A very basic PID controller in C

pid_controller.c

/*
 * pid_controller.c
 *
 *  Created on: Apr 24, 2016
 *      Author: subham roy
 */

#include "pid_controller.h"

double pid(PID_vars *vars, double current_err) {
  
  /* current_error = setpoint - current_process_variable */
  
  vars->_integral_sum += current_err*(vars->_dt);
  
  double output = (vars->Kp)*current_err                     \
                + (vars->Ki)*(vars->_integral_sum)           \
                + (vars->Kd)*((current_err-(vars->_prev_err))\
                              /(vars->_dt)); 
  
  vars->_prev_err = current_err;
  
  /* limit output within output_min and output_max */
  if (output>(vars->output_max))
    output = vars->output_max;
  else if (output<(vars->output_min))
    output = vars->output_min;
  
  return output;
}

pid_controller.h

/*
 * pid_controller.h
 *
 *  Created on: Apr 24, 2016
 *      Author: subham roy
 */

#ifndef __PID_CONTROLLER_H
#define __PID_CONTROLLER_H

typedef struct {
  /* PID controller parameters */
  double Kp;
  double Ki;
  double Kd;
  
  /* max output limits for the PID controller */
  double output_max;
  double output_min;
  
  /* below are session variables for the PID controller */
  double _integral_sum;
  double _prev_err;
  double _dt;
} PID_vars;


#define PID_VARS_INIT(x) PID_vars x = {.Kp=0.0,.Ki=0.0,.Kd=0.0,.output_max=100.0, \
                       .output_min=0.0,._integral_sum=0.0,._prev_err=0.0,._dt=1.0}

/* Function Prototypes */
double pid(PID_vars *vars, double current_err);

#endif