2026-02-19 08:00:15 +00:00
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# Study 01 — TPE v1: Isogrid Mass Minimization (Campaign 01)
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**Parent project:** [isogrid-dev-plate](../../README.md)
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**Context:** [../../CONTEXT.md](../../CONTEXT.md)
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**Status:** Ready to run — not yet started
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**Created:** 2026-02-18
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---
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## 1. Overview
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This is the first optimization campaign for the ACS Stack Main Plate isogrid lightweighting project.
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It uses Optuna TPE (Tree-structured Parzen Estimator) with a budget of 200 trials.
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**Goal:** Find the set of 8 density-field parameters that minimizes total plate mass subject to a
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stress constraint (σ_max ≤ 100.6 MPa, AL7075-T6, SF=5).
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Each trial: Python Brain generates a triangular isogrid rib pattern → NX imports it into the sketch
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→ NX Nastran solves SOL 101 → extractors pull mass from the idealized part and max stress from the OP2.
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---
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## 2. Engineering Problem
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**Client:** ACS — Attitude Control System, spacecraft structural assembly
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**Part:** Main structural plate (AL7075-T6)
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**Challenge:** Remove as much material as possible from the plate interior via an isogrid rib pattern,
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while keeping peak stress within the allowable under the primary axial load case.
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**Load case:** FZ = 1,372.9 N, linear static (SOL 101), Subcase 1
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**Material:** AL7075-T6 — σ_yield = 503 MPa, ρ = 2810 kg/m³, SF = 5 → σ_allow = 100.6 MPa
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The rib pattern is generated in 2 sandbox regions. Ribs automatically cluster around bolt holes
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and the plate perimeter based on physics-inspired density field parameters.
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---
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## 3. Mathematical Formulation
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### Objective
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```
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minimize mass_kg(η₀, α, β, γ_stress, R₀, R_edge, s_min, s_max)
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```
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### Constraint
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```
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subject to σ_max ≤ 100.6 MPa (stress only — no displacement constraint)
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```
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### Penalty
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```
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objective_value = mass_kg + penalty
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penalty = 1e4 × ((σ_max / σ_allow) − 1)² if σ_max > σ_allow
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= 0 otherwise
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```
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### Design Variables
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| Variable | Low | High | Units | Description |
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|----------|-----|------|-------|-------------|
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| `eta_0` | 0.0 | 0.4 | — | Baseline density offset |
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| `alpha` | 0.3 | 2.0 | — | Hole influence scale |
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| `beta` | 0.0 | 1.0 | — | Edge influence scale |
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| `gamma_stress` | 0.0 | 1.5 | — | FEA stress feedback gain |
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| `R_0` | 10 | 100 | mm | Base hole influence radius |
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| `R_edge` | 5 | 40 | mm | Edge influence radius |
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2026-02-20 08:00:17 +00:00
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| `s_min` | 15 | 35 | mm | Min cell size (densest) — manufacturing floor: 15 mm |
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| `s_max` | 40 | 60 | mm | Max cell size (sparsest) — lower bound 40 guarantees s_min < s_max |
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2026-02-19 08:00:15 +00:00
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Fixed parameters (manufacturing constraints + math constants): see `optimization_engine/isogrid/study.py`.
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---
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## 4. Optimization Algorithm
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| Property | Value |
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|----------|-------|
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| Algorithm | Optuna TPE (Tree-structured Parzen Estimator) |
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| Sampler seed | 42 |
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| Direction | Minimize |
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| N startup trials | 10 (random exploration before TPE kicks in) |
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| Trial budget | 200 |
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| Storage | SQLite — `3_results/study.db` |
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| Study name | `isogrid_01_v1_tpe` |
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TPE splits past trials at each parameter into "good" (low objective) and "bad" (high objective) groups,
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then samples from the "good" region. With 8 continuous variables and 200 trials this is well within
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TPE's effective range.
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---
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## 5. Result Extraction
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| Quantity | Extractor | Source |
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|----------|-----------|--------|
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| Mass | `extract_part_mass_material(_i.prt)` | `_temp_part_properties.json` written by `solve_simulation.py` journal |
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| Max von Mises | `extract_solid_stress(op2_file, subcase=1)` | OP2 binary from Nastran solve |
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Mass is extracted from the **idealized part** (`_fem2_i.prt`), not estimated by the Brain.
