Simple C implementation of PID controller. MIT Licence.

pid.c

#include "pid.h"

pid_controller_t new_pid(real_t Kp, real_t Ki, real_t Kd)
{
    pid_controller_t ctrl;
    ctrl.Kp = Kp;
    ctrl.Ki = Ki;
    ctrl.Kd = Kd;
    ctrl.sum_errors = 0.0;
    ctrl.previous_error = 0.0;
    ctrl.previous_error_recorded = 0;
    return ctrl;
}

real_t compute_control_action(pid_controller_t *controller, real_t error)
{
    controller->sum_errors += error;
    real_t ud = 0.;
    if (controller->previous_error_recorded == 1) {
        real_t diff = error - controller->previous_error;
        ud = controller->Kd * diff;
    }
    real_t up = controller->Kp * error;
    real_t ui = controller->Ki * controller->sum_errors;
    controller->previous_error = error;
    controller->previous_error_recorded = 1;
    return up + ui + ud;
}

pid.h

/*
 * Discrete-time PID controller
 * 
 * Author: Pantelis Sopasakis
 *         Queen's University Belfast
 * Date: 18 December, 2019
 * 
 * License: MIT License
 *
 * MIT License
 * 
 * Copyright (c) 2019 Pantelis Sopasakis <p.sopasakis@qub.ac.uk>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 * 
 * Library-free implementation; compile with `gcc -c pid.c` or
 * `clang -c pid.c`
 * 
 * Example of use:
 * 
 * ```
 * real_t control_action;
 * pid_controller_t pid = new_pid(5.0, 0.5, 2.0);
 * control_action = compute_control_action(&pid, 1.0);
 * ```
 */

/**
 * Number type (double, float or other) 
 */
typedef double real_t;

/**
 * PID controller structure
 */
typedef struct
{
    real_t Kp;             /* proportional gain */
    real_t Ki;             /* integral gain     */
    real_t Kd;             /* derviative gain   */
    real_t sum_errors;     /* sum of errors */
    real_t previous_error; /* previous error */
    char previous_error_recorded;
} pid_controller_t;

/**
 * Construct a new PID controller with given parameters
 * 
 * Arguments:
 *  - Kp (real_t): proportional gain
 *  - Ki (real_t): integral gain
 *  - Kd (real_t): derivative gain
 * 
 * Returns:
 *  - Instance of `pid_controller_t`
 */
pid_controller_t new_pid(real_t Kp, real_t Ki, real_t Kd);

/**
 * Compute PID cojntrol action
 * 
 * Arguments:
 *  - controller (pid_controller_t): instance of PID controller 
 *  - error (real_t): current error
 * 
 * Returns:
 */
real_t compute_control_action(pid_controller_t *controller, real_t error);

Isn't the time interval term missing in the integral and derivative gain terms?

@nadukandi no, because this is a discrete-time version of the PID controller; the user is expected to provide appropriate gains for a given sampling time. In other words, Kp, Ki and Kd are not the continuous-time PID gains.

@nadukandi if you want to provide the continuous-time PID gains and the sampling time, you can do the following:

id_controller_t new_pid(real_t Kp, real_t Ki, real_t Kd, real_t sampling_time)
{
    pid_controller_t ctrl;
    ctrl.Kp = Kp;
    ctrl.Ki = Ki * sampling_time;
    ctrl.Kd = Kd / sampling_time;
    ctrl.sum_errors = 0.0;
    ctrl.previous_error = 0.0;
    ctrl.previous_error_recorded = 0;
    return ctrl;
}