ur_onrobot

ROS 2 driver and URDF/xacro description for Universal Robots with OnRobot grippers

This package communicates directly with the OnRobot URCap installed on the UR controller via XML-RPC (port 41414). Because it talks to the URCap independently of the UR robot program, the gripper can be commanded while the robot is in motion — enabling use cases like throwing and catching objects, or adjusting grip during trajectories.

On startup, the driver automatically scans the XML-RPC interface to discover which gripper is connected and what methods are available. This means it works with any OnRobot gripper that exposes the standard URCap API, even models not explicitly tested here.

Supported Grippers

Family	Models	Tested	Notes
FG (Finger Gripper)	2FG7, 2FG14	2FG14 ✓	Width + force + speed control
RG (Robot Gripper)	RG2, RG6	RG6 ✓	Width + force control, safety buttons
VG (Vacuum Gripper)	VG10, VGC10, VGP30	—	Vacuum A/B channel control

The driver should work with any OnRobot gripper connected via the URCap Compute Box or Tool I/O, as it discovers available methods dynamically via system.listMethods.

Features

• UR-Independent Communication: Talks directly to URCap XML-RPC, not through the UR program — gripper moves while robot moves
• Auto-Discovery: Scans XML-RPC API to detect gripper family (FG/RG/VG), model, and capabilities
• Action Server: Standard control_msgs/action/GripperCommand interface
• Preset Services: Quick open/close via std_srvs/Trigger and std_srvs/SetBool
• Status Publishing: Continuous width, busy, object detection, and status topics
• URDF Description: Complete robot + gripper visualization for RViz
• Simulation Mode: Fake hardware for testing without physical robot
• CLI Tool: Standalone command-line introspection and control (no ROS needed)

Installation & Build

Dependencies

sudo apt-get install ros-humble-ur-description
pip install pycurl

Build

cd ~/ros2_ws
colcon build --packages-select ur_onrobot
source install/setup.bash

Usage

Launch Gripper Driver

ros2 launch ur_onrobot ur_onrobot.launch.py

Important: Edit launch/ur_onrobot.launch.py and set the ip parameter to your robot's IP address.

Recommended: Launch via Robot Config YAML

In production, the driver is typically launched as part of a larger robot stack with parameters from a YAML config file:

Node(
    package="ur_onrobot",
    executable="onrobot-ros2",
    name="gripper",
    output="screen",
    parameters=[robot_config],  # YAML file with ip, speeds, presets, etc.
)

Example YAML (robot_config.yaml):

gripper:
  ros__parameters:
    ip: "192.168.178.5"
    status_hz: 5.0
    fg.default_speed: 50
    action.timeout_s: 5.0
    action.feedback_hz: 10.0
    preset.open_width: 55.0
    preset.close_width: 5.0
    preset.default_force: 30.0

Control Gripper via Action

The action server is at ~/gripper_command (under the node's namespace):

# Open gripper (FG/RG: position=width in mm, max_effort=force)
ros2 action send_goal /gripper_command \
  control_msgs/action/GripperCommand \
  "{command: {position: 55.0, max_effort: 30.0}}"

Example output:

Waiting for an action server to become available...
Sending goal:
     command:
  position: 55.0
  max_effort: 30.0

Goal accepted with ID: 2633e5c9f62448af8d7f2bf32bb36679

Result:
    position: 54.60000228881836
  effort: 30.0
  stalled: false
  reached_goal: true

# Close gripper
ros2 action send_goal /gripper_command \
  control_msgs/action/GripperCommand \
  "{command: {position: 0.0, max_effort: 30.0}}"

If using a namespace, prefix accordingly (e.g., /rondor/gripper/gripper_command).

Preset Services (FG/RG only)

# Toggle between open and close
ros2 service call /gripper/preset/toggle std_srvs/srv/Trigger

Example — toggling twice:

requester: making request: std_srvs.srv.Trigger_Request()
response:
std_srvs.srv.Trigger_Response(success=True, message='Preset: open (width=55.0mm)')

requester: making request: std_srvs.srv.Trigger_Request()
response:
std_srvs.srv.Trigger_Response(success=True, message='Preset: close (width=5.0mm)')

# Explicit open (True) or close (False)
ros2 service call /gripper/preset/set_state std_srvs/srv/SetBool "{data: true}"

CLI Tool (No ROS Required)

Introspect the XML-RPC API and control the gripper directly:

# Dump all available methods to JSON and Markdown
onrobot-cli 192.168.178.5

# Move gripper (FG/RG)
onrobot-cli 192.168.178.5 --move --width 30 --force 20 --speed 50

ROS 2 Interface

Published Topics

Topic	Type	Description
~/status	std_msgs/String	JSON snapshot (family, width/vacuum, busy, object, status)
~/busy	std_msgs/Bool	Gripper is currently moving
~/object_detected	std_msgs/Bool	Grip/object detection
~/status_code	std_msgs/Int32	Gripper status code
~/width	std_msgs/Float32	Current width in mm (FG/RG only)
~/vacuum	std_msgs/String	Vacuum state as JSON (VG only)

