Re: ultrasonic distance measurement using 89c51


"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:kk8cs21nhlprtlbef3graee8or504vrlre@xxxxxxxxxx
On Sun, 04 Feb 2007 16:05:48 GMT, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:


"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
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On 4 Feb 2007 02:37:14 -0800, "chetanthegreat"
<chaudhari.chetan@xxxxxxxxx> wrote:

Hey folks!
I am set out to build an ultrasonic distance
measurement module using 89c51.
The range I hav in my mind is about 1 inch to say 25 inches. Is the
speed 89c51 offers enough for such kind of ventures?

---
The velocity of sound in air is about 1100 feet per second, which is
about 76 microseconds per inch. That means that for a two-way path
of 2 inches you've got 152µs to chop up into slices thin enough to
get the resolution you want.

Assuming that you're going to use one of the internal timers to do
the time measurement, that's going to depend on how fast they can
run and the frequency of your clock source. That data should be
available


Just to be a little clear(maybe for my own benefit than others) is that
speed of sound through air is 344 m/s = 13540 in/s. (it doesn't mater how
far the distance is except for atmospheric effects and such)

Assuming a resolution of x in, one has 13540/x hz. So if you want an
accuracy of 1 micron = 1/1000 in

---
1 micron = 1E-6 meter, not 1E-3 inch
---


yeah, I ment a mil ;/ Was converting from speed of light in m/s to in and
guess I got the units mixed up.

then that is 1.35*10^7 or 13 mhz. There is
going to be a point where its useless to get a faster clock but I have no
idea what this is.

Thats pretty resonable for the accuracy. Ofcourse I doubt one could get
that
accuracy but maybe.

---
If it's pretty reasonable then why do you doubt if anyone could get
it?

I never said its reasonable. I said its resonable for accuracy. I guess YOU
didn't CARE to read the SECOND SENTENCE?


If you wanted an accuracy of +/- 0.001" out of 2 inches, that's
+/- 0.05%, which I think is highly unlikely to be attainable since
the air temperature, pressure, and humidity would have to be known
to , say, twice that accuracy in order for the reading to be
believed.



Yes, but you do not know the conditions in which the device will be used so
this is all speculation. You might be entirely right. I was just trying to
give him an estimate on the clock speed that he asked for. What it does show
is that its entirely reasonable because its not like 13Thz or something.
13Mhz for a mil accuracy is pretty reasonable. This means that should easily
be able to get it within reasonable conditions. (I mean get within good
accruacy, maybe 1/100 in)



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