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Servo calibration issues

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Old 09-04-2012, 09:26 PM
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frets24
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Default Servo calibration issues

While setting up the flaps on a current warbird project I found that at full flap deployment there was quite a large descrepancy in the flap angles between the two.

I double checked to insure that I was using the same hole in both of the control horns and both servo arms as well as the arms were indexed at the same position with the flaps retracted. Both are identical in the set-up.

This led me to double check the ailerons and I found that, again with symetrical set-ups, both travelled the same to the down side but one travels 1/8" more than the other to the up side.

I am using Futaba 3004 sevos, Specktrum Dx7 system, each servo pair on a single channel with a standard "Y" cord, no reversers, chokes, limiters, slowdowns or other in-line accessories. The trim and subtrim on both channels in question is null and the reversing function is "normal".

Is it common that servos of the same type have such differences in the calibration? Is there a pot that can be adjusted internally to address the calibration? Being analog, I don't think they can be programmed. What, if any, are the options; short of going dual channel and using sub trim(which I don't have the an extra channel for anyway) or using some type of in-line limiter/adjuster, which I really don't care for?

Maybe grab a dozen servos and measure them to try to "match" a couple of pair?

The flaps can be evened out mechanically by using different index positions and different holes on the arm but the ailerons aren't going to be "fixed" that way because it's just the CCW rotation on one servo that is out of wack with the other.

Thanks!
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Old 09-04-2012, 11:25 PM
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Default RE: Servo calibration issues

Ive used quite a few of the 3004s in the past.I know exactly what your talking about.They were pretty good and affordable at the time.I did find on more than one model when they were used in pairs they didnt travel the same.Even with 2 new or basically the same age.I had one trainer that I always fought to keep the flaps working the same.Swapping servos to mix/match and get 2 that worked the same.Maybe it is the pots in them or just an inherent way they act.They worked ok for a standard entry level servo.They seemed to like the 4cell(4.8volt) packs better.They just didnt work as well moving forward and running 5cell(6volt) packs.I had several of them get really bad.Not failures but they just wouldnt drive out to the ends or one end reliably all of a sudden.I kind of phased them out and went to 3151s.Ive had alot better luck with them.
I have taken the 3004s apart and cleaned the pots with marginal success.

I would think about finding some more reliably accurate servos for applications where they are used as pairs.
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Old 09-05-2012, 05:58 AM
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Default RE: Servo calibration issues

It is not surprising that there is a difference in the throw on servos. In order for them to be close, the components inside the servo would become very expensive as there are several resistors,capacitors and a potentiometer in each one which would have to be identical in value in all the servos; i.e. matched to about 1% or better. This would drive the price to the point where few would buy them. I personally think we get a great bargain in most of the electronics we use in the RC field and, where necessary, not that difficult to pick and match parts where the action of two servos must be close or identical. Compared to just a few years ago, RC electronics are probably the biggest bargain we hobbiests have ever had.
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Old 09-05-2012, 10:33 AM
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Default RE: Servo calibration issues

All servos use an integrated package for its electronics. I would be surprised if there were any difference in response of those. That pretty much leaves the potentiometer as the culprit. I would get rid of those servos if the potentiometer is that bad.
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Old 09-05-2012, 10:46 AM
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Default RE: Servo calibration issues



The short answer is Yes, it is hard to find perfectly matched servos, including digitals. It's why JR offers Match Boxes, Hitec offers a programmer and Smart Fly has Equalizers. It's also why radios with more channels are popular - so that you can get rid of the Y harnesses and the problem by matching servos in the Tx via mated channels,subtrim and end points (after a good mechanical setup).

Idealy, you put the 2 flap servos on separate channels, mate the channels in the Tx and then do a bench setup where you set up the servo centers and end points to match exactly. Then, put them in the airplane and do a final setup/match.

For me, Y harnesses are a last resort for when I'm out of channels. You sound like you might be maxed out as well.

