Safety of automobiles is in great demand. To prevent burst of tires, smart tires are currently under development. Most of the smart tires have integrated or attached sensors to measure deformation or internal pressure of tires during a long period of service. The embedded sensor enables highly precise measurement of road surface friction, and the precise friction measurements make more efficient anti-lock braking system (ABS). In the present study, a new passive wireless strain measurement system utilizing capacitance change of tires is proposed and experimentally investigated. The system consists of external antennas and a strain sensor with an inductance-capacitance (LC) resonant circuit. The wireless system utilizes electromagnetic coupling between two inductors of the antenna and the sensor. This method enables us to use a part of an actual tire as a capacitor of the LC circuit. Deformation of the tire causes a resonant frequency change of the sensor. This resonant frequency change is measured as a change in the phase angle of the load impedance is reflected back to the antenna. Tensile tests are performed and the phase angles of the antenna are measured during the tests. As a result, the method is experimentally shown to be effective as a passive wireless strain monitoring of tires.
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