Inductor–capacitor ( $LC$ ) passive wireless sensors use a transformer with loose coupling between an external readout coil and an inductor that receives power through this inductive coupling. Changes in the sensor are wirelessly and remotely detected by the readout coil, which makes them highly useful in applications that require the sensor to be powered remotely and to occupy a small volume, such as harsh and sealed environments, where physical access to the sensor is difficult. Although the sensor to accomplish this function dates from the 1960’s, its rapid extension over the past decades has benefited from microelectromechanical systems. This paper provides an overview of the status and challenges in the $LC$ passive wireless sensor toward a wireless sensing platform. The basic sensing principles are first categorized into detecting changes of the sensor in response to the capacitance, resistance, inductance, or coupling distance due to the parameter of interest through monitoring the impedance magnitude and phase spectrum. The present state of the art in sensor applications for pressure, strain, temperature, humidity, biochemical, gas, and so on is then reviewed and compared. For emerging applications from many Internet of Things scenarios, geometrical constraints, such as small and non-invasive coils, reduce the magnetic coupling between the sensor and the readout coil, resulting in a limited interrogation distance. Furthermore, an increasing number of applications also require the simultaneous measurement of multiple parameters. Recent efforts to increase the interrogation distance and to extend the measurement of single parameter to multiple parameters are finally outlined. [2016-0093]

LC Passive Wireless Sensors Toward a Wireless Sensing Platform: Status, Prospects, and Challenges

Textile-Based Wireless Pressure Sensor Array for Human-Interactive Sensing

Two dimensional (2D) MXene decorated by the SnO2 nanoparticles has been successfully synthesized by the hydrothermal method. The formed MXene/SnO2 heterojunction based chemirestistive-type sensors exhibit an excellent sensitivity and selectivity to different concentrations of NH3 from 0.5 to 100 ppm at room temperature. The enhanced sensing properties can be attributed to two major factors. On one hand, 2D MXene provides a matrix which has a unique, selective adsorption ability to ammonia and good conductivity that makes the room-temperature sensing of the heterojunction based sensor feasible. On the other hand, the difference in the Fermi level of the 2D MXene and SnO2 drives the charge transfer at the interface of the heterojunctions enriching the electrons on the surface region of SnO2 and consequently enhancing the sensitivity. Furthermore, a wireless sensor made from an inductor-capacitor (LC) antenna and the as-synthesized MXene/SnO2 heterojunctions fabricated by the low-temperature-cofired ceramic technique (LTCC) was preliminary investigated. The wireless sensor shows a even faster response/recovery speeds (

MXene/SnO2 heterojunction based chemical gas sensors

Radio-frequency identification (RFID) sensors are one of the fundamental components of the internet of things that aims at connecting every physical object to the cloud for the exchange of information. In this framework, chipless RFIDs are a breakthrough technology because they remove the cost associated with the chip, being at the same time printable, passive, low-power and suitable for harsh environments. After the important results achieved with multibit chipless tags, there is a clear motivation and interest to extend the chipless sensing functionality to physical, chemical, structural and environmental parameters. These potentialities triggered a strong interest in the scientific and industrial community towards this type of application. Temperature and humidity sensors, as well as localization, proximity, and structural health prototypes, have already been demonstrated, and many other sensing applications are foreseen soon. In this review, both the different architectural approaches available for this technology and the requirements related to the materials employed for sensing are summarized. Then, the state-of-the-art of categories of sensors and their applications are reported and discussed. Finally, an analysis of the current limitations and possible solution strategies for this technology are given, together with an overview of expected future developments.

https://www.mdpi.com/1424-8220/20/7/2135/pdf

Chipless RFID Sensors for the Internet of Things: Challenges and Opportunities

This paper presents a passive wireless sensor for simultaneously and remotely measuring pressure and temperature under harsh environments. The sensor consists of a dual $LC$ (inductor and capacitor) resonant circuit, one without a cavity and the other with a cavity capacitor for temperature and pressure sensing, respectively. The low-temperature co-fired ceramic technology is used to fabricate the sensor, making it suitable for high-temperature harsh environment operations. Experimental results show the prototype sensor has temperature sensitivity of 8.15 kHz/°C and pressure sensitivity of 1.96 MHz/Bar up to 400°C. [2016-0157]

A Wireless Passive Pressure and Temperature Sensor via a Dual LC Resonant Circuit in Harsh Environments

