Simulator

The [sim] extra ships a MuJoCo-backed physics simulator, so you can develop robot programs without a hub on the desk:

$ pipx install 'openbricks[sim]'
$ openbricks sim run examples/full_robot.py --viewer

The sim runs the same script you’d push to the hub — a driver shim maps the openbricks API onto simulated motors and sensors, so ST3032Motor, DriveBase, color sensors, and distance sensors behave like their hardware counterparts.

Commands

$ openbricks sim [robot.assembly.json] [--bricks more.json] [--bin PATH] [--no-download]

Launches the sim, the native desktop application: the Assembly Workbench with LEGO Technic bricks in exact geometry, your own STL parts, and the robot as the top component. The first run downloads the signed build for your platform (about 15 MB) from the release that matches the installed version into ~/.cache/openbricks/sim; it is checked against the same project key that signs firmware images before it runs. OPENBRICKS_SIM_BIN points at a build of your own (cargo build --release in tools/sim). openbricks sim app is the explicit form.

$ openbricks sim workbench [robot.assembly.json] [--bricks more.json] [--port N] [--no-browser]

The same workbench as a page in your browser, for a machine without the native build.

$ openbricks sim preview [--world WORLD] [--x X] [--y Y] [--headless] [--duration S] [--seed N]

Loads the named world (an alias or a path to an MJCF file), splices in the default chassis, and opens the MuJoCo viewer so you can inspect the scene. --headless steps the physics for --duration seconds without opening a window — useful as a smoke test.

$ openbricks sim run SCRIPT [--world WORLD] [--chassis FILE] [--x X] [--y Y] [--yaw DEG] [--viewer] [--no-shim] [--seed N]

Loads the world plus the chassis and executes SCRIPT against the simulated robot. --viewer opens the interactive MuJoCo window; without it the sim runs headless (CI-friendly). --seed makes randomized worlds reproducible.

Run openbricks sim --help for the full, always-current option list.

The sim

openbricks sim opens a window with two tabs. Workbench is the editor described in the next section: the library on the left (your components, the LEGO Technic set, other bricks), the 3D view in the middle, the contents of the component you are editing and the inspector on the right. Drag in the view to orbit, right-drag to pan, scroll to zoom, F to fit; drag a brick to move it on the ground plane in 8 mm steps (shift lifts it), R turns it 90°, S snaps it into the nearest hole, arrows nudge, Delete removes, ⌘Z undoes. Double-click a component to edit its definition in place; every use follows. Open and save robot.assembly.json from the toolbar.

Simulate runs your program on a map with the chassis you assembled. Pick the map (any built-in world), the chassis (a robot.assembly.json, or the build open in the Workbench tab once it is saved) and the program (main.py), then Run. Pause, Resume and Stop do what they say; the speed slider runs the physics slower or faster than wall time; “follow the robot” keeps the camera on the chassis; the program’s prints and errors appear in the log panel below the view. Under the hood the sim starts the MuJoCo runtime as a child process — the same runtime, driver shim and C cores openbricks sim run uses — and draws the run from the poses it streams, with every brick of the chassis in its exact geometry. The chassis is built from the assembly: the wheel, caster and sensor roles place the physics skeleton, the brick-by-brick mass properties become the body’s inertia, and each brick rides along as a visual geom.

The Assembly Workbench

The sim and openbricks sim workbench (the same editor as a page in the browser) read and write one file. The editor is a 3D view of the robot as a tree of components:

Bricks are recorded once, with their geometry, mass and provenance (measured, datasheet, vendor or placeholder). The library that ships in the wheel holds a curated set of popular LEGO Technic parts converted from the LDraw parts library (CC BY 2.0 / 4.0): beams in every common length, bent and L beams, frames, Technic bricks and plates, pins, axles, bushes, connectors, gears, a few rims and tyres, and fairing panels, with BrickLink catalogue weights where known. Servos, boards and wheels are recorded as boxes, cylinders and spheres, and any part you have as a mesh comes in through Import a part from an STL file (binary or ASCII; mm, cm, inch or m; a weighed mass or a density such as PLA 1.24 g/cm³).
Components are lists of bricks and other components, each placed by a position and a roll / pitch / yaw. Drag bricks from the library into the view, move and rotate them with the gizmos, select what you built and Group it: the new component joins the library and can be dropped anywhere, as many times as you like. Double-click an instance to edit its definition in place; every use follows.
Connections. Pins, axles and studs are real features of the LDraw parts, and 4.8 mm bores are recognised as pin holes on every mesh, imported STL files included. Let go of a part near a hole and it snaps: the pin axis aligns to the hole, a pin half centres in its module, an axle keeps its position along the hole. The inspector lists what each part is mated to.
Mass properties are never typed in above the brick level. Volume, centre of mass and the inertia tensor of every LDraw and STL part come from its closed mesh, so a recorded weight becomes a full inertia tensor; components and the robot roll their children up with the parallel-axis theorem. Weight divided by exact volume is shown as a density on every part, which catches a wrong weight or a wrong part at a glance (ABS is about 1.05 g/cm³).
Roles name the parts the simulator binds: the two drive wheels, the caster, the reflectance arrays, the colour sensor, the range sensor and the IMU. From them the page derives the flat ChassisSpec fields (what the simulator receives) with the axle midpoint as the origin, so a build can be run today with openbricks sim run --chassis.

The file the editor reads and writes, robot.assembly.json, stores recorded facts only: bricks, poses, roles, spawn pose. Everything computed is recomputed on load. Open one with openbricks sim robot.assembly.json (or openbricks sim workbench robot.assembly.json in the browser, which also keeps your last draft between visits).

