Crossing Small Voids: Practical Considerations

Niken Syafitri Richard M. Crowder Paul H. Chappell Electronics and Computer Engineering Electronics and Computer Engineering Electronics and Computer Engineering University of Southampton University of Southampton University of Southampton Southampton, UK Southampton, UK Southampton, UK

Abstract—This paper discusses a strategy for a robotic swarm, of assembling all robots into structure, after which the whole consisting of small low cost mobile robots, to cross a void structure climbs, avoids or crosses a void. After crossing or crevice in the environment. To cross the void, the swarm the obstacle, the structure disassembles. Unfortunately, this robots are required to autonomously construct a structure. The individual swarm robots have no knowledge of the swarm size, method has significant problems when the size of the void the void location or its width. In addition all robots have an changes or is unknown. In addition a strategy must be provided equal probability to locate the void and initiate the structures if the swarm cannot cross the obstacle, to maintain the construction. Once all robots have crossed the void, they disas- integrity of the robotic swarm. semble and continue with the task. If the swarm determines that From a swarm perspective, research has fo- there is not enough robots to reach the other side of the void while maintaining the structure’s stability, the structure will retreat and cused on control and application [15], [16], [17], mostly the robots disperse. This paper discusses the algorithm required achieved through simulation. This can leave a lack of un- to cross the void and the resulting simulation results, together derstanding about exactly how real-time interconnections of the practical challenges of implementation such a system using robots to form structures is possible. From the current self- low cost mobile robotic platforms. assembling/reconfiguring modular swarm robotic systems, the approaches for interconnecting units vary quite significantly. I.INTRODUCTION This is because almost every swarm robotics platform or Social insects can perform sophisticated tasks when work- modular robot has significant differences in sensing, communi- ing collaboratively, under unpredictable conditions, which cation and actuation, hence each robot family requires a unique are impossible if they work independently, for example ants interconnection ability. It is important to note that docking crossing voids, bees and wasps building their nests, and recognition and inter-robot communication are as significant termites building nests with complex internal structure [1], [2]. as the physical approaches to docking. The robots need to be Taking the inspiration from nature, especially ant colonies, the able to interconnect and release intelligently. adaptivity of real-world systems to overcome unstructured and In order to maintain the structural integrity and the rigidity unknown environment can be achieved through the application of the structure, we have placed a limit on the size of the void of carefully designed algorithms and mechatronic. This is that can be crossed by a linear simple structure to be no more reflected in the developed of robotic swarms. The characteris- than the equivalent of five robots wide. If the void is wider that tics of a robotic swam include (1) autonomous mobility; (2) this, a far more complex structure is required, the design and limited sensing and communication abilities; (3) simplicity; implementation of which is outside the scope of this paper. (4) decentralized control and coordination mechanism; (5) The paper is structured as follows, the required strategy homogeneity and (6) [3]. This paper considers the is presented in Section II, simulation results are discussed in development of an approach that allows a robotic swarm to Section II-B. Section III considers the design constraints for autonomously cross a void, in particular the algorithm and the a small capable of crossing a small void. The robots design specification. paper concludes with Section IV. While solutions have proposed heterogeneity to solve the II.STRATEGY FOR VOID CROSSING unstructured environment problem [4], [5], it can be costly as a range of robots are required and each type should be In developing the approach to crossing a void by au- present in large numbers. Related approaches to problems such tonomously constructing a self supporting structure, a number as climbing obstacles [6], [7], [8], [9], and avoiding holes or of broad assumptions have been made regarding the robot and traverse a void [10], [11], [12], [13] by assembling swarm its operational environment, namely; robots into a structure or robotic organism have been proposed. • The surface on the environment is smooth. A complimentary approach was reported in [14] discusses • The elevation of both sides of the void are identical, the the development of a vertical self-assembled structure that sides of the void are vertical. mimics the pyramidal tower structure of robots. However, • The robot platform being used is based on our previously many of the reported proposed approaches employ the strategy developed low cost Formica robot [18]. The upgraded (a) The robot that locates the void halts at Fig. 2. The number of robots required in the structure to cross a three robot the edge and waits until two addition robots wide void. couple to it.

