Re: [unclassified] Re: [avr-chat] BLDC control with ATmega128?

[David Brown]

>
> On Feb 3, 2006, at 3:20 AM, David Brown wrote:
>
> > Permanent Magnet Synchronous Motor.  The connections and windings are
> > similar to that of a BLDC, but optomised for running with three sin
> > waves at
> > 120 degree phase differences rather than BLDC trapezoid patterns.
> > Some
> > motors are happy with either type of driving patterns.  BLDC
> > patterns are
> > much easier to generate, while PMSM patterns give finer control (for
> > example, you can lock the motor at a precise angle) and minimal
> > current
> > ripple, but you need a much more powerful controller, and accurate
> > angle
> > feedback (not just a hall-effect controller).
>
> Gotcha.
>
> So, I know you can drive a BLDC with a sinusoidal wave form, as well,
> but I don't know if you need accurate angle sensing to do it.
>

If you are driving out sin waves, you are running it as a PMSM motor, while
if you are driving out trapeziums, you are running it as a BLDC motor.  Of
course, the terminology is not entirely consistent (that would be too
easy!), and some literature refers to PMSM motors as a sub-class of BLDC.
Other literature again refers to them as types of AC motor, since you are
driving alternating polarities into the wires.  The construction of PMSM and
BLDC motors is very similar, there are only subtle differences in the
windings making them more optimal for one type of control or the other, and
it's possible to use either type.  To control them as BLDC, you step round
60 degress at a time, and thus hall-effect sensors are accurate enough.  To
control them as PMSM, you are making many tiny steps, and thus need as
accurate angle feedback as possible.  An encoder is normally used, but it is
possible to use hall-effects and interpolate the position if you are running
with constant speed.  It is also possible to use sensorless control, but
that's harder.

> Even in the PMSM, do you really need 6 PWMs? Aren't three enough,
> because you only need to switch one of the two MOSFETs in each half
> bridge, the other can just be "on"?
>

No, it's not that simple.  With BLDC patterns, at any given time only three
FETs are active - either two on the top and one on the bottom, or vice
versa, and you never have a pair with both the top and bottom side doing
work at the same time.  With PMSM patterns, every FET is active within each
20 KHz (for example) PWM cycle.  The idle state, driving no power and
ignoring dead time (BLDC patterns can ignore dead time, which is nice), is
with a 50% signal on all FETs (with the top and bottom sides out of phase,
obviously).  You could get away with 3 PWMs if you have the necessary
external logic and FET drivers with built-in dead time generation, I
suppose.

Freescale has some useful information and articles that could be helpful:

http://www.freescale.com/webapp/sps/site/homepage.jsp?nodeId=02nQXG&tid=FSH

mvh.,

David


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