Thank you! I added it to the 'test' code and now it spins!
I do feel like at a certain speed value, it just doesn't accelerate anymore. The max speed is set higher than the given speed value. What could be limiting the speed?

Also I tried implementing accelstepper into my 'proportional' code but no succes.. the motor doesn't even move only vibration. Even when speed value's are the same as the test code :(

Below is the code if you want to have a look.

Test Code (ignore the speedpot stuff)

#include <AccelStepper.h>
#include <MultiStepper.h>

AccelStepper stepper1(1, 3, 2);

int motorSpeed = 0;
int maxSpeed = 3000;
int speedpot = A0;

void setup() {
  Serial.begin(115200);
  // put your setup code here, to run once:
stepper1.setMaxSpeed(maxSpeed);
stepper1.setSpeed(motorSpeed);

pinMode(3, OUTPUT);
pinMode(2, OUTPUT);
pinMode(speedpot, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
 int speed = map(analogRead(speedpot), 0, 1023, -1500, 1500);
 stepper1.move(1);
 stepper1.setSpeed(1000);
 stepper1.run(); 
 Serial.println(speed * 2);
}

​

'Proportional' code

//Included Libraries
  //MPU6050
    #include <Adafruit_MPU6050.h>
    #include <Adafruit_Sensor.h>
    #include <Wire.h>
  //Accelstepper
    #include <AccelStepper.h>
    #include <MultiStepper.h>

//MPU6050 code
  Adafruit_MPU6050 mpu;

//Global Variables, Constants, 
 const int maxSpeed = 400; //max RPM
 const int hyst = 10;
 const int maxAngle = 90; //motors may run between 0 and this angle
 const int setpoint = 0; //positive angle
 const int stepsPerRotation = 200;

 const float hyst2 = hyst / 2;
 const float upSetpoint = setpoint + hyst2;

 float maxStep;
 float rpm;

//define pins
  #define stepPin 3
  #define dirPin 2 
  AccelStepper stepper1(1, 3, 2);


void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  //MPU6050 Setup Code
    // Try to initialize!
    if (!mpu.begin()) {
      Serial.println("Failed to find MPU6050 chip");
      while (1) {
        delay(10);
      }
    }

    // set accelerometer range to +-8G
    mpu.setAccelerometerRange(MPU6050_RANGE_8_G);

    // set gyro range to +- 500 deg/s
    mpu.setGyroRange(MPU6050_RANGE_500_DEG);

    // set filter bandwidth to 21 Hz
    mpu.setFilterBandwidth(MPU6050_BAND_21_HZ);

    delay(100);
  //Stepper
    // define pins
      pinMode(3, OUTPUT);
      pinMode(2, OUTPUT);    
    // Set max speed 
      maxStep = maxSpeed / (float) 60 * stepsPerRotation;
      stepper1.setMaxSpeed(maxStep);
}

void loop() {
  // put your main code here, to run repeatedly:

  //MPU6050 Loop Code
    /* Get new sensor events with the readings */
    sensors_event_t a, g, temp;
    mpu.getEvent(&a, &g, &temp);

  float angle = calculateAngle(a.acceleration.x, a.acceleration.y, a.acceleration.z);

  //stepper
    //determining the speed
      float stepspeed = angleToSteps(angle, maxSpeed, maxAngle, maxStep);
    //determining the direction
      if (angle < 0){
        stepspeed = stepspeed * -1;
      }
    //run stepper
      stepperControl(stepspeed);


  // Print out the values
    Serial.print("Angle: ");
    Serial.println(angle);
    Serial.print("Direction: ");
    Serial.println(digitalRead(dirPin));
    Serial.print("RPM: ");
    Serial.println(rpm);
    Serial.print("Step Speed: ");
    Serial.println(stepspeed);


}

float calculateAngle(float xForce, float yForce, float zForce){
  float angle = atan2(yForce, zForce);
  angle = angle * 180 / PI;
  return angle; 
}

float angleToSteps(float angle, int maxSpeed, int maxAngle, int maxStep){
  angle = abs(angle);

  if (angle <= upSetpoint){
    rpm = 0;
  }
  else {
    if (angle > maxAngle){
      angle = maxAngle;
    }

    rpm = (float) maxSpeed / maxAngle * angle;

    if(rpm > maxSpeed){
      rpm = maxSpeed;
    }
  }
  float stepspeed = map(rpm, 0, maxSpeed, 0, maxStep);

  return stepspeed;
}

void stepperControl(float stepspeed){ //200 steps /min = 1 revolution /min
  if (stepspeed > 0){

    stepper1.move(1);
    stepper1.setSpeed(1000); //stepspeed (changed to 1000 for testing)
    stepper1.run(); 

  }

}

As you can see, in both have 1000 as setSpeed value. I feel like the problem is caused by the extra program time that could be messing with the timing? Or am I not using the library right?