Re: Raspberry Pi, Real-Time and Ada

["Rego, P." <pvrego@gmail.com>]

On Saturday, February 8, 2014 5:28:43 AM UTC-3, Stephen Leake wrote:
> 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.

Well... I meant soft real-time and hard real-time; but that's all pure academic.

> But you are optimizing prematurely again; these are times for small
> parts of the overall control cycle. You need to start from the top.
I did not understand.

> How fast does the overal control cycle need to run? Once every second?
> 1000 times a second?
I think this would give a good response; but for the entire control cycle.

> 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. 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; 2) assume override; 3) read sensors (as always); 4) calculate the best landing; 4) command devices and land. Using a complete model you would need to solve some conditions for a 12-dimensions model. With 10Hz I would have only 100 iterations in 1s. And all this should happen before the human sense realizes (so 1s is quite fast for us).

> 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?

No camera for now. But it should be able to have one.

Regards.