A Novel Multiparameter RFID Sensing Characteristic Study Using Varactor Diode and Digital Control Coding Method

This article proposes a multiparameter radio frequency identification (RFID) sensor with two controllable coding structures. The sensor integrates the ability of strain and humidity detection and is suitable for the field of structural health monitoring (SHM). The sensor is equipped with three S-shaped structures and one rectangular structure, two S-shaped structures are used as encoding units, one S-shaped structure is used as a humidity detection unit, and the rectangular structure is used as a strain unit. These structures are made on a Rogers RO3003 dielectric substrate. Controllable coding is achieved by changing the capacitance value of the varactor diode. A single coding unit can achieve a coding value of more than 2 bits. Compared with the traditional coding structure, the coding information capacity is improved, which is more suitable for distributed SHM scenarios that require multiple sensors. The digital control circuit is designed on the flexible printed circuit (FPC), which has the characteristics of bendability. It is mainly composed of a microcontroller, Bluetooth, varactor diodes, and other components, and can realize the wireless control function of the coding unit. A broadband antenna is further designed to match the working frequency band of the sensor. Experimental verification shows that the micro-strain of the strain unit is linearly related to its resonant frequency, and the humidity sensitivity is 193.92 kHz/%RH at a relative humidity (RH) of 33.7%–81.2%. The two encoding units can be controlled wirelessly to achieve 25-bit coding while having the advantages of small size and low cost. In the future, this method can be combined with self-powered devices and Internet of Things (IoT) gateways to achieve long-term state control of multiple sensors, providing a new method for SHM applications.