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Old Nov 27, 2011, 02:23 AM
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New Zealand, Otago, Dunedin
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Servos twitch at full travel?

Hi all,

Just finished my first home built foamy (the free X31 from rcpowers.com) and flew it for the first time today. It flies fine most of the time (got a few circuits and didn't prang it but I've started seeing weird behaviour from the servos and receiver. When testing on the ground with throttle at zero if I throw the servos to full travel the receiver light blinks briefly red, and the servos centre themselves and then return to the correct position (and the receiver reconnects and goes green again) Can anyone shed any light on what might be causing this?

Specs:
Birdie 30A ESC
RCTimer BC-2216 2200kv outrunner
2 x EM 8g / 1.4kg / .12sec Micro Servo
Turningy 2S 1000mAh lipo.
Blitzworks RC 4 channel tx/rx

My initial thoughts (and suggestions from the web) are that the BEC isn't up to the task, but it's a 2A BEC on the ESC and it's only driving two small servos so I don't see why it should have a problem?

Thanks!

Cheers,

Craig.
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Old Nov 27, 2011, 05:39 AM
Bye Bye VP Aug 2010 - Aug 2012
Gerry__'s Avatar
United Kingdom, London
Joined Jan 2005
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Quote:
Originally Posted by OpusMcN View Post
Hi all,

Just finished my first home built foamy (the free X31 from rcpowers.com) and flew it for the first time today. It flies fine most of the time (got a few circuits and didn't prang it but I've started seeing weird behaviour from the servos and receiver. When testing on the ground with throttle at zero if I throw the servos to full travel the receiver light blinks briefly red, and the servos centre themselves and then return to the correct position (and the receiver reconnects and goes green again) Can anyone shed any light on what might be causing this?

Specs:
Birdie 30A ESC
RCTimer BC-2216 2200kv outrunner
2 x EM 8g / 1.4kg / .12sec Micro Servo
Turningy 2S 1000mAh lipo.
Blitzworks RC 4 channel tx/rx

My initial thoughts (and suggestions from the web) are that the BEC isn't up to the task, but it's a 2A BEC on the ESC and it's only driving two small servos so I don't see why it should have a problem?

Thanks!

Cheers,

Craig.
The servos are stalled at the end of travel and are pulling more amps as a result.
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Old Nov 27, 2011, 09:56 AM
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United States, NJ, Point Pleasant Beach
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Originally Posted by Gerry__ View Post
The servos are stalled at the end of travel and are pulling more amps as a result.
+1 to that, your BEC overload protection is kicking in. If your control surfaces moved easier it will stop doing that, but it's not a good solution. A good solution is getting a stronger BEC and removing the reason for binding.
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Old Nov 27, 2011, 11:52 AM
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Forney, TX
Joined Mar 2002
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Yeah, like the others said it sounds like it's binding. If your control surfaces have a max deflection of say 45 degrees before stopping and your servos are set to where they exceed that if pushed to max deflection then they will keep trying to push the surfaces past 45 degrees. It's very bad for the servos, it can cause the BEC to trip and can cause the servos to burn up. Remove the control wires or unbolt the servo arms and check the surface deflection by hand to see if they're binding/ stopping at a certain point. If so, then adjust your EPA (end point adjustment) to back the servos down so they don't hit that point. If your TX doesn't have EPA, then mechanically adjust your wires at the servo and/ or control surface horn to reduce the throws (move to a hole closer to the center of the servo arm at the servo, or farther from the control surface at the control horn).
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Old Nov 27, 2011, 03:52 PM
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Thanks everyone. I'll have to double check all the servos and connections. The control surfaces themselves are just beveled bits of foam held on with tape, so their limit of travel is pretty large (i.e about 180 degrees . It must be a combination of how the servos are connected or mounted or something... I'll go hunting and see what I can find.

Thanks!

Craig.
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Old Nov 27, 2011, 03:54 PM
Bye Bye VP Aug 2010 - Aug 2012
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United Kingdom, London
Joined Jan 2005
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Originally Posted by OpusMcN View Post
Thanks everyone. I'll have to double check all the servos and connections. The control surfaces themselves are just beveled bits of foam held on with tape, so their limit of travel is pretty large (i.e about 180 degrees . It must be a combination of how the servos are connected or mounted or something... I'll go hunting and see what I can find.

Thanks!

Craig.
Can you set the servo end stops on your radio? Can't why you would need so much in the way for travel.
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Old Nov 27, 2011, 05:56 PM
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Can you set the servo end stops on your radio? Can't why you would need so much in the way for travel.
No, the radio is the cheapest thing available that came in a package deal, with nothing apart from reversing switches. It's possible that the dubious quality of the transmitter might be contributing to the problem. Hopefully upgrading to a Turnigy 9X as soon as HK get them in stock.

I'm not trying to get that much travel out the the control surfaces, I'm just saying that the range of possible movement is much bigger than the amount they're being asked to actually move, so the problem probably lies in the servos, or connecting rods, or something like that.

Cheers!

Craig
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Old Nov 28, 2011, 01:19 AM
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Originally Posted by OpusMcN View Post
No, the radio is the cheapest thing available that came in a package deal, with nothing apart from reversing switches. It's possible that the dubious quality of the transmitter might be contributing to the problem. Hopefully upgrading to a Turnigy 9X as soon as HK get them in stock.

