## Support » Pololu 3pi Robot User's Guide » 7. Example Project #1: Line Following »## 7.c. Advanced Line Following with 3pi: PID ControlA more advanced line following program for the 3pi is available in the folder
The technique used in this example program, known as PID control, addresses some of the problems that you might have noticed with the previous example, and it should allow you to greatly increase your robot’s line following speed. Most importantly, PID control uses continuous functions to compute the motor speeds, so that the jerkiness of the previous example can be replaced by a smooth response. PID stands for Proportional, Integral, Derivative; these are the three input values used in a simple formula to compute the speed that your robot should turn left or right. - The
**proportional**value is approximately proportional to your robot’s position with respect to the line. That is, if your robot is precisely centered on the line, we expect a proportional value of exactly 0. If it is to the left of the line, the proportional term will be a positive number, and to the right of the line, it will be negative. This is computed from the result returned by*read_line()*simply by subtracting 2000. - The
**integral**value records the history of your robot’s motion: it is a sum of all of the values of the proportional term that were recorded since the robot started running. - The
**derivative**is the rate of change of the proportional value. We compute it in this example as the difference of the last two proportional values.
Here is the section of code that computes the PID input values: // Get the position of the line. Note that we *must* provide // the "sensors" argument to read_line() here, even though we // are not interested in the individual sensor readings. unsigned int position = read_line(sensors,IR_EMITTERS_ON); // The "proportional" term should be 0 when we are on the line. int proportional = ((int)position) - 2000; // Compute the derivative (change) and integral (sum) of the // position. int derivative = proportional - last_proportional; integral += proportional; // Remember the last position. last_proportional = proportional; Note that we cast the variable Each of these input values provides a different kind of information. The next step is a simple formula that combines all of the values into one variable, which is then used to determine the motor speeds: // Compute the difference between the two motor power settings, // m1 - m2. If this is a positive number the robot will turn // to the right. If it is a negative number, the robot will // turn to the left, and the magnitude of the number determines // the sharpness of the turn. int power_difference = proportional/20 + integral/10000 + derivative*3/2; // Compute the actual motor settings. We never set either motor // to a negative value. const int max = 60; if(power_difference > max) power_difference = max; if(power_difference < -max) power_difference = -max; if(power_difference < 0) set_motors(max+power_difference, max); else set_motors(max, max-power_difference); The values 1/20, 1/10000, and 3/2 represent Please see |

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