Kirk Wallace wrote: > > The whole idea of the differential boards where to reduce the > susceptibility of the system to noise, but the boards themselves seem to > make the effect of the noise worse. > What are you using for the differential driver chip? Is it totem-pole output or open-collector? > I think I need to place ferrite beads or other type of filter on the VFD > inputs and outputs and then revisit the oscilloscope. I don't have much > experience with tracking down noise with an oscilloscope, so if anyone > has some words of wisdom, I would appreciate hearing them. This can be very difficult, as the power cord ground on the scope adds a guaranteed ground loop that can totally confuse the measurement. One way to know you are getting a ground loop through the scope's power plug is when you connect the ground clip of the scope probe to something, with nothing connected to the probe tip, but you see a signal!
Anyway, the diff drivers seem to somehow be the problem. I can't even guess whether it is susceptibility to noise on the power input, noise coupling through the encoder signals, or just the fact that the diff. driver is an amplifier. If the drivers in the encoder are pretty slow, but the drivers in the diff. driver are really fast, then they could be turning a 50 ns glitch with very little amplitude into a strong pulse. I would guess that not only does the Z signal have this, but the A and B have it, too. The digital filtering of the quadrature signals have a defense that only valid quadrature transitions are accepted, but the Z doesn't have that. A filter might be the fix, or more intimate coupling/grounding/shielding between the encoder and diff driver may be the cure. If the encoder has one of those threaded military Cannon or Amphenol connectors with the 4 screws on it, you might make a little box that attaches to the encoder body between encoder and connector and put the diff driver in that. This should help prevent any interference getting into the signals between the two. If the encoder electronics are grounded to the encoder frame, opening this ground could be really helpful. That represents a classic ground loop situation. Don't put capacitors on the VFD output terminals. A series inductance can be helpful, but is not guaranteed to help. A line filter on the input is much more helpful, as capacitors can be used there. I am using a commercial line filter made by "Filter Concepts", probably a Mil-spec unit, rated at 15 A, for my Bridgeport spindle. It is likely overkill, a 6 A unit would most likely handle the 1 Hp fine. I have no filtering, and no shielding, either, on the VFD output. I do have a ground wire in the 4-cond cable, tied to the VFD frame at one end, the machine frame at the other. The line input filter solved the noise issues I was having, with the computer monitor and my computer-controlled air compressor, which was only running off the same breaker panel. That line filter came out of my junk box, no idea what it came out of. Jon ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
