bhanley

Trying to learn about and understand the appeal of digital servos. Would appreciate anyone jumping in to add to the following list or to correct the items in it. Would like to summarze what I have read about digital relative to analog servos.

PROS:
- more accurate centering due to a smaller dead band, ie, consistently repeatable centering
- more accurate movement (again: consistently repeatable for a given stick movement)
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CONS:
- higher current draw - especially when ganged if not perfectly sync'd
- higher price for the same power and/or speed
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COMMON ( to digital and analog servos):
- gear trains ( tend to use the same components at a given price level)
- motors (can get coreles. ferite, etc) in each
- programability ???
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Thanks in advance
Bruce

JohnW

My Feedback: (6)

I'd like to amplify a little on your list.

First, a digital servo is not inherently better than an analog servo. However, a well designed and well made digital servo will be better in many respects to the analog counter part.

Pros - It is more than just centering and accurate movement. A small deadband, regardless of servo type, will give more precise movement. Accuracy is really related more to how linear the pot is, but deadband still plays a part in accuracy as well as precision. When a servo is deflected due to a load, it starts applying a opposite force to compensate. The full rated force of the servo may not be seen until many degrees of deflection from the commanded position. Digitals often shine in this area of "accuracy." Digitals often will have very strong static holding power compared to analogs. Also, they often will deliver their full rated torque at much smaller deflections compared to analog servos. I don't know if it is tight dead band, accelerated amplifier ramp ups, fast refresh rate and a combination of the above that gives digitals this edge.

Cons - High current draw is almost a moot point IMO. Sure, if you gang your servos improperly you will get high draws... for that matter if you do most anything improperly you will have some sort of problem. As for "normal" usage draws, most digitals are only modestly higher than a analog counterpart, say 25% or so. On this, I'd say the con is not the high current draw, but the difficulty and or costs in setting up perfectly ganged servos.

Cons - Higher price. I'd say yes and no to that. While they may seem to have a higher price based on torque and speed, torque and speed only reflect a small part of the "quality" of a servo. When you consider the increased precision and refresh rate of a digital compared to a "equivalent" analog counterpart, I'd say the cost is about a wash.

woodscra

My Feedback: (23)

The faster refresh rate consumes more power. When a digital buzzes it is pulling more current than it would are pure rest, this is more power consumed.

Also, a digital servo when ganged with another and deflected, if they are not exactly aligned they will consume much more power than an analog.

From my experience the digitals when deflected and receiving a significant opposite force can easily draw over 1.5 A per servo. The more this is done, the sooner the battery drains.

They are nice and precise and powerful, but do consume current.

rorywquin

Digital servos are better because of technological improvements (components cost about the same) - but why put Formula 1 equipment on your commuter car ?

If you are a TOC pilot or flying big planes where you need the power/holding/centering - go out and buy digital servos - otherwise buy good quality (ie ball bearing with metal gears or good quality plastic gears) coreless or standard servos and you (Mr. sport flyer) will not be able to note the difference.

There is a big jump in price between the crappy bottom end servos and a digital servo. Even if you are like me and buy top quality standard servos you are still only looking at 50% of the cost of a digital.

I always buy good quality 'non-digital' servos (because they do not add a lot to the overall cost of an aircraft) and IMHO, digital servos are not worth what they cost for what they do and the average pilot will be really hard pressed to tell the difference!

As earlier stated battery power also becomes a problem - extra power / holding (un-needed ??) does not come for nothing!!

How many of you out there really need digital servos???

If we don't buy them will the price come down, will they dissappear off the market or is it a way for us to pay more for what is already cheap technology ????!!!!

Synthetic

My Feedback: (18)

Put it this way...Once you use good digital servos there's a pretty good chance you won't like what you were using much....

JohnW

My Feedback: (6)

Woodscra: You are basically correct. The narrow dead band on most digitals increases precision and holding power at the expense of higher current draws. A properly setup ganged array of digitals should only consume about 25% more than a similar analog counterpart... the key phrase here is "properly setup." The problem is that it can be expensive (matchbox, etc) and very time consuming to setup up ganged arrays. I do consider this a down side to digitals, but one I can live with.

As for high current draws when deflected (high flight loads), I'd prefer the servo held the commanded position and draw more current as opposed to deflecting from my commanded position and draw less current, but that's me.

roywquin: I agree with you to a certain extent. Some applications will not benefit much from digital servos. However, I think most sport flyers could tell the difference between a good servo and a bad on all flight surfaces, regardless of the servo type (digital vs analog.) You hinted to this in your post by saying you use good analogs... why not use a good digital?

