Gazebo reading joints example
We have a gazebo_example_plugins repository folder where we will store the files.
To demonstrate this example, I will use a double pendulum. We will copy the double pendulum model from https://github.com/simbody/simbody/blob/master/examples/Gazebo2Simbody/models/double_pendulum.sdf into a file and name it double_pendulum.world and place it in the world folder. Additionally, we may also refactor a bit to keep the world files and the model files separate. So we will create the double pendulum model in the models folder and have a double pendulum world include the corresponding model.
Add the path where you created the double_pendulum.world to the GAZEBO_RESOURCE_PATH,
export GAZEBO_RESOURCE_PATH=$GAZEBO_RESOURCE_PATH:<path-to-repo-folder>/world
export GAZEBO_MODEL_PATH=$GAZEBO_MODEL_PATH:<path-to-repo-folder>/models
Now, simply running gazebo --verbose double_pendulum.world should add the model into the simulator where the model is simply controlled by gravity.
Now we will create a plugin to list the joint names and link names of the pendulum.
To compile the plugin, from the root folder of the project, run
mkdir build
cd build
cmake ..
make
If the compilation is successful, check if libmodel_info.so is created in the build folder. This is the plugin we will add to the double pendulum model in the double_pendulum.world. Add the following line anywhere between <model name="double_pendulum"> and </model> tags in the .world file, preferably at the end.
<plugin name="model_info" filename="libmodel_info.so"/>Now, we expose the plugin to Gazebo by updating the gazebo plugin path.
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:<path-to-repo-folder>/build
Note that this is not a good practice. A good practice will involve installing the .so files and the world files in an install folder and expose that folder to the users of this library instead of exposing the build folder.
Now, running the Gazebo world,
gazebo --verbose double_pendulum.world
will print the following repeatedly on the terminal,
List of Joints:
- world_to_upper_pin_joint
- upper_to_lower_pin_joint
List of Links:
- upper_link
- lower_link
This is printed repeatedly since the corresponding logic is added in a callback function OnUpdate() within the plugin that is called repeatedly throughout the use of the model in the Gazebo world. However, for typical and correct operations, such data collection of configuration parameters or data that will not be changed, this logic must be implemented in Load() method which is called only once while loading the plugin.
Update: A WorldPlugin was added to the double_pendulum.world. This plugin is called WorldGravity. It switches the direction of gravity in the Gazebo World every 100 counts. A print is shown on the console, while in the simulator, the pendulum acts accordingly to changing gravitational field.