Offboard Mode
Last updated
Last updated
The vehicle obeys position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints provided by some source that is external to the flight stack, such as a companion computer. The setpoints may be provided using MAVLink (or a MAVLink API such as ) or by .
PX4 requires that the external controller provides a continuous 2Hz "proof of life" signal, by streaming any of the supported MAVLink setpoint messages or the ROS 2 message. PX4 enables offboard control only after receiving the signal for more than a second, and will regain control if the signal stops.
:::note
This mode requires position or pose/attitude information - e.g. GPS, optical flow, visual-inertial odometry, mocap, etc.
RC control is disabled except to change modes (you can also fly without any manual controller at all by setting the parameter to 4: Stick input disabled).
The vehicle must be already be receiving a stream of MAVLink setpoint messages or ROS 2 messages before arming in offboard mode or switching to offboard mode when flying.
The vehicle will exit offboard mode if MAVLink setpoint messages or OffboardControlMode are not received at a rate of > 2Hz.
Not all coordinate frames and field values allowed by MAVLink are supported for all setpoint messages and vehicles. Read the sections below carefully to ensure only supported values are used. :::
Offboard mode is used for controlling vehicle movement and attitude, by setting position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints.
PX4 must receive a stream of MAVLink setpoint messages or the ROS 2 at 2 Hz as proof that the external controller is healthy. The stream must be sent for at least a second before PX4 will arm in offboard mode, or switch to offboard mode when flying. If the rate falls below 2Hz while under external control PX4 will switch out of offboard mode after a timeout (), and attempt to land or perform some other failsafe action. The action depends on whether or not RC control is available, and is defined in the parameter .
When using MAVLink the setpoint messages convey both the signal to indicate that the external source is "alive", and the setpoint value itself. In order to hold position in this case the vehicle must receive a stream of setpoints for the current position.
When using ROS 2 the proof that the external source is alive is provided by a stream of messages, while the actual setpoint is provided by publishing to one of the setpoint uORB topics, such as . In order to hold position in this case the vehicle must receive a stream of OffboardControlMode but would only need the TrajectorySetpoint once.
Note that offboard mode only supports a very limited set of MAVLink commands and messages. Operations, like taking off, landing, return to launch, may be best handled using the appropriate modes. Operations like uploading, downloading missions can be performed in any mode.
The following ROS 2 messages and their particular fields and field values are allowed for the specified frames. In addition to providing heartbeat functionality, OffboardControlMode has two other main purposes:
Determines which valid estimates (position or velocity) are required, and also which setpoint messages should be used.
The OffboardControlMode message is defined as shown.
The fields are ordered in terms of priority such that position takes precedence over velocity and later fields, velocity takes precedence over acceleration, and so on. The first field that has a non-zero value (from top to bottom) defines what valid estimate is required in order to use offboard mode, and the setpoint message(s) that can be used. For example, if the acceleration field is the first non-zero value, then PX4 requires a valid velocity estimate, and the setpoint must be specified using the TrajectorySetpoint message.
position (NED)
✓
-
-
-
-
-
position
TrajectorySetpoint
velocity (NED)
✗
✓
-
-
-
-
velocity
TrajectorySetpoint
acceleration (NED)
✗
✗
✓
-
-
-
velocity
TrajectorySetpoint
attitude (FRD)
✗
✗
✗
✓
-
-
none
body_rate (FRD)
✗
✗
✗
✗
✓
-
none
thrust and torque (FRD)
✗
✗
✗
✗
✗
✓
none
where ✓ means that the bit is set, ✗ means that the bit is not set and - means that the bit is value is irrelevant.
The following input combinations are supported:
Position setpoint (position different from NaN). Non-NaN values of velocity and acceleration are used as feedforward terms for the inner loop controllers.
Velocity setpoint (velocity different from NaN and position set to NaN). Non-NaN values acceleration are used as feedforward terms for the inner loop controllers.
Acceleration setpoint (acceleration different from NaN and position and velocity set to NaN)
All values are interpreted in NED (Nord, East, Down) coordinate system and the units are [m], [m/s] and [m/s^2] for position, velocity and acceleration, respectively.
The following input combination is supported:
quaterion q_d + thrust setpoint thrust_body. Non-NaN values of yaw_sp_move_rate are used as feedforward terms expressed in Earth frame and in [rad/s].
The quaterion represents the rotation between the drone body FRD (front, right, down) frame and the NED frame. The trust is in the drone body FRD frame and expressed in normalized [-1, 1] values.
The following input combination is supported:
roll, pitch, yaw and thrust_body.
All the value are in the drone body FRD frame. The rates are in [rad/s] while thrust_body is normalized in [-1, 1].
The following input combination is supported:
xyz for thrust and xyz for torque.
All the value are in the drone body FRD frame and normalized in [-1, 1].
The following MAVLink messages and their particular fields and field values are allowed for the specified frames.
The following input combinations are supported:
Position setpoint (only x, y, z)
Velocity setpoint (only vx, vy, vz)
Acceleration setpoint (only afx, afy, afz)
Position setpoint and velocity setpoint (the velocity setpoint is used as feedforward; it is added to the output of the position controller and the result is used as the input to the velocity controller).
