mimic_joint_gazebo_tutorial

This package is a simple example on how to properly use the mimic joint plugin in Gazebo, while avoiding the gravity bug in Gazebo7.

Tested on ROS Kinetic, Ubuntu 16.04.

Inspired by: ROS Answers: How to do mimic joints that work in Gazebo?

Installation

This repo has the following source dependency. Simply clone it into your catkin workspace:

git clone https://github.com/roboticsgroup/roboticsgroup_gazebo_plugins.git

Running

To see the plugin in action, run:

roslaunch mimic_joint_gazebo_tutorial gazebo.launch

A simple "gripper" will pop up. The right finger (green) is controlled by ros_control, the left finger (red) is controlled by the mimic joint plugin and simply follows the right finger if possible.

• switch to the rqt_joint_trajectory_controller window
• in the "controller manager ns" dropdown box, select "/controller_manager"
• in the "controller" dropdown box, select "gripper_controller"
• press the red button so it becomes green
• control the gripper with the "right_finger_joint" slider

You should see the left finger follow the right finger.

Reproducing the gravity bug

For fun, you can now pause the simulation, lift the model for about a meter or so and resume the simulation. You should see the model dropping to the ground normally.

Now, edit the file launch/gazebo.launch and comment out the following line:

  <rosparam file="$(find mimic_joint_gazebo_tutorial)/config/gazebo/gazebo_controller.yaml" command="load" />

Kill the launch file if it's still running and start it again:

roslaunch mimic_joint_gazebo_tutorial gazebo.launch

Repeat the dropping experiment. Your model should now drop very slowly with the right finger (the one controlled by ros_control) up, as if dangling from a parachute. Congratulations, you've been hit by the gravity bug.

You will also note that the mimic joint plugin doesn't work any more. If you move the right finger, the left doesn't follow any more. This is because the mimic joint plugin expected us to specify PID parameters, and our change to gazebo.launch caused the PID parameters not to be loaded.

If you want, you can edit the following line in the file urdf/mimic_joint_gazebo_tutorial.urdf.xacro:

  <xacro:mimic_joint_plugin_gazebo name_prefix="left_finger_joint"
    parent_joint="right_finger_joint" mimic_joint="left_finger_joint"
    has_pid="true" multiplier="1.0" max_effort="10.0" />

... and change has_pid="true" to has_pid="false".

Now restart Gazebo and drop the model again. The mimic plugin now works, but is also affected by the gravity bug. Note that you actually have to delete the line at the moment, not set the value of hasPID to false.

In case you're wondering what the point is by now: The whole point was to demonstrate why loading the Gazebo PID parameters is important to work around the gravity bug.

Alternatives

For ros_control, you can use a VelocityJointInterface or EffortJointInterface instead of a PositionJointInterface. In that case, you don't need the Gazebo PID parameters (but you'll need PID parameters for your ros_control velocity or effort controller instead).

For the mimic joint plugin, loading the Gazebo PID gains is the only option AFAIK.