Inertial Navigation

Nodes: nav sim | 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]
    • mag Magnetometer option [auto,HDG,3D,indoor,disable]
    • hsrc Altitude preference option [baro,gps,range,vision]
    • bearing Bearing estimation option [vel,yaw]
    • fke Kinetic energy rate filter real [s] 0.5
    • slip Slip estimator 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.