How to measure extremely small amounts of time for LIDAR? - Page 1
Forum section: electronics / beginners

author: codysmith105
author_rank: Contributor
timestamp: October 12, 2014, 05:02:12 am
content: Hello EEV Community,I'm wondering how I could measure the time it takes light to travel small distances, say a few feet, accurately for a LIDAR.How do you poll a signal that fast?Thanks,-Cody

author: Rerouter
author_rank: Super Contributor
timestamp: October 12, 2014, 05:24:54 am
content: One way is to use a very fast XOR gate , 1st input being the source waveform second input being the return pulse, this forms a pulse length to PWM, which you can then smooth and measure with an ADC,

author: codysmith105
author_rank: Contributor
timestamp: October 12, 2014, 06:16:35 am
content: I've foundthis, but I have no idea where or if it's sold.

author: janekm
author_rank: Unknown
timestamp: October 12, 2014, 07:12:30 am
content: Well, how about a time to digital converter?http://www.acam.de/products/time-to-digital-converters/

author: miguelvp
author_rank: Super Contributor
timestamp: October 12, 2014, 08:54:02 am
content: Time of flight for distance is the same thing as what the kinect will do (Microsoft bought upZCamand implemented it in kinect 2). It's been around forever in theory, but now we do have processors that can measure nanoseconds and picoseconds (well, those are more rare) but a nanosecond is just 5GHz (well 1GHz) but you need 5 times the sampling.Pretty much you send an encoded pulse and then listen for it with a camera (or LIDAR) you know the speed of light so the time it takes to get back is half the distance than if you were on the other side (meaning you have to wait for it to reflect), compensate for your pipeline lag and you should have a good measurement on how far each reflected pulse is at.Change the encoding over time and you can measure depth.Btw here are some Grace Hopper nanoseconds for you to virtually have. As you can see they are not too long. Divide them by 1000 and that will give you a picosecond length but since you have to wait for the return signal it's really 2 pico seconds (or two Grace Hopper's grains of pepper). Yeah she carried nanoseconds on her purse, not sure if she did also carry pepperSource of picture:http://blog.americanhistory.si.edu/osaycanyousee/2013/03/finding-female-pioneers-in-mathematics.htmlRest her soul, even if it's been already 12 years since she passed away.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2014, 03:03:44 pm
content: I know a 300 MHz oscilloscope can do this with the appropriate transmitter and receiver with a resolution somewhere below an inch because I have done it. That gives some idea of the bandwidth required. With my fast sampling oscilloscope, I can make electrical TDR measurements down to the millimeter which is spooky.I did not know that the Kinect 2 uses real time-of-flight measurement unlike the earlier Kinect which uses 3D reconstruction. It looks like they get a resolution of 1 to 2 centimeters over a range of 0.5 to 2.5 meters which is impressive for a consumer product.If I were attempting this, I would amplitude modulate a coherent focused light source with a swept frequency, mix the transmitted signal with the received signal, and measure the nulls. This moves all of the high frequency requirements out of the digital realm although the analog RF requirements are not trivial.The alternative of a TDC identified by janekm can also be done in the analog realm with a resolution down to the 50 picosecond range relatively easily which gets you almost down to the inch and also does not require fast digital processing.

author: miguelvp
author_rank: Super Contributor
timestamp: October 12, 2014, 11:39:44 pm
content: Quote from: David Hess on October 12, 2014, 03:03:44 pmAccording to wikipedia, it can detect the player's heart rate.

author: coppice
author_rank: Super Contributor
timestamp: October 13, 2014, 02:28:43 am
content: Quote from: codysmith105 on October 12, 2014, 05:02:12 amWhat you are asking for isn't very fast. Light travels at about 1 foot a nanosencond. Double that for the loop time in a lidar setup. and resolving to a mere 2ns will give you the range in 1 foot increments. When people try to, say, range detail in a 3D scene, and are looking for millimetre resolution, things start to get more interesting. You might look at the high resolution TDC solutions for that kind of application.

