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Example 12

For this example, we will send follow joint trajectory commands for the head camera to search and locate an ArUco tag. In this instance, a Stretch robot will try to locate the docking station's ArUco tag.

Modifying Stretch Marker Dictionary YAML File.

When defining the ArUco markers on Stretch, hello robot utilizes a YAML file, stretch_marker_dict.yaml, that holds the information about the markers. A further breakdown of the yaml file can be found in our Aruco Marker Detection tutorial.

Below is what the needs to be included in the stretch_marker_dict.yaml file so the detect_aruco_markers node can find the docking station's ArUco tag.

'245':
  'length_mm': 88.0
  'use_rgb_only': False
  'name': 'docking_station'
  'link': None

Getting Started

Begin by running the stretch driver launch file.

# Terminal 1
roslaunch stretch_core stretch_driver.launch

To activate the RealSense camera and publish topics to be visualized, run the following launch file in a new terminal.

# Terminal 2
roslaunch stretch_core d435i_high_resolution.launch

Next, run the stretch ArUco launch file which will bring up the detect_aruco_markers node.

# Terminal 3
roslaunch stretch_core stretch_aruco.launch

Within this tutorial package, there is an RViz config file with the topics for transform frames in the Display tree. You can visualize these topics and the robot model by running the command below in a new terminal.

# Terminal 4
rosrun rviz rviz -d /home/hello-robot/catkin_ws/src/stretch_tutorials/rviz/aruco_detector_example.rviz

Then run the aruco_tag_locator.py node.

# Terminal 5
cd catkin_ws/src/stretch_tutorials/src/
python3 aruco_tag_locator.py

The Code

#! /usr/bin/env python3

import rospy
import time
import tf2_ros
import numpy as np
from math import pi

import hello_helpers.hello_misc as hm
from sensor_msgs.msg import JointState
from control_msgs.msg import FollowJointTrajectoryGoal
from trajectory_msgs.msg import JointTrajectoryPoint
from geometry_msgs.msg import TransformStamped

class LocateArUcoTag(hm.HelloNode):
    """
    A class that actuates the RealSense camera to find the docking station's
    ArUco tag and returns a Transform between the `base_link` and the requested tag.
    """
    def __init__(self):
        """
        A function that initializes the subscriber and other needed variables.
        :param self: The self reference.
        """
        hm.HelloNode.__init__(self)

        self.joint_states_sub = rospy.Subscriber('/stretch/joint_states', JointState, self.joint_states_callback)
        self.transform_pub = rospy.Publisher('ArUco_transform', TransformStamped, queue_size=10)

        self.joint_state = None

        self.min_pan_position = -4.10
        self.max_pan_position =  1.50
        self.pan_num_steps = 10
        self.pan_step_size = abs(self.min_pan_position - self.max_pan_position)/self.pan_num_steps

        self.min_tilt_position = -0.75
        self.tilt_num_steps = 3
        self.tilt_step_size = pi/16

        self.rot_vel = 0.5 # radians per sec

    def joint_states_callback(self, msg):
        """
        A callback function that stores Stretch's joint states.
        :param self: The self reference.
        :param msg: The JointState message type.
        """
        self.joint_state = msg

    def send_command(self, command):
        '''
        Handles single joint control commands by constructing a FollowJointTrajectoryGoal
        message and sending it to the trajectory_client created in hello_misc.
        :param self: The self reference.
        :param command: A dictionary message type.
        '''
        if (self.joint_state is not None) and (command is not None):
            joint_name = command['joint']
            trajectory_goal = FollowJointTrajectoryGoal()
            trajectory_goal.trajectory.joint_names = [joint_name]
            point = JointTrajectoryPoint()

            if 'delta' in command:
                joint_index = self.joint_state.name.index(joint_name)
                joint_value = self.joint_state.position[joint_index]
                delta = command['delta']
                new_value = joint_value + delta
                point.positions = [new_value]

            elif 'position' in command:
                point.positions = [command['position']]

            point.velocities = [self.rot_vel]
            trajectory_goal.trajectory.points = [point]
            trajectory_goal.trajectory.header.stamp = rospy.Time(0.0)
            trajectory_goal.trajectory.header.frame_id = 'base_link'
            self.trajectory_client.send_goal(trajectory_goal)
            self.trajectory_client.wait_for_result()

    def find_tag(self, tag_name='docking_station'):
        """
        A function that actuates the camera to search for a defined ArUco tag
        marker. Then the function returns the pose
        :param self: The self reference.
        :param tag_name: A string value of the ArUco marker name.

