Design and simulation of a multi-function MEMS sensor for health and usage monitoring.

Health and usage monitoring as a technique for online test, diagnosis or prognosis of structures and systems has evolved as a key technology for future critical systems. The technology, often referred to as HUMS is usually based around sensors that must be more reliable than the system or structure they are monitoring. This paper proposes a fault tolerant sensor architecture and demonstrates the feasibility of realising this architecture through the design of a dual mode humidity/pressure MEMS sensor with an integrated temperature function. The sensor has a simple structure, good linearity and sensitivity, and the potential for implementation of built-in-self-test features. We also propose a re-configurable sensor network based on the multi-functional sensor concept that supports both normal operational and fail safe modes. The architecture has the potential to significantly increase system reliability and supports a reduction in the number of sensors required in future HUMS devices. The technique has potential in a wide range of applications, especially within wireless sensor networks

Use of self-calibration data for multifunctional MEMS sensor prognostics

This paper proposes a solution to monitor the degradation of a multifunctional microelectromechanical systems (MEMS) sensor (MFS) and to recalibrate the sensor output accordingly. The solution is able to predict the remaining useful life based on the recalibration history. The MFS used is a dual pressure-humidity hybrid sensor where model data have been used to demonstrate the applicability and performance of the proposed method for diagnosis, self-correction, and prognosis

DESIGN AND MICROFABRICATION OF A CMOS-MEMS PIEZORESISTIVE ACCELEROMETER AND A NANO-NEWTON FORCE SENSOR

DESIGN AND MICROFABRICATION OF A CMOS-MEMS PIEZORESISTIVE ACCELEROMETER AND A NANO-NEWTON FORCE SENSOR by Mohd Haris Md Khir Adviser: Hongwei Qu, Ph.D. This thesis work consists of three aspects of research efforts: I. Design, fabrication, and characterization of a CMOS-MEMS piezoresistive accelerometer 2. Design, fabrication, and characterization of a CMOS-MEMS nano-Newton force sensor 3. Observer-based controller design of a nano-Newton force sensor actuator system A low-cost, high-sensitivity CMOS-MEMS piezoresistive accelerometer with large proof mass has been fabricated. Inherent CMOS polysilicon thin film was utilized as piezoresistive material and full Wheatstone bridge was constructed through easy wiring allowed by three metal layers in CMOS thin films. The device fabrication process consists of a standard CMOS process for sensor configuration and a deep reactive ion etching (DRIE) based post-CMOS microfabrication for MEMS structure release. Bulk single-crystal silicon (SCS) substrate was included in the proof mass to increase sensor sensitivity. Using a low operating power of 1.67 m W, the sensitivity was measured as 30.7 mV/g after amplification and 0.077 mV/g prior to amplification. With a total noise floor of 1.03 mg!-!Hz, the minimum detectable acceleration is found to be 32.0 mg for a bandwidth of I kHz which is sufficient for many applications. The second device investigated in this thesis work is a CMOS-MEMS capacitive force sensor capable ofnano-Newton out-of-plane force measurement. Sidewall and fringe capacitance formed by the multiple CMOS metal layers were utilized and fully differential sensing was enabled by common-centroid wiring of the sensing capacitors. Single-crystal silicon (SCS) is incorporated in the entire sensing element for robust structures and reliable sensor deployment in force measurement. A sensitivity of 8 m V /g prior to amplification was observed. With a total noise floor of 0.63 mg!-IHz, the minimum detection acceleration is found to be 19.8 mg, which is equivalent to a sensing force of 449 nN. This work also addresses the design and simulation of an observer-based nonlinear controller employed in a CMOS-MEMS nano-Newton force sensor actuator system. Measurement errors occur when there are in-plane movements of the probe tip; these errors can be controlled by the actuators incorporated within the sensor. Observerbased controller is necessitated in real-world control applications where not all the state variables are accessible for on-line measurements. V

MEMS Accelerometers

Micro-electro-mechanical system (MEMS) devices are widely used for inertia, pressure, and ultrasound sensing applications. Research on integrated MEMS technology has undergone extensive development driven by the requirements of a compact footprint, low cost, and increased functionality. Accelerometers are among the most widely used sensors implemented in MEMS technology. MEMS accelerometers are showing a growing presence in almost all industries ranging from automotive to medical. A traditional MEMS accelerometer employs a proof mass suspended to springs, which displaces in response to an external acceleration. A single proof mass can be used for one- or multi-axis sensing. A variety of transduction mechanisms have been used to detect the displacement. They include capacitive, piezoelectric, thermal, tunneling, and optical mechanisms. Capacitive accelerometers are widely used due to their DC measurement interface, thermal stability, reliability, and low cost. However, they are sensitive to electromagnetic field interferences and have poor performance for high-end applications (e.g., precise attitude control for the satellite). Over the past three decades, steady progress has been made in the area of optical accelerometers for high-performance and high-sensitivity applications but several challenges are still to be tackled by researchers and engineers to fully realize opto-mechanical accelerometers, such as chip-scale integration, scaling, low bandwidth, etc

