A Golay-coded ultrasonic excitation method based on nonlinear frequency modulation

Abstract

The present invention discloses a Golay-coded ultrasonic excitation method based on nonlinear frequency modulation, which is used for combined excitation of adjacent array elements in an ultrasonic sensor array, and can obtain enhanced and high-resolution ultrasonic echo signals. The present invention uses a nonlinear frequency modulation signal as a carrier to modulate the Golay code complementary sequence pair A and B respectively, thereby obtaining an NLFM-Golay coding pair. The coding pair is power-amplified to excite the ultrasonic sensor to generate ultrasonic waves. After the test piece is inspected, the received ultrasonic echoes are matched-filtered and then added together to obtain an ultrasonic detection echo signal. The ultrasonic wave generated by this excitation method significantly improves the echo signal quality when used for test piece inspection. Compared with nonlinear frequency modulation excitation, the signal-to-noise ratio of the echo signal is improved, thereby enhancing the sensitivity of defect detection; compared with Golay code excitation, the distance resolution is improved, thereby facilitating the accurate quantification of defects in the material.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultrasonic detection, and in particular relates to a Golay coding ultrasonic excitation method based on nonlinear frequency modulation. Background Art

[0002] Ultrasonic testing is an important non-destructive testing method and is widely used in the detection of internal defects in materials. When using conventional pulse ultrasonic testing methods to detect defects in non-metallic materials, due to the anisotropy of the materials and the complex internal structure, the sound waves are severely attenuated and scattered during propagation, resulting in serious loss of ultrasonic energy. Therefore, the traditional pulse ultrasonic testing method has the disadvantages of low average transmitted sound power and low echo signal-to-noise ratio. To solve this problem, many scholars (such as Li Changzheng, Wang Rui. Detection of concrete defects using Buck coded signals [J]. Engineering Investigation, 2013, 41 (8): 83-86.) introduced coded ultrasonic excitation technology to increase the average sound power of the excitation signal, thereby improving the penetration ability of the sound wave and the signal-to-noise ratio of the echo signal. Although the emergence of coded ultrasound has alleviated the above contradictions to a certain extent, the sidelobe level is high and the distance resolution is low when phase coding is used as the excitation signal detection; the signal-to-noise ratio is low when frequency coding is used as the excitation signal detection. The existing coding form cannot improve the distance resolution while improving the signal-to-noise ratio, and there is a contradiction between the signal-to-noise ratio and the distance resolution of the detection results.

[0003] To this end, the present invention proposes a Golay-coded ultrasonic excitation method based on nonlinear frequency modulation. When the ultrasonic waves generated by this excitation method are used for non-metallic material specimen detection, the signal-to-noise ratio and distance resolution of the detection results are improved compared with other frequency-coded or phase-coded excitation methods. Summary of the Invention

[0004] To address the problems of weak echo signals, low signal-to-noise ratio, and low distance resolution during ultrasonic detection of defects in high-acoustic-attenuation materials, the present invention proposes a Golay-coded ultrasonic excitation method based on nonlinear frequency modulation. The method is particularly suitable for combined excitation of adjacent array elements in an ultrasonic sensor array, can enhance the echo signal, and improve the distance resolution.

[0005] In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows: a Golay coded ultrasonic excitation method based on nonlinear frequency modulation comprises the following steps:

[0006] S1: Use nonlinear frequency modulation signal s NLFM (t) is used as the carrier to modulate the Golay code complementary sequence pair A and B respectively, and the NLFM-Golay code pairs s1(t) and s2(t) are obtained;

[0007] S2: Power amplify the NLFM-Golay code pair to obtain the excitation signals S1(t) and S2(t) of the coded ultrasonic signal detection system;

[0008] S3: Use the coded ultrasonic signal detection system excitation signals S1(t) and S2(t) to stimulate the ultrasonic sensors respectively, and obtain the sensor response signals h1(t) and h2(t);

[0009] S4: Detect the test piece and obtain echo signals r1(t) and r2(t);

[0010] S5: Perform matched filtering on the echo signals r1(t) and r2(t) to obtain echo signals c1(t) and c2(t);

[0011] S6: The signals c1(t) and c2(t) are summed to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t).

