Secondary effect techniques for automatic test and diagnostic systems

Secondary effect sensing techniques offer several potential advantages over conventional test and diagnostic techniques for nonelectronic systems. These include the ability to detect precursors of failure, and the capability that many failures can be detected without the requirement for disassembly. Most important, the feature that many of the secondary effects can be sensed remotely, that is, by means of a noncontacting sensor, may permit development of standardized sensors which are amenable for cost effective automatic test applications. Secondary effect techniques can be broadly categorized in terms of electromagnetic, mechanical, or chemical. A number of promising techniques in each of these categories are presented. With each sensing technique is included a general description, the failures detectable, and recommendations for future development.