To date, it has been difficult or impossible for most robotic and prosthetic hands to accurately sense the vibrations and shear forces that occur, for example, when a finger is sliding along a tabletop, or when an object begins to fall. Some robots today use fully instrumented fingers, but that sense of touch is limited to that appendage, and its shape or size cannot be changed to accommodate different tasks. The other approach is to wrap a robot appendage in a sensor skin, which provides better design flexibility; however, such skins have not yet provided a full range of tactile information.
A flexible sensor “skin” was developed that can be stretched over any part of a robot’s body or prosthetic to accurately convey information about shear forces and vibration that are critical to successfully grasping and manipulating objects. The bio-inspired robot sensor skin mimics the way a human finger experiences tension and compression as it slides along a surface or distinguishes among different textures. It measures this tactile information with similar precision and sensitivity as human skin, and could vastly improve the ability of robots to perform surgical and industrial procedures. If a robot is dismantling an improvised explosive device, it needs to know whether its hand is sliding along a wire or pulling on it. To hold on to a medical instrument, it needs to know if the object is slipping. This requires the ability to sense shear force, which previous sensor skins have not been able to do well.
Traditionally, tactile sensor designs have focused on sensing individual modalities — normal forces, shear forces, or vibration exclusively. Dexterous manipulation is a dynamic process that requires a multimodal approach. The new skin prototype incorporates all three modalities.
The new stretchable electronic skin is made from the same silicone rubber used in swimming goggles. The rubber is embedded with tiny serpentine channels — roughly half the width of a human hair — filled with electrically conductive liquid metal that won’t crack or fatigue when the skin is stretched, as solid wires would. When the skin is placed around a robot finger or end effector, these microfluidic channels are strategically placed on either side of where a human fingernail would be. As a human finger slides across a surface, one side of the nailbed bulges out, while the other side becomes taut under tension. The same thing happens with the robot or prosthetic finger — the microfluidic channels on one side of the nailbed compress, while the ones on the other side stretch out. When the channel geometry changes, so does the amount of electricity that can flow through them. These differences are measured in electrical resistance, and are correlated with the shear forces and vibrations the robot finger is experiencing.
Placing the sensors away from the part of the finger that is most likely to make contact makes it easier to distinguish shear forces from the normal “push” forces that also occur when interacting with an object, which has been difficult to do with other sensor skin solutions. The physically robust and chemically resistant sensor skin has a high level of precision and sensitivity for light touch applications — opening a door, interacting with a phone, shaking hands, picking up packages, and handling objects, among others. Recent experiments have shown that the skin can detect tiny vibrations at 800 times per second — better than human fingers.