Strain monitoring of automobile tires in-service is quite effective for improving the reliability of tires and ABS (Anti-lock Braking System). In a previous study, the authors proposed a new wireless strain monitoring method which is adopted in the tire itself as a sensor with an oscillator circuit. This method is simple and useful, but it requires a battery to activate the oscillator circuit. 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 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 antenna by using electromagnetic induction. Tensile tests are performed and the phase angles are simultaneously measured. As a result, the method is experimentally shown to be effective as a passive wireless strain monitoring of tires.
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