Jittery servo's? - Page 2 - RCCanada - Canada Radio Controlled Hobby Forum
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Old 02-20-2006, 09:19 PM   #11
Wayne MIller
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Hi Warren,

People give many different explainations on the different threads, but I'll try and boil down what I think it is. I used to build and repair RC equipment many years ago - way back when proportional equipment first came out, when I lived in Saint John, N.B. Rather than get into the circuits, I'll try and keep what I think is happening in simple terms - hope you don't mind.

Typically, a servo has to identify where center is, it does this by a potentiometer (or capacitor feedback) directly connected to the bottom of the output gear of the servo. What happens is that when the servo reaches the point where it should stop, the potentiometer feeds this location back to the servo circuit. This is matched to the input signal from the receiver. If the area on the potentiometer (attached to servo output shaft) matches the receiver signal (which originally comes from the transmitter stick potentiometer position), this would then be the "electrical neutral" for the servo, and the servo stops (no more voltage to the servo motor).

If the servo overshoots this area (due to being too fast etc.) then it gets feedback from the potentiometer, it reverses and tries to go back to find the neutral area, if it overshoots the area in the opposite direction, then it goes back the other way - this back and forth "over shooting" cycle (called "hunting") that may continue for some time - causing what we perceive as jitter.

Sometimes, a manufacturer has delibertly tightened the "nuetral" area to make an extremely accurate servo, unfortunately this can backfire if the servo has to "hunt" for this neutral area. This would be due to the neutral area is so small, and especially if the servo is fast, and it overshoots before it can recognize the neutral zone. The older servos I worked on, did not have IC's and used transistors, capacitors and resistors. Therefore we could fix this "hunting for neutral" by widening the "neutral" zone window a little by adding a resistor.

When you added a slight physical resistance to the servo arm with your finger, you "dampened" (slowed) the servo just a little, and it doesn't overshoot as easily, and therefore it can find neutral easier.

I have the 811's and found that a 6 volt receiver pack gives the servo a little more speed and seems to cause the jittering to be worse.

Typically what I do is use standard servos anywhere where I don't need the speed or torque, such as on the throttle or rudder, and then limit the number of 811's in my planes to elevator and ailerons (sorry I don't fly helicopters, but I wish I could). This helps me a lot, I think this is because the 811's may create a little "electrical noise" when "hunting", which affects the other servos (however I can't prove this) and the fewer 811's you use in an aircraft, the less jitter you get - you may want to give this a try.

Hope the above helps.


Wayne Miller
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Old 02-21-2006, 05:21 PM   #12
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Thanks for the information Wayne. It makes a lot of sense. I've tried a number of different fixes and none seem to work, but when I plugged one of my older 537's into the receiver the jittering was gone. I bought the 811's specifically for the speed and power and it was what was recommended by my hobby supplier as a great servo for the tail of my heli.
I'm thinking I may just put my 537's back in and sell the 811's because I'm pretty nervous about flying with them and what may happen as a result. I just can't see it flying the way it is now. I wonder if Futaba digitals have the same problem? Anyhow thanks for the advice.

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