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m3pi LineFollowing
Line following programs use the reflective sensors under the front of the 3pi to detect a difference in reflectivity, usually contrast between a black line on a white background (or vice versa!). The changes in reflections are used to control the motors to keep the line in the center of the sensors.
Reflection sensors¶
There are 5 sensors on the 3pi, one in the centre, one either side close to the centre and another one either side futher out. This give good resolution near the centre and a wide range.
The m3pi library contains the following function:
float line_position(void);
This reads current state of the sensors and returns a calibrated and normalised floating point number representing where a black line might be with respect to the centre of the 3pi.
- 0.0 = a line is detected in the centre
- 1.0 = a line is detected at right edge
- -1.0 = a line is detected at the left edge, or no line is detected at all
Basic line Following¶
Note that this is a very (very) crude algorithm, with three parameter that affect the behaviour:
- The initial speed of the m3pi
- How far off the centre the m3pi is before corrective action is taken
- How how aggressive the corrective action is
Import program
00001 #include "mbed.h" 00002 #include "m3pi.h" 00003 00004 m3pi m3pi; 00005 00006 int main() { 00007 00008 // Parameters that affect the performance 00009 float speed = 0.2; 00010 float correction = 0.1; 00011 float threshold = 0.5; 00012 00013 m3pi.locate(0,1); 00014 m3pi.printf("Line Flw"); 00015 00016 wait(2.0); 00017 00018 m3pi.sensor_auto_calibrate(); 00019 00020 while (1) { 00021 00022 // -1.0 is far left, 1.0 is far right, 0.0 in the middle 00023 float position_of_line = m3pi.line_position(); 00024 00025 // Line is more than the threshold to the right, slow the left motor 00026 if (position_of_line > threshold) { 00027 m3pi.right_motor(speed); 00028 m3pi.left_motor(speed-correction); 00029 } 00030 00031 // Line is more than 50% to the left, slow the right motor 00032 else if (position_of_line < -threshold) { 00033 m3pi.left_motor(speed); 00034 m3pi.right_motor(speed-correction); 00035 } 00036 00037 // Line is in the middle 00038 else { 00039 m3pi.forward(speed); 00040 } 00041 } 00042 }