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Thread: Have almost all my parts (pics), need servo advice

  1. #21
    Registered HomeCNC's Avatar
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    Just so you know. Limit switches or any software E-stop will NOT stop a run away servo that has lost encoder connection. This is so with the Gecko drives. (Other drives, I don't have experience with).

    Limit switches and software e-stops are only good to stop steps from being sent to the driver. A run away servo must have the power cut! Or in the Gecko's case I was able to hit my "disengage" switch, which faults the drives.

    Thanks

    Jeff Davis (HomeCNC)
    http://www.homecnc.info


    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


  2. #22
    Registered balsaman's Avatar
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    This is true. Estops by law has to be HARD WIRED to cut the power to the machine. Another contact on the estop switch can be used to notify the computer that something has gone teribly wrong.

    Eric

    I wish it wouldn't crash.


  3. #23
    Gold Member Bloy2004's Avatar
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    Ah! How about adding switches directly in series with the motor power cables. I don't want a multitude of wires traveling about the machine, but if this will work for preventing that "runaway", should it ever occur, maybe they can be discreetly hidden. Are there any other 'cons' to doing this?
    While on the subject, are there circuits that could be utilized that would not only cut the power, but instead, brake the motor?

    Balsaman:
    Above you asked "damage to what?"......
    Mainly damage to the machine...or body parts. Dan at Camtronics noted that servo motors are "less forgiving". I think this is what he meant.

    ---------------------------
    A paranoid limit student
    John

    Last edited by Bloy2004; 12-20-2003 at 09:58 AM.


  4. #24
    Registered balsaman's Avatar
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    Use regular limit switches in series to the computer. make sure you hardwire a e-stop pushbutton that cuts all AC power.

    Eric

    I wish it wouldn't crash.


  5. #25
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    My entire e-stop circuit is low voltage. I use a solid state relay to power the PSU, with the ground connected to the e-stop button. If you check Balsamans router thread, you will see his err/res manmager circuit. I have used this as the e-stop input to the computer, and have a 5v line which is switched on with the PSU that goes into this circuit as a "ghost" gecko drive if that makes sense. Again, the ground from that 5v PSU goes to the e-stop button, so when it is pressed, the PC knows it is in e-stop. Also, if any of the drives fault, the software knows about it. The third thing I have implemented is Mariss' idea of using a large resistor to "suck the PSU caps dry" which is levered across the PSU terminals in the event of an e-stop. If you look at the CAD_CAM_EDM_DRO boards file section, you will see it in there as "e-stop.pdf". Of course, I had to alter it a bit to use a low voltage system. This will aid in braking the motors quicker coz the cap isn't still supplying them with power. Hope it helps, but I think I'm rambling a bit! If you check that estop circuit, you will also see how the "latched relay" is used to turn on the power, but cuts out if the power is lost. This will prevent your motors coming back on when you release the e-stop button.



  6. #26
    Gold Member Bloy2004's Avatar
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    I think I understand the use of limit switches through the software, but whats going to stop the motors if communication is lost and I'm not within reach of any mechanical cutoff. I'm sure that during an operation I might run to get a beer or something and if it(the malfunction) occurs during this time, well.....a metal machine can really tear things up.

    -------------
    a guy without 20/20...or that trees and forest thing.
    John

    Last edited by Bloy2004; 12-20-2003 at 10:30 AM.


  7. #27
    Gold Member Bloy2004's Avatar
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    Thanks Kong, I was typing my post while yours was being entered. this answers a lot of questions.



  8. #28
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    Breakout board sounds more expensive than it needs to be, is this optically isolated? Here's one for $5
    http://www.embeddedtronics.com/breakout.html

    Originally posted by ger21
    I would think a good power supply is more like $150-$200. Breakout board = $125
    Decent cables can be a lot more than nothing.
    That box on Jeff's site has a pretty nice case, more $$.

    Gerry


    http://www.embeddedtronics.com/
    Robotics, CNC, and Controllers


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Have almost all my parts (pics), need servo advice

Have almost all my parts (pics), need servo advice