Sawppy Rover Battery Voltage Monitor

Trying to debug a mobile web page without any debug support — no syntax error, no line number, not even console.log() — was extremely frustrating and quite draining. After that experience I was ready for a break from rover work. But before I take that break, I wanted to at least attempt to use the voltage-monitoring provision I wired in to this draft of my Sawppy Rover ESP32 control board.

I encountered a few more mysteries of ESP32 ADC while doing so. I initially set the ADC attenuation at 2.5db, which should have given me a range spanning 1.1V which fits nicely with my 10:1 voltage divider. But the raw ADC values were far below where I thought they should be. My voltmeter read 0.6V on the pin after the divider, which should have been a little more than 2048 out of the ADC’s 12-bit resolution. (0-4095) Instead I got values closer to 1350 and I didn’t understand why yet.

Another mystery was working to convert that value to a voltage reading. I had thought the conversion would be straightforward: look at the raw ADC value, measure the actual voltage with my trusty Fluke meter, and divide them to obtain a conversion coefficient. But I found that the ratio between raw ADC value and voltage measured by the meter was not consistent across the voltage range. The coefficient I calculated from a 5V USB input voltage was different from the coefficient calculated from 7.4V of my 2-cell LiPo battery pack. This was a surprise.

To solve this problem correctly, I should consult Espressif ESP32 ADC documentation on their factory ADC calibration values and how to leverage that work into a more precise value. But what I have today is good enough to roughly monitor battery level. I wanted to keep going and get the rest of the basic infrastructure set up before I ran out of motivation to work on this code.

The small bit of ADC conversion code posted a message containing calculated voltage to a FreeRTOS queue, where multiple tasks can make use of that information. I updated the HTTP server code to peek at values on that queue, and send it to the user interface via websocket in the form of a JSON string. The ESP32 HTTP server sent real data, the Node.js stub server only sent a placeholder value. Sawppy’s browser-side JavaScript was then modified to listen to that message, parse that JSON string, and print that data on the status bar.

With these changes, I could monitor battery voltage level from my touchscreen control. This is enough for the moment. I have a few tasks for the future, using this voltage reading at a few other places. Starting with these two:

  1. If the voltage drops below a certain level, the rover should stop. Or at least use a “limp mode” that runs slower, in order to avoid an ESP32 brownout.
  2. I want the motor control PWM frequency to dynamically adjust according to the battery voltage, with the goal to maintain consistent output voltage to the motors. For example, a command for full speed should always output 6V to the motors. If the battery is drained down to 6V, the PWM frequency would be 100%. If the battery is full, the PWM frequency would be lower than 100% even at “full speed” in order to avoid burning out the motor.

I’ll add those features in a future Sawppy brain coding sprint. Software development will go on pause while I live with the feature set I’ve got on hand for a while. Besides, sometimes you don’t even need the features I already have.

[Code for this project is publicly available on GitHub]

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