基于压缩感知的快速激光超声合成孔径聚焦技术
In the industrial sector, the processing or extended utilization of various metal workpieces can generate assorted surface and internal defects. These imperfections can compromise the mechanical strength of the workpiece, thereby reducing its operational lifespan. Given its high penetration capacity and sensitivity, ultrasonic inspection has gained widespread usage in defect detection. In recent years, there has been an increased focus on imaging technologies in the evolution of defect detection methods. Among these, the synthetic aperture focusing technique (SAFT) is a viable imaging algorithm for ultrasonic inspections. It replaces large-aperture sensors with a series of individual small-aperture sensors, thereby enhancing the lateral resolution. The laser ultrasound synthetic aperture focusing technology (LU-SAFT) is a fusion of SAFT and laser ultrasound technologies, reaping the benefits of both. However, conventional LU-SAFT typically requires small-step scanning of the sample surface to be tested to enhance lateral resolution. This methodology, while effective, results in a prolonged overall detection time, thereby reducing the efficiency of the detection process. This major limitation hinders the practicality of traditional LU-SAFT. In our study, we aim to enhance the scanning efficiency and reduce the scanning duration of the conventional LU-SAFT.
This study presented a LU-SAFT method based on compressed sensing to enhance the scanning efficiency of conventional LU-SAFT. Initially, compressed sensing was employed to retrieve the maximum intensity of the A-scanning signal at the scanning points of the entire field from the maximum intensity of the A-scanning signal at sparse scanning points. Following that, the optimal scanning area of the sample surface was determined. Subsequently, scanning was conducted in this optimal area. Finally, SAFT image reconstruction was conducted for the defect. In the experiment, a pulsed laser was utilized to incite ultrasound on the surface of a defective sample. A laser Doppler vibrometer was employed to detect the ultrasound, and the LU-SAFT method rooted in compressed sensing was applied to identify the defects in the sample. This process served to confirm the feasibility of the proposed method.
The LU-SAFT method is used to scan the detection area based on compressed sensing. A total of 100 points are scanned, taking 0.63 min. Conversely, scanning with the conventional LU-SAFT method, which employs a scan step of 0.05 mm, requires 500 points and takes 3.15 min. When compared to the traditional LU-SAFT scanning process, the LU-SAFT method based on compressed sensing reduces the number of scanning points by 80% and decreases the scanning time by approximately 2.52 min. In the LU-SAFT defect reconstruction image based on compression sensing (Fig. 8), the top of the defect is located at a depth of -3.76 mm, deviating from the actual measurement by 0.01 mm, an error of 0.3%. The lateral position is 0.18 mm, deviating from the actual value by 0.18 mm, with an error of 1.4%. The signal-to-noise ratio corresponds to 71.31 dB. Meanwhile, in the conventional LU-SAFT defect reconstruction image (Fig.8), the top scattering of the defect is positioned at a depth of -3.76 mm and its lateral position remains at 0.18 mm. However, the signal-to-noise ratio is lower at 50.35 dB. Comparing the LU-SAFT defect reconstruction image based on compression sensing with the conventional LU-SAFT defect reconstruction image, it is evident that the depth and lateral positions of the defects in both images are nearly identical to the actual defects. Furthermore, the signal amplitude map of the LU-SAFT defect reconstruction image based on compression sensing (Fig.8) showcases a higher signal-to-noise ratio and requires fewer scanned points than the conventional LU-SAFT defect reconstruction image (Fig.8). From these results, it is clear that the LU-SAFT method based on compression sensing significantly reduces the scanning time of traditional LU-SAFT, thereby enhancing scanning efficiency.
In this study, the principle and processing flow of LU-SAFT based on compressed sensing are analyzed initially. Subsequently, the value of sparse scan points, construction of a dictionary, size of the optimal scanning area, and selection of suitable values are discussed. Finally, experiments are conducted using the parameters obtained from this analysis. The experimental results demonstrate that the LU-SAFT defect reconstruction image based on compressed sensing can enhance scanning efficiency and reduce the scanning time. These findings can offer fresh perspectives and solutions to address the time-consuming scanning process inherent in conventional LU-SAFT.
