High precision stepper control

Discussion in 'General Electronics Chat' started by voxel, Aug 6, 2009.

  1. voxel

    Thread Starter New Member

    Jul 21, 2009
    3
    0
    Help please!

    I am using scavanged Vexta high precision 400 steps per 360° motors (2 phase, 8 wire, at 3.9v, 2.8Ω, 1.4A in bipolar series configuration) driven by the Allegro A4983 chip. The chip carrier however requires a motor voltage of at least 8 volts, which is well above the specified 3.9 for the motors, even in the series configuration.

    Is a resistor in series with the windings neccessary to bring down the voltage or can I rely on the current control of the A4983 to keep things within bounds? Are there any other strategies I should be considering here?

    Any help will be much appreciated.
    Thank you
     
  2. fanie

    Active Member

    Jan 20, 2007
    63
    0
    Doesn't work that way. You want a high voltage because when the motor begins to turn faster and faster, the higher the impedance of the motor gets. If the supply voltage is low, the impedance becomes so high that the current drops too low hence the torque drops off and the motor looses steps.

    Stepper motors rated as low as 1.5V runs off as high as 80 V and even higher. This is done by implementing an active current limiting that protects the motor windings against over current.

    Stepper motors, like LED's are current driven, and not voltage driven.

    There are gazillions of stepper articles you can read up on, most are not too good :D
    Gecko makes a good stepper driver, google for it.

    I use the LMD18245, it has active current limiting and works up to 45V. Almost all these stepper driver chips require uP controllers to be some kind of intellegent drive. You can do microstepping with it as well.

    Microstepping devides each of the 200 full steps into 8 or 10 or 16 or how many ever steps you want for ultra accuracy. 8 micro steps is usually the most sensable, it is more than accurate enough mechanically and offers smooth running without becomming too slow.
     
  3. voxel

    Thread Starter New Member

    Jul 21, 2009
    3
    0
    Thanks for the comments Fanie - very helpful. I see that my idea of how these things actually work has been off target somewhat! I'll follow up the Gecko driver.
     
  4. voxel

    Thread Starter New Member

    Jul 21, 2009
    3
    0
    Thanks for that Alberto - I'm presently using a PICAXE based driver which doesn't have microstepping capability. Your driver is extremely impressive with 24bit control. The mechanical side has a 1mm/rev thread so I would have a much greater precision available. Made my day!
     
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