Accuracy of consumer grade laser distance finders - Page 1
Forum section: electronics / projects

author: raptor1956
author_rank: Frequent Contributor
timestamp: July 01, 2018, 03:42:41 am
content: I had a Leica laser distance finder years ago but haven't seen it in a few years so I've done some poking around and as expected a slew of cheaper units from China with brand names I've never heard of are out there with prices in the $35USD range as well as models from the bigger players (Leica, Bosch, etc) at much higher prices for pretty much the same resolution and accuracy. Now the thing that puzzles me a bit is that not much has improved in the more than decade since I got the Leica as far as accuracy/precision is concerned. At best it's about 1mm with many of the cheaper units more like 2mm and that begs the question -- why don't we have such units capable of 100um or even 10um or 1um. Presumably they are time-of-flight devices so the clock/timer would seem to be the limitation, but I'm not certain of that.So, why don't we have sub mm units by now for the consumer?Brian

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2018, 04:53:57 am
content: I think they mostly use square wave modulated signals and XOR them with the squared up received signal to get a phase, not straight TOF measurement.Probably not with a 1ppm INL ADC, so regardless of the other error sources the ADC is going to be a limiting factor.

author: larsdenmark
author_rank: Regular Contributor
timestamp: July 01, 2018, 06:30:16 am
content: There isn’t a market for submillimeter measurement from a consumer device. Who would benefit from such a feature if it existed? Who would be able to pay for the added cost?Mechanically it would be rather difficult to construct a device that is cheap and has a submillimeter accuracy of the sensor and the outer perimeter of the device.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 01, 2018, 07:42:15 am
content: Even a 10mmaccuracyfrom a time-of-flight device is already quite a feat!Now, there are two types: continuous wave and pulse.Pulse type is more modern and sophisticated and works like a sonar, sending a light ping and measuring the roundtrip time. However, for this to work, you need jitterless detection circuitry (which can separate the higher-density photon train from the background light) and a massively fast counter. If the counter runs at, say, 10GHz, the resolution is still only 15mm. No terahertz counters in the near consumer future... This pulse style is used in super expensive environmental scanners, because when you measure distances of hundreds of meters, the resolution/accuracy of a few cm is great.Continuous wave (which all consumer-priced devices are) is based on sending a light pattern (usually square, sine, or practically a highly distorted square wave; it doesn't matter too much; even a long custom pattern is possible, eliminating crosstalk between multiple devices!), and demodulating the received light with the same signal, so that the measured charge ends up in two wells, fractionally divided between them depending on the phase difference. With differential ADC readout, and combining typically four measurements with 90 degree phase shifts, we get the distance, with a wrap-around issue, which needs to be fixed by taking multiple measurements at different modulation frequencies, which can be compared against each other.Here, a goodresolutionis much easier to achieve, even with low-cost electronics. Modulation frequencies as low as 50MHz can easily give millimeter accuracy in optimal (lab) conditions. Jitter effects average out. Noise can be averaged (as long as the subject isn't moving excessively fast). On the other hand, now the measurement isn't based on the first strong-enough ping, but is an arithmetical mean of all modulated light sensed during the integration period, this means any stray light in the optical path, and multipath light in the room, directly affects the accuracy. Put a finger near to the emitting device, and you can easily have almost -100% measurement error if the optical path is not carefully designed! If your finger is 50 mm away, and the subject is 5000 mm away, the finger reflects 10000 times the amplitude, and there is no lens hood or lens in existence which could completely reject this stray light.Now, in pulse design, it's either or - cause enough stray light, the detection circuitry picks this stray light and you get measurement of 50mm. In continuous design, it's arithmetical mixing. In worst case, you cause just enough stray light to produce measurements that look good but have error beyond your error budget, but you have no way of knowing they have the error.With very narrow laser beams, and very narrow receiving optics, mechanically decoupled from each other, this is not much of an issue, but it becomes an issue even then if we are talking about sub-mm resolution. For 3DTOF, this stray/multipath issue ishuge, 20-50% errors are normal everyday business, but no 3DTOF manufacturer even dares to mention this in their datasheet accuracy figures.Another issue in such systems is that they are very prone to drifts of the differences between the propagation delay in the illumination and demodulation paths. You can design a delay locked loop which tries to automatically correct for the drifts, but these delay lock systems will have drift as well. So, compensation is a big part of ToF design. For example, you measure temperatures of different parts of your system and calibrate thermal coeffs.This is why I predict you won't see a truely millimeter-accuracy (let alone sub-millimeter) laser distance finder at an affordable price for a while. It's not a small engineering/optimization issue; this requires some kind of a breakthrough. If you see that, expect seeing 100GHz CPUs as well!However, what youcanexpect, are low-cost devices that are fairly accurate, but provide new and unimagined possibilities, such as directly measuring tens of thousands 3D points with acceptable accuracy for many uses

