Add docs for HelloNode.get_tf

hello_helpers/README.md

@@ -81,6 +81,20 @@ If you set `custom_full_goal` to True, the dictionary format is string/tuple key
 self.move_to_pose({'joint_arm': (0.5, 0.01, 0.01, 40)}, custom_full_goal=True)
 ```
 
+##### `get_tf(from_frame, to_frame)`
+
+Use this method to get the transform ([geometry_msgs/TransformStamped](https://docs.ros.org/en/noetic/api/geometry_msgs/html/msg/TransformStamped.html)) 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:
+
+```python
+# roslaunch the stretch launch file beforehand
+
+import rospy
+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.
@@ -132,28 +146,6 @@ temp = hm.HelloNode.quick_create('temp')
 temp.stop_the_robot_service(TriggerRequest())
 ```
 
-#### Other
-
-##### TF Listener
-
-Provides a [tf2](http://wiki.ros.org/tf2) listener that buffers [transforms](https://docs.ros.org/en/noetic/api/geometry_msgs/html/msg/TransformStamped.html) under the `self.tf2_buffer` attribute. For example, we can do forward kinematics from the base_link to the link between the gripper fingers, link_grasp_center, using:
-
-```python
-# roslaunch the stretch launch file beforehand
-
-import rospy
-import hello_helpers.hello_misc as hm
-temp = hm.HelloNode.quick_create('temp')
-rate = rospy.Rate(10.0)
-while True:
-    try:
-        t = temp.tf2_buffer.lookup_transform('link_grasp_center', 'base_link', rospy.Time())
-        print(t.transform.translation, t.transform.rotation)
-    except:
-        continue
-    rate.sleep()
-```
-
 ## License
 
 For license information, please see the LICENSE files. 

hello_helpers/src/hello_helpers/hello_misc.py

@@ -159,7 +159,7 @@ class HelloNode:
         rate = rospy.Rate(10.0)
         while True:
             try:
-                return self.tf2_buffer.lookup_transform(to_frame, from_frame, rospy.Time())
+                return self.tf2_buffer.lookup_transform(from_frame, to_frame, rospy.Time())
             except:
                 continue
             rate.sleep()

stretch_core/README.md

@@ -50,7 +50,7 @@ Other ways to stow the robot include using the `stretch_robot_stow.py` CLI tool
 
 See [stretch_driver.launch's TF docs](#TODO) to learn about the full Stretch TF tree. The stretch_driver node, which is part of stretch_driver.launch, is responsible for publishing the "odom" to "base_link" transform if the [broadcast_odom_tf parameter](#broadcastodomtf) is set to true. Odometry for the robot is calculated within the underlying Stretch Body Python SDK within the [update method in the Base class](https://github.com/hello-robot/stretch_body/blob/ea987c3d4f21a65ce4e85c6c92cd5d2efb832c41/body/stretch_body/base.py#L555-L651) by looking at the encoders for the left and right wheels. Note: Odometry calculated from wheel encoders is susceptible to drift for a variety of reasons (e.g. wheel slip, misalignment, loose belt tension, time dilation). A reasonable amount of drift is ~1cm per meter translated by the mobile base. A common remedy is to use a localization library to correct for the drift by integrating other sensors into the calculation of the odometry. For example, [AMCL](http://wiki.ros.org/amcl) is a particle filter ROS package for using Lidar scans + wheel odometry to correct for drift.
 
-If you use [HelloNode](../hello_helpers/README.md#hellonodesrchellohelpershellomiscpy), you can get the odometry using:
+If you use [HelloNode](../hello_helpers/README.md#hellonode), you can get the odometry using:
 
 ```python
 # roslaunch the stretch launch file beforehand
@@ -58,7 +58,7 @@ If you use [HelloNode](../hello_helpers/README.md#hellonodesrchellohelpershellom
 import rospy
 import hello_helpers.hello_misc as hm
 temp = hm.HelloNode.quick_create('temp')
-t = temp.get_tf('base_link', 'odom')
+t = temp.get_tf('odom', 'base_link')
 print(t.transform.translation)
 ```