

For example, when the material of the structure 470 is steel, the acoustic impedance of the steel may be 46 MRayls. On the other hand, if the acoustic impedance of the PZT is 31.5 MRayls, the required acoustic impedance Z _O is 38 MRayls. In this case, the material having an acoustic impedance of 38 MRayls is silver (Ag). Therefore, the cover layer 410 may be preferably made of a silver material.

On the other hand, when the cover layer 410 is formed of silver, it may be most preferable. However, since silver is expensive, it goes without saying that other materials having a difference in acoustic impedance of less than a certain level may be used. For example, when the difference in the acoustic impedance is _{less than 10% from the calculated Z O} , the acoustic impedances may be regarded as similar to each other and replaced.

For example, alumina has an acoustic impedance of 40.6 MRayls. Therefore, the difference in acoustic impedance with silver is about 6.8%, so it has a similar acoustic impedance to each other and is strong, so it can be suitable as a material of the cover layer 410 formed between the structure 470 and the IDT sensor 400. have.

And the thickness of the cover layer 410 may be determined according to the wavelength (λ) of the ultrasonic wave to be excited through the IDT sensor (400). For example, the thickness of the cover layer 410 may be determined to be 1/4 of the wavelength of the ultrasonic wave to be excited through the IDT sensor 400 .

Therefore, if the structure 470 is formed of a steel material, an alumina layer having a thickness of 1/4 of the ultrasonic wave excited through the IDT sensor 400 may be formed as the cover layer 410 . Then, the IDT sensor 400 on which the cover layer 410 is formed may be pressed against the structure 470 with the cover layer 410 interposed therebetween. Then, the ultrasonic wave of the IDT sensor 400 is transmitted to the structure 470 through the alumina layer (cover layer 410), and a response signal to the ultrasonic wave transmitted through the FFT is received and the structure based on the received response signal. A thickness of 470 may be measured. Accordingly, the dry IDT sensor such as the present invention may perform thickness measurement of the structure 470 using ultrasonic waves without a couplant.

Meanwhile, the support layer 420 may be a layer that supports the IDT sensor 400 . For example, the support layer 420 may prevent damage to the IDT sensor 400 when it is pressed against the structure 470 . To this end, the support layer 420 may be formed of a material having a strength greater than or equal to a certain level. For example, the support layer 420 may be formed of an alumina material in the same manner as the cover layer 410 , or may be formed of a steel material unlike the cover layer 410 . That is, it may be formed of a material having a strength greater than or equal to that of the cover layer 410 .

Meanwhile, in FIG. 4c described above, it has been mentioned that the dry IDT sensor according to the embodiment of the present invention may be pressed against the structure 470 . FIG. 4D shows a structure further including at least one fixing part for compressing the dry IDT sensor and the structure so that the dry IDT sensor can be compressed to the structure 470 as described above.

Referring to FIG. 4D , the dry IDT sensor according to an embodiment of the present invention may be formed such that a portion of the cover layer 410 protrudes. The protrusion 450 may be a portion that does not cover the IDT sensor 400 , and at least one or more may be formed around the IDT sensor 400 .

Meanwhile, the protrusion 450 may have a groove in which the attachment part 460 for attaching the cover layer 410 and the structure 470 may be mounted. The attachment part 460 may be coupled to the protrusion 450 through the groove, and may compress the protrusion 450 and the structure 470 .

For example, the attachment part 460 may be formed of a permanent magnet having a magnetic force greater than or equal to a preset level. In this case, when the attachment part 460 is attached to each of the protrusions 450 , the attachment part 460 , that is, the structure 470 formed of steel and the protrusions 450 may be compressed by the magnetic force of the magnets. Then, the cover layer 410 connected to the protrusions 450 may be pressed against the structure 470 due to compression of the protrusions 450 . Therefore, the ultrasonic wave excited by the IDT sensor 400 may be propagated to the structure 470 through the cover layer 410 pressed to the structure 470 , that is, the alumina layer.

5 illustrates a structure monitoring method according to the present invention.

First, as described above, the dry IDT sensor according to the present invention can propagate ultrasonic waves to the structure without a couplant through the cover layer pressed to the structure. To this end, a process of first pressing the dry IDT sensor to the structure may be performed (S500). In this case, the structure thickness measuring apparatus according to an embodiment of the present invention may press the dry IDT sensor to the structure by locating the dry IDT sensor on the surface of the structure to measure the thickness and disposing the attachment part on the protrusion. .

