Mobile Manipulator Shared Code

This repository contains shared code for running experiments with the "Thing" mobile manipulator (shown below). It should work on Ubuntu 18.04, but 20.04 is preferred.

System Information

Hardware

The robot consists of a UR10 manipulator mounted on a Ridgeback omnidirectional mobile base. The base has a Hokuyo UST-10LX laser range finder mounted at the front that provides a two-dimensional scan in a 270 degree arc in front of the robot. The end effector has a Robotiq FT 300 force torque sensor mounted at the wrist to measure the applied wrench, as well as Robotiq 3 finger gripper for manipulation.

Software

The Ridgeback is currently running Ubuntu 20.04 on its onboard computer. The UR10 is running firmware version 3.15.

Documentation

Manuals and datasheets, as well as other documents, can be found in the docs directory.

Installation and Setup

Ensure ROS is installed.

Install Eigen: sudo apt install libeigen3-dev

Clone this repository into the catkin workspace:

cd catkin_ws/src
git clone https://github.com/utiasDSL/mobile_manipulation_central mobile_manipulation_central

Clone the description of the UR10 robot arm into the catkin workspace:

git clone -b melodic-devel https://github.com/ros-industrial/universal_robot.git universal_robot

For kinematics, Pinocchio is required. I prefer to build this outside of the catkin workspace. First, install the dependencies

sudo apt install ros-noetic-eigenpy ros-noetic-hpp-fcl

Then follow the installation directions here (under the "Build from Source" tab), using the cmake command:

cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr/local -DPYTHON_EXECUTABLE=/usr/bin/python3 -DBUILD_WITH_COLLISION_SUPPORT=ON

Ensure that you also modify $PYTHONPATH to include the location of Pinocchio's Python bindings.

Finally, install Python dependencies:

python3 -m pip install -r requirements.txt

Real Hardware Setup

For working with real hardware, install the following packages into the catkin workspace:

• ur_robot_driver - for the UR10 arm.
• robotiq - for the Robotiq 3F gripper. This is a fork of the original (now unmaintained) repo.
• vicon_bridge - required to track the position of the mobile base. May also be useful to track other objects, calibrate the EE pose, etc.

Build the workspace:

catkin build

The ROS master node runs onboard the Ridgeback computer and is started automatically when the Ridgeback is turned on. You need to tell your laptop where to reach the ROS master. First, add to /etc/hosts:

192.168.131.1 cpr-tor11-01

Then, in each terminal where you want to connect to the robot over ROS, run

export ROS_IP=192.168.131.100
export ROS_MASTER_URI=http://cpr-tor11-01:11311

To revert back to default settings (so you can run ROS locally, for example), do:

export ROS_MASTER_URI=http://localhost:11311
unset ROS_IP
unset ROS_HOSTNAME

It is convenient to put the above functions in a script that can be easily sourced.

Connect to the robot via ethernet and set up a new Wired Connection named Thing. In the IPv4 Settings tab, switch to method Manual and enter an address of 192.168.131.100 with netmask 255.255.255.0. Leave the gateway blank. Once done, you should be able to ping the robot at 192.168.131.1.

Usage

URDF files of the robots are used for kinematics and simulation. Compile the xacro files to produce the URDFs:

cd mobile_manipulation_central/urdf
./compile_xacro.sh

One of the main goals of this repo is to facilitate easy development over ROS. We provide ROS interfaces for the base, arm, and combined mobile manipulator system (src/mobile_manipulation_central/ros_interface.py) which provide a standard API to communicate with the robot over ROS. Theses interfaces can be used seamlessly with real hardware or a simulated version of the robot (src/mobile_manipulation_central/simulation_ros_interface.py).

Simulation

This repo provides a basic simulation environment based on PyBullet, a demo of which can be found in scripts/simulation/pyb_simulation.py.

Kinematics

Kinematics based on Pinocchio is provided in both C++ and Python. An example of the kinematics in C++ can be found in src/kinematics_example.cpp

Real Hardware

Some scripts expect the environment variable MOBILE_MANIPULATION_CENTRAL_BAG_DIR to point to the directory where bags are stored (to create or read bag files). Export this variable in your .bashrc.

Vicon is used to track the base position as well as any other objects in the scene. You must be connected to the DSL_DroneNet_5G network.

Start the Vicon bridge, UR10 driver, and gripper driver:

roslaunch mobile_manipulation_central thing.launch

To stream commands to the UR10, you must start the onboard program.

