Re: Raspberry Pi, Real-Time and Ada

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

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

> On Friday, February 7, 2014 6:28:23 AM UTC-2, Stephen Leake wrote:
>> Ok, good.
>> What is your application?
>
> In short, drones. 
>
>> What are your timing requirements?
> For the next application I think I will need achieve 1 ms for I/O
> access in hard RT, and up to 100 us for pure computation for soft RT.

I don't understand; the timing for "soft RT" is stricter than the timing
for "hard RT". What do "soft" and "hard" mean here?

Hmm. Perhaps they just mean "hardware" and "software"; I don't see why
that's a useful distinction.

But you are optimizing prematurely again; these are times for small
parts of the overall control cycle. You need to start from the top.

How fast does the overal control cycle need to run? Once every second?
1000 times a second?

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. 

If the drone is teleoperated, the requirement is the human sensing time;
you push the joystick, you want the plane to respond "quickly". Which
gives control cycle times of 0.1 seconds = 10 Hz. Although 1 second
would be tolerable.

If the drone is autonomous, that analysis does not apply. If the drone's
mission is to take pictures of targets, you can derive timing requirements
from the speed of the drone in flight and the speed of the camera. That
also gives location and pointing accuracy requirements; do you have GPS
on the drone? How does it know where the camera is pointed?

> Indeed. Yes, I will need much faster.

Based on what analysis?

-- 
-- Stephe