Electromagnetic ultrasonic resonance thickness measuring method capable of adaptively regulating and controlling sweep frequency step length

Abstract

The invention discloses an electromagnetic ultrasonic resonance thickness measuring method capable of adaptively regulating and controlling sweep frequency step length, which comprises the following steps of: exciting a continuous sweep frequency signal of a certain frequency band by an electromagnetic ultrasonic probe at a variable frequency step length, forming ultrasonic waves on the surface of a workpiece, penetrating the workpiece, and reflecting on the other surface of the workpiece to measure the thickness of the workpiece. The electromagnetic ultrasonic probe receives the reflected time domain signal, time-frequency conversion is conducted on the time domain signal based on the Fourier transform principle, a spectrogram is obtained, and wall thickness information of the current measuring point is calculated according to the frequency difference. However, due to the fact that the processes of frequency sweeping excitation, ultrasonic wave receiving and data processing of a series of waveforms are slow, the real-time performance of measurement can be affected, according to the electromagnetic ultrasonic resonance thickness measurement method oriented to ultrathin wall thickness measurement, frequency domain segmentation is achieved based on signal amplitude recognition, the frequency sweeping step length is shortened near the resonance frequency, and the measurement accuracy is improved. The step length is increased in other frequency bands, full-band equal-step-length frequency sweeping is replaced by variable-step-length frequency sweeping, the frequency sweeping step number is reduced, and therefore the measurement efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic measurement technology, and specifically relates to an electromagnetic ultrasonic resonance thickness measurement method based on adaptive adjustment of the sweep frequency step size of the target wall thickness, which is used to improve the calculation speed of electromagnetic ultrasonic resonance (EMAR) thickness measurement technology. Background Technology

[0002] Electromagnetic ultrasonic thickness measurement is based on the generation of alternating Lorentz forces on the surface of a workpiece by an electromagnetic ultrasonic sensor, which in turn generates ultrasonic waves inside the part, thereby enabling the measurement of the part's wall thickness. Compared with the traditional piezoelectric ultrasonic thickness measurement method, electromagnetic ultrasonic thickness measurement technology has the advantages of being non-contact, requiring no coupling agent, and being insensitive to the surface quality of the part.

[0003] While electromagnetic ultrasonic pulse thickness measurement can be widely applied to conductor thickness measurement, it still has limitations for measuring ultra-thin components. When the workpiece is thin, the reflected waves overlap, making it impossible to accurately calculate the echo time difference. To overcome these limitations, an electromagnetic ultrasonic resonance thickness measurement method is proposed. Electromagnetic ultrasonic resonance is based on electromagnetic ultrasonic excitation of ultrasonic waves, incorporating a frequency sweep excitation method to achieve a wavelength variation effect. Resonance occurs when the wavelength is an integer multiple of the wall thickness. The frequency value corresponding to the correct wall thickness is determined by peak finding in the frequency domain. This method not only lowers the lower limit of wall thickness measurement but also compensates for the low signal-to-noise ratio of electromagnetic ultrasound. Although the electromagnetic ultrasonic resonance thickness measurement method can measure wall thicknesses below 1 mm, the measurement efficiency based on the frequency sweep method is low, and currently, there is a lack of efficient methods based on frequency sweep measurement.

[0004] Currently, invention patent application No. 201610172021.0 discloses an electromagnetic ultrasonic thickness measurement method based on a digital acquisition circuit system; invention patent application No. 201510582587.6 discloses a method for measuring the thickness of metallic materials using electromagnetic ultrasound with two pulses of defined frequencies; invention patent application No. 201910152431.2 discloses an electromagnetic ultrasonic thickness measurement method based on frequency domain analysis. The above technologies mainly focus on obtaining the time and frequency domain periods of electromagnetic ultrasonic echo signals, and concentrate on solving how to extract the time / frequency interval of the echo signal through various methods, increase the extraction efficiency without changing the original circuit, and improve the detection accuracy.

[0005] The above patents can only improve the measurement accuracy by using electromagnetic ultrasonic thickness measurement methods, but none of them mention a rapid thickness measurement method using electromagnetic ultrasonic resonance technology, and therefore cannot fundamentally improve the real-time performance of thickness measurement. Summary of the Invention

[0006] To address the aforementioned problems in the existing technology, this invention proposes an electromagnetic ultrasonic thickness measurement method for rapid measurement of ultrathin wall thickness.

