Stiffness adjustment of a Series Elastic Actuator in an ankle-foot prosthesis for walking and running: The trade-off between energy and peak power optimization

During walking and running, passive foot prostheses can only do positive work by releasing elastic energy stored in compliant structures. This limited ability to generate positive work can be improved in devices which actively support the push-off. Here, we estimate the peak power and energy requirements of a simulated serial elastic actuator (SEA) for walking and running and compare it with a direct drive setup. The simulations indicate that a serial spring can highly reduce peak power and the energy. Results suggest that optimizing SEA stiffness to obtain minimal peak power is the more general approach as it needs similar energy requirements as observed in optimizing for minimal energy. In contrast, optimization for minimal energy results in clearly higher peak power requirements. For both gaits, the predicted optimal spring stiffness suitable for minimizing peak power increases with speed. For optimizing energy, the stiffness decreases with walking speed and remains nearly constant across speeds in running. A constant stiffness for both gaits is possible. It should be chosen based on the optimized peak power solution for the highest desired speed, where the peak power requirements are most critical.