Ultrasonic thickness measuring method and equipment

Abstract

The invention provides an ultrasonic thickness measuring method and equipment. The method comprises the following steps: transmitting an ultrasonic signal to a test piece by using ultrasonic thickness measurement equipment to obtain an echo signal, carrying out soft threshold-based wavelet filtering processing and Wiener deconvolution filtering processing on the echo signal, then calculating an autoregression coefficient based on a high signal-to-noise ratio signal, reconstructing an original signal through an autoregression spectrum extrapolation model, and finally obtaining the thickness of the test piece. And finally, calculating the thickness of the measured piece according to the time difference of two signals in the reconstructed signals. According to the signal processing method provided by the invention, the thickness of the thin film can be clearly and accurately measured by using the transducer with relatively low frequency, the cost of an ultrasonic thickness measurement system is effectively reduced, and the requirement for measuring the thickness of a thin film material in an industrial environment is met.

Description

Technical Field

[0001] The invention relates to the technical field of acoustics and nondestructive testing, and in particular to an ultrasonic thickness measurement method and equipment. Background Art

[0002] High-resolution ultrasonic thickness measurement equipment is often based on the hardware platform of ultrasonic microscope. Ultrasonic microscope is a non-destructive testing and microscopic imaging equipment widely used in high-tech manufacturing industries such as chip manufacturing, biomedicine, composite materials, and aerospace. Figure 1 As shown in the figure, liquid immersion point-focused ultrasonic transducers are generally used, which can converge the sound beam at one point, so that the energy concentration in the convergence area is enhanced and the width of the sound beam becomes smaller, which can meet the requirements of high sensitivity and high resolution detection.

[0003] With the continuous advancement of manufacturing technology, the thickness of thin film materials has been lower than the half wavelength of the acoustic signal, such as Figure 2 As shown in (c), the signals from different layered structures are superimposed. The superposition of signals will inevitably make it difficult to judge the layered position of the reflected signal, which will affect the accuracy of the thickness measurement and even make it impossible to measure the thickness. By increasing the frequency of the ultrasonic signal, the wavelength can be shorter and the resolution can be higher, but the high-frequency transducer has low penetration ability, high use and maintenance costs, and difficulty in operation. Therefore, in the non-destructive testing field of most high-tech manufacturing industries, ultrasonic transducers with frequencies higher than 200MHz are not widely used. At present, in the field of industrial non-destructive testing, the frequencies of ultrasonic transducers commonly used in ultrasonic microscopes are mainly distributed in the range of 10-200MHz. Moreover, due to the limitation of the diffraction limit of acoustic signals, ultrasonic transducers cannot improve the resolution by continuously increasing the frequency after the frequency is higher than 2GHz. How to improve the resolution of ultrasonic thickness measuring equipment has become a key link in the application and promotion of acoustic high-precision thickness measuring equipment in the production of thin film materials.

[0004] In summary, by improving the resolution of ultrasonic signals through signal processing methods, a lower frequency transducer can be used to measure the thickness of the film, effectively reducing the cost of the ultrasonic thickness measurement system and meeting the needs of measuring the thickness of thin film materials in industrial environments. Summary of the invention

[0005] The present invention describes an ultrasonic thickness measurement method and device, which can solve the above technical problems.

[0006] According to a first aspect, there is provided an ultrasonic thickness measurement method, the method comprising:

[0007] Use ultrasonic thickness measuring equipment to transmit ultrasonic signals to the test piece to obtain the echo signal of the test piece;

[0008] Performing filtering processing on the echo signal using a wavelet filtering method based on a soft threshold;

[0009] The echo signal is transformed from the time domain to the frequency domain, and a low-frequency component of the echo signal is denoised using a Wiener deconvolution filter to separate the echo signals in a superimposed state, and a high signal-to-noise ratio signal is obtained using a frequency window;

[0010] Calculating an autoregressive coefficient based on the high signal-to-noise ratio signal, reconstructing the original signal through an autoregressive spectrum extrapolation model, and transforming the reconstructed signal from the frequency domain to the time domain to obtain a time domain signal;

[0011] The thickness of the measured object is calculated based on the time difference between two adjacent signals in the time domain signal.

[0012] In some embodiments, after filtering the echo signal using a wavelet filtering method based on a soft threshold, discrete wavelet decomposition is performed on the echo signal, which is implemented by the following formula:

[0013]

[0014] Among them, 2 N is the length of the echo signal, J≤N is the number of discrete wavelet decompositions, h Jk (n) is the low frequency filter coefficient, g jk (n) is the high-frequency filter coefficient; the echo signal is decomposed into low-frequency components a Jk and high frequency component d jk Two parts.

[0015] In some embodiments, the method of processing the echo signal by a wavelet filtering method based on a soft threshold is implemented by the following formula:

[0016]

[0017] in, is the high frequency component after noise reduction d jk ,sng(d jk ) is the high frequency component d jk The sign (+-), T is the threshold, and T is set to:

[0018]

[0019] Where L is the length of the echo signal and σ is the standard deviation of the noise.

[0020] In some embodiments, the method of using the Wiener deconvolution filter to perform denoising on the echo signal is completed by the following formula:

[0021]

[0022] Where R(ω) and Y(ω) are the Fourier transforms of r(t) and y(t), respectively, X(ω) is the energy density spectrum of the incident signal x(t), and Q is the noise blocking factor.

