Research on Line Structured Light Calibration Method

Abstract

During the measurement procedure of line structured light 3D scanning system, there is a key about how to calibrate the whole measuring system. In order to solve the calibration problem in the line structured light 3D measured system such as requirements for high accuracy calibration models, complicate calibration procedures and so on. This paper presented a calibration method combining the vertical calibration board with 3D scanning measurement system, this method is simple, easy to operate and the assistant adjustment equipment is unnecessary. Experiment showed that this method can attain relative accuracy about 0.51% which indicates the rationality of this method. Introduction At present, with the development of science and technology, the intelligent non-contact 3D measurement methods have gained wide attention , the line structured light scanning system is applying to precise measurement, based on computer vision technology with reliable, inexpressive and anti-interference. Its processing principle is the deformed of line laser stripe due to the height adjustment of the target object. Capturing the distortion of light pattern, and obtaining the dimensional information of the object surface based on the position relationship between the laser and camera coordinate system. Therefore, the 3D measurement system parameter calibration is one of the key loop of entire detection tasks. Normally, the calibrate methods of structure sensor is drawing method, toothed target calibration method, mechanical adjustment method and so forth. The first two methods both require precise expensive auxiliary equipments, and not suitable for field calibration, besides that, the mechanical adjustment for the factors involved in too many legal, making measurement precision is lower, besides it was proposed based on the calibration reference coplanar method, but this method is to create a three-dimensional world coordinate system in the plane of reference, each time the reference position moves, the positional relationship between the world coordinate system and camera coordinate system must be readjusted [1]. In response to these problems, a vertical scale grid Othello plate is designed as a calibration target system, and improving the traditional calibration methods. Result shows that the improved method can obtain the plane equation of camera coordinate system only single measurement, and the calibration of coordinate system can also be obtained easily. This method doesn’t require expensive precision equipment , simple operation and is suitable for field calibration. Parameter Calibration of Line Structured Light Scanning System Calibration of line structured light 3D scanning system includes two loops, first is the camera spatial location model parameter calibration and the second is the calibration of line structured light plane. (1)Calibration of camera model parameter Camera parameters can be obtained based on the principle of coplanar, non-coplanar and self-calibration. Accounting for this, besides taking into account the simplicity and accuracy of line structured light plane. Article designs the vertical calibration plate as shown in figure 1, and the International Conference on Intelligent Systems Research and Mechatronics Engineering (ISRME 2015) © 2015. The authors Published by Atlantis Press 65 corner of vertical calibration plate also can be extracted as the calibration points needed in experiment, as shown in figure 2. Fig.1 .Vertical calibration chessboard captured by camera Fig.2. Corner extraction of vertical by calibration board Tsai camera calibration based on the radial constraint and Zhang Zhengyou calibration based on planar target are mainly methods of camera calibration[2]. Considering the camera in experiment is general, only the first-order radial distortion is considered in the process of camera calibration, this paper chooses the Tsai calibration method. Firstly most of camera model parameters are solved by the radial collimator constraints, then the effective focal length, nonlinear distortion factor and other parameters are obtained so that the final completion of calibration of the camera[3,4]. (2)Parameter calibration of line structured light plane The design of line structured light scanning system is driven by a stepping motor line lasers, cameras and a rotating platform composed of the entire scanning system platform shown in figure 3.firstly the laser projects the line structure light beam to the target, the deformation line structured light strip can be obtained and captured due to the height modulate of measured object. Among the deformation stripes the location information between line laser and camera and depth data of the measured surface is included, and the three-dimensional information of target object can be obtained based on the cross-ratio invariance principle, after scanning the whole surface using stepper motor the entire three-dimensional contour data can be obtained completely[5]. Fig. 3. 3D scanning prototype used in this experiment