Add unit tests for Butterworth 2nd order low-pass filter

src/lib/mathlib/CMakeLists.txt

@@ -37,6 +37,7 @@ px4_add_library(mathlib
 	math/filter/LowPassFilter2pVector3f.cpp
 )
 
+px4_add_unit_gtest(SRC math/filter/LowPassFilter2pVector3fTest.cpp LINKLIBS mathlib)
 px4_add_unit_gtest(SRC math/filter/MedianFilterTest.cpp)
 px4_add_unit_gtest(SRC math/filter/NotchFilterTest.cpp)
 px4_add_unit_gtest(SRC math/FunctionsTest.cpp)

src/lib/mathlib/math/filter/LowPassFilter2pVector3fTest.cpp

@@ -0,0 +1,115 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2021 PX4 Development Team. 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.
+ *
+ ****************************************************************************/
+
+/**
+ * Test code for the 2nd order Butterworth low-pass filter
+ * Run this test only using make tests TESTFILTER=LowPassFilter2pVector3f
+ */
+
+#include <gtest/gtest.h>
+#include <matrix/matrix/math.hpp>
+#include <px4_platform_common/defines.h>
+
+#include "LowPassFilter2pVector3f.hpp"
+
+using matrix::Vector3f;
+
+class LowPassFilter2pVector3fTest : public ::testing::Test
+{
+public:
+	math::LowPassFilter2pVector3f _lpf{800.f, 30.f};
+	const float _sample_freq = 1000.f;
+	const float _cutoff_freq = 80.f;
+
+	const float _epsilon_near = 10e-3f;
+};
+
+TEST_F(LowPassFilter2pVector3fTest, setGet)
+{
+	_lpf.set_cutoff_frequency(_sample_freq, _cutoff_freq);
+	EXPECT_EQ(_lpf.get_sample_freq(), _sample_freq);
+	EXPECT_EQ(_lpf.get_cutoff_freq(), _cutoff_freq);
+}
+
+TEST_F(LowPassFilter2pVector3fTest, belowCutoff)
+{
+	_lpf.set_cutoff_frequency(_sample_freq, _cutoff_freq);
+
+	const float signal_freq = 10.f;
+	const float omega = 2.f * M_PI_F * signal_freq;
+	const float phase_delay = 10.4f * M_PI_F / 180.f; // Given by simulation
+	const float dt = 1.f / _sample_freq;
+
+	float t = 0.f;
+
+	for (int i = 0; i < 1000; i++) {
+		float input = sinf(omega * t);
+		float output_expected = sinf(omega * t - phase_delay);
+		Vector3f out = _lpf.apply(Vector3f(0.f, input, -input));
+		t = i * dt;
+
+		// Let some time for the filter to settle
+		if (i > 30) {
+			EXPECT_EQ(out(0), 0.f);
+			EXPECT_NEAR(out(1), output_expected, _epsilon_near);
+			EXPECT_NEAR(out(2), -output_expected, _epsilon_near);
+		}
+	}
+}
+
+TEST_F(LowPassFilter2pVector3fTest, aboveCutoff)
+{
+	_lpf.set_cutoff_frequency(_sample_freq, _cutoff_freq);
+
+	const float signal_freq = 100.f;
+	const float omega = 2.f * M_PI_F * signal_freq;
+	const float phase_delay = 108.5f * M_PI_F / 180.f; // Given by simulation
+	const float gain = 0.52f; // = -5.66 dB, given by simulation
+	const float dt = 1.f / _sample_freq;
+
+	float t = 0.f;
+
+	for (int i = 0; i < 1000; i++) {
+		float input = sinf(omega * t);
+		float output_expected = gain * sinf(omega * t - phase_delay);
+		Vector3f out = _lpf.apply(Vector3f(0.f, input, -input));
+		t = i * dt;
+
+		// Let some time for the filter to settle
+		if (i > 30) {
+			EXPECT_EQ(out(0), 0.f);
+			EXPECT_NEAR(out(1), output_expected, _epsilon_near);
+			EXPECT_NEAR(out(2), -output_expected, _epsilon_near);
+		}
+	}
+}