A Synthesis Method for Compact Nonlinear Springs with Custom Torque-Deflection Profiles and Bandwidth for Series Elastic Actuators

Historically, series elastic actuators (SEAs) used linear springs as the torque transmitting element in their drivetrains [1]. Use of linear springs requires tradeoffs between desired actuation bandwidth and measurable torque resolution. Nonlinear transmission elements overcome this limitation, enabling variable torque resolution and bandwidth through their deflection range. Many have recognized the benefits of variable stiffness elements in SEA drivetrains, which has led to multiple actively controlled variable stiffness actuator designs [2] as well as passive mechanisms that change stiffness during actuation [3][4]. The latter devices rely on cams to nonlinearly elongate a metal, linear spring to achieve a desired torque-deflection profile. However, these devices have a large form factor, which limits their use in existing SEA drivetrains.