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magnetometer_compass

README.md

magnetometer_compass

Compass based on a 3-axis magnetometer, attitude readings and possibly also GPS.

Because there is no well established Azimuth message in ROS, this node publishes our custom compass_msgs/Azimuth as well as a few other formats that are capable of carrying orientation information. It also offers the azimuth values in both radians and degrees, because radians are the ROS standard, while degrees are more used in the geographic area. There are tens of possible combinations of the output data formats, so each of the published topics has a boolean parameter that enables it. By default, there is only one enabled output topic.

Explaining some basic terms

So that you know what output is the right for you.

Orientation

  • NED (North-East-Down): Azimuth will be 0 pointing to north, and increases clockwise. This is consistent with the azimuth used in cartography and by tourists.
  • ENU (East-North-Up): Azimuth will be 0 pointing to east, and increases counter-clockwise. This is consistent with REP-103 and robot_localization package.

References for north

  • Magnetic: points towards the magnetic north of Earth (travels in time).
  • Geographic ("true"): points towards the geographic Earth (i.e. the WGS84 North Pole). It is static in time.
  • UTM: points in the north direction on the cartesian UTM grid (similar to Geographic, but it can slightly diverge at the edges of UTM maps). You probably want this azimuth reference for navigation tasks in UTM coordinates.

Magnetic azimuth can be computed directly from the magnetometer and IMU orientation. To compute the other two references, you need to provide the latitude, longitude, altitude and time in addition to the magnetometer and IMU orientation. These are the required inputs to compute magnetic declination and UTM grid convergence, which are the offsets by which geographic and UTM references differ from the magnetic. This is why this compass node subscribes to the GPS fix messages. Until at least a single GPS fix message is received, neither geographic- nor UTM-referenced data are published. If you do not have a GPS receiver, you can alternatively provide these values in parameters.

For the magnetometer to work correctly, it is required to measure its bias. This node listens on the imu/mag_bias topic for this measurement, and until at least one message arrives, the node will not publish anything. If you do not have a node publishing the bias, you can alternatively provide it via parameters. Depending on the application, it may be required to re-estimate the bias from time to time even during runtime.

Node magnetometer_compass and nodelet magnetometer_compass/magnetometer_compass

Subscribed topics

  • imu/data (sensor_msgs/Imu): Output from an IMU or an orientation filtering algorithm. It should have valid contents of orientation and at least roll and pitch should be estimated as well as possible (relative to the gravity vector). These messages should come at the same rate as the magnetometer data (or faster).
  • imu/mag (sensor_msgs/MagneticField): 3-axis magnetometer raw measurements (bias not removed) (disabled by param ~subscribe_mag_unbiased).
  • imu/mag_bias (sensor_msgs/MagneticField): Bias of the magnetometer. This value will be subtracted from the incoming magnetometer measurements. Messages on this topic do not need to come repeatedly if the bias does not change. Disabled by param ~subscribe_mag_unbiased.
  • imu/mag_unbiased (sensor_msgs/MagneticField): 3-axis magnetometer unbiased measurements (enabled by param ~subscribe_mag_unbiased).
  • gps/fix (sensor_msgs/NavSatFix, optional): GPS fix messages from which the latitude, longitude, altitude and current year can be read. These are further used to compute magnetic declination and UTM grid convergence factor if requested.
  • TF: This node requires a (usually static) transform between ~frame and the frame ID of the IMU and magnetometer messages.

Published topics (see above for explanation)

  • imu/mag_unbiased (sensor_msgs/MagneticField, enabled by param ~publish_mag_unbiased, off by default): The magnetic field measurement with bias removed.

  • compass/mag/ned/deg (compass_msgs/Azimuth, enabled by param ~publish_mag_azimuth_ned_deg, on by default): Magnetic azimuth in NED in degrees (the same values you can see on touristic magnetic compasses).

  • compass/mag/ned/rad (compass_msgs/Azimuth, enabled by param ~publish_mag_azimuth_ned_rad, off by default): Magnetic azimuth in NED in radians.

  • compass/mag/ned/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_mag_azimuth_ned_quat, off by default): Magnetic azimuth in NED as a quaternion.

  • compass/mag/ned/imu (sensor_msgs/Imu, enabled by param ~publish_mag_azimuth_ned_imu, off by default): Magnetic azimuth in NED inside an IMU message (only orientation and header fields are valid).

  • compass/mag/ned/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_mag_azimuth_ned_pose, off by default): Magnetic azimuth in NED as a pose (translation will always be zero).

  • compass/mag/enu/deg (compass_msgs/Azimuth, enabled by param ~publish_mag_azimuth_enu_deg, off by default): Magnetic azimuth in ENU in degrees.

