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  1. #1
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    May 2020
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    Unhappy NEMA34 Stepping issues, HELP!!

    Hi everyone. I'm brand new to the boards. Looks like a great community

    I have a project where I'm using 2 NEMA 34 stepper motors to drive a 2 axis machine. To cut a long story short, it's a rig designed to scan artwork in high resolution.

    In testing my stepper motors using Mach3, I've noticed they're very 'crunchy' or seem to be missing steps or badly timed steps. I'm not a big tech guy, and at this point just need them running nice and smooth. I don't need major accuracy or repeatability (for now) as I'm not using it for precision CNC work. I just want smoothness and speed. Motors also seem to stall prematurely if going any faster than in the video.

    See video here (sorry, it's terrible but hopefully enough to inform of setup): https://youtu.be/9tiq8ZpfSUA

    I've spent a fair bit of time trying out different combinations of micro-stepping/speed/acceleration etc. but with no luck yet. I'm thinking it's either the controller, the breakout board OR the parallel port on the PC. I know these are relatively cheap components, but would at least expect smooth operation.

    Parts:
    - Breakout board - https://www.ebay.com.au/i/3629843819...MaAoS4EALw_wcB
    - Motors/drivers/power supply https://www.ebay.com.au/i/1232007868...YaAqXeEALw_wcB

    So far:
    - Measured power supply voltage. 60 volts
    - Driver amps set too low on stepper driver? - NO - set to 6 amps
    - Check wiring for motor - yep, should be good.

    Any help would be much appreciated! Many thanks in advance.

  2. #2
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    Re: NEMA34 Stepping issues, HELP!!

    Hi,
    those steppers have great torque and at 5mH by no means the worse inductance I've seen. They are prone to midband resonance however.
    There are a few posts here about that exact problem. You would probably have more succes with smaller 23/24 size steppers of 350-450
    oz.in and less than 2mH inductance.

    Craig

  3. #3
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    Re: NEMA34 Stepping issues, HELP!!

    Quote Originally Posted by joeavaerage View Post
    Hi,
    those steppers have great torque and at 5mH by no means the worse inductance I've seen. They are prone to midband resonance however.
    There are a few posts here about that exact problem. You would probably have more succes with smaller 23/24 size steppers of 350-450
    oz.in and less than 2mH inductance.

    Craig
    Spot on.
    Mine were 3.5mh 1090oz/in 5.6A and they were as bad. The machine did not perform that well and Iwas set at 7A.
    I've kept everything except the motors.
    I've changed to Nema23 566oz/in 4A with 3mh inductance. It's a HUGE improvement.
    Still running 60V. Set at 4.2A atm. Have no heat problems yet but still tweaking. Can drop it further as it is plenty quick.

  4. #4
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    Re: NEMA34 Stepping issues, HELP!!

    Thanks for the info! Suuuurely there's a way to drive NEMA34s without such a major issue? It seems to be happening at just about all the speeds I run it at and without any load. I can't believe everyone with NEMA34s has jagged movement? Also I don't really have the option to change out the motors - everything measured to fit and I have a deadline (certainly can't wait for the 6 weeks delivery time!) I'd like to treat getting different motors as a last resort. The problem I want to solve is mechanical vibration due to the issue - accuracy or missed steps in themselves are not a problem for my use case.
    Last edited by henhen83; 05-15-2020 at 10:54 AM.

  5. #5
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    Re: NEMA34 Stepping issues, HELP!!

    Hi,
    the technical reason for the vibration is 'inertial mismatch'

    The 34 size servos have a substantial rotor with a lot of rotational inertia. If they are coupled to a low rotational inertia load
    they will exhibit a mid-band resonance. If the same motor was hooked to a much heavier ballscrew with commensurately higher
    rotational inertia the resonance would not ocurr.

    You may have noticed that servo manufacturers offer different model servos but all thne same power, say 750W. One model will be low inertia
    with another model being medium or high inertia. The intention is that the machine builder will select the model required to give the optimum
    inertia ratio.

