Differential Capacitance Detection Research Articles

Study on driving noise of quartz flexible accelerometer torquer

The distributed capacitance inside the quartz flexible accelerometer (QFA) coupled the high frequency voltage excitation signal in the differential capacitance detection circuit to the torquer coil, and superimposes the torquer driving current to form the driving noise. In this study, the values of the distributed capacitance inside the QFA were simulated. According to the formation mechanism of the QFA driving noise, the equivalent circuit model of the driving noise is established, and the driving noise characteristics of the detection circuit with single excitation and double excitation source are analyzed. The theoretical and experimental results show that the electric field coupled driving noise transmission system is a first-order system with high-pass characteristics. The driving noise of the single excitation detection circuit is larger than that of the dual excitation detection circuit (DEDC), and the DEDC can reduce the driving noise by 39.77% when the QFA shell is grounded. The equivalent acceleration of the electric field coupled driving noise is between tens of μg to hundreds of mg, which is one of the important noise sources that affect the measurement accuracy of the QFA. A measure was proposed to suppress the high-frequency driving noise by adding a low-pass filter after the sampling output of the driving current, which can reduce the driving noise to 1.85 μg and effectively reduce the influence of the driving noise on the measurement accuracy of the QFA.

Analysis of Noise-Detection Characteristics of Electric Field Coupling in Quartz Flexible Accelerometer

The internal electric field coupling noise of a quartz flexible accelerometer (QFA) restricts the improvement of the measurement accuracy of the accelerometer. In this paper, the internal electric field coupling mechanism of a QFA is studied, an electric field coupling detection noise model of the accelerometer is established, the distributed capacitance among the components of the QFA is simulated, the structure of the detection noise transfer system of different carrier modulation differential capacitance detection circuits is analyzed, and the influence of each transfer chain on the detection noise is discussed. The simulation results of electric field coupling detection noise show that the average value of detection noise can reach 41.7 μg, which is close to the effective resolution of the QFA, 50 μg. This confirms that electric field coupling detection noise is a non-negligible factor affecting the measurement accuracy of the accelerometer. A method of adding a high-pass filter to the front of the phase-shifting circuit is presented to suppress the noise of electric field coupling detection. This method attenuates the average value of the detected noise by about 78 dB, and reduces the average value of the detected noise to less than 0.1 μg, which provides a new approach and direction for effectively breaking through the performance of the QFA.

Integrated 3-axis tactile sensor using quad-seesaw-electrode structure on platform LSI with through silicon vias

This paper reports a MEMS-on-LSI integrated tactile sensor for robot applications that has both fully differential capacitive 3-axis force sensing and signal processing in a single chip. The MEMS part is composed of a quad-seesaw-electrode structure that senses 3-axis force with fully differential capacitive detection. The LSI part is an original sensor platform LSI that enables signal processing and serial bus communication for various types of sensors. The sensor platform LSI was processed for the MEMS-on-LSI integration after fabrication in a semiconductor foundry; the additional processing formed 300-μm-deep annular-type through silicon vias (TSVs), a bonding ring, and sensor electrodes. By Au-Au thermocompression bonding of the fabricated MEMS and LSI parts, a 2.8-mm-square surface-mountable MEMS-on-LSI integrated tactile sensor was completed. A working test demonstrated that digital packet transmission through TSV and confirmed the working principle of 3-axis force sensing with fully differential capacitive detection. A fully integrated and fully differential 3-axis tactile sensor was achieved on a single chip.

Micro-angle tilt detection for the rotor of a novel rotational gyroscope with a 0.47″ resolution

Differential capacitive detection has been widely used in the displacement measurement of the proof mass of vibratory gyroscopes, but it did not achieve high resolutions in angle detection of rotational gyroscopes due to restrictions in structure, theory, and interface circuitry. In this paper, a differential capacitive detection structure is presented to measure the tilt angle of the rotor of a novel rotational gyroscope. A mathematical model is built to study how the structure’s capacitance changes with the rotor tilt angles. The relationship between differential capacitance and structural parameters is analyzed, and preliminarily optimized size parameters are adopted. A lownoise readout interface circuit is designed to convert differential capacitance changes to voltage signals. Rate table test results of the gyroscope show that the smallest resolvable tilt angle of the rotor is less than 0.47″ (0.00013°), and the nonlinearity of the angle detection structure is 0.33%, which can be further improved. The results indicate that the proposed detection structure and the circuitry are helpful for a high accuracy of the gyroscope.

