trying to use get joint state with my urdf

asked 2012-09-05 21:59:29 -0600

208 ●20 ●27 ●35

updated 2012-09-06 02:27:52 -0600

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- http://ros.org/wiki/joint_state_publisher but istill don't understand how to work it. somehow these things aren't working for me :(

edit retag flag offensive close merge delete

Comments

Simulation or a real robot?

Lorenz ( 2012-09-05 22:51:47 -0600 )edit

Simulation

Nachum ( 2012-09-06 00:22:04 -0600 )edit

add a comment

2 Answers

Sort by » oldest newest most voted

answered 2012-09-06 01:10:00 -0600

Lorenz

22731 ●129 ●202 ●316 http://ias.in.tum.de/p...

updated 2012-09-06 02:37:53 -0600

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:

  <gazebo>
    <controller:gazebo_ros_controller_manager name="gazebo_ros_controller_manager" plugin="libgazebo_ros_controller_manager.so">
      <alwaysOn>true</alwaysOn>
      <updateRate>1000.0</updateRate>
      <interface:audio name="gazebo_ros_controller_manager_dummy_iface" />
    </controller:gazebo_ros_controller_manager>
  </gazebo>

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.

edit flag offensive delete link

Comments

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 ( 2012-09-10 04:03:12 -0600 )edit

add a comment

answered 2012-09-06 01:36:07 -0600

yangyangcv

741 ●21 ●32 ●42 http://www.cnblogs.com...

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.name.resize(7);
joint_state.position.resize(7);
joint_state.name[0] ="base_to_left_link1";
joint_state.name[1] ="left_link1_to_left_link2";
joint_state.name[2] ="left_link2_to_left_link3";
joint_state.name[3] ="left_link3_to_left_link4";
joint_state.name[4] ="left_link4_to_left_link5";
joint_state.name[5] ="left_link5_to_left_link6";
joint_state.name[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
    joint_pub.publish(joint_state);
    broadcaster.sendTransform(odom_trans);

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

    loop_rate.sleep();
}
return 0;
}

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

edit flag offensive delete link

Comments

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 ( 2012-09-06 01:39:57 -0600 )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 ( 2012-09-06 01:46:06 -0600 )edit

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

yangyangcv ( 2012-09-06 01:48:21 -0600 )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 ( 2012-09-06 01:53:37 -0600 )edit

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

Nachum ( 2012-09-06 02:13:02 -0600 )edit

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

Nachum ( 2012-09-06 02:19:20 -0600 )edit

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

Lorenz ( 2012-09-06 02:20:29 -0600 )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 ( 2012-09-06 02:48:21 -0600 )edit

see more comments

trying to use get joint state with my urdf

Comments

2 Answers

Comments

Comments

Question Tools

Stats

Related questions

trying to use get joint state with my urdf edit

Comments

2 Answers

Comments

Comments

Question Tools

Stats

Related questions

trying to use get joint state with my urdf