Hi there, I am working on maze solving with the m3pi robot but sometimes (at low speed around 20%) the robot oscillates in straight line. I just add something to the line following pid function in order to fix it and as it works pretty well I am happy to share it with you.
When you compute the new speed of each wheel :

// Compute new speeds
right = speed+power;
left = speed-power;

you must take in count the actual maximum speed of the robot (MAX) then you will not slow down to much the robot and then make it oscillates :

// Compute new speeds
right = speed+(power*MAX);
left = speed-(power*MAX);

It’s a small contribution but anyway I hope it will help some people .

ps: I am sorry for my English but I am not a native English speaker.

Thanks for sharing your findings. What number is MAX? Can you share/publish your code from within the mbed compiler so we can see exactly what you are doing?

MAX is the maximum speed you want for the robot, and also MIN is the minimum speed. For example if you put MAX = 0.8 and MIN = 0.2 the speed of the robot will be between 20% and 80% of full speed.
Here is the line following pid code I use:

#include "mbed.h"
#include "m3pi.h"
m3pi m3pi;
// Minimum and maximum motor speeds
#define MAX 1.0
#define MIN 0
// PID terms
#define P_TERM 1
#define I_TERM 0
#define D_TERM 20
int main() {
m3pi.locate(0,1);
m3pi.printf("Line PID");
wait(2.0);
m3pi.sensor_auto_calibrate();
float right;
float left;
float current_pos_of_line = 0.0;
float previous_pos_of_line = 0.0;
float derivative,proportional,integral = 0;
float power;
float speed = MAX;
while (1) {
// Get the position of the line.
current_pos_of_line = m3pi.line_position();
proportional = current_pos_of_line;
// Compute the derivative
derivative = current_pos_of_line - previous_pos_of_line;
// Compute the integral
integral += proportional;
// Remember the last position.
previous_pos_of_line = current_pos_of_line;
// Compute the power
power = (proportional * (P_TERM) ) + (integral*(I_TERM)) + (derivative*(D_TERM)) ;
// Compute new speeds
right = speed+power*MAX;
left = speed-power*MAX;
// limit checks
if (right < MIN)
right = MIN;
else if (right > MAX)
right = MAX;
if (left < MIN)
left = MIN;
else if (left > MAX)
left = MAX;
// set speed
m3pi.left_motor(left);
m3pi.right_motor(right);
}
}

Thanks for posting your code. Just a note to anyone trying it, the [ b] [/b] on the relevant line are BBCodes and should be removed if you are using the code.