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trying to use get joint state with my urdf

asked 2012-09-05 21:59:29 -0500

Nachum gravatar image

updated 2012-09-06 02:27:52 -0500

hi i am trying to use the jointstates topic of pr2 for my own urdf file, i am not sure how should i start. how do i get my robot to publish on that topic? do i need to write a code for a publisher of my own? (very new in ros & c++)

thanks everyone

so now i found this link- but istill don't understand how to work it. somehow these things aren't working for me :(

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Simulation or a real robot?

Lorenz gravatar image Lorenz  ( 2012-09-05 22:51:47 -0500 )edit


Nachum gravatar image Nachum  ( 2012-09-06 00:22:04 -0500 )edit

2 Answers

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answered 2012-09-06 01:10:00 -0500

Lorenz gravatar image

updated 2012-09-06 02:37:53 -0500

To get joint states, a node that communicates with your robot's hardware is run on the robot. It reads the position of the encoders in all joints and constructs a sensor_msgs/JointState message that is then published on the /joint_states topic.

If you are running on a real robot, you need to implement such a node. On the PR2 that's the pr2_etherCAT node and you need to implement something similar if you are using a custom robot and want to have joint states.

In simulation, things can be a little easier because you can use the pr2 controller manager that's used on the simulated pr2 for other robots, too The gazebo_ros_controller_manager plugin is responsible for providing the infrastructure for running controllers and for publishing the joint state. You should be able to use it in a custom urdf. I think you will have to add transmission specifications though which seem to be undocumented. To use the controller plugin, add the following to your urdf file:

    <controller:gazebo_ros_controller_manager name="gazebo_ros_controller_manager" plugin="">
      <interface:audio name="gazebo_ros_controller_manager_dummy_iface" />

If you do not want to use that plugin, you need to write your own gazebo plugin that reads the positions of the joints specified in your URDF file, advertises the /joint_states topic and publishes the joint positions as a message of type sensor_msgs/JointState.

The joint_state_publisher node you mentioned in your question essentially publishes a constant value for all non-fixed joint positions, i.e. it does not respect the actual position of joints in simulation. It also provides a gui that lets you to change the joint state values. This node is most useful when you are creating a urdf file to check if all joints are aligned correctly.

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o.k I understood how to run it now. the position and velocitiy look correct but the effort is giving zero when that can't be. did i miss some thing? Thanks

Nachum gravatar image Nachum  ( 2012-09-10 04:03:12 -0500 )edit

answered 2012-09-06 01:36:07 -0500

well, if you want to publish the joint state by yourself, try the following code:

#include <string>
#include <ros/ros.h>
#include <sensor_msgs/JointState.h>
#include <tf/transform_broadcaster.h>

int main(int argc, char** argv) {
ros::init(argc, argv, "myRobot_move_joint");
ros::NodeHandle n;
ros::Publisher joint_pub = n.advertise<sensor_msgs::JointState>("joint_states", 1);
tf::TransformBroadcaster broadcaster;
ros::Rate loop_rate(30);

const double degree = M_PI/180;
double rot4 = 90;

geometry_msgs::TransformStamped odom_trans;
sensor_msgs::JointState joint_state;
odom_trans.header.frame_id = "odom";
odom_trans.child_frame_id = "base_link";;
joint_state.position.resize(7);[0] ="base_to_left_link1";[1] ="left_link1_to_left_link2";[2] ="left_link2_to_left_link3";[3] ="left_link3_to_left_link4";[4] ="left_link4_to_left_link5";[5] ="left_link5_to_left_link6";[6] ="left_link6_to_left_link7";

while (ros::ok()) {
    //update joint_state
    joint_state.header.stamp = ros::Time::now();
    joint_state.position[0] = 0;
    joint_state.position[1] = 0;
    joint_state.position[2] = 0;
    joint_state.position[3] = rot4*degree;
    joint_state.position[4] = 0;
    joint_state.position[5] = 0;
    joint_state.position[6] = 0;

    // update transform
    // (moving in a circle with radius=2)
    odom_trans.header.stamp = ros::Time::now();
    odom_trans.transform.translation.x = 0;
    odom_trans.transform.translation.y = 0;
    odom_trans.transform.translation.z = 0;
    odom_trans.transform.rotation = tf::createQuaternionMsgFromYaw(0);

    //send the joint state and transform

    rot4 += 1;
    if (rot4 > 90) rot4 = 0;

return 0;

this works for my 7DOF robot arm. just change the joint names to your own.

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Isn't this just publishing a constant joint state? It seems to ignore the actual positions of the robot's joint. Also, it seems like ros::spinOnce() is missing so the node should not really behave correctly.

Lorenz gravatar image Lorenz  ( 2012-09-06 01:39:57 -0500 )edit

for me, i have another program getting the robot's actual joint position(running on windows). so by feeding in the corresponding joint positions to the above code, i can publish the joint states to other nodes in ROS. i'm not very clear about the effect of spinonce(), but the above code do work.

yangyangcv gravatar image yangyangcv  ( 2012-09-06 01:46:06 -0500 )edit

and, according to my understanding, we only need the spinonce() when the node subscribe to other node's topic. am i correct?

yangyangcv gravatar image yangyangcv  ( 2012-09-06 01:48:21 -0500 )edit

You are right, it seems like ros::spinOnce is not needed here. I was not sure if node commands such as rosnode info and rosnode kill work without spinOnce. I made a test and apparently they do :)

Lorenz gravatar image Lorenz  ( 2012-09-06 01:53:37 -0500 )edit

thanks. i hope i will be able to run it :)

Nachum gravatar image Nachum  ( 2012-09-06 02:13:02 -0500 )edit

some errors- tried to make it: fatal error: tf/transform_broadcaster.h: No such file or directory compilation terminated.

Nachum gravatar image Nachum  ( 2012-09-06 02:19:20 -0500 )edit

You need to add a dependency on tf in your manifest.xml file.

Lorenz gravatar image Lorenz  ( 2012-09-06 02:20:29 -0500 )edit

@Nachum: seems you have lots to learn about ROS. open a new terminal, cd into a folder which is in your ROS_PACKAGE_PATH, type: roscreate-pkg my_robot_publish_joint_goal roscpp tf sensor_msgs this will create a package which depends on roscpp, tf and sensor_msgs

yangyangcv gravatar image yangyangcv  ( 2012-09-06 02:48:21 -0500 )edit

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Asked: 2012-09-05 21:59:29 -0500

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Last updated: Sep 06 '12