MAVLINKHUD
Download App MAVLink HUD for Android

CONDITION_DELAY

ID: 112

Summary

The CONDITION_DELAY command pauses the mission state machine for a specific number of seconds. Unlike a NAV_DELAY or a loiter with time, which are navigation-level commands, CONDITION_DELAY is a flow-control item that prevents the next mission item from starting until the timer expires.

Status

Supported (All Vehicles)

Directionality

  • RX (Receive): The vehicle receives this command as part of a mission upload.

Mission Storage (AP_Mission)

  • Packet Param 1 (Time): Stored in the internal condition_value field (seconds).
  • Mechanism: When the mission reaches this item, it records the current system time in condition_start.

Execution (Engineer's View)

State Machine Logic

ArduPilot's mission engine treats "Condition" commands as gates.

  1. Verification: Every loop, the verify_wait_delay() function is called (mode_auto.cpp).

  2. Comparison: The function checks if the elapsed time since the command started exceeds the target delay.

    $$ \mathrm{current\_time} - \mathrm{condition\_start} > \mathrm{condition\_value} \cdot 1000 $$

  3. Completion: Once the condition is met, the gate opens, and the mission state machine increments to the next command ID.

Vehicle Behavior during Delay

It is critical to understand that CONDITION_DELAY does not stop the vehicle's movement if it was already performing a navigation task that supports concurrent execution (though typically conditions are placed between nav points).

  • Copter: If placed after a NAV_WAYPOINT, the drone will hover at that waypoint while the delay runs.
  • Plane: The aircraft will continue its previous navigation state (e.g., loitering) while the condition timer counts down.
  • param1 (Time): Delay time in seconds.
  • param2 to param7: Unused.

Theory: Synchronous vs. Asynchronous Delays

In computer science, a delay can be blocking or non-blocking.

  • MAVLink HUD Context: CONDITION_DELAY is a non-blocking delay for the flight controller (it still stabilizes the airframe), but a blocking delay for the Mission Script.
  • Race Conditions: If a GCS sends a "Set Current Waypoint" command while a CONDITION_DELAY is active, the timer is typically reset or abandoned as the mission engine jumps to the new index.

Practical Use Cases

  1. Camera Boot-up:
    • Scenario: A high-end mapping camera takes 10 seconds to initialize after being powered on via a relay.
    • Action: DO_SET_RELAY -> CONDITION_DELAY (10s) -> NAV_WAYPOINT.
  2. Safety Buffer:
    • Scenario: Ensuring a drone has come to a complete, stable hover before dropping a payload.
    • Action: NAV_WAYPOINT -> CONDITION_DELAY (3s) -> DO_GRIPPER.

Key Parameters

  • MIS_OPTIONS: Can affect how the mission engine handles pauses and restarts.

Key Codebase Locations

CONDITION_DISTANCE

ID: 114

Summary

The CONDITION_DISTANCE command creates a proximity-based gate in the mission. It prevents the next mission item from starting until the vehicle is within a specific distance (meters) of the next navigation waypoint.

Status

Supported (All Vehicles)

Directionality

  • RX (Receive): The vehicle receives this command as part of a mission upload.

Mission Storage (AP_Mission)

  • Packet Param 1 (Distance): Stored in the internal condition_value field (meters).

Execution (Engineer's View)

Verification Logic

The mission engine checks the proximity in every loop cycle using verify_within_distance() (mode_auto.cpp).

  1. Distance Calculation: The flight controller calculates the 2D (horizontal) distance between the current GPS coordinate and the coordinate of the NAV_WAYPOINT immediately following the condition.

  2. Comparison:

    $$ \mathrm{dist\_to\_wp} < \mathrm{condition\_value} $$

  3. Completion: As soon as the vehicle crosses the "distance threshold," the condition returns true, and the next command (which is usually a "Do" command like DO_DIGICAM_CONTROL) is executed.

  • param1 (Distance): Distance in meters.
  • param2 to param7: Unused.

Theory: The "Inner Circle"

CONDITION_DISTANCE allows for spatial triggering that is more precise than simple waypoint arrival.

