Robust output-feedback torque controller design for series elastic actuators and its application in multi-level control frameworks

Robust output-feedback torque controller is developed for series elastic actuators (SEAs) in the presence of parameter uncertainties and external disturbances. The strong robustness of the proposed controller results from the filter-based observer which can estimate the velocity signals and the system lumped disturbance. The dynamic surface method is adopted to make the time-domain controller independent of any derivatives of the command reference, making the torque controller an ideal building block for multi-level control frameworks. The semiglobal stability of the closed-loop control system is proven under the assumption that only the state-independent uncertainty is bounded. The experimental results verify the effectiveness of the torque controller, and the implementation of two-level control frameworks, including the impedance control and SEA's load position control, further demonstrates its wide applicability.