Adding passive biarticular spring to active mono-articular foot prosthesis: Effects on power and energy requirement

Current active foot prostheses try to emulate the foot function by using a mono-articular series elastic actuator (SEA). In humans, a bi-articular actuation scheme is used to plantar-flex the foot through Soleus and Gastrocnemius muscles (in plantar-flexion the angle between foot and shank increases). In this study we investigated on using a bi-articular actuation scheme in active foot prostheses (where a passive bi-articular spring is added to an active mono-articular actuator). For design of these robotic systems, motor peak power (PP) and energy (E) requirement are two important factors. Lower PP or E requirement lead to a smaller motor or battery, hence decreasing the weight and increasing the performance capacity of the device. For this purpose, we compared the PP and E requirement of the mono- and the proposed bi-articular actuation models for active foot prostheses. The simulation results showed that for normal running (2.6 m/s), the PP requirement of the proposed bi-articular actuation can decrease about 54% in comparison to the mono-articular actuation. For normal walking (1.55 m/s), the required PP can reduce about 38%. On the other side, the corresponding E requirement can reduce about 7% for running, however, for walking, it can increase about 8%. In addition, it was found out that, if the spring is attached to the shank, the average results of the PP and E requirements are higher than the bi-articular scheme, however the differences were not very noticeable. Thus, our study shows that for designing a powered prosthesis both methods can be used. As the bi-articular spring can also influence the knee joint power of a trans-tibial amputee, the engineering advantages should be taken into account together with the user's comfort.