## 2 Design
### Overview of SMMRob System and Module  
The integrated modular robot system, the SMMRob system, can have a tree-like topology. Most of this system consists of repeated homogeneous robotic modules, called SMMRob modules. Each module has one active docking panel and/or one or multiple passive docking panels. An SMMRob module can use its active docking panel to connect with the passive one of another module. Each module is self-contained. One of the most commonly used modules is the motion module, used as a composite robot joint body, but to increase their variety, other modules may have additional functions, such as having more cameras, lidars, batteries, or more docking panels, e.g., SMMRob sensing modules, power modules, base platforms, etc. The homogeneous SMMRob modules with different payloads can move, sense, manipulate, and dock individually. Furthermore, in this paper, the main configuration that the robot modules can connect together to form is a tree-like structure with a thin body, and such a structure may make use ofthe modules' caterpillar track or joints, as shown in Figure 2. The motion modules are used to build the body and the sensing modules are used for the head. The SMMRob module (the default module, referring to the motion module) consists of a thin rectangular chassis, a track drive unit, a docking mechanism, and a joint actuation mechanism, as well as electronic hardware for sensing, control, communication and localization, as shown in Figure 3. A docking mechanism is installed on the front face of the chassis and there is a passive face on the back of the base. The docking mechanism enables each robot module to link to the tail of another module. Figure 1: Limited capacity of individual robots and enhanced capacity of self-assembling mobile modular robots by mechanically connecting their modules together. In the front view, it can be seen that there are different terrains such as the ground, obstacles, highland, gaps, and stairs blocking single modular robots, whereas these modular robots can assemble into an articular structure to overcome these difficulties, as shown in the top view. The size of each robot module is 296 mm (length) \(\times\) 68 mm (height) \(\times\) 60 mm (width). The chassis provides the base for the track drive unit, which is a differentially driven and can move forward and backward and rotate. A drive pulley for each track is driven by a gear-reduced DC motor (DC 7.4V, KingMax, China) with a magnetic encoder. There are two idler pulleys on each side to support the track and adjust the tension. A summary of parameters such as dimensions, weight, actuation strength, etc., are shown in Table 1.  
The sensing module at the head of the snake-likesystem is equipped with a laser distance sensor (TFmini-S, operating range: 0.1-12 m, Benewake, China), a stereo camera, and a wireless video transmitter. The lidar can measure the distance of the target in front and can even be combined with a pitch and yaw joint to build a sparse point-cloud model of the environment. The stereo camera, providing video with 720P resolution and at 30 \begin{table}
\begin{tabular}{c c} \hline
**Term** & **Value** \\ \hline Dimensions & 296 \(\times\) 68 \(\times\) 60 mm \\ Weight & 900 g \\ Battery life & 120 min (track motion) \\ Max. linear speed & 40 cm/s \\ Max. payload & 8 kg \\ Communication & ZigBee/WIFI/CAN bus \\ Sensor & Camera/UWB/IMU/Encoder \\ Joint torque & 2.7 Nm \\ Range of yaw & \(-\)30\({}^{\circ}\)\(\sim\) +30\({}^{\circ}\) \\ Range of pitch & \(-\)45\({}^{\circ}\)\(\sim\) +45\({}^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 1: A summary table of the parameters of SMRRob modules. Figure 2: A self-assembling mobile modular robot system consisting of a group of self-contained robot modules and modules with a different function forming the head. (**a**) Each homogeneous SMRRob module with a different payload can move, sense, manipulate, and dock individually. (**b**) The robot modules can connect together to form a snake-like high-level system that can move on tracks with joints.