In addition, wave number sensitivity and mode separation degree may be calculated based on the material and thickness of the structure to be inspected (S501).

The frequency and wavelength applied to the IDT sensor may be calculated using the calculated wavenumber sensitivity and mode separation (S502).

The apparatus for measuring the thickness of a structure according to the present invention may use an IDT sensor manufactured according to the calculated frequency and wavelength to detect thinning information of a structure to be inspected (S503).

In addition, the apparatus for measuring the thickness of the structure may generate image information related to the thickness of the structure based on the detected thinning information ( S504 ).

In the conventional method performed by the apparatus for measuring the thickness of the structure of FIG. 1 , PZT is used to excite the bending mode, but in the present technology, the structure is excited using an IDT sensor for excitation in the mode desired by the user. The wavenumber sensitivity is defined by Equation 2 below and is expressed as a value obtained by differentiating the wavenumber by the thickness.

는 파수변화율,

는 두께변화율, f는 주파수이다.In Equation 1 above

is the rate of change of wavenumber,

is the thickness change rate, and f is the frequency.

가 클수록 두께변화에 민감한 파수이며,

는 주파수에 따라 변화하므로, 재질과 두께가 결정되면 주파수에 따른

를 계산해야 한다.

The larger it is, the more sensitive it is to the change in thickness.

is changed with frequency, so when the material and thickness are determined,

should be calculated

In addition, for clear imaging, each mode must be completely separated. This is called “mode separation” and is expressed by Equation 3 below.

와

는 서로 다른 두 모드의 파수이다.In Equation 3 above

Wow

is the wavenumber of the two different modes.

이 클수록 인접 모드간 분리가 잘되며,

이 작으면 인접 모드간 분리가 어려워 영상으로 두께 변화를 구분하기 어렵다.

The larger the value, the better the separation between adjacent modes.

If this value is small, it is difficult to separate the adjacent modes, so it is difficult to distinguish the thickness change in the image.

성분만을 가지는 Steady state response (정상파 응답)를 획득한다. Steady state response를 2D FFT를 수행하여

를 계산하고, 모드를 분리하기 위하여 특정모드를 제거하기 위한

을 곱한다. 모드 분리된

에

을 곱한 후, 2D IFFT를 수행하여

가 최대가 되는

를 계산하고, 영역을 나누어 국소공간 웨이브넘버 필터링을 하여 스무딩한다. 마지막으로, 두께 별 구조물의 분산선도를 계산한 후, 웨이브넘버와 두께의 상관식을 도출하고 이 식을 이용하여 구조물의 두께를 나타낸다.The thickness imaging method of the structure is the same as the conventional method. A signal measured from the LDV through scanning is a three-dimensional matrix of v[x, y, t], and bandpass filtering is performed to extract a single frequency component. To transform a 3D matrix into a 2D matrix, FFT (Fast Fourier Transform) is performed to

Acquire a steady state response (standing wave response) with only components. Steady state response by 2D FFT

In order to calculate , and to remove a specific mode to isolate the mode,

Multiply by mode separated

on

After multiplying by , 2D IFFT is performed

becomes the maximum

is calculated and smoothed by dividing the region by local spatial wave number filtering. Finally, after calculating the distribution diagram of the structure by thickness, a correlation equation between the wave number and the thickness is derived, and the thickness of the structure is expressed using this equation.

6 is an example of the inspection target of the structure thickness measuring apparatus according to the present invention is shown.

The inspection object was a plate with a width of 300 mm, a length of 300 mm, and a thickness of 6 mm, and different specimens were prepared for thinning defects with a width of 40 mm, a length of 40 mm, and a thickness of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm. The thickness ratio of thinning defects is 5%, 10%, 15%, and 20% with the goal of detecting defects of 25% or less compared to the new material.

7A to 7C show wavenumber sensitivity according to thickness for each frequency, and FIGS. 8A to 8C show mode separation according to thickness for each frequency.

Referring to FIGS. 7A to 7C and 8A to 8C , it can be seen that the A0 mode has low wavenumber sensitivity in a thick material, making it unsuitable for a thinning test.

In the S0 mode, the wavenumber sensitivity by thickness increases as the frequency increases. Since the thickness of the specimen to be detected is within 4.8 mm to 6 mm, the effect of the S0 mode is maximized when excitation at 0.422 MHz. In the thickness range of 4.8 mm to 6 mm, the wavenumber sensitivity of A1 mode is higher than that of S0 mode, but the mode separation of A1 mode is low, making signal processing difficult and therefore not suitable for use. Accordingly, in this example, an IDT sensor of S0 mode of 0.422 MHz was used.

