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Servo "waving"

Old 03-26-2014, 04:48 PM
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p3arljam
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Default Servo "waving"

Im stumped and not sure if im explaining this right. I have just finished a Kyosho BF109 as i was testing all the surface controls i noticed that the right aileron would "wave" up and down after i released the sticks. The movement is normal while i move the sticks but when i let go the left aileron returns to neutral and so does the right but it goes up and down on its own then back to neutral. Also this is the second servo this has occurred on i changed it out for a new one along with the servo extension wires.
Old 03-26-2014, 05:37 PM
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woodie
 
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Need more information.... are you using a Y connector to drive both servos or are they plugged into different channels on the receiver? Is the 'waving' a very rapid movement, or is it slower, more gradual movement? What is your power source for receiver and servos? Radio? Have you tried switching the two ailerons connection (ie plug left aileron servo into right aileron port and vice versa) to see if the problem stays with the right aileron or moves to the left aileron?

Detailed information helps people provide better/quicker suggestions for fixes.



Woodie

Last edited by woodie; 03-26-2014 at 05:44 PM. Reason: typo
Old 03-26-2014, 05:38 PM
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Truckracer
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I have seen a few servos do this with a full charged battery. Seems to be worse with heavy ailerons and servos connected with a "Y" harness. Usually putting just a bit of finger pressure on the "waving" surface will stop the oscillation. This could be signaling a problem of some sort or it could be completely normal. If slight pressure stops the motion, I would consider this normal.

More info about your setup would be helpful.
Old 03-26-2014, 06:12 PM
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p3arljam
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I am using a Spketrum DX7S no prior problems with it. My RX battery is a 6V 2500MAH brand new battery i switched the battery same problem. I am using a Y harness and tried switching and the same thing happened on the same servo. if i do soft slow normal flying movements they aileron works completely normal but pulling the stick left or right and releasing it is when it occurs.

Last edited by p3arljam; 03-26-2014 at 06:16 PM.
Old 03-26-2014, 06:23 PM
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woodie
 
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I would recommend eliminating the Y connector, run a standard extension to your right aileron and see if the problem continues. If the problem goes away, the Y connector is probably the culprit or contributing to the problem.

Using Y connectors has caused a lot of problems for people in the past. They don't always cause problems but their track record isn't good. If your receiver is capable of using 2 channels for ailerons, that is a much cleaner and much more easily adjusted setup. You can control total throw on each aileron, adjust differential, use the ailerons as flaperons, etc. Just an all around cleaner setup and it eliminates the problematic Y connectors.

Woodie
Old 03-26-2014, 07:14 PM
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Truckracer
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Originally Posted by woodie View Post
I would recommend eliminating the Y connector, run a standard extension to your right aileron and see if the problem continues. If the problem goes away, the Y connector is probably the culprit or contributing to the problem.

Using Y connectors has caused a lot of problems for people in the past. They don't always cause problems but their track record isn't good. If your receiver is capable of using 2 channels for ailerons, that is a much cleaner and much more easily adjusted setup. You can control total throw on each aileron, adjust differential, use the ailerons as flaperons, etc. Just an all around cleaner setup and it eliminates the problematic Y connectors.