    Its not talked about with steppers, we are by and large amatuers, and inertia ratios are not easy to characterise and use intelligently but the
    same physics applies to servos and steppers.

    Craig

  6. #6
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    Re: NEMA34 Stepping issues, HELP!!

    Quote Originally Posted by joeavaerage View Post
    Hi,
    the technical reason for the vibration is 'inertial mismatch'

    The 34 size servos have a substantial rotor with a lot of rotational inertia. If they are coupled to a low rotational inertia load
    they will exhibit a mid-band resonance. If the same motor was hooked to a much heavier ballscrew with commensurately higher
    rotational inertia the resonance would not ocurr.

    You may have noticed that servo manufacturers offer different model servos but all thne same power, say 750W. One model will be low inertia
    with another model being medium or high inertia. The intention is that the machine builder will select the model required to give the optimum
    inertia ratio.

    Its not talked about with steppers, we are by and large amatuers, and inertia ratios are not easy to characterise and use intelligently but the
    same physics applies to servos and steppers.

    Craig
    Yep.
    Here's my view on it in commoner words lol.
    Basically they are not working hard enough.
    There's very little load on them making them effectively freewheel.
    This is then translated into rotor vibration and sent through the machine.

    I've left one of my 1090oz 5.6A ones on my Z axis and still it says it's 'too light'.
    Got it at 60V, 7.8A and it's freezing cold. It's not too noisy but it stalls early at 2000mm/min.
    When/if I do change my Z I'll get around an 800oz closed loop if I can and toroidal supply.
    Atm the limited speed I get isn't an issue.

  7. #7
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    Re: NEMA34 Stepping issues, HELP!!

    Thanks everyone. This has been very informative. When I designed my machine, I just went with the old 'bigger is better' mentality. In this case, clearly that was an oversight! Looks like back to the drawing board with the motors. In anyone's opinion, would a 4.5nm NEMA 34 such as this one https://www.ebay.com.au/itm/1-Axis-C...IAAOSwjYleIAtf

    Solve my problem? As at the moment I'm on the much chunkier 12nm https://www.ebay.com.au/itm/1-Axis-S....c100008.m2219

    Being able to use the NEMA34 faceplate dimensions would save the headache of redesigning a bunch of things. Would I be able to get much more speed from it or do I just need to go down to a NEMA24/23?
    Last edited by henhen83; 05-17-2020 at 10:19 AM.

  8. #8
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    Re: NEMA34 Stepping issues, HELP!!

    Hi henhen - You could put a flywheel on it to increase the inertial load. What does your machine look like? Peter

  9. #9
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    Re: NEMA34 Stepping issues, HELP!!

    Hi,
    the 34 size 4.5Nm still has 3mH inductance. Do you really need 34 size? I doubut it. you want 23/24 size of 300=450 oz.in and less than
    1.5mH.

    Craig

  10. #10
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    Re: NEMA34 Stepping issues, HELP!!

    Quote Originally Posted by joeavaerage View Post
    Hi,
    the 34 size 4.5Nm still has 3mH inductance. Do you really need 34 size? I doubut it. you want 23/24 size of 300=450 oz.in and less than
    1.5mH.

    Craig
    In the uk it's almost impossible to get decent steppers below 3mh. We just have some of the crap here.
    These are prob a good route for my next options:
    https://www.cnc4you.co.uk/Stepper-Servo-Kit-4Nm

    I have these below and have turned out to be a good investment so far. They replaced my X&Y nema34's, it's a huge improvement.
    https://www.cnc4you.co.uk/Stepper-Mo...H401-03-Nema23

  11. #11
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    Re: NEMA34 Stepping issues, HELP!!


  12. #12
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    Re: NEMA34 Stepping issues, HELP!!

    Hi,

    Object reference not set to an instance of an object.

    This one is still 23 size but lesser torque at 300oz.in but only 0.75mH. Look at the torque/speed curve, it retains 50% of rated holding torque at 1000 rpm
    with only a 48V supply, I imagine it would have 50% torque at 2000rpm with 80V up its chuff!!

    Craig

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