  • compass/mag/enu/rad (compass_msgs/Azimuth, enabled by param ~publish_mag_azimuth_enu_rad, off by default): Magnetic azimuth in ENU in radians.

  • compass/mag/enu/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_mag_azimuth_enu_quat, off by default): Magnetic azimuth in ENU as a quaternion.

  • compass/mag/enu/imu (sensor_msgs/Imu, enabled by param ~publish_mag_azimuth_enu_imu, off by default): Magnetic azimuth in ENU inside an IMU message (only orientation and header fields are valid).

  • compass/mag/enu/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_mag_azimuth_enu_pose, off by default): Magnetic azimuth in ENU as a pose (translation will always be zero).

  • compass/true/ned/deg (compass_msgs/Azimuth, enabled by param ~publish_true_azimuth_ned_deg, off by default): Geographic ("true") azimuth in NED in degrees.

  • compass/true/ned/rad (compass_msgs/Azimuth, enabled by param ~publish_true_azimuth_ned_rad, off by default): Geographic ("true") azimuth in NED in radians.

  • compass/true/ned/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_true_azimuth_ned_quat, off by default): Geographic ("true") azimuth in NED as a quaternion.

  • compass/true/ned/imu (sensor_msgs/Imu, enabled by param ~publish_true_azimuth_ned_imu, off by default): Geographic ("true") azimuth in NED inside an IMU message (only orientation and header fields are valid).

  • compass/true/ned/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_true_azimuth_ned_pose, off by default): Geographic ("true") azimuth in NED as a pose (translation will always be zero).

  • compass/true/enu/deg (compass_msgs/Azimuth, enabled by param ~publish_true_azimuth_enu_deg, off by default): Geographic ("true") azimuth in ENU in degrees.

  • compass/true/enu/rad (compass_msgs/Azimuth, enabled by param ~publish_true_azimuth_enu_rad, off by default): Geographic ("true") azimuth in ENU in radians.

  • compass/true/enu/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_true_azimuth_enu_quat, off by default): Geographic ("true") azimuth in ENU as a quaternion.

  • compass/true/enu/imu (sensor_msgs/Imu, enabled by param ~publish_true_azimuth_enu_imu, off by default): Geographic ("true") azimuth in ENU inside an IMU message (only orientation and header fields are valid).

  • compass/true/enu/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_true_azimuth_enu_pose, off by default): Geographic ("true") azimuth in ENU as a pose (translation will always be zero).

  • compass/utm/ned/deg (compass_msgs/Azimuth, enabled by param ~publish_utm_azimuth_ned_deg, off by default): UTM heading in NED in degrees.

  • compass/utm/ned/rad (compass_msgs/Azimuth, enabled by param ~publish_utm_azimuth_ned_rad, off by default): UTM heading in NED in radians.

  • compass/utm/ned/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_utm_azimuth_ned_quat, off by default): UTM heading in NED as a quaternion.

  • compass/utm/ned/imu (sensor_msgs/Imu, enabled by param ~publish_utm_azimuth_ned_imu, off by default): UTM heading in NED inside an IMU message (only orientation and header fields are valid).

  • compass/utm/ned/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_utm_azimuth_ned_pose, off by default): UTM heading in NED as a pose (translation will always be zero).

  • compass/utm/enu/deg (compass_msgs/Azimuth, enabled by param ~publish_utm_azimuth_enu_deg, off by default): UTM heading in ENU in degrees.

  • compass/utm/enu/rad (compass_msgs/Azimuth, enabled by param ~publish_utm_azimuth_enu_rad, off by default): UTM heading in ENU in radians.

  • compass/utm/enu/quat (geometry_msgs/QuaternionStamped, enabled by param ~publish_utm_azimuth_enu_quat, off by default): UTM heading in ENU as a quaternion.

  • compass/utm/enu/imu (sensor_msgs/Imu, enabled by param ~publish_utm_azimuth_enu_imu, off by default): UTM heading in ENU inside an IMU message (only orientation and header fields are valid).

  • compass/utm/enu/pose (geometry_msgs/PoseWithCovarianceStamped, enabled by param ~publish_utm_azimuth_enu_pose, off by default): UTM heading in ENU as a pose (translation will always be zero).

