LibrePilot/flight/libraries/math/pid.c

/**
 ******************************************************************************
 * @addtogroup OpenPilot Math Utilities
 * @{
 * @addtogroup Sine and cosine methods that use a cached lookup table
 * @{
 *
 * @file       pid.c
 * @author     The OpenPilot Team, http://www.openpilot.org Copyright (C) 2012.
 * @brief      Methods to work with PID structure
 *
 * @see        The GNU Public License (GPL) Version 3
 *
 *****************************************************************************/
/*
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 * for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */

#include "openpilot.h"
#include "pid.h"
#include <pios_math.h>

// ! Private method
static float bound(float val, float range);

// ! Store the shared time constant for the derivative cutoff.
static float deriv_tau   = 7.9577e-3f;

// ! Store the setpoint weight to apply for the derivative term
static float deriv_gamma = 1.0f;

/**
 * Update the PID computation
 * @param[in] pid The PID struture which stores temporary information
 * @param[in] err The error term
 * @param[in] dT  The time step
 * @returns Output the computed controller value
 */
float pid_apply(struct pid *pid, const float err, float dT)
{
    // Scale up accumulator by 1000 while computing to avoid losing precision
    pid->iAccumulator += err * (pid->i * dT * 1000.0f);
    pid->iAccumulator  = bound(pid->iAccumulator, pid->iLim * 1000.0f);

    // Calculate DT1 term
    float diff  = (err - pid->lastErr);
    float dterm = 0;
    pid->lastErr = err;
    if (pid->d > 0.0f && dT > 0.0f) {
        dterm = pid->lastDer + dT / (dT + deriv_tau) * ((diff * pid->d / dT) - pid->lastDer);
        pid->lastDer = dterm; // ^ set constant to 1/(2*pi*f_cutoff)
    } // 7.9577e-3  means 20 Hz f_cutoff

    return (err * pid->p) + pid->iAccumulator / 1000.0f + dterm;
}

/**
 * Update the PID computation with setpoint weighting on the derivative
 * @param[in] pid The PID struture which stores temporary information
 * @param[in] setpoint The setpoint to use
 * @param[in] measured The measured value of output
 * @param[in] dT  The time step
 * @returns Output the computed controller value
 *
 * This version of apply uses setpoint weighting for the derivative component so the gain
 * on the gyro derivative can be different than the gain on the setpoint derivative
 */
float pid_apply_setpoint(struct pid *pid, const float setpoint, const float measured, float dT)
{
    float err = setpoint - measured;

    // Scale up accumulator by 1000 while computing to avoid losing precision
    pid->iAccumulator += err * (pid->i * dT * 1000.0f);
    pid->iAccumulator  = bound(pid->iAccumulator, pid->iLim * 1000.0f);

    // Calculate DT1 term,
    float dterm = 0;
    float diff  = ((deriv_gamma * setpoint - measured) - pid->lastErr);
    pid->lastErr = (deriv_gamma * setpoint - measured);
    if (pid->d > 0.0f && dT > 0.0f) {
        dterm = pid->lastDer + dT / (dT + deriv_tau) * ((diff * pid->d / dT) - pid->lastDer);
        pid->lastDer = dterm; // ^ set constant to 1/(2*pi*f_cutoff)
    } // 7.9577e-3  means 20 Hz f_cutoff

    return (err * pid->p) + pid->iAccumulator / 1000.0f + dterm;
}

/**
 * Reset a bit
 * @param[in] pid The pid to reset
 */
void pid_zero(struct pid *pid)
{
    if (!pid) {
        return;
    }

    pid->iAccumulator = 0;
    pid->lastErr = 0;
    pid->lastDer = 0;
}

/**
 * @brief Configure the common terms that alter ther derivative
 * @param[in] cutoff The cutoff frequency (in Hz)
 * @param[in] gamma The gamma term for setpoint shaping (unsused now)
 */
void pid_configure_derivative(float cutoff, float g)
{
    deriv_tau   = 1.0f / (2 * M_PI_F * cutoff);
    deriv_gamma = g;
}

/**
 * Configure the settings for a pid structure
 * @param[out] pid The PID structure to configure
 * @param[in] p The proportional term
 * @param[in] i The integral term
 * @param[in] d The derivative term
 */
void pid_configure(struct pid *pid, float p, float i, float d, float iLim)
{
    if (!pid) {
        return;
    }

    pid->p    = p;
    pid->i    = i;
    pid->d    = d;
    pid->iLim = iLim;
}

/**
 * Bound input value between limits
 */
static float bound(float val, float range)
{
    if (val < -range) {
        val = -range;
    } else if (val > range) {
        val = range;
    }
    return val;
}