I have seen many posts on the theory of PID. I do understand the theory. but when it comes to tuning the arduino PID library i could not find any good examples. please correct me where I’m wrong. As i understand in the arduino libraryPIDLibrary
I am using the LED brightness control example. the brightness of a LED is measured by a photoresistor according to that the output to the LED is controlled through analogWright to keep the LED in set brightness.
Set point is the value we want the system to stay (brightness), Input is the photoresistor sensor input. the Output is the value given to the LED pin. So for the system to be perfectly stable the Setpoint = Input and the Output =0, right ?? that cant be wright, If the output is 0 the LED turns OFF.
In tuning I’m trying to follow these rules
1.Set all gains to zero.
2.Increase the Kp until the response to a disturbance is steady oscillation.
3.Increase the Td until the oscillations stop (i.e. it’s critically damped).
4.Repeat steps 2 and 3 until increasing the Td does not stop the oscillations.
5.Set Kp and Td to the last stable values.
6.Increase Ti gain until the convergence to the set point occurs with or without overshoot at an acceptable rate.
first of all how to identify steady oscillation ?
In the attached picture the set point is 30 and the kp=2,Ki=0,Kd=0. red colore is Input, Blue is Set pint and Green is Output.
these outputs are confusing. can some one please explain what is going on hear. these responses are without changing the set point.
// From https://github.com/bcomnes/315-lab-microcontroller/blob/master/code/pid_led_set_serial/pid_led_set_serial.ino
#include <PID_v1.h>
const int photores = A0; // Photo resistor input
const int pot = A1; // Potentiometer input
const int led = 9; // LED output
double lightLevel; //variable that stores the incoming light level
// Tuning parameters
float Kp=2; //Initial Proportional Gain
float Ki=0; //Initial Integral Gain
float Kd=0; //Initial Differential Gain
double Setpoint =30; // fixed setpoint
double Input, Output; //These are just variables for storing values
PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, REVERSE); // This sets up our PDID Loop
//Input is our PV
//Output is our u(t)
//Setpoint is our SP
const int sampleRate = 1; // Variable that determines how fast our PID loop runs
// Communication setup
const long serialPing = 500; //This determines how often we ping our loop
// Serial pingback interval in milliseconds
unsigned long now = 0; //This variable is used to keep track of time
// placehodler for current timestamp
unsigned long lastMessage = 0; //This keeps track of when our loop last spoke to serial
// last message timestamp.
void setup(){
lightLevel = analogRead(photores); //Read in light level
Input = map(lightLevel, 0, 1024, 0, 255); //Change read scale to analog out scale
//Setpoint = map(analogRead(pot), 0, 1024, 0, 255); //get our setpoint from our pot
Serial.begin(9600); //Start a serial session
myPID.SetMode(AUTOMATIC); //Turn on the PID loop
myPID.SetSampleTime(sampleRate); //Sets the sample rate
myPID.SetOutputLimits(0,255);
Serial.println("Begin"); // Hello World!
lastMessage = millis(); // timestamp
}
void loop(){
//Setpoint = map(analogRead(pot), 0, 1024, 0, 255); //Read our setpoint
lightLevel = analogRead(photores); //Get the light level
Input = map(lightLevel, 0, 1024, 0, 255); //Map it to the right scale
myPID.Compute(); //Run the PID loop
analogWrite(led, Output); //Write out the output from the PID loop to our LED pin
now = millis(); //Keep track of time
if(now - lastMessage > serialPing) { //If its been long enough give us some info on serial
// this should execute less frequently
// send a message back to the mother ship
Serial.print("Setpoint = ");
Serial.print(Setpoint);
Serial.print(" Input = ");
Serial.print(Input);
Serial.print(" Output = ");
Serial.print(Output);
Serial.print("\n");
if (Serial.available() > 0) { //If we sent the program a command deal with it
for (int x = 0; x < 4; x++) {
switch (x) {
case 0:
Kp = Serial.parseFloat();
break;
case 1:
Ki = Serial.parseFloat();
break;
case 2:
Kd = Serial.parseFloat();
break;
case 3:
for (int y = Serial.available(); y == 0; y--) {
Serial.read(); //Clear out any residual junk
}
break;
}
}
Serial.print(" Kp,Ki,Kd = ");
Serial.print(Kp);
Serial.print(",");
Serial.print(Ki);
Serial.print(",");
Serial.println(Kd); //Let us know what we just received
myPID.SetTunings(Kp, Ki, Kd); //Set the PID gain constants and start running
}
lastMessage = now;
//update the time stamp.
}
}
The code was modified not to take pot input as setpoint.
