On Sun, Oct 11, 2009 at 3:39 AM, Alan Teeder wrote:
> First, thanks for the quick into to Flightgear´s autopilot.
>
> I think that your premise that autopilots need to run at very high frame
> rates is not realistic.
>
> When I first got into autopilots and simulation, back in the 60´s, all of
> our models had as the final element a 0.1 second low pass filter which
> simulated the control surface actuator. This turns out to be a reasonable
> approximation for most actuators and is very simple to implement both in a
> analogue computer (which is all we had then) and in digital ones.
>
> As there is such a filter in the inner loop means that there is no need to
> simulate anything which has a faster response time than this. Therefore
> there is no need to run the autopilot at high frame rates. 10 or 20 per
> second is perfectly adequate.
>
> Many of the outer loops (e.g. the heading mode) of the autopilot can in
> practice be run at much slower frame rates as the response of the aircraft
> is quite slow.
>
I think what Lee(e) may be observing is that when you tune an autopilot to
behave well when the update rate is 60hz, and then fly into an area of
complex scenery + effects the frame rate in FlightGear can drop
substantially. In this situation, the autopilot (which is running at the
same update rate as the graphics) can go unstable, leading to wild
oscillations. Presumably this is an entirely different issue than what Pete
is dealing with, and neither of these contradict anything you have said.
Regards,
Curt.
--
Curtis Olson: http://baron.flightgear.org/~curt/
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