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Help! Servos Move When Radio Connects

dpeach

New member
#1
I power on my servos and they go to, what I assume is, the center position. But, when the radio connects to the receiver, the servos change position.

I am not sure which is the actual centered position. I don't have one of the servo tools that let you run through the whole range of motion, but could do it with a bit of Arduino programming if needed.

But the real question is, is my thinking correct that when they power on that they always move to the centered position? And, if they do, then what is going on when I connect the radio to the receiver?

Here is some testing I have done. This may get confusing, so let me set up my test scenario.

Plane1 (the one with the problematic servos) is a Simple Soarer. It uses receiver1 with the plane1 model on the transmitter.​
Plane2 is an FT Tiny Trainer. It also uses receiver1 with a plane2 model on the transmitter. But it has its own servos.​

I thought that the problem might be something configured in my radio model on the transmitter for plane1. To test, while receiver1 is still in plane1 I switch to the plane2 model and I get the exact same behavior as with the plane1 model (that is, servos move from what I assume is the centered position to another position when the Tx connects to the Rx). Therefore, I conclude that the problem is with the servos and not the radio model since this is not a problem on plane2.

If all that seems confusing, you can just try to answer the bold question above.

Thanks for any light you can shed on this.
 

dpeach

New member
#3
you power the TX first.........then plug in the battery into the aircraft's esc....THIS IS YOUR CENTERED SERVO POSITION
That takes me to the same position I get when powering on the plane first and then the TX after the TX and RX pair up. So I'm getting a consistent position at least. And you've confirmed which of the two positions should be my center position.

Next question then, what is the position that the servos go to when powered on, but not connected to the transmitter? Is that a position I can adjust? Because I would like for that non-connected state to be closer to the connected (centered) position. It puts stress on my push rods and control surfaces any time the TX is not connected.

This is a plane I rebuilt recently. I don't remember it doing this in its previous iteration. But, I also can't definitively say that these are the same servos.

Thanks for the answer.
 

Wildthing

Well-known member
#4
That takes me to the same position I get when powering on the plane first and then the TX after the TX and RX pair up. So I'm getting a consistent position at least. And you've confirmed which of the two positions should be my center position.

Next question then, what is the position that the servos go to when powered on, but not connected to the transmitter? Is that a position I can adjust? Because I would like for that non-connected state to be closer to the connected (centered) position. It puts stress on my push rods and control surfaces any time the TX is not connected.

This is a plane I rebuilt recently. I don't remember it doing this in its previous iteration. But, I also can't definitively say that these are the same servos.

Thanks for the answer.
TX should always be on before you plug in the battery for safety , the motor could unexpectantly go to full throttle and the prop takes your fingers off or worse. Even then the servos will jump back and forth to center themselves when powered up.
 

Aireal Anarchist

Well-known member
#5
That takes me to the same position I get when powering on the plane first and then the TX after the TX and RX pair up. So I'm getting a consistent position at least. And you've confirmed which of the two positions should be my center position.

Next question then, what is the position that the servos go to when powered on, but not connected to the transmitter? Is that a position I can adjust? Because I would like for that non-connected state to be closer to the connected (centered) position. It puts stress on my push rods and control surfaces any time the TX is not connected.

This is a plane I rebuilt recently. I don't remember it doing this in its previous iteration. But, I also can't definitively say that these are the same servos.

Thanks for the answer.
most of my planes do this, turn on TX first :)
 
Last edited:

Ketchup

4s mini mustang
#6
That takes me to the same position I get when powering on the plane first and then the TX after the TX and RX pair up. So I'm getting a consistent position at least. And you've confirmed which of the two positions should be my center position.

Next question then, what is the position that the servos go to when powered on, but not connected to the transmitter? Is that a position I can adjust? Because I would like for that non-connected state to be closer to the connected (centered) position. It puts stress on my push rods and control surfaces any time the TX is not connected.

This is a plane I rebuilt recently. I don't remember it doing this in its previous iteration. But, I also can't definitively say that these are the same servos.

Thanks for the answer.
As people have already said here, the tx should always be turned on first for safety. Also, about what you said with your push rods and control surfaces being stressed. If your control surfaces and pushrods are stressed just because they are not centered, that is a problem. They should be able to always move freely.
 

Merv

Well-known member
#7
I agree with the others, turn the Tx on before plugging in the plane . AND unplug the plane before turning off the Tx.

The idea always have a powered plane under the control of your Tx.
 

LitterBug

Troll Spammer
#8
Make sure the trims and sub-trims are centered on the TX. THAT is the centered position. Ideally you will have it so it flys level with trims at zero and just use the trims to make minor adjustments.

Like others have said, ALWAYS power the TX on first unless you are binding and the binding procedure dictates otherwise. If you are binding or calibrating your ESCs, TAKE THE PROPS OFF FIRST!

