Subject:
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Re: IR scanner
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 1 Dec 1999 03:41:57 GMT
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Viewed:
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672 times
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Your numbers aren't quite correct. The speed of light is
982,080,000 fps or 0.000,000,001,018 seconds per foot. That
yields a value of 0.002036 microseconds for a "radar foot";
12 inches out, 12 inches back. You are NOT going to measure
distance changes in the inch range with any microprocessor
that I know about. You CAN measure range by comparing the
phase angle of the echo to the xmit signal ... but you're
looking at some fairly complex analog circuitry to do it
with any accuracy.
- Nick -
Matthias Jetleb wrote:
>
> In reply to:
> > I may be underestimating the capabilities of current Lego sensors, but I • believe
> > that the time difference of (light across 1 foot) and (light across 10 feet)
> > would be too small to register without some seriously fast processing.
> >
> > [ BTOE check...
> >
> > 186000 mi/s * 5280 feet/mi = 96,720,000 ft/s ==> 1.0339e-08 s/ft
> > means we're talking about hundredths or thousandths of microseconds
> > here.....]
> A couple of quick points here: Your value of 96,720,000 ft/s, while
> accurate and sounds extreme should be considered in it's proper context.
> For the light to travel 1 foot, the object must be 6 inches away (6in there
> + 6in back = 1ft). For a crystal to measure that time period, it must have
> a frequency (not so coincidentally) of 96,720,00 Hz (96.72Mhz). Think about
> it, this is smack in the middle of the FM broadcast band which has been
> well within the frequency capabilities of even the cheapest $20 radios for
> the last quarter century - hardly a stretch of technology. Keep in mind at
> the average $100 cell phone works in the 870Mhz range and satellite
> receivers are in the Ghz area. (For that matter, ham radio has a Ghz Band)
>
> The second point is that even the cheapest auto-focus (not fixed focus)
> cameras use LED range finders - total cost starting at about $120.
>
> My third and final point is that the range computer doesn't have to
> calculate the time taken for the pulse to go and return (the calculation
> takes processor time), it is easier and more accurate to:
>
> Start the timer at 0
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> send a light pulse<-----------------------|Repeat 20 times
> | ^
> turn it off |
> | |
> signal received? No: loop here/Yes: ->|
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> End of loop: check timer value
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> Calculate distance
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> Divide distance by 20
>
> Regards
> Matthias Jetleb
> VA3-MWJ
>
> P.S. Does anyone know what processor Lego is actually using and at what clock
> frequency?
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Message has 1 Reply:
 | | Re: IR scanner
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| (...) That's my fault - I missed the button on the calculator, and so the fps value in my original post was off. (25 years ago, 1-Dec-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: IR scanner
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| In reply to: (...) believe (...) A couple of quick points here: Your value of 96,720,000 ft/s, while accurate and sounds extreme should be considered in it's proper context. For the light to travel 1 foot, the object must be 6 inches away (6in there (...) (25 years ago, 1-Dec-99, to lugnet.robotics)
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