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How much do different stepper motors have in common?

Discussion in 'Electronic Design' started by bigorangebus, May 14, 2007.

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  1. bigorangebus

    bigorangebus Guest

    Ive been looking at a little control project and using stepper
    motors.

    However, I would like to design a control board that will work with a
    number of different steppers without modification (apart from perhaps
    some jumpers to change from uni to bi polar).

    Whilst I am fairly confident I could take a particular stepper spec
    and design a driver, are there common characteristics that would allow
    a wide range of steppers to work with one driver board?

    I should make it clear that I understand how a stepper motor works,
    unipolar, bipolar etc and bridge configurations. However I am
    unclear on whether I can get away without current limiting (perhaps
    only on small motors with higher restistance windings?). It seems if
    I use a resistor that would affect startup torque and need to be
    changed depending on motor used. And if I use current limiting that
    would have to be programmable?

    Also does microstepping always require strict current control? Or can
    you just instead of connecting say 12V to one winding, connect 12V to
    one and a PWM'd 12V to another (to control microstep position).

    Any advice on how to make driver circuits as generic as possible
    gratefullly received!

    Thanks
    Andy
     
  2. Guest

    Unless you are going in for really slow stepping applications, you
    need to drive stepping motors from much higher voltages than the
    manufacturer's rated voltage - which is merely the maximum DC voltage
    you can put across a static coil without over-heating the motor. Some
    kind of current limiting is pretty much essential.

    Stepper motors present such a wide range of coil resistances, coil
    inductances and torque/back emf ranges that a universal controller is
    going to be vastly sub-optimal and over-priced in 99% of applications.
    The moment of inertia of the rotor and the stifness of the magnetic
    coupling - which determines the first resonance frequency of the
    stepper motor - is another variable that your controller is going to
    have to deal with, but at least it is easy enough to make the
    electronics fast enough to cover the commerically available range, and
    slower electronics aren't siginificantly cheaper or more compact.

    Note that the torque rating, in newton.metres per amp, of a stepper
    motor is numerically equal to the back emf rating in volts per radians
    per second.
     
  3. bigorangebus

    bigorangebus Guest

    Thanks Bill, this is useful.

    What do you mean by "fast enough"? Ive heard that stepper resonances
    can be overcome by using higher frequency pulsing...
     
  4. Guest

    Don't top post. "Fast enough" depends both on the back-emf of the
    coils and the inductance of the motor coils, so it varies a lot from
    motor to motor. I've seen people rotate a motor at the target rotation
    speed with an electric drill and monitor the voltage induced across
    the drive coils with an oscilliscope, which can tell you something
    about the back-emf in terms that even a boss can understand, even
    though it doesn't tell you anything about the effects of coil
    inductance.

    Microstepping doesn't make any dramatic difference to the voltage you
    need to put across the motor to make it step reliably at a specific
    rate of rotation.

    Microstepping a stepper motor usually does raise the frequency of
    pulses going to the motor above the first resonance, which does reduce
    the problem quite a lot. There will be a - relatively slow - rotation
    rate for which even the microstepped drive currents will have
    significant harmonic content at the resonant frequency, but it will be
    a much lower amplitude component than it is with half-step and full
    step operation so the amplitude of the wobble on the motor's rotation
    won't build up to anything approaching a full step, so the resonance
    is - even then - most unlikely to cause the motor to lose or gain
    steps.
     
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