通过ros节点发布Twist Messages控制机器人--10

原创博客：转载请表明出处：http://www.cnblogs.com/zxouxuewei/

1.到目前为止，我们已经从命令行移动机器人，但大多数时间你将依靠一个ros节点发布适当的Twist消息。作为一个简单的例子，假设你想让你的机器人向前移动一个1米大约180度，然后回到起点。我们将尝试完成这项任务，这将很好地说明不同层次的ros运动控制。

启动tulterbot机器人：

roslaunch rbx1_bringup fake_turtlebot.launch

2.在rviz视图窗口查看机器人：

rosrun rviz rviz -d `rospack find rbx1_nav`/sim.rviz

3.运行timed_out_and_back.py节点：

rosrun rbx1_nav timed_out_and_back.py

4.通过rqt_graph查看消息订阅的框图：

rosrun rqt_graph rqt_graph

5.分析timed_out_and_back.py节点代码：

#!/usr/bin/env python
 
import rospy
from geometry_msgs.msg import Twist
from math import pi
 
class OutAndBack():
    def __init__(self):
        # Give the node a name
        rospy.init_node('out_and_back', anonymous=False)
        # Set rospy to execute a shutdown function when exiting
        rospy.on_shutdown(self.shutdown)
 
        # Publisher to control the robot's speed
        self.cmd_vel = rospy.Publisher('/cmd_vel', Twist, queue_size=)
 
        # How fast will we update the robot's movement?
        rate = 
 
        # Set the equivalent ROS rate variable
        r = rospy.Rate(rate)
 
        # Set the forward linear speed to 0.2 meters per second
        linear_speed = 0.2
 
        # Set the travel distance to 1.0 meters
        goal_distance = 1.0
 
        # How long should it take us to get there?
        linear_duration = goal_distance / linear_speed
 
        # Set the rotation speed to 1.0 radians per second
        angular_speed = 1.0
 
        # Set the rotation angle to Pi radians ( degrees)
        goal_angle = pi
 
        # How long should it take to rotate?
        angular_duration = goal_angle / angular_speed
 
        # Loop through the two legs of the trip
        for i in range():
            # Initialize the movement command
            move_cmd = Twist()
 
            # Set the forward speed
            move_cmd.linear.x = linear_speed
 
            # Move forward for a time to go the desired distance
            ticks = int(linear_duration * rate)
 
            for t in range(ticks):
                self.cmd_vel.publish(move_cmd)
                r.sleep()
 
            # Stop the robot before the rotation
            move_cmd = Twist()
            self.cmd_vel.publish(move_cmd)
            rospy.sleep()
 
            # Now rotate left roughly  degrees  
 
            # Set the angular speed
            move_cmd.angular.z = angular_speed
 
            # Rotate for a time to go  degrees
            ticks = int(goal_angle * rate)
 
            for t in range(ticks):
                self.cmd_vel.publish(move_cmd)
                r.sleep()
 
            # Stop the robot before the next leg
            move_cmd = Twist()
            self.cmd_vel.publish(move_cmd)
            rospy.sleep()    
 
        # Stop the robot
        self.cmd_vel.publish(Twist())
 
    def shutdown(self):
        # Always stop the robot when shutting down the node.
        rospy.loginfo("Stopping the robot...")
        self.cmd_vel.publish(Twist())
        rospy.sleep()
 
if __name__ == '__main__':
    try:
        OutAndBack()
    except:
        rospy.loginfo("Out-and-Back node terminated.")

6.等以上节点运行完成后。可以运行下一个节点;

rosrun rbx1_nav nav_square.py

查看节点订阅框图：

7.分析nav_square.py节点的源码：

#!/usr/bin/env python
 
import rospy
from geometry_msgs.msg import Twist, Point, Quaternion
import tf
from rbx1_nav.transform_utils import quat_to_angle, normalize_angle
from math import radians, copysign, sqrt, pow, pi
 
class NavSquare():
    def __init__(self):
        # Give the node a name
        rospy.init_node('nav_square', anonymous=False)
 
        # Set rospy to execute a shutdown function when terminating the script
        rospy.on_shutdown(self.shutdown)
 
        # How fast will we check the odometry values?
        rate = 
 
        # Set the equivalent ROS rate variable
        r = rospy.Rate(rate)
 
        # Set the parameters for the target square
        goal_distance = rospy.get_param("~goal_distance", 1.0)      # meters
        goal_angle = rospy.get_param("~goal_angle", radians())    # degrees converted to radians
        linear_speed = rospy.get_param("~linear_speed", 0.2)        # meters per second
        angular_speed = rospy.get_param("~angular_speed", 0.7)      # radians per second
        angular_tolerance = rospy.get_param("~angular_tolerance", radians()) # degrees to radians
 
