178
HVAC&R
RESEARCH
Lee et al. (1996a) used two methods to detect eight different faults (mostly abrupt faults) in a
laboratory test
AHLJ.
The first method uses discrepancies between measured and expected vari-
ables (residuals) to detect the presence of a fault. The expected values are estimated at nominal
operating conditions. The second method compares parameters estimated using autoregressive
moving average with exogenous input
(ARMX)
and
ARX
models with the normal (or expected)
parameters to detect faults. The faults evaluated included complete failure of the supply and
return fans, complete failure of the chilled-water circulation pump, stuck cooling-coil valve,
complete failure of temperature sensors, complete failure of the static pressure sensor, and fail-
ure of the supply and return air fan flow stations. Because each of the eight faults has a unique
signature, no separate diagnosis is necessary.
Lee et al. (1996b) used an
ANN
to detect the same faults described previously (Lee et al.
1996a). The
ANN
was trained using the normal data and data that represented each of the eight
faults. Inputs to the
ANN
were values for seven normalized residuals, and the outputs were nine
values that constitute patterns that represent the normal mode and the eight fault modes. Instead
of generating the training data with faults, idealized training patterns were specified by consider-
ing the dominant symptoms of each fault. For example, supply fan failure implies that the sup-
ply fan speed is zero, the supply air pressure is zero, the supply fan control signal is maximum,
and the difference between the flow rates in the supply and return ducts is zero. Using similar
reasoning, a pattern
of
dominant training residuals was generated for each fault (see Table
4).
A
dominant symptom residual is assigned a value of +1 if the residual is positive and -1 if the
residual is negative; all other residuals are assigned a value of
O.
The
ANN
was trained using the
pattern shown in Table
4.
Normalized residuals were calculated for faults that were artificially
generated in the laboratory
AHU.
The normalized residuals vector at each time step was then
used with the trained
ANN
to identify the fault. Although the
ANN
was successful in detecting
the faults from laboratory data, it
is
not clear how successful this method would be in general
because the faults generated in the laboratory setting were severe and without noise.
Lee et al. (1997) extended the previous work described in Lee et al. (1996b). In the 1997 anal-
ysis, Lee et al. (1997) used two
ANN
models to detect and diagnose faults. The
AHü
is decom-
posed into various subsystems such as the pressure control subsystem, the flow-control
subsystem, the cooling-coil subsystem, and the mixing-damper subsystem. The first
ANN
model is trained to identify the subsystem in which a fault occurs, while the second
ANN
model
is trained to diagnose the specific cause of a fault at the subsystem level.
An
approach similar to
the one used in Lee et al. 1996b is used to train both
ANN
models. Lee et al. (1997) note that this
two-stage approach simplifies generalization by replacing a single
ANN
that encompasses all
considered faults with a number of less complex
ANNs,
each one dealing with a subset of the
residuals and symptoms. Although 11 faults are identified for detection and diagnosis, fault
detection and diagnosis are presented for only one fault in the paper.
Peitsman and Soethout
(1
997) used several different
ARX
models to predict the performance
of an
AHü
and compared the predictions to measured values to detect faults. The training data
for the
ARX
models were generated using
HVACSIM+.
The
AHü
is modeled at two levels. The
first level is the system level, where the complete AHü is modeled with one
ARX
model. The
second level
is the component level, where the
AHU
is subdivided into several subsystems such
as the return fan, the mixing box, and the cooling coil. Each component is modeled with a sepa-
rate
ARX
model. The first level
ARX
model is used to detect a problem and the second level
models are used to diagnose the problem. Most abrupt faults were correctly identified and diag-
nosed, while slowly evolving faults were not detected. There is a potential for a conflict between
the two levels with this approach; for example, the top-level
ARX
model could detect a fault
with the
AHU,
while the second-level
ARX
models do not indicate any faults. Furthermore,
there is a potential for multiple diagnoses at the second level. Peitsman and Soethout (1997)
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