Multi-Robot Relative Pose Estimation and IMU Preintegration Using Passive UWB Transceivers

Achieving collaborative tasks in multi-robot teams requires knowledge of the relative poses of a robot's neighbours. Ultra-wideband (UWB) systems are becoming increasingly popular as a means of inter-robot ranging and communication. A major constraint associated with UWB is that only one pair of UWB transceivers can range at a time to avoid interference, hence hindering the scalability of UWB-based localization and requiring complex scheduling algorithms. In this paper, a novel ranging protocol is proposed that allows all robots to passively listen on neighbouring communicating robots without any hierarchical restrictions on the role of the robots. This is then utilized to allow each robot to obtain more range measurements, broadcast preintegrated inertial measurement unit (IMU) measurements for relative pose state estimation directly on SE2(3), and to propose a simple media-access control (MAC) protocol to avoid interference. Consequently, a simultaneous clock-synchronization and relative-pose estimator (CSRPE) is then formulated using an on-manifold extended Kalman filter (EKF) and is evaluated in simulation using Monte-Carlo runs for up to 7 robots. Additionally, the proposed ranging protocol is implemented in C on custom-made UWB boards fitted to quadcopters, and the proposed filter is then evaluated over multiple experimental trials for 3 quadcopters, yielding up to 55% improvement in localization accuracy when using passive listening.