Digital vs Analog Servos

[Gary Clark 1]

I'm tempted to try some of the Jx digital servos from Bangor because I've heard good things about them but I have never used digital servos before.

Any thoughts on using digital over analog?

Gary

[Jon H]

digi servos have a better holding power and tend to have better accuracy. They often operate faster as well.

Their suitability depends on the model you plan to fly with them. If you have a hot pattern ship, an aerobatic glider (one of those quick ones), a turbine or something high performance like that then sure its a better bet, on a tiger moth or a wot 4..not so much.

I have plenty of 5-10kg digi servos on my warbirds but not deliberately, it just so happens that the spec i use happens to be digital. I have some analogue servos of the same spec and for my use case there is nothing between them in terms of performance.

[Nigel R]

I would rather a good analogue than a cheap digital.

But like Jon says there's nothing in it for me. My recent builds have all sported analogues.

That better holding power comes at the cost of using more mAh.

What model are they for?

Edited By Nigel R on 07/12/2020 20:52:22

[Gary Clark 1]

Thanks guys, seems like digital is unnecessary

They're for my BT P-47 so a medium sized warbird around the 15lb mark.

The other question is the 4 max servos and what they are like?

[Jon H]

A 70 inch P47 will be ok on standard 148 or similar 3kg servos. I flew my pica P40 on the same size on this setup and it was fine. If you want something a little better, 4-5kg digitals or standard servos would be a nice upgrade for flaps and elevator.

A nice all round servo would be the savox 351 https://www.servoshop.co.uk/index.php?pid=SAVSG0351&area=Servo Its not expensive, and a nice little spec bump over a standard servo. I have a few of them dotted around rudders and ailerons on my 80 inch warbirds. The 352 is also very good but a little more money for a little more torque. Both happen to be digital, but its not why i use them, they just happen to offer good performance at a good price. 

As Nigel points out, a cheap servo from a good brand is better than any servo from a cheap brand. Given the nature of the model you are building, i would always go for a savox, futaba or perhaps a hitec servo. I have some 5485's..? i think? anyway the handful of hitecs i have work well but are more of a pain than the savox as they even splined outputs and are less adjustable than the savox which uses futaba arms.

 

I forgot to mention, i run all my warbirds on 5 cell subc 6v nimh batteries through a hd futaba switch. 3000ish mah lasts ages

Edited By Jon - Laser Engines on 07/12/2020 22:23:04

[Tim A]

I generally stick with analog's, although I used to get some digital's from HK, D-MG 16's brought because they are metal geared and were not advertised as digital, great little servo and cheap! Of course NLA.

I try to stick with Analog, Metal geared because most servo's that have failed on me were due to stripped gears 90% of the time caused by knocking control surfaces loading and unloading the car.

Alway's worried that using digital may cause brown out if stalled.

John, Hitec now supply some servos with a 25T output spline, I know because I used them in a Seagull Extra back in February, only problem was arms were to short, acquired some Futaba ones from a friend.

[Nigel R]

Tim i think it is the larger hitec that have 25 spline arms. Standard size are still 24 (?)

Jon 5485 are the standard digital. 485 are equivalent analogue, decent strong standard size units. Both similar torque to the savox and probably what I would buy.

[Allan Bennett]

I use digitals on my flybarless helis because they are able to respond faster to the commands being issued by the gyros. On my flybarred heli I use analogue for the main controls and digital for the tail (because the latter is controlled by a gyro).

On a recent warbird I used digital servos for the first time in a fixed-wing, because of the ability to reverse their direction and slow their speed of operation -- handy for flap servos. Otherwise all my other fixed-wing models are analogue, generally metal-geared.

[Gary Clark 1]

Thanks guys.

Lots of great information there. The Savox one looks like a a 1kg increase in torque for little gain in price (almost a 25% increase on the 148) so those will work well for a bit extra power for the elevator and flaps but will stick with standard analogue in general.