The strategy of small void crossing is centered on maintain- ing the correct number of robots in the structure, to ensure that (b) Once three robots are present, the structure the structures remains stable. Figure 2 shows the construction advances by one robot over the void. of a structure to cross a three robot wide gap. If there are Fig. 1. Key steps in construction the structure. N robots in the structure, np is the number of robots of the structure at the start area, ng as the number of robots in the structure over the void, and no as the number of robots in the version is approximately 8cm × 8cm and weights ap- structure on the finish area, the structural stability a structure proximately 200gm over the void is satisfied when np = ng = no. Thus, an ng- We considered the structure required to cross the void robot void is able to be crossed when the minimum number and concluded that for voids less than five robots wide a of robots in the structure is N = 3ng. structure one robot wide would be satisfactory, this was B. Simulation Results based on consideration of the proposed couplings discussed in Section III-B. In the simulations the physical docking process Figure 3 shows the steps in the completion of a successful was not considered, in addition to the challenges of obtaining void crossing, when the swarm was able to reach the finish and maintains the alignment between individual robots, and area because its size is satisfying the size of the void and the robot structure and the edge of the void. a robot found the target. In this case, eighteen robots have to traverse a 5-robot wide void. Because the minimum robot A. Strategy number required to cross 5-robot void is fifteen robots, the In the simulation a simple environment is used that consists swarm is able to reach the opposite ground. When fifteen of two areas in which the swarm robots and wander, separated robots assembled, the structure was waiting for the sixteenth by the void. The robots are initially placed in the start area, robot to join so that it could moves forward and maintain its and cross the void to the finish area. stability when the front robot decoupled. The proposed strategy is as follows: the robots initially Figure 4 shows the process by which a swarm crosses a conducts a random exploration of the start area until one robot 2-robot wide void, with only three robots available. Unable to detects the void. Once this robot detects the void, it becomes detect the opposite ground, the structure moved backward to the seed robot and broadcasts a message to all members of the the start area and the rear robot dispersed from the structure swarm in range, to initiate the construction of the structure. in sequence. The rear robot of the structure calls the closest robot to Figure 5 shows the frequency of a specific robot becoming joint the structure until the number of robots in the structure the seed robot and show that all robots have the same oppor- passes a preset parameter, normally three, Figure 1. Then the tunity to become the seed. There is also a trend that the robot structure advances across the void and continue calling other becoming the seed depends on its initial position, as proved wandering robots to join to maintain the structural stability in the case of fixed initial positions. The closer the robots to as the structure advances. When a part of the structure has the void, the higher probability to become the seed robot. crossed the void and the number of robots at the finish area Figure 6 shows the simulation time to cross the void for again satisfies the structural stability, the front robot detaches fixed and random initial positions. As expected, it can be seen itself from the structure and returns to a wandering mode. The that the more robots, the longer the time to cross. This time is process of robots joining the structure continues, while the determined by the time to find the void, the time to complete structure continually moves forwards, until there is no robots the task of locating an object in the finish zone, and actual left to join the structure. The individual swarm robots, have no construction and crossing time. prior knowledge of the total number in the swarm, the void’s For the assembly and moving forward time, it is clear location and or its width. In addition any swarm member must that the more robot in the swarm makes the longer time for be able to become the seed robot to initiate construction. the swarm to construct the structure and move forward. The In the case when the number of robots available is not success crossing involves the whole process and the structure enough to cross the void, or if the structural stability criteria requires more time to pass the void and make all robots is not satisfied, the structure will retreat, and the robots will reaching the opposite ground and disassembling to be ready disassemble from the structure and return to re-exploring the to find the target. The factor of the area dimension does not start area. This part of scenario is to prevent the loss or damage affect the required time of the process because there is only to the robots or part of the swarm so that the swarm can slightly differences between the recorded time in five case accomplish other tasks. studies. The area dimension was supposed to influence the (a) Swarm robots at their initial positions. (b) Structure moves forward while recruiting (c) Fifteen robots are kept in the structure for other available robots, while checking for the structural stability, while the leading robots opposite side of the void. leave the structure.

(d) When there is no robots available to add (e) The process continues while the decoupled (f) Task completed as the target has been to the structure, the whole structure moves robots explore the target area. located and the structure is fully disassembled. forward, the leading robot decouples, while maintaining structural stability.

