Added documentation for urscript_interface

Author: fmauch

ur_robot_driver/doc/usage.rst

@@ -214,3 +214,57 @@ are a couple of things to know:
   additional tool configured on the Teach pendant (TP), this should be equivalent to ``tool0`` given that
   the URDF uses the specific robot's :ref:`calibration <calibration_extraction>`. If a tool is
   configured on the TP, then the additional transformation will show in ``base`` -> ``tool0``.
+
+Custom URScript commands
+------------------------
+
+The driver's package contains a ``urscript_interface`` node that allows sending URScript snippets
+directly to the robot. It gets started in the driver's launchfiles by default. To use it, simply
+publish a message to its interface:
+
+.. code-block:: bash
+
+  # simple popup
+  ros2 topic pub /urscript_interface/script_command std_msgs/msg/String '{data: popup("hello")}' --once
+
+Be aware, that running a program on this interface (meaning publishing script code to that interface) stops any running program on the robot.
+Thus, the motion-interpreting program that is started by the driver gets stopped and has to be
+restarted again. Depending whether you use headless mode or not, you'll have to call the
+``resend_program`` service or press the ``play`` button on the teach panel to start the
+external_control program again.
+
+Multi-line programs
+^^^^^^^^^^^^^^^^^^^
+
+When you want to define multi-line programs, make sure to check that newlines are correctly
+interpreted from your message. For this purpose the driver prints the program as it is being sent to
+the robot. When sending a multi-line program from the command line, you can use an empty line
+between each statement:
+
+.. code-block:: bash
+
+   ros2 topic pub --once /urscript_interface/script_command std_msgs/msg/String '{data:
+   "def my_prog():
+
+     set_digital_out(1, True)
+
+     movej(p[0.2, 0.3, 0.8, 0, 0, 3.14], a=1.2, v=0.25, r=0)
+
+     textmsg(\"motion finished\")
+
+   end"}'
+
+Non-interrupting programs
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To prevent interrupting the main program, you can send certain commands as `secondary programs
+<https://www.universal-robots.com/articles/ur/programming/secondary-program/>`_.
+
+.. code-block:: bash
+
+   ros2 topic pub --once /urscript_interface/script_command std_msgs/msg/String '{data:
+     "sec my_prog():
+
+       textmsg(\"This is a log message\")
+
+     end"}'