A knowledge–based expert system was developed to diagnose the vibration problems of turbomachinery. A two–step diagnosis was proposed in this work. The huge knowledge base was divided into two sub–bases, called Knowledge Level I and II. Knowledge Level I was first processed to extract a few possible causes, and only these possible causes were re–evaluated by Knowledge Level II. The main purpose of the two–step diagnosis was to speed up the knowledge processing and to make the development of inference procedure easier. The concept of certainty factor was adopted to handle the problem of reasoning under uncertainty. The possible causes of fault were ranked according to the certainty factors.

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