SoftSense — underwater self-sensing soft gripper

🏆 Project complete (July 2026) — prize winner. The full SoftSense project took home a prize at its competition. This repository is the finished, consolidated record: parametric CAD, FEA campaigns, the grip-texture and actuator studies, print profiles, ESP32 firmware and the Orange Pi controllers — everything in one place. (Formerly gripper-cad; the old GitHub URL redirects. The judge-facing engineering document is SoftSense_Innovations_Documentation.docx.)

A robotic gripper with working single-DOF motion: rotate one input shaft and both fingers open/close symmetrically, splaying outward as they open. The jaws are Fin Ray-style compliant fingers (3D-printed in TPU with a crosshatch micro-post grip texture) that bend and wrap around a grasped object. The gear/linkage drive lives in a flooded enclosure designed for underwater use (drain/flood holes, corrosion-resistant material choices — docs/UNDERWATER.md).

The SoftSense idea: there is no dedicated force sensor. The actuator is the grip-force sensor — motor current → torque → tip force through the calibrated kinematic chain (motor/SENSING.md). The printed drivetrain is the structural limit (T_safe), and the motor's current limit is its protection.

Mechanism (one DOF)

• Two equal spur gears mesh on the centreline. The left gear (A_L) is driven through a 90° straight bevel pair: a 12-tooth bevel gear fused on A_L's +Z face, driven by an integral 6-tooth bevel pinion (2.0 ratio, module 1.8, 25° PA — deliberately coarse, thick teeth from the gear-FEA strength pass) on a vertical Ø15 input shaft, so the drive enters the housing from the bottom while the fingers point up. The mesh counter-rotates the right gear: one shaft moves both fingers as a mirror pair.
• The input shaft's bottom end carries a female Feetech 25T spline socket (Ø5.9 serration, press-fit) — a Feetech STS3250 bus servo presses straight in, no adapter horn. Caliper-verify the spline and print the fit coupon before committing (gripper.py, SPLINE_*).
• Each gear is the crank of a non-parallelogram four-bar linkage; the finger is rigid with the coupler. The link lengths give translate-apart + ~18° outward splay over the travel, clear of any four-bar dead-point.
• Travel: closed (jaw faces ~1.6 mm apart) → open (~60 mm at the base, ~118 mm at the fingertips) at 1× scale.

Fin Ray fingers (TPU)

Each finger is a compliant triangular truss — thin 1.2 mm contact beam, sharply tapered 1.8 mm spine, 14 fine reversed-slant ribs — chosen by multi-shape FEA to grasp universally: it distributes contact pressure along the whole finger on flat/large objects and grips round objects evenly across a wide size range (fea/UNIVERSAL_FINGER.md). The contact face carries a crosshatch micro-post grip texture (1.8 mm posts + 0.54 mm drainage channels) — the empirical winner among textures that actually tile the 10 mm blade, chosen by a wet-grip physics model + sensitivity campaign with an honestly-documented override of the model's raw winner (grip/GRIP_TEXTURE.md §5). Printed in Bambu TPU 95A HF (measured ISO 527 data; ether-based TPU recommended for sustained immersion).

Enclosure (flooded, underwater)

Rounded hollow housing; the vertical input shaft exits through two journal bearings in the bottom wall, surrounded by a mounting flange with 4 × M4 bolt holes. Drain/flood holes let it flood and drain — no trapped air, pressure equalizes with depth. Top slots are sized to the measured arm sweep.

Electronics & control

Where	What
firmware/	ESP32 controller (the shipped one) — Waveshare General Driver for Robots. Boots its own Wi-Fi AP + captive web UI (OPEN/CLOSE, slider, live position/load/voltage/temp, on-flash calibration), drives the STS3250 on the built-in bus-servo port. Stop-on-load (gentle stop-on-contact) ported from the Pi controller.
firmware/gamepad/	DualSense bridge — a laptop-side script + page that turns a PS5 pad into a jog controller via the firmware's /api/jog.
opi/	Orange Pi controllers (3 LTS Wi-Fi-AP appliance + Ethernet-only Orange Pi PC/H3 variant) — the original offline web controller with stop-on-load, vendored pyserial, provisioning + deploy scripts. Superseded by the ESP32 after the Pi died, kept as a complete working alternative.
motor/	The actuator & sensing study: requirements → 12-agent survey → weighted selection (±50% sensitivity). Depth-rated pick: DYNAMIXEL XW540-T260; the built demonstrator runs the Feetech STS3250 direct-spline mount. Power chains per interface in motor/POWER_SUPPLY.md.

