servo glitches

[recon]

I have 2 hs-55 servos in my hummingbird heli. Recently whenever I turn it on one servo glitches like crazy! It will stay in one spot and just vibrate, then I move the stick it will hesitate, vibrate, then move and the same thing when I center the stick. Im going to try switching channels on the rx to see if it has anything to do with the receiver. If not, what else could it be?

[Dave Holmes]

Sounds like the feedback potentiometer wiper is making rotten contact with the static portion. You can fight with it, try to clean the pot (if you can get at the guts), put Stabilant 22(tm) on the contacts etc, but in the end I expect you may have to replace the servo, or risk the bird it is in.

Dave

[recon]

Umm, I think that went in one ear and out the other... lol. What is a feed back potentiometer wiper, and a static portion? Ive only taken one servo apart before and it was to replace the gears...

[airframe]

- Dirty servo pot

Mike

[Dave Holmes]

Sorry Recon, I forget that not everybody is an electronics tech.

A potentioeter is a variable resistor, such as the volume control in a stereo. There is typically a flat curved surface, which is a resistor without insulation on one side, and a a moveable contact touches that. (The wiper)

As the shaft of the potentiometer is rotated moving the wiper (like the volume control) the wiper contacts a diferent place on the static (unmoving) part, and the voltage at the wiper changes. If the wiper or the static contact get dirty or worn contact between the two becomes unreliable and the voltage that is "picked off" by the wiper becomes randomly varying depending on how good the contact is.

The voltage is used as a feedback signal for the electronics in the servo and tells it to drive either forward or back to achieve a balance in the circuit. If the feedback signal becomes unreliable, (jumpy or noisy) the electronics will be contiually trying to reposition the servo motor, causing the servo to jump about, and the feedback potentiometer moves with it, contacting a different spot on the pot, which changes the feedback signal, which induce a change in the drive, which changes the feed back signal....

I'm sure you get the idea.

Try swapping the two servos on the receiver channels. If the same Servo is noisy or chattering, the fault is in the servo. If the faulty operation stays with the reciever channel and the other servo is suddenly noisy, the fault is in the receiver.

Good Luck

Dave

[Lord Kimbo]

Are you running 6 volt Rx pack? Some servos (even if they are rated @ 6 volts) don't like 6 volts.

[recon]

This is in my century hummingbird heli and I have been running it on a 7.2 volt lithium pack for a while now with no problems until now.

[recon]

Ok, I think I found it... Now what? lol :?

[Propworn]

Cleaning is only temporary at best throw the dang thing out and buy a new one. Its liable to mess up at the worst possible time. Not worth risking a heli or plane.

Dennis

[recon]

How long are these potentiometers suppose to last? This servo was bought and installed last October, less than a year ago.

[Propworn]

How long they last depends on the installation, including how hard they are worked. Vibration will effect their life to. You have to remember you are using a servo for mini applications with only 15 oz of torque. If you have upgraded your heli with more powerful motor and battery that results in longer run times as well. I personally would send it off to Hitec and in the mean time buy another. When you get that one back (most of the time they just give you a new one) keep it as a spare.

Dennis

[orenda635]

Could this problem also be caused by the transmitter? I'm having the same problem and I think this is what's causing it.

I first noticed the problem on my Avistar a couple weeks ago. I'm using a Futaba Skysport 4 transmitte with a Futaba 7ch receiver and 4 standard servos. I was told it might be a weak RX battery so I replaced that with a brand new one out of my Tomahawk. It was still doing it. I thought it might be the TX battery. Tried it on my Tomahawk which has brand new never before used servos. It started doing the same thing. Replaced the battery in that. Seemed to stop when I tested it in the garage but when I took it to the field, it started again.

It seems to do it when I move the TX about. I even seem to be causing the problem. (remember that episode of Mr Bean where he's trying to fix the TV)

Any ideas on what's causing this?

[Dave Holmes]

It's definately possible. If the symptoms are unique to one or two servos, the servos are at fault. If the problem moves with the transmitter, the transmitter is at fault.

The controlling device in the transmitter is another pot for each axis of movement. If it is one particular servo that jitters, say the rudder, try swapping it with the elevator (on the ground/bench only!) at the receiver. If the problem moves to the other servo, it is not the servo. If you can make it become momentarily quite by adjusting stick position it would point to the transmitter, and possibly a dirty/worn transmitter pot.

Dave

[orenda635]

I'm guessing there may be dirt or oil that came off my hands that somehow may have gotten in there. The problem is with all servos. I have a funny feeling it may be the radio transmitter itself.

I did a simple test at the field yesterday. I placed the radio on the ground about 10m away from the airplane. When I stood in certain possitions it would start but in other positions it would stop. I could imput contol but it was fighting me. It's almost like the signal is breaking up before it reaches the reciever. The reason it jitters in when I move about is probably due to the natural electrical charge all humans produce that is distorting the signal even more.

[Nuker]

Didn't Recon just win the contest? Now he can order a new servo :D

Bob

[Dave Holmes]

How about it, Recon? Are you going to spend your "mad money" on a servo or two?

Dave