Kirk, See: http://en.wikipedia.org/wiki/Gray_code
Gray codes have the property that every position is a valid value. Each change in position involves a change in only a single bit. Absolute rotary encoders typically use gray code (or something like it). They are available in many resolutions. Ken Kirk Wallace wrote: > I was reviewing a document from Sanyo covering AC servo motors: > > http://motiononline.com/PDF%20Files/Handbook/Handbook.pdf > > It talked about the AC system needing absolute shaft position in order > to duplicate a brushed commutator in software. It was indicated that a > low resolution absolute encoder could be used to determine shaft > position during motor drive initialization. Then a high resolution > incremental encoder could take over for commutation and axis position > feed-back. I would like to replace the encoder on a Yaskawa motor, and > got to thinking about what I could use for an absolute encoder. My first > thought was an optical disk encoder with N bits. Then, I realized that > there will be positions between bits where the data is zero or unstable. > I remembered US Digital had absolute encoders, so I looked at this one: > > http://www.usdigital.com/assets/general/102_mae3_datasheet_1.pdf > > When I saw the output graph, it showed a discontinuity at position 0, so > this would not work either. Then, I envisioned a pair of sine wave > outputs, which would be continuous across all possible outputs. I > believe resolvers have this kind of output. I could use a pair of ADC's > with Schmitt triggers to digitize the analog signals, so that I can get > the data into EMC2. > > I had a plan for a 24 position absolute encoder for a tool changer and > realized that I had the same problem. If the tool changer powers up > between tool positions, I won't be able to determine the changer status > without moving the carousel to scan for a valid output. I don't want to > have any unsolicited changer movement or require the user to remember to > manually initialize the changer, so I suppose I need a continuous > encoder here too. > > Am I understanding the issues correctly? Anybody know of a _cheap_ way > to get a non-mechanical low resolution continuous absolute position? > > (P. S. Maybe use two USD encoders 180 degrees apart, but this is too > expensive and maybe produces two discontinuities.) > ------------------------------------------------------------------------- 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
