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adapting servo for retract

Old 05-01-2008, 01:06 AM
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apaloosa
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Default adapting servo for retract

Pals, i got a memory problem. sometime ago i read in one of the forums about a simple modification on a servo pot [adding 2 resistors] that enables it to run 180 degree usable as proportional retract. i did not write down any note and i lost track of the thread can anyboby refresh my memory. thanks
Old 05-01-2008, 03:28 PM
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A.T.
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Default RE: adapting servo for retract


ORIGINAL: apaloosa Pals, i got a memory problem. sometime ago i read in one of the forums about a simple modification on a servo pot [adding 2 resistors] that enables it to run 180 degree usable as proportional retract. i did not write down any note and i lost track of the thread can anyboby refresh my memory. thanks
Refer to sub section "Servo - Modify for Retracts, Number of Turns, Ganging & Continous Rotation " under "Servo - alterations, calculators, databases, repairs, convert to an ESC or winch & FAQ." at Alan's Hobby, Model & RC FAQ Web Links
Regards
Alan T.

Old 05-02-2008, 12:49 AM
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Campgems
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Default RE: adapting servo for retract

There is a big problem with that mod.

Basicly, servos work by matcing the pulse width they generate to the one the transmitter generates and trys to match the transmitter's pulse. The pot controls the pulse width the servo generates to compair to the one from the transmitter via the receiver. When the pulse width from the transmiter matches the pluse width from the servo, movement stops. If they are not equal the servo moves in the direction to make them equal again and then stops. The pot in the servo is the feedback to change it's pulse width.

So the way the two resisitor mod works its that it is basicly a pot with no movement. It generates a fixed pulse width and it is up the transmitter to find the match now. The servo is going to move from it's position if the transmitter pulse changes. As the servo can't tell how far it has moved, it keeps going until the transmiter pulse changes back to one matchig the fixed servo pulse.

Now, all of the serovos I've torn down for gear replacemsnt have a mechanical hard stop. The mod above is going to keep trying to move the servo even though it is at hard stop. Stalled servo and huge battery drain.

The other problem is that on power on, if the transmitter pulse width doesn't match the servo's fixed pulse width, the servo is off an running but not knowing where to stop. In addition, if the two do happen to match, what ever position the servo was in at power on it it's new neutral position. It can't tell where it's at.

This leaves you with the need to setup limit switches that will stop movement at a pre determined maximum move. I still haven't figured how to home, center, the servo without the pot feedback.

A retract servo probably works like this but with the liimits in place. As it is not a proportional servo, but a binay one. The transmitter needs to have a binary channel to operate it. I know the Futaba 9c has one such channel and the new 10C has two, demending on wherether is it 2.4 or another freaquency if I understand the spec on the 10C

Writing this, the thought came to me that I don't know how a retract servo is going to operate on a proportioinal channel, if at all. I'll do a bit of expermenting tomorrow as I'm just setting up a new plane and I have a retacct servo that I'm going to use "one of these days" I might as well see how it reacts to differnent inputs as long as I'm doing bench work anyway.

Don


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