Kano T et al. (2017), A brittle star-like robot capable of immediatel...

ECB-ART-46017

R Soc Open Sci 2017 Dec 01;412:171200. doi: 10.1098/rsos.171200.

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A brittle star-like robot capable of immediately adapting to unexpected physical damage.

Kano T , Sato E , Ono T , Aonuma H , Matsuzaka Y , Ishiguro A .

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A major challenge in robotic design is enabling robots to immediately adapt to unexpected physical damage. However, conventional robots require considerable time (more than several tens of seconds) for adaptation because the process entails high computational costs. To overcome this problem, we focus on a brittle star-a primitive creature with expendable body parts. Brittle stars, most of which have five flexible arms, occasionally lose some of them and promptly coordinate the remaining arms to escape from predators. We adopted a synthetic approach to elucidate the essential mechanism underlying this resilient locomotion. Specifically, based on behavioural experiments involving brittle stars whose arms were amputated in various ways, we inferred the decentralized control mechanism that self-coordinates the arm motions by constructing a simple mathematical model. We implemented this mechanism in a brittle star-like robot and demonstrated that it adapts to unexpected physical damage within a few seconds by automatically coordinating its undamaged arms similar to brittle stars. Through the above-mentioned process, we found that physical interaction between arms plays an essential role for the resilient inter-arm coordination of brittle stars. This finding will help develop resilient robots that can work in inhospitable environments. Further, it provides insights into the essential mechanism of resilient coordinated motions characteristic of animal locomotion.

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Genes referenced: LOC100893907 LOC583082 srpl

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	Figure 1. Body structure and motion of a brittle star. (a) Overview of a brittle star (Ophiarachna incrassata). Five flexible arms radiate from a central disc. The body of an intact brittle star is radially symmetrical. (b) Micro-computed tomography image of a brittle star. The nervous system is indicated by pink lines. Radial nerves that innervate the arms are connected via a circumoral nerve ring located in the central disc. The method for obtaining the images is provided in appendix A. (c) Photographs of a brittle star autotomizing one of its arms when hypertonic seawater is applied while the tip of the arm is immobilized. (d) Locomotion of brittle stars whose arms have been trimmed or amputated. Seven types of morphology (A–G) were examined. The direction of motion was from left to right. Photographs were taken around every 0.5–1.0 s. The arrows denote the arms that mainly contributed to the locomotion.
	Figure 2. Behavioural experiments. (a) Experimental set-up. The motion of the subjects was monitored using an overhead camera. A mirror was placed beside the subjects to simultaneously monitor their top and side views. (b) Markers on a real brittle star, which were used to calculate the indices.
	Figure 3. Quantitative comparison of the brittle star and the robot. (a) Normalized locomotion velocity. (b) Inter-arm coordination index Eij. The morphology examined is shown at the top of each graph. The numbers at the top of each graph denote the arm pairs, and the arm numbers are shown in the inset. The open circles in the schematics denote arm 1. The bars in the graphs denote the mean and variance.
	Figure 4. Experiments without ground contact of the arms. (a) Experimental set-up for the behavioural experiment. In seawater, the centre of the central disc of a brittle star was fixed on a stage to which a drawing pin was attached; thus, the arms did not touch the ground. (b) Photographs of a brittle star on the stage, which were taken around every 1.0 s. (c) Experimental set-up for the robot experiment. The robot was placed on a stage such that its arms did not touch the ground. (d) Photographs of the robot on the stage, which were taken around every 1.0 s.
	Figure 5. Outline of the decentralized control mechanism inferred from the behavioural experiments. (a) Each arm moves randomly and obtains a response from the environment. (b) If the reaction force assists propulsion, the arm pushes against the ground. (c,d) The arm lifts off from the ground and moves forward when the joint angle at the proximal end reaches a certain threshold. (e) When the reaction force impedes propulsion, the arm does not push against the ground.
	Figure 6. Schematics for the proposed model. (a) Schematic of the body system. (b) Definitions of the vectors d and ri. (c) Evaluation of reaction force from the environment when ai−1>ai>ai+1. The ith arm pushes against the ground when it obtains a reaction force from the left because ai>ai+1. On the other hand, the ith arm does not push against the ground when it obtains a reaction force from the right because ai<ai−1.
	Figure 7. Structure of the robot. (a) Overview of the brittle star-like robot PENTABOT II. The robot consists of a central disc and five arms, each of which has yaw and pitch joints. (b) Detailed structure of each robot arm. Two servomotors are implemented to drive the joints. A fragile material was used for the distal part in order to allow examination of adaptability to damage. (c) Internal structure of remote controller. (d) Markers on the robot, which were used to calculate the indices.
	Figure 8. Results for the robot experiments. (a) Photographs of the distal part of one of the arms being destroyed during locomotion. The robot immediately adapted to the damage and maintained its locomotion. Arrows indicate damaged arms. The direction of motion was from left to right. Photographs were taken every 1.0 s. (b) Photographs of robot locomotion for which the direction of motion and configuration were nearly the same as those in the behavioural experiments shown in figure 1d. The direction of motion was from left to right. Photographs were taken around every 0.5–2.0 s. The arrows denote the main arms that contributed to locomotion. (c) Time evolution of the displacement of the robot. Data for the trials conducted in (b) are shown. The start point was set to the instant at which the robot was powered on. The arrow denotes the time at which the robot was placed on the ground.
	Figure 9. Qualitative explanation of the mechanism of inter-arm coordination. (a) Suppose that arm 1 is oriented towards the direction of motion (black arrow) and that arm 5 strikes the ground (red arrow). (b) The central disc rotates because of the counterbalance torque generated (white arrow). (c) Owing to the displacement of the proximal end of arm 2, the assistive reaction force acting on its tip increases (thick black arrow). (d) The local reflex works such that arm 2 strikes the ground (red arrow). This physical interaction works such that arms 2 and 5 strike the ground simultaneously.

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Arshavskiĭ, [Types of locomotion in Ophiuroidea]. 1976, Pubmed, Echinobase