Subject:
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Shutting off illumination LED on Light Sensor for LIDAR
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 28 Mar 2001 19:10:33 GMT
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Original-From:
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Chris 'Xenon' Hanson <XENON@3DNATURE.nospamCOM>
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Reply-To:
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XENON@3DNATUREihatespam.COM
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Viewed:
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773 times
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Has anyone successfully found a way (short of paint, or cutting
internal wiring) of shutting off the red 'supply light' LED on the
Lego light sensor?
The supply LED is great for active-detection situations, like
line-following, when you need to read the reflectance of an object
at close proximity, and the sensor itself might cast a shadow onto
the target, blocking ambient light.
I wish to use the light sensor in a purely passive mode, detecting
available ambient and direct light from other room sources. It seems
capable of doing this even with the light on, but I feel I am probably
getting less sensitivity because the close proximity of the LED to the
photodiode (I think that's what they're using) will cause some bleed
over, gain, and loss of low-light sensitivity.
I've considered buying another sensor and either painting over the
supply LED, or opening it up and cutting it's wires or something.
Alternately, if I can figure out what the photodiode part is, and
how it's wired, I could just build my own passive-only sensor that
would be level-compatible with the Lego part.
Obviously, if I can figure out a way to disable the supply LED via
software, that would be best, so I can share my development with
others who do not have custom hardware.
I have noticed that configuring the input port the light sensor is
connected to as the wrong type of sensor, say a switch, causes the
Light Sensor's LED to dim in intensity. Perhaps this can be used,
but I think it impairs the ability to actually read the light sensor
value. ;(
(I have also tried stacking a few 1x2 technic blocks with the hole in
them in front of the light sensor to block the emitter, and give the
sensor "tunnel vision" as it were, but I still feel there's some
bleed.)
So here's what I'm using it for right now:
I have mounted both the Light Sensor (with 'tunnel vision' blocks)
and the Lego Light (from Ultimate Accessory) aimed forward on the
front of my bot.
Using NQC, I ensure the light is off, and take a reading from the
light sensor to record ambient light level in that direction. Then,
without moving the bot, I turn on the light and take another light
reading. The difference in the two light levels is basically a LIDAR
distance estimate to the nearest obstacle.
The LIDAR ranging technique is quite accurate, and can detect out
to a distance of a little over 1m (limited by the light intensity of
the lego light, and the sensitivity of the light sensor). I am also
considering using a super high-intensity custom LED in place of the
lego light to increase detection range.
I forget the exact number of discrete range steps I get, I think it's
about 10, giving it a nominal ranging resolution of about 10cm. The step
interval seems non-linear, which would make sense given the spherical
dispersion of light in an expanding sphere.
I need to do more precise testing and calibration, but having a more
sensitive sensor (one not overrun by its own light source) would help.
The LIDAR is good at detecting almost any obstacle, and because it
has both an ambient and an illuminated reading, it is equally good at
detecting surfaces of all colors. It will even detect a (glossy) glass
plate placed in front of it. It is excellent for forward collision
prevention without contact.
Possible avenues of exploration:
Using the RCX IR emitter as the illumination source. Don't know
exactly how to just jam it 'on' like a light connected to an output
port, and don't know it's comparative power. This would give me one
additional output port free if I didn't need the Lego light, a big
bonus. This would make the design more accessible to those who don't
have the Lego Light part from Ultimate Accessory. Also, the annoying
visible strobing of the Lego light would be eliminated, making it
more of a (visibly) passive, 'stealthy' sensor. (This is what the
Caterpillar One bot at http://www.rcx.ic24.net/catone.htm does, though
the example code is very convoluted). This would be helped by turning
off the light sensor's own light supply LED.
Using the lego light sensor's supply light, if I can figure out how
to toggle it on and off under software control. This would also gain
me an extra output port, but the sensor's LED doesn't seem that powerful,
and might impair the sensor range. This too would eliminate an accessory
part and make the design more widely-accessible.
Using a custom-built high-intensity visible light or infrared LED.
Increased detection range, possibly using 'stealthy' non-visible light.
Mounting the emitter/sensor package on stepped 1D or 2D rotation
axes to detect obstacles all around the bot for navigation planning,
or for contact-free 3D scanning(!). One could log both the ambient-light
level, and the calculated LIDAR level to assemble (on a host computer)
a raster image and depth map of the robot's environment. Without really
tricky mechanical or electrical design, this would consume all of the
inputs and outputs of a stock RCX, leaving it unable to move about,
but with very clever design, one could make an autonomous robot rover
that could navigate about, image interesting locations, and send
(one-way) traffic back to a telemetry computer via serial IR with
location info and a snapshot image/3D model of the environs. Almost as
good as Sojourner on Mars.
Much of the inspiration for this comes from the Caterpillar One ('inverse
RADAR' IR-based LIDAR design) Pipe Scanner, Arm Scanner and Remote Probe
from:
http://www.rcx.ic24.net/designs.htm
and the Probe from:
http://web.one.net.au/~bwilliamson/lego/
Plus, I've always wanted to build a LIDAR (LIght Detection And Ranging,
the light form of RADAR) unit.
Comments?
Chris - Xenon
--
Chris Hanson | Xenon@3DNature.com | I've got friends in low latitudes!
New WCS 5 Demo Version! http://www.3DNature.com/demo/
"There is no Truth. There is only Perception. To Perceive is to Exist." - Xen
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