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 inertial reference frame is a problem of paramount importance for realistic application 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 exist some sophisticated optical systems which track the path followed by specific (lighted) points of the robot. Although these systems can provide very precise measurements, they cannot cover larger areas with the same precision. This paper presents a laboratory prototype capable of measuring the position 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 obtained from range data that can be estimated from the travel time of the acoustical wave from the sparking point to three static receivers. The EM wave is used for synchronization. The system provides real time data to operate in a wider space than the optical systems. After the processing and filtering 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 introduced by the use of inclinometers.