Sample rate selection for digital control project

Yes we will be using MEMS sensors, at least at first because they are the "right price". This project is essentially a radio controlled toy.

Can you give me more details on the distinction between "delta-V" and acceleration, or "delta-theta" and rate gyro outputs, and how such sensors are able to get around the Nyquist "problem"?

Thanks Roy

Rate integrating gyros have an electromechanical assembly with a rate input, and a control loop that maintains the gyro wheel centered within the case. The rate output from the system is a filtered version of the rate command to the gyro itself. Nowadays this rate output is integrated for the duration of one sample time, so aside from control loop bandwidth issues it a fairly close measure of the angle through which the gyro has passed in that sample time.

Force balance accelerometers have a force (i.e. acceleration) input to the electromechanical assembly, and a control loop that maintains the proof mass centered within the case. The acceleration/delta-V output is very analogous to the delta-angle output of a rate integrating gyro.

One approach wit MEMS sensors would be to sample your sensors _fast_ (i.e. well above their bandwidth), sum up their readings within the controller's sample period, then decimate at the controller's sample period. It's basically a one-step CIC (cascade integrator comb?) filter.

Another approach is to just sample, and accept a high level of noise. This may be good enough, if you're just looking to keep the thing upright mostly.

Using a CIC filter (or some sort of averager) was our concept as well. I think it may have to wait for the 32 bit processors - I doubt the 8-bit can handle the iteration rates, and the ARMs seem to have some capability to do A/D and perhaps even averaging in the "background" (without requiring direct s/w intervention). We'll probably always be stuck with MEMS sensors, however.

I'm fairly confident we can get something to behave "good enough" with the AVR (similar to the video that Peter posted) from a radio- controlled standpoint. For fully autonomous operations we'll almost certainly need the 32 bit chips.

Thanks again Roy

You want autonomous? On the cheap? Seriously? Consider the difference in processing needed by an RC sailboat and an autonomous one. Controlling rudder and sail set is easy. Deciding what to set to is the hard part.

Jerry

...and they have lots of very smart but inexperienced students, who work 24/7 for weeks on end (with - mind you! - no pay), to get those things implemented.

No need to follow the universities' lead unless you are troubled by scores of idle PhD-level cybernetics students hanging around the front gate.

Not necessarily impossible, though. You could set a number of way-points, let the thing loose and have the autopilot visit them all in sequence.

Rune

Indeed, but I doubt that an 8-bitter that also acts as autopilot and rudder and trim servos is up to the added requirements.

In an R-C boat, the "skipper" controls the rudder and sail positions, appropriate to wind and intended destinations (your waypoints). An autonomous boat needs to sense the wind direction and make those decisions by computation. Updating the waypoints via GPS may not be accurate enough. Will we need onboard video to replace the skipper's eyes?

Jerry

I have no idea what is required to get an airplane to fly. Maybe some sort of 'dithering' of the controls? Lack of precision in individual corrections is compensated by sheer number of corrections?

You questioned the OP's ambition of coming up with an autonomous aircraft, and then compared it with controlling a sailboat. I commented on the OP's ambition for the aircraft and disregarded your comparision with the sailboat.

Anyway, the premise for the comparision is flawed: The aircraft has an on-board power plant while the sailboat does not.

Rune

As Rune mentioned Roy's problem is probably simpler, or at least less subtle.

I suspect that if you know the boat's speed, position and direction, and if you know the wind speed and direction relative to the boat, that you have all the information that you need to at least competently handle the boat -- doing it with real flair would take either lots of programming, or a really good human skipper.

...

A boat moves in two dimensions, an aircraft in three. I thought that a simpler problem might make the difficulty of autonomy clearer. Despite a few years of DARPA-sponsored competition among university teams, no autonomous automobile is robust enough for serious consideration.

So consider an autonomous power boat, then. Power or sail, autonomy is the sticky part.

Jerry

...

Maybe we're using the same word to mean different things. I don't think of an autopilot as creating autonomy. A system that changes course from waypoint to waypoint and avoids unanticipated obstacles is nearly there.

As for wind speed and direction relative to the boat, that changes with the boat's speed through the air. On most tacks, a sail's function is redirecting air that would pass over the side over the stern instead. (When before the wind, a sail slows air that passes over the bow.) Efficient sails are traditionally analyzed as airfoils, but I think of them as turbine blades. Their leading edges should face into the apparent wind and their trailing edges should be just to windward of the stern. Their curves are intentional, not evidence of the difficulty of pulling them flat.

Nevertheless, one could sail adequately with a perfectly flat sail. For every real fore-and-aft sail, there is a flat model that acts similarly. That flat sail nearly aligns with the boom. I think it's easy to see that the best set for that sail bisects the angle between the apparent wind and the centerline of the boat. You might not win races by setting the main boom to bisect the angle between the centerline and the telltale, but you'll get where you want to go and pretty handily at that.

My (analog) RC model boat had radio control for the rudder, but servoed the sail according to the principle above. To keep it simple the boat was cat rigged (one sail, no jib) and to make it simpler yet, the sail was trimmed by rotating the mast-boom assembly with a worm gear located below deck. No sheet, no "ropes" to tangle or go slack. As long as the wind was up. it would go within 45 degrees of it; any closer, and I had to tack to get there. *But the rudder was in my hand.* It may have had an autonomous sail, but it was not an autonomous boat.

Jerry