Adaptive Friction Compensation for Humanoid Robots without Joint-Torque Sensors

This paper presents a novel approach for friction compensation on humanoid robots originally designed for position control in order to enable torque-based control methods on such systems. Due to their design, this kind of robots lacks joint-torque sensors and is equipped with high-reduction gearboxes. Nevertheless, we can still apply torque commands using torque estimation from motor currents. Moreover, the high gear reduction ratio produces high dynamic friction which significantly affects the robots drives and must be taken into account. Considering the LuGre friction model, an adaptive friction compensator based on a second-order sliding mode is developed and illustrated on a humanoid robot. The proposed method only relies on IMU data as well as joint position and velocity measurements from the joint encoders and is applied to the 12 DoFs of the robot's legs for CoM motion. The proposed control approach does not require the FT sensors mounted on the ankles.