Analysis on 3D object measurement based on fringe projection

A coordinate measuring method is presented, which is specially devised to perform the measurement of coordinates with projected fringe techniques of projectors in three dimensions. The system is composed of two parts: one is a target which can move freely in three dimensions, and the other is a stationary two-dimensional array of photodetectors. The mini-projector is tied to the target, and the projected fringe is monitored by the photodetectors. The phase of the photodetectors can be precisely measured with the phase-shifting algorithm, so that, the xyz location of the target can be determined with the geometric model of multilateration using the method of optimization. In this paper, the measuring principle, iterative method, computer simulation and preliminary results are given. The phase-shifting technique has the advantages of high accuracy and noise endurance. The method will provide the basis for follow-up iterative optimization calculation. The experimental results prove that the proposed coordinate measuring method is of high precision.     

基于二进制条纹加相位编码条纹离焦投影的三维测量方法

复杂表面的精密三维测量在工业无损检测中非常重要。二进制条纹离焦投影方法在快速三维测量中有重要的应用前景,但该方法难以实现复杂表面高精度三维测量。为此,提出了基于二进制条纹加相位编码条纹离焦投影的三维测量方法。由于离焦投影滤除了高次谐波和高频噪声,可以克服投影仪的非线性伽马效应,与传统投影正弦条纹方法相比,提高了其测量精度。针对离焦投影时,随着相位编码条纹频率增大,条纹级次判决困难,出现周期错位,导致相位解包裹出错,提出了相移编码方法来解决以上问题。采用相移编码方法校正周期错位,使条纹级次判决准确,进一步提高其测量精度。实验结果表明,其测量精度可以达到0.044 mm,验证了本方法的有效性和实用性。     

基于彩色光栅投影的快速三维测量方法

 针对条纹投影术提取物体高度信息的问题,提出了一种新的基于彩色光栅投影的三维测量方法。对选取G分量为255,R,B分量各取0或255而组成的青、白、黄、绿四色遵循格雷码原理进行编码,然后将G分量作正弦调制形成投影光栅投向被测物体。提取采集到的光栅变形图中G分量,利用傅里叶变换方法得到其初始相位;同时对采集到图像中的R,B分量作阈值迭代分割而G分量自动赋值为255,综合三分量信息得到条纹颜色信息进而获取条纹的周期信息,从而展开相位。全过程仅需投影一幅彩色光栅图就能完成三维测量,实验结果表明该算法易于实现,在测量实时性和精确性上表现良好。     

A composite-structured-light 3D measurement method based on fringe parameter calibration

This paper proposes a novel method for reducing measurement error caused by spectrum overlapping in composite-structured-light 3D measurement systems. For a composite-structured-light 3D measurement system, spectrum overlapping causes parameters of each deformed phase-shifting fringe to change, and therefore leads to phase measurement errors. The proposed fringe parameter calibration method is based on the fact that variations in each deformed fringe's parameters are independent of height and reflectivity of the measured object. Three frames of composite grating are projected on the reference plane, and each carrier channel includes the information of three phase-shifting sinusoidal gratings used in Phase Measuring Profilometry (PMP). With the parameter calculation formulas of PMP, the parameters of fringes demodulated from the same carrier channel can be calculated, and therefore parameter relation coefficients between fringes demodulated from different carrier channels may be obtained. When an object is measured, these relation coefficients can be used to calibrate the parameters of the deformed phase-shifting fringes. A new 3D measurement mathematical model is established to reconstruct the shape of the object. Experimental data proved that the proposed method can effectively restrain the effect of spectrum overlapping and improve measurement accuracy by more than three times.     

A novel fast 3D measurement method based on phase-coded fringe projection

Optical Review - Fringe projection profilometry is widely used for the 3D measurement of real-world objects; however, quickly obtaining high-precision 3D measurements is an issue that needs to be...     

A flexible 3D profilometry based on fringe contrast analysis

This paper proposed a flexible 3D profilometry based on fringe contrast analysis. A series of sinusoidal fringes are projected on the surface of the testing object that is laid in front of the imaging plane of the fringes, and captured through a beam splitter by a CCD camera in the same direction as the axis of the projection. After the fringe contrast obtained using phase-shifting technique, the height of the object can be restored through searching the look-up table of the contrast and the height. In the measurement process, we only need to capture fringe patterns in one position, without changing the imaging plane of the fringes, so it is faster and more convenient than the existing method based on fringe contrast analysis. Our system is simple and flexible by reason of no special devices used. In this paper, the principle of this method, the setup of the measurement system and primary experiment results are given. The experimental results prove this method can restore the height of the complex object accurately and effectively.     

