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This paper describes a fail-safe software-based hazard control system designed for safety-critical operation of a dexterous space robot. This software safety system is fully autonomous, requiring no human intervention to detect or prevent a hazardous situation. Using a system analysis, we show that a computer-based approach is necessary due to issues of time delay, inadequate human reaction time given robot tip velocities, and the complexity of the robot and workspace. A system-wide software design is presented that satisfies the constraints of this analysis. The validation of this safety-critical software system is examined with fault trees constructed to determine combinations of events necessary to cause a hazard. These fault trees are qualitatively and quantitatively evaluated to determine the minimal set of events that can cause a hazard and the probability of a hazard occurring. A sensitivity analysis is conducted to determine which failures, and which combination of failures, most contribute to hazard probabilities. The theoretical analysis and sensitivity results have been presented previously, however this paper extends this work by examining the operating history of the robot, presenting its safety record, and discussing the system failures that have occurred. |
