Line Tracking using PID using 89S52

First I'd like to thank So ChikMan From Hong kong HKU, he is a pioneer in control engineering besides, he has repersented his country many times in Robocon contest, he has a great impact on teaching me the theory and operation of PID Control.

Hi, In robocon 2008, i've tried to implement the well-known algorithm in Control engineering the PID ( Proportional -integeral -derivative), I was having a great experience using the microcontroller from ATMEL 89S52 , plus I've already designed a complete board using it at the past years of the contests , and due to low fund, I've used it again. The problem is that uC doesnt have a PWM Hardware module so you've to write your own Software PWM, also it doesnt have an ADC ( anlogue to digital converter), and i've solved this by using a crude way of doing an ADC which is using a compartor like LM324.

The idea is there are 6 sensors (some people uses 8 or 16,or 32! depends on your complexity of design and speed you want to achieve), These sensors will give you the position of the robot from the line which it follows considring that the output from the sensors are Digital , so you've to define some numeric values to enumrate for the posssibilites of the position of the robot on the line to be tracked.

Consider that 1 is the value on white which means your on the line, and 0 is the value when your away from the line. Consider The sensor arrangement as this xxxxxx, let it be L1L2L3R3R2R1
Consider that L3, R3 have been positioned to be on the line , and the other sensors are seprated by 10mm.
The Pid algorithm calculates the error from the feedback element which is our 6 line sensors , when L3,R3 are on the white line, which means the robot is going straight and it also means the the Error is 0, which is our desired value or what's always called in control engineering text books ( Set Point ).
pseudo code :
error = setpoint- measured value .
If you've a control engineering textbook, you will see many figures of the well-known control loop which is familiar to many engineers.
The measured value should be a predfined numeric values because we dont have an ADC here.
We will define it like in the code.

Code List using 89S52

void init_timer0(void)
{
EA=0; // disable gloabl interrupts
TMOD=0x02; // 0000 0010
TH0=0x9B; // Timer values
TL0=0x9B; //
PWM=0;
ET0=1; // Enable timer interrupt 0
EA=1;
}

The Software PWM Code
// Motor1_2 , is the enable pin of the motor for PWM
// pwm_l , r are the values of the PWM which be sent to the motor , from 0 > 11
void timer0(void) interrupt 1 using 0
{
TR0=0;
PWM++;
TL0=0x9B;TF0=0;
if (PWM==11) PWM=0
if (PWM <= pwm_l) Motor1_2=1; else Motor1_2=0; if (PWM <= pwm_r) Motor2_2=1; else Motor2_2=0; TR0=1; }


// Do this in a 20ms (sampling time) control loop

void Line_Track(void)
{
int mpos;
float error,Rate,Deriv,previous_error;
pwm_r_=11;
pwm_l_=11;
TR0=1;
Walk(1); // move forward
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=0;
}
if ((Sensor_1==1)&&(Sensor_2==1)&&(Sensor_3==1)&&(Sensor_4==1)&&(Sensor_5==1)&&(Sensor_6==1))
{
mpos=0;
}
if ((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==1)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=-1;
}
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==1)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=1;
}
if((Sensor_1==0)&&(Sensor_2==1)&&(Sensor_3==0)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=-2;
}
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==0)&&(Sensor_5==1)&&(Sensor_6==0))
{
mpos=2;
}
if((Sensor_1==1)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=-5;
}
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==1))
{
mpos=5;
}
if((Sensor_1==0)&&(Sensor_2==1)&&(Sensor_3==1)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=-4;
}
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==1)&&(Sensor_5==1)&&(Sensor_6==0))
{
mpos=-4;
}
if((Sensor_1==0)&&(Sensor_2==0)&&(Sensor_3==0)&&(Sensor_4==1)&&(Sensor_5==1)&&(Sensor_6==1))
{
mpos=3;
}
if((Sensor_1==1)&&(Sensor_2==1)&&(Sensor_3==1)&&(Sensor_4==0)&&(Sensor_5==0)&&(Sensor_6==0))
{
mpos=-3;
}
// PID Equations
error=-mpos; // error = 0 - feedback , 0 is the center
error=error*2.2; // KP*ERROR , Kp=2.2
Rate = error-previous_error;
Deriv = Rate * 2.0; // KD = 2.0
previous_error=error;
error=error+Deriv; // PD Terms
// CUT Terms
// error is negative , robot is going to right
if (error < pwm_r =" plus"> 0){
pwm_r_+= ABS(error);
pwm_l_-=ABS(error);
}
if(pwm_r_>11)pwm_r_=11;
if(pwm_l_>11)pwm_l_=11;

}


Videos

Fast line follower
http://www.youtube.com/watch?v=g2_9nDmEmPo

Doing a task
http://www.youtube.com/watch?v=kiqopmLjFjE



Eng.Ahmed Saleh Mohammed
El Shorouk Acadmey
Electronics and communications engineering department