01 Conclusion First: This is a highly integrated mechatronic actuator
Judging merely by appearance, this joint looks like a thick metal cylinder. Once disassembled, however, it reveals itself as a complete robotic motion unit integrating the following compacted subsystems:
- Brushless motor: Converts electric energy into rotational torque
- Reduction mechanism: Converts the motor’s high-speed low torque into low-speed high torque for the joint
- MOS power driver: Supplies controllable current to the three-phase motor
- Main control MCU: Executes motor control, signal sampling, communication and protection logic
- Current sampling: Estimates output torque and serves as the foundation for FOC control
- Magnetic encoder: Detects rotation angle and multi-turn position
- NTC temperature sensor: Monitors the thermal status of the motor
- Communication & debugging interface: Connects to the overall robot control network and supports factory testing & maintenance
- Metal housing & output flange: Bears structural loads during robot movement
This marks the biggest difference between humanoid robot joints and ordinary motors: standard motors only need to rotate, while robot joints must constantly know their position, output torque, operating temperature safety status, and remaining output capacity.
02 Appearance & Structure: Hollow Shaft, Metal Housing and High-Stiffness Output End
Figure 2: The front side of the joint adopts a large-size output end fastened with dense screws, with a through hole reserved in the center.
The most notable exterior feature is the central through hole. This structure is not merely decorative; it delivers highly practical benefits for the overall robot design:
- Power cables, communication wires and sensor cables can follow shorter routing paths.
- Wire harnesses are less likely to be exposed when multiple joints are connected in series.
- The risk of wires being scraped, pulled or caught in mechanisms during large-angle joint movement is reduced.
- For multi-degree-of-freedom parts such as legs, waists and shoulders, the hollow structure helps improve the overall assembly density of the robot.
Figure 3: Multiple sets of interfaces are visible on the side, indicating this joint simultaneously handles power supply, communication, debugging and potential daisy-chain connection.
The metal housing and densely arranged front screws reveal that the joint is designed to withstand substantial structural loads. When a humanoid robot walks, squats or stands up, its joints bear not only motor torque, but also radial forces, axial forces, impact loads and transient reaction torque. Therefore, such modules must be engineered as an integrated structural assembly combining the motor, reducer, bearings and output flange.
03 Overall PCBA Layout: Power, Control, Sensing and Communication Circuits Integrated on a Single Circular Board