Resonant Pressure Sensor Research Articles

Modelling and characterisation of a micromachined resonant pressure sensor with piezoelectric excitation and sensing

A resonant pressure sensor with piezoelectric excitation and sensing is presented, which is realised by combining a micromachined diaphragm and a quartz resonator. The sensor performance is improved by introducing double enhancement beams into the diaphragm. Finite element method is utilised to reveal the validity and optimise the structural dimensions. A self-excitation circuit is designed to drive the resonator into steady oscillation. The sensor chip is fabricated by bulk micromachining technologies and packaged in a metal shell in the inert gas environment for characterisation. The performance characteristics including the short-term drift, temperature characteristic and quality factor are investigated. The experimental results demonstrate that the sensor features a high sensitivity of 100.45 Hz/kPa and a short-term drift <14 ppm in the operating range of −10∼10 kPa at room temperature. Owing to the excellent performance in sensitivity and accuracy, the sensor can be applied for micropressure measurement.

무선 압력센서를 이용한 실시간 맥박수 측정기 개발

Department of Electronics and Control Engineering, Hanbat National University, Daejeon 34158, Korea(Received March 14, 2016; Revised April 19, 2016; Accepted April 22, 2016)Abstract: Among the various physiological information that could be obtained from human body, heartbeat rate is a commonly used vital sign in the clinical milieu. Photoplethysography (PPG) sensor is incorporated into many wearable healthcare devices because of its advantages such as simplicity of hardware structure and low-cost. However, healthcare device employing PPG sensor has been issued in susceptibility of light and motion artifact. In this paper, to develop the real-time heart rate measurement device that is less sensitive to the external noises, we have fabricated an ultra-small wireless LC resonant pressure sensor by MEMS process. After performance evaluation in linearity and repeatability of the MEMS pressure sensor, heartbeat waveform and rate on radial artery were obtained by using resonant frequency-pressure conversion method. The measured data using the proposed heartbeat rate measurement system was validated by comparing it with the data of an commercialized heart rate measurement device. Result of the proposed device was agreed well to that of the commercialized device. The obtained real time heartbeat wave and rate were displayed on personal mobile system by bluetooth communication.Keywords: Heartbeat, Wearable, Wireless power transmission, Pressure sensor, LC resonance !

CMOS-MEMS resonant pressure sensors: optimization and validation through comparative analysis

An optimized CMOS-MEMS resonant pressure sensor with enhanced sensitivity at atmospheric pressure has been reported in this paper. The presented work reports modeling and characterization of a resonant pressure sensor, based on the variation of the quality factor with pressure. The relevant regimes of air flow have been determined by the Knudsen number, which is the ratio of the mean free path of the gas molecule to the characteristic length of the device. The sensitivity has been monitored for the resonator design from low vacuum to atmospheric levels of air pressure. This has been accomplished by reducing the characteristic length and optimization of other parameters for the device. While the existing analytical model has been adapted to simulate the squeeze film damping effectively and it is validated at higher values of air pressure, it fails to compute the structural damping mechanisms dominant in the molecular flow regime, i.e. at lower levels of air pressure. This discrepancy has been solved by finite element modeling that has incorporated both structural and film damping effects. The sensor has been designed with an optimal geometry of 140 × 140 × 8 µm having 6 × 6 perforations along the row and column of the plate, respectively, for maximum Q, with an effective mass of 0.4 µg. An enhanced quality factor of 60 and reduced damping coefficient of 4.34 µNs/m have been obtained for the reported device at atmospheric pressure. The sensitivity of the manufactured device is approximately −0.09 at atmospheric pressure and increases to −0.3 at 40 kPa i.e. in the lower pressures of slip flow regime. The experimental measurements of the manufactured resonant pressure sensor have been compared with that of the analytical and finite element modeling to validate the optimization procedure. The device has been manufactured using standard 250 nm CMOS technology followed by an in-house BEOL metal-layer release through wet etching.

Design and modelling of a micro resonant pressure sensor

A silicon island compliant mechanism supported frequency tunable resonating beam based micro pressure sensor is proposed in this work. The sensor consists of a silicon frame with silicon islands supporting a silicon nitride resonating beam placed diagonally on the island in a square diaphragm made of silicon. The analytical model of the sensor is developed by modelling the diaphragm having islands as non uniform sections and the resonating beam under axial tensile load. The sensor is numerically modelled and its dimensions are optimized using MEMS CAD Tool COVENTORWARE. The performance of the proposed sensor is evaluated through simulation for a pressure range of 0---20 bar. The results demonstrate that the sensor is linear and its characteristics obtained from the analytical and numerical model are in close agreement. The sensitivity of the sensor is compared with the sensitivity of a sensor having islands and the resonating beam placed between two opposite sides of the diaphragm and found to have 5 % increment in sensitivity.

