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Mavros IMU

ROS / ros eco / packages / mavros

  • attitude: The attitude in the aeronautical frame (x-front, y-right, z-down).)
  • attitude_quaternion: The attitude in the aeronautical frame (q: w, x, y, z)
  • highres_imu (105): The IMU readings in SI units in NED body frame
  • raw_imu (27): The RAW IMU readings for a 9DOF sensor, which is identified by the id (default IMU1). This message should always contain the true raw values without any scaling to allow data capture and system debugging.
  • scaled_imu (26): The RAW IMU readings for the usual 9DOF sensor setup.
  • scaled_pressure:

ATTITUDE_QUATERNION_COV(61)

This message not use by mavros and Not support by ardupilot

mavlink mavros topic(message)
ATTITUDE (30) data (sensor_msgs::msg::Imu)
ATTITUDE_QUATERNION(31) data (sensor_msgs::msg::Imu)
HIGHRES_IMU(105) data_raw (sensor_msgs::msg::Imu)
RAW_IMU(27) data_raw (sensor_msgs::msg::Imu)
SCALED_IMU(26) data_raw (sensor_msgs::msg::Imu)
SCALED_PRESSURE

Message Topic mapping

there two imu topic and the code map the priorities mavlink message

  • ~/data: The topic publish/mapping mavlink ATTITUDE or ATTITUDE_QUATERNION if publish by the FCU
  • ~/data_raw: the topic publish/mapping selected mavlink message with priority HIGHRES_IMU or SCALED_IMU or RAW_IMU

Message priority variables

  • has_hr_imu = set to true automaticlay if system send HIGHRES_IMU message, disabled handle/publish RAW_IMU and SCALED_IMU message.
  • has_raw_imu = set to true automaticlay if system send RAW_IMU message;
  • has_scaled_imu = set to true automaticlay if system send SCALED_IMU message, disabled handle/publish RAW_IMU message.
  • has_att_quat : set to true automaticlay if system send ATTITUDE_QUATERNION message, disabled handle/publish ATTITUDE message.

data_raw" vs "data

data_raw: Publish only gyro and accelerometer data: Publish Orientation and gyro

ROS Imu msg

sensor_msgs/msg/Imu.msg

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std_msgs/msg/Header header
geometry_msgs/msg/Quaternion orientation
double[9] orientation_covariance
geometry_msgs/msg/Vector3 angular_velocity
double[9] angular_velocity_covariance
geometry_msgs/msg/Vector3 linear_acceleration
double[9] linear_acceleration_covariance

Covariance

The message covariance filed by parameters

parameter name field name
linear_acceleration_stdev IMU.linear_acceleration_covariance
angular_velocity_stdev IMU.angular_velocity_covariance
orientation_stdev / unk_orientation_cov IMU.orientation_covariance
magnetic_stdev MagneticField.magnetic_field_covariance

The -1 convention in IMU covariance

Each covariance array (orientation_covariance, angular_velocity_covariance, linear_acceleration_covariance) is a 3×3 matrix stored row-major.

Normally you’d put variances (σ²) on the diagonal.

BUT: If a sensor does not provide that measurement, the convention is to set the first element to -1.

Demo: Attitude message (/mavros/imu/data)

set message stream
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ros2 service call /mavros/set_message_interval mavros_msgs/srv/MessageInterval "{message_id: 30, message_rate: 1.0}"

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ros2 topic echo --once /mavros/imu/data
#
header:
  stamp:
    sec: 0
    nanosec: 0
  frame_id: base_link
orientation:
  x: 0.00042285808111321393
  y: -4.743792265771591e-05
  z: -0.7445043500394287
  w: -0.6676174740920542
orientation_covariance:
- 1.0
- 0.0
- 0.0
- 0.0
- 1.0
- 0.0
- 0.0
- 0.0
- 1.0
angular_velocity:
  x: -0.0001230202615261078
  y: 9.356345981359479e-05
  z: -0.00024927849881350994
angular_velocity_covariance:
- 1.2184696791468346e-07
- 0.0
- 0.0
- 0.0
- 1.2184696791468346e-07
- 0.0
- 0.0
- 0.0
- 1.2184696791468346e-07
linear_acceleration:
  x: 0.0
  y: 0.0
  z: 0.0
linear_acceleration_covariance:
- -1.0
- 0.0
- 0.0
- 0.0
- 8.999999999999999e-08
- 0.0
- 0.0
- 0.0
- 8.999999999999999e-08

Covariance explain

  • orientation_covariance: default orientation_stdev is: 1.0
  • angular_velocity_covariance: default angular_velocity_stdev: 0.02*(pi/180)
  • linear_acceleration_covariance: -1 , no orientation sensor data

calculate variance from angular_velocity_stdev parameter

\[0.02^\circ = 0.02 \times \frac{\pi}{180} = 0.00034906585 \ \text{rad/s}$$ **Variance = (stdev)²** $$\sigma^2 = (0.00034906585)^2 = 1.21846968 \times 10^{-7} \ (\text{rad/s})^2\]

Demo: scaled_imu message (/mavros/imu/data_raw)

set message stream
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ros2 service call /mavros/set_message_interval mavros_msgs/srv/MessageInterval "{message_id: 26, message_rate: 1.0}"

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ros2 service call /mavros/set_message_interval mavros_msgs/srv/MessageInterval "{message_id: 26, message_rate: 1.0}"

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ros2 topic echo --once /mavros/imu/data_raw
header:
  stamp:
    sec: 0
    nanosec: 0
  frame_id: base_link
orientation:
  x: 0.0
  y: 0.0
  z: 0.0
  w: 1.0
orientation_covariance:
- -1.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
- 0.0
angular_velocity:
  x: 0.0
  y: 0.0
  z: 0.0
angular_velocity_covariance:
- 1.2184696791468346e-07
- 0.0
- 0.0
- 0.0
- 1.2184696791468346e-07
- 0.0
- 0.0
- 0.0
- 1.2184696791468346e-07
linear_acceleration:
  x: 0.0
  y: 1.202169221539125e-15
  z: 9.81645665
linear_acceleration_covariance:
- 8.999999999999999e-08
- 0.0
- 0.0
- 0.0
- 8.999999999999999e-08
- 0.0
- 0.0
- 0.0
- 8.999999999999999e-08
---

Covariance explain

  • orientation_covariance: -1 , no orientation sensor data
  • angular_velocity_covariance: like data message
  • linear_acceleration_covariance: default value 0.0003 is 9e-8