Improvement of ultrasonic beacon-based local position system using multi-access techniques

The present work shows a Local Positioning System (LPS) for Mobile Robots (MR) using simultaneous emissions in the ultrasonic beacons. In order to solve the problem of the simultaneous emission of the ultrasonic beacons, the well-known technique called Direct Sequence Code Division Multiple Access (DS-CDMA) is used. This technique modulates the ultrasonic signal (50 kHz) with a 127-bit Gold code for every beacon. In this way, it is possible to detect the arrival time of the code, by carrying out the simultaneous correlations with the assigned codes to each beacon. For determining the absolute position, the triangulation technique uses the Time Difference-of-Arrival (TDOA) obtained between a reference beacon and the others. Using this method, certain errors, derived from the delay in the firing of the beacons and the response times of the ultrasonic transducers, are avoided. The existence of a synchronism in the emission of the beacons, that guarantees a periodic and simultaneous emission in all of them, is required, not being necessary to know the emission instant in the mobile robot. This is particularly useful in environments where several robots can coexist.