[0023] In some embodiments, the method of calculating the autoregressive coefficient based on the high signal-to-noise ratio signal and reconstructing the original signal through the autoregressive spectrum extrapolation model specifically includes:

[0024]

[0025] Among them, a k is the autoregression coefficient, for a k The conjugate of is the complex value obtained by extrapolating the autoregressive spectrum, p=HL is the order of the autoregressive spectrum extrapolation, frequencies 1~L are the spectrum of the low-frequency part, and frequencies H~N are the spectrum of the high-frequency part.

[0026] In some embodiments, a method for calculating the thickness of a test piece according to the time difference between two adjacent signals in a time domain signal is as follows:

[0027] Δt×v=2×l

[0028] Among them, Δt represents the interval time between adjacent pulses, v is the propagation speed of ultrasound in the film, and l is the thickness measurement result.

[0029] According to a second aspect, there is provided an ultrasonic thickness measuring device, comprising:

[0030] Ultrasonic transducer, coupling liquid supply system, rolling mill and motor, wherein:

[0031] The rolling mill is used to drive the tested piece to move in the horizontal direction;

[0032] An ultrasonic transducer, used for measuring the thickness of the measured object, wherein the ultrasonic transducer is arranged below the measured object and vertically aligned with the lower surface of the measured object;

[0033] A motor, used for adjusting the distance between the ultrasonic transducer and the lower surface of the measured object;

[0034] The coupling liquid supply system is used to apply the coupling liquid between the ultrasonic transducer and the measured object to ensure that the ultrasonic transducer and the measured object are acoustically coupled.

[0035] In some embodiments, the coupling liquid supply system includes: a liquid storage tank, a filtering device, a pump, and a coupling liquid injection device, wherein:

[0036] A coupling liquid spraying device, used for spraying coupling liquid to a position between the ultrasonic transducer and the lower surface of the measured object;

[0037] A liquid storage tank, used for storing coupling liquid;

[0038] A pump, used for driving the coupling liquid in the liquid storage tank to be transferred to the coupling liquid injection device;

[0039] The filtering device is arranged between the liquid storage tank and the pump and is used to filter impurities from the coupling liquid.

[0040] In some embodiments, the ultrasonic thickness measuring device further includes an air dryer, which is used to air dry the coupling liquid on the surface of the measured object after the thickness measurement is completed.

[0041] In some embodiments, the ultrasonic thickness measuring device further comprises:

[0042] A transceiver selector, connected to the ultrasonic transducer via the first port, the transceiver selector being used for switching the ultrasonic transducer between a transmitting mode and a receiving mode;

[0043] A pulse transmitting unit connected to the second port of the transceiver selector, wherein the pulse transmitting unit is used to excite the ultrasonic transducer to generate a corresponding ultrasonic signal;

[0044] A preamplifier connected to the third port of the transceiver selector, wherein the preamplifier is used to amplify the echo signal received from the ultrasonic transducer;

[0045] An analog signal acquisition unit, used for acquiring the echo signal amplified by the preamplifier;

[0046] Industrial computer, used to control the operation of the entire ultrasonic thickness measurement equipment;

[0047] Among them, the preamplifier, the analog signal acquisition unit, and the industrial computer are connected in series in sequence; when in the transmitting state, the transceiver selector is disconnected from the preamplifier, and the transceiver selector is connected to the pulse transmitting unit, so that the ultrasonic transducer completes signal transmission; when in the receiving state, the transceiver selector is disconnected from the pulse transmitting unit, and the transceiver selector is connected to the preamplifier, the echo signal is amplified by the preamplifier, and the amplified echo signal is collected by the analog signal acquisition unit, thereby completing the collection of the echo signal.

[0048] The present invention provides an ultrasonic thickness measurement method and device. An ultrasonic thickness measurement device is used to transmit an ultrasonic signal to a test piece to obtain an echo signal, and the echo signal is subjected to wavelet filtering based on a soft threshold and Wiener deconvolution filtering. The autoregressive coefficient is then calculated based on a high signal-to-noise ratio signal, and the original signal is reconstructed through an autoregressive spectrum extrapolation model. Finally, the thickness of the test piece is calculated based on the time difference between two signals in the reconstructed signal. Through the signal processing method provided by the present invention, the thickness of the film can be clearly and accurately measured using a lower frequency transducer, which effectively reduces the cost of the ultrasonic thickness measurement system and meets the needs of measuring the thickness of thin film materials in an industrial environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0050] Figure 1 A schematic diagram of the structure of an ultrasonic microscope provided in an embodiment of this specification is shown;

[0051] Figure 2 The following is a diagram showing an echo signal measurement result of a device under test provided by an embodiment of this specification;

[0052] Figure 3 A schematic diagram showing the structure of an ultrasonic thickness measuring device provided in an embodiment of this specification is shown;

[0053] Figure 4 A schematic flow chart of an ultrasonic thickness measurement method provided in an embodiment of this specification is shown;

[0054] Figure 5 A flow chart showing a logic diagram of an ultrasonic thickness measurement method provided in an embodiment of this specification;

[0055] Figure 6 A schematic diagram of a frequency window principle provided by an embodiment of this specification is shown;

[0056] Figure 7 A schematic diagram of a thickness measurement time domain signal result provided in an embodiment of this specification is shown. DETAILED DESCRIPTION

[0057] The solution provided in this specification is described below in conjunction with the accompanying drawings.

[0058] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in conjunction with the accompanying drawings.

