actionlib

To control the gripper (and most other interfaces of the robot), you will need to be familiar with actionlib and the actionlib tutorials.

Do the actionlib beginner tutorials if you have not already.

Gripper interface

Go to the Fetch docs and click on API Overview -> Gripper Interface. The robot runs an actionlib server to control the gripper, so you will need to write an actionlib client to send open/close commands.

If you look at the GripperCommand action linked from the Fetch docs, you will see that the goal msg has just one field, command, which is confusingly of type GripperCommand. No, this is not a circular definition. GripperCommand is also the name of a message as well as an action. You can see what a GripperCommand message looks like by clicking on the control_msgs link in the GripperCommand action page:

From there, you can see a list of all the messages, services, and actions defined in the control_msgs package:

Note that the name GripperCommand is used twice, once as an action and once as a message.

So to create a goal, your code will look something like this:

goal = control_msgs.msg.GripperCommandGoal()
goal.command.position = CLOSED_POS
goal.command.max_effort = max_effort

Write the action client

In the course repo, a skeleton of the gripper code has been written for you in fetch_api/src/fetch_api/gripper.py. Fill in the methods open and close.

Finish the demo

The course repo contains a partially implemented file you can use to test your code in applications/scripts/gripper_demo.py. Fill out the sections that simply print "Not implemented."

If you run the demo with:

rosrun applications gripper_demo.py

You should see the error:

[rosrun] Couldn't find executable named gripper_demo.py below /home/teamN/catkin_ws/src/cse481c/applications
[rosrun] Found the following, but they're either not files,
[rosrun] or not executable:
[rosrun]   /home/teamN/catkin_ws/src/cse481c/applications/scripts/gripper_demo.py

Any ROS Python file with a main function should be marked executable:

chmod +x gripper_demo.py

Now you should be able to run the demo, and see the robot's gripper open and close in the simulator:

rosrun applications gripper_demo.py close
rosrun applications gripper_demo.py open