Elastic Structure Preserving Control for Flexible Joint Robots With Position-Controlled Actuators

This article presents a novel control approach for flexible joint robots that use servo systems to control their motion. Under this configuration, the servo system is understood to have an inner feedback loop that accepts motor positions as inputs and uses torques as outputs. Hence, the soft robot uses motor positions as control inputs instead of torques. To address this system configuration with a reliable control system of high performance, we aim to generalize the elastic structure preserving (ESP) control approach, which previously has been proposed for backdrivable torque-controlled elastic robots, to robots with position-controlled elastic actuators. This scheme results in a dynamic feedback controller that recovers the elastic structure of the uncontrolled robot in the closed loop. At the same time, damping is injected, thus achieving a control system with high compliance and desired energy dissipation. Our results are supported by a rigorous analysis, where local input-to-state stability and output strict passivity can be concluded if the inner feedback loop from the servo system satisfies some assumptions. Experiments on two platforms validate the proposed control scheme and show the overall control system’s performance.