You say battery drain? But if you don't need the extra holding power as you put it, why not use a smaller digital servo that has all the holding power you need, it willl draw less current than the "too large" digital, and you'll save some bucks?

Price? I disagree with the 50% cost difference. Which two servos were you comparing? I say the cost difference is closer to 0%-10%. Take a JR2721 analog coreless, metal gear dual BB, 106 oz-in torque... a very nice analog servo, street price about $95. Now take a JR 8411 digital, coreless, metal gear bual BB, 155 oz, in - very nice digital. It has almost 50% more torque than the 2721 and probably better accuracy too. Cost? $105 street. For an extra $10, or about 10%, the digital gains you accuracy and almost 50% more torque. I just used these two servos as an example, but if you compare other servos and brands you will see similar results.

Synthetic: I agree.

woodscra

My Feedback: (23)

Good information.

It may be that a good flyer could tell the difference between a high-end analog and digital, but Frank Noll, J. Shulman, and CPLR all use analog servos.

If the benefit of digitals were enormous I am sure they would switch. However, what they have works for them.

For ganged applications I am considering analogs for the larger deadband and ease to setup properly. Elminating all buzz from digitals is extremely difficult, so for 90% of those out there, that extra buzz is current draw which means less flight time per charge.

To each his own.

Synthetic

My Feedback: (18)

Dale Earnhardt used an open faced helmet while everyone else used the full faced helmet. He was the best but the full face helmet is a better helmet. It is not always the best use the best!

moses

Anybody have any thoughs on reliability. I would guess that digital servos are a bit more prone to failure.

I have seen quite a few threads complaining about electrical failures involving several brands of digital servos. Heck, they are way more complex - just seems logical that they would be less reliable than an analog servo. On the flip side, the digital servos are more likely to be used on larger planes and be subject to more severe conditions. Prehaps they are actually more relaible given the job that they are asked to do.

I have a set of digital servos in one of my planes. I only have about a dozen flights on them. So far, they work just fine. Battery life has really not been a problem - but I have not been pushing them that hard.

Moses

Synthetic

My Feedback: (18)

Moses, look at it this way. The guys that have to have the most in reliability will use what breaks down the least right? Such as jets and helicopters. What do most jet pilots and heli pilots use for servos? Digitals....

Dyehard

I can't speak for the jet jockeys, but I am an active heli pilot, and have been for close to thirty years. Digitals (at least Futaba digitals) are gaining in popularity amoung heli flyers, but they are far from the norm yet. You still see more heli flyers using analog servos for the same reason the you see more sport airplane pilots using analogs, price. Most flyers, until they become quite competent at flying helis and can actually feel the difference in precision the digitals provide, can't justify spending well over $100 dollars per servo. Especially since they know that sooner or later, probably sooner, those over $100 dollars servos will will be helping to punch a hole in the ground. The so called experts and the well to do certainly use the best that money can buy, but a great deal of the sport flyers still use good coreless analogs.

AJF--2

My Feedback: (119)

Please allow me to jump in here. I have been using Multiplex digitials for years--long before JR or Futaba produced a digitial. On some of my planes I use digitials--on some I don't. It is foolish to use these on my Balsa 1/4 Pup or Cub/Taylorcraft/etc. Battery drain is not a problem--if it was (and at times has been) by field charger fixes that. (If you can afford a digitial servo, then you probably have a field charger) The main reason I use digis is because of the way they center--PERFECTLY EVERY TIME--EVEN IF THERE IS A BIND-(heaven forbid). Can I tell the difference? Probably not. I am not that good of a pilot. Do I enjoy the flawless performance? Absolutely! I can buy the Super Dependable Royal MPX (programable-in other words no reversers)for about $65--which is about the same price as a good high end coreless--why shouldn't I?

Synthetic

My Feedback: (18)

Allen, You are correct in that I also think more heli pilots use analog vs digital due to the fact of heli beginers and those who choose for one reason or another not to invest in higher dollar servos. However, each heli fun fly that I attend when I look at the helis and their servos most of them have digitals. Also, if cost weren't a determining factor then I would say the by far most would have digitals. I had JR 8231's in a Raptor 50 and either JR8411's or Futaba 9252's in all my other helis. I had purchased through a real good deal to get some high end Futaba analog and took out the 8231 digitals and replaced them with the analogs. I could tell the difference before it left the ground. I flew it around for two tanks and landed and removed them and put back in the digitals. Another case of digitals vs digitals. When JR was having problems with their 8411's I didn't want to take the chance of having one go bad on me and crashing I made a decision to switch to Futaba 9252's. Within five minutes of flying I believe I could tell a difference in smoothness between the two and I am staying at least for now with the Futaba's. This is what my experience with digitals has been.

