Abstract
A Musculo-skeletal system or tendon-driven system can adjust the joint stiffness mechanically by inserting nonlinear spring unit (by using some pulley and a linear spring) into the middle of wires. However, robots don't have surplus spaces in their body in general. For setting the units in all tendon-wires, we have designed a compact nonlinear spring unit. In this new unit, a motor is placed in the center of a compression spring and plays a role of the structure as well. This unit not only small but also has a wide adjustment range of stiffness. We made this unit practically and tested its performance.
