1. School of Vehicle and Mobility, Tsinghua University, Beijing 100084, China.
2. Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109 USA.
3. School of Vehicle and Mobility, Tsinghua University, Beijing 100084, China (e-mail: ydg@tsinghua.edu.cn)
4. Toyota Research Institute, Ann Arbor, MI 48105 USA.