Arm Urdf

Francesco Ganci - 4143910 - 2021/2022

Compatible With:

• ROS1 (Noetic)

Starting from this project:

• CarmineD8/robot_urdf on GitHub

What is this?

This ROS package contains a xacro description for a robot made up of

• a mobile base with two wheels
• and a manipulator on the chassis of the robot

THe robot also supports some other functionalities such as laser scan and others.

Structure of the repository

This repository contains a ROS1 package arm_urdf ready to be copied inside a workspace and then used. Here are the main files inside the folder:

.
├── arm_urdf
│   ├── config
│   │   └── motors_config.yaml	<> motors configuration
│   │
│   ├── launch
│   │   ├── run.launch	<> launch the entire model
│   │	│
│   │   ├── gazebo.launch	<> launch Gazebo only with the model
│   │   ├── rviz.launch	<> launch RViz only
│   │	│
│   │   └── cubes_example.launch	<> a little example about URDF
│   │
│   └── urdf
│       ├── test_model.sh	<> helpful for syntax checking, with visual schema
│    	│
│       ├── main.xacro	<> this XACRO contains the entire model, chassis + manipulator
│       ├── macros.xacro	<> main macros
│       ├── params.xacro	<> main parameters of the model
│    	│
│       ├── arm.xacro	<> the XACRO of the manipulator only
│       ├── chassis.xacro	<> the XACRO of the chassis only
│    	│
│       ├── gazebo.gazebo	<> plugins and materials for Gazebo
│       ├── materials.xacro	<> RViz materials data
│       ├── transmission.xacro	<> data about the motorization of the robot
│    	│
│       └── cubes.xacro	<> a very simple model, for understading
└──

How to inspect and test the model

Inspecting the model -- slow way

Here are the steps to check the structure and the syntax of the model:

1. go inside the folder urdf in the package, and open up a shell from there

2. build the URDF file from XACRO; you can use these commands here:

   roscore &
   
   rosrun xacro xacro main.xacro -o model.urdf
   

3. for checking the structure of the robot on the console, run this:

   check_urdf model.urdf
   

   a text screen showing the structure of the robot should appear. Otherwise, the program will complain with some syntax error.

4. (optional) build a graph from the model by the command

   urdf_to_graphiz model.urdf
   # firefox ./robot.pdf

   the command urdf_to_graphviz makes two files when launched: robot.pdf and robot.gv.

Inspecting the model -- fast way

In order to make a quick syntax check, I wrote a small script which does every abovementioned step. This script is located into the package folder urdf alongside with the xacro model. The script checks the syntax of the model and tries to build a graphical representation of the robot structure; if everything works fine, Firefox will show up the generated graph from the urdf/xacro model.

Here are the steps for executing the script:

1. from inside the package folder urdf, run a new shell

2. then, roscore &

3. and finally, ./test_model.sh

   first remember to check if the script is set executable. If not, use this: chmod +x ./test_model.sh.

Launch files

The main launch file is run.launch which runs both Gazebo (paused) and RViz.

If you just want to see the model in Gazebo, run the model with the launch file gazebo.launch. Or, in case you want to run RViz only, the launch file is rviz.launch.

TroubleShooting

Here are some well known errors and troubles with this model

WARN -- root lik with Inertia

This (neglectable) warning is caused by assigning an tag to the so called root link i.e. the very first link defined in the URDF model. When trying to run Gazebo, an error message could appear such like this one:

[ WARN] [1653916723.536752700]: The root link base_link has an inertia specified in the URDF, but KDL does not support a root link with an inertia.  As a workaround, you can add an extra dummy link to your URDF.

For further informations, see this interesting post from the official ROS1 community.

The error should be alreaty solved: see the main.xacro which includes a dummy link before defining every other component of the robot.

WARN -- ignoring data with redundant timestamp

Another warning that sometimes appears in the console, especially while executing the entire model with run.launch is the following:

[ WARN] [1654443200.004133200, 6.204000000]: TF_REPEATED_DATA ignoring data with redundant timestamp for frame link_left_wheel at time 6.200000 according to authority unknown_publisher

There are a lot of discussions about it, but no real solution. However, this error can be ignored. It is caused bu the Gazebo plugin differential_drive_controller.