A Novel Camera-to-Robot Calibration Method for Vision-Based Floor Measurements

Abstract

A novel hand-eye calibration method for ground-observing mobile robots is proposed. While cameras on mobile robots are com- mon, they are rarely used for ground-observing measurement tasks. Laser trackers are increasingly used in robotics for precise localization. A referencing plate is designed to combine the two measurement modalities of laser-tracker 3D metrology and camera- based 2D imaging. It incorporates reflector nests for pose acquisition using a laser tracker and a camera calibration target that is observed by the robot-mounted camera. The procedure comprises estimating the plate pose, the plate-camera pose, and the robot pose, followed by computing the robot-camera transformation. Experiments indicate sub-millimeter repeatability.

Figures (6)

• Figure 1: Mobile robot Rita (adapted from Naab+2023+109+117), used in the proposed experiments. It features a differential drive with a front-mounted caster wheel. A spherically mounted retro-reflector is installed on top within a rotatable mount, maintaining the line of sight to a laser tracker. The nadir-mounted camera remains occluded in this image, with only its active illumination apparent.
• Figure 2: Retrieval of the orientation of the camera inside the robot frame. The plate-absolute-transformation ${}^{abs}H_{plt}$ and the plate-camera-transformation ${}^{plt}H_{cam}$, as well as the robot-absolute-transformation ${}^{abs}H_{rob}$ on the other side, are combined to robot-camera-transformation ${}^{rob}H_{cam}$ (figure adapted from UlrichHillemann2024TRO).
• Figure 3: Stereo measurement of the reflector nest positions in the referencing plate frame. The plate-camera-transformations ${}^{plt}H_{cam,0}$ and ${}^{plt}H_{cam,1}$ are used to calculate the reflector nest positions ${}^{plt}P_{i}$ by triangulation (turquoise arrows) (figure adapted from UlrichHillemann2024TRO).
• Figure 4: Retrieval of the absolute orientation of the referencing plate. Absolute laser-tracker-measurements ${}^{abs}P_{i}$ are matched to the reflector nest positions w.r.t. the referencing plate, yielding the absolute-plate-transformation ${}^{abs}H_{plt}$ (figure adapted from UlrichHillemann2024TRO).
• Figure 5: Referencing plates used in experiments. While they are arbitrarily posed on the ground, a mobile robot Rita can be placed on them. The colored circles mark flush-mounted reflector nests. The dark texture represents the HALCON calibration target with circular marks. The rest of the white area provides space to move the robot for a few decimeters.