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 was interested in it because I work on a class of soft robots that are really adaptable. There's a lot of potential, but they don't operate for long periods of time and you need a pump to operate them. Those are some stumbling blocks, to competing with established robots, which use dc motors and rigid linkages. There's a lot of potential in the area of fluidically actuated soft robots I work in, but the stumbling blocks are repeatedly cited by people who tried to use them as things that limit their potential.
So, I started thinking: organisms have hearts and these hearts are pumps. So if nature is using pumps effectively, that shouldn’t be a fundamental limit to robots either. We should be able to use pumps. Then I thought, how can we make the fluids being pumped in the robots, carry energy like our blood carries energy? We could use them to power electrical systems, but I think it's too far a leap right now to have that fluid operate chemical systems like our bodies do.
I knew that are there liquid batteries, but I’m not an expert in them. However with these basic concepts, I started thinking about flow cell batteries. I hired a person who is an expert in batteries to work with me on implementing flow cells in a fluidically powered robot. And then working with Lyndon Archer, who is a battery expert at Cornell, we came up with a pretty robust, high energy-density robot that uses, not a full flow cell battery, but a half flow cell battery — half of it is flexible and the other half is liquid. And it worked really well right off the bat. We tested the concept by creating an aquatic soft robot inspired by a lionfish, designed by James Pikul.
Tech Briefs: Does that mean that the fluid that makes the robot function also has chemicals in it that allow the flow battery to work?
Prof. Shepherd: That's right. They have ions that have an electrical potential.
Tech Briefs: So, you use the same fluid for the mechanical and electrical functions?
Prof. Shepherd: That’s correct.
Tech Briefs: Could you describe how that works?
Prof. Shepherd: Flow cell batteries are used right now for grid energy storage. If you're generating electricity with solar power, you're not going to generate that electricity at nighttime, so you want to store that energy somewhere and use it later. Flow cell batteries are really good for that because they don't cost too much. But although the energy density isn't as high as you can get with a lithium polymer battery, it's still pretty high. So what we did is identify 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 laboratory setup.
The zinc iodide half flow cell 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. However, we chose to use something more like 150 watt-hours per liter just to demonstrate it without having the ionic concentration too high — the higher it is, the more chance you have for chemical hazards. But certainly safe enough to use in our lab. But it’s still pretty high compared to having no electrical energy potential in the fluid at all.
Tech Briefs: So, you still need a pump to get the fluid moving?
Prof. Shepherd: Yes, the pump is required for two reasons. One, for sure and one still questionable. We need the pump to push the liquid into the actuator to cause it to move. But pumping hydraulic fluid is a good use of the real estate that the fluid takes up. The second part is that flow cell batteries, when they’re used at large scale, start to deplete an ion concentration near the electrodes. Then you move the liquid around so that you replenish those ions. However, it's not clear yet that you actually need to do that for a very small-scale flow cell battery. From our experiments there’s some indication you may not need to.
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: Is the pump electric, and is it run by the flow cells?
Prof. Shepherd: Yes, we use a peristaltic pump run by an electric motor that pushes the liquid through a tube and causes the motion of the fish. There are also electronic controllers that run the sequence that tell the pump how to move.
Tech Briefs: According to your paper, electrodes are distributed throughout the fin area — could you explain the reason for that.
Prof. Shepherd: The more surface area you have for the flow of liquid exposed to the electrodes, the higher the power you can get out. In a flow cell battery, the energy density and power density are decoupled. So you can store a lot of energy, then transport it to the electrodes to power the electronics. The reason the fish move fairly slowly is because the power density isn't very high. We did our best to improve it by increasing the electrode surface area. We still need to increase it more in order to get the system to run faster or perhaps we can use capacitors and other types of electrical engineering techniques to harvest the energy and then release it later at a higher power. However, the direct way to increase the power is to have more electric surface area.
Tech Briefs: Is your work so far basically a lab demonstration?
Prof. Shepherd: Yes, we’re demonstrating the functional use of hydraulic fluids for batteries.
Tech Briefs: One other question I wanted to ask you. You're talking about extending the length of time that the fish could operate. Could you give me some idea of the improvement?
Prof. Shepherd: We have one coin cell battery that operates a boost converter, which takes the lower voltage flow cell battery and increases the voltage high enough to operate the pump. If we kept the coin cell and replaced the zinc iodide solution with plain water, it would be four times less energy dense. So keeping everything the same and replacing the water with the zinc iodide solution will increase the energy density by about four times.
Tech Briefs: Do you have some idea of the timeframe for when this could start actually being put into practice?
Prof. Shepherd: Yes. For example, a lot of time has gone into looking into packaging of lithium polymer batteries to make them safe at higher powers. By way of comparison, changing the boundary conditions for flow cell batteries to be useful on a smaller scale and in more intricate devices, as we’re doing, is new. I would expect a similar time scale for the development and implementation of these things in hydraulically powered machinery is a similar technical challenge. So, probably, probably a decade.
Tech Briefs: Do you foresee other applications besides fish?
Prof. Shepherd: Yes, 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. The buoyant support of water allows you to do more with soft robots. Long duration underwater exploration vehicles monitoring ocean temperatures and pollution and so on, I think is a good use of it.
Another possible application is in space exploration. And then on the terrestrial scale on land, exoskeletal systems for assisting locomotion for stroke patients. A soft exoskeleton might be helpful where you don't want the batteries to run out right away.
An edited version of this interview appeared in the September Issue of Tech Briefs.