AUTOMATIC PUSHBACK FOR FLIGHTGEAR

Version 2.0.1

Copyright (c) 2018 Autopush authors:
 Joshua Davidson http://github.com/Octal450
 Merspieler http://gitlab.com/merspieler
Some of the code (c) FlightGear
Distribute under the terms of GPLv2.


This project aims to develop a generic pushback for JSBSim and YASim
aircraft, with the following characteristics.

1. Do the pushback procedure automatically.
2. Scale to different aircraft with minimum changes to their logic.
3. Use no computer resources in flight.


INSTALLATION

Minimum FlightGear version: 2018.0

NOTE: these steps cover the minimal testing setup. To get the most
realism out of Autopush, you will have to add logic aircraft-side as
described in the last step.

1. Copy the autopush.config.xml.template into your aircraft directory
and the Goldhofert.xml into <your-airfraft>/Models/Autopush/

2. Set up the pushback logic.

Add the following line under <sim><model> of your set.xml

<autopush include="autopush-config.xml"/>

Rename autopush-config.xml.template to autopush-config.xml and edit
the file as described below.

Alternatively, you can copy the contents of
autopush-config.xml.template to under <sim><model><autopush> of your
set.xml and make these modifications there.

Change the aliases to point to the following data:

steer-cmd-norm   nose wheel steering command norm
yaw              nose wheel steering output norm or angle (see below)
chocks           chocks command bool (or leave it until step 7)
available        pushback availability bool (or leave it until step 7)

Replace the MULTIPLIER in yaw-mult with:

max steering angle  if yaw is norm
1.0                 if yaw is degrees
57.3                if yaw is radians

Replace PITCH with the pushback's pitch relative to aircraft. If your
aircraft's ground pitch varies greatly, you can make an alias to
property with calculated relative pitch.

Replace MIN_RADIUS with the minimum allowed turning radius in m of the
center of the aircraft during pushback procedure.

NOTE: on some aircraft the turning radius is different for taxi and
pushback. Please check the aircraft's literature.

Replace STOP_DIST with the stopping distance in m from pushback speed.

Add under <nasal> of your set.xml:

 <autopush>
  <file>Nasal/Autopush/autopush.nas</file>
 </autopush>
 <autopush_driver>
  <file>Nasal/Autopush/driver.nas</file>
 </autopush_driver>
 <dynarr>
  <file>Nasal/Autopush/dynarr.nas</file>
 </dynarr>
 <autopush_route>
  <file>Nasal/Autopush/route.nas</file>
 </autopush_route>

Add under <sim><aircraft-data> of your set.xml:

 <path>/sim/model/autopush/route/show</path>
 <path>/sim/model/autopush/route/show-wingtip</path>

3. Connect the force to FDM.

 3.a. JSBSim:

  3.a.1. Add the following under <external_reactions> of your JSBSim
         XML.

 <force name="tractor" frame="BODY">
  <location unit="M">
   <x>FRONT-X</x>
   <y>FRONT-Y</y>
   <z>FRONT-Z</z>
  </location>
  <direction>
   <x>1.0</x>
   <y>0.0</y>
   <z>0.0</z>
  </direction>
 </force>

Replace FRONT-X, FRONT-Y, FRONT-Z with coordinates of your front
bogey.

  3.a.2. Add the following under <fdm><jsbsim><external_reactions> of
         your set.xml.

   <tractor>
    <magnitude alias="/sim/model/autopush/force-lbf"/>
    <x alias="/sim/model/autopush/force-x"/>
    <y alias="/sim/model/autopush/force-y"/>
    <z alias="/sim/model/autopush/force-z"/>
   </tractor>

 3.b. YASim. Add the following under <airplane> of your YASim xml.

 <thruster x="FRONT-X" y="FRONT-Y" z="FRONT-Z" vx="1" vy="0" vz="0" thrust="100000.0">
  <control-input axis="/sim/model/autopush/force-x-yasim" control="THROTTLE" src0="-1" src1="1" dst0="-1" dst1="1"/>
 </thruster>
 <thruster x="FRONT-X" y="FRONT-Y" z="FRONT-Z" vx="0" vy="1" vz="0" thrust="100000.0">
  <control-input axis="/sim/model/autopush/force-y-yasim" control="THROTTLE" src0="-1" src1="1" dst0="-1" dst1="1"/>
 </thruster>
 <thruster x="FRONT-X" y="FRONT-Y" z="FRONT-Z" vx="0" vy="0" vz="1" thrust="100000.0">
  <control-input axis="/sim/model/autopush/force-z-yasim" control="THROTTLE" src0="-1" src1="1" dst0="-1" dst1="1"/>
 </thruster>

Replace FRONT-X, FRONT-Y, FRONT-Z with coordinates of your front
bogey.

