Replies: 7 comments 58 replies
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You could implement an autopilot which uses elevator to keep alpha constant and uses the throttle to keep altitude constant. |
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do you have a sample of autopilot? or the command that should I add? |
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Take a look in the JSBSim Reference Manual for some examples and explanations - http://jsbsim.sourceforge.net/JSBSimReferenceManual.pdf There are a handful implemented in the JSBSim repo. |
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maybe it is easier to implement a negative ramp of speed with a simple table with two lines...I did that but I do not know how to inform it to the simulator. |
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start_time KCAS_i like <function name="ic/vc-cruise">
<description>Change_in speed according to simulation time</description>
<table>
<independentVar>simulation/sim-time-sec</independentVar>
<tableData>
0.0 177.2
18000 165.0
</tableData>
</table>
</function>Today my simulation is running with <event name="Trim">
<condition> simulation/sim-time-sec ge 0.0 </condition>
<set name="ic/vc-kts" value="177.2"/>
<set name="ic/h-sl-ft" value="30000.0"/>
<set name="simulation/do_simple_trim" value="1"/>
</event>if I replace the line
with
It does not work. |
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The You can't simply change the airspeed at some time. It's read-only: jsbsim/src/models/FGAuxiliary.cpp Line 314 in 375f5be Even if you could, then immediately during the next timestep the aircraft wouldn't be in trim anyway so it would start rotating, translating etc. |
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I will try...need to learn... |
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What is wrong? Everything? Lol... <channel name="Alpha hold">
<!--
The difference between the desired alpha and the actual alpha
is determined, and limited to 1. The output from this component is
the desired alpha rate in percent of maximum.
-->
<summer name="aero/alpha-error">
<input> ap/alpha_setpoint </input>
<input> -aero/alpha-deg </input>
<clipto>
<min>-1</min>
<max> 1</max>
</clipto>
</summer>
<!--
The desired alpha rate is lagged slightly for stability.
-->
<lag_filter name="aero/alpha-error-lag">
<input> aero/alpha-error </input>
<c1> 1 </c1>
</lag_filter>
<!--
Dependent on altitude, the lagged (and limited) altitude error is multipled
by the scheduled gain determined from the table, below. The output from this
component is the absolute climb rate in feet/second. For example, if the desired
climb rate is 100 percent of maximum and the current altitude is 1000.0 ft., then
the output from this component would be 11 ft. sec.
-->
<scheduled_gain name="aero/alphadot-command">
<input> aero/alpha-error-lag </input>
<table>
<independentVar>position/h-sl-ft</independentVar>
<tableData>
0.0 0.12
1000.0 0.11
2000.0 0.10
3000.0 0.096
4000.0 0.093
5000.0 0.086
6000.0 0.078
7000.0 0.069
8000.0 0.061
9000.0 0.053
10000.0 0.045
11000.0 0.037
12000.0 0.028
</tableData>
</table>
</scheduled_gain>
<!--
This component calculates the alpha rate error, taking the difference between
the commanded alpha rate (from the previous component) and actual alpha rate
in degress
-->
<summer name="aero/alphadot-error">
<input> aero/alphadot-command </input>
<input> -aero/alphadot-deg_sec </input>
</summer>
<!--
If the altitude hold autopilot command is ON, then this switch component will
pass through the alpha rate error (from the previous component). Otherwise, it
will pass zero.
-->
<switch name="aero/ap-alpha-hold-switch">
<default value="0.0"/>
<test value="aero/alphadot-error">
ap/alpha_hold == 1
</test>
</switch>
<!--
The windup trigger below assumes the elevator will travel +/-10 per cent. The
elevator, however, does not travel symmetrically. This will need to be addressed
in a fix to the deadband component.
-->
<deadband name="aero/n2-trigger">
<input> propulsion/engine[0]/n2</input>
<width>0.20</width>
</deadband>
<!--
The integrator integrates the hdot error (when the switch component passes that
signal through above when the altitude hold is selected ON). In the situation
where the n2 becomes saturated, the integrator ceases to integrate. The
engine protection is indicated below, with the engine-trigger property being
the trigger to halt integration. When the engine trigger is non-zero (when the
elevator position falls outside the range +/- 10 per cent - a deadband of 20
per cent) then the deadband passes a non-zero value, triggering the anti-engine
logic in the integrator.
The proportional component multiplies the error signal by a constant, providingc
The pid component combines the proportional and integral control
signals. It clips the sum to +/- 1.0.
-->
<pid name="aero/alpha-hold-pid">
<input> aero/ap-alpha-hold-switch </input>
<kp> 100 </kp>
<ki> 2 </ki>
<kd> 20 </kd>
<trigger> aero/n2-trigger </trigger>
<clipto> <min>-1</min>
<max> 1</max> </clipto>
</pid>
<!--
The elevator component flips the sign on the output of the control summer
above and sets the ap/engine_command property.
