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Controlling robots in contact with environment is the important problem in industry applications. In the conventional force control, great many researches have paid attention to develop novel force control systems and implemented force sensors to detect external force. Narrow bandwidth of force sensor has a big influence on the force control system. In order to solve the instability in force control, the velocity feedback gain is enlarged. The system becomes unstable with small velocity feedback gain, and robot's response becomes slow with large one. Since there is trade-off between stability and responsivity, it is thought that force control by robots is difficult. As a result, it is trapped in a vicious circle of narrowing further the frequency band which a force sensor has. Force control is attainable by the construction of the easiest force control architecture by feedback of observer. It is possible to set any bandwidth due to the sampling time and observer gain. This paper shows that bandwidth of force sensing is very important for contact motion control. Sensor-less force control is the one of the fundamental techniques for evolution of human-cooperating robot. The numerical and experimental results show viability of the proposed method.
Katsura et al. (2004) studied this question.