Question about limit switches & shielding


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    Default Question about limit switches & shielding

    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?

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    Member ger21's Avatar
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    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.

    Gerry

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    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



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    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.



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

    [URL="http://www.cnczone.com/forums/cnc_wood_router_project_log/124543-mountaincrafts_router_plasma_build_thread.html"][U]4' x 8' CNC Wood Router Project[/U][/URL]
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    Quote Originally Posted by Mountaincraft View Post
    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?



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    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.



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    Quote Originally Posted by Claytonc View Post
    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?

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    Community Moderator Al_The_Man's Avatar
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    Quote Originally Posted by Claytonc View Post
    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.

    CNC, Mechatronics Integration and Custom Machine Design

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    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.

    Last edited by neilw20; 07-30-2011 at 12:15 PM. Reason: my dumb grammar
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    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.



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    Community Moderator Al_The_Man's Avatar
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    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.

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    Quote Originally Posted by stewi View Post
    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.
    There are no warnings in the docs that came with my Gecko drives or my BOB to mount them on some sort of isolated pad or standoff.... In fact, from what I can see, the BOB standoff pads appear to be part of the larger ground network on the card and is indeed tied to the other ones....

    I've never seen anyone try and isolate their gecko drives when mounting them... They come with four indents for hold down screws and no mention or warnings about using nylon screws or pads between the base of the drives and any conductive surface they might be attached to...

    I can see no reason why I should isolate the heat sink from the can....

    [URL="http://www.cnczone.com/forums/cnc_wood_router_project_log/124543-mountaincrafts_router_plasma_build_thread.html"][U]4' x 8' CNC Wood Router Project[/U][/URL]
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    Quote Originally Posted by Mountaincraft View Post
    There are no warnings in the docs that came with my Gecko drives or my BOB to mount them on some sort of isolated pad or standoff.... In fact, from what I can see, the BOB standoff pads appear to be part of the larger ground network on the card and is indeed tied to the other ones....

    I've never seen anyone try and isolate their gecko drives when mounting them... They come with four indents for hold down screws and no mention or warnings about using nylon screws or pads between the base of the drives and any conductive surface they might be attached to...

    I can see no reason why I should isolate the heat sink from the can....
    There are at least two reasons why you shouldn't tie the heat sink to ground. First, your board has likely an IC voltage regulator on board, which has its own heat sink. This heat sink is not ground, but carries the regulated voltage. Do a continuity test from heat sink to heat sink.
    Second, the power MOSFET's are attached to the heat sink using thermally conductive epoxy or aluminum oxide, which is not electrically conductive (the screw is). If you connect the heat sink to ground and there is a small voltage drop from the IC heat sink to the aluminum heat sink fins, than we created a power source and destroy the MOSFET's.
    I don't know anything about your particular Gecko drive (maybe I get lucky and win one in the raffle), but if the supplier does not particular say the heat sink must be grounded, I wouldn't.



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    Community Moderator Al_The_Man's Avatar
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    Personally I would think it very irresponsible for a drive manuf. to include mounting holes on the product that should not come in contact with ground.
    I would assume the Gecko's are intended for a enclosure back plane mounting which normally should be connected to Earth Ground.
    If this were not the case, then they would be derelict in not supplying insulated stand offs?
    Al.

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    Quote Originally Posted by Al_The_Man View Post
    Personally I would think it very irresponsible for a drive manuf. to include mounting holes on the product that should not come in contact with ground.
    I would assume the Gecko's are intended for a enclosure back plane mounting which normally should be connected to Earth Ground.
    If this were not the case, then they would be derelict in not supplying insulated stand offs?
    Al.
    I'm not disputing the board mounting holes, but the IC heat sinks.
    If there is no continuity between the aluminum cooling fins of IC's and ground, we should not create one.

    Last edited by stewi; 07-30-2011 at 05:38 PM.


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    LOL... I'm not going in and opening the drivers and looking for Triad heat sinks and grounding them, nor doing anything like that with the BOB.. I bought a third party Big ole heat sink (probably 9" x 12" x 2") and mounted all my drives on them as per recommendation from gecko, and I also used metal standoffs to mount the BOB to the same heat sink... The heat sink is then acting as a backboard (as per Al's scenario), which is bolted to the stainless steel box (using long 1/4" bolts as standoffs), on which is a ground post where basically everything else is grounded to...

    Below is a link to an image of the control box.. sorry the WSIWYG editor no longer works for me so I can't attach images.. seeing as I can't remember what the hand code looks like, and I have to use the quick reply window...
    http://www.r8rphan.com/cnc/pix/cnc_build_39.jpg

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    Thanks for clarification. Must be my English. I looked at the Gecko website and realized, that the large heat sink actually contains the mounting holes.
    My only critics on this design is that the heat sink is likely mounted upside down, so your idea of mounting everything to a larger metal surface is quite alright.

    Regards,
    Stefan



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Question about limit switches & shielding

Question about limit switches & shielding