lbrmed_control

LBRMed Control

Configurations of ros controllers which can be used with the hardware and simulation. Provides launch files to load and start a set of default controllers.

config

• lbrmed_control.yaml: Default configuration of controllers.
• lbrmed_sim_hw.yaml: Configuration of fake controllers for the kinematic simulation via ros_control_boilerplate.

launch

• lbrmed_control.launch: Launch file to load and start a set of default controllers.
  • controllers: List of controllers to start immediately.
  • load_controllers: List of controllers to load but not start.
• lbrmed_hw_sim.launch: Launch file to launch the ros_control_boilerplate fake controllers.

Typically, used controllers

• PositionJointInterface_trajectory_controller: Implements a controller which accepts trajectories of joint values, e.g. from MoveIt.
• PositionJointInterface_cartesian_motion_controller: Implements a controller which accepts Cartesian pose commands.
• joint_state_controller: Implements a controller which re-publishes the joint states of the robot.
• force_torque_sensor_controller: Implements a controller which re-publishes the force and torque sensors of the robot.

FZI Cartesian controllers

The config contains entries for the FZI Cartesian controllers. However, this package is intended to be a self-contained build and source dependencies are reduced to a minimum. To install the Cartesian controllers, run the following commands in the workspace root:

wstool init src
wstool merge -t src src/lbrmed-ros/lbrmed_control/.rosinstall
wstool update -t src

Then re-build and -source the whole workspace

Custom controllers

• PositionJointInterface_abs_admittance_controller: Implements an absolute force-based admittance controller. Compared to a normal admittance controller, the robot approaches an object until the total force exceeds a limit and stops. I tried implementing a similar behavior using the FZI Cartesian controllers, but it deviated from the commanded position, no matter how I tuned the parameters. This controller uses good old KDL inverse kinematics and monitors the forces in the ROS control loop. Compared to the FZI version, this implementation is less robust near singularities.
  • Offers abs_admittance_control action:
    • Send a goal velocity velocity and absolute goal force goal_abs_force
    • Robot will measure use the currently measured Cartesian force as a static tare force which is used with the force_limit for safety
    • Robot will start approaching with the constant goal velocity
    • after dynamic_tare_time the robot will measure the current Cartesian force as a dynamic tare force. The measured joint torques change due to dynamic friction compared to the static tare force.
    • If the absolute force abs(tare - measured) exceeds the goal or the safety limit, the robot will stop.
  • Parameters in config/lbrmed_control.yaml
    • force_limit is a safety limit used with the static tare measurement
    • dynamic_tare_time is the time in seconds after which the dynamic tare force is measured