[0012] Furthermore, the time domain function expression of the nonlinear frequency modulation signal in the above step S1 is:

[0013]

[0014] Where a(t) is s NLFM (t) is the amplitude function, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, the fractional bandwidth at -20 dB, and the signal duration;

[0015] Golay code consists of a pair of finite sequences A and B with equal lengths. A code (a0, a1, ..., a N-1 ) and B code (b0, b1, ..., b N-1 ) are expressed as follows:

[0016]

[0017]

[0018] Where a i ,b i is the constituent element of the Golay code sequence, a i ,b i ∈{+1,-1},i=0,1,2…,N-1, N is the length of the Golay code, satisfying G a (j)+G b (j) = 2Nδ(j), where δ(j) represents the impulse function, i.e., code A and code B are complementary;

[0019] With nonlinear frequency modulation signal s NLFM(t) modulates the Golay complementary sequence pair A and B respectively, and obtains the NLFM-Golay code pair s1(t) and s2(t):

[0020] s1(t)=s NLFM (t)*G A (t)

[0021] s2(t)=s NLFM (t)*G B (t)

[0022] Where G A (t) and G B (t) is the oversampled signal of sequence A code and B code.

[0023] Furthermore, the above step S3 includes:

[0024] The NLFM-Golay coded signal pair S1(t) and S2(t) excites the ultrasonic sensor to obtain the sensor response signals h1(t) and h2(t), which are expressed as follows:

[0025] h1(t)=S1(t)*h(t)

[0026] h2(t)=S2(t)*h(t)

[0027] The impulse response h(t) of the ultrasonic sensor is approximated by the Gaussian function model, and the expression is:

[0028]

[0029] Where, e is a natural constant, β is the amplitude coefficient, f0 is the center frequency of the sensor, and s is a coefficient related to the relative bandwidth ρ of the sensor, s = 3.32 / ρ, is the initial phase, T P is the time width of the Golay signal sub-pulse, and the total duration of the Golay code is T = nT P .

[0030] Furthermore, the above step S5 includes the following steps:

[0031] 1) The matched filter function of the NLFM-Golay coded excitation signal is p(t)=p1(t)*p2(t), where p1(t) is the Golay code matched filter. For A and B complementary codes, p 1A (t) and p 1B (t) represents, p2(t) is the matched filter of NLFM signal, respectively

[0032] p 1A (t) = G A (tTP )

[0033] p 1B (t) = G B (tT P )

[0034]

[0035] Where, T P is the time width of the Golay signal sub-pulse, G A (t) and G B (t) is the oversampled signal of Golay code sequences A and B, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, fractional bandwidth at -20 dB, and signal duration;

[0036] 2) The echo signals c1(t) and c2(t) after matched filtering in the NLFM-Golay coded ultrasonic signal detection system are:

[0037] c1(t)=r1(t)*p 1A (t)*p2(t)

[0038] c2(t)=r2(t)*p 1B (t)*p2(t).

[0039] Furthermore, in the above step S6: the echo signals after matched filtering are added to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t), ie, c(t)=c1(t)+c2(t).

[0040] The present invention has the following beneficial effects:

[0041] The present invention proposes a coded excitation method suitable for ultrasonic detection of defects in high-acoustic-attenuation materials. The excitation signal is a composite of a phase-coded signal and a frequency-coded signal, combining the high signal-to-noise ratio of the Golay code and the low sidelobe level of the NLFM signal. When used for specimen defect detection, the method of the present invention can achieve a better signal-to-noise ratio and distance resolution than when using frequency-coded signals or phase-coded signals alone as excitation signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a flowchart of the method of the present invention.

[0043] Figure 2 : is a time-frequency domain waveform diagram of the Golay code complementary sequence A and complementary sequence B in an embodiment of the present invention, Figure 2 (a) and (b) are the time domain waveform and spectrum of Golay code A, respectively. Figure 2(c) and (d) are the time domain waveform and spectrum of Golay code B, respectively.

[0044] Figure 3 This is a time-frequency domain waveform diagram of the NLFM-Golay coding pair in an embodiment of the present invention. Figure 3 (a) and (c) are the time domain waveforms of the NLFM-Golay coding pair. Figure 3 (b) and (d) are the spectrograms of the NLFM-Golay coding pair.

[0045] Figure 4 : is a time-frequency domain waveform diagram of the echo signal of the bottom surface of the test piece using NLFM-Golay coding ultrasonic testing in an embodiment of the present invention. Figure 4 (a) is the time domain waveform of the echo signal of the bottom surface of the test piece detected by NLFM-Golay coding ultrasonic testing. Figure 4 (b) is the spectrum diagram of the echo signal of the bottom surface of the test piece detected by NLFM-Golay coding ultrasonic testing.

