A visco-elastic actuator with linear gas spring and variable damping

Abstract

Series elastic actuators (SEAs) enhance safety and robustness in robotic joints but sacrifice torque and motion bandwidth compared to rigid actuators. Variable viscoelastic actuators (VVEAs) overcome these limitations by integrating an adjustable damping element with a spring, balancing open-loop torque bandwidth with impact safety and robustness. However, most existing VVEA designs rely on mechanical springs that add significant weight. We introduce a lightweight VVEA based on a pneumatic spring design. This actuator achieves linear, offline-adjustable stiffness using antagonistic gas chambers and provides online-adjustable damping via a hydraulic throttle valve. In our design, the increasing force of one chamber is moderated by the decreasing force of the other, resulting in a measured maximal deviation of 12% from the linear model. Additionally, an elastic silicone cast bladder replaces the conventional ring-shaped piston, significantly reducing breakaway force. To evaluate its benefits over standard actuators, we applied optimal control to periodic rest-to-rest motions typical of pick-and-place tasks. Based on mean tracking error, the VVEA outperforms SEAs in 29.1% of trials and rigid actuators in 89.2% of trials. Although increasing damping improves performance, it compromises safety; our optimization study demonstrates the potential of an online variable damping setpoint to balance these trade-offs. These results suggest that VVEAs hold great promise for applications requiring rapid maneuvers and enhanced impact safety and robustness.