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Unread 09-12-2005, 20:32
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Sparks333 Sparks333 is offline
Robotics Engineer
AKA: Dane B.
FRC #1425 (Wilsonville Robotics)
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Re: Alternative to limit switches?

Quote:
Originally Posted by Al Skierkiewicz
As for reed switches, I do not recommend them for this application. They have a bad habit of disengaging with vibration and magnets get knocked off leaving you defenseless. Additionally, the contacts don't always meet properly and high resistance occurs. With very little current being supplied by the RC, there is a source of error there. Microswitches are designed to "wipe" the contacts when they make contact, minimizing this effect.
This is very true. The good news is that reed switches aren't used too much anymore (on account of being mechanical instead of solid-state) and the Hall-Effect sensors have all but replaced them (Could be hyperbolizing, but I see many more hall effect sensors than reed switches). Best of all, they both work about the same: bring 'em into a magnetic field, and they trip, except hall effect sensors output a digital 0-5vdc signal almost 100% of the time (depending on the sensor). So, if you have a real hankering for magnetic switches, halls are the way to go. The ones in the kit last year were decent, but they were designed to count the teeth on a gear of a ferrous metal. Halls in general just respond to magnets. One final note: Some halls (most? I actually don't know) respond to the flipping of a magnetic polarity (aka N, S, N, S, etc.). This is not what you want for limit switch application, but it works rather well for encoder operation (So you don't count a magnet twice). We used a pair of hall-effect sensors on our robot last year in place of encoders. Can't say we used them well, but the scope says they worked great.

Sparks
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Last edited by Sparks333 : 09-12-2005 at 20:35.