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stepper motor speed ramp

Discussion in 'Embedded' started by david austin, Nov 20, 2003.

  1. david austin

    david austin Guest

    This site has a pdf with a neat method to generate timings for a linear
    ramp.
    It is quick enough to do in real time in a timer-comparator ISR on a PIC.

    http://mysite.freeserve.com/stepper

    Dave Austin
     
    david austin, Nov 20, 2003
    #1
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  2. david austin

    Bill Sloman Guest

    A linear ramp isn't all that helpful in accelerating a stepper motor.

    If your maximim stepping rate is higher than the first resonance
    frequency of the stepper motor you are using, you do have to chose
    your acceleration sequence with the resonant frequency in mind, and an
    arbitrary linear ramp probably won't serve. Read Douglas W. Jones on
    the subject.

    http://www.cs.uiowa.edu/~jones/step/physics.html
     
    Bill Sloman, Nov 20, 2003
    #2
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  3. david austin

    david austin Guest

    The method can cope with Bill Sloman's objection by building a
    piecewise-linear acceleration profile.
    My experience is that this is not necessary
    Dave Austin
     
    david austin, Nov 20, 2003
    #3
  4. Well that's all very well, but I've never heard of a stepper motor
    driver that does anything other than linear or s-curve accelerations,
    but then I guess I've only seen about half a dozen :)

    --
    Trevor Barton
    Isotek Electronics Ltd, 9 Clayton Wood Bank, Leeds, LS16 6QZ, UK.
    Tel: +44 (113) 275 1339, Fax +44 (113) 224 9827
    Remove X from before replying.
    Views expressed are my own and not necessarily those of Isotek Electronics Ltd.
     
    Trevor Barton, Nov 20, 2003
    #4
  5. david austin

    Bill Sloman Guest

    I hope your lucky break continues. A lot depends on how dissipative
    your load is. Several of the steppers I've played with had a fairly
    high-Q first resonances, and the starting sequence that worked
    involved enegergising the coils for several cycles of the resonant
    period, until the rotor had settled down aligned with the magnetic
    field, then making the first step period one quarter of the inverse of
    the resonant frequency.

    Do it wrong, and the rotor doesn't rotate, it just sits there buzzing
    at the stepping frequency, which isn't helpful.
     
    Bill Sloman, Nov 20, 2003
    #5
  6. david austin

    Stephen Pelc Guest

    A long, long time ago I did the software for a six axis machine
    that cut patterns on glasses and decanters. The first time we
    tried an S-curve, we hit the low-frequency resonance problem
    and the motors stalled. Both resonant regions are important.

    Stephen
    --
    Stephen Pelc,
    MicroProcessor Engineering Ltd - More Real, Less Time
    133 Hill Lane, Southampton SO15 5AF, England
    tel: +44 (0)23 8063 1441, fax: +44 (0)23 8033 9691
    web: http://www.mpeltd.demon.co.uk - free VFX Forth downloads
     
    Stephen Pelc, Nov 21, 2003
    #6

  7. A linear (or S curve or low jerk etc etc) ramp is just fine for a stepper as
    long as you use microstepping so that you do not exite the resonance. Any
    kludge that limits your flexibility in choosing acceleration or velocity
    would be extremely painful to deal with in a multi axis system...


    Peter Wallace
     
    Peter C. Wallace, Nov 24, 2003
    #7
  8. david austin

    Alan Kilian Guest


    Here's a nice kludge on a 343-axis stepper farm.

    http://taomc.com/stepperarray/stepperarray.htm

    It sure was fun fun fun!!!

    MAN, 100 Amps at 4.25 Volts takes BIG wire.
     
    Alan Kilian, Nov 25, 2003
    #8
  9. david austin

    Rich Webb Guest

    Rich Webb, Nov 25, 2003
    #9
  10. david austin

    Bill Sloman Guest

    Agreed. Microstepping almost always allows you to operate at stepping
    frequencies well above the motor resonances, and if by chance you do
    happen to hit a resonance anyway, the amplitude of the oscillation is
    almost certanly going to be too low to drive the system out of its
    piece-wise linear range.

    I was in involved in a cheapish stepper-motor controller development
    back in 1992, where we used a bottom-of-the-line Transputer chip to
    get the processing band-width to calculate micro-step up-dates in real
    time in a single processor.

    Regular microcontroller chips of that period couldn't hack it. I would
    have preferred to use a decent-sized programmable logic device, but
    the software guys found the Transputer to be much more congenial
    programming environment.

    Knowing the resonant frequencies of your motor-load combination can
    let you use cheaper,dumber controllers, but it takes work, and you
    have to be reasonably confident that these resonance freqencies aren't
    going to change when the buyers find a cheaper stepper motor ....
     
    Bill Sloman, Nov 25, 2003
    #10
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