Luis Moreira wrote:

> Hi Rolf,
> What you are saying is that there is no braking as such, but if I force=
d
> the servo to move before the next pulse the controller would try to get
> it back to the original position. Is that correct?

No.
It only makes the correction with repeated pulses.
The controller takes the difference between the internal monostable
controlled by the pot, and the input pulse, and uses
that difference (error) to drive the H bridge using simple logic.

On the other hand,
modern 'digital' servos perform active servoing between
input pulses.

http://www.futaba-rc.com/servos/digitalservos.pdf
"=95 The second, is that a digital servo sends pulses to the motor at a
significantly higher frequency. This means that, as opposed to the motor
receiving 50 pulses/sec., it now receives 300. Although the length of the
pulses is reduced in a direct ratio to the higher frequency because the p=
ower
is being turned on/off to the motor more frequently, the motor has more
incentive to turn. This also means that not only does the servomotor resp=
ond
faster to the commands, but that increases or decreases in power for
acceleration/deceleration are able to be transmitted to the servomotor
far more frequently. This gives a digital servo an improved deadband, a
faster response, quicker and smoother acceleration/deceleration, and bett=
er
resolution and holding power."

If you want a cheap R/C servo to hold it's position you
MUST drive it with a train of pulses. If there is no
position error, the incremental power draw is minimal.

As other posters have noted, if you want full torque
you need a train of pulses since the motor is only
driven by the pulse width DIFFERENCE, PER PULSE.

I don't know if modern 'digital' servos actively correct
for position error between input pulses. Someone will have
to test one sending in a single pulse and see if it holds
position when perturbed.


> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behal=
f
> Of Robert Rolf
> Sent: 12 July 2005 19:29
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] R/C servos control
>=20
> Most R/C servos have relatively small holding torque
> unless they have a steady pulse stream. Most are
> easily back-driven unless they are a high torque
> model where the higher gear ratio increases the holding
> torque. One can drop the refresh rate from 50 hz to
> something lower if power saving is an issue, although they
> really only draw power if the motor has to move.
>=20
> There is no 'braking' action by the controller.
> Only pulses from the H-bridge driver when there is
> a position error. And the controllers will respond
> to micro second wide pulse width changes so use
> a hardware PWM if you don't want a lot of 'chatter'.
>=20
> I doubt that R/C electronics have changed much since I
> designed the control electronics for C-leg prototypes
> a decade ago but YMMV.
>=20
> Robert
>=20
> Josh Koffman wrote:
>=20
>=20
>>This is just a guess, but I'm guessing it's assuming the gear torque
>>will keep it in place. From my understanding most of the servos are
>>fairly simple - a small DC motor with a gearbox and control/feedback
>>circuit. Adding an electrical brake seems like it would up the
>>complexity quite a bit.

It's certainly doable, but the H-bridge logic would get quite
a bit more complex to prevent 'shoot-through' currents (both
transistors on one H leg turned on).

>>Again, just a theory, don't blame me if I'm wrong!
>>
>=20
>=20
> On 7/11/05, Luis Moreira <Luis.Moreira@jet.uk> wrote:
>=20
>  >> On the R/C servos when you stop at a certain position do you rely o=
n
> the
>  >> high torque generated by the gears to keep the motor braked or does
> the
>  >> controller inside the servo actually breaks the motor electrically?
>  >> If it is the later do you guys have any info on it that you could
> share.

Robert

--=20
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