1. Department of Electrical and Computer Engineering, Laboratory for Robotics and Systems in Engineering and Science (LARSyS); Instituto Superior Técnico, Universidade de Lisboa; Lisboa 1049-001 Portugal
2. Department of Computer Engineering; University of California; Santa Cruz CA 95064 USA
3. Department of Electrical and Computer Engineering, Faculty of Science and Technology; University of Macau; Taipa, Macau China