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Servo Chatter

Old 08-12-2011, 09:06 AM
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chemie
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Default Servo Chatter

I have a servo on a pull-pull set-up. With the clevis' disconnected from the rudder, everything is fine. Connect them and the servo jiggles back and forth. I tried increasing and decreasing the tension...no change. Pull the arm off and the servo is steady. Put the arm and steady. Connect the wires and leave them loose, fine. Attach to cnotrol horns (even loose) and it jiggles.

????
Old 08-12-2011, 09:16 AM
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RCKen
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Default RE: Servo Chatter

Is is a digital or analog servo???

Ok, just to make sure what you said. Even with the pull-pull in place but there is no tension on the servo you get servo chatter?? If this is the case then it sounds like you might have some metal to metal contact that could be causing RF interference that is causing it. Check to make sure you don't have any metal to metal contact in your pull-pull setup. 

You might want to also try a different servo and see if it works with a different servo. If it does, then you have a bad servo.
Old 08-12-2011, 09:51 AM
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CGRetired
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Default RE: Servo Chatter

If you are using metal cable for your pull-pull setup, I might suggest this product:

http://www.thunderboltrc.com/index.p...roducts_id=580

It is a coated kevlar cord. Very strong and does not generate any electromagnetic interference. I use it on my Excelleron 90 pattern plane which has pull-pull on both the rudder and the elevator.

CGr.
Old 08-13-2011, 06:01 AM
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chemie
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Default RE: Servo Chatter

analog; metal wires

setup worked fine for last 2 years so I doubt it is the metal.

I do not have the same servo but will swap in another just to see if that changes things. What is strange is that the servo does not chatter with just the arm even when I move it or hold and jiggle with my hand (but the wires are there...just disconnected from arm).
Old 08-13-2011, 06:24 AM
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chemie
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Default RE: Servo Chatter

OK. This is interesting.

I started to troubleshoot and when I turned everything on, happened to have a different model selected...no chatter. So I cycled through all my models and only the correct one had the chatter. Play some more to find what was different. If I change the sub-trim by about 10 (either way), it stops. Since this is pull-pull, I had to set sub-trim to about +40 to get it neutral but I could stand a change of 10 without mcuh impact. Now it does not chatter on neutral setting. However, if I move it just a small amount, back to the "old neutral", the chatter starts again. Basically, the servo will chatter when it is in a specific single position. Any thoughts on why?
Old 08-13-2011, 07:18 AM
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Default RE: Servo Chatter

This is just a guess, but it sounds like the pot is worn or dirty there in the neutral spot. The side load from the cables may be just enough to make it act up. It's a simple job to take the servo apart and open up the pot to clean it with a dry Q-tip. You didn't mention how expensive a servo this is, so you'll have to decide if it's worth the effort to do that.
Old 08-13-2011, 07:58 AM
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Default RE: Servo Chatter

This probably isn't the problem, but thought I would mention it just in case. This spring while setting up the plane that i built last winter, the flap and airleon servoes were going crazy. I always solder my own servo extensions and figured that a wire was touching by not getting the heat shrink on good enough. So I pulled all the servoes out of the wings and redone all the splices. Put it all back together and same thing. Then remembered that with my radio when you select a different model the radio has to turned off then turned back on. DUH, brain fart big time. I don't know if I have more on my mind now, but it seems like the older I get the more I forget.

Pat
Old 08-13-2011, 03:26 PM
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Default RE: Servo Chatter

The pot is worn at the center point and the servo is hunting for center. When connected to the rudder, the momentum of the rudder carries the rudder through the center point and this sets up the chatter around center. By changing the trim setting, you're moving the center point. The chatter will probably go away when the plane is in the air with come pressure on the rudder.

Brad
Old 08-14-2011, 04:41 AM
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chemie
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Default RE: Servo Chatter

Actually, Brad, the reason I got worried was that the plane had a bad wiggle in the air like the rudder was being pushed around. It did that once before when the same servo had two front screws come out and I thought it was that. I am actually surprised the jiggle is enough to cause the wiggle ;-)

This is a Futaba 3305..can cleaning the pot correct this? Could having too high tension on the pull-pull have caused this?

After pulling off the case, then what?
Old 08-14-2011, 04:50 AM
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Default RE: Servo Chatter

High tension on the pull-pull could possibly do it, but what is more likely is that it puts tension on the servo shaft and bearings where it just doesn't need to be that tight.. certainly not guitar string tight, but not loose enough to have a droop. You should easily be able to replace the servo to see if that is the problem. I'd adjust the tension so that it's not "tight", check it, if it continues, swap out the servo and give that a quick try.

CGr
Old 08-14-2011, 05:11 AM
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jester_s1
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Default RE: Servo Chatter

Agreed with CGRetired. Try a different servo first. To answer you question, cleaning the pot will fix it if the problem is a dirty pot or corrosion on the brush. If it's a worn pot the only option is replacement though. I suppose if a guy were motivated enough to keep using that servo you could move the pot a little to get a fresh spot to be the center, but eventually the wear will get worse and you'll have a glitch every time you go the direction of the wear. I wouldn't risk that for a $35 servo.
Old 08-14-2011, 06:09 AM
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Default RE: Servo Chatter

CGretired gave you some good advice on pull-pull; do not have a lot of tension on the lines, as he said, just enough to not have any droop. Just as important, have a little positive Ackerman (that is where the line not being pulled becomes a little slack as you move off neutral position). Get that Ackerman backward (where the line not being pulled gets tighter the farther from neutral you get) and you can quickly ruin a servo and will increase current drain on the battery.

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