KDL parser: URDF parsing issue

Question

Hey,

This question seems a bit obvious/easy but I am not able to get the KDL parser working with URDF file for making a KDL tree. I am following this tutorial, section 2.1 - KDL parser: From a file. I am not able to pass the correct filename in the function kdl_parser::treeFromFile("filename", tree). I tried with the model urdf name, the relative path to that file, the absolute path; all of them gives the same error while running - Error document empty. Can anyone specify how to pass the URDF file to the function?

ROS: Melodic

Edit:
As mentioned by @pmuthu2s in the comment, I added below line in the launch file.

<param name="robot_description" textfile="$(find exercise2)/model/robots/sim.urdf"/>

Also the code for parsing looks like this:

KDL::Tree lumi_tree;
std::string lumi_robot;
node.param("robot_description", lumi_robot, std::string());
if (!kdl_parser::treeFromFile(lumi_robot, lumi_tree)){
    ROS_ERROR("Failed to construct kdl tree");
    return false;
}

Still getting the same error. Probably I am doing something wrong here. Anyone know how to fix this?

Regards,
Anand

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Originally posted by AnandGeorge on ROS Answers with karma: 56 on 2020-01-30

Post score: 0

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Original comments

Comment by pmuthu2s on 2020-01-31:
Hey, Maybe check this out Error document empty

Comment by AnandGeorge on 2020-01-31:
Added some more details in the question. Still the same error.

Comment by gvdhoorn on 2020-01-31:
Have you checked whether lumi_robot actually contains anything?

And does cat $(rospack find exercise2)/model/robots/sim.urdf show the expected content?

Comment by AnandGeorge on 2020-01-31:
@gvdhoorn - I can see the urdf with that cat command. But the problem here is lumi_robot does not have any value. Thanks for pointing out that. The launch file just launches the robot and starts rviz. I run the executable separately. Is that the reason why it cannot get the URDF from rosparams? I am able to see the URDF with rosparam get /robot_description. I am getting the URDF as std::string lumi_robot; node.param("robot_description", lumi_robot, std::string());

Comment by AnandGeorge on 2020-01-31:
I changed the it to /robot_description as node.param("/robot_description", lumi_robot, std::string());. Now lumi_robot as the content of sim.urdf. Still I am geting Error document empty.

Answer 1

As mentioned in the question, I was trying to get the KDL tree of the robot by loading the URDF using treeFromFile() function with taking the URDF from the params server. I changed it to treeFromString() as the URDF is stored as a string after getting it from the params server. Below is the modified code.

KDL::Tree lumi_tree;
std::string lumi_robot;
node.param("/robot_description", lumi_robot, std::string());
if (!kdl_parser::treeFromString(lumi_robot, lumi_tree)){
    ROS_ERROR("Failed to construct kdl tree");
    return false;
}

Now the KDL tree is formed and is used for further calculations.

Thanks @pmuthu2s and @gvdhoorn for their inputs.

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Originally posted by AnandGeorge with karma: 56 on 2020-01-31

This answer was ACCEPTED on the original site

Post score: 0

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Original comments

Comment by pmuthu2s on 2020-02-03:
No problem