Measuring the 3D-position of a walking vehicle using ultrasonic and electromagnetic waves

The knowledge of the true position of a walking machine (rover) in an inert ial reference frame is a problem of paramount importance for realistic appl ication of mobile robots. The points of the robot follow three-dimensional trajectories even on even terrain. When no position feedback is possible, a good knowledge of the dynamical behaviour of the rover is needed to get an estimation of the position and orientation of the robot. There already exi st some sophisticated optical systems which track the path followed by spec ific (lighted) points of the robot. Although these systems can provide very precise measurements, they cannot cover larger areas with the same precisi on. This paper presents a laboratory prototype capable of measuring the pos ition of a four-legged walking robot using a combination of electromagnetic (EM) and ultrasonic (US) waves produced by a spark-generator, avoiding any physical link between the robot and the environment. The 3D position is ob tained from range data that can be estimated from the travel time of the ac oustical wave from the sparking point to three static receivers. The EM wav e is used for synchronization. The system provides real time data to operat e in a wider space than the optical systems. After the processing and filte ring of the signal, a final precision better than 1 mm is reached in a work range of about 5 m. The track-data obtained by the position meter is used to know the dynamic behaviour of the robot and to study the improvement int roduced by the use of inclinometers. (C) 1999 Elsevier Science S.A. All rig hts reserved.