Multi-Sensor Prognostics using an Unsupervised Health
Index based on LSTM Encoder-Decoder
Pankaj Malhotra, Vishnu TV, Anusha Ramakrishnan
Gaurangi Anand, Lovekesh Vig, Puneet Agarwal, Gautam Shro↵
TCS Research, New Delhi, India
{malhotra.pankaj, vishnu.tv, anusha.ramakrishnan}@tcs.com
{gaurangi.anand, lovekesh.vig, puneet.a, gautam.shro↵}@tcs.com
ABSTRACT
Many approaches for estimation of Remaining Useful Life
(RUL) of a machine, using its operational sensor data, make
assumptions ab out how a system degrades or a fault evolves,
e.g., exponential degradation. However, in many domains
degradation may not follow a pattern. We prop ose a Long
Short Term Memory based Encoder-Decoder (LSTM-ED)
scheme to obtain an unsupervised health index (HI) for
a system using multi-sensor time-series data. LSTM-ED
is trained to reconstruct the time-series corresponding to
healthy state of a system. The reconstruction error is used
to compute HI which is then used for RUL estimation. We
evaluate our approach on publicly available Turbofan Engine
and Milling Machine datasets. We also present results on a
real-world industry dataset from a pulverizer mill where we
find significant correlation between LSTM-ED based HI and
maintenance costs.
1. INTRODUCTION
Industrial Internet has given rise to availability of sensor
data from numerous machines belonging to various domains
such as agriculture, energy, manufacturing etc. These sensor
readings can indicate health of the machines. This has led
to increased business desire to p erform maintenance of these
machines based on their condition rather than following the
current industry practice of time-based maintenance. It has
also been shown that condition-based maintenance can lead
to significant financial savings. Such goal s can be achieved
by building models for prediction of remaining useful life
(RUL) of the machines, based on their sensor readings.
Traditional approach for RUL prediction is based on an
assumption that the health degradation curves (drawn w.r.t.
time) follow specific shape such as exponential or linear.
Under this assumption we can build a model for health
index (HI) prediction, as a function of sensor readings.
Extrap olation of HI is used for prediction of RUL [29, 24,
25]. However, we observed that such assumptions do not
hold in the real-world datasets, mak ing the problem harder
to solve. Some of the important challenges in solving the
prognostics problem are: i) health degradation curve may
not necessarily follow a fixed shape, ii) time to reach s ame
level of degradation by machines of same specifications is
often di↵erent, iii) each instance has a slightly di↵erent
initial health or wear, iv) sensor readings if available are
Presented at 1st ACM SIGKDD Workshop on Machine Learning for Prognostics
and Health Management, San Francisco, CA, USA, 2016. Copyright 2016 Tata
Consultancy Services Ltd.
noisy, v) sensor data till end-of-life is not easily available
b ecause in practice periodic maintenance is performed.
Apart from the health index (HI) based approach as
describ ed above, mathematical models of the underlying
physical system, fault propagation models and conventional
reliability models have also been used for RUL estimation
[5, 26]. Data-driven models which use readings of sensors
carrying degradation or wear information such as vibration
in a b earing have been e↵ect ively used to build RUL
estimation models [28, 29, 37]. Typically, sensor readings
over the entire operational life of multiple instances of a
system from start till failure are used to obtain c ommon
degradation behavior trends or to build models of how a
system degrades by estimating health in terms of HI. Any
new instance is then compared with these trends and the
most similar trends are used to estimate the RUL [40].
LSTM networks are recurrent neural network models
that have been successfully used for many sequence
learning and temporal modeling tasks [12, 2] such as
handwriting recognition, speech recognition, sentiment
analysis, and c ustomer behavior prediction. A variant
of LSTM networks, LSTM encoder-decoder (LSTM-ED)
mo del has been successfully used for sequence-to-sequence
learning tasks [8, 34, 4] like machine translation, natural
language generation and reconstruction, parsing, and
image captioning. LSTM-ED works as follows: An
LSTM-based encoder is used to map a multivariate input
sequence to a fixed-dimensional vector representation. The
deco der is another LSTM network which uses this vector
representation to produce the target sequence. We provide
further details on LSTM-ED in Sections 4.1 and 4.2.
LSTM Encoder-decoder based approaches have been
prop osed for anomaly detection [21, 23]. These approaches
learn a model to reconstruct the normal data (e.g. when
machine is in perfect health) such that the learned model
could reconstruct the subsequences which belong to normal
b ehavior. The learnt model leads to high reconstruction
error for anomalous or novel subsequences, since it has not
seen such data during training. Based on similar ideas,
we use Long Short-Term Memory [14] Encoder-Decoder
(LSTM-ED) for RUL estimation. In this paper, we propose
an unsupervised technique to obtain a health index (HI)
for a system using multi-sensor time-series data, which does
not make any assumption on the shape of the degradation
curve. We use LSTM-ED to learn a model of normal
b ehavior of a system, which is trained to reconstruct
multivariate time-series corresponding to normal behavior.
The reconstruction error at a point in a time-series is used
arXiv:1608.06154v1 [cs.LG] 22 Aug 2016