First of all, let me know how you played with the JSBSim wing-leveler
example - I mean, did you use JSBSim in its standalone mode, or did you
somehow integrate this with JSBSim within FlightGear. I ask, because I have
never tried it within FlightGear, as I have not looked at how to make sure
the FlightGear autopilot and the JSBSim flight control/autopilot features
could be made to work independently - i.e. how to make a choice on which
capability to use. I'd be concerned that they might end up "fighting" each
other.
Yes, I used FlightGear. I did a cut and paste into the c172p.xml file from the example code in the manual. I had to modify it a bit: I rederected the output into the fcs/roll-trim-cmd-norm property (the example uses some property under ap/) I figured that the autopilot should use the trim to control the aircraft, is this correct/reasonable?
I'm pretty sure that the jsbsim autopilot and FlightGear's autopilot are not fighting each other. I haven't activated the FlightGear autopilot at all! And it works great (the jsbsim one), exept for the integrator windup.
Also, can you explain what you mean by integrator windup?
Integrator windup is a problem with all PID controllers. When the actuator, in our case the roll trim, goes into saturation (uses all available trim deflection), and still is unable to bring the wings level, the integrator keeps on integratin the error. Problems arise when the actuator (roll trim) goes out of saturation. By then the integrator has been winding up it's contribution to the PI controller output signal. The integrator then has to unwind, the time it takes to unwind ofcourse depends on how long it has been winding-up. While the integrator is unwinding the controller isn't working as it is supposed to.
I provoked this actuator saturation by pushing the stick to the right, the autopilot tried to counter this with the roll trim. As I pushed the stick further to the right eventually the roll trim was unable to keep the wings level. The roll trim goes into saturation (full deflection), but is still not able to keep the wings level. This is when the intergator begins winding up.
Hope this explains a bit!
I've run tests in
JSBSim with that component and it holds steady for quite a while until I
stopped the test. I am guessing you mean that the aircraft grows a bias over time.
When the wings are level and the actuator (roll trim) stays out of saturation, this PI controller works great. It does not grow a bias as long as the actuator is able to do it's job, it only grows a bias when the actuator does not have enough power (deflection angle) to do it's job.
The solution to this is to stop the intergation when the actuator goes into saturation.
-- Roy Vegard Ovesen
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