Move AlphaFilter out of EKF

AlphaFilter/AlphaFilter.hpp

@@ -0,0 +1,95 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2019-2020 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.
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+ *    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
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+ *
+ ****************************************************************************/
+
+/**
+ * @file AlphaFilter.hpp
+ *
+ * @brief First order "alpha" IIR digital filter also known as leaky integrator or forgetting average.
+ *
+ * @author Mathieu Bresciani <brescianimathieu@gmail.com>
+ * @author Matthias Grob <maetugr@gmail.com>
+ */
+
+#pragma once
+
+template <typename T>
+class AlphaFilter {
+public:
+	AlphaFilter() = default;
+	~AlphaFilter() = default;
+
+	/**
+	 * Set filter parameters for time abstraction
+	 *
+	 * Both parameters have to be provided in the same units.
+	 *
+	 * @param sample_interval interval between two samples
+	 * @param time_constant filter time constant determining convergence
+	 */
+	void setParameters(float sample_interval, float time_constant) {
+		const float denominator = time_constant + sample_interval;
+
+		if (denominator > FLT_EPSILON) {
+			setAlpha(sample_interval / denominator);
+		}
+	}
+
+	/**
+	 * Set filter parameter alpha directly without time abstraction
+	 *
+	 * @param alpha [0,1] filter weight for the previous state. High value - long time constant.
+	 */
+	void setAlpha(float alpha) { _alpha = alpha; }
+
+	/**
+	 * Set filter state to an initial value
+	 *
+	 * @param sample new initial value
+	 */
+	void reset(const T &sample) { _filter_state = sample; }
+
+	/**
+	 * Add a new raw value to the filter
+	 *
+	 * @return retrieve the filtered result
+	 */
+	void update(const T &sample) { _filter_state = updateCalculation(sample); }
+
+	const T &getState() const { return _filter_state; }
+
+protected:
+	T updateCalculation(const T &sample) { return (1.f - _alpha) * _filter_state + _alpha * sample; }
+
+	float _alpha{0.f};
+	T _filter_state{};
+};