Visual Vibrometry and its application to NDT through the use of various video and signal processing techniques

该项目的重点是视觉测振领域及其通过使用各种视频和信号处理技术在超声无损检测中的应用。探索了视觉振动测量作为无损检测的一种方法的概念，利用常规的消费相机从标准帧速率视频中恢复信号。目视测振法有可能成为无损探伤中大面积快速监测的主要方法。因为视觉测振的工作原理是，在役损伤以一种人眼看不到但通过图像处理可见的方式改变振动模式。无损检测的一种常见形式涉及使用超声波来提取底层材料和系统的动态属性。该项目的一个目标是将超声导波的能力与视觉系统的能力结合起来，以便穿透深度部分。特别是探索如何使用大面积目视监测方法来监测结构的状态。对于收集的视频数据，使用强制激励，而不是简单地记录结构的被动响应。一个目标是成功地以显著高于相机宣传的帧速率的频率对结构振动进行成像，并将相机的使用扩展到其硬件规格之外。即使在激发频率超过奈奎斯特抽样限度的情况下，也可以通过利用联合国开发计划署常见的短曝光时间和周期性激发来实现这一点。这将涉及到使用频闪效果和利用相机曝光时间来实现这一点，本质上是欺骗相机系统以比正常情况下高得多的帧速率进行采样。这些情况旨在通过考虑产生的频闪效应来分析，其中自然频率和输入激励分别是可计算的或已知的。最后的挑战将是这些发展是否可以推进到足够远的程度，以允许测量距离有用的超声波频率的位移。为目视测振作为监测和检测零件损伤的一种方法提供依据。为了实现这些目标，开发了一个模拟和后处理工具，作为验证实验测试和预测实验测试行为的一种手段。其中，模拟视频被设计为模拟真实数据，并使用根据执行的实验测试结果定义的噪声模型来调查噪声和强度级别之间的关系，如图1所示。模拟测试视频提供了在物理测试之前测试实验设置的方法，并可用于探索关键摄像机参数对结果的影响。由于摄像机的动态特性对测试的性能影响很大，因此将对摄像机的每个测试参数进行研究和量化。图像质量、帧速率和曝光时间之间的关系是必需的，并且可能通过使用信噪比关系以及所识别的输出位移幅度和相位的精度来量化。通过使用模拟来研究每个摄像机和测试参数的影响，可以探索测试的局限性。例如，识别可以可靠和准确地从视频数据中提取的可能的最小像素位移。

The project focuses on the field of visual vibrometry and its application to ultrasonic non-destructive testing (UNDT) through the use of various video and signal processing techniques. The concept of visual vibrometry as a method of NDE is explored by utilising regular consumer cameras to recover signals from standard frame-rate videos. Visual vibrometry has significant potential to be a major method for rapid large area monitoring within NDE. As visual vibrometry works on the basis that in-service damage alters vibrational modes in a way that is imperceptible to the human eye, but visible through image processing. A common form of NDT involves the use of ultrasonic sound waves to extract underlying material and dynamic properties of a system. One aim of the project is to combine the ability of ultrasonic guided waves, in order to penetrate a part at depth, with the ability of visual systems. In particular, to explore how large area visual monitoring approaches may be employed to monitor the state of structures. For the video data collected, forced excitation is utilised as opposed to simply recording the passive response of the structure One goal is to successfully image structural vibrations at frequencies significantly greater than the advertised frame-rate of the camera and extend the use of the camera beyond its hardware specification. This may be achieved by exploiting short exposure times and periodic excitation common in UNDT, even when the excitation frequency exceeds the Nyquist sampling limit. This will involve using stroboscopic effects and exploiting camera exposure times in order to achieve this and essentially 'cheat' the camera system in to sampling at a significantly higher frame-rate than normally possibly. These cases are intended to be analysed by taking into account stroboscopic effects produced, where natural frequencies and input excitations are either calculable or known, respectively. The final challenge will be whether these developments may be pushed far enough to allow the measurement of displacement from useful ultrasonic frequency. In order to provide the basis for visual vibrometry as a method to monitor and detect damage in parts. To achieve these goals a simulation and post processing tool has been developed as a means to validate experimental testing and to predict the behaviour of experimental testing. Where, the simulated video was designed to imitate real data and used a noise model defined on results from experimental testing performed to investigate the relationship between noise and intensity levels as shown in figure 1. The simulated test video provides a means to test experimental set ups prior to physical testing and may be used to explore the effects of key camera parameters on the results. As the dynamics of the camera are highly influential on the performance of the testing, each of the camera testing parameters will be investigated and quantified. A relationship between image quality, frame-rate and exposure time is required and will likely be quantified by using a signal to noise ratio relationship as well as the accuracy of the identified output displacement amplitude and phase. By exploring the effect of each camera and test parameter using the simulation, the limitations of the testing may be explored. Such as, identifying the smallest possible pixel displacement that may be reliably and accurately extracted from video data.