[0059] In the description of the embodiments of the present application, words such as "exemplary", "for example" or "for example" are used to indicate examples, illustrations or descriptions. Any embodiment or design described as "exemplary", "for example" or "for example" in the embodiments of the present application should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of words such as "exemplary", "for example" or "for example" is intended to present related concepts in a concrete way.

[0060] In the description of the embodiments of the present application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, B exists alone, and A and B exist at the same time. In addition, unless otherwise specified, the term "plurality" means two or more.

[0061] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. The terms "include", "comprises", "has" and their variations all mean "including but not limited to", unless otherwise specifically emphasized.

[0062] In the prior art, ultrasonic microscopes are non-destructive testing and microscopic imaging equipment that use ultrasonic reflection or transmission wave signals to locate and analyze defects, and can interpret the defects and structures inside the inspected part. Figure 1 As shown in the figure, the ultrasonic microscope is centered on the industrial computer, which uniformly controls the collaborative operation of various parts. The industrial computer sends motion instructions to the motion control board and sends signal acquisition instructions to the high-speed analog-to-digital converter; the motion control board drives the high-precision mechanical motion platform, driving the ultrasonic transducer to complete the full coverage scanning of the sample; the ultrasonic signal excitation and acquisition circuit drives the ultrasonic transducer, and completes the automatic acquisition of ultrasonic signals according to the synchronization information of the synchronous encoder; the ultrasonic signal processing software obtains the digitized ultrasonic signal through the high-speed analog-to-digital converter, and obtains the 2D and 3D images inside the sample, or measures the thickness.

[0063] The ultrasonic thickness measuring device proposed in this scheme is based on the ultrasonic microscope and has improved the mechanical structure: Figure 3When measuring the test piece 6, the y-axis motor in the motion platform is replaced by the rolling mill 1, so that the ultrasonic transducer 2 and the rolling mill 1 work synchronously, so as to measure the thickness of the test piece 6 in real time. The ultrasonic transducer 2 is located below the test piece 6, and the coupling liquid spraying device 44 is used to slowly spray water on the head of the ultrasonic transducer 2 to provide coupling liquid and avoid washing the graphite layer. The air dryer 5 is used to blow dry the residual water on the test piece 6 to avoid affecting the subsequent processing.

[0064] Figure 4 A schematic diagram of a process flow of an ultrasonic thickness measurement method provided in an embodiment of this specification is shown, such as Figure 4 As shown, the method comprises the following steps:

[0065] Step S101, using an ultrasonic thickness measuring device to transmit an ultrasonic signal to a test piece to obtain an echo signal of the test piece. Specifically, prepare an ultrasonic thickness measuring device, set parameters for the ultrasonic thickness measuring device, transmit a stable ultrasonic signal to the test piece, and receive an echo signal of the test piece through the ultrasonic thickness measuring device.

[0066] Step S102, the echo signal is processed by a wavelet filtering method based on a soft threshold. Specifically, in order to improve the signal-to-noise ratio of the echo signal, a wavelet filtering method based on a soft threshold can be used to reconstruct the signal after noise reduction. First, a suitable threshold needs to be determined, and the threshold can be set according to the noise level of the echo signal. Then, the set threshold is applied to process each sample point in the echo signal. After the soft threshold processing, the echo signal is de-noised.

[0067] Step S103, transforming the echo signal from the time domain to the frequency domain, using a Wiener deconvolution filter to perform denoising on the low-frequency component of the echo signal to separate the echo signals in a superimposed state, and using a frequency window to obtain a high signal-to-noise ratio signal.

[0068] Specifically, the echo signal after the wavelet filtering in step S102 is first converted from the time domain to the frequency domain, and then the low-frequency component of the echo signal is denoised using the Wiener deconvolution filter to separate the echo signal in the superimposed state. After the echo signal is separated, the frequency window is used to filter out the noise in other frequency ranges, and the signal components in the selected frequency range are retained to obtain a high signal-to-noise ratio signal.

[0069] Step S104, calculating the autoregressive coefficient based on the high signal-to-noise ratio signal, reconstructing the original signal through the autoregressive spectrum extrapolation model, and transforming the reconstructed signal from the frequency domain to the time domain to obtain a time domain signal.

[0070] Specifically, since the bandwidth of the system is limited, in order to improve the time domain resolution of the reflection function r(x), it is necessary to expand the bandwidth of the frequency domain signal R(ω), which can be achieved through the AR model spectrum extrapolation method. AR model spectrum extrapolation can use Burg or Yule-Walker methods to calculate the autoregressive coefficients of high signal-to-noise ratio signals, and then reconstruct the original signal through the autoregressive spectrum extrapolation model, and use the inverse Fourier transform to transform the reconstructed signal from the frequency domain to the time domain.

[0071] Step S105, calculating the thickness of the test piece according to the time difference between two adjacent signals in the time domain signal.

[0072] Specifically, after the reconstructed signal is transformed from the frequency domain to the time domain, the time domain signal is the same as the frequency domain signal, and the two signals in the superimposed state will be obviously separated. The time difference between the two signals can be obtained based on the time domain signal, and finally the thickness of the measured object can be calculated based on the time difference between the two adjacent signals.

[0073] In one implementation, the method for processing the echo signal using a wavelet filtering method based on a soft threshold can be implemented by the following formula:

[0074]

[0075] in, is the high frequency component after noise reduction d jk ,sng(d jk ) is the high frequency component d jk The sign (+-), T is the threshold, and T is set to:

[0076]

[0077] Where L is the length of the echo signal and σ is the standard deviation of the noise.

