My Contribution in Pneumatic Amphibious Soft Robot Project

2nd Place in Undergraduate Project Contest

This was an undergraduate group project under Prof. Chueh's instruction. We designed and fabricated an amphibious soft robot inspired by salamanders. For more detail of our work, please check out the link and the poster pdf document attached below.

Throughout the project, I studied the locomotion physics of animals underwater, designed the entire structure with Inventor, utilized composite materials to optimize outcomes, and analyzed the performance with Tracker. 

Where I contributed...

I designed the entire structure, including a streamlined head and body to reduce drag, serrated front limbs to enhance crawling ability, and smooth plastic sheets as the hind limbs to reduce frictional resistance. The tail is made with waterproof fabric tape combined with plastic sheets to increase rigidity, helping resist underwater pressure and improve swimming capability.

I created a prototype of our soft robot design using Inventor. The design features two series of chambers distributed throughout the entire body. These two series are separated by a composite material made of fabric and silicone.

I used Inventor to design a detachable mold, as shown in the picture below. This design allows for easier demolding and extends the mold's lifespan.

Physics Theory

I conducted a literature review on the locomotion of animals underwater, comparing the body of a fish to an airfoil. Just as an airfoil experiences a pressure difference between its upper and lower surfaces, creating lift, a similar pressure difference exists between the left and right sides of a fish's trunk, generating thrust that propels it forward. Understanding that curvature is key to generating these forces, our focus is on designing a body structure that can deform into an optimal curvature to enhance swimming efficiency.

(K. N. Lucas, 2020, https://doi.org/10.1073/pnas.1919055117)

Material

I combined various materials with silicone to create composite materials, which we applied to our soft robot. For example, we used thick cardboard in the front limbs to strengthen the support of the body during crawling. In the middle of the torso, we used a flexible and tightly woven fabric. After applying silicone to the fabric, the silicone fills the fabric's pores, forming a sealed and elastic layer. This material divide the chambers into left and right halves. Each series of chambers was designed to be connected, allowing them to be inflated through a single inlet.

We tested various fabrics, including elastic and non-elastic materials, with coarse and fine weaves. We found that coarse-weave fabrics tended to trap bubbles in the gaps, even with vacuum techniques. Other fabrics showed minimal differences when combined with silicone, as the elasticity of silicone itself is limited. Since the central plane of the trunk is designed to be constrained, we used fabrics with less elasticity.

In the image, the leftmost piece was made by directly stirring the silicone mixture by hand and pouring it into the mold without performing a vacuum degassing process. The middle piece was made by pouring the mixed silicone into the mold and then placing the filled mold into a vacuum chamber. The rightmost piece was made by first vacuum degassing a cup of stirred silicone mixture, pouring half of it into the mold, vacuuming the mold again, and then pouring the remaining silicone to fill the mold.

In the leftmost result, some small air bubbles can be seen in the final product. The middle one has the most severe voids. This is because the mold has many intricate structures. After the bubbles burst, no new silicone mixture flowed in to fill the voids, leading to the worst outcome. This is why the molding process was adjusted to the method used for the rightmost piece.

Body Structure Design

The original design featured a smooth body surface with chambers distributed throughout the structure. We introduced two key characteristics: "grooved" and "half-inflated." In the grooved design, the chamber walls are separated at the surface, as shown in the figure. The half-inflated design inflates only the chambers on the lower side of the body. We blocked the gaps in the middle of the body to prevent air from flowing to the front, which minimized deformation in the upper body during operation. We designed four structures: "grooved + half-inflated," "smooth + half-inflated," "grooved + fully-inflated," and "smooth + fully-inflated."

We experimented with different chamber sizes and wall thicknesses. If the walls were too thick, the structure deformed too little; if too thin, the walls were prone to holes due to trapped bubbles. We selected a moderate thickness, which is not yet optimal. Further research and experimentation are needed to achieve better performance.

Limbs

After a series of experiments, we found that the grooved structure allows greater bending, so I applied this structure to the limb design as well. The grooves were placed in the middle of the limb, allowing it to bend as if it had an ankle. The constrained side of the limbs was made with a composite material combining paperboard and silicone. This material provides the limbs with greater mechanical strength to support the entire body.

Inspired by the soles of sneakers, I designed a jagged structure at the tip of the limbs. The profile of these jagged edges is a blunt-angled triangle, with one side longer than the other. When the chambers are deflated, the longer side allows the tip to slide and move forward.

The lower limbs were made of plastic sheets. The smooth surface reduces friction between the lower limbs and the ground. However, we agree that the lower limbs should be improved in the future, such as by designing limbs that enable the soft robot to crawl backward.

Tubing Design

In our pneumatic system, which is a closed system, the inlet and outlet of each series of chambers are located in the same spot. The input and output tubes are placed on the top side of the body, rather than the front as in the prototype. This design minimizes the effect of the airflow's momentum, reduces pressure drop, and shortens the distance for fluid to flow back to the tube from the end of the chambers.

Tail

The tail is made with waterproof fabric tape combined with plastic sheets. This combination is strong enough to withstand the thrust forces. According to research, forked, high aspect ratio tails, where the ratio of the height to surface area is much greater compared to paddle-shaped, can create large thrust forces without having a lot of drag-producing surfaces. I tested the swimming ability with three different tail shapes, but their performance was not significantly different. I suspect this is due to a lack of precise measurement tools and the presence of too much error. If the shapes had more pronounced differences, the results might have shown more significant variations.

(J. C. Liao, 2022, https://doi.org/10.1016/j.cub.2022.04.073)

Originally, since our design was inspired by salamanders, our initial model did not include a tail fin structure. However, after testing, the results were not ideal. We determined that this was likely due to the oscillation speed being too slow and the movement not being long enough to create the wave-like motion seen in a salamander's tail. To improve swimming performance, we added a tail fin structure similar to that of a fish.

Head

The head was designed to resemble the exact appearance of a salamander. Its rounded shape reduces form drag. The inner part of the head is not entirely solid; some voids were designed to ensure the robot maintains a consistent density, allowing it to stay horizontal in water.