Offboard 模式

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 MAVSDK (opens new window)) or by ROS 2.

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 OffboardControlMode message. PX4 enables offboard control only after receiving the signal for more than a second, and will regain control if the signal stops.

注解

• 此模式需要位置或位/姿信息 - 例如 GPS、光流、视觉惯性里程计、mocap 等。
• 除了更改模式外， 禁止遥控器控制。
• The vehicle must be already be receiving a stream of MAVLink setpoint messages or ROS 2 OffboardControlMode 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. :::

描述

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 OffboardControlMode 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 (COM_OF_LOSS_T), 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 parameters COM_OBL_ACT and COM_OBL_RC_ACT.

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 OffboardControlMode messages, while the actual setpoint is provided by publishing to one of the setpoint uORB topics, such as TrajectorySetpoint. 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. 像上传、下载任务这样的操作可以在任何模式下执行。

ROS 2 Messages

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:

1. Controls the level of the PX4 control architecture at which offboard setpoints must be injected, and disables the bypassed controllers.
2. Determines which valid estimates (position or velocity) are required, and also which setpoint messages should be used.

The OffboardControlMode message is defined as shown.

# Off-board control mode

uint64 timestamp        # time since system start (microseconds)

bool position
bool velocity
bool acceleration
bool attitude
bool body_rate
bool actuator

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.

desired control quantity	position field	velocity field	acceleration field	attitude field	body_rate field	actuator field	required estimate	required message
position (NED)	✓	-	-	-	-	-	position	TrajectorySetpoint
velocity (NED)	✗	✓	-	-	-	-	velocity	TrajectorySetpoint
acceleration (NED)	✗	✗	✓	-	-	-	velocity	TrajectorySetpoint
attitude (RFD)	✗	✗	✗	✓	-	-	none	VehicleAttitudeSetpoint
body_rate (RFD)	✗	✗	✗	✗	✓	-	none	VehicleRatesSetpoint
thrust and torque (RFD)	✗	✗	✗	✗	✗	✓	none	VehicleThrustSetpoint and VehicleTorqueSetpoint

where ✓ means that the bit is set, ✗ means that the bit is not set and - means that the bit is value is irrelevant.

Copter

• px4_msgs::msg::TrajectorySetpoint (opens new window)

  • 支持以下输入组合：
    • 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.

• px4_msgs::msg::VehicleAttitudeSetpoint (opens new window)

  • 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.

• px4_msgs::msg::VehicleRatesSetpoint (opens new window)

  • 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].

• px4_msgs::msg::VehicleThrustSetpoint (opens new window) + px4_msgs::msg::VehicleTorqueSetpoint (opens new window)

  • 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].

MAVLink Messages

The following MAVLink messages and their particular fields and field values are allowed for the specified frames.

直升机/垂直起降

• SET_POSITION_TARGET_LOCAL_NED (opens new window)

  • 支持以下输入组合：
    • 位置设置值 （仅 x，y，z）
    • Velocity setpoint (only vx, vy, vz)
    • 加速度设定值（仅 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).
  • • PX4 supports the following coordinate_frame values (only): MAV_FRAME_LOCAL_NED (opens new window) and MAV_FRAME_BODY_NED (opens new window).

• SET_POSITION_TARGET_GLOBAL_INT (opens new window)

  • 支持以下输入组合：

    • 位置设定值（仅lat_int，lon_int，alt）

    • Velocity setpoint (only vx, vy, vz)

    • Thrust setpoint (only afx, afy, afz)

      注解

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).

• PX4 支持以下 coordinate_frame 值(仅限)： MAV_FRAME_GLOBAL (opens new window)。

• SET_ATTITUDE_TARGET (opens new window)
  • 支持以下输入组合：
    • 带有推力设定值（SET_ATTITUDE_TARGET.thrust）的姿态和方向（SET_ATTITUDE_TARGET.q）。
    • 带有推力设定值（SET_ATTITUDE_TARGET.thrust）的机身速率（SET_ATTITUDE_TARGET .body_roll_rate ，.body_pitch_rate，.body_yaw_rate）。

固定翼

• SET_POSITION_TARGET_LOCAL_NED (opens new window)

  • 支持以下输入组合（通过 type_mask）：
    • 位置设定值（仅x，y，z；速度和加速度设定值被忽略）。

      • 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. :::

