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Really annoyed - lost chance to fly model due to digital servos

Old 03-29-2015, 09:28 AM
  #1  
solentlife
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Default Really annoyed - lost chance to fly model due to digital servos

So many people keep going on about 4.8v and 6v NiMh use. I have used 4.8v since first ever doing RC. I fly my 58" Extra, my 53" Biplane, Pitts etc. on 4.8v and never had any trouble.

So late Friday I decide to swap out the 4.8 in the Extra for a 5 cell 6.0v pack. All fine at home, but admit it was only a quick switch on flick sticks switch off.

Get to field, do radio check first as usual before fueling up. WTF ? The ailerons are going crazy ... so much I'm waiting for hinges to fail.
The Rx has lost "bind" .... an 8ch FrSky.
I rebind .... still same, ailerons flapping away.
I take out Y lead, separate to different ch's .... better but still having occasional glitch from them.
Now I'm really annoyed as today was maiden with the new 61 engine.

As last resort I take the 4.8 pack out of my biplane, meaning she's unflyable rest of day. I put this in the Extra. All now fine.

Fire up, taxi out for take-off ..... open throttle .... she starts run hits bump, flips tail over nose and rudder hinges break ! I HATE CA hinges !

That's both the biplane and Extra now consigned back to the van.

The servos are digital, rated for 6v, Hitec's. Sadly I cannot replace them quickly as they are half height jobs. I only have full height analogue spares.

This is the second occasion this model has proved a problem with digital servos. I had to change rudder and elevator servos for serious flutter.

This does nothing to change my dislike for digitals !

Nigel
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Old 03-29-2015, 02:49 PM
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Funny, because i've got digitals in almost all my models and i don't have any trouble at all. They are all running either 6v or 8v for the HV types. I'd never buy anything else unless it was a cheap bind and fly type model that came with servos pre-installed.
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Old 03-29-2015, 04:33 PM
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If it was a delivered voltage problem, I wouldn't blame the digital servos. Maybe your battery was going into a motorboating problem. If it was fully charged, I would like to see a oscilloscope trace of the output voltage while your servos are going crazy.

Years ago I had a model act exactly like you are describing. In my case, but maybe not in yours, it turned out to be 1 vibration sensitive cell in my flight battery.

It would act fine, until the motor started, then the servos would go crazy. I took the model home and removed the battery then disassembled it. On a battery tester it would discharge at full current until I tapped on the bad cell. It was so sensitive a tiny tap from a pencil was all it took. Then it was instantly ready to go, until the next tap.
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Old 03-29-2015, 04:44 PM
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solentlife
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The model did not have any vibration as engine was not running.
The cells are fine and pack was in use in another function - I basically trial my packs in less serious consequence work to prove before committing to air.

All went back to good when a 4.8V pack was installed. This is where it is strange.

While at the flight site - another guy had similar with his glider. Same !! Changed to 6v ... his servos went do-ally ! Don't know which he had though, was too annoyed with my own.

Well the model flew fine on 4.8V - so until I swap out the servo's ... that's what she'll keep.

For info :

Rx :
FrSky V8FR-II 8ch HV (3 - 16V)
Servos are 25gr HITEC Digitals ... unfortunately labels are missing so I cannot give model #

Nigel

Last edited by solentlife; 03-29-2015 at 05:40 PM.
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Old 03-29-2015, 05:10 PM
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That's something I've never seen on my own models or anyone elses. I don't doubt that you experienced it but take my word that it's not a common problem.

One thing worth thinking about... A 5 cell NiMh pack fully charged will be about 7V. It's possible that the servos you were using werent happy running with that high an input voltage? A 2s or 3s lipo and a 6V BEC would give a constant input voltage to the servos and avoid exceeding their rating.
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Old 03-29-2015, 05:39 PM
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fhhuber
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I've seen that symptom on a model before... turned out to be a bad extension. Wire broken inside the insulation. Digitals are more likely to display issues with something like that.

There have also been issues when mixing some digitals with some analog servos in the same aircraft. Not sure why that would matter.
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Old 03-29-2015, 05:47 PM
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solentlife
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Originally Posted by JetPlaneFlyer View Post
That's something I've never seen on my own models or anyone elses. I don't doubt that you experienced it but take my word that it's not a common problem.
I know plenty people are happy with Digitals ... but I know also plenty people who have had trouble and wont use them.

