shielded stepper motor cable or not?

[budman68]

Ok, I know you guys know your stuff so I've decided to just come out and ask. I been doing some checking all over the place and I'm getting conflicting stories wherever I look.

I'm putting together a small CNC taig lathe and have simple little stepper motors with 8 wires coming out of them. Do I really need shielded cable for these? I'm reading that I "should" have them but then I read that it will not be an issue as long as I have my limit switches shielded. Is this correct? The cables will not be close together except at the controller box where they'll plug in.

Don't get me wrong, it's not the problem of buying the shielded cable, just trying to understand what the issue actually is.
Thanks so much for your time-
Dave

[Chaoticone]

Dave, I would defiantly use shielded cable if at all possible. The problem is on most Mach machines, there I/O is 5 volts.  A slight bit of noise can bring a value high enough to be seen as high. 24V systema arent nearly as suspect to noise. 20% of 24 is 4.8 volts, 20% of 5 is 1 volt. Not a very big window. The big boys have this same problem as well, but they usually use something to isolate and convert the 5V to 24V for field wireing such as home switches and push buttons. This can be done as well in Mach and on the Hurco I'm working on that is how I will do it with Opto 22 racks. I will still shield everything though. Just too cheap an insurance policy to ignore. 

Brett

[budman68]

Thank you, that makes sense, Brett, will do on the shielding then- 

As for the shielding foil, I ground those together at the controller box and not the motor side to avoid a ground loop, correct? Also, I'm using a plastic project box, should I ground these directly to the AC ground (green wire) coming into the box?

Dave

[RICH]

Dave,
Yes, ground back at the controller box and not the motor side. Metal box would be better, but use it and you can always encase it with Al metal duct tape ( cheap alternative ) on the outside and you have a metal enclosure.

Here is some more confusion or conflict for your understanding.  

A shield is nothing more than a metal enclosure which provides a low reisistive path to ground. Lets use the analogy that the air around us is really a snow storm full of electromagnetic energy ( noise ( signals we can't understand ), TV, radio signals, high and low signals, anything sparking, etc. They are there but we can't see them. A shield will keep what's inside from going outside and vise versa. The shield can be totaly closed 100% or maybe have holes / an open braid say 50% which is open. Now that open braid shield can become closed if the snow is big ( so allowable openings, electricaly speaking are dependent on frequency if your trying to keep radiating ( signal getting out of shield ) or conducting ( signal getting past / into or onto the conductor inside the shield).  An accumilatiion of snow may not bother you while walking outside but if you never shook it off it may overpower your ability to walk. In order for the snow to stick it has to have a source ( something creating it/ motor, pulse frequency, transformer, kid playing with a sparkler ) and addtionaly it needs to stick to you ( conduction, unlikely induction, sometimes a combination of conduction and radiation ( rain and snow mix ). Now if your snow clothes ( shield) lets the snow go easily to ground you don't have to worry about it bothering your walking, but....  watch out if a whole bunch comes down on you at once from a tree ( overload / a motor turning on and generating a lot of noise), right on your butt you go ( computer locks up, signal interference , loss of or addititons to  the steppers). Now since your only walking back and forth and the snow falls equaly on each side of you ( common ground with equal resistance values) no big pile will form and interfere with the small pile of snow getting to the ground ( ground loop ) happens. 
 
No wonder you "E" guys tell us process guys to go back to our cubical!
  RICH

[budman68]

Man, I want a little of what you're smoking, my friend. That was great! I'm still trippin' from reading it 

But seriously, so I can line the inside of the case with a layer of good metallic duct tape and that should help? I have a fan that I was going to have suck cool air in to help keep it icey in there, is that a problem?

Thanks again, Rich- 

Dave

[RICH]

Dave,
If a junction box or just mounting a board inside something ok with lining it. You didn't say, but with a fan, I assume
you talking about a controller box and that one I would make out of metal or at the very least have a metal plate for one side of it. This way you have a nice metal conductor and it's not paper thin. The fan shouldn't be a problem as we have used ones for  PC's.
If the fan has brushes then your generate noise.  May or may not cause any problems. The opening for the fan should have a metal mesh screen which serves to keep something from getting into the controller but it also helps to keep an
electrical mechancal interference (EMI) from being conducted onto say a sensitive board inside. The screen is attached to make contact with the metal box and the box is grounded. The metal screen acts as a shield ( interferes with any electrical field trying to enter from outside the box) and it hopefully goes to ground. The screen openings  ?? 1/2 or 1/4 wavelength of the EMI frequency ??  Been a while since I was up on this stuff so hope I'm not to far off base.
Those "E" electrical guys know all about this stuff.

Maybe I'll post some pictures of RF, EMI pictures just for kicks. ( No it won't look like a snow storm )
RICH

[Chaoticone]

Great analogies on noise Rich. 

Brett

[RICH]

Thanks,
I was lucky on my end and didn't have any problems. Was concerned when I made a switch box for changing the resistor values used for the  Gecko drives. Have different motors among the machines and use the same controller. Also when I added another switch box so i could change between two different boards IE; PC to parallel port or serial port, that
box was also was plastic.

I think we little machines aren't as affected as compared to big machines but you never know. When you have a problem it's a real pain in the neck and solutions  can become " black magic fixes ". I would envision that there will more problems as time goes on because of BOB's , new pc's etc. So hopefully a "snow explanation"  will shed light on a difficult subject.

RICH

[budman68]

Thanks again for the info, Rich, the box has an aluminum front and rear panel (.078) which is nice and the 3 drives (only using 2 at the moment) and the BOB are mounted on an aluminum plate as well with standoffs. The fan is indeed a 12 volt PC fan so no worries about the brushes there and yes, it will have a metal grill over it on the outside of the aluminum rear panel.

Eventually I'll get pics up of this little joke of a project but it's a great learning tool for me. I was one of these guys who bought an all-inclusive Taig CNC because I wanted to make parts right away and not learn about how to do it until I was ready (time). It's nice to be able to work on it when I have some free moments and not have to worry about having to get it done to be able to use it because I'll probably screw it up anyway..... 

Dave