Engineering-level Reliability at Campus Cost

Open-APEX uses PETG for all structural links because it prints reliably on common campus FDM machines with a single slicing profile (0.4 mm nozzle, 0.2 mm layer height). Load-bearing regions use ribs; non-critical regions are hollowed. Total printed mass: 930.41 g. Filament cost for one arm: approximately SGD 15.

All joints use identical 42 mm stepper motors (SGD 8 each) on a shared RS-485 daisy-chain bus. A single trunk carries 24 V power alongside the differential RS-485 pair, reducing harness size, connector count, and wiring mistakes. The brand is not critical -- any equivalent motor meeting the published specs may be substituted.

The digital twin lets users move the end effector directly in 3D. When a target is dragged, the system solves inverse kinematics online and synchronizes the approved joint state to the physical arm. Users think in terms of tool position rather than individual joint angles.

The jog loop creates a rolling short-horizon plan: each jog input creates a small target update, the UI requests a short trajectory from the Flask server, then streams the returned joint frames to both the visual model and the robot simultaneously.

Two execution modes are supported. In Simul mode, visualization and hardware move together frame by frame, minimizing the delay between what the operator sees and what the robot does. In Preview-First mode, the UI plays the motion in the digital twin before sending it to hardware -- useful for teaching and cautious operation.

The controller uses Damped Least Squares (DLS) inverse kinematics to remain stable near singularities. The IK step solved every control cycle:

Joints are always clamped: \(\mathbf{q}_{k+1}=\mathrm{sat}_{[\mathbf{q}_{\min},\mathbf{q}_{\max}]}(\mathbf{q}_{k}+\Delta\mathbf{q})\). For short final-approach motions, a distance-aware target extrapolation strategy prevents erratic proximal joint motion:

A distance-coupled joint regularization term further penalizes large proximal joints during close-range motions:

Open-APEX uses three isolated electrical domains: 24 V actuation, 5 V differential communication, and 3.3 V logic. This limits fault propagation, simplifies debugging, and gives future modules clear connection points.

The 24 V supply enters a hard emergency-stop path before any software-controlled branch. After the E-stop stage, an 8-channel relay module distributes power across the actuation line, electromagnet release line, and status indicators. Three status indicators with NC-safe defaults make system state visible at the hardware level.

The arm-side harness is limited to just 5 conductors: Motor 24 V, Magnet 24 V, RS-485 A, RS-485 B, and Ground. Aviation-style keyed connectors prevent miswiring during maintenance. All grounds return to one power-adapter reference for clean signal integrity.

A magnetic electrical connector links the arm harness to the tool harness. After docking, the electromagnet locks Link-5 to the tool. The system then powers up in a strict sequence:

1. Dock -- Electromagnet engages, physically locking tool module
2. 5 V Enable -- ESP32 Slave boots, establishes Wi-Fi link, reports telemetry
3. 24 V Enable -- High-power tool actuator is powered (if needed)

Undocking reverses the sequence: sensors and actuators stop first, then 24 V is cut, then 5 V, and finally the electromagnet is released. The electromagnet is configured as power-off magnetic / power-on release, so energy is used only for intentional tool release. A watchdog timer limits energisation time to prevent overheating.

The ESP32 Master controls three separate power rails for the end effector (9 V docking magnet, 5 V logic, 24 V tool power), communicating wirelessly with the ESP32 Slave mounted on the tool. This means tool changes never disturb joint-side control.

The software is split into cooperating processes. The Trajectory Server (Flask) accepts a start pose, end pose, and initial joint config over HTTP. It generates a Cartesian path and solves IK to produce a joint trajectory -- kept stateless for on-demand replanning.

The Commander layer publishes joint-angle frames over a ZMQ PUB socket (:5557). Visualizers subscribe to the same channel for real-time mirrored views. Simultaneously, serialised motion frames are streamed to stepper drives over dedicated serial managers. This separates fast visualization traffic from deterministic hardware traffic.

Safety is enforced at every module boundary: the UI validates commands before any hardware frame is sent, the Commander only transmits approved frames, and the physical E-stop remains the final fallback. Because executed frames are also shown in the visual stream, the operator always sees what the robot is about to do.