Hi! I would like to join the discussion about AP tuning. I recently posted a message on the forum (http://www.flightgear.org/forums/viewtopic.php?f=4&t=6853) for b1900d. With enclosed files, action is quite smooth and autopilot seems stable, at least on my conputer. For vertical stabilization (selected altitude holding) I used an approach very similar to the one proposed by LeeE, with a first stage that controls vertical speed (I used a proportional only stage with <pi-simple-controller>). I also managed the configuration in such a way to share as much stages as possible for different modes, using a flag which activates these stages. I also used several noise-spike filters in order to avoid abrupt changes. For the problem of bouncing during transition between modes, I modified the script flightdirector.nas in such a way that when pilot switches on AP or changes mode all internal variables of the autopilot are reinitialized with the current attitude of the plane. In fact, for velocity mode PIDs the memory of the past is only contained in these variables, since there isn't an integrator.
I hope that it can be useful. Giuseppe leee wrote: > > The large initial deflections you can get when switching between > different controller cascades seems to occur because on the first > iteration the controller sees an error but has no 'trend history' > to know how effective its correction is, and so may command the max > correction. As the aircraft responds to the initial correction > though, the controller gets feedback upon which it then gauges the > correction for the next iteration. > > There are a number of ways around this, from limiting the flight > control surface rate, either in the FDM config, or by inserting a > filter between the output from the controller and the control axis > input of the FDM, to filtering the target inputs, to using common > pid controllers in the control hierarchy. > > I ended up using a three controller hierarchy in some aircraft, with > the same lower-level controllers being used by all modes and which > were always engaged; only the top level controllers were switched. > > For example, for altitude-hold I might have the following hierarchy: > > Altitude-hold: generates a target climb-rate in FPS from the > difference between the current altitude and the target altitude. > > Climb-rate-hold: generates a target pitch angle from the > difference between the current climb-rate and the target > climb rate generated by the Altitude-hold controller. > > Pitch-hold: generates an elevator deflection from the > difference betwen the current pitch and the target > pitch generated by the Climb-rate-hold controller. > > but then I'd use the same Climb-rate-hold and Pitch-hold controllers > for the glideslope-hold hierarchy and just switch the Top level > Altitude hold controller off instead of duplicating the entire > hierarchy again. > > Because the lower-level controllers are running continuously and are > not switched on and off they've got some trend history data to be > working with and this seems to ameliorate the problem by > effectively buffering the top-level input change. > > This type of hierarchy isn't without its own problems though; at low > airspeeds you might need the Pitch-hold controller to be pretty > brutal and almost driving into oscillation but at high speeds you > need it to be much more delicate. However, judicious use of > filters and variable gains can usually get you around most > problems. > > There is one type of autopilot problem that's more difficult fix > though: the autopilot controllers and filters should run at a > constant rate and although a <rate> parameter was added and > initially worked, I think it may have become broken at some point > and the rate at which the controllers and filters operate depends > more upon the frame rate than anything else. Because the rate at > which the controllers work is critical, depending upon the gains > used, a controller that might be stable on one system might become > unstable on a slower, or even sometimes a faster system (I've seen > this sort of problems happen both ways). > > LeeE ------------------------------------------------------------------------------ The Planet: dedicated and managed hosting, cloud storage, colocation Stay online with enterprise data centers and the best network in the business Choose flexible plans and management services without long-term contracts Personal 24x7 support from experience hosting pros just a phone call away. http://p.sf.net/sfu/theplanet-com _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
