Gene Heskett wrote: > As above, I am aware that halls have been used to good effect as engine > crankshaft position detectors in automotive ECM apps for quite a few years > now, with minimal failure rates. > > So there appears to be a disconnect between my observations and the rest of > the automotive etc industrial use of hall sensors, and I can't come up with a > sensible explanation for that disconnect. > > The particular devices that failed like Orville's popcorn were made by > Microswitch, and the keyboard itself was a Cherry, These were quite popular. The Hall sensor was in a little plastic box that held the key plunger, spring and magnet. I suspect that the Hall sensor was on a tiny piece of ceramic thick-film hybrid, with a blob of epoxy, or maybe even RTV over the Hall sensor and amplifier chip. This was sufficient protection from the normal environment, but not WD-40. The ceramic wasn't porous, but the epoxy or RTV would have been, or may have reacted with something in the WD-40. specially built for the > Beston Marquee character generator, and had a replacement cost from Beston of > about $500. The active element in the key was fabbed on a 3/8 square ceramic > substrate with a blob of epoxy B covering it, and the key driven magnet slid > onto the bottom of the ceramic when the key was depressed. It may be that > this particular ceramic was slightly porous, and that other makers devices > are better sealed. OK, you describe it about the same! But it looked to be the usual berylium oxide, No, not Beryllium oxide, that stuff is EXPENSIVE, as well as being potentially poisonous. The white ceramic is usually alumina, aluminum oxide, MUCH cheaper.
I'm sure that there are Hall sensors that have much better environmental seals. MicroSwitch themselves made sensors with a little metal cover over the works. If they did it with thin-film conductor traces running under a glass seal, then the things would be true hermetic. Except for defects, the ceramic can be made true hermetic, too, so you should be able to dunk them in WD-40 for years without failure. Certainly what you want to sense gear teeth in an oil bath. Might still be too slow for an encoder, though. Jon ------------------------------------------------------------------------- SF.Net email is sponsored by: The Future of Linux Business White Paper from Novell. From the desktop to the data center, Linux is going mainstream. Let it simplify your IT future. http://altfarm.mediaplex.com/ad/ck/8857-50307-18918-4 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
