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Airflow sensor

Discussion in 'Electronic Design' started by The Al Bundy, May 28, 2004.

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  1. The Al Bundy

    The Al Bundy Guest

    For a project I need to have an airflow sensor to protect a circuit in case
    of a fan failure. However this airflow sensor needs to be fitted on a PCB,
    and if possible only SMD.

    Analog devices has an airflow sensor (TMP-12), but this one is not what I
    need. 1) the ambient temperature is fixed (can be fixed in some way to let
    the device work with a variable ambient temperature), but 2) the resolution
    is bad, max 5deg centigrade deviation. I would need to detect an airflow of
    min. 0.5m/s. That is not possible with this device.

    After a long search I couldn't find any airflow sensor that can be mounted
    on a PCB (and SMD). So my question is if someone knows such sensor or how to
    build one with simple components?

    An idea that I have is to have 2 identical diodes, one self heating due to a
    constant current flow and one for measuring the ambient temperature. When no
    airflow around the self heated diode the forward voltage drop will get
    lower. With an airflow this voltage will go higher (diode cools down). The
    delta voltage with the identical temperature sense diode is a measurement of
    the airflow. Am I correct?

    If someone has any source of information about this subject it would be
    handy:)

    Thanks,
    Al
     
  2. mike

    mike Guest

    It's often better and easier to measure the thing you really care about.
    The Temperature of the sensitive device is probably easier to measure
    than the secondary airflow.
    Anything that has a temperature dependence can be used in a bridge.
    Low thermal mass is better.
    Take a look at hot-wire anemometer designs. Mine has two thermistors.
    The BIG one is about the size of a grain of sand. The LITTLE one needs
    a microscope to see it.
    You can make a sensitive airflow detector from two incandescent
    light bulbs side by side. Bust the glass off one of 'em. But it's
    not rugged enough for general use.
    mike

    --
    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
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  3. Tam/WB2TT

    Tam/WB2TT Guest

    Makes sense. Where you actually have to measure airflow is something like an
    automobile engine. Not positive, but I think the mass airflow sensor
    measures the resistance of a piece of nichrome type wire, with enough
    current going through it to self heat.

    Tam
     
  4. Yes, what you are suggesting should work. I'll suggest an alternative-
    look at the ripple in the fan current if it's a DC brushless fan. You
    can tell if it has seized or gone open. I'm not sure if this method
    has been patented by someone or not..

    Best regards,
    Spehro Pefhany
     
  5. Terry Given

    Terry Given Guest

    without airflow, you are left with adiabatic heating (near enough) so the
    mass of your heatsink soaks up the heat (actually the Rtheta just got real
    high, so the thermal time constant just got a lot bigger, as did the
    steady-state temperature. but the bit your interested in looks adiabatic, ie
    you lose bugger all heat until T is way too high). The mass gives you time,
    which is why a thermal trip works - if, of course, it responds quickly
    enough. But in general heatsinks are big and thermal sensors are small, so
    it works well. I have used thermal switches, NTCs, silicon temperature
    sensors etc - all can do the job. And you can stick the sensor anywhere you
    like, as long as you experimentally determine the corelation between
    Tmeasured and Thotspot.

    Funny story: during development of a 100kW 3-phase motor controller, I had a
    junior engineer playing with ducting around the heatsink fins, to maximise
    cooling. He was using cardboard & masking tape for the ducting. Which worked
    fine until he forgot to turn on the fan (separate supply for this test as we
    only had a mechanical prototype). The unit was 97% efficient, so dumped 3kW
    into its uncooled heatsink, which got hot, and set all the cardboard on
    fire. Production staff came and got him when 3' flames were seen coming out
    of the enclosure, which was unsupervised. Procedural changes followed. And
    all of our techs told him to go to hell when he essentially demanded they
    rebuild his drive - you fucked it, you fix it. He got a bawling out over the
    lack of supervision, and a public one for not being nice to the techs. And
    he had to re-built it himself. I betcha he never made that mistake again. We
    then wired a relay in parallel with the AC fan, that de-energised the main
    contactor if the fan power was off.....and made him stand and watch the
    experiment. And we put up a 5' perspex barrier on top of the 3' wall around
    the R&D test area (but that was more because one of our 400kW destructive
    tests involved rapid disassembly, and production staff got bombarded with
    shrapnel :)

    Cheers
    Terry
     
  6. Terry Given

    Terry Given Guest

    dont Papst use a similar trick in their expensive fans with "oh shit"
    outputs? I have done similar things in the past.

    Cheers
    Terry
     
  7. Maybe they use magnet, pickup and retriggerable monostable or
    something. I don't know how they work, but the fans are available with
    such outputs and AC power.


    Best regards,
    Spehro Pefhany
     
  8. Or just use the tachometer-equipped fans common in many PCs. Motherboard
    fan/temperature monitoring with data available on an I2C buss is
    commonplace these days.
     
  9. Dan Major

    Dan Major Guest

    The best, simplest, most failure-resistant design I've seen was a small
    flap attached to a microswitch. The flap was positioned in the airstream.
    When there was airflow, the flap kept the switch open. When there was no
    airflow, the switch was closed. Mechanical solutions frequently can beat
    electronic ones due to simplicity, cost, ruggedness, and ease of
    implementation.
     
