ekf: add range finder kinematic consistency check

At each new valid range measurement, the time derivative of the distance
to the ground is computed and compared with the estimated velocity.
The average of a normalized innovation squared statistic check is used
to detect a bias in the derivative of distance measurement,
indicating that the distance measurements are kinematically inconsistent
with the filter.

src/modules/ekf2/CMakeLists.txt

@@ -66,6 +66,7 @@ px4_add_module(
 		EKF/mag_control.cpp
 		EKF/mag_fusion.cpp
 		EKF/optflow_fusion.cpp
+		EKF/range_finder_consistency_check.cpp
 		EKF/sensor_range_finder.cpp
 		EKF/sideslip_fusion.cpp
 		EKF/terrain_estimator.cpp

src/modules/ekf2/EKF/CMakeLists.txt

@@ -51,6 +51,7 @@ add_library(ecl_EKF
 	mag_control.cpp
 	mag_fusion.cpp
 	optflow_fusion.cpp
+	range_finder_consistency_check.cpp
 	sensor_range_finder.cpp
 	sideslip_fusion.cpp
 	terrain_estimator.cpp

src/modules/ekf2/EKF/control.cpp

@@ -140,6 +140,7 @@ void Ekf::controlFusionModes()
 			const Vector3f pos_offset_body = _params.rng_pos_body - _params.imu_pos_body;
 			const Vector3f pos_offset_earth = _R_to_earth * pos_offset_body;
 			_range_sensor.setRange(_range_sensor.getRange() + pos_offset_earth(2) / _range_sensor.getCosTilt());
+			_rng_consistency_check.update(_range_sensor.getDistBottom(), getRngHeightVariance(), _state.vel(2), P(6, 6), static_cast<float>(_time_last_imu) * 1e-6f);
 		}
 	}
 

src/modules/ekf2/EKF/estimator_interface.h

@@ -63,6 +63,7 @@
 #include "common.h"
 #include "RingBuffer.h"
 #include "imu_down_sampler.hpp"
+#include "range_finder_consistency_check.hpp"
 #include "sensor_range_finder.hpp"
 #include "utils.hpp"
 
@@ -297,6 +298,8 @@ protected:
 	extVisionSample _ev_sample_delayed_prev{};
 	dragSample _drag_down_sampled{};	// down sampled drag specific force data (filter prediction rate -> observation rate)
 
+	RangeFinderConsistencyCheck _rng_consistency_check;
+
 	float _air_density{CONSTANTS_AIR_DENSITY_SEA_LEVEL_15C};		// air density (kg/m**3)
 
 	// Sensor limitations

src/modules/ekf2/EKF/range_finder_consistency_check.cpp

@@ -0,0 +1,64 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2022 PX4. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file range_finder_consistency_check.cpp
+ */
+
+#include "range_finder_consistency_check.hpp"
+
+void RangeFinderConsistencyCheck::update(float dist_bottom, float dist_bottom_var, float vz, float vz_var, float time_s)
+{
+	const float dt = time_s - _time_last_update_s;
+
+	if ((_time_last_update_s < FLT_EPSILON)
+	    || (dt < 0.001f) || (dt > 0.5f)) {
+		_time_last_update_s = time_s;
+		_dist_bottom_prev = dist_bottom;
+		return;
+	}
+
+	const float vel_bottom = (dist_bottom - _dist_bottom_prev) / dt;
+	const float innov = -vel_bottom - vz; // vel_bottom is +up while vz is +down
+	const float vel_bottom_var = 2.f * dist_bottom_var / (dt * dt);
+	const float innov_var = vel_bottom_var + vz_var;
+	const float normalized_innov_sq = (innov * innov) / innov_var;
+	_vel_bottom_test_ratio = normalized_innov_sq / (_vel_bottom_gate * _vel_bottom_gate);
+
+	_vel_bottom_signed_test_ratio_lpf.setParameters(dt, _vel_bottom_signed_test_ratio_tau);
+	const float signed_test_ratio = matrix::sign(innov) * normalized_innov_sq / (_vel_bottom_signed_gate * _vel_bottom_signed_gate);
+	_vel_bottom_signed_test_ratio_lpf.update(signed_test_ratio);
+
+	_time_last_update_s = time_s;
+	_dist_bottom_prev = dist_bottom;
+}

src/modules/ekf2/EKF/range_finder_consistency_check.hpp

@@ -0,0 +1,66 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2022 PX4. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file range_finder_consistency_check.hpp
+ * @brief Compute statistical tests of the range finder data
+ * using the estimated velocity as a reference in order to detect sensor faults
+ */
+
+#pragma once
+
+#include <mathlib/math/filter/AlphaFilter.hpp>
+
+class RangeFinderConsistencyCheck final
+{
+public:
+	RangeFinderConsistencyCheck() = default;
+	~RangeFinderConsistencyCheck() = default;
+
+	void update(float dist_bottom, float dist_bottom_var, float vz, float vz_var, float time_s);
+
+	float getTestRatio() const { return _vel_bottom_test_ratio; }
+	float getSignedTestRatioLpf() const { return _vel_bottom_signed_test_ratio_lpf.getState(); }
+	bool isKinematicallyConsistent() const { return _vel_bottom_signed_test_ratio_lpf.getState() < 1.f; }
+
+private:
+	float _time_last_update_s{};
+	float _dist_bottom_prev{};
+
+	float _vel_bottom_test_ratio{};
+	AlphaFilter<float> _vel_bottom_signed_test_ratio_lpf{}; // average signed test ratio used to detect a bias in the data
+
+	static constexpr float _vel_bottom_signed_test_ratio_tau = 2.f;
+	static constexpr float _vel_bottom_gate = 3.f;
+	static constexpr float _vel_bottom_signed_gate = 0.1f;
+};