Layout Optimization And Calibration Method Of Cameras In The Measuring System For Large Forgings

With the rapid development of large-scale equipment manufacturing industry in our country, there is an increasing demand for large forgings in nuclear power industry, petrochemical industry and other related industry. An accurate and timely measurement of the dimensions for hot large forgings on the site can provide an effective guidance to formulate a reasonable forging process so as to improve the material utilization and to reduce the production cost. Therefore, to carry out the study of both the on-line measurement technique for the dimensions of forgings and the development of the corresponding measuring instrument will have wide application prospect and huge economic value. In view of the fact that the binocular stereo vision has the advantage of non-contact measurement, good real-time performance, high measuring accuracy and good flexibility, the binocular stereo vision measurement system is used to realize the on-line measurement of the dimensions for hot large forgings in this paper.As for a binocular stereo vision measurement system, each of the links from the camera layout, camera calibration, feature point matching to three-dimensional reconstruction has a direct effect on the final measurement accuracy. The dissertation focuses on the two links including the camera layout and the camera calibration. The main research contents are as follows.1. Some study on the layout optimization method of binocular stereo vision is carried out. A mathematical model of the relationship between the measurement error caused by image point extraction error (resulting from image sampling) and structural parameters, such as focal length, baseline distance, etc. is built. In this model, the constraint of effective field of view (FOV) of the binocular sensor is taken into consideration. Then, the layout optimization is conducted by using the genetic algorithm (GA) to obtain a set of optimal structural parameters. Finally, the distributed regularities of both the comprehensive measurement error of the space points and their measurement errors in each coordinate component under the optimal parameter settings are analyzed. Some advices on the camera layout for the improvement of accuracy are given.2. Research on some existing calibration theories and calibration methods of camera is carried out. Given the measurement demand of the large FOV for the large forgings, the calibration method based on active vision is more appropriate, because it is not necessary to provide a high-precision large calibration target. Aiming at the shortcoming that the existing calibration methods based on active vision have not considered the effect of the precision of motion control platform on the calibration result, an improved active vision based camera calibration method with the projected laser raster as the calibration target is proposed. The camera is controlled to perform several orthogonal translations in different planes by the four dimensional motion control platform, meanwhile, the squareness of the two translation guide rails, which is measured by the dual-frequency laser interferometer, is taken into consideration to build the constraints. In the process of calibration, a laser array is used to project large-scale calibration target. Research on the robust extraction of the calibration feature points offered by the laser raster is also carried out. Finally, all the camera parameters are optimized by means of taking several images of the calibration board in different locations of the measurement FOV.3. Verification experiments on the calibration accuracy of the calibration method proposed in this paper are done. Experiments show that the average value of relative errors between the true value and the measured value is0.0583%. And this level of measuring error can satisfy the actual engineering demands.