[0007] The technical solution of this invention:

[0008] An adaptive electromagnetic ultrasonic resonance thickness measurement method with adjustable sweep step size is proposed. This method uses an electromagnetic ultrasonic probe to excite a continuous sweep signal across a specific frequency band with varying frequency step sizes, generating ultrasonic waves on the workpiece surface. The ultrasonic waves penetrate the workpiece and are reflected on another surface. The electromagnetic ultrasonic probe receives the reflected ultrasonic signal, and an A / D conversion module converts the analog signal into a digital signal. A data processing unit performs time-frequency conversion on the digital waveform based on Fourier transform to obtain a spectrum. The wall thickness information at the current measurement point is calculated based on the frequency difference. However, the process of sweep excitation, ultrasonic wave reception, and data processing is slow, affecting the real-time performance of the measurement. To address this, a proposed electromagnetic ultrasonic resonance thickness measurement method for ultra-thin wall thickness measurement is proposed. This method achieves frequency domain segmentation based on signal amplitude recognition, shortens the sweep step size near the resonant frequency, and increases the step size in other frequency bands. Variable step size sweeping replaces uniform step size sweeping across the entire frequency band, reducing the number of sweeping steps and improving measurement efficiency. The flowchart is shown below. Figure 1 As shown, the method includes the following steps:

[0009] A. Maintain a distance of 1-3mm between the electromagnetic ultrasonic sensor probe and the workpiece being tested;

[0010] B. Based on the thickness and material properties of the workpiece being tested, determine the frequency sweep interval, input an excitation voltage with a carrier frequency of f1 to f2 into the electromagnetic ultrasonic sensor probe, and input the number of sampling points to excite the transmission of ultrasonic signals; the ultrasonic signals are reflected multiple times inside the workpiece being tested, and the superimposed time-domain signals F(t) are received on the electromagnetic ultrasonic sensor probe.

[0011] C. Based on the Fourier transform principle, the time-domain signal F(t) is converted to a frequency spectrum to obtain a spectrum diagram; the frequency difference between two adjacent peaks in the spectrum diagram is calculated, and the thickness of the measuring point is calculated according to formula (1), where Δf is the frequency difference between two adjacent peaks, d is the thickness of the measuring point, and v is the transverse wave velocity of the workpiece being measured.

[0012]

[0013] D. Pre-collect the spectrum signal of the target wall thickness d and the workpiece with different thicknesses within the tolerance range (d-δ~d+δ) as the calibration signal, where δ represents the manufacturing tolerance. At the same time, keep the distance between the electromagnetic ultrasonic sensor probe and the workpiece being tested constant, analyze the amplitude characteristics of the calibration signal, and obtain the amplitudes A1 and A2 of the Nth and N+1th wave peaks, respectively.

[0014] E. Set amplitude thresholds based on the spectral analysis of the calibration signal to select different sampling strategies;

[0015] F. Divide the frequency bands for sweeping according to the set amplitude threshold, and adopt sparse / dense sampling strategies for different frequency bands. Adopt a dense sampling strategy near the Nth and N+1th peaks to reduce the sweeping step size; adopt a sparse sampling strategy at other locations to increase the sweeping step size.

[0016] G. Set the initial frequency for frequency sweeping according to the thickness measurement method and start frequency sweeping. When the amplitude is less than the threshold, adopt a sparse sampling strategy and set a larger frequency sweeping step size to improve frequency sweeping efficiency. When the amplitude is greater than the threshold, adopt a dense sampling strategy and set a smaller frequency sweeping step size to improve measurement accuracy. Repeat the above process until the Nth and N+1th wave peaks are acquired, then end the frequency sweeping.

[0017] H. Based on the frequency spectrum obtained by frequency sweep, calculate the frequency difference between two adjacent peaks, the Nth and N+1th order peaks, and calculate the thickness of the workpiece under test using equation (1).

[0018] Furthermore, it is necessary to set an amplitude threshold to determine the sparse / dense sampling strategy for the frequency sweep step size. The threshold selection strategy is as follows: the threshold for the Nth order peak is A1 / 2, and the threshold for the (N+1)th order peak is A2 / 2. When the frequency amplitude curve reaches A1 / 2 and A2 / 2 with an upward trend, dense sampling is selected; when the frequency amplitude curve reaches A1 / 2 and A2 / 2 with a downward trend, sparse sampling is selected.

