SO-101

Affordable robotic arm with Feetech STS3215 servos

The SO-101 is an affordable 6-DOF robotic arm that uses Feetech STS3215 servo motors. It’s part of the SO follower series and is well-suited for teleoperation and imitation learning tasks.

Overview

The SO-101 features:

• 6 degrees of freedom (5 arm joints + gripper)
• Feetech STS3215 servo motors
• USB serial communication
• Position control mode
• Optional camera integration
• Affordable hardware components

Hardware Specifications

Motors

The SO-101 uses the following motor configuration:

Motor Name	ID	Model	Normalization Mode
shoulder_pan	1	STS3215	Degrees or Range[-100,100]
shoulder_lift	2	STS3215	Degrees or Range[-100,100]
elbow_flex	3	STS3215	Degrees or Range[-100,100]
wrist_flex	4	STS3215	Degrees or Range[-100,100]
wrist_roll	5	STS3215	Degrees or Range[-100,100]
gripper	6	STS3215	Range[0,100]

By default, SO-101 uses degree normalization (use_degrees=True) for backward compatibility with existing datasets and policies.

Installation

Hardware Setup

1. Connect the SO-101 arm to your computer via USB
2. Identify the serial port (usually /dev/ttyUSB0 on Linux or /dev/tty.usbserial-* on macOS)
3. Ensure you have proper permissions to access the serial port:

# On Linux, add your user to the dialout group
sudo usermod -a -G dialout $USER
# Log out and back in for changes to take effect

Software Requirements

The SO-101 robot is included in the LeRobot installation:

pip install lerobot

Configuration

Basic Configuration

Create a configuration for your SO-101:

from lerobot.robots.so_follower import SOFollowerRobotConfig
from lerobot.cameras.opencv import OpenCVCameraConfig

config = SOFollowerRobotConfig(
    robot_type="so101_follower",
    id="so101_main",
    port="/dev/ttyUSB0",
    cameras={
        "top": OpenCVCameraConfig(
            index_or_path=0,
            fps=30,
            width=640,
            height=480
        )
    }
)

Configuration Parameters

port · str · required

Serial port to connect to the arm (e.g., /dev/ttyUSB0)

id · str

Robot identifier for calibration file management

disable_torque_on_disconnect · bool · default: "true"

Whether to disable motor torque when disconnecting

max_relative_target · float | dict[str, float]

Limit the magnitude of relative position changes for safety. Can be a single value for all motors or a dictionary mapping motor names to individual limits.

cameras · dict[str, CameraConfig]

Dictionary of camera configurations

use_degrees · bool · default: "true"

Use degree normalization instead of range[-100, 100]. Set to True for backward compatibility with existing datasets.

calibration_dir · Path

Directory to store calibration files (defaults to ~/.cache/lerobot/calibration/robots/so_follower)

Usage

Basic Connection

from lerobot.robots.so_follower import SOFollower
from lerobot.robots.so_follower import SOFollowerRobotConfig

# Create configuration
config = SOFollowerRobotConfig(
    robot_type="so101_follower",
    id="so101_main",
    port="/dev/ttyUSB0"
)

# Connect to robot (context manager auto-disconnects)
with SOFollower(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": -45.0,
        "elbow_flex.pos": 90.0,
        "wrist_flex.pos": 0.0,
        "wrist_roll.pos": 0.0,
        "gripper.pos": 50.0
    }
    robot.send_action(action)

Calibration

The SO-101 requires calibration before first use:

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

# Follow the prompts:
# 1. Move the arm to the middle of its range of motion
# 2. Press ENTER
# 3. Move each joint through its full range
# 4. Press ENTER when done

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

Calibration data is saved per robot ID. If you have multiple SO-101 arms, use different IDs for each.

Observation Format

The get_observation() method returns a dictionary:

{
    # Motor positions (in degrees or normalized range)
    "shoulder_pan.pos": 0.0,
    "shoulder_lift.pos": -45.0,
    "elbow_flex.pos": 90.0,
    "wrist_flex.pos": 0.0,
    "wrist_roll.pos": 0.0,
    "gripper.pos": 50.0,
    
    # Camera images (if configured)
    "top": np.ndarray  # shape: (height, width, 3)
}

Action Format

Actions follow the same structure as observations:

action = {
    "shoulder_pan.pos": 10.0,   # degrees (if use_degrees=True)
    "shoulder_lift.pos": -30.0,
    "elbow_flex.pos": 60.0,
    "wrist_flex.pos": -15.0,
    "wrist_roll.pos": 0.0,
    "gripper.pos": 75.0  # 0=open, 100=closed
}

robot.send_action(action)

Safety Features

Maximum Relative Target

Limit how much motors can move in a single action:

config = SOFollowerRobotConfig(
    port="/dev/ttyUSB0",
    # Limit all motors to 10 degrees per action
    max_relative_target=10.0
)

# Or set individual limits per motor
config = SOFollowerRobotConfig(
    port="/dev/ttyUSB0",
    max_relative_target={
        "shoulder_pan": 15.0,
        "shoulder_lift": 10.0,
        "elbow_flex": 20.0,
        "wrist_flex": 15.0,
        "wrist_roll": 30.0,
        "gripper": 25.0
    }
)

PID Tuning

The SO-101 configures motors with conservative PID values to prevent shakiness:

• P Coefficient: 16 (default is 32)
• I Coefficient: 0
• D Coefficient: 32

For the gripper specifically:

• Max Torque Limit: 500 (50% to prevent burnout)
• Protection Current: 250 (50% to prevent burnout)
• Overload Torque: 25 (25% when overloaded)

These are set automatically in robot.configure() at /home/daytona/workspace/source/src/lerobot/robots/so_follower/so_follower.py:155.

Motor Setup

If you need to assign motor IDs (for a new arm or replacement motors):

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

# Follow the prompts to connect each motor individually
# Motors will be assigned IDs 1-6 in the order:
# gripper (6) -> wrist_roll (5) -> wrist_flex (4) -> 
# elbow_flex (3) -> shoulder_lift (2) -> shoulder_pan (1)

Teleoperation

The SO-101 is commonly used in leader-follower setups:

from lerobot.robots.so_follower import SOFollower, SOFollowerRobotConfig

# Leader arm (records human input)
leader_config = SOFollowerRobotConfig(
    robot_type="so101_follower",
    id="leader",
    port="/dev/ttyUSB0"
)

# Follower arm (executes actions)
follower_config = SOFollowerRobotConfig(
    robot_type="so101_follower",
    id="follower",
    port="/dev/ttyUSB1",
    max_relative_target=10.0  # Safety limit
)

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

Troubleshooting

Port Permission Denied

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

Robot Not Responding

1. Check cable connections
2. Verify the correct port is specified
3. Try reconnecting the USB cable
4. Check motor power supply

Calibration Issues

# Force recalibration
robot.connect(calibrate=False)
robot.calibrate()  # Will prompt to run new calibration

Motor Shaking

If motors are shaking or oscillating, the PID values may need adjustment. See the source code at /home/daytona/workspace/source/src/lerobot/robots/so_follower/so_follower.py:155 for the configuration method.

Related Documentation

SO-100

Compact version of the SO series

Recording Data

Record demonstrations with SO-101

Teleoperation

Set up leader-follower control

Feetech Motors

Motor bus API reference