Defining state-transition diagrams as part of technical state monitoring
Authors: Zelentsov B. P. 
Published: June 2022

State-transition diagrams are given for the situation of monitoring that takes into consideration explicit and hidden failures.

Aim. To generate state-transition diagrams that are used in the development of the dependability model and associated calculation methods under conditions of periodic monitoring.

Methods. The method is based on the classification of failures by detection, i.e., explicit and hidden failures. Accordingly, facility condition inspection may be continuous and/or periodical. Periodical inspection is conducted at equal intervals. The state-transition diagram is generated taking into consideration the relations between the states and events, i.e., each state can be the cause of a certain event and, at the same time, a state is the consequence of an event. Similarly, each event is the cause of a state change, while at the same time being a consequence of a state. Within the time between two periodic inspections, continuous-time transitions take place due to failures. This process is described by the theory of continuous-time Markov processes. Upon the completion of a periodic inspection operation, the up state ensues if an operable item is subjected to inspection. If an item proves to be down, it is submitted to maintenance. Such transitions in discrete moments in time are described by semi-Markov processes. For a better understanding of the information set forth in the paper, a list is provided of the used terminology referring to state standards.

Results. Under the adopted failure modes within a single inspection period, state transitions are described in continuous time, while after the inspection operation they are described in discrete time. Within each inspection period, a system of differential equations is defined and solved based on the assumption of initial up state. Based on the deduced probabilities, the probabilities of a period without failures and with a failure are calculated along with the mean time of up and down states within the period with a failure. Should failures be detected, maintenance state is initiated. The probability of such transition is within the theory of semi-Markov processes. Based on the probabilities of the semi-Markov process, the mean failure-free periods are calculated. Such model adequately reflects the dependability processes of operated equipment. If the method is not fully observed, the dependability indicators may take significantly different values. Three examples of state-transition diagrams are given: taking into account only explicit failures, only hidden failures and both.

Discussion and conclusions. The presented approach with a constant (regular) inspection period allows building models that adequately reflect processes occurring in actual systems. All operations involved in the implementation of simulated processes are performed based on the theory of continuous-time Markov processes and semi-Markov processes. The operations are performed in matrix form. Such method enables mathematical operations using computers. The presented approach can be used for improving dependability models of technical systems.