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Oscillating servo

Posted: Wed Aug 07, 2019 9:48 pm
by Raythegreat
Hi all,

I have attached a video to show how my aileron servo oscillates when I move fast right and left on my aileron stick.
I have OpenTx 2.2.2 and Taranis plus and all works well.
I can fly all my airplanes with no problems but I would like to know what can cause the small oscillations when a sudden max aileron stick left or right is applied.
I hold the joystick to be sure it does not come from the joystick swinging or oscillating. The servo is a standard servo from Futaba.
Also i have tried a servo tester alone (no TX and Rx) and it does not oscillate.
All other channels don't oscillate, only the aileron channel.
I have also seen this on the left aileron channel on another airplane but the channel number was not the same.

Anyone has an idea what could cause this?

Thanks
RTG

Re: Oscillating servo

Posted: Thu Aug 08, 2019 6:57 am
by Kilrah
Do other S3004's do it if swapped in place? Do all S3004's you have do it or is there one that works? Do you see this with servos other than the S3004?
Could be that this dinosaur doesn't appreciate the 3.3V signal of modern receivers. Also check the supply voltage of your receiver system, could be too high or too low for it.

Re: Oscillating servo

Posted: Thu Aug 08, 2019 5:05 pm
by Raythegreat
Thanks Kilrah,

I will answer your questions:
Do other S3004's do it if swapped in place?
NO not all S3004 do it but I have seen this only on different S3004 a few times.

Do all S3004's you have do it or is there one that works? Do you see this with servos other than the S3004?
I have seen this only on 3 S3004. There were in good condition and practically new. Never on HS-65 or others.

Could be that this dinosaur doesn't appreciate the 3.3V signal of modern receivers. Also check the supply voltage of your receiver system, could be too high or too low for it.

I have a frsky receiver X6R, I believe the voltage is around 5V. I have tried the servo tester and have seen this behavior on one S3004.

You are probable right, the servo loop feedback circuit of these very old servos might be the cause, but they don't all do this.
As an example, I have 4 servos S3004 on my trainer wing and only left ailerons does it. The 3 others right ailerons and 3 flaps don't do this.

RTG

Re: Oscillating servo

Posted: Thu Aug 08, 2019 5:47 pm
by Raythegreat
Hi Kilrah,

OK I have made further tests.
I exchanged the servos from the oscillating S3004 to HS-65 and back to the oscillating S3004.
The same S3004 you saw in the video did not oscillate anymore!

I have seen this behavior before, an S3004 was oscillating and then when outside after flying a few times not oscillating anymore.
My suspicions are now towards the servo extension cable, the connection may be weak and from time to time the weak electrical connections might cause the oscillations but connection remains enough to work (and fly).

I'm not sure about this conclusion but it is the only one I have right now since the S3004 servo is not perfectly working.

I have removed from suspects, TX, RX, speed control. The only things left are servo and cables.

RTG

Re: Oscillating servo

Posted: Thu Aug 08, 2019 6:29 pm
by Kilrah
Raythegreat wrote:
Thu Aug 08, 2019 5:05 pm
I have a frsky receiver X6R, I believe the voltage is around 5V.
Supply is 5V, but I'm talking of the signal pin.
Old receivers used to give out the same voltage on the signal pin as the supply. Everything nowadays only gives 3.3V signal, and some old accessories don't play well with that. Used to already be a problem more than 10 years ago when Futaba started making more "intelligent" receivers that also used 3.3V microcontrollers, some stuff wouldn't work with it.

The fact you see it sporadically with different servos and different signal sources suggest that might be the issue, the signal being just at the boundary of "works" and "doesn't work" based on component tolerances, temperature etc. Could also be that there are batches of S3004's that are newer and got a fix for that.

Re: Oscillating servo

Posted: Fri Aug 09, 2019 1:31 am
by Raythegreat
Yes this makes sense and should be considered as the main suspect, the 3.3V signal on old servos.
I will try to find a way to prove it.

Thanks Kilrah,
Ray