Fig. 3. Eighteen robots crossing a 5-robot wide void with fixed initial position.

(a) Swarm robots at their initial positions. (b) The last free robot joining the structure. (c) The structure moves forward and checks if the opposite zone can be detected

(d) Because the number of robots is not (e) Disassembling of rear robots continues (f) End of simulation enough to reach the opposite ground, the structure moves backward and the rear robot decouples.

Fig. 4. Example of a failed task, where three robots attempt to cross a void, two robots wide. to detach two units from each other and most swarm robotics could not handle that kind of interaction force. The CATOMS [21] system uses controllable electromagnets to provide a secure attachment on demand. This system is effective in that it can correct relatively high angles of misalignment and on-demand operation means connections are only made when appropriate, as opposed to the M-Blocks. Further, these alignment correcting systems make physical communications a possibility between units in contact, especially where the magnets interface. This is similar to the Molecule [22] and EM-cube [23] systems. However, electromagnets require a constant current draw to maintain the structure, which is an issue for small swarm units. An adaption of electromagnets is to use electro-permanent magnets [24] but these have a high current draw when switching polarity and still present all the noise issues of a permanent magnet solution. Other systems use mechanical actuators to achieve their interconnections include Roombot [25], ATRON [26] and M- Fig. 5. The frequency of individual robot that becomes see robots in a in TRAN [27]. Whilst these systems have the potential to be thirty robot swarm, the higher numbered robots are positioned in the starting area closer to the void. The results are from thirty simulation. fairly complex due to numerous moving parts at small scales, they tend to also have the benefits of strength and reliability. The foot-bot is one of three variations of robotic systems time because free robots are still exploring the area and the developed in the Swarmanoid project [4] and incorporates distance between them and the structure could be affected by a scissored actuator that can orient itself around the entire the zone size. circumference of the circular bot. This actuator slots into the Retreat and disassembling time is only occurred when the complementary encompassing track on another. This allows swarm failed to cross the void because of unsatisfying number for interconnection with numerous configurations of track according to the void size. There is a trend that the more the alignment between bots. Docking recognition is handled via a robots in the swarm and the larger the void size, the longer 2D-force sensor on the gripper. However, these units cannot the time required to retreat and disassemble. connect if on any kind of uneven surface next to each other. For the exploration time at the target zone when the swarm The foot-bots uses a top mounted camera to provide a 360o succeed passing the void, it depends only to the random view of local area but still rely on the eye-bots to undertake movement of all robots, so there is no trend of increased most detection of the environment for them. or decreased graphic. However, the wide area of exploration The PolyBot [28] and CONRO [29] systems use a slot and causes the exploration takes a longer time. pin to interlock with an SMA actuator that holds the pins in the holes. These pin connections also allow serial communications III.ROBOTDESIGN and docking recognition between docked units. For such In the development of a robot suitable for this task, the applications where some misalignment must be tolerated to main challenge is to ensure that the robots can assemble handle uneven terrain, these communication systems are not and disassemble autonomously, hence to date effort has been appropriate. Further, SMA actuators have slow deactivation, concentrated on this problem. There is main problems the which is not ideal for controlled and on-demand disassemble communication requirements and the coupling itself. In the of units. A significant current is required to heat the alloy for work being considers, it should be noted that the robot actuation. has relatively low computing and communication capability, together with a low mass. B. Proposed Solution A. Previous Work The design specification can be summarized as follows: A wide range of design for robotic assemble-disassemble 1) Individual robots to be captured and released, on de- techniques have been published, of particular note are MIT’s mand, as rapidly as possible. M-Blocks [19] where permanent magnets are embedded in 2) The interconnection system must be able to tolerate large the edges and faces of cubes to passively align and aid the degrees of misalignment. cubes’ movement. This system is effective as it provides a 3) The system has minimal power draw throughout use, zero power draw with minimal complexity. The Crystal Bot particularly when in the assembled mode, and during [20] uses hall effect sensors in a similar fashion to M-Block docking. for docking recognition which complements the permanent 4) Size of the docking system must be comparable with the magnet solution. Further, the system relies on inertial impulse mobile platform. (a) Time for the swarm to locate the void. (b) Time for the swam to start construction, (c) Time for the swarm to successfully cross and then retreat because the number of swarm the void. robots presents is less that the number re- quired to cross the void (i.e 15).

