A method is described whereby faults in control systems may be detected on the basis of the generation and crosscomparison of band-limited signals from dissimilar sources. While signals measured at different points in the system may differ widely when viewed over a wide bandwidth, over limited pass bands there are shown to exist simple relationships which can be verified by means of appropriate filtering and comparison logic. The band-limiting filters employed are, in effect, used to monitor the propagation of information, originating in the structure of external inputs, through the system.
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