Humanoid Arm Joints: Breaking Through Pain Points, Defining the Future

As the core power unit of humanoid robots, the performance and reliability of the arm joints directly determine the overall machine's capabilities. However, the current development of humanoid arm joints faces multiple challenges, urgently requiring the search for optimal solutions that balance breakthrough innovation with performance.

I. Confronting Legacy Pain Points of Collaborative Joints

Current humanoid arm joint design largely follows the technological path of collaborative robots but has also inherited their inherent drawbacks:

• Low Design Maturity: Collaborative arms have a relatively short application history, resulting in chaotic joint specifications and numerous internal components. Designers often struggle to master all part domains, leading to a disconnect between selection and design, ultimately compromising product stability.

• Low Standardization: Extensive non-standard customization and component R&D scatter design and supply chain focus. The lack of unified electromechanical interfaces means parts from different suppliers are not interchangeable, and lengthy custom cycles introduce unreliable factors.

• Low Functional Safety: The safety function logic of the joints themselves has not yet converged into unified standards and solutions, posing potential risks.

II. Brake Strategy: Precise Trade-offs, Optimal Hybrid Braking

The inclusion or exclusion of brakes is a critical decision point for humanoid arm joints. Traditional servo brakes activate upon power loss to stop and record position, resuming upon power-up for safety. However, their application in humanoid arm scenarios requires re-evaluation:

• Advantages:

  • Energy Saving & Stability: Humanoids often stand by in a "standing" state. Power-off braking saves energy and maintains posture.

  • Safety Essential: Provides critical safety assurance during load-bearing operations.

  • Maintenance Convenience: Allows arbitrary locking of arm posture during maintenance.

• Disadvantages:

  • Increases weight, size, and length.

  • Requires continuous power to remain open during normal operation.

  • Practical benefits can sometimes be limited.

Comprehensive Conclusion: Adopting a "Hybrid Braking" strategy is most reasonable:

• Gravity Axes (e.g., shoulder pitch, elbow joint): Use pin-type brakes to meet the core needs of power-off locking and safe load-bearing.

• Non-Gravity Axes: Use soft brakes (electrical braking) to avoid unnecessary weight and size burden.

III. Functional Characteristics: Integration, Compactness, Robustness, Flexibility

The ideal integrated joint module for humanoid arms must satisfy the following core characteristics:

1. High Integration (Universal Foundation):

   • Drive-Control Integration: Driver and controller integrated onto a single circuit board.

   • Mechatronic Integration: Deep integration of motor, reducer, driver, and (optional) sensors into an independent unit. Pre-installed power and communication interfaces minimize internal cabling (drive-control board can support separate removal/installation).

2. Extreme Compactness & Light Weight (Core Competitiveness):

   • Utilize hollow structures for internal cable routing to prevent cable entanglement and aging.

   • Minimized outer diameter and axial dimensions.

   • Lightweight structure for easy deployment and maintenance.

3. Exceptional Load Capacity (Core Competitiveness):

   • High Torque Density: Achieves high rated torque output at low mass.

   • High Load Capacity: Withstands large permissible torque, inertia, bending moments, and radial/axial forces.

   • Excellent Braking Performance: Rapid and reliable response from physical brakes (gravity axes) or electrical soft braking (non-gravity axes).

   • Wide Temperature Operation & Heat Dissipation: Broad operating temperature range, optimized heat dissipation design, high heat resistance rating.

   • Ultra-Long Service Life: Excellent durability ensures long-term reliable operation.

4. Flexible Custom Interfaces (Scenario Adaptability):

   • External flange designed as an independent part, supporting customized multiple mounting hole patterns and datums for seamless adaptation to diverse robot body configurations and application scenarios.

5. Multiple Transmission Directions (Scenario Adaptability):

   • Supports rotary joint designs for horizontal transmission or 90-degree vertical transmission.

6. Sensitive Perception Expansion (Intelligence Foundation):

   • Optionally equipped with rich sensors (position, force/tactile, temperature, vision, IMU, etc.) to build environment and self-state perception as needed, laying the hardware foundation for intelligence.

IV. Reflection: The Art of Balance

There is a tendency in current humanoid arm joint design to excessively pursue "light, small, and cheap." It is crucial to recognize clearly that, under equivalent output performance, extreme lightweighting, miniaturization, and low cost often come at the expense of critical functionality or performance (such as heat dissipation capability, load margin in extreme conditions, space for sensor integration, long-term reliability). Designers must make scientific trade-offs between competing priorities, finding the optimal balance point that meets core requirements and long-term reliability.

The evolutionary path for humanoid arm joints is a journey of confronting historical pain points, breaking through technical bottlenecks, and precisely defining requirements. By enhancing standardization and maturity, adopting innovative hybrid braking strategies, and maintaining a sober balance between cost and function while pursuing ultimate performance, humanoid robots can truly obtain stable, reliable, and powerful "limbs," advancing towards broader application horizons. Though small, the joint is the crucial fulcrum supporting the future of humanoids.