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Carbon Nanotubes Introduce Tire Wear Monitoring into Cars

An inexpensive printed sensor capable of monitoring the tread of car tires in real time has been invented by Electrical Engineers at Duke University. This new invention warns drivers when the rubber meeting the road becomes dangerously thin.

An illustration of how the novel tread sensor works. The sensor is placed on the inside of the tire, where the tire wall and tread interferes with an electric field that arcs between two electrodes. That interference can be measured to determine the thickness of the rubber with millimeter accuracy. Credit: Duke University

This device, if adopted, will enhance vehicle performance, increase safety and also reduce fuel consumption. The Engineers hope that the tire wear sensor could become the first of many that possibly will disrupt the $2 billion tire and wheel control sensor market.

By partnering with Fetch Automotive Design Group, the Duke Researchers succeeded in demonstrating a design employing metallic carbon nanotubes (small cylinders of carbon atoms only one-billionth of a meter in diameter) capable of tracking millimeter-scale changes in tread depth with 99% accuracy. With two patents pending, the team is now moving towards establishing industry collaborations in order to bring the technology to a tire that is commonly used by all individuals.

With all of the technology and sensors that are in today's cars, it's kind of crazy to think that there's almost no data being gathered from the only part of the vehicle that is actually touching the road. Our tire tread sensor is the perfect marriage between high-end technology and a simple solution.

Aaron Franklin, Associate Professor, Electrical and Computer Engineering, Duke University

Franklin and his colleagues have presented a report on their sensor design in a paper published June 9th in IEEE Sensors Journal.

The technology depends on the well-understood mechanics of how electric fields work together with metallic conductors. The core of the sensor is developed by placing two tiny, electrically conductive electrodes extremely close to each other. An electric field develops between the electrodes when an oscillating electrical voltage is applied to one electrode and when the other electrode is grounded.

While most of this electric field directly passes between the two electrodes, it was observed that some of the field arcs between them. Placing a material on top of the electrodes, results in it interfering with this so-called "fringing field." It is now possible to determine the thickness of the material covering the sensor by measuring this interference via the electrical response of the grounded electrode.

It is considered to be more than enough to encompass the several millimeters of tread detected in the existing tires although there is indeed a limit as to how thick a material this setup is capable of detecting. With evidence of sub-millimeter resolution, the new technology could effortlessly tell drivers when it is time for them to buy a new set of tires or provide details about uneven and often unsafe tire wear by connecting several sensors in a grid in order to cover the width of the tire.

Tests have also established that the metal mesh embedded within tires does not upset the working of the new sensors.

When we pitch this idea to industry experts, they say to each other, 'Why haven't we tried that before? It seems so obvious once you see it, but that's the way it is with most good inventions.

Aaron Franklin, Associate Professor, Electrical and Computer Engineering, Duke University

While it is possible to produce the sensor from a wide range of materials and methods, the paper describes how the team enhanced performance by exploring a wide range of variables starting from sensor size and structure to substrate and ink materials. Printing of electrodes developed from metallic carbon nanotubes on a flexible polyimide film provided best results. The metallic carbon nanotubes, besides providing best results, are also adequately durable in order to survive the harsh environment within a tire.

It is possible to print the sensors on almost anything with the help of an aerosol jet printer, and this can also be done on the inside of the tires themselves. Though it is not yet certain that direct printing will be the ideal manufacturing approach, whatever approach is eventually used, Franklin stated that the cost of the sensors should be less than a penny each once they are developed in large quantities.

Franklin's team also aims at exploring other automotive applications for the printed sensors, such as maintaining tabs on the thickness of brake pads or the air pressure present inside tires. This is regular with a key trend in the automotive sector toward employing embedded nanosensors.

Recently, other tire tracking products hit the market. For instance, Pirelli, a tire company, recently launched a system for electronically tracking each tire -- when it was installed, when it was last serviced, how many miles it has gone, etc. – and using an algorithm in order to estimate the wear and tear of the tire. Even though it is not the same thing as physically, actively measuring tire tread, it points out that the market is now open for such details.

However, the technology is not limited to cars.

This setup could be used with just about anything that isn't metallic or too thick. Right now we're focusing on tires, but really anything you'd rather not have to cut apart to determine its thickness could be monitored by this technology in real time.

Aaron Franklin, Associate Professor, Electrical and Computer Engineering, Duke University

"Noninvasive Material Thickness Detection by Aerosol Jet Printed Sensors Enhanced Through Metallic Carbon Nanotube Ink," Joseph B. Andrews, Changyong Cao, Martin A. Brooke and Aaron D. Franklin. IEEE Sensors Journal, June 5th, 2017.

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