Modulation/demodulation of Red & Green laser pointer and laser sensor

[Bob May]

Don't remember the name but they were in the City of Industry. Enter on the
left (as you look at the front of the building) and there were two stations
for looking at the video tapes that the customers brought in.

 

[Guest]

Rather than modulating with a swept frequency, just use a constant
high frequency, for easier detection; and use the direction and time
between hits to determine the angle. For example, if the sender
does r degrees per second and the time between illuminations
from the right and then the left [OP says sending head rotates 180
left, 180 right, repeatedly] is t, the receiver is r*t/2 degrees
clockwise from the counterclockwise stop. Example: r=180, t=.3
gives 27 degrees. For extra credit, do least squares analysis on
successive hits. If you don't want to use two sensors (to enable
whether hit is left or right) alternate pairs of 1-second sweeps
with pairs of 2-second sweeps, or some such arranglement.

-jiw

Thank you Jiw.
Using direction and time is very good idea!.

If we assume that the receiver is mobile (ie. can move) then am I right that
sender and receiver must be in sync so that the receiver can start a
timer/counter when sender start scanning and stop the timer/counter when it
receive the laser hit?

I have one more issue; the photodiodes and phototransistors that I have seen
so far have upto 120 degree view angles. I'm thinking to position multiple
sensors at different orientations/directions, to make the receiver omni
directional ie. can receive the laser signal from any direction. Is there
any better, more efficient method to receive laser pulse from (almost) any
direction ?

Regards

Rahgu

 

[Curt Welch]

Rather than modulating with a swept frequency, just use a constant
high frequency, for easier detection; and use the direction and time
between hits to determine the angle. For example, if the sender
does r degrees per second and the time between illuminations
from the right and then the left [OP says sending head rotates 180
left, 180 right, repeatedly] is t, the receiver is r*t/2 degrees
clockwise from the counterclockwise stop. Example: r=180, t=.3
gives 27 degrees. For extra credit, do least squares analysis on
successive hits. If you don't want to use two sensors (to enable
whether hit is left or right) alternate pairs of 1-second sweeps
with pairs of 2-second sweeps, or some such arranglement.

-jiw

Thank you Jiw.
Using direction and time is very good idea!.

If we assume that the receiver is mobile (ie. can move) then am I right
that sender and receiver must be in sync so that the receiver can start a
timer/counter when sender start scanning and stop the timer/counter when
it receive the laser hit?

He was talking about having the laser scan back and forth in a sweeping
motion, not in a circle. You could even do it with a full 360 deg sweep.
If you do that, then the receiver doesn't need to be directional, and
doesn't need to be in sync with the transmitter. It simply looks at timing
of the pulses created when the beam hits the receiver. It will create an
uneven duty cycle between each pulse. So, you receive a pulse, and start a
counter to see how long it takes before you receive the next pulse, then do
that again to see how long it takes to receive the third pulse. You then
look at the ratio between those two times to determine where in the sweep
pattern the receiver is located.

If the spacing is even, the receiver is in the middle of the sweep. If
it's uneven, it's to one side or another relative to the difference in
timing.

Now, without more help, this doesn't tell you which side of the sweep you
are on. But I assume you are doing this to allow the receiver to locate
itself. So you are going to need at least two transmitters. If you use
three then I suspect it could always solve for the correct location using
all the data. And actually, if two transmitters are pointed so the
mid-line of the sweep doesn't cross, I think you can do it with only two
transmitters and always solve for the correct location.

Or, if you are modulating the laser, and could modulate it with one
frequency when scanning in one direction, and modulate it with another
frequency when scanning in the opposite direction, then the receiver could
tell which direction it was sweeping and correctly solve where in the sweep
pattern it was located.

Another possible option is to use multiple receivers side by side and see
which one gets hit first to understand which direction the beams are
sweeping.

I have no idea however if any of this will produce the accuracy you need.

 

[[email protected]]

Another way to this is to make an "angle of arrival sensor" Depending
on the angle the incident light makes with light makes with the
aperature the position of the focused spot will change. You can detect
this by placing a camera at the focal plane of the lens. There are
other ways to do this with wavefront sensing etc as well.

john muth

 

[Bob May]

They started the tape and immediately heard the noise of digital data which
confused them and the volume was loud. The software engineer was somewhere
else but I was there and told them to just turn down the volume. Later on,
in the audio section, the engineer had come back in (they heard the noise
all over the front part ot the factory) and he told them to turn up the
volume. The guys were amazed that they heard the sounds of the audio
section so clearly (no background noise as the audio was all digitally made,
probably another first). We got the glass master about 4pm. that day.

 

[Guest]

Another way to this is to make an "angle of arrival sensor" Depending
on the angle the incident light makes with light makes with the
aperature the position of the focused spot will change. You can detect
this by placing a camera at the focal plane of the lens. There are
other ways to do this with wavefront sensing etc as well.

john muth

Hi John,

Thank you for the new ideas.
I want to be able to measure relative angle between transmitter (sender) and
receiver where they may be located 100 meters (or more) apart from each
other and receiver is mobile (moving). I think determination of the position
of the focus sounds a bit difficult (I may be totaly wrong if there is a
simple method for this).

I do not have any clue about your other suggested method of "wavefront
sensing" . Can you give us a bit info abut this method and the other methods
that you've summarized as "etc" ?

Once again, thank you for your contribution.

Regards,

Rahgu

 

[Guest]

. . .
4.) Would it make any difference if I use either Red or Green laser
pointers?

5.) Is there any advantage or disadvantages using IR instead of Red or Green
laser?

Thx.
Rahgu

 

[Louis Boyd]

5.) Is there any advantage or disadvantages using IR instead of Red or Green
laser?

Advantages for infrared:
Silicon detectors are most sensitive around 800-900 nm
Sunlight is weaker.
If you're working long distance (>100m) atmospheric scatter will be less.
powerful lsaers (> 50mw) are more readily available in the IR,
particularly between 800 and 840 nm.
Humans cant see it.


Disadvantage of infrared.
Alignment and focusing can't be done visually. you need do do that with
an image intensifer or a video camera.
Humans can't see it.