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Ranging

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Skakabop

New Member
Hello everyone,

I will build a mobile device, and i need a range meter. I've built Ultrasonic Range Meter before with a microcontroller. But it won't be useful because it doesn't compute the distances beyond 10 meters, sound dissolves much and since it's a mobile device, maybe it won't detect returning signal(which i'm not sure).

Thus, i'm thinking about laser detection or infrared, ranging minimum 10 cm to like 40-50 meters, with centimetric tolerance, though i don't have much information about them.

What can you suggest me about this ?

Thanks.
 

Skakabop

New Member
Thanks for your answers, but i'm thinking of building a range meter, not buying one. Is it impossible to make when it's beyond 10 meters ?
 

Sceadwian

Banned
Laser interferometry isn't easy.
 

crutschow

Well-Known Member
Most Helpful Member
Thanks for your answers, but i'm thinking of building a range meter, not buying one. Is it impossible to make when it's beyond 10 meters ?
Not impossible, just difficult. The simplest way to measure distance with a laser it to have the laser emit a short pulse of light and measure the time it takes to return from the target. Light travels about 0.3m/ns, so going from 10cm to 50 meters would require being able to measure time intervals between the transmit pulse and it's return from about 0.6ns to 300ns, not a trivial task. And obviously the laser pulse width has to be very short so the trailing edge of the pulse doesn't overlap the return signal.

So unless you have the expertise to design and build circuitry that can generate and detect sub ns pulses, I won't recommend trying it.
 

Mr RB

Well-Known Member
Actually it's a little easier than that; you modulate the laser at high frequency then measure the phase shift between the sent waveform and the received waveform.

But it's still NOT a trivial project, given the very small received signal amplitude and especially if he needs anything like fine resolution.
 

crutschow

Well-Known Member
Most Helpful Member
Actually it's a little easier than that; you modulate the laser at high frequency then measure the phase shift between the sent waveform and the received waveform.

But it's still NOT a trivial project, given the very small received signal amplitude and especially if he needs anything like fine resolution.
The zero-crossing phase shift times you have to detect are the same as you would measure for the returned pulse time. The difference is, you modulate the laser with a high frequency signal (several MHz) instead of sending out pulses. Don't see that one technique is significantly easier than the other.
 

Skakabop

New Member
Thanks all to responses, i see it's really difficult to range with laser since light is too fast, and since i don't have GHz processor, i cannot do it via phase shifting.

I saw some projects with angle calculations, though i don't completely know how it works, will it be useful ? And what can you suggest me about infrared ranging ? Is it effective ?
 

crutschow

Well-Known Member
Most Helpful Member
I saw some projects with angle calculations, though i don't completely know how it works, will it be useful ? And what can you suggest me about infrared ranging ? Is it effective ?
The angle calculations use triangulation to calculate the distance. The transmitter and receiver are placed some distance apart. The receiver has to have a very directional detector so you can measure the angle between the transmitted beam and the received beam. The detector can be a single detector that you mechanically move until you center the light on the detector, or you use a row of multiple detectors and sense which detector sees the brightest image.

From the angle and the distance between the transmitter and receiver, you can use trigonometry to calculate the distance to the target. The accuracy is determined by how far apart the transmitter and detector are apart and how accurately you can measure the angle.

Don't know how you would use IR to detect distance except as described above, with an IR transmitter and detector.

None of these schemes are easy to implement. That's why there's not much that's commercially available for a low price. The laser tape measures seem to be the cheapest.
 

Mr RB

Well-Known Member
The zero-crossing phase shift times you have to detect are the same as you would measure for the returned pulse time. The difference is, you modulate the laser with a high frequency signal (several MHz) instead of sending out pulses. Don't see that one technique is significantly easier than the other.
I have to disagree there. :)

Measuring a period that might only be a few nS long is a pain, even if the signals are cleaned up.

But if you have the sent and returned signals, cleaned up a bit to be square waves you only have to AND them together and you get a nice easy linear voltage output that represents the difference in phase, which represents the time of flight.

To me that is MUCH easier than measuring the period.
 
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