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InfraRed Refective Long Distance Object Sensing

Discussion in 'Electronic Design' started by Mark, Sep 30, 2004.

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  1. Mark

    Mark Guest

    I am relatively new to Infra-Red Electronics and I need help please. I
    need to "see" how fast an Aluminum Bar (unpainted/unpolished) passes
    in front of a panel. The bar is about 25mm (1") in diameter and it can
    pass in front of the panel at different distances from it, ranging
    from 0 to about 250mm (10"). I need to do this with a row of Photo
    transistors (I cant use Ultrasonic's in this application) and a
    Microprocessor using the 10 bit ADC feature. My initial thoughts were
    to mount the photo transistors in parallel mounted Black tubes,
    pointing towards the target area, so that they had a limited viewing
    angle, flood the area in front of the panel with Infra Red light, see
    the reflection, let the Microprocessor decide if it is a valid
    reflection or not, and then calculate the speed in the usual way. The
    panel will be in daylight, but not pointing directly at any bright
    light source. Cost is an issue. so this precludes me from using very
    expensive devices.

    I would welcome any advice people can give me, like is what I am
    suggesting feasible? what size LED's/Photo-Transistors to use
    (3mm/5mm), what wavelength of Infra-red light would yield the best
    results, what would be a good starting length for the Black Tubes,
    recommendations of which device's to use, is it worth flashing the
    Infra-Red light source, to help with ignoring the ambient light
    problem, would I need to amplify the output of the photo transistor or
    will a simple resistor up to 5v be good enough with a 10 bit DAC.
    Sorry for so many questions, but as I say, I have not done much work
    with Infra-Red before.

    Thanking you in anticipation
    Mark in Spain
    (remove the X to reply)
     
  2. Hi Mark,

    What is the maximum speed that the bar (specimen) will traverse the
    aperture or measurement window?

    Will the specimen surface profile very in texture from specimen to
    specimen?

    Will the specimen always arrive in a fixed path?

    If the device has to be outdoors you should chop your source to remove
    DC effects of the sun. The black body or source should be a halogen
    bulb chopped at a frequency high enough to yield several cycles during
    the time that the specimen will be present.

    You will have to be able to accurately detect the presence and absence
    of the
    Specimen as it traverses a fixed window or aperture so you may need to
    have your device first validate the specimen then set a read mode
    where you detect the leading edge of the specimen, start a counter,
    and then terminate count upon detection of trailing edge as it passes
    out of view.

    I have designed coating systems where I had to open a shutter to
    evaporatively coat a small 25mm lens or mirror on a spinning table
    inside a vacuum chamber but I had the benefit of an absolute position
    encoder for triggering. You need to be able to trigger your "time of
    flight" counter upon specimen confirmation, which is going to be a
    little tricky.
     
  3. Dave Garnett

    Dave Garnett Guest

    Can you back illuminate ? ie put the sources on the other side and look at
    when the objects interrupt the beam. If you haven't got enough room for a
    source on the far side, then perhaps a retro reflector (plastic device like
    a bicycle reflector).

    Depending on the trajectory/geometry of the bar, why do you need more than
    two sensors ?

    If you are operating outdoors or in an artificially lit environment the
    using pulsed sources with synchronous detection is a good idea, as is using
    infrared filters.

    Dave
     
  4. John Fields

    John Fields Guest

    ---
    I suggest that you forget the reflective scheme and use beam-breaking
    to do what you want.

    But... what is the range of speeds you need to detect and how far
    apart were you planning on putting the IR detectors?
     
  5. Mark

    Mark Guest

    Beam breaking is not an option, I cannot get anything behind the
    target object. The speed I guess is about 10-30 mph (I will verify
    that with my customer later), I cannot attach anything to the target,
    the path that the target takes is constant but varies in distance from
    the panel by the amount already mentioned. I will be using about 20
    sensors at a set distance from each other to read the speed of the
    target (for reasons that is too complicated to explain at this stage),
    the target is not a constant width/shape, but I was going to use an
    averaging process (compute the peak refected light value, as being the
    centre (the taget is always round/ovoid)) with all the sensors I have.
    Accuracy needs to be within 1 or 2 mph, I will be running the
    Microprocessors at 20Mhz, 1 for each sensor). Hope that clarify's
    things a bit better.
     
  6. Jim Miller

    Jim Miller Guest

    obtaining the reflection from the aluminum bar will be difficult. 3M makes a
    reflective tape which acts as a retroreflector. can you apply this to the
    aluminum?

    jtm

    Beam breaking is not an option, I cannot get anything behind the
    target object. The speed I guess is about 10-30 mph (I will verify
    that with my customer later), I cannot attach anything to the target,
    the path that the target takes is constant but varies in distance from
    the panel by the amount already mentioned. I will be using about 20
    sensors at a set distance from each other to read the speed of the
    target (for reasons that is too complicated to explain at this stage),
    the target is not a constant width/shape, but I was going to use an
    averaging process (compute the peak refected light value, as being the
    centre (the taget is always round/ovoid)) with all the sensors I have.
    Accuracy needs to be within 1 or 2 mph, I will be running the
    Microprocessors at 20Mhz, 1 for each sensor). Hope that clarify's
    things a bit better.
     
  7. John Fields

    John Fields Guest

    ---
    Yeah, OK.

    What I'd do would be to plan on using an individual IRLED for each
    station, (instead of a light curtain) and collimating its output into
    a nice, tight beam. That'll get more power on the target and give you
    a better signal to noise ratio than if stuck the LED at the end of a
    piece of tubing, and you might be able to get away with using just an
    IR phototransistor in a tube and and a comparator as the receiver
    instead of having to modulate the IR beam and use an IR receiver to
    detect it. If you want to do it that way you can get some nice
    Fresnel lenses to do the collimating from http://www.fresneltech.com.
     
  8. Mark

    Mark Guest

    OH shit, lenses etc will most probably not be an option, we are
    looking to manufacture quite a few of these, and my subcontractor may
    be able to solder boards on his flow soldering machine, in the shed he
    has in his back garden, but glueing lenses into situ will be beyond
    his reproducable scope. Just to clarify things a little further, the
    black plastic tubes were in fact going to be a row of holes drilled
    with a vertical pillar drill, at equi-distant spaces in a black
    plastic bar, with the holes drilles to an exact depth, which the
    Photo-Transistors would be mounted in.
     
  9. How would you get "behind" the aluminum bar to create an interruptable
    beam and what if the bar changes profile longitudinally creating a
    false profile?

    Using the reflective approach permits distinguishing a face profile
    reflection from the bar wall. If the bar arrives in varying angles
    reflection is out unless he is familiar with using an array, a Fresnel
    lens, or a huge integrating sphere!

    If there is enough space between bars
     
  10. Rich Grise

    Rich Grise Guest

    Put two bars alongside each other, so each PHT corresponds to an LED.
    Pulse each LED at its own center frequency, and just watch. The maxima/
    minima thing seems to be something that you were going to have to work
    out anyway, right? 'Cause I have no clue about that. But it seems I've
    seen LEDs/PHDs with a little collimating lens built into the package.

    You say the distance will be constant, but varying? What does that mean?

    Cheers!
    Rich
     
  11. Rich Grise

    Rich Grise Guest

    Actually, aluminum's a pretty decent reflector in its own right. ;-)
    Especially if there's a black background.
    Might want to watch the skew angle, but shininess could narrow your
    response pattern.

    Cheers!
    Rich
     
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