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measuring distance between two cars using infrared circuits

Discussion in 'Electronic Design' started by [email protected], Jan 21, 2005.

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  1. Guest

    hello,
    i want to construct a circuit which will be able to measure distance
    between two moving cars.The two cars are needed to maintain a fixed
    distance between each other.
    if the distance between them increases or decreases the circuit
    should be able to detect this change and notify the amount of change
    in the distance to both cars.
    i want to construct this circuit using infrared LEDS ,so if any one
    can help me out with this(circuit idea) please mail it to me .
     
  2. Usenet is not a write-only medium. You post your questions here and you
    get your answers here, otherwise the only person that gains anything
    from it is you. That's not the way a public forum works. If you'd
    rather work this issue via e-mail, then feel free to e-mail your
    question to as many people as you like.

    Now to the question at hand. You need to specify the distance, weather,
    and other operating conditions that you expect this to work in. You are
    not likely to get this to work in the bright sun without allot of
    effort.
     
  3. Andrew Holme

    Andrew Holme Guest

    It might be easier with microwaves than infra-red:

    Measure relative velocity using the Doppler effect. By integrating
    this, you get a running estimate of the change in distance. Weird
    things might happen when you go around corners!

    Unfortunately, absolute measurement of short distances using
    electromagnetic waves is difficult / impossible due to the speed of
    light.
     
  4. peterken

    peterken Guest

    microwave is indeed more reliable
    using a bursting microwave gives an indication of absolute distance and
    speed between objects

    timing between start of burst and start of reception of it is a measure for
    absolute distance
    doppler frequency gives relative speed
     
  5. The SoL is a nanosecond per foot. A nanosecond isn't a particularly
    difficult thing to measure. Gate delays of modern semiconductor
    processes are in the few picosecond range.
    No need for doppler measurements. Differentiate the distance
    calculations.
     
  6. I can't help you with an electronic way of doing the task you require but I
    just happen to have worked with some computer software and hardware that can
    do this task very well.

    What you are looking for is a "Stereo Vision Pair" (two cameras feeds) and
    some software that works out the distance between the cameras and a selected
    object. The software is a little complex but I have written some code that
    does the trick with simple math. This is all done using trigonometry.
    The most complex part of the software is object tracking, once you have
    zero'd onto the object you can take the displacement of the object from the
    two image frames and use trigonemetry to workout the distance between the
    cameras (which are normaly 90 degree and 6 degree with a spacing of 60mm) .
    It's very processor intensive but there are a number of algorithms that are
    required to do this task optimally. Once you have this all working you can
    even tell the size of objects and distance from with great accuracy (down to
    about 1mm depending on the total distance range required.).

    I have a technique and system I developed from scratch, If you are
    interested I could give you a head start with information on object
    tracking, stereo vision metrics and optimizing algorithms for image
    processing. My system is developed using the C programming language and on
    the WIN32 API.

    The only down side to this all is that the software is rather complex when
    you put all the components together.

    Hope this may help.
     
  7. Jeroen

    Jeroen Guest

    Mercedes-Benz uses a radar device in their adaptive cruise control system on
    their S class cars. I think it's not easy to build such device yourself from
    scratch.

    Jeroen
     
  8. Mark Jones

    Mark Jones Guest

    Or use something much slower than light - ultrasonics.
     
  9. mike

    mike Guest

    One can only imagine what you're trying to do.
    If you're gonna use the information to CONTROL the cars, you're
    absolutely (fill in the blank with your favorite expression of dementia).
    You're quite likely to get sued by the estate of the person(s) you kill
    using your contraption...assuming you survive. Suggest you get a
    different hobby.
    mike

    --
    Return address is VALID.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    http://nm7u.tripod.com/homepage/te.html
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    http://www.geocities.com/SiliconValley/Monitor/4710/
     
  10. One interesting, and perhaps simpler approach, is to take the same
    approach that the sharp IR distance measurement devices use.

    These are cheap ($10) electronics gizmos that take 5V, and output an
    analog voltage that corresponds to the distance. They are not well
    suited to your application, because their maximum range is something
    like 80cm, but the scheme they use may be adaptable.

    What they do is have an infrared LED, and a sensing device, which
    consists of an array of sensors. The LED and the sensors are separated
    by a fixed distance (maybe 3cm?), and arranged so that the returning IR
    falls onto a particular sensor according to the angle. Using this, they
    can sense the angle of return of the IR, and compute distance from that.

    Seems like a similar scheme, albeit at a larger scale, might be usable.

    Oddly enough, this is the scheme that bees' 'compound eyes' use. They
    don't have a 'continuous' set of receptors like we do focused by a lens.
    They sense changes with a set of cone shaped segments.

    Feynman goes into detail about the compound eye in "The Feynman Lectures
    on Physics", volume I, ch 36. He makes some typically clever
    calculations, and determines that given a bee's eye, the maximal
    resolution vs the diffraction of light at the wavelength they care about
    will predict the shape of these cones. The formula he comes up with is

    sigma = sqrt(lambda * r)

    where sigma is the diameter of a segment at the tip, r is the length of
    the segment, and lambda is the wavelength of light to be seen.

