Visual Self-Calibration of Pan-Tilt Kinematic Structures

With the increasing miniaturization of robotic devices, some actuators are not provided with absolute position sensing, thus making the state of the system unknown at startup. In this paper we present a vision based method for the automatic calibration of serial pan-tilt kinematic structures with a perspective camera on the end-effector. Examples of such systems are surveillance cameras and humanoid robot heads. The method is based on prospective motions of one joint in the kinematic chain to induce image motion in the camera. The analysis of the induced homography allows the computation of the angle of the other joint. The method can be iterated on more axes to calibrate longer serial chains composed of rotational joints. The method requires calibrated cameras, but runs completely automatic. We have implemented and validated the method in a small humanoid robot head.