Disturbance torque observer-based variable impedance control for compliant stair-descending of transformable wheel mechanism

This study presents a disturbance torque observer -based variable impedance control method to mitigate the disturbance force acting on the 2 -degree -of -freedom (DOF) transformable wheel mechanism. Compared to the previous study, the numerical solution of forward kinematic problem is calculated over the entire operating ranges of joint angles and the viability of dynamic model for the proposed transformable wheel is verified via the simulation study with the PD -based feedback. By tabularizing the numerical solution of kinematics, the complexity of forward kinematic problem of transformable wheel has been reduced to implement the disturbance torque observer in real time. In combination with the disturbance torque observer, the variable impedance control method is suggested, whose stiffness can be adjusted for mitigating the disturbance force acting on the transformable wheel and recovering its desired shape before the next step. The stability of proposed variable impedance control is proved with Lyapunov method without constraint on the stiffness of impedance model so that the adjustment of stiffness can be heuristically constructed. The extensive experiments using the proposed variable impedance control equipped with disturbance torque observer are carried out to demonstrate its excellent ability to mitigate the disturbance force acting on the transformable wheel during its stair -descending.