Objectives In the scene of motion control application, small deformation of equipment will be caused for the fatigue and other reasons, therefore the more intensive observation and the recovery methods are needed. Taking into account the photogrammetric properties of the equipment and the high-information of the high frame-frequency camera, the deformation detection of the motion control axis is realized by combining the high frame-frequency camera with the photogrammetric method.
Methods The indoor deformation detection environment is established using high frequency charge coupled device camera with 240 frame/s, motion controller and motion control axis that need to be detected.(1)Fixed the checkerboard calibration board, the motion of the high frame frequency camera are controlled by the motion controller, and continuously shoots the calibration board. (2) The corner coordinates of calibration plate are extracted, and the accurate positioning of corner coordinates based on the geometry and motion characteristics of corner are proposed. Corner geometry characteristic: The corner points of a checkerboard in the same direction locate in the same line.Motion characteristic: The coordinate variation between the corresponding corner points of two pictures of the calibration plate taken at different times is equal.(3)Calculating the camera coordinate sequence and restoring the high-density point trajectory of the camera motion. n pictures of the calibration plate are taken using the camera from different angles to calculate the elements of interior orientation of camera. The exterior orientation elements of camera for a new picture about the calibration plate, and the camera center coordinates can be solved by the homography between the pixel coordinates and the world coordinates, the camera high-density point of motion trajectory can be recovered.
Results (1) The maximum projection error of corner detection is less than 0.5 pixel, and the extracted corner is stable. (2) The camera reprojection error is less than 0.4 pixel, when the resolution of the object is in the range of 1.13—2.04 mm, and it meets the requirements of deformation detection accuracy. For the camera moving along the straight line to collect the calibration plate image, 400 frames of the calibration plate image are randomly selected from the acquisition calibration plate image for reprojection, the maximum re-projection error is about 0.3 pixel, which can meet the requirements of deformation detection accuracy.(3)Comparing the deformation detection method with a 3D laser scanner with higher accuracy and normal industrial camera, the recovered motion trajectory of the high frequency camera is consistent with the scanning trajectory of the 3D laser scanner. The actual detection of this method shows that the deformation is 0.98 mm at the left end of the horizontal axis 48.016 cm and at the upper end of the vertical axis 3.810 cm is 2.00 mm. The inclination of the horizontal axis is 0.012° in the Y direction and in the Z direction is 0.370°. The inclination of vertical axis is 0.021° in the X direction and 0.509° in the Z direction. The deformation of the control axis is not detected by the normal industrial camera. The inclination of the horizontal axis in the Y direction is 0.015° and is 0.375° in the Z direction, the inclination of vertical axis is 0.022° in the X direction and 0.527° in the Z direction.
Conclusions The experimental results show that data is collected by the high frame frequency camera can reflect the details of the deformation of control axis, the actual deformation position and deformation quantity can be accurately obtained, and realize the sub-millimeter deformation detection. It effectively makes up for the low frame rate of normal industrial camera in deformation detection, and has certain practical value in industrial deformation detection.
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