(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

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


I can't find a specification for pointing accuracy, which does not
inspire confidence.

Ditto for


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


(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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