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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.")