Researchers at Northwestern University and Israel's Tel Aviv University have overcome a major barrier to achieving a low-cost solution for advanced robotic touch. The authors argue that the problem that has been lurking in the margins of many papers about touch sensors lies in the robotic skin itself.
In the study, inexpensive silicon rubber composites used to make skin were observed to host an insulating layer on the top and bottom surfaces, which prevented direct electrical contact between the sensing polymer and the monitoring surface electrodes, making accurate and repeatable measurements virtually impossible. With the error eliminated, cheap robotic skins could allow robots to mimic human touch, allowing them to sense an object's curves and edges, necessary to properly grasp it.
In a paper published in the journal Advanced Electronic Materials, an interdisciplinary team of researchers that pairs electrical engineers with polymer materials scientists sheds light on this problem and provides a path forward with practical steps for validating electrical contacts, which might unknowingly be obscuring device performance, according to Northwestern's Matthew Grayson, a professor of electrical and computer engineering at the McCormick School of Engineering.
"A lot of scientists misunderstand their sensor response because they lump together the behavior of the contacts with the behavior of the sensor material, resulting in inconsistent data," Grayson said. "It turns out, if you are not aware of this problem, you can publish papers which no one can reproduce. Our work identifies the exact problem, quantifies its extent both microscopically and electrically, and gives a clear step-by-step trouble-shooting manual to fix the problem."
The rubber that can be used for typical robotic skin, called an elastomer, is flexible, lightweight and inexpensive, and when electrically conducting fillers like carbon nanotubes are added to the mix, the resulting composite becomes an ideal candidate for a touch sensor, whose resistance changes locally when pressed. But to receive electrical signals, the sensors need to be electrically contacted, and the researchers detected a thin insulating layer ever-present in such composites which could drastically change the behavior of the contacts. Just by sanding down the ultrathin insulation layer, the team was able to achieve a much stronger electrical contact and calibrate the thickness of the insulating layer both electrically and microscopically.
"All interesting things happen at the interface," said co-author and professor at Tel Aviv University Noa Lachman. "This publication not only shows the importance of sensor interfaces, but also the importance of working at the nexus between two different disciplines: materials science and electrical engineering. Materials experts suspected the presence of this insulating external layer in conductive polymer composites for years but couldn't understand its electrical effects. Each of us has one piece of the puzzle, but only together can we get the whole picture."
Robotics in particular can be tricky in part because it requires so many types of expertise. The polymer materials scientist designing the functional electronic material for a robot, for example, does not have the same training and skills as the electrical engineer whose electronics will process the sensor signals. Grayson said the "contact preparation" challenge was precisely where the conversation about this research began.
"That's why our collaboration with Tel Aviv is essential - they know the materials science that we don't know," Grayson said. "We rely on them to prepare the materials we are studying, then we take and study the material before turning around to help the Tel Aviv materials scientists characterize their materials better."
Producing new materials - and then reproducing them - requires consistency across many different variables that are often difficult or even impossible to control. In exposing the question of reproducibility in much of the literature on touch sensing, Grayson challenges the research community to hold itself to a higher standard with the quality check described in the paper. As awareness of this problem spreads among researchers, new publications can be more rigorously relied upon to advance the field with new capabilities.
The paper was supported by the U.S. National Science Foundation (NSF ECCS-1912694, NSF DMR-1720139), Northwestern through the Crown Family Fund, Leslie and Mac McQuown, Tel Aviv University through the Center for Nanoscience & Nanotechnology, and the U.S.-Israel Binational Science Foundation (BSF grant number 2018732). Additional publication support came from the Israel Innovation Project of Northwestern and Northwestern's department of electrical and computer engineering.
