Fixedwing Configuration

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Revision as of 12:20, 15 September 2013 by Mbina (talk | contribs) (added general AHRS info; minor changes;)
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Airframe Configuration
Modules
Subsystems
Airframe Configuration
Autopilot generation
Fixedwing Configuration
Rotorcraft 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>

Attitude and Heading Reference System (AHRS)

Next the filter for attitude estimation needs to be selected, where the tratidtional approach is to use infrared sensors (see blow). To select the type of AHRS use the subsystem-tag as follows:

File: conf/airframes/myplane.xml 
  <firmware name="fixedwing">
    <target name="ap"  board="tiny_2.11"/>
     ...
    <subsystem name="ahrs" type="int_cmpl_quat"/>
  </firmware>

Infrared Sensors

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

File: conf/airframes/myplane.xml 
  <firmware name="fixedwing">
    <target name="ap"              board="tiny_2.11"/>
     ...
    <subsystem name="ahrs"     type="infrared"/>
  </firmware>
  <modules>
    <load name="infrared_adc.xml"/>
  </modules>

See the infrared module page for more details on configuration.

Control loops

The control loops can be divided in two largely independent groups:

  • The vertical loop (sw/airborne/firmwares/fixedwing/guidance/guidance_v.c)
  • The horizontal loop (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. Currently you can choose between no type (see below) and the types adaptive, new and energy.


File: conf/airframes/myplane.xml 
  <firmware name="fixedwing">
    <target name="ap" board="tiny_2.11"/>
     ...
    <subsystem name="control" />  <!-- Standard fixed wing control loops -->

    <!-- Different control loop types can be enabled instead (Use only one) -->
    <subsystem name="control" type="adaptive"/>  <!-- fixed wing control loops with adaptive horizontal control, standard vertical control -->
    <subsystem name="control" type="new"/>       <!-- new fixed wing control loops with merged auto pitch and auto throttle, adaptive horizontal control -->
    <subsystem name="control" type="energy"/>    <!-- Since v4.1.0 -->

  </firmware>

XML Parameters

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="35" unit="deg"/>
  <define name="MAX_PITCH" value="5" unit="deg"/>
 </section>

NOTE: automatic unit conversion using unit="deg" is supported since v3.9, if you have an older version set it in radians or using value="RadOfDeg(35)"

Infrared

The INFRARED section describes the configuration of the infrared sensors. For additional configuration to change the defaults, see the infrared module page.

The only mandatory definitions are the sensor neutral readings and how the IR sensors are mounted.

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"/>
   <define name="HORIZ_SENSOR_ALIGNED" value="1"/>
 </section>

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.

In the example above the horizontal sensor is connected to the airframe in aligned orientation. The other possibility is tilted. Define either

  • HORIZ_SENSOR_ALIGNED: ir1 is along the lateral axis (The axis that passes through the plane from wingtip to wingtip) and ir2 along the longitudinal one.

or

  • HORIZ_SENSOR_TILTED: the sensors are tilted by 45 degrees; ir1 is along rear-left -- front-right, and ir2 along rear-right -- front-left.

If the airframe construction allows choose an aligned sensor orientation since this gives the best stabilization response results.

Gyro

This section only applies to versions prior to v3.9 when using a gyro with IR sensors. 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="20" unit="deg"/>
    <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.


IMPORTANT: Previous to v3.9 some of the gains need to be set with a negative sign: COURSE_PGAIN, ROLL_ATTITUDE_GAIN, ROLL_RATE_GAIN, PITCH_PGAIN

NOTE: automatic unit conversion using unit="deg" is supported since v3.9, if you have an older version set it in radians or using value="RadOfDeg(20)"

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_">
    <!-- outer loop proportional gain -->
    <define name="ALTITUDE_PGAIN" value="0.1" unit="(m/s)/m"/>
    <!-- outer loop saturation -->
    <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="30" unit="deg"/>
    <define name="AUTO_PITCH_MIN_PITCH" value="30" unit="deg"/>

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%.


IMPORTANT: Previous to v3.9 some of the gains need to be set with a negative sign: ALTITUDE_PGAIN, AUTO_THROTTLE_PGAIN, AUTO_PITCH_PGAIN, '

NOTE: automatic unit conversion using unit="deg" is supported since v3.9, if you have an older version set it in radians or using value="RadOfDeg(20)"

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.

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