Cheers!
LitterBug
 

Bricks

Well-known member
#9
As people have already said here, the tx should always be turned on first for safety. Also, about what you said with your push rods and control surfaces being stressed. If your control surfaces and pushrods are stressed just because they are not centered, that is a problem. They should be able to always move freely.

Exactly you need to mechanically change the throws so you do not over drive your control surfaces. When setting up control linkage have the transmitter on and receiver so the servos are centered before any linkage is installed, check trims so they are zeroed. You want to try and install the servos so there is no subtrim needed to center the servos to the servo arms, not always possible. Be sure all control surfaces are free moving thru the entire range you want the surface to move.

There are ways to get better servo resolution but will not get into that here it is how I set up all my planes..
 
#10
Thank you for the input everyone. Here is a video of what is happening:

Trims are centered. And I am powering TX before plane in the video.

What I mean by stressed is that when powering up the plane this particular servo jumps to about 90 degrees of throw from the centered position. That is much more than I would be giving it as input from the radio.

I hope the video helps explain what is happening. It just seems like it should not make this leap when powering up.

If I power it up incorrectly (plane before radio) then the servo goes to the extreme position and rests there until powering up the radio.
 

Bricks

Well-known member
#12
What you are showing us there is an incorrect servo setup, not saying cannot get it to work this way but not the right way to do it. You want to use as much of the servo travel as you can to the control surfaces this what is called resolution. The more resolution the more precise movements of the transmitter control stick to the servos. You do this mechanically by using different holes in the servo arm and the control horn. You want to get this as close as possible first then if need be go into your transmitter and adjust end point just to where the servo does not stall.

The way I set up my servos is first go into servo travel and turn them all the way up, my Spektrum is 150. Then take my servo arms and mount them so they are at 90 degrees to the servo, normally can mount the servo so this works ( some times space does not allow ) if not then I will use sub trim to center the servo arm. Next mount the servo, now set up the control rod what I want is to use as much of the servo throw when controlling the surface as I can. This may mean using the inner most hole on the servo arm and the most outer hole or visa versa on the control surface or even changing servo arms or control horns to utilize as much of the servo travel that I can. This a good start to set up servos correctly.
 

Merv

Well-known member
#13
Next question then, what is the position that the servos go to when powered on, but not connected to the transmitter? Is that a position I can adjust?
This depends on your model & brand of radio.

If I understand what is going on. @dpeach is not giving any input on the video. The Tx is on, and the servo reacts to just being pugged in. Could this have something to do with his fail safe.

I'm not sure how to set the failsafe on your brand of Rx, on mine it is set when you bind. You could try rebinding to see if it makes a differance.
 
#14
reduce your servo throw using your TX rates or end points features...
Here's another video showing what my actual travel is that is controlled by my radio. I have the endpoints dialed down to -85 and +80 on my radio outputs (this has been true this whole episode, not something new). I'm not going to the extreme travel distance that the servo jumps to when it is powered on.


@Bricks, thank you for the info about getting more resolution out of my servos.

@Merv, I guess it could be failsafe. But it does this to a much greater degree on one servo than on the other. And you are right that I was not giving any input in the first video. When I shot that video, I had not moved any servos from the time I had unplugged the receiver to when I plugged it in for the video. So it is jumping at power up even though the servo was already in the centered position.

TX is Taranis Q X7. RX is FrSky V8FR-II.

It just seems odd to me that one servo is reacting so differently to being powered on than the other one. And, on my other plane (different servos, same receiver) when powered on, if the servos were still in the centered position from being powered off the last time, they will jump only about 10 degrees.

None of this makes the plane unusable. But, I was getting ready to make a change to some things (move the servos and shorten the pushrods), so I thought I would try and figure out this extreme jump at power up.
 

Bricks

Well-known member
#15
When powering up the receiver and radio the servos goes to there extreme values until it boots up that is why you need to set up your servos and mechanical throws correctly. Other wise you end up with what you have stalling out the servos and bending pushrods.
 

Wildthing

Well-known member
#18
Do you have something you suggest I read about properly setting up the servos? Because I'm completely baffled as to why this one is jumping so dramatically.
If none of the other servos jump, swap the leads around on the rx and try again to see if it follows it.
 
#19
Sorry, I've been gone a couple of days.
If none of the other servos jump, swap the leads around on the rx and try again to see if it follows it.
That's a good suggestion. I tried that and the same servo is jumping even on a different channel. The other servo (this is a 3 channel setup) acts the same as it always has regardless of which channel.

Therefore, it seems that it is a servo issue and not something in the receiver or transmitter setup.
 

Wildthing

Well-known member
#20
Sorry, I've been gone a couple of days.

That's a good suggestion. I tried that and the same servo is jumping even on a different channel. The other servo (this is a 3 channel setup) acts the same as it always has regardless of which channel.

Therefore, it seems that it is a servo issue and not something in the receiver or transmitter setup.
Pop in one new servo then