On Wed, 2011-05-11 at 23:00 -0500, Jon Elson wrote: > Well, I think I have figured out most of what this encoder can do. It > seems to have a 16-bit binary count per revolution plus a 16-bit signed > count of revolutions, and these are battery-backed. It also has a > 10-bit count of absolute angle per quadrant. > > If the backup battery doesn't save the high-res position, then it starts > off counting from zero, and then jumps back to zero when it passes the > index mark the first time. There is a bit to indicate it has found the > index mark. > > So, I think I can see how one could use this with EMC, but it is just a > little bit crude to deal with homing if you don't want to deal with the > battery. > > What I am thinking is the 10-bit absolute angle could be sent to the > bldc component for initial commutation alignment. > The high-res angle part could be used, but the driver would have to > watch for the jump when it passed the index the first time, and fudge > the count with an offset so it didn't cause a following error. This > fudging would be crude, as the encoder might only be sampled at 1 KHz. > I guess the last velocity could be used to figure out about how fast you > were moving each sample. Then, when the homing sequence searched for > the index, the fudging offset could be removed when the rotation sign > bit changed, and the system would be perfectly aligned at the index mark > of the encoder. Only the 16-bit angular count would be used, and the > driver could sign-extent the value for multiple rotations. > > There are two things I'm trying to accomplish with all this. First, if > you don't use a backup battery, the encoder count jumps suddenly when it > passes the index position the first time after power on. Second, there > doesn't seem to be a command you can send to the encoder to zero the > count at index. And, since the encoder is only sampled at some rate > (about 10 KHz is the maximum) then you are likely to miss the exact > moment when the index pulse is passed when homing, so you have to just > accept that it happened recently, and the encoder count WAS zero at that > moment. > > One other concern I have is that if the motor was to be rotated while > encoder power was off, I believe the battery-backed count would no > longer be correct. This seems to be a serious flaw in this serial > absolute encoder scheme. Now, maybe the encoder actually re-zeroes > EVERY time it passes the index position, so what would usually be a > small error won't accumulate. (If you moved it more than a turn, then > the count of turns would certainly be off.) > > > So, anybody have any comments on this? There probably are quite a > number of machines out there with these encoders on them, all newer than > the very first series of AC "red cap" motors. > > > Jon How serious is the problem of using the battery, i.e. what is the expected life of the battery? Is the a real maintenance problem? Of course, the next question is accessibility to replace the battery and the cost.
How do Fanuc people deal with this in the field? D > > ------------------------------------------------------------------------------ > Achieve unprecedented app performance and reliability > What every C/C++ and Fortran developer should know. > Learn how Intel has extended the reach of its next-generation tools > to help boost performance applications - inlcuding clusters. > http://p.sf.net/sfu/intel-dev2devmay > _______________________________________________ > Emc-developers mailing list > Emc-developers@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-developers ------------------------------------------------------------------------------ Achieve unprecedented app performance and reliability What every C/C++ and Fortran developer should know. Learn how Intel has extended the reach of its next-generation tools to help boost performance applications - inlcuding clusters. http://p.sf.net/sfu/intel-dev2devmay _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