3D shape measurement of larger complex objects based on fringe cycle correction

The grating fringe on the reference plane is broadened in the intersecting axis system because of oblique-angle projection. In order to solve this problem, we study the theoretical model of the temporal phase unwrapping method based on the fringe cycle correction. We also study the 3D shape measurement theoretical model of the larger complex objects after considering the coordinate deviation and lens distortion. Experimental results demonstrate that the fringe cycle on the reference plane can be corrected to a constant value, the lens distortion can be corrected, and 3D shape of larger complex objects can be accurately measured.     

BP neural network applied to 3D object measurement based on fringe pattern projection

In the fringe projection profilometry, the traditional triangle method, such as Fourier-transform profilometry (FTP), is difficult to recover the stepped shape object from the deformed fringe pattern. In order to solve this problem, the neural network is introduced to deal with this kind of fringe patterns and gain the three-dimensional (3D) information of the measured object. By training the network, the relationship between the deformed fringe pattern and the height of the object can be obtained, and thereby the height of the object can be obtained. Furthermore, the object can be reconstructed perfectly without knowing the optical parameters of the experiment system. An obvious merit of this network method is that it can recover the 3D object in a short time and only need one deformed fringe pattern. Computer simulations and experiment validate the feasibility of the method.     

基于四步相移光栅投影的三维形貌测量系统

为了实现对具有明显跳变面物体的轮廓测量,建立了四步相移光栅投影的三维形貌测量系统,并对所采用的四步相移测量算法进行研究。在介绍四步相移原理的基础上以有明显跳变面的三棱锥橡皮块为例,建立针对明显跳变面的测量算法。通过对4幅相移图算术求和并除以4得到平均灰度图;将平均灰度图二值化处理定位明显跳变面或阴影部分;将二值化图像与包裹相位图卷积,而后对包裹相位图进行解包裹,从而得到正确的连续相位,最终获取物体三维形貌。根据上述算法测量三棱锥的三维形貌并计算其体积,测量实验表明:被测物的体积平均相对精度为0.47%。     

Performance analysis of a 3D full-field sensor based on fringe projection

This article analyzes the measurement performance of a 3D full-field imaging system based on the projection of grating and active triangulation. We first explore the exact mathematical relationship that exists between the height of an object's surface, the phase and the parameters of the experimental setup, which relationship can be used to obtain the precise shape of an object. We then investigate in detail the influence on the measurement results of the introduction of an inaccuracy into the determination of the system's parameters. Finally, using simulated data, we conduct experiments to evaluate the measurement performance.     

A novel color fringe projection based Fourier transform 3D shape measurement method

A novel Fourier transform 3D shape measurement method based on color fringe projection is proposed in order to solve the spectrum overlapping and phase unwrapping problems existed in Fourier transform profilometry (FTP). The R and G components of the color fringe are set to two sinusoidal patterns with different frequencies and the B component is set to the average value of R or G component. Then this pattern is projected to the object and the deformed fringe image is captured. Three gray patterns are separated from the color fringe, the background and high frequency noise can be eliminated using our method and the accurate unwrapped phase can be got. Only one shot color pattern is projected to get the 3D information of the object. Experiment results show that the 3D information of an object can be obtained rapidly and accurately.     

Highly accurate film thickness measurement based on automatic fringe analysis

This paper describes an automated multiple-beam interferometry for measuring a calibrated step height of (27.00 ± 3.00) nm nominally. The fringes captured from the multiple-beam Fizeau–Tolansky interferometer were thinned by using a written program to obtain accurate measurement. We claim that multiple-beam interferometry based on automatic fringe thinning process can provide a real time solution for calibrating step heights precisely and with high accuracy. The uncertainty budget of the multiple-beam interferometry method due to incomplete parallelism of the incident beam and the inhomogeneity of the reflecting layers was calculated automatically with a written ray tracing program. The uncertainty budget in multiple-beam interferometry was estimated to be of the order of 3.00 nm.     

Background and amplitude encoded fringe patterns for 3D surface-shape measurement

This paper presents a new fringe projection method for surface-shape measurement that uses background and amplitude encoded high-frequency fringe patterns. The background and amplitude, combined as a codeword, identify the wrapped phase fringe order to partially unwrap the phase to a low frequency. The low-frequency wrapped phase map is then directly used to reconstruct the surface based on geometry constraints without requiring additional images as in other temporal phase-unwrapping methods. Measurements performed on a double-hemisphere, mask, and manikin head, using projected fringe patterns with 48 periods, demonstrated the ability of the method to perform 3D shape measurement with only four projected patterns and captured images, using a single camera and projector.     

Evaluation of absolute phase for 3D profile measurement using fringe projection

A new method of absolute phase evaluation for three-dimensional (3D) profile measurement using fringe projection is presented, which combines the gray code and the phase shift technique. Two kinds of fringe patterns are projected onto the object surface respectively, one is sinusoidal intensity distribution used for phase demodulation and the other is gray code fringe pattern for unwrapping. These images are acquired by camera and stored into computer. The absolute phase is obtained by analyzing these images. The validity of this method is verified experimentally. The method is superior to other phase unwrapping methods.     

