Micromachined sensor structures with linear capacitive response

High-performance flexible pressure sensors are highly desirable in health monitoring, robotic tactile, and artificial intelligence. Construction of microstructures in dielectrics and electrodes is the dominating approach to improving the performance of capacitive pressure sensors. Herein, we have demonstrated a novel three-dimensional microconformal graphene electrode for ultrasensitive and tunable flexible capacitive pressure sensors. Because the fabrication process is controllable, the morphologies of the graphene that is perfectly conformal with the electrode are controllable consequently. Multiscale morphologies ranging from a few nanometers to hundreds of nanometers, even to tens of micrometers, have been systematically investigated, and the high-performance capacitive pressure sensor with high sensitivity (3.19 kPa-1), fast response (30 ms), ultralow detection limit (1 mg), tunable-sensitivity, high flexibility, and high stability was obtained. Furthermore, an ultrasensitivity of 7.68 kPa-1 was successfully achieved via symmetric double microconformal graphene electrodes. The finite element analysis indicates that the microstructured graphene electrode can enhance large deformation and thus effectively improve the sensitivity. Additionally, the proposed pressure sensors are demonstrated with practical applications including insect crawling detection, wearable health monitoring, and force feedback of robot tactile sensing with a sensor array. The microconformal graphene may provide a new approach to fabricating controllable microstructured electrodes to enhance the performance of capacitive pressure sensors and has great potential for innovative applications in wearable health-monitoring devices, robot tactile systems, and human-machine interface systems.

Skin mountable flexible pressure sensors are crucial for the next generation technologies. Capacitive pressure sensors bring several advantages and hence, they have been preferred over piezoelectric and piezoresistive alternatives. Flexible electrode and dielectric material are the fundamental components of the capacitive flexible pressure sensors. Hence, solution processable silver nanowires (AgNWs) are highly promising as electrode material or filler material in dielectric layer, owing to their superior inherent conductivity and flexibility. This article reviews recent progress on fabrication of flexible capacitive pressure sensors with AgNWs used as electrode and dielectric material. Performance of the sensors based on AgNWs is comprehensively discussed and emerging applications, as well as future outlook, are presented in detail.

Capacitive pressure sensors measure changes in pressure typically by the deflection of a flexible conducting membrane towards a fixed electrode. The deflection in the membrane produces a quadratic change in capacitance, which often yields higher sensitivity to changes in pressure compared to piezo-resistive pressure sensors, which measures the resistance changes proportional to the applied pressure. However, residual stresses in the membrane can provide a substantial resistance to deformation compared to the driving force created by the applied pressure, which decreases the sensitivity at low pressures and produces a nonlinear signal. If the membrane is made compliant enough to increase sensivitiy, pull-in of the membrane can occur, reducing the effective pressure range of the capacitive manometer type pressure sensor. Hence, these type of sensors are typically not used to measure very low pressure differences over several hundred Pascals. To overcome this limitation, a capacitive pressure sensor was developed that operates in a peeling mode while under applied electrostatic actuation, which counters the residual stresses. The changes in capacitance can be detected if the pressure is just enough to overcome the interfacial electrostatic pressure. This type of pressure sensor can potentially be used for very low differential pressure differences, well below 100 Pa, over ~ 1 kPa range.

"This thesis is concerned with the electro-mechanical characterisation of MEMS capacitive pressure sensors and the determination of models to describe their behaviour. The finite element software package ANSYS is validated as a tool for the examination of the electro-mechanical behaviour of capacitive pressure sensors. It is then used to incorporate the effects of applied load, voltage and residual stress into a model of the sensor structure. The capacitive pressure sensor is modelled both in 3D and also in 2D axis-symmetric modes. The error between the 2D and 3D models is quantified. The behaviour of structures under electrostatic actuation is investigated. A means of extracting the capacitance from the sensor structure is developed and also a m.eans of modelling contact between structural entities in the system is proposed. The optical profiling method. Scanning White Interferometry is introduced and utilised to measure the behaviour of sample capacitive pressure sensors under electrostatic actuation. The behaviour of classical structures under electrostatic actuation is investigated and the electrostatic pull-in behaviour of fixed-fixed beams is investigated using both analytical and TEA methods. The output from the ANSYS models developed is also compared to the commercially available numerical analysis software packages SUGAR and Coventorware. Capacitance and the effect of fringing fields are also addressed to a lesser extent. A sensitivity analysis was also conducted on the geometric structure of the sensor and the critical dimensions such as diaphragm diameter and thickness were investigated. The capacitance output range for the sensor was found to be non-linear and in the region of 390 to 415 femto Farads in a pressure range from 0-0.14 MPa."

