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Getting more accuracy out of a stepper motor

Discussion in 'Electronic Design' started by Michael Brown, Aug 26, 2007.

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  1. Hey all - I'm a bit new to using steppers, but I've got a quick
    question about this stuff because I know so little :)

    Is there a well-known method to getting high accuracy AND speed out of
    a stepper? I'm talking like 100 deg/sec at a resolution of around
    40,000 steps (0.009 degree accuracy). The application is for small
    camera PTZ mounts.

    I'm assuming some reduction gearing would be involved, and possibly
    using optical encoding (just from what I've learned googling). Anyone
    got any insight on this? I'm not aware of any setups that can get this
    kind of speed and accuracy, but I may be just out of the loop.

    Thanks!

    -Mike
     
  2. Ok after reading a little more I guess you wouldn't need any optical
    encoding with a stepper motor, that looks like a technique that only
    applies to dc motors.. does that sound correct? I've been reading a
    little about microstepping controllers, but have yet to determine if
    stepper motors are even capable of high speed with gear reduction
    used...
     
  3. A stepper motor is just that. The variance may be in number of steps per
    revolution and the speed at which it makes them. Any required changes
    have to be in external mechanics, and those have to be spring loaded so
    there is no free play when changing directions.
    And there is NO such thing like 'half a step'.
    So be conservative and design your setup for worst case probability.

    HTH

    Stanislaw.
     
  4. Rob

    Rob Guest

  5. John Larkin

    John Larkin Guest


    Half-stepping drive is common, as is microstepping. You can buy a
    1/256 step microstepping driver, and get very smooth motion with no
    perceptable stepping, but 1/256 step position accuracy probably isn't
    achievable.

    John
     
  6. Jeff L

    Jeff L Guest

    I also agree that 1/256 step position accuracy would be pretty hard to do,
    and would move with any change in torque load on the motor. AC servo motors
    with optical encoder feedback are much better at maintaining really small
    increments, but are costly. They are basically a stepper with really large
    steps and optical position feedback. They also don't "cog" when turned
    manually.

    I still find microstepping noisy and resonance prone. AC servo drives are
    really smooth and can be very quiet - often the high precision ball screws
    they drive and the high precision linear ball slides make more noise. Even
    large ones rated for 7.5 HP are quiet. The linear motors we have are however
    very loud, but they accelerate at around 4 G and move at 40 to 50" per
    second.

    One of our solder paste printers uses microstepping, and achieves a 3 sigma
    accuracy (the new ones are 6 sigma!) of 0.001" with 1.8deg per step steppers
    driving ball screws with about 1/2" pitch. That's after accounting for the
    X,Y, Z, Theta table axis, in which the z axis moves about 4" or 10 cm after
    the vision processor camera reads the fiducials (which is not perfect), and
    the camera head moves in on linear motors (basically a long, flat stepper -
    electrically they are the same) with a slat pitch of about 1 mm (about
    0.040"), which gives a stepping pitch of about 0.5 or 0.25 mm. I figure the
    steppers must achieve a repeatable ~1/20 step, and the linear motors 3 or 4
    times better (the linear motors are sitting on air cushions). Other then the
    zero setting home position sensors, there is no feedback for mechanical
    position, which I think is quite bad in this precision application,
    especially when the motors get old or abused (like getting jammed up and the
    motor field continues to rotate, while the rotor stays put - we all know
    what strong alternating magnetic fields do to magnets) and start slipping
    from core demagnetization. Steppers in the range of 70 to 500 W are not
    cheap. Platens for linear motors are around $3k


    For the original poster, take a look at floppy drives for the stepper
    actuated head, and look at modern high speed optical drive laser moving
    mechinisims.
     
  7. Benj

    Benj Guest

    Stepper motors have limitations on speed and accuracy. Speed is a
    function of the maximum steps per second the motor (and driver) can
    do. It is limited by the induction of the motor coils. There are
    tricks to force more current through the windings faster to get higher
    speed, but there are limits and steppers are not especially fast as
    motors go.

    You'd think that stepper accuracy is limited to the number of steps
    per revolution. And for the most part it is. However, if you add a
    step between two given steps (by energizing both coils at once) you
    double the number of steps. which works pretty well. You can carry
    that further by what is termed "micro-stepping" Here you essential
    vary the voltage between the two simultaneously energized coils to
    produce fine variations between two steps. However, as someone else
    noted you lose torque. The bottom line is that with a simple set of
    driver currents (say set by series resistors) you can get maybe 16
    microsteps. BUT don't expect any torque. And friction will kill most
    accuracy. I've got that working pretty well if you are driving a
    MIRROR on the end of the stepper shaft. But if there are gears or
    friction, you'll need a huge stepper to generate the torque.
    Optical encoding (really an analog motor technique) can be combined
    with variable current half-stepping to create stable and repeatable
    micro-stepping. The accuracy is set by the encoder and the current
    drivers supply the torque. It's sort of a hybrid stepper and DC motor
    with optical encoder system.

