Subject:
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Re: Robot navigation - barcode stations?
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Newsgroups:
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lugnet.robotics.rcx
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Date:
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Sun, 11 Mar 2001 05:18:05 GMT
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Viewed:
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1469 times
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On Sat, 10 Mar 2001 10:45:33 GMT, "Chris Page" <chris@starforge.co.uk>
wrote:
> When a robot needs to fix its absolute position, it looks for the brightest
> light source using the standard light sensor (so don`t try this in a
Actually, if you are trying this, be aware that LEDs are typically
mounted on a panel that faces the user (more or less) and as such the
rounded housing of most LEDs acts as a lens which projects most of the
light out toward the user rather than sideways into the enclosure
housing where it's wasted. You can see this clearly by viewing an LED
at different angles. The problem here is this:
| (1)
V
* <-- (2)
The Robot (*) sees light source (2) as being closer because light
source (2) is pointed right at it while light source (1) is viewed at
a low angle and gets little light even though light source (1) is
actually the closest point. Use an LED with a flat lens or wide beam
characteristics or place a small piece of wax or tissue paper over it
to diffuse the light more evenly in all directions, albeit with a loss
of brightness.
> light source using the standard light sensor (so don`t try this in a
> brightly light area, unless you use IR LEDs I suppose..). Once it has
Bad assumption. Infra Red light is a form of radiated heat and
incandescent light bulbs get quite hot (they make good IR sources). An
easy way to work around this is to use flashing LEDs - Power them
directly from an AC source so they flicker at 60Hz. You won't see it,
but if your robot sees an IR light source that is steady - it's a
light bulb. If it flickers, it's an LED.
Proramming for this is a bit more difficult. Let's say that your light
sensor shows an ambient reading of 50. What you are looking for is a
signal riding on that ambient light. If the sensor looks around and
finds a source that reads 70 and it's steady, then it's a light bulb.
If it looks around and it sees 50 everywhere except somewhere where
it's pulsating between 50 and 52, then it's an LED.
Lastly, you made a reference to using a laser for this project. You
don't actually need one. Lasers (or narrow-beam LED's) are used in
Point-of-Sale terminals for three reasons:
1) Actual contact would contaminate the sensor.
2)The bar code is sometimes quite small in relation to the package
which can make lining it up, face-down, directly over the sensor
rather time-consuming.
3) You can't easily control the speed at which the code is swiped over
the scanner (look at how many times credit cards have to be re-swiped
for example).
Typical POS barcode scanners are kept behind glass, away from the
product. They scan at a controlled rate and at various angle over a
large area to locate and correctly orient the bar code.
Since you aren't (likely) taking your barcode markers from the frozen
food section, you don't have much risk of contaminating the senor by
(near) contact-scanning. You are in control of how fast the scanner is
moved by the robot and you chose the orientation of the bar codes
ahead of time. Your environment is much more controlled that the
typical supermarket checkout.
Matthias Jetleb
VA3-MWJ
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Message is in Reply To:
 | | Robot navigation - barcode stations?
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| Hi, Some time ago, on this or another robotics list, someone was talking about navigation and location sensing techniques for RCX based robots. After some thinking, reading through the literature and messing with some homebrew sensors I have one (...) (24 years ago, 10-Mar-01, to lugnet.robotics.rcx)
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