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// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
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/// @file LowPassFilter.cpp
/// @brief A class to implement a second order low pass filter
/// Author: Leonard Hall <LeonardTHall@gmail.com>
#include <px4_defines.h>
#include "LowPassFilter2p.hpp"
#include "math.h"
#ifndef M_PI_F
#define M_PI_F 3.14159f
#endif
namespace math
{
void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
{
_cutoff_freq = cutoff_freq;
if (_cutoff_freq <= 0.0f) {
// no filtering
return;
}
float fr = sample_freq/_cutoff_freq;
float ohm = tanf(M_PI_F/fr);
float c = 1.0f+2.0f*cosf(M_PI_F/4.0f)*ohm + ohm*ohm;
_b0 = ohm*ohm/c;
_b1 = 2.0f*_b0;
_b2 = _b0;
_a1 = 2.0f*(ohm*ohm-1.0f)/c;
_a2 = (1.0f-2.0f*cosf(M_PI_F/4.0f)*ohm+ohm*ohm)/c;
}
float LowPassFilter2p::apply(float sample)
{
if (_cutoff_freq <= 0.0f) {
// no filtering
return sample;
}
// do the filtering
float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
if (!PX4_ISFINITE(delay_element_0)) {
// don't allow bad values to propagate via the filter
delay_element_0 = sample;
}
float output = delay_element_0 * _b0 + _delay_element_1 * _b1 + _delay_element_2 * _b2;
_delay_element_2 = _delay_element_1;
_delay_element_1 = delay_element_0;
// return the value. Should be no need to check limits
return output;
}
float LowPassFilter2p::reset(float sample) {
float dval = sample / (_b0 + _b1 + _b2);
_delay_element_1 = dval;
_delay_element_2 = dval;
return apply(sample);
}
} // namespace math