Setting up a mill control with conflicting grounds?

[Brian H.]

Hi everyone,

I need some help with one of my projects. Here's my current situation - I am in the process of converting a Sherline mill to CNC. Most of the mechanical work is done and I am working on figuring out appropriate electronics. I am using an Ah-Ha! interface card* in a 75MHz Pentium, which I want to use to drive a PMDX-126 breakout board. The manual for the Ah-Ha! controller says that certain pins need to be grounded at the breakout board end, presumably for voltage reference; it seems that the controller card does not reference the PC ground or +5V rail for those I/O lines. The PMDX board, having completely isolated power, also depends on certain pins to be grounded - at the PC end, again for voltage reference. So, each component wants to reference the other. Connecting both sets to an earth or chassis ground seems like it would be inviting all kinds of noise, and it seems to me that just connecting all the grounds to each other would leave them floating, which at the very least, is not ideal.

To be honest, this is my first go-around with 'back end' CNC stuff - I'm familiar with G-code, CAD, CAM, basic machine parameters, and such, that being my day job - but I don't know that much about the hardware that makes it all work. I've tried to read and understand as much as I can, but this is a little beyond my current knowledge...

If anyone has any suggestions, I'd be glad to hear them!

Brian

*Yes, the Ah-Ha! card and its associated PC are both antiquated technology. Personally, I find that to be part of the appeal. I may upgrade to linuxcnc (possibly on a raspberry pi) at a future date, but this will serve as a good introduction to hobby CNC. Oh, and the hardware was given to me for free, so there's that - I have heard that Ah-Ha! made very high-quality stuff.

[MaxHeadRoom]

The normal procedure when connecting Earth Grounds is to connect them to a central star point ground together with the service ground, this also includes the motor frames etc, The P.C. power supply is already connected to earth ground therefore the commons from the PC ports will be at E.G. The Siemens PDF also explains about the importance of equi-potential bonding.

http://www.automation.siemens.com/doconweb/pdf/840C_1101_E/emv_r.pdf?p=1

Max.

[robosilo]

typically stepper drives have isolated inputs and error outputs so you shouldn't need to worry about the ground plane for their power supplies. It seems odd to have only some of the I/O on the breakout board be isolated. But if you're really worried about it you could use an isolation transformer to power the breakout board and then have the ground line connected to the same ground line as your PC. If the breakout board has a DC input you could always tap power from the PC power supply to power the board. That would basically guarantee that the ground plane is the same. I used a dynamotion board to upgrade my little taig and it's powered and controlled by the USB port so i don't need to worry about ground planes(not to brag). I also made an optical isolator array for all inputs and used solid state relays for high isolated high voltage control of spindle motor and coolant pump. Solid state relays are cheap on ebay. They can be run off 5v outputs and draw very little current making them ideal for projects like these.

[MaxHeadRoom]

... and it's powered and controlled by the USB port so i don't need to worry about ground planes(not to brag). I also made an optical isolator array for all inputs and used solid state relays for high isolated high voltage control of spindle motor and coolant pump. Solid state relays are cheap on ebay. .

Any Optos fed or powered off of the P.P. or USB 5vdc is going to be at earth ground potential, due to the PC.P.S. common connected to earth ground.
Also the high voltage AC out is also usually at earth ground via the grounded neutral so in reality there is no galvanic isolation.
Just voltage transition.
Max.

[Brian H.]

Thanks for the replies and links. I haven't had much time to devote to this project lately, but I did finally figure out where I was going wrong.

The P.C. power supply is already connected to earth ground therefore the commons from the PC ports will be at E.G.

In my particular situation, the documentation for the breakout board assumes, naturally enough, that I will be using the parallel port for my I/O. That is not the case, however, and I discovered that all the I/O from my interface card on the PC is isolated. Therefore, I cannot ground those pins on the PC side. Instead, all the necessary pins will be connected to a common grounding point, on the breakout board end.

Thanks again!
Brian