WANG Zhong, YANG Tong-yu, WANG Lei, FU Lu-hua, LIU Chang-jie
(National Key Laboratory of Precision Testing Techniques and Instrument, Tianjin University, Tianjin 300072, China)
Abstract： In the laser displacement sensors measurement system, the laser beam direction is an important parameter. Particularly, the azimuth and pitch angles are the most important parameters to a laser beam. In this paper, based on monocular vision, a laser beam direction measurement method is proposed. First, place the charge coupled device (CCD) camera above the base plane, and adjust and fix the camera position so that the optical axis is nearly perpendicular to the base plane. The monocular vision localization model is established by using circular aperture calibration board. Then the laser beam generating device is placed and maintained on the base plane at fixed position. At the same time a special target block is placed on the base plane so that the laser beam can project to the special target and form a laser spot. The CCD camera placed above the base plane can acquire the laser spot and the image of the target block clearly, so the two-dimensional (2D) image coordinate of the centroid of the laser spot can be extracted by correlation algorithm. The target is moved at an equal distance along the laser beam direction, and the spots and target images of each moving under the current position are collected by the CCD camera. By using the relevant transformation formula and combining the intrinsic parameters of the target block, the 2D coordinates of the gravity center of the spot are converted to the three-dimensional (3D) coordinate in the base plane. Because of the moving of the target, the 3D coordinates of the gravity center of the laser spot at different positions are obtained, and these 3D coordinates are synthesized into a space straight line to represent the laser beam to be measured. In the experiment, the target parameters are measured by high-precision instruments, and the calibration parameters of the camera are calibrated by a high-precision calibration board to establish the corresponding positioning model. The measurement accuracy is mainly guaranteed by the monocular vision positioning accuracy and the gravity center extraction accuracy. The experimental results show the maximum error of the angle between laser beams reaches to 0.04° and the maximum error of beam pitch angle reaches to 0.02°.
Key words： monocular vision; laser beam direction; coordinate transformation; laser displacement sensor
CLD number： TH212 Document code： A
Article ID： 1674-8042（2017）04-0354-10 doi： 10.3969/j.issn.1674-8042-2017-04-008
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基于单目视觉的激光光束方向标定研究
王仲， 杨桐郁， 王磊， 付鲁华， 刘常杰
（天津大学 精密测试技术及仪器国家重点实验室， 天津 300072）
摘要:在由激光位移传感器组成的测量系统中， 激光光束的方向是一个关键参数。 方位角和俯仰角对于一条激光光束是最为重要的两个参数。 本文中提出一种基于单目视觉的激光光束方向测量方法。 首先， 将CCD相机放置于基础平面上方， 保持相机光轴与基础平面接近于垂直状态， 并利用误差为10 μm的圆孔型标定板建立单目定位模型。 然后将激光光束发生装置放置在基础平面上并保持位置固定， 同时在基础平面上放置特制靶块， 使激光光束可以投射到靶块斜面上并形成一个激光光斑。 在基础平面上方放置的CCD相机可以清晰的采集到激光光斑、 靶块斜面的图像， 应用相关算法提取出光斑质心的二维图像坐标。 沿激光光束方向以相等间距移动靶块， 通过CCD相机采集每移动一次靶块在当前位置下的光斑、 靶块图像。 利用相关的转换公式， 结合靶块本身固有参数， 将光斑质心图像二维坐标转换为基础平面下的空间三维坐标。 由于靶块的移动， 会得到靶块不同位置下激光光斑质心的三维坐标， 将这些三维坐标拟合成空间直线表征待测激光光束。 拟合直线得俯仰角即为待测激光光束的俯仰角。 实验中， 应用高精度仪器对靶块参数进行测定， 并使用高精度标定板标定相机内外参数建立相应的定位模型。 测量精度主要通过单目视觉定位精度、 光斑重心提取精度来保证。 结果显示， 待测光束的俯角最大误差达到0.02°， 光束间夹角的最大误差为0.04°。
关键词:单目视觉； 激光光束方向； 坐标转换； 激光位移传感器
引用格式：WANG Zhong, YANG Tong-yu, WANG Lei, et al. Calibration of laser beam direction based on monocular vision. Journal of Measurement Science and Instrumentation, 2017, 8(4)： 354-363. ［doi： 10.3969／j.issn.1674-8042.2017-04-008］
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