Prototype Design of a Rubik Snake Robot Xin Zhang and Jinguo Liu
Abstract This paper presents a reconfigurable modular mechanism—Rubik Snake robot, which can change its configurations by changing the position relationship of modules. The geometric characteristics and design process of the module are described in detail, and the module’s features ensure Rubik Snake robot to own a strong ability of transformation and manipulation. Through the structural statics and dynamics analysis, the mechanical properties of the system are verified to meet the requirements of missions. Furthermore, a platform was fabricated, and a configurations experiment identified the rationality of the platform. Two potential space applications of Rubik Snake robot: reconfigurable structures and reconfigurable mechanisms, are introduced to fulfill the future space station missions. We hope our study will give insights into the reconfigurable modular robot applications on future space application. Keywords Reconfigurable modular mechanism Structural statics and dynamics analysis
Geometric characteristics
1 Introduction The reconfigurable modular robot is an important branch of the robot family. Compared with the conventional fixed robots, reconfigurable modular robots have their own outstanding features: adaptability, flexibility and robustness [1]. X. Zhang J. Liu (&) State Key Laboratory of Robotics, Shenyang Institute of Automation (SIA), Chinese Academy of Sciences, Shenyang, China e-mail:
[email protected] X. Zhang e-mail:
[email protected] X. Zhang University of Chinese Academy of Sciences, Beijing, China © Springer International Publishing Switzerland 2016 X. Ding et al. (eds.), Advances in Reconfigurable Mechanisms and Robots II, Mechanisms and Machine Science 36, DOI 10.1007/978-3-319-23327-7_50
581
582
X. Zhang and J. Liu
Reconfigurable modular robots can transform their configurations to adapt themselves to the unpredictable environment, so they are adapted to some unstructured environment such as: ruin rescue, planetary exploration and on-orbit servicing [2, 3]. The ability of changing shape directly determines whether the reconfigurable modular robot can fulfill the requirement of tasks, so the research on configurations is the core question of reconfigurable modular robots. For example, a reconfigurable modular robot can transform into a snake shape to pass through a narrow tunnel and reorganize as a multi-legged robot to traverse the rough terrain [4]. The whole reconfigurable modular robot is composed of homogeneous or heterogeneous modules, so the whole performance of locomotion and manipulation is highly dependent on the characteristics of modules. The hardware design of modules is one of the most important sections, and many factors need to be considered in the course of conception design, such as: degrees of freedom, structural dimension and connection [5]. The existing reconfigurable modular robots can be divided into three categories: chain architecture, lattice architecture and hybrid architecture [6]. The chain robots generally have a chain or tree topology and the structures are relative simple, and each module have an independent ability of locomotion. However, the motion controlling is arduous to realize. The lattice robots generally are connective or filled array topology in 2D or 3D space, and the structures are relatively complicated, and the implementation of lattice systems still faces difficulties. The hybrid robots can reconfigure into both chain and lattice structures and provide high versatility in reconfiguration. Corno [7], PolyBot [4] AMOEBA-I [8] and Molecubes [9] are typical chain systems; Cystalline [10], Miche [11] and Odin [12] are typical lattice systems; Hybrid systems such as: M-TRAN [2], SuperBot [13] and Sambot [14]. In brief, in the past two decades, a variety of reconfigurable modular robot platforms have been developed and lots of breakthrough improvements have been made in the field of reconfigurable modular robots. In the aspect of applications, Goeller [15] proposed the concept of modular robots for on-orbit satellite servicing. As we know, general space missions have strict requirements, such as compactness and lightness, robustness, versatility, and adaptability [1]. Some platforms are designed for space exploration, such as Corno, PolyBot and SuperBot. Some potential space applications need to be studied profoundly. In this paper, we presented a reconfigurable modular mechanism named Rubik Snake Robot with a strong ability of transformation. Inspired by the toy, Rubik’s Transformable Snake, we designed the module and analyzed the mechanical properties of our robot. Then the platform was fabricated and the configuration experiment identified the rationality of our platform. Finally the conception applications for the future space station tasks were introduced. This paper is organized as follows. Section 2 introduces the origin of our design and the principle of reconfiguration and the geometric characteristics of modules. Section 3 introduces the design of the 3D model, the structural statics and dynamics analysis, driving torque analysis and the platform and the configuration experiment. Finally, the conclusion and future works are provided in Sect. 4.
