OMX

Open Manipulator-X modular robotic arm by ROBOTIS

The OMX (Open Manipulator-X) is a modular 6-DOF robotic arm developed by ROBOTIS. It uses Dynamixel servo motors and is designed as an educational and research platform with a focus on accessibility and expandability.

Overview

The OMX is designed for:

• Education and research
• Manipulation tasks
• Teleoperation and imitation learning
• Modular hardware expansion
• Open-source development

Features

• 6 degrees of freedom (5 arm joints + gripper)
• Dynamixel XL430-W250 and XL330-M288 servos
• Pre-calibrated from factory
• USB serial communication (U2D2 adapter)
• Extended position control
• Current-based gripper control
• Open-source design by ROBOTIS

Hardware Specifications

Motor Configuration

Motor Name	ID	Model	Description
shoulder_pan	11	XL430-W250	Base rotation
shoulder_lift	12	XL430-W250	Shoulder pitch
elbow_flex	13	XL430-W250	Elbow joint
wrist_flex	14	XL330-M288	Wrist pitch
wrist_roll	15	XL330-M288	Wrist rotation
gripper	16	XL330-M288	Gripper (current-controlled)

Motor IDs start at 11 (not 1) as per ROBOTIS Open Manipulator-X standard configuration.

Control Modes

• Arm joints: Extended Position Mode (allows >360° rotation)
• Gripper: Current-Based Position Mode (force-limited grasping)

Installation

Hardware Setup

1. Connect Dynamixel U2D2 adapter to OMX
2. Connect U2D2 to computer via USB
3. Power the servos (typically 12V)
4. Set up serial port permissions:

# Linux: Add user to dialout group
sudo usermod -a -G dialout $USER
# Log out and back in

Software Installation

pip install lerobot

Configuration

Basic Configuration

from lerobot.robots.omx_follower import OmxFollower, OmxFollowerConfig
from lerobot.cameras.opencv import OpenCVCameraConfig

config = OmxFollowerConfig(
    robot_type="omx_follower",
    id="omx_main",
    port="/dev/ttyUSB0",  # U2D2 port
    cameras={
        "wrist": OpenCVCameraConfig(
            index_or_path=0,
            fps=30,
            width=640,
            height=480
        )
    }
)

Configuration Parameters

port · str · required

Serial port for Dynamixel U2D2 adapter (e.g., /dev/ttyUSB0 or /dev/tty.usbserial-*)

id · str

Unique identifier for this robot (used for calibration files)

disable_torque_on_disconnect · bool · default: "true"

Disable motor torque when disconnecting

max_relative_target · float | dict[str, float]

Safety limit for maximum position change per action. Can be a single value or per-motor dictionary.

cameras · dict[str, CameraConfig]

Dictionary of camera configurations

use_degrees · bool · default: "false"

Use degree normalization (default is normalized range [-100, 100])

calibration_dir · Path

Directory for calibration files (defaults to ~/.cache/lerobot/calibration/robots/omx_follower)

Usage

Basic Connection

from lerobot.robots.omx_follower import OmxFollower, OmxFollowerConfig

config = OmxFollowerConfig(
    robot_type="omx_follower",
    id="omx_main",
    port="/dev/ttyUSB0"
)

with OmxFollower(config) as robot:
    # Get current state
    obs = robot.get_observation()
    print(f"Current positions: {obs}")
    
    # Send action
    action = {
        "shoulder_pan.pos": 0.0,
        "shoulder_lift.pos": 30.0,
        "elbow_flex.pos": -45.0,
        "wrist_flex.pos": 20.0,
        "wrist_roll.pos": 0.0,
        "gripper.pos": 50.0
    }
    robot.send_action(action)

Factory Calibration

The OMX comes pre-calibrated from ROBOTIS. LeRobot uses factory default values:

robot = OmxFollower(config)
robot.connect(calibrate=True)

# Automatic factory calibration applied:
# - homing_offset: 0
# - range_min: 0
# - range_max: 4095
# - drive_mode: NON_INVERTED

# Calibration saved to:
# ~/.cache/lerobot/calibration/robots/omx_follower/{id}.json

Unlike Koch or SO robots, the OMX doesn’t require manual calibration. Factory defaults work out of the box.

