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DPRG: Line Sensors

Subject: DPRG: Line Sensors
From: Clay Timmons ctimmons at asic.sc.ti.com
Date: Tue Jul 1 08:54:37 CDT 1997

> I am working on the mechanics of my line following robot.  I have
> questions for all the successful line followers.  
> 
> What specific device is used to detect a line or not? Is it something
> that can be purchased at BG Micro or Tanners? Which is prefered?
> 
> How close to the floor does it need to be mounted to detect the line?
> 
> Does there need to be an illuminator (especially if IR)?
> 
> Is cross talk between sensors a problem, requiring a something to
> separate the detectors?
> 
> Regards,
> Kip
> 

Kip,

I did much tinkering with sensors for line following.
What I found to work best is a high brightness red LED
and a NPN phototransistor.  Mount them at a narrow angle
to each other 10 - 30 degrees, not 45 degrees.  Narrow
helps give a better depth of field. Space them
about 1/4" to 3/4" away from the floor.  Distance from the
floor affects how narrow of field of view.  Closer gives
a sharper response and will detect thin lines better.
You will see the spot from the red LED and can visually
adjust.  Placing them too close to the floor can cause
rubbing if the floor is not even or the robot tilts.

The last design parameter is how much space between detector
pairs.  This depends on the thickness of the line you are
trying to follow.  If you have a 1" thick line then two
sensors just over an inch apart work best.  More than two
sensors can be usefull fo smoother steering and to keep
>from loosing the line alltogether.  Cross talk is not a
problem unless the sensors are extremely close together
and too far above the floor.

Ambient light is a major problem.  If your robot has 
digital inputs for the sensor then you must adjust a 
trip point somewhere between no light reflected and
light reflected.  The phototransistor is sensitive
to IR, and visible light.  Thus the trip point is
biased by the amount of ambient light.  A little bit
of shroud around the sensors to give them shade helps
bunches.  Also beware of flourescent lights which 
give a nice 60 herts flicker which your sensors will
pick up.

Both devices are available at Tanners.  Use a current
limit resistor on the LED and a pullup resistor on the
phototransistor.

Let me know if you have any more questions,

- -Clay Timmons   ctimmons at asic.sc.ti.com

 



   

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