我有一个由刚体控制的玩家。Z轴上的移动在游戏中保持恒定，以使玩家向前移动。然而，这意味着刚体会不断加速，因此它在游戏开始时的速度比游戏结束时的速度慢得多。下面是我的脚本：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.InputSystem;
using UnityEngine.SceneManagement;

public class Controller : MonoBehaviour
{
    [Tooltip("Rigidbody component attached to the player")]
    public Rigidbody rb;
    public float forwardMax;
    public float slowBy;

    private float movementX;
    private float movementY;
    private float gravity = -9.81f;
    private bool isJumping = false;
    private bool isSlowing = false;
    private bool isSpeeding = false;
    private float speedX = 100;
    private float speedY = 150000;
    private float speedZ = 60;

    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody>();
    }
    void Update()
    {
        // if(!controller.isGrounded)
        // {
        //     Vector3 gravityVector = new Vector3(0.0f, gravity, 0.0f);
        //     rb.AddForce(gravityVector * Time.deltaTime);
        // }
    }
    void OnCollisionEnter(Collision collision)
    {
        // SceneManager.LoadScene(1);
    }

    // Update is called once per frame
    void OnMove(InputValue movementValue)
    {
        Vector2 movementVector = movementValue.Get<Vector2>();

        movementX = movementVector.x;
        movementY = movementVector.y;
    }

    void OnJump()
    {
        isJumping = true;
    }

    void CalculateMovement()
    {
        if(rb.velocity.z > 20)
        {
            rb.drag = 20;
        }
        else
        {
            rb.drag = 0;
        }

        Vector3 movement = new Vector3(movementX * speedX, 0.0f, speedZ);
        if(isJumping)
        {
            movement.y += Mathf.Sqrt(speedY * -3.0f * gravity);
            isJumping = false;
        }
        rb.AddForce(movement);
        Debug.Log("Speed is " + rb.velocity.z);
    }
    void FixedUpdate()
    {
        CalculateMovement();
    }

}

有没有办法保持前进速度不变？当玩家跳跃时问题更严重。
首先，我试着钳制正向（z轴）矢量，但没有效果;然后，我试着在正向速度超过某个数值时，将反向矢量加到总和上，但这会导致它一直加速和减速;然后，我试着在rigidbody上使用drag，但效果相同。