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'[OT] R/C servos control'
2005\07\12@082805 by Luis.Moreira

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Hi guys
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.
Best regards
               Luis


2005\07\12@092036 by Josh Koffman

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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!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

On 7/11/05, Luis Moreira <spam_OUTLuis.MoreiraTakeThisOuTspamjet.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.

2005\07\12@142852 by Robert Rolf

picon face
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.MoreiraKILLspamspam@spam@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.

2005\07\12@211304 by John Ferrell

face picon face
You need a steady stream of appropriate pulses to move and to hold a
position. If you expect rated torque specs, you must meet the pulse rate
specs as well. The nominal operating range is 1ms to 2ms with 1.5ms the
center. All servos are not equal. Repeatable centering is a big issue with
high end RC airplane folks.
John Ferrell
http://DixieNC.US

{Original Message removed}

2005\07\13@025151 by Luis.Moreira

picon face
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.moreiraspamKILLspamjet.uk
tel. 01235464615
JET PSU Department
UKAEA Culham Division
J20/1/55, Culham Science Centre
Abingdon
Oxfordshire
OX14 3DB


{Original Message removed}

2005\07\13@031342 by PicDude

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face
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:
{Quote hidden}

> {Original Message removed}

2005\07\13@033536 by Robert Rolf

picon face
Luis Moreira wrote:

> 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?

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.

www.futaba-rc.com/servos/digitalservos.pdf
"• 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 power
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 respond
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 better
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 removed}

2005\07\13@125802 by David P Harris

picon face
Robert Rolf wrote:

> Luis Moreira wrote:
>
>> 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?
>
>
> 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.

I hadn't realized this is how they worked... very elegant.  So, if I
want to get to another position quickly, I could exaggerate the position
to maximize the error to increase the movement per pulse, and then sent
the correct position to zero in on it.  This would have to take into
account the characteristics of the servo, but could give better
performance.

David


2005\07\13@135410 by John Ferrell

face picon face
Send the correct value to the position and let the electronics handle speed
& overshoot problems.
BTW, the servo chips are not available to the public, just authorized
service centers and the factory.

John Ferrell
http://DixieNC.US

{Original Message removed}

2005\07\13@142028 by David P Harris

picon face
John Ferrell wrote:

> Send the correct value to the position and let the electronics handle
> speed & overshoot problems.
> BTW, the servo chips are not available to the public, just authorized
> service centers and the factory.
>
> John Ferrell
> http://DixieNC.US

Are the electronics sophisticated enough to do 2nd order correction?
David


2005\07\13@143155 by John Ferrell

face picon face
The more you spend, the better they are. The digital versions sometimes
"sing" around a position maintaining it.

You should be aware that they are not rated for continuous duty, they are
intended for glorified & expensive toys. I draw the line at $90 for a servo,
but I may weaken if I think my skills could show a difference!

John Ferrell
http://DixieNC.US

{Original Message removed}

2005\07\13@144059 by olin piclist

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David P Harris wrote:
> I hadn't realized this is how they worked... very elegant.  So, if I
> want to get to another position quickly, I could exaggerate the position
> to maximize the error to increase the movement per pulse, and then sent
> the correct position to zero in on it.  This would have to take into
> account the characteristics of the servo, but could give better
> performance.

Probably worse performance.  The servo control mechanism is presumably
already optimized for the servo characteristics.  Unless it's a poor
implementation, the best thing is to tell it directly where you want the
motor to be, and let it decide how to get it there.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\07\13@154354 by David Minkler

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face


Olin Lathrop wrote:

{Quote hidden}

Without independent feedback of position information, you would almost
certainly get worse performance.  I think the key here is 'could'.  You
could also increase the frame rate or the supply voltage (within limits
in both cases) and achieve improved performance.  Too much of a good
thing isn't a good thing.

Dave


2005\07\13@165024 by Robert Rolf

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David Minkler wrote:
{Quote hidden}

Given that my application required all the performance
we could get from a cheap servo (least latency),
I ended up running it at 300 hz, which was near the upper
bound for the electronics seeing the pulses.

Fortunately the duty cycle (changes) was fairly low so the servo
stayed cool. And we also tried raising the supply voltage,
and it helped, but not enough to warrant the extra
regulator (frame rate made a bigger difference).

