Hello,
Just to add my stone to this thread, most microchip/onboard controler manufacturer provide some thick documentation about there chips.
The most used in inexpensive BL ESC is the infamous Atmel Mega8, Acccording Mr del castillo (need quote) they didn't use it because they need more processing power to drive some motors and add more features.
http://www.atmel.com/dyn/resources/p...ts/doc8012.pdf
Quote:
AVR444: Sensorless control of 3-phase
brushless DC motors
Features
• Robust sensorless commutation control.
• External speed reference.
• Overcurrent detection/protection.
• Basic speed controller included.
• Full source code in C.
• Source code can be adapted to a new motor by changing parameters.
• Several I/O pins/peripherals not used for motor control. Can be used for other
purposes, such as status LEDs, communication or user input.
• Works without modification on the pin and source code compatible devices
ATmega48, ATmega88 and ATmega168.
bla bla
The magnitude of the back-EMF is directly proportional to the motor speed. This
makes it extremely difficult to detect zero-crossings at low speed, since the signal to
noise ratio is very small. The sensorless commutation scheme presented in this
application note will thus not work during startup and at very low speeds. A number of
strategies for sensorless startup of brushless DC motors have been proposed over
4 AVR444
8012A-AVR-10/05
the years. These differ in complexity and computational complexity, and there does
not seem to be one solution that fits all. Furthermore, many of these startup methods
are patented.
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Quote:
BEMF vs. Hall Sensors
An important physical difference concerning determination of the rotor position via BEMF or
using Hall sensors is that for the BEMF the change of the magnetic flux in time within the coil
gives the BEMF where for the Hall sensor the magnetic flux is sensed. Because of this, for sensorless
commutation based on the measurement of the BEMF the rotor has to move before one
can determine its position. In contrast to that, Hall sensors always give a valid signal representing
a position. The position of the rotor is represented by a three bit vector with a resolution of
60° within the electrical period. For most BLDC motors, the Hall sensors are mounted within the
120° scheme that is direct compatible to the BEMF. Nevertheless, there are BLDC motors with
Hall sensors mounted within a 60° scheme resulting in a different Hall signal pattern. Those 60°
Hall sensor BLDC motors are not taken into account here.
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