[0078] It can be seen from this embodiment that, assuming that the frequency of the incident signal remains unchanged during propagation, the echo signal has a large waveform distortion during propagation, and the frequency and amplitude will continue to decrease. The traditional Wiener deconvolution is not suitable for processing the echo signal of the ultrasonic scanning microscope. Using discrete wavelets to preprocess the filtered ultrasonic signal can eliminate signal distortion during transmission and improve the processing effect of Wiener deconvolution.

[0079] In some embodiments, after the echo signal is processed by the wavelet filtering method based on the soft threshold, the signal may be further subjected to discrete wavelet decomposition, which is implemented by the following formula:

[0080]

[0081] Among them, 2 Nis the length of the echo signal, J≤2 is the discrete wavelet decomposition series. In this scheme, the discrete wavelet decomposition series of J=2 or J=3 can be used, and h Jk (n) is the low frequency filter coefficient, g jk (n) is the high-frequency filter coefficient, and the echo signal is decomposed into low-frequency components (approximately) a J' and high frequency components (details) d jk The two parts are divided into two parts, and then the low-frequency component obtained by decomposition is used as the input signal for subsequent processing.

[0082] It should be understood that since the filtered signal will be downsampled and only half of the sampling points are retained, the discrete wavelet decomposition reduces the time domain resolution of the signal, and the decomposition operation decomposes the signal into different frequency bands, and the bandwidth of each frequency band is halved. Halving the frequency band doubles the frequency domain resolution of the signal, so the frequency components of the signal can be analyzed more accurately.

[0083] In one embodiment, the method of using the Wiener deconvolution filter to denoise the echo signal is completed by the following formula:

[0084]

[0085] Where r(t) refers to the reflectivity of the ultrasonic signal, R(ω) and Y(ω) are the Fourier transforms of r(t) and y(t), respectively, where y(t) refers to the echo signal after filtering, or the echo signal after filtering and wavelet decomposition; X(ω) is the energy density spectrum of the incident signal x(t); Q is the noise blocking factor.

[0086] In one embodiment, the original signal is reconstructed by the autoregressive spectrum extrapolation model, which can be performed by the following formula:

[0087]

[0088] In the formula, a k is the autoregression coefficient, for a k The conjugate of is the complex value obtained by extrapolating the autoregressive spectrum, p=HL is the order of the autoregressive spectrum extrapolation, frequencies 1~L are the spectrum of the low-frequency part, and frequencies H~N are the spectrum of the high-frequency part, that is, frequency H to the Nyquist frequency of the ultrasonic scanning microscope probe (ultrasonic scanning microscope probe center frequency x2).

[0089] In one implementation, the method for calculating the thickness of the measured object according to the time difference between two adjacent signals in the time domain signal is calculated by the following formula:

[0090] Δt×v=2×l

[0091] Among them, Δt represents the interval time between adjacent pulses, v is the propagation speed of ultrasound in the film, and l is the thickness measurement result.

[0092] refer to Figure 5 , Figure 5 A flow chart of an ultrasonic thickness measurement method provided in an embodiment of this specification is shown as follows: Figure 5 As shown, the method includes:

[0093] Step S201: Use ultrasonic thickness measuring equipment to transmit an ultrasonic signal X(t) to a test piece to obtain an echo signal Y(t) of the test piece.

[0094] For example, the ultrasonic thickness measuring device used can adopt the high-speed analog-to-digital conversion card of Acquisition AL8xGTe, with a sampling frequency of 1GHz (i.e., 109 times per second), an accuracy of 8bit, and the length of the RF signal of the echo signal determines the number of sampling points of the ultrasonic device. For an echo signal with a data length of 10 microseconds, 10,000 sampling points are collected within 10 microseconds, and the thickness of each layer of ultrasonic echo data is a sound path of 10-9 seconds. The measured object can be a thin film, such as a graphene film, a perovskite film, a transition metal disulfide film, a hexagonal boron nitride film, a silicene film, and a germanene film.

[0095] Figure 2 The echo signal result of a DUT can be displayed according to Figure 2 To understand the relationship between the echo signal and the multi-layer structure of the device under test. Figure 2 As shown in (a), when scanning and imaging complex multi-layered specimens and performing nondestructive testing, especially complex multi-layered structures such as thin films, due to the continuous advancement of specimen manufacturing technology, the thickness of the layered structure is often lower than the wavelength of the acoustic signal of the point-focused transducer, so in many cases, signal superposition will occur. For example, the echo signal s(t 1 ), s(t 2 ) and s(t 3 ),like Figure 2 As shown in (b), when the thickness of the thin film layer is greater than the wavelength of the point-focused transducer, the echo signal s(t 1 ), s(t 2 ) and s(t 3 ) have better separation, such as Figure 2 As shown in (c), when the film thickness is smaller than the wavelength of the transducer, the echo signal s(t 1 ), s(t 2 ) and s(t 3 ) overlap each other, and the thickness cannot be accurately measured at this time.

[0096] Step S202: performing soft threshold-based wavelet filtering on the echo signal.

[0097] Specifically, this step is completed by calculating the formula of the wavelet filtering processing method based on the soft threshold described in the above embodiment:

[0098]

[0099] in,

[0100] Exemplarily, σ is set in this scheme to be each wavelet coefficient d jk Divide the median by 0.6745.

