Citation: Bai, L.; Yan, H.; Li, J.; Shan,
J.; Hou, P. Detachable Soft Actuators
with Tunable Stiffness Based on Wire
Jamming. Appl. Sci. 2022, 12, 3582.
https://doi.org/10.3390/
app12073582
Academic Editors: Giovanni
Boschetti and João Miguel da Costa
Sousa
Received: 10 March 2022
Accepted: 30 March 2022
Published: 1 April 2022
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Article
Detachable Soft Actuators with Tunable Stiffness Based on
Wire Jamming
Long Bai, Hao Yan *, Jiafeng Li , Jiefeng Shan and Penghao Hou
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China;
16116352@bjtu.edu.cn (L.B.); 18116014@bjtu.edu.cn (J.L.); 21121271@bjtu.edu.cn (J.S.); 20126009@bjtu.edu.cn (P.H.)
* Correspondence: hyan@bjtu.edu.cn
Abstract:
The integration of variable stiffness materials and structures into soft robots is a popular
trend, allowing soft robots to switch between soft and rigid states in different situations. This
concept combines the advantages of rigid mechanisms and soft robots, resulting in not only excellent
flexibility but also tunable stiffness for high load capacity and fast and precise operation. Here,
a stiffness-tunable soft actuator based on wire/fiber jamming structure is proposed, where the fiber-
reinforced soft actuator is responsible for the bending motion, and the jamming structure acts as
a stiffness-tunable layer controlled by vacuum pressure. The primary design objective of this study is
to fabricate a jamming structure with wide-range stiffness, universal adaptability and high dexterity.
Thus, the behaviors of wire/fiber jamming structures with different layouts, materials and wire
arrangements are analyzed, and a theoretical model is developed to predict the effect of geometric
parameters. Experimental characterizations show that the stiffness can be significantly enhanced
in the bending direction, while the stiffness is smaller in the torsion direction. Additionally, by
integrating Velcro strips into the design, a quick and detachable scheme for the stiffness-tunable soft
actuator is achieved. Application examples exhibit high load capacity and good shape adaptability.
Keywords:
wire/fiber jamming; tunable stiffness; detachable design; soft actuators; fiber reinforcement
1. Introduction
Soft machines fabricated with soft materials have been rapidly developed in recent
years. They have proven to have great potential for applications in industrial robot manip-
ulators, medical devices, biomimetic robots, wearable exoskeletons and aerospace struc-
tures [
1
–
5
]. The Young’s modulus of soft materials is defined in the order of
10
4
–10
9
Pa [1]
.
Compared with conventional rigid mechanisms, soft machines and actuators can handle
fragile or irregular objects without massive computations or sensing systems, and interact
with human bodies safely [
6
], but still carry with drawbacks due to their low stiffness
and hyper-elastic properties. Soft actuators are unsuitable for working in areas with large
load capacity, high-speed motion and precise control. Many soft robots can be stiffened
by simply increasing air pressure, reducing size, adding rigid components or using higher
hardness materials [
7
–
9
]. However, these solutions have limited effect and may lead to
reduced flexibility.
In order to address these drawbacks, researchers have focused on developing stiffness
tunable materials and structures such as shape memory alloys (SMAs), shape memory
polymers (SMPs), low melting point alloys and jamming structures [
10
–
14
]. Among these
stiffness-tunable schemes, jamming structures are favored by scholars owing to their advan-
tages of structure simplicity, ease of fabrication and wide range of stiffness variation. Two
main types of jamming structures, granular and laminar jamming structures, have been ex-
tensively studied [
15
]. Granular jamming consists of particle elements such as grains, coffee
grounds, hollow spheres or interlocked granular particles in an airtight
envelope [13,16–18]
.
The jamming principle is to change the particles from a flowing state to a solid-like state by
Appl. Sci. 2022, 12, 3582. https://doi.org/10.3390/app12073582 https://www.mdpi.com/journal/applsci
