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 04:24:49 GMT
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Original-From:
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dave madden <DHM@MERSENNEantispam.COM>
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Viewed:
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573 times
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=>From: "Matthias Jetleb" <jetleb@netcom.ca>
=>...
=>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.
[My summary: the timescale involved in measuring the travel time
of light over smallish distances is not *that* far removed from the
realm of ordinary experience -- example used was radio frequencies]
However, radios don't try to do much more per signal cycle than stay
locked to the carrier, typically with a phase-locked loop. The carrier
is changing rapidly, but the signal is not. In any case, (AFAIK)
radios have a dedicated circuit to implement the PLL -- nobody uses a
general purpose computer to track the carrier and extract a signal.
=>The second point is that even the cheapest auto-focus (not fixed focus)
=>cameras use LED range finders - total cost starting at about $120.
I believe IR auto-focus uses an edge-detecting transform on part of
the field of view, and adjusts the focus to achieve the sharpest
edge. Ultrasonic auto-focus, OTOH, uses the round-trip time of a
sound pulse.
=>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:
=>[ascii art: measure time to send & receive N pulses ]
This still requires the ability to accurately control, if not measure,
time periods on the order of 10ns. That's possible -- barely -- on a
current PC, but interrupt latencies make it impractical or impossible
to measure arbitrary distances using these methods.
=>P.S. Does anyone know what processor Lego is actually using and at
what clock frequency?
It's a Hitachi H8 (HD6433292) running at 16MHz, according to Russ
Nelson's web page at http://www.crynwr.com/lego-robotics/.
regards,
d.
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Message has 1 Reply:
 | | Re: IR scanner
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| That STILL is NOT the correct value for the speed of light! It is 186,000 * 5280 = 982,080,000,000 feet per second (approximately). To get 96,720,000 you must have multiplied 186,000 * 520. - Nick - (...) (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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