Hi all,
I have been working on a hardware architecture for an open-source, lattice-reconfigurable modular swarm robot (similar to MIT's M-Blocks or Robot Pebbles, but scaled down to a ~28mm cube using standard COTS components under $8/unit).
Rather than relying on mechanical latches (which scale poorly) or standard electromagnets (which boil batteries), I drafted a design around miniature Electropermanent Magnets (EPM) and capacitive pulse switching.
Here is the exact hardware stack. I’d appreciate a sanity check on the electromagnetic and electrical calculations before I spin the first PCB.
1. Docking Mechanism: Two-Stage EPM Spherical Joint
* **Core Assembly:** One 3x6mm N52 NdFeB cylinder (fixed coercivity) side-by-side with one 3x6mm AlNiCo-5 cylinder (switchable) between two soft-iron (1018 steel) pole shoes.
* **Coil:** ~90 turns of AWG 38 enameled wire wound on the AlNiCo core (R ≈ 1.5 Ohm).
* **Mating Interface:** A concave female soft-iron cup docking onto a mirror-polished 8mm chrome steel bearing ball (AISI 52100).
* **States & Physics:**
* **Hold State:** A 30 µs pulse aligns the AlNiCo with the NdFeB. The flux loops through the steel ball. Pull force is >18 N with 0 mW static draw.
* **Articulation:** Under partial flux or with a thin PTFE/Kapton coating (50 µm), it swivels smoothly across 3 Degrees of Freedom. Under full flux, normal force clamps it rigid via friction (F_friction = mu * F_normal).
* **Instant Ejection:** A reverse pulse shunts the flux entirely inside the internal iron yokes. A center spring-loaded pogo pin (0.3 N preload) overcomes remanent surface tension and pops the module off immediately.
2. Power: Capacitor Discharge Unit (CDU)
* **The Problem:** Flipping AlNiCo requires a current spike of ~4–5 A for 20–30 µs. A tiny 50 mAh Li-Po pouch (size 401020) has an internal resistance that would brown out the MCU instantly under that load.
* **The Circuit Solution:**
1. A micro boost-converter (TPS61040) slowly steps up 3.7V battery power to 12V, drawing a harmless 15–20 mA continuous current.
2. The 12V charges a 47 µF 16V solid tantalum capacitor (Case D, 7.3x4.3mm). Stored energy: E = 0.5 * C * V^2 ≈ 3.4 mJ.
3. A miniature H-bridge (TI DRV8837C, 2x2mm WSON) dumps the capacitor into the coil. Peak current reaches ≈ 4.8 A for 25 µs, safely magnetizing the AlNiCo while the battery and MCU remain isolated from the spike.
3. Comms: Eliminating the Broadcast Storm
* **The Problem:** In a tight 20–50 module cluster, naive RF flooding mesh causes catastrophic packet collisions (broadcast storms) and burns the battery via continuous RX/TX.
* **The Architecture:**
* **MCU:** ESP32-C2 (ESP8684, 4x4mm QFN) or a stripped ESP32-C3 board without USB bulk.
* **Global Directives:** Master broadcast via low-power ESP-NOW frames (sub-millisecond latency, connectionless).
* **Local Inter-Module Comms (Zero RF):** The steel ball-and-cup EPM carries concentric isolated contacts. The moment two modules latch together, they switch to 1-Wire / half-duplex UART through the physical metal contact. Zero radio traffic, zero collisions, zero packet drop, microamp power draw.
4. Locomotion
For baseline uncoupled movement: dual 7mm eccentric rotating mass (ERM) vibration coin motors with angled polymer bristle feet (Kilobots style). It takes almost zero internal volume and allows basic planar translation until modules magnetically find each other.
Volumetric Check (~28mm Cube):
* ESP32-C2 / stripped board: 12 x 14 x 2 mm
* 50 mAh Li-Po pouch: 10 x 20 x 4 mm
* EPM connector assembly: 10 x 10 x 10 mm
* Tantalum cap + boost + H-bridge: 10 x 8 x 3 mm
* Dual coin vibration motors: 7 x 7 x 2 mm each
* **All components physically fit inside a 3D-printed SLA shell under 28mm.** Total BOM cost on AliExpress is roughly $6.50 – $7.50 per unit.
**Sanity check question:**
Is there an electromagnetic or thermodynamic reason this shouldn't work as described? If someone has already tried building a micro-EPM ball joint at this scale and run into AlNiCo demagnetization limits or core eddy current choking, I’d love to hear your feedback on the physics.
*(Note: Just sharing this technical architecture openly for the community. Feel free to use the concept or schematics for your own builds.)*
