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 09:22:44 GMT
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Viewed:
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565 times
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In lugnet.robotics, Matthias Jetleb writes:
> 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)
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 967,200,000...).
It is not practical with any microprocessor to go down to these sort of ranges
(I am using 100MHz processors, driving fibre optic connections at work, and
delays are just 'see-able' by the processors, through 30m (100ft) cables -
also the velocity in a fibre is slightly slower than in air, so the real
position will be worse).
> The second point is that even the cheapest auto-focus (not fixed focus)
> cameras use LED range finders - total cost starting at about $120.
These don't use LED rangefinders in the sense you mean. They either use
ultrasonic, or actually detect the return light, and 'focus' the lens to get
distance. Not the same as measuring the 'pulse' time.
> 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
The processor is the Hitachi H8, at (I think) 16Mhz, and you are going to be
several orders of magnitude away from acheiving this measurement.
Best Wishes
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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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