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Stepper Motor Speed Problem

Discussion in 'Electronic Design' started by sudheervemana, Oct 25, 2004.

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  1. Hi all,

    I am developing an stepper motor position control board in the
    devicenet network.I want the motor to run in the microstepping (up to
    1/64) mode to an target speed of 2500RPM for an stepper motor with 200
    step resolution.

    In the case of Fullstep, my motor can have 200
    steps/revolution.Suppose if i want to rotate in the microstep
    mode(1/64)(the motor can have 12,800 steps/rev)to an target speed of
    2500 RPM,does the controller clock frequency 40Mhz is enough for my
    controlling or else i should get the controller with some more higher
    clock freq.Why because i am not able to achieve the desired speed in
    the 1/64 microstepping mode.

    Clear me regarding the speed of an stepper motor.Suppose
    if my motor is an 1.8 deg step motor where it can rotate up to 200
    steps /rev in full step mode,if i want to achieve the target speed of
    2500RPM with fullstep which is (2500*200*60) 3000000 pulses/sec,my
    timer freq which is 40Mhz is greater than the 3MHZ.Whereas in the case
    of microstepping(1/64) the stepper motor can rotate 12,800 steps
    /rev.now if want to reach an target speed of 2500RPM by using
    microstepping(1/64) the target velocity will be (2500*12800*60)1280Mhz
    which is less than my timer freq 40Mhz.Is this the problem for my
    speed,or else my interpretation is wrong.Please clear me regarding
    this speed of an stepper motor in the microstepping mode.

    With Regards,
    Sudheervemana.
     
  2. Paul Burke

    Paul Burke Guest

    30M if you do the sums right, but it's wrong anyway. DIVIDE RPM by 60,
    not MULTIPLY!!!!
    2500 * 400 / 60 = 8333 pulses/ sec.
    1920M if you do the sums properly, but still wrong.


    2500 * 200 * 64 / 60 = 533333 pulses / rev.

    Easy slip to make, but I think it's back to school on the sums front.

    Paul Burke
     
  3. BFoelsch

    BFoelsch Guest

    Big problem here. 2500 RPM = 41.66666 revs/second

    41.666 revs/second x 200 pulses/rev = 8.333 kHz, NOT 3 MHz.

    Re-work your calculations using the correct speed formula and the answers
    will be much more manageable.
     
  4. CFoley1064

    CFoley1064 Guest

    Subject: Stepper Motor Speed Problem
    Hi, Sudheervemana. Apart from doing a quick check on your math, you should be
    aware that most all real world microstepping stepper controllers drop some and
    then all of the microsteps as speed increases, and go to straight
    half-stepping. There are dedicated controller ICs available which do this job
    for you. And also, 1/64th microstepping is kind of unrealistic as far as
    positional accuracy on even mil-spec stepper motors. Except at _very_ low
    speeds, you won't get _any_ additional torque, either. And you haven't even
    looked at driver efficiency losses with switching speeds that high.

    As far as it goes, a controller of this type will generally get motion commands
    and then internally calculate a motion profile prior to the move. This profile
    can be ON-OFF, trapezoidal, or any number of other profiles. Commands are also
    issued for minimum speed (to avoid mid-range resonance), and other things.

    The Guru Don Lancaster once said:

    "An hour on the net is worth a week in the lab."

    You might want to do some reading before you spend a lot more time on this.

    And you should seriously consider purchasing a one-chip solution for your
    controller instead of rolling your own, unless you're making tens of thousands
    of these. Of course, if it's a one-off, just buy a board. There are so many
    good products out there at such reasonable prices that reinventing the wheel
    just isn't worth it.

    Good luck
    Chris
     

  5. Hi all,

    Thankyou very much for your reply,yesterday i have
    miscalculated the speed and afraid that my controller freq is not
    enough for my controlling.The problem is with my motor whose max RPM
    is 500.Anyway thankyou very much for all of your suggestions.

    With Regards,
    Sudheervemana.
     
  6. john jardine

    john jardine Guest

    That's about 1/2Meg micro steps/sec. I've had 120k/sec being sent from a
    10MHz PIC so a 40MHz setup should be fine.
    Max step rate is pro-rata with how much machine code command parsing, you
    wish to perform after each of the the incoming 'step' commands.
    regards
    john
     
  7. Joerg

    Joerg Guest

    Hello Sudheervemana,
    That is pretty typical with steppers, they just won't run very fast. It
    seems you should look at other solutions, either motors with much
    coarser steps or maybe a geared DC motor with servo.

    Regards, Joerg
     
  8. BobGardner

    BobGardner Guest

    You can run steppers at 1000s of steps per sec in 'current mode'. I think this
    is where the LR time constant in the stepper coils is just brute force swamped
    out by a strong driver. MicroMo site sells current mode controllers and rates
    their motors in this mode
     
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