Towards an Active Ankle-Foot Prosthesis Powered by Dielectric Elastomer Actuators in Antagonistic Pairs

The field of lower limb prosthetics is heading towards active devices, considering the complications that arise from their passive counterparts, such as higher metabolic consumption and abnormal gait patterns. Prosthetic devices, especially those ones subject to high impact, must exhibit a certain level of compliance and shock absorbance. Due to their inherent compliance, soft actuators are promising for applications in bionics. Among these, Dielectric Elastomer Actuators (DEAs) stand out, by virtue of their high energy density, high actuation strains, and fast response, making them suitable for applications as artificial muscles. In this work, we assessed the application of DEAs in an antagonistic pair configuration to an ankle-foot prosthesis. First, we modeled the nonlinear viscoelastic behavior of a single pair of coupled planar actuators. Then, artificial muscles constituted of several stacked DEAs were dimensioned to completely meet the ankle torque-angle curves, and therefore cover the range of ankle torsional stiffness in each phase of the gait cycle. A model-based approach combined with a proportional-integral (PI) controller performed well in simulations to reproduce the torque of the ankle joint by controlling the applied voltage in the antagonist muscle. The final structure enables remarkably lightweight prostheses when compared to conventional active transtibial prostheses.