I.    INTRODUCTION

The first passive LC pressure wireless sensors were suggested by Collins in 1967 involving two flat spiral coils; the end result was a small pressure sensor that was implanted in the eye [1]. Nevertheless, they have not attracted significant attention prior to the 1990’s [2] due to the emergence of micro-electro-mechanical-system (MEMS) technology. Passive wireless sensing has been widely used in industrial and medical applications due to its miniaturization, battery-independent and physical independent characteristics [3]. Examples of such applications include tire pressure monitoring in automotive industry [4], eye pressure testing in medicine [5], and pressure sensing at high temperature [6]. Recently, DeRouin et al. proposed a single spiral coil LC wireless humidity sensor based on the parasitic capacitor as the detecting capacitance [7]. A single sided FR-4 fiberglass printed circuit board (PCB) was used to fabricate the sensor. The PCB isn’t throwaway, however. The proposed paper-based technology therefore works well since it is disposable. One additional benefit is that the paper-based LC sensors do not need a power source. A contactless, and range-insensitive readout approach is a promising improvement in LC-based sensor technology [8-9]. Not only will this approach enhance the sensor's performance, it will also overcome the drawbacks of traditional touch oriented sensor system. Those drawbacks such as friction, contamination or regular maintenance are avoided with the contactless sensing while a distance insensitive readout ensures that variations in distances between the sensor coil and the reader coil does not result in coordinating gaps, not affect the sensor's performance.[12]

II.                 WORKING PRINCIPLE

Based on the idea that resonant frequency fluctuations in an LC circuit are influenced by humidity levels and other environmental factors, the LC wireless humidity sensor functions [10]. These sensors rely on an LC circuit's resonant frequency relationship. The resonant frequency can be calculated by the following formula-

where ‘fo’ is the resonant frequency (in Hz), ‘L’ is the inductance (in henry, H), ‘C’ is the capacitance (in farads, F). Any change in the surrounding environment that alters these two parameters will cause a shift in the resonance frequency, which can be detected and used to gather the measured physical property. A parasitic capacitance of the circuit is utilized. As humidity changes, the capacitance of the sensor changes, which in turn alters the resonant frequency of the LC circuit. For instance, changes in the percentage of relative humidity could affect the material used inside the sensor which, due to its dielectric properties would alter capacitance or inductance and accordingly the resonance frequency would change. LC-based sensors can be designed for contactless operation. It features a loop-based control system with multiple bio-sensors such as heart rate, blood pressure, pulse oximeter, and body temperature sensors, which are provided by a microcontroller chip located onboard. There’s also a dedicated mobile app that accompanies the device, making it remotely controllable with health alerts and mask maintenance reminders among several other health management features.

The schematic circuit of the actual readout system is shown in Fig. 1.

  Fig.1 Equivalent Circuit of Readout System

In the above circuit diagram, L 0 is the self-inductance of the readout coil and R 0 is the resistance of the coil. The parasite capacitor C0 is connected parallel to the read-out coil. Ls, Cs and Rs are the inductance, the capacitance and the resistance of the single-spiral coil sensor, respectively. The system possesses only one series resonant frequency and two parallel resonant frequencies [11].

III.              FORMULA

In this section, we will describe the expressions used for the calculation of inductance.

A.     MODIFIED WHEELER FORMULA

Wheeler [12] derived several formulas for planar spiral inductors, but are applicable to discrete inductors; several of these formulas can be given a slight modification to reach a formula that is efficient for planar spiral integrated inductors.

where ϕ represents the fill ratio,  is the average diameter, μ =  is the magnetic permeability through a given proportionality, which is the product of the permeability of the free space (μ0) and the permeability of the particular medium (μr), and K1 and K2 are layout-dependent parameters that have varying values for square, hexagonal, and octagonal shapes given in table 1 [13].

TABLE 1: Coefficients for modified wheeler expression

IV.              SENSOR DESIGN

In this section, we will describe the sensor elements of our proposed LC sensor.

A.     SENSOR COIL

An LC wireless humidity sensor coil was made of single sided FR-4 fiberglass printed circuit board (PCB). The area of the designed sensor is 2cm×2cm. The coil is made of silver. The design considerations taken into account for designing the sensor coil for the LC sensor is given in Table 2.                              

 TABLE 2: Sensor coil design considerations

The designed sensor coil is given in below in figure 2.

Fig. 2. Sensor Coil

The sensor coil is fabricated using – “Voltera V-One”, the PCB printing machine.

The Voltera V-One enables you to create prototype circuit boards with two layers right at your workspace. You input Gerber files, and the printed circuit boards are produced. The dispenser applies a conductive ink based on silver to print your circuit in real time.

1. READOUT SYSTEM

These sensor can be contactless read out with the help of a read-out coil which is inductively coupled to the coil on the sensor head [13]. In comparison with the sensor change due to the relative humidity yielding a poor accuracy of humidity detection, it is clear that the relative position between two coils effects significantly on the system readout resonant frequency.

The design of readout coil using HFSS on FR4 substrate was made in which 8 turns of the coil are made first. The width of the coil and the spacing between the turns are kept as 0.3mm. Then coil was connected with microstrip whose dimensions were calculated by Microstrip calculator. The microstrip is tapered from one end for the purpose of impedance matching (50ohm).

For calculating the dimensions of microstrip, we have taken into account two different frequencies of 1GHz and 0.7GHz, but considered 1GHz frequency to obtain the length and width of microstrip. The design of the readout coil is shown in fig. 3.

Fig. 3. Readout Coil Design

The fabricated readout coil of the LC sensor is given in the figure 4.

Fig. 4. Fabricated Readout System

V.                 RESULTS

The simulated result of designed sensor and readout circuit is obtained. We have designed the LC sensor in HFSS software and get the simulation results at 384.66 MHz with return loss value of -12.29 dB and 732.58 MHz with -9.41dB. After placing the sensor on readout circuit coil then we get the frequency shift on 425.58 MHz with return loss value -13.87 dB and 791.06 with return loss value of -10.56dB. That means here dielectric constant changed with the material. Sensor was on polymide and readout circuit was on FR4. Due to dielectric constant, humidity will also get changed with material therefore it is showing the frequency shift.

Fig. 5. Simulation Result of the designed sensor and readout circuit

VI.              CONCLUSION