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Re: [taigtools] Re: Microstepping and accuracy/resolution

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  • Paul J. Ste. Marie
    [The original bounced, but I added some more thoughts before resending] ... Usually I think of full step positions as where the armature aligns with the
    Message 1 of 12 , Sep 30, 2011
      [The original bounced, but I added some more thoughts before resending]

      On 9/29/2011 4:34 PM, Mars Bonfire allegedly wrote:
      > "hmm...you could run both phases at max current...you're get 44% more
      > torque at the 45 degree marks than at the 90 degree marks." I
      > believe that is the basis of "you can get higher torque if you only
      > full step" idea. Basically, what you refer to as the "45 degree"
      > marks are the full step positions.

      Usually I think of "full step" positions as where the armature aligns
      with the field magnets, but looking at Wikipedia that's commonly
      described as "wave drive".

      > .... I am not sure what you mean by
      > the comment about "not so good dynamically" unless you are referring
      > to resonance...if you are speaking about mid band resonance, I agree
      > that half steps help to prevent it.

      Resonance is one effect. The motion is also going to be much more jerky
      (which is what will drive the resonance). I think may be more prone to
      losing steps, but with multiple poles on the rotor, that might not be
      true. In the simple cases with a small number of poles on the rotor, you
      would only get the full torque when the armature is halfway between the
      poles, and if the resistance to the motors is actually greater than what
      one phase can generate, then the motor could hold a position, but it
      cannot advance it.

      With a large number of poles on the rotor, the angle between the next
      pole and the field is always very small, so much less of an effect, but
      you still lose torque while the current in one phase reverses. Instead
      of the magnetic field vector tracing out a circle through one step
      cycle, the head of the vector traces out a square--the field strength
      pulses.

      What I can't quite figure out is if the opposing torque is greater than
      the torque delivered during the low point in the cycle, will that cause
      a step to be lost, or will the rotor fall back and then be pulled
      forward when the field regains full strength? I think the answer to
      that depends on the moment of inertia, etc.
    • Mars Bonfire
      Paul, I sense a meeting of the minds on a complex subject...difficult on email, easier standing together in front of a white board (chalkboard if you prefer)
      Message 2 of 12 , Oct 1, 2011
        Paul,

        I sense a meeting of the minds on a complex subject...difficult on email, easier standing together in front of a white board (chalkboard if you prefer) in my experience. Thanks for a simulating interchange.

        Steve


        --- In taigtools@yahoogroups.com, "Paul J. Ste. Marie" <taig@...> wrote:
        >
        > [The original bounced, but I added some more thoughts before resending]
        >
        > On 9/29/2011 4:34 PM, Mars Bonfire allegedly wrote:
        > > "hmm...you could run both phases at max current...you're get 44% more
        > > torque at the 45 degree marks than at the 90 degree marks." I
        > > believe that is the basis of "you can get higher torque if you only
        > > full step" idea. Basically, what you refer to as the "45 degree"
        > > marks are the full step positions.
        >
        > Usually I think of "full step" positions as where the armature aligns
        > with the field magnets, but looking at Wikipedia that's commonly
        > described as "wave drive".
        >
        > > .... I am not sure what you mean by
        > > the comment about "not so good dynamically" unless you are referring
        > > to resonance...if you are speaking about mid band resonance, I agree
        > > that half steps help to prevent it.
        >
        > Resonance is one effect. The motion is also going to be much more jerky
        > (which is what will drive the resonance). I think may be more prone to
        > losing steps, but with multiple poles on the rotor, that might not be
        > true. In the simple cases with a small number of poles on the rotor, you
        > would only get the full torque when the armature is halfway between the
        > poles, and if the resistance to the motors is actually greater than what
        > one phase can generate, then the motor could hold a position, but it
        > cannot advance it.
        >
        > With a large number of poles on the rotor, the angle between the next
        > pole and the field is always very small, so much less of an effect, but
        > you still lose torque while the current in one phase reverses. Instead
        > of the magnetic field vector tracing out a circle through one step
        > cycle, the head of the vector traces out a square--the field strength
        > pulses.
        >
        > What I can't quite figure out is if the opposing torque is greater than
        > the torque delivered during the low point in the cycle, will that cause
        > a step to be lost, or will the rotor fall back and then be pulled
        > forward when the field regains full strength? I think the answer to
        > that depends on the moment of inertia, etc.
        >
      • luv2bsailin
        If you want to really get a feel for how stepper motors work, set some up and play around. Put a big knob on it and get a feel for the magnetic springyness
        Message 3 of 12 , Oct 2, 2011
          If you want to really get a feel for how stepper motors work, set some up and play around. Put a big knob on it and get a feel for the magnetic "springyness" between steps. Grab the knob and try to stop the motor at various speeds. See how fast it can go and still recover from a stall. Above a certain speed it will lock up and just sit there and screech until the drive signal is removed. If you put a heavier flywheel on it the behavior changes drastically. Try different drives and settings, etc.
          Steppers do have some quirky behaviors which can be hard to understand or predict with pure theory until you start playing with them.
          Jim


          --- In taigtools@yahoogroups.com, "Mars Bonfire" <mbonfire@...> wrote:
          >
          > Paul,
          >
          > I sense a meeting of the minds on a complex subject...difficult on email, easier standing together in front of a white board (chalkboard if you prefer) in my experience. Thanks for a simulating interchange.
          >
          > Steve
          >
          >
          > --- In taigtools@yahoogroups.com, "Paul J. Ste. Marie" <taig@> wrote:
          > >
          > > [The original bounced, but I added some more thoughts before resending]
          > >
          > > On 9/29/2011 4:34 PM, Mars Bonfire allegedly wrote:
          > > > "hmm...you could run both phases at max current...you're get 44% more
          > > > torque at the 45 degree marks than at the 90 degree marks." I
          > > > believe that is the basis of "you can get higher torque if you only
          > > > full step" idea. Basically, what you refer to as the "45 degree"
          > > > marks are the full step positions.
          > >
          > > Usually I think of "full step" positions as where the armature aligns
          > > with the field magnets, but looking at Wikipedia that's commonly
          > > described as "wave drive".
          > >
          > > > .... I am not sure what you mean by
          > > > the comment about "not so good dynamically" unless you are referring
          > > > to resonance...if you are speaking about mid band resonance, I agree
          > > > that half steps help to prevent it.
          > >
          > > Resonance is one effect. The motion is also going to be much more jerky
          > > (which is what will drive the resonance). I think may be more prone to
          > > losing steps, but with multiple poles on the rotor, that might not be
          > > true. In the simple cases with a small number of poles on the rotor, you
          > > would only get the full torque when the armature is halfway between the
          > > poles, and if the resistance to the motors is actually greater than what
          > > one phase can generate, then the motor could hold a position, but it
          > > cannot advance it.
          > >
          > > With a large number of poles on the rotor, the angle between the next
          > > pole and the field is always very small, so much less of an effect, but
          > > you still lose torque while the current in one phase reverses. Instead
          > > of the magnetic field vector tracing out a circle through one step
          > > cycle, the head of the vector traces out a square--the field strength
          > > pulses.
          > >
          > > What I can't quite figure out is if the opposing torque is greater than
          > > the torque delivered during the low point in the cycle, will that cause
          > > a step to be lost, or will the rotor fall back and then be pulled
          > > forward when the field regains full strength? I think the answer to
          > > that depends on the moment of inertia, etc.
          > >
          >
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