Robert


2005\07\13@180945 by David Minkler

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face
Robert,

That was my experience as well.  I'm surprised you were able to operate
as high as 300Hz.  It's amazing how much performance you can get for
around $10US.  I got enough torque to break a bell crank.

Dave


Robert Rolf wrote:

> David Minkler wrote:
>
<snip>

{Quote hidden}

2005\07\14@030325 by Luis.Moreira
picon face
Hi John,
That would explain why I can not find the datashettes for the chips in
the servo I opened up.
Regards
               Luis

Luis Moreira
EraseMEluis.moreiraspam_OUTspamTakeThisOuTjet.uk
tel. 01235464615
JET PSU Department
UKAEA Culham Division
J20/1/55, Culham Science Centre
Abingdon
Oxfordshire
OX14 3DB


{Original Message removed}

2005\07\14@035657 by Luis.Moreira

picon face
Hi Guys
Thanks for all the info, if you have more please keep it coming.
In my application I need to move, not necessarily to a position, but in
a direction, another sensor will tell me when I reached it (switch or
similar), and then I need to stop at that position until I need to move
again. Mechanically I want to use the servo, but I think I need to throw
away the electronics in it and build my one controller, hence the need
for info on the servos. When I looked at ways to control a DC motor one
of the ways to brake it was to turn on both bottom or top transistors on
the H bridge and that should short the motor and brake it. Any views on
that technique welcomed.
Best regards
               Luis




{Original Message removed}

2005\07\14@042813 by David P Harris

picon face
David P Harris wrote:

> I hadn't realized this is how they worked... very elegant.  So, if I
> want to get to another position quickly, I could exaggerate the
> position to maximize the error to increase the movement per pulse, and
> then sent the correct position to zero in on it.  This would have to
> take into account the characteristics of the servo, but could give
> better performance.
> David
>
So, is this the simplest cicuit that would work?:

                                V
                             +--+--+
                             |     |
                             R     R
                             |     |
                             +-Mot-+
                             C     C
          +-----one-shot----B       B----+
          |                  E     E     |
          |                  +--+--+     |
pulses-----+                     G        |
          |                              |
          +------------------------------+
where the duration of the one-shot is dependant on the servo position.


If no pulse, both transistors are off, no movement.
If pulse = on-shot, both transistors on, no movement.
If pulse longer that one-shot, motor moves one way.
If pulse is shorter than oneshot, motor moves the other way.

It is wasteful as if both transistors are on they just sink current
without any work being done.

David

2005\07\14@044508 by Howard Winter

face
flavicon
picon face
Luis,

On Thu, 14 Jul 2005 08:56:54 +0100, Luis Moreira wrote:

>...<
> When I looked at ways to control a DC motor one
> of the ways to brake it was to turn on both bottom or top transistors on
> the H bridge and that should short the motor and brake it. Any views on
> that technique welcomed.

Regenerative braking certainly works - I have a couple of battery-powered drills, and when you release the
trigger they stop as if the motor has seized!  The largest, an 18V DeWalt, is heavy enough and stops so fast
that the torque reaction gives the thing a kick that you can't stop by hand.  You could look at providing a
reverse-voltage to try to stop if more quickly, but I think the effort and complexity isn't worth the tiny
improvement you might get.

The only thing to watch is the rating of the transistors - you need to be generous when it comes to this sort
of thing because they will see some impressive current spikes.

Cheers,


Howard Winter
St.Albans, England


2005\07\14@051011 by Luis.Moreira

picon face
Hi Howard
Thanks for the tip.
I am still looking into it, so I will probably try it at some stage to
see how well it works and then improve it to my needs.
I definitely like the idea of braking the motor this way.
If you have any links for this subject can you let me know, as I would
like to learn a bit more about the theory behind it.
Best regards
               Luis



{Original Message removed}

2005\07\14@052222 by Howard Winter

face
flavicon
picon face
Luis,

On Thu, 14 Jul 2005 10:09:50 +0100, Luis Moreira wrote:

> I definitely like the idea of braking the motor this
way.
> If you have any links for this subject can you let me
know, as I would
> like to learn a bit more about the theory behind it.

Sorry, I haven't!

Cheers,


Howard Winter
St.Albans, England


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