  10. Objection! I have seen this solution in the old Ampex VTRs, and it was a frequent
    source of failures.
    The things would get stuck, damaged by maintenance (you just needed to touch it),
    and usually cause severe damage to overheating to the equipment if they did
    not work.
    Tacho is good.
    And proportional too:

    CPU core: +1.66 V (min = +1.53 V, max = +1.73 V)
    I/O: +3.33 V (min = +3.13 V, max = +3.43 V)
    +5V: +4.83 V (min = +4.70 V, max = +5.23 V)
    +12V: +12.07 V (min = +11.39 V, max = +12.44 V)
    CPU Fan: 4787 RPM (min = 4687 RPM, div = 2)
    P/S Fan: 2824 RPM (min = 2700 RPM, div = 2)
    CPU Temp: +57.6°C (limit = +63°C, hysteresis = +65°C)
    SYS Temp: +38.9°C (limit = +45°C, hysteresis = +48°C)
    VIA686a Temp:
    +26.3°C (limit = +38°C, hysteresis = +40°C)

    Ain't Linux COOL :)
    This was lm-sensors.
    JP
     
  11. The temperature data is probably as helpful as the tach data. If a fan
    stops, you have time to shut down before the temp rises. But if the
    airflow is blocked (and the fan keeps going), you'll be glad you had
    overtemp alarms too.

    Interesting question: If the airflow is blocked, what happens to the fan
    speed? Vacuum cleaners speed up when you block the hose. If the P/S or
    CPU fans do so as well to a significant degree, maybe a fan overspeed
    alarm could be added to the code.

    --
    Paul Hovnanian mailto:p
    note to spammers: a Washington State resident
    ------------------------------------------------------------------
    It's easier said than done.
    .... and if you don't believe it, try proving that it's easier done than
    said, and you'll see that it's easier said that `it's easier done than
    said' than it is done, which really proves that it's easier said than
    done.
     
  12. I have tried, power supply fan does not seem to speed up if you
    cover the outlet..., not 100% though, but the reason is likely
    that these motors are synchronous somehow.
    In it is a 3 pole stator, with the magnets rotating around it.
    There are no brushes, so the 3 stator windings likely are driven by
    some small chip (3 phase rotating?).
    (I opened one up for repair, some small PCB is in it).
    JP
     
  13. Tam/WB2TT

    Tam/WB2TT Guest

    If there is no overlap between the blades, he could shine a light through
    the fan at a light detector. for a 4 bladed fan, that would give 4 pulses
    per revolution. Amplify it and rectify it.

    Tam
     
  14. The Al Bundy

    The Al Bundy Guest


    OK, I agree with this. The only thing is that I can't use any mechanical
    sensing device such like a DC fan, flap and switch or anything like. The
    reason why is that it must be placed on a PCB without any adjustments for
    calibrating the sensor, and it must be as small as possible (if possible
    even no through-hole device)

    I will just put something together with a self heating diode and a measuring
    diode and see how it reacts. Will post the results:)

    Thanks everyone for the ideas!
    Al
     
  15. Richard Lamb

    Richard Lamb Guest

    A google search for 'Electro fluidic autopilot truned up the
    Sport Aviation index at:

    http://www.cozybuilders.org/ref_info/sportavi80.html

    August 1980 issus

    The project uses two thermosistors as an airflow sensor.

    Includes the complete schematics and quite a bit of theory...


    Bext of luck,


    Ricahrd Lamb
     
  16. The Al Bundy

    The Al Bundy Guest

    Sounds interesting! Only the small problem is that I never heard about this
    magazine.. and august 1980 is already some time ago, so I cant get it in a
    shop anymore:)

    If someone can scan these pages I would be very happy!

    EAA Sport Aviation: August 1980

    I guess these pages:
    p16...p24 : Fine Tuning The Electro-Fluidic Autopilot.. . by Doug Garner

    Thanks,
    Al
     
  17. Wouldn't be fairly easy to detect a failed fan by the power it draws?
    You could put it in a vacuum system and call it a Pirani-style pressure
    gauge. The thermal conductivity of a gas depends on the pressure. Other
    metals used in the Pirani gauge include tungsten, nickel, iridium, and
    platinum.
     
  18. Richard Lamb

    Richard Lamb Guest

    Sorry Al. It's NEVER been on the new stand.

    Sport Aviation is (one of) the private magazines published by the
    Experimental Aircraft Association.

    Chapters world wide - probably one near you.

    Or try www.EAA.ORG ?


    Richard
     
  19. Terry Given

    Terry Given Guest

    depends entirely on the fan. cheaper ones, yes. better ones, perhaps not - I
    have tested Papst fans that draw the same current when stalled as running. I
    have tested other fans that catch fire when stalled.
    Cheers
    Terry
     
  20. Terry Given

    Terry Given Guest

    I looked at a little AC drive (50kW or so) in our service dept. once, that
    kept tripping on over-temperature fault. The IP56 (ish) drive was in a milk
    powder manufacturing plant. The heatsink was an extrusion with 1.6mm slots
    spaced about 5mm, and had 1.6mm Al plates glued in place for the fins (we
    later changed to mechanical staking as it was better. And Aussie stopped
    selling us the weapons-grade Al powder we had used for 10 years to allegedly
    improve the thermal conductivity of the epoxy - later tests showed the Al
    did bugger all). So the fin-to-fin gap was about 3.4mm. Except this drive -
    the entire heatsink was a solid block of baked milk powedr & Al - water
    blasting wouldnt get rid of it, we had to replace the whole damned heatsink.
    Other than that, the drive worked - yay for thermal protection circuitry.

    Cheers
    Terry
     
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