[0101] Step S203, determining whether to perform discrete wavelet decomposition (DWT) processing on the echo signal after the soft threshold-based wavelet filtering.

[0102] Specifically, whether to perform discrete wavelet decomposition on the echo signal depends on the actual situation. In some cases, the echo signal is relatively simple or the noise level is low, so discrete wavelet decomposition is not required to avoid the computational burden. For complex echo signals, discrete wavelet decomposition can be used to perform multi-resolution analysis on the echo signal to achieve observation and processing of signals at different scales.

[0103] For the echo signal that needs to be processed by discrete wavelet decomposition, the discrete wavelet decomposition formula described in the above embodiment can be used to implement it:

[0104]

[0105] For example, in order to reduce the non-zero coefficients, this embodiment can select the wavelet basis functions such as daubechies and Coiflet, which are similar in shape to the waveform of the ultrasonic signal, as the mother wavelet. The signal can be decomposed n times using the wavelet basis function, 2 N is the length of the echo signal to be decomposed. The decomposition scale is determined according to the actual situation such as the number of sampling points and the noise size. If the decomposition scale is too high, the time domain resolution will be reduced, and if it is too low, the noise cannot be effectively suppressed. Therefore, in this embodiment, the decomposition level is set to J=2 or J=3. The low-frequency in-band signal (i.e., the low-frequency component of the echo signal) stores a lot of information of the useful signal. Therefore, by selecting the same mother wavelet function and performing a single-branch reconstruction on the original signal, these low-frequency components can be extracted and retained as the input signal for subsequent processing.

[0106] For the echo signal that does not need discrete wavelet decomposition processing, go directly to the next step.

[0107] Step S204: transform the echo signal from the time domain to the frequency domain, and then use a Wiener deconvolution filter to perform denoising on the low-frequency component of the echo signal.

[0108] Specifically, the echo signal can be transformed from the time domain to the frequency domain using Fourier transform. It is easy to understand that the echo signal here refers to the low-frequency component of the echo signal after the soft threshold-based wavelet filtering process, or the low-frequency component of the echo signal obtained after the soft threshold-based wavelet filtering process and then the discrete wavelet decomposition process. Then, the low-frequency component of the echo signal is subjected to the Wiener deconvolution filter denoising process, and the processing process is completed by the Wiener deconvolution formula described in the above embodiment:

[0109]

[0110] Where R(ω) and Y(ω) are the Fourier transforms of r(t) and y(t), respectively. r(t) refers to the reflectivity of the ultrasonic signal, and y(t) refers to the low-frequency component of the echo signal after the soft threshold wavelet filter processing, or the low-frequency component of the echo signal after the soft threshold wavelet filter processing and then the discrete wavelet decomposition. X(ω) is the energy density spectrum of the incident signal x(t), and the incident signal x(t) is Figure 5 The ultrasound device pulse waveform x(t) shown in FIG. 1 needs to be converted from the time domain to the frequency domain (i.e., Figure 5 x = FFT(x) shown in ); Q is the noise blocking factor. Specifically, the noise blocking factor Q refers to the quality factor of a system or component at a specific frequency, which describes the response characteristics of the system near its resonant frequency. The noise blocking factor Q can be estimated by spectrum analysis, experimental measurement and simulation, adaptive algorithm and other methods. In this example, the value of Q can be set to |X(ω)| 2 1% of maximum value.

[0111] Step S205, calculating the frequency drop window of the deconvolution spectrum to obtain a high signal-to-noise ratio signal.

[0112] Specifically, the main frequency range of the signal of interest can be determined based on prior knowledge or signal characteristics. Then a frequency window function is designed, which is 1 (or close to 1) in the selected frequency range and 0 (or close to 0) in other frequency ranges. The window types used include but are not limited to rectangular windows, triangular windows, and Gaussian windows. Finally, the frequency domain signal is multiplied by the frequency window function, thereby retaining the signal components in the selected frequency range and filtering out the noise in other frequency ranges to achieve attenuation compensation for the echo signal.

[0113] For example, Figure 6As shown, the -3dB bandwidth can be used as a frequency reduction window. Figure 6 In the graph, the vertical axis represents the amplitude response Y(ω) of the filter, which reflects the gain or attenuation of the filter at different frequencies. The horizontal axis represents the frequency ω. The maximum value of the curve in the figure corresponds to the maximum gain of the filter at a certain frequency. This point is the center frequency of the filter. The 3dB bandwidth is the frequency range between the two -3dB points. Within this range, the response of the filter is relatively flat and the signal can pass well. Beyond this range, the response of the filter drops rapidly and the signal is significantly attenuated.

[0114] Step S206: Calculate the autoregressive coefficient of the autoregressive spectrum extrapolation (AR) model based on the high signal-to-noise ratio signal.

[0115] The autoregressive coefficient of the high signal-to-noise ratio signal can be calculated by using methods such as Burg or Yule-Walker. Specifically, it is necessary to first determine the order p of the AR model according to the needs of the specific application. Exemplarily, this embodiment determines the order p=HL of the AR model. Specifically, H and L refer to the frequency of the echo signal, and the frequencies 1 to L are the spectrum of the low-frequency part of the echo signal, and the frequencies H to N are the spectrum of the high-frequency part of the echo signal. Then, the autoregressive coefficient of the high signal-to-noise ratio signal is calculated by using methods such as Burg or Yule-Walker.

