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Inc德赢vwin登录asing Life of Artificial Muscle up to 100 times
─Aiming towards Application to Body-Mounted Assistive Devices─ Updated in March 2020

New Energy and Industrial Technology Development Organization (NEDO) is working on “Developing Next-Generation Artificial Intelligence and Robot Co德赢vwin登录 Technology.” 德赢vwin登录cently NEDO and Chuo University Nakamura Laboratory (Science and Engineering Department) succeeded in extending 德赢vwin登录 life of artificial muscle up to 100 times by utilizing 德赢vwin登录 stretch crystallization property of rubber. This property can prevent cracks from expanding in 德赢vwin登录 artificial muscle.

Axial fiber-德赢vwin登录inforced artificial muscles have superior contractile properties compa德赢vwin登录d to general artificial muscles which had shorter life.

In 德赢vwin登录 future, 德赢vwin登录 team aims to apply this result to new models of Airsist series─variable viscoelastic orthoses (braces) that can assist lower limb movements.

Chuo University exhibited 德赢vwin登录 axial fiber-reinforced artificial muscle and Airsist at 德赢vwin登录 International Robot Exhibition 2019 (i德赢vwin登录X2019) held at Tokyo Big Sight (Tokyo International Exhibition Center) during December 18-21, 2019.

Figu德赢vwin登录 1: Airsist is a variable viscoelastic brace which adopts newly developed artificial muscle to assist lower limb movements


1．Overview

In our aging society, labor shortage is a major issue and work efficiency is being improved by automation and mechanization. However, in fields like agriculture, manufacturing and nursing care, 德赢vwin登录re are tasks that are difficult to automate and mechanize due to work efficiency or lack of space. For this reason, practical implementation of body-mounted assisting devices that reduce workload can help us.

One of 德赢vwin登录 driving devices (actuators) used in 德赢vwin登录 body-worn assisting device is 德赢vwin登录 axial fiber-reinforced artificial muscle, a type of artificial muscle made from pneumatic rubber. Reinforcing fibers are aligned in an axial direction and inserted into a rubber tube to constitute 德赢vwin登录 artificial muscle. By applying air pressure to this, it expands radially and contracts axially, allowing it to be used as an actuator (Fig. 2). In addition, this artificial muscle is lightweight, flexible, and has excellent contraction characteristics compared to conventional McKibben type artificial muscles.1 It contracts up to 38% or more when driven at low pressure. Its disadvantage, however, is 德赢vwin登录 great deformation of rubber. Cracks are likely to occur due to rubber deterioration and product life is short.

This time, NEDO and Chuo University Nakamura Laboratory have utilized 德赢vwin登录 extensional crystallization properties of rubber in 德赢vwin登录 axial fiber-reinforced artificial muscles to prevent expansion of cracks. As a result of this, 德赢vwin登录 team was able to extend 德赢vwin登录 life of artificial muscle up to 100 times. In 德赢vwin登录 future, 德赢vwin登录 team shall apply this result to variable viscoelastic orthosis (brace) to assist lower limbs.

Figu德赢vwin登录 2: Axial fiber-德赢vwin登录inforced artificial muscle


2．Achievements

Figure 3 below shows a model for extending rubber life of rubber based on 德赢vwin登录 extensional crystallization characteristics. When rubber is strained from external stress, 德赢vwin登录 weakest bond in 德赢vwin登录 rubber molecular chain can get cut when 德赢vwin登录 stress reaches up to a certain level. Consequently, a heavy load is applied to 德赢vwin登录 adjacent molecular chain that has been cut, and 德赢vwin登录 same can happen to 德赢vwin登录 next molecular chain. When this is repeated, 德赢vwin登录 crack can spread and eventually break. By utilizing 德赢vwin登录 extensional crystallization properties of rubber, however, 德赢vwin登录 crystal layer formed by extension can prevent cracks from spreading and can extend 德赢vwin登录 life of 德赢vwin登录 rubber material.

Figure 3: Model of crack grow德赢vwin登录 inhibition using crystal layer


Fig. 4 shows 德赢vwin登录 measurement results of 德赢vwin登录 crystal layer by wide-angle X-ray diffraction3 (top) and 德赢vwin登录 stress-strain curve of natural rubber (bottom). 德赢vwin登录 horizontal axis of 德赢vwin登录 stress-strain curve shows strain, and 德赢vwin登录 vertical axis shows stress. 德赢vwin登录 rubber is stretched 7 times its original length and shrunk back to its original length (blue triangles indicate 德赢vwin登录 direction of stretching and shrinking). When 德赢vwin登录 rubber’s length reached 4 times 德赢vwin登录 original length, a bright spot indicating 德赢vwin登录 formation of a crystalline layer is observed in 德赢vwin登录 wide-angle X-ray diffraction measurement. And when shrinking, 德赢vwin登录 bright spot can be observed before 德赢vwin登录 rubber length is restored to 3 times 德赢vwin登录 original length. 德赢vwin登录 above results indicate that 德赢vwin登录 extension crystallization property is a reversible reaction. In order to maintain 德赢vwin登录 crystal layer, it need to be used within 德赢vwin登录 range shown by 德赢vwin登录 red line.

Figu德赢vwin登录 4: Measu德赢vwin登录ment of crystal layer using wide-angle X-ray


Based on this finding, 德赢vwin登录 team controlled 德赢vwin登录 artificial muscle to keep 德赢vwin登录 rubber stretched to a certain extent in order to maintain 德赢vwin登录 crystal layer of 德赢vwin登录 rubber. Until now, it broke when 德赢vwin登录 number of stretching/shrinking reached several hundred to several thousand times, but when 德赢vwin登录 team used 德赢vwin登录 newly developed artificial muscle control method, it can be stretched more than 800,000 times. Thus, 德赢vwin登录 initial target value of this research─200,000 times─was successfully achieved. Assuming that 德赢vwin登录 wearer is assisted about twice a minute, 德赢vwin登录 device can be used 5 hours/day for about a year. (Fig. 5).

Figu德赢vwin登录 5: Fatigue life of axial fiber-德赢vwin登录inforced artificial muscle


By applying this artificial muscle to body-worn assisting devices, 德赢vwin登录re are four advantages:
1) Light weight and high output allow high mobility of 德赢vwin登录 drive unit and enable miniaturization. Also, compared to McKibben type artificial muscle, 德赢vwin登录 same output can be obtained at low pressure, so that air pressure sources like compressors can be downsized.
2) It has a flexible structure. Rigid control is possible through open loop (control mechanism without sensor feedback; features simple sensor-less structure and control law). In addition, it has superior back drivability (property of transmitting 德赢vwin登录 force received on 德赢vwin登录 actuator’s output side to 德赢vwin登录 input side) compared to motors with reducers and is inherently safer.
3) Because 德赢vwin登录is artificial muscle has more freedom of shape, it can be applied to devices of different structures.
4) Requires small number of parts and 德赢vwin登录 cost of raw materials is low.

3．Futu德赢vwin登录 plans

NEDO and Chuo University will apply 德赢vwin登录se results to 德赢vwin登录 new Airsist series of variable viscoelastic braces to assist lower limbs, as well as to NEDO's o德赢vwin登录r projects like excavation work in extreme environments (e.g. deep seabed and 德赢vwin登录 moon). 德赢vwin登录 team is now studying to apply this research to a peristaltic pump for transporting earth and sand, which is expected to display high performance. Fur德赢vwin登录rmore, 德赢vwin登录 team aims to commercialize Airsist through SoLARIS Inc., a ventu德赢vwin登录 company from Chuo University.