Woodie
Agree 100%
Old 03-26-2014, 07:36 PM
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dirtybird
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The waving you are seeing is called servo hunting. Its usually caused by a worn potentiometer. It also could be caused by worn gears. Try changing the mechanical connection so you have to retrim with sub trim and see if it goes away. If not you may have to send it in for repair or replace it if it bothers you. It wont affect your flying.
Old 03-26-2014, 08:00 PM
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Truckracer
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Originally Posted by dirtybird View Post
The waving you are seeing is called servo hunting. Its usually caused by a worn potentiometer. It also could be caused by worn gears. Try changing the mechanical connection so you have to retrim with sub trim and see if it goes away. If not you may have to send it in for repair or replace it if it bothers you. It wont affect your flying.
Back a few years when "Y" harnesses were common and single servos such as on ailerons were not dedicated to their own receiver channel, this problem was very common. The scenario is usually the same ... a fairly free, heavy aileron connected to a servo with a fairly tight deadband. The servo is connected through a "Y" harness to the other aileron servo and the receiver. Add to that a freshly charged Nixx battery common at the time with a bit more than normal voltage and there ya go ... a bouncing aileron. Sometimes when one would start to bounce, due to the voltage changes across the "Y" harness, the other aileron would start to wag as well. No worn out servo, nothing really wrong, etc. The results were almost repeatable given certain servos. Remove the weight of the aileron from the servo with a bit of finger pressure and the wagging would stop. Most of us just ignored the problem after awhile as it caused no problems in flight. Most times, the problem would just go away after the radio was on for a bit and you rarely saw the problem after the first flight of the day. Recharge and the problem would come back for a bit.

Today with stronger servos and independent, one channel per servo setups the problem is much less common. Certainly worn servos can also cause this problem but as stated in my first post, if light finger pressure stops the wagging, there probably isn't a problem. As Woodie suggested, getting rid of the "Y" harness is highly advisable.
Old 03-26-2014, 08:30 PM
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p3arljam
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I removed the Y harness plugged the aileron into AIL channel and AUX channel on RX problem solved no more Y harness for me from now on.Thanks to all who replied.
Old 03-26-2014, 08:36 PM
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Truckracer
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Glad it was a simple problem. We all learn the same lesson sometime along the way. Back when flying smaller planes there was rarely a problem with "Y" harnesses.
Old 03-27-2014, 07:47 AM
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dirtybird
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I just use cheap servos with wide deadband. I use Y connectors. I never have a problem but I dont fly IMAC.
I used to be the Kraft rep for the Seattle area. We never had a problem with Kraft servos. In those days we only used one servo for the ailerons. I remember building a Goldberg Ultimate. I had one servo for all four ailerons.
I have a Lazy Ace now that I have flown for 10 years. I use a Y connector on both the top wing and the bottom wing. Two channels for four servos. I have a standard Futaba servo in all positions. The deadband of those is 13usec.
I dont remember how many flights it has. I have gone thru 2 sets of wings and three fuselages. It has all of the original servos and I have never had a problem.
If your flying has more than sport flying requirements, then you will just have to put up with more problems.
Hunting will not affect your flying but it will drain your battery.
Old 03-27-2014, 08:36 AM
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Tony Iannucelli
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I flew Kraft for a decade. The KPS14 and KPS15 servos were the standard back then. We might think they were good, but they were not even up to the standards of the $10 servos available today, in my opinion. Backl then, at $40 a piece, we thought we couldn't get anything better. I remember testing centering and travel on them. They were off 5-10% on right-left travel, and at least 3% on centering. Compared to the quality servos available today, it's understandable using a 'Y" back then would be no problem. We probably wouldn't have been able to tell the difference. We didn't even have SMT motors in the servos back then, let alone more than a few transistors. No comparison to today's technology, again, in my opinion.

I was just in the process of setting up the radio for a quarter scale plane on the bench, just checking servos. The "Y" was driving two Hitecs 7954s crazy, followed by two Ace 1213s. Separate links 'mated' to second channels completely eliminated all issues and put those servos dead on.

So, if I were you, I'd follow everyone's advice and don't use the 'Y's and if you have any doubt, swap out the offending servo. It's not worth potentially losing your plane.
Old 03-27-2014, 10:06 AM
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mikes68charger
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Another Reason not to use a Y harness, I had a Top Flite Gaint P47 and the ail keep gliching a little in the turns. I replaced the servos and it still happend. Then I was going to land, but did a sweet 20foot up speed pass, and out of no were the freaking P47 rolled right on to its back, I still had control as I was giveing elevator trying to save it but it was going to hit me and at that size, no thankyou. It turned out to be a short int he Y harness..


