How to use publish and use multiple sonar range_msg data (using rosserial) on a single topic?

edit

Hi,

I am learning ROS by developing a differential drive collision avoidance robot. (Specifically the Parallax ARLO Robot). I have written a code that can publish one ping sensor's range value to a node, and then have the Arduino subscribe to that node and read the data and execute it's if loop based on that data.

My question is, how do I publish multiple range messages to a topic and decide which ultrasonic sensor value should be used when subscribing to that topic?

Below is the current working code. What I'm unsure how to figure out is: in the callback function, the distance (this is the front sensor) is equal to range_msg.range. So what if I have multiple range messages, and don't know how to define which distance belongs to each sensor?

For example,

distance_left = range_msg.range
distance_right = range_msg.range

and so on is all I know how to do as of now. I know how to publish multiple messages to that range_msg but I my understanding of subscribing is still very weak. Thank you very much for your help, and I am very new to this ros community so please let me know if there are ways I can improve my question.

#include <ArloRobot.h>      // Include ARLO Library
#include <SoftwareSerial.h> // Include SoftwareSerial Library
#include <Servo.h>
#include <ros.h>
#include <ros/time.h>
#include <sensor_msgs/Range.h>

// Create Arlo and Serial Objects
ArloRobot Arlo;                                       // Arlo Object
SoftwareSerial ArloSerial(12, 13);                    // Serial in I/O 12, 
Servo servo;

ros::NodeHandle  nh;

sensor_msgs::Range range_msg;

long duration;
int distance;

/**** Motor Callback Function ****/
void motor_cb( const sensor_msgs::Range& range_msg){
  distance = range_msg.range; 

  if (distance < 30) {
    Arlo.writeMotorPower(0,0);
  }
  else {
    Arlo.writeMotorPower(30,30);
  }
}

ros::Publisher pub_range1("/ultrasound1", &range_msg);
ros::Publisher pub_range2("/ultrasound2", &range_msg);
ros::Subscriber<sensor_msgs::Range> sub_range("/ultrasound1", motor_cb);

char frameid[] = "/ultrasound";

void setup()
{
  servo.attach(9);
  nh.initNode();
  nh.advertise(pub_range1); // Front Sensor
  nh.advertise(pub_range2); // Back Sensor
  nh.subscribe(sub_range);

  ArloSerial.begin(19200);
  Arlo.begin(ArloSerial);

  Arlo.writeMotorPower(60, 60);
  delay(3000);
  Arlo.writeMotorPower(0,0);
}



long range_time;

void loop()
{
  if ( millis() >= range_time ){
    range_msg.range = getRange_Ultrasound(14);
    pub_range1.publish(&range_msg);
    pub_range2.publish(&range_msg); 

    range_time =  millis() + 50;
  }

  nh.spinOnce();
}


/**** Get Ultrasonic Ranges ****/
float getRange_Ultrasound(int pin_num)
{
  pinMode(pin_num, OUTPUT);
  digitalWrite(pin_num, LOW);
  delayMicroseconds(2);
  digitalWrite(pin_num, HIGH);
  delayMicroseconds(10);
  digitalWrite(pin_num, LOW);

  pinMode(pin_num, INPUT);
  duration = pulseIn(pin_num, HIGH);

  return duration/58;
}