asq

Brushless Motor control : 

Brushless Motor control Advantages: High efficiency and performance Less risk of interference as there are no brushess or gears Numerous sources of motors with different power output and rpms per Volt. However, to use such motors the direct current of the batteries has to be converted to an triphasic alternating current) with power output controled so that the speed of the motors can be accurately controled. The BL-Ctrl board is a sensorless driver for brushless DC current motors.

Slide 2: 

The control of brushless motors is tri-phasic in groups of PWM pulses. The PWM frequency determines the height of the phase-voltage . The phase voltage on the motor (as group of PWM pulses) determines the rotations per minute (RPMs) of the motor. A motor generates a countercurrent when turning (like a generator). the resultant RPMs will be the balance between current, coutercurrent and power output. At any time 2 of the 6 FETs are operational to power the motor. The time of commutation (the time at which the 2 FETs must be switched off and the next two switched on to go to the next phase), is determined by voltage measurement (actually comparison) of the unpowered phase of the three motor wires. Power is measured by the voltage drop over a defined shunt on the PCB.

Mosfet : 

Mosfet The poweroutput stage consists of an N-Channel and a P-Channel MosFet for each of the three phases. The P-Channhel Mosfet requires a transistor to be able to switch the gate potential over a controler output pin

Recognition of the rotor position : 

Recognition of the rotor position This simple part of the circuit allows measuring the position of the rotor in the brushless motor. From the voltages of each of the motorphases a virtual star is calculated (averaging). The controler compares the filtered voltages of the phasses with the average voltage and uses this to calculate the timepoint for cummutation, which is the point in time at which the FET stage is switched to the next phase.

Power Measurement : 

Power Measurement There is a shunt on the PCB, consisting of a wider copper trail which is lengthened a bit, similar to the layout in this drawing. The voltagedrop over the shunt allows the controller to calculate the power used, and to reduce power if amperage is excessive. The software does not suddenly cut power, but gradually decreases it. This ensure that the aircraft does not suddenly drops out of the sky if the ESC is close to its maximum power output

Micro controller : 

Micro controller The Atmel Atmega8 is used as microcontroler. It has some important features required for use in such speed controlers: the analog comparators with multiplexers. In addition it has serial interfaces and and an I2C interface integrated into the chip. The Atmega8 is cheap (<2EUR), is easy to program, requires limited peripherals and is fast enough for our required RPMs.