Overview

The stretch4_body repository contains the core Python software stack that allows developers to interact with the hardware of Stretch 4 robots. The repository for Stretch 3 and below can be found in the stretch_body repo. This repo provides a robust, soft real-time capable framework for managing low-level motor communication, subsystem coordination, autonomous behaviors, and a high-level API for user applications. This repository is intended to be imported by other code that needs access to these features.

This package can be installed by:

python3 -m pip install -U hello-robot-stretch4-body

Architecture Block Diagram

At its heart, the architecture is built around a Client-Server model. A dedicated RobotServer runs as a background daemon managing the physical hardware at 100Hz, executing safety monitoring, self-collision detection, and hardware command multiplexing. Developers build their applications using the RobotClient, which asynchronously communicates with the server over ZeroMQ. This decouples user scripts from strict hardware timing constraints and allows for safe, concurrent control of the robot.

graph TD
    ClientCode["User Application RobotClient"]
    Server["Robot Server 100Hz Loop"]

    Subsystems["Hardware Subsystems"]
    Arm
    Lift
    Omnibase
    PowerPeriph
    EndOfArm

    Behaviors["Behaviors"]
    Sentries["Sentries Safety Monitors"]
    SafeMotions["Safe Motions Collision Avoidance"]
    Routines["Routines Autonomous Actions"]

    Workers["Background Workers"]
    LineSensorLoop["Line Sensor Loop"]
    CollisionLoop["Self Collision Loop"]
    EOALoop["End Of Arm Loop"]

    ClientCode -->|ZeroMQ Commands and Status| Server

    Server --> Behaviors
    Behaviors --> Sentries
    Behaviors --> SafeMotions
    Behaviors --> Routines

    Server --> Subsystems
    Subsystems --> Arm
    Subsystems --> Lift
    Subsystems --> Omnibase
    Subsystems --> PowerPeriph
    Subsystems --> EndOfArm

    Server --> Workers
    Workers --> LineSensorLoop
    Workers --> CollisionLoop
    Workers --> EOALoop

Technical Primers

For an in-depth understanding of how specific parts of the system are designed, refer to the following technical primers:

Installation

1. pip3 install -e .
2. stretch_body_server --launch

Note: The C++ shared libraries for transport and SCSerial will compile automatically via Meson during the pip install.

If you want to install the object detection dependencies:

pip3 install -e .[object_detection]

Troubleshooting Editable Installs

If you make a C++ syntax error or typo in the source files and attempt to run a command while in editable mode (e.g., launching stretch_body_server), you may encounter an obscure Python exception instead of the actual C++ compiler error message:

subprocess.CalledProcessError: Command '['ninja']' returned non-zero exit status 1.

Because meson-python editable builds run quietly in the background on import, it drops the standard output of the C++ compiler natively, hiding your C++ syntax error. To see the actual compiler output and locate the line where C++ failed, prepend your command with the verbose flag:

MESONPY_EDITABLE_VERBOSE=1 stretch_body_server --launch

Custom User End-of-Arm Tools

Stretch 4 supports dynamic user-defined custom end-of-arm tools. Users can define, process, register, and switch to their own tools without modifying the core software stack.

1. Directory Structure

Custom tools should be placed in your fleet's user_tools directory:

• If environment variables HELLO_FLEET_PATH and HELLO_FLEET_ID are set: <HELLO_FLEET_PATH>/<HELLO_FLEET_ID>/user_tools/
• Otherwise (fallback): ~/stretch_user/user_tools/

Create a subdirectory named after your tool (e.g., user_eoa_mytool):

> user_eoa_mytool
    > meshes
        my_tool_mesh.stl      # Visual/Collision mesh files
    user_eoa_mytool.urdf      # Tool URDF file describing joints & links
    user_eoa_mytool.py        # Optional custom Python driver class

If your tool has custom driver code, the main Python file must match the tool directory name (e.g., user_eoa_mytool.py) or be named tool.py, and contain a class matching the tool name in PascalCase (e.g., class UserEoaMytool).

2. Mesh Preprocessing and Registration

Once your files are in place, process the tool using the automatic registration utility. This script simplifies visual meshes, generates collision meshes, and appends the default baseline configuration (including serial devices, joint exclusion, and collision management) to stretch_user_params.yaml:

stretch_configure_tool --add_user_tool

The tool will prompt you to select your custom tool subdirectory, process its URDF/meshes, and generate the parameters.

3. Switching to Your Tool

To switch your robot to use the custom tool:

stretch_configure_tool --quick --tool user_eoa_mytool

This updates stretch_user_params.yaml to make user_eoa_mytool the active tool. The RobotClient, stretch_status, and stretch_system_check utilities will automatically recognize, load, and poll your custom tool.