Application of pulse compression technique in steel plate corrosion detection with SH guided wave EMATs
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摘要:
将Barker码脉冲压缩技术应用于水平剪切(SH)导波电磁超声换能器( EMAT)，对提高在役钢板腐蚀检测回波的信噪比(SNR)和空间分辨率，以及实现大范围在线快速扫查具有重要的工程应用价值。建立基于Barker码信号激励的钢板SH导波传播有限元模型，结合实验分析与数值计算，分析了Barker码序列长度、码元长度、EMAT设计参数、提离等因素对脉冲压缩后缺陷回波的信噪比和波包宽度的影响，并与传统单一频率脉冲串激励方式进行对比实验。结果表明：与单一频率脉冲串激励方式相比，采用Barker码脉冲压缩技术，可以将缺陷波的SNR提高5.8 dB；当EMAT提离为3.0 mm时，经过脉冲压缩后的缺陷回波SNR>8.7 dB，而采用单一频率脉冲串激励方式对应的SNR接近于0 dB；当Barker码信号序列长度为13位、码元长度为15 μs时，能检测深度为1 mm、直径为20 mm的圆孔，且SNR>25.4 dB。
Abstract:The application of Barker code pulse compression technique in a shear horizontal (SH) guided wave electromagnetic acoustic transducer (EMAT) has significant value in engineering application as it enhances the signal-to-noise ratio (SNR) and spatial resolution of the corrosion echo detected in in-service steel plates, and helps to realize large-scale and rapid on-line scanning. In this paper, a finite element model of SH-guided wave propagation in a steel plate excited by Barker code signals is established. Then, combined with experimental analysis and numerical calculations, the effects of Barker code sequence length, subpulse length, EMAT design parameters, and lift-off on the SNR and wave packet width of the pulse-compressed echoes are analyzed and compared with those of the traditional tone-burst excitation method. The results show that the SNR of the defect echo processed with the Barker code pulse compression technique is 5.8 dB higher than that with the tone-burst excitation method. When the EMAT liftoff is 3.0 mm, the SNR of the defect echo after Barker code pulse compression is greater than 8.7 dB, while that with the tone-burst excitation method is approximately 0 dB. When the sequence length and subpulse of the Barker code signals are respectively 13 and 15 μs, a circular hole with a depth of 1 mm and a diameter of 20 mm can be detected, the SNR of which is greater than 25.4 dB.
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图 1 基于洛伦兹力的PPM-EMAT换能机理
Figure 1. PPM-EMAT conversion mechanism based on Lorentz force
图 3 SH导波在钢板中的频散曲线
Figure 3. Frequency dispersion curve of SH guided wave propagation in steel plate cross-section
图 4 钢板横截面的SH导波传播云图
Figure 4. Contour plots of SH guided wave propagation in steel plate cross-section
图 5 13位Barker码脉冲压缩及旁瓣抑制
Figure 5. Pulse compression and side-lobe suppression of 13-bit Barker code
图 6 不同序列长度Barker码对应的激励信号、超声波信号、脉压信号和经旁瓣抑制后的脉压信号
Figure 6. Excitation signals, ultrasonic signals, pulse compressed signals, pulse compressed signals after side lobe suppression when Barker code signals with different sequence lengths is used
图 7 不同码元长度Barker码对应的激励信号、超声波信号、脉压信号和经旁瓣抑制后的脉压信号
Figure 7. Excitation signals, ultrasonic signals, pulse compressed signals, pulse compressed signals after side lobe suppression when Barker code signals with different subpulse lengths is used
图 8 永磁体对数对旁瓣抑制后脉压信号的主瓣宽度和PSL的影响
Figure 8. Effect of permanent magnet pairs on main lobe width and PSL of pulse compressed signals after side lobe suppression
图 11 含预制缺陷的钢板试样示意图
Figure 11. Schematic diagram of steel plate with prefabricated defects
图 12 无同步平均时，不同Barker码序列长度对应的旁瓣抑制后的脉压信号
Figure 12. Pulse compressed signals after side lobe suppression in correspondence with the Barker codes of different sequence lengths when there is no synchronous average
图 13 无同步平均时，不同Barker码码元长度对应的旁瓣抑制后的脉压信号
Figure 13. Pulse compressed signals after side lobe suppression in correspondence with the Barker code with different subpulse durations when there is no synchronous average
图 14 单一频率正弦脉冲串激励时，同步平均次数对缺陷波SNR和波包宽度的影响
Figure 14. Effect of the synchronous averaging number on the SNR and packet width of the defect echo when the tone-burst excitation is used
图 15 EMAT提离为1.5 mm和同步平均次数为128时，单一频率正弦脉冲串激励对应的超声波信号
Figure 15. Received ultrasonic signals with tone-burst excitation, when life-off is 1.5 mm, and synchronous average is 128
图 16 EMAT提离对无同步平均、旁瓣抑制后脉压信号及128次平均的单一频率信号的缺陷波信噪比的影响
Figure 16. Effect of EMAT lift-off on the SNR of the defect echoes from the pulse-compressed signals after side-lobe suppression with no synchronous average and averaged single frequency signals at 128
表 1 有限元建模参数
Table 1. Parameters in finite element model
变量 数值 变量 数值 钢板长度ws/mm 300 纵波波速/（m·s−1） 5960 钢板宽度ls/mm 100 横波波速/（m·s−1） 3260 钢板高度hs/mm 5.6 钢板密度/（kg·m−3） 7932 永磁体间距d/mm 7 圆孔直径/mm 20 永磁体长度wm/mm 20 圆孔深度/mm 3 
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表 2 采用不同Barker码激励信号和无同步平均时，2 mm深度圆孔对应的缺陷波SNR
Table 2. SNR of the defect echo from 2 mm deep circular hole with different Barker code signals and without synchronous average
Barker激励信号 缺陷波SNR/dB 序列长度/位 码元长度/μs 5 10 17.7 7 10 20.7 13 5 16.3 13 10 26.5 13 15 31.4
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