On Friday 30 April 2004 13:59, Jon Berndt wrote: > I've made some more progress in building an example autopilot using the > JSBSim flight control components. I already have a wing leveler for the > C-172, but I added an altitude hold module last night. The idea - in words > - behind the altitude hold concept (at least the way I have implemented it) > is that once a target altitude has been set, one can determine the error > between where you are and where you want to be. This error term is limited > to 100, filtered with a slight lag, and then multiplied by 0.1 in order to > get a commanded HDOT (time derivative of altitude, or rate of climb) of 600 > ft/min.
This is a slightly unusual way of doing it, normally the commanded HDOT would be limited to 600 ft/min instead of the altitude error. But this approach works great too. > This error is run through the altitude hold switch, and either this > quantity or zero is passed to a proportional-integral controller (PI). The > output from these two components is summed, multiplied by an appropriate > gain, and the signal is sent to the elevator. I have a plot online of > altitude versus time as the C-172 is commanded to fly (from takeoff) to 800 > feet, then 850, then 600, then 2000 ft: > http://www.jsbsim.org/JSBSimAltHoldAP.pdf From the plot it looks like the altitude hold performs very well. But if you try another test where you control the throttle in such a way that the aircraft is unable to hold a 600 ft/min vertical speed. I think you will see that the integrator will wind-up as the "HDot Error" value never reaches zero. > > The autopilot is configured in JSBSim as follows: > > <COMPONENT NAME="Altitude Error" TYPE="SUMMER"> > INPUT -position/h-sl-ft > INPUT ap/altitude_setpoint > CLIPTO -100 100 > </COMPONENT> > > <COMPONENT NAME="Alt Error Lag" TYPE="LAG_FILTER"> > INPUT fcs/altitude-error > C1 0.25 > </COMPONENT> > > <COMPONENT NAME="HDot Command" TYPE="PURE_GAIN"> > INPUT fcs/alt-error-lag > GAIN 0.1 > </COMPONENT> > > <COMPONENT NAME="HDot Error" TYPE="SUMMER"> > INPUT fcs/hdot-command > INPUT -velocities/h-dot-fps > </COMPONENT> > > <COMPONENT NAME="AP Alt Hold Switch" TYPE="SWITCH"> > <TEST LOGIC="DEFAULT" VALUE="0.0"> > </TEST> > <TEST LOGIC="AND" VALUE="fcs/hdot-error"> > ap/altitude_hold == 1 > </TEST> > </COMPONENT> > This integrator will start winding whenever the elevator is saturating and still unable to achieve the commanded climb rate. > <COMPONENT NAME="Integral" TYPE="INTEGRATOR"> > INPUT fcs/ap-alt-hold-switch > C1 0.0041 > </COMPONENT> > > <COMPONENT NAME="Proportional" TYPE="PURE_GAIN"> > INPUT fcs/ap-alt-hold-switch > GAIN 0.035 > </COMPONENT> > > <COMPONENT NAME="Control Summer" TYPE="SUMMER"> > INPUT fcs/integral > INPUT fcs/proportional > CLIPTO -1.0 1.0 > </COMPONENT> > > <COMPONENT NAME="Elevator" TYPE="PURE_GAIN"> > INPUT fcs/control-summer > GAIN -1.0 > OUTPUT ap/elevator_cmd > </COMPONENT> > > -- end -- > > I've got some ideas for future enhancements, including a scheduled target > rate-of-climb, so that the aircraft does not try and achieve 600 ft/min > near its service ceiling or something silly like that. Also to be added is > an automatic cutoff or safety feature, and perhaps the use of throttle to > control altitude as appropriate. I guess I really need to read up on > specific A/P operation, but this is presently being modeled to give the > ability for JSBSim aircraft to fly automatic batch runs for testing, etc. > > I am going to include this in the JSBSim automatic flight document soon, > and will have a block diagram with this, too. > > Jon > > > _______________________________________________ > Flightgear-devel mailing list > [EMAIL PROTECTED] > http://mail.flightgear.org/mailman/listinfo/flightgear-devel -- Roy Vegard Ovesen _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
