HUANG et al.: REMAINING USEFUL LIFE PREDICTION 689
model and Si's model are linear. It leads to the fact that both of
their models cannot trace the dynamics of nonlinear degradation
trajectories well.
From the above review of the related work, it can be found
that, though there is some research focusing on nonlinear degra-
dation trajectories or degradation models adaptive to historical
degradation data for RUL prediction, separately, few works con-
centrate on both of them simultaneously. As stated before, an
adaptive drift helps to take historical degradation data into ac-
count, and a nonlinear structure can make predicted degradation
trajectories close to actual degradation trajectories. The combi-
nation of an adaptive drift and a nonlinear structure contributes
to promoting the accuracy of RUL prediction. Accordingly, we
make three major contributions in this paper. First, we propose
a novel nonlinear heterogeneous Wiener-process-based model
with an adaptive drift for RUL prediction, and utilize a state-
space-based method to characterize our model. Second, we de-
velop an on-line filtering algorithm based on Bayes' theorem
to update the hidden degradation drift, and then apply the ex-
pectation-maximization (EM) algorithm, and a novel Bayes'-
theorem-based smoother to estimate the unknown parameters
once a new measurement becomes available. Last, we derive
an explicit analytical distribution for the predicted RUL incor-
porating the complete distribution of the estimated degradation
drift.
The rest of this paper is organized as follows. In Section II,
we formulate the problem. In Section III, we develop an on-line
filtering algorithm to estimate the degradation drift, and apply
the EM algorithm to re-estimate the unknown parameters. In
Section IV, we derive an explicit analytical distribution for the
predicted RUL. In Section V, a simulation, and a case study are
used to demonstrate the effectiveness of our model, and com-
pare the performance of our model with that of Wang's model,
and Si's model. Section VI concludes this paper.
II. P
ROBLEM FORMULATION
To take the complete historical degradation data into consid-
eration, as well as maintain the nice properties of the WP model,
we consider Wang's model, given as [30]
(1)
(2)
In Wang's model, (2) was the discretized conventional WP
model, and the authors constructed an updating procedure for
the degradation drift
using a random walk model as in (1),
with the initial assumption that
followed a s-normal distri-
bution. Hence,
evolved as a time-dependent random variable.
Because
was unobservable, it had to be estimated once a new
measurement became available. Each time a new measurement
came out, they updated the posterior distribution of
using the
prior distribution and the new measurement by a recursive filter.
When the next measurement was available, the posterior distri-
bution at the current CM point became the prior distribution for
the next CM point. Thus, they established a link between the
historical degradation data and the predicted degradation levels
by incorporating the historical degradation data into the estima-
tion of
. Once the degradation drift was estimated, they used a
linear function with respect to time
to predict the evolution of
the degradation level.
However, in reality, nonlinear degradation trajectories are
encountered frequently, such as the degradation trajectories of
bearings [32], batteries [33], and gyros [34]. Thus, the linear
prediction used in the model described by (1) and (2) cannot
trace nonlinear degradation trajectories well. Additionally,
the conventional WP model is a time homogeneous process,
which means that the transition probability between two given
degradation states at any two CM points depends only on the
time interval. But not all degradation processes satisfy this
constraint. For example, the typical lifetime of the bearings
[35] can be roughly divided into two stages: one is the normal
stage, and the other is the defective stage. At the first stage, the
bearings degrade slowly, whereas the defects propagate quickly
at the second stage. This example indicates that equally long
time intervals may give rise to different degradation increments.
To take nonlinearity and heterogeneity into consideration, we
incorporate a nonlinear structure into the WP model, given as
(3)
where
is usually assumed to be 0. The specific form of
can be determined by domain experts, or inferred from
the lifetime data of other identical systems. It is notable that
there are three advantages in (3). First, in contrast with the con-
ventional WP model, (3) can track nonlinear degradation trajec-
tories better by adjusting
to fit the historical degradation data.
Second, the nonlinear function
can characterize a time
heterogeneous process. Third, as pointed out in [13], (3) retains
the nice mathematical property of the WP model that the explicit
distribution of the predicted RUL can be obtained analytically
once the form of
is specified.
Remark 1: There are two reasons for choosing a time-trans-
formation function instead of a more general one, e.g., a non-
linear drift used in [36], to characterize nonlinear degradation
trajectories. First, the explicit distribution of the predicted RUL
for the former can be obtained analytically, while the distribu-
tion of the predicted RUL for the latter is related to solving
the Fokker-Planck-Kolmogorov equation with boundary con-
straints, and the explicit analytical solution can be hardly ob-
tained. Second, members of a population of identical systems
have some common properties, so the specific form of
can be inferred from the lifetime data of other identical systems.
Subsequently,
can approach an individual degradation
trajectory through the adjustable
. Consequently, a time-trans-
formation function suffices to characterize nonlinear degrada-
tion trajectories.
To better characterize practical degradation processes and ob-
tain the predicted RULs more accurately, we take nonlinearity,
heterogeneity, and the complete historical degradation data into
consideration simultaneously by combining (1) with (3). Ac-
cording to the Euler discretization, our complete model at the
th CM point is formulated as
(4)