[ros2_control] Combined hw_interface for arm and gripper

edit

We have a robot that's a little unusual in that the manufacturer combined gripper control and arm joint control into a single, unified API. Thus, we can't really instantiate 2 objects to control gripper and arm separately. We aren't sure how to control the gripper and the arm in a combined fashion.

Our first attempt has a controllers.yaml like this:

joint_trajectory_controller:
  type: joint_trajectory_controller/JointTrajectoryController

# The gripper controller just relays position commands
gripper_controller:
  type: forward_command_controller/ForwardCommandController

joint_trajectory_controller:
  ros__parameters:
    joints:
      - joint_1
      - joint_2
      - joint_3
      - joint_4
      - joint_5
      - joint_6
      - joint_7

gripper_controller:
  ros__parameters:
    joints:
      - finger_joint
    interface_name: position

Our custom interface inherits like this:

class OurMultInterfaceHardware : public hardware_interface::BaseInterface<hardware_interface::SystemInterface

When launching on hardware, we can see from some print statements that OurMultiInterfaceHardware gets instantiated twice, which will not work with the manufacturer's API. It works fine in simulation.

The important parts from the launch file:

robot_controller_spawner = Node(
    package="controller_manager",
    executable="spawner.py",
    arguments=[robot_controller, gripp, "-c", "/controller_manager"],
)

gripper_controller_spawner = Node(
    package="controller_manager",
    executable="spawner.py",
    arguments=[gripper_controller, "-c", "/controller_manager"],
)

nodes_to_start = [
    control_node,
    robot_state_publisher_node,
    rviz_node,
    joint_state_broadcaster_spawner,
    robot_controller_spawner,
    gripper_controller_spawner,
]

I think the crux of the question is, does each controller that gets spawned require its own hardware interface?