[0046] Figure 5 This is a comparison diagram of the time domain waveform and sidelobe peak value of the echo signal of the bottom surface of the ultrasonic inspection test piece using Golay code, NLFM and NLFM-Golay code in an embodiment of the present invention. Figure 5 (a) and (b) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when the Golay code is used for excitation. Figure 5 (c) and (d) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when the NLFM signal is used for excitation. Figure 5 (e) and (f) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when the NLFM-Golay signal is used as excitation. DETAILED DESCRIPTION

[0047] The specific forms of the present invention are further described in detail below with reference to the accompanying drawings and examples. It should be noted that the present invention can also be applied through other equivalent embodiments, and the embodiments described in the following examples are only used to illustrate the basic concept of the present invention, and are not intended to limit the present invention.

[0048] like Figure 1 The figure shows a flow chart of the method of the present invention, which is a Golay coded ultrasonic excitation method based on nonlinear frequency modulation, comprising the following steps:

[0049] S1: Use nonlinear frequency modulation signal s NLFM (t) is used as the carrier to modulate the Golay code complementary sequence pair A and B respectively, and the NLFM-Golay code pairs s1(t) and s2(t) are obtained;

[0050] S2: Power amplify the NLFM-Golay code pair to obtain the excitation signals S1(t) and S2(t) of the coded ultrasonic signal detection system;

[0051] S3: Use the coded ultrasonic signal detection system excitation signals S1(t) and S2(t) to stimulate the ultrasonic sensors respectively, and obtain the sensor response signals h1(t) and h2(t);

[0052] As a preferred embodiment of the present invention,

[0053] S4: Detect the test piece and obtain echo signals r1(t) and r2(t);

[0054] S5: Perform matched filtering on the echo signals r1(t) and r2(t) to obtain echo signals c1(t) and c2(t);

[0055] S6: The signals c1(t) and c2(t) are summed to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t).

[0056] As a preferred embodiment of the present invention, the time domain function expression of the nonlinear frequency modulation signal in step S1 is:

[0057]

[0058] Where a(t) is s NLFM (t) is the amplitude function, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, the fractional bandwidth at -20 dB, and the signal duration;

[0059] Golay code consists of a pair of finite sequences A and B with equal lengths. A code (a0, a1, ..., a N-1 ) and B code (b0, b1, ..., b N-1 ) are expressed as follows:

[0060]

[0061]

[0062] Where a i ,b i is the constituent element of the Golay code sequence, a i ,b i ∈{+1,-1},i=0,1,2…,N-1, N is the length of the Golay code, satisfying G a (j)+G b (j) = 2Nδ(j), where δ(j) represents the impulse function, i.e., code A and code B are complementary;

[0063] With nonlinear frequency modulation signal s NLFM (t) modulates the Golay complementary sequence pair A and B respectively, and obtains the NLFM-Golay code pair s1(t) and s2(t):

[0064] s1(t)=s NLFM (t)*G A (t)

[0065] s2(t)=s NLFM (t)*G B (t)

[0066] Where G A (t) and G B (t) is the oversampled signal of sequence A code and B code.

[0067] As a preferred embodiment of the present invention, in step S2), the NLFM-Golay coding is amplified by a power amplifier to obtain the coded ultrasonic signal detection system excitation signals S1(t) and S2(t).

[0068] As a preferred embodiment of the present invention, in step S3, the NLFM-Golay coded signal pair S1(t) and S2(t) excite the ultrasonic sensor to obtain sensor response signals h1(t) and h2(t), which are expressed as follows:

[0069] h1(t)=S1(t)*h(t)

[0070] h2(t)=S2(t)*h(t)

[0071] The impulse response h(t) of the ultrasonic sensor is approximated by the Gaussian function model, and the expression is:

[0072]

[0073] Where, e is a natural constant, β is the amplitude coefficient, f0 is the center frequency of the sensor, and s is a coefficient related to the relative bandwidth ρ of the sensor, s = 3.32 / ρ, is the initial phase, T P is the time width of the Golay signal sub-pulse, and the total duration of the Golay code is T = nT P .

[0074] As a preferred embodiment of the present invention, in step S4, the amplified NLFM-Golay coded signal is used to excite the ultrasonic sensor to detect the test piece, thereby obtaining detection echo signals r1(t) and r2(t).

