A General Framework for Minimizing Energy Consumption of Series Elastic Actuators With Regeneration

Abstract

The use of actuators with inherent compliance, such as series elastic actuators (SEAs), has become traditional for robotic systems working in close contact with humans. SEAs can reduce the energy consumption for a given task compared to rigid actuators, but this reduction is highly dependent on the design of the SEA’s elastic element. This design is often based on natural dynamics or a parameterized optimization, but both approaches have limitations. The natural dynamics approach cannot consider actuator constraints or arbitrary reference trajectories, and a parameterized elastic element can only be optimized within the given parameter space. In this work, we propose a solution to these limitations by formulating the design of the SEA’s elastic element as a non-parametric convex optimization problem, which yields a globally optimal conservative elastic element while respecting actuator constraints. Convexity is proven for the case of an arbitrary periodic reference trajectory with a SEA capable of energy regeneration. We discuss the optimization results for the tasks defined by the human ankle motion during level-ground walking and the natural motion of a single mass-spring system with a nonlinear spring. For all these tasks, the designed SEA reduces energy consumption and satisfies the actuator’s constraints.

Simulation of Stand-to-Sit Biomechanics for Design of Lower-Limb Exoskeletons and Prostheses with Energy Regeneration

“…Analogous to electric and hybrid electric vehicles [13][14], regenerative braking can extend the operating durations of robotic lower-limb prostheses and exoskeletons by converting the otherwise dissipated joint biomechanical energy during negative work movements into electrical energy via backdriving the onboard actuator-transmission system. Whereas previous investigations have focused exclusively on level-ground walking [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][33][34][35], this research computed the lower-limb joint biomechanical energy available for electrical regeneration during stand-to-sit movements. The sitting and standing movement biomechanics of nine participants were experimentally measured.…”

“…Although the hip performed the most net negative mechanical work, the knee joint should instead be considered for regenerative powertrain system designs given the increased structural complexity of the biological hip (i.e., more degrees-of-freedom). Coincidentally, most lower-limb biomechatronic devices with energy regeneration have involved knee-centered designs [5,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][33][34][35]. Using previously published maximum energy regeneration efficiencies (i.e., approximately 37 %) [16], robotic knee prostheses and exoskeletons could theoretically regenerate 0.06 ± 0.03 J/kg of electrical energy per stand-to-sit movement.…”

“…regenerative powertrains have focused exclusively on level-ground walking [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][33][34][35]. However, geriatric and rehabilitation patients often exhibit slower walking speeds (e.g., 24 % speed reduction from 25 to 75 years age) and take fewer steps/day (e.g., 75 % reduction from 60 to 85 years age) [3], therein limiting the potential for energy regeneration during level-ground walking.…”

“…Analogous to electric and hybrid electric vehicles [13][14], several biomechatronic knee designs have incorporated regenerative powertrains to convert the otherwise dissipated joint biomechanical energy during legged locomotion into electrical energy for recharging the onboard batteries while providing negative mechanical work for controlled system deceleration [5,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. These energy-efficient powertrains can enable lighter onboard batteries and/or extend the operating durations between recharging.…”

Robust Optimal Design of Energy Efficient Series Elastic Actuators: Application to a Powered Prosthetic Ankle

“…Elastic collisions can also improve energy efficiency in applications subject to periodic impacts such as bipedal locomotion [6], [7]. In addition, compared to rigid actuators, SEAs can reduce energy dissipated by the actuator for periodic tasks [8], [9].…”