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Issues with a stepper motor drive

Discussion in 'Electronic Design' started by chetanthegreat, Feb 17, 2008.

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  1. Hello folks,

    I am facing some issues regarding a stepper motor drive. The stepper
    motor I have is a 20KG 12V DC L/R Full step driven one. I need it to
    drive a Lead screw which eventually builds up pressures of the likes
    of 400Kg cm in a cylinder.
    I have attached a gear box of ratio 12:1 to the stepper and tried
    boosting the torque. With the direct coupling the pressure used to
    reach around 100-150 Kg cm but even with the gearbox which I presume
    would boost the torque by a factor of 12 the pressure fails to rise
    above 200.
    And the stepper slips if I try to run it at high RPMs (15RPM ) even
    without any load.
    What can I do to find a solution to these 2 problems? What could be
    going wrong in this whole exercise?
    Thanking you in anticipation.
  2. Guest

    you dont say what you are driving the stepper with. its not so easy to
    get performance from a stepper.
  3. Nico Coesel

    Nico Coesel Guest

    Using a stepper is wrong in the first place. You'll need a proper
    servo controller and a servo motor. The next thing you need to do is
    determine the maximum amount of torque required to achieve your goals.
    However, I guess the required torque depends highly on the amount of
    friction in the lead screw so it may be difficult to calculate the
    amount of torque precisely. You may want to get rid of the lead screw
    and opt for a system with a more constant friction.
  4. Guest

    The problem with the gear box could be as simple as friction losses
    thriugh the gear train.

    The problem with the stepper motor seem to be that you haven read the
    application notes carefully enough.

    A stepper motor is just a synchronous AC motor. The torque you get out
    of it is proportional to the AC current that you drive through motor
    coils,and the faster the motor rotates, the harder it is to drive
    current through the motor coils.

    You have to allow for the back emf of the motor - this can be found
    from the manufacturers data sheet, though some of them assume you know
    that the back emf 0f a motor - in volts per radian per second - is the
    same as the torque constant (when expressed in newton metres per
    ampere) and you also have to remember that the AC impedance of the
    coil is the vector sum (root means square sum) of the coil resitance
    and the coil inducance. The inductive impedance increases in direct
    proportion to the stepping rate.

    This means that you have to apply a lot more than twelve volts to a
    twelve volt motor to get its rated torque when it is spinning fast,and
    the drive circuit has to be designed in such a way that the motor
    won't burn out if you happen to stall it.


    Nico Coesel is not entirely right to tell you not to use a stepper
    motor for your job - the right stepper motor with the right drive
    could do it - but a brushless DC motor (which is just a stepper motor
    with a built-in stepper drive) is a lot easier to use.
  5. Benj

    Benj Guest

    Everyone has given you great advice. Steppers are difficult to drive
    fast and tend to lose torque when you do. One simple way to improve
    high speed performance is to use a much higher voltage power supply
    and drop the current with a resistor. Lots of lost power but faster
    performance. Generally for what you are doing, you don't want a
    stepper at all! Get an nice strong DC motor and use some servo scheme
    to monitor the shaft position. Usually it's some kind of digital
    encoder. You can buy these as total systems. The beauty of this is
    the DC motor can have huge torque and the encoder does not "slip".
    Downside is complexity and expense. A stepper can be simpler and
    cheaper but you really have to over-rate the torque spec to make it
    reliable. I had to increase the motor size on a device I built that
    only moved a drill X-Y with leadscrews and didn't build up any
    pressure at all! It also used high speed, high voltage drivers so you
    could get some speed out of the thing.
  6. Tim Wescott

    Tim Wescott Guest

    -- snip --
    Be careful with your terminology, though. Many brushless motors don't
    come with a built-in drive, even ones marketed as "brushless DC".

    I wouldn't characterize a brushless DC motor as 'just a stepper with
    built-in drive' -- there are physical differences in the way they're
    built that makes each one better for certain tasks. When you get down
    to the bottom, though, a brushless motor and a stepper are both
    synchronous AC machines.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  7. Tim Wescott

    Tim Wescott Guest

    You've been given a lot of good advice. Something that has only been
    alluded to, however, is that you need to figure out what torque is
    necessary on your lead screw to develop the pressure you need, then
    figure out what gear box to use to turn your available torque into the
    torque you need.

    Gear boxes (and lead screws) have gear ratios (or lead/turn ratios), and
    ideal gear boxes will, indeed, multiply torque by the gear ratio while
    an ideal lead screw will transform a torque into a force in a
    predictable way (force = 2 * pi * torque / pitch, if I'm not mistaken,
    but check).

    But gear boxes and lead screws also exhibit friction. Lead screws (and
    worm gears) have _lots_ of friction. You need to check not only the
    gear ratio or lead, but the efficiency of the gear box and lead screw --
    basically, an 80% efficient gear box will deliver your input torque,
    times your gear ratio, times 0.8.

    So check the efficiency of the lead screw, or better yet get an
    indicating torque wrench and _measure_ the torque it takes to develop
    the pressure you want. Then use a gearbox with a known efficiency and
    enough of a ratio to develop the torque you need with the amount of
    torque that you can trust to get out of whatever motor you use, whether
    you stick to your stepper or go to another sort of motor (a brushed DC
    motor is much lower tech, by the way, and may be easier to implement if
    you're doing the drive electronics).

    Good luck. If you succeed, you'll come out of this smarter than you
    were going in.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  8. Thanks for the help people!
    Thing is I am kind of stuck with the stepper I bought for this
    application. And for the first few rotations I don't need precise
    movement or much torque but the last revolution is highly demanding as
    far as torque and precision is concerned.
    Today, I tried hanging weight on the cross bars handle attached to the
    lead screw and found out that at around 20-25 Kg weight the lever
    moves in order to develop the ultimate 400 Kg/cm pressure in the
    gauge. As the distance from the centre was 16 cm, the calculated
    torque needed would come around 20 / 25 x 16 Kg cm. Thus I am hoping
    that a 60 Kg Stepper with a 12:1 gearbox will be suffice to drive the
    lead screw in.
    Please comment if you could point out a flaw in my logic.
  9. Nico Coesel

    Nico Coesel Guest

    Perhaps you could use a counterweight which is equal to half the
    maximum pressure you need on the rod (assuming the cylinder is
    standing). This way the stepper engine needs to deal with half the
  10. Guest

    First you need to fing the torque needed at the motor shaft at the
    speed you need. That means you'll need to measure it or try guessing.
    Stepper performance depends on the drive, you seem to keep ignoring
    that, so you need a motor/drive pakage, ie. you have to buy one or
    guess. Then look at the torque/speed curve published for said package
    to see if it exceeds your minimum requirement. Repeat untill you have
    the best match.
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