[0075] As a preferred embodiment of the present invention, step S5 includes the following steps:

[0076] 1) The matched filter function of the NLFM-Golay coded excitation signal is p(t)=p1(t)*p2(t), where p1(t) is the Golay code matched filter. For A and B complementary codes, p 1A (t) and p 1B (t) represents, p2(t) is the matched filter of NLFM signal, respectively

[0077] p 1A (t) = G A (tT P )

[0078] p 1B (t) = G B (tT P )

[0079]

[0080] Where, T P is the time width of the Golay signal sub-pulse, G A (t) and G B (t) is the oversampled signal of Golay code sequences A and B, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, fractional bandwidth at -20 dB, and signal duration;

[0081] 2) The echo signals c1(t) and c2(t) after matched filtering in the NLFM-Golay coded ultrasonic signal detection system are:

[0082] c1(t)=r1(t)*p 1A (t)*p2(t)

[0083] c2(t)=r2(t)*p 1B (t)*p2(t).

[0084] As a preferred embodiment of the present invention, in step S6, the matched filtered echo signals are added to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t), that is, c(t)=c1(t)+c2(t).

[0085] The present invention is described below with a specific embodiment. A 10-bit Golay code with a center frequency of 4 MHz and a time width of 2.5 μs is used as an example. The complementary sequences A and B are:

[0086] A={1,1,-1,1,-1,1,-1,-1,1,1}

[0087] B={1,1,-1,1,1,1,1,-1,-1}

[0088] Its time-frequency domain waveform is as follows Figure 2 As shown, Figure 2 (a) and (c) are the time domain waveforms of the complementary sequences A and B, respectively. Figure 2 (b) and (d) are the spectra of complementary sequences A code and B code, respectively.

[0089] The time domain function expression of the NLFM signal used is: Taking an ultrasonic sensor with a center frequency f0 of 4 MHz and a fractional bandwidth of 7 MHz at -20 dB as an example, the coefficients e0 = 12804043.45; e1 = 4.272177775 × 10 13 ;e²=-4.080977223×10 20 ;d1=-4979791.62; d2=3.166972711×10 12 ;T P =0.23μs.

[0090] The Golay complementary sequence pair A and B are modulated by NLFM signal respectively, and the time-frequency domain waveforms of NLFM-Golay code pair s1(t) and s2(t) are obtained as follows: Figure 3 As shown, Figure 3 (a) and (c) are the time domain waveforms of the NLFM-Golay coding pair. Figure 3 (b) and (d) are the spectrograms of the NLFM-Golay coding pair.

[0091] A signal generator was used to generate Golay code, NLFM signal and NLFM-Golay signal with a center frequency of 4MHz and an amplitude of 300mV. After amplification by a power amplifier, the amplitude reached 150V. The ultrasonic sensor was used to perform ultrasonic testing on a 6mm thick nitrile rubber specimen. The time-frequency domain waveforms of the bottom echo signal obtained when the NLFM-Golay code was excited are shown in the following figure: Figure 4 As shown, Figure 4 (a) is the time domain waveform of the bottom wall echo signal during NLFM-Golay coding excitation. Figure 4 (b) is the spectrum of the bottom echo signal when NLFM-Golay coded excitation is used. The time domain waveform and sidelobe peak of the bottom echo when Golay code, NLFM signal and NLFM-Golay signal are shown in the figure. Figure 5 As shown. Figure 5 (a) and (b) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when the Golay code is excited. Figure 5 (c) and (d) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when the NLFM signal is excited. Figure 5(e) and (f) are the time domain waveforms and peak sidelobe levels of the bottom wall echo signal when excited by the NLFM-Golay signal.

[0092] Depend on Figure 5 Comparative analysis shows that the signal-to-noise ratio of the echo signal detected by NLFM-Golay coding is about 26dB, which is about 6dB higher than that of the echo detected by NLFM signal, thereby improving the sensitivity of defect detection. The main lobe width of the echo signal detected by NLFM-Golay coding at -6dB is 0.18μs, which is 0.12μs shorter than that of the echo detected by Golay code at -6dB, concentrating the energy and improving the penetration ability of the sound wave. The peak sidelobe level of the echo signal detected by NLFM-Golay coding is -20dB, which is about 5.5dB lower than that of the echo signal detected by Golay code, facilitating the accurate quantification of defects in the material. Comprehensive analysis shows that although the ultrasonic waves generated by the NLFM-Golay coded excitation method cannot achieve better signal-to-noise ratio and main lobe than any other coded excitation method when used for specimen detection, they are significantly improved compared with the ultrasonic waves generated by pulse excitation. Moreover, the NLFM-Golay coded energy proposed in the present invention is mainly concentrated around the main lobe, which is suitable for defect detection of high acoustic attenuation materials. The synthetic signal has a high signal-to-noise ratio and improves the suppression performance of material background noise.

