Fixedwing Configuration

This page describes configuration options specific to the fixedwing firmware in the airframe file.

Firmware and Hardware definitions

Select your Board

Make sure you use the fixedwing firmware and choose the correct board, e.g.

File: conf/airframes/myplane.xml

<firmware name="fixedwing"> <target name="sim" board="pc"/> <target name="ap" board="twog_1.0"/> ... </firmware>

Infrared Sensors

To use the IR sensors for attitude estimation add the attitude infrared subsystem:

File: conf/airframes/myplane.xml

<firmware name="fixedwing"> <target name="ap" board="tiny_2.11"/> ... <subsystem name="attitude" type="infrared"/> </firmware>

The ADC channels are already default to the correct ADCs for your board, but you can of course change the assignments:

File: conf/airframes/myplane.xml

<firmware name="fixedwing"> <target name="ap" board="tiny_2.11"/> ... <subsystem name="attitude" type="infrared"> <configure name="ADC_IR1" value="ADC_1"/> <configure name="ADC_IR2" value="ADC_2"/> <configure name="ADC_IR_TOP" value="ADC_0"/> <configure name="ADC_IR_NB_SAMPLES" value="16"/> </subsystem> </firmware>

Control loops

The control loops can be divided in two largely independent groups : the vertical ones and the horizontal ones (standard files sw/airborne/firmwares/fixedwing/guidance/guidance_v.c and sw/airborne/firmwares/fixedwing/stabilization/stabilization_attitude.c ). Those loops can be commanded at different levels by either the R/C transmitter or the autonomous navigation routine.

Just specify the appropriate subsystem in your firmware section. You can currently choose between no type (see below) and the types adaptive and new.

File: conf/airframes/myplane.xml

<firmware name="fixedwing"> <target name="ap" board="tiny_2.11"/> ... <subsystem name="control"/> </firmware>

XML Parameters

Commands

The commands lists the abstract commands you need to control the aircraft. In our example, we have only three:

<commands>
 <axis name="THROTTLE" failsafe_value="0"/>
 <axis name="ROLL"     failsafe_value="0"/>
 <axis name="PITCH"    failsafe_value="0"/>
</commands>

Each command is also associated with a failsafe value which will be used if no controller is active, for example during initialization of the autopilot board. The range of these values is [-9600:9600]. For "THROTTLE", the range is [0, 9600] and in the corresponding servo definition the neutral and min are usually the same (see below). Note that these commands do not necessarily match the servo actuators. For example, the "ROLL" command is typically linked to two aileron actuators.

Servos

The above commands get translated to the servos here. In the example below we use two elevons and a motor. (Elevons are surfaces used for both pitch and roll as on a flying wing.) These servos are listed in the servos section:

<servos>
 <servo name="THROTTLE"         no="0" min="1000" neutral="1000" max="2000"/>
 <servo name="ELEVON_LEFTSIDE"  no="1" min="2000" neutral="1500" max="1000"/>
 <servo name="ELEVON_RIGHTSIDE" no="2" min="1000" neutral="1500" max="2000"/>
</servos>

Names are associated to the corresponding real physical connector to which a servo is connected on the autopilot board. For example no="2" means connector two on the board. Also the servo neutral value, total range and direction are defined. Min/max/neutral values are expressed in milliseconds. The direction of travel can be reversed by exchanging min with max (as in "ELEVON_LEFTSIDE", above). The standard travel for a hobby servo is 1000ms - 2000ms with a 1500ms neutral. Trim can be added by changing this neutral value. Absolute servo travel limits can be increased or reduced with the min/max values. The "THROTTLE" servo typically has the same value for the neutral and min.