Fig. 6. The simulation time of both fixed and random initial positions 5-robot void.

D. Proposed Communication Each robot is provided with six infra red LED receivers and transmitters arranged around the base giving 360o coverage. To simplify communication requirements, when required, the robot will broadcast a signal at the discrete frequency (i.e 400Hz, 500Hz, etc) depending on the robot’s state and its requirement, typical messages will include void located and I am the rear robot. This received signal will be processed on using a conventional signal processing approach to determine message. In addition the rear transmitters will provide specific ranging and alignment information during the docking process.

IV. CONCLUSION Fig. 7. The prototype interconnection mechanism. The robot to the left is coupling to the right hand robot, as the mechanism open, the fingers connect In practice a small robot platform cannot cross the void with the loop and pull the robots together, until the end of the worm gear individually, but will be able to complete the task by cooper- touches the cross member, locking the two robots together. atively by constructing a simple line structure, inspired by ant behavior. In this paper, we introduced a strategy, together with 5) The integrity of the structure must be maintained, with the proposed design of a low cost robot, which as a robotic minimum flex in the structure. swarm will be able to autonomously cross a void. 6) The robot is capable of discriminating between the The simulations have demonstrated the robustness of the environment’s surface and the void. strategy, with various sizes of swarm traversing various sizes of 7) Communication is limited to general broadcast from one void, or it retreated back to the initial area when the swarm size robot to the complete swarm, it addition the information is not satisfying the stability criteria. Loss a few of individuals content of any message is restricted to a robot’s individ- robots does not make the overall strategy fail as the same ual state. controller in each robot, all robots in the proposed strategy have the same opportunity to be the seed or to find the target so it does not matter if a few number of robots meet the failure C. Proposed Coupling to operate. The strategy also shows the flexibility, any number The design used in this work is based on a worm-screw of robots can be added into the swarm and it performs well based actuator driving geared fingers which open and hooks according to the given main task - finding the target, the swarm onto ring on the rear of the leading robot, Figure 7. Once the is also able to cope with the different size of voids. And finally, fingers are opened by the worm-screw the whole assembly will the strategy also accommodates the scalability, any size of lock the two robots together with very little free movement. robotics swarm can perform well. If the number of robots is Due to the size of the loop, this approach can accommodate not enough, robots can retreat to the initial area. If the number initial misalignment, and does not require any force to be of robots is far enough, they can traverse any size of void applied to the robot being couple to, hence minimizing the through a temporary self-assembled dynamic structure. risk of pushing the seed robot over the edge of the void. We In further works, this strategy will be implemented in have undertaken a FEA analysis of this 3D printed design to practical using specific platform as robots require strong confirm that the loop will not fail under the wight of robots connection mechanism to assemble each other. By applying during the construction. a homogeneous robotic swarm, the cost of implementation in practical can be reduced. In the case of large void crossing, a [13] V. Trianni, E. Tuci, C. Ampatzis, and