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Jun 11, 2007, 07:13 PM
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critterhunter's Avatar
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Discussion

Servo Returns To One Of Two Positions After Movement


I've been tracking down an on and off issue since last fall. This first showed up last fall with an HS81 servo in one of my elevons. It would always return to one or two positions based on your stick movement. It's always the same two spots, never a random new one. I swapped out that servo and the problem seemed to go away until this spring when it showed up in the same plane again. I replaced the faulty servo a few times until the condition went away. I thought the problem must be elsewhere (what are the odds of bad pots in so many servos?) and thought it might have been a loose servo compartment that I noticed was producing servo slop. When the problem once again appeared I thought maybe it's the RX, but swapping that out didn't help. Stuck another servo in and all was fine. Now the problem is showing up on another build (the elevator returns to one of the two spots based on stick movement of the elevator). This is with a different RX on that plane but I might have used one of the suspect servos from the Stryker. So, what is the most likely cause of this? I haven't ruled out a dirty servo pot since I haven't isolated those servos, but I'm leaning toward a problem with my JR Sport 600SX radio. Ideas or suggestions? If it's a dirty pot in the radio does anybody have any tips on cleaning it and with what?
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Jun 11, 2007, 09:20 PM
Dance the skies...
Tom Frank's Avatar
Quote:
Originally Posted by critterhunter
I've been tracking down an on and off issue since last fall. This first showed up last fall with an HS81 servo in one of my elevons. It would always return to one or two positions based on your stick movement. It's always the same two spots, never a random new one. I swapped out that servo and the problem seemed to go away until this spring when it showed up in the same plane again. I replaced the faulty servo a few times until the condition went away. I thought the problem must be elsewhere (what are the odds of bad pots in so many servos?) and thought it might have been a loose servo compartment that I noticed was producing servo slop. When the problem once again appeared I thought maybe it's the RX, but swapping that out didn't help. Stuck another servo in and all was fine. Now the problem is showing up on another build (the elevator returns to one of the two spots based on stick movement of the elevator). This is with a different RX on that plane but I might have used one of the suspect servos from the Stryker. So, what is the most likely cause of this? I haven't ruled out a dirty servo pot since I haven't isolated those servos, but I'm leaning toward a problem with my JR Sport 600SX radio. Ideas or suggestions? If it's a dirty pot in the radio does anybody have any tips on cleaning it and with what?
I've experienced a servo having a different neutral point depending on whether it was deflected one way or the other. But based on what you have said, I'm betting it's the pot in your radio stick assembly needing replacement or adjustment. I don't think they are user servicable.
Jun 12, 2007, 09:20 AM
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critterhunter's Avatar
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I swapped the servo to the rudder port on the RX and it still does the same thing, so I'm guessing it has to be the servo and not the TX stick...since the other servo that was in the rudder port didn't show this problem.
Jun 12, 2007, 12:12 PM
that's gonna hurt...
Double centering is a common problem for small servos used in DLGs as well as inexpensive servos in general. After tearing out a couple of bad servos, I bought a servo tester and now break in and test all servos on the tester before installation. I got a deal on the GWS tester and it has worked out well.

Bruce
Jun 12, 2007, 12:50 PM
H.A.L. 9000
checkenbach's Avatar
HS 81's are notorious for this, get another servo. FMA makes a great servo cycler to check servos before installation. Chris
Jun 12, 2007, 01:09 PM
Dance the skies...
Tom Frank's Avatar
Quote:
Originally Posted by critterhunter
I swapped the servo to the rudder port on the RX and it still does the same thing, so I'm guessing it has to be the servo and not the TX stick...since the other servo that was in the rudder port didn't show this problem.
Oh, OK, I agree, but sooooo many bad servos!? I had a Hitec HS-55 servo that did that once, too. It was replaced free under warranty by the LHS along with another that was bad out of the box. I never bought any more.
Jun 12, 2007, 01:24 PM
Hitec/Multiplex USA
MikeMayberry's Avatar
There is more at work here than just the servo... Yes some servos will center better than others and you pay for this with coreless motors, digital ciruits, etc.

Some things to consider about issues like this are the linkage geometry, radio end points & D/R. First you want to make sure that you are using full travel in the radio and not turning the EPA or D/R down to get the desired travel... use the linkage for this with the shortest servo arm possible. The control horn length on the surface should 1:1 with the servo horn if possible.