1 引言
在工业领域中,不同金属材料工件的加工或长期使用均有可能使工件表面、内部产生不同类型的缺陷,从而影响工件的机械强度和缩短工件的使用寿命。为了确保生产安全,必须将其检出。传统的光学检测手段,例如人工观测法、激光扫描法等[1-3],均难以获取材料内部的缺陷信息。超声检测具有穿透能力强、灵敏度高的优点,在缺陷检测领域被广泛应用[4]。传统的超声检测通常使用压电换能器进行超声的激发和检测,声耦合剂如水或甘油用于换能器和试件之间的超声传输[5],然而耦合剂的使用可能会对被测样品产生影响,例如使用水作耦合剂会对样品表面造成腐蚀等。激光超声检测[6-10]是超声检测中的一种新型的检测方法,其通过激光远程激发超声波来实现对样品的检测,克服了耦合剂引入的附加影响。此外,激光超声检测还具有高空间分辨率的特点,这对于微小缺陷(直径不超过0.5 mm)的检测和成像非常有利[11-12]。
近些年的缺陷检测,不论是基于传统超声还是基于激光超声的检测,都更加关注成像算法。合成孔径聚焦技术(SAFT)[13-30]是一种适合超声检测的成像算法,其基本原理是用一系列单一的小孔径传感器代替大孔径传感器,以达到提高横向检测分辨率的目的。该技术最早被应用在雷达领域[24]。目前的SAFT大致可以分为时域SAFT[23-24,20]和频域SAFT[13-22,25,30]。时域SAFT基于延时叠加和全范围动态聚焦,仅通过简单的累计求和便可获得分辨率较高的成像结果[31]。
激光超声合成孔径聚焦技术(LU-SAFT)[32]是一种将SAFT与激光超声相结合的技术。LU-SAFT兼具SAFT与激光超声技术的优势,例如:远距离获取成像数据,激发多种模式的超声波等。为了获得高空间分辨率和高信噪比的LU-SAFT缺陷图像,传统的LU-SAFT通常需要在被测样品表面进行高空间分辨率扫描,而这需要非常长的扫描时间,从而严重限制了传统LU-SAFT的实用性[33]。
压缩感知(CS)[35-36]是信号处理的一个新领域,其目标是用尽可能少的测量值重构信号。压缩感知假设一个信号可以用适当的基函数在另外一个域中以稀疏的形式表示,通过在基空间中进行信号重构便可重构出完整的原始信号。针对传统LU-SAFT扫描过程耗时长的问题,人们将压缩感知运用到LU-SAFT中,以提升传统LU-SAFT的扫描效率,缩短扫描时间。Park等[37]将二分搜索和压缩感知技术运用到表面缺陷波场的重建中,解决了表面缺陷的定位加速问题;之后,孙强等[38]又进一步运用二分搜索和压缩感知技术解决了体内缺陷的定位加速问题。但是到目前为止,尚未有人运用压缩感知技术来提升LU-SAFT的扫描效率从而解决传统LU-SAFT扫描过程耗时长的问题。
为了解决传统LU-SAFT扫描效率低的问题,笔者将压缩感知与LU-SAFT相结合。在实验上,首先将脉冲激光入射到含有缺陷样品的表面激发超声波信号,利用激光测振仪探测超声波信号;然后基于压缩感知和稀疏扫描得到的A扫信号确定最优检测区域,进而对最优检测区域进行扫描;最后利用最优检测区域内所有扫查数据进行SAFT图像的重建,从而验证算法的正确性。此外,通过选取合适的稀疏扫描点数和最优检测区域,减小了干扰信号对SAFT重建图像的影响并缩短了扫描时间,从而提高了传统LU-SAFT的扫描效率和重建图像的信噪比。
2 基本原理及处理流程
2.1 基于压缩感知的LU‐SAFT原理
在基于压缩感知的LU-SAFT原理中,假设样品表面共有
式中:
式中:
为了获得空间分辨率尽可能高的SAFT图像,
式中:
当使用LU-SAFT对样品内部缺陷进行成像时,由于激光超声的方向性,样品表面全场的扫描点A扫信号的最大强度呈双峰分布,在双峰附近的小扫描区域内激光超声信号较强,而在其他扫描区域内激光超声信号较弱,并且越靠近双峰的扫描点对SAFT成像的贡献越大[32]。两个峰对应的检测位置即为本次实验的最优扫描位置,最优扫描位置附近激光超声信号较强的小扫描区域即为本次实验的最优扫描区域。因此,压缩感知恢复出样品表面全场的扫描点A扫信号最大强度后输出两个峰对应的检测位置,输出的位置即为样品表面的最优扫描位置,以最优扫描位置为中心拓展形成的小扫描区域即为样品表面的最优扫描区域。
2.2 基于压缩感知的LU‐SAFT检测流程
传统的LU-SAFT技术主要使用体横波和体纵波对内部缺陷进行成像。由于扫描点处探测到的A扫信号中除了包含体横波和体纵波等目标信号外,还包含表面波和掠面纵波等干扰信号。这些干扰信号的幅值通常都比目标信号大,它们的存在会影响压缩感知算法的结果。因此,在得到扫描点处A扫信号后需要使用窗函数来进行模式选择。
