[1] Univ Calif Berkeley, Dept Mech Engn, Berkeley, CA 94706 USA
[2] Beijing Inst Technol, Sch Mech Engn, Beijing 100081, Peoples R China
[3] Univ Michigan, Dept Mech Engn, Ann Arbor, MI 48109 USA
[4] Tsinghua Univ, Dept Comp Sci, Beijing 100084, Peoples R China