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    DVD Stepper motor thread.

    Hi everybody. I have a stepper motor from an old DVD rom unit, which has 20 steps per turn. The thread bar for linear movement has 3 MM. of advance per turn, which results in 0.15mm of advance per step, 0.01875mm with 1/8 micro stepping by using the A3967 driver. My question is, how is that possible if distance between tracks in a dvd disc is less than 1 micron (<0.001mm)? And how can I get that displacement of 1 micron? Hope any of you could help me. Thank you very much in advance.

    Best regards

    •   Alt8th December 2016, 20:47

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  2. #2
    FvM
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    Re: DVD Stepper motor thread.

    CD and DVD drive are using a combination of coarse radial positioning by the stepper motor and fine positioning by an electrodynamic drive (coil + permanent magnet). A second electrodynamic drive is moving the lens up and down to adjust the focus.


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    •   Alt8th December 2016, 22:58

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    Re: DVD Stepper motor thread.

    Hi,

    And the code on a CD is self-clocking, I assume on the DVD, too.
    Therefore the code itself tells where a data bit is located. No need for very precise mechanical positioning/timing.

    Klaus


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    •   Alt9th December 2016, 02:07

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    Re: DVD Stepper motor thread.

    Your observation is true.
    Unless it is operated in 1/16 or 1/32 microstepping mode , the movement cannot be less than a micrometer.


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    Re: DVD Stepper motor thread.

    Quote Originally Posted by FvM View Post
    CD and DVD drive are using a combination of coarse radial positioning by the stepper motor and fine positioning by an electrodynamic drive (coil + permanent magnet). A second electrodynamic drive is moving the lens up and down to adjust the focus.
    In my CD drive (LG, old defunct drive), the optical assembly is mounted between two permanent magnets and there are two sets of coils (four wires are seen). The beam can be rotated about y axis (assume that the stepper drive is the x axis) and also can be moved up and down (z axis) by manipulating the currents through these two coils.

    So I see that one drive (two magnets and two coils) is sufficient to tilt and move up and down. I am not sure about the polarities of the magnets, but I guess it is a quadrupolar setup (but I am often wrong).


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