OpenMuscle FlexGrid is a modular 60-sensor pressure-sensing platform designed for advanced forearm-based gesture tracking and biomechanical research.
This device is part of the OpenMuscle ecosystem and represents a leap forward from the original 12-sensor OM12 prototype by introducing high-density, flex-rigid PCBs and better sensor modularity.
📌 For documentation, firmware, and assembly guidance, visit the OpenMuscle Hub.
V4 (current, validated). Fabricated at JLCPCB in June 2026 (10-unit batch, single-side SMT), brought up 2026-06-23 and in use since. Two V4 bands were assembled, flashed with FlexGridV4-Firmware v4.0.1 and shipped as the first OpenMuscle starter kit in October 2026. V4 is the best-performing FlexGrid revision to date and the one to build. It adds an on-device microSD card socket for local data logging, a discrete RGB status LED on three PWM channels, and a reoriented slimmer board profile that hugs the forearm. It carries forward the V3 power and interconnect architecture, with the FFC tail width corrected from 11.1 mm to 10.5 mm per the Wurth 687120183722 datasheet, the ADC matrix-row caps reduced from 2.2 uF to 100 pF (resolves V3's 22 ms RC bleed in silicon rather than via post-fab cap removal), all resistors normalized to 0603, and several footprint cleanups. Manufacturing strategy: single-side SMT at JLCPCB plus hand-soldering of bottom-side parts and through-hole connectors. The design files are now at board revision V4-2 (2026-11-03): USB presence sensing for the firmware, vias moved out of the SMD pads, USB data lines re-routed as a pair. V4-2 has not been fabricated yet, and its BOM, placement and Gerber files are still the June 2026 as-ordered set. Full status, bring-up findings and the fab checklist are in the V4 README.
V3 (previous, validated 2026-05-13). First revision to use a 20-pin ZIF FFC connector for the flex-to-rigid interconnect (Wurth 687120183722). Two boards populated and tested end-to-end. Sensor matrix delivers clean single-cell press detection across all 60 sensors with ~22% carryover decaying to noise floor within 3 columns. V3 is the first FlexGrid generation usable as a real ML training data source. Firmware: FlexGridV3-Firmware v0.1.7.
→ For V4 hardware files, see KiCad/OM-FlexGrid V4/ and the V4 README. For V3 see KiCad/OM-FlexGrid V3/ and the V3 README. Firmware: V4 bands run FlexGridV4-Firmware; V3 boards run FlexGridV3-Firmware.
| Revision | Status | Notes |
|---|---|---|
| V4 | 🟢 Validated (current) | microSD + RGB LED + slimmer reorient; 10.5 mm FFC tail; 100 pF ADC caps; fabbed 2026-06, in starter kits since 2026-10; design files at V4-2 (USB sensing, vias out of pads, USB pair; not yet fabbed) |
| V3 | 🟢 Validated (previous) | 20-pin FFC interconnect, integrated stiffened FFC tail, ~140 Hz raw scan |
| V2 | 🟢 Tested | FFC connector exploration, fabric harness testing |
| V1 | 🟢 Tested | First production revision |
| V0 | 🟢 Tested | Original prototype |
→ Full cross-version comparison: KiCad/REVISIONS.md. Manufacturing process used across revisions: docs/JLCPCB_WORKFLOW.md.
FlexGrid uses a flexible pressure sensor matrix with rigid breakout interfaces to detect volumetric muscle contractions in real time. It supports:
- 🧠 Biomechanical signal analysis
- 🦾 Prosthetics and exosuit research
- 🎛️ Machine learning data pipelines
Design files are organized by board type and version:
📂 KiCad/ — all design files, organized by revision.
| Revision | Path | Description |
|---|---|---|
| V4 (current) | KiCad/OM-FlexGrid V4/ |
Validated. microSD socket, RGB status LED, slimmer reorient, 10.5 mm FFC tail, 100 pF ADC caps. See V4 README. |
| V3 (previous) | KiCad/OM-FlexGrid V3/ |
Validated. 20-pin ZIF FFC interconnect. See V3 README. |
| V2 | KiCad/OM-FlexGrid V2/ |
Tested. FFC exploration + fabric harness. See V2 README. |
| V1 | KiCad/OM-FlexGrid V1/ |
First production revision. |
| V0 | KiCad/OM-FlexGrid V0/ |
Original prototype. |
Each revision folder contains both the flex (OM-FlexGrid-Flex/) and rigid (OM-FlexGrid-Rigid-PCB/) board projects, plus their gerber bundles.
BOMs live next to their KiCad project for each revision:
- V4 Rigid (JLCPCB-ready):
KiCad/OM-FlexGrid V4/OM-FlexGrid-Rigid-PCB/jlcpcb/production_files/BOM-OM-FlexGrid-Rigid-PCB.csv(LCSC part numbers, machine-readable, generated by the JLCPCB KiCad plugin). This is the June 2026 as-ordered V4 set; the V4-2 additions (R19, R20, R21) are not in it yet. The hand-soldered parts list (microSD socket, FFC connector, battery JST, pin headers, plus single-side-strategy BOT-side SMT) is documented prose-style in the V4 README. - V3 Rigid:
KiCad/OM-FlexGrid V3/OM-FlexGrid-Rigid-PCB/OM-FlexGrid-Rigid-PCB.csv(KiCad export). - Flex (V3 and V4): no electronic-component BOM. The flex tail integrates the FFC connector pads directly; the only off-board parts are the Velostat sheet and the mating Wurth 687120183722 connector populated on the rigid board.
Older revision BOMs are kept under BOM/ for history.
For V4: the build video covers hand-soldering the parts JLCPCB does not populate, BRINGUP.md in the firmware repo covers flashing, and the band reference explains every button and light pattern. Recording, training and inference software lives in OpenMuscle-Software. For the wider picture see the OpenMuscle Hub and the videos on YouTube Open Muscle or TURFPTAx Youtube.
We welcome improvements to sensor design, firmware, or real-world testing. Please fork the repo and submit pull requests!
Open Muscle is an open-source prosthetic sensor initiative designed for accessibility, innovation, and community collaboration.