PID synchronization.

Hello everyone
Few weeks ago i posted a topic about synchronization of two actuators. I received many useful answers, so i decided to implement PID controller.
How it works: I have two actuators, one is master, second is slave (follower). One button is for extension (when we press this button and keep him HIGH, the master starts to move forward) another one is for retraction. Basically, with buttons we are controlling only master, and slave is always following the master (so its always a liitle bit behind of master). Actuators are electrical with potentiometers. They are controlled by one arduino MEGA and two H-bridges (one H-bridge per actuator).
So it works pretty fine now, but I can still see some oscillations. The problem is, when master is stopped, and slave have reached his position, sometimes he begins to move forward and backwards constantly. I wonder what may be wrong in my code. Some useful information: The values from both potentiometers are not the same, to be more precisly, value from potentiometers, when master and slave are fully extended, are not the same (they are oscillating between 443-445).
I would be very grateful for feedback and if u could take a look.
Thanks!

Scheme:

Code:

#include <PID_v1.h>      //include PID library (załącz bibliotekę regulacji PID (http://playground.arduino.cc/Code/PIDLibrary))
  double value, value1;  //values, which will be readed from potentiometers of master and slave (wartosc, ktora bedzie odczytana z potencjometru mastera)
  const int buttonPos = 42;  //button for extension of the master (przycisk do wysuwu mastera z przypisaniem pinu)
  const int buttonPos2 = 43; //button for retrack the master (przycisk do wsuwu mastera)
  double value_imp, value1_imp;        //values, which will be scalled from master and slave (wartosc, ktora zostanie przeskalowana z mastera i slave)
  double Setpoint, Input, Output; //arguments of PID regulator (argumenty funkcji regulatora PID)
  double consKp=1.00, consKi=0.00, consKd=0.00;  //parameteres of PID (nastawy)
  double junk, junk1;
  int pidZeroLimit = 1, pidOutputMinValue = 150, pidOutputValue = 0; //min. speed for slave, initial speed 
  PID myPID(&Input, &Output, &Setpoint, consKp, consKi, consKd, DIRECT); //initialization of pid function (zainicjowanie funkcji)
void setup() 
{
pinMode (2,OUTPUT);   //PWM master
pinMode (3,OUTPUT);   //PWM follower
pinMode (22,OUTPUT);  //extension of master wysuw master 
pinMode (23,OUTPUT);  //retract of master wsuw mastera 
pinMode (24,OUTPUT);  //extension of slave 
pinMode (25,OUTPUT);  //retract of slave 
pinMode (42,INPUT);   //button for extension 
pinMode (43,INPUT);   //button for retract 
analogWrite(2,150);   //speed of master 
junk = analogRead(A0);
Setpoint = analogRead(A0);      //Setpoint - value from master pot., that we want to Input to maintain (wartosc z pot. mastera, do której będzie dążył slave) 
junk1 = analogRead(A1);
Input = analogRead(A1);  //Input - value from slave pot., that we are trying to control (to wartosc z potencjometru slave'a, którą chycemy kontrolować)

myPID.SetTunings(consKp, consKi, consKd); //dictate the dynamic behavior of the PID (ustawienie nastaw regulatora)
myPID.SetOutputLimits(0, 255);  //configurate the speed limits (ustawienie limitów prędkości)
Output = 0; 
myPID.SetMode(AUTOMATIC); 
Serial.begin(9600);
}

void loop() 
{ 
  
  value = analogRead(A0);    //read the value from master pot. (sczytaj wartosc z potencjometr mastera)
  value_imp = map(value, 944, 29, 255, 0);  //scale these value to receive lesser resolution (zeskaluj wartosci (994 - max wysuw, 29 - max wsuw) na 255-0) 
  Setpoint = value_imp; //make a Setpoiunt from scalled value of pot. master (ustaw setpoint, czyli wartosc, jaka input bedzie musial osiagnac)
  value1 = analogRead(A1);    //make the same cycle for slave
  value1_imp = map(value1, 944, 29, 255,0);    
  Input = value1_imp;   
  myPID.Compute(); // 
  