4. Add the pushback model to your Model XML:

 <model>
  <name>Pushback</name>
  <path>MODEL</path>
  <offsets>
   <x-m>FRONT-X</x-m>
   <y-m>FRONT-Y</y-m>
   <z-m>FRONT-Z</z-m>
  </offsets>
 </model>

Replace FRONT-X, FRONT-Y, FRONT-Z with coordinates of your front
bogey. Replace MODEL with one of the following file names:

Autopush/Goldhofert.xml  for Goldhofer towbarless pushback

Edit the "SETTINGS" part in the beginning of the Goldhofert.xml to match
your setup.

5. Add gui/dialogs/autopush.xml to your aircraft's menu (see
   FlightGear documentation for editing the menu).

6. Launch the simulator and try Autopush. After making sure it works,
   complete the support by implementing some logic in your aircraft
   and connecting the rest of the interface.

CAUTION.
1. Make sure to HAVE A BACK-UP COPY of your work.
2. If anything is not clear in the instruction below, do not guess --
   contact the authors.

 7.1. When pushback is connected ("/sim/model/autopush/connected"),
      nose wheel steering must actuate at different speed, depending
      on its rollspeed. The exact dependency is different per aircraft
      and used pushback visual, and for most aircraft the only way to
      find it is trial and error until it "looks right":
      - the input must be taken from pushback steering command norm
        (set by autopush-config.xml, default:
        "/controls/flight/rudder");
      - the output must be written to pushback orientation in degrees,
        radians or norm (set by autopush-config.xml, default:
        "/gear/gear[0]/steering-norm").

 7.2. Set or alias (in autopush-config.xml) the availability property
      "/sim/model/autopush/available". It must be false if e.g. the
      aircraft is moving too fast, the front wheel is not touching
      the ground, the aircraft is damaged or is outside of an airport
      etc.

 7.3. Make Autopush remove wheel chocks by setting the alias in
      autopush-config.xml to the chocks property. (default:
      "/controls/gear/wheel-chocks").

 7.4. Optionally, Autopush may be made visible in multiplayer by
      duplicating the properties used in its model.xml into some
      unused MP properties. See the list in

https://sourceforge.net/p/flightgear/flightgear/ci/next/tree/src/MultiPlayer/multiplaymgr.cxx

      around line 215, and then using those properties in the
      "SETTINGS" part of Autopush model.xml.

   After making these changes check the Autopush model.xml's settings
   and make sure they match the setup. Launch the simulator and try
   Autopush.


TUNING

There should normally be no need to change the coefficients, because
the inertia of different aircraft is already taken into account. But
the gear setup may differ enough to require some tuning.

If these settings are changed during simulation, they take effect
after disconnecting and reconnecting the pushback.

Pushback (/sim/model/autopush):

Coefficient  Unit                    Description

K_p          ((km/h)/s)/(km/h)       Proportional coefficient of PID.
                                     Defines the start/stop throttle
                                     and immediate response to speed
                                     difference deltaV = V_set - V.

F_p          (km/h)/s                Proportional clipping.

K_i          ((km/h)/s)/((km/h)*s)   Integral coefficient of PID.
                                     Defines how fast the steady
                                     throttle is ramped during the
                                     push.

F_i          (km/h)/s                Integral clipping.

K_d          -((km/h)/s)/((km/h)/s)  Differential coefficient of PID.
                                     Stabilizes the approaching of
                                     steady throttle.

F_d          (km/h)/s                Differential clipping.

Pushback driver (/sim/model/autopush/driver):

Coefficient  Unit        Description

F_V          km/h        Max towing speed in auto mode.
K_psi        1/deg       Amount of steering per heading correction.
F_psi        1/deg       Max steering per heading correction.
K_deltapsi   -1/(deg/s)  Amount of steering compensation per heading
                         derivative.
F_psi        1/(deg/s)   Max amount of steering compensation per
                         heading derivative.