Turtlebot sensor_state all 0s with Gazebo on Fuerte

edit

I am trying to access the bumps wheeldrops sensor from the Turtlebot sensor_state topic. However, the bump sensor is never activated. In fact, if I execute rostopic echo /turtlebot_node/sensor_state, all the sensor_state values are permanently set to 0. In addition, I noticed that the /odom Twist parameter remains 0 even when the robot is moving. Here is my launch file:

<launch>
    <arg name="use_sim_time" default="true"/>
    <arg name="gui" default="true"/>

    <!-- start gazebo with an empty plane -->
    <param name="/use_sim_time" value="true" />

    <node name="gazebo" pkg="gazebo" type="gazebo" args="-u $(find turtlebot_gazebo)/worlds/1box.world" respawn="false" output="screen"/>

    <include file="$(find turtlebot_gazebo)/launch/robot.launch"/>

    <!-- start gui -->
    <group if="$(arg gui)">
        <node name="gazebo_gui" pkg="gazebo" type="gui" respawn="false" output="screen"/>
    </group> 

</launch>

And here is the simple program that is running as a node:

#include <ros/ros.h>
#include <turtlebot_node/TurtlebotSensorState.h>
#include <geometry_msgs/Twist.h>
#include <ros/rate.h>
#include <iostream>
#include <stdio.h>                        

using namespace std;    
uint8_t bumper;

void bumperCallback(const turtlebot_node::TurtlebotSensorState::ConstPtr& msg);

int main (int argc, char** argv){       

    ros::Time start, end;
    ros::init(argc, argv, "bumper");    
    ros::NodeHandle nodeH;  
    ros::Rate loop_rate(10); //10 Hz.
    geometry_msgs::Twist twist;

    //Bumper subscriber.
    ros::Subscriber bumperSub = nodeH.subscribe<turtlebot_node::TurtlebotSensorState>("/turtlebot_node/sensor_state", 1000, bumperCallback);
    //Moving.
    ros::Publisher movingPub = nodeH.advertise<geometry_msgs::Twist>("cmd_vel", 1000);

    //Twist set to navigate forward.
    double forwardSpeed = 0.15;
    twist.linear.x = forwardSpeed; 
    twist.linear.y = 0;
    twist.linear.z = 0;
    twist.angular.x = 0; 
    twist.angular.y = 0; 
    twist.angular.z = 0;

    start = ros::Time::now(); //Time when the robot starts moving.
    while(ros::ok){
        if(bumper != 0){
            end = ros::Time::now(); //Time when robot bumped into a wall.
            ROS_INFO("BOOM OUCH");

        }
        else{ //Go forward until the robot bumps into a wall.
            ROS_INFO("Going forward");      
            movingPub.publish(twist);
        }
        loop_rate.sleep();
        ros::spinOnce();
    }

    return 0;
}

//Fetch the sensor state containing the bumper state.
void bumperCallback(const turtlebot_node::TurtlebotSensorState::ConstPtr& msg){

    bumper = msg->bumps_wheeldrops;
    ROS_INFO("Bumper is %d .", bumper);     
}

I verified that Gazebo is indeed publishing on "/turtlebot_node/sensor_state" and that my own node is subscribed to it. How can I solve this problem? I've researched similar topics, but I don't see anything wrong with my files based on these.

Note that the following warning is being reported by Gazebo, but I think it's irrelevant:

Warning [RaySensor.cc:206] ranges not constructed yet (zero sized)

[Edit] I just noticed these additional errors. I had not seen them before because the RaySensor warning is being displayed thousands of times. Gazebo seems to be running fine, though...

Warning [parser.cc:348] Gazebo SDF has no gazebo element
Warning [parser.cc:291] DEPRECATED GAZEBO MODEL FILE
On July 1st, 2012, this formate will no longer by supported
Convert your files using the gzsdf command line tool
You have been warned!

Error [ColladaLoader.cc:1392] No Opaque set
[ INFO] [1375303568.841199057]: imu plugin missing <serviceName>, defaults to /default_imu
[ INFO] [1375303568.842760080]: imu plugin missing <xyzOffset>, defaults to 0s
[ INFO] [1375303568 ...