Hardware

Mechanical Structure

Degrees of Freedom and Joint Definitions

DoF Definition

KAI Hand has 21 kinematic degrees of freedom: 20 active DoF and one passive DoF. It also incorporates 16 compliant DoF—three in each of the four fingers and four in the thumb.

Joint Definitions

Finger	Joint name	Control index	Definition
Thumb	thumb_cmc_1	0	Active. Axis remains normal to the palm. Positive motion closes toward the first web space; external force can drive the joint in the positive direction through its compliant mechanism.
	thumb_cmc_2	1	Active. Axis remains parallel to the palm. Positive motion closes toward the palm; compliant under external force in the positive direction.
	thumb_mcp_1	2	Active. Positive motion closes toward the first web space; compliant under external force in the positive direction.
	thumb_mcp_2	3	Active. Positive motion closes toward the first web space; compliant under external force in the positive direction.
	thumb_dip	—	Passive and coupled to thumb_mcp_2; compliant under external force in the positive direction.
Index	index_mcp_roll	4	Active. Positive motion is toward the thumb; compliant under external force in the negative direction.
Index	index_mcp_pitch	5	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Index	index_pip	6	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Index	index_dip	7	Active tendon-driven joint. Positive motion closes toward the palm; compliant under external force in the positive direction.
Middle	middle_mcp_roll	8	Active. Positive motion is toward the thumb; compliant under external force in the negative direction.
Middle	middle_mcp_pitch	9	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Middle	middle_pip	10	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Middle	middle_dip	11	Active tendon-driven joint. Positive motion closes toward the palm; compliant under external force in the positive direction.
Ring	ring_mcp_roll	12	Active. Positive motion is toward the thumb; compliant under external force in the positive direction.
Ring	ring_mcp_pitch	13	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Ring	ring_pip	14	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Ring	ring_dip	15	Active tendon-driven joint. Positive motion closes toward the palm; compliant under external force in the positive direction.
Little	pinky_mcp_roll	16	Active. Positive motion is toward the thumb; compliant under external force in the positive direction.
Little	pinky_mcp_pitch	17	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Little	pinky_pip	18	Active. Positive motion closes toward the palm; compliant under external force in the positive direction.
Little	pinky_dip	19	Active tendon-driven joint. Positive motion closes toward the palm; compliant under external force in the positive direction.

Thermal-Balanced Design

All joints except the four tendon-driven distal joints—index_dip, middle_dip, ring_dip, and pinky_dip—use worm-gear transmission. When KAI Hand maintains a grasp, the transmission mechanically holds contact force instead of requiring continuous motor stall torque. This substantially reduces actuator heating during sustained grasps and helps extend actuator service life.

Multimodal Sensing

Joint Position Sensing

Joint-angle sensors measure position changes caused by both commanded motion and externally induced compliant motion.

Tactile Sensing

Parameter	Specification
Minimum detectable force	≤ 0.05 N
Measurement range	0–20 N
Permissible overload	≥ 250% of calibrated full scale（50 N）
Force resolution	≤ 0.1 N
Sensing points	Thumb: 44; index finger: 58
Update rate	100 Hz
Nonlinearity error	≤ ±0.5% FS
Repeatability error	≤ ±1% FS
Drift	≤ 1% FS/hour; ≤ 0.5% FS/°C
Hysteresis	≤ ±1% FS
Noise	RMS ≤ 0.2% FS
Sensor-element life	1,000,000 cycles

Vision Sensing

KAI Hand can be equipped with an optional wrist-mounted stereo depth camera. The currently specified model is the Orbbec Gemini 305.

Electrical and Communication Interfaces

Power Requirements

Supply voltage: 12 V DC. Maximum current: 20 A. Support for 12–48 V wide-input operation is planned.

Communication

CAN and CAN FD are currently supported. RS-485 and EtherCAT support is planned.

Wiring Diagram

• KAI Hand is powered through the red and black power wires, which connect to the supplied power adapter via an XT30 connector.
• The communication board provides a standard 2-pin CAN interface. Connect KAI Hand to the host computer using the supplied communication harness and USB-to-CAN FD adapter.

Status Indicators

System Integration

Mechanical Integration

Design an adapter based on the wrist-mount drawing. Fasten KAI Hand to the adapter with M3 screws, then connect the adapter to the target robot or test fixture.

Refer to the model repository for the 3D files.

Wrist Vision Integration

If a wrist vision sensor is required, design an adapter with an integrated camera bracket. Fasten both KAI Hand and the vision sensor to the adapter, then mount the assembly to the target system.

Integration Safety

• Never install or remove mechanical or electrical connections while power is applied.
• Before starting an integrated system, verify that every mechanical and electrical connection is secure.
• When fastening KAI Hand to an adapter, select screw lengths that cannot contact the wrist communication board. Contact may cause a short circuit.