Laterally Driven Resonant Pressure Sensor with Etched Silicon Dual Diaphragms and Combined Beams.

A novel structure of the resonant pressure sensor is presented in this paper, which tactfully employs intercoupling between dual pressure-sensing diaphragms and a laterally driven resonant strain gauge. After the resonant pressure sensor principle is introduced, the coupling mechanism of the diaphragms and resonator is analyzed and the frequency equation of the resonator based on the triangle geometry theory is developed for this new coupling structure. The finite element (FE) simulation results match the theoretical analysis over the full scale of the device. This pressure sensor was first fabricated by dry/wet etching and thermal silicon bonding, followed by vacuum-packaging using anodic bonding technology. The test maximum error of the fabricated sensor is 0.0310%F.S. (full scale) in the range of 30 to 190 kPa, its pressure sensitivity is negative and exceeding 8 Hz/kPa, and its Q-factor reaches 20,000 after wafer vacuum-packaging. A novel resonant pressure sensor with high accuracy is presented in this paper.

The analysis of the self-oscillation system for resonant pressure sensor

In this paper a detailed analysis on the self-oscillation system that is used for the resonant pressure sensor is presented. The stability and transient response of the system are investigated through a theoretical model, which is verified by experiment. This analysis provides an approach to optimize the parameters of the control system to get a short settling time and a stable system in the full pressure scale. The sensor chip is fabricated using a commercially available silicon-on-insulator wafer. By optimizing the parameters, the self-oscillation system for the sensor works well and the setting time is less than 300 ms in full pressure scale. The calibration result shows that the resonant pressure sensor has a nonlinearity of 0.045 %FS, a hysteresis error of 0.14 %FS, and a repeatability error of 0.18 %FS.

Effects of the van der Waals Force on the Dynamics Performance for a Micro Resonant Pressure Sensor

The micro resonant pressure sensor outputs the frequency signals where the distortion does not take place in a long distance transmission. As the dimensions of the sensor decrease, the effects of the van der Waals forces should be considered. Here, a coupled dynamic model of the micro resonant pressure sensor is proposed and its coupled dynamic equation is given in which the van der Waals force is considered. By the equation, the effects of the van der Waals force on the natural frequencies and vibration amplitudes of the micro resonant pressure sensor are investigated. Results show that the natural frequency and the vibrating amplitudes of the micro resonant pressure sensor are affected significantly by van der Waals force for a small clearance between the film and the base plate, a small initial tension stress of the film, and some other conditions.

A Resonant Pressure Sensor Capable of Temperature Compensation with Least Squares Support Vector Machine

Resonant pressure sensors are widely used in pressure monitoring due to high accuracies, long-term stabilities and quasi-digital outputs, which, however, suffers from the key issue of temperature drifts. This paper presents a resonant pressure sensor capable of temperature compensation leveraging the compensation algorithm of least squares support vector machine. Double “H” type resonators were arranged in a differential output pressure sensor where the pressure under measurement was translated to resonant frequency output. In comparison to two conventional algorithms which are support vector machine and polynomial fitting, the new algorithm based on least squares support vector machine can effectively improve the precision of temperature compensation for the developed micro sensors. Experimental results showed that the maximum temperature drift was reduced from 8901.4 Pa (without compensation) to 2.0 Pa (with compensation) in the full temperature and pressure scale (temperature range of −40°C to 70°C and pressure range of 20 kPa to 260 kPa), validating the effectiveness of the new algorithm in temperature compensation.

Wireless Telemetry of Stainless-Steel-Based Smart Antenna Stent Using a Transient Resonance Method

A radio-frequency (RF) interrogation method for sensor-integrated “smart” stent implants is reported. This device, formed with a 2-cm-long inductive antenna stent and a micro capacitive pressure sensor, both based on a medical-grade stainless steel, serves as a wireless resonant pressure sensor designed to detect changes in local blood pressure for early detection of post-stenting complications. Wireless telemetry of the devices deployed in vascular model and graft is demonstrated by reading the decay of resonance in the stent excited with pulsed RF fields using two- and single-antenna setups at source powers of 4-20 dBm, showing the read ranges of up to 2.75 cm that are 2 ×-3 × larger than those measured with the conventional impedance-phase method. The devices immersed in saline are also successfully interrogated. The obtained results suggest that the proposed technique is a promising alternative path to wireless monitoring of the carotid artery through implanted smart stents.