    This may actually affect the minimum resolution of your detector. Say
    your detector is w from the source, and you are projecting IR which has
    a wavelength of lambda. You presumably wish to detect differences at
    distances of about 10m. Say the resolution is deltaD. Then

    deltaD = 0.5*w * (tan(t1) - tan(t2))

    where t1 is the angle at the farther distance, and t2 is the angle at
    the closer distance. Then by a little trig, we have

    deltaD = 0.5*w*(1 + tan(t1)*tan(t2)) * (tan (t1-t2))

    since by Feynman's formula,

    sigma = sqrt(lambda * r)

    if the angle of the opening is (t1 - t2), and the internal length is r,
    we have

    sigma = r * tan(t1 - t2)

    so

    r^2 * tan(t1-t2)^2 = lambda * r

    and

    tan(t1-t2) = sqrt(lambda/r)

    thus, by the formula above,

    deltaD = 0.5*w*(1 + tan(t1)*tan(t2)) * sqrt(lambda/r)

    Now, assuming that w is 2m, and we want to detect differences at 10m,
    tan(t1) is about 10. Thus

    deltaD = 101 * sqrt(lambda/r)

    For lambda = 1um, and a detector of 2cm in length, that means the
    minimum distance resolution that can be detected is

    deltaD = 101 * sqrt(1e-6/20e-3) = 750mm

    before diffraction causes problems. This is 7% of the total distance!
    However, if you use ultraviolet, then you can make the opening much
    smaller, since the wavelength is smaller. For UV at lambda = 10e-7, you
    can detect

    deltaD = 101 * sqrt(10e-7/20e-3) = 255mm

    which is a bit better.

    At smaller angles, the (1 + tan(t)^2) factor gets smaller. Thus, at 5m,
    we have 180mm, and at 2m, we have 35mm for IR with a 2cm r.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  11. John Larkin

    John Larkin Guest

    Well, radar was invented around 1939.

    John
     
  12. John Fields

    John Fields Guest

     
  13. Mac

    Mac Guest

    It is not difficult at all. It just requires bandwidth. Before anybody
    jumps on my case about detecting short CW pulses, let me point out that
    short CW pulses have a LOT of bandwidth, and the shorter they are the more
    bandwidth they have.

    The same rules would apply to a modulated IR signal. There is no way the
    OP is going to get any kind of high resolution ranging using IR alone
    because there is just not enough bandwidth. (Some laser diodes have more
    than enough bandwidth to do this, but I don't think they put out enough
    power)

    Ultrasound might work well. You could have a transponder on the back of
    the car in front and a range-finder on the front of the car in back.

    --Mac
     
  14. Mac

    Mac Guest

    This won't work.

    I suggest you try to think of a different approach. It seems to me as
    though radar is the best approach.

    Ultrasound might work, but high frequency ultrasound attenuates rapidly in
    air.

    You might be able to use two LED's on the rear of the car in front, and a
    video camera on the car in back. The distance would be calculated from the
    angular separation of the LED's. The LED's would have to be mounted with a
    carefully measured separation.

    Good luck. You're going to need it.

    --Mac
     
  15. Rich Grise

    Rich Grise Guest

    People have mentioned triangulation. Does a license plate retroreflect
    laser pointer light?

    Thanks,
    Rich
     
  16. keith

    keith Guest

    Why? If better than a few inch resolution is needed, which doesn't seem
    to be the case here, use interferometry or as another poster said,
    stereoscopic vision. The guts of a few optical mice may do a decent
    stereoscope.

    I'd think ultrasonics would be dicy in a noisy environment.
     
  17. keith

    keith Guest

    Ok, what's the bandwidth of a kHz modulated ~2GHz carrier (wherever there
    is some free bandwidth). It should be trivial to measure the round-trip
    delay to withing a nS, which is about six inches. At a kHz,
    that gives us a distance measuremnt every millisecond, which should be
    enough for distance and differentiate to give a relative velocity
    number.
    Since it *is* done, I'm not sure why you contend that it can't.
    It's *is* done without any transponder, which would make the idea useless.
     
  18. Mac

    Mac Guest

    Are you talking about on/off modulation of a 2GHz carrier at a 1KHz
    rate? How long is the "on" time?
    Please elaborate. What is done? Are you saying that there is an IR diode
    based system that can give precise range information? I am very interested
    in this system.
    I admit that the transponder is not essential. It just makes it easier to
    detect the signal, and increases the range over which the system would
    work.

    --Mac
     
  19. keith

    keith Guest

    Yes, pick your poision.
    I was referring more to the RADAR range finders that Merc is using for
    "smart" speed control. IR diodes may have different problems (ambient
    noise, etc).
    It also adds an unknown and significant delay into the path.
     
  20. Mark Jones

    Mark Jones Guest


    The design of an ultrasonic rangefinder would be trivial in
    comparison to a laser rangefinder.

    But of course, the OP may do what he likes.
     
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