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 .
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.
