90 Degree Turns Using PID

Robotics and electronics discussion Software and microcontrollers
1

I have built a line follower using the Pololu - QTR-8RC Reflectance Sensor Array. It runs pretty well enough on curves and straight lines, but is unable to precisely navigate 90 degree turns. I mean that is does turn around the 90 degree turns, but wobbs a lot.
The program is below. Please help me out with these 90 degree turns with the pololu qtr-8rc sensor array.

#include <QTRSensors.h>
#include <AFMotor.h>

AF_DCMotor m_left(2, MOTOR12_1KHZ ); 
AF_DCMotor m_right(3, MOTOR34_1KHZ ); 

float Kp = 0.1; //0.2
float Kd = 0.28;


int base_speed = 255; // vary base motor speed according to voltage
int bot_position = 0;
int error = 0;
unsigned int sensors[8];
int proportional = 0;
int derivative = 0;
int last_proportional = 0;

#define NUM_SENSORS 8         
#define TIMEOUT 2500          //waits for 2500 us for sensor outputs to go low
#define EMITTER_PIN 2    
#define DEBUG 0

QTRSensorsRC qtrrc((unsigned char[]) {A0,A1,A2,A3,A4,A5,9,10} ,NUM_SENSORS, TIMEOUT, 
EMITTER_PIN);
unsigned int sensorValues[NUM_SENSORS];

void setup() {
Serial.begin(9600);
auto_calibration();
bot_stop();
delay(1000);
}

void loop() {
pid_calc();
motor_drive();
}

void auto_calibration() {
for (int i = 0; i < 170; i++){
qtrrc.calibrate(QTR_EMITTERS_ON);
delay(20);
     if(i < 50) left();
else if (i > 50 && i < 120) right();
else if (i > 120 && i < 170) left();
}

if (DEBUG) {
for (int i = 0; i < NUM_SENSORS; i++){
qtrrc.calibratedMinimumOn[i];
}

for (int i = 0; i < NUM_SENSORS; i++){
qtrrc.calibratedMaximumOn[i];
  }
}
}

void pid_calc(){
  bot_position = qtrrc.readLine(sensors);
  //Serial.println(bot_position);
 proportional = bot_position - 3400;
 derivative = proportional - last_proportional;
 last_proportional = proportional;

  error = (Kp * proportional) + (Kd * derivative);
 //Serial.println(error);
}

void motor_drive(){
 int left_motor_speed; 
 int right_motor_speed;  

 left_motor_speed = base_speed - error;
 right_motor_speed = base_speed + error;

if(left_motor_speed < 0) left_motor_speed = 0;
   else if(left_motor_speed > 255) left_motor_speed = 255;

if(right_motor_speed < 0) right_motor_speed = 0;
   else if(right_motor_speed > 255) right_motor_speed = 255;
  
  Serial.print(left_motor_speed);
  Serial.print(" ");
  Serial.println(right_motor_speed);
  
  m_left.setSpeed(left_motor_speed); 
  m_right.setSpeed(right_motor_speed);
   m_left.run(FORWARD); 
 m_right.run(FORWARD);      
}

void left(){
 m_left.setSpeed(55); 
 m_right.setSpeed(55);
 m_left.run(BACKWARD); 
 m_right.run(FORWARD);  
}

void right(){
  m_left.setSpeed(55); 
  m_right.setSpeed(55);
  m_left.run(FORWARD); 
  m_right.run(BACKWARD);  
}

void bot_stop(){
  m_left.setSpeed(0); 
  m_right.setSpeed(0);
  m_left.run(FORWARD); 
 m_right.run(FORWARD);  
}
2

I wanted to know that how can I use the QTR-8RC Reflectance Sensor Array like an ordinary digital sensor array. I want to use the array as an ordinary digital sensor array without the Pololu qtr sensor array because I am not able to define special cases using the library for right and acute angled turns. Please help me out with this.

3

Hello.

I merged your two posts, since it seems like they are part of the same problem.

It sounds like you might have your PID tuned to follow the line well, but that same tuning is not working well for 90 degree angles. If so, it could make sense to handle the two cases separately and it should be possible to use the sensor array like you mentioned to detect which case the robot should be dealing with. The QTRRCExample sketch we include with our QTR library shows where the individual calibrated sensors values are stored (in the sensorValues array). It should be possible to compare the calibrated values to some threshold value (which will depend on your surface and possibly background lighting) to determine whether or not any particular sensor can see the line.

-Nathan