On May 26, 2012, at 9:31 AM, Kirk Wallace wrote: > On Sat, 2012-05-26 at 02:26 -0600, Jeshua Lacock wrote: >> 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. > > I had a similar problem with my tool changer carousel. The carousel > would randomly shift to a random position. I used unshielded CAT5 for > the encoder. While looking at the signal with my oscilloscope, I recall > noticing the signal was close to a Volt when logically 0 with noise > adding about a half Volt of AC. My guess is that the AC would on > occasion push the voltage high enough to trigger the encoder inputs. My > next thought was to add a pull down resistor from the input terminal to > the input ground to pull the signal below the trip point. I sized the > resistor such that the logic 1 was still high enough to be well above 4 > Volts. This solved the problem, but I also added a small capacitor to to > help filter some of the AC. Basically, many input circuits have such > high input impedance that any EMI can induce a significant voltage > change on the cable. Loading or decreasing the impedance at the input > can reduce the interference. Studying cable termination for RS-422/485 > may provide a more formal description of how this should be done and > really should apply to any cabling not just RS-422. > > See Section 4 here: http://www.ti.com/lit/an/slla070d/slla070d.pdf
Hi Kirk, Interesting. So the pull down resistor would go on the encoder ground? Was it connected at the control box end? I don't have much electricity knowledge (no surprise there), so could you recommend what size the resistor would be? What size capacitor? Was it connected between the encoder DC +5v and ground? Was it connected at the encoder end or the control box end? Thanks, 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
