Magpie Gripper

Design and manufacture of the MAGPIE hand — version 2

A 3D-printed parallel-jaw gripper with a depth camera in its palm, built from two smart servos and about $455 of off-the-shelf parts.

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Two AX-12A servos drive a four-bar linkage — crank and rocker — that keeps the fingers parallel through their whole travel, an OpenRB-150 board controls them, and an Intel RealSense D405 looks out from the palm. Because the servos are current-limited, the gripper can be told how hard to squeeze; because the camera is between the fingers rather than on the wrist, it can still see the object at the moment of contact.

This repository is the hardware: models, bill of materials, a photographed step-by-step build, and the record of what changed from version 1. The software that drives it lives in correlllab/MAGPIE.

Version 2 is a simplification. Fewer parts, fewer fasteners, and less to get wrong: the three-piece finger became one piece, the camera moved from slots to holes so it lands in the same place every time, the controller's standoffs are printed into the base plate, and the two-piece camera cover became one part that snaps on. It weighs 390 g on the bench scale, against 414 g for the hand in the paper. What changed, and why →

Actuation	2 × Dynamixel AX-12A, four-bar linkage per finger
Control	OpenRB-150, one USB-C cable
Sensing	Intel RealSense D405 in the palm
Mass	≈ 390 g (weighed)
Parts cost	≈ $455, of which the camera is $272
Printed	9 pieces, ≈ 140 cm³, PLA at 30 % gyroid infill
CAD	Onshape, live document · STLs in CAD/

Build one

1 · Print. Everything in CAD/, PLA, 30 % gyroid infill with 2 mm borders and organic supports. The rocker and the crank are the parts that break, so give them at least 50 % infill.

	Part	Qty	Size	File
	Base plate	2	133 × 83 × 13 mm	Base Top.stl, Base Bottom.stl
	Servo crank	2	64 × 25 × 8 mm	Crank.stl
	Servo rocker	2	59 × 14 × 8 mm	Rocker.stl
	Finger	2	87 × 71 × 18 mm	Finger V4 - covered.stl
	Camera protector	1	54 × 54 × 7 mm	Camera Cover.stl
	Wire cover	1	90 × 40 × 14 mm	Wire Cover.stl

Note

Base Top.stl and Base Bottom.stl are currently the same file, byte for byte — it is the plate carrying the printed OpenRB-150 standoffs. If version 2 really does use one plate twice, the BOM (which prices them separately) and the assembly guide (which says the plates are different sizes) should say so; otherwise the second export is missing.

2 · Buy. The bill of materials lists both versions. For V2: two AX-12A servos, an OpenRB-150, a RealSense D405, six 5 mm bearings, M2/M3 hardware, and grip tape.

3 · Assemble. The assembly guide — eleven steps, photographed, from taping the fingers to setting the servo IDs. Also available as a PDF to print.

The fingers are the experiment

Grip is where a jaw gripper is won or lost, so the finger is the part that has been through the most versions. All four are in CAD/ and interchangeable:

• V1 — a separate coupler and finger, so the contact face could be printed in TPU.
• V2 — a cast silicone finger, with the mould to make it.
• V3 — finger and coupler as one part; simpler to print and assemble, and it needs grip tape to match the older grip.
• V4 — V3 plus a cover that takes the play out of the joint and stiffens it.

Driving it

Force control, the palm camera, grasp planning and the behaviour-tree/PDDL layer are in correlllab/MAGPIE — a Python package, no ROS. Once the gripper is wired and its servo IDs are set, that is where to go next.

What is in here

CAD/                      STL models — gripper mechanism and every finger variant
Documentation/
  assembly.md             the build, step by step, on GitHub
  Magpie Assembly Guide.pdf   the same guide, to print
  BOM.md                  bill of materials, V1 and V2
  MagpieDesignChanges.md  what changed from V1, and why
  figures/                the guide's figures
  assembly_photos/        every build photo, full resolution
  design_photos/          CAD comparisons, old against new
  renders/                part renders, generated from the STLs
  paper/                  the paper this hand comes from
tools/                    regenerate the renders and the figures

License

MIT — see LICENSE. The models, the bill of materials and the guide are yours to print, modify and sell; the software that drives the gripper, correlllab/MAGPIE, is MIT too.

Built in the Correll Lab at the University of Colorado Boulder.

Citation

The hand this one descends from — its four-bar linkage, its force characterisation and the manipulation pipeline built on it — is described in A versatile robotic hand with 3D perception, force sensing for autonomous manipulation (RSS 2024 Workshop on Perception and Manipulation Challenges for Warehouse Automation, Daejeon, Korea). A copy is kept here at Documentation/paper/2402.06018-magpie.pdf, recompressed to keep a clone small — arXiv has the original.

@inproceedings{correll2024versatile,
  title     = {A versatile robotic hand with {3D} perception, force sensing for
               autonomous manipulation},
  author    = {Correll, Nikolaus and Kriegman, Dylan and Otto, Stephen and
               Watson, James},
  booktitle = {RSS Workshop on Perception and Manipulation Challenges for
               Warehouse Automation},
  address   = {Daejeon, Korea},
  year      = {2024},
  eprint    = {2402.06018},
  archivePrefix = {arXiv},
  primaryClass  = {cs.RO},
  url       = {https://arxiv.org/abs/2402.06018}
}