BioInspired Design Final Project: Biomimetic Robotic Starfish
Our final deliverable encompasses the modeling and development of a biomimetic robotic starfish. This project was completed to gain a deeper understanding of the species’ locomotive patterns and ability to safely navigate obstacles and rough terrain. Our design sought to preserve the primary physical features of common starfish (Asterias Rubens), chiefly the radial symmetry of their legs and multiple tube feet. We also hoped to preserve the disjointed, multi-gate locomotive pattern of their tube feet in our design. We chose to maintain these particular physiological features, while foregoing others such as the tube feet’s ability to suction, due their respective importance in establishing the sinusoidal movement patterns displayed by starfish. The exploration of how the radial symmetry of the starfish legs in conjunction with the disjointed movement of their feet contribute to this sinusoidal movement pattern was the key goal of our project. Understanding these two factors are of critical importance to the understanding of the locomotive ability of starfish, which we hope will inform future robotic models. These future models could be utilized in seafloor exploration throughout a multitude of ecosystems due to their ability to scale in size and engage in omnidirectional movement. Additionally, contraining the robot’s movement to the seafloor requires a lower energy expenditure than a swimming model that must contend with tides or currents (Ishida et al. 2022). These energy savings would enable long term monitoring of ocean ecosystems to establish trends in water temperature and salinity, as well as the level of oxygen, nutrients, and pollution present in the ecosystem. Other robots, such as the jelly-bots we explored earlier in the term may be able to monitor and establish trends in this category as well. However, because the jelly bots are at the mercy of the tide, they may not be able to monitor specific areas over a long enough time frame to establish reliable trend data. A starfish robot would afford scientists the ability to move the robot into place, and then establish a stationary monitoring device. However, unlike a typical stationary monitoring device, once sufficient data is collected the robot would be able to relocate to another location at a cheap energy cost and without disturbing its environment.
Figure 1: Starfish internal anatomy showing contents of central disk (Left), final robot design showing servo motors and circuit board (center), and final robot design showing feet (right)
Background on Relevant Physiology:
Our design sought to maintain two physical features of the starfish: the radial symmetry of their legs, and their multiple tube feet. We identified the tube feet as the most likely feature to cause sinusoidal movement patterns, and wished to explore how the radial symmetry of the starfish affected this movement pattern. The majority of starfish species have five arms arranged radially around the center of their body, however, some species such as Pycnopodia helianthoides can have as many as 24 limbs. These limbs are arranged radially around a central disk which surrounds and protects the starfish’s central organs. We preserved this in our design, which utilized the central disk to house the mechanical components of our robot, such as its servos and circuit board, which will be described in further detail later in the report. Heydari et al. describe how the starfish’s nervous system communicates a dominant direction of motion, while the tube feet actuate individually (2020). We constrained our starfish’s direction of motiong by orienting all the foot slots in our starfish legs in the same direction. This approximately mimics the communication of a dominant direction of motion by the starfish’s CNS. However, this constrains the feet’s movement to a single direction, which impacts our robots ability to engage in omnidirectional motion. Since our project is only an initial investigation of the sinusoidal locomotive patterns of starfish we decided it would be best to focus our attention on the actuation of the feet, rather than the ability for omnidirectional movement. Our robot’s tube feet are not individually actuated. Instead, in an attempt to mimic the disjointed motion exhibited by real starfish, we actuated the feet on each leg with four separate servos. This allowed the feet to move in a disjointed, yet non-random fashion. Ideally, each foot would be actuated individually; however, this undertaking was outside the scope of our project. Another alteration made was replacing the suction function of the tube feet with a strong adhesive. This removed the burden of installing a hydraulic actuator to initiate suction movements and further allowed us to focus on the movement of the feet.
Figure 2: Starfish tube feet with suction (left) vs. Final foot design with adhesive tape (right)
Our Design in Context of Previous Research:
Several soft body starfish inspired robots have been developed in the past (Otake 2002, Jin et al. 2014, Shepard et al. 2011, Mao et al. 2014). Their soft body designs enabled researchers to generate movement by deforming its legs in a sequence which produced motion. These designs were able to produce and replicate the motion of starfish; however, the mechanism for movement used in each study, that being the deformation of the model’s legs, was achieved without the inclusion of tube-feet along the bottom of the legs (Poungrat et al. 2017). One study conducted by Ishida et al. in 2022 investigated only the movement of the tube feet. Their design utilized soft active suction discs in conjunction with soft pneumatic linear actuators in order to produce locomotion over flat surfaces, stepped terrain, and in confined spaces. Another study conducted by Zou et al. in 2020 created a soft body starfish robot with the ability to manipulatd the length of its legs in order to explore the function of a newly invented flexible valve. However, their robot was not able to generate motion through this process. During our research, we were unable to identify any study that utilized multiple feet arranged under radially symmetric legs in order to create a robot capable of generating locomotion. We hope to fill this gap in research with our design. Unlike the soft body robots described in previous reports, we decided to use a rigid body for our starfish in order to isolate the movement of the feet. This decision was informed by a study conducted by Heydari et al. in 2020, which was able to generate the sinusodal movement pattern observed in nature using a mathematical model which treated the body and legs of the starfish as a rigid body. This ensures that any sinusodal behavior we generate originates from the starfish’s feet, rather than deformation of its legs. Therefore, since we sought to investigate specifically how movements of the starfish’s many feet cause the sinusoidal movement, we decided that our work would focus specifically on generating movement using actuation of multiple feet.