ur_modern_driver - Refactored

A new driver for the UR3/UR5/UR10 robot arms from Universal Robots. It is designed to replace the old driver transparently, while solving some issues, improving usability as well as enabling compatibility of ros_control.

Improvements

• A script is only running on the robot while a trajectory is actually executing. This means that the teach pendant can be used to move the robot around while the driver is connected.

• The driver exposes the same functionality as the previous ur_driver:

  • Action interface on /follow_joint_trajectory for seamless integration with MoveIt

  • Publishes robot joint state on /joint_states

  • Publishes TCP force on /wrench

  • Publishes IO state on /ur_driver/io_states (Note that the string /ur_driver has been prepended compared to the old driver)

  • Service call to set outputs and payload - Again, the string /ur_driver has been prepended compared to the old driver (Note: I am not sure if setting the payload actually works, as the robot GUI does not update. This is also true for the old ur_driver )

• Besides this, the driver subscribes to two new topics:

  • /ur_driver/URScript : Takes messages of type std_msgs/String and directly forwards it to the robot. Note that no control is done on the input, so use at your own risk! Intended for sending movel/movej commands directly to the robot, conveyor tracking and the like.

  • /joint_speed : Takes messages of type trajectory_msgs/JointTrajectory. Parses the first JointTrajectoryPoint and sends the specified joint speeds and accelerations to the robot. This interface is intended for doing visual servoing and other kind of control that requires speed control rather than position control of the robot. Remember to set values for all 6 joints. Ignores the field joint_names, so set the values in the correct order.

• Added support for ros_control.

  • As ros_control wants to have control over the robot at all times, ros_control compatibility is set via a parameter at launch-time.
  • With ros_control active, the driver doesn't open the action_lib interface nor publish joint_states or wrench msgs. This is handled by ros_control instead.
  • Currently two controllers are available, both controlling the joint position of the robot, useable for trajectroy execution
    • The velocity based controller sends joint speed commands to the robot, using the speedj command
    • The position based controller sends joint position commands to the robot, using the servoj command
    • I have so far only used the velocity based controller, but which one is optimal depends on the application.
  • As ros_control continuesly controls the robot, using the teach pendant while a controller is running will cause the controller on the robot to crash, as it obviously can't handle conflicting control input from two sources. Thus be sure to stop the running controller before moving the robot via the teach pendant:
    • A list of the loaded and running controllers can be found by a call to the controller_manager rosservice call /controller_manager/list_controllers {}
    • The running position trajectory controller can be stopped with a call to rosservice call /universal_robot/controller_manager/switch_controller "start_controllers: - '' stop_controllers: - 'pos_based_pos_traj_controller' strictness: 1" (Remember you can use tab-completion for this)

• Added activation modes: Never, Always and OnStartup. Sets wether a call to the ur_driver/robot_enable service is required at startup, at startup + on errors or never. Is intended as a safety feature to require some sort of manual intervention in case of driver crashes and robot safety faults.

• Low Bandwidth Trajectory Follower mode of execution. (only works when ros_control is set to false)In this mode the real-time control loop for the robot is shifted from the client PC running the driver to URscript executed on the URControl PC of the robot with the following features:

