Fault Detection and Diagnostics for Rooftop Air Conditioners

Faults
External
Driving
conditions
RTU
Model
Measurements
-
+
Expectations
Residual
distribution
Figure 1.2 Rooftop system with a normal performance model
Gray-box modeling approaches use lumped system parameters and some semi-empirical
expressions. Strictly speaking, there are no pure physical models in engineering; most of socalled
physical models are gray-box models. Although easier to build and faster to solve
than a physical model, a gray-box model may not be practical for commercial HVAC
FDD.
Black-box models, which can overcome the shortcomings of physical models, use
empirical input/output relationships that are fit to training data. There are many black-box
modeling approaches. Different approaches have different characteristics and interpolating
abilities. Generally speaking, in the mathematical point of view a black-box model can not
be expected to have good extrapolating ability. However, in the physical point of view,
most real systems normally would not change dramatically and have major linear
components, so black-box models can have some extrapolating abilities near the training
data range. What’s more, different approaches have different extrapolating abilities and
different parameters and structure of the same approach have different interpolating and
extrapolating abilities. This report reviews some relevant literature, and then documents
comparisons between several modeling approaches, and proposes an improved modeling
approach.
5