sensors: Calculate and publish magnetometer inconsistency

src/modules/sensors/sensors.cpp

@@ -621,8 +621,8 @@ Sensors::run()
 
 	/* advertise the sensor_preflight topic and make the initial publication */
 	preflt.accel_inconsistency_m_s_s = 0.0f;
-
 	preflt.gyro_inconsistency_rad_s = 0.0f;
+	preflt.mag_inconsistency_ga = 0.0f;
 
 	_sensor_preflight = orb_advertise(ORB_ID(sensor_preflight), &preflt);
 
@@ -686,6 +686,7 @@ Sensors::run()
 			if (!_armed) {
 				_voted_sensors_update.calc_accel_inconsistency(preflt);
 				_voted_sensors_update.calc_gyro_inconsistency(preflt);
+				_voted_sensors_update.calc_mag_inconsistency(preflt);
 				orb_publish(ORB_ID(sensor_preflight), _sensor_preflight, &preflt);
 
 			}

src/modules/sensors/voted_sensors_update.cpp

@@ -62,6 +62,7 @@ VotedSensorsUpdate::VotedSensorsUpdate(const Parameters &parameters, bool hil_en
 	memset(&_last_baro_timestamp, 0, sizeof(_last_baro_timestamp));
 	memset(&_accel_diff, 0, sizeof(_accel_diff));
 	memset(&_gyro_diff, 0, sizeof(_gyro_diff));
+	memset(&_mag_diff, 0, sizeof(_mag_diff));
 
 	// initialise the corrections
 	memset(&_corrections, 0, sizeof(_corrections));
@@ -1210,4 +1211,51 @@ void VotedSensorsUpdate::calc_gyro_inconsistency(sensor_preflight_s &preflt)
 	}
 }
 
+void VotedSensorsUpdate::calc_mag_inconsistency(sensor_preflight_s &preflt)
+{
+	float mag_diff_sum_max_sq = 0.0f; // the maximum sum of axis differences squared
+	unsigned check_index = 0; // the number of sensors the primary has been checked against
 
+	// Check each sensor against the primary
+	for (unsigned sensor_index = 0; sensor_index < _mag.subscription_count; sensor_index++) {
+
+		// check that the sensor we are checking against is not the same as the primary
+		if ((_mag.priority[sensor_index] > 0) && (sensor_index != _mag.last_best_vote)) {
+
+			float mag_diff_sum_sq = 0.0f; // sum of differences squared for a single sensor comparison against the primary
+
+			// calculate mag_diff_sum_sq for the specified sensor against the primary
+			for (unsigned axis_index = 0; axis_index < 3; axis_index++) {
+				_mag_diff[axis_index][check_index] = 0.95f * _mag_diff[axis_index][check_index] + 0.05f *
+								     (_last_sensor_data[_mag.last_best_vote].magnetometer_ga[axis_index] -
+								      _last_sensor_data[sensor_index].magnetometer_ga[axis_index]);
+				mag_diff_sum_sq += _mag_diff[axis_index][check_index] * _mag_diff[axis_index][check_index];
+
+			}
+
+			// capture the largest sum value
+			if (mag_diff_sum_sq > mag_diff_sum_max_sq) {
+				mag_diff_sum_max_sq = mag_diff_sum_sq;
+
+			}
+
+			// increment the check index
+			check_index++;
+		}
+
+		// check to see if the maximum number of checks has been reached and break
+		if (check_index >= 2) {
+			break;
+
+		}
+	}
+
+	// skip check if less than 2 sensors
+	if (check_index < 1) {
+		preflt.mag_inconsistency_ga = 0.0f;
+
+	} else {
+		// get the vector length of the largest difference and write to the combined sensor struct
+		preflt.mag_inconsistency_ga = sqrtf(mag_diff_sum_max_sq);
+	}
+}

src/modules/sensors/voted_sensors_update.h

@@ -142,7 +142,12 @@ public:
 	/**
 	 * Calculates the magnitude in rad/s of the largest difference between the primary and any other gyro sensor
 	 */
-	void calc_gyro_inconsistency(struct sensor_preflight_s &preflt);
+	void calc_gyro_inconsistency(sensor_preflight_s &preflt);
+
+	/**
+	 * Calculates the magnitude in Gauss of the largest difference between the primary and any other magnetometers
+	 */
+	void calc_mag_inconsistency(sensor_preflight_s &preflt);
 
 private:
 
@@ -263,6 +268,7 @@ private:
 
 	float _accel_diff[3][2];	/**< filtered accel differences between IMU units (m/s/s) */
 	float _gyro_diff[3][2];		/**< filtered gyro differences between IMU uinits (rad/s) */
+	float _mag_diff[3][2];		/**< filtered mag differences between sensor instances (Ga) */
 
 	/* sensor thermal compensation */
 	TemperatureCompensation _temperature_compensation;