[0019] Furthermore, in step F, it is necessary to set the sweep step size based on the frequency information. The selection of the step size includes the following steps:

[0020] A. The principle for determining the sweep step size of dense sampling is: for the wall thickness tolerance range of the calibrated workpiece is d-δ~d+δ, where δ represents the manufacturing tolerance, that is, the wall thickness range of the workpiece being measured is distributed within d-δ~d+δ, then the required measurement resolution of the workpiece being measured is Δd=δ / 10, and its frequency difference Δf1 is as shown in equation (4). It can be seen that for the selection of the dense step size of the workpiece being measured with different thicknesses, in order to ensure measurement accuracy, the larger the thickness d, the smaller the frequency difference Δf1. Therefore, based on the maximum thickness value measured, the minimum value of frequency change Δf1 is calculated, and the minimum value of frequency change is set as the sweep step size of dense sampling.

[0021]

[0022]

[0023]

[0024] Where d is the thickness of the measuring point, v is the transverse wave velocity of the workpiece being measured, Δd is the required measurement resolution of the workpiece being measured, f1 is the maximum first-order resonant frequency within the thickness tolerance range of the thin plate workpiece being measured, f1' is the minimum first-order resonant frequency within the thickness tolerance range of the thin plate workpiece being measured, and Δf1 is the minimum value of the frequency change, i.e. the selected dense sampling step size.

[0025] B. The principle for determining the sweep step size of sparse sampling is: to ensure that the sweep step size does not cross the resonant frequency, the peak values ​​in the frequency-amplitude plots of the Nth and (N+1)th order peaks in the calibration signals of different thicknesses are respectively [A 11 A 12 A 13 ,…,A 1n ], [A 21 A 22 A 23 ,…,A 2n Find the rising half-peak and falling half-peak corresponding to each resonant frequency. The resonant frequencies corresponding to the half-peaks are respectively... Where n represents the number of thin plates selected for calibration, superscript 1 indicates the rising edge, and superscript 2 indicates the falling edge; then the sparse frequency selection rule is: and Smaller values.

[0026] Furthermore, the workpiece is made of metal or alloy.

[0027] The beneficial effects of this invention are:

[0028] 1. This invention proposes a variable step size sweep frequency method, which sets an amplitude threshold, reduces the number of sampling points, and improves the efficiency of thickness measurement.

[0029] 2. This invention can be applied to in-machine thickness measurement and processing compensation, and can measure the thickness of ultra-thin wall workpieces in real time, realizing the integration of processing and measurement, thereby better improving the uniformity of workpiece wall thickness. Attached Figure Description

[0030] Figure 1 The variable step size sweep frequency thickness measurement method proposed in this invention includes a sparse / dense step size calibration step and a variable step size step during the measurement process;

[0031] Figure 2 To measure the overlapping time-domain signal F(t) obtained from the thin-walled component;

[0032] Figure 3 The frequency amplitude curve is obtained by electromagnetic ultrasonic resonance thickness measurement method;

[0033] Figure 4 This is a time-domain signal used to calibrate the speed of sound. Detailed Implementation

[0034] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and technical solutions.

[0035] The theoretical thickness of the aluminum alloy sheet to be tested is 0.4 mm, with upper and lower tolerances of +0.01 mm and -0.01 mm, respectively. Based on the principle of electromagnetic ultrasonic resonance, the accurate thickness of the target aluminum alloy sheet is obtained through a variable step size sweep frequency mode.

[0036] A. Sound Velocity Calibration: The sound velocity of the material being measured is calibrated using a calibration block of known thickness. Here, a 15mm thick aluminum alloy sample block is selected. The electromagnetic ultrasonic sensor probe is mounted on the machine tool spindle or clamping fixture, maintaining a 1mm distance between the electromagnetic ultrasonic probe and the calibration block. An excitation pulse signal is sent to the thickness measuring probe via the electromagnetic ultrasonic excitation module. The echo signal reflected by the 15mm calibration block is as follows: Figure 4 As shown, the sound speed calibration calculation process is as follows:

[0037]

[0038] B. Resonance frequency amplitude calibration: Calculate the electromagnetic ultrasonic resonance frequencies f1 and f2 based on the target wall thickness of 0.4 mm.