I'm not trying to get that much travel out the the control surfaces, I'm just saying that the range of possible movement is much bigger than the amount they're being asked to actually move, so the problem probably lies in the servos, or connecting rods, or something like that.

Cheers!

Craig
Yep, get rid of the radio for one that you can limit servo travel through programming.
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Old Nov 28, 2011, 05:31 PM
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Weird, I think it must be an issue with the servos themselves. I've disconnected everything and just running the bare servos on a bench they still cause the problem. It's also only happening when I really throw the control from one extreme to the other, if I slowly go from full down to full up on the controls it's fine, flick the stick from one extreme to the other and the receiver drops out and the servo reset....

My Bixler arrived last night so I plugged the wings into the this setup to test and those servos work fine with the everything else the same.

Might have to invest in a new pair of servos, and maybe not buy the absolute cheapest ones available this time?

Cheers.

Craig
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Old Nov 28, 2011, 05:56 PM
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Originally Posted by OpusMcN View Post
Weird, I think it must be an issue with the servos themselves. I've disconnected everything and just running the bare servos on a bench they still cause the problem. It's also only happening when I really throw the control from one extreme to the other, if I slowly go from full down to full up on the controls it's fine, flick the stick from one extreme to the other and the receiver drops out and the servo reset....

My Bixler arrived last night so I plugged the wings into the this setup to test and those servos work fine with the everything else the same.

Might have to invest in a new pair of servos, and maybe not buy the absolute cheapest ones available this time?

Cheers.

Craig
This is exactly what I said at the beginning. If you drive the servos until that can't physically go any further they will pull a lot of amps. Some servos are worse than others.

There's no way you could need the amount of travel you have described either.
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Old Nov 28, 2011, 06:02 PM
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United States, NJ, Point Pleasant Beach
Joined Mar 2009
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Quote:
Originally Posted by OpusMcN View Post
Weird, I think it must be an issue with the servos themselves. I've disconnected everything and just running the bare servos on a bench they still cause the problem. It's also only happening when I really throw the control from one extreme to the other, if I slowly go from full down to full up on the controls it's fine, flick the stick from one extreme to the other and the receiver drops out and the servo reset....

My Bixler arrived last night so I plugged the wings into the this setup to test and those servos work fine with the everything else the same.

Might have to invest in a new pair of servos, and maybe not buy the absolute cheapest ones available this time?

Cheers.

Craig
If you have a watt meter, measure how many amps does volt meter, look for a voltage drop from your BEC with the servos connected. Then try different servos or a different battery. I suspect the problem to be there. Bare servos should not stall your setup, no matter how fast you move them. That was one of the points I was trying to make earlier.
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Old Nov 28, 2011, 08:18 PM
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Originally Posted by Gerry__ View Post
This is exactly what I said at the beginning. If you drive the servos until that can't physically go any further they will pull a lot of amps. Some servos are worse than others.

There's no way you could need the amount of travel you have described either.
Sorry Gerry, I misunderstood your first post. I was looking for some physical obstruction outside the servo (like other people suggested), not a limit on the actual servo itself.

As I said, I don't need (and I'm not asking for!) that amount of travel, that's just what the hinge will easily swing through (if you remove the control rod) so it's not a physical limit on the control surfaces. They're only moving through +/- 30-ish degrees at full travel of the servo.

I'm guessing I'm just paying the price for cheap transmitter (with no servo limit) and cheap servos?
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Old Nov 28, 2011, 08:43 PM
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Originally Posted by Gedexas View Post
If you have a watt meter, measure how many amps does volt meter, look for a voltage drop from your BEC with the servos connected. Then try different servos or a different battery. I suspect the problem to be there. Bare servos should not stall your setup, no matter how fast you move them. That was one of the points I was trying to make earlier.
I'll check with my meter and see what the voltage is doing.

It's definitely dropping something with bare servos and fast movement on that setup. I've tried different batteries with no change, and different servos work fine. The high speed movement thing is confusing me, I guess either the tx/rx is overshooting the value when it's moved fast, or the momentum inside the servo is forcing it to travel too far?

Cheers.
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Old Nov 28, 2011, 08:49 PM
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United States, NJ, Point Pleasant Beach
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Originally Posted by OpusMcN View Post
I'll check with my meter and see what the voltage is doing.

It's definitely dropping something with bare servos and fast movement on that setup. I've tried different batteries with no change, and different servos work fine. The high speed movement thing is confusing me, I guess either the tx/rx is overshooting the value when it's moved fast, or the momentum inside the servo is forcing it to travel too far?

Cheers.
It's the servos drawing more than your BEC/battery is capable of. There is a slight chance that your servos are shorting out somewhere, but I doubt that. The most likely thing is a weak ESC/BEC (if it's in a single package).
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Old Nov 30, 2011, 11:18 AM
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I've had a similar problem with my micro boomerang; I've noticed that the rudder will sometimes lock into a hard right causing the plane to bank losing altitude. This has only ever happened during flight; luckily it hasn't caused a crash. When I go to inspect the plane i notice that the rudder is still locked, against the elevator tab and when I push the control stick to the left it has no effect (no sound), however as I push it the right it will "unlock" and move to its neutral position. How do you reckon I'd fix this?
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