Thruster Controller

The team chose to use brushless DC motors due to their small size and high efficientcy, (relatively low loss) Brushless DC, or BLDC have no electrical losses due to the rotor as this is made up of high power permanent magnets. The motor is rotated by applying current to three coils set around the rotor at 120 degree offsets. This type of setup insures that the magnets will be centered on one coil while between the other two. this way there will always be at least one coil that can pull the motor so short bursts of speed are nearly always at maximum and there are virtually no dead spots.

We start by experimenting with BLDC motors and controllers using an evaluation board from Atmel corp, it uses a microprocessor simular to the one we used on the position sensor. After experimenting we will decide what we like , dislike about this approach and suggest improvements with our own design.

Eventually we were sucessful writing the code to commutate the motor, it was driven via interrupts on the comparator inputs from the hall sensors. However being busy moving the motor at speeds of 8000 rpm and higher left communications rather slow, so we considered a second microprocessor, to aid communications, cost increase only $5 or using an available BLDC commutation controller, to offload the actual motion of the motor to logic, and the micro to providing PWM, and comm support. We choses the latter as we found a controller from Allegro micro systems that also had 3A mosfet bridges built in. THe unused PWM channels were diverted to controlling 2 servos, one for planes and onefor rudders.

The thruster design is to have a port and starboard thruster, each with its own plane and rudder. The new controller shown on top, was a low power 3A BLDC controller with RS485 port, motor temperature sensor, and servo controller. in a 1" x 2" package.