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can clock to control a stepper motor ?

Discussion in 'Sensors and Actuators' started by oatcakeandy, Jan 21, 2013.

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  1. oatcakeandy

    oatcakeandy

    8
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    Nov 20, 2012
    hi all,
    i'm thinking of building a barn door telescope mount, i have already built a circuit from the internet but it does not turn the motor at the required rate (1rpm).
    so i was thinking, would it be easy to send a pulse from a clock mechanism, so that the second hand controls the speed of the stepper and giving the correct RPM ?

    may be a daft question but i'm a newbie at all this and i'm also a tight arse.

    thanks

    Andy
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,363
    2,758
    Jan 21, 2010
    1 RPM from a stepper motor will require many more than 1 pulse per second.

    I would suggest that you build a 555 oscillator that runs at the required frequency to drive a stepper motor at 1 RPM. (you will need some more circuitry to convert the pulse train to signals suitable to control the stepper).

    You will also need to make this variable over a small range to trim the oscillator to the correct frequency (it will change a little with temperature and battery voltage). The easiest thing to do is to adjust it until the stars stop moving.

    Are you planning to use this for photography, or just to make it easier to view without continual readjustment?
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    A good way to generate an accurate frequency to control a stepper motor is with a crystal oscillator and a frequency divider.

    One rotation for a stepper motor requires a large number of step commands. The number depends on the stepper motor and is clearly stated in the data sheet for the stepper motor. Assuming you have a stepper motor that rotates 3 degrees per step, one rotation will require 120 steps, which for 1 RPM is 2 steps per second. Each step command is usually divided into four phases of equal duration, so you will need a clock frequency of 8 Hz.

    You can get 8 Hz from a CD4060 using a 32.768 kHz watch crystal. Google those keywords and you'll find a circuit. If you need some other frequency, you can change the division ratio of the CD4060, with some limitations.

    You will then need a stepper motor controller that can generate the necessary phases in the right sequence, or you can use a circuit made from flip-flops. You'll need a driver that will provide the required current to the stepper motor. These subjects are covered on many web sites, and Googling those keywords will get you started.

    You may find it better to run the stepper motor at a higher speed than 1 RPM and gear it down, to avoid the little jumps that occur on each step.

    This whole issue has already been discussed on another thread:
    https://www.electronicspoint.com/help-small-project-precise-1rpm-motor-and-circut-t256056.html

    You might want to compare notes with the guy who started that thread.
     
  4. oatcakeandy

    oatcakeandy

    8
    0
    Nov 20, 2012
    thanks for the replies

    Yes Steve i would try to use it for photography.
    looking at the other tread mentioned it looks harder than i first thought.

    what about a normal motor that i could vary the power to adjust the speed, would that work ?


    regards

    Andy
     
  5. poor mystic

    poor mystic

    1,061
    30
    Apr 8, 2011
    As I recall, a very similar project, but for a professional telescope, is showcased in "The Art Of Electronics" with all considerations discussed.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    That is discussed in the other thread. I don't think a DC motor would be accurate enough.
     
  7. poor mystic

    poor mystic

    1,061
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    Apr 8, 2011
    :)
    The professionals who wrote the book I mentioned used a simple phase-locked loop design (and installed it at Mauna Loa if I remember correctly).
    The PLL drove a stepper motor.
     
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