CRAFT-Wrist

CRAFT-Wrist

A Direct Rigid Transmission 2-DoF Wrist Extension for Tendon-Driven Hand

Yujie Pang1,2 · Sadman Sakib1 · Mohammad Abdullah Al Faruque1

1University of California, Irvine  ·  2UCInspire

Paper Assembly guide Bill of materials Print files Control API (coming soon) SolidWorks
CRAFT-Wrist system overview showing the CAD model, physical prototype, and wrist moduleFig. 01 / system
(left) CAD design (middle) Physical assembly (right) Close-up of the wrist
Bending±20°left–right
Axial rotation−18° → +80°front–back
InterfaceDrop-inCRAFT Hand compatible

Motivation

Limitation of No-wrist Hand

A robot arm can move the hand into different places, but confined tasks often need a smaller local orientation correction near the fingers.

Value of CRAFT-Wrist

CRAFT-Wrist is mounted between the arm and the tendon-driven CRAFT Hand. Its two direct-drive axes add local orientation control while keeping the hand itself unchanged.

Mechanical Design

CRAFT-Wrist wrist workspace showing radial–ulnar deviation and flexion–extension ranges
Fig. 02 / wrist workspace±20° deviation · +80° flexion / −18° extension

The wrist carries the distal hand as an independent module, so the existing hand mechanics, electronics, and finger-control stack remain intact.

01

Concentric wrist center

Bending and axial rotation share a compact wrist center instead of stacking a long serial chain.

02

Direct, rigid transmissions

Two XC430-T240BB-T servos create short, deterministic load paths with position control and current limiting.

03

Unmodified hand interface

An independent frame carries the distal hand while leaving the CRAFT Hand's mechanics and finger-control stack unchanged.

Degrees of freedom2
ActuationDirect drive
Bending range±20°
Axial rotation−18° to +80°
Actuators2 × XC430-T240BB
InterfaceCRAFT Hand compatible
ControlPosition / current-limited

Demonstrations

Nail hammering

Aligns the hammer head with the nail axis before impact and restores alignment between strokes while the arm drives the strike.

Arm suppliesgross motionCRAFT-W supplieslocal orientation

Coupling Characterization

Experimental setup

The wrist was fixed at 15°, 30°, 45°, and 60° while finger motor 2 followed the same outward-and-return sweep.

The finger reached its prescribed travel at every posture. The comparison therefore asks how wrist posture changes motor loading, rather than whether the hand can still move.

4wrist postures
300 mApeak return current
17%of 1750 mA limit
Motor 2 current traces during outward and return finger sweeps at wrist postures of 15, 30, 45, and 60 degrees
Motor 2 current / 516 readbacksSolid: outward · dashed: return

Result

Return loading was similar at 15° and 30°, increased at 45°, and was largest at 60°, where peak current reached 300 mA. No over-current event occurred.

Interpretation

The separation between outward and return traces is consistent with tendon friction, routing hysteresis, or local stick–slip motion.

Limit

With one sweep per posture and no current-to-torque calibration, these measurements remain preliminary rather than a complete mechanical identification.

BibTeX

@misc{pang2026craftwrist,
      title={A Direct Rigid Transmission 2-DoF Wrist Extension for Tendon-Driven Hand}, 
      author={Pang, Yujie and Sakib, Sadman and Al Faruque, Mohammad Abdullah},
      year={2026},
      url={https://arxiv.org/abs/2609.22681}, 
}