B-dot Finite-Difference Controller

B-dot detumbling controller implemented as a WebAssembly Component guest plugin for the orts simulator. This is the first end-to-end example of the Phase P plugin architecture: a spacecraft attitude controller written in Rust, compiled to a .wasm Component, loaded by the orts host via wasmtime + Pulley interpreter, and driven through the PluginController trait.

How it works

The guest implements the orts:plugin/controller WIT interface:

1. init(config) — accepts a JSON config blob to set the gain, max moment, and sample period
2. update(observation) — receives the spacecraft state + epoch at each sample tick, queries the host’s geomagnetic field model via the host-env.magnetic-field-eci import, computes the finite-difference dB/dt in the body frame, and returns a Command::MagneticMoment(m) to the host
3. The host applies the command to a CommandedMagnetorquer actuator and integrates the next zero-order-hold segment

The control law is the standard finite-difference B-dot:

m = -k · (B_body(t) - B_body(t - Δt)) / Δt

clamped per-axis to ±max_moment. On the first sample, the command is zero (no previous measurement).

Building

# Install cargo-component if not already:
cargo install cargo-component

# Add the WASM target (cargo-component uses wasip1 adapter internally):
rustup target add wasm32-wasip1 --toolchain 1.91.0

# Build the guest component:
cargo +1.91.0 component build --release

The output is target/wasm32-wasip1/release/orts_example_plugin_bdot_finite_diff.wasm (~70 KB).

Running the simulation sweep

From the orts workspace root:

cargo run --example wasm_bdot_simulate --features plugin-wasm --release

This sweeps gain × initial angular velocity (3×3 = 9 conditions) over 600 s of simulated time and writes CSV files here. Each CSV has columns t, omega_x, omega_y, omega_z, omega_mag.

Plotting

cd plugins/bdot-finite-diff
uv run plot.py

Produces two plots:

Gain Sweep

Detumbling rate for different controller gains (k = 1e3, 1e4, 1e5) at each initial angular velocity magnitude.

Initial Angular Velocity Sweep

Detumbling trajectories for different initial tumble rates at each gain setting.

Oracle test

The host-side oracle test (orts/tests/oracle_plugin_wasm_bdot.rs) loads this guest and compares its trajectory against the native orts::attitude::BdotFiniteDiff controller. Both implementations compute the same finite-difference B-dot law but via independent code paths (the guest uses a hand-rolled quaternion rotation and calls host-env.magnetic-field-eci for the magnetic field, while the native controller uses nalgebra and calls TiltedDipole::field_eci directly). The oracle asserts that the final quaternion and angular velocity agree within 1e-12 tolerance.

cargo test -p orts --features plugin-wasm --test oracle_plugin_wasm_bdot

Configuration

The guest accepts a JSON config blob via init:

{
  "gain": 1e4,
  "max_moment": 10.0,
  "sample_period": 1.0
}

All fields are optional (defaults are shown above). The host passes this string at WasmController::new(pre, label, config).