T.  Menard, Grioli, G., and Bicchi, A., ?A real time robust observer for an agonist antagonist variable stiffness actuator?, in IEEE International Conference on Robotics and Automation (ICRA2013), Karlsruhe,  Germany, May 6-10, 2013 , 2013, pp. 3988 - 3993 [Online]. Available: 10.1109/ICRA.2013.6631139