# Pneumatic Soft Actuator with Anisotropic Soft and Rigid Restraints for Pure in-Plane Bending Motion
###### Abstract  
A variety of soft robots with prospective applications has been developed in recent years. As a key component of a soft robot, the soft actuator plays a critical role and hence must be designed carefully according to application requirements. The soft body may deform in undesired directions if no restraint is endued, due to the isotropy of the pure soft material. For some soft robots such as an inchworm-like biped climbing robot, the actuation direction must be constrained with the appropriate structure design of the soft actuator. This study proposes a pneumatic soft actuator (PSA) to achieve pure in-plane bending motion with anisotropic soft and rigid restraints. The in-plane bending pneumatic soft actuator (2D-PSA) is developed with a composite structure where a metal hinge belt is embedded into the soft material. The design method, material choice, and fabrication process are presented in detail in this paper. Tests are conducted to measure the actuating performance of 2D-PSA in terms of the relationship between the bending angle or force and the input air pressure. Dynamic response is also measured with a laser tracker. Furthermore, a comparative experiment is carried out between the presented 2D-PSA and a general PSA, with results verifying the effectiveness of the presented 2D-PSA. A robot consisting of two serially-connected 2D-PSAs and three pneumatic suckers, which can climb on a flat surface mimicking a snake's locomotion, is developed as an application demo of the presented 2D-PSA. Its locomotion capability presents the in-plane performance and mobility of 2D-PSA. soft robot; pneumatic actuator; composite structure; soft and rigid restraint; in-plane bending 2020 IEEE/RSJ International Conference on Robotics and Automation (ICRA), Vol., pp., pp. freely in some DOFs and are restrained in others. Thus, both soft and rigid restraints are employed to design the structure. Common soft actuators are constructed as monolithic structures from soft materials, such as silicone rubbers [2; 3], electroactive polymers [6; 7; 8], shape memory alloys [9; 10], elastomers [11; 12], hydrogels [13], or composites [14; 15]. Actuation methods include a variety of stimuli, such as electrical charges [6; 7; 8; 9; 10], chemical reactions [2; 16; 17], and pressurized air or fluids [12; 18]. Although their size, structure, material, and stimuli generally differ, their actuation motion can be divided into three basic types: bending, extending/contracting, and twisting [19]. These actuators can be configured into soft robotic systems to fulfill designated applications. However, the majority of current soft robots are designed and fabricated for specific movements. The literature includes some exceptions, such as the application of a rapid mechanical programmable method to change the bending radius and bending axis by modifying the mechanical structure [20]. A bipolar network made of soft materials was studied both experimentally and numerically to evaluate the precise role of geometry [21]. However, these studies were able to achieve their target motions, but not address the design method to make them anisotropic rigorously. In other words, these robots were able to move as required, but were also able to be forced to deform passively by unexpected loads. The problem related to variable stiffness design and control is regarded as the core challenge of soft robotics [22]. Two main approaches are summarized in this paper: the use of active actuators arranged in an antagonistic manner and the use of semi-active actuators that can change their elastic properties ([PERSON]'s modulus). Actuators with variable and controllable deformability and compliance have been successfully applied to develop many robot systems [23], like a medical robot [24; 25] and a wearable robot [26], as well as to achieve robot manipulation [27] and locomotion [28; 29]. The cited works have used active, active-passive, or semi-active solutions to achieve variable stiffness. However, variable stiffness usually needs complex and active control.