Project: Laser range finder... big distance!
Sniper King
Posts: 221
this is something I have wanted to do for a really long time.· I ave had a few claiming to hit 1000 yrds but in reality at 1000 yrds it had to be a white building facing you in the dark.· So I started looking for the "build your own laser Range Finder" sites.· Their are some clever rangefinder plans bassed on angular measurement and even some good hardware ideas to support that.· But alas...· I want the real thing and I want miles not yards!!!
OK optics, i think i am good.· Two childrens (for now) reflecting telescopes.· One for transmit and one for recieve.... Columnated.... ready for laser and detector.· Now...· How do we amplify our received laser energy?· Think the propeller could handle the speed of light?·
What do you say?
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
OK optics, i think i am good.· Two childrens (for now) reflecting telescopes.· One for transmit and one for recieve.... Columnated.... ready for laser and detector.· Now...· How do we amplify our received laser energy?· Think the propeller could handle the speed of light?·
What do you say?
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
Comments
Regards,
Xander
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| To know recursion, you must first know recursion.
| Current projects and ramblings: I'd Rather Be Building Robots
The speed of light is 299,792,458 meters per second... so taking the inverse, that's about 3.33ns per meter.
Running at 80MHz, the resolution of the Propeller is 12.5ns which would give you a distance resolution of roughly 1.88 meters.
The value of 1.88 meters is half of what you would expect from dividing 12.5ns by 3.33ns, because you apply the "round trip" distance in this circumstance.
"Two childrens (for now) reflecting telescopes.· One for transmit and one for recieve...."
I like this idea, but why not use a laser (transmit) and only one reflecting telescope (receive)?
"Now...· How do we amplify our received laser energy?"
There are several ways this could be done... most commonly you would modulate the laser (transmitter) at a specific frequency or carrier.
On the receiver look for the presence or non-presence of that carrier frequency... apply the same principles that you would for radio when detecting a "carrier", but apply them to the laser beam instead.
·
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
(Links)
···· Detector· ' 940nm +/- 50nm
·····Laser· ' 30mw 904nm
Add a opAmp· to the detector and a amplitude modulated 38khz signal and we have a rangefinder hardware.· Make the output binary and now we can feed it into the Propeller.· For best results i would imagine a assembly routine to handle the speed at its finest.
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
The telescope for the TX portion is to get as much laser energy as possible in the smallest area we can.· Doing a little research brought me to a page where somebody did just that and had great results.· As for infrared, I'm sure I am not going to get lense transmission data from XYZ toy/telescope company.· That brings me to mirrors.· One small· convex lense from Edmund scientific designed for IR·and the front surface mirror of the telecope and we have a TX/RX pair that is reasonably priced for the expected ranges we could get.··
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
Post Edited (Sniper King) : 7/1/2008 7:03:58 AM GMT
"Add a opAmp· to the detector and a amplitude modulated 38khz signal and we have a rangefinder hardware."
An opAmp by itself may not be enough... you may need an LC·tuned·to 38kHz·excited by the IR receiver, and then use an opAmp.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
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I see having the detector on.· Turn on the laser and 'listen' for a return.· In assembly i can imagine this is only going to take a few lines of code...
In spin it would be something like this...
Outa[noparse][[/noparse]laser]:=1
repeat I from 0 to 500_000_000
if ina[noparse][[/noparse]detector]==1
··· range:=i
·' Do some math to get actual range in meters
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
I think a better approach would be to use continuously-modulated light and run the outgoing modulation and incoming modulation through a phase comparator. The phase difference will give the distance. Even better would be to modulate the light at a much higher frequency with a maximal-length sequence from an LFSR. Assuming a phase detector can be programmed into the Propeller that's fast enough to lock onto the returning signal, this approach should yield better results when the return signal is weak. The Prop's video output may be a key factor it making this work.
-Phil
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'Still some PropSTICK Kit bare PCBs left!
To explain to others - typical laser pointer has a beam diameter of let say 5 mm and a divergence of 1.2 mRad - At line of sight maximum to the horizon on the ground, a distance of 2.89 miles, the beam would be over 18 feet in diameter.
John
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
-Phil
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'Still some PropSTICK Kit bare PCBs left!
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
Another way to do this would be to convert the incoming signal back to a digital clock train. Then you'd feed the digital signal, together with the original clock into one of the Propeller's counters, using the LUT logic mode configured as an exclusive OR gate. In this mode the counter will increment only when the two signals are different. The amount the counter increments over several cycles will then be proportional to the phase shift between the two signals.
I've never done any of this, so there's a good dose of speculation here. It's your ball now. Good luck!
-Phil
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'Still some PropSTICK Kit bare PCBs left!
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
-Phil
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'Still some PropSTICK Kit bare PCBs left!