  //sterowanie masterem (master controlling)
  if(digitalRead(buttonPos) == HIGH) //extension if button is pressed (wysuw, jeśli przycisniemy przycisk)
  {
    digitalWrite(22,HIGH); //exntension is HIGFH
    digitalWrite(23,LOW); //retract is LOW
    //digitalWrite(24,HIGH);
    //digitalWrite(25,LOW);
  }
  else if(digitalRead(buttonPos2) == HIGH) //retract if button is pressed
  {
    digitalWrite(22,LOW); 
    digitalWrite(23,HIGH); 
    //digitalWrite(24,LOW); 
    //digitalWrite(25,HIGH);
  }
  else //if nothing is pressed do nothing (jesli nic nie przycisniete to nic nie rob)
  {
   digitalWrite(22,LOW); 
   digitalWrite(23,LOW);
  }
  
  //slave
  int deltaAnalogValue = Setpoint - Input;  //Measure the difference between setpoint and input (Mierz roznice pomiedzy setpoint a input)
  if (Output > pidZeroLimit) //if output is greater than limit than (jesli output jest wiekszy od obszaru tolerancyjneg)
  {
   pidOutputValue = pidOutputMinValue + abs(Output) ; //add the output distance to minimum speed (dodaj do predkosci docelowej predkosc minimalna oraz output)
   //the greater is output, the actuator should exten/retract faster (im wekszy output tym szybciej silownik powinien sie wysuwac/wsuwac)
   pidOutputValue = constrain(pidOutputValue, 0, 255); //the value shouldnt be greater than 255 or lesser than 0 (wartosc ta nie moze przekroczyc 0-255)
   if (deltaAnalogValue > 0) //if the difference is >0: (jesli wartosc roznicy przekracza 0, to:)
   {
     digitalWrite(24,HIGH); //extend the master 
     digitalWrite(25,LOW); //
     analogWrite(3, pidOutputValue); //speed of slave (Przypisz do pwmu slave'a predkosc)
   }
   
  }
  else if(Output < pidZeroLimit) //same as above but for slave
  {
    pidOutputValue = pidOutputMinValue + abs(Output);
    pidOutputValue = constrain(pidOutputValue, 0, 255);
    if (deltaAnalogValue < 0)
    {
      digitalWrite(24,LOW);
      digitalWrite(25,HIGH);
      analogWrite(3, pidOutputValue); //pwm slave
    }
  } 
  else
  {
   digitalWrite(24,LOW);
   digitalWrite(25,LOW);
   pidOutputValue = 0; 
  } 
  Serial.println("Output");
  Serial.println(Output);
  Serial.println("Input");
  Serial.println(Input);
  Serial.println("Setpoint");
  Serial.println(Setpoint);
}

I think part of your problem is modifying the Output based on the relation between Input and Setpoint. I would just set the limits of Output to +/-255 and use the sign for direction and magnitude for speed.

I would also use the analogRead() values directly rather than map them to a smaller range.

I would put the slave motion right after calculating the PID rather than calculating the PID, acting on the buttons, and then acting on the PID. You will also want to tune the PID parameters. Right now only ‘P’ is used.

#include <PID_v1.h>      //include PID library (załącz bibliotekę regulacji PID (http://playground.arduino.cc/Code/PIDLibrary))

const int buttonPos = 42;  //button for extension of the master (przycisk do wysuwu mastera z przypisaniem pinu)
const int buttonPos2 = 43; //button for retrack the master (przycisk do wsuwu mastera)

double Setpoint, Input, Output; //arguments of PID regulator (argumenty funkcji regulatora PID)
double consKp = 1.00, consKi = 0.00, consKd = 0.00; //parameteres of PID (nastawy)

PID myPID(&Input, &Output, &Setpoint, consKp, consKi, consKd, DIRECT); //initialization of pid function (zainicjowanie funkcji)

void setup()
{
  pinMode (2, OUTPUT);  // PWM master
  pinMode (3, OUTPUT);  // PWM slave
  pinMode (22, OUTPUT); //extension of master wysuw master
  pinMode (23, OUTPUT); //retract of master wsuw mastera
  pinMode (24, OUTPUT); //extension of slave
  pinMode (25, OUTPUT); //retract of slave
  pinMode (42, INPUT);  //button for extension
  pinMode (43, INPUT);  //button for retract
  analogWrite(2, 150);  //speed of master

