What effect does microstepping have on torque? - Page 2


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Thread: What effect does microstepping have on torque?

  1. #13
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    Mid-band resonance cannot occur while the drive is in current mode. Increasing the supply voltage increases the 'corner speed' of the motor where the change from current mode to voltage mode occurs.

    Mariss



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    Default TO Moderator and Administrator

    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.

    This is the type info that does not need to be buried somewhere in some post, where one has to search and maybe find it.

    Is there a possibility that the Moderators and/or Administrator could insert info like this into a special CNC INFORMATION forum which members cannot post to?

    This info is invaluable to members for reference and also to new members who are making an effort to learn about CNC fundamentals. I think it would be a resource which is invaluable, and would be a definative source on steppers and micro-stepping. It would also be great to have such posts on drivers, leadscrews, etc.

    Jerry



  3. #15
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    Testing your motor and drive for mid-band resonance:

    1) Stet the motor on its side on a hard, flat surface. This can be a desktop; make sure there are no papers under the motor. It must be a hard surface.

    2) Run the motor up to a speed where the drive is in voltage mode. This speed will be 5 to 15 revs per second depending on your motor inductance and supply voltage.

    3) Pivot the motor up 1/2" or so from the desktop using one corner of the motor's mounting flange as a fulcrum. Keep the fulcrum flange corner in contact with the desktop.

    4) Rotate the motor back down sharply (with a bang) onto the desktop and press down on it firmly.

    5) The motor will immediately break out in mid-band resonance if the drive doesn't have compensation. It will make a warbling or growling sound and then probably stall in a second or two if you continue to press down firmly on it.

    Nothing interesting at all will happen if the drive has mid-band compensation.

    Mariss



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    I have a couple of questions, maybe someone can comment.

    I have an assortment of step drivers, and many of the high end ones have step resolutions of up to 256 microsteps/step. What are the practical applications of such high resolution? Are there any situations in a typical CNC set up that would require or benefit from these high resolutions ( above 16 ustep/step )?

    Some of these drivers have on the fly or programmable resolution selection. They can change resolutions at any time by recieving a binary coded signal from the indexer. Has anyone ever implemented this feature with MachXX or TurboCNC? How would you ( could I ) do this? I would like to have my drives switch at some RPM from micro to half step if it is practical, but I dont have the skill or time to write a custom application.

    Thanks

    Halfnutz

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


  5. #17
    Registered Xerxes's Avatar
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    1/256 stepping could be totally inaudible at low speeds. Stepper accuracy is still only +/- half step so it wouldn't give much more benefit over 1/8 step.

    Variable step size CNC control would be cool though. Would help those with 8000 CPR encoders on servos.



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    My stepper drives have a setting for 1/25,000 microstep! I would guess this kind of fine movement might be used to position mirrors or maybe dispense chemicals through a piston pump. Something like that. Certainly not on a cnc scale.

    Changing the microstep setting on the fly might allow very quick rapids and then slower higher-resolution cutting moves. Hard to say on the accuracy though.

    Scott



  7. #19
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    A typical step motor has a +/-5% non-accumulative tolerance specification. This means a full step will be 1.8 degrees, give or take 5% and the error is cyclic, meaning it cancels to zero for 1 full revolution. +/-5% means a 10% error band so the motor's accuracy is 1/2,000th of a revolution and practice bears this out.

    Any microstep resolution beyond 10 gives no additional accuracy, just empty resolution. The only uses for higher resolution are slightly smoother motion below 10 full-steps per second or the drive is used closed-loop.

    Mariss



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    Thanks for reply's. I geusse that brings up the next question though, how can I ( we ) set up a closed loop stepper system? Are there any examples of a closed loop stepper system anywhere that could be implemented with standard CNC H.W. and S.W.? Ive been looking everywhere but cant find an application described thoroughly enough to use as a design guide. I have a couple of awesome 900 oz/in PacSci steppers with encoders on them but I cant figure out how to utilize them.

    Halfnutz

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


  9. #21
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    I'm working on a step-motor PID closed-loop algorithm that will be implemented in the G-Rex firmware. About 6-months or so; it's a tough nut to crack but it will be worth it.

    Mariss



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    Well,it will be worth it. Im sick of going slow, and trashing work trying to go fast.

    I,m sure if you do it (Marrise), it will be done well. I will continue poking around in the meantime. I know error checking and zero detection are being done, but the applications I know of are custom programs run on indexer platforms.

    Thanks

    Halfnutz

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


  11. #23
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    I have been experimenting with PID stepper control, too. One encountered problem is the cyclic error of the motor or the encoder causing lack of torque at some angles. I got around it using a cyclic look up table to correct encoder readings.

    Mariss, do G-Rex have some kind of compensation for cyclic encoder error? Or is an expensive and very accurate encoder the solution?



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    Quote Originally Posted by Mariss Freimanis View Post
    which is summed to the loop and adds about 70 degrees of phase margin.

    Mariss

    interesting. hey, i remember the terms gain margin and phase margin from my control systems class (i've forgotten 90% of what i learned in school)

    is this the same thing?



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What effect does microstepping have on torque?

What effect does microstepping have on torque?