Hello,
I am using the code below to heat up a yoghurt maker. My heater is an 8 ohm 25 watt resistor (on a pc heat-sink). The thermometer used is a DS18b20. I got the code off the internet (http://www.instructables.com/id/Cheap-Arduino-Controled-Yogurt-Maker/) and it works fine. The only problem is that I would like to adjust the parameters to have a more steady temperature (43 degrees Celsius) but I don’t know what the 3 PID variables (Kp, Ki, Kd) are responsible for. I asked the person who made it, but I found his answer wasn’t precise… Could somebody precisely tell me what they do?
// Include necessary libraries
#include <OneWire.h>
#include <DallasTemperature.h>
#include <SD.h>
#include <PID_v1.h>
// Setup vars
long intervalTemp = 1000; // interval between temperature measurements / PID computation
int minOn = 1; // minimum time for the relay to be on
int maxOn = 60; // maximum time for the relay to be on, IE, one cicle
long intervalSD = 5000; // interval between saving data on the SD card
double Setpoint = 43; // temperature to be manteined
int relayPin = 3; // pin where the relay is connected
const int chipSelect = 10; // pin CS (SD card) (optional)
int led1 = 4;
int led2 = 5;
// Internal vars (don't change)
long previousMillisTemp = 0;
long previousMillisSD = 0;
unsigned long windowStartTime;
float tempAtual = 0; // current temp
int stat = 0; // 0 = relay off / 1 = relay on
static char tempBuff[15];
File datafile;
// PID parameters (input, output, setpoint, Kp, Ki, Kd, direction)
double Input, Output;
PID myPID(&Input, &Output, &Setpoint,35,30,5, DIRECT);
// Configures thermometer
OneWire oneWire(2); // pin where the themometer is connected
DallasTemperature sensors(&oneWire);
DeviceAddress insideThermometer;
void setup() {
sensors.begin();
sensors.getAddress(insideThermometer, 0);
//sensors.setResolution(insideThermometer, 11); //set thermometer do max resolution
pinMode(relayPin, OUTPUT);
pinMode(10, OUTPUT);
pinMode(led1, OUTPUT);
pinMode(led2, OUTPUT);
// PID
windowStartTime = ceil(millis()/1000);
myPID.SetOutputLimits(minOn, maxOn);
myPID.SetMode(AUTOMATIC);
Serial.begin(9600);
if (!SD.begin(chipSelect)) {
Serial.println("SD card initialized.");
// Create a new file
char filename[] = "DATA00.TXT";
for (uint8_t i = 0; i < 100; i++) {
filename[4] = i/10 + '0';
filename[5] = i%10 + '0';
if (! SD.exists(filename)) {
// only open a new file if it doesn't exist
datafile = SD.open(filename, FILE_WRITE);
break; // leave the loop!
}
}
if (! datafile) {
Serial.print("Logging to: ");
Serial.println(filename);
}
}
Serial.print("Found ");
Serial.print(sensors.getDeviceCount(), DEC);
Serial.println(" thermometers.");
Serial.print("Device 0 Resolution: ");
Serial.println(sensors.getResolution(insideThermometer), DEC);
Serial.print("Set temperature: ");
Serial.println(Setpoint);
Serial.println("##################");
}
void loop() {
unsigned long currentMillis = millis();
// Checks current temp and prints it on serial
if((currentMillis - previousMillisTemp > intervalTemp)||(currentMillis - previousMillisTemp < 0)) {
// Updates previous millis
previousMillisTemp = currentMillis;
// Gets temperature
sensors.requestTemperatures();
tempAtual = sensors.getTempCByIndex(0);
// Computes PID
Input = tempAtual;
myPID.Compute();
// Controls relay
unsigned long now = ceil(millis()/1000);
if(now - windowStartTime > maxOn) { // maximumOn reached
windowStartTime += maxOn;
}
if(Output >= now - windowStartTime) { // output defines how much of the time >minOn and <maxOn the relay will be on, the remaining time it will be off.
digitalWrite(relayPin, HIGH);
digitalWrite(led1, HIGH);
digitalWrite(led2, LOW);
stat = 1;
} else {
digitalWrite(relayPin, LOW);
digitalWrite(led2, HIGH);
digitalWrite(led1, LOW);
stat = 0;
}
// Prints current temperature on serial
Serial.print("Temp: ");
Serial.print(sensors.getTempCByIndex(0));
Serial.print("C | Output: ");
Serial.print(Output);
Serial.print(" | Heat: ");
if(stat == 0) {
Serial.println("off");
} else {
Serial.println("on");
}
}
// Saves data on the SD card
if((currentMillis - previousMillisSD > intervalSD)||(currentMillis - previousMillisSD < 0)) {
// Updates previous millis
previousMillisSD = currentMillis;
// Creates string to save on file
String dataString = "";
dtostrf(tempAtual, 5, 2, tempBuff);
dataString.concat(tempBuff);
dataString.concat(",");
dataString.concat(stat);
// If the file is available, write to it:
if (datafile) {
datafile.println(dataString);
datafile.flush();
}
}
}
/* Code ends here! */
Thanks!