CD/DVD Motor on BLHeli_S Controller Under Oscilloscope

I dug up a brushless motor salvaged from a laptop optical drive and wired it up so I could connect it to a cheap brushless motor controller running BLHeli_S firmware. This firmware supports multiple control protocols, but I’ll be sending classic RC servo control pulses generated by an ESP32 I had programmed to be a simple servo tester.

I saw what looked like a WS2812-style RGB LED on the motor controller circuit board and was disappointed when I didn’t see it light up at all. I had expected to see different colors indicating operating status. Instead, the firmware communicates status by pulsing the motor coils to create a buzzing tone. I found an explanation of these beeps on this Drones StackExchange thread titled “When I power up my flight controller and ESC’s, I hear a series of beeps. What do all of the beeps mean?” The answer says the normal sequence is:

  • Powered-Up: A set of three short tones, each higher pitch than the previous.
  • Throttle signal detected (arming sequence start): One longer low pitch tone.
  • Zero throttle detected (arming sequence end): One longer high pitch tone.

(The author of that answer claimed this information came from BLHeli_S manual, but the current edition I read had no such information I could find. Was it removed? If so, why?)

Once the “arming sequence end” tone ends, I could increase my throttle PWM setting to spin up the little motor. It spins up quite enthusiastically! It responded exactly as expected, speeding up and slowing down in response to change in motor control PWM signal.

Oscilloscope Time

Once I verified the setup worked, I added my oscilloscope probes to the mix. I wanted to see what the motor power waveforms looked like, and how it compares against what I saw from an old Western Digital hard drive.

That might be a bit jumbled, so here are the phases separately. We can see they are in a sequence, one after another. They all peak at battery power level, and there’s a transition period between peaks. The transitions don’t look as smooth as the Western Digital controller and I don’t have a center wye tap to compare voltage levels against.

Next experiment: try the same controller with different motors.

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