ardupilot/libraries/AC_PID/AC_PID.cpp

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// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
/// @file AC_PID.cpp
/// @brief Generic PID algorithm
#include <AP_Math.h>
#include "AC_PID.h"
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const AP_Param::GroupInfo AC_PID::var_info[] PROGMEM = {
// @Param: P
// @DisplayName: PID Proportional Gain
// @Description: P Gain which produces an output value that is proportional to the current error value
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AP_GROUPINFO("P", 0, AC_PID, _kp, 0),
// @Param: I
// @DisplayName: PID Integral Gain
// @Description: I Gain which produces an output that is proportional to both the magnitude and the duration of the error
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AP_GROUPINFO("I", 1, AC_PID, _ki, 0),
// @Param: D
// @DisplayName: PID Derivative Gain
// @Description: D Gain which produces an output that is proportional to the rate of change of the error
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AP_GROUPINFO("D", 2, AC_PID, _kd, 0),
// @Param: IMAX
// @DisplayName: PID Integral Maximum
// @Description: The maximum/minimum value that the I term can output
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AP_GROUPINFO("IMAX", 3, AC_PID, _imax, 0),
AP_GROUPEND
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};
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float AC_PID::get_p(float error) const
{
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return (float)error * _kp;
}
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float AC_PID::get_i(float error, float dt)
{
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if((_ki != 0) && (dt != 0)) {
_integrator += ((float)error * _ki) * dt;
if (_integrator < -_imax) {
_integrator = -_imax;
} else if (_integrator > _imax) {
_integrator = _imax;
}
return _integrator;
}
return 0;
}
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float AC_PID::get_d(float input, float dt)
{
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if ((_kd != 0) && (dt != 0)) {
float derivative;
if (isnan(_last_derivative)) {
// we've just done a reset, suppress the first derivative
// term as we don't want a sudden change in input to cause
// a large D output change
derivative = 0;
_last_derivative = 0;
} else {
// calculate instantaneous derivative
derivative = (input - _last_input) / dt;
}
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// discrete low pass filter, cuts out the
// high frequency noise that can drive the controller crazy
derivative = _last_derivative + _d_lpf_alpha * (derivative - _last_derivative);
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// update state
_last_input = input;
_last_derivative = derivative;
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// add in derivative component
return _kd * derivative;
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}
return 0;
}
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float AC_PID::get_pi(float error, float dt)
{
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return get_p(error) + get_i(error, dt);
}
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float AC_PID::get_pid(float error, float dt)
{
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return get_p(error) + get_i(error, dt) + get_d(error, dt);
}
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void AC_PID::reset_I()
{
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_integrator = 0;
// mark derivative as invalid
_last_derivative = NAN;
}
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void AC_PID::load_gains()
{
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_kp.load();
_ki.load();
_kd.load();
_imax.load();
_imax = abs(_imax);
}
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void AC_PID::save_gains()
{
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_kp.save();
_ki.save();
_kd.save();
_imax.save();
}
void AC_PID::set_d_lpf_alpha(int16_t cutoff_frequency, float time_step)
{
// calculate alpha
float rc = 1/(2*PI*cutoff_frequency);
_d_lpf_alpha = time_step / (time_step + rc);
}