ardupilot/libraries/PID/PID.cpp

// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-

/// @file	PID.cpp
/// @brief	Generic PID algorithm

#include <cmath>

#include "PID.h"
#include <AP_HAL/AP_HAL.h>
#include <AP_Math/AP_Math.h>

extern const AP_HAL::HAL& hal;

const AP_Param::GroupInfo PID::var_info[] = {

    // @Param: P
    // @DisplayName: PID Proportional Gain
    // @Description: P Gain which produces an output value that is proportional to the current error value
    AP_GROUPINFO("P",    0, 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
    AP_GROUPINFO("I",    1, 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
    AP_GROUPINFO("D",    2, PID, _kd, 0),

    // @Param: IMAX
    // @DisplayName: PID Integral Maximum
    // @Description: The maximum/minimum value that the I term can output
    AP_GROUPINFO("IMAX", 3, PID, _imax, 0),

    AP_GROUPEND
};

float PID::get_pid(float error, float scaler)
{
    uint32_t tnow = AP_HAL::millis();
    uint32_t dt = tnow - _last_t;
    float output            = 0;
    float delta_time;

    if (_last_t == 0 || dt > 1000) {
        dt = 0;

		// if this PID hasn't been used for a full second then zero
		// the intergator term. This prevents I buildup from a
		// previous fight mode from causing a massive return before
		// the integrator gets a chance to correct itself
		reset_I();
    }
    _last_t = tnow;

    delta_time = (float)dt / 1000.0f;

    // Compute proportional component
    _pid_info.P = error * _kp;
    output += _pid_info.P;

    // Compute derivative component if time has elapsed
    if ((fabsf(_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 {
			derivative = (error - _last_error) / delta_time;
		}

        // discrete low pass filter, cuts out the
        // high frequency noise that can drive the controller crazy
        float RC = 1/(2*M_PI*_fCut);
        derivative = _last_derivative +
                     ((delta_time / (RC + delta_time)) *
                      (derivative - _last_derivative));

        // update state
        _last_error             = error;
        _last_derivative    = derivative;

        // add in derivative component
        _pid_info.D = _kd * derivative;
        output                          += _pid_info.D;
    }

    // scale the P and D components
    output *= scaler;
    _pid_info.D *= scaler;
    _pid_info.P *= scaler;

    // Compute integral component if time has elapsed
    if ((fabsf(_ki) > 0) && (dt > 0)) {
        _integrator             += (error * _ki) * scaler * delta_time;
        if (_integrator < -_imax) {
            _integrator = -_imax;
        } else if (_integrator > _imax) {
            _integrator = _imax;
        }
        _pid_info.I = _integrator;
        output                          += _integrator;
    }

    _pid_info.desired = output;
    return output;
}

void
PID::reset_I()
{
    _integrator = 0;
	// we use NAN (Not A Number) to indicate that the last 
	// derivative value is not valid
    _last_derivative = NAN;
    _pid_info.I = 0;
}

void
PID::load_gains()
{
    _kp.load();
    _ki.load();
    _kd.load();
    _imax.load();
}

void
PID::save_gains()
{
    _kp.save();
    _ki.save();
    _kd.save();
    _imax.save();
}