Original Goals For Sawppy ROS Melodic Project

Since a member of the Sawppy builder community has stepped up to deliver a ROS Melodic software stack, I’ve decided to abandon my own effort because it would mean duplicating a lot of effort for no good reason. I will write down some thoughts about the project before I leave it behind. It’s not exactly a decent burial, but it’ll be something to review if I ever want to revisit the topic.

Move to ROS Melodic

My previous ROS adventures were building the Phoebe Turtlebot project, which was based on ROS Kinetic. I wanted to move up to the latest long term service release, ROS Melodic, something Rhys has done as well in the Curio project.

Move to Python 3

I had also wanted to move all of my Python code to Python 3. ROS Kinetic was very much tied to Python 2, which reached end-of-life at the beginning of 2020. It was not possible to move the entire ROS community to Python 3 overnight, but a lot of work for this transition was done for ROS Melodic. Python 2 is still the official release for Melodic, but they encourage all Python modules to be tested against Python 3 and supposedly all of the core infrastructure has been made to be compatible with Python 3. Looking over the Curio project, I saw nothing offhand indicating a dependency on either Python version, so I’m cautious optimistic it is Python 3 compatible.

Conform to ROS Project Structure

I originally thought I could create a Sawppy ROS subdirectory under Sawppy’s main Github repository, but decided to create a new repository for two reasons:

  1. ROS build system Catkin imposes its own directory structure, and
  2. Existing name “Sawppy_Rover” does not conform to ROS package naming recommendations. Name must be all lowercase to avoid ambiguity between case-sensitive and case-insensitive file systems. https://www.ros.org/reps/rep-0144.html

Rhy’s Curio project solves all of these concerns.

Conform to ROS Conventions

Another motivation for a rewrite of my Sawppy code was to change things to fit ROS conventions for axis orientation and units:

  • Sawppy had been using +Y as forward, ROS uses +X as forward.
  • Sawppy had been using turn angle of positive degrees as clockwise, ROS uses right hand rule along +Z axis meaning counter-clockwise.
  • Math functions prefer to work in radians, but older code had been written in terms of degrees. Going with ROS convention of radians would skip a lot of unnecessary conversion math.
  • One potential source of confusion: “angular velocity” flips direction from “turn direction” when velocity is negative, old Sawppy code didn’t do that.

Rhy’s Curio project appears to adhere to ROS conventions.

All of that looks great! Up next on this set of notes, my original intent to practice better Python coding style with my project.

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