One thing worth thinking about... A 5 cell NiMh pack fully charged will be about 7V. It's possible that the servos you were using werent happy running with that high an input voltage? A 2s or 3s lipo and a 6V BEC would give a constant input voltage to the servos and avoid exceeding their rating.
The Rx and Servo's are rated for high voltage use .. (5 cell). In fact the Rx is as per usual Full Range FrSky rated to 16V.

To get a SBEC of sufficient rating to cover possible amps max is about same as buying a couple of analogue servo's and sort it totally ! With the SBEC - still need a decent battery to supply the SBEC .. so another expense.

Using FrSky - I am able to live with greater voltage fluctuations than many ... so NiMh is not an issue at all.

Nigel
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Old 03-29-2015, 05:55 PM
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Originally Posted by solentlife View Post
The Rx and Servo's are rated for high voltage use .. (5 cell). In fact the Rx is as per usual Full Range FrSky rated to 16V.
It wasnt the Rx i was thinking about, it was the voltage rating of the servos. For standard (non-HV) servos maximum voltage is usually 6V, your battery will have been supplying 7V.. this could possibly explain the problem. having said that i know many 6V servos will take 7V without issue but possibly not all of them do.
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Old 03-29-2015, 06:05 PM
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My experience with supplying "excess" voltage (5 cell NiCd for 4.8v rated) to a servo says that isn't the issue. They either work or burn up very fast.

Most "4.8v rated" servos actually mean they can handle 4 cell Alkaline. 6.0 v nominal and almost 8 V for fresh batteries. The only time I've had a 4.8v rated servo fail on 5 cell NiCd was a really cheapie 9 gram being used to turn a pilot head inside a canopy (virtually zero load)

The original power for modern style servos in RC was 4 cell "dry cell" (also 1.5 v/cell nominal and up to 2.0 v full)and the major makers never quit making their standard line of servos to handle that.
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Old 03-29-2015, 06:19 PM
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Originally Posted by fhhuber View Post
The original power for modern style servos in RC was 4 cell "dry cell" (also 1.5 v/cell nominal and up to 2.0 v full).
The maximum voltage for a standard alkaline dry cell battery is 1.6-1.65V.. so thats 6.6V max for four. Pretty close to the 7V you would get from a 5s MiHh but it's still conceivable (though admittedly not very likely) that that extra 0.4v might push them over the edge.

Just grasping at straws here as i cant think of any other explanation short of two faulty servos, which seems unlikely.
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Old 03-29-2015, 08:43 PM
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Originally Posted by JetPlaneFlyer View Post
The maximum voltage for a standard alkaline dry cell battery is 1.6-1.65V.. so thats 6.6V max for four. Pretty close to the 7V you would get from a 5s MiHh but it's still conceivable (though admittedly not very likely) that that extra 0.4v might push them over the edge.

Just grasping at straws here as i cant think of any other explanation short of two faulty servos, which seems unlikely.
No dry-cell will maintain even 1.5v when loaded, even a light load. That's why rechargeable compare on-load favorably to dry-cells. As the load goes up, the dry-cell loses out significantly to the rechargeable.

Anyway when servos quote 6v - I always understood that to be in same vein as 4.8v - that is nominal voltage.

Anyway - thought I'd pass on what happened.

Replacements are on way - high speed 0.09sec metal gear jobs.. analogue, so problem will be history ...

Nigel
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Old 03-30-2015, 03:41 AM
  #12  
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I am with you Nigel I dont do precision flying like 3D or Pattern Flying, so Analogue Servos are good enough for me, Digital Servos can sometimes be very Touchy, needing some kind of an assist / resistance, to centre properly, For me, Digital servos are not worth the extra Money spent on them, I just Fun Fly Take care, Chellie
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Old 03-30-2015, 04:08 AM
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I'm not saying anything
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Old 03-30-2015, 08:36 AM
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Originally Posted by CHELLIE View Post
I am with you Nigel I dont do precision flying like 3D or Pattern Flying, so Analogue Servos are good enough for me, Digital Servos can sometimes be very Touchy, needing some kind of an assist / resistance, to centre properly, For me, Digital servos are not worth the extra Money spent on them, I just Fun Fly Take care, Chellie
Thanks Chellie ....

As I maintained in another thread - my personal experience and others I have seen, indicates exactly that - digitals are 'touchy' and can play up when even slightest thing is not quite as they need. Analogues are much more tolerant, and please to others - that does not mean sloppy or bad installations.

