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Manipulators and frames

A manipulator is the moving arm, and here also the rail it rides on. An end-effector is what touches the product (a parallel-jaw gripper). A workcell is the fixed surroundings: shelf, gravity-fed lanes, stock tray, cameras. A pose is position plus orientation. A frame is a named coordinate system bolted to part of that world (tool0, shelf, lane_02).

Software never says “move to (1.2, 0.3, 0.8)” without also saying in which frame. Think of frames as nested local origins in a 3D scene graph.

Why it matters here​

combeanie restocks cans and bottles from a rear stock tray into gravity-fed refrigerated shelf lanes. The robot is a rail + 6-DoF arm + parallel-jaw gripper. Planning goals target semantic datums (grasp_center, lane frames), not arbitrary camera pixels. Wrong frames look like successful plans that collide or miss the product.

This project​

PiecePath
Assembled robotros_ws/src/restocker_description/urdf/robot.xacro
Rail…/urdf/rail.xacro (rail_base → carriage)
Arm…/urdf/arm.xacro (official ur_description UR10e meshes and chain to tool0)
Gripper…/urdf/gripper.xacro (grasp_center, fingers)
Beanie…/urdf/beanie.xacro (decorative shoulder_link cap, visual only)
Workcell…/urdf/workcell.xacro (shelf, lanes, overhead camera)
Geometry source of truth…/config/workcell_geometry.yaml, gripper_geometry.yaml
MoveIt groupsrestocker_moveit_config/config/restocker.srdf (manipulator, arm, gripper)

Teach these names: world, rail_base, carriage, base_link, tool0, grasp_center, lane_01…lane_06, *_camera_optical_frame.

Coordinate frame tree · world (metres, REP-103)

Both branches start at world. Arrows show ancestry; grouped siblings do not form a chain.

Robot branch
rail_base → carriage → base_linkprismatic rail + UR10e root
shoulder_link → … → wrist_3_link → tool0official ur_description chain
tool0 children: gripper · wrist_camera_linkgripper → grasp_center / fingers; camera → optical frame
Shelf branch
shelfstatic world → shelf transform
Separate children of shelf
lane_01 … lane_06six sibling frames · +Y downhill
roller_bedfixed shelf child
overhead_camera_linkfixed shelf child
overhead_camera_optical_framechild of camera link · image stamps
RootStructureKinematics / sensorsEnd-effector
world
`-- rail_base
    `-- carriage          (rail_joint, prismatic +X)
        `-- base_link
            `-- … shoulder_pan_joint → wrist_3_joint …
                `-- flange → tool0 → grasp_center / wrist camera

world
`-- shelf → lane_01 … lane_06
         → overhead_camera_optical_frame

Grasp planning targets grasp_center; MoveIt solves to tool0. Units are metres and radians (REP-103). Authoritative object poses live in world; transform observations at the acquisition stamp, never “latest,” and never invent identity transforms.

The UR10e is selected because its 1.30 m reach covers the rail-to-stock and pre-grasp envelope; the smaller UR3e and UR5e do not without moving the cell. The standard upstream joint names are a cross-stack contract; there are no joint_1…joint_6 compatibility aliases.

References​