LowPassFilter2p.cpp

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#include "LowPassFilter2p.hpp"

#include <px4_platform_common/defines.h>

#include <math.h>

namespace math
{

void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
{
    _cutoff_freq = cutoff_freq;

    // reset delay elements on filter change
    _delay_element_1 = 0.0f;
    _delay_element_2 = 0.0f;

    if (_cutoff_freq <= 0.0f) {
        // no filtering
        _b0 = 1.0f;
        _b1 = 0.0f;
        _b2 = 0.0f;

        _a1 = 0.0f;
        _a2 = 0.0f;

        return;
    }

    const float fr = sample_freq / _cutoff_freq;
    const float ohm = tanf(M_PI_F / fr);
    const 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)
{
    // 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;
    }

    const 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)
{
    const float dval = sample / (_b0 + _b1 + _b2);

    if (PX4_ISFINITE(dval)) {
        _delay_element_1 = dval;
        _delay_element_2 = dval;

    } else {
        _delay_element_1 = sample;
        _delay_element_2 = sample;
    }

    return apply(sample);
}

} // namespace math