  • It works only with /follow_joint_trajectory action for MoveIt integration. It is mutually exclusive with ros_control
  • It only implements "position + velocity" based control - it uses coarse positions and velocities calculated by MoveIt and performs cubic interpolation of joint positions (Bezier' curve) of positions. The positions are set using servoj command of URScript.
  • It is much more resilient to connectivity problems than other methods of ur_modern_driver. It is resilient to additional load on client PC, latency of network and kernel of the PC the driver runs on. This makes it much better suitable for development/research quick iteration loops without separate dedicated physical setup.
    • Other methods of controlling the robot by ur_modern_driver are very fragile and require low-latency kernel, separated wired network and ideally no other network activity on the PC where ur_modern_driver runs.
    • Low Bandwidth Trajectory Follower will never make dangerous "catch-up" when move is delayed. Other methods prioritise execution time of a move which sometimes might lead to situation where robot gets out of control and speeds up to catch-up - often resulting in Protective Stop. For Low Bandwith Trajectory Follower predictability of the move has priority over move execution time.
    • The amount of TCP/IP communication between the driver and UR robot (URControl PC) is two orders of magnitude (around 100x) less with Low Bandwidth Trajectory Follower than with other methods- coarse MoveIt generated trajectory is sent to robot and cubic interpolation of the move is calculated by the URScript program run on URControl PC.
    • Due to communication optimisations and Low Bandwidth Trajectory Follower works reliably even over WiFi connection (!) which is impossible for other methods.
  • Time flow for the URScript program might be independent from "real time" and can be controlled to speed up or slow down execution of planned moves.
  • The Low Bandwidth Trajectory Follower requires 3.0+ version firmware of the robot (servoj command must support lookahead_time and servoj_gain parameters)
  • There are several parameters that you can use to control Low Bandwidth Trajectory Follower's behaviour:
    • use_lowbandwidth_trajectory_follower - should be set to true to enable the follower
    • time_interval - time interval (in seconds) this is 'simulated' time for interpolation steps of the move. Together with servoj_time it can be used to change speed of moves even after they are planned by MoveIt. Default value is 0.008
    • servoj_time - time interval (real time) for which each interpolation step controls the robot (using servoj command). See below on examples of setting different time parameters. Default value is 0.008 (corresponds to expected 125Hz frequency of UR robot control)
    • servoj_time_waiting - time in seconds (real time) of internal active loop while the robot waits for new instructions in case of delays in communication. The smaller value, the faster robot restarts move after delay (but more stress is put on URControl processor). Default value is 0.001 (1000 Hz check frequency)
    • max_waiting_time - maximum time in seconds (real time) to wait for instructions from the drive before move is aborted. Defaults to 2 seconds.
    • servoj_gain and servoj_lookahead_time - useful to control precision and speed of the position moves with servoj command (see URScript documentation for detailes)
    • max_joint_difference - maximum allowed difference between target and actual joints - checked at every trajectory step Here are some examples of manipulating the time flow for Low Bandwidth Trajectory Follower mode. You can use other settings but you should do it on your own risk.
  • Default mode: servoj_time = 0.008, time_interval = 0.008 : interpolation time flows with the same speed as real time - moves are executed as planned
  • Slow-motion mode: servoj_time = 0.008, time_interval = 0.004 : interpolation time flows 2x slower than real time, so the move is executed 2x slower than planned. Requires configuring MoveIt to accept much slower moves than expected (otherwise MoveIt cancels such move mid-way)
  • Fast-forward mode: servoj_time = 0.004, time_interval = 0.012 : interpolation time flows 3x faster than real time, so the move is 3x faster than planned. Might violate limits of the robot speed so use carefully and make sure you gradually increase the speed in-between to check how safe it is.

NOTE! In case you use Low Bandwidth Trajectory Follower and you experience MoveIt to cancel robot moves prematurely because of too long move duration, you should increase tolerance of duration monitoring of MoveIt trajectory execution You can find the configuration usually in trajectory_execution.launch.xml in generated moveit config - there are parameters that configure scaling and margin for allowed execution time among others. The relevant parameters are trajectory_execution/allowed_execution_duration_scaling (default 1.2) and trajectory_execution/allowed_goal_duration_margin (default 0.5). The first one is factor that scales execution time, the second is margin that is added on top of the scaled one. You can increase either of those values to make moveit executor more "tolerant" to execution delays. There is also another parameter: trajectory_execution/execution_duration_monitoring. You can set it to false to disable duration monitoring completely.

Installation

As the driver communicates with the robot via ethernet and depends on reliable continous communication, it is not possible to reliably control a UR from a virtual machine.

Just clone the repository into your catkin working directory and make it with catkin_make.

Note that this package depends on ur_msgs, hardware_interface, and controller_manager so it cannot directly be used with ROS versions prior to hydro.

Usage

The driver is designed to be a drop-in replacement of the ur_driver package. It won't overwrite your current driver though, so you can use and test this package without risking to break your current setup.

If you want to test it in your current setup, just use the modified launch files included in this package instead of those in ur_bringup. Everything else should work as usual.

