Improve magnetometer inconsistency check

To check directional difference between the magnetometer field vectors
instead of vector component difference.

ROMFS/px4fmu_common/init.d/airframes/4250_teal

@@ -47,7 +47,7 @@ then
 	param set CAL_MAG0_ROT 0
 	param set CAL_MAG_SIDES 63
 	param set SENS_BOARD_ROT 0
-	param set COM_ARM_MAG 1.5
+	param set COM_ARM_MAG_ANG -1
 	param set COM_ARM_EKF_AB 0.0032
 
 	# circuit breakers

msg/sensor_preflight.msg

@@ -2,7 +2,7 @@
 # Pre-flight sensor check metrics. These will be zero if the vehicle only has one sensor.
 # The topic will not be updated when the vehicle is armed
 #
-uint64 timestamp		   # time since system start (microseconds)
-float32 accel_inconsistency_m_s_s  # magnitude of maximum acceleration difference between IMU instances in (m/s/s).
-float32 gyro_inconsistency_rad_s   # magnitude of maximum angular rate difference between IMU instances in (rad/s).
-float32 mag_inconsistency_ga       # magnitude of maximum difference between Mag instances in (Gauss).
+uint64 timestamp # time since system start (microseconds)
+float32 accel_inconsistency_m_s_s # magnitude of maximum acceleration difference between IMU instances in (m/s/s).
+float32 gyro_inconsistency_rad_s # magnitude of maximum angular rate difference between IMU instances in (rad/s).
+float32 mag_inconsistency_angle # maximum angle between magnetometer instance field vectors in radians.

src/modules/commander/PreflightCheck.cpp

@@ -45,6 +45,7 @@
 #include "rc_check.h"
 
 #include <math.h>
+#include <mathlib/mathlib.h>
 
 #include <parameters/param.h>
 #include <systemlib/mavlink_log.h>
@@ -203,6 +204,8 @@ out:
 // return false if the magnetomer measurements are inconsistent
 static bool magConsistencyCheck(orb_advert_t *mavlink_log_pub, vehicle_status_s &status, bool report_status)
 {
+	bool pass = false; // flag for result of checks
+
 	// get the sensor preflight data
 	uORB::SubscriptionData<sensor_preflight_s> sensors_sub{ORB_ID(sensor_preflight)};
 	sensors_sub.update();
@@ -210,25 +213,24 @@ static bool magConsistencyCheck(orb_advert_t *mavlink_log_pub, vehicle_status_s
 
 	if (sensors.timestamp == 0) {
 		// can happen if not advertised (yet)
-		return true;
+		pass = true;
 	}
 
 	// Use the difference between sensors to detect a bad calibration, orientation or magnetic interference.
 	// If a single sensor is fitted, the value being checked will be zero so this check will always pass.
-	float test_limit;
-	param_get(param_find("COM_ARM_MAG"), &test_limit);
+	int32_t angle_difference_limit_deg;
+	param_get(param_find("COM_ARM_MAG_ANG"), &angle_difference_limit_deg);
 
-	if (sensors.mag_inconsistency_ga > test_limit) {
-		if (report_status) {
-			mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Compass Sensors inconsistent");
-			set_health_flags_healthy(subsystem_info_s::SUBSYSTEM_TYPE_MAG, false, status);
-			set_health_flags_healthy(subsystem_info_s::SUBSYSTEM_TYPE_MAG2, false, status);
-		}
+	pass = pass || angle_difference_limit_deg < 0; // disabled, pass check
+	pass = pass || sensors.mag_inconsistency_angle < math::radians<float>(angle_difference_limit_deg);
 
-		return false;
+	if (!pass && report_status) {
+		mavlink_log_critical(mavlink_log_pub, "Preflight Fail: Compass Sensors inconsistent");
+		set_health_flags_healthy(subsystem_info_s::SUBSYSTEM_TYPE_MAG, false, status);
+		set_health_flags_healthy(subsystem_info_s::SUBSYSTEM_TYPE_MAG2, false, status);
 	}
 
-	return true;
+	return pass;
 }
 
 static bool accelerometerCheck(orb_advert_t *mavlink_log_pub, vehicle_status_s &status, uint8_t instance,

src/modules/commander/commander_params.c

@@ -600,15 +600,14 @@ PARAM_DEFINE_FLOAT(COM_ARM_IMU_GYR, 0.25f);
 
 /**
  * Maximum magnetic field inconsistency between units that will allow arming
+ * Set -1 to disable the check.
  *
  * @group Commander
- * @unit Gauss
- * @min 0.05
- * @max 0.5
- * @decimal 2
- * @increment 0.05
+ * @unit deg
+ * @min 3
+ * @max 180
  */
-PARAM_DEFINE_FLOAT(COM_ARM_MAG, 0.15f);
+PARAM_DEFINE_INT32(COM_ARM_MAG_ANG, 30);
 
