Re: Raspberry Pi, Real-Time and Ada

[Stephen Leake <stephen_leake@stephe-leake.org>]

"Rego, P." <pvrego@gmail.com> writes:

>> For spacecraft, this is based on a stability and pointing accuracy
>> analysis; if the control cycle were slower, the spacecraft would not be
>> stable, or would not point accurately enough. 10 Hz is usually adequate. 
>> What does the control cycle in a drone do? I suspect the plane is
>> basically stable by the design of the wings and tail, and the control
>> system is just running the steering.
>> Based on what analysis?
>
> Actually it depends highly on your design. 

Yes it does; that was my main point.

>My perception is that as
> smaller is the drone, faster you should be able to control it. For
> example, wonder a quadcopter 10m high, falling down, maybe because you
> (controller) lost communication. An autonomous recover-to-ground
> algorithm should in 1s: 1) identify communication is lost; 

Assume no communications in 3 cycles = lost; that leaves 7 cycles.

>2) assume
> override; 3) read sensors (as always); 4) calculate the best landing;

That all takes less than 1 cycle.

> 4) command devices and land. 

No problem; each of the 7 remaining cycles keeps the quadcopter level,
and moves it one step closer to the ground.

> Using a complete model you would need to solve some conditions for a
> 12-dimensions model. 

If that takes longer than 100 ms, you need a faster on-board computer.

> With 10Hz I would have only 100 iterations in 1s.

10 iterations; 10 Hz = 10 cycles per second = 100 ms per cycle.

>And all this should happen before the
> human sense realizes (so 1s is quite fast for us).

Right.

So 10 Hz is adequate. 5 Hz is probably ok as well, if your computer can
only manage that.

This is a quad-copter, which is stabilized solely by controlling the
rotor blades, and thus needs active control. 

I was talking earlier about a wing-and-tail design, which requires no
active stabilization; paper airplanes manage to fly :).

-- 
-- Stephe