UltraSonic Sensor Trial with tank

#include <NewPing.h>
#include <Servo.h>
#define TRIGGER_PIN  12  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     11  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 250 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.
Servo left,right; // 360 servo
NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.
int led=13;
int pos=0;
int pos1=0;
void setup() {
Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
pinMode(13, OUTPUT);
left.attach(6);//left side
right.attach(5); //right side
}
void loop() {
Serial.print(sonar.ping_cm()); // Send ping, get distance in cm and print result (0 = outside set distance range)
Serial.println("cm");
if (sonar.ping_cm() >= 15 && sonar.ping_cm() != 0) //move forward
{
 digitalWrite(led, LOW);
 for (pos1 = 95; pos1 <= 360; pos1 += 1) {
  right.write(pos1);
  left.write(-pos1);
  }
}
else if (sonar.ping_cm() > 0 && sonar.ping_cm() <= 15 ) // move left
{
 for(pos1=95; pos1<=360; pos1 +=1){
  right.write(pos1);
  left.write(pos1);
 }
}
else //move backward
{
 digitalWrite(led,HIGH);
 for (pos1 = 95; pos1 <= 360; pos1 += 2){
  right.write(-pos1);
  left.write(pos1);
 }
}
}