public float sp = 20;

       private float timeToChangeDirect;
       private float timeToReset;
       private int dir = 1;

       void Start()    
       {
           timeToReset = 60 / sp;
           timeToChangeDirect = timeToReset;
       }

    void Update ()    
       {
           timeToReset = 60 / sp * 2;

           transform.RotateAround(transform.parent.position, Vector3.forward * dir, sp * Time.deltaTime);

           if (timeToChangeDirect > 0) timeToChangeDirect -= Time.deltaTime;
           else
           {
               timeToChangeDirect = timeToReset;
               dir *= -1;
                  
           }
    }

void Update()
       {
           transform.position = transform.parent.position;
       }