8.1 Remaining useful life
Remaining useful life (RUL) which is also named as
remaining service life, residual life, or remnant life means
remaining time before happening a failure. It is
essential to mention that the definition of a failure is
crucial to the interpretation of RUL. Yan et al. (2004)
employed a logistic regression model to calculate the
probability of failure for given condition variables and an
ARMA time series model to trend the condition variables
for failure prediction. A predetermined level of failure
probability was used to estimate the RUL. Ao et al.
(2004) described the use reliability of Chinese tractors, as
assessed by measuring working hours until failure
occurred in an agricultural field.
8.2 Prognostics incorporating maintenance policies
The aim of machine prognosis is to provide decision
support for maintenance actions. As such, it is natural to
include maintenance policies in the consideration of the
machine prognostic process. This makes the situation
more complicated since extra effort is needed to describe
the nature of maintenance policies. Compared to
conventional maintenance, mathematical models
applicable to the CBM scenario are much smaller (Scarf,
1997). The optimization of the maintenance policies
regarding to some main criteria such as risk, cost,
reliability and availability is the main idea of prognostics
incorporating maintenance policies.
9 Condition monitoring interval
Condition monitoring can be divided to continuous
and periodic types. Expensive cost and producing large
volume of data because of including noise with raw
signals are two limitations of continuous monitoring.
Periodic monitoring, therefore, is used due to its being
more cost effective. Diagnostics from periodic
monitoring are often more accurate due to the use of
filtered and/or processed the data. Of course, the risk of
periodic monitoring is the possibility of missing some
failure events that occur between successive inspections
(Goldman, 1999).
Christer and Wang (1996) derived a simple model to
find the optimal time for next inspection based upon the
wear condition obtained up to current inspection. The
criterion is to minimize the expected cost per unit time
over the time interval between the current inspection and
the next inspection time. Okumura (1997) used a
delay-time model to obtain the optimal sequential