High resolution quartz flexure accelerometer based on laser self-mixing interferometry.

As common high-precision inertial sensors, quartz flexure accelerometers have a wide application prospect in low-cost inertial navigation systems. To ameliorate their resolution performance restricted by differential capacitance detection, we proposed a modified type of quartz flexure accelerometer based on an emerging optical technique named laser self-mixing interferometry, which is utilized to sense the displacement of a quartz pendulous reed, and then an equal and opposite force is accordingly produced to maintain the reed motionless relative to the inertial frame. The configuration and working principle of the improved accelerometer have been introduced and analyzed. The preliminary experiments indicate that its bias stability reaches 0.75-0.85 μg, which shows some progress when compared to the traditional type. Further improvements are mainly limited by the characteristics of the laser diode and the multiple reflections from the pendulous reed.

Micromachined Tri-Axis Capacitive Accelerometer Based on the Single Mass

A novel micromachined tri-axis capacitive accelerometer based on the single mass is proposed in this paper. The presented accelerometer contains a single proof mass acting as the sensitive element which is suspended by two flexible beams above the substrate, and an array of movable and fixed electrodes is utilized. To match the sensitivity design requirement, the differential capacitive detection is utilized, and the cross-sensitivity problem of three orthogonal axes is avoided ideally. The accelerometer structure is based on the EPI-poly process to realize the 18 microns thickness. Thus the detection capacitance, sensitivity and reliability are improved, the mechanical noise is decreased. To reduce the thermomechanical stress’ effect on the offset, a special anchor-middle structure is designed. Simulation results show that the offset is insensitive to the thermomechanical stresses and the sensor has a good consistency in three axes which has been validated by the testing results.

A MEMS differential viscometric sensor for affinity glucose detection in continuous glucose monitoring

Micromachined viscometric affinity glucose sensors have been previously demonstrated using vibrational cantilever and diaphragm. These devices featured a single glucose detection module that determines glucose concentrations through viscosity changes of glucose-sensitive polymer solutions. However, fluctuations in temperature and other environmental parameters might potentially affect the stability and reliability of these devices, creating complexity in their applications in subcutaneously implanted continuous glucose monitoring (CGM). To address these issues, we present a MEMS differential sensor that can effectively reject environmental disturbances while allowing accurate glucose detection. The sensor consists of two magnetically driven vibrating diaphragms situated inside microchambers filled with a boronic-acid based glucose-sensing solution and a reference solution insensitive to glucose. Glucose concentrations can be accurately determined by characteristics of the diaphragm vibration through differential capacitive detection. Our in vitro and preliminary in vivo experimental data demonstrate the potential of this sensor for highly stable subcutaneous CGM applications.

Lengthwise Detection of High Linearity Micromachined Gyroscope

This paper introduces the basic working principle of capacitive micromachined gyroscope. A novel differential lengthwise capacitance comb detection structure is put forward. And it also analyzes the linearity of the differential lengthwise capacitive comb detection structure compares to the transversal detection. By measuring the scale factor, it verifies the high linearity of lengthwise differential capacitive detection structure. The linearity insures the linearity of conversing section of the detect system’s displacement and the capacitive variation. And this will greatly improve the linearity and stability of micromachined gyroscope.

