Research On Calibration Method Of Line-structured Optical Overlap-free Field-of-view Measurement System Based On Plane Mirror

At present,vision measurement research is moving toward high precision,non-contact real-time measurement,mostly for small object measurement,and there are many good research results.However,there are relatively few methods for visually measuring the extent of the large field of view.Increasing the number of cameras can effectively increase the field of view,but the most important problem is that fields of view no longer overlap between cameras,which makes it impossible to calibrate them systematically and affects subsequent measurements.Therefore,in order to realize the visual measurement of large field of view,this paper solves the problem of no overlapping field of view between cameras with the help of a plane mirror.Secondly,the line structured light measurement system in non-contact measurement is widely used in practical measurement with the advantages of high robustness and high accuracy.Therefore,this paper selects the line structured light as the active light source to adapt to different intensity of light environment.This paper combines a plane mirror to build a line structured optical vision measurement system to achieve a wide range of measurements with left and right symmetric fields of view,such as non-destructive inspection of the inner contour of railroad rails,train wheel pair measurement and other realistic application scenarios where symmetric measurements exist,and investigates the global calibration method of this system.The study includes the following points:First,a line structured light vision measurement system based on plane mirrors is built.The two cameras are placed in reverse parallel relationship with an angle of about 180°,the plane mirror is placed within the field of view of the left camera,and the checkerboard grid target is placed in front of the right camera.Adjust the position of each device: the camera on the left captures a virtual image of the chessboard pattern on the flat mirror,the camera on the right captures the chessboard target,and the reflection of the flat mirror projects the optical surface of the laser onto the chessboard target on the right.Second,the camera calibration part is based on a planar mirror with no overlapping field of view.The two camera positions do not have a common field of view and their optical axes are nearly parallel.Using the image characteristics of a plane mirror,a camera with nonoverlapping fields of view is transformed into a camera with overlapping fields of view,and the plane mirror coordinate system is used as an intermediate coordinate system to gradually derive the transformation relationship between two cameras,thereby solving the problem of cameras with non-overlapping fields of view being unable to observe the same target.The experimental accuracy of camera calibration with non-overlapping fields of view reached 0.49 mm,which confirms the effectiveness of the global camera calibration method proposed in this work.Third,the light plane calibration part based on plane mirrors.Using plane mirror reflection,the line laser can form two optical planes and appear in the field of view of two cameras respectively,and then the two optical planes are calibrated.And the threedimensional spatial relationship between the two optical planes is a symmetric incidenceoutput relationship with respect to the normal,and there are strict constraints between the two planes.Compared with the traditional linear structure optical measurement system that requires one-to-one matching of camera and laser,this paper can satisfy the measurement of two cameras with only one line laser and mathematically constrain the two planes,thus improving the accuracy of the line-structure optical planes to a certain extent and reducing the experimental cost at the same time.In this work,only one laser is needed to make two sets of linear structured optical sensors,which reduces manual errors and experimental costs to some extent,while improving the accuracy of the calibration of the linear structured optical planes.The effectiveness of the calibration method is verified by simulation experiments and physical experiments.Compared with the simplistic invariant method,the RMS error of the simplistic invariant method is about 0.0540 mm,and the method adopted in this paper is 0.0547 mm,which indicates that the proposed calibration method has a high accuracy.At this point,the global calibration of the whole system constructed in this paper is completed.In summary,the results show that the application of this measurement system can be extended to tunnel profile detection,train wheel pair measurement,and obstacle detection around vehicles.It has a very wide range of applications and prospects.