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Milan
Milan

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I Built an Open-Source Simulink Alternative That Runs in Your Browser

#python #opensource #webassembly #svelte

What if you could build and simulate block-diagram models in your browser — for free, with no installation?

That's what I've been working on for the last 27 months.

The Problem

System simulation — control systems, vehicle dynamics, signal processing — is dominated by MATLAB/Simulink. It's excellent software, but:

  • Expensive: €2,000-15,000 per seat per year
  • Closed: Proprietary file formats, vendor lock-in
  • Heavy: Complex installation, license servers, IT involvement
  • Isolated: Awkward Python integration, poor CI/CD support

There's no real open-source equivalent at the system simulation level.

The Solution: PathSim + PathView

PathSim is a pure-Python framework for dynamical system simulation:

pip install pathsim
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from pathsim import Simulation, Connection
from pathsim.blocks import Integrator, Amplifier, Adder, Scope

# Build a simple feedback system
integrator = Integrator()
gain = Amplifier(-1.5)
summer = Adder()
scope = Scope()

sim = Simulation(
    blocks=[integrator, gain, summer, scope],
    connections=[
        Connection(summer, integrator),
        Connection(integrator, gain),
        Connection(integrator, scope),
        Connection(gain, summer[1])
    ]
)

sim.run(duration=10.0)
scope.plot()
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PathView is a visual block-diagram editor that runs entirely in your browser via Pyodide/WebAssembly:

Try it now: view.pathsim.org

No signup. No server. Your data never leaves your machine.

Key Features

  • 18+ numerical solvers — from basic Euler to implicit BDF for stiff systems
  • Hierarchical subsystems — compose complex models from reusable components
  • Event handling — hybrid continuous/discrete systems (check the bouncing ball)
  • Algebraic loop resolution — automatic detection and solving
  • Browser execution — full simulation in WebAssembly (Pyodide), no backend needed

Real-World Adoption

  • JSBSim (NASA's flight dynamics simulator) adopted PathSim for system-level modeling
  • MIT Plasma Science & Fusion Center uses PathSim for nuclear fusion fuel-cycle simulation
  • 300+ GitHub stars, 3,000+ monthly downloads

Python in WebAssembly vs. Server Side

Running a simulation framework in the browser via Pyodide is interesting. You might assume that a dedicated backend serever hosted somewhere on the web wins over Pyodide. But it turns out, when you have a decently performant CPU, the 30% loss of Pyodide over antive Python still places you way ahead of off the shelf server performance.

What's Next

  • Model library — 50+ curated, validated simulation models
  • Code generation — C code from PathSim models (early stage)

Try It

I'd love to hear your feedback. What simulation problems are you working on? What would make PathSim useful for your work?

I'm Milan Rother, a simulation engineer. If you need help with simulation — from Simulink migration to custom model development — reach out at milanrother.com.

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