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 02:33:03 GMT
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Viewed:
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605 times
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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...
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> 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
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turn it off |
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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 3 Replies:
 | | Re: IR scanner
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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 (...) (25 years ago, 1-Dec-99, to lugnet.robotics)
| | | Re: IR scanner
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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 (...) (25 years ago, 1-Dec-99, to lugnet.robotics)
| | | Re: IR scanner
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| (...) Um. Your frequencies are out somewhere... There is a factor of ten 'slip' in the figures being used here. In fact your oscillator needs to be at ten times this frequency for a 1ft/Hz measurement. (the figure in the first line should be (...) (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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| (...) 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 (...) (25 years ago, 30-Nov-99, to lugnet.robotics)
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