-->
<pure_gain name="propulsion/engine[0]/n2">
<input> aero/alpha-hold-pid </input>
<gain> -1 </gain>
<output> ap/engine[0]_cmd </output>
</pure_gain>
<pure_gain name="propulsion/engine[1]/n2">
<input> aero/alpha-hold-pid </input>
<gain> -1 </gain>
<output> ap/engine[1]_cmd </output>
</pure_gain>
</channel>
|
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how to link with real engine? like the elevator cmd? It appear that is wrong at last part. |
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after finishing the autopilot I will submit a new PR |
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That would be most welcome ! Especially since we have very little examples of autothrottle control systems in our examples. The only example we have at the moment is systems/rpm_governor.xml which is used by the AH1S helicopter to constrain the main rotor to a constant RPM, and it is driving an electric engine (hence it has no lag to manage such as there is for a turbine or a piston engine). |
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Regarding the autothrotlle, I think that you should start simple and drop all the I guess that you don't plan to simulate asymmetric thrust so all you need from the autopilot is one thrust lever position (say <channel name="Alpha hold">
<summer name="aero/alpha-error">
<input> ap/alpha_setpoint </input>
<input> -aero/alpha-deg </input>
<clipto>
<min>-1</min>
<max> 1</max>
</clipto>
</summer>Here you may not want to clip your <pid name="aero/alpha-hold-pid">
<input> aero/ap-alpha-hold-switch </input>
<kp> 100 </kp>
<ki> 2 </ki>
<kd> 20 </kd>
<trigger> aero/n2-trigger </trigger>
<clipto> <min>-1</min>
<max> 1</max> </clipto>
</pid>The same applies to your PID: since you want a throttle command, the output value should be between 0 and 1. So the min/max values of Also I'd suggest to start setting The So your channel should look like this (:warning: this is untested code !): <channel name="Alpha hold">
<!-- Compute the error between the target alpha and the actual value -->
<summer name="aero/alpha-error">
<input> ap/alpha_setpoint </input>
<input> -aero/alpha-deg </input>
<clipto>
<min>-5</min>
<max> 5</max>
</clipto>
</summer>
<pid name="aero/alpha-hold-pid">
<input> aero/alpha-error </input>
<kp> 100 </kp>
<ki> 0 </ki>
<kd> 0 </kd>
<clipto>
<min> 0 </min>
<max> 1</max> </clipto>
</pid>
<!-- Forces the auto throttle value to 0 when ap/alpha_hold is different than 1.0
This allows to disable the autothrottle. -->
<switch name="ap/throttle-cmd">
<default value="0.0"/>
<test value="aero/alpha-hold-pid">
ap/alpha_hold == 1
</test>
</switch>
<!-- Drives engine #0 throttle position -->
<summer name="fcs/throttle-pos-norm[0]">
<input> fcs/throttle-cmd-norm[0] </input>
<input> ap/throttle-cmd </input>
<clipto>
<min> 0 </min>
<max> 1.0 </max>
</clipto>
</summer>
<!-- Drives engine #1 throttle position -->
<summer name="fcs/throttle-pos-norm[1]">
<input> fcs/throttle-cmd-norm[1] </input>
<input> ap/throttle-cmd </input>
<clipto>
<min> 0 </min>
<max> 1.0 </max>
</clipto>
</summer>
</channel>Also note that if you want to change the sign of |
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Wow...Fantastic..However I am not able to turn-off the autopilot to hold alpha... May be a problem in a switch |
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In other words you would have two PID controllers, the first one would control alpha by looking at the alpha-error and driving the elevator to zero the alpha-error. The second PID controller would control altitude by looking at the altitude-error and driving the throttle to zero the altitude-error. Pretty much the way aircraft carrier landings are performed 😉 |
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first plot is with altitude hold and alpha free |
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for my use, the solution with alpha and engine plus altitude elevator solves the problem.. It is very stable...But I do not know if you would like me to PR that. |
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Yeah, let's go for a PR with your stable controllers even if it does not meet aircraft carrier landing standards. |
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OK Good If you have any idea how to kill those sawtooth oscillations just let know. I will talk to my control friends also and see if it is easy to solve. |
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See the final result with alpha controlled by engine and altitude by elevator.
|
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In terms of constant alpha during cruise during a long flight, your original question, if it's to do with optimizing fuel mileage take a look at the following:
|
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OK...what do you suggest to keep the longest flight? |
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I am not interested to have a high Mach...the speed can be the optimum also |
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How to keep the angle of attack in a long flight? The fuel consumption decrease the aircraft weight so it changes the lift required and changes the angle of attack due to constant speed.
I have the L/D_max KCAS for heavy and light aircraft and it varies along the flight. Instead of KCAS constant, I would like to keep the alpha during the 5 hours flight to have the L/D_max condition.
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