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'[EE] Electromechanics and RC servos'
2011\11\13@162920 by

Hi Group,

I have this RC airplane hobby which fr I use servos for operating the
control surfaces and the engine speed.

Recently I have decided to buy my first digital servos, which is much
smaller than the old traditional one, however, has the same torque with
faster speed to move the arms. In fact, with an analogue one I can move the
arms slightly out of it's position while it was nearly impossible with the
digital one.

Anyway, I have read that the digital one takes more power and drains down
the battery quicker, so I have started to do some measurements to see if
this true as it did not make any sense (same torque needs same input power,
right?). As a results it turned out that while at 'no load idle' the
digital one requires 2/3rd less power, however, on full power a traditional
Futaba 3003 takes about 600mA (I tried to push the arm out of the desired
position), the digital one take about 1A (so almost twice as much). My best
guess is the excess of power is needed to hold the arms tightly in

- How to calculate required power for electromechanical components like
this? So if they say the torque is 3.5 kg / 1 cm arm length for both
analogue and digital, but the analogue gets out of the way by x degrees on
full stress, can you calculate from this the required power?

- Can we actually explain the excess of power in need with this behavior?
(how tightly holds the arms at full power)

- Do you think I would need to make a data acquisition device to see how
much power it takes or is it enough to measure the charge of the battery
somehow and the time spent in the air to be able to tell it in a less
accurate way and make some calculations as well?

Many thanks,
Tamas

-- int main() { char *a,*s,*q; printf(s="int main() { char *a,*s,*q;
printf(s=%s%s%s, q=%s%s%s%s,s,q,q,a=%s%s%s%s,q,q,q,a,a,q); }",
q="\"",s,q,q,a="\\",q,q,q,a,a,q);
As far as I know, analog servos have only Gain, so the further it is from position, the more current flows.  Digital servos have PID, or at least PI, and will provide max current for pretty much any position difference.  Current vs. torque will depend on the efficiency of the motor.

Kerry

Tamas Rudnai wrote:
{Quote hidden}

>
On 11/13/2011 4:29 PM, Tamas Rudnai wrote:
> Hi Group,
>
> I have this RC airplane hobby which fr I use servos for operating the
> control surfaces and the engine speed.
>
I also have the RC Airplane obsession. Health issues have kept me grounded for a while.

The analog servos only try to achieve position during the pulse cycle. The digitals are running at max rate regardless of the command pulse rate.

If you can drive an analog at the max rate then the power required will likely be the same.

-- John Ferrell W8CCW
"The man who complains about the way the
ball bounces is likely to be the one who dropped it."
On 13 November 2011 23:58, Kerry Wentworth <kwentworthskunkworksnh.com>wrote:

> As far as I know, analog servos have only Gain, so the further it is
> from position, the more current flows.  Digital servos have PID, or at
> least PI, and will provide max current for pretty much any position
> difference.  Current vs. torque will depend on the efficiency of the motor.
>

Do you mean as the small digital I have is faster therefore need more power
for the same torque or the pure type of the motor?

Thanks,
Tamas

{Quote hidden}

>
At 22.29 2011.11.13, you wrote:
>Hi Group,
>
>I have this RC airplane hobby which fr I use servos for operating the
>control surfaces and the engine speed.
>
>Recently I have decided to buy my first digital servos, which is much
>smaller than the old traditional one, however, has the same torque with
>faster speed to move the arms. In fact, with an analogue one I can move the
>arms slightly out of it's position while it was nearly impossible with the
>digital one.
>
>Anyway, I have read that the digital one takes more power and drains down
>the battery quicker, so I have started to do some measurements to see if
>this true as it did not make any sense (same torque needs same input power,
>right?).

Nope:

Power (W) = Torque (Nm) * speed (m/s), as you said this one is faster, so...

{Quote hidden}

You have to use the formula above, and factor in the efficiency of the motor.

>- Can we actually explain the excess of power in need with this behavior?
>(how tightly holds the arms at full power)

The motor stalls, thus the efficiency goes to ~0.

>- Do you think I would need to make a data acquisition device to see how
>much power it takes or is it enough to measure the charge of the battery
>somehow and the time spent in the air to be able to tell it in a less
>accurate way and make some calculations as well?

Depends if you problem is autonomy or peak power absorption.

By the way, which brand/model is this digital servo motor?

>Many thanks,

You're welcome.

