Future robot systems are required to focus on developing more flexible systems for use with human operator. These robots must operate more slowly and be more compliant to safe user interaction. This paper proposes an improved design methodology for the humanrobot interaction system. The main contribution of the work described in this paper is the design of dual disturbance observers combined with modal space design for the twin direct-drive motor system. Two of the same direct-drive motors are required and connected by wire rope mechanism. Since both direct-drive motors have almost the same value of the friction effect and other nonlinearities, it is easy for the proposed system to compensate these deterioration effects. The proposed force control system is independently controlled in acceleration dimension by realizing the system based on common mode and differential mode. In the common mode, it is utilized for control of vibration suppression and wire rope tension. Moreover, the bandwidth and the stiffness of mechanism can be regulated by changing the wire rope tension control. In the differential mode, the purity of human external force with compensation of friction force is obtained. This mode is useful for control of the interaction force of human. The effectiveness of the proposed method is verified by experimental results.
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