Prototype Design of a Rubik Snake Robot
583
Fig. 1 The inner structure of the Rubik’s Transformable Snake toy
2 Design Origin Over millions of years of evolution, human beings gradually reached the dominant position in nature, because we invented a variety of tools inspired by the nature. In the field of robots, researchers also have invented some advanced robots inspired by biological organisms and behavior [16]. The idea of our design origins from the popular toy—Rubik’s Transformable Snake, which consists of 24 right isosceles. Figure 1 shows the inner structure of Rubik Snake toy, and the prisms are connected by spring bolts which allow a rotation between the two connected units. Moreover, each unit can adopt 4 different positions each with an offset of 90° [17]. By being rotated, the Rubik’s Transformable Snake toy can be reassembled into a wide variety of geometric shapes as shown in Fig. 2. We will take advantage of its transformation ability to design a similar reconfigurable mechanism.
Fig. 2 The Rubik’s Transformable Snake toys consisted of 24 prisms can transform into many different configurations: a the linear structure, b the frame pattern, c the cuboid shape, d the triangle shape, and e the ball shape
584
X. Zhang and J. Liu
Fig. 3 The geometry of modules
2.1
Geometric Characteristic
The module is the basic element of robots, and the geometry of modules has an important role in the design of the reconfigurable modular robot and directly determines the locomotivity and reconfigurability of the robot. Generally, the modules are geometric structures with certain volume [18]. In Fig. 3, the modules of the robot are regular triangle prism structures, and the two bases of the module are isosceles right triangles, namely, △adf and △bce. The length of lab, lbc and lce is l. The rotated axis Si of two adjacency modules is perpendicular to the contact surfaces □dcef and intersecting at the center of □abef, point O. Each module has an actuated DOF. The entire structure of the robot is capable of being reconfigured by rotating the modules. The design of the robot will be discussed in the next paragraph.
3 System Design According to the relationship of geometry of modules, we imitate the mechanism of Rubik toys to design the module. One of the most important steps is to select a suitable actuator, which can fulfill the precise control of rotation. For the purpose of saving interior volume and easy controlling, we choose the digital servo motor as the actuator, which integrates a DC motor, a gear box, an angle sensor and a control circuit. The other step is designing the docking mechanisms, which can roughly be divided into two categories: mechanical connector and electromagnetic connector. We choose the mechanical flange as the connector, which can guarantee the stability of the connection, and the process of connection needs to operate manually. Other components are the shells, and the model is shown in Fig. 4: (a) is the module, (b) is the digital servo motor, (c) is the mechanical flange and (d) is a robot with three modules. The flange has two interfaces: a loop surface and a round surface. The loop surface is connected with the shells of the first module and the round surface is connected with the interface of the servo motor of the second module. The maximum torque of actuator is 1764 N mm, and the material of the flange and shells are made of aluminum alloy.
Prototype Design of a Rubik Snake Robot
585
Fig. 4 The 3D model of the robot
3.1
Model Analysis
As our robot is designed to work in outer space, we needn’t consider the effect of gravity. But we can’t ignore the gravity in the earth, as the number of modules increase, and the influence of gravity on earth will be greater. So the number of modules should be considered, and we designed a prototype with three modules. Before the fabrication of the prototype, the performance of our prototype should be validated. In other words, we should check whether our design is reasonable. Therefore, we will analyze the mechanical properties of our prototype from two aspects of statics and dynamics.
3.1.1
Structural Statics Analysis
The structural statics analysis of key components is an important part in designing the whole system. In the process of forming a configuration, the docking mechanism transmits the torque from the actuator to the next module, so the mechanical flange is the key component which suffers a greater force. We input the 3D model into the finite-element software and adopted the finite-element method to analyze the key component. The process of analysis was as follows: The small surface of the flange was fixed; then the output torque of the actuator was acting on the big surface. The highest torque of the actuator is 1764 N mm. The finite element model and the analysis results are given in Fig. 5. The maximum stress of flange is 5.63 MPa, and the maximum strain of flange is 0.0125 mm. The yield stress of aluminum alloy is 195 MPa, so the flange can meet the use requirement.
