相位编码脉冲压缩方法在空气耦合超声检测信号处理中的应用

Application of Phase Coded Pulse Compression Method to Air-coupled Ultrasonic Testing Signal Processing

由于声波在空气介质中的衰减及气固界面巨大声阻抗差导致空气耦合超声透射信号信噪比差,大大影响检测结果的可靠性及缺陷定量精度。针对上述问题,基于雷达系统相位编码脉冲压缩技术的基本原理讨论匹配滤波器的设计及其在空气耦合超声检测中的实现,分析不同巴克码信号、带宽及载波周期数对脉冲压缩效果的影响,并以玻璃纤维复合材料(Glass fiber reinforced polymer,GFRP)试样为对象,对比用单频正弦及相位编码脉冲压缩方法获得的透射超声信号信噪比及C扫描缺陷定量精度。试验结果表明选择换能器中心频率作为13位巴克码信号带宽时,脉冲压缩信号主瓣脉宽最小、幅值最大;相位编码脉冲压缩后超声信号信噪比较单频信号提高了12.4 dB,缺陷边缘更清晰且定量精度较单频信号提高了12%。说明在空气耦合超声检测中采用相位编码脉冲压缩技术能显著提高接收信号信噪比及缺陷定量精度。

The experiment reliability and defect quantification accuracy of air-coupled ultrasonic testing are highly affected by the bad SNR（signal to noise ratio） of the through transmitted signal because of the high damped medium property in air and giant mismatch of acoustic impedance in air-solid interface. To deal with these handicaps, the matching filter design and its implementation in air-coupled ultrasonic inspection based on the theory of phased Coded pulse compression are discussed. Influences of different sequences, bandwidth and cycles of Barker code on pulse compressed results are analyzed either. SNR of the through-transmitted signal and defect quantification accuracy of C-scan results on glass fiber reinforced polymer（GFRP） specimen are compared using single frequency excitation and pulse compression method, respectively. The results show that the smallest time width and peak amplitude of the main lobe are acquired using central frequency of the transducer as bandwidth of Barker code. Using phase coded pulse compression method, the SNR of the through-transmitted signal is increased about 12.4 dB; the quantification accuracy is increased about 12% compared to the single frequency excitation, and the edge of the defect is better highlighted. The results represent that the SNR of the through transmitted signal and defect quantification accuracy are apparently increased by using phase Coded pulse compression method in air-coupled ultrasonic testing.