Understanding linear stepper motors

  1. Hi guys,

    1.
    In this video:


    Could someone explain me how the linear stepper motor responsible for drawing/injecting the syringe works?
    I mean, the threaded rod going through the motor is suppose to turn around its axis in order to advance linearly and in this mechanism there's a rail to the right/left to make sure it dosen't.
    So how does it still move linearly ?

    2.
    With external stepper motors could someone please explain if the external linear nut rotates and moves along with the threaded rod or does the threaded rod simply rotates without moving linearly like a non-captive motor and the nut rotates and moves along it?

    Example:
    http://www.haydonkerk.com/LinearAct...size_8_external_stepper_motor_linear_actuator

    3.
    Is there a way to reduce step angle from the typical 1.8 deg per step to 1.4 deg without using gears?
    Like when using linear stepping motors, is there a way to control that via the threaded rod?

    Thanks a lot!
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Baluncore

    Baluncore 2,516
    Science Advisor

    Sorry, no youtube here.

    Where the feed screw is the shaft of the motor, the motor is fixed. The nut is constrained to prevent rotation while still being free to travel axially along the guides as the stepper motor screw shaft rotates.

    A stepper motor with 1.8° steps has very fine control. The flexibility of the mounting structure and the pitch of the screw will set the sensitivity in microlitre per step. A gearbox would increase the resolution but would require an improved structure. There are other ways to improve resolution such as to reduce the diameter of the syringe or change the pitch of the thread. Most ways of changing the step rate will require an improved structure.

    For fine metering of a fluid it is an advantage to rotate the piston in the syringe as it is advanced. That prevents steps due to a sticky seal, but requires a very different drive topology.
     
  4. See, I don't get the nut part being constrained to the shaft to prevent rotation of this it move linearly WITH the shaft ? Is a bearing being used?

    I'm talking about exclusively changing the 1.8 step as a resolution of 1.4 is needed.
     
  5. Baluncore

    Baluncore 2,516
    Science Advisor

    Is it a ball screw?
    http://en.wikipedia.org/wiki/Ball_screw

    1.8° per step is 200 steps per turn. That is decided by the way the stepper motor is built.
    1.4° per step needs 257.142857 steps per turn which is impossible because there must be an even number of magnetic poles in the motor.
    So you must change the diameter of the syringe or the pitch of the screw.
     
    1 person likes this.
  6. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    Hi dislect,

    The two actuators in the video are definitely linear ballscrews, it's an interesting application actually where Nippon applies two of their "Linear Stepper Motors"; one motor is stationary and the shaft moves up and down, the other motor moves up on down on the "stationary" (e.g. vertically contrained) shaft. Both utilize what looks like a parallel linear stage to constrain rotation of the motor.

    http://www.nipponpulse.com/articles/view/tin-can-and-linear-stepper-stirrer-and-syringe-applications

    Regarding your question about changing the number of steps per rotation, the best way to achieve this is with a motor controller capable of "micro-stepping" which allows many intermediate steps per each full step. See here for an explanation: http://www.nmbtc.com/step-motors/engineering/full-half-and-microstepping/

    I've seen microstepper drives that support up to 64,000 microsteps per revolution on a 200 step/rev motor, they allow for much finer resolution positioning although this sometimes comes at the cost of motor torque/speed and positioning repeatability.
     
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