Ultra-Compact Harmonic Joint Module with a 6 mm Hollow Shaft

Meet the HONPINE HPJM harmonic joint module, an ultra-compact integrated robot joint actuator designed for applications where installation space, cable routing, and motion-control precision are critical. Despite its compact size and high level of integration, this module features a 6 mm through-hole for internal cable routing. It is suitable for humanoid robots, drones, optical platforms, and precision automation systems that require compact, high-performance rotary joints.

In this article, Theodore Li, Technical Director at HONPINE, introduces the key specifications, structural design, and engineering advantages of the HPJM harmonic joint module.

HONPINE HPJM Harmonic Joint Module: Key Specifications

The HONPINE HPJM harmonic joint module, identified in the engineering drawing as HPJM-RE30-40-PRO-XXX-2E-EC, combines a compact hollow-shaft design, dual-encoder feedback, and a highly integrated mechatronic architecture (MEA).

Its key structural advantages include a 6 mm through-hole for cable routing, an integrated motor and drive system, and a dual-encoder configuration designed to support precise motion control.

Why Choose the HONPINE HPJM Harmonic Joint Module?

1. Highly Integrated Mechatronic Design

The HPJM integrates a harmonic reducer, frameless torque motor, 16-bit encoder, and FOC (field-oriented control) drive into a compact housing measuring Ø57 × 60.5 mm.

Instead of separately selecting and integrating a motor, reducer, encoder, and drive, customers receive a highly integrated joint module designed to simplify system development. This can reduce component-matching work, shorten integration time, and accelerate robot development.

For robot manufacturers, this integrated architecture helps streamline mechanical and electrical design while reducing the complexity of joint-level assembly and commissioning.

2. 6 mm Through-Hole for Internal Cable Routing

One of the HPJM's key features is its Ø6 mm hollow shaft, which provides a through-hole for routing cables and, where the application design permits, pneumatic lines through the center of the joint.

Internal routing helps reduce external cable interference and can minimize cable twisting or snagging during joint rotation, depending on the routing arrangement and permitted rotation range.

This feature is particularly valuable in humanoid robots and compact robotic systems, where multiple cables must pass through tightly constrained joint structures. Compared with a non-hollow design of similar dimensions, a through-hole configuration offers greater flexibility for internal cable routing.

3. Dual EtherCAT Interfaces for Daisy-Chain Networking

The engineering drawing identifies two EtherCAT connectors: EtherCAT_Input and EtherCAT_Output, using X1002WVS-04L connectors.

This input/output configuration supports daisy-chain networking, allowing multiple joint modules to be connected in sequence rather than requiring a separate point-to-point communication cable for every joint.

For humanoid robots with more than 20 joints, a daisy-chain architecture can help reduce cabling complexity and simplify system wiring.

The PRO version also includes a separate firmware download interface, allowing individual module firmware updates without relying exclusively on the main communication connection.

Actual network topology, supported node count, and firmware-update procedures should be confirmed against the selected model and controller configuration.

4. Rear-Mounted Electrical Interfaces

The power connector, communication interfaces, and firmware download port are concentrated on the rear face of the module. The drawing identifies the power connector as A1257WV-S-2P.

This rear-mounted layout keeps the output flange and radial surfaces free from protruding connectors and external cable outlets. It can simplify mechanical integration, reduce interference with surrounding structures, and facilitate enclosure design.

For robot joints that require large angular movements or installation inside a compact arm housing, this arrangement provides greater flexibility in connector placement and cable management.

The final design should still account for connector clearance, cable bending radius, thermal management, and the intended rotation range.

5. Symmetrical Mounting Flanges for Flexible Installation

The HPJM features symmetrical mounting interfaces on both ends, helping simplify mechanical integration across different robot joint positions.

According to the supplied drawing information, the mounting configuration includes Ø38 mm and Ø37.5 mm bolt-circle patterns, eight M2.5 threaded holes, and three M2 through-holes.

This symmetrical arrangement can make the same joint module adaptable to different installation orientations, potentially supporting shoulder, elbow, wrist, and hip joint applications, subject to the mechanical requirements of each joint.

For robot manufacturers, a common mounting interface can help reduce the number of mechanical variants, simplify spare-parts management, and improve modularity across a robotic platform.

6. Low Backlash and High Shock-Torque Capability

The HPJM uses harmonic transmission technology, which is known for its low-backlash characteristics. The supplied specifications indicate an output positioning accuracy of approximately 0.67 arcmin and a shock-torque capability of approximately 1.8 times the rated torque.

These characteristics can be beneficial in robotic applications involving frequent direction changes, acceleration and deceleration, and transient mechanical loads.

However, backlash, positioning accuracy, repeatability, and lost motion are different performance indicators. The actual output performance depends on the selected model, load conditions, control configuration, and measurement method. Similarly, the allowable shock torque must be interpreted according to the manufacturer's rating conditions and should not be treated as a continuous operating torque.

For applications such as humanoid robots and precision automation equipment, engineers should evaluate these parameters together with rated torque, peak torque, torsional stiffness, speed, and thermal performance.

Applications of the HONPINE HPJM Harmonic Joint Module

Humanoid Robots

Humanoid robots require compact joint modules that can fit within limited limb space while supporting precise motion control and integrated cable routing. The HPJM's compact housing, hollow shaft, and integrated drive architecture make it a candidate for shoulder, elbow, wrist, and hip joint designs, subject to the torque and dynamic requirements of each joint.

Drones and Compact Robotic Systems

In compact robotic platforms and selected drone-related mechanisms, installation space and component integration are important design considerations. An integrated harmonic joint module can simplify the assembly of rotary mechanisms where its weight, torque, speed, and power requirements match the application.

Optical Platforms and Precision Automation

Optical positioning systems and precision automation equipment may require compact rotary drives with controlled motion and repeatable positioning. The HPJM can be evaluated for these applications based on the required angular accuracy, vibration characteristics, motion profile, and environmental conditions.

The HONPINE HPJM harmonic joint module combines a compact hollow-shaft structure, integrated motor and drive components, dual-encoder architecture, and EtherCAT connectivity in a single rotary joint solution.

Its 6 mm through-hole helps accommodate internal cable routing, while its rear-mounted connectors and symmetrical mounting interfaces provide flexibility for mechanical integration. These features make the HPJM worth considering for humanoid robots, compact robotic systems, optical platforms, and precision automation applications.

If you are developing a compact robot joint or looking for an integrated harmonic joint actuator, contact HONPINE for detailed specifications, engineering drawings, and model selection support.