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advice on cheap pressure sensors

Discussion in 'Electronic Design' started by Ben, Oct 13, 2005.

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  1. Ben

    Ben Guest

    Hi all,

    I'd much appreciate any advice on my search for a cheap and simple pressure
    sensor. I need to make a device which can measure how hard a baby is
    squeezing a silicon tube, around 2cm thick. I figure the best way is to
    connect the tube via a pneumatic tube to a pressure sensor. So what I need
    is a pressure sensor that does not have to be accurate in absolute terms,
    but needs to be relatively sensitive, so it can measure changes in grip
    strength. I reckon if it went up to 1000 kPa that would be more than
    enough. Preferably it would come with an integrated USB interface and
    drivers which provided an API. Alternatively, if you know a good one but it
    just outputs a voltage, perhaps you could also recommend a good cheap and
    cheerful USB A to D converter?

    So far the best thing I've found is this, which you can combine with a USB
    interface from the same supplier:
    http://store.pasco.com/pascostore/showdetl.cfm?&DID=9&Product_ID=51374
    &Detail=1
    But I think it will only interface using their proprietary software - it's
    for classroom use, after all. But this gives you a basic idea of what I'm
    after.

    Other solutions to the basic problem also gratefully received!

    Cheers,

    Ben
     
  2. Tim Wescott

    Tim Wescott Guest

    That's an absolute pressure sensor; you want a differential one with the
    atmosphere as the reference pressure.

    Motorola used to make pressure sensors as a board-level part. I assume
    these were spun off into Freescale although they may be On semiconductor
    parts now. You'd have to build a little board to interface to an ADC.

    Or try these folks:
    http://www.phidgets.com/modules.php...ns&file=index&req=viewarticle&artid=22&page=1
     
  3. Guest

    Freescale's parts are good, I'm sampling them now. They have a version
    with integrated hose barb, very convenient. The ADC-1R2 from
    Superlogics is a serial-connected ADC which is very suitable for
    reading this type of device; I'm not sure if Superlogics has a USB
    version, but the ADC-1R2 is easily connected via USB-serial converter.
     
  4. xarvia

    xarvia Guest

    Hi,

    Freescale have a good variety of pressure sensors, they are about
    $10-$15US from digi key (www.digikey.com). Digi key also has a category
    for pressure sensors with 900 products and a decent parametric search
    function:

    http://www.digikey.com/scripts/DkSearch/dksus.dll?Criteria?Ref=282967&Site=US&Cat=33489888

    As for connecting to a computer, personally I use a Microchip PIC with
    RS232, if you want USB and do decide to use a microcontroller for A/D
    conversion I'd still use RS232 and buy an off the shelf RS232-USB
    converter. Playing with USB in microcontrollers seems more bother than
    it's worth.

    Good luck!
     
  5. Ben

    Ben Guest

    Hi all,

    thanks very much for all your advice. Those Phidgets look perfect for
    me... I'm not averse to getting my soldering iron out once in a while
    but it looks like I could avoid it completely with those and save a bit
    of time. They are dead cheap too.

    Cheers,

    Ben
     
  6. Guest

    Dr. Ben, an inexpensive solution might be to salvage the pressure
    sensor and electronic from an inexpensive, broken, blood pressure
    monitor. (Usually the compressor on these little boxes fails well
    before its other components.)

    Off-the-shelf solutions are available, but tend to be costly.

    Harry C.
     
  7. Frank S.

    Frank S. Guest

    Put a piece of black anti static foam (like the kind you put IC's in)
    between 2 pieces of metal (I used PCB material)
    That is the cheapest way I know about.

    Frank
     
  8. The answer really depends a lot on volume, financial requirements, and
    exact specifications. If you are are looking for a one-off system and
    willing to spend the money, you can buy an integrated pressure sensor
    (Motorola makes some for instance) with voltage output and a data
    aquisition board (www.windaq.com or National Instruments). This solution
    would be rather expensive (my guess is on the order of $150 or more) and
    it may not be terribly accurate, but it would get you up and running fast
    and may have adequate accuracy for your purposes.

    There are a few things you didn't mention in your post in terms of
    requirements:

    1) How much resolution do you need? (I presume that "accuracy" must be to
    at least +/- 1/2 of the resolution)
    2) How much full-scale range do you need?
    3) How many samples per second do you need? (presumably this would be on
    the order of 1 per ms, and not a major requirement)
    4) You mention that you do not care about absolute values, so it sounds
    like you don't care about an offset in the result. But do you care about
    an offset that changes over long periods of time or over temperature?
    5) Do you care about absolute gain (i.e. Does a specific pressure change
    have to correlate to a specific output change)?
    6) If you do not need an absolute gain, do you need to ensure that the
    gain doesn't change over long periods of time or over temperature?
    7) Can you calibrate against a known accurate sensor either at production
    or at any time in the future?

    Accuracy is going to be a function of three things: the pressure sensor,
    the A/D converter, and the signal processing between the two. The easiest
    solution I can think of for the A/D conversion and computer interface that
    doesn't include buying a premade board is to buy an 8-bit microcontroller
    that has a built in USB interface and A/D converter. However, make sure
    that the microcontroller's A/D converter is adequate for your required
    task, otherwise you'll have to buy an external one (if you are trying to
    get very accurate/high resolution #'s at relatively slow speed (< 10k
    samples a second) you may be best off with an external sigma delta A/D,
    but it may be overkill).

    While integrated pressure sensors are very easy to use, it may be cheaper
    and more accurate to get a dedicated current bridge style sensor. This
    will require more sophisticated signal processing, possibly aided by
    digital calibration in the microcontroller. I only recommend going this
    route if you are forced to by the specifications of the project or if you
    are going for very high volume (and it actually saves money).

    The signal processing could range from virtually nothing to a carefully
    designed current source, filters, and precision operational amplifier
    setup, and again will vary completely on the requirements of your project.

    good luck,
    dan
     
  9. What about a salt water filled manometer? The salt water forms one plate of
    a capacitor. The other plate is metal foil surrounding the plastic tube
    manometer. Pressure causes the salt water to rise in the tube increasing
    the capacitance. Put that capacitance into a square wave generating
    circuit, 555 for example, and then measure the changes in output frequency.

    R
     
  10. Steve

    Steve Guest

  11. Don Foreman

    Don Foreman Guest

    1000 KPa? These are baby gorillas? Bionic babies?
     
  12. Are you measuring pressure or force? Pressure will depend on the surface
    area acted upon which will depend on the child's grip.

    Force could be easily measured with a strain gauge attached to a
    suitably designed spring. Something like a metal rod bent into a long,
    narrow U shape, 2 cm across. If you can't control the direction that the
    baby will squeeze, place two such springs at right angles inside the
    tube, measure the strain in each axis and sum their vector quantities in
    software.
     
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