High-speed three-dimensional shape measurement with inner shifting-phase fringe projection profilometry

Research background

Structured light three-dimensional (3D) reconstruction has been widely used in medicine, aerospace and other industrial fields. As one of the 3D reconstruction methods of structured light, phase-shifting profilometry (PSP) has attracted much attention due to its high accuracy and robustness. PSP is used to obtain the 3D shape of objects by collecting and solving the fringe patterns projected onto the object surface with phase information. The phase information of PSP has periodic phase ambiguity of (0-2π). The traditional method requires additional fringe patterns to determine the fringe period, which undoubtedly increases the time cost and limits the area of usage scenarios. With the increased of application scenarios such as dynamic object reconstruction and real-time object scanning in industry, traditional PSP cannot meet the requirements.

Research progress

In recent years, researchers have proposed lots of methods to improve the dynamic measurement capability of PSP. However, it is always difficult to ensure both accuracy and measurement efficiency simultaneously. In order to solve this problem, Dr. Liu Fei's group in Chongqing University proposed an inner shifting-phase coding fringe projection profilometry. Related research results were published in Chinese Optics Letters Vol. 20, Issue 11 in 2022, under the title of "Shichao Yang, Fei Liu, et al. High-speed three-dimensional shape measurement with inner shifting-phase fringe projection profilometry", and was selected as the cover.

This method embeds fringe order decoding information and wrapped phase information into three fringe patterns. The wrapped phase and fringe order are obtained only by 3 fringe patterns, which are needed 12 patterns in the traditional method. The flow chart of the 3D reconstruction of inner shifting-phase coding fringe projection profilometry is shown as follows:

It is seen from the flow chart that there are mainly four steps: acquiring the wrapped phase, calculating the background intensity and shifting phase, unwrapping the phase, and reconstructing the 3D model. Compared with the traditional method, which requires additional multiple fringe patterns to unwrap the phase, this method simultaneously obtains the wrapped phase and fringe order from three patterns. The number of projected patterns is only one-third of that of the traditional method. A standard sphere precision measurement experiment has been conducted in the paper. the experimental result proves that the two methods have the same precision.

In order to verify the performance of the proposed method in measuring dynamic objects, a 3D measurement experiment of high-speed vibration of aircraft models was prepared. The aircraft wing vibrates at high speed under the blowing of the fan. The real-time 3D data of the aircraft model and the dynamic trajectory information of the three points on the wing are given by the inner shifting-phase coding fringe projection profilometry proposed in this paper. The experimental results show that the proposed method is applied to high-speed dynamic 3D measurement. This method is simple and effective, which provides a new idea for 3D dynamic measurement research.

Future outlook

The inner shifting-phase fringe projection profilometry proposed by researchers has solved the contradiction between high accuracy and high dynamics of 3D measurement, but it is still insufficient for the measurement of reflective parts such as metals. In the future work, researchers will conduct in-depth research on the 3D measurement of highly reflective objects.

三维动态测量，高精度与高动态双赢

研究背景

结构光三维重建在医学、航空航天和其他工业领域都得到了广泛的应用。相移轮廓术作为主流三维测量方法之一，因其高精度和高鲁棒性广受关注。相移轮廓术通过采集、解算投影到物体表面的，有相位信息的条纹图像，获取物体三维形貌数据。

但相移轮廓术相位信息带有0~2π的周期性相位模糊，传统方法需要额外投射图像确定条纹周期，不但增加了时间成本，也限制了其使用范围。随着工业上动态物体重建和实时物体扫描等应用场景增加，传统的相移轮廓术已经不能满足使用需求。近年来，国内外研究人员为了提高相移轮廓术的动态测量能力做了大量研究，但仍难以同时满足高精度与高动态的需求。

基于内相移编码的条纹投影轮廓术

针对这个问题，重庆大学刘飞副教授课题组提出了基于内相移编码的条纹投影轮廓术，相关成果发表在Chinese Optics Letters 2022年20卷第11期上（Shichao Yang, Fei Liu, et al. High-speed three-dimensional shape measurement with inner shifting-phase fringe projection profilometry），并被选为当期封面。

封面内容展示的是论文中的代表性实验：飞机模型高速振动三维测量。主要包括三个元素：飞机模型、风扇和光源。光源将光条投影在飞机模型表面，飞机模型在风扇的吹动下高速振动。通过内相移编码算法，实时动态测量飞机三维模型。

事半功倍，实现高精度高动态双赢

基于内相移编码的条纹投影轮廓术三维重建的流程如图1所示。该方法主要分为以下四个步骤：获取包裹相位、计算背景光强和相移相位、展开包裹相位、三维重建。与传统方法需要额外的多张条纹图去展开包裹相位相比，该方法可通过3张条纹图同时获取包裹相位及条纹周期，达到传统方法12张条纹图的效果。投射图片数量仅为传统方法的1/4。同时，标准球精度测量对比实验发现两种方法的精度相当。

此外，为了验证该方法的三维动态测量性能，该课题组还开展了飞机模型高速振动三维测量实验。如图2所示，飞机模型在风扇的吹动下高速振动。通过提出的内相移编码算法，给出了飞机模型实时的三维数据，以及飞机机翼上三个点的轨迹动态信息。实验结果证明该方法能够应用于高速动态三维测量领域。并且此方法操作简单，效果良好，为三维动态测量研究提供了一个新思路。

综上所述，该课题组提出的基于内相移编码的条纹轮廓术方法较好的解决了三维测量高精度与高动态之间的矛盾，但对于金属等反光零件的测量还有不足。在后续的工作中，该课题中将对强反光物体的三维测量进行深入的研究。