With power and a constant-position signal applied, it's difficult to move the 
servo arm, even on a small little hobby servo.  This is sort of the braking 
action, but without any specific braking device.  But remove power or the 
(PWM) position signal and you can move it around easily.

Cheers,
-Neil.



On Wednesday 13 July 2005 01:51 am, Luis Moreira scribbled:
> Hi Rolf,
> What you are saying is that there is no braking as such, but if I forced
> the servo to move before the next pulse the controller would try to get
> it back to the original position. Is that correct?
> Best regards
> 		Luis
>
> Luis Moreira
> luis.moreira@jet.uk
> tel. 01235464615
> JET PSU Department
> UKAEA Culham Division
> J20/1/55, Culham Science Centre
> Abingdon
> Oxfordshire
> OX14 3DB
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Robert Rolf
> Sent: 12 July 2005 19:29
> To: Microcontroller discussion list - Public.
> Subject: Re: [OT] R/C servos control
>
> 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.
>
> 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'.
>
> I doubt that R/C electronics have changed much since I
> designed the control electronics for C-leg prototypes
> a decade ago but YMMV.
>
> Robert
>
> Josh Koffman wrote:
> > 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.
> >
> > Again, just a theory, don't blame me if I'm wrong!
>
> On 7/11/05, Luis Moreira <Luis.Moreira@jet.uk> wrote:
>  >> On the R/C servos when you stop at a certain position do you rely on
>
> 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.


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