In this paper, Poly-crystalline silicon carbide (poly-sic) Micro-electromechanical systems (MEMS) capacitive pressure sensor operating in harsh environment in touch mode is proposed, The principle of the paper is to design, obtain analytical solution and compare the results with the simulation for a circular diaphragm deflection before and after touch point. The sensor demonstrated a high temperature sensing capability up to 400°C, the device achieves a linear characteristic response and consists of a circular clamped-edges poly-sic diaphragm suspended over sealed cavity on a silicon carbide substrate. The sensor is operating in touch mode capacitive pressure sensor, The advantages of a touch mode are the robust structure that make the sensor to withstand harsh environment, near linear output, and large over-range protection, operating in wide range of pressure, higher sensitivity than the near linear operation in normal mode, The material is considered to be used for harsh environment is SiC (Silicon Carbide), Because of SiC owing excellent electrical stability, mechanical robustness, and chemical inertness properties and the application of pressure sensors in harsh environments are, such as automotive industries, aerospace, oil/logging equipments, nuclear station, and power station. We are simulating MEMS capacitive pressure sensor to optimize the design, improve the performance and reduce the time of fabricating process of the device. The proposed touch mode MEMS capacitive pressure sensor demonstrated diaphragm ranging from 150 μm to 360 μm in diameter, with the gap depth from 0.5 μm to 7.5 μm and the sensor exhibit a linear response with pressure from 0.05 Mpa to 10 Mpa.

F1exible pressure sensors are important in applications of flexible electronics. Although extensive efforts have been made to improve the performance of pressure sensors through materials fabrication or circuitry design, the effect of loading mode on the sensor response has rarely been discussed before. In this study, an appropriate constitutive relationship of the composite material that is used as the dielectric layer in capacitive pressure sensors was characterized via tensile tests, thereby ensuring the stresses in the film sensor under a pressure can be simulated accurately by the finite element analysis. The stresses under the pressure head, as a major factor to affect the capacitance variation, were further fitted by a polynomial equation in terms of the variation of the capacitance of the film sensor with a designed experiment for both measuring the capacitance and force. The equation supports an empirical model that reveals the capacitance of the film sensor can be estimated directly when the stress in the sensor is obtained. The model helps to develop a new pressure sensor applied in flexible devices.

A highly sensitive and wide-range pressure sensor based on orientated and strengthened TPU nanofiber membranes fabricated by a conjugated electrospinning technology

Porous dielectric materials based wearable capacitance pressure sensors for vital signs monitoring: A review

Flexible pressure sensors have drawn considerable attention for their potential applications as electronic skins with both sensitivity and pressure response range. Although the introduction of surface microstructures effectively enhances sensitivity, the confined volume of their compressible structures results in a limited pressure response range. To address this issue, a biomimetic kapok structure is proposed and implemented for constructing the dielectric layer of flexible capacitive pressure sensors employing 3D printing technology. The structure is designed with easily deformable concave and rotational structures, enabling continuous deformation under pressure. This design results in a significant expansion of the pressure response range and improvement in sensitivity. Further, the study purposively analyses crucial parameters of the devised structure that affect its compressibility and stability. These include the concave angle θ, height ratio d1/d2, rotation angle α, and width k. As a result, the ultimate pressure sensors demonstrate remarkable features such as high sensitivity (≈2.38kPa-1 in the range of 0-10kPa), broad detection range (734kPa), fast response time (23ms), and outstanding pressure resolution (0.4% at 500kPa). This study confirms the viability of bionic structures for flexible sensors, and their potential to expand the scope of wearable electronic devices.

Capacitive pressure sensors received significant attention in line with advancements in wearable electronics. However, in the era of the wearable electronics, fabricated sensors fail to fulfill the absolute requirements. Significant portion of the previously reported capacitive pressure sensors suffer from excessive weight, lack of air permeability, and washing stability due to the use of separate electrode layers. A low‐cost, lightweight, parallel plate capacitive sensor with a unique seamless monolithic design that allows sensors to circumvent aforementioned problems is hereby demonstrated. The seamless monolithic capacitive sensor (MCS) with a density of as low as 10.78 mg cm−3 is fabricated using 3D masking technique on a commercially available melamine foam with silver nanowire electrodes and a protective thin layer of polydimethylsiloxane coating. In addition to its unique design, the MCSs show high sensitivity (up to 1.285 kPa–1), fast response/recovery time (18 ms/53 ms), very low‐pressure sensing ability (0.5 Pa) with ultra‐high mechanical (42 000 cycles), and washing stabilities (10 cycles) along with high air permeability. Moreover, finite element analysis simulations are conducted to reveal the deformation mechanism differences between the monolithic and classical design. Finally, in order to demonstrate their true potential, fabricated MCSs are utilized in real‐time body motion monitoring, word and proximity detection.

