1. Vrije Universiteit Brussel, Department of Mechanical Engineering, Pleinlaan 2, 1050 Brussel, Belgium , Italian Institute of Technology, Robotics, Brain and Cognitive Sciences Department, Via Morego 30, 16163 Genova, Italy
2. Vrije Universiteit Brussel, Department of Mechanical Engineering, Pleinlaan 2, 1050 Brussel, Belgium
3. Arizona State University, Department of Mechanical and Aerospace Engineering, Tempe, Arizona 85287-6106, USA,
4. Arizona State University, Department of Mechanical and Aerospace Engineering, Tempe, Arizona 85287-6106, USA