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[DPRG]Wall-E -> 1GHZ computers laser rangefinding

Subject: [DPRG]Wall-E -> 1GHZ computers laser rangefinding
From: Ted Larson ted at larsonland.com
Date: Fri Jul 25 00:43:26 CDT 2008

Ray,

The way you deal with background noise....i.e. sunlight....is to modulate
the signal at a known carrier frequency, and send pulses of the modulated
carrier.  So, run it at like 56khz, and then 1 to 2ms pulses of it.  Most
people use an op-amp setup sensitive to the carrier frequency, and then look
for the edges of your 1 to 2ms pulses.  This is how IR remote controls work,
and sonar systems work, and most optoelectronic systems work.  This is what
Ken is referring to when he talks about the frequency you are going to
modulate the signal at;  The carrier frequency you are modulating the light
at.

There is a reason most laser ranging systems don't use visible light, and
are infrared.  It is too hard to separate a bounced signal from background
noise.  Again, typically IR systems use optical filters for IR only, and
then analog filters on the output of the photodiode matched to the modulated
carrier frequency, to eliminate the background.  Even with all this, it is
still difficult to get good reflections, from various substances the
transmitter might encounter.  The worlds cheapest optical IR filter, is a
piece of exposed photographic film.  Go buy a cheap roll of BW 35mm film,
pull it out of the can outside, cut off a piece...bingo...IR filter.

So...the first thing you are going to need to do is figure out how to
modulate your laser at a carrier frequency.  When you hook your photodiode
up the way you are describing, you should see your nice modulated carrier
coming out the other side when you turn it on continuously.  Then, you will
need to build an amplifier/filter circuit for the photodiode that is
sensitive to that carrier frequency only, and you will be ready to see it
work when you pulse the laser.

Just my 2 cents...

-Ted





-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf
Of ray xu
Sent: Thursday, July 24, 2008 1:17 PM
To: dprglist at dprg.org
Subject: RE: [DPRG]Wall-E -> 1GHZ computers laser rangefinding


I will be modulating it at low frequency; like 1 or 2 kHz, but not below
70Hz.  I am also not sure that if the unwanted light is in the same
wavelength (I don't have a light meter); but I'm assuming there is because
the sun light is triggering my sensor.  

The sensor I'm using is a PNZ334; I noticed on my scope that it is a hybrid
of digital and analog.  Most of the time its either 5 volts or none; but if
a VERY tiny laser beam is detected, its somewhere in between.  

And yes, the difference between the sun's (or even my bedroom lamp) energy
vs. my laser's energy is very big. 

I am also not familiar about electronics filters, but I barely know them.  

___________________
Ray Xu
rayxu at tx.rr.com
DPRG member
OOPic group member
Seattle Robotics group member
My Blog


-----Original Message-----
From: Kenneth Maxon [mailto:kmaxon at qwest.net] 
Sent: Wednesday, July 23, 2008 7:19 PM
To: ray xu; dprglist at dprg.org
Subject: RE: [DPRG]Wall-E -> 1GHZ computers laser rangefinding



Ray,

What frequency range are the laser and receiver pair.  In general this will
be a problem with most visible light laser systems as the sun (and many
other sources) put out energy across a large portion of the spectrum.   So
adding high-Q notch filter can help if the sensitivity of the receiver
extends beyond the frequency range of the signal to be sampled, however if
the interference is on the same frequency range as the signal to be
detected, then the problem is more complex and a simple optical filter *may*
not help.

In the paragraph above I use the word *may* not help as there are cases
where it can.   If the broad spectrum that is bleeding across the frequency
of interest is of a much lower amplitude than the signal returned to the
senor, then an optical filter can indeed help.   In this case an appropriate
optical filter can be used to drop the transmission of ALL frequencies
reaching the sensor.  This helps because all frequencies can be lowered to
the point where the undesired broad band noise no longer triggers or is
measurable by the sensor.

If the sensor has an analog output and modulation is the method of choice,
then an electronic filter *May* be of use.  The limiting issue here again
will be how much the returned signal stimulates the sensor vs how much the
background noise stimulates the sensor.  (SNR = Signal to Noise Ratio)  If
there is significant difference between the two, then an electronic band
pass filter and a modulated source will be able to detect the signal.  (This
of course assumes the sensor is not being used near saturation point,
otherwise both methods will be necessary (optical and electronic))...

-Kenneth

-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of ray xu
Sent: Wednesday, July 23, 2008 3:48 PM
To: dprglist at dprg.org
Subject: RE: [DPRG]Wall-E -> 1GHZ computers laser rangefinding


My receiver is picking up ambient light.  Are there any ways to filter that
out?  Should I modulate my laser and put a bandpass filter on the receiver
end?

___________________
Ray Xu
rayxu at tx.rr.com
DPRG member
OOPic group member
Seattle Robotics group member
My Blog





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