  • Waypoint Acceptance: Standard waypoints use a radius (e.g., WP_RADIUS) to determine when to turn.
  • Condition Precision: By using CONDITION_DISTANCE, you can trigger an action (like a laser fire or camera snap) at a much tighter distance (e.g., 1 meter) than the navigation waypoint's acceptance radius (e.g., 5 meters).

Practical Use Cases

  1. Precise Photo Capture:
    • Scenario: A surveyor needs a photo taken exactly as the drone passes over a marker.
    • Action: NAV_WAYPOINT (Marker) -> CONDITION_DISTANCE (0.5m) -> DO_DIGICAM_CONTROL.
  2. Obstacle Proximity Actions:
    • Scenario: A drone needs to turn on high-intensity landing lights as it approaches a narrow dock.
    • Action: NAV_WAYPOINT (Dock) -> CONDITION_DISTANCE (10m) -> [DO_SET_SERVO](/mission-planning/do-commands.html#DO_SET_SERVO) (Lights ON).

Key Parameters

  • WPNAV_RADIUS: (Copter) The horizontal radius used for waypoint completion; often used in conjunction with CONDITION_DISTANCE.

Key Codebase Locations

CONDITION_YAW

ID: 115

Summary

The CONDITION_YAW command forces the vehicle to rotate to a specific heading. This command is a "blocking" mission item; the mission will not advance to the next item until the aircraft has successfully achieved the target yaw angle within a small tolerance.

Status

Supported (ArduCopter and QuadPlane Only)

Directionality

  • RX (Receive): The vehicle receives this command as part of a mission upload.

Mission Storage (AP_Mission)

  • Angle (Param 1): Target angle in degrees.
  • Rate (Param 2): Rotation speed in deg/s.
  • Direction (Param 3): -1 for Counter-Clockwise, 1 for Clockwise.
  • Relative (Param 4): 0 for Absolute (North-up), 1 for Relative (Offset from current heading).

Execution (Engineer's View)

ArduCopter Implementation

In Copter, the command triggers ModeAuto::do_yaw (mode_auto.cpp).

  1. Request: The command is passed to the auto_yaw controller, which sets a FIXED yaw target.

  2. Tracking: The autopilot calculates the shortest path (or the path requested by the direction parameter) to the new heading.

  3. Completion: The verification function ModeAuto::verify_yaw() checks if the current heading matches the target:

    $$ |\text{Yaw}_{current} - \text{Yaw}_{target}| < 2^\circ $$

  4. Priority: Note that if a subsequent NAV_WAYPOINT command has a different yaw requirement (like "Face Next Waypoint"), it will override the CONDITION_YAW once the condition is met and the mission advances.

  • param1 (Angle): Target angle in degrees [0-360].
  • param2 (Rate): Desired turn rate in deg/s.
  • param3 (Direction): -1: CCW, 1: CW.
  • param4 (Relative): 0: Absolute, 1: Relative.
  • param5 to param7: Unused.

Theory: The Singularity of North

Yaw management in ArduPilot must account for the 360/0 degree wrap-around.

  • Angular Error: The autopilot uses the wrap_180() function to calculate the smallest error.
  • Magnetic vs. True: Mission angles are typically relative to Magnetic North unless the system has been configured with a high-precision GPS (Moving Baseline) or a specific declination offset.

Practical Use Cases

  1. Fixed-Camera Alignment:
    • Scenario: A drone is inspecting a solar panel array. The camera is fixed (no gimbal).
    • Action: CONDITION_YAW is used to point the entire drone airframe at the panels before the next survey leg.
  2. Antenna Tracking:
    • Scenario: A drone needs to point a high-gain directional antenna back at the GCS for a data burst.
    • Action: CONDITION_YAW targets the bearing to the Home point.

Key Parameters

  • ATC_SLEW_YAW: Limits the maximum rate of change for yaw to prevent mechanical stress on the frame.
  • WP_YAW_BEHAVE: Determines how the drone handles yaw during navigation between CONDITION_YAW commands.

Key Codebase Locations