Resonance analysis and unified compliance controller design for elastic joint robots

In this paper, resonance frequency and a unified compliance controller with on-line gravity and friction observer compensation for elastic joint robots based on dual-encoder and the torque sensor arrangement is proposed and analyzed. In terms of the elastic joint system, a schematic diagram is described as two inertia linear spring damping systems. And then, the elastic joint could be acted as a fourth-order model (torque to motor position). Furthermore, the frequency diagrams of the torque input to the motor velocity output will be achieved by logarithmic sweep, and the resonance frequency including ARF (Anti-Resonance Frequency) and NTF (Natural Torsional Resonance Frequency) will be analyzed. Aiming at light elastic joint, a unified position and impedance controller will be achieved with dual loop construction: an outer-loop position/impedance control, and an inner-loop torque control. Moreover, it is proved by a Lyapunov function that the control law is globally asymptotically stabilizes at any reference configuration. Finally, the simulation and experimental studies reveal a better effect for the unified compliance in contrast to proportional-derivative (PD) control.