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'More servo Questions!'
1997\02\02@212625 by Steve Hardy

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I know how to drive a servo using PWM.  My question is how does
the servo work internally?  Is it a DC motor?  I presume there
is some sort of active driver inside to perform the feedback
function and interpret the PWM.  Does anyone know the history
of development of these devices e.g. why did they settle on this
1-2ms pulse thingy?  Just curious...

> From: Clyde Smith-Stubbs <spam_OUTclydeTakeThisOuTspamHTSOFT.COM>
> Thus spake Jean-Francois Joly (.....jolyalKILLspamspam@spam@CAPITALNET.COM):
>
> > But the PIC can supply enought power to send pulses to the pulse pin?
> > And then I can connect the power and ground pin to a battery or something?
>
> Yes, the servo can be run off the same 5V supply you use for the PIC. It needs
> 4.5V-6V as a rule.

Would one have problems with commutator noise etc?

{Quote hidden}

Perhaps some units 'gear the pot down' as you intimate so one can
achieve several output rotations for one pot rotation.  This would be
useful for screw drives (ever sat in a 747 and watched how the flaps
are extended?  Reminds me of a lathe leadscrew, sounds like a windscreen
wiper motor is driving it!).

Regards,
SJH
Canberra, Australia

1997\02\03@011348 by Clyde Smith-Stubbs

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Thus spake Steve Hardy (hardyspamKILLspamSWENG.STORTEK.COM):

> I know how to drive a servo using PWM.  My question is how does
> the servo work internally?  Is it a DC motor?  I presume there
> is some sort of active driver inside to perform the feedback
> function and interpret the PWM.  Does anyone know the history
> of development of these devices e.g. why did they settle on this
> 1-2ms pulse thingy?  Just curious...

They have a dc motor, which is geared down by a series of usually
about 3 gear sets in series, so there is a fairly high gear
ratio. The control is done by a chip (there are several, all
similar, that are used). Basically it works a little like a PLL -
there is a monostable controlled by the feedback pot; the incoming
pulse triggers the monostable, and if the monostable times out before
the end of the control pulse, the motor driver is switched on in one
direction, and vice versa. So the stable position is where the
duration of the monostable equals the duration of the control pulse.

Consequently the repetition rate of the control pulse is not
especially critical, and if you remove the signal, the monostable
never fires, and the motor is never driven. This is why a servo with
power applied but no signal is "soft".

--
Clyde Smith-Stubbs    | HI-TECH Software,       | Voice: +61 7 3354 2411
.....clydeKILLspamspam.....htsoft.com      | P.O. Box 103, Alderley, | Fax:   +61 7 3354 2422
http://www.htsoft.com | QLD, 4051, AUSTRALIA.   |
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