Position setpoint and velocity setpoint and acceleration (the velocity and the acceleration setpoints are used as feedforwards; the velocity setpoint is added to the output of the position controller and the result is used as the input to the velocity controller; the acceleration setpoint is added to the output of the velocity controller and the result used to compute the thrust vector).
The following input combinations are supported:
Position setpoint (only lat_int, lon_int, alt)
Velocity setpoint (only vx, vy, vz)
Thrust setpoint (only afx, afy, afz)
:::note Acceleration setpoint values are mapped to create a normalized thrust setpoint (i.e. acceleration setpoints are not "properly" supported). :::
Position setpoint and velocity setpoint (the velocity setpoint is used as feedforward; it is added to the output of the position controller and the result is used as the input to the velocity controller).
The following input combinations are supported:
Attitude/orientation (SET_ATTITUDE_TARGET.q) with thrust setpoint (SET_ATTITUDE_TARGET.thrust).
Body rate (SET_ATTITUDE_TARGET .body_roll_rate ,.body_pitch_rate, .body_yaw_rate) with thrust setpoint (SET_ATTITUDE_TARGET.thrust).
The following input combinations are supported (via type_mask):
Position setpoint (x, y, z only; velocity and acceleration setpoints are ignored).
Specify the type of the setpoint in type_mask (if these bits are not set the vehicle will fly in a flower-like pattern): :::note Some of the setpoint type values below are not part of the MAVLink standard for the type_mask field. :::
The values are:
292: Gliding setpoint. This configures TECS to prioritize airspeed over altitude in order to make the vehicle glide when there is no thrust (i.e. pitch is controlled to regulate airspeed). It is equivalent to setting type_mask as POSITION_TARGET_TYPEMASK_Z_IGNORE, POSITION_TARGET_TYPEMASK_VZ_IGNORE, POSITION_TARGET_TYPEMASK_AZ_IGNORE.
4096: Takeoff setpoint.
8192: Land setpoint.
12288: Loiter setpoint (fly a circle centred on setpoint).
16384: Idle setpoint (zero throttle, zero roll / pitch).
The following input combinations are supported (via type_mask):
Position setpoint (only lat_int, lon_int, alt)
Specify the type of the setpoint in type_mask (if these bits are not set the vehicle will fly in a flower-like pattern):
:::note The setpoint type values below are not part of the MAVLink standard for the type_mask field. :::
The values are:
4096: Takeoff setpoint.
8192: Land setpoint.
12288: Loiter setpoint (fly a circle centred on setpoint).
16384: Idle setpoint (zero throttle, zero roll / pitch).
The following input combinations are supported:
Attitude/orientation (SET_ATTITUDE_TARGET.q) with thrust setpoint (SET_ATTITUDE_TARGET.thrust).
Body rate (SET_ATTITUDE_TARGET .body_roll_rate ,.body_pitch_rate, .body_yaw_rate) with thrust setpoint (SET_ATTITUDE_TARGET.thrust).
The following input combinations are supported (in type_mask):
Position setpoint (only x, y, z)
Specify the type of the setpoint in type_mask:
:::note The setpoint type values below are not part of the MAVLink standard for the type_mask field. ::
The values are:
12288: Loiter setpoint (vehicle stops when close enough to setpoint).
Velocity setpoint (only vx, vy, vz)
The following input combinations are supported (in type_mask):
Position setpoint (only lat_int, lon_int, alt)
Specify the type of the setpoint in type_mask (not part of the MAVLink standard). The values are:
Following bits not set then normal behaviour.
12288: Loiter setpoint (vehicle stops when close enough to setpoint).
The following input combinations are supported:
Attitude/orientation (SET_ATTITUDE_TARGET.q) with thrust setpoint (SET_ATTITUDE_TARGET.thrust). :::note Only the yaw setting is actually used/extracted. :::
Offboard mode is affected by the following parameters:
Time-out (in seconds) to wait when offboard connection is lost before triggering offboard lost failsafe (COM_OBL_RC_ACT)
Specify modes in which RC loss is ignored and the failsafe action not triggered. Set bit 2 to ignore RC loss in Offboard mode.
The following resources may be useful for a developer audience:
Controls the level of the at which offboard setpoints must be injected, and disables the bypassed controllers.
and
:::note Before using offboard mode with ROS 2, please spend a few minutes understanding the different that PX4 and ROS 2 use. :::
+
In order to save resources, this mode is disabled by default. If you want to use it you need to manually add vehicle_thrust_setpoint and vehicle_torque_setpoint to the list of , and manually recompile the firmware.
PX4 supports the following coordinate_frame values (only): and .
PX4 supports the following coordinate_frame values (only): .
PX4 supports the coordinate frames (coordinate_frame field): and .
PX4 supports the following coordinate_frame values (only): .
PX4 supports the coordinate frames (coordinate_frame field): and .
PX4 supports the coordinate frames (coordinate_frame field): .
Mode to switch to if offboard control is lost (Values are - 0: Position, 1: , 2: Manual, 3: , 4: ).
Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to . This is not enabled for offboard mode by default.
The amount of stick movement that causes a transition to (if is enabled).
Typically developers do not directly work at the MAVLink layer, but instead use a robotics API like or (these provide a developer friendly API, and take care of managing and maintaining connections, sending messages and monitoring responses - the minutiae of working with Offboard mode and MAVLink).