[0039]

[0040]

[0041] C. Considering the frequency range of the electromagnetic ultrasonic sensor and ensuring that the sweep frequency interval includes at least two resonant frequencies, the sweep frequency interval is determined to be 3–8.5 MHz based on the calculated f1 and f2. The sweep frequency step size is set to 0.1 MHz, and the spectrum is obtained. After three repetitions, the average value of the resonant peaks is taken, which is the amplitude calibration signal. The average peak value of the first-order resonant frequency is A1 = 6.973, and the average peak value of the second-order resonant frequency is A2 = 5.019. The corresponding step size switching gate is set to half of the resonant frequency, i.e., A1 / 2 = 3.4865 and A2 / 2 = 2.5095. The rising and falling edge frequencies corresponding to A1 / 2 are 3.8037 MHz and 3.8863 MHz, respectively, and the rising and falling edge frequencies corresponding to A2 / 2 are 7.6354 MHz and 7.6643 MHz, respectively.

[0042] D. Calculation of dense step size. The actual measured workpiece thickness is 0.4mm, with upper and lower tolerances of +0.01mm and -0.01mm respectively. Based on the set threshold, the minimum frequency difference corresponding to this threshold is f0 MHz, and the large step size is set to f0 / 2MHz. The required measurement resolution for this workpiece is 0.001mm. According to formula (4), the minimum frequency difference corresponding to the measurement error should be calculated as follows: The dense step size is set to Δf1, and the calculation process is shown in equation (8).

[0043]

[0044] E. Sparse step size calculation: Set the initial frequency to 3MHz. According to calibration requirements, a smaller calibration step size results in more accurate calibration. Select a calibration step size of 1000Hz. The echo is presented in the form of voltage V. When V≥A1 / 2, i.e., 3.4865, f... 11 =3.8037MHz,f 12 =3.8863MHz, calculate the frequency difference when the voltage value passes through A1 / 2 twice to ensure that the sweep step size does not cross the resonant frequency. ΔF1 = 3.8863MHz - 3.8037MHz = 82600Hz. When V ≥ A2 / 2, that is, ΔF2 7 = 0.664 - 3.MH, take the smaller value between ΔF1 and ΔF2, so the sparse step size is 28900Hz.

[0045] F. Begin frequency sweep measurement, starting from 3MHz. When the amplitude is less than 3.8037MHz, adopt a sparse sampling strategy and increase the sweep step size to 28900Hz to improve sweep efficiency. When the amplitude is greater than A1 / 2, i.e., 3.4865, adopt a dense sampling strategy and decrease the sweep step size to 9641Hz to improve the accuracy of thickness measurement. When the amplitude is less than 3.4865, increase the sweep step size again to 28900Hz and continue the sweep process. When the amplitude is greater than A2 / 2, i.e., 2.5095, decrease the sweep step size to 9641Hz. When the amplitude is less than 2.5095, increase the sweep step size again to 28900Hz to 8.5MHz to complete the spectrum diagram.

[0046] G. Based on the frequency sweep spectrum, the frequency difference between two adjacent peaks is calculated to be Δf, which is 3.79MHz. The thickness of the workpiece is calculated to be 0.408mm using formula (1).

[0047] Using the above methods, the number of sampling points was reduced from 570 to 196 compared to dense sampling across the entire frequency band.

