Digital vs Analogue servos

[Basil]

Hello, I assume the std servo is an analogue type !!!!!!.  Whats the difference, ( Not to technical). Can you use a 'Digital' servo in an ordinary set up?

Bas

[John Rickett 102]

Digital and analogue servos can be interchanged. For most applications an analogue servo will be all you'll need unless you want to fly zippy models. Digital servos are generally faster and have greater holding power - this can be good and bad, if you stall a digital servo it will draw a very high current trying to reach its commanded position and may well burn out.

 

A programmable servo will be digital, programmable can be useful in some applications if you want servo reversing, end point adjustment, servo slow etc and for whatever reason can't achieve it in the transmitter.

 

For most scale models, gliders and sports models, an analogue servo will perform just fine.  

 

[toto]

Hi Bas,

 

My understanding is that standard servos are analogue. Digital servos can be used in " standard " set ups as you say ... but come into their own if using telemetry etc. ..... my usual qualification applies ..... I'm relatively inexperienc3d myself so wait for the more technically correct answer by others.

 

I believe that they can be a little more accurate, programmable and quicker but not ,aybe as much as to be appreciated by novice fliers.

 

I started off buying digital servos for allmy models but have also used analogue servos ( Hitec ) in my Domino trainer. Some of the older hands at one of my clubs told me I was wasting money on digital servos for the type of model and level of flying that I was doing ....... I'm sure more qualified answers will follow.

 

Cheers

 

Toto

[Nigel R]

 

1 hour ago, Basil said:

Whats the difference

 

To the average sport flyer, next to no detectable difference, I would say.

 

That said. In like for like cases, a digital version of an standard analogue servo will more accurately hold its position when force is applied and (usually) has higher torque - but you pay for this in terms of extra power being consumed by the servo.

 

If you already have some, just use them.

 

Fancy servos that can be individually reversed or have their speed adjusted or whatever, are always digital. And expensive.

 

42 minutes ago, toto said:

but come into their own if using telemetry etc

 

Why are they better if you use telemetry?

Or indeed how do they affect telemetry?

[john stones 1 - Moderator]

I find this often had discussion confusing. A servos a link in the chain, as is the gap between hinge lines, slop free linkages, building light/straight, having a reliable engine that throttles well, it's part of the package. If you stall an anologue servo it's bad news, extra current used is easily managed, they're Red Herrings, old wives tales. Can I tell if I upgrade a link in the chain ? Of course I can, and I very much think most of you could. Do you need them ? Entirely up to you to decide that one. Negatives ? They cost more.

[Nigel R]

Some cost about the same. Savox standards spring to mind.

 

Difference is, max current draw is higher, accuracy under load is better, can you tell? A small bet you couldn't, in a "proper" scientific test, double blind, etc etc.  Humans are pretty good at convincing ourselves we've made excellent choices . "Digital' can be used as a pretty good sales pitch. Much better than "those old analogue things". Etc.

[toto]

Hi Nigel,

 

I was under the impression that digital servos can feedback info to the transmitter via telemetry.  I've maybe picked that up wrong.

 

Toto

[Philip Lewis 3]

Some SBUS servo's can feedback telemetry and being SBUS they are all digital, most digital servo's however aren't SBUS and have no telemetry capability I don't think that even all SBUS servo's have telemetry capability just some that do.

[john stones 1 - Moderator]

Yep, and some digis in smaller sizes are very reasonable prices, science and modelling moved on massively over the years, as we're all witness to. Would I rubbish analogue servos, no, I still use them and digis.

 

[toto]

Thanks for clarifying that Philip.

 

I just read for myself that the digital signal used is between the servo and receiver only ..... not the transmitter ..... sorry for the red herring Bas.

 

Toto

[Frank Skilbeck]

On a 2.4 system or 35mhz PCM system the servo positions are sent to receiver as a digital stream. The receiver decodes this and each channel then outputs a pulse width signal, the servo position being determined by the length of the pulse, typically 900 to 2100 micro seconds. This is updated ever 22 millisecond for analogue servos and can be more frequently for digital servos, ever 10 milliseconds. If you select the higher rate on analogue servos it can cause them to overheat with potential burnout. Note SBUS servos if fed direct from the SBUS port receiver a digital stream and you have to program the channel into the servo.

 

BTW if you use a gyro or flight controller then beware they may update the servos at a faster rate which could cause analogue servos to burn out.

[Basil]

Thank you gentlemen for your feedback. I do not use Digital types, just plain old analogue for the sort of model I build . I raised the subject  mainly to find out the differences for future reference.

I dont see the sense in buying a much more expensive product just for the fun of it. Horses for courses !!!!

Many thanks Bas

[Nigel R]

14 hours ago, toto said:

I was under the impression that digital servos can feedback info to the transmitter via telemetry. 

