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Wireless data transmission

Discussion in 'Electronic Design' started by Grumps, Jan 14, 2010.

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  1. Grumps

    Grumps Guest

    I'm trying to find a way of transferring power and data between two rotating
    devices (1500 rpm). The two devices rotate along a common spindle and are
    separated by 5mm.
    Connecting power between these two is easy by the use of some form of
    commutator arrangement (you know, a couple of brushes and concentric copper
    rings), but I don't know if this is a good approach to send the data as
    well. I can envisage noise appearing on the data due to dirty brushes or
    copper rings, or should I just try this and add more brushes for the data
    The data rate is less than 1Mbps.
    Ideally an inexpensive solution.
    Thanks for any ideas.
  2. Tim W

    Tim W Guest

    Air bourne optical link for the data? I thinking 3 or 4 TX leds with common
    drive spaced around the axis and one photoreceiver on the other end. Repeat
    in reverse if duplex is required.

    Or use the brushes and perhaps use a balanced current driven signal with
    plenty of error detection. Carbon brushes might be better than small contact
    area wipers.
  3. Sylvia Else

    Sylvia Else Guest

    I'd be thinking of transferring the power by means of a transformer
    consisting of two coils sharing a common axis, about which one is
    rotating and the other isn't. I can't see why data wouldn't pass the
    same way. Perhaps not so effecient in transferring power, but no wear.

  4. Grumps

    Grumps Guest

    Thanks for the idea. I was thinking of some form of optical interface too.
    Ooh, I like that idea! Thanks.
  5. Grumps

    Grumps Guest

    Thanks. I know of one patent which covers this approach, so it's not going
    to be good for the finished product.
  6. Jules

    Jules Guest

    Interesting. There might be issues with channeling the light so that the
    receiver can always pick it up; 1Mbps @ 1500rpm works out at about
    42000 bits each revolution, so any 'dead zones' would cock things right up.

    Maybe the spindle that everything's mounted on could be hollow? That way
    you could have one TX LED and one detector mounted inside, with one
    part rotating relative to the other.


  7. 1 Mbps is pretty need a carrier up at least 27Mhz, which would
    be my first thought, since its essentially cowboy territory. Anything
    goes below a half watt or so.

    ...but then you ned all that mod/demod kit.. Yuk.

    probably time to grab a couple of WiFi chips and pics and get
  8. well that's just using the M part of teh EM wave instead of the E part.
    Frequencies and modulation schema are the same for either.
  9. Grumps

    Grumps Guest

    691,200Hz at present. I may want to increase this (x2 or x4) later.
    Any idea of the data rates that these use.
  10. Grumps

    Grumps Guest

    I can try.
    This is my current favourite solution.
  11. about 50 baud ;-)
  12. amdx

    amdx Guest

    If this is a one of, you can pickup a junked VCR for next nothing and
    use the rotory transformer from the video head assembly. It has good
    and is very stable. I'm not sure about the frequency response or the power
    level it is capable. Opps, your 5mm spec. means you can't use the whole
    assembly. Oh well the ferrite pieces are there.
    Anyone have thoughts about it?
  13. How much space on each 'wheel' do you have and what sort of weight can
    it carry before causing some sort of imbalance... ?

    I'm thinking of xbee modules... but you might need a smart sort of
    microcontroller on each do do the data caputre/transmit - but maybe you
    have that already?

    Then again, I've just looked and they're 250Kb/sec max. however there
    are then things like this:

    up to 2Mb/sec, but you will need some sort of smart device to push data
    into it and get it out again...

  14. Grumps

    Grumps Guest

    Thanks, that looks a good little module. My system could easily support that
    But I'm tending to the optical approach.
  15. I once worked with a slowly rotating system using 20 mA current loop
    at 9600 bit/s through two sets of rings and the third pair of rings
    carrying 220 V at a few amperes.

    Originally it used a half duplex (command/Ack) protocol, but since
    both the command or the acknowledge message could be lost, due to
    individual bit errors at that data rate, it created quickly quite a
    nasty backlog of messages due to retransmissions. I wrote a broadcast
    (no ACK) protocol with heavy error correction coding (ECC) and after
    that the system worked OK.
    At that data rate several consecutive bits will be lost due to dirt
    etc. so you also would have to use bit interleaving to convert burst
    errors into random errors, that can be readily corrected by the ECC.

    Exactly for the same reason ECC and interleaving is used for instance
    in CDs or digital radio links.

    Adding 30-50 % of ECC bits with interleaving should help lot with the
    reliability and since the data rate is only about 1 Mbit/s and hence
    the wavelength is in order of 200-300 m, ring assemblies less than a
    few meters in diameter should not be a problem.
  16. Grumps brought next idea :
    An infra red link could do that.
  17. amdx

    amdx Guest

    Nice picture.
  18. Grumps

    Grumps Guest

    Ah, slip rings. That term escaped me, thanks.
    Yes, 3 rings would be enough.
  19. Every spinning VCR head assembly works this way, at 20-40MHz IIRC.
  20. 4Mbps Fast IrDA should do the data rate, and the chips and transceivers are

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