Someone mentioned bad leads or connection. Well the problem occurred whether Y lead or separate Rx sockets. Problem dissapeared when 4.8v put back. The 6v pack has been subjected to load and physical abuse since and no indications of problems with the pack.

I make up my own packs, having had a factory JR pack destroy a Royal Models P51 Mustang years ago. The welded cell tag broke away from one of the cells and I lost all control. Since then I have always made my own packs. Never had any fail in decades of doing this. I test and match my cells, fit, test, then wrap securely to stop any movement or possibility of problems. They then get subjected to load and 'abuse' before installation. I make sure the pack lead is locked to prevent any stress or pull on its connections to the pack. When connected to flight switch harness - the connection is taped to prevent unplugging.

I reckon I take more care in packs than manufacturers do !

If I was willing to pay out significant sums - I'm sure I could live with digitals. But the question remains - why if I can get good response, control with analogues ? My names not Wargo, Prettner etc. and I stopped flying Comps years ago. I really cannot tell difference in flight between a same speed digital and analogue. If the analogue is poor in centering or position resolution - that's a different matter. Corona servos - the bag full I bought were bad for that. Same size Turnigy were seriously better and cheaper.

I freely admit I budget buy. But the digitals I have had to swap out from this Extra were not bought by me - they were fitted by previous owner. They are not 'my budget' gear. I'm not surprised he crashed the model to be honest. And he's no beginner to RC. He presently flies a 2.1m Sbach .....

Nigel
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Old 04-01-2015, 09:32 PM
  #15  
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It could also be that the servos were not reliably seeing the edges of the pulses as the voltage was too high relative to the input signal level. This isn't an uncommon problem; we've run into it with Hitec before.

The pulse coming out of most modern receivers is usually 3.2V, the level the CPU is running on inside. The servo is looking for a pulse that is usually 70% or so of the supply voltage. Do a little math - that means a 3.2V signal is fine with 4.8V, but when you bump it up to 6+V it's way down, and only might catch the pulses now and then, and they might look like very different lengths.

If you want to keep using those servos and that 6V pack, insert an SPMCP in the line for each problem servo. It will boost the incoming signal level up to the supply rail, and your servos will again be happy.

There are some servos that don't like higher voltages no matter what. It could be that's the problem too, in which case you either use 4.8V or swap out the servos.

Andy
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Old 04-01-2015, 10:01 PM
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solentlife
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New servos ordered ....

Once fitted - then the 5 cell pack will be refitted.

Nigel
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Old 04-02-2015, 05:42 AM
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Originally Posted by AndyKunz View Post
It could also be that the servos were not reliably seeing the edges of the pulses as the voltage was too high relative to the input signal level. This isn't an uncommon problem; we've run into it with Hitec before.

The pulse coming out of most modern receivers is usually 3.2V, the level the CPU is running on inside. The servo is looking for a pulse that is usually 70% or so of the supply voltage. Do a little math - that means a 3.2V signal is fine with 4.8V, but when you bump it up to 6+V it's way down, and only might catch the pulses now and then, and they might look like very different lengths.

If you want to keep using those servos and that 6V pack, insert an SPMCP in the line for each problem servo. It will boost the incoming signal level up to the supply rail, and your servos will again be happy.

There are some servos that don't like higher voltages no matter what. It could be that's the problem too, in which case you either use 4.8V or swap out the servos.

Andy
Hi Andy
I did some testing on that very subject last year. I setup my home made digital pulse generator to drive servos. Then varied the pulse output voltage of the pulse generator from zero to where the servo would start to respond. Also varied the battery voltage to the servo.

I've got a limited selection of servos, some old Futaba 3003's, some Hitech servos, including the 645MG's plus a few other brand servos. All of them started working with the pulse level above 1.0 Volts DC, and kept working on up to when the pulse level voltage matched the DC voltage applied to the servo.

There are a lot of other servos out there, so it's very likely your results will vary.
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Old 04-02-2015, 05:50 AM
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Originally Posted by solentlife View Post
Thanks Chellie ....

As I maintained in another thread - my personal experience and others I have seen, indicates exactly that - digitals are 'touchy' and can play up when even slightest thing is not quite as they need. Analogues are much more tolerant, and please to others - that does not mean sloppy or bad installations.

Someone mentioned bad leads or connection. Well the problem occurred whether Y lead or separate Rx sockets. Problem dissapeared when 4.8v put back. The 6v pack has been subjected to load and physical abuse since and no indications of problems with the pack.