 

Well... I guess probably the more complete / correct picture is - "it depends."

 

(apologies for slightly 'devil's advocate' question earlier BTW)

 

Standard size digital sport servos like Bas is talking about, I'll wager not, they'll be on a standard RX/servo PWM interface. "Digitals" in this context have no extra smarts over and above analogue servos, only the position feedback/control method is digital.

 

Fancier expensive (and certainly digital) servos can provide health status via a digital bus. Not - AFAIK - widely implemented (yet), probably manufacturer specific (e.g. Futaba SBUS... Futaba servos, Futaba RX, Futaba TX), certainly top drawer product line features (after all, why would any manufacturer put this latest stuff on their basic product lines).

 

What use is that telemetry? Temperature, position, current draw, probably you can determine and set limits and alarms. Proving out the kit install chain during first flights, health warning limits for future flights. Large models? Yes. Very fast stuff? Yes. Operating at public demos? Sure, why not. But. Investment in time setting it up, and you probably need to know how to interpret the data. Maybe the servos know enough to do sensible default warnings, I couldn't say.

 

Sport flyer, club size model? I'm not seeing it yet myself. It's another thing to set up and monitor. Sometimes there is a place for that stuff, sometimes you want quick and simple. Those of us grabbing a 3S foamie from the boot? Probably not...

[Nigel R]

4 minutes ago, Basil said:

Thank you gentlemen for your feedback. I do not use Digital types, just plain old analogue for the sort of model I build . I raised the subject  mainly to find out the differences for future reference.

I dont see the sense in buying a much more expensive product just for the fun of it. Horses for courses !!!!

Many thanks Bas

 

For reference, the "standard" Savox servo is a digital:

 

https://servoshop.co.uk/index.php?pid=SAVSG0351PLUS&area=Servo

 

and not particularly expensive.

 

Futaba standard servo now also digital:

 

https://servoshop.co.uk/index.php?pid=FUT051026711&area=Servo

 

End of day, if you want a standard servo, buy a standard servo, don't worry about the buzzword.

 

Edited March 1 by Nigel R

[GrumpyGnome]

Isn't there some consideration of frame rate on Spekky radios that means you can't mix and match......? I could well be misremembering; and if not, it could well apply to other brands.

Edited March 1 by GrumpyGnome

[John Lee]

You can certainly mix and match analogue & digital servos. But if you do so you cannot take advantage of the digital servos ability to accept a higher frame rate, the whole system must operate to the lowest common denominator otherwise you may burn out the analogue servos.

 

That means in practice for mixed digital/analogue servos the frame rate must be left at the default 22ms for Spektrum (or 17ms for Jeti, those are the 2 brands I am familiar with) rather than a faster rate being selected under the 'Frame rate' options in the menu.

[Nigel R]

Some analogue servos do not like the 11ms frame rate, do not like it one little bit, magic smoke kind of not like.

 

 

 

[DocPrinter]

I have seen frame rates for digital servos goes of 333Hz, which is more like 9ms. Certinally relevant to RC cars like 1/12th carpet racers. Easily hitting 50kph, 13.9m/s, or 0.28m of travel between servo updates on a 50Hz PWM signal. The track lane is not much more than 1.5m wide, and so reliably hitting apexes on hairpins and chicanes warrents faster responce from the servo.

 

I'm yet to fly my plane (NW UK weather and family birthdays conspiring to keep me off the field!) but struggle to see why this would help fly an docile by design plane flown line of sight at more than 100m away. Raise speed, or bring it close and the argument improves.

 

That said if you have any sort of electronic stabaliser, flight controller, or advanced failsafe on board then these could certinally make use of faster response & speed servos.

 

"Digital" servos may also grant many other benefits other than responce & speed. For exxmple; accuracy, repeatability, and potentially reliability. Starting with the latter brushless and hall sensors over brushed dc and potentiometer position sensing strengthens two weak points of traditional analogue servos. Repeatability is one I hadn't given much thought too but it the ExpressLRS world there was a real fuss about temperature issues effecting the Tx and Rx circuitry. Anyone ever tested if the analogue servo goes to the same posions up high on a cold day vs your ~18 to 20C workshop? Sure, if your setup process is consistent, not an issue but changing servos even within make/model could add unnecessary unknowns to your re-maidens.

 

Anyway, as with all things you pay your money and take your choices. A set of brushless, hall sensor, programmable servos would cost more than many to most aerobatic plane kits ready to fly. Technology is slowly creaping down the price chain, but too slowly in my mind. PWM is archaic, and only recently being superceded by canbus (simpler wiring, improved reliability), even so only really at the highest price points. I don't know much about sbus, but generally steer clear of proprietary formats where possible.

Edited 11 hours ago by DocPrinter
Adding 50Hz clarigication to original example.