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
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--
Roy Vegard Ovesen
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