Torque-Control

This repository provides a torque controller for the KUKA youBot arm as well as a Service to generate trajectories which can then be executed by the torque controller. The Torque Controller has been tested under the following setup:

• Ubuntu 12.04 with ROS-Hydro

Watch the video demonstrating the RPG youBot Torque Controller:

More information: Master Thesis, More software from the Robotics and Perception Group

Before You Use The Controller!

In order to use the torque controller, you will have to change the controller gains of the motor controllers of the KUKA youBot arm. Otherwise, the controller will not work. To change these gains, you can use the joint configurator application. Please read Section 4.3 of the Master Thesis to find out how to tune the gains. Our values are listed in Table 4.1--however, these values are most likely different for every robot.

Installation

Dependencies

To compile everything you need to install the youBot driver.

sudo apt-get install ros-hydro-youbot-driver

The Torque Controller makes use of the pr2_msgs and the brics_actuator packages. You can install them by running

sudo apt-get install ros-hydro-pr2-msgs
sudo apt-get install ros-hydro-brics-actuator

Additionally, you need an adapted version of the ros wrapper of the driver which you have to clone into the src folder of your catkin workspace

cd catkin_ws/src
git clone https://github.com/uzh-rpg/youbot_driver_ros_interface.git

The latter is an adapted version of the mas-group/youbot_driver_ros_interface. It is important to use the adapted package instead of the original one since the torque controller will not work properly otherwise! The adapted version enables sending torque messages to the youbot_oodl and disables gripper sensor readouts. This was necessary because the gripper position readout is blocking the youbot_oodl.

Main Installation

You can download the actual Torque Controller by running

cd catkin_ws/src
git clone https://github.com/uzh-rpg/rpg_youbot_torque_control.git

Then, you can simply catkin_make it

cd catkin_ws
catkin_make

Test the Torque Controller with the Trajectory Generator

An axample on how to use the torque controller is provided in the torque_example package. To test it, you first have to launch the youbot_ros_driver_interface by

roslaunch youbot_driver_ros_interface youbot_driver.launch

Then, you can launch the provided launch file

roslaunch torque_example torque_example.launch

which starts the Torque Controller and a Trajectory Generator Service. Then you can start the exapmple by running

rosrun torque_example circle_traj

This will place the gripper to a start position using the existing position control. The node then asks you if you are ready to execute a trajectory with the torque controller. If you are type yes in the console where you started the circle_traj node. The gripper should then follow a circular trajectory.

Basic Usage

Torque Controller

The Torque Controller node is organized as an Action Server and can be launched as follows:

<launch>

<node name="torque_control" pkg="torque_control" type="torque_control" cwd="node" output="screen"/>

</launch>

Subscribed Topics

• joint_states (sensor_msgs::JointState)

  The joint state message as published by the youbot_oodl.

Published Topics

• arm_1/arm_controller/position_command (brics_actuator/JointPositions)

  The joint positions commands to be sent to the youbot_oodl.

• arm_1/arm_controller/torques_command (brics_actuator/JointTorques)

  The joint torque commands to be sent to the youbot_oodl.

Action Subscribed Topics

• torque_control/goal (trajectory_msgs/JointTrajectory)

• This message is created by calling the trajectory_generator service. The trajectory must consist of values for joint positions, velocities and acceleration for all 5 arm joints.

• torque_control/cancel (actionlib_msgs/GoalID)

  Cancels an action with a specific ID.

Action Subscribed Topics

• torque_control/feedback (torque_control/torque_trajectoryActionFeedback)
• torque_control/status (actionlib_msgs/GoalStatusArray)
• torque_control/result (torque_control/torque_trajectoryActionResult)

Services

• turn_gravity_compensation_on
• turn_gravity_compensation_off

Trajectory Generator

The trajectory generator node can be launched as follows:

<launch>

<node name="trajectory" pkg="trajectory_generator" type="trajectory_service" output="screen" />

</launch>

I provides four different services to generate trajectories for the KUKA youBot arm. In the following, JS denotes Joint Space and CS denotes the Cartesion Space. If a feasible trajectory for the input parameters are found, it is returned in a trajectory_msgs/JointTrajectory message.

Services

• From_JS_2_JS (trajectory_generator/JStoJS)

  Generates a joint space trajectory to move all the joints from the start joint space position to the desired end joint space position. The joints will perform a quadratic profile in position.

• From_JS_2_CS (trajectory_generator/JStoCS)

  Generates a joint space trajectory to move the gripper in a straight line from the start joint space position to the Cartesian end position. This is helpful to reach a Cartesian end-effector position from the current manipulator configuration.

• From_CS_2_CS (trajectory_generator/CStoCS)

  Generates a joint space trajectory to move the gripper in a straight line from the Cartesian start to the Cartesian end position.

• Circular_Trajectory (trajectory_generator/Circle)

  Generates a circular joint space trajectory around a center point.

Examples on how to use the different services can be found in the tester.cpp file.