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[Editors note: Bill sent this email to our list server in the last couple of weeks. Bill has done a great job in pointing out some important information about using Infrared detection circuits. Its very much worth reading!]
From: bill harrison <bill@sinerobotics.com>
Date: Thursday, November 05, 1998 1:17 PM
Subject: Re: Sharp GP1U5 Based IR Sensors.
I've run into many of the problems of the
Sharp GP1U5 family of IR detectors. They were originally designed as data receivers
and as such are excellently designed. But they have been used successfully as object
detector sensors. In using them for object detect we have to keep a couple of
things in mind:
First, and I think you have this one covered
(but others have not done this), is that you have to have a second modulation for the
detector to work. The detector sorta "re-zeroes" its self (probably via
caps and resistors). If it sees a steady state 40 kHz signal (assuming a 40 kHz
module, there are other frequencies available), it'll do fine for a short while and
then start having a lot of "glitches". If the signal has a second
modulation (833 Hz works well) then the state is changed often enough to maintain a good
signal.
A sort of trick, is that if you do the
software/hardare filtering technique of turning the IR LED on, check detector, turn the IR
LED off, check the detector, and repeat, this does as well as a second modulation (it's
actually a second modulation if you think about it). Then you just check if you see
enough "offs" with the IR LED off and enough "ons" with the IR LED on.
This is pretty, easy if you use a loop, and has worked pretty successfully in some
of my robots. I just go into a loop in which I get one "off" reading and
one "on" reading. I count good readings (if off, a good reading is off, if
on, a good reading is on). Then I allow it to have a few bad (as this will
inevitably happen), of something like 5% of the total count ("good" count must
be within 5% of the maximum count). You can play with the allowable bad count for
your situation (keep it
big if you prefer to see, and keep it small if you prefer to not see).
The second problem that occurs is that it takes
the Sharp detector some discrete time to stabilize a good output signal. I seem to
remember that it was something like 300 uSec. (I haven't figured this out in a while
as I just patch in the software that works now).
So what you do is to change the state of the IR
LED, go into a delay of 300 uSec (that's micro), then take a reading. You would do
the same every time you need a reading.
You can play with the delay time, too short and
the data gets unreliable due to not stabilizing fully, and too long you'll start seeing
"glitches" (just as bad) (and your routine could start taking too long, Karl
Lunt recommended using 80 cycles of readings). Note that "too long" is
*way* longer than 300 uSec.
Another problem most face is that this type of
sensor has a practical range limit of 2 to 6 inches (depending on a lot of things, though
I have gotten one to operate at well over 20 feet), so most see it as not too
sensitive and try to boost up the IR LED power. This is what I did when I started.
Actually the detector is *so* sensitive that it almost won't work as an object
detector and boosting the IR power only complicates things. (in fact Radio Shack now
sells a non-Sharp module under their same part number that they used to sell the Sharp
modules, but the new ones are so sensitive they are almost impossible to get to work).
I've found that the low output old-fasioned IR
LEDs running at only about 3 mAmp (that's mili) works pretty well. Build a
breadboard setup with a couple of 555 timers to run it (dual modulation) and just play
with it (put a pot on the IR power).
If the IR LED is too bright you'll run into
problems of it bouncing off of everything and the Sharp detector has about a 135 degree
field of view, as well as some sensitivity in pretty much all directions (problem with
such high sensitivity level). So you'll "see" something all the time.
I use those foil stickers that write-protect floppy disks to cover the holes and
slits on the detector. I also put some shrink tubing over the back of the IR LEDs.
Another problem is that things that look opaque
can actually be transparent to IR. I was surprised to find that IR can pass through
typical black electrical tape for example (though it makes it dimmer). IR is also
pretty good at going through paper. Some pretty solid looking white plastics can be
totally clear to IR. I did find several things that were pretty good at blocking IR,
but the best was any sort of metal, even a very thin coating on plastic works great (try
using aluminum foil, just don't touch it to other electronics (besides grounding it)).
Yet another consideration is that the detector
acts as a sort of "integrator of area", therefore you can get the same
detect with a very bright small spot of IR reflection, *or* a large area of dim IR.
So this type of detector has trouble with things like table legs unless it's right
in front of the detector (taking up a large portion of it's field of view). But this
works pretty well for flat surfaces such as maze walls.
So when you point the IR LEDs down a tube you
are limiting the area that the light will shine on, which greatly effects the detectors
sensitivity. Due to other considerations, I haven't seen this used very successfully
yet (except one setup in which I used lenses). You are probably better off just
letting the IR LED shine on everything in front of it. There is still enough
resolution for such things as right or left. The detector works better in this mode.
A consideration on those cards that glow when
IR is shined on them, the only real use I have for them is telling if the IR LED is on or
not by putting directly in front of it. Otherwise they are too insensitive to be
useful (I have been in situations in which I absolutely could not tell if the IR LED was
on using one of those cards due to the low power level needed for the design).
An alternative that works pretty well is to use
one of those hand held video cameras. They see "near" IR (what we use for
this type of detector) just like regular light, so you can actually see the IR through the
view finder. You can just look at the LED with the camera to see if it's on or if
you want to know something like the beam angle (you just get in front of the IR LED and
move around to see the edge of the disk in which you can still see it lit).
As a last note, be sure to ground the case on
those Sharp detectors. They can generate interference by being an antenna for
any electromagnetic waves around the detector. Also you can put developed exposed
color negative film in front of the detector to help filter optical noise.
Sharp also sells a module that is just the
same, but with added resistance to optical and electromagnetic noise (GP1U78Q for
example).
I've found many of my robots work fine in my
shop only to have them behave very strangely at remote competitions. This was due to
interference at the competition that wasn't in my shop (some types of fluorescent lights
can play havoc).
Hope
this was of help, and that you don't have to go though all that I did :-)
Thanx
to Eastside Robotics (which I belong) for the information.
Cheers,
Bill Harrison
http://www.sinerobotics.com