Hand-eye 3D pose estimation for a drawing robot

In order to draw pictures on a plane, it is important for a robot arm to know the pose of the end-effecter relative to the world frame which is settled on the workplace. Visual servoing is frequently used in the hand-eye configuration of robot arms. However, 3D pose is preferred for a drawing robot. It is not only required to obtain the three dimensional position of the end effecter so that the pen can touch the paper without drilling a hole, but also required to get the direction so that the pen is always perpendicular to the drawing plane. In this paper, we proposed a monocular camera vision system for a 6DOF drawing robotic arm to estimate 3D pose of the end effecter robustly. First, a coplanar asymmetric polygonal landmark, which is a rectangle with a corner cut off to remove rotation ambiguity, is introduced. Second, the corner points of the landmark are indirectly obtained by intersection of the edges obtained using RANSAC. After that, corner points are filtered by KALMAN filter to reduce detection errors. Third, the end effecter's three dimensional pose relative to the world frame is estimated through PnP algorithm and the kinematics of robotics arm. Experiments show that the 3D vision system has been successfully applied in our robotic arm to draw a circle. The stability and robustness of hand-eye is improved in real time drawing usage.