Sail Wing FMU

A Functional Mock-up Unit (FMU) implementation for a rigid wing sail, compatible with FMI 3.0 standard. This FMU models the aerodynamic behavior and actuator dynamics of a sail wing and is designed to be integrated into a larger sail wing control system.

Features

• FMI 3.0 Compliant: Co-simulation FMU with standard FMI interface
• Aerodynamic Modeling: 2D rigid wing model with bilinear interpolated Cx/Cy coefficients
• Actuator Dynamics: Rate-limited boom and flap control with position limits
• Cross-Platform: Supports Linux and Windows (x86_64)
• Rust Implementation: Using fmu_from_struct macro for automatic FMU generation
• Comprehensive Testing: Unit tests and visualization tools for force and coefficient analysis

Model Parameters, Inputs, and Outputs

Parameters

• wing_length_m (m): Wing span length
• wing_chord_m (m): Wing chord length
• boom_speed_deg_per_s (deg/s): Maximum boom rotation rate
• flap_speed_deg_per_s (deg/s): Maximum flap deflection rate

Inputs

• apparent_wind_speed_ms (m/s): Wind speed relative to the wing
• apparent_wind_angle_rad (rad): Wind direction relative to the wing
• boom_angle_order_rad (rad): Commanded boom angle
• flap_angle_order_rad (rad): Commanded flap deflection angle

Outputs

• boom_angle_rad (rad): Actual boom angle
• flap_angle_rad (rad): Actual flap deflection angle
• force_x_N (N): Aerodynamic force in X direction (body frame)
• force_y_N (N): Aerodynamic force in Y direction (body frame)
• force_z_N (N): Aerodynamic force in Z direction (body frame, always 0)

Building

Standard Build

cargo build --release

FMU Generation

Generate cross-compiled FMU for Linux and Windows:

./build_fmu.sh

Output: dist/SailWing.fmu

Testing

# Run all tests
cargo test

# Run specific test
cargo test test_sail_wing_do_step

# Generate aerodynamic coefficient plots (Cx/Cy vs. angle of attack)
cargo test test_aero_database_raw_plots -- --nocapture

# Generate force analysis plots (Forces vs. boom angle for different flap angles)
cargo test test_force_analysis_plots -- --nocapture

Test plots are saved to tests/output/:

• raw_cx_coefficient_plot.png: Cx coefficients vs. angle of attack
• raw_cy_coefficient_plot.png: Cy coefficients vs. angle of attack
• force_fx_analysis.png: X-forces vs. boom angle
• force_fy_analysis.png: Y-forces vs. boom angle

Python Simulation

A simple Python test script is available for testing the generated FMU:

# Run simple simulation test (requires fmpy)
python tests/simple_simulation.py

Model Physics

Aerodynamic Database

The aerodynamic coefficients are hardcoded in src/aerodatabase.rs with data for:

• Flap angles: 0°, 10°, 20°, 30°
• Angle of attack: 0° to 360° (varying resolution)
• Coefficients: Cx (chord-aligned) and Cy (perpendicular to chord)

The database uses bilinear interpolation for intermediate values with proper wrapping at 360°/0°.

Coordinate Systems

• Wing coordinates: X along chord, Y perpendicular to chord
• Body coordinates: X forward, Y starboard
• Forces are transformed from wing to body frame using the boom angle

Actuator Dynamics

• Boom: Unlimited rotation (wraps at 360°), rate-limited by boom_speed_deg_per_s
• Flap: Limited to ±30° (max defined in aerodynamic database), rate-limited by flap_speed_deg_per_s
• Both actuators use simple rate limiting with position clamping

Project Structure

.
├── src/
│   ├── lib.rs              # Main FMU struct and physics implementation
│   └── aerodatabase.rs     # Aerodynamic coefficient database and interpolation
├── tests/
│   ├── integration_tests.rs    # Comprehensive test suite
│   └── simple_simulation.py    # Python FMU validation script
├── build_fmu.sh            # Cross-compilation script for FMU generation
├── Cargo.toml              # Rust dependencies and build configuration
└── README.md               # This file

Dependencies

Runtime

• fmu_from_struct: FMU macro generation for Rust structs
• serde, serde_json: Serialization support (for future config file support)

Development

• more-asserts: Enhanced test assertions
• plotters: Plot generation for analysis and visualization
• cross: Cross-compilation for Linux and Windows targets

License

See LICENSE.txt for details.