Commercial Capacitive Sensor Research Articles

A Capacitance to Digital Converter Using Continuous Time ΔΣ Modulator for Microphone-Based Auscultation

This paper presents a novel Capacitance to Digital Converter (CDC) for ultra-low power microphone-based auscultation. The proposed CDC utilizes a continuous-time Capacitance to Voltage Converter (CVC) inside a delta-sigma (ΔΣ) modulator to perform the Delta operation. The multi-bit feedback in the embedded CVC with a capacitive DAC reduces the voltage swing at its output, which permits a larger capacitive signal gain and relaxes the noise requirements. The multi-bit feedback also allows a larger signal swing at the CVC output, and improves the dynamic range. The proposed CDC was fabricated in 0.18 μm CMOS technology and was measured with a commercial capacitive sensor. Measurement results demonstrate that the proposed CDC consumes only 6.3 nJ while achieving 13-bit resolution. This corresponds to a FoM of 0.84 pJ/conversion-step.

Novel Capacitive-Based Sensor Technology for Augmented Proximity Detection

In this paper, the authors developed a capacitive-based sensor technology for high-range proximity detection, composed of multiple active units. The single sensor unit consists of a multi-layer compliant structure with a coplanar plate capacitor configuration. A prototype, composed of two single active units, was designed using, for each one, conductive electrodes printed on a polyimide layer and positioned in the middle of soft polymeric insulating substrates, either acting as shock-absorber and for sensor electrical and mechanical protection. For an in-depth theoretical analysis of the physical capacitive principle, a conditioning electronic circuit (i.e. data collection, conversion, amplification and filtering modules) was built in order to collect a digital voltage output proportional to the variable sensed capacitance. Experiments were performed to characterize the single capacitive sensor and the multi-unit sensor technology (i.e. curve of response, dynamic response, accuracy and precision). The results confirmed the outcomes originally achieved through FEM simulations and the effectiveness of the technology in high-range detection. Using a combination of two active sensor units, an operating range was achieved of up to 15 times the performance of a single commercial capacitive sensor with comparable dimensions.

Accuracy of capacitive sensors for estimating soil moisture in northeastern Brazil

Commercial capacitive sensors are used to determinate soil moisture, but the probes can be expensive. The objective of this study was to calibrate the YL-69 sensor and compare its accuracy with two commercial ECH2O probes. The sensors were used to determinate soil moisture in an Entisol Orthent and an Oxisol Ustox, representative soils of Northeast Brazil. The greenhouse study was conducted at the Federal Rural University of Pernambuco, Northeast Brazil, with two soils (Entisol Orthent and Oxisol Ustox). The experimental plots consisted of 5 replicates of 5 L pots, daily soil moisture was monitored from saturation to air dried. The soil textural classes were determined using the hydrometer method and the textural triangle of the Brazilian classification. After calibration, the YL-69 sensor performed well in both soils, obtaining a coefficient of determination (R2) value > 0.90 and root mean square error (RMSE) ≤0.01 m3 m−3; however, this sensor did not perform well if not calibrated. As such, the YL-69 sensor performed as well as the EC-5 and 5TE probes. The low cost of the YL-69 sensor makes it an attractive replacement for the EC-5 and 5TE sensors in determining the water content in Entisol Orthent and Oxisol Ustox.

Accurate displacement-measuring interferometer with wide range using an I2 frequency-stabilized laser diode based on sinusoidal frequency modulation

We propose the use of the sinusoidal frequency modulation technique to improve both the frequency stability of an external cavity laser diode (ECLD) and the measurement accuracy and range of a displacement-measuring interferometer. The frequency of the ECLD was modulated at 300 kHz by modulating the injection current, and it was locked to the b21 hyperfine component of the transition 6-3, P(33), 127I2 (633 nm) by the null method. A relative frequency stability of 6.5 × 10−11 was achieved at 100 s sampling time. The stabilized ECLD was then utilized as a light source for an unbalanced Michelson interferometer. In the interferometer, the displacement and direction of the target mirror can be determined using a Lissajous diagram based on two consecutive and quadrant-phase harmonics of the interference signal. Generally, the measurement range of the interferometer by the proposed method is limited by the modulation index and the signal-to-noise ratio of the harmonics. To overcome this drawback, suitable consecutive harmonic pairs were selected for the specific measurement ranges to measure the displacement. The displacements determined in the specific ranges by the proposed method were compared with those observed by a commercial capacitive sensor. From the comparison, the proposed method has high precision to determine the displacement. The measurement range was also extended up to 10 m by selecting a suitable modulation index and suitable consecutive pairs of harmonics.

Investigation of the influence of the main error sources on the capacitive displacement measurements with cylindrical artefacts

Capacitive displacement sensors are usually used with flat-targets in dimensional metrology applications, which require measurements with nanometer-level uncertainties. The use of capacitive sensors has recently expanded to cover the measurement of cylindrical artefacts (roundness, straightness and cylindricity). The error produced by the curved shape leads to the increase of the nonlinearity, since the sensing range between the targets and the sensitive part shrinks. This phenomenon cannot be ignored for applications requiring a nanometer-level uncertainties. In the context of LNE's (French National Metrology Institute NMI) on-going development of a machine for form measurement with an uncertainty of a few nanometres (<5 nm) using capacitive sensors, an experiment has been developed to characterize the behaviour of two commercial capacitive sensors. The experiment enabled the evaluation of the major error sources (axial and radial error motions as well as the deviation/tilt of the capacitive sensors) which influence capacitive displacement measurements. The research was completed with a flat-target and cylindrical artefacts whose diameter values varied between 50 and 200 mm. For radial error motion, both experimental and theoretical results were compared and were found to agree within 2 nm, especially when the radial error motions were small (<50 μm). Finally, the polynomial fitting methods using terms up to 4 and 6 resulted in a deviation between measured displacements and fit of 0.005%FS (FS: Full Scale corresponds to the working range of 90 μm).

Fluorescent Optosensor for Humidity Measurements in Air

A fiber-optic sensor for relative-humidity (RH) monitoring in environmental samples is described based on the adiabatic photoreaction that produces an intramolecular charge-transfer excited state, which is the basis of the sensor response. The sensitive membranes are obtained immobilizing a highly fluorescent dye, 4-[2-(pyrazin-2-yl)-1,3-oxazol-5-yl]benzenamine (pzoxba; formerly called appzox), in hydroxypropylcellulose (HPC). The composition of the sensing films was optimized to a final ratio pzoxba/HPC of 1.8⋅10−5 mol g−1 with a 100-μm membrane thickness. The optode response spans from 1.68 to 100% RH, with a detection limit of 0.56% (Table 2). Typical response times (t90) to 0 – 100% relative humidity are 1 – 2 min, the relative standard deviation for repeated measurements being 0.77 – 1.8%. The optode is insensitive to typical organic vapor interferents of commercial capacitive sensors (see Table 3) as well as to molecular oxygen, an important quencher of other luminescence-based optical sensors. The proposed optode was successfully applied and validated for continuous monitoring of the relative humidity level in environmental samples.