[Gary Clark 1]

Secondary question, anyone have experience of the 4 max servos? They have a standard size, metal geared servo putting out 8kg+ for £11

I really like 4 max stuff so expect these to be good but that seems quite cheap

[Nigel R]

Savox et al are known quantities. YMMV.

It is not all about torque and speed. Myself I would put bearings, single or dual, above metal gears. A high output fast servo running single plain bearing? Not for me.

Centring accuracy, overshoot, jitter, deadband, stall torque and current are not often quoted. Compare what e.g. hitec are prepared to share on their spec pages, they do at least list deadband.

With cheap servos the specs may also be a bit creative...

[Jon H]

Posted by Nigel R on 09/12/2020 07:09:06:

Savox et al are known quantities. YMMV.

It is not all about torque and speed. Myself I would put bearings, single or dual, above metal gears. A high output fast servo running single plain bearing? Not for me.

Centring accuracy, overshoot, jitter, deadband, stall torque and current are not often quoted. Compare what e.g. hitec are prepared to share on their spec pages, they do at least list deadband.

With cheap servos the specs may also be a bit creative...

^ What he said!

I have never used a 4max servo, but given the sorts of models i fly i would not take the risk myself and i stick to savox, hitec and futaba.

As i wouldnt use it myself its a big disingenuous to recommend giving it a go.

I look at it like this. How much will your P47 cost? Not just in terms of money but in the build time, the effort etc. Is it worth risking all that to save 3 quid on an unknown servo?

Edited By Jon - Laser Engines on 09/12/2020 08:57:38

[Piers Bowlan]

Posted by Nigel R on 09/12/2020 07:09:06:

Savox et al are known quantities. YMMV.

It is not all about torque and speed. Myself I would put bearings, single or dual, above metal gears. A high output fast servo running single plain bearing? Not for me.

Centring accuracy, overshoot, jitter, deadband, stall torque and current are not often quoted. Compare what e.g. hitec are prepared to share on their spec pages, they do at least list deadband.

With cheap servos the specs may also be a bit creative...

+1

[Rich Griff]

Gary, would that be Bangor North Wales, and you in the cockpit in a hawk?

[David Davis 2]

I've heard that it's not wise to mix digital and analogue servos in the same model. Is this true?

[Frank Skilbeck]

No problem mixing digital and analogue servos, but if your radio has a fast response setting you can only use that with all digital, analogue servos don't like the faster refresh rate but digitals are OK at the standard refresh rate.

For dual surfaces, e.g. ailerons, flaps, dual elevators then stick with the same type so the surfaces move in unison.

[Nigel R]

DD, some (older) analogues require the 22ms frame rate. Some can deal with 11ms. Safe option is to use the slower 22ms rate. I've not heard of a digital than needs the slow rate.

I think you are a Spektrum user? 22ms is the default setting, it is fixed when you bind the RX.

[Roger Jones 3]

Posted by Gary Clark 1 on 07/12/2020 20:09:04:

I'm tempted to try some of the Jx digital servos from Bangor because I've heard good things about them but I have never used digital servos before.

Any thoughts on using digital over analog?

Gary

None of our servos are truly 'digital' as they all use an analogue potentiometer to determine the output arm position and the input is a constantly variable width (therefore analogue) pulse.

The only time I've noticed much difference in 'precision' is in large high power helis. And anyway I was using Multiplex 'digital' servos that were four times the price of the cheap JR analogue servos I was using previously.

Increased 'holding power' is marketing BS. The servos gears will strip or the servo amplifier will go up in smoke when you force ANY servo, be it analogue or so-called 'digital' out of it's present position.

[Peter Miller]

Ihave used 4 Max mini metal geared digital servos. Very good indeed. I trust 4 Max products for my motors and ESCs.

[Nigel R]

Posted by Roger Jones 3 on 11/01/2021 08:23:04:

None of our servos are truly 'digital' as they all use an analogue potentiometer to determine the output arm position and the input is a constantly variable width (therefore analogue) pulse.

