Biology

This post will cover the biology of the brittle stars and how it inspires the design of a legged robot that can move even after taking damage. 

The brittle star is also known as the serpent star and it belongs to the phylum Echinodermata and class of Ophiuroidea. Brittle stars have one circular disc body and five arms that are tough and long, reaching 3 feet (Kano, 2017). One unique ability of brittle stars is their ability to regrow amputated arms. The mouth is located on the underside and feed on plankton and scavengers (Brittle star, 2018). Their limbs move in a coordinated fashion, even though they lack a brain. To move, a central arm is chosen as its lead arm and the other four limbs move in coordinated fashion in a direction parallel to the lead arm. The arms are controlled by a circumoral nerve ring that surrounds its body and connects to nerves along each arm (Kano, 2017). To change directions, another one of its arms is chosen as the lead arm (Astley, 2012).

Interestingly enough, the brittle stars can move even without a set of eyes. The skin of the brittle star is covered in crystal-like structures. These structures act like photoreceptors and are connected by a network of nerves. These crystal structures are believed to help them react to light and move away from it (Guglielmi, 2018).

Video 1 below shows how brittle stars generally move. The next section will discuss how brittle stars inspired the design of robots that can withstand physical damage to its limbs, and still function.

Problem

Currently, designing robots that can respond to physical damage to itself is a tough problem. Robots that are tasked to work in extreme environments, such as disaster areas, can lead to unexpected damages to the robot. The ability to design a robot that can operate after damage to it can be very handy in rough environments. Previous methods that account for damage to robots include trial and error methods, but they can be slow. Researchers from Japan wanted to develop a method that allows robots to respond in real-time, similar to how brittle stars respond to missing arms in the ocean. This research work falls in line with the lessons and readings of Module 3, when we learned about the biomechanics of insects and their bio inspired robots. The difference between this robot and the robots we were exposed to in class, is that this robot aims to adapt to damage to its limbs, rather than focusing on what type of terrain it can move across.

Technology

The brittle star, as mentioned in the biology section, does not have a brain or central nervous system. A simple network of nerves helps the species coordinate their arms for locomotion. This decentralized control in brittle stars inspired a similarly decentralized control algorithm for a five limb robot called Pentabot, designed by Kano and his colleagues. The control algorithm assigns rhythmic and non-rhythmic arm movements depending on its environment (Kano, 2017) . A live study on brittle stars themselves was used to develop the algorithm. As an experiment, Kano et. al performed surgery on the arms of the brittle star and studied how they adapted to the loss of limbs during locomotion. By observing and mimicking the live brittle stars, similar movements were implemented into their robot.

Kano and his colleagues then applied their control algorithm to their five-armed robot, Pentabot. Each arm of the robot has two degrees of freedom, or two joints that can rotate in different planes. To picture the movement of each arm, place your arm flat against a desk; you are able to bend at the elbow (bicep curls for example), but also move your hand in a windshield like manner against the desk. These two rotational movements are the degrees of freedom that an arm from the Pentabot has.

To move the Pentabot, each arm initially moves randomly to detect the reaction forces from the ground. If this force is beneficial to its intended direction, the robot will push against the ground and stop after the arm has moved a preset angle. However, if the force is not beneficial to its movement, the arm will not move. This action within each arm is described in Figure 2 above and shown in Video 2 below.

Damaged Limb Locomotions

Most importantly, to test the ability to move without a limb, a portion of the robots arm was cut and observed. Once damaged, the robot only controls its remaining limbs to coordinate movement. This experiment simulating damaged limbs can be seen in the video below. The Pentabot is able to move even with only two limbs remaining.

Conclusion

The results of the Pentabot technology shows potential in developing robots to survey disaster sites. While the results of this study is promising, the experiment only considered flat surfaces. The next step to ensure this device can be used in disaster environments is to incorporate soft-limb robotics or limbs that can climb obstacles. Moreover, incorporating swarm behavior that was covered in Module 5, would allow for multiple robots to cooperate and search disaster sites more quickly.

References

Astley, Henry C. “Getting around when you’re round: quantitative analysis of the locomotion of the blunt-spined brittle star, Ophiocoma echinata.” Journal of Experimental Biology 215.11 (2012): 1923-1929.

“Brittle Star.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 12 Oct. 2018, http://www.britannica.com/animal/brittle-star.

Guglielmi Giorgia. “How brittlestars ‘see’ without eyes.” Nature. 24 January 2018.

Kano, Takeshi, et al. “A brittle star-like robot capable of immediately adapting to unexpected physical damage.” Royal Society open science 4.12 (2017): 171200.

Lauren Sumner-Rooney, Imran A. Rahman, Julia D. Sigwart, Esther Ullrich-Lüter. Whole-body photoreceptor networks are independent of ‘lenses’ in brittle stars. Proceedings of the Royal Society B: Biological Sciences, 2018; 285 (1871): 20172590 DOI: 10.1098/rspb.2017.2590

Orienstein, David. “Five-limbed brittle stars move bilaterally, like people.” Brown University. 10 May, 2012.

Media

Ho, Leonard. “Brittle Stars are covered with thousands of light sensors.” Advanced Aquarist. 05 Feb, 2018. 

Kano, Takeshi; Sato, Eiki; Ono, Tatsuya; Aonuma, Hitoshi; Matsuzaka, Yoshiya; Ishiguro, Akio (2017): video S5 from A brittle star-like robot capable of immediately adapting to unexpected physical damage. The Royal Society. Media.

Kano, Takeshi; Sato, Eiki; Ono, Tatsuya; Aonuma, Hitoshi; Matsuzaka, Yoshiya; Ishiguro, Akio (2017): video S4 from A brittle star-like robot capable of immediately adapting to unexpected physical damage. The Royal Society. Media.