Hideo Kitagawa Takashi Ohno Yuki Ueno Kazuhiko Terashima Differential-Drive Steering System using Planetary Gearing for Omnidirectional Mobile Robot. 171-176 2009 ICINCO-RA conf/icinco/2009ra db/conf/icinco/icinco2009-ra.html#KitagawaOUT09