Joints for humanoid robots

By R. David Whitby, Contributing Editor | TLT Worldwide July 2025

Mechanisms are being worked on to obtain high torque, precision and compact multi-axis movement.
Side view of the top half of a humanoid robot with joints
Several human joints, particularly the wrist, shoulder and ankle, are remarkable in having three rotational degrees of freedom (RDoF). This ability would be very useful for humanoid robots to mimic human movement and dexterity.

Mechanical designs that attempt to combine several degrees of freedom in a single unit have been tried in recent years. One type comprises a sphere and carefully placed friction wheels or omni-wheels to transfer power. Although these achieve unrestricted movement with three RDoF, they are structurally complicated and tend to have uncontrolled slippage, making them less than energy efficient. Another mechanism uses high frequency vibrations generated by magnetostrictive or piezoelectric devices to induce rotational movements. However, these systems are not good at achieving high torque at low speeds and are difficult to manufacture. Designs that use spherical gears and a slider mechanism have an expanded range of motion, but the strength of the magnetic coupling in these mechanisms governs the torque that can be transmitted.

Conventional joints in many current robots use a series of single-axis rotational joints, each driven by its own motor and gearbox. To achieve multiple degrees of freedom the joints must be linked, which increases the length and bulk of the robot’s arms and can limit movements in confined spaces.

In 2021, engineers at Yamagata University and Tohoku University in Japan published a paper1 describing a novel ball joint mechanism, which they called ABENICS. This mechanism has a single compact joint using a cross-spherical gear and monopole gears to achieve three RDoF (pitch, roll and yaw). The engineers postulated that ABENICS gears enable complex, shoulder-like movement in a much smaller volume, which makes them ideal for uses which have spatial constraints, such as robots. They claimed that ABENICS gears are able to deliver high torque, with precise and slippage-free motion in all three rotational axes. The design permits reliable and continuous positioning without the need for external orientation sensors, although integrating an inertial measurement accelerometer might further enhance accuracy.

In the initial design, the cross-spherical gear and monopole gears were fabricated from polyetheretherketone (PEEK). It was found that this mechanism suffered from slippage between the gears, causing energy loss due to friction, while the interaction of slipping and engagement between the gears uniquely determined its attitude.

Then, in mid-2024, the engineers presented a second ABENICS design2 to the 2024 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM), July 15-18, 2024, in Boston, Mass. In the new mechanism, the gears are made from Alumigo Hard and manufactured using a five-axis-computer numerical control (CNC) machine. Alumigo Hard is an aluminium alloy known for its exceptional hardness. The strength and hardness of the new gears improves the capability of the mechanism to tolerate substantial loads, thereby ensuring a robust performance.

The new design also includes an inertial measurement unit (IMU) sensor to measure the orientation of the output cross-spherical gear.2 The sensor provides real-time orientation feedback and improved control. It is placed at the center of a specially crafted box and located within the hollow structure of the cross-spherical gear. The existence of backlash in several gear pairs, including the inner-worm gear and pinion gear, helical gear pair, pinion-monopole gear pair and monopole gear-cross-spherical gear pair, has an impact on the ultimate orientation of the spherical gear. As a result, the inclusion of an IMU sensor has proven essential, giving a critical feedback mechanism for real-time orientation data and enabling accurate positioning of the spherical gear to its predetermined setpoint.2

A search of the internet has found there are currently no commercial products using ABENICS gears as of April 2025. Although it has demonstrated impressive capabilities in research prototypes, it has not yet been widely adopted in commercial, medical or industrial applications, even though it is recognized for future commercial use, especially in fields that demand high torque, precision and compact multi-axis movement, such as surgical robotics and manufacturing and service robots.

REFERENCES
1. Abe, K. Tadakuma, K. and Tadakuma, R. (2021), “ABENICS: Active ball joint mechanism with three-DoF based on spherical gear meshings,” IEEE Transactions on Robotics, 37 (5), pp. 1806-1825, doi: 10.1109/TRO.2021.3070124.
2. Selvamuthu, M. G., Abe, K. Tadakuma, K. and Tadakuma, R. “Metal ABENICS: Metallic spherical gear mechanism with orientation correction using embedded IMU sensor,” presentation to the 2024 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM), July 15-18, 2024, in Boston, Mass.
 
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.