MODULE 12 / DIGITAL WORKSHOP
Bring BDX-R into simulation
Connect the CAD and robot-description lessons to the full BDX-R model. Inspect joints, mass, collision geometry and runtime assumptions before asking it to walk.
By the end, you can…
- Obtain an exact source revision
- Inspect model counts, units and joint mappings
- Explain why raw XML and a complete runtime behave differently
Your first physics laboratory
Robot descriptions and frames
Keep provenance attached to the robot
A model is useful when you can say where it came from. The course baseline uses the Deploy repository at commit d4f3671a44f8ca2c02e31b23607aa8d7fdc076a3. A commit identifies source content. It does not prove that every exported controller was trained with exactly those same files. Save the commit, the XML path and later the policy hash with your result.
Cloning a repository creates a local Git working copy. Checking out the named commit deliberately holds it at one historical state. Git may call this a detached HEAD; that is expected for inspection. Keep experiments in separate scripts and avoid editing the reference files while establishing the baseline.
Count what the model controls
The selected bdxr.xml has fourteen actuators: five joints per leg plus neck pitch and three head joints. Its modeled total mass is 17.749 kg in the audited snapshot. That mass is an input to the simulation, not a measured weight of your future build. The complete design also has ears, while this runtime map does not provide the full sixteen-actuator configuration.
A floating base introduces positions and velocities in addition to actuated joints. A free body uses a position and quaternion, so its number of position coordinates differs from its velocity coordinates. Do not assume the first fourteen entries in qpos are fourteen motor angles. Use the joint names and their addresses. This is the kind of small indexing mistake that can make a plausible-looking program command the wrong motion.
Appearance and contact are different jobs
A detailed mesh can make the robot recognizable. Contact geometry determines where collisions are solved. A hidden or simplified shape may therefore affect balance even when the decorative surface looks correct. Use the viewer’s geometry/contact controls to inspect both layers, and examine XML contact flags rather than assuming every visible triangle participates in contact.
Imagine giving a shoe a decorative ridge. If that ridge also becomes a collision surface, it may change the support pattern. Conversely, a beautifully modeled sole with contact disabled cannot support the robot. Collision simplification is a modeling decision to document and evaluate, not merely a way to make graphics faster.
Startup is part of the experiment
The runtime sets an initial base height, joint positions, standing gains and a two-second settling interval before policy gains take over. Starting too low can place geometry in contact incorrectly. A fall during settling tells a different story from a fall after many policy decisions.
The Deploy simulation backend also transforms actuator definitions into position-control behavior using gains from policy metadata. Opening the XML in the standalone viewer shows the model, but it does not reproduce that transformation or the learned policy. The raw XML inspector reports a 0.002-second time step; the audited walking backend explicitly overrides it to 0.005 seconds. Record the effective runtime setting, not only the static file. Begin with inspection. The walking lesson later adds the entire control loop in a dedicated simulation-only adapter.
WORDS YOU NOW KNOW
- Floating base
- A body free to translate and rotate rather than being attached to the world.
- Collision geometry
- Shapes used to decide and solve physical contact.
- Provenance
- The source, revision and settings that identify how an artifact was made.
YOUR CALL
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Primary references for this module. Use the version and hardware revision named in the lesson.
Pinned Deploy simulation backendMuJoCo modeling guideDownload model inspector