Online Safety Monitoring Using Safety Modes

Abstract — Robotic systems have to carry out more and more complex tasks, including ones where humans can be endangered. Residual design faults in such systems, as well as the inevitability of physical faults and interac-tion faults during operation, motivate the use of safety monitors to prevent catastrophic failures. In this paper, we consider the design of such safety monitors for multi-functional robotic systems. We present an approach and a formalization of the process for determining safety rules. It consists in identifying safety modes, according to the different tasks carried out by the monitored system. In practice, each safety mode is related to one or several functional modes and is specified by a permissiveness vector that defines the authorized domains of variation of key physical variables. The set of safety modes can be partially ordered according to their authorization vectors and can thus be represented as a directed acyclic graph. This graph is used to automatically build a model representing safety modes and their transitions, which can be implemented in an independent safety monitor. A case study has been carried out on a mobile manipulator robot, working in a factory alongside humans