Odometry and calibration methods for multi-castor vehicles

We are developing a mobile robot capable of emulating general 6-degree-of-freedom spacecraft relative motion. The omni-directional base uses a trio of active split offset castor drive modules to provide smooth, holonomic, precise control of its motion. Encoders measure the rotations of the six wheels and the three castor pivots. We present a generic odometric algorithm using a least squares framework which is applicable to vehicles with two or more castors and apply it to our unique vehicle configuration. As the accuracy of odometry algorithms depends on the accuracy to which the model parameters are known, a method to perform calibration on the physical robot is needed. We present a geometric calibration method based solely on internal sensor measurements. We present a range of simulation results comparing our odometry results to other algorithms under various systematic and non-systematic errors. We evaluate the ability of our calibration method to accurately determine the true values of our system parameters. The odometry algorithm was also implemented and tested in hardware on our robotic platform. The results presented in the paper validate the calibration and odometry algorithms in both simulation and hardware.