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Filter: LowPass: use calc_lowpass_alpha_dt in apply and prefix all class varables with _ #27582

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Filter: LowPass: use calc_lowpass_alpha_dt in apply and prefix all class varables with _ #27582

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35 changes: 8 additions & 27 deletions libraries/Filter/LowPassFilter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9,7 +9,6 @@
#pragma GCC optimize("O2")
#endif
#include "LowPassFilter.h"
#include <AP_InternalError/AP_InternalError.h>

////////////////////////////////////////////////////////////////////////////////////////////
// DigitalLPF
Expand All @@ -24,33 +23,15 @@ DigitalLPF<T>::DigitalLPF() {
// add a new raw value to the filter, retrieve the filtered result
template <class T>
T DigitalLPF<T>::apply(const T &sample, float cutoff_freq, float dt) {
if (is_negative(cutoff_freq) || is_negative(dt)) {
INTERNAL_ERROR(AP_InternalError::error_t::invalid_arg_or_result);
_output = sample;
return _output;
}
if (is_zero(cutoff_freq)) {
_output = sample;
return _output;
}
if (is_zero(dt)) {
return _output;
}
float rc = 1.0f/(M_2PI*cutoff_freq);
alpha = constrain_float(dt/(dt+rc), 0.0f, 1.0f);
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should this constrain be added to calc_lowpass_alpha_dt?

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@IamPete1 IamPete1 Jul 18, 2024 •
edited
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See: #20911

The constrain will never trigger because we have already checked both values are > 0.

_output += (sample - _output) * alpha;
if (!initialised) {
initialised = true;
_output = sample;
}
return _output;
_alpha = calc_lowpass_alpha_dt(dt, cutoff_freq);
return apply(sample);
}

template <class T>
T DigitalLPF<T>::apply(const T &sample) {
_output += (sample - _output) * alpha;
if (!initialised) {
initialised = true;
_output += (sample - _output) * _alpha;
if (!_initialised) {
_initialised = true;
_output = sample;
}
return _output;
Expand All @@ -59,9 +40,9 @@ T DigitalLPF<T>::apply(const T &sample) {
template <class T>
void DigitalLPF<T>::compute_alpha(float sample_freq, float cutoff_freq) {
if (sample_freq <= 0) {
alpha = 1;
_alpha = 1;
} else {
alpha = calc_lowpass_alpha_dt(1.0/sample_freq, cutoff_freq);
_alpha = calc_lowpass_alpha_dt(1.0/sample_freq, cutoff_freq);
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note that we are already using the calc_lowpass_alpha_dt function in this case.

}
}

Expand All @@ -74,7 +55,7 @@ const T &DigitalLPF<T>::get() const {
template <class T>
void DigitalLPF<T>::reset(T value) {
_output = value;
initialised = true;
_initialised = true;
}

////////////////////////////////////////////////////////////////////////////////////////////
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8 changes: 4 additions & 4 deletions libraries/Filter/LowPassFilter.h
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Original file line number Diff line number Diff line change
Expand Up @@ -59,18 +59,18 @@ class DigitalLPF {
CLASS_NO_COPY(DigitalLPF);

void compute_alpha(float sample_freq, float cutoff_freq);

// get latest filtered value from filter (equal to the value returned by latest call to apply method)
const T &get() const;
void reset(T value);
void reset() {
initialised = false;
_initialised = false;
}

private:
T _output;
float alpha = 1.0f;
bool initialised;
float _alpha = 1.0f;
bool _initialised;
Comment on lines +72 to +73
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I thought we were trying to canonicalise the other way, to remove leading underscores over time

};

// LPF base class
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