Saturday, November 7, 2015

FLL - Trash Trek: The Color Sensor and the Field Mat...


     This year’s FLL Field Mat is of nice quality; teams need to be aware that the high gloss of the mat may cause faulty readings for the color sensors. Depending on the type of light source and angle of that light source, some lighting situations may cause bright reflections on black lines causing the color sensor to read the line as a color value less than black.

Students at Brics~2~Bots Academy ran a test on the FLL Trash Trek field mat, under normal house lighting, to see how an EV3 Color sensor would respond to the mat and this is what they observed…
  1. When line following with one color sensor and using Compare Reflected mode detecting the black line, lighting could be an issue where glare is being reflected off the mat depending on the angle of the lighting. You might get various readings at different parts of the same line. However, using Compare Reflected mode detecting the White bordering the Black line seems to work fine. For those who plan to use Compare Color mode it seems to work fine as long as the robot’s motor power is 30 or less and detects the line at an angle of about 20 degrees; more than that or greater power may cause it to miss or go off the line. For a smoother moving robot (less zig zag) use a power combination of 30 and 10.
  2. When using duel color sensors, the power of the True Value motor can be as high as 40-45, while
    the other motor has a power of 10-20. This allows the robot to move in a smooth straight line.
  3. This year’s mat is very colorful, so when detecting a white line or a black line you need to be aware of the part of the mat the robot has to move across to get to that White line or black line. 
Moving over a light colored area to detect a white line, in Compare Color Mode, the robot might stop or will hesitate over white (duh!) and light colors, but tends to move on to detect the pure white area. 

When detecting black in Compare Color Mode there seems to be no problem as long as the motor power was 50 or less; greater power can cause the robot to over shoot the line; it is moving too fast to have the sensor read and respond to the line.

Moving over dark colored areas to detect a white line, in Compare Color Mode, the robot might stop or hesitate.  When detecting black in Color Mode the robot might hesitate over dark colors and stop on black spots of .5 cm thickness. The thin black lines did not prove to be a problem with motor power of 50 or less. 

When detecting a white line, in Compare Color Mode, the sensor will read light colors mixed in and may hesitate or even stop. 

If you use the Compare Reflected Mode to detect white and moving over a multi-colored area, use GREATER THAN/EQUAL TO the threshold and a motor power of 50 or less. 

If you use the Compare Reflected Mode to detect black and moving over a multi-colored area, use LESS THAN/EQUAL TO the threshold and again a motor power of 50 or less.

It is recommended that teams run similar tests using different lighting situations before you start programing your missions. Note, we did not test the NXT Color sensor, but we suspect that the results would be similar.

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