Note the following important tips:

• Reverse the servo direction by exchanging min/max
• Trim should always be adjusted mechanically if possible to avoid asymmetrical travel
• Any reduction of the total travel range should be done mechanically to maintain precision
• Many servos will respond well to values slightly outside the normal 1000-2000ms range but experiment carefully as the servo may not operate reliably outside this range and may even suffer permanent damage.
• Board connector numbering starts with zero (0) not with one
• Servos are also known under the synonym actuators

The servos are then linked to the commands in the command_laws section:

<command_laws>
 <let var="aileron"            value="@ROLL  * 0.3"/>
 <let var="elevator"           value="@PITCH * 0.7"/>  
 <set servo="THROTTLE"         value="@THROTTLE"/>
 <set servo="ELEVON_LEFTSIDE"  value="$elevator + $aileron"/>
 <set servo="ELEVON_RIGHTSIDE" value="$elevator - $aileron"/>
</command_laws>

Sign conventions for flight dynamics

where the third line is the simplest: the throttle servo value equals throttle command value. The other lines define and control the pitch/roll mixing. Elevon values are computed with a combination of two commands, ROLL and PITCH. This mixer is defined with two intermediate variables aileron and elevator introduced with the let element. The @ symbol is used to reference a command value in the value attribute of the set and let elements. In the above example, the servos are limited to +/- 70% of their full travel for pitch and 30% for roll, only in combination can the servos reach 100% deflection. Note that these numbers should add up 100% or more, never less. For example, you may want 100% travel available for pitch - this means if a roll is commanded along with maximum pitch only one servo will respond to the roll command as the other has already reached its mechanical limit. If you find after tuning that these numbers add to less than 100% consider reducing the surface travel mechanically.

Note that the signs used in the description follow the standard convention.

Manual

The rc_command sections links the channels of the RC transmitter (defined in the Radio Control file) to the commands defined above:

<rc_commands>
  <set command="THROTTLE" value="@THROTTLE"/>
  <set command="ROLL"     value="@ROLL"/>
  <set command="PITCH"    value="@PITCH"/>
</rc_commands>

This example looks trivial since the channel values have the same name than the commands.

RC commands in Auto

To control servos or other servo signal compatible devices by RC in Auto1 or Auto2, define them in the <auto_rc_commands> section. If you have an airframe with a dedicated rudder (YAW channel) then it is still controllable in auto mode via RC. This is the default behavior and is equivalent to setting the YAW command in auto_rc_commands:

<auto_rc_commands>
  <set command="YAW" value="@YAW"/>
</auto_rc_commands>

To disable this behavior (meaning no RC control of the rudder in auto) define an empty auto_rc_commands section:

<auto_rc_commands>
</auto_rc_commands>

Autopilot Only Commands

For certain missions it might be required to control servos (payload) from the autopilot (gcs) at all times (even during manual flight). These commands should not be in the <rc_commands> block but in the special <ap_only_commands> block. This allows for instance the pantilt operator to keep working when in manual flight, or safety logic to automatically close cameras below a certain altitude during manual landings.

<ap_only_commands>
  <copy command="PAN"/>
  <copy command="TILT"/>
  <copy command="SHOOT"/>
</ap_only_commands>

Auto1

The next section, named AUTO1, gives the maximum roll and pitch (in radians) allowed for the augmented stability mode.

<section name="AUTO1" prefix="AUTO1_">
 <define name="MAX_ROLL" value="RadOfDeg(35)"/>
 <define name="MAX_PITCH" value="RadOfDeg(35)"/>
</section>

Infrared

The INFRARED section describes the configuration of the infrared sensors.

The first definitions are relative to the electronic neutral of the sensors (a sensor here is a pair of thermopiles). A perfect sensor should give 512 if it measures the same value on both sides.

<section name="INFRARED" prefix="IR_">
  <define name="ADC_IR1_NEUTRAL" value="512"/>
  <define name="ADC_IR2_NEUTRAL" value="512"/>
  <define name="ADC_TOP_NEUTRAL" value="512"/>

These neutrals are tuned with the "cupboard test": Put the sensor in a close box (a cupboard) and read the values of the IR_SENSORS message (ir1, ir2 and vertical). Set the neutrals (they are subtracted from the measurement) to get null values. E.g. if you read 5 for the ir1 value with ADC_IR1_NEUTRAL equal to 512, change the latter to 517.

