TrussOrbit: Edge-Orbit Sequence Modeling for Truss Metamaterial Inverse Design

Inverse design of periodic truss metamaterials requires accurate stiffness matching, physically valid generation, and a representation that spans diverse topologies. We present TrussOrbit, a structural language model conditioned on nine effective-stiffness components. Its Edge Orbit Truss Representation (EOTR) encodes node geometry and connectivity through Pmmm symmetry orbits. A unified data pipeline consolidates four public sources into 1.49 million canonical structures with common finite-element labels. We introduce Setwise Marginal Credit Optimization (SMCO) to optimize stiffness matching and binary validity across fixed-size candidate groups through sequence-level marginal credit. On the unified benchmark, TrussOrbit achieves the lowest NMAE, NRMSE, and MAPE among the compared models while covering the full dataset; it generates candidates 6.6 times as fast as the fastest external baseline. Representation and policy-objective ablations show complementary gains: EOTR shortens sequences and reduces invalid generation, while SMCO improves stiffness matching over supervised fine-tuning in both single-candidate and best-of-16 evaluation.