My decision to follow ROS conventions for my Micro Sawppy rover control software also meant taking on some of the challenges of adapting manual control conventions to an automation-centric world. I remain optimistic that this short term pain will prove worthwhile long term, but it’ll be some time before I know for sure.
I chose an ESP32 as my rover brain, thanks to its feature set being a good match for my ambition to design a low-cost entry-level rover. The cost-cutting decision to move hardware from Raspberry Pi to ESP32 has a big impact on the software environment. From Python running on a general purpose Linux-based operating system to C running on a bare-bones FreeRTOS environment optimized for embedded systems. While I’ve written a C version for Sawppy’s alternate Arduino control scheme, I couldn’t port my old code straight across due to my new ROS-centric focus.
This is far from the first attempt at a ROS-friendly Sawppy rover code base. I tried to do it once myself before aborting that effort due to unneeded duplication. I looked at Rhys Mainwaring’s rover software stack for ROS Melodic to see if I could adapt it to Micro Sawppy’s ESP32. Logic-wise, it follows all the ROS conventions I wanted to follow, but it also used all the ROS software frameworks I won’t have on an ESP32. I also wanted more abstraction between rover chassis math and actuator code. In ROS this is the job of
ros_control which Rhys plans to adopt, but that is part of the ROS infrastructure I won’t have on bare ESP32 FreeRTOS.
So I went back to something I created as part of my aborted effort: a Jupyter notebook for working through my rover math. Jupyter has since evolved into JupyterLab but it didn’t seem to have any problem opening up my old notebook. Thanks to all the markdown documentation I had written alongside my Python code in the Jupyter notebook, I could quickly get back up to speed on my thinking on the project. Reviewing my code months later, I even found and fixed a bug that slipped through earlier! I’m not sure this is the most elegant fix, my Python code is not very “Pythonic” because I still think like a C programmer. It is technically a bug, but today it is a feature because I’m porting it to C that will compile for ESP32. My plan was to build a FreeRTOS task that takes a desired rover chassis movement command similar to a ROS
cmd_vel message, and outputs data similar to a
ros_control message… but then my plan changed.