        # Publisher to control the robot's speed
        self.cmd_vel = rospy.Publisher('/cmd_vel', Twist, queue_size=)
 
        # The base frame is base_footprint for the TurtleBot but base_link for Pi Robot
        self.base_frame = rospy.get_param('~base_frame', '/base_link')
 
        # The odom frame is usually just /odom
        self.odom_frame = rospy.get_param('~odom_frame', '/odom')
 
        # Initialize the tf listener
        self.tf_listener = tf.TransformListener()
 
        # Give tf some time to fill its buffer
        rospy.sleep()
 
        # Set the odom frame
        self.odom_frame = '/odom'
 
        # Find out if the robot uses /base_link or /base_footprint
        try:
            self.tf_listener.waitForTransform(self.odom_frame, '/base_footprint', rospy.Time(), rospy.Duration(1.0))
            self.base_frame = '/base_footprint'
        except (tf.Exception, tf.ConnectivityException, tf.LookupException):
            try:
                self.tf_listener.waitForTransform(self.odom_frame, '/base_link', rospy.Time(), rospy.Duration(1.0))
                self.base_frame = '/base_link'
            except (tf.Exception, tf.ConnectivityException, tf.LookupException):
                rospy.loginfo("Cannot find transform between /odom and /base_link or /base_footprint")
                rospy.signal_shutdown("tf Exception")  
 
        # Initialize the position variable as a Point type
        position = Point()
 
        # Cycle through the four sides of the square
        for i in range():
            # Initialize the movement command
            move_cmd = Twist()
 
            # Set the movement command to forward motion
            move_cmd.linear.x = linear_speed
 
            # Get the starting position values
            (position, rotation) = self.get_odom()
 
            x_start = position.x
            y_start = position.y
 
            # Keep track of the distance traveled
            distance = 
 
            # Enter the loop to move along a side
            while distance < goal_distance and not rospy.is_shutdown():
                # Publish the Twist message and sleep  cycle
                self.cmd_vel.publish(move_cmd)
 
                r.sleep()
 
                # Get the current position
                (position, rotation) = self.get_odom()
 
                # Compute the Euclidean distance from the start
                distance = sqrt(pow((position.x - x_start), ) +
                                pow((position.y - y_start), ))
 
            # Stop the robot before rotating
            move_cmd = Twist()
            self.cmd_vel.publish(move_cmd)
            rospy.sleep(1.0)
 
            # Set the movement command to a rotation
            move_cmd.angular.z = angular_speed
 
            # Track the last angle measured
            last_angle = rotation
 
            # Track how far we have turned
            turn_angle = 
 
            # Begin the rotation
            while abs(turn_angle + angular_tolerance) < abs(goal_angle) and not rospy.is_shutdown():
                # Publish the Twist message and sleep  cycle
                self.cmd_vel.publish(move_cmd)
 
                r.sleep()
 
                # Get the current rotation
                (position, rotation) = self.get_odom()
 
                # Compute the amount of rotation since the last lopp
                delta_angle = normalize_angle(rotation - last_angle)
 
                turn_angle += delta_angle
                last_angle = rotation
 
            move_cmd = Twist()
            self.cmd_vel.publish(move_cmd)
            rospy.sleep(1.0)
 
        # Stop the robot when we are done
        self.cmd_vel.publish(Twist())
 
    def get_odom(self):
        # Get the current transform between the odom and base frames
        try:
            (trans, rot)  = self.tf_listener.lookupTransform(self.odom_frame, self.base_frame, rospy.Time())
        except (tf.Exception, tf.ConnectivityException, tf.LookupException):
            rospy.loginfo("TF Exception")
            return
 
        return (Point(*trans), quat_to_angle(Quaternion(*rot)))
 
    def shutdown(self):
        # Always stop the robot when shutting down the node
        rospy.loginfo("Stopping the robot...")
        self.cmd_vel.publish(Twist())
        rospy.sleep()
 
if __name__ == '__main__':
    try:
        NavSquare()
    except rospy.ROSInterruptException:
        rospy.loginfo("Navigation terminated.")