A study of failure models in feedback control systems

Feedback control systems have a peculiar behavior that allows them to compensate for disturbances in the controlled application. This paper investigates whether this resilience also extends to disturbances originating from faults in the controller itself. The question of what kind of failure model is more effective in this type of system is addressed, with three different models being studied: arbitrary failure, fail-silent, and fail-bounded. The study is conducted essentially by experimental fault injection in the controller of one of the best known and most demanding of the benchmarks used in the control systems area: an inverted pendulum. The considered failure models are compared according to criteria based on the quality of the control action. Other insights gained from the experiments made are described, for instance on how to significantly increase dependability at a very low cost in the feedback controllers, and on the need for a different kind of real-time scheduling algorithm.