In this investigation we put forth a simple model to quantify the capacity of series elasticity to increase peak power output from an actuator. Using a linear bandwidth limited force source, we show how series spring stiffness and source stroke length effect power output to an inertial load. We show that an appropriate spring constant increases the peak power delivered to the inertial load over a limited stroke. The series elasticity changes the actuator operating point along its force-velocity curve and therefore effects the actuator work output over a fixed stroke length. For single shot motions, the peak power can increase by a factor of radic2 for appropriate series spring stiffness. We also present an example of how this work relates to human ankle function and it implies for the design of robotic or prosthetic ankles

Metabolic studies have shown that there is a metabolic cost associated with carrying load. Several leg exoskeletons have been developed by various groups in an attempt to augment the load carrying capability of the human. Previous research efforts have not fully exploited the passive dynamics of walking and have largely focused on fully actuated exoskeletons that are heavy with large energy requirements. In this paper, a lightweight, underactuated exoskeleton design is presented that runs in parallel to the human and supports the weight of a payload. Two exoskeleton architectures are pursued based on examining human walking data. A first architecture consists of springs at the hip, a variable impedance device at the knee, and springs at the ankle. A second architecture replaces the springs at the hip with a non-conservative actuator to examine the effect of adding power at desired instances throughout the gait cycle. Preliminary studies show that an efficient, underactuated leg exoskeleton can effectively transmit payload forces to the ground during the walking cycle

In this paper, we seek to understand how leg muscles and tendons work mechanically during walking in order to motivate the design of efficient robotic legs. We hypothesize that a robotic leg comprising only knee and ankle passive and quasi-passive elements, including springs, clutches and variable-damping components, can capture the dominant mechanical behavior of the human knee and ankle during level-ground walking at self-selected speeds. As a preliminary evaluation of this hypothesis, we put forth a simple leg model that captures the gross features of the human leg musculoskeletal architecture. We vary model parameters, or spring constants, damping levels and times when clutches are engaged, using an optimization scheme where errors between model joint behaviours and biological joint mechanics are minimized. For model evaluation, kinetic and kinematic gait data are employed from a single participant walking across a level-ground surface at a self-selected gait speed (1.3 m/sec). With only a single hip actuator, we find good agreement between model predictions and experimental gait data, suggesting that knee and ankle actuators are not necessary for level-ground robotic ambulation at self-selected gait speeds. This result is in support of the idea that muscles that span the human knee and ankle mainly operate eccentrically or isometrically, affording the relatively high metabolic walking economy of humans