(2009.07.08) kirk...@pdx.edu:
>[..]
>> My experience with off the shelf servos is they won't re-point with single
>> degree accuracy but i haven't tried the more expensive digital servos.
>
> Remember, our linkage ratio is not 1:1. The servo travels (180,,)  
> degrees. The fin requires (,36,50) degrees. That's how we get our torque 
> AND our accuracy.

Yes, increased angular resolution is an advantage of the external linkage.
I know you've been working on anti-backlash designs, so i expect the
actuator control will be precise.

In the single servo design the collective accuracy is a function only of
the linkage. A DC offset in the servo neutral will presumably be handled
by the controller, so there is no problem.

On the other hand, 180/50 ~4:1 so the per-servo torque requirements are
still about the same.

However, i'm willing to give up on this and concede defeat. Let us be
linked in ;)


>> Every servo i've ever taken apart had a sub-50 cent potentiometer in it.
>> Will that A) work. B) work under vibration, C) work reliably, D) work to
>> sub-degree precision? Honestly i think the answers are: Sometimes. No,
>> no, & no.
>
> I think you were working with crap $12 servos. I'll bring some real  
> servos for you to inspect. Do you think this guy could do this with an  
> unreliable vibration-prone servo?

(One of the servos was 35 $, still 30 cent pot.)

> http://www.youtube.com/watch?v=AqZ-mCd0HhM

That is a very impressive video.

I don't think the helicopter vibration environment is as harsh as ours.

The question of servo quality is still interesting.

Potentiometers tend to be noisy. They get noisy all of the sudden.
They temperature drift like carbon resistors. They get noisy in high
vibration environments. They are not reliable compared to standard
semiconductors.

I would feel better if Futaba made a servo with an optical or magnetic
encoder, but i have not been able to find one.

I would feel better if any hobby servo manufacturer specified pointing
accuracy, but i can't find that either.

--

As far as pointing accuracy goes, if the servo is only a degree or two
off, the controller can compensate. The maximum penalty will be somewhat
increased drag.

The only serious problem then is servo failure.

So i guess i should figure out how long a hobby servo has to operate
while on a shake table to prove it's a reliable device, and then try it.


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