A fault
in a multivariate system is usually attributed to abnormal
changes of only a small part of variables. For such a fault, the fault
detection index that is defined using all variables may not have a
good detection performance, due to the amplification and masking effects
caused by fault-free variables. To overcome this problem, this paper
proposes a multigroup fault detection and diagnosis (FDD) scheme for
multivariate systems. This scheme consists of two main parts: A method
for the grouping of variables, and a method to use variable groups
for online FDD. In the variable grouping method, the closely correlated
variables are grouped together, because the close correlations among
variables are proved to be advantageous to FDD. In the online FDD
method, a key group to FDD is adaptively selected for every new sample,
and then FDD is performed in the key group using two types of fault
detection indices that take into account the intragroup and intergroup
variable correlations, respectively. Because online FDD is carried
out only in one variable group, the multigroup FDD scheme has two
advantages. First, the fault detection capability is improved by reducing
the amplification and masking effects caused by variables in other
groups. Second, fault diagnosis becomes easier because the search
scope of faulty variables is narrowed down to members of the key group.
These two advantages are illustrated with two case studies.