Through comparison, it can be clearly seen that the granularity of the meta module proposed by our group is optimal. It is composed of only two basic modules, with regular and compact shape and strong motion ability. Although such a comparison is unfair, because the structural characteristics of the unit modules and the different target tasks will lead to a certain differences in the design of the meta-modules, it can be seen that the superiority of the mutual assistance. In order to meet the carrier weight and space requirements of launch vehicle and reduce the potential for mechanical failure, the module was designed to be extremely simple, with only two L-shaped surfaces and a revolute joint. Hence, stronger mobility means that more modules should be combined and two modules are far from enough. As shown in previous description of Mover and Tumbler, both of them have motion defects. However, through mutual assistance, full mobility is achieved, and the granularity of the meta modules remains at the size of two modules. Therefore, mutual assistance is of great significance to the research of non-mobile modules, which is a further improvement on the meta-module method.
2. Self-assembly of Space Module
Based on the premise assumption that it is necessary to place the cargo in a centralized and regular manner during the rocket launch process, the initial stacking form of meta-modules is shown in Figure 11, Figure 12 and Figure 13 . Although they are stacked together, the meta-modules are separated from each other, which is not fundamentally different from the scattered distribution, and will not affect the self-assembly process.
In the self-assembly process, the required meta-modules are driven out from the library in turn and move to the assembly position assigned to them. During the movement, there will be corresponding meta-modules to assist them to change direction or rotate. When all meta-modules reach the target positions, joint postures are adjusted uniformly to ensure reliable connection surfaces. Finally, the assembly configuration can be assembled, and then the quadruped robot, humanoid robot and Super redundant manipulator are formed by adjusting the joint angles.
The situations contained in the experiments is comprehensive. Firstly, it contains all the motion forms of the meta-modules, including the tumbling motion of Tumbler, the movement of Mover and the tumbling motion of Rotated Mover. Secondly, it contains all forms of mutual assistance, including Mover helping Tumbler to change its forward direction and Tumbler helping Mover to rotate. Finally, it contains various positions and postures in the target configurations, including horizontal or vertical placement and different rotation angles. Hence, the above experiments are a strong proof for the effectiveness of the self-assembly method proposed in this paper.
Conclusion
The concept of meta-modules mutual assistance is proposed inspired by biological cooperation and mutual assistance, which makes non-mobile modules no longer need to seek a balance between granularity and mobility. Then an assembly planner is designed to achieve autonomous assembly while avoiding assembly conflicts and positional blockage. Some experiments demonstrate the effectiveness of the method. Through this method, the self-manufacturing of various robots can be realized, which will make a good start for the development of the unmanned and adaptive robotic system of the lunar base.
In next work, hardware experiments will be refined and an attempt will be make to apply to a specific task, and then the assembly strategy will be further optimized to reduce the constraint of the configuration using the method of configuration decomposition and component assembly. In addition, the method of meta-module mutual assistance has high application value in the fields of self-reconfiguration and self-repair, which is worth further exploration.
Acknowledgements
This study was supported by National Outstanding Youth Science Fund Project of National Natural Science Foundation of China (Grant no. 52025054).
Conflict of interest
The authors declare no conflicts of interest.