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This paper presents a position-based impedance control scheme for force tracking of a wall-cleaning unit installed on the wallclimbing mobile platform ROPE RIDE. It is important to maintain a contact force between the wall-cleaning unit and various types of walls in order to guarantee good cleaning performance. For this purpose, the target stiffness of impedance model as well as the reference position command for the wall-cleaning unit are tactfully adjusted according to the force tracing error between the real and reference contact forces. In comparison with existing impedance control methods, the proposed impedance control scheme is not only simple to implement but also robust against external disturbances such as varying trajectory or stiffness of walls as well as model uncertainties of cleaning unit. As a result, the proposed impedance control scheme can promise acceptable force tracking error in the steady state even for various trajectories of wall and varying reference forces. The stability and performance of proposed impedance control scheme are theoretically examined and the extensive experiments using the wall-cleaning unit are carried out to prove the force tracking capability of the proposed impedance control scheme.