How to control a robot arm using ultrasonic sensor and Arduino

I'm working about arduino and HC_SR04. I searched most of the documents, but they didn't solve our problem.

My question is that how to read a value that is taken from library <NewPing>.

#define echopin 11 //set echopin
#define trigpin 12 //set trigpin

#include <Servo.h>;

Servo robotArm;

#include <NewPing.h>

#define MAX_DISTANCE 400

NewPing sonar(trigpin, echopin, MAX_DISTANCE);

int distance;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  robotArm.attach(9); //attach our servo
  robotArm.writeMicroseconds(150);
}

void loop() {
  // put your main code here, to run repeatedly:
  robotArm.write(90); //always set to servo 90 to position it to the middle
  //codes of ultrasonic sensor
  distance=digitalRead(echopin);
  if (distance <= 20) //if ultrasonic sensor detects on obstacle less than 20 cm in 90 degree angle
  {
    robotArm.write(0); //dervo rotates at full speed to the right
    delay(60);
  }

  else
  {
    robotArm.write(90); //else servo stays at 90 degree angle
    delay(60);
  }
  Serial.print(distance); //print distance
  Serial.println("cm"); //print distance unit cm
}

Instead of distance=digitalRead(echopin); try using distance = sonar.ping_cm().