Servos - Analogue vs Digital

[Nigel Heather]

I understand the different technologies and the theoretical pros and cons.  What I’m more interested is at the budget end mini-size servos is there much real-world difference in practice.

 

For example, specifically I’m looking at these two for a 47” fun fly type aircraft :-

 

https://www.4-max.co.uk/servo-emax-ES3004.html

https://www.4-max.co.uk/servo-emax-ES3054.html

 

Both are mini-sized, metal gear, bearing.

 

Bonus question, what is the real-world difference between bearing and plain bush at the budget end of the market?

 

Cheers,

 

Nigel

 

 

 

[Peter Jenkins]

Hi Nigel

 

For a budget end 47" fun fly, I'd opt for the analogue plain bearing solution.  Why?  You aren't going to be flying precision aerobatics with a fun fly so you don't need the exact holding power of a digital servo.  Theoretically, a ball bearing bushed output shaft will not wear as much as a plain bushed shaft and will help in accuracy of servo movement but there are other factors, like the electronics and the accuracy of the gears that will also have a bearing on the servo performance.  Finally, unless you are a very good pilot you will not notice the difference in the servo performance versus your skill in flying.  The single most important issue though is how accurately the servos centre.  I've had poorly centreing digital servos and accurately centring analogue servos.  You have to try them to see how well they centre.  Also, metal geared servos tend to have more slop than nylon gear servos.  If you are flying electric then metal gear servos add little to performance whereas in an IC engine model they are more robust.

 

Hope that helps.

[Nigel Heather]

@Peter JenkinsMany thanks for the wisdom.

 

From that particular supplier, not checked others, they don’t do a servo with plastic gears and a ball bearing output, you would have to go with a plain bushed output. 

[Peter Jenkins]

Plastic gears and plain bushing would be fine for an electric fun fly of 47" span.  However, check how well the servos centre.  Best way to do that is to secure a balsa stick about 4" long to the servo arm and then secure the servo either on the model or on the bench with a piece of paper under the sweep of the balsa pointer.  Select full servo movement one way and allow it to centre - mark the balsa stick position on the paper and then move the servo the other extreme and allow to centre.  Clearly, you want the balsa pointer to return to the same position from either direction.  If the servo doesn't do that then you will suffer from the aircraft never quite being in trim as the your control surface will always end at different centre positions having deflected one way and then the other.  You can reduce this error a bit by using the inner hole on the servo arm and the other hole on the control surface provided you get the control surface movement you want.  Move the position of the clevis on the control surface to adjust the control movement.

[Brian Cooper]

Be careful. 

Regrettably there are too many (cheap) mini servos, digital and analogue, with their quoted performance figures being somewhat optimistic (to say the least). 

 

Fun Fly models have large flying surfaces with large throws, and they are renowned for their tight manoeuvrability.   If the model is going to be powered "properly" and flown to its ragged limits, it will need good servos which are equal to the task. 

 

Bearings on the output shafts are better. 

Plain bearings will soon develop a floppy output arm due to wear.  This will create a "vague" feeling to the controls, and the model will handle like a wet dishcloth. 

 

However, if the model is not going to be flown aggressively, the servos don't need to be overly good.

It's horses for courses.  

Just keep that little echo alive:  Buy cheap, buy twice. 

[Cuban8]

I recall paying around £10 or more for standard Futaba servos in the mid 70s (the ones with the exposed pin connectors and twisted wires) - that's equivalent to over £50 in 2023 prices. Smaller ones (not that much smaller actually) at the time would be a crazy cost in in today's money. Worth paying a bit more for known brands from reputable retailers who can't afford getting a bad name by selling junk.

I bought some Corona brand mini servos from one of the well known model shops (on-line sale) and they've been fine in a small autogyro.

[Geoff S]

One advantage of metal gears is their robustness and I usually pick them for that reason.  I've inadvertently damaged servos with plastic gears by accidentally catching the control surfaces when handling the model.  I think a funfly with large ailerons, elevator and rudder is more vulnerable than 'normal' models so I would choose metal gears for that reason alone.

 

As Cuban8 says, a few years ago any servo other than a 148 size was staggeringly expensive by comparison - well double 🙂   9 gram servos seem to work well in any other than extreme models - particularly electrically powered ones where vibration is much less than glow or petrol.

[David Davis 2]

Slight diversion from the theme of the original post but i have a question for your combined wisdom?

 

Is it advisable to mix digital and analogue servos in the same airframe?

 

If not why not?

[Wookman]

This is no problem.

Just take care not to provide an analogue servo with 9 milisecond update rate from the rx.

[David Davis 2]

20 minutes ago, Wookman said:

This is no problem.

Just take care not to provide an analogue servo with 9 milisecond update rate from the rx.

 

I don't understand this answer.

 

[john stones 1 - Moderator]

16 minutes ago, David Davis 2 said:

 

I don't understand this answer.

 

https://www.bing.com/search?FORM=AFSCWO&PC=AFSC&q=Frame+rate+on+RC+servos

[David Davis 2]

I just want to know whether it is advisable to mix digital and analogue servos in the same airframe. If not why not?

[John Lee]

Wookman has given the answer - it is not a problem.

 

BUT, in doing so you cannot take advantage of one of the benefits of Digital servos. Digital servos can accept and process their position information at about twice the speed of analogue servos. On most decent radios you will be able to set the speed of this information, typically it is something like 'frame rate'.

 

Digital servos can accept 9 or 10 millisecond updates (Frame rate), whereas the maximum that analogue servos can process is about 17-22 milliseconds. If you try and operate analogue servos using the higher frame rate they can burn out.

