A Bipedal Wheel-Legged Robot with High-frequency Force Control by Qausi-Direct Drive: Design and Experiments

Under-actuated systems like bipedal wheel-legged robots have dynamics coupled with force control, which cannot be separated or integrated out. Joint force control is necessary for these robots to realize model based controllers. In this paper, we design a bipedal wheel-legged robot prototype with proprioceptive quasi-direct drive (QDD) actuators to realize high-frequency force control and good back-drivability to deal with unexpected impacts. We compare the control bandwidth, back-drivability (impact mitigation capability) of three commonly used force-control actuation paradigms including geareddown motor with integrated torque sensor, QDD, and SEA (series elastic actuation) in details. Detailed actuator calibration reveals good current-torque linearity, thereby simplifying the control problem. We also use EtherCAT-based communication network and real-time main controller to ensure high control frequency. We derive the robot dynamics model, establish the simulation model, and design a LQR controller, in order to demonstrate our proposed design. Moreover, in the real-world experiments, we use a velocity-based balance controller together with position joint controller to show preliminary mobility, including balancing, rotating, and forward motion.