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Using urdf with robot_state_publisher

Description: This tutorial gives a full example of a robot model with URDF that uses robot_state_publisher. First, we create the URDF model with all the necessary parts. Then we write a node which publishes the JointState and transforms. Finally, we run all the parts together.

Keywords: robot_state_publisher urdf

Tutorial Level: INTERMEDIATE

Create the URDF File

Here is the URDF file for a 7-link model roughly approximating R2-D2. Save the following link to your computer: model.xml

Some Key Features of the URDF

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Publishing the State

Now we need a method for specifying what state the robot is in. To do this, we must specify all three joints and the overall odometry. Create a package to include the source code below (throughout this example, we assume package name "r2d2" and node name "state_publisher"):

   1 #include <string>
   2 #include <ros/ros.h>
   3 #include <sensor_msgs/JointState.h>
   4 #include <tf/transform_broadcaster.h>
   5 
   6 int main(int argc, char** argv) {
   7     ros::init(argc, argv, "state_publisher");
   8     ros::NodeHandle n;
   9     ros::Publisher joint_pub = n.advertise<sensor_msgs::JointState>("joint_states", 1);
  10     tf::TransformBroadcaster broadcaster;
  11     ros::Rate loop_rate(30);
  12 
  13     const double degree = M_PI/180;
  14 
  15     // robot state
  16     double tilt = 0, tinc = degree, swivel=0, angle=0, height=0, hinc=0.005;
  17 
  18     // message declarations
  19     geometry_msgs::TransformStamped odom_trans;
  20     sensor_msgs::JointState joint_state;
  21     odom_trans.header.frame_id = "odom";
  22     odom_trans.child_frame_id = "axis";
  23 
  24     while (ros::ok()) {
  25         //update joint_state
  26         joint_state.header.stamp = ros::Time::now();
  27         joint_state.name.resize(3);
  28         joint_state.position.resize(3);
  29         joint_state.name[0] ="swivel";
  30         joint_state.position[0] = swivel;
  31         joint_state.name[1] ="tilt";
  32         joint_state.position[1] = tilt;
  33         joint_state.name[2] ="periscope";
  34         joint_state.position[2] = height;
  35 
  36 
  37         // update transform
  38         // (moving in a circle with radius=2)
  39         odom_trans.header.stamp = ros::Time::now();
  40         odom_trans.transform.translation.x = cos(angle)*2;
  41         odom_trans.transform.translation.y = sin(angle)*2;
  42         odom_trans.transform.translation.z = .7;
  43         odom_trans.transform.rotation = tf::createQuaternionMsgFromYaw(angle+M_PI/2);
  44 
  45         //send the joint state and transform
  46         joint_pub.publish(joint_state);
  47         broadcaster.sendTransform(odom_trans);
  48 
  49         // Create new robot state
  50         tilt += tinc;
  51         if (tilt<-.5 || tilt>0) tinc *= -1;
  52         height += hinc;
  53         if (height>.2 || height<0) hinc *= -1;
  54         swivel += degree;
  55         angle += degree/4;
  56 
  57         // This will adjust as needed per iteration
  58         loop_rate.sleep();
  59     }
  60 
  61 
  62     return 0;
  63 }

Launch File

This launch file assumes you are using the package name "r2d2" and node name "state_publisher".

   1 <launch>
   2         <param name="robot_description" command="cat $(find r2d2)/model.xml" />
   3         <node name="robot_state_publisher" pkg="robot_state_publisher" type="state_publisher" />
   4         <node name="state_publisher" pkg="r2d2" type="state_publisher" />
   5 </launch>

Viewing the Results

Run rviz, choosing odom as your fixed frame (under Global Options). Then choose "Add Display" and add a Robot Model Display and a TF Display (see http://www.ros.org/wiki/rviz/UserGuide).

Wiki: urdf/Tutorials/Using urdf with robot_state_publisher (last edited 2013-02-11 07:20:36 by tleyden)