Re: ultrasonic distance measurement using 89c51


"John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:f9kbs2hk0o3tbsuvu3vgdsr6ojfhk5pte8@xxxxxxxxxx
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 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.

I'm not sure how accuracy one can do this using passive components but maybe
its worth a shot? If one can get a precise current source and charge up a
capacitor and then read the voltage precisely then the voltage will be
proportional to the distance(charge rate would depend on C and R which can
be used to keep the voltage in range of an ADC if needed).

Also, since your distances measured is fairly short maybe you can use an
laser? (not sure if it works with your application but I thought I'd mention
it. )








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