Servo problem - help needed

[Birgir]

Hello

Can anyone explain to me what is happening with my servo?

When I release the transmitter stick suddenly from full up or down the servo fluctuates around the centre position for a moment before it stops in the centre.

Should I be concerned about this, or should I just ignore it?

This does not bother me when flying.  I’m not that good a pilot.  There is no gyro to blame.

Any comments welcomed.

Thanks  - Birgir

 

Edited July 29 by Birgir

[Philip Lewis 3]

I would replace that straightaway, the simple answer is it shouldn't do that, so replace it with a new one that doesn't.

[RottenRow]

Is that happening when you let the tx stick move back by itself? Ie when you take your thumb off of it when it is at full deflection?

 

If so, I would watch the tx stick and see if it goes past the centre position and ‘bounces’ backwards and forwards a couple of times before becoming stationery.

 

If it is doing this, then your servo is simply following the stick movement.

 

Adjustment of the stick tension might help if this is the case.

 

Brian.

[Martin Harris - Moderator]

It might be due to the weight of the control surface if it’s particularly free moving. Invert the model and try it - if the overshoot then happens from down elevator it would support this. 
 

If this seems to be the case, confirm by swapping with one of the “good” servos to see if it’s due to the relatively low holding power of these servos around the commanded position but if in doubt, change it. 

[Paul De Tourtoulon]

I have only seen this on rubbish or false futaba  servos sold on eboot,  have you tried another servo at its place and what happens when you disconnect the kwik link. Could the fault be the TX stick, as said above.

 

I wouldn't fly with them.

[Brian Cooper]

Change it. 

Elevator servos have a lot of work to do, and the workload is increased by being in the strong blast from the propeller.

 

From experience, if any servos are going to fail, the elevator servos are the most likely to fail first. 

Ideally, replace it with a better servo.... especially if there is only one servo on that function. 

[Engine Doctor]

I have experienced this with servos running aftermarket Y leads and or extension leads . Also as Paul says Fake Futaba servos in the 3003 spec  abound . are you sure they are genuine? Check the sound of them do they sound smooth and quiet or noisy ? if noisy then suspect. Look at bottom of case it should have well moulded " made in Taiwan"  the fake one used to have shiny plastic cases but we have seen some lately with the matt finish . Only safe check if unsure is to remove the bottom cover and look at the internals and if you not familiar with them compare against a known genuine servo.

The best giveaway is the often the low price they sell sell for , around £ 15 for 5 delivered ! 

This can occasionally  happen with a freshly charged battery . Are you letting the tx stick go and centring itself ? If you are it could be the signal emitting from the Tx causing this . Try centring quickly with your thumb on the stick , Never let the stick bounce back to centre on its own as this can damage the pots in the gimble .

[John Lee]

Looks like the feedback loop that controls the servo's position is underdamped. It's not adjustable & may be a sign that something is about to fail more comprehensively. As others have said - bin it.

[Cuban8]

Before consigning the servo to the bin be absolutely certain that this problem isn't being caused by a bad servo extension connector. I've seen this many times with my own models and in fact I was talking to a flying mate over the weekend who was experiencing the same thing with a model he'd not used for some time. The way that the servo arm is behaving fits that type of failure exactly.....has the servo lost torque and can easily be pushed off centre - (feel with your finger) typical symptoms of a bad connection I've found in the past. If you operate the servo for a couple of minutes in situ,  it may well start to improve - another symptom of a bad connection. Never seen the issue with a servo plugged directly into an RX but I suppose there is a possibility that it could occur - easy to see with a quick spray of contact cleaner on the servo plug.

IIWY, I'd take the servo out and bench test it direct from a servo tester or other RX......if I was a betting man I reckon it'll work OK. In that case replace the servo extension and carry on as normal.

Naturally, if no extension is being used and you're sure that the servo plug is clean and  it still plays up when being driven directly, then that points to the servo itself. Genuine Futaba servos, even the standard budget ones are incredibly reliable and I can only recall a couple of failures (excluding gears and worn pots) in many, many years.

 

Hope this is of some use.

Edited July 30 by Cuban8

[Andy Stephenson]

I have seen this exact effect with a flat battery which is dropping its voltage as soon as the load of the servo motor comes in.

[Birgir]

Thanks everyone for your thoughts and good suggestion.

 

The most secure way is to bin the servo, and I totally understand this suggestion. But being a sportsman, I find that a bit too easy. (Most likely I would also make this suggestion if someone asked me).