So... you must maximize the mechanical and electronic advantage for the servo or you're fighting a losing battle. Long horns and low EPA/D/R will seriously affect the centering capabilities of any servo.

Note that the HS-85 which is the same size as the 81 will be a better choice if looking for more precision. Also, the new digital HS-5082/5085's are due out soon and will offer improved performance for this size of servo. http://www.hitecrcd.com/images/Digital_Micro_Servos.pdf


Mike.
Jun 12, 2007, 04:34 PM
Registered User
Quote:
Originally Posted by critterhunter
I swapped the servo to the rudder port on the RX and it still does the same thing, so I'm guessing it has to be the servo and not the TX stick...since the other servo that was in the rudder port didn't show this problem.
Was servo still connected to same elevator/rudder/aileron etc when you swapped the lead? may well be the linkage. One hinge out of line and hinged surface will never center correctly, especially film hinges where hinge line is not ironed straight or there is a small warp in either surface which prevents surface from ever centering. Disconnect servo and move surface to extreme both sides and ensure it does not stick near center one way or the other and neither part is is seen to flex when released. Use of Pull-Pull systems (refer my web page) eliminates many binding issues.
Regards
Alan T.
Alan's Hobby, Model & RC FAQ Web Links
Jun 13, 2007, 11:08 AM
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critterhunter's Avatar
Thread OP
Out of six servos (with one yet to be tested) I found four with dirty pots. I've already cleaned and fixed one and have yet to try the others. No wonder I was having problems with figuring out the HS81 in my Stryker. I was starting to think it was an RX or TX problem. I tested them independent of any control linkages.
Jun 13, 2007, 12:52 PM
That thing's operational!
birdlives1955's Avatar
I'm pretty sure the HS-55 that I just installed in my Mig-29 foamy does this too. It'll end up in slightly different positions based on the direction of travel. Too bad, I have heard about centering problems with servos but never experienced it until now.

What's really a drag is that I've already closed up the fuselage and it's almost ready to fly! I certainly don't want to take the time and effort to open up one of the intakes and take the servo out now, so I think I'm just going to try and fly it that way. The difference isn't that big, and maybe there linkage is loose enough that it'll just sort even out in the wind during flight.
Jun 13, 2007, 04:17 PM
Can you spot the giraffe?
daleksw's Avatar
There's a particular problem with the HS81.

The shaft that drives the potentiometer has a flat ground on one side, so the shaft is D-shaped. The hole in the final output gear is the same shape so that it fits snugly on the pot shaft and the pot rotates when the output rotates.

Trouble is, the centre of the output gear is quite soft plastic (even on the 81-MG) and it's easily damaged, allowing the output to move without moving the pot. Sadly, a fair no. of HS81s seem to ship pre-damaged from the factory in this way. I recently binned a set of gears from a new from the box HS81-MG with exactly this problem. Replace the final output gear, being careful not to damage it, and the problem vanished.

David
Jun 14, 2007, 09:23 AM
Registered User
critterhunter's Avatar
Thread OP
Grand total: Out of 6 servos 4 or 5 had dirty pots. A few of these servos were almost new. I've cleaned and fixed 3 or 4 of them. One was giving me a new problem...no movement at all and getting hot. I broke it down and found the motor was very tight. Pulled the gears off and plugged the servo in, allowing the motor to spin since it was trying to find the neutral point and obviously couldn't since the gears were not in to move the pot. Put a few drops of oil on it's brass bushings and it ran it's self smooth. Fixed that one. After breaking the pot apart on these and cleaning it with brake cleaner and a Q-tip, I am also slightly bending the metal "feelers" up a little for better contact with the other surface. The ones that didn't "take" on the first cleaning are going to be broke down and scrubbed better.
Jun 16, 2007, 08:58 PM
Registered User
I just replaced the gears on an HS-81 servo. I'm definately having this problem with 2 different centering positions. Does anyone know what the 5th screw INSIDE the servo case does?

Its definately posible I damaged the output gear during assembly as I didn't recognize the D shaped mating surface at 1st. Strange thing is that I get drift in the center position with minor stick movement.

I'm thinking the screws may be too tight and the friction is binding up the servo.
Any input greatly appreciated.
Whorges, MEEP!


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