Kano T et al. (2019), Flexible Coordination of Flexible Limbs: Decent...

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Front Neurorobot 2019 Aug 23;13:104. doi: 10.3389/fnbot.2019.00104.

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Flexible Coordination of Flexible Limbs: Decentralized Control Scheme for Inter- and Intra-Limb Coordination in Brittle Stars'' Locomotion.

Kano T , Kanauchi D , Ono T , Aonuma H , Ishiguro A .

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Conventional mobile robots have difficulties adapting to unpredictable environments or performing adequately after undergoing physical damages in realtime operation, unlike animals. We address this issue by focusing on brittle stars, an echinoderm related to starfish. Most brittle stars have five flexible arms, and they can coordinate among the arms (i.e., inter-arm coordination) as well as the many bodily degrees of freedom within each arm (i.e., intra-arm coordination). They can move in unpredictable environments while promptly adapting to those, and to their own physical damages (e.g., arm amputation). Our previous work focused on the inter-arm coordination by studying trimmed-arm brittle stars. Herein, we extend our previous work and propose a decentralized control mechanism that enables coupling between the inter-arm and intra-arm coordination. We demonstrate via simulations and real-world experiments with a brittle star-like robot that the behavior of brittle stars when they are intact and undergoing shortening or amputation of arms can be replicated.

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References [+] :

Arshavskiĭ, [Coordination of arm movement during locomotion in Ophiuroidea]. 1976, Pubmed, Echinobase

	Figure 1. Body and nerve structure of a real brittle star (Ophiarachna incrassata). (A) Overview of a brittle star. Five flexible arms radiate from a central disc. (B) Micro-computed tomography image of a brittle star. The nervous system is indicated by pink and orange lines. Radial nerves that innervate the arms (orange lines) are connected via a circumoral nerve ring located in the central disc (pink lines).
	Figure 2. Snapshots of locomotion of real brittle stars under various situations: (A) an intact brittle star on a flat terrain, (B) a brittle star with five shortened arms on a flat terrain, (C) a brittle star with two arms on a flat terrain, (D) a brittle star with one arm on a flat terrain, and (E) an intact brittle star on a terrain with several square objects. Red and blue arrows denote points where the arms exploit and avoid objects, respectively.
	Figure 3. Snapshots of motion of real brittle stars when only fraction of the body remains. (A) Locomotion of a brittle star with one arm and its proximal end of the disc on a terrain with a square object. (B) Motion of an arm without the disc. The arm was physically stimulated with an ink brush to induce motion.
	Figure 4. Schematic for anatomical structure of an arm.
	Figure 5. Schematic of the body model. Definition of the joint angle is shown in the magnified view.
	Figure 6. A conceptual diagram of the proposed control mechanism.
	Figure 7. Feedback mechanism: (A) exploitation and (B) avoidance. In the upper figures, muscles that contract due to the local reflexive mechanism are depicted in red and blue for the distal and proximal segments, respectively. This local reflexive mechanism was implemented in the proposed body model (Figure 5), as shown in the lower figures. The blue and red arrows represent the torques to bend the joints, as shown in the inset. The definition of the unit vector ni,j is also shown in the inset.
	Figure 8. Snapshots of the simulated brittle star: (A) an intact brittle star on a flat terrain, (B) a brittle star with five shortened arms on a flat terrain, (C) a brittle star with two arms on a flat terrain, (D) a brittle star with one arm on a flat terrain, and (E) a brittle star with one arm on a flat terrain with several circular objects. Black and white arrows denote points where the arms exploit and avoid objects, respectively. Mass points on and off the ground are colored by green and purple, respectively.
	Figure 9. Snapshots of the simulated brittle star in the case where the arms are intact and the body is on a flat terrain with σy = 1.2.
	Figure 10. Brittle star-like robot developed. (A) Overview of the robot. (B) CAD image for the arrangement of servo motors and sensors within an arm. (C) Proximal end sensor. (D) Back view and bottom view of the sensor. (E) Sensing mechanism.
	Figure 11. Snapshots of the brittle star-like robot (A) with five flexible arms, (B) with five shortened arms, (C) with two flexible arms, and (D) with one flexible arm. All experiments were performed on a flat terrain.