PID controller algorithm for sr90 digital controller

algorithmpid controller

I am trying to implement PI control algorithm in c#. I found some help in internet and i understand the formula how to use. I have measured value(PV)= 20 °C and set point (SP)= 80°C; I would like to update every 2 sec in time. once the measured value reach to setpoint the controller has to switch off like that.

I found this code link to pid algorithm

but what i need is how to calculate gain factors Kp and Ki gains as well as what about dt. I am neglecting Derivative. So please can anyone explain me how to calculate those gain factors. I am using shimaden SR90 digital controller(i bought this). I found some factors in the manual but i am not sure can i use those gain factors.

Edited for doubts.

I have tried your code like this, i am not sure this is the correct way or not.

int temperature;
//int setpoint; // i have commented these because we never used.
//int status = 0;
int pulses = 0;
//int integral = 4;
//int derivative = 5;
int upperTemp = 1000; //100.0C
int upperLimit = 100; //pulses per time frame // i can take 100 pulses per sec.


 runtime //make a timer that runs and updates temperature value...lets say every second
{
  read temperature; //every second
  temperature *= 10; //multiply by 10 for decimal place, but still keeping in integer
  pulses = map(temperature, upperTemp, 0, 0, upperLimit);

  //take this value of pulses to update the timer interval
   I didn't understand the above line if i am right then here i need to call the       
   timer like 
   if(pulses> somevalue)
     call timer
   else
     no call
  }

//borrowed from arduino
long map(long x, long in_min, long in_max, long out_min, long out_max)
{
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

I have some question here
1) This method seems to be different than previous because we never used gains or error in this method?
2) I understand that "upperTemp" is the setpoint value.

I have tried with simple example like this

 int temperature = 20;
 int pulses = 0;
 int upperTemp = 800; //80.0C
 int upperLimit = 100; //pulses per time frame // i can take 100 pulses per sec.

 read temperature; //every second which is 20 in this case
 temperature *= 10; //multiply by 10 for decimal place, 20*10 =200;
  pulses = map(temperature, upperTemp, 0, 0, upperLimit);// map(200,800,0,0,100)

 //then map function will return pulses = 75;

//then here i don't understand that much but i am thinking like that

if(pulses> 75)
  switch on;
else 
   switch off;

I am not sure that i am doing right! can you suggest me some more here.

Best Answer

The key to good engineering is to keep it simple. In your case, you can use something very simple to turn on/off a relay to control a heater. A psuedo code:

int temperature;
int setpoint;
int status = 0;
int pulses = 0;
int integral = 4;
int derivative = 5;
int upperTemp = 1000; //100.0C
int upperLimit = 100; //pulses per time frame
int counter = 0;

//NO PULSING
runtime //make a timer that runs and updates temperature value
{
  read temperature; 
  temperature *= 10; //still keeping in int for faster processing
  status = ((setpoint - temperature)*integral)/derivative; //EX: (800-400)*4/5 = 320
                                                           //EX: (800-780)*4/5 = 16
  status = abs(status); //get absolute value, this for for within range
                        //(800-850)*4/5 = -40 -> ABS(-40) = 40
  if(status < 16)  //from example, 16 means that value is within 2*C, you can always 
    Relay Off;     //change this value to something else (e.g: (800-795)*4/5 = 4
  else             //4 gives 0.5*C tolerance range...which is a bit tight for temp.
    Relay On;      //anyway, play around with this value;
                   //you can also change integral and derivative if you wish.        
}

//WITH PULSING
runtime //make a timer that runs and updates temperature value...lets say every second
{
  read temperature; //every second
  temperature *= 10; //multiply by 10 for decimal place, but still keeping in integer
  pulses = map(temperature, upperTemp, 0, 0, upperLimit); //when temperature increases, 
                                                          //pulses decreases...vice versa
  //take this value of pulses to update the timer interval
  //timer's interval = pulses per second or minute
}

timer
{
  if(Relay On)
    Relay Off;
  else
    Relay On;   
}

//borrowed from arduino
long map(long x, long in_min, long in_max, long out_min, long out_max)
{
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

//PWM method: still need the runtime with pulsing from above. Let's say you get 75 pulses, this means that the relay will be on for 75% of the duration of each pulse and off for the other 25%. Let's say that you want to have a frequency of 1s (1000mS). Relay will be on for 750mS and off for 250mS. If the temperature is very far away from setpoint, then it might even be on for 100% of the time.

//choose entire duration by tick_interval*percentage: 100mS*100 = 10000mS or 10seconds. Back to example. If you have 75 pulses, then relay will be on for 7.5 seconds and off for 2.5 seconds. 

timer //example 100mS per tick
{
  if(counter > 100) //has reached the end of time period
    counter = 0;
  if(counter > pulses) //pulses from above is now the duty cycle
  {
    if(Relay is On)
      Relay off;
  }
  else
  {
    if(Relay is Off)
      Relay on;
  }
  counter++; //increment by 1
}

//this method is more flexible than using a fixed number (30 minutes) because different heater controllers respond differently

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