 

So you mix analogue & digital servos in the same airframe you have to operate the whole system to accommodate the lowest common denominator, the analogue frame rate (which is the default in most, if not all, radio systems). In doing so the digital servos will be less responsive than they can be if you use all digital servos and the higher frame rate.

 

In practice you will have to be an exceptional flyer with an immaculately set up model to detect the difference.

 

 

Edited January 24, 2023 by John Lee

[David Davis 2]

Thanks for the answer John Lee.

[Manish Chandrayan]

David what Tx are you using?

Modern Tx's have the ability to select the frame rate from the Tx so if you use a mix of digital and analogues you can select 20ms frame rate on the Tx and things will work out. 

Edited January 24, 2023 by Manish Chandrayan

[Mike Blandford]

Just for information, if you have UNI firmware running on a FrSky receiver, you may set individual servo outputs to be 9mS or 18mS.

 

Mike

[john stones 1 - Moderator]

I find pretty much every servo thread mystifying, we talk of maximising performance from engines, props, rest of the radio gear, straight correctly balanced airframes, gaps in moving surfaces kept to minimum, and please don't mention your fuel of choice, you'll start a war.

"I've used XY or Z standard servos for 104 years" never done me any harm. 😉 Well of course we have, standard servos came  with the package when we bought our sets, that doesn't negate "are there better choices" and believe it or not, there are, someone who's never tried ain't your best source of advice coz if they've always used standards, how would they know ?

 

Unless you fly precision aerobatics, or are just an average Joe or Jill, you won't notice a difference ? Untrue, of course you'll notice a difference, some servos perform better than others, the feel of your model in flight will be better and you will notice it. You will definitely notice a better centering servo, after an aileron input, it'll go back to neutral and not induce roll, on a rudder input, it'll go back to neutral and you'll not drift offline on your landing approach, The whole flight will  be better because models always IN trim, not rocket science is it.

 

Another one ? on I.C i use metal gear because of vibration, but on throttle a nylon gears O.K, curious is that, vibrations not affecting that servo then, or are MGs designed for a purpose other than combating vibration ?

 

So am I knocking standard servos ? No but I am knocking the idea that anything other than standards is "Overkill" How much for your TX, a grand ? well that's just Overkill. Old wives tales.

[Martin Dance 1]

Reading this thread reminded me of just how servos can affect the 'feel' of a model in flight. Many years ago I flew an HB/ Sprengbrook radio. This had linear servos which were quite slow. I replaced this set with a Skyleader set with Logictrol linear servos, these were substantially quicker. The new set transformed the 'feel' and handling of the model which had previously had the HB radio in it. So I would agree that quicker servos with better centring can transform even a sport model. Mixing digital and analogue servos, bear in mind earlier comments about frame rate. A digital servo running on 22ms, or thereabouts is not going to be able to give of its best, so probably no point in mixing the two types. Metal gears I suspect it is a good idea to use metal geared servos when you start using smaller servos 20g or less simply because the gear train is stronger and unlikely to be damaged in hanger accidents.

[Mike Blandford]

As usual you need to consider the complete system, not just the servos. If the Tx/Rx combination doesn't get data to the servo frequently enough then having a digital servo with data being sent every 9mS, or less, may well not gain a quicker response.

 

As a specific answer to the OP, I've used a lot of EMAX servos and they have all performed well.

 

Mike

[John Lee]

5 hours ago, john stones 1 - Moderator said:

Unless you fly precision aerobatics, or are just an average Joe or Jill, you won't notice a difference ?.....

 

In my comment above where I said 'In practice you will have to be an exceptional flyer with an immaculately set up model to detect the difference' I was referring only to detecting the difference between the two frame rates, not the different types of servo. I stand by that.

 

At the slower frame rate the servos will still be told their position 50 times a second, just not the 100 times a second of the 10ms setting (accepting Mike's comments about the whole system above). The other benefits of digital servos, in particular their ability to hold an accurate position, is not otherwise affected by the slower frame rate.

[john stones 1 - Moderator]

Fine John, my comments a general one about what's often said by numerous, not a dig at you or any other individual.

[Robin Colbourne]

Digital servos tend to consume battery power more quickly, so in lightweight gliders such as 'Le Fish', they recommended analogue servos despite the fact that they have to work hard on the big control surfaces.  That was the case when the model was designed around 2006; whether its still the case now, I couldn't say.
Le Fish - Voltige Tres Pres du Relief (Aerobatics very close to the ground)

 

There is some interesting info for the more technically minded here, comparing digital and analogue servos on an oscilliscope and servo tester from page 69 on this document:  ToolkitRC ST8 Servo Tester Review

[Frank Skilbeck]

One other thing to note is that some gyro/flight stabliser units can have a higher refresh rate than from the transmitter, so if using analogue servos on the gyro controlled surfaces, e.g. ailerons, elevator or rudder, then ensure you have programmed the gyro unit for the slower refresh rate.

[Torsten Spitzner]

On 24/01/2023 at 14:18, Mike Blandford said:

Just for information, if you have UNI firmware running on a FrSky receiver, you may set individual servo outputs to be 9mS or 18mS.

 

Mike

Uni-Rx is a great bit of FW.

Thank you Mike B...

[Don Fry]

One point, of value, I haven't read;

I think, if good centering is wanted, a good digital will beat a good analogue, nothing to do with speed, fast servos from end to end come in both flavours, but a digital will seek and hold that centre quicker. 
I once owned a Junior 60, rumbled about on 128 servos ( huge things, predecessors to the ancient 148 servo. The rudder 128 shaft bearing wore out, and I replaced it with a 20 g fast digital. 
That plane was odd to fly from that moment, tail wagging like a dog after a mouse.