The brick library

$ openbricks bricks fetch [--dest DIR] [--force]
$ openbricks bricks convert NUMBER [NUMBER ...] [--out FILE] [--weights FILE] [--ldraw DIR]
$ openbricks sim workbench --bricks FILE

The wheel ships the curated Technic set; the whole LDraw library (every LEGO part ever catalogued, 145 MB to download, about 600 MB unpacked) is one command away. bricks fetch unpacks it into ~/.cache/openbricks/ldraw (or $OPENBRICKS_LDRAW_DIR), and bricks convert turns any part numbers — the LEGO design ids printed on the parts, 3648 for the 24-tooth gear — into a bundle file that openbricks sim --bricks (and openbricks sim workbench --bricks) adds to the library. Converted parts without a weight carry a volume estimate at 1.05 g/cm³ and are flagged until you weigh them; pass --weights with a JSON of {"3648": {"g": 1.62}} to record real ones.

LEGO® and Technic are trademarks of the LEGO Group, which does not sponsor or endorse openbricks. The geometry is the LDraw community’s work; the bundle carries its attribution.

Describing your robot

The default chassis is a 60 mm-wheel, 150 mm-axle box with every down-facing sensor 60 mm ahead of the axle. A real robot differs, and those differences decide whether a mission script’s numbers work: --chassis FILE loads a JSON object of ChassisSpec fields (metres, kilograms, degrees) that describe the robot the script was written for. Fields not given keep the defaults.

{
  "wheel_radius": 0.0432,   "axle_length": 0.135,
  "body_length": 0.16,      "body_width": 0.12,
  "line_sensor_x": 0.06,
  "line_sensor_2_x": -0.03,  "line_sensor_2_y": 0.0,
  "color_sensor_x": 0.06,   "color_sensor_y": 0.184,
  "pos_x": -0.547,          "pos_y": -0.15,        "yaw_deg": 90
}

wheel_radius / axle_length size the chassis at load time. The DriveBase(wheel_diameter_mm=…, axle_track_mm=…) in the script resizes it again at adoption, so the script’s geometry always wins — set them here so a preview shows the same robot.
line_sensor_x places the first reflectance-array site (chassis_line) ahead of the axle; line_sensor_2_x / _y place the second (chassis_line2, default 30 mm behind the axle on the centre line, the same height). Reflectance arrays bind these sites in construction order within one run: the first QTRArray / QTRLineSensor / QTRChannel the script constructs reads chassis_line, the second reads chassis_line2, and a third raises RuntimeError (two sites is the chassis’s limit). The counter resets when the shim is installed for a run, so every sim run starts with both sites free. color_sensor_x / _y / _z place the centre colour camera (chassis_cam_down, the no-mux TCS34725) in the chassis frame (the floor is at -(wheel_radius + 0.005)); color_sensor_yaw / _pitch aim it (default straight down; a sensor on the robot’s left flank reading bricks beside the line is yaw 90, pitch 0 at brick height); color_sensor_fov is the cone it integrates (degrees, 0 = one ray) and color_sensor_range how far it sees. The left/right down pair rides 18 mm either side of (color_sensor_x, color_sensor_y).
pos_x / pos_y / yaw_deg are the spawn pose; --x / --y / --yaw on the command line override them one at a time. yaw_deg is counter-clockwise from +X seen from above (0 = facing +X).

What the shim simulates

Firmware class	Sim binding
`ST3032Motor` / `ST3215Motor`	The first two servo ids become the chassis wheels, the third and fourth kinematic task shafts (a gripper motor that turns but pushes nothing). A `DriveBase` always gets the physical wheels for the pair it adopts, whatever order the script constructed its motors in, and re-constructing a motor for a servo id yields the same motor — both firmware rules.
`DriveBase`	The firmware engine over an emulated `st_bus`; `use_gyro(True)` reads the chassis’s true yaw.
`ICM45686` / `BNO055`	Ground-truth chassis heading; the ICM’s bias estimator reports calibrated at once.
`TCS34725`	The firmware driver class over a synthesised raw read: the centre camera (no mux) or the left/right pair (mux channels 1 / 0) casts along its own axis — optionally a cone, with a range — and the first geom hit (a mat texel, a LEGO brick’s material) gives the reflectance; `rgb()` / `ambient()` are the driver’s channel-over-clear arithmetic, so white reads about (85, 85, 85) and a blue brick has the largest `b`, as on the robot.
`QTRLineSensor` / `QTRArray` / `QTRChannel`	The firmware driver over a reflectance model: one element per array position (`QTRLineSensor(channels=8)` gives the eight-channel front layout, exactly as on the hub), spread left-to-right from the site the array bound at construction — the first array a run constructs reads `chassis_line`, the second `chassis_line2`, a third raises `RuntimeError` — each element averaging the floor over a 3 mm spot so an edge reads as a gradient, which is what makes `50 - reading[i].ambient()` proportional. `load_calibration("/qtr_front.cal")` and `calibrate()` need no file — the sim’s reflectance is born normalised.
Distance sensors	A forward ray from the `chassis_dist` site.

Nothing above has a load: task motors don’t grip, and a prop is only pushed when the chassis body drives into it.

Notes

The sim needs the [sim] extra (mujoco, numpy). Without it, openbricks sim … prints an install hint instead of crashing.
The wheel carries the firmware package (openbricks.drivers.*, openbricks.parameters, …) since 3.6.0, so a plain pipx install 'openbricks[sim]' runs hub-style scripts; earlier releases needed a repo checkout for that.
Firmware-only users never need the simulator — it’s strictly host-side tooling.