I think Phil is on the right track here... "Another way to do this would be to convert the incoming signal back to a digital clock train. Then you'd feed the digital signal, together with the original clock into one of the Propeller's counters, using the LUT logic mode configured as an exclusive OR gate. In this mode the counter will increment only when the two signals are different. The amount the counter increments over several cycles will then be proportional to the phase shift between the two signals."
The modulated frequency·at 38kHz is ok, the wavelength at this frequency is slightly less than 5 miles. (4.9 miles) ... in other words, after every 4.9 miles the phase offset should complete 360 Deg.··If you are going further than 2.45 miles (<-round trip) then you could introduce a second frequency with a longer wavelength as a way to get a course adjustment reading, and then go with a shorter wavelength for finer readings.·
Still think you might be limited to about 1.9 meters resolution.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
I will put together an Eagle layout of the schematic.· If any really good EEs out there can check me that would be helpful.
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
-Phil
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'Still some PropSTICK Kit bare PCBs left!
something like - charge a cap with a series of pulses and let it decay-·to remove error.
I seem to remember a pulse that looks like a 3-4-5 triangle with the hyp's on the x -axis.·Used a flip-flop ....
The Bushnell's· use this i am almost sure....
let me do a bit of digging- might be from an old issued patent from the USPTO site....
At least I know there are old patents (read dead) with schematics on
how to do this with clocks much slower than the propeller.... .
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read the counter - then wait until·(laser_out:=1 and laser_in:=1) read the counter again to get flight time?
the article was not for this purpose of course - I think it had to do with timing in an RC circuit ....
even has the pulses from my minds eye ...
from the patent:
"A method has been developed to obviate the need for such high frequency clocks in a laser range finder. This method, developed by Laser Tech, Inc. of Colorado, is described in application for U.S. patent Ser. No. 08/375,945, entitled LASER RANGE FINDER HAVING SELECTABLE TARGET ACQUISITION CHARACTERISTICS AND RANGE MEASURING PRECISION, filed Jan. 19, 1995, which is incorporated herein by reference. In the Laser Tech circuit, a series of 30 or more individual laser pulses are emitted each time a trigger switch is engaged. A charging circuit is triggered with the emission of each pulse whereby a capacitor is charged at a first, relatively rapid rate during the time of flight for each pulse. After each return pulse is detected, a discharging circuit is triggered which discharges the capacitor at a second, much slower rate. A microcontroller times the discharge period and calculates a range based upon the time of discharge by the use of a calibrated formula. In one example, the capacitor discharge rate was 1000 times slower than the charging rate, thus allowing an 8 MHz crystal oscillator to be used as a timer clock source to yield an accuracy of .+-.1 meter at a 1000 meter range. An automatic noise threshold circuit sets a minimum threshold noise level which allows reliable detection of reflected laser pulses and a dithering circuit selectively provides increased accuracy."
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
This may be a little over simplified but it is close I think.··
Fire a pulse. at the same time start charging a capacitor very fast.··· When you detect· your pulse after bouncing off your target, stop charging the capacitor.· Using a much larger resistor, discharge the capacitor and measure the time.· Kind of like an RC Circuit x 1.5.
Am I crazy or it that all it will take?
This got really easy (NEVER say easy) if it is true.
I am getting excited!
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group
·
That method will work, but there are a few things, actually several things that I would be concerned with.
·
First of all your using somebody's patent that is there to provide proof of prior art to persuade someone else from using their design.
·
One thing that I think you will have problems·with in the·patent design·is in·trying to compensate for how temperature·and humidity·will affect the capacitor.· By charging/discharging the capacitor at different rates you can sort of compensate for this, but that only assumes that the charge and discharge rate·have a similar response to one another which may not always be the case.
·
I don't believe that it would be completely necessary to initially charge a capacitor very fast at the beginning.·
Really, all that you need to do, and this would compensate for temperature/humidity variations as well, is to...
·
1)·Turn on a laser while pulling the trigger and at the same time charge a capacitor to a very specific voltage.
2) Discharge the capacitor for a specific amount of time at a specific rate and note the voltage
3) Recharge the capacitor back to the very specific voltage used in step #1 and turn the laser off
4) While the receiver still "sees" the laser, discharge the capacitor at the same rate as in step #2 and note the voltage when the receiver·no longer· "sees" the laser
·
The purpose for step #2 is for calibration to determine a specific voltage drop over a known amount of time and discharge rate that may drift due to temperature.· Step #4 uses information obtained from step #2 to solve for an unknown amount of time, which later gets translated into a distance.
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Beau Schwabe
IC Layout Engineer
Parallax, Inc.
Post Edited (Beau Schwabe (Parallax)) : 7/2/2008 6:25:20 AM GMT
Timeline of rangefinder operation:
I would imagine a voltage comparator circuit would be more accurate and temperature stable too.
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·- Was it pin 11 or 26?· Hmmm....··I think the smell of smoke tells the whole story.· Must be 26.
Michael King
Application Engineer
R&D
Digital Technology Group