Hi,
I recently built myself a simple robot, consisting of: an Arduino 328 board, motor-driver board, two DC-motors (with tank treads), batteries. See picture, you'll get the idea 
Picture of my project:
hxxp://ttjcrew.com/forum/download/file.php?id=2 (change the xx)
After finishing my robot I found this amazing tutorial on how to build an autonomous car (over ttjcrew.com). The tutorial was for a car with one DC-motor for speed (forward/backward) and one servo for steering. The tutorial provided a code for this to work.
Eager to do the same I ordered the a GPS module (the one used in the tutorial, EM-406A). The only design change on my project was the lack of a servo, but i figured I could use the two motors for steering (like a tank). However I am no expert on Arduino so I did not manage to rewrite the provided code to use two DC-motors for steering instead of a servo.
So I was wondering if any of you know how to modify this code (ttjcrew all rights reserved) for my project:
/* GPS Car v0.1 by Eric Barch (ttjcrew.com) */
#include <SoftwareSerial.h>
#include "Printfloat.h"
#include <nmea.h>
#include <Servo.h>
#undef abs
#undef round
int wpt = 0;
float dest_latitude;
float dest_longitude;
NMEA gps(GPRMC); // GPS data connection to GPRMC sentence type
/* BEGIN EDITABLE CONSTANTS SECTION */
//These define the positions for your steering servo
#define CENTER_SERVO 95
#define MAX_LEFT_SERVO 85
#define MAX_RIGHT_SERVO 108
//When the car is within this range (meters), move to the next waypoint
#define WPT_IN_RANGE_M 12
//These pins are your h-bridge output. If the car is running in reverse, swap these
#define MOTOR_OUTPUT_ONE 3
#define MOTOR_OUTPUT_TWO 4
/* DEFINE GPS WPTS HERE - Create more cases as needed */
void trackWpts() {
switch(wpt) {
case 0:
dest_latitude = 40.756054;
dest_longitude = -73.986951;
break;
case 1:
dest_latitude = 37.775206;
dest_longitude = -122.419209;
break;
default:
dest_latitude = 0;
dest_longitude = 0;
break;
}
if (gps.gprmc_distance_to(dest_latitude,dest_longitude,MTR) < WPT_IN_RANGE_M)
wpt++;
}
/* END EDITABLE CONSTANTS SECTION */
Servo steering;
float dir; // relative direction to destination
int servo_pos = CENTER_SERVO;
void setup() {
Serial.begin(4800);
pinMode(MOTOR_OUTPUT_ONE, OUTPUT);
pinMode(MOTOR_OUTPUT_TWO, OUTPUT);
steering.attach(9);
}
void loop() {
trackWpts();
trackGPS();
}
void trackGPS() {
if (dest_latitude != 0 && dest_longitude != 0)
{
if (Serial.available() > 0 ) {
char c = Serial.read();
if (gps.decode(c)) {
// check if GPS positioning was active
if (gps.gprmc_status() == 'A') {
// calculate relative direction to destination
dir = gps.gprmc_course_to(dest_latitude, dest_longitude) - gps.gprmc_course();
if (dir < 0)
dir += 360;
if (dir > 180)
dir -= 360;
if (dir < -75)
hardLeft();
else if (dir > 75)
hardRight();
else
driveToTarget();
}
else //No GPS Fix...Wait for signal
stop();
}
}
}
else
stop();
}
void driveStraight() {
steering.write(CENTER_SERVO);
digitalWrite(MOTOR_OUTPUT_ONE, LOW);
digitalWrite(MOTOR_OUTPUT_TWO, HIGH);
Serial.println("Driving straight...");
}
void driveToTarget() {
Printfloat(dir);
servo_pos = map(dir, -75, 75, MAX_LEFT_SERVO, MAX_RIGHT_SERVO);
steering.write(servo_pos);
digitalWrite(MOTOR_OUTPUT_ONE, LOW);
digitalWrite(MOTOR_OUTPUT_TWO, HIGH);
Serial.print("Driving at ");
Serial.print(dir);
Serial.print(". - ");
Serial.print(gps.gprmc_distance_to(dest_latitude,dest_longitude,MTR));
Serial.print("m to go");
Serial.print("\n");
}
void hardLeft() {
steering.write(MAX_LEFT_SERVO);
digitalWrite(MOTOR_OUTPUT_ONE, LOW);
digitalWrite(MOTOR_OUTPUT_TWO, HIGH);
Serial.print("Driving hard left. - ");
Serial.print(gps.gprmc_distance_to(dest_latitude,dest_longitude,MTR));
Serial.print("m to go");
Serial.print("\n");
}
void hardRight() {
steering.write(MAX_RIGHT_SERVO);
digitalWrite(MOTOR_OUTPUT_ONE, LOW);
digitalWrite(MOTOR_OUTPUT_TWO, HIGH);
Serial.print("Driving hard right. - ");
Serial.print(gps.gprmc_distance_to(dest_latitude,dest_longitude,MTR));
Serial.print("m to go");
Serial.print("\n");
}
void stop() {
steering.write(CENTER_SERVO);
digitalWrite(MOTOR_OUTPUT_ONE, HIGH);
digitalWrite(MOTOR_OUTPUT_TWO, HIGH);
Serial.print("Stopped.");
Serial.print("\n");
}
I don't know if this code is any good, but they proved it worked over at ttjcrew.com. Anyways, I really want to finish my project 