        :returns transform: The docking station's TransformStamped message.
        """
        pan_command = {'joint': 'joint_head_pan', 'position': self.min_pan_position}
        self.send_command(pan_command)
        tilt_command = {'joint': 'joint_head_tilt', 'position': self.min_tilt_position}
        self.send_command(tilt_command)

        for i in range(self.tilt_num_steps):
            for j in range(self.pan_num_steps):
                pan_command = {'joint': 'joint_head_pan', 'delta': self.pan_step_size}
                self.send_command(pan_command)
                rospy.sleep(0.2)

                try:
                    transform = self.tf_buffer.lookup_transform('base_link',
                                                                tag_name,
                                                                rospy.Time())
                    rospy.loginfo("Found Requested Tag: \n%s", transform)
                    self.transform_pub.publish(transform)
                    return transform
                except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException):
                    continue

            pan_command = {'joint': 'joint_head_pan', 'position': self.min_pan_position}
            self.send_command(pan_command)
            tilt_command = {'joint': 'joint_head_tilt', 'delta': self.tilt_step_size}
            self.send_command(tilt_command)
            rospy.sleep(.25)

        rospy.loginfo("The requested tag '%s' was not found", tag_name)

    def main(self):
        """
        Function that initiates the issue_command function.
        :param self: The self reference.
        """
        hm.HelloNode.main(self, 'aruco_tag_locator', 'aruco_tag_locator', wait_for_first_pointcloud=False)
        self.static_broadcaster = tf2_ros.StaticTransformBroadcaster()
        self.tf_buffer = tf2_ros.Buffer()
        self.listener = tf2_ros.TransformListener(self.tf_buffer)
        rospy.sleep(1.0)
        rospy.loginfo('Searching for docking ArUco tag.')
        pose = self.find_tag("docking_station")

if __name__ == '__main__':
    try:
        node = LocateArUcoTag()
        node.main()
    except KeyboardInterrupt:
        rospy.loginfo('interrupt received, so shutting down')

The Code Explained

Now let's break the code down.

#!/usr/bin/env python3
Every Python ROS Node will have this declaration at the top. The first line makes sure your script is executed as a Python3 script.

import rospy
import time
import tf2_ros
import numpy as np
from math import pi

import hello_helpers.hello_misc as hm
from sensor_msgs.msg import JointState
from control_msgs.msg import FollowJointTrajectoryGoal
from trajectory_msgs.msg import JointTrajectoryPoint
from geometry_msgs.msg import TransformStamped
You need to import rospy if you are writing a ROS Node. Import other python modules needed for this node. Import the FollowJointTrajectoryGoal from the control_msgs.msg package to control the Stretch robot. Import JointTrajectoryPoint from the trajectory_msgs package to define robot trajectories. The hello_helpers package consists of a module the provides various Python scripts used across stretch_ros. In this instance we are importing the hello_misc script.

def __init__(self):
    """
    A function that initializes the subscriber and other needed variables.
    :param self: The self reference.
    """
    hm.HelloNode.__init__(self)

    self.joint_states_sub = rospy.Subscriber('/stretch/joint_states', JointState, self.joint_states_callback)
    self.transform_pub = rospy.Publisher('ArUco_transform', TransformStamped, queue_size=10)

    self.joint_state = None
The LocateArUcoTag class inherits the HelloNode class from hm and is instantiated.

Set up a subscriber with rospy.Subscriber('/stretch/joint_states', JointState, self.joint_states_callback). We're going to subscribe to the topic "stretch/joint_states", looking for JointState messages. When a message comes in, ROS is going to pass it to the function joint_states_callback() automatically.

rospy.Publisher('ArUco_transform', TransformStamped, queue_size=10) declares that your node is publishing to the ArUco_transform topic using the message type TransformStamped. The queue_size argument limits the amount of queued messages if any subscriber is not receiving them fast enough.

self.min_pan_position = -4.10
self.max_pan_position =  1.50
self.pan_num_steps = 10
self.pan_step_size = abs(self.min_pan_position - self.max_pan_position)/self.pan_num_steps
Provide the minimum and maximum joint positions for the head pan. These values are needed for sweeping the head to search for the ArUco tag. We also define the number of steps for the sweep, then create the step size for the head pan joint.

self.min_tilt_position = -0.75
self.tilt_num_steps = 3
self.tilt_step_size = pi/16
Set the minimum position of the tilt joint, the number of steps, and the size of each step.

self.rot_vel = 0.5 # radians per sec
Define the head actuation rotational velocity.

def joint_states_callback(self, msg):
    """
    A callback function that stores Stretch's joint states.
    :param self: The self reference.
    :param msg: The JointState message type.
    """
    self.joint_state = msg
The joint_states_callback() function stores Stretch's joint states.

