Multi-mode High-speed Actuator using Smart Soft Composite

Abstract

Soft morphing is an emerging technology for applications in various industrial fields, such as wearable devices and biomimetic soft robots, because of its advantages in adaptability to various environmental conditions by mimicking the soft motions of nature. Various smart materials-based actuators have been developed to generate active soft morphing in structures, but their limited performance in terms of actuated morphing shapes and actuating speeds have prevented them from being used more widely. This work presents a soft composite actuator capable of achieving flexible and complex motions, using a shape-memory alloy (SMA). The anisotropic material properties of the composite, considered a major defect in composite structures, were accentuated using a scaffold structure, so that the actuator could generate more diverse motions, even in a simple, lightweight structure. The composite characteristics depend on the scaffold structure embedded in the actuator, so actuator motion could be designed according to the scaffold structure. The scaffold structure was easy to fabricate using a three-dimensional (3D) printer. SMA wires were also included to generate a force for actuation

these components were combined as a composite using a soft polymer. The actuating characteristics of the actuator depended on the type of scaffold: symmetric, anti-symmetric, and asymmetric scaffolds were evaluated, and four different modes of actuation were realized. A design methodology was proposed to permit more diverse and complex motions than the four basic modes of actuation, and was implemented in a turtle mimetic robot as an example application. The motion of a marine turtle flipper was analyzed and simplified into three sections. The required motion for each was matched with an appropriate scaffold design and the three scaffold structures were combined into a single actuator module. This flipper actuator was capable of mimicking two different swimming gaits of the marine turtle with a single actuator, depending on the current pattern applied. The locomotion characteristics of the two swimming gaits were evaluated in terms of efficiency and swimming speed. An actuator design to increase actuating speed and deformation magnitude was also developed, which extended the range of actuating performance for the SMA-bending actuator. Use of a bundle of SMA wires with a small radius instead of a single, thick SMA wire improved the cooling efficiency of the wire and increased the actuating speed, up to 35 Hz. Also, because the natural frequency of the actuator could be controlled by the scaffold structure design, the resonance effect was used actively to increase the actuating deformation. With this, a scaffold design methodology to achieve the required natural frequency of the actuator was developed and confirmed with experimental data. Actuating performance underwater was also evaluated and a model to predict the appropriate actuator length for the best performance was proposed. As an application of the high-speed actuator, a fish mimetic robot capable of 10 Hz fin flapping was developed and its speed was measured. The actuator was also applied to flying wings to mimic the flapping motion of birds or insects by adding a mechanism for passive rotation.