What effect does microstepping have on torque? - Page 3


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  1. #25
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    The attached gif green trace shows an MCG IH23014 motor's cyclic error. The green trace scale is 0.0625 degree per volt and the motor speed is just under 1 rev per second (yellow sync trace).

    The peak to peak error amplitude is 1.3V for a 0.081 degree P to P cyclic error. It is approximately sinusoidal (as expected), giving a +/-0.04 degree non-accumulative error. This is twice as good as the guaranteed +/-0.09 degree (+/-5% of full step) motor specification.

    The test was run with a G201, the motor equipped with a 1,000 line encoder and a G100 generating the step pulse rate.

    The encoder was taken to the G100 and the G100 axis lead-lag register was ported to a G100 DAC output.

    The 10Hz, 100mV "waviness" can be attributed to the encoder accuracy limits.

    Mariss

    Attached Thumbnails Attached Thumbnails What effect does microstepping have on torque?-tek00013-gif  


  2. #26
    Registered Xerxes's Avatar
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    How can you be sure it is stepper error and not encoder error? Even small runout of encoder codewheel will make cyclic sinusoidal error.



  3. #27
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    This is a calibrated (and at $800 a very expensive) Canon instrument-grade laser encoder. It ouputs a sin/cos interpolatable to an accurate 50,000+ counts per revolution (+/- 0.003 degrees spec'ed). Not what you would ordinarily hang on the end of a $60 motor.

    Mariss



  4. #28
    Registered Xerxes's Avatar
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    A very nice encoder indeed :-)
    I'd wish those sin/cos encoders were more common.



  5. #29
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    What's nice about even quasi-sine encoders is they completely eliminate PID dithering or hunting when a motor is stopped or turning slowly. The improvement is very noticable. Also you can apply a micro-position adjust summing signal to move the motor to any location between the quadrature limits.

    Mariss



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    I assume you always mount that encoder before performing the "Mariss PImp slap mid band resonance test" ????? :-)


    Bill



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    USDigital encoders with mylar disks yes. The Canon encoder no. The pain of breaking the chromed glass disk would be more than I could bear.:-)

    I like the technical sounding name for that test. May I borrow it?

    Mariss



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    Sure you can borrow it it, I would be honored, I would think it should be come an acronym.

    One of my favorites for other uses is about firearms and gear

    A.T.A.S. = All Tactical and Shot (well not shot hehe)

    "Mariss PImp slap mid band resonance test" ????? :-)

    MPSMBRT ... It may take some effort to make the test consistant, times when there is no coffee in the building on a monday morning for example there may be more ft lbs of energy delivered to the motor :-)

    Bill



  9. #33
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    Because English cannot understand it, I use an interpreter.
    I cannot express it definitely.


    By my experience, there is the case that objective torque does not come out by the rule electric current depending on the form of the driver IC in the case of the uni-polar motor.
    There is the case that an electric current spreads in the reverse aspect from a theory.
    When I called the driver IC maker, and it heard it, it was an answer to say to an electric current of the about √ double when I set it.
    I am careful to temperature rises (less than 80 degrees C), and please increase electric currents slowly.
    The temperature reaches equilibrium about 2 hours later.
    When a motor is installed to a metal frame, radiation of heat is simpleness.
    When a sound changed suddenly, the motor has magnetic saturation をの possibility and cannot expect more than it.
    A loss by the contents of this sentence is responsibility of your own. (I do not guarantee it)



  10. #34
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    So the conclusion here may be that microstepping is good and useful at low speeds but when the speed increases (maybe higher than 5 steps per second?) it has a very negative effect in the Torque. is that right?

    I've found this table, but from what i understood in this post, this valid for higher speeds than 5steps per secnd.

    Microsteps/full step Holding Torque/Microstep

    1 100.00%
    2 70.71%
    4 38.27%
    8 19.51%
    16 9.80%
    32 4.91%
    64 2.45%
    128 1.23%
    256 0.61%



  11. #35
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    Quote Originally Posted by CJL5585 View Post
    I believe the above posts by Mariss on microstepping motor operation should be made a sticky note which should be available under a stepper motor operation forum.
    Jerry
    Hi Jerry and all,
    I am sorry my post is not directly related to the content of the thread but i 100% agree with Jerry about keeping this valuable information from Mariss sticky.
    Mariss, why not you write a thesis and present it in the Automation world.
    Regards

    http://free3dscans.blogspot.com/ http://my-woodcarving.blogspot.com/
    http://my-diysolarwind.blogspot.com/


  12. #36
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    bbp,

    Your table of torque versus microstep resolution is somewhat misinterpreted. Holding torque is defined as the torque a motor exerts when its shaft is displaced +/- 1.8 degrees from its rest position.

    The relationship between a stopped motor's torque versus its shaft angle is sinusoidal over the span of 1 full step and your table reflects this relationship.

    Let's say you have a stopped 300 in-oz (about 2.1 Nm) motor. You apply a torque load that displaces the shaft 1/10 of a step (0.18 degrees) from its unloaded rest position. The applied torque equals 300 in-oz times sine (90 / 10) or 47 in-oz (1/3 Nm).

    This is different than holding torque. Remove the 47 in-oz load and the motor returns to its original rest position. Exceed the 300 oz-in holding torque of the motor and it will jump to an adjacent pole location 7.2 degrees (4 full steps) away. It will not return to the original location when the load is removed.

    What this means practically is the motor will always be 0.18 degrees behind where you think it is with a continuous 47 in-oz load and it will be 1.8 degrees behind if you apply a continuous 300 in-oz load. Exactly the same numbers apply for a full-step drive as well. Remove the load and the motor will spring to the zero error location whether it's being microstepped or full-stepped.

    ---------------------------------------------------------------------

    Things get more interesting at higher speeds. The motor's inductance introduces a 90 degree phase lag in winding current versus driving voltage. This results in an unloaded motor being 1.8 degrees behind where you think it is at higher speeds and 3.6 degrees behind when it's loaded to just short of stalling. This behavior is drive-independent and the error is "reeled-in" when the motor slows down again.

    Mariss

    Last edited by Mariss Freimanis; 10-20-2011 at 10:57 AM.


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What effect does microstepping have on torque?

What effect does microstepping have on torque?