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. > > Does this position change show up on the LinuxCNC DRO display, or are you reading it with a mechanical indicator? One possible cause is that interference is being read differently by the USC board and the G320, leading to a difference in position between the two. But, the PID algorithm should cause LinuxCNC to command a move to null out the error. If LinuxCNC got false counts due to interference, then you would get a discrepancy between displayed and actual machine position. > 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. > > Yes, keep these separated. it is best practice to ground the shield only at the end where the signal is being received, ie. the control box. > 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. > > Yes, this may help. but, a short length of exposed wire may not be the cause, unless it is really close to noisy power wires. Noise on the cable shield has a MUCH higher ability to couple noise, as it contacts the wire intimately for the entire length. > Would I want to ground the power wire shield to the power supply ground or > the encoder ground? Do you think this might help? > > Shields for the power wires should NOT be grounded to the signal-sensing components, but to a different shield location. > Anyone have any other suggestions? > > I wish I could see magnetic fields! Well, with an oscilloscope, you really CAN sense them. With a DVM, you may also be able to. Find something scrap with a coil of fine wire. A few hundred turns of tightly coiled wire can be connected to your DVM on AC Volts mode, and will pick up these fields. Then, you can move it around and see where you get a high reading. Near the motor output wires of the Gecko 320 drives should give several Volts, at least!
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