Embedded joint-space control of a series elastic humanoid

Abstract

This paper provides an overview of the embedded joint-space control approach developed for THOR, a new series elastic humanoid. The 60 kg robot features electromechanical linear series elastic actuators (SEAs), enabling low-impedance control of each joint in the lower body via linear to rotary and parallel mechanisms. We present a distributed joint impedance control framework that leverages a custom dual-axis motor controller to track position, velocity, and torque setpoints for each pair of joints. The required actuator forces are tracked using an inner force control loop combining feedforward and PID control with a model-based disturbance observer (DOB). Unlike previous approaches, we utilize an inverse plant model based on the open-loop actuator dynamics to simplify tuning of the cascaded controller by decoupling DOB estimates from the inner loop gains. The effectiveness of the proposed approach is verified through trajectory tracking and dynamic walking experiments conducted on the THOR humanoid utilizing a complementary optimization-based whole-body controller. † M. A. Hopkins and A. Leonessa are with the Terrestrial Robotics, Engineering & Controls Lab at Virginia

Compliance Shaping for Control of Strength Amplification Exoskeletons with Elastic Cuffs

“…When applied to rejecting disturbances in a closed-loop force-control plant, the DOB must cancel out the transmission friction and output acceleration inputs which act alongside the force set-point to influence the spring force [28]. It is also possible to directly reject disturbances in the open loop model [29], [30], which has shown to be an equivalent approach [31]. These force disturbance observers improve impedance rendering accuracy when used in a cascaded impedance controller [32].…”

Team VALOR's ESCHER: A Novel Electromechanical Biped for the DARPA Robotics Challenge

“…A custom dual‐axis motor controller handles low‐level joint level impedance control of the linear SEAs, including parallel actuated joints on ESCHERs legs (Hopkins, Ressler, Lahr, Hong, & Leonessa, ; Ressler, ). These motor controllers allow for custom algorithms to be quickly implemented, and they are more compact than commercially available options.…”

“…Developing software in a custom framework enabled fine‐grained control of how to address challenges, and built up in‐team expertise in key areas. While this approach used to develop software for THOR resulted in a mature, well‐tested motion subsystem demonstrating high‐fidelity force control (Hopkins et al., ) and robust walking and balancing (Hopkins et al., ), other components struggled to integrate and test in a timely fashion. Delaying integration of initial path planning and perception systems in order to extend decentralized development time compounded the effort required to eventually integrate.…”

“…Upper body trajectories are tracked using a high‐gain velocity controller for each DOF, while lower body trajectories are tracked using the outer loop low‐gain joint impedance controller presented in Hopkins et al. () using actuator velocity feedback. This approach significantly improves velocity tracking because of the collocation of the motor and incremental encoder, and due to velocity feedback being actuator‐independent for parallely actuated joints.…”

“…A disturbance observer (DOB) based on the plant dynamics was implemented on the current feedback loop to reduce errors resulting from factors such as stiction and dynamic coupling of the actuators. This results in excellent torque tracking, a critical component for successful stabilization of the CoM (Hopkins et al., ).…”