How to control a Robotiq 2F-85 in ROS via the UR Tool I/O

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This started out as a question but I ended up solving the problem myself! So I thought I'd post it here in case anyone is looking for the solution. So far this has only been tested in Melodic.

The setup is this: A Robotiq 2F-85 gripper connected to a UR arm via the Robotiq GRP-ES-CPL-062 Gripper Coupler. Control is via the Robotiq ROS driver, and NOT via the Robotiq URCap. The benefit here is that the UR controller takes care of the 24VDC supply that the gripper requires, along with the RS485 communications, all while eliminating the ugly and complicated external cable. Communications in the background is handled by a virtual com port set up by socat.

On the UR computer:

• Install the rs485 URCap from the official Universal Robots GitHubrepo. This URcap creates a socat interface on the UR computer.
• Install the external control URCap from the official Universal Robots GitHub repo. This allows ROS control of the UR arm.
• Give the UR a static IP address (say, 192.168.131.40)

In Installation>General>Tool I/O:

• set the tool output voltage to 24V
• enable “communication Interface” and use defaults

On the ROS computer:

• install the official UR ROS driver: https://github.com/UniversalRobots/Un...
• install the robotiq driver being maintained by Clearpath: https://github.com/clearpathrobotics/...

In the UR launch file (ur5e_bringup.launch):

• define the UR's static IP address
• set tool communication to true. This creates a virtual device link using socat.
• change the tool_device_name to /tmp/ttyTool (the default is /dev which introduces all sorts of permissions nonsense)
• download the calibration parameters from the arm and save them to the computer. Point the launch file at that path.

In the robotiq launch file (robotiq_action_server.launch):

• set respawn=true
• set the comport to /tmp/ttyTool

Startup/Launch order:

• power on the arm, release brakes, etc
• roslaunch ur_robot_driver ur5e_bringup.launch (the UR launch file you customized)
• wait 10-15 seconds for the UR driver to finish launching
• roslaunch robotiq_2f_gripper_control robotiq_action_server.launch (The robotiq launch file you customized)
• wait for the robotiq driver to start. It may take a dozen respawns before it finally makes a connection. Or it might take one. You'll see lots of modbus exception errors, but it should eventually connect. After that the light on the gripper will turn blue and the connection should be pretty solid. (if anyone would like to take a crack at making the rs485 URCap + robotiq driver combo more robust, I'm sure everyone would sing your praises!)
  • start the external_control program on the UR