> If i were to pick a root cause: inadequate testing.

If you are saying that we didn't throw enough things out of airplanes
then I couldn't agree more. :)

What do you suggest would count as proper testing?  Someone (alright,
me) can work on building tests before we even make the thing so that
we can do a really good round of testing as soon as we have a
prototype.

On Wed, Jul 8, 2009 at 3:45 PM, <rq1...@q7.com> wrote:
> (2009.07.08) kirk...@pdx.edu:
>> Quoting rq1...@q7.com:
>>> If i were doing it i'd use 4 mechanically independent servos. I estimate
>>> the reliability of doing this is about 1/2 of the single servo system
>>> being proposed. (Can we put the cool linkage drawing on the wiki page?)
>>>
>>> Despite the reduced reliability, i still think the 4 independent scheme
>>> is a winner.
>>>
>>>   * Reliability is still very high
>>>
>>>   * Mechanically ready for full flight control
>>>
>>>   * Mechanically simpler
>>>
>>>   * Same system that must be developed down the road anyway
>>
>> Add higher cost to the list.
>
> Certainly it might cost more. Probably no more than 400 $ extra.
>
> Keep in mind that the per-servo torque requirements might be
> considerably reduced in the 4 independent scheme.
>
>
>> The reason I was pushing for a linked system is because there were
>> reservations on the team about what could go wrong if the micro messes
>> up and the servos go out of sync. Keep in mind we have NEVER
>> successfully flown an ARM micro in 3 airborne tests.
>
> Yeah, what's up with all the ARM failures? (See Doug's question below.)
>
> The linked system is safer with respect to controller failure.  A
> controller failure in the independent system has the potential to make
> the flight path into a large radius circle, possibly at high spin rate,
> not good.
>
>
>> I will be happy to machine both the linked single servo system and the
>> four servo system, so multiplying the work required is not a big
>> problem. I really want to take baby steps on this, and I think the
>> additional  insurance is worth the work. We'll do the independent
>> version, but I don't think we should do that one first.
>
> I appreciate what you're saying.
>
> Since you're doing the work, you should do it the way that seems best to
> you.
>
> The linkage will be hard to get working well, but if it does work well
> it will be a thing of beauty.
>
>
>>> In the 4 independent scheme, the servos must be individually trimmed. To
>>> do this i would consider an absolute magnetic shaft encoder:
>>
>> Why add a shaft encoder when the existing servo positioning system gives
>> minute of angle precision? I think the additional sensor violates the
>> KISS principle. Modern digital servos are strong, fast, and accurate. I
>> plan to align the fins the same way RC helicopter blades are aligned;
>> using an inclinometer style pitch gauge.
>
> 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.
>
> --
> I just checked what i think is the Futaba site
>
>   http://www.futaba-rc.com
>
> I can't find a specification for pointing accuracy, which does not
> inspire confidence.
>
> Ditto for
>
>   http://www.rc.futaba.co.jp
>
> 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.
>
> Clearly doing anything but buying and using an off the shelf servo is
> way more work than desirable. I'd like someone to prove me wrong and
> show that off the shelf servos are just fine. Please.
>
> I did notice these guys, who are interesting
>
>   http://www.openservo.com/
>
>
> (2009.07.08) daus...@gmail.com:
>> What has been the root-cause of each of the three failed ARM flights? A
>> robust controller solution would seem to be a primary factor for the
>> roll-control project, with either a single- or a multi- servo approach.
>
> If i recall, first was either a blown GPIO pin or an odd reset due to a
> wiring problem.  Second was a blown GPIO.  Third was a firmware bug.
>
> If i were to pick a root cause: inadequate testing.
>
>
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