        值为：

        • 292：滑动设定值。 这会将 TECS 配置为空速优先于高度，以便在没有推力时使无人机滑行（即控制俯仰以调节空速）。 这相当于设置 type_mask 为 POSITION_TARGET_TYPEMASK_Z_IGNORE，POSITION_TARGET_TYPEMASK_VZ_IGNORE，POSITION_TARGET_TYPEMASK_AZ_IGNORE。
        • 4096：起飞设定值。
        • 8192：降落设定值。
        • 12288：悬停设定值（以设定值为中心绕圈飞行）。
        • 16384：空闲设定值（油门为0， 横滚 / 俯仰为0）。
  • PX4 支持坐标系指定 (coordinate_frame 字段): MAV_FRAME_LOCAL_NED (opens new window) 和 MAV_FRAME_BODY_NED (opens new window)。

• SET_POSITION_TARGET_GLOBAL_INT (opens new window)

  • 支持以下输入组合（通过 type_mask）：
    • 位置设定值（仅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):

注解

The setpoint type values below are not part of the MAVLink standard for the type_mask field. :::

    值为：
    - 4096：起飞设定值。
    - 8192：降落设定值。
    - 12288：悬停设定值（以设定值为中心绕圈飞行）。
    - 16384：空闲设定值（油门为0， 横滚 / 俯仰为0）。

• PX4 支持以下 coordinate_frame 值(仅限)： MAV_FRAME_GLOBAL (opens new window)。

• SET_ATTITUDE_TARGET (opens new window)

  • 支持以下输入组合：
    • 带有推力设定值（SET_ATTITUDE_TARGET.thrust）的姿态和方向（SET_ATTITUDE_TARGET.q）。
    • 带有推力设定值（SET_ATTITUDE_TARGET.thrust）的机身速率（SET_ATTITUDE_TARGET .body_roll_rate ，.body_pitch_rate，.body_yaw_rate）。

无人车

• SET_POSITION_TARGET_LOCAL_NED (opens new window)
  • 支持以下输入组合(在 type_mask 中)：
    • 位置设置值 （仅 x，y，z）
      • Specify the type of the setpoint in type_mask:

注解

The setpoint type values below are not part of the MAVLink standard for the type_mask field. ::

    值为：

    - 12288：悬停设定值（无人机足够接近设定值时会停止）。
* Velocity setpoint (only `vx`, `vy`, `vz`)

• PX4 支持坐标系指定 (coordinate_frame 字段): MAV_FRAME_LOCAL_NED (opens new window) 和 MAV_FRAME_BODY_NED (opens new window)。

• SET_POSITION_TARGET_GLOBAL_INT (opens new window)

  • 支持以下输入组合(在 type_mask 中)：
    • 位置设定值（仅lat_int，lon_int，alt）
  • Specify the type of the setpoint in type_mask (not part of the MAVLink standard). 值为：
    • 下面的比特位没有置位，是正常表现。
    • 12288：悬停设定值（无人机足够接近设定值时会停止）。
  • PX4 支持坐标系（corrdinate_frame字段）：MAV_FRAME_GLOBAL (opens new window)。

• SET_ATTITUDE_TARGET (opens new window)

  • 支持以下输入组合：
    • 带有推力设定值（SET_ATTITUDE_TARGET.thrust）的姿态和方向（SET_ATTITUDE_TARGET.q）。 :::note 实际仅使用/提取了偏航设置。

Offboard参数

Offboard mode is affected by the following parameters:

参数	描述
COM_OF_LOSS_T	丢失 Offboard 连接时的等待超时时间（以秒为单位），然后将触发 offboard 丢失的失效保护措施 (COM_OBL_ACT 和 COM_OBL_RC_ACT)
COM_OBL_ACT	Mode to switch to if offboard control is lost when not connected to RC (Values are - 0: Land, 1: Hold, 2: Return ).
COM_OBL_RC_ACT	Mode to switch to if offboard control is lost while still connected to RC control (Values are - 0: Position, 1: Altitude, 2: Manual, 3: Return , 4: Land).
COM_RC_OVERRIDE	控制多旋翼（或者多旋翼模式下的 VOTL）的摇杆移动量来切换到 位置模式。 默认情况下未启用此功能。
COM_RC_STICK_OV	导致发射机切换到 位置模式 的摇杆移动量（如果 COM_RC_OVERRIDE 已启用）。
COM_RCL_EXCEPT	Specify modes in which RC loss is ignored and the failsafe action not triggered. Set bit 2 to ignore RC loss in Offboard mode.

开发者资源

Typically developers do not directly work at the MAVLink layer, but instead use a robotics API like MAVSDK (opens new window) or ROS (opens new window) (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).

以下资源可能对开发者有用：

• Offboard Control from Linux
• ROS/MAVROS Offboard Example (C++)
• ROS/MAVROS Offboard Example (Python)