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Land Detector Configuration

The land detector is a dynamic vehicle model representing key vehicle states from ground contact through to landed. This topic explains the main parameters you may wish to tune in order to improve landing behaviour.

Auto-Disarming

The land-detector automatically disarms the vehicle on landing.

You can set COM_DISARM_LAND to specify the number of seconds after landing that the system should disarm (or turn off auto-disarming by setting the parameter to -1).

Multicopter Configuration

The complete set of relevant landing detector parameters are listed in the parameter reference with the prefix LNDMC (these can be edited in QGroundControl via the parameter editor).

:::tip Information about how the parameters affect landing can be found below in Land Detector States. :::

Other key parameters that you may need to tune in order to improve landing behaviour on particular airframes are:

  • MPC_THR_HOVER - the hover throttle of the system (default is 50%). It is important to set this correctly as it makes altitude control more accurate and ensures correct land detection. A racer or a big camera drone without payload mounted might need a much lower setting (e.g. 35%).

    :::note Incorrectly setting MPC_THR_HOVER may result in ground-contact or maybe-landed detection while still in air (in particular, while descending in Position mode or Altitude mode). This causes the vehicle to "twitch" (turn down the motors, and then immediately turn them back up). :::

  • MPC_THR_MIN - the overall minimum throttle of the system. This should be set to enable a controlled descent.

  • MPC_LAND_CRWL - the vertical speed applied in the last stage of autonomous landing if the system has a distance sensor and it is present and working. Has to be set larger than LNDMC_Z_VEL_MAX.

MC Land Detector States

In order to detect landing, the multicopter first has to go through three different states, where each state contains the conditions from the previous states plus tighter constraints. If a condition cannot be reached because of missing sensors, then the condition is true by default. For instance, in Acro mode and no sensor is active except for the gyro sensor, then the detection solely relies on thrust output and time.

In order to proceed to the next state, each condition has to be true for a third of the configured total land detector trigger time LNDMC_TRIG_TIME. If the vehicle is equipped with a distance sensor, but the distance to ground is currently not measurable (usually because it is too large), the trigger time is increased by a factor of 3.

If one condition fails, the land detector drops out of the current state immediately.

Ground Contact

Conditions for this state:

  • no vertical movement (LNDMC_Z_VEL_MAX)
  • no horizontal movement (LNDMC_XY_VEL_MAX)
  • lower thrust than MPC_THR_MIN + (hover throttle - MPC_THR_MIN) * (0.3, unless a hover thrust estimate is available, then 0.6),
  • additional check if vehicle is currently in a height-rate controlled flight mode: the vehicle has to have the intent to descend (vertical velocity setpoint above LNDMC_Z_VEL_MAX).
  • additional check for vehicles with a distance sensor: current distance to ground is below 1m.

If the vehicle is in position- or velocity-control and ground contact was detected, the position controller will set the thrust vector along the body x-y-axis to zero.

Maybe Landed

Conditions for this state:

  • all conditions of the ground contact state are true
  • is not rotating (LNDMC_ROT_MAX)
  • has low thrust MPC_THR_MIN + (MPC_THR_HOVER - MPC_THR_MIN) * 0.1
  • no freefall detected

If the vehicle only has knowledge of thrust and angular rate, in order to proceed to the next state the vehicle has to have low thrust and no rotation for 8.0 seconds.

If the vehicle is in position or velocity control and maybe landed was detected, the position controller will set the thrust vector to zero.

Landed

Conditions for this state:

Fixed-wing Configuration

Tuning parameters for fixed-wing land detection:

  • LNDFW_AIRSPD_MAX - the maximum airspeed allowed for the system still to be considered landed. Has to be a tradeoff between airspeed sensing accuracy and triggering fast enough. Better airspeed sensors should allow lower values of this parameter.
  • LNDFW_VEL_XY_MAX - the maximum horizontal velocity for the system to be still be considered landed.
  • LNDFW_VEL_Z_MAX - the maximum vertical velocity for the system to be still be considered landed.
  • LNDFW_XYACC_MAX - the maximal horizontal acceleration for the system to still be considered landed.
  • LNDFW_TRIG_TIME - Trigger time during which the conditions above have to be fulfilled to declare a landing.

:::note When FW launch detection is enabled (FW_LAUN_DETCN_ON), the vehicle will stay in "landed" state until takeoff is detected (which is purely based on acceleration and not velocity). :::

VTOL Land Detector

The VTOL land detector is 1:1 the same as the MC land detector if the system is in hover mode. In FW mode, land detection is disabled.