A two-staged residual for resilient external torque estimation with series elastic actuators

Collaborative robots driven by Series Elastic Actuators (SEA) exploit physical compliance, which enables joint torque sensing to accurately estimate external contact torques based on residual momenta. The deflection measurement precision for the compliant element determines the torque accuracy. It is affected by support clearances, assembly tolerances, stiffness calibration errors as well as unmodelled non-linearity. Moreover, the stiffness value amplifies deflection errors leading to larger torque estimation errors, which limits the benefits of integrated joint torque sensing for enhanced external torque estimation with higher stiffness SEAs. Accordingly, this paper newly proposes a two-staged approach of the residual based external torque estimation for SEA driven robots. The proposed method augments the residual calculation by the motor-side dynamics, uses both, motor and link side angular measurements, but entirely waives the need to model the spring dynamics for the external torque estimation. Consequently, this improves accuracy and sensitivity in wide-ranging stiffness applications. The performance of the two-staged residual is verified by experiments with the SEA of the WALK-MAN humanoid robot.