 

I’m sure that the servos are original Futaba S3003 bought from Pegasus Models in Norwich.

I tested the model upside down as Martin suggests and moving the sticks quickly without letting them go as RottenRow and Engine Doctor suggested.  This did not eliminate this behaviour in either case.

 

Regarding extension leads as Engine Doctor and Cuban8 have pointed out, I have a confession to make.  This model has two receivers and a Dual Receiver unit that connects both receivers to all the servos.  The extension leads are JR but the servos are Futaba. So, the extension leads theory sounds appealing to me.

While testing the servo the behaviour seems to diminish as Cuban8 did forecast, that also supports his suggestion.

 

Engine Doctor also mention that a similar behaviour can happen just after the batteries are recharged.  This might also play a role because I just chanced from a five batteries rechargeable Overlander battery pack to four Duracell Optimum non-rechargeable batteries which might be a bit more powerful.  The reason for this change is that I’m using rechargeable batteries in my Futaba 6J Tx and when I put this Duracell Optimum in it lasted and lasted much longer than I expected.

 

But I do worry a bit because I don’t know how these batteries behave when they are close to empty.  Do they empty very suddenly and fall rapidly below working voltage or is it ok to expect noticeable fall in voltage that will alert that battery change is needed?

 

B.

[EvilC57]

I had exactly this problem with an aileron servo in a model a few years ago. The servo for the other aileron was OK, but one would 'bounce' back & forth if you let the TX stick spring back to the middle. It turned out to be caused by a duff servo extension lead. Changing the servo made no difference, but I changed the lead going through the wing and the problem went away.

[Phil Green]

It can also happen when the battery voltage is too high for the servo - higher voltage, faster transit speed, overshoot, overcorrect, repeat...  🙂
The PID should allow for supply variations but many dont

 

[Simon Chaddock]

Birgir

I can understand the desire to analyse the problem.

First simply disconnect the servo from its linkage and check if servo still oscillates. If it does then the problem is with the servo and should not be used. If removing the linkage stops the servo oscillation then the problem is down to the linkage in conjunction with that servo. If a replacement servo does not oscillates with the linkage added then the original servo is "weak" and should not be used.

Logical testing in sequence can identify a problem that cannot be traced with sophisticated test equipment.     

[Martin Harris - Moderator]

That takes me back to my apprenticeship when they taught us logic faulting at training school. A test lamp, circuit diagrams and the mk1 brain was all that we were allowed. 

[John Stainforth]

Absolutely don't bin the servo until you have proved that it is at fault. I agree, from experience, with what others have said, that the problem is most likely to be poor electrical connections. When I was living half the year in the US and half the year here, with models in both places, about half the models would develop these problems in my absence. The cure is simple: just unplug all the servo leads from the RX and separate all the connectors, and then reconnect - perhaps several times, to remove the oxides that have built up on the contacts, that cause the problem.

Edited July 30 by John Stainforth
typo

[Cuban8]

The first time that I encountered this was with the original  Hitec servos I'd fitted in my Goldberg Chippy (ailerons). Getting on for thirty years ago now, but I remember becoming cheesed off with getting all set up and ready to fly and then an aileron would do the bouncing around centre trick. Some weeks it would and some weeks it wouldn't. Swapped servos and that seemed OK for a while and then the same thing would happen again.

Through a process of elimination the fault was being caused by poor servo extension contacts and in fact even after I upgraded the model to 2.4 and replaced the well used Hitec servos with Futabas, I have had 'the bounces' a couple of times but easily cured with a good spray of contact cleaner on the extension connectors.

[Birgir]

 

Hi everyone

 

I did the servo test as Cuban8 suggested. I pulled the servo lead from the Dual-receiver module and connected it strait to the receiver, the same receiver, and Vola! No bouncing at all.

When I connected it again as it was previously, the bouncing was minimal.  The unplugging and re-plugging might have cleaned the connectors a little, proving John’s theory.

I also must admit that the inside of this old Keil Kraft Mini Super has a very nice smell coming from previous fuel leaks. Everything also has a nice oily touch, based om the same history. 😊

 

B.

[Engine Doctor]

Hi quick update. I have just had exactly this issue while assembling a wing for a club mate. Two aileron servos  F 3003 on a "y" lead. One servo perfect , powerful centering well other one was weak and flapping around center. He has supplied some decent HD extension leads . I removed the plugs a few times and they now both work perfect.  Obviously its the plug orsocket connectors not making a proper connection.

