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Timing Critical Wireless System

Discussion in 'Electronic Design' started by [email protected], May 13, 2007.

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  1. Hi

    I just need an advice on a possible wireless solution for my system.

    My task is to collect responses from test-subjects in medical research
    studies. This is done with different response pads with push-buttons.
    Since the test subject is inside a scanner during the examination it
    would be practical to have battery driven response pads with a
    wireless transmission solution instead of cabling.

    But my biggest problem with such a solution I guess is the timing
    issue because timing in these studies are often critical, and
    deviation in the delay from one response to the other more than a
    10-100 ms caused by signal delay in the system is not accepted.

    Does anyone have a suggestion on a ready to use solution for such a
    timing critical wireless system?

    -Transmission length is maximum 7-10 meters (30 feet???) from sender
    to receiver
    -This is inside a faradays cage, meaning little RF noise in the room,
    but for regulative reasons only the common used frequency bands
    (open) should be used.
    -Up to 3-4 independent transmitters should be able to work with the
    receiver simultaneously

    My idea is that some alarm systems probably are wireless with strict
    requirements, and not to forget high-end gaming devices that use a
    wirless solution (if that excist...)

    I'm greatful for any suggestions or advice.

    Stian SS
     
  2. Donald

    Donald Guest

    Whats the scanner ?
     
  3. An RF solution shouldn't have too much trouble with the latency
    requirements. The Faraday cage and scanner make me think that avoiding
    interference with the equipment might be your biggest problem. Have you
    considered an IR link?

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  4. Guest

    Again - what type of scanner?

    The shielding strongly suggests an MRI scanner, in which case the
    problem will not just be rf interference with the scanner.

    Two other problems will be:

    1) The scanner transmits very high power pulses of rf whiich could
    have a bad effect on your circuit.

    2) There will be a VERY powerful magnetic field which can easily turn
    seemingly harmless items such as batteries with steel casings into
    dangerous projectiles.

    You would be better off using a fibre-optic system where the button
    interrupts the light path between two optical fibres. All electrical
    and electronic components could then be outside the shielded
    enclosure.

    Alternatively, thin plastic tubes and a pneumatic bulb under each
    button and a pressure switch at the other end could work.

    John
     
  5. MooseFET

    MooseFET Guest

    Do you need to react quickly or merely know exactly when the pad was
    pressed? You can have the circuit that it connected to the button
    have an internal clock that it records at the instant of the press.
     
  6. Yes we are talking about an MRI scanner here, but to answer your
    questions:

    1: The response pads will be at the subjects hands, meaning outside
    the MR-bore not not so much affected by the shifting gradients and RF
    transponders - but it's not unusual to have electronics like LCD
    projectors/displays etc. located insider the bore mounted to he RF
    transponder coil.

    2: The RF interference of the scanner is in the frequency range 60 Mhz
    and 120 Mhz, if we're going for a solution outside the frequencies RF
    interference should not be a to big problem.

    So do anyone have any experience with wireless system with minimum
    latency?

    Thanks in advance!
     
  7. whit3rd

    whit3rd Guest

    How about TV remote controls? There's lots of handheld controllers
    to choose from, the number of available buttons is adequate,
    and receivers for computer (or other) connection are common
    enough; see www.lirc.org for more info.
     
  8. joseph2k

    joseph2k Guest

    Possible but also maybe not suitable due to EMC and high magnetic field
    compatability. Proper engineering should recommend a technology compatible
    with any type of medical scanner.
    Since controlled and preferably low latency is requires reliable detection
    with sub-millisecond total response time. This leads to continuous
    transmission at 10,000 symbols per second through the "push button" and a
    sturdy, reliable, low required force mechanism no matter how done. After
    that, RF, IR, and visible light are all reasonable choices. Air (or other
    fluids) will not be fast enough. The fiber interrupter looks like the best
    bet, followed by (perhaps moving mirror) free path optic interrupter (no
    wires of any kind, but much harder to setup and has other usage problems).
     
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