Flexible capacitive pressure sensors are becoming increasingly significant in wearable electronic devices. The conventional form of pressure sensors, however, focuses primarily on microstructure design. While this approach can enhance sensitivity, it can also compromise the reliability of the sensor device. Here, we present a flexible pressure sensor according to the capacitance effect using the high dielectric P(VDF-TrFE)/MXene polymer as the capacitive pressure sensor dielectric layer. The P(VDF-TrFE)/MXene film has a high dielectric constant, making it an ideal choice for the dielectric layer of the flexible capacitive pressure sensor, resulting in rapid response and high sensing sensitivity. We also examine the impact of dielectric layer thickness on the sensor’s performance and find that increasing the thickness from 0.5 to 1.5 mm leads to a continuous improvement in sensitivity, reaching a maximum sensitivity of 0.42 kPa−1 and a wide sensing amplitude (3 Pa–25 kPa). Moreover, the response time of the capacitive pressure sensor can reach 47 ms. In addition, the sensor can be utilized to monitor human physiological information such as respiration and heartbeat. Its flexibility makes it ideal for application in sports, particularly for monitoring the movement posture of football players. Our findings can drive the wider adoption of pressure sensors in football motion monitoring.

This paper put forward the design and modeling pertaining to a wireless pressure sensor that can monitor blood pressure within the coronary artery with the help of a capacitive pressure sensor. To adjust the resonance frequency (26.78–27.09 MHz) with regards to this sensor’s applied pressure (0–30 KPa), sensor’s dimension as well as human blood pressure (0–220 mmHg), a variable capacitor in the pressure sensor has been put forward. The shape of the capacitive pressure sensor was designed to look like an Ellipse and a circle, which were assessed for their performance with regards to capacitive sensitivity and diaphragm deflection. Also, the diaphragm thickness pertaining to the capacitive pressure sensor with Ellipse and circular shapes was altered (0.1μm to 0.5μm) while the cross-section area was kept constant at 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . As per the results, an invasive sensor with circular and Ellipse capacitive pressure shape that had smaller size (1) mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> displayed high sensitivity. The sensitivity readings for Ellipse and circular shape capacitive pressure sensor were 7.73 ff mmHg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 9.94 ff mmHg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively. Diaphragm deflection was simulated based on the COMSOL Multiphysics software, while MATLAB was employed for simulation with regards to changes in resonance frequency, capacitance, and capacitive sensitivity.

This paper describes initial development and testing of a novel high temperature capacitive pressure sensor system. The pressure sensor system consists of two 4H-SiC 11-stage ring oscillators and a SiCN capacitive pressure sensor. One oscillator has the capacitive pressure sensor fixed at one node in its feedback loop and varies as a function of pressure and temperature while the other provides a pressure-independent reference frequency which can be used to temperature compensate the output of the first oscillator. A two-day repeatability test was performed up to 500C on the oscillators and the oscillator fundamental frequency changed by only 1. The SiCN capacitive pressure sensor was characterized at room temperature from 0 to 300 psi. The sensor had an initial capacitance of 3.76 pF at 0 psi and 1.75 pF at 300 psi corresponding to a 54 change in capacitance. The integrated pressure sensor system was characterized from 0 to 300 psi in steps of 50 psi over a temperature range of 25 to 500C. The pressure sensor system sensitivity was 0.113 kHzpsi at 25C and 0.026 kHzpsi at 500C.

Capacitive pressure sensors have several advantages in areas such as robotics, automation, aerospace, biomedical and consumer electronics. We present mathematical modelling, finite element analysis (FEA), fabrication and experimental characterization of ultra-low cost and paper-based, touch-mode, flexible capacitive pressure sensor element using Do-It-Yourself (DIY) technology. The pressure sensing element is utilized to design large-area electronics skin for low-cost prosthetic hands. The presented sensor is characterized in normal, transition, touch and saturation modes. The sensor has higher sensitivity and linearity in touch mode operation from 10 to 40 kPa of applied pressure compared to the normal (0 to 8 kPa), transition (8 to 10 kPa) and saturation mode (after 40 kPa) with response time of 15.85 ms. Advantages of the presented sensor are higher sensitivity, linear response, less diaphragm area, less von Mises stress at the clamped edges region, low temperature drift, robust structure and less separation gap for large pressure measurement compared to normal mode capacitive pressure sensors. The linear range of pressure change is utilized for controlling the position of a servo motor for precise movement in robotic arm using wireless communication, which can be utilized for designing skin-like structure for low-cost prosthetic hands.

Attempts have been made to introduce microstructures or wrinkles into the elastomer surface to increase the sensitivity of the elastomer. However, the disadvantage of this method is that when a force is applied to the pressure sensor, the contact area with the electrode is changed and the linear response characteristic of the pressure sensor is reduced. The biggest advantage of the capacitive pressure sensor using an elastomer is that it is a characteristic that changes linearly according to the change in pressure, so it is not suitable to introduce microstructures or wrinkles into the elastomer surface. A method of increasing the sensitivity of the capacitive pressure sensor while maintaining the linearity according to the pressure change is proposed. We proposed a bubble-popping PDMS by creating pores inside the elastomer. The sensitivity of the pressure sensor made of the bubble-popping PDMS was approximately 4.6 times better than that of the pressure sensor without pores, and the pressure sensor made of the bubble-popping PDMS showed a high linear response characteristic to the external pressure change. These results show that our pressure sensor can be used to detect applied pressures or contact forces of e-skins.