[0116] Step S207, reconstructing the original signal through an autoregressive spectral extrapolation (AR) model, and transforming the reconstructed signal from the frequency domain to the time domain to obtain a time domain signal.

[0117] Specifically, this step reconstructs the original signal through the autoregressive spectrum extrapolation model reconstruction formula described in the above embodiment, that is:

[0118]

[0119] In the formula, a k is the autoregression coefficient, for a k The conjugate of is the complex value obtained by extrapolating the autoregressive spectrum, p=HL is the order of the autoregressive spectrum extrapolation, frequencies 1~L are the spectrum of the low-frequency part, and frequencies H~N are the spectrum of the high-frequency part, that is, frequency H to the Nyquist frequency of the ultrasonic scanning microscope probe (ultrasonic scanning microscope probe center frequency x2).

[0120] Next, the reconstructed signal can be transformed from the frequency domain to the time domain using the Fourier transform:

[0121]

[0122] Where Y(t) is the Fourier transform of Y(ω), and y(w) refers to the reconstructed frequency domain signal. After the frequency domain signal y(w) is Fourier transformed, the time domain signal Y(t) of the reconstructed signal is obtained.

[0123] For example, Figure 7 The following is a diagram showing the effect of this scheme on the measurement of the thickness of the negative electrode film of a lithium battery. (a) Three original A-scan signals reflected from the negative electrode of the battery. (b) The time domain signal processing result after the reconstruction of the original signal by this scheme. As can be seen from Figure (a), due to the low frequency and long wavelength of the original signal, the echo signals are superimposed on each other. Therefore, it is impossible to accurately find the time when the echo is generated from the echo signal and calculate the thickness of the film. Furthermore, in Figure (b), after the signal reconstruction process, the superimposed signals are separated, and the position of the echo signal can be accurately determined, thereby realizing super-resolution thickness measurement of the film material. The above comparison shows that this scheme can achieve super-resolution measurement of film thickness without increasing the operating frequency of the ultrasonic equipment, thereby realizing low-cost ultrasonic thickness measurement of thin film materials.

[0124] Step S208, calculating the thickness of the test piece according to the time difference between adjacent signals.

[0125] Specifically, the thickness of the measured object is calculated according to the thickness calculation formula described in the above embodiment, that is:

[0126] Δt×v=2×l

[0127] Among them, Δt represents the interval time between adjacent pulses, v is the propagation speed of ultrasound in the film, and l is the thickness measurement result.

[0128] For example, continue to refer to Figure 7 After the original signal is processed, only the reflected signals of the upper and lower surfaces are retained, and the rest of the irrelevant signals are basically filtered out. The time point t at the center of the two signals is taken. 1 =129us and t 2 =159us, by calculating the time difference △t=0.03us between adjacent signals, the thickness of the film can be calculated to be 0.06715mm by applying the thickness calculation formula shown in this embodiment.

[0129] The present invention also provides an ultrasonic thickness measuring device, Figure 3 FIG. 2 shows a schematic diagram of the structure of an ultrasonic thickness measuring device provided in an embodiment of this specification. Figure 3 As shown, the device includes: a rolling mill 1, an ultrasonic transducer 2, a motor 3 and a coupling liquid supply system 4.

[0130] Specifically, the rolling mill 1 is used to drive the measured piece 6 to move in the horizontal direction. It is easy to understand that the rolling mill 1 driving the measured piece 6 to move can be one of the manufacturing processes of the measured piece 6, and the thickness of the measured piece 6 can be measured during the manufacturing process.

[0131] The ultrasonic transducer 2 is used to measure the thickness of the measured object 6. Specifically, the ultrasonic transducer 2 can be arranged below the measured object 6 and vertically aligned with the lower surface of the measured object 6.

[0132] The motor 3 is used to adjust the distance between the ultrasonic transducer 2 and the lower surface of the test piece 6. Specifically, the motor 3 can drive the ultrasonic transducer 2 to move along a direction perpendicular to the test piece 6, thereby maintaining a suitable distance between the ultrasonic transducer 2 and the test piece 6.

[0133] The coupling liquid supply system 4 is used to apply coupling liquid between the ultrasonic transducer 2 and the test piece 6 to ensure acoustic coupling between the ultrasonic transducer 2 and the test piece 6 to optimize the energy transmission and detection effect of the ultrasonic wave.

[0134] Specifically, while the rolling mill 1 drives the measured piece 6 to move in the horizontal direction, the ultrasonic transducer 2 disposed below the measured piece 6 measures the thickness of the measured piece 6. During the measurement process, the coupling liquid supply system 4 provides coupling liquid to the position between the ultrasonic transducer 2 and the measured piece 6 to ensure that the ultrasonic transducer 2 forms an acoustic coupling with the measured piece 6. In addition, during the measurement process, the motor 4 adjusts the distance between the ultrasonic transducer 2 and the lower surface of the measured piece 6 to ensure a good thickness detection effect.

[0135] It can be seen from the ultrasonic thickness measuring equipment provided in this embodiment that the synchronous operation of the rolling mill 1 and the ultrasonic transducer 2 can realize the thickness measurement of the workpiece 6 during the manufacturing process. During the measurement process, the coupling liquid supply system 4 provides coupling liquid to the ultrasonic transducer 2, which can optimize the energy transmission and detection effect of the ultrasonic wave.

[0136] In one embodiment, continue to refer to Figure 3 The coupling liquid supply system 4 includes: a liquid storage tank 41 , a filtering device 42 , a pump 43 and a coupling liquid injection device 44 .

