Combining a stable 2-D vision camera and an ultrasonic range detector for 3-D position estimation

The authors present a robot control model combining a stable 2-D vision camera with an ultrasonic range sensor mounted in a three-fingered robot gripper. This sensor configuration is an alternative to more complex and costly 3-D vision systems for industrial robots. In this application the vision system gives the direction to found objects or features. Since the vision system is only 2-D, the distance from the camera to the object or feature is needed to estimate the correct object parameters. This distance is given by the ultrasonic range sensor, mounted in the robot gripper in such a way that it measures the distance parallel or approximately parallel to the optical axis of the camera system. The information derived can be used for object recognition, part dimension inspection, determination of position relations among objects and general workplace manipulation. Since a grasp is relatively loose an estimation error or about ± 1-2 mm can be allowed. The calibration of the system and some experimental results are outlined