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Nav2 Stack Using Simple Commander Python API

In this tutorial, we will work with Stretch to explore the Simple Commander Python API to enable autonomous navigation programmatically. We will also demonstrate a security patrol routine for Stretch developed using this API. If you just landed here, it might be a good idea to first review the previous tutorial which covered mapping and navigation using RViz as an interface.

The Simple Commander Python API

To develop complex behaviors with Stretch where navigation is just one aspect of the autonomy stack, we need to be able to plan and execute navigation routines as part of a bigger program. Luckily, the Nav2 stack exposes a Python API that abstracts the ROS layer and the Behavior Tree framework (more on that later!) from the user through a pre-configured library called the robot navigator. This library defines a class called BasicNavigator which wraps the planner, controller and recovery action servers and exposes methods such as goToPose(), goToPoses() and followWaypoints() to execute navigation behaviors.

Let's first see the demo in action and then explore the code to understand how this works!

Warning

We will not be using the arm for this demo. We recommend stowing the arm to avoid inadvertently bumping it into walls while the robot is navigating.

Execute:

stretch_robot_stow.py

Setup

Let's set the patrol route up before you can execute this demo in your map. This requires reading the position of the robot at various locations in the map and entering the co-ordinates in the array called security_route in the simple_commander_demo.py file.

First, execute the following command while passing the correct map YAML. Then, press the 'Startup' button:

ros2 launch stretch_nav2 navigation.launch.py map:=${HELLO_FLEET_PATH}/maps/<map_name>.yaml

Since we expect the first point in the patrol route to be at the origin of the map, the first coordinates should be (0.0, 0.0). Next, to define the route, the easiest way to define the waypoints in the security_route array is by setting the robot at random locations in the map using the '2D Pose Estimate' button in RViz as shown below. For each location, note the x, and y coordinates in the position field of the base_footprint frame and add it to the security_route array in simple_commander_demo.py.

Finally, Press Ctrl+C to exit out of navigation and save the simple_commander_demo.py file. Now, build the workspace to make the updated file available for the next launch command. 

cd ~/ament_ws/
colcon build

See It In Action

Go ahead and execute the following command to run the demo and visualize the result in RViz. Be sure to pass the correct path to the map YAML: Terminal 1:

ros2 launch stretch_nav2 demo_security.launch.py map:=${HELLO_FLEET_PATH}/maps/<map_name>.yaml

Code Breakdown

Now, let's jump into the code to see how things work under the hood. Follow along in the code to have a look at the entire script.

First, we import the BasicNavigator class from the robot_navigator library which comes standard with the Nav2 stack. This class wraps around the planner, controller and recovery action servers.

from stretch_nav2.robot_navigator import BasicNavigator, TaskResult

In the main method, we initialize the node and create an instance of the BasicNavigator class called navigator.

def main():
    rclpy.init()

    navigator = BasicNavigator()

Then, we set up a path for Stretch to patrol consisting of the coordinates in the map reference frame. These coordinates are specific to the map generated for this tutorial and would not be suitable for your robot. To define coordinates that work with your robot, first command the robot to at least three random locations in the map you have generated of your environment, then read the base_link x and y coordinates for each of them from the RViz TF plugin. Plug them in the security_route list. Keep in mind that for this demo, the robot is starting from [0.0, 0.0] which is the origin of the map. This might not be the case for you.

    security_route = [
        [0.0, 0.0],
        [1.057, 1.3551],
        [1.5828, 5.0823],
        [-0.5390, 5.6623],
        [0.8975, 9.7033]]

Next, we set an initial pose for the robot which would help AMCL localize the robot by providing an initial estimate of the robot's location. For this, we pass a PoseStamped message in the map reference frame with the robot's pose to the setInitialPose() method. The Nav2 stack recommends this before starting the lifecycle nodes using the "Startup" button in RViz. The waitUntilNav2Active() method waits until precisely this event.

    initial_pose = PoseStamped()
    initial_pose.header.frame_id = 'map'
    initial_pose.header.stamp = navigator.get_clock().now().to_msg()
    initial_pose.pose.position.x = 0.0
    initial_pose.pose.position.y = 0.0
    initial_pose.pose.orientation.z = 0.0
    initial_pose.pose.orientation.w = 1.0
    navigator.setInitialPose(initial_pose)

    navigator.waitUntilNav2Active()

Once the nodes are active, the navigator is ready to receive pose goals either through the goToPose(), goToPoses() or followWaypoints() methods. For this demo, we will be using the followWaypoints() method which takes a list of poses as an argument. Since we intend for the robot to patrol the route indefinitely or until the node is killed (or the robot runs out of battery!), we wrap the method in an infinite while loop with rclpy.ok(). Then, we generate pose goals with the security_route list and append them to a new list called route_poses which is passed to the followWaypoints() method.

    while rclpy.ok():

        route_poses = []
        pose = PoseStamped()
        pose.header.frame_id = 'map'
        pose.header.stamp = navigator.get_clock().now().to_msg()
        pose.pose.orientation.w = 1.0
        for pt in security_route[1:]:
            pose.pose.position.x = pt[0]
            pose.pose.position.y = pt[1]
            route_poses.append(deepcopy(pose))

        nav_start = navigator.get_clock().now()
        navigator.followWaypoints(route_poses)

Since we are utilizing an action server built into Nav2, it's possible to seek feedback on this long running task through the action interface. The isTaskComplete() method returns a boolean depending on whether the patrolling task is complete. For the follow waypoints action server, the feedback message tells us which waypoint is currently being executed through the feedback.current_waypoint attribute. It is possible to cancel a goal using the cancelTask() method if the robot gets stuck. For this demo, we have set the timeout at 600 seconds to allow sufficient time for the robot to succeed. However, if you wish to see it in action, you can reduce the timeout to 30 seconds.

        i = 0
        while not navigator.isTaskComplete():
            i += 1
            feedback = navigator.getFeedback()
            if feedback and i % 5 == 0:
                navigator.get_logger().info('Executing current waypoint: ' +
                    str(feedback.current_waypoint + 1) + '/' + str(len(route_poses)))
                now = navigator.get_clock().now()

                if now - nav_start > Duration(seconds=600.0):
                    navigator.cancelTask()

Once the robot reaches the end of the route, we reverse the security_route list to generate the goal pose list that would be used by the followWaypoints() method in the next iteration of this loop.

        security_route.reverse()

Finally, after a leg of the patrol route is executed, we call the getResult() method to know whether the task succeeded, canceled or failed to log a message.

        result = navigator.getResult()
        if result == TaskResult.SUCCEEDED:
            navigator.get_logger().info('Route complete! Restarting...')
        elif result == TaskResult.CANCELED:
            navigator.get_logger().info('Security route was canceled, exiting.')
            rclpy.shutdown()
        elif result == TaskResult.FAILED:
            navigator.get_logger().info('Security route failed! Restarting from other side...')

That's it! Using the Simple Commander API is as simple as that. Be sure to follow more examples in the nav2_simple_commander package if you wish to work with other useful methods exposed by the library.