Core concept
At the heart of this project is a modular actuator design that combines precision torque control with affordability. Each actuator module weighs only 150g yet delivers 2.5Nm of torque at 12A, with a compact 160mm segment length. This power-to-weight ratio enables dynamic movements like jumping, running, and precise manipulation tasks.All hardware designs, CAD files, and electronics schematics are released under the BSD 3-Clause License, allowing you to build, modify, and share your own robots.
Key features
Dual-stage timing belt transmission
The actuator modules use a 9:1 dual-stage timing belt transmission (3:1 reduction per stage) with:- Conti Synchroflex AT3 GEN III timing belts (3mm tooth pitch)
- First stage: 150mm belt, 50 teeth, 4mm width
- Second stage: 201mm belt, 67 teeth, 6mm width
- Heavy-duty polyurethane construction with steel cord reinforcements
High-resolution sensing
Each actuator includes a Broadcom AEDT-9810-Z00 optical encoder providing:- 5000 pulses per revolution per channel
- 20,000 counts per revolution at the microcontroller
- 5V two-channel quadrature output with index pulse
- 26mm codewheel diameter
- Only 5g weight
3D printed components
Most structural components are 3D printed using SLA, Polyjet, or Multijet printers for:- Rapid prototyping and customization
- Precise tooth profiles for timing belt pulleys
- Lightweight construction (core 3D printed parts: 11.3g)
- Easy replication without expensive tooling
The tooth profile of pulleys must be precise and concentric. Stereolithography or multi-jet printers are strongly recommended over FDM printing for optimal performance.
Brushless motor design
The T-Motor Antigravity 4004 300kV motor provides:- 24 magnets / 12 pole pairs / 18 slots
- 3-phase WYE configuration
- 45mm rotor diameter
- 53g weight
- Designed for RC aircraft, adapted for robotic applications
Open-source electronics
Custom electronics enable precise torque control: Micro Driver Board v2- Dual brushless motor driver
- Performs torque control at 10kHz per motor
- Miniaturized design: 51mm x 50mm, 13g
- 6-layer FR4 PCB
- Input voltage: 5V-32V (robots operate at 24V)
- SPI communication at 1kHz
- Central controller for multi-actuator systems
- Coordinates up to 12 motors
- Ethernet communication with host computer
- Onboard IMU interface
- 19g weight
Robot platforms
The project documentation includes complete designs for multiple robot platforms:Solo12 - Quadruped 12DOF
12 degrees of freedom, 2.5kg total weight, 45cm x 30cm x 6cm dimensions. Features 4 identical 3DOF legs with Hip AA, Hip FE, and Knee joints. Capable of dynamic locomotion including trotting, jumping, and recovering from falls.
Solo8 v2 - Quadruped 8DOF
8 degrees of freedom, 1.9kg total weight including battery placeholders. Lightweight body structure with 4 identical 2DOF legs. Designed for research in simplified quadrupedal locomotion.
Bolt - Biped 6DOF
6 active degrees of freedom, 1.34kg total weight. Each leg has 3 active DOF plus passive ankle joint. Extended leg segments (200mm vs 160mm) for human-scale experiments. Line-contact feet for yaw stability.
TriFingerEdu
9 torque-controlled degrees of freedom for manipulation research. Three identical finger modules mounted on an aluminum frame structure. Each finger weighs 930g assembled. Includes optional camera system and barrier for contained manipulation tasks.
Additional platforms
The project also provides designs for:- 3DOF legs - modular leg assembly used in Solo12 (485g each)
- 2DOF legs v2 - simplified leg design for Solo8
- Biped legs - extended length legs for the Bolt platform
- NYU Finger - single-finger manipulation platform
- Leg test stands - for individual actuator and leg testing
- Dual motor testbed - for characterizing actuator performance
Technical specifications
Actuator module core v1
| Specification | Value |
|---|---|
| Weight | 150g |
| Segment length | 160mm |
| Output torque | 2.5Nm at 12A |
| Gear ratio | 9:1 (dual stage) |
| Encoder resolution | 20,000 counts/rev at MCU |
| Core component weight | 95g |
| Off-the-shelf components | Motor (53g), encoder (5g), bearings (16g), timing belts (4.5g) |
Bill of materials approach
The project emphasizes:- Commercially available components - motors, encoders, bearings, and fasteners from established suppliers
- Minimal machined parts - only 3 small aluminum and steel parts require machining (motor shaft, motor pulley, center pulley)
- Standard tooling - assembly requires only basic tools plus Helicoil installation equipment
- Global sourcing - documentation includes multiple suppliers for each component with worldwide shipping
Assembly approach
The project provides comprehensive step-by-step instructions:- Motor preparation - modifying motor housing and installing components
- Motor shaft preparation - machining and codewheel installation
- Encoder preparation - assembly and alignment procedures
- Pulley preparation - center and output pulley assembly
- Shell preparation - 3D printed housing preparation with Helicoil inserts
- Actuator assembly - complete module integration
- Testing procedures - verification and calibration methods
Electronics architecture
The electronics follow a distributed control architecture:- Master Board - communicates with host PC via Ethernet, coordinates multiple Micro Driver boards via SPI
- Micro Driver Boards - perform real-time torque control (10kHz), communicate encoder values and motor status
- IMU - Lord Microstrain 3DM-CX5-25 provides base state estimation (±900°/sec gyro, ±20G accelerometer)
- Power distribution - 24V supply distributed through the robot structure
- Wiring harness - custom lengths for each platform with detailed routing documentation
Design philosophy
The Open Robot Actuator Hardware project prioritizes: Accessibility - Complete documentation with ordering information, assembly instructions, and troubleshooting guides. Multiple supplier options for global access. Modularity - Core actuator modules remain identical across platforms. Only shell structures change for different applications. Transparency - All design files available: Solidworks CAD, Eagle electronics, STL files for 3D printing, and technical drawings for machined parts. Practicality - Designs tested through extensive research use. Known issues documented with solutions. Calibration tools and test fixtures included. Performance - Torque-controlled actuators enable advanced control strategies: impedance control, force control, and dynamic movements impossible with position-controlled systems.Getting started
To build a robot using this hardware:- Choose a platform - Select the robot design that fits your research needs
- Source components - Use the detailed bills of materials with supplier information
- Fabricate custom parts - 3D print structural components and machine the three required metal parts
- Order electronics - Contact PCB manufacturers (BetaLayout or MacroFab) to replicate the custom boards
- Follow assembly guides - Use step-by-step instructions for each actuator module and platform
- Install software - Access the companion software repository for control interfaces
- Calibrate and test - Use provided calibration tools and procedures
The Solo12 platform is now commercially available from PAL Robotics as both a complete robot and a component kit, offering a turnkey option for labs without fabrication capabilities.
Community and support
The Open Dynamic Robot Initiative provides:- Webpage - open-dynamic-robot-initiative.github.io
- Forum - Active community discussion at odri.discourse.group
- YouTube Channel - Video demonstrations and tutorials
- Academic paper - arxiv.org/pdf/1910.00093.pdf
- Software wiki - Comprehensive control software documentation
Next steps
Build your first actuator
Follow detailed instructions to assemble a complete actuator module
Choose a platform
Explore the available robot platforms and select one for your application
Electronics overview
Learn about the custom motor driver and master board electronics
Order components
Access complete bills of materials with supplier information