Parameters

  • All the publish_* parameters mentioned above.
  • Please note that you cannot combine both ~subscribe_mag_unbiased and ~publish_mag_unbiased set to true. Such configuration is invalid and the node will not start.
  • ~frame (string, default base_link): Frame into which the IMU and magnetometer data should be transformed.
  • ~low_pass_ratio (double, default 0.95): The azimuth is filtered with a low-pass filter. This sets its aggressivity (0 means raw measurements, 1 means no updates).
  • ~initial_mag_bias_x (double, no default, optional): Magnetometer bias in the X axis.
  • ~initial_mag_bias_y (double, no default, optional): Magnetometer bias in the Y axis.
  • ~initial_mag_bias_z (double, no default, optional): Magnetometer bias in the Z axis.
    • If you specify any of the ~initial_mag_bias_* params, the node does not need to receive the bias messages.
  • ~initial_lat (double, no default, optional): Latitude in degrees.
  • ~initial_lon (double, no default, optional): Longitude in degrees.
    • If you specify both ~initial_lat and ~initial_lon, the node does not need to receive the GPS fix messages.
  • ~initial_alt (double, default 0): Altitude in meters (it is usually okay to omit it and use the default).
  • ~initial_year (int, no default, optional): If set, overrides the current time for declination computation.
  • ~magnetic_declination (double, no default, optional, radians): If this parameter is set, the magnetic models are ignored and this value for declination is forced. This can be useful either if you know the value in advance or in simulation.
  • ~utm_grid_convergence (double, no default, optional, radians): If set, forces this value of UTM grid convergence.
  • ~magnetic_models_path (string, defaults to a pre-installed directory): Path where WMM magnetic models can be found. If set to empty string, the models will be searched in a default folder of GeographicLib. Environment variables GEOGRAPHICLIB_MAGNETIC_PATH or GEOGRAPHICLIB_DATA influence the location of this folder. You usually do not need to use other than the preinstalled models. But if you do, specify the path to the custom models directory here.
  • ~magnetic_model (string, defaults to autodetection by year): Name of the magnetic field model to use. If omitted, an automated decision is made based on the current year (or ~initial_year, if set). This model is used for computing magnetic declination.
  • utm_zone (int, optional): If set, forces using this UTM zone instead of determining the proper one.
  • keep_utm_zone (bool, default true): If true, the first determined UTM zone will be used for all future conversions.

Nodelet visualize_azimuth_nodelet and node visualize_azimuth

This node visualizes Azimuth messages in RViz converting them to a pose.

Subscribed topics

  • ~azimuth (multiple types supported): The azimuth to visualize. Any type supported by compass_conversions package can be used: compass_msgs/Azimuth, geometry_msgs/Quaternion, geometry_msgs/PoseWithCovarianceStamped or sensor_msgs/Imu. If other types than compass_msgs/Azimuth are used, either the resolved topic name must contain the azimuth type identification (e.g. end with mag/enu/imu), or you must provide parameters ~input_reference and ~input_orientation.
  • gps/fix (sensor_msgs/NavSatFix, optional): GPS fix messages from which the latitude, longitude, altitude and current year can be read. These are further used to compute magnetic declination and UTM grid convergence factor.
  • utm_zone (std_msgs/Int32, optional): Optional UTM zone updates.

Published topics

  • ~azimuth_vis (sensor_msgs/MagneticField, enabled by param ~publish_mag_unbiased, off by default): The magnetic field measurement with bias removed.

Parameters

  • max_rate (double, optional): If specified, visualization messages frequency will be at most this value [Hz].
  • magnetic_declination (double, radians, optional): If set, forces this value of magnetic declination.
  • utm_grid_convergence (double, radians, optional): If set, forces this value of UTM grid convergence.
  • magnetic_models_path (string, default "$PACKAGE/data/magnetic"): Path where WMM magnetic models can be found. If set to empty string, the models will be searched in a default folder of GeographicLib. Environment variables GEOGRAPHICLIB_MAGNETIC_PATH or GEOGRAPHICLIB_DATA influence the location of this folder.
  • magnetic_model (string, optional): If set, forces using the given WMM model instead of determining the proper one by year. Example value is "wmm2020".
  • utm_zone (int, optional): If set, forces using this UTM zone instead of determining the proper one.
  • keep_utm_zone (bool, default true): If true, the first determined UTM zone will be used for all future conversions.
  • initial_lat (double, degrees, optional): If set, use this latitude before the first navsat pose is received.
  • initial_lon (double, degrees, optional): If set, use this longitude before the first navsat pose is received.
  • initial_alt (double, meters, optional): If set, use this altitude before the first navsat pose is received.
  • ~input_orientation (str, 'enu' or 'ned', default: unspecified): ENU or NED orientation to be used to interpret input messages (in case orientation cannot be derived either from message contents or topic name).
  • ~input_reference (str, 'magnetic', 'geographic' or 'UTM', default: no change): North reference to be used to interpret input messages (in case reference cannot be derived either from message contents or topic name).
  • ~input_variance (double, optional, rad^2): If specified, this variance will be used in the output messages if variance cannot be determined from the input messages (e.g. for QuaternionStamped).