1. Introduction
Information fusion, when applied to fault diagnosis and defect inspection, revolves around two main ques- tions [1–3]:
(1) How to acquire precise and reliable information cues about potential faults by incorporating com- plementary, and possibly redundant, multiple sensors.
(2) How to fuse decisions that are derived based on
multi-sensor data, which can be imprecise, and conflicting.
In the context of engine diagnosis, the first question is concerned with extracting fault-reveling engine features from multiple sensors, and describing them in a coherent representation scheme. Furthermore, since information obtained from the sensors is inherently incomplete, uncertain, and imprecise, it is imperative that a fusion mechanism be devised so as to minimize such impreci- sion and uncertainty. The effectiveness of such mecha- nism depends to a large extent on how redundant and complementary are the information cues obtained from the sensors. It is equally important to decide at what level of abstraction the fusion process is to take place, e.g., at the measurement level, at the feature level, and/or at the decision level. Generally speaking, acquir-
ing precise and certain engine quality descriptors can be achieved by fusing sensory data at the feature level. Refs. [4–6] present some examples of this type of data fusion.
The second question is concerned with the quality of decisions made with respect to engine diagnosis. It is
quite conceivable that information obtained from differ- ent sensors leads to different and possibly conflicting decisions. The challenge in this case is how to detect conflicts among the sensors and how to fuse their deci- sions into one coherent decision. Addressing this issue constitutes the main scope of the paper. In formulating the multi-sensor decision fusion, the paper assumes the
scenario of two sensor modalities used to monitor the
quality of single piston engines in real-time. One modal- ity measures acoustics emitted by the engine; the other modality measures engine vibration. The two modalities are typically employed in the final stage of an engine assembly line. The acoustic modality monitors timing related performance, while the vibration modality mon- itors performance indices related to valve clearance. Fusing the two modalities can lead to reliable decisions with respect to the presence and absence of faults.
There has been a substantial amount of research work conducted in the area of decision fusion, most of which is built around Bayes theory. The basic strategy is that if the prior probabilities and conditional probabilities are determined in advance, then the posteriori probabilities