The list of control system options for soft robots now includes analog and air-driven.
Researchers in robotics, engineering, and materials science at Rice University and Harvard University showed this week that it is possible to make non-electronic circuits that can be programmed to control the actions of soft robots by decoding the information in bursts of compressed air.
Undergraduate student at Rice University, Colter Decker, who is the study’s lead author and published the findings in the Proceedings of the National Academy of Sciences, said, “Part of the beauty of this system is that we’re really able to reduce computation down to its base components.” He said that because electronic control systems have been developed and improved over many years, it was easier to add pneumatic computation by recreating computer circuitry “with analogs to pressure and flow rate instead of voltage and current.”
Decker, a senior mechanical engineering major, primarily used rubber bands and plastic drinking straws to build his soft robotic control system. Even though it was simple, tests showed that the system’s air-driven logic gates could be set up to do what are called “Boolean operations,” which are the core functions of modern computers.
The goal, according to Decker, “was never to completely replace electronic computers.” It is possible that the technology shown in the paper “would be much cheaper, safer for use, and much more durable” than conventional electronic controls, he said, adding that there are many situations where soft robots or wearables only need to be programmed for a few simple movements.
Decker started working in Daniel Preston’s lab as a freshman; he is now an assistant professor of mechanical engineering at Rice. After winning a competitive summer research fellowship that would allow him to work for a few months in the lab of Harvard chemist and materials scientist George Whitesides, Decker became interested in developing fluidic control systems.
The two research groups worked together on the project for several months, and Decker ended up with nine co-authors on the study, including co-corresponding authors Preston and Whitesides.
Two parts were developed by Decker and his colleagues: an actuator resembling a piston that converts air pressure into mechanical force and a valve that has two states: off and on. Plastic drinking straws, flexible plastic tubing, rubber bands, parchment paper, and thermoplastic polyurethane sheets were used to make the parts, which could all be put together using a hot iron or a desktop heat press.
The research team demonstrated how the two elements could be combined into a single device, a bistable valve, which uses air pressure as both an input and an output and functions as a switch. The switch must be turned between the off and on states at a specific air pressure. The rubber bands that hold the valves shut are programmed to change the amount of pressure needed for activation by adding or removing rubber bands. Decker showed that the circuits could be used to control a pneumatic cushion, a soft robot in the shape of a hand, and a shoebox-sized robot that could walk a set number of steps, pick up an object, and then go back to where it started.
According to Preston, the integration of both digital and analog control in the same system architecture is the biggest accomplishment in this work. Having both enables digital programming of the pneumatic control circuits with the “ones and zeros you would expect to find in a conventional computer. However, analog capabilities, or continuous things, are also an option, “said he. “That lets us add new features that weren’t possible before and really simplifies the architecture of the whole system.”
Preston said Decker’s success wasn’t a fluke, even though it’s rare for an undergraduate to be the lead author of a study in a respected journal like the Proceedings of the National Academy of Sciences.
In his case, Colter has actually advanced to essentially what I would say is the level of a Ph.D. student in terms of some of his output as an undergraduate researcher, according to Preston. “The undergraduates at Rice are truly top-notch,” Preston said.
The Harvard researchers Haihui Joy Jiang, Samuel Root, Jonathan Alvarez, Jovanna Tracz, and Lukas Wille, Rice University’s Anoop Rajappan, and the Worcester Polytechnic Institute’s Markus Nemitz are additional co-authors.
The study was paid for by the National Science Foundation (grant numbers 2144809, 2011754, and 2025158), the Department of Energy (grant number DE-SC0000989), the Rice University Academy of Fellows, and the Harvard University Center for Nanoscale Systems.
