A Compliant Mechanism with Progressive Stiffness for Robotic Actuation

Softer series elastic actuators display higher torque resolution and better torque transparency, which facilitates soft physical interaction of robots with their environment. Stiffer series elastic actuators favour fast, strong and powerful physical interaction between the robot and the environment. Passive compliant elements with progressive stiffness characteristic could render the actuator inherently soft at low interaction torque levels and much stiffer at higher interaction torque levels, remedying common design trade-offs of linear series elastic actuators. This paper proposes a compact solution, which realises such a passive elastic element without complex nonlinear transmissions. The solution is solely based on a number of radially oriented linear springs. The paper presents two distinct implementations using i) an assembly of concentrated tension springs and ii) a monolithic arrangement of distributed flexure beams. Detailed theoretical analyses and experimental prototype validations lead to design guidelines and confirm a cubic polynomial torque-deflection characteristic paired with low hysteresis.