But Im currently working on a Bryon F16 and have 2 nice futaba servos 3305 in the full flying tails stabs, and When i go from full up were I let go of the stick, I can see one side over shoots just a tiny bit.

As they are not on a Y, I can only assume that the one servo is eather going bad or just not as good as the other right?
Old 03-27-2014, 10:36 AM
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essyou35
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I have this issue if I attempt to run servos on 6V instead of 5.
Old 03-27-2014, 10:44 AM
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Lone Star Charles
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I have a 95cc Extra that uses a pair of older Futaba FP S131S servos - one on each elevator half. They are Y connected to a Futaba R517FS receiver. After not flying the airplane for well over a year, I noticed the same problem that was described by the OP. One of the elevators would track the control signal perfectly while the other would oscillate for about three or four damped oscillations before tracking the control signal. This would occur at center, or off center. I tried replacing the Y connector as well as unplugging and plugging the connectors back into their respective slots and nothing seemed to work. I never figured out what the problem was. I still don't know. In any event, and since the elevators did track somewhat correctly, I flew the airplane for a few flights just to "shake down" all the remaining components and see what else might not work properly. After about four or five twelve minute flights, the elevators started working in perfect simultaneous operation - no oscillations. That was over six months ago. It still performs properly.

I still don't know what caused the oscillations in the first place, although I suspect it had something to do with the internal electronics of the servo and that something was off in the servo position feedback. I do not think that the problem was caused by the Y connector.

I may be way off on this, but as long as the wires and connectors can handle the added load, and as long as the servos are properly matched, I don't see why Y connectors wouldn't make a viable solution to controls that require two (or more) servos. I realize that there are a lot of advantages to use separate channels for each servo, but as far as Y's being a bad thing, it just doesn't make sense to me.
Old 03-27-2014, 10:45 AM
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Are they rated for 6V (5 cell) operation?
Pete
Old 03-27-2014, 11:19 AM
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dirtybird
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Originally Posted by Lone Star Charles View Post
I have a 95cc Extra that uses a pair of older Futaba FP S131S servos - one on each elevator half. They are Y connected to a Futaba R517FS receiver. After not flying the airplane for well over a year, I noticed the same problem that was described by the OP. One of the elevators would track the control signal perfectly while the other would oscillate for about three or four damped oscillations before tracking the control signal. This would occur at center, or off center. I tried replacing the Y connector as well as unplugging and plugging the connectors back into their respective slots and nothing seemed to work. I never figured out what the problem was. I still don't know. In any event, and since the elevators did track somewhat correctly, I flew the airplane for a few flights just to "shake down" all the remaining components and see what else might not work properly. After about four or five twelve minute flights, the elevators started working in perfect simultaneous operation - no oscillations. That was over six months ago. It still performs properly.

I still don't know what caused the oscillations in the first place, although I suspect it had something to do with the internal electronics of the servo and that something was off in the servo position feedback. I do not think that the problem was caused by the Y connector.

I may be way off on this, but as long as the wires and connectors can handle the added load, and as long as the servos are properly matched, I don't see why Y connectors wouldn't make a viable solution to controls that require two (or more) servos. I realize that there are a lot of advantages to use separate channels for each servo, but as far as Y's being a bad thing, it just doesn't make sense to me.
What happened to you is you had a dirty potentiometer. When you moved it back and forth a few times it wiped the dirt off and htings were OK again.
A servo is a very complex device. In order to get it to start and stop where you want it to you have to create velocity and acceleration feed back and apply it to the proper place in the electronics. If you know calculus, its the first and second derivative. Electronic parts have a tolerence. Sometimes the tolerence will add in the wrong way and you will get an oscillation. It certainly is better now than it used to be with integrated circuits.
If you want tight dead band servos you will have to put up with servo hunt sometimes, with or without a Y connector.
A voltage fluctuation caused by the other servo can upset the delicate balance of the servo and cause a servo to hunt. But its not the Y connector, its the servo that is the problem.
Think of my Lazy Ace. If I dont use Y connectors I will have to use four channels and have a rats nest of wires. And it will have to have a complicated Transmitter with 4 available mixes.
Old 03-27-2014, 11:36 AM
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I had the exact same problem on a Dirty Birdy. Tried many servos, always the same surface did that. Finally I replaced the servo extension with a heavy gauge extension and the problem went away. As soon as I saw the title of your thread I knew what you were talking about.