With kind regards,
Mario

>Tamas
>
>--
>int main() { char *a,*s,*q; printf(s="int main() { char *a,*s,*q;
>printf(s=%s%s%s, q=%s%s%s%s,s,q,q,a=%s%s%s%s,q,q,q,a,a,q); }",
>q="\"",s,q,q,a="\\",q,q,q,a,a,q); }
>
On 14 November 2011 02:07, John Ferrell <jferrell13triad.rr.com> wrote:

> I also have the RC Airplane obsession. Health issues have kept me
> grounded for a while.
>

Sorry to hear that, get well soon!

> The analog servos only try to achieve position during the pulse cycle.
> The digitals are running at max rate regardless of the command pulse rate..
>
> If you can drive an analog at the max rate then the power required will
> likely be the same.
>

That what I have read in many forums but that does not make sense to me to
be honest: In my mind if the command comes at every 20ms in an analogue
servo then I can push the arm a bit further in the non powered period
simply as I have more time for that, however, then the servo still needs to
push the arm back to the desired position, right? So in my mind it need to
work harder to get the position back there fore still needs the same power
for the same torque -- or at least this is what I did not get :-)

Thanks,
Tamas

>
> --
> John Ferrell W8CCW
> "The man who complains about the way the
> ball bounces is likely to be the one who dropped it."
>
>
>
>
On 14 November 2011 07:43, Electron <electron2k4infinito.it> wrote:

> At 22.29 2011.11.13, you wrote:
> Power (W) = Torque (Nm) * speed (m/s), as you said this one is faster, so..
>

That explains this then! Thanks!

By the way, which brand/model is this digital servo motor?
>

I have measured two analogue: Futaba 3003 and Hitec HS-55 (this latter one
is a micro one with a very weak torque, not so fast either therefore
consumes less power). The digital I have bought is the Ripmax New Power
XLD-09HMB. I have never used digital neither any Ripmax products but it
seems to be ok so far -- did not test it in the air yet.

Thanks
Tamas

{Quote hidden}

> >-
At 08.54 2011.11.14, you wrote:
>On 14 November 2011 07:43, Electron <electron2k4infinito.it> wrote:
>
>> At 22.29 2011.11.13, you wrote:
>> Power (W) = Torque (Nm) * speed (m/s), as you said this one is faster, so..
>>
>
>That explains this then! Thanks!
>
>By the way, which brand/model is this digital servo motor?
>>
>
>I have measured two analogue: Futaba 3003 and Hitec HS-55 (this latter one
>is a micro one with a very weak torque, not so fast either therefore
>consumes less power). The digital I have bought is the Ripmax New Power
>XLD-09HMB. I have never used digital neither any Ripmax products but it
>seems to be ok so far -- did not test it in the air yet.

Thanks, and where can one purchase them at good price? I'm located in Europe,
but couldn't find it at Mouser/Digikey/etc...

Cheers,
Mario

{Quote hidden}

>> >--
At 08.47 2011.11.14, you wrote:
{Quote hidden}

However, you need to remember that the efficiency of the motor is maximum
when it is free to spin at half speed that it would reach without any load.

Any more or less load than this, reduces the efficiency (or so I recall at
least).

The digital motor is always stalled, while the "vibration" of the analog
one, driven with low frequency, allows it to be more efficient, but the
price you pay is that vibration / lack of precision.

{Quote hidden}

>> -
On 14 November 2011 08:14, Electron <electron2k4infinito.it> wrote:

> Thanks, and where can one purchase them at good price? I'm located in
> Europe,
> but couldn't find it at Mouser/Digikey/etc...
>

I am buying these in specialized RC model shops.

I have bought this digital one from here (the price was right and they are
nice people and fast delivery):
http://www.phoenixmp.com/acatalog/Servos.html

But there are many more online:
http://www.servoshop.co.uk/
http://www.sussex-model-centre.co.uk/

Towerhobbies is probably one of the biggest but they are in the US (you can
always delivery from there or even from Hong Kong ;-)
http://www.towerhobbies.com/

eBay of course:

A good shop which is close to my home is:
http://www.sloughrc.com/

You need to check it though if they deliver to your country and what is the
condition for that. But I am pretty sure there are Italian local shops too,
plus Germany is really good at RC airplanes which might be a bit closer to
you?

Tamas

{Quote hidden}

>
At 09.59 2011.11.14, you wrote:
{Quote hidden}

Thanks for the many infos.. yes now that I think of it, in Italy there are
some similar shops, like this one:

http://www.robot-italy.com/index.php?cPath=85&osCsid=c5796497da38a33688425de54e0bacf7

I have an use for a servo, better if digital.. but first of all I have to measure
the needed torque, then I'll see what's the best deal I can find here in Italy, in
the EU or elsewhere, e.g. in the links you provided.

Thanks again. :-)

Mario

{Quote hidden}

>>

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