Research on various types of soft robots has been conducted recently. Soft robots with flexible mechanisms are expected to contribute significantly to the development of robotics, not only in industrial applications but also in various other applications, such as those involving robot-human interactions. Flexible mechanisms cannot necessarily be realized using flexible materials alone; they can also be realized by adopting new approaches to conventional mechanisms. It is also desirable for sensors and other components to be flexible so that they do not reduce the flexibility of the mechanism. The development of these mechanisms and elemental technologies will lead to further development of soft robots. This special issue will feature papers on the structures and applications of soft mechanisms and elements as well as on design and analysis methods and control techniques.

Wearable power-assisted robots driven by artificial rubber muscles should be designed with a wide range of assistance capabilities to support various tasks while minimizing the sense of restraint when worn. Ideally, to realize a compact and lightweight power-assisted robot, the size of the artificial rubber muscle should be maintained while increasing the amount of movement. This study aims to develop an artificial rubber muscle that can achieve a wide range of assistance through rope-based force transmission. An artificial rubber muscle is proposed that can transmit force through a human-like pulling motion using a rope. This McKibben-type artificial rubber muscle is enhanced with a power transmission mechanism. The proposed artificial muscle has two layers: an inner layer that is deformed by air pressure to grasp the rope and an outer layer that pulls the rope by contraction. Each layer moves stepwise by supplying compressed air to realize the pulling motion of the rope. In this study, a prototype of the proposed artificial muscle was fabricated, and its operational principle was confirmed experimentally. Additionally, the force transmission performance was verified by comparing the contraction force of the artificial muscle with the tensile force of the rope.

Recently wearable power assist robots have been developed in order to solve labor shortage and reduce work load. It is desirable that the wearable power assist robots should be lightweight and flexible from the viewpoint of reducing burden on wearers and maintaining safety. Pneumatic actuators are used in various wearable power assist devices because of high power-weight ratio and flexibility. The authors had developed the portable air supply system for retrieving and re-compressing exhausted air using the variable volume tank constructed with elastic material. In addition, the authors had developed the hollow cylindrical-shaped variable volume tank with the reduced internal volume to downsize and improve energy characteristics. The purpose of this study is to construct deformation model to design the hollow cylindrical-shaped variable volume tank with desired characteristics. The effectiveness of the proposed model is verified by comparing the results of the calculated and the measured values. Finally, the proposed model is applied to the multi-layered hollow cylindrical-shaped variable volume tank.

Since abnormal posture of the trunk due to aging decreases walking ability, the use of a trunk device is required to maintain and improve posture. Wearable assistive devices for the purpose of extending healthy life expectancy must be lightweight, have a simple mechanism, and have a drive system that cooperates with human movements. In our previous study, we developed an adjustable stiffness mechanism to correct the posture of the trunk. In this study, we propose a rotation mechanism that can support trunk rotation during walking and posture correction. The mechanism is a simple structure consisting of a bearing mechanism to reduce rotational resistance and a plastic block connected to pneumatic soft actuator bellows. Using a pneumatic soft actuator provides high adaptability to sudden human movements. This paper explains the outline of the proposed mechanism and the mathematical model of the actuator, followed by a description of the rotational stiffness adjustment method to support trunk rotation movements. Finally, the validity of the proposed mechanism and the effectiveness of the stiffness adjustment method are described based on the results of evaluation experiments.

The purpose of this study is to develop a soft robot hand that can grasp an object, which has various shape or characteristic. By combining bending motion of a soft actuator and vacuum suction using a vacuum ejector, the developed finger can both grasp objects and suck object surfaces. In addition, as a vacuum ejector is introduced in order to generate negative pressure, both bending and suction movements can be realized by using only a positive pressure source. In this paper, we describe the control method for bending and suction pressure in order to keep simultaneously the desired value and then discuss the method for switching grasp/suction movements according to an object.

The purpose of this study is to develop the ankle wearable device that constructed with artificial muscles and the pneumatic friction clutch to train leg motion directly. The developed clutch is constructed with the air bellows, the friction member and the wire. Pull-out of the wire is constrained by the friction force of the friction member increased owing to compressed foce from the bellows . As the artificial muscle can be kept at initial length, this friction clutch reduces displacement dependence of an artificial muscle. In this paper, the structure of tthe pneumatic friction clutch is described and then the influence of the device on gait are confirmed experimentally.

The purpose of this study is to develop a soft robot hand that can grasp any shape or characteristic object due to the expansion of the actuator applied positive pressure and vacuum adsorption due to negative pressure. The actuator made of soft material has advantage to grasp a complex shape or fragile objects and can pick up a flat plate and objects with large radius by vacuum adsorption. In addition, by generating negative pressure from positive pressure source, it is possible to operate this a system with a single pressure source. In this paper, we describe the outline of the soft fingers and then the performance of this finger is verified experimentally.

McKibben type artificial rubber muscle has high affinity with human body due to mechanical flexibility and light-weight. It is widely used for personal care robots as the reason for above. Therefore, several models of artificial muscle based on energy conservation law to control the artificial muscle had been proposed in previous studies. However, parameters in the previous study are determined from trial-and-error to decrease the modeling error, but these determined parameters do not necessarily show the actual values of physical property of material. Purpose of this study is to construct a model from which the characteristic of McKibben type artificial muscle can be calculated based on only design parameters and actual rubber physical property. In this paper, first of all we propose expansion plane model to consider balance state of several forces. Subsequently, elastic force owing to rubber material is modeled based on Mooney-Rivlin model. And then the precision of proposed model is confirmed experimentally.

