Thomas Wimböck Christian Ott Alin Albu-Schäffer Andreas Kugi Gerd Hirzinger Impedance control for variable stiffness mechanisms with nonlinear joint coupling. 3796-3803 2008 IROS http://dx.doi.org/10.1109/IROS.2008.4651079 conf/iros/2008 db/conf/iros/iros2008.html#WimbockOAKH08