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

Abstract

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.Index Terms-UHF-RFID RSSI and phase measurements, Kalman filtering, Electromagnetic coupling I. INTRODUCTIONP ASSIVE UHF RFID tags are increasingly supporting sensing and localization functions [1], [2], [3], [4] in addition to the usual identification function. For that purpose, specific sensor functionalities are frequently included in the microchip or, as an alternative, sophisticated electromagnetic designs exploiting specific properties of the antennas [5], [6], are used. Falls within this second design methodology the use of electromagnetic coupled pairs of tags, closely spaced, which are sensitive to small reciprocal displacements. In fact, because of mutual coupling, small changes in the relative distance modify the phase and the Received Signal Strength Indicator (RSSI) of backscattered signals [7], [8], [9]. This kind of tag is conceived for structural health monitoring being able to measure (or estimate) the enlargement of cracks in structures. They require a strong electromagnetic coupling between the two tags in order to achieve high sensitivity in detecting the movement but, the stronger the coupling, the worse the communication, so a trade-off between sensing and communication needs is essential for their functioning [10],