Micro Sawppy Beta 3 Running With Spektrum Radio Control

I had to iron out a few bugs, but I now have a FreeRTOS task running on my ESP32 interpreting radio control receiver signals. It uses the RMT peripheral to count duration of servo control signals sent by my Spektrum SR300 radio control receiver. This allows me to use my Spektrum DX3E transmitter to send commands to an ESP32. From there it was a relatively straightforward translation to the ROS-ish joystick message format I used on this project. Once everything was connected and working, I could drive Micro Sawppy Beta 3 (MSB3) and watch that little rover scoot along.

The potentiometers on my DX3E are far superior than those on the wired joystick I used for testing earlier. Not a surprise given their price difference was two orders of magnitude. The cheap wired joystick was practically a digital on/off direction pad like an old Atari 2600 joystick, making it very difficult to make gradual adjustments. In contrast making tiny changes in a DX3E is trivial, and I could finally explore MSB3 partial steering and partial throttle response.

The good news is that proportional steering worked well. Even though MSB3 is capable of turning about arbitrary radii, including a zero-radius turn in place, I’ve found that confused people on Sawppy V1. So the human-operated joystick drive logic on MSB3 constrained minimum turning radius to pivoting about one of the middle wheels. I have yet to verify its capability to steer inside that radius, a task for the future via autonomy operation. But right now, with the DX3E I found it easy to control turning radius of MSB3 and it appears all my Ackermann steering calculation code and their execution are working correctly. This is great.

The bad news is that proportional throttle control did not work well. I had some preliminary results on TT gear motor speed ranges that made me worried, but that was done individually instead of all six wheels working together like they would be on MSB3. I had hoped that six wheels working together will exhibit better low speed behavior than I saw individually, and sadly I have proven this is not the case. Trying to get a bit more usable torque, I dropped the PWM frequency down to 1Hz. It gave the rover an audible whine and while it improved low end torque slightly, this rover is never going to be a rock crawler. The minimum sustainable speed is far faster than scale, and hampers the ability for this chassis to climb obstacles.

But despite that disadvantage, MSB3 still does quite well running across the uneven surface of my back yard. Including its ability to drop down to tile pavers and then climbing back up onto grass. This freshly-edged transition presented a cliff almost the diameter of a TT gear motor wheel. In the department of rough terrain capability, MSB3 still has quite a big of advantage over other robots driven by TT gearmotors, thanks to the rocker-bogie suspension geometry inherited from the bigger Mars rovers.

Driving MSB3 using my Spektrum radio was fun, but this is not the main goal of the project. The goal is to use ESP32’s wireless communication capabilities directly, without support of external radio equipment.

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