ecto/vcad/assembly-physics

vcad Assembly & Physics Simulation

Create assemblies from parts, connect them with joints, and run physics simulations via the vcad MCP tools.

Assembly Structure

An assembly is defined inside create_cad_document alongside the parts:

{
  "parts": [...],
  "assembly": {
    "instances": [...],
    "joints": [...],
    "ground": "base-1"
  }
}

Instances

Each instance places a part in the assembly:

{
  "id": "arm-1",
  "part": "arm",
  "name": "Lower Arm",
  "position": {"x": 0, "y": 0, "z": 50},
  "rotation": {"x": 0, "y": 0, "z": 0}
}

• id — unique identifier (referenced by joints)
• part — name of a part in the parts array
• position — initial position in mm (optional)
• rotation — initial rotation in degrees (optional)

Joints

Joints connect two instances and define degrees of freedom:

{
  "id": "shoulder",
  "name": "Shoulder Joint",
  "parent": "base-1",
  "child": "arm-1",
  "type": "revolute",
  "axis": "y",
  "parent_anchor": {"x": 0, "y": 0, "z": 50},
  "child_anchor": {"x": 0, "y": 0, "z": 0},
  "limits": [-90, 90],
  "state": 0
}

• parent — instance ID, or null for world/ground
• child — instance ID
• type — joint type (see below)
• axis — "x", "y", "z", or custom {x, y, z} vector
• parent_anchor / child_anchor — connection points in local coordinates
• limits — [min, max] (degrees for revolute, mm for slider)
• state — initial angle or position

Ground

Set "ground" to the instance ID of the fixed/base part. This anchors the kinematic chain.

Joint Types

Physics Simulation Tools

The gym-style tools simulate assembly dynamics using phyz:

Simulation Workflow

create_cad_document (with assembly)
  → create_robot_env (document, end_effector_ids)
    → gym_reset (env_id)
      → gym_step (env_id, action_type, values) [loop]
        → gym_close (env_id)

create_robot_env

{
  "document": "<IR document from create_cad_document>",
  "end_effector_ids": ["gripper-1"],
  "dt": 0.004167,
  "substeps": 4,
  "max_steps": 1000
}

Returns: env_id, num_joints, action_dim, observation_dim.

gym_step

{
  "env_id": "sim_1",
  "action_type": "torque",
  "values": [0.5, -0.3]
}

Action types:

• "torque" — apply torque in Nm to each joint
• "position" — set target position (degrees for revolute, mm for slider)
• "velocity" — set target velocity (deg/s or mm/s)

The values array must have one entry per joint.

Returns:

• observation — joint positions, velocities, end effector poses
• reward — scalar reward signal
• done — true if episode ended (max steps or termination)

gym_reset / gym_observe

Both return the observation object:

{
  "joint_positions": [0.0, 0.0],
  "joint_velocities": [0.0, 0.0],
  "end_effector_poses": [
    {"position": {"x": 0, "y": 0, "z": 100}, "orientation": {"x": 0, "y": 0, "z": 0, "w": 1}}
  ]
}

Complete Example: 2-DOF Robot Arm

{
  "parts": [
    {
      "name": "base",
      "primitive": {"type": "cylinder", "radius": 25, "height": 10},
      "material": "steel"
    },
    {
      "name": "arm",
      "primitive": {"type": "cube", "size": {"x": 10, "y": 10, "z": 80}},
      "material": "aluminum"
    }
  ],
  "assembly": {
    "instances": [
      {"id": "base-1", "part": "base"},
      {"id": "lower-arm", "part": "arm", "position": {"x": 0, "y": 0, "z": 10}},
      {"id": "upper-arm", "part": "arm", "position": {"x": 0, "y": 0, "z": 90}}
    ],
    "joints": [
      {
        "id": "shoulder",
        "parent": "base-1",
        "child": "lower-arm",
        "type": "revolute",
        "axis": "y",
        "parent_anchor": {"x": 0, "y": 0, "z": 10},
        "child_anchor": {"x": 5, "y": 5, "z": 0},
        "limits": [-90, 90]
      },
      {
        "id": "elbow",
        "parent": "lower-arm",
        "child": "upper-arm",
        "type": "revolute",
        "axis": "y",
        "parent_anchor": {"x": 5, "y": 5, "z": 80},
        "child_anchor": {"x": 5, "y": 5, "z": 0},
        "limits": [0, 135]
      }
    ],
    "ground": "base-1"
  }
}

After creating the document, run a simulation:

1. create_robot_env with end_effector_ids: ["upper-arm"]
2. gym_reset to get initial observation
3. gym_step with action_type: "torque" and values per joint
4. Repeat step 3 for your control loop
5. gym_close when done

Tips

• Every assembly needs a ground instance — this is the fixed reference
• Joint anchors are in the instance's local coordinate frame
• Revolute limits are in degrees, slider limits in mm
• State outside limits is clamped automatically
• Use position action type for precise pose control, torque for dynamic simulation
• end_effector_ids must reference valid instance IDs in the assembly
• Forward kinematics propagates transforms from ground through the joint chain