Wait a minute! We are talking about two things: stepper motor can be
positioned without measuring its position while servo motor uses some
position measurements (in model servos for example a potentional meter that
rotates together with the shaft so that the servo receives a pulse with
modulated signal which converted to voltage and then it will compare with
the one accumulated by the pot meter. In that case the shaft moves between
120 degree while the motor inside makes several tans or several hundreds of
rotations (depending on the gear used for that servo housing). So yes, that
way you can use a brushless motor just as anything else to position. But
with stepper motor you can make a small rotation by its nature, so that 1
step is X degree which is known by your controller therefore do not have to
measure and could be calculated how many steps needed to achive the Y
degree. If you disassemble an old floppy disk you will even see a threaded
shaft that moves the head up and down. That shaft is powered with a stepper
motor and the controller knows how many staps needed to position to cylinder
Z -- the position of the head is not monitored except if data cannot be read
it went back to park position and tried to repositioning again, that caused
that strange noise with the damaged floppies.

Tamas


On 16/08/06, Mike Singer <znatok@gmail.com> wrote:
>
> Herbert Graf wrote:
> > > Servos in HDD are also "brushless", position quite precisely under
> PID.
> >
> > Very true, but I wouldn't consider them "motors" per se, which is what
> > the thread was about.
>
> Yes, you may call them servo drive, that's very close in meaning  to
> motor, at least for my level of English.
>
>
> > A brushless motor isn't very good for positioning since it doesn't have
> > a great deal of "holding power" at stall.
>
> It depends on efficiency of close loop. (Encoders provide positioning
> information back)
>
>
> > I'm sure you could devise some PID system to
> > turn a brushless motor into a pseudo servo,
>
> Google says that brushless motors are often called servo motors. I
> think, you meant that steppers are usually better for positioning than
> brushless (servo) motors.
>
>
> > but frankly a stepper is just so much easier I don't really see the
> point.
>
> I'm sure (but I can't prove it) that the very brushless (servo) motors
> are used in CDs and DVDs to precisely position head, not steppers, for
> example.
>
>
> Best regards,
> MS.
>
>
> PS
> Herb, I remember I was somewhat wrong to you a couple of years ago, my
> apologies.
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