Building a system using YAML

This tutorial explains how to define mechanical systems using YAML configuration files. YAML is the recommended approach for complex models with many components, since it separates geometry data from simulation code.

Overview

The YAML workflow has three steps:

1. Write a YAML file — define materials, points, segments, and other components in a structured text file
2. Load with load_sys_struct_from_yaml — parses the YAML and calls the same Julia constructors used in the Julia tutorial
3. Compile and simulate — same as the Julia path: SymbolicAWEModel → init! → next_step!

The YAML loader does as little as possible: it parses YAML, converts string enum values, resolves material references, and calls constructors. All defaults and derived calculations happen in the component constructors.

YAML file structure

A YAML file can contain any of these top-level blocks:

Each block uses a headers + data format:

points:
  headers: [name, pos_cad, type, extra_mass]
  data:
    - [anchor, [0, 0, 0], STATIC, 0.0]
    - [mass, [0, 0, -50], DYNAMIC, 1.0]

The headers row defines column names. Each data row is a list of values matching those headers. Missing trailing columns default to nothing.

Alternatively, you can use a dict format where each row is a dictionary:

points:
  data:
    - {name: anchor, pos_cad: [0, 0, 0], type: STATIC}
    - {name: mass, pos_cad: [0, 0, -50], type: DYNAMIC, extra_mass: 1.0}

Minimal example

Here is a complete YAML file for a simple two-point tether:

# simple_tether.yaml

points:
  headers: [idx, pos_cad, type, wing_idx, transform_idx, extra_mass]
  data:
    - [1, [0, 0, 0], STATIC, nothing, 1, 0.0]
    - [2, [0, 0, -50], DYNAMIC, nothing, 1, 1.0]

segments:
  headers: [idx, point_i, point_j, type, l0, diameter_mm,
            unit_stiffness, unit_damping, compression_frac]
  data:
    - [1, 1, 2, BRIDLE, 50.0, 5.0, 100000, 50.0, 0.001]

transforms:
  headers: [idx, elevation, azimuth, heading,
            base_pos, base_point_idx, rot_point_idx]
  data:
    - [1, -80, 0, 0, [0, 0, 50], 1, 2]

Load and simulate:

using SymbolicAWEModels

set = Settings("system.yaml")
set.v_wind = 0.0

sys = load_sys_struct_from_yaml("simple_tether.yaml";
    system_name="simple_tether", set=set)
sam = SymbolicAWEModel(set, sys)
init!(sam)

for _ in 1:100
    next_step!(sam)
end

Materials and references

Materials allow you to define physical properties once and reference them across segments. When a segment's unit_stiffness column contains a string instead of a number, it is treated as a material reference.

materials:
  headers: [name, youngs_modulus, density, damping_per_stiffness]
  data:
    - [dyneema, 55e9, 724, 0.00077]

segments:
  headers: [idx, point_i, point_j, type, l0, diameter_mm,
            unit_stiffness, unit_damping, compression_frac]
  data:
    # 'dyneema' in unit_stiffness triggers material lookup
    - [1, 1, 2, BRIDLE, 5.0, 5.0, dyneema, nothing, 0.01]
    # Explicit stiffness (no material lookup)
    - [2, 2, 3, BRIDLE, 5.0, 1.0, 100000, 50.0, 0.01]

When a material is referenced, derived properties are calculated automatically:

• unit_stiffness = youngs_modulus * pi * (diameter_mm/2000)^2
• unit_damping = damping_per_stiffness * unit_stiffness

Component reference

Points

points:
  headers: [idx, pos_cad, type, wing_idx, transform_idx,
            extra_mass, body_frame_damping, world_frame_damping,
            area, drag_coeff]

Segments

Two formats are supported:

With explicit stiffness:

segments:
  headers: [idx, point_i, point_j, type, l0, diameter_mm,
            unit_stiffness, unit_damping, compression_frac]
  data:
    - [1, 1, 2, BRIDLE, 5.0, 5.0, 100000, 50.0, 0.01]

With material reference:

segments:
  headers: [idx, point_i, point_j, type, l0, diameter_mm,
            unit_stiffness, unit_damping, compression_frac]
  data:
    - [1, 1, 2, BRIDLE, 5.0, 5.0, dyneema, nothing, 0.01]

Pulleys

pulleys:
  headers: [idx, segment_i, segment_j, type]
  data:
    - [1, 3, 4, DYNAMIC]

Tethers

tethers:
  headers: [idx, segment_idxs, winch_point_idx]
  data:
    - [1, [1, 2, 3], 1]

Winches

winches:
  headers: [idx, tether_idxs]
  data:
    - [1, [1]]

Transforms

transforms:
  headers: [idx, elevation, azimuth, heading,
            base_pos, base_point_idx, rot_point_idx]
  data:
    - [1, -80, 0, 0, [0, 0, 50], 1, 2]

Loading workflow

The full loading workflow for a model with aerodynamics:

using SymbolicAWEModels, VortexStepMethod

set_data_path("data/2plate_kite")
set = Settings("system.yaml")
vsm_set = VortexStepMethod.VSMSettings(
    joinpath(get_data_path(), "vsm_settings.yaml"))

struc_yaml = joinpath(get_data_path(),
    "quat_struc_geometry.yaml")
sys = load_sys_struct_from_yaml(struc_yaml;
    system_name="2plate_kite",
    set=set,
    vsm_set=vsm_set)

sam = SymbolicAWEModel(set, sys)
init!(sam)

After compilation, a cache file (model_*.bin) is saved. Subsequent loads skip the expensive symbolic compilation and deserialize the cached model instead. Force a rebuild with remake_cache=true.

YAML vs Julia

Both paths produce the same SystemStructure type and are equally capable. They can be freely mixed — for example, load a YAML model and then modify component fields in Julia before simulation.