Wiring Sensitivity on Textile Fiber System
The fiber handling system for a new $35 million nonwoven plant began suffering apparently random shutdowns several months after the system was commissioned and production was underway. Fiber transport and blending equipment would stop without warning and halt the entire line, but on examination all machinery and controls were in good order and awaiting a restart command - which was always effective.
By careful monitoring and immediate response to events, WHY traced the failure to the high speed communication bus on the PLC system. The normally reliable architecture uses a solid-conductor, twisted-pair cable with a braided shield, terminated in a connector with a nine-pin plug. This bus connected the central PLC to several remote I/O racks, all of them located in freestanding electrical panels surrounded by fiber opening and blending machinery.
Painstaking examination of each of the cable connectors revealed a single strand of shielding braid which was not captured under the termination clamp inside a particular connector. Under certain conditions of vibration, the strand would make momentary contact with a grounded point inside the connector. The instantaneous change of reference potential would cause a communication error and halt execution of the main control program. However, the problem strand would immediately rebound away from the point of contact, leaving the system halted and ready for restart, clearing the problem but without leaving a smoking gun to identify the source.
A new version of the manufacturer's connector was found with no grounded surfaces near the cable terminals. Each of the cable ends and shields was carefully trimmed and reconnected, and the problem was corrected.
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