[0048] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. An adaptively adjustable sweep frequency step size electromagnetic ultrasonic resonance thickness measurement method, characterized in that, Includes the following steps: A. Maintain a distance of 1-3mm between the electromagnetic ultrasonic sensor probe and the workpiece being tested; B. Based on the thickness and material properties of the workpiece being tested, determine the frequency sweep interval, input an excitation voltage with a carrier frequency of f1 to f2 into the electromagnetic ultrasonic sensor probe, and input the number of sampling points to excite the transmission of ultrasonic signals; the ultrasonic signals are reflected multiple times inside the workpiece being tested, and the superimposed time-domain signals F(t) are received on the electromagnetic ultrasonic sensor probe. C. Based on the Fourier transform principle, the time-domain signal F(t) is converted to a frequency spectrum to obtain a spectrum diagram; the frequency difference between two adjacent peaks in the spectrum diagram is calculated, and the thickness of the measuring point is calculated according to formula (1), where Δf is the frequency difference between two adjacent peaks, d is the thickness of the measuring point, and v is the transverse wave velocity of the workpiece being measured. D. Pre-collect the spectrum signal of the target wall thickness d and the workpiece with different thicknesses within the tolerance range (d-δ~d+δ) as the calibration signal, where δ represents the manufacturing tolerance. At the same time, keep the distance between the electromagnetic ultrasonic sensor probe and the workpiece being tested constant, analyze the amplitude characteristics of the calibration signal, and obtain the amplitudes A1 and A2 of the Nth and N+1th wave peaks, respectively. E. Set amplitude thresholds based on the spectral analysis of the calibration signal to select different sampling strategies; F. Divide the frequency bands for sweeping according to the set amplitude threshold, and adopt sparse / dense sampling strategies for different frequency bands. Adopt a dense sampling strategy near the Nth and N+1th peaks to reduce the sweeping step size; adopt a sparse sampling strategy at other locations to increase the sweeping step size. G. Set the initial frequency for frequency sweeping according to the thickness measurement method and start frequency sweeping. When the amplitude is less than the threshold, adopt a sparse sampling strategy and set a larger frequency sweeping step size to improve frequency sweeping efficiency; when the amplitude is greater than the threshold, adopt a dense sampling strategy and set a smaller frequency sweeping step size to improve measurement accuracy. Repeat the above process until the Nth and N+1th wave peaks are acquired, then end the frequency sweep. H. Based on the frequency spectrum obtained by frequency sweep, calculate the frequency difference between two adjacent peaks, the Nth and N+1th order peaks, and calculate the thickness of the workpiece under test using equation (1).

2. The electromagnetic ultrasonic resonance thickness measurement method with adaptive frequency sweep step size according to claim 1, characterized in that, In step E, an amplitude threshold needs to be set to determine the sparse / dense sampling strategy for the frequency sweep step size. The threshold selection strategy is as follows: the threshold for the Nth order peak is A1 / 2, and the threshold for the (N+1)th order peak is A2 / 2. When the frequency amplitude curve reaches A1 / 2 and A2 / 2 with an upward trend, dense sampling is selected; when the frequency amplitude curve reaches A1 / 2 and A2 / 2 with a downward trend, sparse sampling is selected.

3. The electromagnetic ultrasonic resonance thickness measurement method with adaptive frequency sweep step size according to claim 2, characterized in that, In step F, the sweep step size needs to be set according to the frequency information. The selection of the step size includes the following steps: A. The principle for determining the sweep step size of dense sampling is: for the wall thickness tolerance range of the calibrated workpiece is d-δ~d+δ, where δ represents the manufacturing tolerance, that is, the wall thickness range of the workpiece being measured is distributed within d-δ~d+δ, then the required measurement resolution of the workpiece being measured is Δd=δ / 10, and its frequency difference Δf1 is as shown in equation (4). It can be seen that for the selection of the dense step size of the workpiece being measured with different thicknesses, in order to ensure measurement accuracy, the larger the thickness d, the smaller the frequency difference Δf1. Therefore, based on the maximum thickness value measured, the minimum value of frequency change Δf1 is calculated, and the minimum value of frequency change is set as the sweep step size of dense sampling. Where d is the thickness of the measuring point, v is the transverse wave velocity of the workpiece being measured, Δd is the required measurement resolution of the workpiece being measured, f1 is the maximum first-order resonant frequency within the thickness tolerance range of the thin plate workpiece being measured, f1' is the minimum first-order resonant frequency within the thickness tolerance range of the thin plate workpiece being measured, and Δf1 is the minimum value of the frequency change, i.e. the selected dense sampling step size. B. The principle for determining the sweep step size of sparse sampling is: to ensure that the sweep step size does not cross the resonant frequency, the peak values ​​in the frequency-amplitude plots of the Nth and (N+1)th order peaks in the calibration signals of different thicknesses are respectively [A 11 A 12 A 13 ,…,A 1n ], [A 21 A 22 A 23 ,…,A 2n Find the rising half-peak and falling half-peak corresponding to each resonant frequency. The resonant frequencies corresponding to the half-peaks are respectively... Where n represents the number of thin plates selected for calibration, superscript 1 indicates the rising edge, and superscript 2 indicates the falling edge; then the sparse frequency selection rule is: and Smaller values.

4. The electromagnetic ultrasonic resonance thickness measurement method with adaptive frequency sweep step size according to any one of claims 1-3, characterized in that, The workpiece is made of metal or alloy.

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