Question about limit switches & shielding

[Claytonc]

I understand now (after wiring) that limit switches should use shielded cable. If the wires going to the limit switches/touchplates do not run parallel with the stepper motor wires do they still need to be shielded? I'm using NC connections for all. If so why?

[ger21]

It really depends on the specific application, and sometimes can't be answered until you try it. Every machine can act differently.

I don't use shielded cable, and all my wires run in the same carrier - Router, steppers, and switches. And no problems.

[dfmiller]

Gerry,
Are you running a G540? The inputs are optically isolated on them. So you don't really have many issues. You might have to put in some debounce.
The Commercial Prox switches I am using on my G540 come with non shielded molded cables. That must means that many commercial applications don't use shielding. I could not easily locate shielded cables.


Another issue can be if your cables are flexing the standard foil shielding will break down over time and lose it's effectiveness.

I would suggest using twisted pair and wiring them using a pair or wires instead of a daisy chain configuration or a huge loop you might be OK.

As Gerry says it all depends.

Dave

[Claytonc]

I ordered some wire anyway. I would rather eliminate any potential problems now rather than try and debug and fix them later.
I have a small CNC (12x12 Zentoolworks) that I've been running for a while with unshielded limit swithches and have had no problems at all. But for this new build I would rater be safe that sorry
Thanks for the advice.

[Mountaincraft]

I'm using Cat 5 cable and have no issues...

[Claytonc]

Originally Posted by Mountaincraft I'm using Cat 5 cable and have no issues...

I was going to use cat 5 but just didn't have any laying around. Not only that but I didn't want all the extra wires. I was able to order 60' for $22.25 shipped. Its only 2 conducter w/drain but its much smaller and is going to work well. As it is now, the 14ga speaker wires I'm using for my limit switches are running threw some black split tubing and its pretty tight. This will loosen things up a bit and make this a more (hopefully) reliable machine in the long run. I plan on using this for all limit switches, E-Stop, and touchplate connections.

I do have a question about the drain wire though.. Should it be connected to the power supply ground?

[stewi]

Twisted wires should prevent stray signals through the motor fields. However, if you have ESD problems and fry your electronic by touching a limit switch, you should consider shielding.

[Mountaincraft]

Originally Posted by Claytonc I was going to use cat 5 but just didn't have any laying around. Not only that but I didn't want all the extra wires.

I like having the extra wires there.. It seems that philosophy has come in handy more than once when I 'add' something later, and the 'spare' wiring is already in the vicinity...

I do have a question about the drain wire though.. Should it be connected to the power supply ground?

I've pretty much grounded everything together electrically Power supply, BOB, heat sinks, computer, second computer power supply, motor wiring shields, etc.. all tied to a post on the CNC control box can (directly or indirectly)...

Doesn't your BOB have common grounding connections?

[Al_The_Man]

Originally Posted by Claytonc I do have a question about the drain wire though.. Should it be connected to the power supply ground?

The shields should be taken back to the common Earth Ground point, whether the P.S. common is at earth ground or not, as in your PC Power supply.
Al.

[neilw20]

If you use shielded wire only connect the shield at ONE end of the cable.
This implies you can't use the the shield as a signal return.
Best to earth the shield at the controller common 0v point or the machine frame.
The strategy with the shield (mostly) is that if it is connected at ONE end then it is not possible for current in adjacent wires to induce voltage along it's length.
Keep in mind that the shield can/does act as a radio antenna, so earthing it is important.

There are other issues with the capacitance of the shield to the wires, but for most applications, like limit switches this is usually not a problem.

If the CNC is a plasma cutter, you can induce large voltages due to the high currents involved, so much greater care in selecting ground points is needed.

[stewi]

Power supply, BOB, heat sinks, computer, second computer power supply, motor wiring shields, etc.

Grounding heat sinks? You are looking for trouble! They are not always ground.

[Al_The_Man]

Although for quite some time, it has always been recommended to ground one end of the shield, there is an IEC paper out there that now stresses the practice of earth bonding conductors to all metallic parts of the machine, when this is done to eliminate ground loops, it states if practiced, the shield can be grounded at both ends.
Details can be found on the Siemens and Kollmorgen sites.
BTW, unless you are using a Laptop, it is pretty certain your PC P.S. is connected to Earth Ground through the M.B. Your PC case should be bonded to the central E. GND point.
Al.