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Old 09-05-2012, 03:04 PM
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Default RE: Servo calibration issues

Dirty bird,
The servos are new/previously unused so I suppose I could swap 'em out at the LHS although they are centering and holding just fine.

Thud,
With all of the accessories and programable servos I was inclined to believe that was the case. Just inherent in the nature of the devices. As for mating the channels; You're right, I'm maxed out on this one; standard 4 + gear flaps and lights =7ch Might be time to seriously consider a 12 ch. I dont mind "Y" cords so much but I do shy away from other in line devices, though most of the reversers that I have seen have an adjuster in them to match the servos...Wouldn't help in this case though. I don't need to reverse anything in the aileron or flap chain due to the installation already acounting for throw and direction.
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Old 09-05-2012, 03:19 PM
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Default RE: Servo calibration issues


ORIGINAL: Thud_Driver



The short answer is Yes, it is hard to find perfectly matched servos, including digitals. It's why JR offers Match Boxes, Hitec offers a programmer and Smart Fly has Equalizers. It's also why radios with more channels are popular - so that you can get rid of the Y harnesses and the problem by matching servos in the Tx via mated channels, subtrim and end points (after a good mechanical setup).

Idealy, you put the 2 flap servos on separate channels, mate the channels in the Tx and then do a bench setup where you set up the servo centers and end points to match exactly. Then, put them in the airplane and do a final setup/match.

For me, Y harnesses are a last resort for when I'm out of channels. You sound like you might be maxed out as well.

Yes there are differences between servos. Matching devices are required to prevent servos ,when in parallel, from fighting each other. But those differences are hard to see. You wont see any difference when in full deflection.
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Old 09-05-2012, 03:54 PM
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Default RE: Servo calibration issues

[/quote]
You wont see any difference when in full deflection.
[/quote]


Sorry, but I've found this not to be true for most every pair I've matched. After bench matching and installation, I put "stick" extensions on the surfaces roughly 10-12" long and do a visual mechanical match and then an electronic match for neutral and end points. Surprising how much difference you find that way.
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Old 09-05-2012, 05:25 PM
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Default RE: Servo calibration issues

I can't remember who showed me or where I saw that method of using sticks, or in my case drinking straws, to make a "tattle-tail" (mis-spelling intended) for control surfaces. Works great.
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Old 09-05-2012, 08:54 PM
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Default RE: Servo calibration issues


ORIGINAL: Thud_Driver
You wont see any difference when in full deflection.
[/quote]


Sorry, but I've found this not to be true for most every pair I've matched. After bench matching and installation, I put ''stick'' extensions on the surfaces roughly 10-12'' long and do a visual mechanical match and then an electronic match for neutral and end points. Surprising how much difference you find that way.
[/quote]

You must use cheap servos
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Old 09-06-2012, 10:12 AM
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Default RE: Servo calibration issues

JR digitals - 3421, 8411, 8611, 8231, etc. plus some Futaba, Hitec & Airtonics digitals. The analogs go on the gear and brakes in the jets.
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Old 09-07-2012, 06:53 AM
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ORIGINAL: Thud_Driver

JR digitals - 3421, 8411, 8611, 8231, etc. plus some Futaba, Hitec & Airtonics digitals. The analogs go on the gear and brakes in the jets.
I tested the 8611 several years ago as a part of testing I did for an article. Its deadband is 3 to 5us.
If driven from a y-harness there should be just barely discernible difference with a 12" stick mounted on the control surface. If you are driving the servos from separate channels the difference is due to the difference in the channels and not the servos.
I also tested the linearity of several servos. I was surprised how good they were,even the low cost servos.
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Old 09-07-2012, 04:52 PM
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Default RE: Servo calibration issues

Yeah, it's a funny thing.  Recently had 2 JR DS3421's go into a jet.  Mated elevator channels, Airtronics SD-10G.  After a little subtrim to match the arms, they were as perfect as I've seen.  In the airplane, I got them mechanically so they were dead matches at center and full down on the sticks, but ended up with a 5% difference in the Up EPAs.  Too much time to figure out where the error came from, just fixed it in the radio.
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