The next lines define the installation of the horizontal and vertical sensors. The vertical sensor must give a positive value when the temperature under the aircraft is higher than the temperature above. The two channels of the horizontal sensor must give positive values when it is warmer on the right side and the rear side. To adjust these signs, use the following declarations:

  <define name="IR1_SIGN" value="-1"/>
  <define name="IR2_SIGN" value="-1"/>
  <define name="TOP_SIGN" value="-1"/>

Then, define how the horizontal sensor is connected to the airframe, orientation aligned or tilted. In the aligned case, ir1 is along the lateral axis (The axis that passes through the plane from wingtip to wingtip) and ir2 along the longitudinal one. In the tilted case, the sensors are tilted by 45 degrees; ir1 is along rear-left -- front-right, and ir2 along rear-right -- front-left. The parameter "value" in the aligned and tilted definition has no effect! If the airframe construction allows choose an aligned sensor orientation since this gives the best stabilization response results.

For help with orientation of Previous Versions of Infrared Sensor Boards try here : http://paparazzi.enac.fr/wiki/Previous_Infrared_Sensors

  <define name="HORIZ_SENSOR_ALIGNED" value="1"/>
 or
  <define name="HORIZ_SENSOR_TILTED" value="1"/>

The three axis must give similar values for similar contrasts. The following factors can be used to scale these values. For example with an horizontal tilted sensor, the following ratios are usually needed:

  <define name="LATERAL_CORRECTION" value="0.7"/>
  <define name="LONGITUDINAL_CORRECTION" value="0.7"/>
  <define name="VERTICAL_CORRECTION" value="1."/>

Default values are 1.

It may be hard to align the horizontal sensor with the aircraft. A tuning in flight will be needed to adjust the following neutrals. Adjust the roll neutral to fly straight. Adjust the pitch neutral to fly level with the desired throttle.

  <define name="ROLL_NEUTRAL_DEFAULT" value="-2.5" unit="deg"/>
  <define name="PITCH_NEUTRAL_DEFAULT" value="6" unit="deg"/>

An asymmetric (left/right, front/rear) correction can be added with a last set of factors.

  <define name="CORRECTION_UP" value="1."/>
  <define name="CORRECTION_DOWN" value="1."/>
  <define name="CORRECTION_LEFT" value="1."/>
  <define name="CORRECTION_RIGHT" value="1."/>
</section>

These corrections are set on the angles. You don't have to set these as they are set to 1. per default, but adjust them if needed.

Gyro

Defines the type of gyro installed, each axis neutral, and any required temperature compensation. If the gyro has two axes, the pitch neutral is defined as well. Many gyros output their internal temperature and require a temperature-dependent linear correction be made to the neutral value. No correction is done for the temperature in this example.(ADC_TEMP_SLOPE=0).

<section name="GYRO" prefix="GYRO_">
 <define name="ADC_ROLL_COEFF" value="1"/>
 <define name="ROLL_NEUTRAL" value="500"/>
 <define name="ADC_TEMP_NEUTRAL" value="476"/>
 <define name="ADC_TEMP_SLOPE" value="0"/>
</section>

Horizontal Control

<section name="HORIZONTAL CONTROL" prefix="H_CTL_">
   <define name="COURSE_PGAIN" value="-0.4"/>
   <define name="ROLL_MAX_SETPOINT" value="0.35" unit="radians"/>
   <define name="ROLL_ATTITUDE_GAIN" value="-7500."/>
   <define name="ROLL_RATE_GAIN" value="-1500"/>
   <define name="PITCH_PGAIN" value="-8000."/>
   <define name="ELEVATOR_OF_ROLL" value="1250"/>
 </section>

The outer loop acts on the route. It will produce a roll command from a course setpoint and a course measurement. The COURSE_PGAIN parameter is the factor multiplied by the course error (in radian) to get a roll setpoint (in radian). So if the plane is expected to go north (course=0) and is actually flying to 57 degrees (course=1 radian, i.e. ENE), with a gain of -0.4, a roll of -0.4 (23 degrees) will be set for the lower control loop.

The ROLL_ATTITUDE_GAIN is used to compute a ROLL command from the roll error (setpoint minus measurement). If a gyro in installed, the ROLL_RATE_GAIN to keep a null roll rate. So these two gains provide a P-D controller.

The graphical representation of the control loops can help you to visualize the effect of each gain.