M. Dorigo, “Evolutionary similar strategy is being developed by implementing a complex Swarm Robotics: a theoretical and methodological itinerary from individual neuro-controllers to collective behaviours,” structure to reduce the deflection effect of the structure so that in The Horizons of , P. Vargas, robots can safely reach the opposite area. E. Paolo, I. Harvey, and P. Husbands, Eds., MIT Press, 2014, no. January 2011, pp. 153–178. [Online]. Available: ACKNOWLEDGMENT http://mitpress.mit.edu/books/horizons-evolutionary-robotics%5Cnhttp: //laral.istc.cnr.it/Pubblicazioni/English/Journals/2010trianni-etal-her.pdf The authors acknowledge the for funding of the scholarship [14] L. Cucu, M. Rubenstein, and R. Nagpal, “Towards self-assembled for Niken Syafitri from the Directorate General of Resources structures with mobile climbing robots,” in Proceedings of the 2015 IEEE International Conference on Robotics and , 2015, pp. for Science, Technology and Higher Education Ministry of 1955–1961. Research, Technology and Higher Education of Indonesia. [15] E. Carpanzano, D. Dallefiate, and F. Jatta, “A Modular Framework for the Development of Self-reconfiguring Manufacturing Control Systems,” REFERENCES in Proceedings of the 2002 IEEE/RSJ Intl. Conference on Intelligent Robots and Systems, 2002. [1] D. M. Bonabeau E. and T. G., : From Natural to [16] J. M. Montanier and P. C. Haddow, “Adaptive self-assembly in swarm Artificial Systems. Oxford University Press, New York, 1999. robotics through environmental bias,” in Proceedings of the IEEE [2] M. Brambilla, E. Ferrante, and M. Birattari, “Swarm robotics: A review Symposium Series on Computational Intelligence - IEEE ICES: 2014, from the swarm engineering perspective.” Swarm Intelligence, vol. 7, 2014, pp. 187–194. no. 1, pp. 1–41, 2012. [Online]. Available: http://link.springer.com/10. [17] K. H. Yoo, Y. Lee, H. J. Cho, B. H. Yoo, and D. H. Kim, “Swarm 1007/s11721-012-0075-2 Robotics: Self Assembly, Physical Configuration, and Its Control,” in [3] G. Beni, “From Swarm Intelligence to Swarm Robotics,” in Swarm SICE-ICASE International Joint Conference 2006, 2006, pp. 4276–4279. Robotics WS 2004, E. Sahin and W. Spears, Eds. Springer-Verlag [18] R. Crowder and K.-P. Zauner, “A project-based biologically-inspired Berlin Heidelberg, 2005, vol. 3342, pp. 1–9. [Online]. Available: robotics module,” IEEE Transactions on Education, vol. 56, no. 1, 2013. http://link.springer.com/content/pdf/10.1007/978-3-540-30552-1 1.pdf [19] J. W. Romanishin, K. Gilpin, S. Claici, and D. Rus, “3D M-Blocks: [4] M. Doriigo, D. Floreano, L. M. Gambardella, F. Mondada, S. Nolfi, Self-reconfiguring robots capable of locomotion via pivoting in three T. Baaboura, M. Birattar, M. Bonani, M. Brambilla, A. Brutschy, dimensions,” in 2015 IEEE International Conference on Robotics and D. Burnier, A. Campo, A. L. Christensen, A. Decugniere,` G. D. Automation (ICRA), 2015, pp. 1925–1932. [Online]. Available: http: Caro, F. Ducatelle, E. Ferrante, A. Forster,¨ J. Guzzi, V. Longchamp, //ieeexplore.ieee.org/lpdocs/epic03/wrapper.htm?arnumber=7139450 S. Magnenat, J. M. Gonzales, N. Mathews, M. M. de Oca, R. O’Grady, [20] D. Rus and M. Vona, “A basis for self-repair robots using C. Pinciroli, G. Pini, P. Retornaz,´ J. Roberts, V. Sperati, T. Stirling, self-reconfiguring crystal modules,” Intelligent Autonomous Systems, A. Stranieri, T. Stutzle,¨ V. Trianni, E. Tuci, A. E. Turgut, and F. Vaussard, pp. 2194–2202, 2000. [Online]. Available: http://citeseerx.ist.psu.edu/ “Swarmanoid: A Novel Concept for the Study of Heterogeneous Robotic viewdoc/download?doi=10.1.1.28.8956&rep=rep1&type=pdf Swarms,” IEEE Robotics & Automation, no. December, pp. 60–71, 2013. [21] S. C. Goldstein, “,” Computer, no. June, pp. 99– [5] S. Kernbach, F. Schlachter, R. Humza, J. Liedke, S. Popesku, S. Russo, 101, 2005. T. Ranzani, L. Manfredi, C. Stefanini, R. Matthias, C. Schwarzer, [22] C. McGray and D. Rus, “Self-reconfigurable molecule robots as 3D B. Girault, P. Alschbach, E. Meister, and O. Scholz, “Heterogeneity for metamorphic robots,” in Proceedings. 