Dave
Old 03-27-2014, 01:03 PM
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Truckracer
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Lone Star, when using a "Y" harness with some of the newer receivers that have low voltage decoders, the two servos connected in parallel can load the signal pulse voltage down enough that one or both servos will not work at all or at best, very intermittent. This was very noticeable with some of the early and expensive Futaba 14 Ch. receivers when used with certain Hitec servos. Hitec came out with a modification for their servos that resolved the problem. Still, when more than one servo is connected through the "Y" harness with these receivers, the signal level is still loaded to a level where there is little headroom. Its just not good practice to use a "Y" harness with these receivers.

On your 517 receiver with a "Y" harness, you probably wouldn't have this problem as the pulse voltage is higher. My guess you just had one servo that had a slightly dirty pot and with use it cleaned itself up enough to operate normal.
Old 03-27-2014, 01:08 PM
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Truckracer
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Originally Posted by dirtybird View Post
Think of my Lazy Ace. If I dont use Y connectors I will have to use four channels and have a rats nest of wires. And it will have to have a complicated Transmitter with 4 available mixes.
Seems that I have read you have one of those fancy transmitters! I always thought the wiring got simpler when dedicating a receiver channel per servo. I can set up (4) channels dedicated to ailerons in just a few short minutes on most of the newer transmitters as it has become almost standard practice for larger airplanes.
Old 03-27-2014, 02:29 PM
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dirtybird
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Originally Posted by Truckracer View Post
Seems that I have read you have one of those fancy transmitters! I always thought the wiring got simpler when dedicating a receiver channel per servo. I can set up (4) channels dedicated to ailerons in just a few short minutes on most of the newer transmitters as it has become almost standard practice for larger airplanes.
Yes I have one of those "fancy" transmitters.But I didn't have when I got the lazy ace. Why change something that works?
If I had one of those Futaba receivers that puts out a critical low voltage, I would get rid of the receiver.
Old 03-27-2014, 04:28 PM
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Dave McDonald
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Waving (not jittering) servos are usually the result of a poor or corroded connection. The old Hobbico extensions were notorious for causing this problem. Here's an old 2006 thread about it.
http://www.rcuniverse.com/forum/rc-r...centering.html

So did the bad Y-harness come from Hobbico?
Old 03-27-2014, 04:59 PM
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p3arljam
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It was a JR Y harness after removing it and plugging the ailerons directly into the RX problem solved. It does not mean it was a bad Y harness it could be the voltage but not having to use a Y harness is just one less thing that can go wrong.
Old 03-27-2014, 05:05 PM
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dbsonic
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Agree with David and others regarding parasitics and drive strength/headroom. And some of the rx signal wire voltage swings are not actually full swing of the battery supply. I believe that is the case for many of the newer Futaba rx. There was a thread on it over the radio manufacturers forum.
Old 03-27-2014, 05:25 PM
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Truckracer
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Originally Posted by Dave McDonald View Post
Waving (not jittering) servos are usually the result of a poor or corroded connection. The old Hobbico extensions were notorious for causing this problem. Here's an old 2006 thread about it.
http://www.rcuniverse.com/forum/rc-r...centering.html

So did the bad Y-harness come from Hobbico?
Some of the newer HD Hobbico (now Tactic brand) extensions are some of the best on the market. I have tested just about every common brand and these test with the best of them. These use the same gold plated connector pins that Hitec uses on much of their stuff. Lots of junk extensions still available out there but there are more and more choices for good ones also. Just because a brand was bad a few years ago doesn't mean it still is today.

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