Enhanced STag Marker System - Materials and Methods for Flexible Robot Localisation

摘要

Accurate localisation is a key requirement for the utilisation of autonomous mobile robots. Fiducial localisation is an approach which utilises the relative positioning of markers physically deployed across an environment, to determine a position estimate for a robot. Fiducial markers are strictly designed, with very limited flexibility in appearance. This often results in a "trade-off" between visual customisation, library size, and occlusion resilience. Further, many fiducial localisation approaches vary in their position estimation over time, leading to instability. The Stable Fiducial Marker System (STag) was designed to address this limitation with the use of a 2-stage homography detection. Through its combined square and circle detection phases, it can refine detection stability without resorting to temporal filtering. In this work, we explore the utility of STag as a basis for a stable mobile robot localization system. Key marker restrictions are addressed in this work through the development of three new chromatic STag marker types. The Hue/Greyscale STag marker set addresses constraints in customisability, the High-Capacity STag marker set addresses limitations in library size, and the High-Occlusion STag marker set improves resilience to occlusions. Each of these is designed to be compatible with the existing STag detection system, requiring only preprocessing steps for enhanced detection. They are assessed against their respective aims, showing clear benefits to their utility when compared with the existing STag markers. Further, we explore the viability of various materials for the fabrication of markers, to be used in outdoor and low-light conditions. This includes the exploration of static materials such as acrylic and wood and explores "active" materials which induce effects such as retro-reflectance and photo-luminescence. Marker detection rates are experimentally assessed across extensive lighting conditions; with active markers further assessed on the practicality of their effects. To encapsulate this work, we have developed a full end-to-end deployment for fiducial localisation under the STag system. It is shown to function for both on-board and off-board localisation, with deployment in practical robot trials. As a part of this contribution, the associated software for marker set generation/detection, physical marker fabrication, and end-to-end localisation has been released as an open-source distribution.