pms67/pid

PID controller implementation written in C.

pid's Issues

Question about function

Hello， I would like to ask how the finally function 'TestSystem_Update' in the file 'PID_Test.c' comes from and what is the specific significance. Thank you~

Issue with D component

Code now (bi-linear transform):

pid->differentiator = -(2.0f * pid->Kd * (measurement - pid->prevMeasurement)
                        + (2.0f * pid->tau - pid->T) * pid->differentiator)
                        / (2.0f * pid->tau + pid->T);

if you assume tau = 0, e[n] - e[n-1] = 0, then you'll get: D[n] = D[n-1], so it keeps always the previous value (and doesn't go to 0).
if you assume tau >>T, e[n] - e[n-1] = 0, then you'll get: D[n] = -1*D[n-1], so it keeps oscillating (and doesn't go to zero).

After reviewing other sources, I suggest using Backward-Euler transform s→(z-1)/z, then you'll get:

D[n] = K[d]/(tau + T)*(e[n] - e[n-1]) + tau/(tau + T)*D[n-1] // classical form
D[n] = K[d]/(tau + T)*(measurement[n-1] - measurement[n]) + tau/(tau + T)*D[n-1] // derivative on measurement

in this case it works as expected, for both "test cases" from above, if e[n] - e[n-1] = 0 then D[n] = 0

PID phlips lab

Pid

#include <stdio.h>

#define KP 2.0
#define KI 0.5
#define KD 0.1

float setpoint = 25.0;
float error, last_error, integral, derivative;
float temperature[3], pwm_output[3];

void read_temperature()
{
// read temperature from sensors and store in temperature array
}

void update_pid()
{
// calculate error value
error = setpoint - temperature[0];

// calculate integral term
integral += error;

// calculate derivative term
derivative = error - last_error;

// calculate PID output
pwm_output[0] = KP * error + KI * integral + KD * derivative;

// limit PWM output to a safe range
if (pwm_output[0] > 255) pwm_output[0] = 255;
if (pwm_output[0] < 0) pwm_output[0] = 0;

// update last_error
last_error = error;

// set PWM output to thermoelectric cooler
}

int main()
{
// initialize integral and last_error
integral = 0.0;
last_error = 0.0;

while (1)
{
read_temperature();
update_pid();
}

return 0;
}

License clarification

Can you clarify which license applies to this code, please?

antiwindup should be extended to a test, whether the output of P,I and D will be limited by the output range

Basically the integration can be stopped when a PID(timestep+1) would be limited by the all 3 parts. This is resulting into calculating all 3 parts and throwing away the part for the integration when a limit would occur.

Add the link to your video in the README

Hello Mr Phil,

I think you should add the link to your video in the README. There is some useful information in the video https://www.youtube.com/watch?v=zOByx3Izf5U.

Best,
Vincent