Inertial Navigation

Nodes: nav sim com | Module: ahrs | Parameters: AHRS

The AHRS module estimates different navigation parameters, based on sensor readings from other modules. The estimations are performed with MEKF algorithm, and the dead-reckoning (in case of GPS lost) is based on airspeed sensor readings and current wind estimator values.

The current implementation is capable to provide bandwidth of up to 8KHz for primary loop controls (stabilization), and 100Hz for outer loop controls (navigation). The update rate is adjusted automatically, based on available sensors data rate.

The concept of distributed architecture, as the basic concept of APX system, assumes, that all sensors monitor their health and supply data to consumers (AHRS) if possible. Then AHRS automatically choose the best option for the data source, and reconfigure itself to adopt to current conditions.

The source for inertial sensors is automatically selected, based on availability, thus making redundancy possible.

The data source is chosen with the following priorities (high to low):

• local node sensors;
• other LAN sensors with explicit priorities;
• sensor data fixes from ground control;

Estimated parameters

• bodyframe angles;
• global position coordinates;
• bodyframe velocities;
• filtered gyros with compensated drift;
• linear accelerations;
• wind speed and direction (correlator);
• airspeed fused with other sensors;
• vertical speed fused with other sensors;
• compensated variometer, used for gliding with thermals;
• vehicle energy, used for TECS control;
• slip angle to aid maneuvers and coordinated turns;
• trajectory tangent with different dynamic models;
• sensors malfunction monitor and validation;

Configuration parameters

• AHRS
  

  Inertial navigation

  • rebias Reset imu bias command
  • reset Full reset command
  • mode Operation mode option [off,local,primary,secondary,failsafe,auxillary]
    

    The priority of data, published by the module. All priorities, except local are broadcasting data in network.

  • align
    

    Bodyframe parameters

    • roll real [deg]
    • pitch real [deg]
    • yaw real [deg]
  • offset
    

    Bodyframe sensors positions

    • imu
      

      IMU position

      • x real [m]
      • y real [m]
      • z real [m]
    • gps
      

      GPS position

      • x real [m]
      • y real [m]
      • z real [m]
    • rng
      

      Rangefinder position

      • x real [m]
      • y real [m]
      • z real [m]
    • flow
      

      Flow focal point position

      • x real [m]
      • y real [m]
      • z real [m]
    • ev
      

      Vision focal point position

      • x real [m]
      • y real [m]
      • z real [m]
  • gyro
    

    Vehicle angular rate

    • lpf Lpf cut-off frequency word [Hz] 30
    • nf Notch filter frequency word [Hz]
    • nfbw Notch filter bandwidth word [Hz] 20
    • dlpf Derivative lpf frequency word [Hz] 30
  • acc
    

    Vehicle acceleration

    • lpf Lpf cut-off frequency word [Hz] 30
  • agl
    

    AGL sensor

    • min Minimum altitude real [m] 0.3
    • max Maximum altitude word [m] 50
    • spd Ground speed limit word [m/s] 100
    • gnd Ground clearance real [m] 0.1
    • aid Maximum agl for range height source word [m]
  • air
    

    Airdata parameters

    • airspeed Airspeed source option [sensor,vel]
    • err Airspeed measurement error real [m/s] 20
    • proc Process noise real 0.001
    • bias Process noise for bias real 0.01
    • lpf Energy derivative lp filter real 0.01
    • baro Barometer sensor option [sensor,off]
    • slip Slip source option [acc,sensor]
    • stab Stability detector sensitivity word 100
      

      Will emit warning on GCS and apply either reverse ailerons or negative elevator. Lower values make detector more sensitive. The stability is estimated based on rate of turn and energy flow.

  • mag Magnetometer option [auto,HDG,disable]
  • hsrc Default altitude preference option [baro,gps,range,vision]
  • bearing Bearing estimation option [vel,yaw]