Research On The Light Pen Vision Measurement System

With the advantages of portability, simplicity, large measurement range and high precision, the portable vision measurement machines (PVMM), as a uniquely portable coordinate measurement system developed in recent years, have been widely used in large-scale manufacturing. Compared with the usual vision measurement systems, PVMM can be used in light discontinuity situations, such as measuring the internal structure in products and complex surface. In order to realize the large-scale measurement of key components with complex structure, on the basis of the existing research, this paper develops a monocular light pen vision measurement system and a binocular light pen vision measurement system. The key issues in the light pen systems and their solutions are also well discussed.According to the system structure and the measuring process, this paper introduces the design of the system in hardware selecting, the principles in monocular and binocular vision measurement, and the optimization and development of the measuring algorithms. The main innovations of the work include:firstly, propose a camera calibration method to obtain the camera’s internal parameters. Secondly, we develop a type of hand-held light-pen with retro-reflective feature points on it. Positions of the feature points on the light pen are unknown, so we need to calibrate the spatial coordinates of these points. Thirdly, considering that the probe tip needs to be changed depending on different measuring requirements, the position of the probe tip center should be recalculated every time we reassemble the light pen. That means a real-time and flexible program is necessary to calibrate the location of the probe tip. In our system, we propose a probe tip center calibration method to improve the system’s flexibility. At the last, during the measurement, the light pen is employed to measure a point on the object surface manually, while the pose (position and orientation) relative to the fixed camera is determined by studying the correspondences between the2D image coordinates and the3D coordinates of the feature points. Based on the position and orientation, the3D coordinate of the contact point can be calculated. During the calibration process, we first get a rough initial estimation of the unknown quantities, and then these quantities are optimized by the bundle adjustment method. Principles are different in monocular and binocular light pen vision measurement systems and they will be described separately in this thesis, In the experiment part, we also compare the precision of the two systems. Experiments show that in a100mmx120mm field, the position errors of42points are less than0.02mm measured by the monocular system, and that is less than0.02mm by the binocular system. The measuring systems are portable, efficient and high-precision.