author: codysmith105
author_rank: Contributor
timestamp: October 13, 2014, 04:42:42 am
content: Thanks for the replies guys.Ideally I'd love to cross into the inch resolution scale with light. I did the math, I know that I'm looking to sample in the 1 GHz range, as miguelvp indicated.So, the biggest problem I see is actually sampling that fast. I suppose I see two routes, build something that goes that fast or buy an IC that does it. My background is in CS not EE, but I do love to tinker. I guess I'm saying that I'm inexperienced but really willing to try to make things.Anyway, does anyone have a good example (link to) a suitable IC?Or could anyone elaborate on how one would build something that can sample at this rate?Essentially, I'm still scratching my head here.Thanks,-Cody

author: kolbep
author_rank: Frequent Contributor
timestamp: October 13, 2014, 04:58:27 am
content: I think I read somewhere that laser rangefinding does not use time-of-flight. But instead is an optical alignment thing. Where the further an object is away from the unit, the returning beam is further away from the center of the sensor adjacent to the laser emitter?but I am not too sure about that...

author: jeremy
author_rank: Super Contributor
timestamp: October 13, 2014, 08:17:45 am
content: Quote from: kolbep on October 13, 2014, 04:58:27 amIt can be done both ways. The device you are talking about is here:http://en.wikipedia.org/wiki/Position_sensitive_deviceand it can be used for extremely accurate distance measurements (easily below 1mm)

author: codysmith105
author_rank: Contributor
timestamp: October 13, 2014, 08:40:36 am
content: I've seen this method, the camera (or sensing device) is offset from the laser a certain amount and basically wherever the point from the laser is on the CCD is triangulated to determine distance.It's actually looking like a easier to implement method at this point.

author: miguelvp
author_rank: Super Contributor
timestamp: October 13, 2014, 09:05:09 am
content: Quote from: codysmith105 on October 13, 2014, 08:40:36 amI was going to propose that to compensate from the slow camera frame rate. If you know when it's coming back you can make it so it's on the threshold, but it will get complicated soon and you will need many frames and offsets to the laser to get the right response.Also think about the pulse encoding so that if you run multiple pulses you can tell which one is which when you capture them back. And if you are able to do a spread spectrum kind of pulse then the interference of other signals won't interfere, although I'm pretty certain that not many natural processes will emit coherent lightStill playing with delays to a known threshold will give you a brute force approach to get accurate measurements, other that it will take more time so your frame rate suffers.

author: jeremy
author_rank: Super Contributor
timestamp: October 13, 2014, 05:28:26 pm
content: Quote from: codysmith105 on October 13, 2014, 08:40:36 amThe trouble is that as you increase the distance you want to measure, your alignment becomes more and more sensitive to error. Ask anyone who has worked with optical instrumentation and they will tell you that alignment is hard, even when it isn't supposed to be.You will reach the point where you can't even machine your mirror holders at such a low tolerance after a few metres (depending on the size of the detector; this is just a guess). It is good for extremely high resolution over a small distance though.

author: codysmith105
author_rank: Contributor
timestamp: October 13, 2014, 05:40:35 pm
content: I knew alignment would be an issue. I'm thinking about doing something likethis. That way I don't have to actually generate coherent light, it's kind of a point and shoot method.

author: sca
author_rank: Regular Contributor
timestamp: October 13, 2014, 05:47:15 pm
content: More on the parallax method athttp://nikita.argia.net/kmaxon/page/side/art9_137.htmsca

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2014, 07:31:39 pm
content: Quote from: codysmith105 on October 13, 2014, 04:42:42 amKeep in mind that you need to sample with a resolution of 1 nanosecond per foot (or 85 picoseconds per inch) but that does not have to mean a sample rate of 1 GHz (or 11.8 GHz). The time delay counters mentioned in the replies generally do this very thing. An all digital solution might use a centroid timing TDC or transition midpoint timing TDC with a 16 bit 80 MS/s converter to deliver 10 picosecond resolution getting you down to almost 1/10th of an inch. I would consider that a pretty high tech solution but there are simpler and slower TDC designs which have as good or better resolution.QuoteIt requires a little more hardware for counting but the Charge Time Measurement Unit (CTMU) in some recent PIC microcontrollers can making timing measurements down below 1 nanosecond and will certainly get you into the foot range.http://ww1.microchip.com/downloads/en/AppNotes/CTMU%2001375a.pdfhttp://www.ecnmag.com/articles/2009/07/low-cost-high-resolution-time-measurement-applicationI am not recommending this particular solution but it is in the class of solutions I would consider first.QuoteI would start by not sampling at 1 GHz.