I'm needing to pass variables from my timerIsr() function to my loop() function. I have little experience with the Arduino language, I'm mostly familiar with Python so this has been quite difficult for me.

Essentially I'm creating a PID controller to maintain RPM of a fan motor if friction is applied to it. I'm implementing the PID control in the loop() function but I need the rotation variable information from my timerIsr() function. I've tried to return the variable info but it doesn't match up to the actual fan speed. I think it has something to do with difference delay/timer times operating at different instances but I'm unsure.

Here's what I have so far

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

int counter=0;

const int IN1 = 11;
const int IN2 = 10;
volatile float pot=0;
const int POT = 0;
const int ENA = 6;

double Setpoint, Input, Output;
double Kp=2, Ki=5, Kd=1;

PID myPID(&Input, &Output, &Setpoint, Kp, Ki, Kd, DIRECT);


void docount()  // counts from the speed sensor
{
  counter++;  // increase +1 the counter value
} 

void timerIsr()
{
  Timer1.detachInterrupt();  //stop the timer
  Serial.print("Motor Speed: "); 
  int rotation = (counter / 30);  // divide by number of holes in Disc
  Serial.print(rotation,DEC);  
  Serial.println(" Rotation per second"); 
  counter=0;  //  reset counter to zero
  Timer1.attachInterrupt( timerIsr );  //enable the timer
  Serial.print("pot = ");
  Serial.print(pot);
  Serial.print("\n");

  //return rotation;
}

void setup() 
{
  Serial.begin(9600);
  
  pinMode(IN1, OUTPUT);
  pinMode(IN2, OUTPUT); 
  
  Timer1.initialize(1000000); // set timer for 1sec
  attachInterrupt(0, docount, FALLING);  // increase counter when speed sensor pin goes High
  Timer1.attachInterrupt( timerIsr ); // enable the timer
  

  myPID.SetMode(AUTOMATIC);
} 

void loop()
{
  //Input = timerIsr();
  pot=analogRead(POT)/4.01569;
  //Setpoint = sp;

  //Serial.print(Input);
    
  analogWrite(ENA, pot);
  digitalWrite(10, HIGH);  // set rotation of motor to Clockwise
  digitalWrite(11, LOW);
  
  delay (250);
}

As you can see I've commented some stuff out that's related to the PID library. Right now I'm just trying to pass the rotation variable to the loop() function. Thanks in advance.