External Parameters Optimization And High-precision Pose Recognition In Multi-camera Measurement

The digital image correlation(DIC)method has been developed into the most active threedimensional deformation measurement technology in the field of photo-mechanics with the advantages of simple optical setup,low environmental requirements,and non-contact full-field measurement.However,due to the constraints of camera spatial resolution,image noise and analysis methods,the measurement accuracy and measurement range of the DIC method cannot meet the needs of most scientific research and engineering measurement.Further improving the measurement accuracy of the DIC method and expanding the measurement range have become a hot research topic that most scholars are focusing on.The DIC method based on the principles of two-camera and multicamera vision can be applied to the three-dimensional shape and deformation measurement of the full surface,especially the complex surface,of materials or structures at multiple scales.In multi-camera measurement,the camera position and pose,i.e.the external parameters of cameras,are important factors affecting the measurement accuracy.By means of external parameters optimization and realtime camera pose correction,high-precision multi-camera measurement systems are established to fulfill the needs of large-range and high-precision three-dimensional deformation measurement.The baseline distance and the angle of optical axis of cameras determine the camera position and pose.Starting from the mathematical model of stereo-vision principle,the relationships between these parameters and the measurement error are analyzed,and the optimal range of external parameters is found for the two-camera measurement.Combined the external parameters optimization and actual measurement requirements,two-camera measurement systems are established to achieve highprecision three-dimensional deformation measurement of the small-scale complex deformation and interfacial slip.With the human-computer interaction calibration method for small view field,the three-dimensional DIC method is utilized to record and analyze the propagation process and motion characteristics of the Lüders band for the first time,and the experimental study of Lüders behavior is extended to the three-dimensional small scale.The three-dimensional DIC method is also applied to study the bond behavior of the carbon fiber reinforced polymer-to-steel interface.The gradual debonding process of the surface is directly observed,and a more accurate interfacial bond-slip relationship than the existing methods is obtained.To solve the problems of limited view field and large measurement error in two-camera measurement of columnar components,a discrete-layout multi-camera three-dimensional deformation measurement system is established,among which multiple sets of two-camera subsystems are used to measure different parts of the columnar surface,to realize the full-surface deformation measturement of columnar components.For the camera pose change will directly affect the measurement accuracy,the speckle-based calibration method of external parameters is used to realize the real-time camera pose correction,which can avoid the measurement error caused by camera pose change.With the pre-calibrated internal parameters and correlation matching results,the relative orientation algorithm is conducted to calculate the relative external parameters between cameras.The change experiment of camera pose and the drop test of a ductile cast iron vessel describe the necessity and effectiveness of real-time camera pose correction.A discrete-layout multi-camera system is established for the axial compression test of basalt fiber reinforced polymer confined concrete cylinder,the real-time camera pose recognition verifies that a good experimental environment and a stable system installation can effectively decrease camera pose change,and the strain distribution during the deformation and failure process of the cylindrical surface are intuitively recorded.Also,a discrete-layout multi-camera system is utilized to accurately capture the full-surface buckling mode of square hollow section steel column reinforced with carbon fiber reinforced polymer,providing an intuitive three-dimensional measurement method for the study in buckling behavior of steel members.In the continuous-layout multi-camera three-dimensional deformation measurement system,multiple cameras are unified into a global coordinate system to form an overall three-dimensional measurement system,and the full-surface complete deformation of objects with large curvature and with large slenderness can be obtained.The camera layout and overlap of view field determine the position and pose between cameras,which directly affect the measurement accuracy.The experimental study on optimal layout of continuous-layout multi-camera system is conducted,and the four-point bending experiment,translation experiment of glass plate and three-point bending experiment of rubber beam all verify the system accuracy.Taking the grouted sleeve as an example,a continuous-layout multi-camera system for complex surface with large curvature is established to measure the three-dimensional shape and deformation of grouted sleeve.Compared with the results of two-camera measurement,strain gauge data and finite element analysis,the system accuracy is verified.It provides a new high-precision means for the experimental study on the connection properties of the grouted sleeve.Simultaneously,in order to improve the measurement accuracy of surface with large slenderness,a multi-camera system with continuous arrangement is designed and constructed for the flexural experiment of coral concrete beam.The full-surface deformation distribution and crack propagation are visually recorded and the DIC results are in good agreement with the results of displacement meters.A reliable three-dimensional measurement approach is provided for the components with large slenderness.Based on the actual measurement requirements,high-precision two-camera measurement system and multi-camera measurement systems under multiple scales are established via external parameters optimization and camera pose recognition,which greatly improve the measurement accuracy of the DIC method and extend the measurement range of measured object.To some extent,the threedimensional deformation measurement requirements for engineering materials and structures are fulfilled.