Make Life Accessible - Motor Suite Test - blog 8

There is an Atmel doc here http://www.atmel.com/Images/doc32126.pdf

page 15 (figure 4.9) looks like they can use a Hall sensor, but just a single one presumably to compute difference between the estimated position and the actual position.

I'd originally thought Doug's system used the typical commutation method but sounds like FOC is a controlled variant of sinusoidal drive like stepper motors.

Hall sensors don't provide 60 degrees of resolution in a BLDC as far as I understand (or maybe there are variations). They will latch approx every 180 degrees, i.e. at repeatable positions per revolution, so you know as it rotates when it passes a certain physical position - that would give you the accurate position and velocity you mention) just that there are three of them so they all operate approx 60 deg out of phase. I could be wrong - I'm no expert on BLDC nor on how flexible FOC is - e.g. if it can (or needs to) resolve the error more frequently than every revolution, nor on how slow a speed is needed by Doug.

Of course it does rotor position estimation. Accurate position and velocity are essential for FOC algorithm to work. At medium and high speed, a BEMF observer in d/q reference frame is usually used. At low speed, some other techniques like high frequency injection or open-loop start-up are needed to spin the motor until BEMF is sufficiently high for the BEMF observer. Usually 5% of the nominal speed is enough for proper operation in sensorless mode. I didn't check how things are implemented in Kinetis Motor Suite though. Of course things are easy if you have proper sensor (either encoder or resolver), but i'm afraid hall effect sensors won't improve situation much. 60 electrical degrees of resolution they provide is simply not enough for FOC to work. Don't know if low speed operation is needed for Doug's project.

I'm guessing it must still be performing some measurement (if not back-emf then some other characteristic) otherwise there is no closed-loop. It is still likely that that measurement (whatever it is) is impacted at slow speeds/standstill, but it is just a guess. I don't know anything about FOC : (

Or is it treating it like a stepper motor, i.e. not closed loop at all?

All true, but applies to BLDC motor with six-step commutation (sometimes called block commutation). But you are dealing with PMSM motor and Field Oriented Control (FOC) with all three phases driven at the same time with sinusoidal current. From quick scrolling through Kinetis Motor Suite user's guide it seems that KMS does not support six-step trapezoidal control.

Hi Doug,

The hall sensors should always allow you to know which phase in the commutation cycle you're at (i.e. even with a stall), whereas with the sensorless method you won't know at very slow speeds/standstill.

Good luck with your project, and super-interested to hear more about the control algorithms. I used a BLDC chip for driving a DC motor a while back, and used the sensor feedback for speed/position determination. However it was an old-school method of control (using analog integration of pulses to determine speed). Modern methods using microcontrollers/DSPs are far more advanced. The FOC method kulky64 mentions is quite new I think, it may well use things other than back EMF for sensorless control, but it is beyond what little I know about BLDCs. While the hall sensors may not help with that method (I have no idea), if back EMF is being used then at slow speeds there could well be issues.

I tried to write up some short motor notes a while back, here and here - in case any of that is useful or gives ideas.