  Multiple LiDARs have progressively emerged on autonomous vehicles for
rendering a wide field of view and dense measurements. However, the lack of
precise calibration negatively affects their potential applications in
localization and perception systems. In this paper, we propose a novel system
that enables automatic multi-LiDAR calibration without any calibration target,
prior environmental information, and initial values of the extrinsic
parameters. Our approach starts with a hand-eye calibration for automatic
initialization by aligning the estimated motions of each sensor. The resulting
parameters are then refined with an appearance-based method by minimizing a
cost function constructed from point-plane correspondences. Experimental
results on simulated and real-world data sets demonstrate the reliability and
accuracy of our calibration approach. The proposed approach can calibrate a
multi-LiDAR system with the rotation and translation errors less than 0.04
[rad] and 0.1 [m] respectively for a mobile platform.
