On Saturday, May 26, 2012 08:30:46 PM Jeshua Lacock did opine:
Sorry for the late reply, I rebooted this box because it was getting laggy early this morning, an the whole darned nvidia-current chain took a dump on me. Just now got it back among the living. > On May 26, 2012, at 5:44 AM, gene heskett wrote: > > On Saturday, May 26, 2012 07:12:02 AM Jeshua Lacock did opine: > >> Greetings, > >> > >> I am now setting up my new X axis servo with an AMT 102 encoder. > >> > >> The motor is normally dithering between 0.0003 and 0.0006 inches. > >> > >> But every now and then - for no obvious reason it jumps up to 0.02+ > >> inches and doesn't attempt to return to the correct location. It just > >> jumps randomly. > >> > >> I think it must be interference. > >> > >> The cable is double shielded and the shields are grounded to the > >> encoder ground the length of the cable outside my enclosure. I am > >> suspecting the interference is inside the enclosure where there are > >> unshielded encoder and power wires. > > > > You should have a 'star' ground system, where some bolt in the main > > housing, which is connected to the 3rd pin ground of the powerline, > > and all shielding is connected to that, as are all the common ground > > wires from the rest of the system. Connecting the other end of the > > shielding to the encoders ground will make a ground loop, and when > > you allow that current to flow on your shielding, all bets are off. > > So connect the shielding at that designated bolt and nowhere else. > > Connect the commons from the motor supplies to this same bolt. > > Anything that needs grounded should also be grounded at that bolt and > > no place else. That bolt should also be the only place an earthen > > ground is connected. > > Hi Gene, > > OK, so I just checked and the entire aluminum enclosure is grounded to > the same earth ground (3rd prong of the AC outlet) as the PSU for the > servos. That should be good, but if the grounds aren't to exactly the same point, strange things can happen. > If I understand you correctly, you are suggesting the encoder cable > shield be grounded to the same earth ground? Yes. That said, my only encoder, an ABI model, is running on a 4 wire plus shield cable identical to what I drive my motors with, using 3 of its 4 wires for the A/B/I signals, the 4th one for 5 volts from the C1G, and the shield for the ground return. In this case, the pcb for the photo- interrupters, while mounted solid to the end of the lathes spindle housing, the ground on the pcb doesn't connect to the lathe, nicely satisfying the open shield at the far end. The motors are wired with the exact same cable, but the shielding is trimmed back and not connected to the motor frame, only to a ground buss common to the drivers. I've had no hint of any noise pickup so far, and inspecting the signals with a real scope shows they are very clean. > Jon had recommended that I > ground them to the DC encoder ground if I understood him correctly. > Currently the encoder cable shield is only connected at the enclosure > side and to the encoder power DC ground. > > >> I was thinking of carefully wrapping aluminum foil around the power > >> wires and grounding them, then shrink-wrapping or covering the > >> aluminum with electrical tape. > > > > Probably will not work because of the difficulty of making a good > > connection to the foil that lasts more than a few minutes. When > > dealing with alu, the only reliable connection is called a gas tight > > connection. alu is a VERY reactive metal in the presence of the O2 in > > the air, oxidizing rapidly enough when exposed to the air by > > scratching it, a film of oxide which is a perfect insulator with a > > breakdown voltage of 40 volts or so, will be formed in 0.001 seconds. > > Give it some chemical help and its good to around 500 volts! > > Hmm, my aluminum enclosure is years old and conducts electricity just > fine. > Touch it gently. I'll bet you have to grind through the oxide film with the tip of the probe before it makes contact. Either that or it has some sort of a plated finish. ;) Cadmuim plated perhaps? > > Use shielded wire for critical signals inside the cubical too. > > That would basically mean I would have to rewire the entire box, so > hopefully I don't have to resort to such drastic measures. Ouch! The twisted pair idea will help, you might give that a shot, if they are long enough to have the slack to be twisted & still reach. > > >> Would I want to ground the power wire shield to the power supply > >> ground or the encoder ground? Do you think this might help? Since that is the fallback that hopefully keeps one alive if a short develops, defnitely. > > See above, all grounds should 'radiate' like a star from one common > > point. do not use a shield to take a ground someplace, run a separate > > piece of normal wire that is connected to this common bolt. > > I actually have all my earth ground connected to a terminal block, and > that terminal block is connected to the enclosure. Do you think that is > essentially the same thing? Pretty close for the girls we go with if all the screws are tight. Put the small signal stuff out on one the end of the bar, and the earth ground in the center, and put the high current stuff on the other side of the earth ground so the distance the higher currents have to travel is minimized. > >> Anyone have any other suggestions? > >> > >> I wish I could see magnetic fields! Short of that, at times like this > >> I wish I had a magnet implant like this guy that can feel magnetic > >> fields: > >> > >> http://www.wired.com/gadgets/mods/news/2006/06/71087?currentPage=all > > > > Chuckle, that would be nice. I understand that pigeons sense that > > somehow. But they don't speak english... > > Here is a really cool video visualizing what magnetic fields would look > like if we could see them: > > http://www.todayandtomorrow.net/2008/06/02/magnetic-movie/ > Very little of that, perhaps some of the last one, are anywhere near accurate. Most of it resembles a time exposure of someone randomly waving a wand with an LED on the end of it. No real image of a magnetic field there. Take a little toy bar magnet, and lay it in the debris under your bench grinder to give you a better idea. Also, you've seen the solar flares on the sun, most of which are very highly constrained by the suns own magnetic field, many thousands of times stronger than this ball of rock has. > Cheers, > > Jeshua Lacock > Founder/Engineer > 3DTOPO Incorporated > <http://3DTOPO.com> > Phone: 208.462.4171 > > > ------------------------------------------------------------------------ > ------ Live Security Virtual Conference > Exclusive live event will cover all the ways today's security and > threat landscape has changed and how IT managers can respond. > Discussions will include endpoint security, mobile security and the > latest in malware threats. > http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> FORTUNE PROVIDES QUESTIONS FOR THE GREAT ANSWERS: #4 A: Go west, young man, go west! Q: What do wabbits do when they get tiwed of wunning awound? ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
