The capacitive sensors are widely used for liquid level measurement in different applications, but the contact type capacitive sensors suffer from frequent maintenance and drift of the sensor output. The present paper presents a simple noncontact IDT fringing field capacitance sensor for liquid level measurement. The sensor can be made using flexible /non-flexible substrate, and it can be used to measure the liquid level of the vessel for both planar and non-planar surfaces. Theory of level measurement using the sensor has been discussed. Four different IDT sensor (1-n-1) configurations such as 1-1-1, 1-3-1, 1-5-1 and 1-11-1 have been designed, analysed, simulated (finite element method), and fabricated to measure water and insulating transformer oil levels from 0 to 40 mm. The analytical expressions are derived for 1-n-1 configuration to estimate the value of the capacitance and the maximum field penetration depth of the sensors. The capacitance value of all the four sensors varies almost linearly with the variation of liquid levels. Experimental results show that the sensitivity of the 1-5-1 IDT sensor is 40 fF/mm. Performances of the sensors were compared with other capacitive sensors reported in the literature. The sensor is capable to detect the water level with a resolution of 0.1 mm.

A digital interfacing circuit (DIC) based on sliding discrete Fourier transform (SDFT) bins has been proposed for lossy capacitive sensors. The proposed interfacing circuit consisting of an analog sensing circuit and an analog-to-digital converter (ADC)-field-programmable gate array (FPGA)-based DIC. The sliding DFT bin-1 is employed as a quadrature signal generator to obtain in-phase and quadrature signals of the input supply voltage provided to the analog sensing circuit. The output sinusoidal signal of analog sensing circuit is correlated with the in-phase and quadrature signals separately and then averaged with SDFT bin-0 for computing sensing capacitance and resistance values. The proposed method is implemented in FPGA and tested for capacitance measurement range of 11 pF–5.27 nF with the maximum relative percentage error of 1.36%. The nonlinearity error is < 0.37% FS and FSS. Moreover, the suitability of the proposed interface for humidity measurement has been explored and validated experimentally with saturated salt solutions. Furthermore, the interdigitated capacitance is fabricated and tested with the proposed digital interface for the capacitance range of 30–122 pF, which finds an application in capacitive touchpad.

Moisture in food grains, including chickpea and mustard seeds, plays a crucial role in their storage and processing, thus ensuring food quality. It helps in the improvement of preservation techniques. Moisture in these materials is an age-old problem; hence, it is important to monitor it in real time. The conventional gravimetric method is manual and time-consuming; some online electrical techniques are available in which grains are considered as a dielectric material, but they are relatively complex and costly. This present work describes a nondestructive concentric fringing field (CFF) capacitive sensor to detect moisture (4–33% by absolute weight) of chickpea grain and (12–30% by absolute weight) mustard seed. First, the proposed CFF sensor was modeled, and then three distinct concentric sensors were designed, simulated, fabricated, and experimentally validated to determine moisture in chickpea grains and mustard seeds. The capacitance values of all the sensors approximately linearly varied with the changes in the moisture of the grains. The average sensitivity of the most sensitive sensors was close to 20 fF/% wt for chickpeas and 31 fF/% wt for mustard seeds. The proposed sensor is sensitive, nondestructive, easy to use, inexpensive, and fast.

In this article, printed circuit board (PCB) based improved reconfigurable interdigital (IDT) multi-sensor structures are proposed for sensing of capacitance (C) and resistance (R) thereby physical quantities such as humidity, moisture content (MC), liquid level, and temperature (T) can be measured. The structures of IDT capacitive sensor such as 1-1-1, 1-3-1, 1- 5-1, 1-7-1, 1-9-1 have been modified to have resistive sensor along with capacitive sensor, which are reconfigurable structures. The proposed improved IDT sensor structures for measuring C and R with single PCB are: 1-1-1, 1-2-1, 1-4-1, 1-6-1, 1- 8-1. This modification introduces third terminal in the sensor structure. The proposed structures can be used for C and R measurements, which implies the measurement of physical quantities. The fabricated IDT sensor is tested for humidity, MC, liquid level, and T measurements. The maximum sensitivity was observed during the measurements with 1-2-1 multi-sensor IDT structure, which are found to be 1.39 pF/%relative humidity (RH) and 4.55 mΩ/°C. The sensor is also tested in laboratory for non-contact measurement of MC of chickpea with range of 6% to 27%. The variation of ΔC = 648.20 fF with a sensitivity S = 31.27 fF/%MC. The proposed IDT multi-sensor offers acceptable reliability, repeatability, and sensitivity.

This article proposes a use case of sliding discrete Fourier transform (SDFT) in the Capacitive and Resistive sensors interface. The input and output signals of the analog sensor circuit are passed through the analog to digital converters and then the quadrature signal of the input excitation signal is generated through SDFT Bin-1, which can extract only fundamental frequency by filtering all other harmonics. The Sliding DFT is a tuned filter which can filter out the high frequency components, especially second harmonic present in the correlated signal of analog front end and quadrature component of the excitation signal. The digitized correlated signal is passed to the SDFT Bin-0, which can extract only the dc present in the correlated signal. The averaged output from SDFT Bin-0 is further utilized to calculate the capacitance (C) and resistance (R) values of the sensors under test. The proposed use case in designing the interface is applicable for one measurement at a time either C or R sensing. Experimental validation of the proposed use case has been carried out by FPGA implementation of the algorithm. FPGA implementation is validated for the C range spanning from 54.28 fF to 5600 pF. The maximum absolute relative (AR) percentage error observed is 1.27% for the measured range of 20 pF to 5600 pF and 1.92% for the measured range of 54.28 fF to 10 pF. The maximum AR percentage error observed for R measurement in the range of 10 k to 4 M is 1.157%. The application of the proposed interface include smart devices, touch sensors, and automotive electronics.