Actuator Control for the NASA-JSC Valkyrie Humanoid Robot: A Decoupled Dynamics Approach for Torque Control of Series Elastic Robots

This paper discusses the actuator-level control of Valkyrie, a new humanoid robot designed by NASA’s Johnson Space Center in collaboration with several external partners. We focus on several topics pertaining to Valkyrie’s series elastic actuators including control architecture, controller design, and implementation in hardware. A decentralized approach is taken in controlling Valkyrie’s many series elastic degrees of freedom. By conceptually decoupling actuator dynamics from robot limb dynamics, we simplify the problem of controlling a highly complex system and streamline the controller development process compared to other approaches. This hierarchical control abstraction is realized by leveraging disturbance observers in the robot’s joint-level torque controllers. We apply a novel analysis technique to understand the ability of a disturbance observer to attenuate the effects of unmodeled dynamics. The performance of our control approach is demonstrated in two ways. First, we characterize torque tracking performance of a single Valkyrie actuator in terms of controllable torque resolution, tracking error, bandwidth, and power consumption. Second, we perform tests on Valkyrie’s arm, a serial chain of actuators, and demonstrate its ability to accurately track torques with our decentralized control approach.