Action Server

Action	Type	Description
~/gripper_command	control_msgs/action/GripperCommand	FG/RG: position=width_mm, max_effort=force. VG: position=vacuum_percent

Services (FG/RG only)

Service	Type	Description
~/preset/toggle	std_srvs/Trigger	Toggle between open/close presets
~/preset/set_state	std_srvs/SetBool	True=open, False=close

Parameters

Parameter	Type	Default	Description
ip	string	required	Robot/gripper IP address
gid	int	-1 (auto)	Gripper ID (auto-scans 0-3)
status_hz	double	5.0	Status publishing rate
action.timeout_s	double	5.0	Action timeout
action.feedback_hz	double	10.0	Action feedback rate
fg.default_speed	int	50	FG gripper speed (1-100)
vg.apply_both	bool	true	Apply vacuum to both channels
vg.threshold_gripped	double	20.0	VG grip detection threshold
preset.open_width	double	60.0	Open preset width (mm)
preset.close_width	double	0.0	Close preset width (mm)
preset.default_force	double	30.0	Preset force

Executables

Entry Point	Description
onrobot-ros2	Main ROS 2 driver node
onrobot-cli	Standalone CLI tool (no ROS)
gripper-joint-state-publisher	Converts width to joint states for URDF visualization
gripper-simulator	Simulates gripper for testing without hardware

URDF/Xacro

Use in Your Launch File

from launch.substitutions import Command, FindExecutable, PathJoinSubstitution
from launch_ros.substitutions import FindPackageShare

robot_description_content = Command([
    PathJoinSubstitution([FindExecutable(name="xacro")]),
    " ",
    PathJoinSubstitution([
        FindPackageShare("ur_onrobot"),
        "urdf", "ur_with_onrobot.xacro"
    ]),
    " robot_ip:=192.168.X.X",
    " ur_type:=ur10e",
    " tf_prefix:=robot_",
])

Xacro Arguments

Argument	Default	Description
robot_ip	192.168.1.1	Robot IP address
ur_type	ur10e	UR model (ur3e, ur5e, ur10e, ...)
tf_prefix	``	TF prefix for all links
gripper_prefix	gripper_	Prefix for gripper links
use_fake_hardware	false	Enable simulation mode

How It Works

1. The OnRobot URCap must be installed on the UR controller (via the Teach Pendant)
2. The URCap exposes an XML-RPC server on port 41414 of the robot's IP
3. This driver connects to that XML-RPC endpoint and calls system.listMethods to discover all available gripper methods
4. Based on the available methods, it auto-detects the gripper family (FG, RG, or VG) and configures itself accordingly
5. Commands are sent via XML-RPC independently of the UR robot program, meaning the gripper can be controlled in parallel with robot motion

Package Contents

• ur_onrobot/ — Python package
  • UR_onrobot.py — Low-level XML-RPC helper with auto-discovery
  • Onrobot_UR_ROS2_driver.py — ROS 2 node with action server
  • gripper_joint_state_publisher.py — Width → joint state converter
  • gripper_simulator.py — Fake gripper for simulation
• urdf/ — Robot + gripper URDF/xacro descriptions
  • ur_with_onrobot.xacro — Robot-specific version (requires binary patches, recommended)
  • ur_with_onrobot_generic.xacro — Generic version (works with stock packages)
  • onrobot_2fg14.xacro — Gripper macro
• srdf/ — Semantic robot description (collision exclusions)
  • ur_with_onrobot.srdf.xacro — Robot-specific SRDF (ur5e_manipulator, ur10e_manipulator)
  • ur_with_onrobot_generic.srdf.xacro — Generic SRDF (ur_manipulator)
• launch/ — Launch files
• meshes/ — STL files for visualization
• config/ — Robot-specific controller configurations (ur5e, ur10e)

URDF/SRDF Variants Explained

Robot-Specific (Recommended):

• Uses proper robot names (ur5e, ur10e) matching the actual hardware
• Creates correct MoveIt planning groups (ur5e_manipulator, ur10e_manipulator)
• Eliminates "semantic description mismatch" warnings
• Requires: Binary patches to /opt/ros/humble/share/ur_moveit_config/
• Apply patches: Run the included apply_binary_patches.sh script

Generic (Stock Compatible):

• Always uses name="ur" regardless of robot type
• Planning group is always "ur_manipulator"
• Works without system modifications
• Use when: Shared systems, CI/CD, or cannot modify /opt files

Tested Hardware

• Universal Robots UR5e, UR10e
• OnRobot 2FG14 gripper
• OnRobot RG6 gripper (driver works, STL meshes not yet included)

TODO

• Add RG6 gripper visualization meshes from UOsaka-Harada-Laboratory/onrobot (MIT License)
• Create onrobot_rg6.xacro description file

Credits

• STL Meshes: The 2FG gripper visualization meshes were adapted from juandpenan/onrobot_2FG7_gripper_description (MIT License), originally generated from OnRobot official CAD files using the fusion2urdf tool
• Driver Development: Neuro Information Technology Lab, Otto von Guericke University Magdeburg

License

MIT

Maintainer

Dominykas Strazdas
DoStraTech

Used at the Neuro Information Technology (NIT) Lab, Otto von Guericke University Magdeburg.