Px4 Offboard Mode

PX4 Offboard 模式技能

由外部计算机通过 MAVLink 发送位置/速度/姿态目标，实现完全自主控制

何时使用

当需要以下帮助时使用此技能：

• 实现自主避障导航
• 运行高级路径规划算法
• 集成视觉里程计/VIO
• 多机编队控制
• 实时轨迹跟踪

模式特性

控制原理

offboard_mode:
  source: external_computer (ROS2/MicroXRCE-DDS or MAVLink)
  command_type: position / velocity / attitude / thrust
  update_rate: 10 Hz (minimum)
  
  safety:
    timeout: 1.0s  # 无命令超时应退出
    must_armed: true
    must_position_lock: true

命令类型

与任务模式对比

ROS2 接口

MicroXRCE-DDS (推荐)

# 订阅话题
/fmu/out/vehicle_odometry: 车辆里程计
  - position: [x, y, z] (m)
  - velocity: [vx, vy, vz] (m/s)
  - q: quaternion [w, x, y, z]
/fmu/out/vehicle_status: 飞行器状态
/fmu/out/timesync_status: 时间同步状态

# 发布话题
/fmu/in/vehicle_command: 车辆命令
  - command: SET_POSITION / SET_VELOCITY / SET_ATTITUDE
  - param1-7: 根据命令类型不同
  - target_system: 1
  - target_component: 1
  
# 时间同步
/timesync/control_in: 时间同步输入

MAVROS (备选)

# 订阅话题
/mavros/state: 连接状态
/mavros/local_position/pose: 位置
/mavros/setpoint_position/local: 位置设定点
/mavros/setpoint_velocity/cmd_vel: 速度设定点
/mavros/setpoint_attitude/attitude: 姿态设定点

# 发布话题
/mavros/cmd/command: 命令

开发技能

Offboard 连接技能

name: offboard_connection_skill
description: Offboard 通信连接建立
type: setup

interface:
  inputs:
    - topic: /fmu/out/vehicle_status
    - topic: /fmu/out/vehicle_odometry
  outputs:
    - topic: /fmu/in/vehicle_command
    
params:
  connection_type: {type: enum, values: [udp, serial, wifi]}
  fcu_url: {type: string, default: "udp://:14540@"}
  protocol: {type: string, default: "v2.0"}
  
preconditions:
  - timesync_established
  - vehicle_odometry_received
  - local_position_estimate_valid
  
connection_sequence:
  1. Establish UDP/Serial connection
  2. Wait for timesync
  3. Verify position estimate
  4. Arm vehicle
  5. Start setpoint stream
  6. Request mode switch to OFFBOARD

位置控制技能

name: offboard_position_control_skill
description: Offboard 位置控制
type: control

interface:
  inputs:
    - topic: /fmu/out/vehicle_odometry
  outputs:
    - topic: /fmu/in/vehicle_command
      
params:
  setpoint_rate: {type: int, default: 20}  # Hz
  timeout: {type: float, default: 1.0}    # s
  
command_format:
  type: SET_POSITION
  frame: MAV_FRAME_LOCAL_NED
  position: [x, y, z]  # m
  yaw: {type: float, unit: rad}
  
control_loop:
  - step: 1
    action: stream_position_setpoint
    rate: 20Hz
  - step: 2
    action: monitor_timeout
    threshold: 1.0s
  - step: 3
    action: emergency_rtl on_timeout

速度控制技能

name: offboard_velocity_control_skill
description: Offboard 速度控制
type: control

params:
  setpoint_rate: {type: int, default: 20}  # Hz
  timeout: {type: float, default: 1.0}    # s
  
command_format:
  type: SET_VELOCITY
  frame: MAV_FRAME_BODY_NED
  velocity: [vx, vy, vz]  # m/s
  angular_velocity: [yaw_rate]  # rad/s
  
velocity_limits:
  vx_max: 5.0  # m/s
  vy_max: 5.0  # m/s
  vz_max: 3.0  # m/s
  yaw_rate_max: 90.0  # deg/s

轨迹控制技能

name: offboard_trajectory_control_skill
description: Offboard 轨迹跟踪控制
type: control

interface:
  inputs:
    - topic: /trajectory (Trajectory)
    - topic: /fmu/out/vehicle_odometry
  outputs:
    - topic: /fmu/in/vehicle_command
    
params:
  trajectory_type: {type: enum, values: [ Jerk, Snap, Polynomial ]}
  execution_time: auto  # 实时跟踪
  
trajectory_message:
  type: trajectory_msgs/Trajectory
  points:
    - position: [x, y, z]
      velocity: [vx, vy, vz]
      time_from_start: t

配置参数

关键参数

安全设置

safety:
  offboard_timeout: 1.0  # s
  RC_loss_action: RTL
  datalink_loss_action: RTL
  battery_low_action: RTL
  
failsafe:
  enable_offboard_failsafe: true
  min_setpoint_rate: 10  # Hz

测试与验证

连接测试流程

# 1. 启动 MicroXRCE-DDS Bridge
ros2 launch micro_ros_ddsx agent.launch.py

# 或启动 MAVROS
ros2 launch mavros px4.launch fcu_url:="udp://:14540@"

# 2. 检查连接状态
ros2 topic echo /fmu/out/vehicle_status

# 3. 发送测试 setpoint
ros2 topic pub /fmu/in/vehicle_command <msg> -r 20

# 4. 切换到 Offboard 模式
ros2 service call /mavros/set_mode std_srvs/srv/Trigger "{custom_mode: 'OFFBOARD'}"

典型测试序列

test_sequence:
  1. arm_vehicle: 解锁飞控
  2. takeoff_to_3m: 起飞到3米
  3. switch_to_offboard: 切换到Offboard模式
  4. send_position_setpoints: 发送位置目标
  5. verify_tracking: 验证轨迹跟踪
  6. switch_back_to_position: 切换回位置模式
  7. land: 降落

故障排除

相关技能

• px4-position-mode: 位置模式（Offboard的替代）
• px4-mission-mode: 任务模式（离线规划）
• px4-ros2: ROS2 集成基础
• px4-vision-nav: 视觉导航集成
• px4-mc-tuning: 参数调优