NNS, a knowledge-based on-line system for an assembly workcell

Performing complex assembly tasks requires decisional capabilities available on-line, in order to handle various events (sensor conditions, part arrivals, failure detection...), that occur during execution. In this paper, we describe NNS, a complete on-line system for multi-robot assembly workcells, that deals with problems of execution monitoring, action scheduling, and failure diagnosis and recovery. NNS is distributed in different decisional layers with their own reasoning methods and state representation; it uses an "execution model" that embeds various knowledge bases built off-line. This approach fits well with requirements of flexibility and efficiency.