On the same theme my car had a similar issue with the electrical connector plug from the gearbox . Jagyar issed a bulletin about stating both plug and socket shoukd have the same coloured plating on the pins/ sockets. This only affects cars of certian year as the plug and socket used to have gold plated pins. Later models probably to cut costs had one half of the plug and socket silver coloured plating , poss nickel and the other half gold plated. The two metals eventually react  and cause a bad connection/resistance resulting in gearbox fault. 

Virtually all our servo leads now have silver coloured pins and sockets but we dont know what the plating is. A good push in and out a few times should wipe the connection clean . I was wondering if there is a lubricant for these type of plugs ? Any electronics wiz's out know ?

[John Lee]

There are loads of specialist electrical contact cleaners. I always keep a can of this handy (note that although it is made by WD40 it is NOT standard WD40):

 

https://www.wd40.com/products/contact-cleaner/

[Nigel R]

Spray type Switch Cleaner or Contact Cleaner would be all I'd use here. As you've stated ED this kind of thing (extension cable issues) is down to very slow reactions between dissimilar metals and/or oxidisation. Nothing for it other than to unplug, spray clean, plug, re-test.

 

Similar net effect can result from dirty feedback pot inside the servo as well (apologies for teaching any grandmas to suck eggs there). Within the last couple of months I had an (admittedly old and well loved) Hitec servo that was suffering from that - open case, squirt of cleaner, close up case, and it was back to normal.

 

Whilst you can get conductive grease, I'm not sure I'd want to try and apply it on a servo plug! It's more for electricals working in environments where moisture is a concern.

[Cuban8]

Be careful of what is used on small signal connectors such as used in our radio systems. Products like Contact Lubricants aren't really ideal -  I've used them extensively in the past on much heavier duty power contactors etc at work where the environment and useage  is much different to that in our models or in large connectors where the contact force is very high. BTW, dielectric greases are non-conductive (a dielectric) and are intended to be used on connectors that are designed to displace the lubricant and form a tight metal to metal contact surrounded and sealed by the grease.

Better to use a  cleaner that's designed to clean and degrease and won't leave any residue. I use automotive brake cleaner where the last thing you need is any contamination on pads & discs etc. If a conector remains problematic after that, then best to replace it.

I'm pretty sure that most of our problems are caused by the type of plating on the crimps - the old genuine Futaba types way back in the day were, I believe, gold flashed but given how everything has been cheapened I don't know whether that's still the case. The pattern leads (Futaba type, JR type etc) that are all over the internet are an unknown quantity - those that I've bought are well made with good crimping but whether the material used for the crimps and any plating is of the top notch, I don't know.

Edited August 1 by Cuban8

[Phil Green]

[flackjacket]

Not a rant, just something I muse over from time to time:

Personally I think the standard R/C servo plug is well overdue for an update.   They're just so flimsy and insubstantial, and not properly polarized or latched.

Back in the day™ servos used less current yet the connectors were generally much better than todays little bits of bent tin in a plastic sleeve - although there were many 'standards'- companies like Skyleader, Fleet, Staveley, Macgregor used high quality, electrically robust connectors, often made by SLM and often having round, gold or silver-plated pins. 

When I see huge models like the Vulcan at Elvington LMA I despair when I see the servos being connected up.  On this very forum you'll frequently see 'power-supply' and 'switch' posts talking about 5 Amp servo currents.  IMHO the current standard connector just isnt up to the job.

 

Another thought is that when the 'Futaba standard' connector was introduced, the pin headers in receivers used to be thinner pins than the standard .025" usually found in todays gear - early servo sockets (the connector  on the servo is a socket, not a 'servo plug') when fitted onto .025" headers will be opened up slightly, reducing contact force if returned to the original thinner pins.  Most modern gear if not all uses .025" pins but if a 'strained' socket was to be used on an older header plug - maybe an older receiver or extension lead - the reduced contact force could be a problem.

Again, not a rant, just me musing...   😉

[/flackjacket]

 

Edited August 1 by Phil Green

[Don Fry]

My fear on wiring is cheap plugs. Nowadays, I keep servo plugs to a minimum, no extension leads, cut and solder an extension wire into the gap. Wire and solder is cheap.

[Birgir]

Considerable cleaning work ahead...