High-Robust Force Control for Environmental Stiffness Variation Based on Duality of Two-Inertia System

The conventional force control has proposed load-side torque control based on the resonance ratio control (RRC) enhanced by an instantaneous state observer using the acceleration sensor; however, environmental stiffness variation ( $K_e$ variation) is not considered in such systems. This paper evaluates the robust stability of the conventional method based on open-loop characteristics and the stability analysis results indicate that the conventional method becomes unstable during $K_e$ variation. To realize superior high-robust stability, this paper proposes a high-robust force control based on the duality of the two-inertia system. This paper constructs an equivalent RRC (ERRC) combining the motor-side velocity control and the load-side velocity observer and demonstrates a torque-velocity duality. The results confirm that the load-side torque control based on ERRC is robust against $K_e$ variation through the analysis of open-loop characteristics, and illustrate that the ERRC the same as the conventional RRC proposed in position or velocity controls. The effectiveness of the proposed control method is validated through numerical simulations and experiments.