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Pretty part of the week: Lian Xin XDMD camera autofocus coils (aussie.zone)
submitted 1 year ago by to c/electronics@discuss.tchncs.de
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Source: https://lcsc.com/product-detail/Vibration-Motors_Lian-Xin-Technology-XDMD-YB200-08_C47118014.html

Applying current changes the vertical position. You would glue a lens onto this and place it above your camera sensor.

Machine-translated page from the datasheet:

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[–] [S] 4 points 1 year ago* (6 children)

Woah. I thought it was 3mm, not 3cm. This this is huge.

Maybe you could also use it for laser experiments, not just cameras? Variable laser focus or for tuning distances to match.

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  • [–] 2 points 1 year ago (5 children)

    There looks to be no feedback mechanism, this would make it less useful for laser applications, as drift would not let it keep position well enough. Very neat though.

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  • [–] 3 points 1 year ago (4 children)

    Depending on how its built you might be able to do adjustment and sensing with a single coil. Similar to how you can do current sensing in motors or how you can send and receive with the same coil in a metal detector.

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  • [–] [S] 1 point 1 year ago

    Similar to how you can do current sensing in motors

    I do not think there are not enough variables here to make this work. Reading coil current won't give you any information you don't already know if you're already controlling coil current. You need to be reading at least one more variable that is somehow related to coil position.

    Turning off the coil drive and shorting the coil temporarily to measure current is unlikely to give you anything but the same current you were driving it with (minus some losses). Ie still not an extra variable.

    you can send and receive with the same coil in a metal detector.

    This introduces new variables! Reflected signal magnitudes (and distortions & phase delays) that depend on distance to a nearby metal object that you intentionally install near the coil (eg the metal casing).

    Not sure how easy or reliable this would be to do in practice. I have my doubts but I could be wrong :)

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  • [–] 1 point 1 year ago (2 children)

    I'm skeptical, but I kind of want one to mess around with. (How fast can it move, how accurate, etc)

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  • [–] 1 point 1 year ago* (last edited 1 year ago) (1 child)

    I never worked with these before, but these coils are apparently called "Voice Coil Motors" (VCM). The one from this post is big but its the same tech that is used on a smaller scale in phones.

    I went and found a "driver" for these things and took a look at the datasheet to understand how a VCM is typically controlled. This thing is 1.5mm x 0.8mm small so something you might actually find in a phone.

    The driver chip has built in current sensing and the current going through the coil directly corresponds to lens position so just from that, the driver knows where the lens is positioned.

    https://www.ti.com/lit/ds/symlink/drv201.pdf#page=8

    The device is used to control the current in the voice coil motor (VCM). The current in the VCM generates a magnetic field which forces the lens stack connected to a spring to move. The VCM current and thus the lens position can be controlled via the I2C interface and an auto focus function can be implemented

    Because the lens stack is spring mounted it can lead to "ringing" which basically means bouncing that continues after the desired position was set. So the chip apparently has built in logic for compensating for that so it more smoothly reaches the desired position without overshooting.

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  • [–] [S] 2 points 1 year ago

    The ringing comp is interesting: it's completely synthetic (sensorless). The chip models what it thinks the ringing should be based off some physical parameters (that can be configured over I2C), then drives the coil in a way that it hopes would counteract the ringing, but never actually knows if it has succeeded.

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