def send_command(self, command):
    '''
    Handles single joint control commands by constructing a FollowJointTrajectoryGoal
    message and sending it to the trajectory_client created in hello_misc.
    :param self: The self reference.
    :param command: A dictionary message type.
    '''
    if (self.joint_state is not None) and (command is not None):
        joint_name = command['joint']
        trajectory_goal = FollowJointTrajectoryGoal()
        trajectory_goal.trajectory.joint_names = [joint_name]
        point = JointTrajectoryPoint()
Assign trajectory_goal as a FollowJointTrajectoryGoal message type. Then extract the string value from the joint key. Also, assign point as a JointTrajectoryPoint message type.

if 'delta' in command:
    joint_index = self.joint_state.name.index(joint_name)
    joint_value = self.joint_state.position[joint_index]
    delta = command['delta']
    new_value = joint_value + delta
    point.positions = [new_value]
Check to see if delta is a key in the command dictionary. Then get the current position of the joint and add the delta as a a new position value.

elif 'position' in command:
    point.positions = [command['position']]
Check to see if positionis a key in the command dictionary. Then extract the position value.

point.velocities = [self.rot_vel]
trajectory_goal.trajectory.points = [point]
trajectory_goal.trajectory.header.stamp = rospy.Time(0.0)
trajectory_goal.trajectory.header.frame_id = 'base_link'
self.trajectory_client.send_goal(trajectory_goal)
self.trajectory_client.wait_for_result()
Then trajectory_goal.trajectory.points is defined by the positions set in point. Specify the coordinate frame that we want (base_link) and set the time to be now. Make the action call and send the goal. The last line of code waits for the result before it exits the python script.

def find_tag(self, tag_name='docking_station'):
    """
    A function that actuates the camera to search for a defined ArUco tag
    marker. Then the function returns the pose
    :param self: The self reference.
    :param tag_name: A string value of the ArUco marker name.

    :returns transform: The docking station's TransformStamped message.
    """
    pan_command = {'joint': 'joint_head_pan', 'position': self.min_pan_position}
    self.send_command(pan_command)
    tilt_command = {'joint': 'joint_head_tilt', 'position': self.min_tilt_position}
    self.send_command(tilt_command)
Create a dictionaries to get the head in its initial position for its search and send the commands the the send_command() function.

for i in range(self.tilt_num_steps):
    for j in range(self.pan_num_steps):
        pan_command = {'joint': 'joint_head_pan', 'delta': self.pan_step_size}
        self.send_command(pan_command)
        rospy.sleep(0.5)
Utilize nested for loop to sweep the pan and tilt in increments. Then update the joint_head_pan position by the pan_step_size. Use rospy.sleep() function to give time for system to do a Transform lookup before next step.

try:
    transform = self.tf_buffer.lookup_transform('base_link',
                                                tag_name,
                                                rospy.Time())
    rospy.loginfo("Found Requested Tag: \n%s", transform)
    self.transform_pub.publish(transform)
    return transform
except (tf2_ros.LookupException, tf2_ros.ConnectivityException, tf2_ros.ExtrapolationException):
    continue
Use a try-except block to look up the transform between the base_link and requested ArUco tag. Then publish and return the TransformStamped message.

pan_command = {'joint': 'joint_head_pan', 'position': self.min_pan_position}
self.send_command(pan_command)
tilt_command = {'joint': 'joint_head_tilt', 'delta': self.tilt_step_size}
self.send_command(tilt_command)
rospy.sleep(.25)
Begin sweep with new tilt angle.

def main(self):
    """
    Function that initiates the issue_command function.
    :param self: The self reference.
    """
    hm.HelloNode.main(self, 'aruco_tag_locator', 'aruco_tag_locator', wait_for_first_pointcloud=False)
Create a funcion, main(), to do all of the setup for the hm.HelloNode class and initialize the aruco_tag_locator node.

self.static_broadcaster = tf2_ros.StaticTransformBroadcaster()
self.tf_buffer = tf2_ros.Buffer()
self.listener = tf2_ros.TransformListener(self.tf_buffer)
rospy.sleep(1.0)
Create a StaticTranformBoradcaster Node. Also, start a tf buffer that will store the tf information for a few seconds.Then set up a tf listener, which will subscribe to all of the relevant tf topics, and keep track of the information. Include rospy.sleep(1.0) to give the listener some time to accumulate transforms.

rospy.loginfo('Searching for docking ArUco tag.')
pose = self.find_tag("docking_station")
Notify Stretch is searching for the ArUco tag with a rospy.loginfo() function. Then search for the ArUco marker for the docking station.

if __name__ == '__main__':
    try:
        node = LocateArUcoTag()
        node.main()
    except KeyboardInterrupt:
        rospy.loginfo('interrupt received, so shutting down')
Declare LocateArUcoTag object. Then run the main() method.