Twisting and tumbling in the murkiest waters, seals easily chase down fleeing shoals of fish. Their secret? Whiskers, that like antennae are tuned to the vibrations of their prey as they slice through the water.
Inspired by the seal’s superlative hunting skills, researchers at the University of Illinois at Urbana-Champaign and Illinois’ Advanced Digital Sciences Centre in Singapore built a set of robotic whiskers. When partnered with high-tech mathematical modelling, the whiskers produced detailed, colour-coded images revealing the speed and direction of fluids flowing over them.
The work has “great potential to be a useful, if unconventional, sensing system”, according to lead author Cagdas Tuna. It was published in the journal Bioinspiration and Biomimetics in August.
Whiskers are specialised for tactile sensing. They’re often found on animals that prefer the cover of darkness, such as nocturnal or underground dwellers.
For instance, a rat brushes its whiskers over an object to learn about its surface. At the base of each hair whisker lies a pressure-sensitive follicle lined with nerves. These project to the somatosensory cortex, a part of the brain responsible for sensing touch. Rats and mice have around 25,000 nerves innervating their whiskers; seals have a whopping 300,000.
Tuna and his colleagues kept their robotic whiskers relatively simple. Each set comprised five nickel titanium wires, threaded into plastic straws to expand the area exposed to the flow and increase signal strength. As with animal whiskers, there was a sensor at each wire’s base, in the form of a strain gauge that measured how much each one bent as water or air flowed over them.
The team tested their system using the airflow from a standard household hairdryer fitted with a slit-shaped nozzle. The nozzle could be rotated to change the angle of flow.
The simple materials used in the whiskers are “not so new”, says Kuniharu Takei, an engineer at Japan’s Osaka Prefecture University. The advance is the nifty mathematical modelling used to build up an image of the airflow’s angle and intensity as the whiskers bent in the breeze.
The resulting images were similar to those produced by computerised tomography, or CT scans. Tuna says coloured pixels denote flow strength and shape “with high accuracy”. Does a seal tracking a fish with its whiskers ‘see’ fluid flow by building maps like these in its brain? We don’t know. But, just like reading a weather map, a human could easily look at these images and identify the strength and direction of the airflow.
But the real application is robots. Takei sees a day when small agile robots, bristling with whiskers, are sent into emergency zones to pinpoint and mend gas leaks, for instance.
So far, researchers have only built sensing whiskers that track flow from a stationary position. But animals use their whiskers as they move about. Tuna says he’s working on a way to better emulate how a seal hunts. He’s already developed a model that can track moving objects from their flow pattern – “maybe not as good as seals”, he adds, but it’s a start.