Hi Chris, The frequency of the noise as measured on the ground appears to be a ringing signal repeating at roughly 600kHz. I guess my previous measurement of 80kHz was wrong. I’m not sure why it would be different. I’m not 100% sure that it’s a consistent 600kHz, and might actually be several signals overlaid. The noise events appear to ring, as they decay from a larger sinusoidal spike, with about 6 sinusoidal pulses in total until they taper off.
The wires are generally bundled, so that may be a factor. The housings on the motors, drivers, and supplies are grounded to the machine. Only the encoders and motor to drive cables are shielded, and they’re shielded on only one end. The remaining wires are simply primary cable and are not grounded. Only some of the signal wires are twisted pairs. Thanks! Matt > On Aug 27, 2020, at 1:24 PM, Chris Albertson <albertson.ch...@gmail.com> > wrote: > > Can you measure the frequency of the noise? If this is a digital scope it > might be able to show a spectra. Lacking that you can eyeball-estimate the > period or width of any spikes. > > The other bit of information we don't have is how the wiring is laid out. > Best practice would be to use tightly twisted pairs where the current in > each of the two members of the pair is equal and also to never bundle power > and signal wires. The field around a twisted pair is close to zero > after about 10 cable-diameters so if a twisted power cable is 3mm thick, > keep other wires 30mm away. > > Are all the housing on the motors, drivers, and supplies grounded. This > would be to a protective ground Also are the wires shielded and grounded > on one and only one end? > > If the noise is determined to be a relative high frequency then ferrite > cores on the wires help but they no nothing for 120 Hz. > > > > On Thu, Aug 27, 2020 at 10:00 AM Matthew Herd <herd.m...@gmail.com> wrote: > >> I spent some time the week before last investigating the noise issue that >> I’ve been having on my Bridgeport BOSS mill. The system architecture >> involves a single phase 240V input, which powers a VFD for the 2HP spindle >> motor, a 120V step down transformer to power some devices on the machine >> (spray mister, accessory lights, cabinet ventilation fans), and a >> 72V/24V/12V DC power supply to power the USC board and the three Gecko 203V >> stepper drivers. First, I disconnected every device from the DC power >> supply by disconnecting the positive terminal and disconnected one of the >> 240V AC wires to the VFD. I measured the noise from the ground bolt to the >> ground pin of the 12V output of the power supply. Noise was nominal, >> perhaps 20-40mV peak to peak at 50ms/div (as I recall). Reconnecting the >> VFD gave approximately 600mV of noise peak to peak. This seemed rather >> surprising given the fact that I’m essentially measuring the same wire >> across about 18". >> >> I installed a Rasmi VFD filter and I re-wired the grounds. Instead of 2 >> grounds which were connected via the machine, I ran all grounds back to the >> ground bolt in the power cabinet. Previously, the 72V ground to the drives >> was only wired directly from the power supply to the drives. It was >> isolated from the other power and logic grounds, but is now common. The >> only ground that was not run to the power cabinet ground was the stepper’s >> step and direction signal ground. This ground is connected at the USC >> board to a ground which is common with the 12V power input ground that >> powers the USC board (and is therefore run back to the main ground bolt). >> Upon powering up everything I discovered that the spindle index encoder >> noise had increased from around 1.4V peak to peak to more like 7V peak to >> peak. I’m shocked that a ground wiring change could impact this >> measurement so dramatically. The VFD only noise is now measuring >> approximately 400mV p-p at the power supply 12V ground, which I believe is >> acceptable. However, I’m sure I could find higher spikes if I reduced the >> time scale based on my previous test results. I got some conflicting >> results when trying to isolate the cause, so I rechecked my measurements >> today. Rechecking my measurements at the 12V output ground on the power >> supply, I found: >> >> VFD only: >> ~300mV >> >> USC only: >> ~40mV >> >> USC and 24V devices (mostly SSRs for pneumatic valves): >> ~40-50mV >> >> USC, 24V devices, and Geckos with NO 72V input (i.e. only step & direction >> inputs) >> ~40-50mV >> >> USC, 24V devices, and Geckos with 72V input (i.e. ready to move motors) >> ~1-1.2V >> >> Now I tested the 72V wire at the gecko’s power input and found noise of >> approximately 5V. >> >> Then I removed the wire at one of the two 72V supply outputs (it has dual >> 72V outputs) and measured at the + terminal with two other geckos powered >> from the other 72V output. Noise was minimal (though difficult to measure >> as my scope wouldn’t let me scale to see the ripple as clearly — at most >> it’s probably 1-2V). This leads me to the conclusion that either the >> supply somehow struggles under load or the Geckos are introducing >> inordinate noise. I’m doubtful the supply is the culprit because the noise >> appears to be a much higher frequency than would be expected from a simple >> rectified power supply with filtering capacitors. The geckos are in the >> control cabinet with the USC. Would it help to move them closer? >> >> Thanks, >> Matt >> >> _______________________________________________ >> Emc-users mailing list >> Emc-users@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/emc-users >> > > > -- > > Chris Albertson > Redondo Beach, California > > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
