Sensor Fusion

The sensor_fusion.h header file interfaces with the sensor fusion algorithm running on MetaMotion boards. When using the sensor fusion algorithm, it is important that you do not simultaenously use the Accelerometer, Gyro, and Magnetometer modules; the algorithm configures those sensors internally based on the selected fusion mode.

To activate the sensor fusion algorithm, first set the fusion mode and data ranges, then subscribe to and enable the desired output data, and finally, call mbl_mw_sensor_fusion_start.

Mode

The sensor fusion algorithm has 4 fusion modes, listed in the below table:

Mode	Description
NDoF	Calculates absolute roeintation from accelerometer, gyro, and magnetometer
IMUPlus	Calculates relative orientation in space from accelerometer and gyro data
Compass	Determines geographic direction from th Earth’s magnetic field
M4G	Similar to IMUPlus except rotation is detected with the magnetometer

The mode is set with mbl_mw_sensor_fusion_set_mode and written to the board by calling mbl_mw_sensor_fusion_write_config. Before writing the configuration, you can also set the acceleration and rotation ranges of the accelerometer and gyroscope respectively.

#include "metawear/sensor/sensor_fusion.h"

void configure_sensor_fusion(MblMwMetaWearBoard* board) {
    // set fusion mode to ndof (n degress of freedom)
    mbl_mw_sensor_fusion_set_mode(board, MBL_MW_SENSOR_FUSION_MODE_NDOF);
    // set acceleration rangen to +/-8G, note accelerometer is configured here
    mbl_mw_sensor_fusion_set_acc_range(board, MBL_MW_SENSOR_FUSION_ACC_RANGE_8G);
    // write changes to the board
    mbl_mw_sensor_fusion_write_config(board);
}

Data

The sensor fusion algorithm provides raw acceleration, rotation, and magnetic field values along with quaternion values and Euler angles. Furthermore, the source of acceleration can be separated into gravity and linear acceleration and both values are also provided. Keep in mind that each sensor fusion mode has different sets of available data and produces it at different rates.

Mode	Acc	Gyro	Mag
NDoF	100Hz	100Hz	25Hz
IMUPlus	100Hz	100Hz	N/A
Compass	25Hz	N/A	25Hz
M4G	50Hz	N/A	50Hz

Also note that the units and type casting of the sensor fusion data is different for each type of data..

Data	Units	Casted Data
Acceleration	g	MblMwCorrectedCartesianFloat
Rotation	deg/s	MblMwCorrectedCartesianFloat
Magnetic Field	uT	MblMwCorrectedCartesianFloat
Quaternion	None	MblMwQuaternion
Euler Angles	degrees	MblMwEulerAngles
Linear Acc	g	MblMwCartesianFloat
Gravity	g	MblMwCartesianFloat

#include <cstdio>

#include "metawear/core/datasignal.h"
#include "metawear/core/data.h"
#include "metawear/core/types.h"

void stream_quaternion(MblMwMetaWearBoard* board) {
    auto quaternion = mbl_mw_sensor_fusion_get_data_signal(board,
            MBL_MW_SENSOR_FUSION_DATA_QUATERION);
    mbl_mw_datasignal_subscribe(quaternion, [](const MblMwData* data) -> void {
            MblMwQuaternion* quaternion = (MblMwQuaternion*) data->value;
            std::printf("{w: %.3f, x: %.3f, y: %.3f, z: %.3f}\n",
                    quaternion->w, quaternion->x, quaternion->y, quaternion->z);
    });

    mbl_mw_sensor_fusion_enable_data(board, MBL_MW_SENSOR_FUSION_DATA_QUATERION);
    mbl_mw_sensor_fusion_start(board);
}