This paper presents an integrated wireless passive sensor for remotely monitoring both temperature and relative humidity. The sensor consists of a coil and a capacitor to form an inductor–capacitor ( $LC$ ) resonant circuit, which oscillates electrically at its resonant frequency. The inductor was a single-layer planar spiral copper inductor and the capacitor was fabricated by the silicon-on-glass process, which utilizes graphene oxide films as the sensing material. The change of the capacitance due to environmental humidity variation shifts the resonant frequency, while environmental temperature affects the resistance and capacitance of the $LC$ circuit and changes the resonant frequency and quality factor. By monitoring the real portion magnitude maximum of the impedance and the resonant frequency for the sensor, it is possible to get the capacitance and resistance from which the temperature and humidity can be extracted. The results presented here show that the sensitivity of the passive wireless sensor is about −17.80 kHz/%RH and $7.32~\Omega $ /%RH at 25 °C from 55%RH to 95%RH, and it is about −7.69 kHz/°C and $6.27~\Omega $ /°C at 65%RH from 10 °C to 40 °C. [2014-0311]

Simultaneous Remote Sensing of Temperature and Humidity by LC-Type Passive Wireless Sensors

This paper presents a high-sensitivity passive wireless humidity sensor system with a portable telemetry unit for applications in sealed environments. A complementary metal oxide semiconductor (CMOS) interdigital capacitive humidity sensor die was attached to an organic substrate (FR-4) on which a fixed planar spiral copper inductor was fabricated. The variable capacitor and the fixed inductor were wire bonded to form an inductor–capacitor ( LC ) tank circuit. The resonant frequency of the sensor tank is dependent on the sensor capacitance, which changes in response to the humidity. The sensitivity of the capacitive sensor was improved significantly using graphene oxide as a sensing material. The package-level integration was achieved by employing the embedded inductor on an organic packaging substrate. The LC -type sensor is interrogated wirelessly using our homemade portable telemetry unit, which is based on a standing wave ratio bridge to measure the real part of the readout coil impedance. Measurements show a sensitivity of −18.75 kHz/%RH over a range of 15%–95% RH. The implemented telemetry unit addresses the need for a low-cost, portable, and universal reader of the LC -type passive wireless sensors. [2013-0188]

An LC -Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

This paper presents an approach to extend the remote distance of inductive-capacitance (LC) passive wireless sensors via strongly coupled magnetic resonances. The use of an intermediate resonator (repeater) for enhancing the magnetic resonance results in multiple peak frequencies of a readout system. A coefficient matrix method is used here to analyze the effects of the repeater theoretically, including frequency splitting and sensitivity decay. The circuit simulation results agree well with the analytical analysis. An interrogation approach to avoid sensitivity decay is proposed by summing the two peak frequencies. A varactor-based LC passive voltage sensor is wirelessly measured with three different repeaters and without the repeater, respectively. It is shown that the remote distance of the passive wireless sensor with the repeater can be improved to 180% without sensitivity decay.

https://iopscience.iop.org/article/10.1088/0960-1317/24/12/125021/pdf

Extending the remote distance of LC passive wireless sensors via strongly coupled magnetic resonances

This paper presents an innovative design of miniature LC-type passive wireless multi-parameter sensor. The sensor consists of multiple planar inductors on a single substrate and variable capacitors forming resonant LC tanks with separated resonant frequencies. The multiple parameters of interest could be remotely measured by tracking the changes in the peak frequencies of the impedance phase at an external readout coil which is magnetically coupled to the sensor. In order to make sure that the discernible peaks can be observed, the partly-overlapped planar embedded inductors in a multi-layer organic substrate are used and respectively in parallel connected with MEMS variable capacitors. The sensor in this stacking configuration achieves multi-parameter telemetry without any significant increase in size compared to the previous single parameter counterpart. EM simulation and tentative experiments both have been carried out, and the results validate the proposed design.

Design of LC-type passive wireless multi-parameter sensor

Inductor-capacitor (LC) passive wireless sensors have been used in situations where wired connections are impossible. When this scheme is applied to implanted or sealed fields, the sensors are required to be scaled down to small dimensions, which results in the limited remote distance. Here we present an approach to enhance the remote readout distance of the LC sensors by a cyclic-scanning repeater. The resonant frequency of the repeater is cyclic-scanned so as to detect that of the LC sensor, and the readout distance is enhanced by strong magnetic coupling. The proposed full-spectrum-enhancing approach has the simplified circuits when compared with other readout enhancement ways. An LC humidity sensor, as an example, is then monitored both with and without the proposed repeater. We have demonstrated that the readout distance with the cyclic-scanning repeater can achieve 3 times long as that without a repeater, while keeping the same for their signal intensity and sensitivity. Overall, this approach with the cyclic-scanning repeater enables a longer detection range and broader applications in the implanted or sealed fields.

Cong Zhang

Papers

Simultaneous Remote Sensing of Temperature and Humidity by LC-Type Passive Wireless Sensors

An LC -Type Passive Wireless Humidity Sensor System With Portable Telemetry Unit

Extending the remote distance of LC passive wireless sensors via strongly coupled magnetic resonances

Design of LC-type passive wireless multi-parameter sensor

A Cyclic Scanning Repeater for Enhancing the Remote Distance of LC Passive Wireless Sensors