The background for this post is that I am currently visiting ISIR, and I’ve started a new project working with the panda robot by Franka Emika. Unfortunately, we only have a single physical robot, and we are 2 PhDs using it for different experiments. To minimize downtime, I am, hence, setting up a simulator for it, and, to facilitate reusability, I decided to go with a Docker based setup.
The image is available on GitHub and called panda_sim. You can clone it, build it, and see how it works for you. While I tried to keep ROS out of this, the current model still loads the gazebo ros plugin for control. If I get around to it, I will remove this dependency in a future version, to get it completely ROS independent.
I am currently working on a ROS based image that runs a Gazebo server and a ROS controller for the robot with gripper. This way, you can easily spin up a simulator for your robot behavior. You can even do that in parallel for some epic deep reinforcement learning action.
Preparing the Robot Model
To use Panda in the simulator, we need to convert the existing model into .sdf format. While Gazebo can work with .urdf files, this requires a parallel ROS installation, which we try to avoid. Internally, the .urdf is converted to .sdf anyway, so we might as well supply .sdf and save the dependency.
First, we need to get the model from franka_ros which is located in the `franka_description` folder. It is a .urdf model, so in order to use it in Gazebo, we need to add some additional information such as joint inertia, or that the arm should be attached rigidly to the world frame. Erdal Pekel also has a tutorial how to bring Panda into Gazebo (using ROS). I used his numbers and suggestions to modify the files.
Next, as we are ripping out the model from an existing ROS package, we will also need to update the paths in the .urdf. In particular, I removed `$(find franka_description)/robots/` in both the `panda_arm.urdf.xacro` and the `panda_arm_hand.urdf.xacro`, and changed the `robot_name` to panda. I also changed the `description_pkg` value to `panda_arm_hand` or `panda_arm` in the `panda_arm.xacro`, depending on the model; this name of the package needs to match the name of the model folder (see below). In `hand.xacro` the value for description_pkg is hardcoded, so I introduced the description_pkg variable, and set it appropriately.
Converting .xacro to .sdf
After doing all the necessary modifications, we need to convert the .xacro to a .urdf. Docker can again be incredibly helpful, as we can spin up a throw away ROS instance, do our conversion, and save the result on the host:
docker run -v <path/to/modified>/franka_description:/xacro osrf/ros:kinetic-desktop-full rosrun xacro xacro --inorder /xacro/robots/panda_arm_hand.urdf.xacro > panda_arm_hand.urdf
Next, we want to convert the model to a .sdf. This is again solved in an easy one liner using docker:
docker run -v <path/to/generated/urdf>:/urdf gazebo:latest gz sdf -p /urdf/panda.urdf > model.sdf
Assembling the Model
Finally, all that is left is to assemble the pieces into a full Gazebo model of Panda. For this we create a new folder called `panda` and copy the meshes folder and the model.sdf in there. We then create a `models.config` to describe the model to Pandas as follows
Here is the folder structure:
panda_arm_hand |__meshes | |__collision | | |__files from franka_description | |__visual | |__files from franka_description |__model.config |__model.sdf
Copying this folder into ~/.gazebo/models will make it available to Gazebo. To pack it into a docker image, I wrote a small script in `build.sh` that will construct the above model and then build a docker image with the models already installed. The script will create a tmp folder where it stores the fully constructed models, so you can also run the script and get just the models, if that’s what you need.
Thank you for reading, and happy coding! If you liked this article, and would like to hear more ROS, Gazebo, or Panda related stuff, consider giving this post a like, or leaving me a comment 🙂