BLDC BEMF

[vinodkaruvat]

Hello Friends,

I am trying to build my own ESC (Brushless DC controller) and I managed to get the motor running starting from a ramp to a steady PWM duty Cycle and Commutation period and these are the BEMF waveforms that I see. My question is, where is the "zero crossing event" and why is it different ,somehow inverted,from the regular BEMF waveforms displayed on various documents and notes. I have attached the images in this thread.

In the 2 images, how can i look for '0' crossing. Its like high and then falling. It should be the verse and should begin at a much lower voltage valu and rise up to almost peak or so(depending on the duty cycle).

Here-in, it is almost impossible to detect '0' crossing. Also, in my scheme I am PWMing the upper MOSFETS, with the lower ones being used/made high during relevant commutations. Kindly help me out pls.

[muvideo]

hello,
if you want to peek inside a working BLDC ESC firmware check this:
https://github.com/sim-/tgy/blob/master/tgy.asm
or this: https://github.com/bitdump/BLHeli

Reversing the schematic of a cheap model ESC built around an Atmega8
is pretty easy and can be a good starting point.

As for the zero crossing detection, afaik model ECS measure open leg voltage
again a "virtual neutral" generated using 3 resistors star-connected to the 3 phases.