Does anyone have any references/examples for detailed circuitry for time-of-flight laser distance measurements - schematics, appnotes etc.?
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asked Nov 5, 2010 at 22:53
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\$\begingroup\$ Oh wow. RF Engineering here we come! \$\endgroup\$– Connor WolfCommented Nov 6, 2010 at 3:21
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\$\begingroup\$ I'm interested in distances in the 1-5 metre range - I've seen a few neat tricks for pulse-width to voltage conversion but no full schems. GHz bandwidths aren't necessary as you're looking at changes in pulse edge delay. You can buy handheld laser tape measures that have ~2mm accuracy for $100, and I somewhat doubt that they 300GHz electronics inside. \$\endgroup\$– mikeselectricstuffCommented Nov 7, 2010 at 11:34
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5\$\begingroup\$ All the handheld "Laser" tape measures you find commonly are a sonar tape measure with a laser pointer stuck to it. \$\endgroup\$– Connor WolfCommented Nov 8, 2010 at 1:20
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\$\begingroup\$ Oh, it's OT, but your user profile webpage link is misspelled, mikeselectricstuff. (Ans I can't find any way to send a PM) \$\endgroup\$– Connor WolfCommented Nov 8, 2010 at 1:22
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3\$\begingroup\$ It's very likely that any solution for this sort of thing is going to use interferometry. As I understand it, most short-distance "Time-of-flight" measurement systems actually modulate the laser at a high frequency, and use the phase-difference generated by the period of time the laser takes to travel the distance to actually compute how far the object is away. \$\endgroup\$– Connor WolfCommented Nov 8, 2010 at 1:27
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3 Answers
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I also wondered how these affordable devices work.
Searching for 'laser distance measurement patent' I came across: http://www.freepatentsonline.com/3733129.html . Note that this patent was filed already in 1973 so I could imagine that today's laser distance measurement devices use some other method.
From what I understood is this using the target as one of the reflectors of the laser cavity and then modulates the laser trying to find a resonance frequency which seems to be related to the wavelength of an electromagnetic wave with this frequency (e.g. 1 meter seems to correspond to 150 MHz, bringing this down to a more 'manageable' frequency range).
I have to admit that I did not fully understand the details in the patent abstract though.
I'm not convinced though how well this actually works with 'real world surfaces'...I would think that e.g. a wall surface alone is not reflective enough to keep the system lasing.
EDIT: I found another page here: http://www.acuitylaser.com/resources/principles-measurement.shtml . Looking at the 'Time of flight' section, they are a bit more specific: the reflected laser light is focused onto a light diode whose (inverted) signal is then used to modulate the laser. This will form an oscillator. The phase shift is determined by the distance to be measured (apart from fixed delays inside the device), so the frequency of this 'oscillator' is determined by the distance and can be measured.
(Again, the time it takes to travel to a object at e.g. 1.5 m distance and back is 3m/3E8 m/s = 10ns which corresponds to an oscillation frequency of 100 MHz).
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4\$\begingroup\$ I think you'll find that the 150MHz component is not part of the lasing as such. Apparently, the power of the entire laser is modulated at that frequency, and the idea is to treat the optical path as a delay line, and use that delay to set a frequency of oscillation. Dead clever. An object 1m away gives 6.667ns of delay, 150MHz. An object 2mm further away, 6.680 ns, or 149.7MHz. Trying to measure the actual time of flight would require high performance electronics, but make the thing into an oscillator, and you can easily distinguish 149.7MHz from 150MHz. Brilliant! \$\endgroup\$– JustJeffCommented Nov 7, 2010 at 21:30
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2\$\begingroup\$ Basically, any "Laser" tape measure you buy at Home depot is actually an ultrasonic sonar ranger with a laser pointer stuck to it. \$\endgroup\$ Commented Nov 8, 2010 at 1:24
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1\$\begingroup\$ Yes - there are a lot of cheap ultrasonics with pointers, but at around $100 there are some real laser ones, e.g. from Bosch. I just bought one one Ebay to investigate. \$\endgroup\$ Commented Nov 8, 2010 at 9:27
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\$\begingroup\$ lucidscience.com/pro-laser%20spy%20device-1.aspx \$\endgroup\$– tybluCommented Dec 25, 2010 at 18:05
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2\$\begingroup\$ Wow, that lucid science link has the theory of operation completely wrong. the window pane is modulating the reflected laser light. It's pretty much just a photophone but with a laser instead of the sun. The movement of the window glass is amplitude-modulating the received laser light. \$\endgroup\$ Commented Jan 22, 2011 at 20:34
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Contact Maxim-IC and ask for their whitepaper number "HFRD40". It describes a laser rangefinder using time-of-flight. Straightforward to build. It basically uses a time-to-voltage converter (charge a cap) to measure the TOF of a bunch of laser pulses (to average out noise) then uses an ADC to measure the voltage on the cap. Their whitepaper has a complete design. They sent me one of their test units a while back, because I was looking at building one for a scanning lidar system, but ran out of time. You can see some more discussion here: http://forums.trossenrobotics.com/showthread.php?t=4357
answered Jan 22, 2011 at 18:34
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\$\begingroup\$ This technique also works for single shot ranging. \$\endgroup\$ Commented Jan 23, 2011 at 21:35
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2\$\begingroup\$ Thanks - eventually got this doc from Maxim. It's based around their MAX3806 chip. Still pretty complex, and uses very short 5 watt (!) IR laser pulses. I was mostly interested in the Rx front-end, so this is a useful starting point. As I'm only interested in shorter ranges I'm wondering if a simpler approach is possible by putting the TOF path in the loop of an amplifier, to make it oscillate at a TOF-proportional frequency. \$\endgroup\$ Commented Jan 29, 2011 at 10:08
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\$\begingroup\$ The Maxim doc also references this academic paper which looks interesting: herkules.oulu.fi/isbn9514269667/isbn9514269667.pdf \$\endgroup\$ Commented Jan 29, 2011 at 10:14
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12\$\begingroup\$ Yeah, I'm the guy that works at Maxim who manages this product. Send me an email to: mike.roberts@ maxim-ic.com and I will forward this ev kit paper to you. Mike \$\endgroup\$– user8503Commented Mar 6, 2012 at 16:46
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You can look at current state of the art for Lunar Distance measurement http://en.wikipedia.org/wiki/Apache_Point_Observatory_Lunar_Laser-ranging_Operation I think that this materials are public property. There is another link there to Single Photon Avalanche Photodiodes
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2\$\begingroup\$ This is actually a harder problem, because you need sub-nanosecond timing resolution that is good over second-scale spans. \$\endgroup\$ Commented Nov 8, 2010 at 16:14