If you would like to run this package to connect to the hardware, you only need to run the following launch file.

roslaunch ur_modern_driver urXX_bringup.launch robot_ip:=ROBOT_IP_ADDRESS

Where ROBOT_IP_ADDRESS is your UR arm's IP and XX is '5' or '10' depending on your robot. The above launch file makes calls to both roscore and the launch file to the urXX_description so that ROS's parameter server has information on your robot arm. If you do not have your ur_description installed please do so via:

sudo apt install ros-<distro>-ur-description

Where is the ROS distribution your machine is running on. You may want to run MoveIt to plan and execute actions on the arm. You can do so by simply entering the following commands after launching ur_modern_driver:

roslaunch urXX_moveit_config ur5_moveit_planning_execution.launch
roslaunch urXX_moveit_config moveit_rviz.launch config:=true

---

If you would like to use the ros_control-based approach, use the launch files urXX_ros_control.launch, where XX is '5' or '10' depending on your robot.

Note: If you are using the ros_control-based approach you will need 2 packages that can be found in the ur_driver package. If you do not have the ur_driver package in your workspace simply copy these packages into your workspace /src folder:

• urXX_moveit_config
• ur_description

The driver currently supports two position trajectory controllers; a position based and a velocity based. They are both loaded via the launch file, but only one of them can be running at the same time. By default the velocity based controller is started. You can switch controller by calling the appropriate service:

rosservice call /universal_robot/controller_manager/switch_controller "start_controllers:
- 'vel_based_pos_traj_controller'
stop_controllers:
- 'pos_based_pos_traj_controller'
strictness: 1"

Be sure to stop the currently running controller either before or in the same call as you start a new one, otherwise it will fail.

The position based controller should stay closer to the commanded path, while the velocity based react faster (trajectory execution start within 50-70 ms, while it is in the 150-180ms range for the position_based. Usage without ros_control as well as the old driver is also in the 170ms range, as mentioned at my lightning talk @ ROSCon 2013).

Note that the PID values are not optimally tweaked as of this moment.

To use ros_control together with MoveIt, be sure to add the desired controller to the controllers.yaml in the urXX_moveit_config/config folder. Add the following:

controller_list:
 - name: /vel_based_pos_traj_controller #or /pos_based_pos_traj_controller
   action_ns: follow_joint_trajectory
   type: FollowJointTrajectory
   default: true
   joints:
      - shoulder_pan_joint
      - shoulder_lift_joint
      - elbow_joint
      - wrist_1_joint
      - wrist_2_joint
      - wrist_3_joint

Using the tool0_controller frame

Each robot from UR is calibrated individually, so there is a small error (in the order of millimeters) between the end-effector reported by the URDF models in https://github.com/ros-industrial/universal_robot/tree/indigo-devel/ur_description and the end-effector as reported by the controller itself.

This driver broadcasts a transformation between the base link and the end-effector as reported by the UR. The default frame names are: base and tool0_controller.

To use the tool0_controller frame in a URDF, there needs to be a link with that name connected to base. For example:

<!-- Connect tool0_controller to base using floating joint -->
<link name="tool0_controller"/>
<joint name="base-tool0_controller_floating_joint" type="floating">
  <origin xyz="0 0 0" rpy="0 0 0"/>
  <parent link="base"/>
  <child link="tool0_controller"/>
</joint>

Now, the actual transform between base and tool0_controller will not be published by the robot_state_publisher but will be taken from this driver via /tf.

NOTE: You need an up-to-date version of robot_state_publisher that is able to deal with floating joints, see: ros/robot_state_publisher#32

Compatibility

Should be compatible with all robots and control boxes with the newest firmware.

Tested with:

• Real UR10 with CB2 running 1.8.14035
• Real UR5 with CB2 running 1.8.14035
• Simulated UR3 running 3.1.18024
• Simulated UR5 running 3.0.16471
• Simulated UR5 running 1.8.16941
• Simulated UR5 running 1.7.10857
• Simulated UR5 running 1.6.08725

Credits

Please cite the following report if using this driver

@techreport{andersen2015optimizing,
  title = {Optimizing the Universal Robots ROS driver.},
  institution = {Technical University of Denmark, Department of Electrical Engineering},
  author = {Andersen, Thomas Timm},
  year = {2015},
  url = {http://orbit.dtu.dk/en/publications/optimizing-the-universal-robots-ros-driver(20dde139-7e87-4552-8658-dbf2cdaab24b).html}
  }

The report can be downloaded from http://orbit.dtu.dk/en/publications/optimizing-the-universal-robots-ros-driver(20dde139-7e87-4552-8658-dbf2cdaab24b).html