Noobie to arduino need help with incubator project

Hey all

trying to get pid working in this using dht22 but cant get it to work any input heres my code

// Example testing sketch for various DHT humidity/temperature sensors
// Written by ladyada, public domain

#include "DHT.h"
#include <PID_v1.h>

#define DHTPIN 12     // what pin we're connected to

// Uncomment whatever type you're using!
//#define DHTTYPE DHT11   // DHT 11
#define DHTTYPE DHT22   // DHT 22  (AM2302)
//#define DHTTYPE DHT21   // DHT 21 (AM2301)

// Connect pin 1 (on the left) of the sensor to +5V
// NOTE: If using a board with 3.3V logic like an Arduino Due connect pin 1
// to 3.3V instead of 5V!
// Connect pin 2 of the sensor to whatever your DHTPIN is
// Connect pin 4 (on the right) of the sensor to GROUND
// Connect a 10K resistor from pin 2 (data) to pin 1 (power) of the sensor

// Initialize DHT sensor for normal 16mhz Arduino
DHT dht(DHTPIN, DHTTYPE);
// NOTE: For working with a faster chip, like an Arduino Due or Teensy, you
// might need to increase the threshold for cycle counts considered a 1 or 0.
// You can do this by passing a 3rd parameter for this threshold.  It's a bit
// of fiddling to find the right value, but in general the faster the CPU the
// higher the value.  The default for a 16mhz AVR is a value of 6.  For an
// Arduino Due that runs at 84mhz a value of 30 works.
// Example to initialize DHT sensor for Arduino Due:
//DHT dht(DHTPIN, DHTTYPE, 30);

int heater = 3;

//Define Variables we'll be connecting to
double Setpoint, Input, Output,t;

//Specify the links and initial tuning parameters
PID myPID(&Input, &Output, &Setpoint,2,3,0, DIRECT);

void setup() {
  Serial.begin(9600);
  Serial.println("DHTxx test!");

  dht.begin();
// from here is heater control.
   pinMode (heater, OUTPUT);
  
    //initialize the variables we're linked to
  Input = t;
  Setpoint = 24.0;

  //turn the PID on
  myPID.SetMode(AUTOMATIC);

  
}

void loop() {
 
// Wait a few seconds between measurements.
  delay(2000);

  // Reading temperature or humidity takes about 250 milliseconds!
  // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
  float h = dht.readHumidity();
  // Read temperature as Celsius
  float t = dht.readTemperature();
  // Read temperature as Fahrenheit
  float f = dht.readTemperature(true);
 
  // Check if any reads failed and exit early (to try again).
  if (isnan(h) || isnan(t) || isnan(f)) {
    Serial.println("Failed to read from DHT sensor!");
    return;
  }

  // Compute heat index
  // Must send in temp in Fahrenheit!
  float hi = dht.computeHeatIndex(f, h);

  Serial.print("Humidity: ");
  Serial.print(h);
  Serial.print(" %\t");
  Serial.print("Temperature: ");
  Serial.print(t);
  Serial.print(" *C ");
  Serial.print(f);
  Serial.print(" *F\t");
  Serial.print("Heat index: ");
  Serial.print(hi);
  Serial.println(" *F");
 
//from here is heater control.
//analogWrite(heater, 100);

//if (t <= 31)
//     {analogWrite(heater, 254);}
     
//else if (t >= 33)
         // {analogWrite(heater, 20);}
         
//else {analogWrite(heater, 100);}
     
   
  myPID.Compute();
  analogWrite(heater,Output); 
 
}

Sean,

          not sure what you are using for a heater but with a system such as yours I doubt PID will work very well at all.  Firstly these sensors are very slow as typified by the 250 mS response time and the 2 seconds between readings.  Secondly there is the thermal lag of the heater and the thermal mass of the system.  All in all the system becomes very unresponsive.

An alternative may be to use PWM to control the power to / from the heater and build in control of the on / off based on trial runs.  For example if the heater is required to turn off at 18 degrees C and the temperature over shot to 20 degrees then you could actually turn if off at 16 degrees in the program.  You could log the temperature over a period of time using Coolterm with a log file.

I appreciate this is not the answer you asked for but it is an alternative approach.

Bob

PID will not work any less well because the system is slow - you just need to take that into account when calculating the PID loop constants.

Almost all low cost commercial temperature controllers use modified PID control loops - they work equally well with time constants from a few tens of seconds to hours or days.

@ Sean - is the any documentation for the PID library you are using ?

When you say it doesn't work - what exactly is happening ?

MK

This is Library I'm useing http://playground.arduino.cc/Code/PIDLibrary also have this http://brettbeauregard.com/blog/2011/04/improving-the-beginners-pid-introduction/

What what is happening is with code I have input as t.so I want to read t as temp and I have set point at 24 and my output is analog write to gate pin of a MOSFET that controls a 55 watt 12 volt bulb for heater when I run setup temp just keeps climing up to max heat about 39 deg c also when running and I serial read it freezes a lot not every time but 4 out of 5 times.

I think you may have several problems:

The PID code you are using is designed to be called much more often than its sample time which is set to 100mS by default. If your slow sensors mean that you are only calling it perhaps every 250mS you should set the sample time in the PID code to at least 2500 mS. But because your system is very slow I would be tempted to set the sample time longer to perhaps 10 or 20 seconds (10000 or 20000).

To tune your system set the I and D terms to zero and the proportional term to 25  - this will mean that you get full power out when the error is 10C.

Plot the temperature against the heater power and see if it does what you expect.

Does your heater respond to the analogWrite correctly ?

Are your serial prints working OK - what device do you send them to.

MK

Hey sorry so late getting back to you thanks for all the help have it up and running now and its working good with your info used.

Hey sorry so late getting back to you thanks for all the help have it up and running now and its working good with your info used.

Hey.. I am interested to yr project.. can i get de programming code from u? Your reply will be highly appreciated.