An instability indicator for expert control

An instability indicator is proposed for use in supervisory/expert control systems. The indicator-which consists of a trend indicator, a rectifier, and a smoother-observes the plant output. The trend indicator generates the difference between two time averages of different final horizons and is implemented as a pair of discrete moving-average or first-order exponential filters. The rectifier is an absolute value or square operation, while the smoother is a moving-average or exponential filter, with a longer horizon/time constant. The instability alarm is triggered on the basis of the amplitude of the indicator output.