Kirk Wallace wrote: > On Sun, 2008-07-20 at 16:30 -0400, Kenneth Lerman wrote: >> What do you mean by "encoder is in an invalid position". Every position >> read from an absolute encoder is valid in the sense that it reflects the >> actual position of the encoder within its tolerance. > > I don't think it does. Every edge from on/off to off/on has an unstable > region, which can be cleaned up with hysteresis, which requires motion > (?). The only time the data is valid, is when all bits are cleanly > within a stable area, off any edges.
That statement is not correct. When you read the data, even if it is at the edge, you will get either a zero or a one. Since only one of the sensors can be at the edge, all of the other bits will have "stable" values. An important feature of a Gray code is that the difference in location between that "unstable" bit being a zero and it being a one is one part in 256 (for our eight bit encoder). That should not be significant. > >> If you used a 256 count encoder for your 24 positions, each tool >> position would correspond to 256/24 = 10-2/3 counts. So if the encoder >> read from 0-10, that would be position zero. 11-22 would be position >> one, etc. >> >> On power up, you would read the encoder and that would tell you what >> position you were at. Assuming that the tool positions corresponded to >> the start of each range, 0 would correspond to position 0, 10 would >> correspond to tool position 1. 21 would correspond to tool position 2, >> etc. To allow for some "slop", you would probably set things up so that >> 255, 0, 1 were tool position 0. 9, 10, 11 were tool position 1, etc. >> >> If you found that you were between positions, it wouldn't really matter. >> The first time you seek to a tool, you would still know where you are >> and where you have to go. > > Bingo. You need motion to do that, it just took me a while to realize I > could deal with that on the first tool change. Or, use the spiffy > magnetic sensor. > >> A true absolute encoder is a static device. You can read a value without >> moving it. >> >> Ken > > Thanks for your help. That's why I posted the question. > -- Kenneth Lerman Mark Kenny Products Company, LLC 55 Main Street Newtown, CT 06470 888-ISO-SEVO 203-426-7166 ------------------------------------------------------------------------- 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
