"Arild Jensen" wrote: > REPLY > Why indeed? Lew you bring up an very contemporary topic. Also a > hotly debated issue. <VBG> > > It used to be devices were designed to FAIL SAFE. Especially in > motor > controls etc.
My soap box rant was directed specifically at motor control designs. > Thomas Kobelt of Kobelt Engineering gave a presentation to the > Vancouver chapter of Marine Engineers. <snip> > What is the difference you might well ask? Fail safe neans if a > failure occurs the device reverts to a safe configuration. When discussing closed loop servo drives, the only thing that makes any sense is a "where is, as is" failure. IOW, if power fails, the drive stops dead "where is, as is". > However with the advent of digital control we have seen a number of > situations where a momentary failure in a system might cause the > controls to change from the mode they were currently in. As a result > the momentary failuer caused the digital controls to CHANGE whatever > they were controlling to some other state. Not swift. >FAIL in MODE on the other hand means the controls would stop in >whatever > position they were in immediately before the power failure. > When power is resumed the controls woudl be exactly where they had > been > left. . > > What I had mentioned to Norm was using latching relays for cabin > LIGHT > CIRCUITS. That technology has been around for ever, and is a valid application of a latching relay. Back when I sold it, GE called theirs low voltage lighting control. Lighting at 120V or 277V and control at 24 VDC. It was primarily aimed at multi-story high rise buildings. Gasoline shortages in the 70s peaked some interest, but not a lot. Don't even know if GE still makes theirs. > On the other hand, configuring the lights to drop out and stay off > even when the power resumed (such a a momentary outage from for > example a reclosure in the utulity power during a storm) is not good > either. The term "Lunch bucket" mean anything to you? A slang expression for an emergency battery operated lighting pack mounted on the wall above eye level with a couple of sealed beam lamps aimed down at the egress aisles of a building. Lew _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [EMAIL PROTECTED] To unsubscribe send an email to [EMAIL PROTECTED] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
