Analyzing the Performance Limits of Articulated Soft Robots Based on the ESPi Framework: Applications to Damping and Impedance Control

In situations of harsh impacts, damping injection directly on the link of an articulated soft robot is challenging and usually requires high actuator torques at the moment of impact. In this work, we discuss the underlying reasons and analyze the performance limitations arising in the implementation of basic impedance elements, such as springs and dampers, through the elastic structure preserving impedance (ESPi) control framework. Using the insights obtained, we present a way to design impedance controllers with a damping design based on dynamic extensions. Inspired by the design of shock absorbers and the muscle-tendon model, the presented damping layout requires substantially smaller actuator torques in situations where the robot is subject to harsh impacts. The implementation is facilitated through the ESPi control framework resulting in a physically intuitive impedance design. The resulting closed-loop system can be interpreted as an interconnection of passive Euler Lagrange systems, which again, yields a passive system. The design's passive nature ensures stability in the free motion case and enables the robot to interact robustly and safely with its environment. The work focuses on robotic systems with no inertial coupling between the motor and link dynamics. Experimental results, obtained with the presented design on a dedicated series elastic actuator (SEA) test bed, are reported and discussed.