 /**
  * Enable RC stick override of auto modes

src/modules/sensors/sensors.cpp

@@ -673,7 +673,7 @@ Sensors::run()
 				preflt.timestamp = hrt_absolute_time();
 				_voted_sensors_update.calc_accel_inconsistency(preflt);
 				_voted_sensors_update.calc_gyro_inconsistency(preflt);
-				_voted_sensors_update.calc_mag_inconsistency(preflt);
+				_voted_sensors_update.calcMagInconsistency(preflt);
 				orb_publish(ORB_ID(sensor_preflight), _sensor_preflight, &preflt);
 			}
 		}

src/modules/sensors/voted_sensors_update.cpp

@@ -50,6 +50,7 @@
 
 using namespace sensors;
 using namespace DriverFramework;
+using namespace matrix;
 
 VotedSensorsUpdate::VotedSensorsUpdate(const Parameters &parameters, bool hil_enabled)
 	: _parameters(parameters), _hil_enabled(hil_enabled)
@@ -60,7 +61,7 @@ VotedSensorsUpdate::VotedSensorsUpdate(const Parameters &parameters, bool hil_en
 	memset(&_last_accel_timestamp, 0, sizeof(_last_accel_timestamp));
 	memset(&_accel_diff, 0, sizeof(_accel_diff));
 	memset(&_gyro_diff, 0, sizeof(_gyro_diff));
-	memset(&_mag_diff, 0, sizeof(_mag_diff));
+	memset(&_mag_angle_diff, 0, sizeof(_mag_angle_diff));
 
 	// initialise the publication variables
 	memset(&_corrections, 0, sizeof(_corrections));
@@ -1205,34 +1206,25 @@ void VotedSensorsUpdate::calc_gyro_inconsistency(sensor_preflight_s &preflt)
 	}
 }
 
-void VotedSensorsUpdate::calc_mag_inconsistency(sensor_preflight_s &preflt)
+void VotedSensorsUpdate::calcMagInconsistency(sensor_preflight_s &preflt)
 {
-	float mag_diff_sum_max_sq = 0.0f; // the maximum sum of axis differences squared
+	Vector3f primary_mag(_last_magnetometer[_mag.last_best_vote].magnetometer_ga); // primary mag field vector
+	float mag_angle_diff_max = 0.0f; // the maximum angle difference
 	unsigned check_index = 0; // the number of sensors the primary has been checked against
 
 	// Check each sensor against the primary
-	for (int sensor_index = 0; sensor_index < _mag.subscription_count; sensor_index++) {
-
+	for (int i = 0; i < _mag.subscription_count; i++) {
 		// 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)) {
+		if ((_mag.priority[i] > 0) && (i != _mag.last_best_vote)) {
+			// calculate angle to 3D magnetic field vector of the primary sensor
+			Vector3f current_mag(_last_magnetometer[i].magnetometer_ga);
+			float angle_error = AxisAnglef(Quatf(current_mag, primary_mag)).angle();
 
-			float mag_diff_sum_sq = 0.0f; // sum of differences squared for a single sensor comparison against the primary
+			// complementary filter to not fail/pass on single outliers
+			_mag_angle_diff[check_index] *= 0.95f;
+			_mag_angle_diff[check_index] += 0.05f * angle_error;
 
-			// 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_magnetometer[_mag.last_best_vote].magnetometer_ga[axis_index] -
-								      _last_magnetometer[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;
-
-			}
+			mag_angle_diff_max = math::max(mag_angle_diff_max, _mag_angle_diff[check_index]);
 
 			// increment the check index
 			check_index++;
@@ -1241,16 +1233,10 @@ void VotedSensorsUpdate::calc_mag_inconsistency(sensor_preflight_s &preflt)
 		// 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);
-	}
+	// get the vector length of the largest difference and write to the combined sensor struct
+	// will be zero if there is only one magnetometer and hence nothing to compare
+	preflt.mag_inconsistency_angle = mag_angle_diff_max;
 }

src/modules/sensors/voted_sensors_update.h

@@ -141,7 +141,7 @@ public:
 	/**
 	 * Calculates the magnitude in Gauss of the largest difference between the primary and any other magnetometers
 	 */
-	void calc_mag_inconsistency(sensor_preflight_s &preflt);
+	void calcMagInconsistency(sensor_preflight_s &preflt);
 
 private:
 
@@ -261,7 +261,7 @@ 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) */
+	float _mag_angle_diff[2];	/**< filtered mag angle differences between sensor instances (Ga) */
 
 	/* sensor thermal compensation */
 	TemperatureCompensation _temperature_compensation;