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Servo jitter/judder

Discussion about Tx, Rx, Servo's, Batteries, Chargers, and all the other things we like to talk about..
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StephenB
Posts: 176
Joined: 26 Dec 2018, 08:45
Location: Hungary

Servo jitter/judder

Post by StephenB »

Working on the Spatz installing the aileron servos, these are Turnigy BB380 Max MG 4Kg servos, get good reviews on HK and recommended for sailplane usage. I've had these in stock for well over a year (so out of guarantee) but were bench tested with my servo tester and all appeared ok. Install the first one in the wing and connect it up and it's juddering all over the place, almost as if the thing is alive! I've recorded a short video that demonstrates the problem but don't see any means of uploading it.

Is the servo goosed?

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: Servo jitter/judder

Post by Peter Balcombe »

Stephen, I think that for videos, you have to have them in a web repository somewhere & then insert a link in the forum post.

When the servo jitters, is it still being driven from a servo tester, or from a radio Rx?
If all that you have done since testing ok with a servo tester, is put the servo in the wing, then check for solid connections/try to go back to connection setup when previously working.
If running from 2.4G Radio then extra lead length to get to wing servo position shouldn’t be a problem, but if using 35MHz then you could have interference pick-up.

Servos have been known to be very twitchy from new, but to suddenly go after wing installation is a bit unlucky.
Peter

Martin Gough
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Joined: 03 Mar 2017, 23:21
Location: Cheshire

Re: Servo jitter/judder

Post by Martin Gough »

FrSky transmitter's are know to cause jitter when the radio is at close range to the model
if thats what your using.

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Barry_Cole
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Joined: 10 Mar 2015, 09:30
Location: Hampshire UK

Re: Servo jitter/judder

Post by Barry_Cole »

Martin Gough wrote:
15 Sep 2020, 13:44
FrSky transmitter's are know to cause jitter when the radio is at close range to the model
if thats what your using.
I have been using FrSky for over 7 years, and never had any servo jitter at close range. They will sometimes go into failsafe with the Tx very close to the Rx, well less than 1 Mtr in my experience.

:D :D :D :D

BC

Martin Gough
Posts: 92
Joined: 03 Mar 2017, 23:21
Location: Cheshire

Re: Servo jitter/judder

Post by Martin Gough »

Been using mine for years and been jitter free bar one model which seemed to do it.
Walk away a few meters and it stopped.
Only real issue Ive had was my transmitter turned off while flying and I couldn't get it going again,
believe something on the early boards burns out so bought a new one and a jeti radio for my better
models which I really don't want to risk.

SP250
Posts: 146
Joined: 18 Mar 2015, 06:19
Location: North Shropshire

Re: Servo jitter/judder

Post by SP250 »

I've had some brands of servo jitter about when the battery voltage was too low as well. Mainly Hitec.
Putting into the model and using long extensions can drop the voltage enough to cause a problem which isn't there with the same battery and a servo tester out of the model. Done that too.

John M

StephenB
Posts: 176
Joined: 26 Dec 2018, 08:45
Location: Hungary

Re: Servo jitter/judder

Post by StephenB »

Thanks for the many replies.

I'm using Frsky Tx and Rx. The servo jitters both on the servo tester (short cable run) and powered direct from the Rx on the extension leads. I have charged the batteries, taken the wing outside and moved down the garden to eliminate any possible interference. Still jitters.

I'm reluctantly accepting it's faulty but the frustrating thing is that it holds my build schedule up! I wondered if there was a fix for the servo - cleaning the pot??

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Barry_Cole
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Location: Hampshire UK

Re: Servo jitter/judder

Post by Barry_Cole »

They seem to be in stock at HK EU warehouse so you may be able to get some fairly soon..

https://hobbyking.com/en_us/turnigytm-b ... s_products

BC

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Peter Balcombe
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Joined: 18 Mar 2015, 10:13
Location: Clevedon, North Somerset, U.K.

Re: Servo jitter/judder

Post by Peter Balcombe »

Stephen,
If you have more servos from the same batch then if you haven’t already done it, it’s probably worth checking all the others & even replace your aileron servo with another ‘good one’ in order to see if that also goes haywire with longer leads. Worth checking how extensive the problem is before re-ordering ;)

Note that my earlier reference to interference was only relevant with 35MHz, as the long leads needed for 1/4 scale-ish aileron servo cable extensions could act as 35MHz aerials & feed back into the Rx.
You have 2.4GHz, so not relevant.

StephenB
Posts: 176
Joined: 26 Dec 2018, 08:45
Location: Hungary

Re: Servo jitter/judder

Post by StephenB »

I've identified the problem.

Having resigned myself to the probability that the servo was indeed goosed, I dug it out. I'd mounted it by hot glueing to my improvised servo tray and before bonding it in I wrapped the servo in masking tape, all this was dug out and discarded but just for confirmation I connected to the servo tester for one last ghost of a chance. It worked!

Squeezing the servo case between thumb and forefinger brought back the jitters, so it would seem that a tight wrap of masking tape put undue pressure on the case and internal components. I therefore decided to mount it using only the screws and fabricated another servo tray to suit, aileron was then connected, knobs twiddled and all worked flawlessly.

I've often mounted servos by bonding and pre-wrapping with either masking or insulation tape, never had an issue. I guess the case on these particular Turnigy servos are a touch fragile, maybe a 4Kg servo of these dimensions for £16 has to compromise somewhere.

Before and after pictures:
Attachments
IMG_0487.jpeg
IMG_0488.jpeg

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