I don't know what parameter I should enter here.

edit

Trying to use this function to get (x,y,z) out of a bag file. However, I don't know what I should enter as "joint_name" input. Function won't work unless i supply this input.

I try to use: get_joint_data(somefile.bag, ?). I don't know what to put there. base_link, base_footprint, start_of_device don't work.

def get_joint_data(bagFile, joint_name, convert_to_sec=True):
    """
        Function that filters bag files to obtain data from a joint that is 
        published to the /tf topic.
        Only x-, y- and z-coordinates are returned
        if convert_to_sec is set to True (default), then the first timestamp 
        will be taken as zero seconds, and the following timesteps that are 
        retured will be relative to this timestamp.
    """
    # INITIALIZATION
    x = np.array([])
    y = np.array([])
    z = np.array([])
    all_t = np.array([])
    bag = rosbag.Bag(bagFile) # Initialize rosbag object
    first = True # True on first iteration to take first timestamp

    # Add message values to collections
    for topic, msg, t in bag.read_messages(topics=['/tf']):

        joint = msg.transforms[0].child_frame_id
        translation = msg.transforms[0].transform.translation

            if joint == joint_name: 

                # Get timestamp in seconds

    t = msg.transforms[0].header.stamp
        t_sec = t.to_sec()
        if (first and convert_to_sec):
            t_0 = t_sec
            first = False
        all_t = np.append(all_t, t_sec-t_0)        

        # Get x, y and z coordinates
            pose = [translation.x , translation.y, translation.z]

            x = np.append(x, pose[0])
            y = np.append(y, pose[1])
            z = np.append(z, pose[2])

    pose = [x,y,z]

    return pose, all_t