Magnetometer

A magnetometer is a device for detecting and measuring magnetic fields (in Teslas). The device can be used in conjunction with a 3-axis accelerometer to produce orientation independent accurate compass heading information.

MetaWear CPro, MMC, and MMR boards have a BMM150 geomagnetic sensor for sensing magnetic fields.

Functions communicating with the magnetometer are defined in the magnetometer_bmm150.h.

Power Modes

The BMM150 magnetometer has 4 recommended power modes:

Setting	ODR	Average Current	Noise
LOW_POWER	10Hz	170µA	1.0µT (xy axis), 1.4µT (z axis)
REGULAR	10Hz	0.5mA	0.6µT
ENHANCED_REGULAR	10Hz	0.8mA	0.5µT
HIGH_ACCURACY	20Hz	4.9mA	0.3µT

Constants identifying these power modes are defined by the MblMwMagBmm150Preset enum. For most use cases, MWL_MW_MAG_BMM_150_PP_LOW_POWER is the recommended power mode.

// Use low power mode
MetaWear.mbl_mw_mag_bmm150_set_power_preset(device.board, MWL_MW_MAG_BMM_150_PP_LOW_POWER);

B Field Sampling

B field sampling measures the magnetic field strength for the XYZ directions. To enable B field sampling, call mbl_mw_mag_bmm150_enable_b_field_sampling before starting the magnetometer.

Field strength is represented by the MblMwCartesianFloat struct and is in units of micro Teslas (µT). The x, y, and z fields contain the B field strength in that direction.

console.log('Get mag signal.')
let mag = MetaWear.mbl_mw_mag_bmm150_get_b_field_data_signal(device.board);

console.log('Set up stream.')
MetaWear.mbl_mw_datasignal_subscribe(mag, ref.NULL, MetaWear.FnVoid_VoidP_DataP.toPointer((ctx, pointer) => {
    var data = pointer.deref();
    var value = data.parseValue();
    console.log('epoch: ' + data.epoch + ' mag: ' + value.x + ' ' + value.y + ' ' + value.z)
}))

console.log('Start accelerometer.')
MetaWear.mbl_mw_mag_bmm150_enable_b_field_sampling(device.board);
MetaWear.mbl_mw_mag_bmm150_start(device.board);