On the use of (lockable) parallel elasticity in active prosthetic ankles.

New challenges arise when investigating the use of active prostheses for lower limb replacement, such as high motor power requirements, leading to increased weight and reduced autonomy. Series and parallel elasticity are often explored to reduce the necessary motor power but often the effect on the energy consumption of the prosthesis is not directly investigated, as the mechanical power properties are examined yet the motor and gearbox dynamics and efficiencies are not considered. This paper presents the investigation of a parallel elasticity compared to a series elastic actuation system used in an active ankle prosthesis. Using a matched electromechanical model of the actuator shows that the electrical efficiency can be influenced using parallel elasticity. The optimal configuration depends on the motor characteristics (dynamic behavior) and limitations, which should always be taken into account when designing optimal series and parallel springs. It has been shown that adding parallel elasticity allows to reduce the required gear ratio and thus associated friction and inertial losses. Allowing the parallel elasticity to be lockable can further influence the behavior and allow for a more versatile actuator.