[0093] The above is a preferred embodiment of the present invention. It should be noted that the preferred embodiment is merely an illustration of the present invention and is not intended to be limiting. For those skilled in the relevant art, equivalent embodiments and modifications that do not depart from the principles and spirit of the present invention are also within the scope of protection of the present invention.

Claims

1. A Golay coded ultrasonic excitation method based on nonlinear frequency modulation, characterized in that: The following steps are involved: S1: Use nonlinear frequency modulation signal s NLFM (t) is used as the carrier to modulate the Golay code complementary sequence pair A and B respectively, and the NLFM-Golay code pairs s1(t) and s2(t) are obtained; S2: Power amplify the NLFM-Golay code pair to obtain the excitation signals S1(t) and S2(t) of the coded ultrasonic signal detection system; S3: Use the coded ultrasonic signal detection system excitation signals S1(t) and S2(t) to stimulate the ultrasonic sensors respectively, and obtain the sensor response signals h1(t) and h2(t); S4: Detect the test piece and obtain echo signals r1(t) and r2(t); S5: Perform matched filtering on the echo signals r1(t) and r2(t) to obtain echo signals c1(t) and c2(t); S6: The signals c1(t) and c2(t) are summed to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t).

2. The Golay coded ultrasonic excitation method based on nonlinear frequency modulation according to claim 1, characterized in that: The time domain function expression of the nonlinear frequency modulation signal in step S1 is: Where a(t) is s NLFM (t) is the amplitude function, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, the fractional bandwidth at -20 dB, and the signal duration; Golay code consists of a pair of finite sequences A and B with equal lengths. A code (a0, a1, ..., a N-1 ) and B code (b0, b1, ..., b N-1 ) are expressed as follows: Where a i ,b i is the constituent element of the Golay code sequence, a i ,b i ∈{+1,-1},i=0,1,2…,N-1, N is the length of the Golay code, satisfying G a (j)+G b (j) = 2Nδ(j), where δ(j) represents the impulse function, i.e., code A and code B are complementary; With nonlinear frequency modulation signal s NLFM (t) modulates the Golay complementary sequence pair A and B respectively, and obtains the NLFM-Golay code pair s1(t) and s2(t): s1(t)=s NLFM (t)*G A (t) s2(t)=s NLFM (t)*G B (t) Where G A (t) and G B (t) is the oversampled signal of sequence A code and B code.

3. The Golay coded ultrasonic excitation method based on nonlinear frequency modulation according to claim 1, characterized in that: The step S3 comprises: The NLFM-Golay coded signal pair S1(t) and S2(t) excites the ultrasonic sensor to obtain the sensor response signals h1(t) and h2(t), which are expressed as follows: h1(t)=S1(t)*h(t) h2(t)=S2(t)*h(t) The impulse response h(t) of the ultrasonic sensor is approximated by the Gaussian function model, and the expression is: Where, e is a natural constant, β is the amplitude coefficient, f0 is the center frequency of the sensor, and s is a coefficient related to the relative bandwidth ρ of the sensor, s = 3.32 / ρ, is the initial phase, T P is the time width of the Golay signal sub-pulse, and the total duration of the Golay code is T = nT P .

4. The Golay coded ultrasonic excitation method based on nonlinear frequency modulation according to claim 1, characterized in that: The step S5 comprises the following steps: 1) The matched filter function of the NLFM-Golay coded excitation signal is p(t)=p1(t)*p2(t), where p1(t) is the Golay code matched filter. For A and B complementary codes, p 1A (t) and p 1B (t) represents, p2(t) is the matched filter of NLFM signal, respectively p 1A (t)=G A (t-T P ) p 1B (t)=G B (t-T P ) Where, T P is the time width of the Golay signal sub-pulse, G A (t) and G B (t) is the oversampled signal of Golay code sequences A and B, j is the imaginary unit, e0, e1, e2, d1, d2 are coefficients related to the sensor center frequency, fractional bandwidth at -20 dB, and signal duration; 2) The echo signals c1(t) and c2(t) after matched filtering in the NLFM-Golay coded ultrasonic signal detection system are: c1(t)=r1(t)*p 1A (t)*p2(t) c2(t)=r2(t)*p 1B (t)*p2(t)。 5. The Golay coded ultrasonic excitation method based on nonlinear frequency modulation according to claim 1, characterized in that: The step S6 includes: adding the echo signals after matched filtering to obtain the NLFM-Golay coded ultrasonic signal detection system echo signal c(t), that is, c(t)=c1(t)+c2(t).

Images