stretch_tutorials/ros2/intro_to_hellonode.md

Introduction to HelloNode

HelloNode is a convenience class for creating a ROS 2 node for Stretch. The most common way to use this class is to extend it. In your extending class, the main funcion would call HelloNode's main function. This would look like:

import hello_helpers.hello_misc as hm

class MyNode(hm.HelloNode):
    def __init__(self):
        hm.HelloNode.__init__(self)

    def main(self):
        hm.HelloNode.main(self, 'my_node', 'my_node', wait_for_first_pointcloud=False)
        # my_node's main logic goes here

node = MyNode()
node.main()

There is also a one-liner class method for instantiating a HelloNode for easy prototyping. One example where this is handy in sending pose commands from iPython:

# roslaunch the stretch launch file beforehand

import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.move_to_pose({'joint_lift': 0.4})

Attributes

dryrun

This attribute allows you to control whether the robot actually moves when calling move_to_pose(), home_the_robot(), stow_the_robot(), or other motion methods in this class. When dryrun is set to True, these motion methods return immediately. This attribute is helpful when you want to run just the perception/planning part of your node without actually moving the robot. For example, you could replace the following verbose snippet:

# launch the stretch driver launch file beforehand
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
actually_move = False
[...]
if actually_move:
    temp.move_to_pose({'translate_mobile_base': 1.0})

to be more consise:

# launch the stretch driver launch file beforehand
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
[...]
temp.dryrun = True
temp.move_to_pose({'translate_mobile_base': 1.0})

Methods

move_to_pose(pose, blocking=False, custom_contact_thresholds=False, duration=2.0)

This method takes in a dictionary that describes a desired pose for the robot and communicates with stretch_driver to execute it. The basic format of this dictionary is string/number key/value pairs, where the keys are joint names and the values are desired position goals. For example, {'joint_lift': 0.5} would put the lift at 0.5m in its joint range. A full list of command-able joints is published to the /stretch/joint_states topic. Used within a node extending HelloNode, calling this method would look like:

self.move_to_pose({'joint_lift': 0.5})

Internally, this dictionary is converted into a JointTrajectory message that is sent to a FollowJointTrajectory action server in stretch_driver. This method waits by default for the server to report that the goal has completed executing. However, you can return before the goal has completed by setting the blocking argument to False. This can be useful for preempting goals.

When the robot is in position mode, if you set custom_contact_thresholds to True, this method expects a different format dictionary: string/tuple key/value pairs, where the keys are still joint names, but the values are (position_goal, effort_threshold). The addition of a effort threshold enables you to detect when a joint has made contact with something in the environment, which is useful for manipulation or safe movements. For example, {'joint_arm': (0.5, 20)} commands the telescoping arm fully out (the arm is nearly fully extended at 0.5 meters) but with a low enough effort threshold (20% of the arm motor's max effort) that the motor will stop when the end of arm has made contact with something. Again, in a node, this would look like:

self.move_to_pose({'joint_arm': (0.5, 40)}, custom_contact_thresholds=True)

When the robot is in trajectory mode, if you set argument duration as ts, this method will ensure that the target joint positions are achieved over ts seconds. For example, the below would put the lift at 0.5m from its current position in 5.0 seconds:

self.move_to_pose({'joint_lift': 0.5}, duration=5.0)

home_the_robot()

This is a convenience method to interact with the driver's /home_the_robot service.

stow_the_robot()

This is a convenience method to interact with the driver's /stow_the_robot service.

stop_the_robot()

This is a convenience method to interact with the driver's /stop_the_robot service.

get_tf(from_frame, to_frame)

Use this method to get the transform (geometry_msgs/TransformStamped) between two frames. This method is blocking. For example, this method can do forward kinematics from the base_link to the link between the gripper fingers, link_grasp_center, using:

# launch the stretch driver launch file beforehand

import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
t = temp.get_tf('base_link', 'link_grasp_center')
print(t.transform.translation)

get_robot_floor_pose_xya(floor_frame='odom')

Returns the current estimated x, y position and angle of the robot on the floor. This is typically called with respect to the odom frame or the map frame. x and y are in meters and the angle is in radians.

!!! note To get the robot pose with respect to the odom frame we need to launch stretch_driver along with the broadcast_odom_tf parameter set to True. To do this execute the command: ros2 launch stretch_core stretch_driver.launch.py broadcast_odom_tf:=True

# launch the stretch driver launch file beforehand

import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
t = temp.get_robot_floor_pose_xya(floor_frame='odom')
print(t)

main(node_name, node_topic_namespace, wait_for_first_pointcloud=True)

When extending the HelloNode class, call this method at the very beginning of your main() method. This method handles setting up a few ROS components, including registering the node with the ROS server, creating a TF listener, creating a FollowJointTrajectory client for the move_to_pose() method, subscribing to depth camera point cloud topic, and connecting to the quick-stop service.

Since it takes up to 30 seconds for the head camera to start streaming data, the wait_for_first_pointcloud argument will get the node to wait until it has seen camera data, which is helpful if your node is processing camera data.

quick_create(name, wait_for_first_pointcloud=False)

A class level method for quick testing. This allows you to avoid having to extend HelloNode to use it.

# launch the stretch driver launch file beforehand

import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.move_to_pose({'joint_lift': 0.4})

Subscribed Topics

/camera/depth/color/points (sensor_msgs/PointCloud2)

Provides a point cloud as currently seen by the Realsense depth camera in Stretch's head. Accessible from the self.point_cloud attribute.

# launch the stretch driver launch file beforehand

import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp', wait_for_first_pointcloud=True)
print(temp.point_cloud)

/stretch/joint_states (sensor_msgs/JointState)

Provides the current state of robot joints that includes joint names, positions, velocities, efforts. Accessible from the self.joint_state attribute.

print(temp.joint_state)

/mode (std_msgs/String)

Provides the mode the stretch driver is currently in. Possible values include position, trajectory, navigation, homing, stowing.

print(temp.mode)

/tool (std_msgs/String)

Provides the end of arm tool attached to the robot.

print(temp.tool)

Subscribed Services

/stop_the_robot (std_srvs/Trigger)

Provides a service to quickly stop any motion currently executing on the robot.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.stop_the_robot_service.call_async(Trigger.Request())

/stow_the_robot (std_srvs/Trigger)

Provides a service to stow the robot arm.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.stow_the_robot_service.call_async(Trigger.Request())

/home_the_robot (std_srvs/Trigger)

Provides a service to home the robot joints.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.home_the_robot_service.call_async(Trigger.Request())

/switch_to_trajectory_mode (std_srvs/Trigger)

Provides a service to quickly stop any motion currently executing on the robot.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.switch_to_trajectory_mode_service.call_async(Trigger.Request())

/switch_to_position_mode (std_srvs/Trigger)

Provides a service to quickly stop any motion currently executing on the robot.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.switch_to_position_mode_service.call_async(Trigger.Request())

/switch_to_navigation_mode (std_srvs/Trigger)

Provides a service to quickly stop any motion currently executing on the robot.

# launch the stretch driver launch file beforehand

from std_srvs.srv import Trigger
import hello_helpers.hello_misc as hm
temp = hm.HelloNode.quick_create('temp')
temp.switch_to_navigation_mode_service.call_async(Trigger.Request())