8 Prognostics
Compared with diagnostics, the literature on
prognostics is much smaller. Machine prognostic
includes two main types of prediction. The most
familiar one is the prediction of remaining time before
occurrence of a failure indicating current and past/future
condition of operating profile of a machine. The time
left before observing a failure is usually called
“remaining useful life” or RUL. In many situations,
especially when a fault or a failure has catastrophic
consequences (e.g. nuclear power plant), it is desirable to
predict the chance that a machine operates without a fault
or a failure up to some future time (for example, the next
inspection), given the machine’s current condition and its
past operational profile. In the general maintenance
context, the probability that a machine operates without
fault until next inspection interval is a good reference in
helping to determine whether or not the inspection
interval is appropriate.
Most of the papers in the literature of machine
prognostics discuss only the former type of prognostics,
namely RUL (Remaining Useful Life) estimation. Only
a small number of papers address the second type of
prognostics (Araiza et al., 2002; Farrar et al., 2003). In
the following sections, it is tried to discuss RUL
estimation, prognostics that incorporate maintenance
actions or policies, and the determination of the
appropriate condition monitoring interval.