Reflectance sensor array

Yozh has a built-in array of reflectance sensors, pointed down. These sensors can be used to detect filed borders, for following the line, and other similar tasks.

Basic usage

linearray_on()

linearray_off(): Turns reflectance array on/off. By default, it is off (to save power).

linearray_raw(i): Returns raw reading of sensor i. One can use either indices 0…7 or (preferred) named values bot.A1 = 0 … bot.A8 = 7. Readings range 0-1023 depending on amount of reflected light: the more light reflected, the lower the value. Typical reading on white paper is about 50-80, and on black painted plywood, 950. Note that black surfaces can be unexpectedly reflective; on some materials which look black to human eye, the reading can be as low as 600.

Calibration

Process of calibration refers to learning the values corresponding to black and white areas of the field and then using these values to rescale the raw readings.

calibrate(): Calibrates the sensors, recording the lowest and highest values. This command should be called when some of the sensors are on the white area and some, on black.

linearray_cal(i): Returns reading of sensor i, rescaled to 0-100: white corresponds to 0 and black to 100. It uses the calibration data, so should only be used after the sensor array has been calibrated.

sensor_on_white(i): Returns True if sensor i is on white and false otherwise. A sensor is considered to be on white if calibrated value is below 50.

sensor_on_black(i): Returns True if sensor i is on black and false otherwise.

Line following

A common task for such robots is following the line. To help with that, Yozh library provides the helper function.

line_position_white()

Returns a number showing position of the line under the robot, assuming white line on black background. The number ranges between -5 (line far to the left of the robot) to 5 (line far to the right of the robot). 0 is central position: line is exactly under the center of the robot.

Slightly simplifying, this command works by counting how many sensors are to the left of the line, how many are to the right, and then taking the difference. It works best for lines of width 1-2cm; in particular, electric tape or gaffers tape (1/2” or 3/4”) works well.

This command only uses the central 6 sensors; rightmost and leftmost sensor (A1 and A8) are not used.

If there is no line under these sensors, the value returned by this command will be close to 0, but can not be relied on.

line_position_black(): Same as above, but assuming black line on white background.