Design and fabrication of a differential capacitive three‐axis SOI accelerometer using vertical comb electrodes

AbstractA differential capacitive three‐axis silicon‐on‐insulator (SOI) accelerometer using vertical comb electrodes fabricated by using surface‐micromachining technique has been developed. The accelerometer structures consist of only the device layer of a SOI wafer without lower or upper electrodes. The vertical comb electrodes of this device have structures different from conventional vertical comb electrodes. The bottom faces of both movable and fixed electrodes are in the same plane at their initial positions but their heights are different. Three‐axis acceleration can be sensed with differential capacitance detection by using only these vertical comb electrodes. The device structures were successfully fabricated using self‐alignment and Deep‐RIE processes. As an initial result, the capacitance changes against three‐axis acceleration were observed and the device sensitivities to three‐axis accelerations were measured. The capacitance sensitivities of X, Y and Z‐axis were 1.01, 0.898 and 0.989 fF/G, respectively. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

High Resolution Differential Capacitance Detection Scheme for Micro Levitated Rotor Gyroscope

A differential capacitance detection circuit aiming at detection of rotating angle in a novel levitation structure is presented. To ensure the low non-linearity and high resolution, noise analysis and non-linearity simulation are conducted. In the capacitance interface, an integral charge amplifier is adopted as a front end amplifier to reduce the parasitic capacitance caused by connecting wire. For the novel differential capacitance bridge with a coupling capacitor, the noise floor and non-linearity of the detection circuit are analyzed, and the results show that the detecting circuit is capable of realizing angle detection with high angular resolution and relative low non-linearity. With a specially designed printed circuit board, the circuit is simulated by PSpice. The practical experiment shows that the detection board can achieve angular resolution as high as 0.04° with a non-linearity error 2.3%.

A Study on a Diode-Bridge Type Differential Capacitance Detection Circuit

AbstractDifferential capacitance sensor with movable electrodes is usually applied for accelerometers, pressure gauges, etc. because of its high sensitivity and resolution. Two axis inclination sensor with differential capacitance structure using MEMS (Micro Electro Mechanical Systems) technology has recently been designed for industrial and environmental purposes. The initial capacitance of the sensor should be very low, as few pF, therefore, a C-V converter circuit, which is able to detect very small capacitance variations is required. In this study, the particular principle of a “diode-bridge type differential capacitance detection circuit” has been analyzed. After the mathematical simulations performed by using PSpice simulator, the detection of the capacitance variations in fF order resolution are considered.

A microfabricated wall shear-stress sensor with capacitative sensing

A silicon-based micromachined, floating-element sensor for low-magnitude wall shear-stress measurement has been developed. Sensors over a range of element sizes and sensitivities have been fabricated by thin-wafer bonding and deep-reactive ion-etching techniques. Detailed design, fabrication, and testing issues are described in this paper. Detection of the floating-element motion is accomplished using either direct or differential capacitance measurement. The design objective is to measure the shear-stress distribution at levels of O(0.10 Pa) with a spatial resolution of approximately O(100 /spl mu/m). It is assumed that the flow direction is known, permitting one to align the sensor appropriately so that a single component shear measurement is a good estimate of the prevalent shear. Using a differential capacitance detection scheme these goals have been achieved. We tested the sensor at shear levels ranging from 0 to 0.20 Pa and found that the lowest detectable shear-stress level that the sensor can measure is 0.04 Pa with an 8% uncertainty on a 200 /spl mu/m/spl times/500 /spl mu/m floating element plate.

Pulse Enhanced Electrosorptive Detection of Ions at Silver in Ion Chromatography

Abstract Differential double-layer capacitance measurements applied to the silver-aqueous interface may serve as the basis of detection for surface-active ions in ion chromatography. There is a caveat to this approach, though, and that is that the electrode is subject to gradual fouling due to the accumulation of adsorbed matter. We report here on the incorporation of potential pulsing sequences to counter this damaging effect. Two approaches have been taken: 1) inclusion of a periodic potential cleaning pulse in the conventional, fixed-potential double-layer capacitance measurement scheme, and 2) utilization of a potential-pulse/coulometric measurement technique. the potential-pulse/coulometric procedure entails measurement of the change in charge required to charge the electrical double layer in response to specific adsorption of analyte. Its response-concentration behavior is similar to that observed for differential capacitance detection. the use of a combined, dual-potential (sequential) capacitive/a...