[0137] The coupling liquid spraying device 44 is used to spray the coupling liquid to the position between the ultrasonic transducer 2 and the lower surface of the test piece 6. Specifically, the coupling liquid spraying device 44 can be used to slowly spray the coupling liquid to the head of the ultrasonic transducer 2 so that the space between the ultrasonic transducer 2 and the test piece 6 is filled with the coupling liquid while avoiding flushing the test piece 6.

[0138] The liquid storage tank 41 is used to store coupling liquid, and the coupling liquid includes but is not limited to water, glycerin, and silicone oil. Preferably, water is used as the coupling liquid.

[0139] The pump 43 is used to drive the coupling liquid in the liquid storage tank 41 to be transmitted to the coupling liquid injection device 44 .

[0140] The filtering device 42 is disposed between the liquid storage tank 41 and the pump 43 and is used to filter the coupling liquid to filter out unnecessary impurities.

[0141] It can be seen from this embodiment that the coupling liquid is slowly sprayed toward the head of the ultrasonic transducer 2 by the coupling liquid spraying device 44, thereby ensuring that the coupling liquid is evenly filled in the gap between the transducer and the object under test, thereby avoiding poor coupling or damage to the surface of the object under test caused by rapid flushing.

[0142] In one embodiment, continue to refer to Figure 3 The ultrasonic thickness measuring device further includes an air dryer 5, which is used to air dry the coupling liquid on the surface of the measured object 6 after the thickness measurement is completed, so as to prevent the coupling liquid from affecting the subsequent processing of the measured object 6. Specifically, the air dryer 5 can be arranged on one side of the ultrasonic transducer 2 and on one side of the moving path of the measured object 6, so as to ensure that the surface of the measured object 6 can be dried in time after the measurement.

[0143] In one embodiment, continue to refer to Figure 3 The ultrasonic thickness measuring device also includes: a transceiver selector 7, a pulse transmitting unit 8, a preamplifier 9, an analog signal acquisition unit 10 and an industrial computer 11.

[0144] The transceiver selector 7 is connected to the ultrasonic transducer 2 , and is used to control the transmitting and receiving states of the ultrasonic transducer 2 .

[0145] The pulse transmitting unit 8 is used to excite the ultrasonic transducer 2 to generate corresponding ultrasonic signals.

[0146] The preamplifier 9 is used to amplify the echo signal received from the ultrasonic transducer 2 .

[0147] The analog signal acquisition unit 10 is used to acquire the echo signal amplified by the preamplifier 9 .

[0148] The industrial computer 11 is used to control the operation of the entire ultrasonic thickness measuring device, such as sending an ultrasonic signal transmission instruction to the transceiver selector 7, the transceiver selector 7 switches to the transmission mode, and the pulse transmission unit 8 excites the ultrasonic transducer 2 to generate a corresponding ultrasonic signal; sending an echo signal reception instruction to the transceiver selector 7, the transceiver selector 7 switches to the reception mode; sending an echo signal amplification instruction to the preamplifier 9, the preamplifier 9 turns on the switch to amplify the echo signal; sending an analog signal acquisition instruction to control the analog signal acquisition unit 10 to acquire the echo signal, etc.

[0149] Specifically, the transceiver selector 7, the preamplifier 8, the analog signal acquisition unit 10, and the industrial computer 11 are connected in series in sequence to realize a complete data processing flow from signal selection and transmission, amplification, acquisition to final processing. When in the transmitting state, the transceiver selector 7 is disconnected from the preamplifier 9, and the transceiver selector 7 is connected to the pulse transmitting unit 8, so that the ultrasonic transducer 2 completes signal transmission; when in the receiving state, the transceiver selector 7 is disconnected from the pulse transmitting unit 8, and the transceiver selector 7 is connected to the preamplifier 9, and the echo signal is amplified by the preamplifier 9, and the amplified echo signal is collected by the analog signal acquisition unit 10, thereby completing the echo signal collection.

[0150] In one implementation, the industrial computer 11 may be externally connected to a display screen 12 , and the display screen 12 is used to monitor the operating status of each device and set the operating parameters of each device.

[0151] As can be seen from this embodiment, the industrial computer 11 can perform precise time and logic control on each component to ensure the synchronization and consistency of ultrasonic emission, reception and signal processing. The transceiver selector 7 can quickly switch modes under the command of the industrial computer to ensure seamless connection of the emission and reception process. The preamplifier 9 is used to amplify the weak echo signal received from the ultrasonic transducer, improve the signal-to-noise ratio, and enhance the sensitivity and accuracy of the detection.

[0152] It is worth noting that those skilled in the art should easily realize that, in combination with the units and algorithm steps of each example described in the embodiments disclosed herein, the present application can be implemented in the form of hardware or a combination of hardware and computer software. Whether a function is executed in the form of hardware or computer software driving hardware depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0153] Those skilled in the art should be aware that in one or more of the above examples, the functions described in the present invention can be implemented by hardware, software, firmware or any combination thereof. When implemented by software, these functions can be stored in a computer-readable medium or transmitted as one or more instructions or codes on a computer-readable medium.

[0154] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made on the basis of the technical solution of the present invention should be included in the scope of protection of the present invention.

