A Portable Device for Passive LC Sensors Readout With Low-Coupling Enhanced Sensitivity

Abstract

Passive sensors composed of inductor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}$ </tex-math></inline-formula> and capacitor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${C}$ </tex-math></inline-formula> (LC sensors) have significant importance for applications where reliable wireless monitoring of temperature, pressure, relative humidity, strain, and pH is needed. In this article, we propose a method suitable for implementation on the portable device for wireless readout of LC sensors, which eliminates the need for costly and bulky laboratory equipment (impedance analyzers and vector network analyzers). Our approach is based on an electrical circuit of voltage divider formed by a primary inductor and a reference resistor. The resonant frequency of the LC sensor is detected from the processing of the measured voltage across the reference resistor. The readout device is realized in the form of a printed circuit board (PCB) compatible with the widely available Arduino boards. Wireless readout of the ten LC tags with the proposed device and with the laboratory impedance analyzer, showed that, in frequency range 1–10 MHz, relative differences in estimated values are lower than 4%. The distinguishing feature of the presented method is its enhanced sensitivity in the case of very weak coupling between primary and sensing inductor, which was achieved using numerical differentiation approximation of the measured voltage across the reference resistor. An additional contribution of the proposed device is demonstrated with the simultaneous readout of multiple tags. Finally, the proposed device was characterized with respect to the noise impact of the two power supply realizations (dc adapter and the battery).