Relative Positioning of the Inspection Robot Based on RFID Tag Array in Complex GNSS-Denied Environments

High-precision location information is the key to efficient, safe, and automated inspection. A novel relative positioning method based on radio frequency identification (RFID) tag array is proposed and demonstrated, and the high-precision relative position of the inspection robot is realized by configuring only a small tag array in the inspection area. First, an array of reference tags is deployed on the walls around the inspection area. Second, in the offline stage, the inspection robot patrols along the designated route, and uses a system that integrates a mobile robot with an RFID reader and antenna to obtain the phase measurement value of the reference tag array (RTA) and build a fingerprint database. Finally, in the online stage, the inspection robot continuously collects the phase information of the RTA during the inspection process, compensates the error of the online data based on the offline fingerprint database, and selects the appropriate reference tag to participate in the positioning based on the geometric dilution of precision (GDOP) theory. We built our test platform using the commercial RFID reader Impinj Speedway R420, and deployed a set of smaller RFID tag arrays with a number of
$3\times 6$
and a spacing of 40 cm between tags in the conference room of our university. The results showed that the new design was less affected by the inspection trajectory and obstacles, and could assist the inspection robot to achieve high-precision relative positioning, with a positioning error of no more than 6 cm in complex scenarios.