I make up my own packs, having had a factory JR pack destroy a Royal Models P51 Mustang years ago. The welded cell tag broke away from one of the cells and I lost all control. Since then I have always made my own packs. Never had any fail in decades of doing this. I test and match my cells, fit, test, then wrap securely to stop any movement or possibility of problems. They then get subjected to load and 'abuse' before installation. I make sure the pack lead is locked to prevent any stress or pull on its connections to the pack. When connected to flight switch harness - the connection is taped to prevent unplugging.

I reckon I take more care in packs than manufacturers do !

If I was willing to pay out significant sums - I'm sure I could live with digitals. But the question remains - why if I can get good response, control with analogues ? My names not Wargo, Prettner etc. and I stopped flying Comps years ago. I really cannot tell difference in flight between a same speed digital and analogue. If the analogue is poor in centering or position resolution - that's a different matter. Corona servos - the bag full I bought were bad for that. Same size Turnigy were seriously better and cheaper.

I freely admit I budget buy. But the digitals I have had to swap out from this Extra were not bought by me - they were fitted by previous owner. They are not 'my budget' gear. I'm not surprised he crashed the model to be honest. And he's no beginner to RC. He presently flies a 2.1m Sbach .....

Nigel
Hi Nigel

Just a wild a**d thought. Have you tried running your receiver and digital servos with a higher powered receiver battery such as an LiFe or A123 battery pack.

Just in case your 5 cell receiver battery voltage sagged to far with the digital servos you've used.

I've put my oscilloscope on one of my digital servos. That thing pulled a pretty high level short duration pulsing DC current out of the battery pack. It was something around 5 Amps or so.

I also ran across some really bad receiver switch problems from 72 Mhz radios purchased in the 2000 era. All of my old receiver switches had contact resistance of over one ohm. Real pieces of ****. Also scared the heck out of me.

A lot of my club members are using those digital servos, including some very high powered units on their giant scale models, with 60 cc and 150 cc gassers up front. Their receiver power is a combination of several two cell A123 battery packs running everything. The couple of guys with those 150 cc gassers are running a pair of 4S2P totalling 9200 Mah A123 packs as primary/backup power. I did some quick calculations on one of those 150 cc gassers. The average current being pulled out of the big A123 battery packs during the 10 minute flights came out to around 3.7 Amperes. Who knows how high the peak current was.

Backup power, IMHO, something absolutely required when running models worth a substantial fraction of the cost of a new car. They are also running Spektrum or JR 2.4 Ghz equipment, and have had absolutely flawless performance from their A123 battery packs.
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Old 04-02-2015, 08:49 AM
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Denny, appreciate the post.

But the same servos are fine with 4.8v NiMh pack. No problems at all.

Andy may have a point about signal voltage level. But to be honest - I shall swap out the servos, go with someone I'm more confident with.

That's the matter at end of day - what works and each of us feel confident with. I think we have all been at flight line, seen a guy with a set-up that we would leave at home to be corrected. He flies, probably survives but its not 'comfortable'.

Nigel
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Old 10-20-2015, 04:33 PM
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Thought I'd add final note ....

Model now has flown some months with a 1800 3S LiPo via SBEC to the Rx ... and flies superb.

Nigel
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Old 10-20-2015, 05:21 PM
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Always glad to hear of success, Nigel!

Andy
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Old 10-20-2015, 08:01 PM
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Originally Posted by AndyKunz View Post
Always glad to hear of success, Nigel!

Andy
Cheers

Info : I actually saved a few grams in weight for an increase in potential power.

Nigel
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Old 10-20-2015, 11:07 PM
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Funny, isn't it? How the things that always work right without ever glitching for some people have never worked right once for others.

Someday someone is going to get a PhD when they write their thesis explaining why that happens...
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Old 10-21-2015, 01:40 AM
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Originally Posted by FlyWheel View Post
Funny, isn't it? How the things that always work right without ever glitching for some people have never worked right once for others.

Someday someone is going to get a PhD when they write their thesis explaining why that happens...
That's not involved in this case, but having worked in the electronics service industry for 44 years before retiring, we found that 20% of our customers had 90% of the problems.
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Old 10-21-2015, 03:29 AM
  #25  
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Originally Posted by kyleservicetech View Post
That's not involved in this case, but having worked in the electronics service industry for 44 years before retiring, we found that 20% of our customers had 90% of the problems.

We used to call that "EBCK"
Error
Between
Chair and
Keyboard

There was one guy I had to take a magic marker and write "Any Key" on his space bar because he couldn't find the Any Key.

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