Vertical Control

  <section name="VERTICAL CONTROL" prefix="V_CTL_">
   <define name="ALTITUDE_PGAIN" value="-0.1" unit="(m/s)/m"/>
   <define name="ALTITUDE_MAX_CLIMB" value="3." unit="m/s"/>

These lines are associated with vertical control loops contained in sw/airborne/firmwares/fixedwing/guidance/guidance_v.c. These are outer loop parameters that calculate a desired climb rate based on altitude error. Here, if the altitude error is 10m, the climb setpoint will be set to 1m/s. ALTITUDE_MAX_CLIMB is a bounded value (in m/s) so that the outer loop does not calculate too large of a climb rate

   <define name="AUTO_THROTTLE_NOMINAL_CRUISE_THROTTLE" value="0.65" unit="%"/>
   <define name="AUTO_THROTTLE_MIN_CRUISE_THROTTLE" value=".4" unit="%"/>
   <define name="AUTO_THROTTLE_MAX_CRUISE_THROTTLE" value="1" unit="%"/>
   <define name="AUTO_THROTTLE_LOITER_TRIM" value="1000" unit="pprz_t"/>
   <define name="AUTO_THROTTLE_DASH_TRIM" value="-2500" unit="pprz_t"/>
   <define name="AUTO_THROTTLE_CLIMB_THROTTLE_INCREMENT" value="0.15" unit="%/(m/s)"/>
   <define name="AUTO_THROTTLE_PGAIN" value="-0.008" unit="%/(m/s)"/>
   <define name="AUTO_THROTTLE_IGAIN" value="0.25"/>
   <define name="AUTO_THROTTLE_PITCH_OF_VZ_PGAIN" value="0.35" unit="rad/(m/s)"/>

These lines are associated with vertical rate control loops contained in sw/airborne/firmwares/fixedwing/guidance/guidance_v.c and are used by default in most cases. The default vertical control law is for the vertical rate to be managed by a combination of throttle and pitch.

   <define name="AUTO_PITCH_PGAIN" value="-0.1"/>
   <define name="AUTO_PITCH_IGAIN" value="0.025"/>
   <define name="AUTO_PITCH_MAX_PITCH" value="0.5"/>
   <define name="AUTO_PITCH_MIN_PITCH" value="-0.5"/>

These lines are associated with vertical control loops contained in sw/airborne/firmwares/fixedwing/guidance/guidance_v.c but are not used in default. The non-default vertical control law is for the vertical rate to be managed by the pitch.

  <define name="THROTTLE_SLEW_LIMITER" value="2" unit="s"/>

THROTTLE_SLEW_LIMITER is the required time is seconds to change throttle from 0% to 100%.

The graphical representation of the control loops can help you to visualize the effect of each gain.

Misc

<section name="MISC">
 <define name="NOMINAL_AIRSPEED" value ="12." unit="m/s"/>
 <define name="CARROT" value="5." unit="s"/>
 <define name="KILL_MODE_DISTANCE" value="(1.5*MAX_DIST_FROM_HOME)"/>
 <define name="CONTROL_RATE" value="60" unit="Hz"/>
</section>

• The "NOMINAL_AIRSPEED" is mainly used in the simulator.
• "CARROT" gives the distance (in seconds, so ground speed is taken into account) between the carrot and the aircraft.
• "KILL_MODE_DISTANCE" is the threshold distance to switch the autopilot into KILL mode (defined descent with no throttle)
• "CONTROL_RATE" is the rate of the low level control loops in Hertz (60 or 20).

Simu

Values from this section can be used to tweak the software in the loop (SITL) simulation.

<section name="SIMU">
 <define name="WEIGHT" value ="1."/>
 <define name="YAW_RESPONSE_FACTOR" value ="1."/>
 <define name="ROLL_RESPONSE_FACTOR" value ="15."/>
</section>

• "YAW_RESPONSE_FACTOR" adapts the aircraft's turn rate corresponding to a bank angle; a larger value increases the turn radius
• "ROLL_RESPONSE_FACTOR" is basically your aileron efficiency; a higher value increases roll agility

If you want to use JSBSim as SITL simulator, you have to make some definitions in this section as well; see here.