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Author Topic: Digital servos - why?  (Read 5543 times)

inertia

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Digital servos - why?
« on: December 12, 2015, 10:40:47 AM »

As a follow-on from a topic about brown-out possibly due to a digital servo, I'd like to make a few observations and invite comments on the subject of digital servos.

Firstly, I hope we don't get the stage where all RC manufacturers offer us nothing else but "digital" servos - in the same way that it's nigh on impossible to get a new VHF set with more than two channels these days. They do have their place in applications such as RC cars and helicopters - and the bigger ones which fit into monster scale RC aeroplanes - but for the vast majority of RC model boats the rudder is the only function which uses a servo and so I can't see any need for so much extra torque and speed.
 
After all if a standard non-digital servo will traverse from end-to-end in less than 0.2 seconds then why would you want it to go any faster when it's just steering a boat? For the gyro-control of a helicopter tail-rotor servo then by all means fit a high-speed one, but you'd never need (or be able) to move the rudder on a four foot scale powerboat as fast or as often as that.

As for torque, I've never seen any figures published of the loads on model boat rudders but I have doubts that they are anything like as high as some folk imagine. It's not as if water is a particularly dense medium to push aside, is it? A standard Futaba 3003 has a torque of 3.2Kg.cm - that's a push of nearly five pounds available at the edge of a standard output disc. Pick up a five pound weight and ask yourself if you really think you need even that much force to move a boat rudder. Winch servos are a different matter altogether but most of them are run from their own high-voltage power supply and so "brown-out" would not be a problem.

I'm not saying here that digital servos aren't inherently worth the money (typically more than twice the price of a similar non-digital one) but I am expressing some reservations about using them in place of a standard servo in 99% of typical model boat applications.

Over to you, chaps.

Dave M
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C-3PO

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Re: Digital servos - why?
« Reply #1 on: December 12, 2015, 10:51:52 AM »

Would be really interested in an explanation of the difference (benefits/drawbacks) between the old fashioned analogue servo and new digital ones re what is the difference with the internal workings/circuit etc of both as I do not have a clue of the difference.

Do you have to have a special TX/RX to use them?

Hopefully others may also find it interesting if someone (perhaps Dave M) can explain - thanks in advance.

C-3PO
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Andyn

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Re: Digital servos - why?
« Reply #2 on: December 12, 2015, 11:30:04 AM »

They are incredibly useful when you're pelting along at over 40mph, especially for a multi where quick servos will make or break your race, and an S3003 will not turn a gas boat sized rudder, simple as, but not so much useful as you say for a springer. However, if like me you're buying your servos from the far east the price difference between digital and analogue is negligible, so there is no real reason to get the analogue. In theory they'd be better built drivetrain wise to cope with the extra power, in practice I'm not so sure. Having said that, the only servos I've ever had fail on me have been analogue, and funnily enough have been Futaba S3003's.
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ballastanksian

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Re: Digital servos - why?
« Reply #3 on: December 12, 2015, 11:30:56 AM »

When Inertia steps into the local telephone box and changes out of his lycra Electronics wizard suit and into his usual toga and top hat, he becomes Mr Milbourne.

Taking your points regarding torque,the only other applications on a boat other than the rudder that could be high torque are valves and associated fittings on steam plant, and I have read that these when well fitted and sited to suit the valves are not over stressed.

In a non steam setting, I can only think of a Radar unit driven by a flexishaft causing slight torque problems, most of our turrets and the like are free rotating and so should move well enough. Ooh, and perhaps valves for Submarines!

The introcudtion of the 2.4 control systems was a real boon to most areas of the hobby*, but I cannot see the need for one exclusive type of servo except for the manufacturers to keep their markets afloat with new kit. I cannot imagine anyone on here who hasn't got a tub of servos having been given/collected them through the years, so if the majority of us are only operating rudders, then we should be OK for a while :-))

*Sadly 2.4 does not suit the operations of submarines due to the wavelength.
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ballastanksian

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Re: Digital servos - why?
« Reply #4 on: December 12, 2015, 11:32:03 AM »

Hm, I hadn't thought of fast running boats.
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inertia

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Re: Digital servos - why?
« Reply #5 on: December 12, 2015, 12:43:57 PM »

C-3PO
There's a very good description of the difference between the two types of servo here http://www.rchelicopterfun.com/rc-servos.html
You don't need to change either the Rx or Tx to use a digital servo but the caveat about having a larger-capacity battery pack or high-current BEC should be well heeded.

Andy
You may have failed to notice that I said 99% of model boat applications! After all, this is Modelboat Mayhem and there's always some contrary b*gger who wants to take issue - hence my bets are usually hedged! High-stress applications such as multi-boat and big offshore racers are clearly an exception as regards torque, but I still doubt that the difference between the speed of the two types could mean the difference between winning and losing a race. We're talking here about less than a tenth of a second end-to-end difference in transit time, and I'm not convinced that anyone could react quickly enough to take advantage of that tiny fraction of a second - except for maybe a Formula 1 racing car driver or a fast-jet pilot.