图 1. 基于压缩感知的LU-SAFT处理流程图
Fig. 1. Flow chart of LU-SAFT method based on compression sensing
第一步:首先在目标检测区域内随机扫描
第二步:比较目标检测区域内全场的扫描点A扫信号最大强度,输出最大值S。若S值大于噪声值的两倍,则认为该检测区域内有缺陷,继续进行后续步骤;若S值小于等于噪声值的两倍,则认为该检测区域内没有缺陷,停止后续步骤。
第三步:根据目标检测区域内全场的扫描点A扫信号最大强度,确定样品表面的最优扫描位置。
第四步:以最优扫描位置为中心向左右两边拓展
第五步:在最优扫描区域内进行精确扫查并使用最优扫描区域内所有的扫查数据进行SAFT图像重建。
3 实验装置及参数
实验样品与装置如
图 2. 实验样品及装置示意图。(a)铝合金样品结构示意图;(b)实验装置图
Fig. 2. Diagram of experimental sample and setup. (a) Diagram of aluminum sample structure; (b) diagram of experimental setup
实验样品中缺陷的直径为0.5 mm,当有10个扫描点覆盖到缺陷时,对于检测缺陷的空间分辨率来说是足够的。因此,使用传统LU-SAFT以逐点扫描的方式对检测区域进行扫描时,选择0.05 mm作为扫描步长。本次实验中,检测区域的长度为25 mm,以0.05 mm为步长进行逐点扫描时,需要扫描500个点。首先在检测区域内规划好这500个扫描点的位置,并将这500个扫描点的位置分别记为1~500;然后使用随机函数randperm(500)随机生成500个1~500范围内的整数,接着选取生成的前40个随机数作为稀疏扫描点的位置,再根据确定的稀疏扫描点位置规划好随机扫描时每一步的扫描步长。随机扫描时的扫描步长确定后,固定激发、探测光距离
4 结果与讨论
前文已经介绍过,在基于压缩感知的LU-SAFT的处理过程中,需要通过在样品表面随机扫描
4.1 的取值
假设样品表面全场的扫描点个数为
式中:
图 3. 缺陷SAFT重建图像的信噪比和扫描时间随 的变化
Fig. 3. Variation of signal-to-noise ratio and scanning time of defective SAFT reconstructed images with
式中:
综合考虑SAFT缺陷图像的信噪比、扫描时间和稀疏采样定理后,本文选择缺陷SAFT重建图像的信噪比刚好为饱和值时的
4.2 字典 的构建
样品表面全场的扫描点A扫信号最大强度受方向性影响呈双峰分布,只需要确定双峰对应的检测位置(即最优扫描位置)即可,并不需要确定双峰的强度,因此可以只考虑样品表面全场的扫描点A扫信号最大强度的相对强度分布,而不用考虑它们的绝对强度,此时可使用缺陷位于样品内各个位置时样品表面全场的扫描点A扫信号最大强度的相对强度分布作为基构成字典
接下来用文献[32]的模型构建字典
式中:
式中:
式中:
如
式中:
式中:
将一个直径为0.5 mm的全反射缺陷分别设置在2、4、6、8 mm深度时,样品表面全场的扫描点A扫信号最大强度的相对强度分布如
图 5. 缺陷深度分别为2、4、6、8 mm时样品表面全场的扫描点A扫信号最大强度的相对强度分布
Fig. 5. Relative intensity distribution of A-scanning signal maximum intensities at the scanning points of the full field of the sample surface obtained with the defect setting at a depth of 2, 4, 6, 8 mm, respectively
4.3 的取值
在基于压缩感知的LU-SAFT的处理流程中,最优扫描区域是以最优扫描位置为中心向左右两边拓展
图 6. 缺陷SAFT重建图像的信噪比和扫描时间随 的变化
Fig. 6. Variation of signal-to-noise ratio and scanning time of defective SAFT reconstructed images with
