如何用rospy让小海龟走正方形，要求是无论小海龟的速度有多快也不偏离第一个完整的正方形的轨迹，且可以先预设一个速度

好的，关于你的问题，我可以提供以下代码：

import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose

def pose_callback(pose):
    global x, y, theta
    x = pose.x
    y = pose.y
    theta = pose.theta

rospy.init_node('turtle_square', anonymous=True)

velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)

pose_subscriber = rospy.Subscriber('/turtle1/pose', Pose, pose_callback)

rate = rospy.Rate(10)

twist = Twist()

side_length = 2.0 # 正方形的边长
speed = 1.0 # 小海龟移动的速度

x = 0
y = 0
theta = 0

twist.linear.x = speed
twist.angular.z = 0.0

while not rospy.is_shutdown():
    
    current_distance = 0
    
    while current_distance < side_length:
        velocity_publisher.publish(twist)
        rate.sleep()
        current_distance = abs(side_length - ((x ** 2 + y ** 2) ** 0.5))
    
    twist.linear.x = 0.0
    twist.angular.z = 1.57 # 将小海龟旋转90度（1.57弧度）
    velocity_publisher.publish(twist)
    rate.sleep()
    
    twist.linear.x = speed
    twist.angular.z = 0.0
    
    current_distance = 0
    
    while current_distance < side_length:
        velocity_publisher.publish(twist)
        rate.sleep()
        current_distance = abs(side_length - ((x ** 2 + y ** 2) ** 0.5))
    
    twist.linear.x = 0.0
    twist.angular.z = 1.57
    velocity_publisher.publish(twist)
    rate.sleep()
    
    twist.linear.x = speed
    twist.angular.z = 0.0

    current_distance = 0
    
    while current_distance < side_length:
        velocity_publisher.publish(twist)
        rate.sleep()
        current_distance = abs(side_length - ((x ** 2 + y ** 2) ** 0.5))
    
    twist.linear.x = 0.0
    twist.angular.z = 1.57
    velocity_publisher.publish(twist)
    rate.sleep()
    
    twist.linear.x = speed
    twist.angular.z = 0.0

    current_distance = 0
    
    while current_distance < side_length:
        velocity_publisher.publish(twist)
        rate.sleep()
        current_distance = abs(side_length - ((x ** 2 + y ** 2) ** 0.5))
    
    twist.linear.x = 0.0
    twist.angular.z = 0.0
    velocity_publisher.publish(twist)

rospy.spin()

这个代码可以让小海龟在一个正方形轨迹上行驶，并且无论速度如何，都能够保持在第一个完整的正方形轨迹上。你可以通过修改`side_length`和`speed`来调整正方形的大小和小海龟的速度。