  Output = 0;

  myPID.SetTunings(consKp, consKi, consKd); //dictate the dynamic behavior of the PID (ustawienie nastaw regulatora)
  myPID.SetOutputLimits(-255, 255);  //configurate the speed limits (ustawienie limitów prędkości)
  myPID.SetMode(AUTOMATIC);
  Serial.begin(9600);
}

void loop()
{
  Setpoint = analogRead(A0);
  Input = analogRead(A1);
  myPID.Compute();

  Serial.print("Setpoint: ");
  Serial.print(Setpoint);
  Serial.print(", Input: ");
  Serial.print(Input);
  Serial.print(", Output :");
  Serial.println(Output);

  //slave
  int slaveSpeed = 0;
  if (Output > 0) {
    slaveSpeed = Output;
    digitalWrite(24, HIGH); // extend the slave
    digitalWrite(25, LOW);
  }
  else {
    slaveSpeed = -Output;
    digitalWrite(24, LOW);
    digitalWrite(25, HIGH);  // retract the slave
  }
  
  // Should not be needed
  slaveSpeed = constrain(slaveSpeed, 0, 255);

  if (slaveSpeed == 0) {
    digitalWrite(24, LOW);
    digitalWrite(25, LOW);
  }
  
  analogWrite(3, slaveSpeed); //speed of slave (Przypisz do pwmu slave'a predkosc)


  //sterowanie masterem (master controlling)
  if (digitalRead(buttonPos) == HIGH) //extension if button is pressed (wysuw, jeśli przycisniemy przycisk)
  {
    digitalWrite(22, HIGH); //extend the Master
    digitalWrite(23, LOW);
  }
  else if (digitalRead(buttonPos2) == HIGH) //retract if button is pressed
  {
    digitalWrite(22, LOW);
    digitalWrite(23, HIGH); //retract the Master
  }
  else //if nothing is pressed do nothing (jesli nic nie przycisniete to nic nie rob)
  {
    digitalWrite(22, LOW);
    digitalWrite(23, LOW);
  }
}

Hi, thanks for feedback!
Your method doesnt work for me, since you try to write output value to the speed of actuator motor, so i can hear as pwm is working, but move of the cylinder is so slow, that it cant reach destination of master. However, after i write some random speed to the pwm speed of slave, its oscillating contionously, moving forward and backwards without stop. I suppose, its working like that, because of oscillation of potentiometers values (for example, when its fully extended, its values oscillate between 943-947).

empire180:
(for example, when its fully extended, its values oscillate between 943-947).

That is fairly normal. You could perhaps arrange for your program to ignore small changes.

...R

empire180:
The values from both potentiometers are not the same, to be more precisly, value from potentiometers, when master and slave are fully extended, are not the same (they are oscillating between 443-445).

Without looking into your code: I guess your PID "thinks" that the slave is off, when really it's just seeing the normal fluctuations of the measurement.

So maybe something like (Pseudocode, just to get the idea)

if(abs(masterposition - slaveposition) > 2){
//use PID-Output
}else{
//use last Output
}
{/code]

lg, couka

empire180:
Hi, thanks for feedback!
Your method doesnt work for me, since you try to write output value to the speed of actuator motor, so i can hear as pwm is working, but move of the cylinder is so slow, that it cant reach destination of master. However, after i write some random speed to the pwm speed of slave, its oscillating contionously, moving forward and backwards without stop. I suppose, its working like that, because of oscillation of potentiometers values (for example, when its fully extended, its values oscillate between 943-947).

Slow response and oscillations can both be caused by your complete lack of PID tuning. Modifying the Output in random ways is NOT the same as tuning the PID.

double consKp = 1.00, consKi = 0.00, consKd = 0.00; //parameteres of PID (nastawy)

You have to TUNE these constants for your system. Try increasing the P constant until you get the speed you want. Then increase the others to to eliminate overshoot and offset.

I don’t understand your “follow” function - why cannot the same setpoint be used for both motors?

If the PID of the second motor depends on two analog values, the jitter and noise on these inputs sums up and can make the PID jitter as well. I’d also add RC low pass filters to these inputs, to reduce noise.