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  1. #1
    Join Date
    Aug 2005
    Posts
    6

    Servo Motor question

    This may be a silly question but I am new to this. How come nobody is using servo motors on these X3 conversions? I thought a closed loop system was surperior to stepper motor systems.

    Jason

  2. #2
    Join Date
    Sep 2004
    Posts
    412
    Cost.... usually.

    Servo's cost more, and the controllers for them.

    Alot of people are of the opinion that with the new stronger steppers. You don't miss steps and don't need to worry as much about the servos.

    I have servos for my x2, but they are in a box somewhere, waiting for my garage to be unpacked.

  3. #3
    Join Date
    May 2005
    Posts
    240
    One thing I don't understand about servos is how they keep a position. Do they just do a balancing act or what?

  4. #4
    Join Date
    Sep 2004
    Posts
    412
    There is an encoder on the end of the shaft.

    It's a two part device. One part consists of a disc with ultra small index marks. Then other is the encoder which reads where those marks are and if they have moved.

    If it detects a move that wasn't expected the driving software will readjust the servo back to where it's suspose to be.

  5. #5
    Join Date
    May 2005
    Posts
    240
    The thing I don't get is wouldn't this lead to inaccuracies? Or is the encoder that sensitive to where that movement wouldn't matter?

  6. #6
    Join Date
    Sep 2004
    Posts
    412
    I'll let you know when I get mine going.

    *grins*

    However I believe that you set the amount of backlash with in your system. Then it takes that into account.

  7. #7
    Join Date
    Dec 2003
    Posts
    24216
    With say a 1000 line quadrature encoder the quadrature edge pulses are often detected to increase the resolution x4 so now you have 4000 counts/turn. say with gearing or ball screw, 1 inch represents 4 turns of the encoder, this will then result in a resolution of 16000 counts/inch = 0.0000625/count.
    If the position error is 1 or 2 pulses, then you can see the kind of error or accuracy you would have.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
    Join Date
    May 2006
    Posts
    22
    I have servos on my X3 and they are working really well. The motors cost quite a bit more but the drives are about the same (assuming quality parts like Geckos). I really overkilled it on mine, have 400oz on X & Y and 800oz on the Z. I feared it would not be enough torque to lift the Z in rapids without a spring attachment but it has enough to lift the head plus a buddy off the ground hanging on for dear life!

  9. #9
    Join Date
    Jun 2005
    Posts
    866
    Your steppers will only miss steps when they are not able to supply the necessary torque. Get motors powerful enough, and it will be okay. With ballscrews, the need is greatly reduced still. You will see the kits available all make use of 23 frame motors, and I believe have good results.

  10. #10
    Join Date
    May 2005
    Posts
    240
    Mac why haven't you installed a gas spring on it? That was the easiest part for me.

    I wouldn't dare leave the Z unlocked without the gas spring. Do you have to lock it?

  11. #11
    Join Date
    May 2006
    Posts
    22
    LOL
    I should have, could of used smaller servos!

  12. #12
    Join Date
    May 2006
    Posts
    22
    The Z gib is tight enough to keep it from falling (usually...).

  13. #13
    Join Date
    May 2005
    Posts
    240
    Heh

    I was scared to install the Z conversion and ballscrew without the gas spring. I figured it'd just slam down if the gib wasn't locked. I installed the gas spring before anything else.

  14. #14
    Join Date
    Apr 2005
    Posts
    403
    JL123,

    Don't forget, although the effeciency of the ball screw and ballnut system works against the holding force of the Z axis, there is enough to prevent the head from moving too rapid. I suspect that the downward travel would simulate closer to creeping down vs. slamming down.

  15. #15
    Join Date
    Jun 2004
    Posts
    445
    Hi there,

    I'm keen to use servos on my next mill (which is still many many months away...), but one of the biggest problems I have found so far while just looking around, is that servo motors are difficult to find. I know people use DC motors from places like the Surplus Centre (I think that's what it is called) and add encoders, but what other options are there?

    I've come across 'proper' servo motors, but those are expensive. Most of the servos I've seen have been from electronic component suppliers and they run well into the hundreds of Pounds each - too much for my hobby use.

    Where on earth are reasonably priced, decent servo motors found (for something like an X3)?

    Regards
    Warren
    Have a nice day...

  16. #16
    Join Date
    Apr 2006
    Posts
    402
    This seems to be the general German solution:
    http://www.dunkermotoren.de/default.asp?lang=2

  17. #17
    Join Date
    Jun 2005
    Posts
    866
    I think that DeskCNC sells them, also Homecnc.info sells some. Smaller DC motors with encoders are pretty much it though

  18. #18
    Join Date
    Apr 2006
    Posts
    844

    Wink

    Quote Originally Posted by Deviant
    Cost.... usually.

    Servo's cost more, and the controllers for them.

    Alot of people are of the opinion that with the new stronger steppers. You don't miss steps and don't need to worry as much about the servos.

    I have servos for my x2, but they are in a box somewhere, waiting for my garage to be unpacked.

    could your post your servo x2??
    thank you..hehe

  19. #19
    Join Date
    Sep 2004
    Posts
    412
    My x2 never got servo'd. Everytime I start, life kicks me in the butt.

    I think I'm close now, but looking at ordering a taig to remove the possibility of another conversion delay.

    But, I have almost everything for it.
    x2 mill.
    3 250oz servos with 250ppr encoders.
    3 gecko 320s
    1 Dual output 25v 400a toroid
    1 600v 35a bridge rectifier
    120000 UF 80v Capacitor.

    I have all the materials to complete Steele's conversion.

    Need to pickup a break out board and plate to mount everything on.

    Albeit, I'm torn on the taig. I like the travel and the fact that it's ready to go out the door. More or Less. But it looks dinky in all the pictures. More so compared to the x3. I really won't to get something made.

    I think I've invested over 11k in trying to make something now. Between the lathes, mills, building my old shop running power. Then I changed jobs and moved. Got everything in the garage now on my table waiting for the chips to start flying.

  20. #20
    Join Date
    Jan 2006
    Posts
    481
    Quote Originally Posted by MacCNC
    I have servos on my X3 and they are working really well. The motors cost quite a bit more but the drives are about the same (assuming quality parts like Geckos). I really overkilled it on mine, have 400oz on X & Y and 800oz on the Z. I feared it would not be enough torque to lift the Z in rapids without a spring attachment but it has enough to lift the head plus a buddy off the ground hanging on for dear life!

    Hi MacCNC

    what are the chances of some pictures of your setup
    and some details on the conversion.

    where did you buy the servos from ??

    I think everyone is after DC servo motors but the biggest killer is the expense

    cheers

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