P1 - Simulate Motor + Drive

[erickmu1]

Description:

• Photon is an RC car and thus uses ackermann steering.
• A first iteration will concern getting Photon to move using the differential drive controller.
• The second iteration will search for an ackermann steering Gazebo plugin.

Goals:

• Add diff_drive_controller
• Add ackermann steering Gazebo plugin

Acceptance Criteria:

• Photon moves as expected when controlled using rqt_steering

Notes:

• If no ackermann steering plugin exists, then we will consider implementing one!

[kishya]

https://answers.ros.org/question/252478/how-to-build-a-package-from-source-on-ubuntu-mate-1604-lts-and-ros-kinetic/

http://wiki.ros.org/ackermann_steering_controller

http://wiki.ros.org/steer_bot_hardware_gazebo

http://wiki.ros.org/steer_drive_controller

[erickmu1]

As noted in PR #13, the current integration of Ackermann steering is unstable. What is common is if you tell Photon to go fast or make a turn, then it flips on it's side eventually (in Gazebo), and soon after it clips to the origin, inside the ground.

I believe this is in part due to how ROS does not seem well suited to simulate small robots (which we notice by how Photon shakes and slips in place - without being told to move). This behaviour is (my guess) probably caused by slightly off/wrong inertia values or by numerical instability.

For a complete ackermann steering integration, Photon should be able to be controlled seamlessly using (eg) rqt_robot_steering. Moreover, the virtual objects (steers and virtual rear wheel) should be invisible in RViz and Gazebo and have small (since you can't use zero) inertias as to not affect the physical properties of Photon. I also point out that looking more into ackermann configurations and talking with the Mech team about plausible values will most likely stabilize how Photon behaves (potentially a better CAD model with bevelled wheels rather than just cylinders).

Nonetheless, note that the current version (#13) also publish the odom topic.