Interaction with the robot is primarily done using the feedback topics (of type sensor_msgs/JointState):

/ridgeback/joint_states
/ur10/joint_states

and the velocity command topics

/ridgeback/cmd_vel  # geometry_msgs/Twist
/ur10/cmd_vel       # std_msgs/Float64MultiArray

For Python nodes this is abstracted away using the interfaces in src/mobile_manipulation_central/ros_interface.py.

Scripts

There are some convenient scripts in the scripts directory:

• control/home.py sends the robot to a particular home configuration, which is taken from home.yaml.
• control/gripper.py opens and closes the gripper.
• control/sine_trajectory tracks a sinusoidal trajectory with a single joint.

Tests

Unit tests can be found in the test directory. Currently there are only Python unit tests, which can be run using pytest.

Robot specifications

Ridgeback limits

With reference to this file, the limits are:

Joint	Velocity	Acceleration
x	1.1 m/s	2.5 m/s²
y	1.1 m/s	2.5 m/s²
yaw	2.0 rad/s	1.0 rad/s²

UR10 limits

With reference to the UR10 datasheet as well as the onboard UR10 configuration files (for acceleration), the joint limits are:

Joint	Position (rad)	Velocity (rad/s)	Acceleration (rad/s²)
1	± 2π	± ⅔π	± 40
2	± 2π	± ⅔π	± 40
3	± 2π	± π	± 40
4	± 2π	± π	± 40
5	± 2π	± π	± 40
6	± 2π	± π	± 40

IP addresses

• Ridgeback: 192.168.131.1
• UR10: 192.168.131.40
• Gripper: 192.168.131.18
• F/T sensor: 192.168.131.14

Ridgeback lights

The lights on the corners of the Ridgeback mean different things depending on the color. The main ones are:

• Green: plugged in (not necessarily charging)
• Flashing red: e-stop
• Front white, back red: ready to drive
• Flashing yellow: battery level is below 24V, charge soon

Misc notes

• The Ridgeback controller automatically publishes a zero command (i.e. brake) to the Ridgeback when a new connection is made; i.e., when something new subscribes or publishes to the cmd_vel topic. This appears to be due to this line.
• Start or stop a particular ros_control controller using:

  rosrun controller_manager controller_manager start <controller_name>
  rosrun controller_manager controller_manager stop <controller_name>
  

• The PS4 controller for the Ridgeback can be manually turned off by holding the PS button for about 15 seconds (until the blue light turns off).

Gripper control

• The gripper typically uses real-time ethernet (Modbus TCP) for communication. If this fails for some reason, it is also possible to communicate using Modbus RTU over a USB connection. This requires the Robotiq User Interface GUI, which only runs in Windows.

Check Ridgeback battery voltage

Ensure you have diagnostics_msgs installed (sudo apt install ros-noetic-diagnostic-msgs). Then:

rosrun mobile_manipulation_central battery_voltage.py

Battery voltage should be between 22V (very low charge) and 27.6V (recently charged) (see here). If the voltage is approaching 22V, stop experiments and plug in the robot.

Ridgeback Packages

• High-level functions: ridgeback
• Low-level functions: ridgeback_robot
• Motor driver: puma_motor_driver

Known Issues

• It appears that the on-board battery is wearing down. The battery voltage on start-up (after charging) is now only about 25V, instead of the nominal full charge of 27.6V. It is worth checking this regularly to assess the battery health over time. The battery has been replaced; a full charge is now approximately 25.6V.
• Occasionally when starting the robot the connection to the UR10 cannot be made. The ur_robot_driver node (launched as part of thing.launch) will complain with something along the lines of

  could not connect to robot at address 192.168.131.40
  

  Thus far the only known solution is to restart the UR10 until the problem goes away (i.e. the connection is established properly). There is no need to restart the Ridgeback. It appears that this is a startup issue; the connection is generally very stable once established. The ethernet cable between the base and arm computers does not seem to be at fault: it was replaced and the issue was seen again.
• Related to the above, it is possible that the connection from the laptop to the Ridgeback will also not be available, despite the base (and possibly the arm) appearing to be powered on normally. Restarting the base eventually resolves the problem. It is possible that the ethernet cable between the base's network switch and the arm's computer is wearing out and will need to be replaced.
• Occasionally after starting the arm, one may get protective stops after every small movement of the arm, due to base deviation from desired path. So far, it appears that restarting the arm resolved the problem.

Todo

• Add Kalman filter for Ridgeback base and UR10 joints