Robert F. Shepherd is Associate Professor of Mechanical and Aerospace Engineering at Cornell University in Ithaca, NY. He is leading a team exploring the use of hydraulic fluids in soft robots to also serve as a source of energy.
Tech Briefs: What motivated this project?
Professor Robert F. Shepherd: I work on soft robots that are really adaptable but they don’t run for long periods of time and you need a pump to operate them. So, I started thinking: organisms have hearts and these hearts are pumps. Perhaps we could use the fluids being pumped in the robots to carry energy like our blood carries energy. Working with Lyndon Archer, a battery expert at Cornell, we came up with a robust, high-energy-density robot that uses a half-flow cell battery — half of it is flexible and the other half is liquid. We tested the concept by creating an aquatic soft robot inspired by a lionfish.
Tech Briefs: How does the system work?
Shepherd: We identified a flow cell chemistry that was pretty high in energy density, could still be used as a hydraulic liquid, and was safe to use in our lab. The zinc iodide battery chemistry we came up with could have an energy density of 300 watt-hours per liter if we used a high enough ion concentration. We chose to use 150 watt-hours per liter to demonstrate it without having the ionic concentration too high — the higher it is, the more chance for chemical hazards.
Tech Briefs: You still need a pump to move the fluid?
Shepherd: Yes. A peristaltic pump run by an electric motor pushes the liquid into the actuator, causing the “fish” to move. Flow cell batteries start to deplete the ion concentration near the electrodes, then when the liquid is moved around, the ions are replenished. The fluid has an electrical potential in it because the positive and negative ions are separated from each other, so as they’re moving near the electrodes, they cause electrons to flow through the electronic circuits and power them.
Tech Briefs: When could this energy system be put into practice?
Shepherd: A lot of time has gone into packaging lithium polymer batteries to make them safe at higher powers. In comparison, changing the boundary conditions for flow cell batteries to be useful on a smaller scale in more intricate devices, as we’re doing, is new. I expect that a similar time scale to develop and implement these things in hydraulically powered machinery is a similar technical challenge. So, probably it’s a decade away.
Tech Briefs: What other forms could this take besides fish?
Shepherd: The fish was just a way to show the vision of our work. It is, though, one of the more useful practical applications for soft robots. Long-duration underwater exploration vehicles monitoring ocean temperatures and pollution are a good use of it. Another possible application is in space exploration. On the terrestrial scale, exoskeletal systems for assisting locomotion for stroke patients.
An edited version of this interview appeared in the September Issue of Tech Briefs.