    Hope this helps.
     

  8. Jeez, folks... all one needs to do is look back some years at the line
    printers that were around. They were controlled by stepper motors, as
    well as by both linear and rotary optical encoders, and they had some
    fairly good accuracy.

    For heavier applications like machine tool bed transitions, sure the
    task will be more difficult, but still quite doable.

    Cincinnati Milacron, as well as any good Japanese CNC machine tool
    maker achieves 10,000th inch accuracies EVERY DAY! Guess what their
    drive mechanisms are?
     
  9. Fred Bloggs

    Fred Bloggs Guest

    Will you ever fix your NNTP time?
     
  10. Yes, and disk drive (HDD) positioners, back 20+ years ago. But no
    more, except for your <$10 FDD. Too slow and too inaccurate.
    *Closed-loop* AC servos even on low-end machine tools like Haas, let
    alone high end machine tools such as Okuma. The only stepper is in the
    $10 floppy disk drive (which Haas charges $1.000 for).


    Best regards,
    Spehro Pefhany
     
  11. Guest

    Stepping motors are just synchronous motors. The motor coils generate
    a magnetic field that varies - more or less sinusoidally around the
    circumference of the motor, and the rotor lines itself up with that
    magnetic field.

    If the currents through the two coils are held constant, and you try
    and rotate the rotor, you will have to apply a progressively
    increasing torque to move it out of alignment with that magnetic
    field, up to a certain angle, where the restoring force will start
    decreasing, falling to zero at twice that angle after which the rotor
    will experiece an increasing force in the opposite direction, driving
    it to the next stable alignment.

    The peak restoring force is the drop-out torque of the stepper motor.

    In a synchronous motor, you modulate the the currents through the two
    coils to create a rotating magnetic field that drags the rotor around
    with it. The resisitng torque that has to be overcome to allow the
    rotor to rotate causes the rotor to lag the field by an angle that
    depends on the ratio of the resisting torque to the drop-out torque.

    When a synchronous motor is used as a stepping motor, the current
    through the coils is switched on and off to create square wave
    approximations the ideal sinusoidal modulation.

    Microstepping drives generate stair-case approximations to the ideal
    sinusoids.

    http://www.cs.uiowa.edu/~jones/step/physics.html

    In practice, not all stepping motors produce smooth rotation when
    driven by sinusoidal fields - an early example of a 1024-microstep
    microstepping drive written up in the then Journal of Physics E:
    Scientific Instruments (now Mreasurment Science and Technology) ended
    up being used to generate some 800 not too evenly spaced microsteps,

    ESCAP still seems to sell stepping motors designed for microstepping

    http://www.boxdoerfer.de/turbodsc.pdf

    Getting stepping motors to rotate fast involves modulating the current
    through the coils fast enough to rotate the magnetic field at the
    appropriate rate, despite the inductance of the coils. This means
    driving the coils with a voltage very much higher than the
    manufacturers ticket voltage, which is just the rated current through
    the coils multiplied by the resistance of the coils.

    I've used 60V on a nominally 5V motor, and this wasn't exceptionally
    high.
     
  12. John Larkin

    John Larkin Guest

    If you grab a typical classic Slo-Syn type stepper motor and connect
    it to a microstepper drive, the accuracy won't be good, because the
    tooth profiles were optimized for torque in full-step mode. If you
    program slow, smooth motion, you'll get nonlinear angular motion,
    maybe even a zero-velocity flat spot every 4 steps. Some people make
    steppers that are optimized for microstepping, but even these probably
    can't do 1/256 accurately. Not to mention static friction, which no
    open-loop system can deal with.

    I once wrote an n/c compiler for a Whitney punch press. It would slam
    around 12-foot long steel sheets at impressive speeds and whack them
    with a hydraulic punch, a hole a second or so, and the building would
    shake when we did the bigger holes in 1/8 inch steel. It used dc drive
    motors and linear encoders and was rarely more than a couple of mils
    (that's thous! not mm) off.

    John
     

  13. Will you ever learn how to quote who you are responding to?
     
  14. Robert Baer

    Robert Baer Guest

    You are incorrect; there is a "half-step".
    "Ordinary" steps are made when one winding is energizes, a
    "half-step" is made when two windings are energized.
    Now the torque of a half-step *is* less than that of a full step...
     
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