DIY CNC Mill - Random Limit Switch Errors

Sorry, I tried uploading pictures of my BOB several times, but it wouldn't do it. I think I got it to work finally.

NWE, even though I have the BOB right next to the contactor and 12v/24v power supplies, I have never had any issues from those. I have consistently been able to run programs without issue with everything except the VFD on. The contactor is part of my Estop circuit, and it only gets turned on before I run a program or off if I hit the Estop. I have only ever experienced issues when the VFD is on.

I experimented yesterday and learned some. I tried connecting it to earth ground, but that did nothing. I tried disconnecting everything from the BOB except for the 24v coming in and the parallel port and USB from the PC (I switched the VFD to manual control). LinuxCNC ran the program with out any trips even though the VFD was running, so the BOB itself can't be the issue. I tried reconnecting one of the stepper driver pulse/direction signal wires (the steppers were completely off) and ran the program again, and it did trip, even though that wire is shielded and grounded.

I will disconnect the second shielding grounding at the motor. I will also try adding ferrites on the VFD wires first. I think we have a couple lying around that we also pulled from other things.

I forgot to mention, VFDs radiate interference out of both incoming and outgoing power wires. Outgoing is by far the worst, but incoming side also quite bad. Thanks for the BOB picture. I circled the power wires going past BOB. Those certainly radiate interference when the VFD is running, everything all the way back to the breaker panel and beyond is affected to a certain extent. I prefer to keep all that AC wiring a minimum of 12" from the BOB.

How long is your parallel port cable? Is it shielded? The longer it is, the more interference it can collect. If it is shielded then I would think no worry.

zippoffs wrote:

I have only ever experienced issues when the VFD is on.

I tried disconnecting everything from the BOB except for the 24v coming in and the parallel port and USB from the PC (I switched the VFD to manual control). LinuxCNC ran the program with out any trips even though the VFD was running, so the BOB itself can't be the issue. I tried reconnecting one of the stepper driver pulse/direction signal wires (the steppers were completely off) and ran the program again, and it did trip, even though that wire is shielded and grounded.


 

Sounds like effects of a ground loop. You have more than one ground connection from point A to point B at one or more points anywhere on the system. You want exactly one ground circuit from each 'groundable' piece to one single point. It is called single point ground aka star connection. Redundant ground paths create ground loops which powerfully amplify interference effects from the vfd, they behave as you described above.

Potential sources of redundant grounds:
1. shield grounded at both ends
2. motor cord have ground wire
3. additional frame ground to the machine when the motor is already in electrical contact with the machine
4. the machine may be in physical conductive contact with another machine that is also grounded
5. is the machine on a steel workbench that has a grounded outlet?

Interference mitigation can be a bit of an art because of the numerous variables involved. Usually not everything has to be perfect, you can cheat some, but if the BOB is within inches of any AC when there is also a VFD nearby then I say you will have trouble at least randomly.

BOB is probably grounded by parallel cable to pc. Check with ohm-meter. In that case do not ground BOB additionally. Also any shielded wires connected to BOB must be connected to BOB's frame ground but not to anything else.

Frame ground and DC voltage negative wire frequently not the same and might not be connected together, in that case shields preferably connect to frame ground, not DC-

Wow, that's a lot to take in. I may have a ground loop to the machine frame. There is a ground wire in the spindle cable that is grounded to the motor frame, which is in contact with the rest of the mill. But I also do have a ground wire on the frame going to the ground bus in the breaker box. I will remove that.

There is continuity from the BOB end of the parallel cable to the PC when the PC power is disconnected, so it probably is shielded.

I currently have all the grounds and shielding grounds running to the ground bus in the breaker box. Are you saying this is causing issues? What would be the BOB's frame ground?

I have most of the electronics mounted to a piece of sheet metal, which is connected to main ground by the green/yellow terminal block by the contactor. Should I not do this?

Connecting all grounds to the buss bar sounds great.

One way to track down redundant grounds is to disconnect the known grounds then take an ohm reading to find out whether it is still connected to ground via an unexpected route.

Like I said, you can cheat some. I am not real strict on my projects about having no redundant grounds, but it is one of the things I work on if there are interference problems.

In your case, I would suggest concentrating on the shield grounds right at the BOB as that seems to be a pain point what with it acting up as soon as you so much as connect stepper signals and start the vfd.

Please, did you try moving those AC wires that are parallel to the BOB and within inches of it?

A couple years ago I bought two BOBs and never did anything with them. Just now dug them out for a closer look. So I have two different BOBs, both look different from yours. It seems to me, if your BOB had a ground screw that had direct connection to the metal shell of the parallel cable, that would be a prime location to connect the shields of any cables connected to the BOB. That is what I am referring to as frame ground.

zippoffs wrote:

I have most of the electronics mounted to a piece of sheet metal, which is connected to main ground by the green/yellow terminal block by the contactor. Should I not do this?

That should be fine. I mount a lot of stuff to DIN rail like that on top of metal plate, seems to me sheet metal should be about the same. I try to keep low voltage DC circuits completely separate from AC circuits. Sometimes it is inconvenient to do so then I run them together a bit like you did. And then sometimes it just doesn't work then I have to separate them.

It seems as soon as there are some electronics directly connected to a PC's parallel port or PCIe slots a bit like these BOBs work, things get a lot more sensitive. Lately I've been using mostly ethernet connected 'BOBs' Mesa electronics or Beckhoff etherCAT because I rarely have interference troubles with those. (ethernet has very good isolation)

I used a BOB some years ago on a cnc plasma cutting table retrofit, I was happy with it. I probably went to the extreme shielding that BOB. Mounted it inside a die cast aluminum box that cost more than the BOB. All shielded cords to the die cast box, and no AC power into it. You should not need to go to that extreme. Plasma cutters make more interference than most VFDs.

zippoffs wrote:

Do the 220v lines going from the breaker box to the VFD also emit interference?

Yes.
Not sure how I overlooked this part of your question previously.

zippoffs wrote:

Wow, that's a lot to take in.

 

Sorry, I think I made too big a deal about some of this. That is a nice mill you've got. With a few tweaks it'll be fine.