Observation Format

obs = robot.get_observation()
# Returns:
{
    # Joint positions (normalized [-100, 100] by default)
    "shoulder_pan.pos": 0.0,
    "shoulder_lift.pos": 30.0,
    "elbow_flex.pos": -45.0,
    "wrist_flex.pos": 20.0,
    "wrist_roll.pos": 0.0,
    "gripper.pos": 50.0,  # 0=open, 100=closed
    
    # Camera images (if configured)
    "wrist": np.ndarray  # shape: (height, width, 3)
}

Action Format

action = {
    "shoulder_pan.pos": 10.0,
    "shoulder_lift.pos": 25.0,
    "elbow_flex.pos": -50.0,
    "wrist_flex.pos": 15.0,
    "wrist_roll.pos": 0.0,
    "gripper.pos": 75.0
}

robot.send_action(action)

Advanced Features

Operating Modes

Extended Position Mode (all joints except gripper):

• Allows unlimited rotation beyond 360°
• Prevents assembly issues from limiting range
• Automatically configured in configure()

Current-Based Position Mode (gripper only):

• Position control with current limiting
• Safe grasping without excessive force
• Prevents motor burnout
• Compliant for leader arm teleoperation

Safety Limiting

config = OmxFollowerConfig(
    port="/dev/ttyUSB0",
    # Limit all joints to 15 units per action
    max_relative_target=15.0
)

# Or set per-motor limits
config = OmxFollowerConfig(
    port="/dev/ttyUSB0",
    max_relative_target={
        "shoulder_pan": 20.0,
        "shoulder_lift": 15.0,
        "elbow_flex": 25.0,
        "wrist_flex": 20.0,
        "wrist_roll": 30.0,
        "gripper": 30.0
    }
)

Teleoperation

Leader-Follower Setup

from lerobot.robots.omx_follower import OmxFollower, OmxFollowerConfig

# Leader arm (human controls)
leader_config = OmxFollowerConfig(
    robot_type="omx_follower",
    id="omx_leader",
    port="/dev/ttyUSB0"
)

# Follower arm (mimics leader)
follower_config = OmxFollowerConfig(
    robot_type="omx_follower",
    id="omx_follower",
    port="/dev/ttyUSB1",
    max_relative_target=15.0  # Safety limit
)

with OmxFollower(leader_config) as leader, \
     OmxFollower(follower_config) as follower:
    
    while True:
        # Read leader position
        obs = leader.get_observation()
        
        # Extract joint positions
        action = {k: v for k, v in obs.items() if k.endswith(".pos")}
        
        # Send to follower
        follower.send_action(action)

Motor Setup

If you need to configure motor IDs (for custom builds or replacements):

robot = OmxFollower(config)
robot.connect(calibrate=False)
robot.setup_motors()

# Follow prompts to connect motors one at a time:
# 1. Connect only gripper motor (ID 16)
# 2. Connect only wrist_roll motor (ID 15)
# 3. Continue in reverse order...
# Motors configured in reverse order from 16 to 11

For standard ROBOTIS OMX, motor IDs are pre-configured and this step isn’t necessary.

Differences from Koch

The OMX and Koch robots are similar but have key differences:

Feature	OMX	Koch
Motor IDs	11-16	1-6
Calibration	Factory defaults	Manual calibration
Source	ROBOTIS	Tau Robotics
Default normalization	Range[-100,100]	Range[-100,100]
Motor models	XL430-W250, XL330-M288	Same

Both use the same software structure and Dynamixel protocol.

Troubleshooting

U2D2 Not Detected

# Check if device is connected
ls /dev/ttyUSB*

# Set permissions
sudo chmod 666 /dev/ttyUSB0

# Or add to dialout group permanently
sudo usermod -a -G dialout $USER

Motors Not Responding

1. Check power supply (~12V)
2. Verify cable connections
3. Ensure motor IDs are 11-16 (not 1-6)
4. Try power cycling

Gripper Issues

The gripper uses current-based control:

# If gripper is too weak or strong, check the configure() method
# See: /home/daytona/workspace/source/src/lerobot/robots/omx_follower/omx_follower.py:137
# Current limits are set in Operating Mode configuration

Wrong Motor IDs

If motors have IDs 1-6 instead of 11-16:

# Option 1: Use setup_motors() to reassign IDs
robot.setup_motors()

# Option 2: Manually set IDs using Dynamixel Wizard
# (ROBOTIS software tool)

Implementation Details

• Motor bus: DynamixelMotorsBus
• Protocol: Dynamixel Protocol 2.0
• Normalization: MotorNormMode.RANGE_M100_100 (default) or DEGREES
• Factory calibration: All offsets = 0, range = [0, 4095]
• Source: /home/daytona/workspace/source/src/lerobot/robots/omx_follower/omx_follower.py
• Config: /home/daytona/workspace/source/src/lerobot/robots/omx_follower/config_omx_follower.py

Related Documentation

Robot Overview

See all supported robots

Koch

Similar Dynamixel-based arm

Dynamixel Motors

Motor bus API reference

Recording Data

Collect training data

External Resources

• ROBOTIS Open Manipulator-X
• ROBOTIS e-Manual
• Open Manipulator GitHub
• Dynamixel Documentation