Fail=safe monitor for two signal valves - monitors signals from main and redundant channels and triggers alarm when different

摘要

The monitor, for redundant systems secured against component failure, is made fail-safe by superimposing a rectagular-wave voltage on the error signal formed as the difference between the signals in the two channels being monitored. The rectangular voltage drives two triggers associated one with each channel. The triggers drive a transistor bridge whose a.c. diagonal contains a transformer supplying two relays each with two sets of change-over contacts. The bridge's d.c.diagonal contains a d.c. source connected via two sets of change-over contacts belonging to different relays.