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. If I understand you correctly, you are suggesting the encoder cable shield be grounded to the same earth ground? 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. > 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. >> Would I want to ground the power wire shield to the power supply ground >> or the encoder ground? Do you think this might help? > > 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? >> 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/ 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
