Safety function in control laws against deviations from normal operating-range

This paper proposes safety function in control laws to reduce the frequency of events that a physical value in a control system deviates from its normal operating-range. This safety function is realized mainly by the safety ability to suppress the fluctuations of transient responses after a device failure in order to prevent the deviations from the normal operating-range on the basis of dependability function to maintain the stability and the control performance even if the system is not in the normal operation. The safety function in control laws can supplement ordinary safety-related systems in risk reduction according to international standards on safety such as IEC 61508. In order to establish the meaning of the safety function, this paper also proposes its assessment framework, which gives the frequency of the deviations.