TSINGHUA SCIENCE AND TECHNOLOGY
ISSN 1007-0214 11/20 pp582-586
Volume 10, Number 5, October 2005
Multivariate Statistical Process Monitoring Using Robust
Nonlinear Principal Component Analysis
ZHAO Shijian
(Äüt),
XU Yongmao (#^#Γ
Department of Automation, Tsinghua University, Beijing 100084, China
Abstract: The principal component analysis (PCA) algorithm is widely applied in a diverse range of fields for
performance assessment, fault detection, and diagnosis. However, in the presence of noise and gross errors,
the nonlinear PCA (NLPCA) using autoassociative bottle-neck neural networks is so sensitive that the
obtained model differs significantly from the underlying system. In this paper, a robust version of NLPCA is
introduced by replacing the generally used error criterion mean squared error with a mean log squared error.
This is followed by a concise analysis of the corresponding training method. A novel multivariate statistical
process monitoring (MSPM) scheme incorporating the proposed robust NLPCA technique is then
investigated and its efficiency is assessed through application to an industrial fluidized catalytic cracking
plant. The results demonstrate that, compared with NLPCA, the proposed approach can effectively reduce
the number of false alarms and is, hence, expected to better monitor real-world processes.
Key words: robust nonlinear principal component analysis; autoassociative networks; multivariate statistical
process monitoring (MSPM); fluidized catalytic cracking unit (FCCU)
Introduction
Multivariate statistical process monitoring (MSPM)
has achieved great success in the past decade, mainly
due to the adoption of powerful chemometric
techniques such as principal components analysis
(PCA) and projection to latent structures (PLS)
[1
'
2]
.
PCA has been developed and successfully applied to
deal with data sets from industrial processes with a
large number of highly correlated variables. However,
these data sets are usually also contaminated by noise
and gross errors. The basic concept behind PCA is to
project the data set onto a lower dimensionality
subspace. Noise will be suppressed to a certain extent
by discarding some latent variables, mainly due to the
Received: 2004-02-26; revised: 2004-05-11
* Supported by the National High-Tech Research and Development
(863) Program of China (No. 2001AA413320)
* * To whom correspondence should be addressed.
E-mail: xym-dau@mail.tsinghua.edu.cn; Tel: 86-10-62785845
influence of random noise.
Unfortunately, although PCA is good for linear or
almost linear problems, it fails to deal well with the
significant intrinsic nonlinearity associated with
real-world processes. Hence, nonlinear extensions of
PCA have been investigated by different researchers.
One of the preferred methods is that proposed by
Kramer
[3]
for its simplicity, which employs a five-layer
autoassociative network with a bottle-neck layer to
achieve the dimensionality reduction.
Data collected from industrial processes are
normally contaminated by noise and gross errors,
which severely affects the model accuracy. Kramer
[4]
proposed a robust version of NLPCA and illustrated
that it could be utilized to detect gross errors,
compensate for missing values, and estimate the
desired value in one pass. However, the model
demands that the data should all be for the normal
operating condition (NOC). Furthermore, massive
samples are required for network training, which