9 is image information generated by using a signal detected from an examination target.

As shown in FIG. 9 , it is confirmed that all of the 20% thinned part, 15% thinned part, 10% thinned part, and 5% thinned part are distinguished from those around them.

Claims (13)

Interdigital transducers (IDT) sensors that generate a predetermined wave so that the surface wave propagates along the structure;
a cover layer formed between the structure and the IDT sensor and bonded to the surface of the structure instead of a couplant;
a scanning unit for scanning vibrations generated in the structure;
a signal collecting unit for measuring the vibration signal of the structure scanned from the scanning unit; and
a signal processing unit for calculating the thickness of the structure based on the vibration signal measured by the signal collection unit;
The frequency and wavelength of the surface wave generated by the IDT sensor are,
It is calculated based on wavenumber sensitivity and mode separation determined according to the material and thickness of the structure,
The cover layer is
It is formed of a material according to the acoustic impedance calculated according to the following equation,

Z _O is an acoustic impedance required for the cover layer, Z _I is an acoustic impedance of a piezoelectric element (PZT), and Z _L is an acoustic impedance according to the material of the structure. According to claim 1,
The wavenumber sensitivity is defined by the following equation,

in the above formula

is the rate of change of wavenumber,

is a thickness change rate, and f is a frequency. 3. The method of claim 2,
The mode separation degree is defined by the following equation,

Wow

is a wavenumber of two different modes. According to claim 1,
The signal processing unit,
performing bandpass filtering to extract a single frequency component of the vibration signal measured by the signal collection unit;
Fast Fourier Transform (FFT) is performed to transform the vibration signal in a three-dimensional matrix form into a two-dimensional matrix form to obtain a steady state response signal, and after separating a preset mode, a local spatial wave After performing the number filtering technique and calculating the distribution diagram for each thickness of the structure, the thickness of the structure is calculated by calculating the correlation between the wave number and the thickness. 5. The method of claim 4,
The signal processing unit
A structure thickness measuring apparatus, characterized in that by using the calculated distribution diagram for each thickness of the structure, the wavenumber sensitivity and mode separation according to the frequency are calculated. delete According to claim 1, wherein the cover layer,
When the material of the structure is a steel material, the structure thickness measuring device, characterized in that formed of an alumina material. According to claim 1, wherein the cover layer,
Structure thickness measuring apparatus, characterized in that it has a thickness corresponding to 1/4 of the wavelength of the wave generated by the IDT sensor. According to claim 1,
The IDT sensor is
Further comprising a support layer supporting the IDT sensor on which the cover layer is formed,
The support layer is
Structure thickness measuring device, characterized in that formed of a material having a strength greater than or equal to that of the cover layer. detecting information related to the material and thickness of the structure;
calculating wavenumber sensitivity and mode separation corresponding to the structure by using the detected material and thickness;
detecting thinning information related to the structure using an IDT sensor designed according to the calculated wavenumber sensitivity and mode separation degree; and
Based on the sensed thinning information, comprising the step of generating image information related to the thickness of the structure,
Between the IDT sensor and the structure,
It is formed of a material having an acoustic impedance calculated based on the acoustic impedance according to the material of the structure according to Equation 1 below, and a cover layer bonded to the surface of the structure is formed instead of a couplant,
[Equation 1]

Z _O is an acoustic impedance required for the cover layer, Z _I is an acoustic impedance of a piezoelectric element (PZT), Z _L is an acoustic impedance according to the material of the structure. 11. The method of claim 10,
The cover layer is
It is formed of a material having an acoustic impedance within a preset error range from the acoustic impedance calculated by Equation 1,
When the material of the structure is steel,
A method for measuring the thickness of a structure, characterized in that it is formed of an alumina material. 11. The method of claim 10, wherein the cover layer,
A method for measuring the thickness of a structure, characterized in that it has a thickness corresponding to a length of 1/4 of a wavelength of a frequency excited through the IDT sensor. 11. The method of claim 10,
The wavenumber sensitivity is defined by Equation 2 below,
[Equation 2]

in the above formula

is the rate of change of wavenumber,

is the thickness change rate, f is the frequency,
The mode separation degree is defined by Equation 3 below,
[Equation 3]

Wow

is the wavenumber of two different modes.

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