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

This paper presents 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 (direction vector from camera) 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 in order 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 is measuring the distance parallel or approximately parallel to the optical axis of the camera system. The sensor data are used for position control and can be used for object recognition, part dimension inspection, determination of position relations among objects and general workpiece manipulation. Since a grasp is performed by opening the gripper wider, with respect to the object, and then closing the grip, we can allow an estimation error of about ±1-2 mm. The calibration of the system is presented shortly, and some practical experiments are presented