Passive joint stiffness in the hip and knee increases the energy efficiency of leg swinging

In the field of minimally-actuated robots, energy efficiency and stability are two of the fundamental criteria that can increase autonomy and improve task-performance capabilities. In this paper, we demonstrate that the energetic cost of leg swinging in dynamic robots can be reduced without significantly affecting stability by emulating the physiological use of passive joint stiffness, and we suggest that similar efficiency improvements could be realized in dynamic walking robots. Our experimental model consists of a two-segment dynamically swinging robotic leg with hip and knee joints. Closed-loop control is provided to the hip using neurally inspired, nonlinear oscillators that do not override the leg’s natural dynamics. We examined both linear and nonlinear, physiologically based stiffness profiles at the hip and knee and a hyperextension-preventing hard stop at the knee. Our results indicate that passive joint stiffness applied at one or both joints can improve the energy efficiency of leg swinging by reducing the actuator work required to counter gravitational torque and by promoting kinetic energy transfer between the shank and thigh. Energetic cost reductions (relative to the no-stiffness case) of approximately 25% can be achieved using hip stiffness, provided that the hip actuation bias angle is not coincident with gravity, and cost reductions of approximately 66% can be achieved using knee stiffness. We also found that constant stiffness combined with a limit on knee hyperextension produces comparable results to the physiological stiffness model without reS.A. Migliore School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA L.H. Ting · S.P. DeWeerth ( ) Department of Biomedical Engineering, Emory University/Georgia Institute of Technology, Atlanta, GA, USA e-mail: [email protected] quiring complex implementation techniques. Although this study focused on the task of leg swinging, our results suggest that passive-stiffness properties could also increase the energy efficiency of walking by reducing the cost of forward leg swing by up to 66%. We also expect that the energetic cost of walking could be further reduced by adding stiffness to the ankle to assist in the propulsive portion of stance phase.