In a pneumatic power assist robot, it has been technical issue that peripheral pneumatic elements such as air source and air valve increase whole weight of air system. Pneumatic mechanical elements had been developed in this study. The developed air source, which can retrieve compressed air from an actuator by using the variable volume tank, is effective to decrease energy consumption. The developed valve regulates air flow rate by constricting air tube put in valve. The developed mechanism can realize weight reduction of the air system.

The purpose of this study is to develop a holding device which has versatility and human friendliness. A conventional granular jamming gripper has some technical issues such as difficulty in holding small objects, gravitational influence on granular. Therefore, we develop the hybrid jamming holding device composed of granular and porous materials and a thin sheet. This hybrid structure can improve holding performance by compensating for a mutual weaknesses. In addition, a bending actuator is installed on the holding device to grasp an object. In this paper, we describe principle and structure of the device and then the grasping performance is confirmed experimentally.

The purpose of this study is to verify the effect of changing the material of only end on the contraction force characteristics of McKibben type artificial rubber muscle. In previous studies, it was thought that the hysteresis characteristics of artificial muscles were influenced by the material properties of the ends. In this paper, we made artificial muscles with different material properties at the ends. The contraction force characteristics were compared with the characteristics of artificial muscles composed of a single material and the calculated values of the artificial muscle model. From the comparison, the effect of changing the material properties at the ends was verified.

Stability and efficiency of walking can be obtained according to trunk motions such as side flexion and rotation. In this study, spine type wearable device which can correct a posture and assist these trunk motions was developed to support exercise therapy for people declined in ability of walking. Proposed device has advantage of high affinity with human body owing to adjustable stiffness mechanism. In this paper, the mechanism of device, design method of artificial muscle based on rubber property and assisting method are described. The mechanism of device imitates spine structure to realize adjustable stiffness characteristics. We applied an artificial muscle model to simplify of mechanism and control method. Finally, the effectiveness of proposed mechanism and assisting method was confirmed from experiment result.

Lack of muscle strength due to aging causes reduction of stable gait motion. Rehabilitation for the recovery of trunk motion during walking and muscle strength are important to obtain stability of walking. In this study, we developed wearable device which can correct a posture and assist trunk rotation to support exercise therapy for people declined in ability of walking. Proposed device is composed of some resin parts with built-in ball bearing structure and pneumatic soft actuator such as McKibben type artificial muscle and bellows. The mechanism of proposed device imitates spine structure to realize adjustable stiffness characteristics. In this paper, the mechanism for rotation assist and drive principle of device are described. The extension characteristic of pneumatic bellows is confirmed to estimate assisting torque. Finally, the validity of proposed mechanism was confirmed from experiment result.

We proposed mathematical model of McKibben type artificial rubber muscle, which considers the hysteresis characteristics, in previous work. The expansion force generated in the radial direction of the artificial muscle was calculated under the cylindrical pressure receiving area in previous model and the shape difference of the pressure receiving area was large from the actual artificial muscle. In this study, accuracy of this model is improved by calculating the pressure receiving area as a combination of a cylindrical shape at center and hemispherical shape at both ends. In this paper, outline of proposed model is discussed and then this model is evaluated by comparing with calculated value of previous model and actual measured value.

In this study, the variable friction dumper driven with pneumatic actuator is developed to correct body movement. The developed device is constructed with friction members which are made with rubber, and pneumatic artificial rubber muscle, nylon belt. By controlling inner pressure of rubber muscle or changing number of rubber muscles, nylon belt pull-out force generated from friction force between friction members can be changed. It is assumed that this pull-out force is used to correct movement. In this paper, structure and principle of this device are described and then the characteristics are discussed.

The purpose of this study is to estimate force characteristic of short-length McKibben type artificial rubber muscle using artificial muscle model. In previous works, we proposed mathematical model of McKibben type artificial rubber muscle considered the hysteresis characteristics and estimated the force characteristic of long-length McKibben type artificial rubber muscle based on proposed model. However, the force characteristics of short-length McKibben type artificial rubber muscle can not be estimated because it can not be represented as the virtual pantograph in the previous model. Thus, the virtual pantograph structure is augmented based on the ratio of diameter to length for applying to artificial muscles of various lengths. In this paper, the augmented model was compared with the force characteristics of actual McKibben type artificial rubber muscles of various lengths. From the results, it was confirmed that proposed model can estimate the force characteristics of short-length McKibben type artificial rubber muscle.

Kyphosis causes decline in respiratory and circulatory functions owing to compresses of internal organs. The purpose of this study is to develop a posture keep assist device. The device is constructed with multiple resin components and McKibben type artificial rubber muscle. This device at initial state has a flexibility because the resin components are separated. On the other hand, the resin component can be connected by pressurized artificial muscle. The shape of device in which all resin components are connected imitates spine-curve to fit human back. So, the proposed device can retain entire range of trunk. Target pressure to correct posture is determined from the mathematical model of artificial muscle. In this paper, the structure of device is described, and control method to retain posture based on artificial muscle model is discussed. Finally, effectiveness of the device is verified from experiments in which subject wears this device actually. We reveal correcting trunk posture by using the device from the results.