```
/**
* PID Controller
*
* The PID controller is a linear control algorithm that has three terms:
* - Proportional: A simple scaling of the error value by a gain kP
* - Integral: Integration of the error value over time, then multipled by gain kI
* - Derivative: Rate of change of the error value over time, multiplied by gain kD
*
* Terms of the controller can be removed by setting their gain to 0, creating a PI (kD = 0)
* or PD (kI = 0) controller. Depending on the control problem at hand, some terms may not
* increase the performance of the system, or may have a negative effect.
*
* For a more mathematical expanation of the PID Controller, see https://en.wikipedia.org/wiki/PID_controller
*
* Limitations of this implementation:
* - Since this implementation is just for demonstration, the pid_step function takes the
* dt as a parameter, and it can be provided by the user in main(). This allows deterministic
* experimentation with the algorithm, rather than using time(NULL) which would make the function
* non-deterministic.
*
* Inputs: e(t) - Current error at time t. For example, how far a servo is off the desired angle
* Output: u(t) - Controller output at time t.
*/
#include <stdio.h>
struct pid {
// Controller gains
float kP;
float kI;
float kD;
// State variables
float lastError;
float integral;
};
float pid_step(struct pid* controller, float dt, float error) {
// Calculate p term
float p = error * controller->kP;
// Calculate i term
controller->integral += error * dt * controller->kI;
// Calculate d term, taking care to not divide by zero
float d = dt == 0 ? 0 : ((error - controller->lastError) / dt) * controller->kD;
controller->lastError = error;
return p + controller->integral + d;
}
int main() {
printf("PID Controller Example\n");
struct pid controller = {
.lastError = 0,
.integral = 0
};
// Take the controller gains from the user
printf("Please enter controller gains in format kP, kI, KD. For example, \"1.2 2.1 3.2\"\n> ");
scanf("%f %f %f", &controller.kP, &controller.kI, &controller.kD);
printf("Using kP: %f, kI: %f, kD: %f\n", controller.kP, controller.kI, controller.kD);
// How often the pid_step algorithm expects to be called. In a real life scenario this would
// be provided by calling time(NULL) - last_time, or by calling the function reliably at X Hz (using a timer or RTOS etc)
// For demonstration of this algorithm though, it is defined below as 1 second, allowing easy testing of integral
// and derivative terms.
float time_step = 1;
float error_value;
while (1) {
printf("Enter error value\n>");
scanf("%f", &error_value);
float output = pid_step(&controller, time_step, error_value);
printf("Output: %f\n", output);
}
}
```