Close Motion Estimation of UHF-RFID Tagged Objects Based on Electromagnetic Coupling

We consider the close relative motion of two UHF RFID tags and the problem is the estimation of the distance between the two tags, by measuring the Received Signal Strength Indicator (RSSI) and the phase of the RFID signal backscattered by one of the two tags. Due to the short distance considered, and hence to the electromagnetic coupling between the antennas, the measured RSSI and phase present a complex and ambiguous dependence on the distance between the tags. The problem is solved through two approaches, based respectively on a Multi-Hypothesis Extended and a Multi-Hypothesis Unscented Kalman Filter (MHEKF and MHUKF). The availability of phase and RSSI allows to mitigate the ambiguity in the problem and to estimate the distance without any information on its initial value. Simulation and experimental results show the effectiveness of the approach, with the MHUKF presenting slightly better performances compared to the MHEKF. The proposed setup can be applied in manufacturing, robotics, safety and in any context where the variable distance between two close objects should be monitored.