What effect does microstepping have on torque? - Page 5


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Thread: What effect does microstepping have on torque?

  1. #49
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    Ooops



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    I hope this question makes sense.

    So if i add a flywheel to a motor the problem gets a lot more complicated. If i run a stepper from 0 to 200 rpm slowly increasing speed with no flywheel, it will hit resonance, for example, at 100 rpm as discussed above. Now if i add a flywheel to it, it seems to work quite well and powers thru the previous resonance.

    It would seem like a tank circuit at the 100 rpm with the flywheel decreasing the Q of the circuit. Does back emf add to the issue at that particular speed? As the frequency continues to increase (motor goes faster) X subscript L (reactive inductance) would continue to increase and it would seem the problem would just get worse. ie, current decreasing.

    From my limited knowledge of this topic, that being ELI leads the ICE man, as in Voltage leads current in an inductor and vice versa for capacitance, and since you need two reactive elements to create a tank circuit (resonance) is it inductance plus mechanical, or inductive and capacitive, or inductive and back emf causing the phase shift? A simple inductor has of course a much simpler phase plot than the inductor in that spinning stepper from 99 to 101 rpm.

    Has much testing gone on into increasing the step distance dynamically as RPM increases? 200 steps up to 100 rpm, the 100 steps to 200 rpm, etc. Inertia would carry the rotor rather than trying to pulse every step. Or even written pole technologies for steppers?

    Lotsa questions.



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    Brian.

    Interesting questions.

    Adding a flywheel is a losing proposition because it adversely affects your system's torque to inertia ratio. The inertia of the flywheel limits your system's ability to accelerate / decelerate. Furthermore, it masks the instability you are trying to cure by only shifting the speed at which it manifests itself.

    Don't bother yourself with the series resonant LC tank circuit. It has absolutely no practical application. Back EMF is also in my mind an unconstructive perspective from which to view things.

    Some specifics of what you are trying to do would be helpful.

    Mariss



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    Just to understand actually why it would resonate at that 100 rpm and why adding that flywheel, even not very heavy, has a profound effect on that point. Also, what is the max speed i should run that motor at.

    Should i never exceed that 100 rpm in using that motor and driver combination? As you say, resonance will not be eliminated, only shifted by adding rotational inertia and loads. For example if a gantry was coasting and no load was on the motor, it would be like the motor was not connected to anything and would run into problems if the software requested at that moment an increase in speed.

    I can definitely see the benefit of electronically 'seeing' that oscillation point and phasing accordingly. DSP is ideal for this app.



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    I couldn't live with the notion of limiting a stepper's RPM below its resonance.
    To get away from the mechanical damper, I bought a G540.
    Out of curiosity I drove my Keling stepper with the G540 and got over 4000 RPM with no load and no resonance bump. It may not have had much useful torque at that RPM, but it was cool to watch that stepper spin.



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    That's a great controller.



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    Adding a flywheel, will kill the dynamic performance of the stepper motor. These will always be optimized by the manufacturer for minimal rotational inertia. Resonance problems are much easier handled by the drives, like Mariss and I are doing. Once high-res microstepping is used, the resonance problems will arise from the inherent discontinuous sin/cos wave profile, as the total of (abs)current in both motor windings has a maximum at every 45degrees (cos45+sin45 = 1,41 and a minimum at every 90 degrees (cos0+sin0 = 1).
    Correcting wave profiles for this, eliminates already a lot of resonance. Only with advanced drives it is possible to have this opportunity. Then it is also possible to correct for discontinuities in the magnet-tooth geometry. That is what you experience when turning an unconnected stepper motor by hand: you feel the 50 tooths, the same will happen if the motor is electrically driven.
    This is also an answer to the question of microstepping vs full stepping and torque loss. Yes there is torque loss with microstepping (and also with half-stepping), at passing the 45degrees points of the si/cos wave. This is to be compared with full stepping, which is hopping with 90degrees through the sin/cos table and always has the same max. torque.



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    As soon as you pass about 800 rpm, resonance will not be heard anymore. I drive a Nema23 PaSci doublestack motor with 120 VDC until 14400 rpm, when torque is almost zero. I dont do that often, it is dangerous.



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    Quote Originally Posted by BrianTee View Post
    Ooops
    Sorry, I am completely new here, did not know Mariss is from geckodrive



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    Quote Originally Posted by Hendrikus View Post
    As soon as you pass about 800 rpm, resonance will not be heard anymore. I drive a Nema23 PaSci doublestack motor with 120 VDC until 14400 rpm, when torque is almost zero. I dont do that often, it is dangerous.
    Its just getting past that 800 rpm every time in practical operation is the trick.



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    Quote Originally Posted by Hendrikus View Post
    Sorry, I am completely new here, did not know Mariss is from geckodrive
    Oh no no. I posted a wrong message there is my oops. Text, so confusing at times. Sorry.

    Mariss is working on some cool stuff.



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    Red face

    Quote Originally Posted by BrianTee View Post
    Its just getting past that 800 rpm every time in practical operation is the trick.
    You are right, in real world CNC you pass by everytime the resonance speed. Sorry, I am past these problems already for some time; the control that I developed has no resonance problems.



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What effect does microstepping have on torque?

What effect does microstepping have on torque?