Stereo-Vison Based 3D Deformation And Attitude Measurement Of Models In Wind Tunnel Test

With the development of aerospace and military equipment,the design of aircraft is getting more and more sophisticated,and the aerodynamic characteristics of the aircraft surface are becoming more and more complex.Currently,wind tunnel testing is the main method to study and verify the aerodynamic characteristics of the aircraft surfaces.To study the aerodynamic characteristics of the aircraft surfaces,the 3D deformation and attitude of aircraft models need to be measured accurately during wind tunnel test.Owing to the advantage of non-contact,having no influence on wind tunnel flow field,flexible system configuration and high accuracy,the stereo-vision measurement technology is considered to be the best method for measuring the 3D deformation and attitude of models in wind tunnel tests.However,considering the complicated measurement environment of domestic wind tunnels,it is difficult to directly apply this technology in domestic wind tunnels.The reason is that it is hard to obtain accurate measurement results for the stereo-vision under the environment of large FOV(field of view),strong vibration and high-speed measurement.To solve these problems,this dissertation is dedicated to propose a flexible calibration method for stereo-vison system in large FOV,to develop a globally optimal camera external parameters self-calibration method,and to develop a low-speed-camera-array-based highspeed 3D measurement method.Based on the above research content,a stereo-vision based 3D deformation and attitude measurement system can be formed,and this system can be applied to measure the 3D deformation and attitude of models in wind tunnel test.The research work is concluded as follows.Aming at the problem of large scale high-precision calibration target is difficult to manufacture and not suitable for wind tunnel on-site calibration,a flexible calibration method for stereo-vison system in large FOV is proposed.To meet the automatic matching requirements of corresponding points in uncalibrated camera images,a ring-coded taget is designed and the ring-coded target decoding algorithm is proposed.When the matching relationship of the corresponding points is established,the classical 8-point algorithm is used to estimate the pose between the images.Then,the objective function with the smallest reverse projection error will be built,and the 3D coordinates of the reference points on the target and the internal parameters of the camera can be solved simultaneously by leastsquares method.At last,caculating the external parameters of the stereo vision measurement system by constructing the cost function with the smallest reverse projection error of the multi-camera image feature points.By using this method,the stereo vision system can be calibrated,when the 3D coordinates of the target points are unknow.The accuracy experiment results demonstrate that the proposed method can calibrate the stereo-vision system with large FOV accurately.In order to achieve robust measurement of stereo vision measurement system in vibration environment,an unconstrained optimization camera external parameters selfcalibration algorithm is proposed.The rotation matrix is parameterized by using non-unit quaternion,and the camera external parameters self-calibration problem is formulated into an unconstrained optimization problem.Surprisingly,the polynomial system arising from its first-order optimality condition is of odd-degree,thus assuming two-fold symmetry,a nice property that can be utilized to improve speed and stability of a Grobner basis solver.As verified by experiment results,the proposed algorithm is accurate,with different point configurations and camera poses.Aiming at the problem of poor measurement accuracy of stereo vision system when the measurement speed is high,a low-speed-camera-array based high-speed 3D measurement method is proposed.To solve the problem of the low resolution of high-speed camera during high-speed image acquisition,a high-speed image acquisition method based on low-speed camera array is proposed.Combined with the principle of multi-view geometry,a highspeed 3D measurement method based on low-speed camera array is developed.Aiming at the problem of uneven distribution of measurement error in the camera array system,a bundle adjustment based calibration method is proposed.The experimental results show that high-precision 3D measurement can be realized by using the proposed method,and it has unique advantages in cost,large FOV and high-speed measurement.In order to meet the 3D deformation and attitude measurement requirements of the models in wind tunnel test,based on the above three key technologies,the stereo-vision based 3D deformation and attitude measurement system of model in wind tunnel is developed,and it is promoted and applied to measure the model's deformation and attitude in wind tunnel test.The developed system was successfully applied to the deformation and attitude measurement of the model in the low-speed wind tunnel of CARDC.The accuracy tests in the wind tunnel showed that when the field of view was 2m(width)× 1.5m(high)× 1m(depth),and the camera resolution was 1380×1040 pixels,the deformation accuracy of the developed measurement system is 0.15 mm,the measurement accuracy of angle is 0.015°,which meet the deformation and attitude measurement requirements of models in low-speed wind tunnel;In the supersonic wind tunnel of CARDC,the stereo vision measurement system was placed inside the test section to measure the vibration of model.The measurement results showed that when the measurement range was 1200 mm(length)× 600 mm(width),and the camera resolution was 1020×600,the system's deformation measurement accuracy is 0.06 mm,and the stereo-vision measurement system can obtain more measurement data than the contact sensor;The developed system has also been applied in other large FOV,strong vibration and high-speed measurement environments.In China Second Heavy Machinery Group Deyang Wanhang Die Forging Co.,Ltd,the ram speed of steam hammer was measured by the developed system.The experimental results show that when the measurement range is 3000mm(width)× 2400mm(height)× 1000mm(deep),the camera resolution is 1280×1024,the displacement measurement accuracy is 0.15 mm,and the camera external parameter self-calibration algorithm can effectively eliminate the influence of vibration on the stereo vision measurement system.At last,aiming at the automatic docking requirements of large barrel components,by using the camera external parameter self-calibration algorithm,a large barrel components automatic docking system with only one camera is proposed.The experimental results show that the proposed method can meet the automatic docking requirements of large barrel components.In summary,the dissertation addresses the major needs of stereo-vision based 3D deformation and attitude measurement of models in wind tunnel.An in-depth study was conducted including: a flexible calibration method for stereo-vision measurement system in large FOV is proposed;an unconstrained optimization camera external parameter selfcalibration algorithm is developed;and a low-speed-camear-array-based high-speed 3D measurement method is built.Based on the above technique,a stereo-based 3D deformation and attitude measurement system is developed.The successful implement of this dissertation has potential applications that it can provide a comprehensive and effective method for 3D deformation and attitude measurement of the model in wind tunnel test,and has great application and promotion potential.