OnesArm

A modular serial robotic arm for multi-scenario tasks
v2 stepper/CAN platform, 24 joint layouts, and mortise-and-tenon interfaces

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About the project

OnesArm (Chinese project title: 珠联“臂”合) is a modular serial robotic arm. v2 is the current platform. It combines 42BYGH47 stepper motors with FLE-42-20 planetary reducers, STM32F103C8T6 CAN joint modules, an STM32F407ZGT6 bus controller, a 350 W supply and a 4040 aluminium platform. The links use 5 mm 5052 aluminium sheet and 50 × 46 mm carbon-fibre tubes, joined at standardised M5 nodes.

The arm is assembled from interchangeable joint, link and end-effector modules through T/N mortise-and-tenon interfaces. For the three-module configuration, the base-adjacent orientation is fixed and the other two modules choose one of two perpendicular channels, giving

N₃ = 3! × 2² = 24

joint layouts. The 24 assemblies are modelled in SolidWorks.

The earlier v1 reference build, with 15 kgf·cm PWM servos and a Raspberry Pi Pico W, is retained in the repository for comparison. The project was developed by 郭沫晗 (Guo Mohan) at Guyuan No. 1 Middle School, Ningxia. The report documents a first prize at the 38th National Youth Science and Technology Innovation Competition (2024).

Platform note. v1 and v2 use separate electronics. Select the matching drive and board set before assembly.

Key features

v2 stepper/CAN drive chain — 42BYGH47 motors, FLE-42-20 1:20 reducers, magnetic encoder closed-loop control and STM32F103C8T6 CAN joint modules.
v2 aluminium/carbon structure — 5 mm 5052 links, 50 × 46 mm carbon tubes, M5 nodes, a 4040 platform and a 350 W supply, with SolidWorks, STEP and STL sources included.
24 reconfigurable layouts — three interchangeable modules, two perpendicular channels, and a fixed base-adjacent orientation.
T/N mortise-and-tenon interface — a T node on top and an N node below, clamped by an M5 screw, inspired by dovetail and half-lap timber joints.
v1 reference build — 15 kgf·cm PWM servos, Raspberry Pi Pico W, five module boards and a 16P gold-finger interconnect.
Multi-scenario end effectors — parallel gripper, HC-SR04 ultrasonic probe, MQ gas sensor and pen module share the same interface.

System architecture

v2 current platform

System architecture. End effector, link module, joint module and 4040 platform, with the CAN bus and 350 W supply.

Subsystem	v2 implementation
Controller	STM32F407ZGT6 CAN bus controller; STM32F103C8T6 CAN module in each joint
Drive	42BYGH47 stepper + FLE-42-20 planetary reducer, 1:20; magnetic encoder closed-loop
Bus and power	CAN via TJA1050; 350 W DC supply; XT60 input; LM2596-5.0 logic rail
Structure	5 mm 5052 aluminium sheet + 50 × 46 mm carbon tubes; M5 nodes; 4040 aluminium platform
End effectors	Parallel gripper, HC-SR04 ultrasonic probe, MQ gas sensor, pen module

Figure 1 — v2 long links. X-X and X-Y links in 5 mm 5052 aluminium with 50 × 46 mm carbon tubes and M5 connection nodes.

Figure 2 — v2 drive module and platform. 42BYGH47 stepper, FLE-42-20 reducer, CAN board and the 4040 aluminium working platform.

v1 reference build

Subsystem	v1 implementation
Controller	Raspberry Pi Pico W, RP2040, Wi-Fi
Drive	15 kgf·cm PWM digital servo
Bus	16P 2.54 mm gold-finger interconnect
Boards	Base, Joint A, Joint B, Holder/Sensor, Expand Controller
Supply	5–9 V servo rail; 5 V logic

Figure 3 — v1 reference build. Servo-driven arm render and the Joint Module PCB with PWM outputs, status LEDs and a servo signal selector.

Theory and design

Modular combinations

The modular interface is a connecting face – connecting channel – connecting face (FCF) chain. An X-X or X-Y long link has 6 faces × 4 channels, a short link has 4 × 4, and a joint has 2 × 4. The per-module connection counts are:

Base: 3 methods.
X-X / X-Y long link: 2 × 3 × 3 = 18 methods.
Short link: 2 × 2 × 3 = 12 methods.
Joint: Pro side 4 × 1 × 1 = 4, Con side 1 × 4 × 1 = 4, total 8 per joint module.
General end link: 2 × 2 or 2 × 3 methods.