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The NX journal writes a JSON temp file after each solve; the extractor reads it back.
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> ⚠️ NX FEM model uses AL6061 material properties (E≈68.98 GPa, ρ=2.711e-6 kg/mm³).
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> Mass extraction is therefore slightly low (~4% underestimate vs AL7075-T6).
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> Tracked as Gap G-01 in CONTEXT.md.
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---
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## 6. Study Structure
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```
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studies/01_v1_tpe/
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├── README.md ← You are here
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├── STUDY_REPORT.md ← Post-run results (fill after campaign)
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├── check_preflight.py ← Quick validation before running
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├── run_optimization.py ← Main optimization loop
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│
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├── 1_setup/ ← Model files (working copies — modified by NX each trial)
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│ └── model/
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│ ├── ACS_Stack_Main_Plate_Iso_Project.prt
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│ ├── ACS_Stack_Main_Plate_Iso_project_fem2_i.prt ← CRITICAL
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│ ├── ACS_Stack_Main_Plate_Iso_project_fem2.fem
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│ ├── ACS_Stack_Main_Plate_Iso_project_sim2.sim
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│ └── adaptive_isogrid_data/
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│ ├── geometry_sandbox_1.json ← Sandbox boundary + holes (from NX)
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│ ├── geometry_sandbox_2.json
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│ ├── rib_profile_sandbox_1.json ← Written per trial (current)
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│ └── rib_profile_sandbox_2.json
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│
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├── 2_iterations/ ← Per-trial logs (auto-created at runtime)
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│ ├── trial_0001/
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│ │ ├── params.json ← Sampled design variables
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│ │ ├── results.json ← mass, stress, SF, objective
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│ │ ├── rib_profile_sandbox_1.json ← Rib geometry (copy)
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│ │ └── rib_profile_sandbox_2.json
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│ └── trial_NNNN/
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│ └── ...
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│
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└── 3_results/ ← Optimization database + summary outputs
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└── study.db ← Optuna SQLite (created on first run)
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```
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---
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## 7. Quick Start
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```bash
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# Step 1: Verify everything is ready
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C:\Users\antoi\anaconda3\envs\atomizer\python.exe \
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projects/isogrid-dev-plate/studies/01_v1_tpe/check_preflight.py
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# Step 2: Launch
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C:\Users\antoi\anaconda3\envs\atomizer\python.exe \
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projects/isogrid-dev-plate/studies/01_v1_tpe/run_optimization.py
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```
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See [../../playbooks/01_FIRST_RUN.md](../../playbooks/01_FIRST_RUN.md) for full step-by-step including
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monitoring, failure handling, and post-run analysis.
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---
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## 8. Expected Runtime
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| Component | Estimate |
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|-----------|----------|
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| Brain (triangulation + pockets) | ~3–10 s |
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| NX import journal | ~15–30 s |
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| NX remesh + Nastran solve | ~60–90 s |
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| Extraction (mass + stress) | ~1–3 s |
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| **Total per trial** | **~90–120 s** |
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| **200 trials** | **~8–10 hours** |
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Actual per-trial time will vary with mesh complexity (pocket count affects remesh time).
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---
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## 9. Success Criteria
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| Criterion | Target |
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|-----------|--------|
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| Mass reduction vs baseline | > 10% (aspirational: > 20%) |
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| Feasibility rate | > 80% of trials (σ ≤ 100.6 MPa) |
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| Best trial SF | ≥ 5.0 (σ_max ≤ 100.6 MPa) |
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| Convergence | Best mass stable over last 50 trials |
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Baseline solid plate mass: TBD (Gap G-02 — run `extract_part_mass_material` on unmodified model).
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---
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## 10. Results
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> **[Campaign 01 not yet started]**
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>
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> Fill in after run. See [STUDY_REPORT.md](STUDY_REPORT.md) for the result template.
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| Metric | Value |
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|--------|-------|
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| Status | 🔴 Not started |
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| Trials completed | 0 / 200 |
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| Best mass | — |
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| Best trial | — |
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| Best σ_max | — |
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| Best SF | — |
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| Feasibility rate | — |
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| Runtime | — |
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