Kenneth Lerman wrote: > > Jon Elson wrote: >> Kirk Wallace wrote: >>> On Mon, 2008-07-28 at 00:33 -0500, Jon Elson wrote: >>> ... snip >>> >>>> you might be >>>> able to cobble up a substitute by making a disc with a narrow >>>> slot and one of those U-shaped optical sensors from a floppy drive. >>> I have thought about this, but I wonder how accurate these are. If you >>> want to resolve .0002", I think you typically need angular accuracy >>> to .16 degrees. I guess I could try it to find out. >>> >> I came up with .36 degrees. A good-sized disc with a >> narrow-enough slot should work. Basically, you are making a >> 1000 line resolution encoder, but with only one line. > > Is that true? Since you always approach the home position in the same > direction, the width of the slot shouldn't matter at all. You should > just be looking at the leading edge. > > Ken >
Exactly - which leads to another interesting possibility. If you make a disk that is as close as possible to 180 degrees "on" and 180 degrees "off", you can mount two sensors at 90 degrees to each other. The result is a quadrature signal, with four counts per rev. Since each sensor still has only one rising edge per rotation, either sensor can be used as the index. (You would only need two hardware inputs, one of which would be split in HAL to drive both phase A and the index of the encoder, the other would drive phase B). This would be a great candidate for a cheap lathe threading encoder. The CVS version of EMC2 can use a single pulse per rev encoder for threading, but this simple 4 count quadrature encoder would provide about 16x better performance during load induced speed changes, for very little extra cost. Regards, John Kasunich ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