For F joint modules, N_LL long links and N_LS short links, the FCF connection-method total is:

$$\displaystyle N_{com} = \sum_{i=0}^{N_{LL}} \sum_{j=0}^{N_{LS}} 8^{F} \cdot 18^{N_{LL}} \cdot 12^{N_{LS}}$$

The three-module assembly fixes the base-adjacent orientation and gives the remaining two modules two perpendicular channels each:

$$N_3 = 3! \cdot 2^{3-1} = 24$$

The 24 combinations are modelled as SolidWorks assemblies.

Figure 4 — FCF abstraction. Every connection is a face, a channel and another face. Long links have six faces, short links four, and joints two.

Kinematics and workspace

The analysed configuration is AyCxBy-D, a 4-DOF chain. Report Table 3-20 gives:

i	dᵢ (mm)	aᵢ (mm)	αᵢ	Report range
1	95	0	90°	0°
2	0	113.5	90°	0–180°
3	21.34	88.5	90°	0–180°
4	0	152	0°	0–180°

The standard DH transform is:

$$T_i^{i-1} = \begin{bmatrix} c\theta_i & -s\theta_i c\alpha_i & s\theta_i s\alpha_i & a_i c\theta_i \\ s\theta_i & c\theta_i c\alpha_i & -c\theta_i s\alpha_i & a_i s\theta_i \\ 0 & s\alpha_i & c\alpha_i & d_i \\ 0 & 0 & 0 & 1 \end{bmatrix}, \qquad T_4^0 = T_1^0 T_2^1 T_3^2 T_4^3$$

The workspace is the set of tool positions over the joint ranges:

$$\mathcal{W} = {, \mathbf{p}(q) \mid q_i \in [q_{i,\min}, q_{i,\max}] ,}, \qquad \mathbf{p}(q) = [T_4^0(q)]_{1:3,4}$$

The link-length sum is 113.5 + 88.5 + 152 = 354 mm. With the model offsets, the sampled workspace reaches about 450 mm.

Figure 5 — Workspace comparison. Four of the 24 layouts: AyCxBy-D, AyCyBx-D, AyCxBx-D and AyCyBy-D. Simulation samples from the kinematic model.

v2 finite element analysis

The v2 XY long link is evaluated as a static assembly. The base connection is fixed and the service load is applied at the connection nodes. The model combines the 5 mm 5052 aluminium plates with the 50 × 46 mm carbon tube.

The finite element solution is:

$$K u = f$$

where K is the stiffness matrix, u the nodal displacement vector and f the nodal load vector. The same load path can be checked with the cantilever relations:

$$M = FL, \qquad \sigma = \frac{Mc}{I}, \qquad \delta = \frac{FL^3}{3EI}$$

The v2 study reports a displacement-magnitude (URES) contour with a maximum of:

$$u_{\max} = 1.388 \times 10^{-2}\ \text{mm}$$

Figure 6 — v2 finite element analysis. Static displacement contour of the XY long link. The base is fixed and the connection nodes carry the service load; the URES legend is in millimetres.

Assembly selection

The v2 board uses the absolute joint-velocity integral as the assembly-selection index:

$$E_{Sum} = \sum_{i=1}^{F} \lvert \int_0^t \dot{\theta}_i(\tau),d\tau \rvert$$

The selection compares E_Sum with the workspace envelope. The workflow is:

Figure 7 — v2 assembly-selection workflow. The effort index and the workspace pre-processing path.

v1 reference static analysis

The v1 static study uses 9400A photopolymer resin and a 10 N load on each joint top face. The joint housings and the full assembly are evaluated in SolidWorks Simulation.

Figure 8 — v1 reference static analysis. Mesh and stress result for the joint housing under the 10 N load case.

Repository structure

OnesArm/
├── BOM/                    summary + manufacturing checklists
├── electronics/            Gerber, BOM and silkscreen outputs
├── mechanical/
│   ├── source/             SolidWorks parts and assemblies
│   ├── output/             STEP, STL and fabrication exports
│   └── analysis/fea/       exported FEA images
├── firmware/               STM32CubeIDE projects + MicroPython libraries
├── software/               MATLAB kinematics scripts
├── docs/
│   ├── en/ zh-CN/ fr/      long-form documentation
│   ├── report/             2025 report and defence deck
│   ├── archive/            2024 report and research log
│   └── references/         citations and provenance
├── media/
│   ├── photos/project/     v2 renders, v1 build photos, PCB and test views
│   ├── photos/reference/   report figures and key figures
│   ├── gifs/               reference-build demonstrations
│   ├── videos/             original MP4 files
│   └── source/             editable poster files
├── data/                   data and log directory
├── scripts/verify_theory/  pure-Python kinematics and dimensional checks
├── README.md README.zh-CN.md README.fr.md
├── MIGRATION_NOTES.md CHANGELOG.md CONTRIBUTING.md
├── LICENSE.md LICENSE-docs.md
├── .gitignore .gitattributes
└── .github/