Arduino Uno Avoidance Robot: One of two motors not working - arduino

Question

I'm trying to code an Arduino avoidance robot as a learning project. When I run the following code only one motor (the left one) works. If I comment out the if statement that uses the checkDistance() function and just loop the forward() function both motors work fine. I'm using an Adafruit Motorshield v1.2, and I believe this is the motor: Link. I've tried powering directly through USB, and also through external power supply (4 x AAs), and have the same issue. Any pointers would be appreciated. Thank you!
#include <AFMotor.h>
#define trigPin 3
#define echoPin 2
AF_DCMotor left_motor(2, MOTOR12_64KHZ); // create motor #1, 64KHz pwm
AF_DCMotor right_motor(1, MOTOR12_64KHZ); // motor #2\
int maxDistance = 30;
//int minDistance = 0;
void setup() {
Serial.begin(9600); // set up Serial library at 9600 bps
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
left_motor.setSpeed(255);
right_motor.setSpeed(255);
}
void forward(int delayTime) {
left_motor.run(FORWARD);
right_motor.run(FORWARD);
delay(delayTime);
}
void backward(int delayTime) {
left_motor.run(BACKWARD);
right_motor.run(BACKWARD);
delay(delayTime);
}
void stop(int delayTime) {
left_motor.run(RELEASE);
right_motor.run(RELEASE);
delay(delayTime);
}
void right(int delayTime) {
left_motor.run(FORWARD);
right_motor.run(BACKWARD);
delay(delayTime);
}
void left(int delayTime) {
left_motor.run(BACKWARD);
right_motor.run(FORWARD);
delay(delayTime);
}
int checkDistance(){
long duration, distance;
/* The following trigPin/echoPin cycle is used to determine the
distance of the nearest object by bouncing soundwaves off of it. */
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
//Calculate the distance (in cm) based on the speed of sound.
distance = duration/59.1;
return distance;
}
void loop() {
int currentDistance = checkDistance();
Serial.print(currentDistance);
if (currentDistance >= maxDistance){
Serial.println(" cm. No immediate obstacles detected. Moving forward.");
forward(2000);
stop(1000);
}
else {
Serial.println(" cm. Obstacle detected. Changing direction.");
right(1500);
stop(1000);
}
}

Answer 1

If I were trying to debug this, I would first test each of the motor functions without any logic check from the sensor. (e.g. do both wheels turn and in the correct direction when left, right, backward, etc are called just from loop()). That will give confidence that it is not a wiring or other hardware problem. If they don't work then sorting that out give you a place to start.
If they do work, then it must be the values with the sensor, right? How did you determine the maxDistancevalue? If it is too high then this line:
if (currentDistance >= maxDistance){
may never evaluate to true. You have some log statements in there so you have some sense of the value of currentDistance. Does it seem to be reflecting accurate values when you place objects at varying distances from the sensor?
Anyway... some thoughts.