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). ------------------------------------------------------------------------- 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
