The motion control of humanoid robot joints requires high torque, high precision and high efficiency motor solutions. The mainstream choice in the market is the frameless torque motor, which has a relatively mature technology and can meet the high torque requirements of robot joints.
However, as an emerging solution, the axial flux motor has gradually attracted the attention of the industry due to its high torque density.
In the industry chain of humanoid robots, the OEM has the "definition right" of joints, which is specifically manifested in mastering three points: Humanoid robot degree of freedom design: determine the number of joints and their range of motion Humanoid robot joint solution design: select the appropriate drive mode and motor type Joint layout position design: optimize the robot's motion performance and structural compactness.
Based on the scale effect and specialized division of labor, the possibility of OEMs to conduct full vertical integration is low, and "independent design, outsourced production" will be a more likely cooperation model. Frameless torque motor: a mature mainstream solution 02 Frameless torque motor is a direct-drive motor without the housing, bearings and encoder, which is specially designed for integration into robot joints or other mechanical structures. High torque output: By optimizing the magnetic circuit design, the torque capacity is improved, which is suitable for joints that require high torque. Compact structure and customizability: easy to integrate into robot joints of different sizes and shapes. Fast response speed and no cogging effect: improve the smoothness and precision of robot movement.
However, due to limited market demand in the past, the industrial chain of frameless torque motors has not been fully developed, resulting in high costs and limited market promotion. However, with the rapid development of the humanoid robot industry, the production scale of frameless torque motors is expanding, and manufacturing costs are expected to decrease. Axial flux motor: an emerging high torque density option 03 Compared with the traditional radial flux motor, the axial flux motor can provide higher torque density due to its unique magnetic circuit design. Higher torque density: Compared with the radial flux motor, the torque density of the axial flux motor can be increased by 30%, which is of great significance for the compact design of humanoid robot joints. Shorter axial length: helps to reduce the volume of robot joints and improve the flexibility of the overall structure. High efficiency: In high power density application scenarios, the energy efficiency performance of the axial flux motor is better. Although axial flux motors have many advantages, there are still some limitations in heat dissipation:
1. Due to the compact structure of the motor, the heat conduction path between the stator and the rotor is short, and the heat dissipation design needs to be optimized.
2. Complex manufacturing process: The production of axial flux motors involves precision machining and assembly processes, and the manufacturing difficulty is relatively high.
3. The industrial chain is not yet mature: The current supply chain of axial flux motors is relatively limited, and large-scale production still needs time.
Market Trends and Development Prospects 04 Rotary joint solutions currently include three categories: rigid drives, elastic drives, and quasi-direct drive drives. Among them, elastic joints are relatively less used due to their complex control algorithms and high hardware costs. Rigid joint solution: A large reduction ratio reducer can be used to provide greater torque and higher precision, but a force sensor is added, which is more expensive and has poor impact resistance. Quasi-direct drive solution: The torque can be controlled through the current loop, eliminating the large reduction ratio reducer, but the output torque is small. It was first used in quadruped robots. In recent years, with the advancement of technology, it has been widely used in humanoid robots. In addition, the reducers of humanoid robots mainly use harmonic and planetary reducers: harmonic reducers are suitable for joints that emphasize high torque, high precision, and small space; planetary reducers are more suitable for occasions that require high efficiency, low cost, and space insensitivity. Choice of sensing technology 05 High-precision, high-cost solutions usually use force sensors, while low-cost solutions prefer current loop control. Current loop control: The torque is estimated by current, with an accuracy error of about ±10%, and it is incompatible with large reduction ratio reducers. Force sensor solution: Higher accuracy, compatible with large reduction ratio reducers, but higher cost. With the development of the industry, low-cost control solutions will prefer current loops, while high-end robots will use force sensors to improve control accuracy.
The choice of motors for humanoid robot joints is undergoing technological evolution. Frameless torque motors still dominate with their mature design and high stability, while axial flux motors are gradually gaining attention due to their high torque density and high efficiency. With technological advances, hybrid motors that combine the advantages of both may appear in the future. In addition, rotary joint technology, reducer solutions and sensor solutions are also being continuously optimized to drive the humanoid robot industry towards higher performance and higher efficiency.