Effects of unidirectional parallel springs on required peak power and energy in powered prosthetic ankles: Comparison between different active actuation concepts

Peak power, peak torque and energy requirements are amongst the main issues for powered prosthetic ankles. Including series elastic actuators (SEA) can reduce peak power and energy requirements by reducing motor speed in comparison to a Direct Drive (DD). Parallel elastic actuation concept (PEA, the parallel spring can be compressed and elongated) can reduce the peak power even more by reducing required motor torque. However, the energy consumption increases in comparison to SEA. In this paper, we investigated if unidirectional parallel springs (UPS) that act only at special ankle angles could combine the positive features of both SEA and PEA concepts in terms of motor peak power and/or energy reductions. Therefore, the motor peak power and energy requirements are compared between active actuation concepts SEA, SEA+PS, SEA+UPS, DD+PS and DD+UPS. For the minimum motor peak power requirements, we found that the reduction was similar between PS and UPS. However, the corresponding energy requirements were less or similar in UPS for corresponding speeds. For the minimum energy requirements, it was found that the UPS reduced the energy requirements but increased the corresponding required peak power for the majority of speeds in comparison to PS. The ultimate goal of combining positive characteristics of SEA and PEA was identified mainly for lower walking and running speeds.