Abstract
This paper presents and discusses a method to calibrate a specially built laser triangulation sensor to scan and map the surface of hydraulic turbine blades and to assign 3D coordinates to a dedicated robot to repair, by welding in layers, the damage on blades eroded by cavitation pitting and/or cracks produced by cyclic loading. Due to the large nonlinearities present in a camera and laser diodes, large range distances become difficult to measure with high precision. Aiming to improve the precision and accuracy of the range measurement sensor based on laser triangulation, a calibration model is proposed that involves the parameters of the camera, lens, laser positions, and sensor position on the robot arm related to the robot base to find the best accuracy in the distance range of the application. The developed sensor is composed of a CMOS camera and two laser diodes that project light lines onto the blade surface and needs image processing to find the 3D coordinates. The distances vary from 250 to 650 mm and the accuracy obtained within the distance range is below 1 mm. The calibration process needs a previous camera calibration and special calibration boards to calculate the correct distance between the laser diodes and the camera. The sensor position fixed on the robot arm is found by moving the robot to selected positions. The experimental procedures show the success of the calibration scheme.
Highlights
In the last decade, the spread of 3D scanning devices has been increasing progressively in industry, mainly for the inspection and quality control of processes that use robotic and machine vision systems, which need motion control within an unknown workspace [1,2]
There is research focused on robot calibration in order to increase the accuracy of the robot end-effector positioning by using measures expressed in a 3D digitizer coordinate system [21]
Where, A0P = matrix representing the position of the scanned object point (P) in the robot base coordinate system (0); A01 = matrix representing the position of Joint 1 (1) in the robot base coordinate system (0); A2S = matrix representing the position of the sensor (S) in the Joint 2 coordinate system (2); (S)
Summary
The spread of 3D scanning devices has been increasing progressively in industry, mainly for the inspection and quality control of processes that use robotic and machine vision systems, which need motion control within an unknown workspace [1,2]. An experimental procedure has been conceived to calibrate the relative position between the vision sensor coordinate system and the robot base coordinate system consisting of moving the robot manipulator to different poses for the digitization of a standard sphere of known radius [10]. A calibration routine is presented to acquire surface 3D maps from a scanner specially built with a vision camera and two laser projectors to transform these coordinates into object coordinates expressed in the robot controller for surface welding. Results have shown that in the application the scanning sensor based on triangulation can generate 3D maps expressed in the robot base coordinates with acceptable accuracy for welding, with the values of positioning errors smaller than 1 mm in the working depth range
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