Electrical engineers at Duke University have invented an inexpensive printed sensor that can monitor the tread of car tires in real time, warning drivers when the rubber meeting the road has grown dangerously thin. If adopted, the device will increase safety, improve vehicle performance, and reduce fuel consumption.
The Duke researchers have demonstrated a design using metallic carbon nanotubes that can track millimeter-scale changes in tread depth with 99 percent accuracy. With two patents pending, the researchers are in the process of establishing industry collaborations.
The technology relies on the well-understood phenomenon of how electric fields interact with metallic conductors. The core of the sensor is formed by placing two small, electrically conductive electrodes very close to each other. By applying an oscillating voltage to one and grounding the other, an electric field forms between the electrodes.
While most of this field passes directly between the two electrodes, some of the field arcs between them. When a material is placed on top of the electrodes, it interferes with this so-called “fringing field.” By measuring the interference through the electrical response of the grounded electrode, it is possible to determine the thickness of the material covering the sensor. While there is a limit to how thick a material this setup can detect, it is more than enough to encompass the several millimeters of tread found in today's tires. And with evidence of sub-millimeter resolution, the technology could easily tell drivers when it's time to buy a new set of tires or give information about uneven and often dangerous wear by connecting many sensors in a grid to cover the width of the tire. Tests also proved that the metal mesh embedded within the tires does not disrupt the operation of the sensors.
The researchers optimized performance by exploring different variables from sensor size and structure to substrate and ink materials. The best results were obtained by printing electrodes made of metallic carbon nanotubes on a flexible polyimide film. Besides providing the best results, the metallic carbon nanotubes are durable enough to survive the harsh environment inside a tire.
The sensors can be printed on most anything using an aerosol jet printer — even on the inside of the tires. And, while it is not yet certain that direct printing will be the best manufacturing approach, whatever approach is ultimately used, it is estimated that the sensors should cost far less than a penny apiece once they're being made in quantity.
The group also wants to explore other automotive applications for the printed sensors, such as keeping tabs on the thickness of brake pads or the air pressure within tires. This is consistent with a key trend in the automotive sector toward using embedded nanosensors.