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2018, 11:08:07 am
content: Quote from: Siwastaja on July 01, 2018, 07:42:15 amWhich 2D sensor even uses phase difference of a continuous signal? Once you are at the level of sophistication of making a CCD ASIC I don't really see any point in doing that way, except maybe choosing a suboptimal method for patent avoidance. Electronically shuttered pulse integration or direct pulse detection with time-to-amplitude conversion make far more sense.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 01, 2018, 12:58:17 pm
content: Quote from: Marco on July 01, 2018, 11:08:07 amUm, everyone? TI, Melexis, Espros come to mind, but haven't looked at the others (if there is any)... Or maybe we have a terminology issue here? How these sensors work is, they demultiplex the photon-induced charge between two wells in sync with the illumination modulation signal, then after the integration time is over, read the two wells out (either differentially, or as two separate single-ended operations). We call this continuous wave measurement. It's "electronically shuttered", and rejects DC ambient light which falls equally in both wells (given perfectly symmetrical demultiplexing, which doesn't happen in reality, hence one compensation step more required to reduce error in strong ambient light).This allows us to take arbitrarily long integrations to collect enough light for good SNR and eliminate modulation/demodulation jitter from affecting the measurement, which would be a huge thing in single-pulse detection... In theory at least. In practice, integration time is limited by the motion blur of the subjects, and the ambient DC light saturating the wells or causing nonlinear distortion. We use up to 4-5 ms per correlation sample/quad so about 20ms total. Inifinitely short illumination pulse of infinite illumination power would be infinitely nice but it'd have an infinite cost.I have only read about prototypes of direct pulse delay detecting 3DTOF sensors, I know the manufacturer we work with is actively developing one. But I don't know if any exists on the market (if they do, they are insanely expensive), and they are designed for significantly longer ranges and less resolution (for the reasons given above). I have a feeling they might be a big thing in maybe a decade, but maybe in automotive applications with hundreds of meters of range where resolution is far less than centimeters.I'm happy with how thesesensorsare performing. Camera level... not so much, which is why we developed our own. The optical design is far more challenging, because you cannot sense a wide field-of-view at once without having huge amount of unwanted scattered light. At the same time, the wide field of view is the strongest and the weakest point in 3DTOF. 3DTOF camera manufacturers traditionally report the raw sensor performance numbers (such as +/-0.5% distance measurement accuracy), which is active lying. I've seen independent tests that easily show a 50% error in such products. (And that's fairly easy to demonstrate yourself; when you get a chance to see a working 3DTOF camera, just put your hand near the illumination source, outside the image area and see what happens.Some additional material I have written (not an exact technical report on the inner working of distance measurement, but an overview on 3DTOF issues and proposed solutions, to try to prevent patent trolling without becoming patent trolls ourselves) :https://www.pulurobotics.fi/web/content/1540?unique=80707103be17ef664616d21f6f1d017fa0d77e6d&download=true

author: CopperCone
author_rank: Super Contributor
timestamp: July 01, 2018, 03:29:16 pm
content: I think there is some market for accurate laser distance meter in the field of construction because you want to see how buildings move away from each other over the years in some places, but its not really something you would do unless you are putting up something that's like six stories or more I think, and it would be a pretty professional service. Maybe surveyors have that kind of equipment.I can't see even the most advanced finishing contractors need more then like 1mm of accuracy on a laser distance measurement in a room or something, everything will get hand fitted anyway because of wood warping and stuff, they go to a different method of dimensional sizing like sanding or planing.I could maybe see some kind of useage if you make like, lasers that can be put into odd areas and give accurate and trustworthy measurements, like if you can slide out a head to measure something like a molding... but lol, usually the floor or wall will be bent, and you use caulking to seal stuff anyway. You would really need some advanced stuff.. I would use it if you could tell me the curvature of the wall and account for it, without a lot of complications, otherwise I would just bring a smaller saw into the house to do it by careful iteration. Homes and wood are really sloppy.Corporate wise for consumer or prosumer you will get this:1) why develop the feature if its not useful2) it will confuse customers3) liability from calibration etcFor professional its expensive enough that the sales rep can explain it to the people buying the equipment. But it would cost at least 1000$.

author: Marco
author_rank: Super Contributor
timestamp: July 01, 2018, 04:15:53 pm
content: I did not know they used a repetitive signal, I assumed they just increased the light strength for a single pulse to get better SNR with the shutter based method.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 01, 2018, 05:28:01 pm
content: A time of flight sensor for light with an resolution of 1mm is indeed quite a feat.For the electronis this means measuring time differences in the 10ps time range.This is a resolution still on the limits of what can be done with affordable electronics.The evolution is terefore in making this stuff cheaper and smaller.https://www.aliexpress.com/wholesale?SearchText=VL53L0XThere are measurement systems with much higher resolution such as:https://en.wikipedia.org/wiki/InterferometryBut these work on different principles such as phase shiftsIf you want to know more about high accuracy measurement, look into "CMM" which stands for Coordinate measurement system. It would of course be silly to expect a hand held device here.https://duckduckgo.com/html?q=CMM+%22laser%22Nicon builds laser based CMM's:https://www.nikonmetrology.com/en-us/product/lc60dxAnother very interesting measurement technique is fotogrammetryhttps://duckduckgo.com/html?q=fotogrammetry