Software-in-the-Loop (SIL) & Hardware-in-the-Loop (HIL) with ModelScript

This guide documents ModelScript's unified SIL and HIL architecture based on eFMI-style deterministic code generation, WebAssembly, FMI Layered Standards (FMI-LS-BUS), and browser-native Soft HIL.

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Architecture Overview

                      Modelica Plant Model (.mo)
                                 │
                                 ▼
                 DAEBuilder Linear Memory Arena
                   ├── Fixed-step RK4 solver
                   └── Neural ROM Surrogates (rom-trainer)
                                 │
       ┌─────────────────────────┴─────────────────────────┐
       ▼                                                   ▼
1. BROWSER SOFT HIL                                 2. HARD REAL-TIME HIL
(Chrome / Edge Web APIs)                           (Linux RT-PREEMPT / WAMR)
- AudioWorklet real-time clock                      - SCHED_FIFO Priority 99
- WebSerial (SLCAN / COBS / JSON)                   - mlockall (Zero page faults)
- Dedicated Web Worker sandbox                      - Linux SocketCAN (can0)
- 10 Hz – 200 Hz control loops                      - > 1 kHz, < 50 µs jitter
- Testing: Arduino, STM32, ESP32, USB-CAN           - Testing: Production ECUs, Inverters

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1. FMI-LS-BUS (Layered Standard for Network Communication)

ModelScript implements the Modelica Association FMI-LS-BUS standard:

• FmiLsBusManifest: Describes CAN, CAN-FD, LIN, and Ethernet buses and frames.
• generateFmiLsBusXml: Generates standard fmi-ls-bus.xml bundled inside the FMU archive.
• FmiLsBusCodec: High-performance bit-level encoder and decoder supporting Intel (little-endian) and Motorola (big-endian) bit alignments, linear scaling (physical = raw * factor + offset), signed/unsigned integers, and float32 data types.

Example: Defining CAN Frames in TypeScript

import { FmiLsBusFrame, FmiLsBusCodec, generateFmiLsBusXml } from "@modelscript/exchange";

const engineFrame: FmiLsBusFrame = {
  name: "EngineStatus",
  id: 0x120, // CAN ID 288
  length: 8,
  cycleTime: 0.01, // 10 ms (100 Hz)
  signals: [
    {
      name: "engineSpeed",
      valueReference: 1,
      startBit: 0,
      bitLength: 16,
      byteOrder: "littleEndian",
      factor: 0.25, // 1 LSB = 0.25 rpm
      offset: 0,
      unit: "rpm",
    },
    {
      name: "throttlePosition",
      valueReference: 2,
      startBit: 16,
      bitLength: 8,
      byteOrder: "littleEndian",
      factor: 0.392, // 0-100%
      offset: 0,
      unit: "%",
    },
  ],
};

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2. In-Browser "Soft HIL" (WebSerial, WebUSB, WebHID)

For desktop and browser-based testing, ModelScript provides the WebSerialParticipant (packages/exchange/src/cosim/participants/web-hardware.ts):

Supported Hardware Adapters & Microcontrollers

• USB-to-CAN Adapters (SLCAN): CANable, candleLight, USBtin, Zubax Babel.
• Microcontrollers (USB-UART / CDC): STM32 Nucleo, ESP32, Arduino, Teensy, Raspberry Pi Pico.
• Framing Protocols:
  • slcan: Standard ASCII CAN protocol (t12080102030405060708\r).
  • cobs: Consistent Overhead Byte Stuffing for binary frames.
  • json: Line-delimited JSON objects ({"rpm": 3200, "throttle": 45}\n).

Using WebSerialParticipant in Co-Simulation

import { CoSimOrchestrator, CoSimSession, WebSerialParticipant } from "@modelscript/exchange";

// Create hardware participant
const hwParticipant = new WebSerialParticipant({
  id: "ecu_serial",
  baudRate: 115200,
  protocol: "slcan",
  busFrames: [engineFrame],
});

// Add to co-simulation session
session.addParticipant(hwParticipant);

Unthrottled Browser Pacing via AudioWorkletClock

Browsers throttle setInterval to 1 Hz when tabs are backgrounded. ModelScript's AudioWorkletClock (packages/exchange/src/cosim/audio-worklet-clock.ts) leverages the high-priority OS audio thread (44.1 kHz) to drive the simulation loop, so it is not subject to background-tab timer throttling.

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3. Hard Real-Time HIL on Linux RT-PREEMPT

For hard real-time execution with physical ECUs over CAN:

1. Compile Model to Standalone C / WASM: ModelScript's C generator produces a static memory model with an embedded zero-allocation integrate_rk4 solver.
2. Compile Neural ROM Surrogates to C: Use exportROMToC(trainedRom) from @modelscript/simulate to generate zero-dependency static C arrays for surrogate evaluation in < 20 µs.
3. Deploy with Standalone Runner: Use hil-runner-template.c (packages/exchange/src/fmu/hil-runner-template.c):
   • Compiles with gcc -O3 hil-runner-template.c model.c -lm -o model_hil.
   • Runs with sudo ./model_hil can0 under SCHED_FIFO priority 80 with mlockall.

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4. Shared-Memory IPC & Zero-Overhead Local Agent Proxy

For multi-rate co-simulation between compiled FMUs, ModelScript provides a high-throughput Shared-Memory IPC Protocol (packages/exchange/src/cosim/ipc/fmu-shm-protocol.ts):

• Zero-Copy Memory-Mapped Ring Buffers: Uses POSIX shared memory (/dev/shm on Linux) and named shared memory segments on Windows with cache-line-aligned (64-byte) atomic variables.
• Spinlock & Futex Synchronization: State exchanges achieve > 500,000 steps/sec with sub-2µs latency, bypassing TCP/loopback network stack overheads.
• Local Agent Proxy (local-agent-proxy.ts): Couples proprietary or legacy simulation executables into ModelScript's CoSimOrchestrator via shared-memory participants with dead-reckoning extrapolation and interface Jacobian negotiation.