Servo problems.

[Peter Futers]

when i connect servo to rudder servo goes berserk, ie banging from end to end travel, have checked all leads ok.

its only when i connect the push rod to the rudder.

rudder very floppy, no binding

 

any ideas?

 

[J D 8]

  Duff servo?   Try another.   Out of interest what is push rod made of and type of radio used.

[Peter Futers]

not a duff servo, disconnect pushrod servo ok, push length aout 6 inch metal, take push rod off servo ok

radio is radiolink at10ii rx rd12ds.

 

[Richard Wills 2]

what servo is it?

 

[Martin Harris - Moderator]

What happens if you push gently on the servo arm with the pushrod disconnected?  Is it an analogue or digital servo?

[Peter Futers]

HI Martin thanks for getting back to me.

10g analogue, bought them from Airtech before he closed down due to poor health.

Had bought them for a different project but never used.Don't know the make got unboxed boxes lost servos are glued to side panel to the rear of the fuselage,

As to your ? disconnect servo perfect good resilience to trying to move from any point of its travel, it only happens when  i connect the pushrod to the servo arm, switch on two waggles servo goes mad mad mad!

 

does this give you any clue?

regards

PETER

 

 

[john stones 1 - Moderator]

If you can, make a non metal pushrod up, or fit a plastic clevice ?

To decide if the metal is the issue.

 

[Peter Futers]

Just taken the model down to try something  found that if i gently touch  the p/rod it all calms down?

clue thier ?

[PatMc]

Is it an electric model with BEC or does it have a separate Rx battery ?

If the latter it could be that the Rx battery is either needing a fresh charge or duff.

 

[Martin Harris - Moderator]

...or exceeding the power capability of a BEC?

[john stones 1 - Moderator]

3 minutes ago, Peter Futers said:

Just taken the model down to try something  found that if i gently touch  the p/rod it all calms down?

clue thier ?

Sounds like pushrod needs a tweak to line up better, is it buzzing with pushrod on ?

[Peter Futers]

NO buzzing very straight rod, just tried plastic clevis to servo, clevis on rudder  wood,

fault applies with bec or 4,8v from external source 

 

losing my patience with this fault.

 

in many many years of building never had this agro . 

[Richard Wills 2]

what is air frame, do you have a picture of the rudder and fin?

 

[PatMc]

1 hour ago, Peter Futers said:

Just taken the model down to try something  found that if i gently touch  the p/rod it all calms down?

clue thier ?

Sounds like what Martin was hinting at earlier, the wiper's probably light in feedback potentiometer or there's a bad connection. I'd be inclined to bin the servo unless you're prepared to open it up & look for an obvious bad connection first.

[Simon Clark]

All servos and pushrods / control surfaces act as a closed loop control circuit and any control system can go unstable under certain circumstances if the gains are too high and there is not enough damping. This is a slightly simplistic way of looking at it and the reality and associated maths is very complicated. It is possible that the mass of your rudder coupled with the mechanical throws that you have are hitting a resonance point and causing control instability. By gently touching the pushrod, you are introducing a small amount of damping which takes it away from the unstable point. Some servos will suffer more than others and with tolerances even two examples of nominally the same type may react very differently. It is safest just to swap the servo out for a different and better type but you could try changing the mechanics of your system a little by moving the pushrod onto a different hole on the control horn or servo arm - yes this will change the overall rates but to a certain extent you can compensate by using servo end point adjustment and rates etc.

 

Good luck

Simon

[GrumpyGnome]

Rearranging the mechanical linkage may fix the issue on the ground, but surely once the airflow hits the surface, the load in the servo is going to change, and it could be back to square one, with potentially worse consequences...... swap the servo out.

Edited April 27 by GrumpyGnome

[Nik Harrison]

Does the same thing happen if you use a servo tester to drive rudder instead of Rx?

[RottenRow]

From reading the above, it sounds like the servo is in the rear of the fuselage, with an extension lead to the receiver.

 

If that’s the case, try plugging the servo straight into the receiver (temporarily move the receiver). I had an issue with a dodgy connector in an extension lead causing servo oscillation, though not quite from end to end as you report.

 

I assume you have already tried another servo, not just assumed it was ok because it appeared to be ok with the pushrod disconnected. It does sound like a faulty servo, possibly the additional inertia caused by the pushrod and rudder causing it to overshoot and oscillate.

 

Brian.

[Martin Harris - Moderator]

The odd thing here is the description of “banging from end to end” - when I’ve encountered similar scenarios it’s been more of an oscillation around the neutral point - and on a control surface subject to gravity e.g. an elevator. 
 

If the servo is a direct, rather than indirect type then perhaps it’s simply the weight of the rod putting a load on a dodgy potentiometer and causing a false position reading?

 

 I’d second the advice to try a different servo if that hasn’t been tried. 

[Phil Green]

A video would help us to understand whats happening... 🙂

Edited April 27 by Phil Green

[Max Z]

--

Edited April 27 by Max Z
invalid suggestion, false logic.

[Cuban8]

1 hour ago, Martin Harris - Moderator said:

The odd thing here is the description of “banging from end to end” - when I’ve encountered similar scenarios it’s been more of an oscillation around the neutral point - and on a control surface subject to gravity e.g. an elevator. 
 

If the servo is a direct, rather than indirect type then perhaps it’s simply the weight of the rod putting a load on a dodgy potentiometer and causing a false position reading?

 

 I’d second the advice to try a different servo if that hasn’t been tried. 

This sounds like the most likely reason. If it's just one servo acting up in this way then I'd advise you to cut your losses and chuck it in the bin. BTW, as they sound to be  unbranded 'bargains' can you really trust the others? Not worth getting your blood pressure up about TBH and certainly not worth risking a model.

Annoying, but there have been cases of rubbish servos finding their way on to the market - thankfully, not quite so much of a problem as of late.

[Engine Doctor]

Read older posts re didgy or fake servos. 

The suggestion that an extension lead could be faulty is very valid . Try the servo on a servo tester or plug directly into Rx without the ex lead and compare. Also try it in another Rx output incase Rx  channel is at fault. Its just a case of isolating the cause.

[Ron Gray]

I had a similar experience the other day. I had flown my Giant Revolver and fuelled it up fir a second flight but when I checked the controls both elevators appeared to be ‘lazy’ in other words there was a delay between a Tx stick input and the servos operating, not all the time but more so if I moved the stick quickly back and forth. Note that there is 1 servo for each elevator half and both exhibited the same issue. I changed the Rx batteries but still had the same problem I also swapped Tx to no avail. Back home I swapped the Rx for a different one, same problem then removed the servos and used a servo tester which showed both servos were toast! Quite why they both went at the same time is a mystery as no others were affected! BTW they were New Power servos which had been in the model since I first assembled it 8 years ago.

[Brian Cooper]

A bad (rudder) servo.

Replace it with a good one. . Ideally, test it thoroughly before committing it to flight. 

 

In service, it is not unusual for the elevator servos to wear out before the others.