[1] Department of Bioengineering, Robotic Graduate School of Engineering, Tohoku University, 6-6-1 Aoba-yama, Sendai 980-8579, Japan
[2] Intelligent Systems Institute, Field Robotics Research Group, National Institute of Advanced Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan