A Complete Guide to Humanoid Robot Joint Modules: Design Challenges, Selection Strategies & Key Component Considerations

The wide range of motion required by humanoid robots, from upper limbs and waist to lower limbs, drives the need for three main types of joint modules: planetary, harmonic, and linear. Modern joint modules adopt a highly integrated design, combining core components like reducers, motors, drivers, and encoders into a single unit. Leveraging all-hardware FPGA drive architectures, they achieve exceptional torque loop response and dynamic performance. However, integrating so many high-performance parts into a compact package and making them work reliably is a challenging systems engineering feat.

Designing and manufacturing high-performance robotic joints is a complex task that merges precision mechanics, motor drive, sensing technology, control algorithms, and thermal management. The core difficulties can be broken down into five interconnected levels. Understanding these challenges is a prerequisite for effective joint selection.

1. High-Density Integration: Tackling the “Compact vs. Powerful” Dilemma
The joint must integrate a motor, reducer, encoder, driver, brake, torque sensor, and cooling structure into an extremely limited space. This demands exceptional electromechanical design skills, forcing constant trade-offs and optimization between power density, weight, volume, and structural strength—a task often compared to “fitting a race car engine inside a wristwatch.”

2. Demands for High Performance & Reliability in Core Components
The performance of each core component directly determines the final capability of the joint module:

• Motor: Requires high torque density, low speed ripple, and fast response, placing extreme demands on materials (e.g., high-grade permanent magnets) and manufacturing processes.

• Reducer: Manufacturing harmonic or RV reducers represents the pinnacle of precision mechanics. Their gear tooth design, material heat treatment, and assembly accuracy (micrometer level) directly determine joint stiffness, backlash, and lifespan. This technology was long dominated by foreign giants and remains a key area for domestic industry advancement.

• Encoder: As the joint’s “sense of position,” the resolution (e.g., 23-bit+) and noise immunity of an absolute encoder directly impact system positioning accuracy and control stability. High-resolution magnetic encoders are becoming the preferred choice in many advanced joint designs due to their ability to deliver high accuracy and strong contamination resistance in compact spaces, though their own design, calibration, and signal processing are highly challenging.

• Torque Sensor: Key to enabling true force control, it must maintain high accuracy, low latency, and overload capacity under dynamic loads.

3. Precise & Robust Control Algorithms
A robotic joint is not a simple “position servo.” Achieving human-like compliant and agile motion requires support from complex algorithms: including high-bandwidth multi-loop control (current, speed, position); and advanced algorithms like friction compensation, resonance suppression, and disturbance observers to overcome system non-linearities. Particularly, force/position hybrid control, which allows the robot to “sense force” during environmental interaction, places extreme demands on control system response speed and stability.

4. Thermal Management & Long-Term Reliability Engineering
High power density inevitably generates significant heat. Ensuring the joint doesn’t overheat and derate under sustained heavy loads through efficient cooling design (e.g., liquid cooling, heat pipes, phase-change materials) is a critical engineering challenge. Furthermore, joints must achieve long-term reliable operation in industrial, medical, or outdoor scenarios through redundancy design, online fault diagnosis, and rigorous environmental/life testing (e.g., targeting tens of thousands of hours MTBF).

5. Balancing Supply Chain & Cost Control
A secure, controllable supply chain for core components (especially high-performance reducers and high-end encoders) is crucial. The domestic supply chain is progressing rapidly, but matching the top international levels in product consistency, reliability, and large-scale cost control still requires time, accumulation, and verification.

These design challenges directly influence joint selection strategies for specific products. Let’s look at how a humanoid robot like Tesla’s Optimus configures its joints in practice.

Joint Selection Strategy in Humanoid Robots: Insights from Optimus

Humanoid joint design requires a global trade-off between motion balance, dynamic control performance, and load capacity, often leading to a hybrid use of rotary and linear joint modules. Taking Optimus as an example:

• Rotary Actuators: Used primarily in areas requiring large rotation ranges and high torque, such as the wrists, shoulders, waist, and hips. Optimus uses a total of 14 rotary degrees of freedom across these 4 areas. The “motor + reducer” solution meets both rotation range and torque requirements, often combined with torque sensors for fine end-effector control.

• Linear Actuators: Deployed mainly in 5 areas: wrists, elbows, ankles, hips, and knees, also totaling 14 degrees of freedom. They are commonly used in joints requiring direct push/pull motion or specific linear actuation.

Example for Reference Selection
Here are typical configurations of a high-performance joint module for your selection reference:
This module uses an ultra-thin harmonic reducer for a very compact structure. It is equipped with a high-torque-density brushless torque motor and a high-precision multi-turn absolute magnetic encoder. Through meticulous design, its volume and weight are optimized to be among the best in its torque class.