That said, these boats aren't festooned with a bank of digital servos like a model aircraft is, and they certainly don't rely on the BEC output from an electronic speed controller. It's the high current used which seems to cause the most problems, along with maybe poor battery choice and maintenance. Cater for that properly and you should have none.

Ballastanksian
A decent high-torque non-digital servo should be suitable for valves etc; they don't need the speed of a digital servo so why pay for it? As for the availability of alternatives, who would have thought even five years ago that we would all effectively be shunted over to buying 2G4 kit, even if we didn't want it? I sympathise with you submarine blokes, but you represent a minuscule fraction of a percent of all RC modellers - certainly compared to the hoards of flyers and car racers who think 2G4 is the dog's best bits and that digital servos go along with it (for some obscure reason). If the manufacturers decide to "rationalise" i.e. move over entirely to digital servos, then the days of 1A BECs and 700maH Rx packs will be numbered. Let's hope they don't.
DM
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Re: Digital servos - why?
« Reply #6 on: December 12, 2015, 01:15:07 PM »

You may have failed to notice that I said 99% of model boat applications! After all, this is Modelboat Mayhem and there's always some contrary b*gger who wants to take issue - hence my bets are usually hedged!

That said, these boats aren't festooned with a bank of digital servos like a model aircraft is

the days of 700maH Rx packs will be numbered.


Someone's cheerful this morning.


I didn't fail to notice at all, I was giving an example of why they're needed. I think you'll find most proper racing boats are fitted with digital servos for both throttle and rudder, and they do indeed make a difference. Perhaps even more so with the throttle, as multi racers blip their throttles round turns. Fast boats also make up far more than 1% of model boating.


Who on earth still uses 700mah battery packs, and why are they still sold? I remember getting one in the pack with my first Futaba radio and slinging it in the bin - duly replaced by a 2000mah pack...
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inertia

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Re: Digital servos - why?
« Reply #7 on: December 12, 2015, 01:52:36 PM »

Andy
Believe it or not the last Futaba set I bought had a 700maH pack included (a 6J - about three years ago). They must have had a lot of them surplus. I take your points about big gas boats and I won't argue with you about small percentages.
I'm only cheerful because it's my 'Sgt Pepper' birthday tomorrow.......and I've had a very special prezzie from those nice people at Component Shop  :-)) (It's NOT a digital servo!)
DM
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JimG

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Re: Digital servos - why?
« Reply #8 on: December 12, 2015, 02:00:20 PM »

Digital servos are not just about speed there main advantage is that they are better at holding position. Put a large rudder (eg tug) behind a large prop with plenty of power and there will be considerable force trying to move it from its position. "Analogue" servos (still digital) have a much poorer resistance to this due to the way the electronics send power to their motor, low power when near their final position. "Digital"  servos are still giving full power to the motor at all postions so will resist movement better.. A friend found this when he replaced standard servos in a plane with standard sized digital servos and found that it needed much less movement of the control surfaces, they had been getting blown back in flight. Note that these were not the very large surfaces found on many modern 3D planes.

Jim
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Andyn

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Re: Digital servos - why?
« Reply #9 on: December 12, 2015, 02:23:18 PM »

Better over the hill than buried under it mate :-))
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unbuiltnautilus

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Re: Digital servos - why?
« Reply #10 on: December 12, 2015, 02:24:46 PM »

Andy
Believe it or not the last Futaba set I bought had a 700maH pack included (a 6J - about three years ago). They must have had a lot of them surplus. I take your points about big gas boats and I won't argue with you about small percentages.
I'm only cheerful because it's my 'Sgt Pepper' birthday tomorrow.......and I've had a very special prezzie from those nice people at Component Shop  :-)) (It's NOT a digital servo!)
DM


Is it a hat that says 'STOP ASKING STUPID QUESTIONS AND GO AWAY'?..cos they don't work %)
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inertia

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Re: Digital servos - why?
« Reply #11 on: December 12, 2015, 03:46:20 PM »

UBN
There speaks a man from the model trade...(almost another line from Sgt Pepper there).  Yes - I've noticed that, too. It wasn't the sign on my workshop cupboard door which says "Keep Calm and **** Off" either. All will be revealed in the fullness of time  8)

Jim
That's something which hadn't occurred to me. That said, there's no reason not to use a separate Rx battery pack in something the size of a tug - especially one that big and powerful.
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Subculture

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Re: Digital servos - why?
« Reply #12 on: December 12, 2015, 05:43:27 PM »

Most servos are digital anyway, even if they're classed as analogue. Open up most modern servos, and you'll find a microcontroller with h-bridge electronics, the old analogue chips are no longer being used because they either out of production or more expensive to use. The microcontrollers are just loaded with different firmware. You can get digital servos which aren't really any more thirsty than their old analogue counterparts. I like the little 10 gram digital servos sold by hobbyking, couple of quid a piece, and powerful enough for most models.
 http://www.hobbyking.com/hobbyking/store/__16260__HobbyKing_8482_HKSCM9_6_Singlechip_Digital_Micro_Servo_1_6kg_0_07sec_10g.html

PeachyPM

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Re: Digital servos - why?
« Reply #13 on: December 13, 2015, 12:31:09 AM »


Someone's cheerful this morning.


