Research On Monocular-Vision-Based Onsite Measurement Of Large Workpiece

In aircraft manufacturing, automobile making, ship building, and some other modern digital manufacturing fields, there are increasing demands for the precision measurement of the 3D shapes and geometric sizes of large-scale workpieces on the working spot. The traditional measurement techniques and equipments cannot meet the manufacturing requirements in these situations. Thanks to the advantages of small size, light weight, large measurement space and high portability, the vision-based coordinate measuring systems have been increasingly studied and have exhibited broad application prospects. This study focused on the key techniques of monocular-vision-based coordinate measuring system for large-scale workpieces. The main contents are as follows:1.Based on the analysis of the controlling technology of LED luminance, a target controlling system was designed. With the designed controlling system, coincident target feature images can be formed by the light pen at different distances in the large span area. A technique of Pulse Width Modulation(PWM) with Multi-level Voltage Automatic Switch was adopted, and it can solve the problem of close target point with insufficient adjustable resolution and low light intensity of distant target point. In order to control LEDs’ intensity, “one main and multi servo method” was proposed, and it can solve the problem of the lack of existing chip’s independent control capability. The communication between controller and computer was established, and achieves an adaptive control for the LEDs’ intensity.2.The principles of imaging system and gray distribution of target spot have been studied, and an active defocused blurred image system have been designed. Based on results of theoretical analyses and many experiments, the imaging parameters were rationally set. The circular aperture was designed and manufactured, and it can solve the problem of target spot with tails. With the imaging technique, target spot size and gray distribution were improved, and serves well as the input the accurate determining the target spot centroid. A target feature image generation technique was implemented, which combined an adaptive control of LED light intensity and an active defocused blurred image method. Thus, high-quality target feature images can be formed in the range of 2~10 m.3.In order to calibrate light pen for monocular-vision-based coordinate measuring system, a complete on-site light pen calibration method based on 1D calibration bar was proposed in this paper. It needed only the camera of the system, several auxiliary target points with the same visual features of the light pen targets, and two or more cone holes with known distance(s). The target point calibration and the probe tip center calibration were jointly implemented. Since the auxiliary target points were taken into consider, light pen calibration could be done with good precision. In order to recover the absolute metric coordinates of the target points, cone holes with known distances came into play. The proposed method requires simply taking several images of the light pen for the calibration, so it is easier to implement and suitable for onsite use.4.A prototype monocular-vision-based coordinate measuring system, namely Free Probe was designed with above key theories and techniques. According to the actual application requirements, the hardware platform was designed. As for the software system, the fuction of light pen calibration, camera calibration and coordinate measurement have all been implemented.In order to verify the validity and practicability of the proposed techniques, a set of measurement experiments was conducted using Free Probe, and reasonable results have been obtained.