We discuss a way to achieve a fail-safe human/robot contact system. Most of our discussion is based on the human pain tolerance, evaluated for the purpose of establishing the human safety space. First we review our previous work on the human-oriented design of a safe robot and the procedure of covering a robot with a viscoelastic material to achieve both impact force attenuation and contact sensitivity, keeping within the human pain tolerance limit. The safe robot design is verified through a demonstration that the robot exerts a contact force much less than the human pain tolerance and gives no pain to humans. Next, we propose a more efficient human/robot system which attains velocity reduction on the robot side activated by the incipient contact detection at the surface and gives the human side an interval margin for the reflexive withdrawal motion to avoid the more severely interactive situation. The experimental result shows the effectiveness of the velocity reduction of the robot in a fail-safe manner.
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