Repository map

Path	What it is
gripper.py	Source of truth — parametric build123d generator + four-bar solver + Fin Ray finger + enclosure. Env: GRIPPER_OPEN 0–1 (pose), GRIPPER_FINGER_SCALE 0.6–2.5, GRIPPER_SCALE (whole-gripper scale-up).
docs/TESTING_AND_SIMULATION.md	How everything was tested & simulated — judge-facing account of every simulation, its physics, what it proves, fidelity honesty, reproduce commands. Start here for "how do you know it works?".
docs/	Engineering guides: assembly, BOM, DFM, underwater, printing, print profiles, materials, testing, scale-up (SCALE_UP.md), the overnight honest-framing fixes (OVERNIGHT_FIXES.md).
fea/	Finger FEA campaign — universal-finger study, scalability, underwater crush, decision log (~90 runs), 3D corotational contact solver (CPU+GPU).
grip/	Grip-texture campaign — wet-grip surrogate model, sweep + ±50% sensitivity, Tier-2 contact FEA, literature validation gate, decision log.
motor/	Actuator/sensing/integration campaign — requirements, survey, selection, drivetrain gear FEA (T_safe), force-via-current sensing, electrical, ROV integration, bench-test plan, FMEA, mounting-interface dossiers + 7 modelled adapters (motor/cad/adapters/), power supply.
parts/ + output/ + print_plates/	Per-part STEP/STL exports (with MANIFEST.md), material-labelled print set, and plate STLs.
variants/scale_1.5x/, variants/scale_2.0x/	Scaled-up variants (fully re-simulated; parts + plates + 3MF print-ready). See docs/SCALE_UP.md.
profiles/	Importable Bambu Studio filament + process profiles (TPU 95A HF on P1S, 0.4 mm hardened nozzle).
derived/	Regenerable assemblies: static poses (gripper_{closed,mid,open}.step) + interactive slider STEPs.
renders/	Final animations, heroes, FEA montages, adapter snapshots.
motor/cad/output/system_assembly_T2_*.step	Full integrated system (gripper + canister + servo + shaft + seal + penetrators), one STEP per servo option.
firmware/, opi/	Controllers — see table above.
scripts/, regen.sh	Export/plate/render helpers; ./regen.sh rebuilds all derived artifacts.
SoftSense_Innovations_Documentation.docx	The competition documentation deliverable.

Print & build

25 printed parts, zero purchased hardware, zero fasteners — the only tool is a soldering iron. 9 structural parts (PA12-GF rigid set + TPU fingers), the one-piece input_pinion_shaft (bevel pinion + Ø15 shaft + STS3250 spline socket), 8 heat-stake pins and 8 retaining caps (PETG-HF): each journal pin is retained by a cap melted over its stud into a thermal-rivet head wider than the bore. Print settings + rationale: docs/PRINT_PROFILE_P1S_TPU.md, docs/PRINTING.md, docs/DFM.md.

Regenerate / re-pose

source /home/andre/.cad-venv/bin/activate          # build123d + OCP toolchain
STEP=/home/andre/.claude/skills/cad/scripts/step
GRIPPER_OPEN=0.7 python $STEP gripper.py -o derived/gripper_open70.step   # any pose 0..1
GRIPPER_SCALE=1.5 python $STEP gripper.py -o derived/gripper_15x.step     # scaled build
python gripper.py                                  # numeric kinematic self-check
./regen.sh                                         # full rebuild (poses + parts + plates + renders)

The FEA solver has a CuPy GPU backend (GRIPPER_FEA_GPU=1), but at these mesh sizes (≤25k DOF) CPU is faster — the GPU only wins past ~100k DOF.

Interactive viewer

Open derived/gripper_interactive.step in CAD Explorer and drag the open slider (= turning the shaft); mesh and edges move together live.

Coordinate convention

X = jaw open/close (right +), Y = toward fingertips (up +), Z = depth = revolute & gear axes. Units: mm.

Assumptions / caveats

• Off-centre drive. A single shaft drives both jaws symmetrically only if the two gears mesh each other, so the input enters at the left pivot (A_L); the right-angle bevel stage redirects it to the vertical bottom-exit shaft without changing the finger kinematics.
• Fin Ray-style, not Festo's patented variant — the generic adaptive-compliant slanted-rib truss principle, not the Fin Ray Effect® tooth-shape variant.
• Gear tooth forms are representative (straight-flank, clean meshing pitch circles), not production involutes — coupon-tunable for backlash/contact before a real run. The STS3250 spline socket dims are research-inferred: caliper-verify a real servo + print the fit coupon first.
• Real compliant grip (wrap-around) is a TPU material behaviour — the CAD shows the undeformed finger; the motion model opens/closes it rigidly.
• Dimensions were inferred from one reference image, not measured hardware.

Honest framing on the published headlines

• The finger-FEA 12 N target is a stress-probe load used to rank finger designs, not an operating force the drivetrain delivers. The printed right-angle stage caps input torque at T_safe — quantified on the crown-era teeth at ≈ 0.013 N·m (radial 2D) / 0.034 N·m (single-station 2D), mapping to a per-finger operating band of 0.14 – 0.73 N. The shipped bevel pair was upsized (module 1.0 → 1.8, ~+90 % tooth thickness) by the same strength pass and should raise that ceiling, but the only bench-grade number is the printed-coupon torque-to-failure test in motor/BENCH_TEST.md. Run motor/scripts/drivetrain_force_envelope.py for live numbers.
• The "5.7 – 8.6× safety margin" was computed with linear tetrahedra that volumetrically lock near ν = 0.5; the locking-stable P2 re-run puts the corrected margin nearer 3.8 – 5.7× at the 12 N probe — and ≈ 120 – 365× at the actual drivetrain-deliverable force. Design call unchanged; the absolute number is corrected (docs/OVERNIGHT_FIXES.md).
• The Tier-1 grip-texture model is rank-only, its literature gate is a sufficient-condition check, and the shipped crosshatch won only after a documented geometric-tileability override of the model's raw winner (grip/GRIP_TEXTURE.md §5, grip/GRIP_MODEL.md).
• T_safe is a 2D upper bound; the right-angle stage is genuinely 3D and straight-flank teeth will gall/edge-load in PA12-GF rather than roll cleanly. Bench coupons are the ceiling that counts.

License

CC BY-NC-ND 4.0 — you may view, download, build, and share this work with attribution, for non-commercial purposes, without distributing modified versions. Commercial use and derivative redistribution rights are reserved; contact the author to arrange a commercial license. See LICENSE.