Design of an Articulated Robotic Leg with Nonlinear Series Elastic Actuation

An articulated leg for the use in a running robot is presented. It is driven by series elastic actuation with a highly compliant spring at the knee joint to exploit periodic energy storage and passively support a running motion. The spring is connected with the knee motor by a cable pulley system, which allows the advantageous placement of the motor in the hip joint and enables us to use a compression spring instead of a heavier torsional element. Additionally, the pulley system creates a nonlinear spring characteristic at joint level which can be shaped by altering the cable tension. This nonlinearity and the inertial effects associated with it substantially increase the effective damping in the unloaded leg and allow precise foot-placement during the flight phase. Still, damping of the loaded leg during stance phase is kept minimal for highly efficient energy recovery.