Dyehard

Sure, if you go to a fun fly, you are likely to see a high percentage of high end equipment. Usually only the more enthusiastic flyers attend fun flys, if you go to enough if them in any given area, you start to see a lot of the same faces. Those with that much enthusiasm tend to spend more on their hobby. I attended the XFC in Dayton with a friend, as soon as we got back he started buying everything needed to try to duplicate Jason Kraus' helicopter. Far more flyers don't attend fun flys than do. I go to several fields within a hundred mile radius to fly, and still don't see most flyers wanting to spend that kind of money. Given that they didn't cost any more, I'm sure that most people would opt for digitals, by the time that most flyers can consistantly do loops, rolls and auto, they can feel the difference that digitals provide. Maybe in time the price will come down, if so they will probably become the norm.

Synthetic

My Feedback: (18)

Allen, I think you are correct.

JohnW

My Feedback: (6)

Woodscra - I agree and I'm not sure the benefit is enormous, but digitals do out perform in most cases the analog counterpart. As many have pointed out, there are other factors to consider. For example, on my 35% I run digitals on my elevators, but I have analogs on the ganged rudder setup. As you pointed out, I'm sure many top notch pilots use analogs, but I doubt many use them exclusively.

I gotta believe that even a bad pilot can tell the difference between a good servo and a bad servo. The range in servo deadband, accuracy, gearlash, etc is enormous from brand to brand, model to model, etc. I have some popular servos I have tested with an accuracy error under 0.1 degrees. Others popular brands/models I have tested had an accuracy error over 3 degrees. This is a factor of 30 times worse accuracy. I gotta believe just about anyone will "feel" that in the air. If nothing else, it will be nearly impossible to trim the plane with the low quality servo. However, I'll would agree that it may take a very good pilot to tell the difference between a good analog and a good digital.

Moses - I'd be willing to bet that every servo has probably caused a problem for someone at some time. The only problem I know of related to digitals that actually has something to do with the digital part of the servo (not gear train, etc.) was from one particular company. While that company is popular, they do not make high end radio equipment, they make mid range equipment. To the best of my knowledge, the major radio players have not had issues with their digitals any different than issues you might expect from an analog servo.

GMPheli

JR has had a major problem with one of their digitals (I forget the number) when used on heli's. It is bad enough for them to recall them. Strange thing is they seem to work fine in planes. Now the mid priced digitals that you are talking about (Hitec I assume) has had most of it's problem on planes when they are ganged together. Ask most heli pilots about Hitec digitals and they have had good experiences with them (no reason to gang). The JR digitals fail when the motor shaft breaks away from the armature of the motor. People seem to think that they fail in helis and not planes because heli's stress them more. Jr is recalling them and replacing the motors with one that has less torque. This has upset some people as now you have a lesser rated servo. FWIW

Alan Angus

JohnW

My Feedback: (6)

Your talking about the JR8411 series, specifically the SA I believe. The problem as you described it is a mechanical issue when the motor shaft breaks away... this has nothing to do with the servo being digital. Giant scale aerobatic planes can easily max out the torque on this servo. I'd also think that a poorly configured ganged setup would cause the shafts to break. Since this does not seem to be happening on planes, I doubt it was direct servo loads that caused the failures, more likely stress due to vibrations from the heli being fed back into the servo.

The Hitec series of digitals actually had a programming error that would cause the servo to go full swing and stay there until you power cycled the servo. I have personally experienced that problem and it could be reproduced on the bench. It had nothing to do with ganging the servos. It wasn't the pot either. It was the software they used to run the servo. In other words, it was directly related to the servo being digital and needing software (Hitec digitals are programmable.)

I don't want to get into a Brand A vs Brand B and both the JR and Hitec issue have been beat to death. Both companies have resolved the above issues. It was just that someone asked about reliability as it pertains to digitals. The point I was making is that there is only one case that I know of where a digital had a common problem because it was a digital servo, and that was the Hitec digitals. Otherwise, the problems with the digitals are really no different or more common than similar problems with analogs.