当难度增加时，C＃_infinite亚军游戏不会逐渐增加速度

Question

我使用C＃在Unity 2018上制作了一个3D无限赛跑游戏，每次升级时我的角色都会加速。但问题是他并没有逐渐增加，相反，他从正常到非常快，很难控制他。这是我的代码：

    public class PlayerController : MonoBehaviour
    {
        private CharacterController controller;
        private float speed = 2.0f;
        private Vector3 moveVector;
        private float vertivalVelocity = 0.0f;
        private float gravity = 12.0f;
        private float animationDuration = 2.0f;
        private bool isDead = false;
    // Use this for initialization
    void Start ()
    {
        controller = GetComponent<CharacterController>();
    }

    // Update is called once per frame
    void Update ()
    {

        if (isDead)
            return;
        if(Time.time < animationDuration)
        {
            controller.Move(Vector3.forward * speed * Time.deltaTime);
            return;
 }

        moveVector = Vector3.zero;

        if(controller.isGrounded)
        {
            vertivalVelocity = -0.5f;
        }
        else
        {
            vertivalVelocity -= gravity * Time.deltaTime;
        }

        //x - Left and Right
        moveVector.x = Input.GetAxisRaw("Horizontal") * speed;
        //y - Up and Down

        //z - Foward and Backward
        moveVector.z = speed;

        controller.Move((moveVector * speed) * Time.deltaTime);
    }
    public void SetSpeed(float modifer)
    {
        speed = 2.0f * modifer;
    }

    //it is being called everytime our capsule hits something

     private void OnControllerColliderHit(ControllerColliderHit hit)
    {
        if (hit.point.z > transform.position.z + 0.1f && hit.gameObject.tag == "Enemy")
            Death();
    }

    private void Death()
    {
        isDead = true;
        GetComponent<Score>().OnDeath (); 
    }