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Line follow sensor spacing ??

I am a bit embarassed to be asking this but... With everything I have done with Walter, I have never played with a line-follow set up. Now, the electronics I have down solid but I am a little fuzzy on the theory of opperation. I intend to use 4 sensors. Where do they go?

 

Lets assume the robot is directly centered above the black line on a white background... Should all 4 sensors be looking at white? This set-up seems that it would allow for 2 "steps" or "degrees" of steering --I.e. one sensor sees the line and turn a little or 2 sensors see the line and turn a lot.

--or--

The 2 inside sensors see the line when the robot is centered and the robot turns when an outside sensor sees the line or when an inside sensor looses the line.

Which is better? Any other thoughts? 

BTW-- the robot will be going very slowly when the line-follow mode is engaged.

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There have been contest robots that use a variety of small reflective IR sensors, like the familiar QRD1114, also the Sharp GP2S28 used in Budgetbot CBA line follower module (discontinued), or the Sharp GP2S40. These types of sensors are typically mounted very close to the line being detected, with a 3/8 inch or less, and are more useful in high speed smooth surface contest floors.

David Cooks Sandwich line follower uses CdS cells in the pattern mentioned.  Also David Andersons SR04 also mounts a CdS paired with a single LED mounted on each of the moving grippers. With only 2 sensors, the robot is first calibrated to the line to match the lighting, then follows the line allowing the grippers to stay on the outsides always. These, with calibration, can be mounted at a greater distance, perhaps an inch from the detected surface, but react in a slightly slower manner. SR04 never missed a line that I recall.

 Forgot to add video line followers too, using a CMUcam or AVRcam to track a line from a greater height. But that's $100+. 

 CdS cells probably the cheapest, easiest, but may have a slow reaction time.

My line following robot had the following setup:
0-x-0--0-x-0

The 0's are the CdS cells, and the x's are white LED's. By doing it this way, the robot will have 2 outer sensors and 2 inner sensors. You can center the robot in the inner sensors, and the outer sensors are so it will have a wider field of view of the line.