Researchers at The University of Texas at Dallas and Virginia Tech have created an undersea vehicle inspired by the common jellyfish that runs on renewable energy and could be used in ocean dives for rescue and surveillance missions.
The robotic jellyfish, dubbed RoboJelly, feeds off hydrogen and oxygen gases found in water.
“We’ve created an underwater robot that doesn’t need batteries or electricity,” said Dr. Yonas Tadesse, assistant professor of mechanical engineering at UT Dallas and lead author of the study. “It feeds off hydrogen and oxygen gasses, and the only waste released as it travels is more water.”
Fabrication of fuel-powered jellyfish. (a) Computer-aided design of molding set-up, A = cope, B = distributor, C = vehicle internal support structure, D = drag and E = bottom plate. Parts A and D are the mold used for forming the silicone bell. (b) Hydrogen-fuel-powered Robojelly shown out of water (one fuel pipe is shown). The pins with reflective discs used for deformation tracking can be seen on the bell.
Schematic diagram of the heat transfer in a MWCNT-wrapped SMA (Shape Memeory Alloy) composite actuator. (a) Actuator with distinct material and (b) actuator lumped within a control volume shown in green.
Artificial muscles powered by a renewable energy source are desired for joint articulation in bio-inspired autonomous systems. In this study, a robotic underwater vehicle, inspired by jellyfish, was designed to be actuated by a chemical fuel source. The fuel-powered muscles presented in this work comprise nano-platinum catalyst-coated multi-wall carbon nanotube (MWCNT) sheets, wrapped on the surface of nickel–titanium (NiTi) shape memory alloy (SMA). As a mixture of oxygen and hydrogen gases makes contact with the platinum, the resulting exothermic reaction activates the nickel–titanium (NiTi)-based SMA. The MWCNT sheets serve as a support for the platinum particles and enhance the heat transfer due to the high thermal conductivity between the composite and the SMA. A hydrogen and oxygen fuel source could potentially provide higher power density than electrical sources. Several vehicle designs were considered and a peripheral SMA configuration under the robotic bell was chosen as the best arrangement. Constitutive equations combined with thermodynamic modeling were developed to understand the influence of system parameters that affect the overall actuation behavior of the fuel-powered SMA. The model is based on the changes in entropy of the hydrogen and oxygen fuel on the composite actuator within a channel. The specific heat capacity is the dominant factor controlling the width of the strain for various pulse widths of fuel delivery. Both theoretical and experimental strains for different diameter (100 and 150 µm) SMA/MWCNT/Pt fuel-powered muscles with dead weight attached at the end exhibited the highest magnitude under 450 ms of fuel delivery within 1.6 mm diameter conduit size. Fuel-powered bell deformation of 13.5% was found to be comparable to that of electrically powered (29%) and natural jellyfish (42%).
Fuel-powered shape memory alloy muscle synthesis
The structure of artificial muscle comprises of commercially available SMA actuators wrapped with a composite of MWCNT sheets overlaid with catalytic platinum black powder. When hydrogen and oxygen fuel are delivered to the composite, the platinum catalyst initiates a reaction and the resulting exothermic heat activates the SMA. Thermal energy transferred by the chemical reaction causes strain in the SMA due to the phase transformation. MWCNT has a thermal conductivity as high as 150 ± 15 W m−1 K−1 which enhances the heat transfer to SMA. The response time of this actuator is dependent on the thermodynamic reaction during injection of fuel and the evacuation in the cooling phase. A schematic diagram of the artificial fuel-powered muscle is illustrated in figure 5. The mechanism for driving these actuators with hydrogen and oxygen is shown in this figure. When hydrogen is exposed to finely divided catalytic platinum in the presence of oxygen, combustion becomes thermodynamically favorable. This combustion is used to provide the heat required for SMA to transition between the low temperature martensite phase and the high temperature austenite phase. An additional benefit of using purely hydrogen and oxygen as reactants is that the sole product is water.
Schematic diagram of the hydrogen-fuel-powered SMA actuator (dimensions are not to scale).
We report on a hydrogen-fuel-powered artificial jellyfish bell segment. The fuel-powered muscle comprises platinum nanoparticles wrapped with multi-wall carbon nanotube (MWCNT) sheets on the surface of nickel–titanium alloy. A mixture of oxygen and hydrogen produces an exothermic reaction as it encounters the platinum. This reaction activates the nickel–titanium (NiTi)-based shape memory alloy actuator (SMA). MWCNT serves as a support for the platinum powder and enhances the heat transfer due to the high thermal conductivity between the composite and the SMA. Entropy-based modeling of the fuel-powered muscle was presented and numerical simulations were performed to study the characteristics of the actuator. The experimental characterization indicated that the high frequency of actuation did not return the bell segment to the relaxed state. The optimum cyclic condition was found to be 0.1 Hz. The by-product of the actuation was water vapor, which does not adversely affect the environment. The fuel-powered Robojelly was able to deform 13.5% at the inflection point while the electrically powered version deformed 29% and the natural animal deforms 42%. Limitations in deformation are due to both the manufacturing technique of the vehicle and in the MWCNT coatings on the SMA.