Axial Flux vs Frameless Torque: Comparing Motor Approaches for Humanoid Robot Joints

Axial-flux motors and frameless torque motors are the two core technical routes for humanoid robot joint drive today; they differ markedly in topology, performance and industrialization progress, and respectively suit different joint needs and development stages.

In terms of topology, the flux direction of an axial-flux motor is parallel to the motor shaft; the stator and rotor are arranged as two or more opposing discs with the air-gap field distributed axially, shortening the magnetic path and reducing reluctance losses compared with a conventional radial motor. The frameless torque motor omits the housing, bearings and output shaft of a conventional motor, keeping only the stator and rotor assemblies, and can be embedded directly inside a joint and integrated with the reducer; its working principle resembles a direct-drive motor but with better torque density.

At the performance level, the core advantage of the axial-flux motor is ultra-high power density and a flat structure. Its power density can reach 10kW/kg and its torque density 30Nm/kg, its axial size is reduced by 50% versus conventional solutions, and its efficiency is above 90% across a 20%-95% load range, making it suitable for the frequent start-stop conditions of humanoid robots and better fitting the space requirements of built-in joints. The advantages of the frameless torque motor centre on high integration and industrial maturity: it is designed in one piece with the joint structure, raising space utilization by over 40% and enabling zero-backlash transmission with peak torque above 500Nm; meanwhile its supply chain is well developed and batch supply has been achieved, making it the current mainstream deployment solution.

Comparing key indicators, the axial-flux motor leads in power density, torque density and axial size, but costs more and is harder to cool, and its industrialization is still at an early stage; the frameless torque motor has low mass-production cost and a mature supply chain, but its radial size is relatively large and its power density is relatively low.

In terms of selection logic, load-bearing lower-limb joints such as the hip and knee better suit the high torque density of axial-flux motors. In the short term, frameless torque motors remain the market mainstream; in the medium and long term, axial flux is expected to form a differentiated replacement in high-torque lower-limb scenarios, and hybrid drive combining the two may also become the industry's endgame. Jiuju has mature technical accumulation in core processes such as stator-rotor magnetizing and precision assembly for both motor types, supporting precision control and mass-production rollout of joint motors and helping advance the industry's technology.

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