Fault-tolerant control system evaluation by fault-injection method

The fault tolerance of control system in this paper is based on cold spare standby and software redundancy. The control system is a microprocessor modular system. It consists of standard nonredundant modules and specific additional modules, such as bus interfaces and a supervision unit. The supervision unit performs the validation of the active microprocessor unit, performs the actualisation of the spare unit and controls the bus interfaces transfer directions. This unit can switch the control from the failed active unit to the spare unit. The evaluation of the fault tolerance of this control system is performed by a fault-injection method. The disturbing sugnal is injected to active bus lines performing fault conditions. The fault-injection system is based on personal computer that also records the tested system reaction. The fault-detection coverage and the fault-recovery coverage are evaluated by using the probability of the fault occurance and the registered number of faults. The control sysrem is tested for nonredundant and redundant software. This experimantal method is therefore one of the ways how to compare the quality of some fault tolerant methods. In our case it was shown that fault recovery coverage could easy be increased by using software redundancy.