CRAFT Hand: Tendon-Driven Hybrid Robotic Hand

• CRAFT Hand is a tendon-driven, anthropomorphic robotic hand featuring a hybrid hard-soft compliance design enabling robust and repeatable contact-rich manipulation.
• Its design combines soft TPU joints with rolling-contact articulation and rigid PLA links to balance impact absorption and load transfer while achieving high dexterity with 15 actuators.
• Teleoperated via vision-based hand tracking, the hand delivers precise control for delicate tasks at a cost below $600, and its open-source release supports extensive research reproducibility.

The CRAFT hand is a tendon-driven, anthropomorphic robotic hand featuring a hybrid hard-soft compliance architecture, optimized for robust, contact-rich manipulation while maintaining compact form factor and high kinematic repeatability. Its design distinctively applies soft polyurethane (TPU) at joints and rigid polylactic acid (PLA) for links, leveraging rolling-contact articulation to concentrate impact absorption and ensure repeatable flexion paths. With 15 actuators entirely remote from the lightweight fingers and teleoperation integration via vision-based hand tracking, CRAFT achieves notable dexterity, strength, and durability at a cost below \$600—all open-sourced with simulation and tele-operation support (Lin et al., 12 Mar 2026).

1. Mechanical Design and Anthropomorphic Architecture

CRAFT’s structure is anchored by 15 tendon-driven degrees of actuation, realized through Dynamixel XL330-M288-T servos, all mounted within a compact forearm module. The resulting palm width is 95 mm, finger length 103 mm (198 mm tip-to-wrist), and mass totals 800 g. Each of the four fingers comprises three active degrees of freedom: metacarpophalangeal (MCP) flexion/extension, MCP abduction/adduction, and pip/distal interphalangeal (PIP/DIP) flexion, for a total of 12 DoFs; the thumb employs a 2-DoF carpometacarpal (CMC) and a 1-DoF MP/IP coupling for a further three active DoFs. Passive DoFs arise from tendon compliance and snap-fit interfaces.

All phalangeal segments and the palm are rigid PLA to maximize load transfer. At each PIP and DIP (and thumb MP/IP), compliant joints are engineered from ≈ 85A shore TPU. On these soft joints, dual hardened rolling surfaces enforce singular, repeatable flexion trajectories. Kinematically, each finger is modeled as a serial 3-link manipulator with one 2-DoF MCP and a single DoF coupled PIP/DIP arrangement. The bidirectional linkage enforces $\theta_3 = \theta_2$ at all tendon tensions. Denavit–Hartenberg parameters formalize joint positions as: $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$ with $l_1 + l_2 \approx 103 \ \mathrm{mm}$. The rolling-contact design constrains $\theta_2$ along a predetermined arc.

2. Hybrid Compliance Characterization

CRAFT’s architecture separates load-bearing link stiffness from impact-absorbing joint compliance. The joint stiffness matrix and link stiffness are defined as: $$K_{\mathrm{joint}} = \frac{\partial \tau}{\partial \theta}, \quad K_{\mathrm{link}} = \frac{\partial F}{\partial \delta}$$ where $\tau$ is the joint torque vector, $\theta$ the joint angles, $F$ an applied external force, and $\delta$ the resulting link deflection. Empirically, $K_{\mathrm{joint}} \ll K_{\mathrm{link}}$, ensuring that soft TPU at the joints absorbs impact. When a force $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$0 is applied at a PIP, the angular deflection is

$$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$1

with $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$2 the local moment arm.

Dynamic transmission between PIP and DIP is governed by the bidirectional linkage such that: $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$3 for coupling tendon tension $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$4, guaranteeing identical torque at both joints. Passive elastic bands supply return torque $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$5 in the absence of tendon tension, with $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$6 tuned to ensure neutral repositioning.

3. Tendon Routing, Actuation, and Teleoperation Control

Tendon routing utilizes low-friction metal dowel pins in the PLA shell, optimized for minimal transmission losses. Each finger incorporates three tendons: two for MCP flexion/extension, two for MCP abduction/adduction, and one for coupled PIP/DIP flexion (with a total of three tendons per finger, allocated across the function pairs). Preload is maintained at ~2 N to prevent slack, and each servo delivers up to 2.1 N·m stall torque (12 V) at a 100 Hz control rate.

Teleoperation employs a vision pipeline that tracks the human operator’s wrist pose (FrankMocap, 20–30 Hz) and hand joints (HaMeR, 20–30 Hz) with a single RGB camera. Operator joint angles $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$7 are mapped to robot limits $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$8 through: $$(\alpha_{i-1}, a_{i-1}, d_i, \theta_i) = \left\{ \begin{aligned} & (0,0,0,\theta_1), \ & (0,l_1,0,\theta_2), \ & (0,l_2,0,\theta_3) \ \end{aligned} \right.$$9 After an exponential moving average smoothing step, commands are transmitted at ~50 Hz via ROS.

4. Empirical Evaluation and Teleoperated Manipulation

Structural evaluation, benchmarked against a rigid LEAP hand with identical actuation, demonstrates CRAFT hand’s mechanical efficacy:

Test Type	CRAFT	LEAP (rigid)
Pull-out Strength	15.29 N (15° slip)	8.67 N
Repeatability	$l_1 + l_2 \approx 103 \ \mathrm{mm}$0 rad mean error	$l_1 + l_2 \approx 103 \ \mathrm{mm}$1 rad
Holding Endurance	~50% lower current draw	Baseline

The increase in pull-out strength results from mechanical tendon advantage, while holding endurance is enhanced by joint compliance and tendon friction-mediated passive braking.

In teleoperation with 10 users and 10 trials per hand, CRAFT outperforms LEAP in manipulating fragile and low-friction objects:

Object	Success Rate (CRAFT)	Success Rate (LEAP)	Completion Time (CRAFT, s)	Completion Time (LEAP, s)
Ball	80%	30%	7.4	24.1
Glass	100%	100%	–	–
Egg	90%	40%	–	–
Raspberry	100%	60%	–	–
Chip	100%	60%	38.5	63.3

CRAFT is 3–5× faster on average. Passive joint compliance attenuates misaligned contact forces, reducing operator cognitive overhead in fragile manipulations.

The hand achieves all 33 grasp types in the Feix taxonomy, including power, precision, and lateral grips. The hybrid compliance preserves lateral stiffness for abduction-heavy grasps, while joint softness allows precise surface conformity for pinches.

5. Cost Structure and Open-Source Ecosystem

CRAFT’s total assembly cost remains below \$l_1 + l_2 \approx 103 \ \mathrm{mm}$2180), PLA/TPU filament (\$l_1 + l_2 \approx 103 \ \mathrm{mm}$315), dowel hardware (\$l_1 + l_2 \approx 103 \ \mathrm{mm}$440), and consumables (\$15). Complete design assets—including CAD, firmware, ROS drivers, robot description formats (URDF, MuJoCo XML), vision-teleop nodes, and simulation environments—are released under open-source terms at http://craft-hand.github.io/.

6. Context and Significance

The CRAFT hand demonstrates that targeted application of hybrid hard-soft compliance, achieved through judicious material selection and tendon actuation, can deliver anthropomorphic dexterity, mechanical robustness, and practical teleoperation performance without excessive cost or form factor penalty. Its open-source availability supports reproducibility and extension in research settings. The use of rolling-contact TPU joints as compliance elements, distinct from single-pivot elastomers, exemplifies a design approach optimized for repeatable, contact-rich manipulation (Lin et al., 12 Mar 2026).