The only time I've noticed much difference in 'precision' is in large high power helis. And anyway I was using Multiplex 'digital' servos that were four times the price of the cheap JR analogue servos I was using previously.

Increased 'holding power' is marketing BS. The servos gears will strip or the servo amplifier will go up in smoke when you force ANY servo, be it analogue or so-called 'digital' out of it's present position.

1) Digital always refers only to the amp. Worth noting there are servos that run from a digital bus, where the input is a digital value. Granted, still use a feedback pot, but I'm not aware of a method of turning a physical position into a digital value without going via an analogue sensor, so I guess that's as much digitalness as we will get in a servo.

2) ok

3) analogues can exert full torque only when displaced from commanded position by a particular amount; below that amount the torque exerted will be less. A digital amp can exert full torque with the smallest detected displacement, if that is how the amp is programmed. So, not strictly marketing BS. At least within the bounds of that small distance. In real world application, a fixed torque will be required and the digital will hold a position closer to that commanded, vs the analogue which will be further from commanded. The end result the digital will be more accurate under high load. This may or may not be of any particular concern or noticable for any given airframe or heli.

[extra slim]

Just my two penneth..

if there are two servo's

1) 10kg 0.12sec at 6v Metal gear, BB- Analogue

2) 10kg 0.12sec at 6v metal gear BB- Digital

Which one is preferable?

In my experience, it is more about quality and performance of the product, which is easy said, and more difficult to ascertain.. I have had "cheap digi's" which have performed brilliantly, and some, far from brilliant, but the same can be said of major brand products. I have had centering issues with well respected manufacturers servos in the past, but equally, I have had great centering... Sadly, it is a suck it and see, and "based on personal experience" is the only way to go, as both cheap and expensive, all have their friday afternoon products.. In the past the "cheapo" did tend to have more friday afternoon items, but nowadays it feels like the gap has closed..

[Capt Kremen]

Agree 'extra slim'.

Consistency and quality control in many, if not most, model items (and other general products), seems to be very hit and miss. You buy a 'WizzFast 800' electric motor, fantastic, buy another a while later as you're so impressed and it's meh! Same for LiPo, ESC, Servo even some ARTFs. Very much a batch issue and if you're unlucky, you're put off whilst your club mate gets a good 'un and of course swears by 'Bloggs Batteries' and so on.

[Geoff S]

As I neither build nor fly models which demand fast, super precise control movements (and my hand/eye co-ordination supports that!) I've always avoided digital servos because of their potential for high instantaneous current draw. So I'd choose the analogue servo every time. More competent 'on the edge' model pilots may choose differently.

Geoff

[Brian Cooper]

In very basic terms, an analogue servo will produce "up to" torque. Whatever its quoted torque figure may be, it tends to "ramp up" its muscles when the Tx stick is moved. If it meets a resistance, it will ramp up a bit higher to overcome it. . All of this takes time, but in the grand scheme of things, most people won't notice the "mushy" time lag because it all happens quite quickly.

A digital servo gives ALL of its torque at every stage of the way. Full power, no delay, as soon as a stick is touched. . This instant, strong response is noticeable and appreciated when flying high performance models in a spirited, high performance way.

A nice feature to have on any servo is bearings on the output shaft. Some have one, some have two and too many have none. . Those with none soon develop "floaty" shafts and this can be an annoyance on some models, but might be less noticeable on lightly loaded, low performance models.

Digital servos all tend to have ballraced output shafts.

It's horses for courses.

[Nigel R]

"Which one is preferable?"

The one that doesn't go wrong! And with solid construction that does not degrade over time, as a secondary need.

 

"This instant, strong response is noticeable and appreciated when flying high performance models in a spirited, high performance way."

Agreed. But. As you noted, the mush only comes in when things are at or approach peak load... so I would add some nuance...

"This instant, strong response is noticeable and appreciated when flying high performance models with controls that need to move a long way, accurately and quickly, when flying in a spirited, high performance way."

 

Edited By Nigel R on 11/01/2021 12:38:36