由于激光超声的方向性,在包含最优扫描位置的小区域内激光超声信号较强,而在其他区域内激光超声信号较弱[30]。当最优扫描区域位于激光超声信号较强区域时,缺陷SAFT重建图像的信噪比随着
4.4 基于压缩感知的LU‐SAFT成像结果及讨论
基于以上分析所得参数进行实验,获得了样品表面扫描点处A扫信号的最大强度,如
图 7. 样品表面扫描点处A扫信号的最大强度。(a)稀疏扫描点处A扫信号的最大强度;(b)压缩感知恢复出的全场的扫描点A扫信号最大强度;(c)样品表面全场的扫描点A扫信号最大强度
Fig. 7. Maximum intensity of the A-scanning signal at the scanning points on the sample surface. (a) Maximum intensity of the A-scanning signal at the sparse scanning points; (b) maximum intensity of the A-scanning signal at the whole field scanning points recovered by compressive sensing; (c) maximum intensity of the A-scanning signal at the whole field scanning points on the sample surface
实验中获得的B扫图和SAFT缺陷图像如
图 8. 实验中获得的B扫图和SAFT缺陷图像。(a)对最优扫描区域进行扫描后得到的B扫图;(b)全场B扫图;(c)基于压缩感知的LU-SAFT缺陷重建图像;(d)传统LU-SAFT缺陷重建图像;(e)基于压缩感知的LU-SAFT缺陷重建图像的信号幅值图;(f)传统LU-SAFT缺陷重建图像的信号幅值图
Fig. 8. B-scanning and SAFT defect images obtained in the experiment. (a) B-scanning image obtained by scanning the optimal scanning area; (b) full-field B-scanning image; (c) LU-SAFT defect reconstruction image based on compressive sensing; (d) traditional LU-SAFT defect reconstruction image; (e) signal amplitude graph of LU-SAFT defect reconstruction image based on compressive sensing; (f) signal amplitude graph of traditional LU-SAFT defect reconstruction image
文中选择缺陷SAFT重建图像信噪比最大时的
按照基于压缩感知的LU-SAFT方法对检测区域进行扫描,共扫描了100个点,耗时0.63 min;而扫描同一检测区域,以0.05 mm为步长按照传统LU-SAFT逐点扫描的方式进行扫描,则需要扫描500个点,耗时3.15 min。相比于传统LU-SAFT的逐点扫描过程,按照基于压缩感知的LU-SAFT方法进行扫描,扫描点数减少了80%,扫描时间缩短了约2.52 min。
从以上结果可以看出,基于压缩感知的LU-SAFT方法相比传统LU-SAFT可以缩短扫描时间,提升扫描效率。
5 结论
首先对基于压缩感知的LU-SAFT原理及处理流程进行分析,然后对流程中稀疏扫描点数
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Article Outline
何志同, 应恺宁, 戴鹭楠, 倪辰荫. 基于压缩感知的快速激光超声合成孔径聚焦技术[J]. 中国激光, 2024, 51(2): 0201004. Zhitong He, Kaining Ying, Lunan Dai, Chenyin Ni. Fast Laser Ultrasonic Synthetic Aperture Focusing Technology Based on Compressed Sensing[J]. Chinese Journal of Lasers, 2024, 51(2): 0201004.