Capacitive micromachined ultrasonic transducer for ultra-low pressure measurement: Theoretical study

Ultra-low pressure measurement is necessary in many areas, such as high-vacuum environment monitoring, process control and biomedical applications. This paper presents a novel approach for ultra-low pressure measurement where capacitive micromachined ultrasonic transducers (CMUTs) are used as the sensing elements. The working principle is based on the resonant frequency shift of the membrane under the applied pressure. The membranes of the biased CMUTs can produce a larger resonant frequency shift than the diaphragms with no DC bias in the state-of-the-art resonant pressure sensors, which contributes to pressure sensitivity improvement. The theoretical analysis and finite element method (FEM) simulation were employed to study the relationship between the resonant frequency and the pressure. The results demonstrated excellent capability of the CMUTs for ultra-low pressure measurement. It is shown that the resonant frequency of the CMUT varies linearly with the applied pressure. A sensitivity of more than 6.33 ppm/Pa (68 kHz/kPa) was obtained within a pressure range of 0 to 100 Pa when the CMUTs were biased at a DC voltage of 90% of the collapse voltage. It was also demonstrated that the pressure sensitivity can be adjusted by the DC bias voltage. In addition, the effects of air damping and ambient temperature on the resonant frequency were also studied. The effect of air damping is negligible for the pressures below 1000 Pa. To eliminate the temperature effect on the resonant frequency, a temperature compensating method was proposed.

Fabrication of a graphene-based pressure sensor by utilising field emission behavior of carbon nanotubes

In this paper, a miniature graphene-based field emission pressure sensor is reported for the first time, in which carbon nanotubes have been applied as the cathode and reduced graphene oxide sheets have been utilized as the flexible anode. Since, graphene is the thinnest (one atom thick) known structure with a uniform thickness, graphene based membrane is a promising candidate for pressure sensors which allows both high sensitivity and very small chip area, simultaneously. A maximum sensitivity of about 21.9μA/A/Pa is achieved at the bias voltage of 5 V for the fabricated field emission pressure sensor, which is about two orders of magnitude higher than the previously reported graphene based pressure sensors. The fabricated structure has been also used as a graphene based resonator in which emission current is applied to detect the mechanical resonance, efficiently. The measured resonance frequency is measured about 10 MHz, which is shifted to lower frequencies by increasing the ambient pressure. The measured sensitivity, for the resonant operation mode, is about 11.32 Hz/Pa (S = Δf/ΔP). Owing to the inherent low mass of graphene, the presented structure can be entitled as a promising resonant pressure sensor.

A Wireless Passive LC Resonant Sensor Based on LTCC under High-Temperature/Pressure Environments.

In this work, a wireless passive LC resonant sensor based on DuPont 951 ceramic is proposed and tested in a developed high-temperature/pressure complex environment. The test results show that the measured resonant frequency varies approximately linearly with the applied pressure; simultaneously, high temperature causes pressure signal drift and changes the response sensitivity. Through the theoretical analysis of the sensor structure model, it is found that the increase in the dielectric constant and the decrease in the Young’s modulus of DuPont 951 ceramic are the main causes that affect the pressure signal in high-temperature measurement. Through calculations, the Young’s modulus of DuPont 951 ceramic is found to decrease rapidly from 120 GPa to 65 GPa within 400 °C. Therefore, the LC resonant pressure sensor needs a temperature compensation structure to eliminate the impact of temperature on pressure measurement. Finally, a temperature compensation structure is proposed and fabricated, and the pressure response after temperature compensation illustrates that temperature drift is significantly reduced compared with that without the temperature compensation structure, which verifies the feasibility the proposed temperature compensation structure.

Multi-field coupled free vibration for a micro resonant pressure sensor

The micro resonant pressure sensor is attractive because it outputs the frequency signals. As the dimensions of the sensor decreases, the effects of the van der Waals forces and the electrostatic forces on its dynamics performance should be considered. It is a multi-field coupled dynamics problem. Here, the dynamic equations of the micro resonant beam for the micro resonant pressure sensor under multi-field coupled situations are deduced. Using these equations, the effects of the van der Waals force or the electrostatic force on the natural frequencies of the micro resonant beam are investigated. The effects become more obvious under certain conditions. The results are useful to improve design about dynamics performance for the micro resonant pressure sensor.

Lower Bounds on the Frequency Estimation Error in Magnetically Coupled MEMS Resonant Sensors.

MEMS inductor-capacitor (LC) resonant pressure sensors have revolutionized the treatment of abdominal aortic aneurysms. In contrast to electrostatically driven MEMS resonators, these magnetically coupled devices are wireless so that they can be permanently implanted in the body and can communicate to an external coil via pressure-induced frequency modulation. Motivated by the importance of these sensors in this and other applications, this paper develops relationships among sensor design variables, system noise levels, and overall system performance. Specifically, new models are developed that express the Cramér-Rao lower bound for the variance of resonator frequency estimates in terms of system variables through a system of coupled algebraic equations, which can be used in design and optimization. Further, models are developed for a novel mechanical resonator in addition to the LC-type resonators.

