Add Franka CELLO variant: non-decoupling ring brakes, base bearing, 3D-printed trigger spring

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franka/brake version/README.md

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+# CELLO (CIIRC's version of GELLO) — Bearing-Isolated, Passive-Braked GELLO Variant for the Franka Emika Panda
+
+**CELLO** is low-cost GELLO leader variant for the **Franka Emika Panda**, that improves joint stability via **frictional joint regularization** and offloads structure-borne loads from motors.
+
+---
+
+## Why
+
+Stock GELLO joints for the Panda rely on springs/rubber bands that degrade, and many links load the motor output directly. In practice this leads to joint sag, play, and fragile handling. **CELLO replaces spring regularizers with compact, adjustable friction brakes** and moves structure-borne loads onto a **concentric ball bearing**, improving stability and longevity while preserving encoder fidelity.
+
+---
+
+## What’s new
+
+- **Passive brake (non-decoupling).** Thin **ring brake** mounted in parallel with the joint axis (not in the drive path).  
+  Two printed rings + **TPU pads** (≈ Shore 95A) + symmetric **preload** (2× M3×25) → **adjustable drag** so links “stay put.” **Encoder sync preserved.**
+
+- **Bearing-isolated base joint.** Concentric **ball bearing** carries radial/axial loads; the motor primarily transmits torque. (Inspired by [**FACTR**](https://github.com/JasonJZLiu/FACTR_Hardware))
+
+- **Printable trigger torsion spring.** Spiral spring biases the trigger open; easy to reprint/replace.
+
+- **Tightened tolerances & reinforced interfaces.** Modern printers → snug fits, reduced backlash.
+
+- **Parametric, modular brakes.** Same core geometry for **axial** (solid hub) and **radial** (hollow-center) variants.
+
+---
+
+## Key features
+
+- **Six brakes total:** 3 axial (L1–L2, L3–L4, L5–L6) + 3 radial (Base–L1, L2–L3, L4–L5).  
+- **Materials:** PETG (structure/brake shells) and **TPU 95A** (pads).  
+- **Serviceability:** replaceable pads; tool-adjustable preload.  
+- **Encoder-safe:** friction regularization is parallel to the joint—no clutch in series.
+
+---
+
+## Build notes
+
+- Use the supplied `.3mf` projects (orientations, copy counts, painted supports).  
+- Print **pads** in TPU with the seam at the **outer edge** to avoid contact ridges.  
+- Tune each brake by tightening both M3×25 preload screws **symmetrically** until the link **holds its own weight** with minimal drift while still driving smoothly.
+
+---
+
+## Full assembly instructions & bill of materials
+
+Assembly guide (with photos) and BOM:  
+<https://docs.google.com/document/d/1AONti0UWSNlmaY5SlAWqmKt7b-Y6IRfGCKrLpYsYRNI/edit?usp=sharing>
+
+---
+
+## Acknowledgments
+
+Built on the ideas and ecosystem of [**GELLO**](https://github.com/wuphilipp/gello_mechanical) . Load-path choices were informed by [**FACTR**](https://github.com/JasonJZLiu/FACTR_Hardware). Contributions and adapters for other robots are welcome.