On an FTA (fault tree analysis) basis, we deal in the paper with several issues on securing human safety in a human/robot coexistence system, where the system is developed for simulating and simplifying the environment of work/tool handling operations in production lines. The result of our FTA reveals three main factors which lead to hazardous situations, the factors being human errors, abnormal robot motions, and outsiders entrance. Based on the FTA, we develop fail-safe analog circuits (FSAC's) which are effectively used in a parallel process of dual signal lines starting from two kinds of dual sensor signals (dual force sensor signals as well as dual joint sensor signals) and ending up with an FSAC which allows one of the dual analog control signals to pass through the gate only when they were checked the same with acceptable deviation, for the purpose of enhancing the diagnostic capability in 1 out of 2 logic. We equip the robot with mechanical safeguarding measures, a sphere-shaped joint, mechanical stoppers, and viscoelastic coverings to eliminate all hazardous parts around the robot links. Then, we claim that human intention becomes a very important factor when the robot is equipped with a tool/work which is hazardous, and propose to provide the robot with a function of inferring human-intended path patterns using a modified HMM. We also demonstrate that human/robot intention mismatch leads the robot to making an emergency stop.
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