Claims

1. An ultrasonic thickness measurement method, characterized in that: The method comprises: Use ultrasonic thickness measuring equipment to transmit ultrasonic signals to the test piece to obtain the echo signal of the test piece; Performing filtering processing on the echo signal using a wavelet filtering method based on a soft threshold; The echo signal is transformed from the time domain to the frequency domain, and a low-frequency component of the echo signal is denoised using a Wiener deconvolution filter to separate the echo signals in a superimposed state, and a high signal-to-noise ratio signal is obtained using a frequency window; Calculating an autoregressive coefficient based on the high signal-to-noise ratio signal, reconstructing the original signal through an autoregressive spectrum extrapolation model, and transforming the reconstructed signal from the frequency domain to the time domain to obtain a time domain signal; The thickness of the measured object is calculated based on the time difference between two adjacent signals in the time domain signal.

2. The method according to claim 1, characterized in that After filtering the echo signal using a wavelet filtering method based on a soft threshold, the echo signal is subjected to discrete wavelet decomposition, which is achieved by the following formula: Among them, 2 N is the length of the echo signal, J≤N is the number of discrete wavelet decompositions, h Jk (n) is the low frequency filter coefficient, g jk (n) is the high-frequency filter coefficient; the echo signal is decomposed into low-frequency components a Jk and high frequency component d jk Two parts.

3. The method according to claim 1, characterized in that The method of filtering the echo signal using a wavelet filtering method based on a soft threshold is implemented by the following formula: in, is the high frequency component after noise reduction d jk ,sng(d jk ) is the high frequency component d jk The sign (+-), T is the threshold, and T is set to: Where L is the length of the echo signal and σ is the standard deviation of the noise.

4. The method according to claim 1, characterized in that: The method of using the Wiener deconvolution filter to denoise the low-frequency component of the echo signal is completed by the following formula: Where R(ω) and Y(ω) are the Fourier transforms of r(t) and y(t), respectively. r(t) refers to the reflectivity of the ultrasonic signal, X(ω) is the energy density spectrum of the incident signal x(t), and Q is the noise blocking factor.

5. The method according to claim 1, characterized in that The method of calculating the autoregressive coefficient based on the high signal-to-noise ratio signal and reconstructing the original signal through the autoregressive spectrum extrapolation model specifically includes: Among them, a k is the autoregression coefficient, for a k The conjugate of is the complex value obtained by extrapolating the autoregressive spectrum, p=HL is the order of the autoregressive spectrum extrapolation, frequencies 1~L are the spectrum of the low-frequency part, and frequencies H~N are the spectrum of the high-frequency part.

6. The method according to claim 1, characterized in that The method for calculating the thickness of the measured object according to the time difference between two adjacent signals in the time domain signal is as follows: Δt×v=2×l Among them, Δt represents the interval time between adjacent pulses, v is the propagation speed of ultrasound in the film, and l is the thickness measurement result.

7. An ultrasonic thickness measuring device, characterized in that: The ultrasonic thickness measuring device comprises: an ultrasonic transducer, a coupling liquid supply system, a rolling mill and a motor, wherein: The rolling mill is used to drive the tested piece to move in the horizontal direction; An ultrasonic transducer, used for measuring the thickness of the measured object, wherein the ultrasonic transducer is arranged below the measured object and vertically aligned with the lower surface of the measured object; A motor, used for adjusting the distance between the ultrasonic transducer and the lower surface of the measured object; The coupling liquid supply system is used to apply the coupling liquid between the ultrasonic transducer and the measured object to ensure that the ultrasonic transducer and the measured object are acoustically coupled.

8. The ultrasonic thickness measuring device according to claim 7, characterized in that: The coupling liquid supply system comprises: a liquid storage tank, a filtering device, a pump and a coupling liquid injection device, wherein: A coupling liquid spraying device, used for spraying coupling liquid to a position between the ultrasonic transducer and the lower surface of the measured object; A liquid storage tank, used for storing coupling liquid; A pump, used for driving the coupling liquid in the liquid storage tank to be transferred to the coupling liquid injection device; The filtering device is arranged between the liquid storage tank and the pump and is used to filter impurities from the coupling liquid.

9. The ultrasonic thickness measuring device according to claim 7, characterized in that: The ultrasonic thickness measuring device further comprises an air dryer, which is used to air dry the coupling liquid on the surface of the measured object 6 after the thickness measurement is completed.

10. The ultrasonic thickness measuring device according to any one of claims 7 to 9, characterized in that: The ultrasonic thickness measuring device also includes: a transceiver selector, connected to the ultrasonic transducer, and used to control the transmitting and receiving states of the ultrasonic transducer; A pulse transmitting unit, used for exciting the ultrasonic transducer to generate a corresponding ultrasonic signal; a preamplifier, used to amplify the echo signal received from the ultrasonic transducer; An analog signal acquisition unit, used for acquiring the echo signal amplified by the preamplifier; Industrial computer, used to control the operation of the entire ultrasonic thickness measurement equipment; Among them, the transceiver selector, preamplifier, analog signal acquisition unit, and industrial computer are connected in series in sequence; when in the transmitting state, the transceiver selector is disconnected from the preamplifier, and the transceiver selector is connected to the pulse transmitting unit, so that the ultrasonic transducer completes signal transmission; when in the receiving state, the transceiver selector is disconnected from the pulse transmitting unit, and the transceiver selector is connected to the preamplifier, the echo signal is amplified by the preamplifier, and the amplified echo signal is collected by the analog signal acquisition unit, thereby completing the collection of the echo signal.

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