1998 IEEE/RSJ International Increasing Performance and Reliability of Self-Reconfigurable Multi- Conference on Intelligent Robots and Systems. Innovations in Theory, Robot Organisms,” IROS11, Workshop on Reconfigurable Modular Practice and Applications (Cat. No.98CH36190), vol. 2, 1998, pp. 0–5. Robotics, 2011. [Online]. Available: http://arxiv.org/abs/1109.2288 [23] B. K. An, “EM-Cube: Cube-shaped, self-reconfigurable robots sliding [6] T. Kuyucu, I. Tanev, and K. Shimohara, “Hormone-Inspired Behaviour on structure surfaces,” in Proceedings - IEEE International Conference Switching for the Control of Collective Robotic Organisms,” Robotics, on Robotics and Automation, 2008, pp. 3149–3155. vol. 2, no. 3, pp. 165–184, 2013. [Online]. Available: http: [24] A. D. Marchese, H. Asada, and D. Rus, “Controlling the locomotion //www.mdpi.com/2218-6581/2/3/165/ of a separated inner robot from an outer robot using electropermanent [7] P. Levi, E. Meister, and F. Schlachter, “Reconfigurable swarm robots magnets,” in Proceedings - IEEE International Conference on Robotics produce self-assembling and self-repairing organisms,” Robotics and and Automation, 2012, pp. 3763–3770. Autonomous Systems, vol. 62, no. 10, pp. 1371–1376, 2014. [Online]. [25] M. Vespignani, E. Senft, S. Bonardi, R. Moeckel, and A. J. Ijspeert, Available: http://dx.doi.org/10.1016/j.robot.2014.07.001 “An experimental study on the role of compliant elements on the [8] H. Li, H. Wei, J. Xiao, and T. Wang, “Co-evolution framework of locomotion of the self-reconfigurable modular robots Roombots,” swarm self-assembly robots,” Neurocomputing, vol. 148, no. 2015, pp. in 26th IEEE/RSJ International Conference on Intelligent Robots 112–121, 2015. [Online]. Available: http://dx.doi.org/10.1016/j.neucom. and Systems:, 2013, pp. 4308–4313. [Online]. Available: http: 2012.10.047 //dx.doi.org/10.1109/IROS.2013.6696974 [9] V. Vonasek,´ M. Saska, L. Winkler, and L. Peucil,ˇ “High-level [26] M. Jorgensen, E. Ostergaard, and H. Lund, “Modular ATRON: mod- motion planning for CPG-driven modular robots,” Robotics and ules for a self-reconfigurable robot,” in 2004 IEEE/RSJ International Autonomous Systems, vol. 68, pp. 116–128, 2015. [Online]. Available: Conference on Intelligent Robots and Systems (IROS) (IEEE Cat. http://dx.doi.org/10.1016/j.robot.2015.01.006 No.04CH37566), 2004, pp. 2068–2073. [10] R. Bruni, A. Corradini, F. Gadducci, A. Lluch Lafuente, and A. Vandin, [27] S. Murata, E. Yoshida, A. Kamimura, H. Kurokawa, K. Tomita, and “Modelling and analyzing adaptive self-assembly strategies with S. Kokaji, “M-TRAN : Self-Reconfigurable Modular,” IEEE/ASME Maude,” Science of Computer Programming, vol. 99, pp. 75–94, 2015. Transactions on Mechatronics., vol. 7, no. 4, pp. 431–441, 2002. [11] R. O’Grady, R. Groß, A. L. Christensen, and M. Dorigo, “Self-assembly [28] M. Yim, Y. Zhang, K. Roufas, D. Duff, and C. Eldershaw, “Con- strategies in a group of autonomous mobile robots,” Autonomous Robots, necting and disconnecting for chain self-reconfiguration with polybot,” vol. 28, no. 4, pp. 439–455, 2010. IEEE/ASME Transactions on Mechatronics, vol. 7, no. 4, pp. 442–451, [12] R. O’Grady, A. L. Christensen, and M. Dorigo, “SWARMORPH: 2002. Multirobot Morphogenesis Using Directional Self-Assembly,” IEEE [29] A. Castano and P. Will, “Mechanical design of a module for recon- Transactions on Robotics, vol. 25, no. 3, pp. 738–743, 2009. figurable robots,” in Proceedings of the 2000 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2000), vol. 3, 2000, pp. 2203–2209.