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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › ok...stupid question of the day
09-07-2006 02:51 PM  14 years ago
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rpat

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ok...stupid question of the day
I run them, but I don't understand what makes a digital servo a "digital". I know that most of them are faster, and have more torque....but why the digital part?
Rich
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09-07-2006 02:53 PM  14 years ago
freakyreef

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Because they are not analog servos. Does this help?Walk on water long enough, eventually you will get your feet wet.
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09-07-2006 02:55 PM  14 years ago
z11355

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New England

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the difference is in the electronic chip that is onboard.

the servo electronics operate by comparing the current position of the output
shaft w/ the commanded position and then driving the output shaft so
that the difference is zero.

you can do that via the older analog comparator or the newer digital
comparator.
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09-07-2006 03:01 PM  14 years ago
JamesonMalpezzi

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lll

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--
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09-07-2006 03:10 PM  14 years ago
rpat

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OK I ask stupid questions and I get stupid answers LOL. So then I guess that the digital chip responds faster to the input than the older analogs......I get it.
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09-07-2006 03:12 PM  14 years ago
Raffy

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Digital or analog does not make too much of a difference until you are into advanced flying.
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09-07-2006 03:19 PM  14 years ago
freakyreef

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Digital or analog does not make too much of a difference until you are into advanced flying.
I noticed a big difference in centering when I switched to digitals over the analog. It seemed to hover more stable than before. Of course, maybe I was just looking for a difference.
Walk on water long enough, eventually you will get your feet wet.
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09-07-2006 03:22 PM  14 years ago
Mark C

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OK I ask stupid questions and I get stupid answers LOL. So then I guess that the digital chip responds faster to the input than the older analogs......I get it.
Not quite. It is not the speed of the response. In fact Analog servos can respond just as fast.

What makes the difference is in the response/behavior around the control point. Analog servos by their nature are limited to ther analog design are only able to apply minimal amounts of power when the servo is slightly off its control point. The are able to apply more and more power as they are forced further from their control point.

Digital servos however, use a program that will apply full power if necessary to drive the servo to it's controlled position - even if it is only very slightly off position.

Cool?


Mark C.
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09-07-2006 03:39 PM  14 years ago
srslamalot

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Now I have a question about digital servos (btw, I don't own any) I thought I read someplace that when a digital servo isn't being powered, there isn't any resistance, and they can be easily moved. Is that true, and if yes, why?
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09-07-2006 04:15 PM  14 years ago
BarracudaHockey

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Jacksonville FL

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Digital servos have a high speed amplifier that updates the motor position many more times per second than an analog amp. The good side is they center better and develop max torque must faster in the rotation than analog servos.

The down side is the constant updating uses more power so it drains the battery and of course they drain the wallet faster.

Heres a white paper from Futaba.
http://www.futaba-rc.com/servos/digitalservos.pdf
Andy
AMA 77227
http://www.jaxrc.com
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09-07-2006 04:38 PM  14 years ago
rpat

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barracuda,
That white paper is excellant. every one reading this thread should go and read it.
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09-07-2006 05:22 PM  14 years ago
rpat

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ok while we are on the subject I want to know why you are not to use 6 volts on them. Is it because the processor is set up for the 4.8 volts, and you can burn out the processor?
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09-07-2006 05:32 PM  14 years ago
helo_chris

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goodlettsville, tn

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If anyone missed it here is an interesting little piece from the xtremelink web site. The difference is in how they position, analog servos use a voltage reference, center is say .75, full right would be 1 and full left would be .5. These aren't actually what they are just used to illustrate. For reason previously stated this limits the holding power, ie torque, because of the voltage tolerance for a given position. In digitals, the position is referenced as point based on a bit point which does not vary, updated several hundred times a second. Basically it goes and stays where it is told, exactly where.
An interesting fact -

Digital servos do not "hum" by nature. With our system, they make no noise while idle unless there is a load on the control arm. The reason why other radios systems cause these types of servos to hum is the pulse conversion stability. "Normal" (non-digital) servos do not respond unless there is at least 3us (three microseconds) of change in the servo pulse width. This "slop" of 3us is very common in standard radio systems. Digital servos are so accurate that they respond to as little as a 1us change in the servo pulse width. This means that a standard radio system is causing a digital servo to move around constantly while just sitting still in what is considered to be an idle position. Our radio system has a servo pulse width variation of +/-10ns.
"There is a fine line between cutting edge and bleeding edge.."
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09-07-2006 05:33 PM  14 years ago
helo_chris

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ok while we are on the subject I want to know why you are not to use 6 volts on them. Is it because the processor is set up for the 4.8 volts, and you can burn out the processor?
Yes, the processors used are 5 volt chips, 6 would be ok, but at 7 on a fully charged pack you would burn it up.
"There is a fine line between cutting edge and bleeding edge.."
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09-07-2006 06:08 PM  14 years ago
rpat

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helo,
I guess that what you say is true. the digitals will humm with the slightest load on them, but if they are sitting there by their selves on say a work bench they are quiet. So take the case of our servos in the helis....they sit there and hum because of the force of lets say the collective pressing on the servo arm.
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09-07-2006 06:15 PM  14 years ago
helo_chris

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Not really, they are humming because the signal they are receiving from the radio is wider than the minimum signal necessary for the servo to change position. Basically as far as the servo is concerned the position it is getting is actually several positions, so it hums to center in that spot. Like the page was saying if the signal is narrower than the dead band of the servo, there is no hum."There is a fine line between cutting edge and bleeding edge.."
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09-07-2006 06:27 PM  14 years ago
BarracudaHockey

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Jacksonville FL

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The higher the voltage the narrower the dead band. There's servos that are designed for 6v packs but they have a wider dead band (neutral area) to begin with. With the 4.8 volt rated servos run on higher voltages the dead band becomes very narrow and the servo constantly tries to center, and hence chatters more and wears faster.

6v regulated is ok on 4.8 v servos but most of us couldn't tell the difference in flight between 6 volt packs and regulated pack to 5.1 or 5.3 volts.
Andy
AMA 77227
http://www.jaxrc.com
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09-07-2006 08:45 PM  14 years ago
rpat

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Helo,
ok I buy that , but it would seem that the servo should be humming all of the time then, but they don't.
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09-07-2006 08:49 PM  14 years ago
BarracudaHockey

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Jacksonville FL

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They hum if theres a load on them, the weight of the links themselves is a load. Same with my planes, they buzz just from the weight of the aileron. As the gear sets get worn in a little it seems that they buzz less.Andy
AMA 77227
http://www.jaxrc.com
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09-07-2006 09:14 PM  14 years ago
Helinutnz

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below 42 South

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well I don't have any "figures" like the scientists on here (thats a compliment and not a dig!) but I do know that after running analogue servos next to digital servos for years that there is a difference in accuracy. Put a good analogue servo on a large control surface like say a big elevator and then fit the digital. The elevator returns exactly to centre whereas the analogue always seems to be happy a few degrees out when released to centre. It isn't much but it does show up. Digitals centre better (read good quality ones) and I'll pay for that especially on a heli.
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