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Dex to Standard Wrist

This tutorial will guide you through the steps of replacing a gripper with a Dex Wrist (3-DoF) for one with a Standard Wrist (1-DoF).

Parts and Tools Required

Please note that this procedure does not require any additional parts or tools apart from the ones that were shipped with the robot:

Removing Dex Wrist Gripper

Here we describe removing the Dex Wrist gripper. Please ensure that the robot is turned off before proceeding.

First, inspect the parts and ensure that you have everything you need for the procedure.

Dex Wrist

Now, remove the cable clip by unscrewing the M2.5x8mm bolts and then unplug the Dynamixel cable out of the wrist pitch servo (pink).

Remove the cable

Next, rotate the wrist yaw joint so that the wrist pitch servo body is accessible. Detach the pitch servo from the mounting bracket by unscrewing the four M2.5x4mm screws (C) with the T8 Torx wrench.

Remove screws

Slide the wrist module out horizontally so that the bearing unmates from its post.

Remove mating

Then, lower the wrist module vertically away from the mounting bracket.

Remove from mounting bracket

Lastly, detach the wrist mount bracket (A) from the bottom of the tool plate by removing the M2x6mm bolts (B) using a T6 Torx wrench.

Remove tool plate

Attaching Standard Wrist Gripper

Here we describe attaching the Standard Wrist gripper.

First, note where the forward direction is on the wrist yaw tool plate. This is indicated by the additional alignment hole that is just outside the bolt pattern shown pointing down in the image.

Alignment hole

Then, route the Dynamixel cable through the center of the standard gripper mounting bracket and install the bracket with the eight screws and T6 Torx wrench. Make sure the forward marking on the bracket matches the forward marking on the wrist yaw.

image alt text

Now, affix the four screws, with the shorter two going to the servo side, to hold the gripper to the bracket. Lastly, route the Dynamixel cable through the back of the gripper and plug it securely into the servo.

image alt text

Software Instructions

Once the hardware has been replaced, it's time to make the software changes for Stretch to recognize the Standart Wrist gripper. Turn on the robot and follow the instructions below.

To revert the changes in stretch_configuration_params.yaml, first download the dex_to_standard_configure_params.py script using: 

cd ~/Desktop
wget https://raw.githubusercontent.com/hello-robot/stretch_tutorials/master/stretch_tool_share/dex_to_standard_configure_params.py

Then, change execution permissions and execute it in a terminal as below: 

chmod +x dex_to_standard_configure_params.py
python3 dex_to_standard_configure_params.py

Ensure that the changes were written correctly to stretch_params: 

stretch_params.py | grep robot.tool

This should return the tool as tool_stretch_gripper.

Next, to ensure the correct gripper is recognized by ROS, we need to update the URDF. For this, first open the stretch_description.xacro file like below.

cd ~/catkin_ws/src/stretch_ros/stretch_description/urdf
gedit stretch_description.xacro

Then, replace the contents of the file with that of the default stretch_description.xacro by copying and pasting the contents from the file in the link to the open file.

Lastly, to generate the updated URDF, execute the following commands in a terminal.

rosrun stretch_calibration update_urdf_after_xacro_change.sh
cd ~/catkin_ws/src/stretch_ros/stretch_description/urdf
./export_urdf.sh

You can ensure that the gripper is functional by homing the Dynamixel servos with the following commands:

stretch_gripper_home.py

stretch_wrist_yaw_home.py

If you encounter any issues, please contact support.