Problems using PID to control DC Motor?

Other Hardware Robotics
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Okay, sorry, I was messing things up. Now I'm able to tune the PID values using the pots. That's okay.
Problem is: I'm trying to use the Ziegler–Nichols method to tune the PID loop. So I try to get a steady oscillation by varying Kp. I had a hard time doing that just by looking at the Serial monitor, so I tried using Python to plot the data in real time. With Ki and Kd both equal to zero, I get something like this:

653×551 49 KB

That doesn't look like a steady oscillation to me. As Kp approaches zero, the system oscillates less, but I can't see a significant difference by rising it's value. I tried varying Kd and Ki also, without sucess. I just can't see any real difference.

That's the complete code I'm using:

#include <PID_v1.h>

#define SpeedSetPoint 200

double Input, Output, Setpoint; //PID Variables

int potKp = 2;
int potKd = 3;
int potKi = 4;
float Kp = 0, Kd = 0, Ki =0;


PID myPid(&Input, &Output, &Setpoint, 0, 0, 0, DIRECT); 

//RPM Counter
unsigned long start;
const byte encoderPinA = 2;//A pin -> interrupt pin 0
const byte encoderPinB = 4;//B pin -> digital pin 4
volatile long pulse;
volatile bool pinB, pinA, dir;
const byte ppr = 25, upDatesPerSec = 2;
const int fin = 1000 / upDatesPerSec;
const float konstant = 60.0 * upDatesPerSec / (ppr * 2);
int rpm=0;

int outputValue=0;

//DC Motor Pins
int pwm = 5;
int motorA1 = 10;
int motorA2 = 9;

void setup(){
  Serial.begin(19200);
  Input = 25;
  Setpoint = SpeedSetPoint;
  myPid.SetMode(AUTOMATIC);

  attachInterrupt(0, readEncoder, CHANGE);
  pinMode(encoderPinA,INPUT_PULLUP);
  pinMode(encoderPinB,INPUT_PULLUP);

  pinMode(pwm, OUTPUT);
  pinMode(motorA1, OUTPUT);
  pinMode(motorA2, OUTPUT);

  //Start DC Motor
  digitalWrite(pwm, HIGH);
  digitalWrite(motorA1, HIGH);
  digitalWrite(motorA2, LOW);
}


void loop() {
  
  Kp = analogRead(potKp)/10.0;
  Ki = analogRead(potKi)/10.0;
  Kd = analogRead(potKd)/10.0;
  
  myPid.SetTunings(Kp, Ki, Kd);

  
  
  if(millis() - start > fin)
  {
    start = millis();
    rpm = pulse * konstant;
    rpm = abs(rpm);
    Serial.println(rpm);
    pulse = 0;
  }//End RPM reading

  Input = rpm;
  myPid.Compute();
  outputValue = Output;
  //outputValue = map(outputValue, 0, 400, 180, 255);
  analogWrite(pwm, outputValue);

  //Serial.print("RPM = ");
  //Serial.println(rpm);
  //Serial.print("\t");
  //Serial.print("Kp = ");
  //Serial.print(Kp);
  //Serial.print("\t");
  //Serial.print("Ki = ");
  //Serial.print(Ki);
  //Serial.print("\t");
  //Serial.print("Kd = ");
 // Serial.println(Kd);
  
}

void readEncoder()
{
  pinA = bitRead(PIND,encoderPinA);
  pinB = bitRead(PIND,encoderPinB);
  dir = pinA ^ pinB;          // if pinA & pinB are the same
  dir ? --pulse : ++pulse;    // dir is CW, else CCW
}

Any tips? I really don't know what to do.