A silicone robotic jellyfish
Jellyfish swim by contracting their bell to expel water, and they are among the swimmers with the lowest known energy cost. This project built a robot that imitates that mechanism: four pneumatic silicone tentacles covered by a membrane that forms a closed bell. A working prototype was made along with its drive electronics. The estimate of its advance, about 6 cm per pulse, comes from a calculation and not from a measurement.
Context
The jellyfish's efficiency is not explained by the jet alone. As it relaxes, the bell refills and the vortices that form at its rim produce a second push, which according to the literature contributes about 30% of the distance traveled. That effect requires the bell to be closed. Several earlier jellyfish robots use separate arms and lose it.
The project continues the jellyfish robot design developed earlier in the lab, which took the Pacific sea nettle (Chrysaora fuscescens) as its model and defined a tentacle that curls when air is injected, in a way similar to the animal's bell.
Tentacles
Each tentacle is an Ecoflex part with a row of chambers. The mold is 3D printed from a parametric model, which allows the chamber size to be changed and the tentacle to be scaled.
Parametric model of the tentacle
When pressurized, the chambers expand. A thin, inextensible sheet at the base, of paper or plastic film, keeps that side from lengthening, and the tentacle curls toward it.
Contraction sequence of a tentacle as air is injected
Bell
The shell has four channels that house the tentacles, with the air inlet toward the center. An umbilical runs from the center to the control station.
Shell model with the four channels for the tentacles
Actuation
Each tentacle has a 12 V three-way valve. Energized, it connects the tentacle to the air source at 12 psi. De-energized, it opens it to the atmosphere. Earlier work had established 15 psi as the safe limit for the material.
Three-way valve: air inlet, outlet to the tentacle and vent to atmosphere
An Arduino drives the valves through a 3 × 3 cm board made for the project. The design provides for an onboard Arduino Nano and accelerometer, coated in the same silicone to protect them from water.
Valve drive board, both sides
Results
The prototype was built and actuated in a tank. Propulsion was estimated from a momentum balance of the jet and from drag on the projected area, using areas and times taken from video. The result is an advance of about 6 cm per pulse, with a loss of about 4 mm to drag.
What is missing
There is no measurement of speed or force, and the calculation ignores precisely the vortex effects that motivate the design. The thesis reports no data from the accelerometer intended to measure the motion.
The underlying obstacle is actuation. Pneumatics requires alternating between high and low pressure, and that ties the robot to an external source: an onboard tank would be heavy and hard to refill. The author notes that an autonomous version needs another kind of actuator, and that the manufacturing process has to be more repeatable to obtain more working prototypes in less time.
How it fits in Robiolab
This is the second project in the lab's jellyfish-robot line and one of those that consolidated its work with molded elastomers. The tentacle, a row of chambers over an inextensible layer, is the same type of actuator the group used years later in wearable devices such as the finger rehabilitation actuator. The constraint it documents, dependence on an external air source, remains the main limit of pneumatic soft robots.