Development of high-performance twisted nylon actuators and their applications for soft robots

Soft actuation technology underpins high compliance, integration, and multifunctionality in soft robots, and has received growing attention in recent years. Among various soft actuators, twisted artificial muscles based on nylon fibers, also known as twisted nylon actuators (TNAs), stand out for their ability to generate linear contraction and extension, offering advantages such as compact structure, low driving voltage, and favorable output linearity. It is critical to enhance the performance of TNAs to broaden their applicability in areas such as soft manipulators, bioinspired dexterous hands, and miniature mobile robots. Previous work has explored methods to improve either the deformation or the output force of TNAs, but achieving both large deformation and high output force remains challenging. To this end, developing effective design strategies that address both aspects is essential.

Now, the researchers from the Lab of Advanced Actuation Technologies (LAAT) at Harbin Institute of Technology propose performance enhancement strategies for TNAs to address the critical demand for high-performance actuators. These approaches enable the simultaneous realization of large deformation and high output force, significantly improving the actuation performance. The novel high-performance TNAs demonstrate strong application potential in soft robots.

“We propose three enhancement strategies inspired by the chromosome construction process from DNA to chromatin.” says Jin Sun, the first author of this paper. The first two strategies respectively improve deformation and output force, while the third integrates the two to achieve both larger deformation and greater force output. These strategies are validated through the proposed three novel types of TNAs.

Firstly, a twisted and ultra-coiled nylon actuator (TUNA) with a dual-level helical structure is presented, which achieves a contraction ratio of 60.2% in the vertical direction and nearly 100% horizontally, while also enabling external energy storage. “TUNA can be elongated under load force in non-actuation state, storing elastic potential energy that is subsequently released as kinetic energy,” Jin Sun explains. This capability is demonstrated by pulling a slider at a speed of 4 m/s and propelling a miniature basketball to a height of 131 cm.

Then, a parallel-twisted method is introduced to fabricate a stronger TNA, named parallel twisted nylon actuator (PTNA). The output force of PTNA reaches 11.0 N, achieving 12.1% contraction under a load of 15 N (over 10,000 times its weight). “We further construct the dual-level helical structure through the parallel-twisted method, and the output force is improved by 439.7% compared with TUNA,” says Jin Sun.

Additionally, these TNAs have been applied to drive several soft robots, including bionic elbows capable of rapidly throwing objects, a jumping robot, and a soft finger with multiple modes of motion. “We develop two 3D-printed bionic elbows inspired by the shooting motion of basketball players, one is capable of rotating 102.0°, and the other can shoot a miniature basketball to a displacement exceeding 130 cm. Moreover, we propose a jumping robot that demonstrates a rapid jump of over 15 times its body height,” adds Jin Sun. Furthermore, the proposed soft finger exhibits contracting, precise bending, and twisting motions. The contracting motion illustrates a contraction of 15.6% under a load of 2 kg. The bending motion can be utilized to precisely track desired trajectories (tracking errors less than 2.0%). The twisting motion can be used to grip a cylindrical object with a diameter of 25 mm.

This research proposes multiple performance improvement strategies for TNAs, which substantially improve both their deformation capability and force output. The various soft robots designed provide a strong foundation for the practical deployment of TNAs. In the future, the researchers plan to investigate the low temperature actuation method and rapid response techniques to facilitate the integration of TNAs into assistive medical equipment, wearable devices and mobile robots.

Sources: https://spj.science.org/doi/10.34133/research.0642