DRO drifting

[GT Mills]

I am experiencing DRO drift as well. Mine is a little different, but I will try the grounding improvements suggested here. I might try a frame ground from the meter to the mill.

My problem shows up intermittently. It is a 3-axis glass scale Chinese type, costs $300 just purchased on Amazon. First attempt to get it installed and working failed. The DRO head was faulty and completely screwed up. The vendor replaced the head. Now I am frustrated with the zero base changing. The vendor is asking if I want the scales replaced next, since I thought they may have been damaged when delivered with the first head that was ruined in shipping when I discovered this problem.

I set X, Y, Z zero. I mill some cuts for 30 minutes or so. Everything will remain OK for a while. At some point when I shut down the mill to change tools or something, the X axis changes .010 - .015" at the moment I shut off the VFD, and the readings change with the spindle speed. Then, later on, the Y axis will change also when I shut down by a few thou. As I continue to work for the next hour or so I see the Z axis has also drifted a bit. The Z scale is not attached to the mill, it is sitting on the bench because I haven't bothered to install it yet. But I keep it plugged in because the head doesn't seem to work right w/o all three plugged in.

Depending on where the mill table is positioned, I get different drifting results. At times, all I do is reduce the mill speed with the VFD and this changes the zero on the DRO head, sometimes just X, then sometimes both X & Y. This is what led me to think the scale was messed up.

It seems that the longer the scale, the more the drift.

Any comments, thoughts, ideas greatly appreciated.

 

[SamI]

If this drift occurs when you alter your VFD could it be electrical noise? I must admit i know nothing of these things so can't offer any more advice but it could be worth considering.

 

[GT Mills]

The vendor first sent me a new head, then sent me a third head and three scales. I now have three heads (one head is definitely screwed up internally and in a state of constant alarm) and two complete sets of three-axis scales. STILL had problems, no matter what combination I set it up with both on the mill and with the scales on the bench, the head attached or on a plastic bucket, plugged into various circuits coming off the main breaker panel, maming sure all earth grounds and frame grounds ohm-ed out perfectly! WTF? Today I did something I had not tried before: ELIMINATE the third prong (110v) safety frame ground by running it through a 3 prong-to-2 prong plug adapter. Hot dang, it WORKED!!! Who would have guessed that REMOVING the safety frame ground would get rid of this exasperating problem? SO now I have two brand new, working, 3-axis DRO's with scales, and a third DRO head that seems to have a short circuit which I believe can be repaired, all for just $300. God works in mysterious ways. Now I just need a lathe or another mill to install this other DRO kit on, haha.

 

[mksj]

VFD electrical noise is a common problem/issue both EMI and RFI, the degree of interference can vary and also DRO's susceptibility. Grounding and having a good electrical filter or using a screened transformer for the sockets can help reduce these problems. I typically use inline noise filters on some of my VFD installs and pay particular attention to grounding. Grounding and shielding of cables is not always straight forward, but the VFD electrical noise can play havoc with low level signals. I had an issue with a MachTach where the display went haywire if the sensor cable shield was grounded to earth, and completely disappeared when the shield was floating. A noise filter or isolation transformer may help to reduce the interference, but erratic readings in the absence of electrical noise suggests a component issue in the head unit, in particular if both scales share similar problems. I did hear of a recent issue with the earlier Easson 12B/C DRO's were there was a firmware programming error that caused reading errors form the scales.

 

[homebrewed]

I've had problems with noise affecting my DROs as well. They are the capacitive type, which are known to be more susceptible to noise. I tried a line filter to no avail, and many variations on grounding, also with little improvement over the long term. The best solution for my DROs was to open the sensor boxes up and solder more capacitors between Vcc and ground (probably not a viable solution for everyone). The remote-readout design places the batteries in the display unit so the power has to travel through the cables -- which appear to act as magnificent antennae for picking up noise, even IF they are shielded.

I'll have to try the 2-wire scheme to see if that helps further reduce my noise problem. Thanks for the update!

 

[GT Mills]

VFD electrical noise is a common problem/issue both EMI and RFI, the degree of interference can vary and also DRO's susceptibility. Grounding and having a good electrical filter or using a screened transformer for the sockets can help reduce these problems. I typically use inline noise filters on some of my VFD installs and pay particular attention to grounding. Grounding and shielding of cables is not always straight forward, but the VFD electrical noise can play havoc with low level signals. I had an issue with a MachTach where the display went haywire if the sensor cable shield was grounded to earth, and completely disappeared when the shield was floating. A noise filter or isolation transformer may help to reduce the interference, but erratic readings in the absence of electrical noise suggests a component issue in the head unit, in particular if both scales share similar problems. I did hear of a recent issue with the earlier Easson 12B/C DRO's were there was a firmware programming error that caused reading errors form the scales.

Thank you very much for your reply! It is welcome news to read that someone else was able to isolate & solve a similar problem by floating the earth ground instead of connecting it. It never occured to me that the problem may be tied to the VFR rather than the motor itself. I had tried a line filter, but the one I had on hand (integrated into a fancy power surge strip) didn't fix the problem, and I after that I wasn't really sure another type might help.