How to increase ball speed gradually by using time. I am already write the code but it was not working . Anyone can help me out from this problem.

using System.Collections; using UnityEngine; using UnityEngine.UI;

public class Ball_Control : MonoBehaviour { public string seconds; public string minutes;

 private float startTime;
 public Text timermints;
 public Text timerseconds;
 private float timerTime;
 private float stopTime;
 private bool isRunning = false;

 private float start_Vol_Time;
 public Text timermintsv;
 public Text timersecv;
 private float timer_vol_Time;    
 private bool vol_isRunning = false;

 //Get a reference to the rigidbody attached to the gameObject
 Rigidbody rb;

 private float vol_v=8f;

 // Start is called before the first frame update
 void Start()
 {
     TimerReset();
     startTime = Time.time;

     VolReset();
     start_Vol_Time = Time.time;

     //Get shortcut to rigidbody component
     rb = GetComponent<Rigidbody>();

     // Pause ball logic for 2.5 seconds, delay launch
     StartCoroutine (Pause ());        
     


 }
 
 // Update is called once per frame
 void Update()
 {            
     timerTime = stopTime + (Time.time - startTime);
     int minutesInt = (int)timerTime / 60;
     int secondsInt = (int)timerTime % 60;        
     if (isRunning)
     {
         timermints.text = (minutesInt < 10) ? "0" + minutesInt : minutesInt.ToString();
         timerseconds.text = (secondsInt < 10) ? "0" + secondsInt : secondsInt.ToString();            
     }

     timer_vol_Time =Time.time - start_Vol_Time;
     int mintsv = (int)timer_vol_Time / 60;
     int secv = (int)timer_vol_Time % 60;
     if (vol_isRunning)
     {
         timermintsv.text = (mintsv < 10) ? "0" + mintsv : mintsv.ToString();
         timersecv.text = (secv < 10) ? "0" + secv : secv.ToString();
     }
     /*int m=1;         
     if (mintsv==m)
     {
         Debug.Log("ms="+m);             
         //vol_v= vol_v+2;
         //Debug.Log("vol=" + vol_v);
         m++;
         Debug.Log("ms="+m);
     }*/
     //If ball goes too far left...
     if (transform.position.x < -9f)
     {
         VolReset();
         TimerStop();
         transform.position = Vector3.zero;
         rb.velocity = Vector3.zero;
         //Give Left player a point
         Score_Board.instance.GiveLeftscore();
         StartCoroutine(Pause());
     }
     //If ball goes too far Right...
     if (transform.position.x > 9f)
     {
         VolReset();
         TimerStop();
         transform.position = Vector3.zero;
         rb.velocity = Vector3.zero;

         //Give Left player a point
         Score_Board.instance.GiveRightscore();
         StartCoroutine(Pause());
     }
 }

 IEnumerator Pause()
 {        

     //Wait for 2.5 seconds
     yield return new WaitForSeconds(2.5f);
     //Call function that launches the ball
     LaunchBall();
     TimerStart();
     VolStart();
 }    

 void LaunchBall()
 {
     transform.position = Vector3.zero;

     // ball chooses a direction
     //Flip a coin, determine direction in x-axis
     int xDirection = Random.Range(0, 2);

     //Flip a coin, determine direction in y-axis
     int yDirection = Random.Range(0, 3);

     Vector3 launchDirection = new Vector3();

     //Check results of one coin toss
     if (xDirection == 0)
     {

         launchDirection.x = -vol_v;
     }
     if (xDirection == 1)
     {

         launchDirection.x = vol_v;
     }

     //Check results of second coin toss
     if (yDirection == 0)
     {

         launchDirection.y = -vol_v;
     }
     if (yDirection == 2)
     {

         launchDirection.y = vol_v;
     }

     rb.velocity = launchDirection;
 }

 // When we hit something else...
 void OnCollisionEnter(Collision hit)
 {
     //If it was the top or bottom of the screen...
     if (hit.gameObject.name == "Top_Collider")
     {

         float speedInXDirection = 0f;

         if (rb.velocity.x > 0f)
             speedInXDirection = vol_v;

         if (rb.velocity.x < 0f)
             speedInXDirection = -vol_v;

         rb.velocity = new Vector3(speedInXDirection, -vol_v, 0f);
     }

     if (hit.gameObject.name == "Bottom_Collider")
     {

         float speedInXDirection = 0f;

         if (rb.velocity.x > 0f)
             speedInXDirection = vol_v;

         if (rb.velocity.x < 0f)
             speedInXDirection = -vol_v;

         rb.velocity = new Vector3(speedInXDirection, vol_v, 0f);
     }

     //If it was one of the bats
     if (hit.gameObject.name == "Paddle(L)")
     {

         rb.velocity = new Vector3((vol_v + 4), 0f, 0f);


         //Check if we hit lower half of the bat...
         if (transform.position.y - hit.gameObject.transform.position.y < -0.5)
         {

             rb.velocity = new Vector3(vol_v, -vol_v, 0f);
         }
         //Check if we hit lower half of the bat...
         if (transform.position.y - hit.gameObject.transform.position.y > 0.5f)
         {

             rb.velocity = new Vector3(vol_v, vol_v, 0f);
         }
     }
     if (hit.gameObject.name == "Paddle(R)")
     {

         rb.velocity = new Vector3(-(vol_v+4), 0f, 0f);


         //Check if we hit lower half of the bat...
         if (transform.position.y - hit.gameObject.transform.position.y < -0.5f)
         {

             rb.velocity = new Vector3(-vol_v, -vol_v, 0f);
         }
         //Check if we hit lower half of the bat...
         if (transform.position.y - hit.gameObject.transform.position.y > 0.5f)
         {

             rb.velocity = new Vector3(-vol_v, vol_v, 0f);
         }
     }
 }
 public void VolStart()
 {
     if (!vol_isRunning)
     {
         print("VOL_START");
         vol_isRunning = true;
         start_Vol_Time = Time.time;
     }
 }
 public void TimerStart()
 {
     if (!isRunning)
     {
         print("START");
         isRunning = true;
         startTime = Time.time;
     }
 }
 public void VolReset()
 {
     print("VOL_RESET");
     vol_isRunning = false;
     timermintsv.text = timersecv.text = "00";
 }
 public void TimerReset()
 {
     print("RESET");
     stopTime = 0;
     isRunning = false;
     timermints .text= timerseconds.text = "00";

 }
 
 public void TimerStop()
 {
     if (isRunning)
     {
         print("STOP");
         isRunning = false;
         stopTime = timerTime;
     }
 }        

}