On Tuesday 12 February 2008 17:21, Robin van Steenbergen wrote: > LeeE wrote: > > This is just for info and probably mainly directed at Roy > > Vegard Ovesen but it might also interest other people > > interested in setting up autopilot systems. > > > > As I've mentioned previously, I've had some problems tuning PID > > controllers, specifically controllers that maintain a specified > > pitch i.e. input is a target pitch and the output is an > > elevator-trim deflection. The problem is that the controller > > needs to be fairly 'brutal' in it's control at low speeds > > during take-off, to initiate rotation and prevent > > over-rotation, but this brutality tends to lead to oscillation > > at higher speeds > > Not to be particularly picky, but: > > Real-world aviation doesn't use the autopilot for take-off, > because of this very issue, which is present in real-world analog > controllers as well. AP is only engaged at acceleration altitude, > when the gear and flaps are up (clean). Using the AP for climbing > out in an unclean configuration at a very low speed will require > a high pitch reference and possibly induce stall. > TO/GA is sometimes used, though, but the TO/GA setting is a fixed > pitch reference, and only engaged after rotation, not on the > runway. One of the main reasons for that is also that the pilots > still have the ultimate control over the aircraft in case of an > engine failure. > > The 'strength' versus 'instability' issue is inherent to PID > control mechanisms, if you build a strong controller it will tend > to oscillate and jitter in situations where small control inputs > are required, because strong controllers are designed for large > control changes. It's a classic duality dilemma in control > engineering, and possibly the only way to compensate for it is to > implement adaptive parameters to the control system (e.g. the > 'strength' of the control laws reduce as the airspeed increases > or the angle of attack reduces).
Thanks for your comments - they fit with what I'm finding & thinking. Sure, in piloted aircraft takeoff is handled by the pilot, who is able to cope with a wide range of possible failures at this critical phase of flight, but I'm trying to work towards a UAV control framework, from takeoff roll to landing stop, so that's why I'm messing about with automated takeoffs, landings and scripted flight etc. I also find auto takeoffs, landings and scripted flight useful for general FDM config & controller tuning for piloted aircraft - it eliminates variations due to the pilot, in this case me:) LeeE ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
