On Sep 30, 2004, at 9:28 PM, Anand Dhuru wrote:

> I  found this fascinating thread that describes modifying a CDROM=20
> motor for
> uase in RC aircraft.
>
> http://www.rcgroups.com/forums/showthread.php?s=3D&threadid=3D127606
>
> Although I am not into RC, the idea of being able to use a compact=20
> CDROM
> motor seems very interesting.

Yeah, me too.  An acquaintance of mine was soliciting broken CD drives
at a party for this purpose, and brought a plane with such a motor
to show off.  Pretty impressive.  As described, you're adding complexity
and power to get increased torque...

> How complex would [a three phase motor controller] have to be if no=20
> direction, or speed control is required? In short, how simple can a
> PIC based DC brushless controller be?
>
I don't know if you can get away with no speed control.  I'd think that
you would need to "ramp up" the speed in sync with the motor rotation to
get to reasonable RPMs.  Otherwise it might sit there and vibrate.

A lot of the cost and complexity of commercial ESCs in providing very
high power; they're designed to provide 10 to 25 amps to each phase.
I'd think that if you didn't need so much power, you could get away
with simpler designs (but then, you wouldn't need to rewind your
stator and replace the magnets, either, and it wouldn't be as much
FUN.)  An 8-pin PIC and three half-Hbridges (lm293D or something else
at even lower power) ought to do it.

This sort of motor control is bread and butter for the low-end DSP
market.  You should be able to find LOTS of app notes on motor
controllers.  Looks like microchip even has a special "Picdem MC"
board for evaluation PICs for motor control purposes.

  AN857 =96 =93Brushless DC Motor Control Made  Easy=94 (DS00857)
  AN885 =96 =93Brushless DC Motor Fundamentals=94  (DS00885)
  AN899 =96 =93Brushless DC Motor Control Using  PIC18FXX31=94 (DS00899)
  AN901 =96 =93Using the dsPIC30F for Sensorless  BLDC Control=94 (DS0090=
1)

Motor control tips n tricks:
   http://ww1.microchip.com/downloads/en/DeviceDoc/41233A.pdf

I should probably mention that down inside the CD drive to start with,
there is a single chip that does a lot of this, with some unknown
amount of involvement from the drive's CPU...  Disk drives, too.

BillW


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