586
X. Zhang and J. Liu
Fig. 5 The model and analysis results: a the geometric model of the flange, b the finite element model of the flange, c the stress distribution of the flange, and d the strain distribution of the flange
Fig. 6 First six modes
3.1.2
Structural Dynamics Analysis
Modal analysis provides a powerful tool for the structural design and performance evaluation of various products and the results are often used as effective standard product performance evaluation, so the modal analysis is very necessary to the structural design of our module. Each module has an actuator, and the different speed will result in different vibrations. The range of the actuator speed is definite, furthermore, the range of operating frequency is given. We should guarantee the inherent frequencies of the module structure to avoid the resonance. We assume that
Prototype Design of a Rubik Snake Robot Table 1 First six modes and description
587
Modes
Frequency (Hz)
Vibration description
1 2 3 4 5 6
0.0041323 3889.2 4102 4519.2 4872 6514.8
Figure Figure Figure Figure Figure Figure
6a 6b 6c 6d 6e 6f
the two joints are not rotating at the same time, and we also adopted the finite-element software to analyze the shell structure and calculated the models of the first six modes. The corresponding inherent frequencies and vibrations are shown in Table 1. The operating frequency of our actuator is 0.85–3.4 Hz, so our prototype can work well.
3.1.3
Driving Torque Analysis
The mechanical strength of key components can meet the operating requirements. We should evaluate whether our actuator can supply enough power. We adopt the multi-bodied dynamics software to analyze the driving torque of the system. To improve the computational efficiency, we should remove some insignificant components to simplify the model. The process of analysis was as follows: Firstly, we imported the model into the software. Secondly, the environmental and material parameters were set consistent with the actual circumstances. Thirdly, the constraint settings depended on the assembly relation of system. We added two rotating drives to the connecting surfaces of actuator and modified the angular velocity as 30 °/s as shown in Fig. 7. The time of acceleration was 2 s; the whole time of simulation was 15 s, and it was divided into 1000 steps. The first drive controlled the second and the third modules, and the second drive only controlled the third module. We can get the value of output torque of drives. From Fig. 8, the maximum value of the first drive was 37.22 N mm, and the maximum value of the second drive was 223.72 N mm. Both change regulation of torque followed the periodic curve. The highest torque of our actuator was 1764 N mm, so our actuator can fulfill the operating requirements and our prototype can work steadily.
Fig. 7 The drives of model: a the first drive and b the second drive
588
X. Zhang and J. Liu
Fig. 8 The output torque of drives
3.2
Experiment
The model analysis identified that our design can function properly with good safety and reliability, so we developed the platform of our reconfigurable modular robot as shown in Fig. 9. In order to further validate our actuator whether can supply enough power to the system, we complied the program of controlling the actuator to accomplish some configurations. In Fig. 10, the configuration (a) was the initial configuration and we controlled the rotation of the third module and obtained the configuration (c). And then we controlled the rotation of the second module and obtained the configuration (e). In the process of changing configurations, we observed that the changes were smooth. The experiment result identified that the prototype structure was an available platform.
Prototype Design of a Rubik Snake Robot
589
Fig. 9 The platform of Rubik Snake
Fig. 10 The configuration experiment
4 Conclusion and Future Works This paper presented a reconfigurable modular mechanism which was inspired by the toy—Rubik’s Transformable Snake. Compared to other reconfigurable modular systems, our system was also a transformable structure. The module geometry and design were described in detail. Furthermore, the experiment prototype was designed, and the mechanical performance analysis of system identified that our prototype can meet the operating requirement. For the future space station tasks, two corresponding conception applications of our robot are proposed. The outstanding distinction from other reconfigurable modular robots is our robots can not only be used as a reconfigurable mechanism but also be used as a transformable structure as shown in Fig. 11. They can be used as space manipulators for space station assembly and maintenance: moving equipment and supplies around the station and servicing instruments and other payloads attached to the space station. Meanwhile, they can transform into some standard geometric structures, such as: cuboid, cube and prism serving as the basic elements in the construction of the space station.