Vacuum-packaged Resonant Pressure Sensor with Dual Resonators for High Sensitivity and Linearity

This paper presents a vacuum-packaged resonant pressure sensor featured with high sensitivity and linearity using dual resonators. The frequency difference between these two resonators is used to represent pressure, so that the nonlinear responds of each resonator is eliminated and the sensitivity of the sensor is also improved at the same time. The sensitivities of the dual resonators were matched through improved diaphragm uniformity and controlled anchor undercut, which leaded to a better linear performance. With this method, the nonlinear error is reduced from 1.3% F.S. to 0.2% F.S. over the pressure range from 0 to 250kPa. Furthermore, to achieve better performance, the micro resonators were wafer-level vacuum packaged with anodic bonding to provide the needed low damping environment and the stable reference pressure. Test results show that the pressure in the vacuum-packaged micro chamber of the sensor is less than 5Pa and kept for 6 months with no drop.

A high-performance LC wireless passive pressure sensor fabricated using low-temperature co-fired ceramic (LTCC) technology.

An LC resonant pressure sensor with improved performance is presented in this paper. The sensor is designed with a buried structure, which protects the electrical components from contact with harsh environments and reduces the resonant-frequency drift of the sensor in high-temperature environments. The pressure-sensitive membrane of the sensor is optimized according to small-deflection-plate theory, which allows the sensor to operate in high-pressure environments. The sensor is fabricated using low-temperature co-fired ceramic (LTCC) technology, and a fugitive film is used to create a completed sealed embedded cavity without an evacuation channel. The experimental results show that the frequency drift of the sensor versus the temperature is approximately 0.75 kHz/°C, and the responsivity of the sensor can be up to 31 kHz/bar within the pressure range from atmospheric pressure to 60 bar.

Potential application of graphene nanomechanical resonator as pressure sensor

A pressure sensor consisting of graphene nanomechanical resonator and Si/SiO2 pressure membrane is demonstrated. The resonance frequency of resonator as function of external pressure smaller than 100kPa is simulated to be linear using the finite element method. The obtained pressure sensitivity for a single-layer graphene resonator can reach 26,838Hz/kPa, bigger than that of conventional resonant pressure sensors by two orders of magnitude. Moreover, the sensitivity can be further improved by increasing the ratio of side length to thickness of pressure membrane, and reducing the thickness and built-in strain of the resonator.

A noncontact wireless passive radio frequency (RF) resonant pressure sensor with optimized design for applications in high-temperature environments

A noncontact wireless passive pressure sensor based on alumina ceramic for pressure measurement is presented in this paper. A faithful pressure signal in harsh environment is captured through wireless sensing, and a novel antenna design method is developed to increase the measurement distance between the antenna and the sensor. The sensor is fabricated using a novel no-co-fired technology, and the properties of the alumina ceramic and platinum ensure the feasibility of the sensor in high-temperature environments. The experimental results show that the coupled distance between the antenna and the sensor can be up to 5.5 cm, and the designed sensor, featuring improved structural parameters, has a high responsivity (15.5 kHz kPa−1) in a pressure environment at room temperature. The sensor can be coupled with the antenna at 850 °C, which verifies the feasibility in high-temperature environments.

A SOI-MEMS Based Resonant Barometric Pressure Sensor with Differential Output

This paper presents a laterally driven resonant barometric pressure sensor fabricated using SOI-MEMS technology. In this device, pressure under measurement causes a deflection of a pressure-sensitive silicon square diaphragm, which is further translated to stress build up in H style doubly-clamped micro beams, leading to resonant frequency shift. In device fabrication, SOI-MEMS fabrication processes were utilized, where a new modified buffered hydrofluoric acid (BHF) solution was used to remove the buried oxide layer and release the suspended resonant beams. Experimental results recorded a device resolution of 10Pa, a nonlinearity of 0.04% and a temperature coefficient of-0.04% F.S/°C in the range of-40°C to 30°C. The long-term stability error of the proposed device was quantified as 0.05% F.S over the past 3 months.

Research for the Resonator Structure Process Technology of Resonant Pressure Sensors

The fabrication of butterfly-shape resonator is key for high precision resonator, for requiring suspend on the silicon substrate. This paper is focused on the technology of making butterfly-shape resonator. the variety of structure design can be used to make butterfly-shape resonator have been analyzed, the structure of butterfly-shape resonator is obtained, and for reducing the etch surface roughness, KOH etching conditions, such as composition, concentration, and temperature of etch solution, have been done. Combining with above testing results, the structure design and optimization KOH etching technology are obtained ,based on the technology, using the boron etch stop technique , the silicon butterfly-shape resonator has been done, it can be used effectively in the fabrication of the silicon resonant sensor.