designing a laser rangefinder

[erique82]

Hi, i'm wondering if anyone here has ever attempted/successfully
designed a rangefinder? i'm designing one currently based on phase
measurement, but i'm just not so sure if it will work out. I greatly
appreciate ideas and suggestions . My questions are:
1. is it ok to use a normal laser pointer as a transmitter? if i do
need to modulate it, then to what frequency should i modulate it to as
so an electronic receiver circuit can detect it?
2. i'll be using two APD photodiodes as detectors. One for the
transmitted wave, the other for the received wave. The original laser
beam is splitted by a splitter, of course. my question is... is this
method ok?
3. can i use a pll to compare these two phases (output from the
receiver circuits) and then use a microcontroller to calculate the
distance? or should i use a normal oscilloscope?


any other simple designs or suggestions? i'd appreciate if anyone could
share a circuit diagram with me. thanx

 

[Rene Tschaggelar]

Hi, i'm wondering if anyone here has ever attempted/successfully
designed a rangefinder? i'm designing one currently based on phase
measurement, but i'm just not so sure if it will work out. I greatly
appreciate ideas and suggestions . My questions are:
1. is it ok to use a normal laser pointer as a transmitter? if i do
need to modulate it, then to what frequency should i modulate it to as
so an electronic receiver circuit can detect it?

Give it a try.

2. i'll be using two APD photodiodes as detectors. One for the
transmitted wave, the other for the received wave. The original laser
beam is splitted by a splitter, of course. my question is... is this
method ok?

Most diode lasers do have a monitor diode built in.
Use this as reference and omit the splitter.

3. can i use a pll to compare these two phases (output from the
receiver circuits) and then use a microcontroller to calculate the
distance? or should i use a normal oscilloscope?

What kind of phase range would you wish to measure ?

any other simple designs or suggestions? i'd appreciate if anyone >could
share a circuit diagram with me. thanx

Build the receiver first, then decide on the optics
to achieve the range you have in mind.

Rene

 

[Ingeniur]

Hi, i'm wondering if anyone here has ever attempted/successfully
designed a rangefinder? i'm designing one currently based on phase
measurement, but i'm just not so sure if it will work out. I greatly
appreciate ideas and suggestions . My questions are:
1. is it ok to use a normal laser pointer as a transmitter? if i do
need to modulate it, then to what frequency should i modulate it to as
so an electronic receiver circuit can detect it?

Depends on the distance - how far do you want to go? A mile or so and you
need CO2 - to teh street then maybe HeNe - in the lab then a pointer.
You really need to understand Interferometers if you are doing this work.
The better coherence a laser has the more chance of success. You will need a
MM Interfeometer. Also a Bragg Cell probably an RF down-convertor (and
amplifier) and a PLL.

Ing

 

[erique82]

I hope to achieve range of tens of metres. Most probably less than 30
metres. I'm using a normal laser pointer, trying to cut cost here.

 

[Marc Popek]

your only hope is an impulsed laser pulse and apd and counter,,,,,

There is 0 chance for you to fabricate a hand held interferometer for laser
range finding.. a pulse timer perhaps..

Good luck

Marco

 

[erique82]

For the time being, i'm not targetting on building a hand-held unit. I
just want to build a system that actually works based on what i have.
gosh, any help anyone?

 

[Don Foreman]

I hope to achieve range of tens of metres. Most probably less than 30
metres. I'm using a normal laser pointer, trying to cut cost here.

You didn't mention accuracy goal. You might be able to do this with
triangulation.

Triangulation includes use of things like camera autofocus chips.

At 30 meters, round trip distance is 60 meters so max modulation
frequency without phase redundancy in the range of interest is 5 MHz.
One degree of phase resolution is then 2 inches total distance, or 1
inch of range. It's also about half a nanosecond at carrier frequency
-- but phase is preserved thru hetrodyning so you could down-convert
before you do the phase measurement. You'll need a pretty stable
master oscillator with low phase noise. This need not be costly if
you have the time to fool with it.

 

[erique82]

I don't quite understand though. Currently i'm already designing a
receiver circuit consisting of transimpedance, differentiator and
buffer circuits. The output is then connected to an oscilloscope to
obtain the phase. I'm building 2 receiver circuits: 1 for the initial
signal (gotten from the spitter) and the other one is the reflected
signals. So all i have to do is just compare the phases (theoretically)
and obtain the distance. Correct?

 

[quietguy]

Unless you just want the joy of designing and building, it would be
easier to just buy one of these off the shelf - about $30 or so these
days at your local hardware store

David

 

[Rene Tschaggelar]

I don't quite understand though. Currently i'm already designing a
receiver circuit consisting of transimpedance, differentiator and
buffer circuits. The output is then connected to an oscilloscope to
obtain the phase. I'm building 2 receiver circuits: 1 for the initial
signal (gotten from the spitter) and the other one is the reflected
signals. So all i have to do is just compare the phases (theoretically)
and obtain the distance. Correct?

Yes, theoretically.
Your received signal is likely to vanish in the noise.
That is where the optics comes in.

Rene

 

[erique82]

Ditto. Finding the appropriate optics wouldn't be much of a problem. As
for noise, i just need to design a good receiver circuit. I wouldn't
have to worry to much about noise if i were to use a reflector as a
target? at a range around 30 metres, I guess that would probably
minimize the noise. I'll let u guys know the outcome

 

[Ingo Cyliax]

I hope to achieve range of tens of metres. Most probably less than 30
metres. I'm using a normal laser pointer, trying to cut cost here.

There was an article in Nuts-n-Volts a little while ago that described
a TOF range finder setup. The author used a laser diode modulated with
a square wave, ran the colimated beam through a hole in a mirror to the
target. the return light was reflected in the mirror with hole and
focused onto a photo diode.

The received signal was then compared to the transmit signal with an
XOR gate, the output being a pulse, where the width represents the
time of flight.

This is from memory, since I don't have the article in front of me.
You'll need a fast scope (since 1ns aprox. 1ft), also I suspect the
cleaner you can get the rise time for the transmit and receive, the
more accurate measurements you'll get. One of my co-workers is trying
to recreate this setup.

Of course, there other schemes you can use. Optical interferometry
is very cool, as someone else mentioned, you'll need lasers with
long coherence length (like HeNe), although some of the communication
solid state lasers aren't too bad. You'll also need a way to frequency
shift the transmitter in order to do absolute ranging to targets.

Typically, laser rangefinder use pulsed time of flight, some sort
of modulation, or optical triangulation.

Later, -ingo