590
X. Zhang and J. Liu
Fig. 11 The conception of space applications of Rubik Snake robots: a assembling the foldable solar panels and b working as the geometric structure elements in constructing space station
For future applications, the core question is how to accomplish a desirable configuration for the assigned mission. And our research will focus on the configuration analysis of the robot, meanwhile, we will improve our design with the ability of anti-water. We will implement a set of theories on target configurations in the underwater condition. Therefore, we can ignore the effect of gravity, and the number of modules will be not limited. Acknowledgments This research is supported by the National Natural Science Foundation of China (51175494, 61128008) and the State Key Laboratory of Robotics Fund.
References 1. Yim, M., Roufas, K., Duff, D., Zhang, Y., Eldershaw, C., Homans, S.: Modular reconfigurable robots in space applications. Auton. Robot 14, 225–237 (2003) 2. Murata, S., Yoshida, E., Kamimura, A., Kurokawa, H., Tomita, K., Kokaji, S.: M-TRAN: Self-reconfigurable modular robotic system. IEEE ASME T. Mech. 7, 431–441 (2002) 3. Gao, Y., Liu, J.: China’s robotics successes abound. Science 345, 523 (2014) 4. Yim, M., Zhang, Y., Roufas, K., Duff, D., Eldershaw, C.: Connecting and disconnecting for chain self-reconfiguration with PolyBot. IEEE ASME T. Mech 7, 442–451 (2002) 5. Yim, M., Shen, W.M., Salemi, B., Rus, D., Moll, M., Lipson, H., Klavins, E., Chirikjian, G.S.: Modular self-reconfigurable robot systems [grand challenges of robotics]. IEEE Robot. Autom. Mag. 14, 43–52 (2007) 6. Murata, S., Kurokawa, H.: Self-reconfigurable robots, shape-changing cellular robots can exceed conventional robot flexibility. IEEE Robot. Autom. Mag. 14, 71–78 (2007) 7. Castano, A., Behar, A., Will, P.M.: The Conro modules for reconfigurable robots. IEEE ASME T. Mech 7, 403–409 (2002) 8. Liu, J., Ma, S., Wang, Y., Li, B.: Network-based reconfiguration routes for a self-reconfigurable robot. Sci. China Ser. F Inf. Sci. 51, 1532–1546 (2008) 9. Zykov, V., Mytilinaios, E., Desnoyer, M., Lipson, H.: Evolved and designed self-reproducing modular robotics. IEEE T. Robot 23, 308–319 (2007) 10. Rus, D., Vona, M.: Crystalline robots: self-reconfiguration with compressible unit modules. Auton. Robot 10, 107–124 (2001)
Prototype Design of a Rubik Snake Robot
591
11. Gilpin, K., Kotay, K., Rus, D., Vasilescu, I.: Miche: modular shape formation by self-disassembly. Int. J. Robot. Res. 27, 345–372 (2008) 12. Lyder, A., Garcia, R.F.M., Stoy, K.: Mechanical design of odin, an extendable heterogeneous deformable modular robot. In: IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 883–888. IEEE Press, Nice (2008) 13. Salemi, B., Moll, M., Shen, W.M.: SUPERBOT: a deployable, multi-functional and modular self-reconfigurable robotic system. In: IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 3636–3641. IEEE Press, Beijing (2006) 14. Wei, H., Chen, Y., Tan, J., Wang, T.: Sambot: a self-assembly modular robot system. IEEE ASME T. Mech 16, 745–757 (2011) 15. Goeller, M., Oberlaender, J., Uhl, K., Roennau, A., Dillmann, R.: Modular robots for on-orbit satellite servicing. In: International Conference on Robotics and Biomimetics, pp. 2018–2023. IEEE Press, Guangzhou (2012) 16. Pfeifer, R., Lungarella, M., Iida, F.: Self-organization, embodiment, and biologically inspired robotics. Science 318, 1088–1093 (2007) 17. Rubik’s Snake. http://en.wikipedia.org/wiki/Rubik%27s_Snake 18. Ding, X., Lu, S.: Fundamental reconfiguration theory of chain-type modular reconfigurable mechanisms. Mech. Mach. Theory 70, 487–507 (2013)
