# A Mechatronics-Embedded Pneumatic Soft Modular Robot Powered via Single Air Tube
## 1 Introduction  
The concept of modular robots has been proposed since the 1980s to solve the universal property and flexibility problems of robots with fixed configurations [1]. A modular robot consists of unified modules with the same structure and function to realize reconfigurable configurations for adapting to various tasks and changeable environments. The unified module has communication, controller, actuator, sensor and connecting mechanisms, which can connect with each other to form different configurations. Many researchers have developed rigid modular robots, including Molecules [2], Yamor [3], Polybot [4], ATRON [5], CONRO [6], M-TRAN [7], SuperBot [8], UBUot [9, 10], and Seremo [11]. These modular robots can alter their structure by connecting the different faces of their modules. However, rigid modular robots have finite degrees of freedom, and rigid materials are dangerous for fragile objects and operating personnel. Therefore, the use of soft materials to replace rigid materials to manufacture modular robots is a novel research direction. Soft modular robots have advantages including infinite degrees of freedom and various configurations [12]. Soft modules are manufactured using soft materials, such as silica gel or rubber, which have characteristics of flexibility and continuous deformation. Therefore, many researchers have studied soft modular robots, including omnidirectional soft robots [13], rotary soft module and bending soft module [14], three types of pneumatically actuated soft modules (SoBL) [15], the soft modular robot with connecting part [16], elastomeric hollow cubes with magnetic connecting mechanisms [17], bidirectional bending modules [18], pneumatic actuators with four chambers [19], multi-spherical modular soft robots [20; 21; 22; 23], bending-type fluidic elastomer actuators [24], modular soft robots with voice coil actuators [25], soft-bodied caterpillar-like modular robots [26], and vacuum-powered soft pneumatic actuator (V-SPA) modules [27; 28]. Some researchers have concentrated their attention on the structure of the soft module. [PERSON] presented the rotary soft module and bending soft module, which can be combined with other actuators as part of a complete manipulator system [14]. [PERSON] designed three types of pneumatically actuated soft modules, which can connect with each other through a unified connecting part [15]. [PERSON] manufactured and presented a pneumatic soft modular robot with a novel connecting mechanism to achieve a docking function [16]. [PERSON] designed a chamber's structure of soft module, which has a bidirectional bending ability [24]. Meanwhile, a number of researchers have concentrated on the soft modular robot's bionic motion. [PERSON] proposed a reconfigurable omnidirectional soft robot based on the locomotion of caterpillars. The pneumatic soft robot is capable of moving with three DOFs (degree of freedoms) on a plane [13]. [PERSON] proposed a new soft modular robot to simulate replicating morphogenetic movements of an embryo, which provide tangible models of cell behavior [17]. [PERSON] manufactured a soft module with four chambers and analyzed the gait of a soft hexapod robot [19]. [PERSON] presented a multi-sphere modular robot to realize an obstacle avoidance motion [21]. [PERSON] proposed a soft-bodied caterpillar-like modular robot to simulate caterpillar motion [25]. Nevertheless, most soft modular robots are actuated by inflating air pressure. Therefore, each chamber of the soft module needs to be connected with an air tube, which provides air pressure to actuate the deformation of the module. However, air tubes incur drag and intertwining problems that affect the motions of the soft modular robots. Moreover, the problems cannot be solved by reducing the number of chambers, which directly affects the deforming ability of the soft robot. Therefore, it is critical to design the soft modular robot with a shared air source and guarantee the deformations. A few researchers have investigated the gas source sharing issue.