基于干涉条纹图像对比法测量微位移

研究了基于迈克尔逊干涉条纹对比法测量微位移的实验。用He-Ne激光器、反射镜和分束镜组成的干涉光路,其中一个反射镜固定在被测物体上,通过被测物体的移动带动反射镜移动使干涉光路发生变化,从而导致干涉条纹的改变。在形成干涉条纹的位置利用线阵CCD采集干涉条纹图像,用序列图像对比的方法对图像进行处理和计算,得出被测物体的微位移。实验表明利用序列图像对比的方法测量微位移,方法切实可行、测量准确度高、测量的精度能够达到微米量级。     

3-D object profilometry based on direct fringe displacement analysis

A simple, fast and accurate method for 3-D object profilometry is proposed and verified by simulation and experiment. The profilometry method is based on the principle that the height data of a measured object can be obtained from the relevant fringe displacement, which in turn can be obtained by determining the precise position of the fringe center on the reference plane and on the object's surface. With the aid of a fringe skeleton extraction technique, the 3-D information can be acquired even from those images in which some fringes are squeezed, interrupted or shadowed.     

Innovative calibration technique for fringe projection based 3D scanner

A new approach to 3D scanners calibration, based on Fourier Transform Profilometry method, is proposed and verified by experiments. Commonly used calibration techniques require that a calibration plane is placed in different and known positions in the scanner measuring volume; an optimization procedure iteratively change the value of some calibration parameters in order to minimize the discrepancy between the height distributions of the measured planes and the reference ones. The main limit of this type of calibration techniques is the necessity to guarantee a high accuracy in the placement of the planes used as calibration references. The innovation of the method proposed in this paper is the complete removal of any device for the accurate plane placement by assigning the task of the plane positions estimation to the camera, which is part of the scanner. The well known camera calibration algorithm proposed by Zhang permits at the same time to calibrate the camera and to estimate the positions and orientations of the plane used for the calibration itself. The knowledge of the plane position allows to use them as a reference of the scanner calibration. The obtained results show that this method provides accuracy values for the scanner parameters estimation comparable with other calibration techniques, but the advantage is that no particular device is needed.     

光电编码器单莫尔条纹测速方法

为了提高编码器的测速精度,研究了基于希尔伯特-黄变换的光电编码器单莫尔条纹测速方法。首先,利用AD采集编码器的单路莫尔条纹光电信号,并将信号序列通过EMD变换,滤除直流分量;然后,利用希尔伯特变换求出信号的相位变化,并通过差分运算求取信号的瞬时频率;最后,结合编码器的具体参数求取编码器的转速。实验结果表明:对某21位编码器进行测速实验,测速误差均方差由0.022 4 rad/s降低到0.013 4 rad/s。此方法测速稳定性高,抗干扰能力强,可用于速度精度要求较高的测速场合。     

一种基于结构光条纹投影的微小物体测量系统

为了通过结构光投影的方法测量微小物体,构建了一套微小物体三维形貌测量系统,视场范围可达1.8 cm×1.6 cm。这套测量系统利用了Light Crafter 4500数字投影组件的高速投影、立体显微镜的低畸变缩放、远心镜头的大景深与低畸变成像的特性。先利用立体显微镜对Light Crafter 4500投影的相移条纹图进行低畸变缩小,再投影到待测物体表面,采用配有远心镜头的相机同步记录受到物体表面形貌调制而发生形变的条纹,利用三步相移法计算出条纹对应的截断相位图,再根据可靠路径跟踪相位展开算法求取连续的相位分布,重建被测物体的三维表面形貌。实验成功重建了以BGA芯片为代表的微小物体表面三维形貌。实验结果表明,系统测量精度达到11 μm,系统的有效深度测量范围为700 μm。     

基于正弦和三角波条纹投影的三维测量方法

在现有的针对复杂物体表面形貌的三维测量方法中, 为了完成绝对相位的测量, 通常需要处理至少6幅条纹图像, 限制了测量速度。提出了采用2幅正弦条纹和2幅三角波条纹图来获得物体三维形貌的方法。利用两步相移正弦条纹和两步相移三角条纹得到截断相位, 再利用两步相移三角波条纹得到条纹级次, 减少了投影条纹幅数, 提高测量速度。在得到条纹级次时, 计算三角波条纹强度调制和强度对比度, 与计算相位相比, 可以减少数据处理的时间, 进一步提高测量速度, 同时能减小物体表面反射率的影响, 提高了测量精度。测量最大高度为39 mm的阶梯状标准块, 得到的最大绝对误差和最大的RMS误差分别为0.045 mm和0.041 mm。验证了该方法的有效性和实用性, 在高速实时的复杂形貌三维测量中有广泛的应用前景。