I didn't fail to notice at all, I was giving an example of why they're needed. I think you'll find most proper racing boats are fitted with digital servos for both throttle and rudder, and they do indeed make a difference. Perhaps even more so with the throttle, as multi racers blip their throttles round turns. Fast boats also make up far more than 1% of model boating.


Who on earth still uses 700mah battery packs, and why are they still sold? I remember getting one in the pack with my first Futaba radio and slinging it in the bin - duly replaced by a 2000mah pack...
I use 3 700mah battery packs and I'm not the only one, in mine and my kids club 500's which all have a servo to steer & a servo to action a micro switch for immediate full throttle, club500's might not be "proper" racing boats, but at least they don't stink, pollute  the environment, shatter ear drums, frighten wild life and make a boating lake completely inaccessible to any other branch of the hobby whilst there on it!
Ps and they're fun too!
 %%
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Martin [Admin]

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Re: Digital servos - why?
« Reply #14 on: December 13, 2015, 08:04:59 AM »

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PeachyPM

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Re: Digital servos - why?
« Reply #15 on: December 13, 2015, 09:49:09 AM »


Some interning thoughts on Utube: 
https://www.youtube.com/results?search_query=Digital+servos+-+why%3F
Wow! That's 10 minutes of my life I won't get back!
That first u-tube video should have a warning on it - "this video is in American speed!"  %%
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malcolmfrary

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Re: Digital servos - why?
« Reply #16 on: December 13, 2015, 09:17:45 PM »

For a digital servo to work, it has to respond to a signal that consists of a pulse with a measured analogue length.  The difference probably happens at the output end - the driver drives the motor flat out until the requested position is achieved, and is then forcibly stopped.  This as opposed to an analogue device which has to have the same front end but causes the motor to progressively sloe as the position is neared to avoid overshoot and hunting.
This flat out driving whenever the output is off-position accounts for the digital type needing more current.
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Subculture

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Re: Digital servos - why?
« Reply #17 on: December 13, 2015, 10:48:20 PM »

Digital servos vary. Some pulse at a fairly high rate and sing, e.g. Hitec servos tend to do this. Some of the Hobbyking digital servos feature a lower pulse rate and tend to hum or growl a bit more. Some digital servos have full PID, but a few I think have more basic PD loops like the old analogue chips.

Finally I find some digital servos are thirstier than others, with most i have encountered not significantly hungrier than analogue counterparts. As in all things YMMV.

Andyn

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Re: Digital servos - why?
« Reply #18 on: December 14, 2015, 10:16:40 PM »

club500's might not be "proper" racing boats, but at least they don't stink, pollute  the environment, shatter ear drums, frighten wild life and make a boating lake completely inaccessible to any other branch of the hobby whilst there on it!
Ps and they're fun too!

......... Club 500's are as dull as ditch water, some fast electrics are just as noisy (below 80db noise limit), any model boat can frighten wildlife and if people look out for each others models there is no problem having scale boats in with the fast stuff, our club regularly has scale or yachts sailing in the middle with fast stuff going round it.

As for pollution, it'll take more Co2 emissions to charge a brace of nimhs or lipos than I'll ever chuck out in a ten minute run. I presume your car is electric too?

The Op is infamous for these words - Suit yourself.
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PeachyPM

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Re: Digital servos - why?
« Reply #19 on: December 15, 2015, 12:01:40 AM »

He he! You took your time! (But it was worth the wait) :}
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inertia

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Re: Digital servos - why?
« Reply #20 on: December 15, 2015, 08:54:38 AM »

"Infamous"?? Aw, come on, Andy - I know we've had our differences but that's a bit strong  http://dictionary.reference.com/browse/infamous    <:(
DM
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PeachyPM

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Re: Digital servos - why?
« Reply #21 on: December 15, 2015, 11:45:33 AM »

club 500's are as dull as ditch water,
Andy, its all relative, have a look at these out of control nutters!  %%


https://www.facebook.com/bridmodelboats/videos/762239577236447/



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unbuiltnautilus

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Re: Digital servos - why?
« Reply #22 on: December 15, 2015, 02:36:12 PM »

most racing is ditch water dull from the outside, I have watched OMRA at Weymouth racing big D Class powerboats..ditch water dull...1 Metre yachts at Gosport..ditch water dull ( and much loud protesting!), ELECTRA guys racing little self righting things...funny on the first turn when they all rolled over, then back again...then dull again. However, three sistern floats, very basic rules and five little 19" yachts, thrilling fun to take part in at Canoe Lake, Ditch water dull to watch though!
Dont get me started on F1, The Grand National or the Vondee Globe >>:-(



 ok2
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david48

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Re: Digital servos - why?
« Reply #23 on: December 15, 2015, 05:20:37 PM »

All a bit of topic ,but you want to try watching small bore pistol shooting at the Comenwealth Games , it makes the previous post the most exciting thing ever .
David
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malcolmfrary

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Re: Digital servos - why?
« Reply #24 on: December 15, 2015, 05:35:48 PM »

How many small bores have they shot?


i'll get me hat.
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