Pot-less servos

[Dean Pappas]

Mike,
A stepper motor is a kissing cousin to the present generation brushless motors. Several sets of windings are popwered in rotating sequence, kinda like a 3-phase AC motor. If the sequence is reversed, it rotates the other way. Each run through the sequence corresponds to an angle. 100-position per rev steppers are common, but we would likely use a coarser one and gears. The motor can hold position one of two ways: steppers "cog" by design, so when powered down, there is a certain amount of breakout torque required to turn the output shaft. If you power the winding(s) that correspond to the present position, then you get great holding torque, but at currents similar to a DC motor at stall. If a critical torque figure, that is related to the desired motor speed, is never exceeded, then control is open loop: you count steps, both left and right forever. You need a home position (usually optical like John said) to make the position information absolute, rather than relative. For the kind of reliability we need, one home position, near neutral will not do! You end up needing a position feedback element, anyway.
Dean

-----Original Message-----
From: Michael Ramsey [mailto:michaelr at flying-models.com]
Sent: Thursday, March 27, 2003 9:32 AM
To: discussion at nsrca.org
Subject: Re: Pot-less servos


It's just a motor right? But a step motor moves in increments of measure. Is
that technology "pot-less". I've only heard about them, but have wondered
how they might work for R/C use.

Michael...


----- Original Message -----
From: "Dean Pappas" <d.pappas at kodeos.com>
To: <discussion at nsrca.org>
Sent: Wednesday, March 26, 2003 5:05 PM
Subject: RE: Pot-less servos


What about stepper motors?

-----Original Message-----
From: Michael Ramsey [mailto:michaelr at flying-models.com]
Sent: Wednesday, March 26, 2003 5:03 PM
To: discussion at nsrca.org
Subject: Re: Pot-less servos


Does anyone know anything about "step" motors?

Michael...


----- Original Message -----
From: "Bill Glaze" <billglaze at triad.rr.com>
To: <discussion at nsrca.org>
Sent: Wednesday, March 26, 2003 5:40 PM
Subject: Re: Pot-less servos


> Remember the Bonner Mailbox?  Didn't have a potentiomenter; didn't need
one!
> Bill Glaze
>
> Dean Pappas wrote:
>
> > Hi John,
> > Pot-less servos are our history! The old KPS-9 had a variable cap, and
F&M sold a servo back in the same time period with a variable L-R
multivibrator, called the Magnevac. But you remember that, don't you!
> > Back as a fresh-out I wedged an optical unit into the variable cap
housing of a KPS-9.
> > I have no idea where it is, but I don't think I tossed it.
> > Dean
> >
> > -----Original Message-----
> > From: John Ferrell [mailto:johnferrell at earthlink.net]
> > Sent: Tuesday, March 25, 2003 6:19 PM
> > To: discussion at nsrca.org
> > Subject: Re: New Futaba wc?
> >
> > Pot-less servos may be nearer than you think. Clarostat has rotary hall
> > effect devices available now.
> >
> > John Ferrell
> > 6241 Phillippi Rd
> > Julian NC 27283
> > Phone: (336)685-9606
> > Dixie Competition Products
> > NSRCA 479 AMA 4190  W8CCW
> > "My Competition is Not My Enemy"
> >
> > ----- Original Message -----
> > From: "Koenig, Tom" <Tom.Koenig at actewagl.com.au>
> > To: <discussion at nsrca.org>
> > Sent: Tuesday, March 25, 2003 5:52 PM
> > Subject: RE: New Futaba wc?
> >
> > >
> > > Thanks Dean...now I'll be disappointed for a while. Maybe they will do
it
> > > when they introduce their 'potless' servo's.<G> I too have been
waiting
> > for
> > > these! I guess I'll keep dreaming.
> > >
> > > -----Original Message-----
> > > From: Dean Pappas [mailto:d.pappas at kodeos.com]
> > >
> >
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