Piezoresistive Carbon Nanofiber-Based Cilia-Inspired Flow Sensor

Evolving over millions of years, hair-like natural flow sensors called cilia, which are found in fish, crickets, spiders, and inner ear cochlea, have achieved high resolution and sensitivity in flow sensing. In the pursuit of achieving such exceptional flow sensing performance in artificial sensors, researchers in the past have attempted to mimic the material, morphological, and functional properties of biological cilia sensors, to develop MEMS-based artificial cilia flow sensors. However, the fabrication of bio-inspired artificial cilia sensors involves complex and cumbersome micromachining techniques that lay constraints on the choice of materials, and prolongs the time taken to research, design, and fabricate new and novel designs, subsequently increasing the time-to-market. In this work, we establish a novel process flow for fabricating inexpensive, yet highly sensitive, cilia-inspired flow sensors. The artificial cilia flow sensor presented here, features a cilia-inspired high-aspect-ratio titanium pillar on an electrospun carbon nanofiber (CNF) sensing membrane. Tip displacement response calibration experiments conducted on the artificial cilia flow sensor demonstrated a lower detection threshold of 50 µm. Furthermore, flow calibration experiments conducted on the sensor revealed a steady-state airflow sensitivity of 6.16 mV/(m s−1) and an oscillatory flow sensitivity of 26 mV/(m s−1), with a lower detection threshold limit of 12.1 mm/s in the case of oscillatory flows. The flow sensing calibration experiments establish the feasibility of the proposed method for developing inexpensive, yet sensitive, flow sensors; which will be useful for applications involving precise flow monitoring in microfluidic devices, precise air/oxygen intake monitoring for hypoxic patients, and other biomedical devices tailored for intravenous drip/urine flow monitoring. In addition, this work also establishes the applicability of CNFs as novel sensing elements in MEMS devices and flexible sensors

PDMS Flow Sensors With Graphene Piezoresistors Using 3D Printing and Soft Lithography

This paper reports the fabrication and characterization of a flexible piezoresistive flow sensor comprising a polydimethylsiloxane (PDMS) cantilever with a serpentine graphene nanoplatelets (GNP) strain gauge embedded at the cantilever base. A facile and cleanroom-free processing work flow involving a combination of high-resolution powder bed fusion and soft lithography was used to fabricate PDMS cantilevers (aspect ratio 20) with 150 µm × 150 µm microchannels on its surface. A high gauge factor of 55 (up to 5 times higher than reported in comparable piezoresistive flow sensors) was achieved using drop-casted GNP ink as the piezoresistive sensing element in the aforementioned microchannels. Finally, the use of the PDMS-graphene cantilever as an airflow sensor with enhanced sensitivity (20 times more than comparable piezoresistive cantilever sensors), low hysteresis, good repeatability, and bidirectional sensing capability was demonstrated

Low power strain sensor based on MOS tunneling current.

Sensors, such as pressure sensors, accelerometers and gyroscopes, are very important components in modern portable electronics. A limited source of power in portable electronics is motivating research on new low power sensors. Piezoresistive and capacitive sensing technologies are the most commonly utilized technologies, which typically consume power in the µW to mW range. Tunneling current sensing is attractive for low power applications because the typical tunneling current is in the nA range. This dissertation demonstrates a low power strain sensor based on the tunneling current in a metal-oxide-semiconductor (MOS) structure with a power consumption of a couple of nano-Watts (nW) with a minimum detectable strain of 0.00036%. Both DC and AC measurements were used to characterize the MOS tunneling current strain sensor. The noise level is found to be smallest in the inversion region, and therefore it is best to bias the device in the inversion region. To study the sensitivity in the inversion region, a model is developed to compute the tunneling current as a function of strain in the semiconductor. The model calculates the tunneling current due to electrons tunneling from the conduction band of the semiconductor to the gate (ECB tunneling current) and the tunneling current due to electrons tunneling from the valence band of the semiconductor to the gate (EVB tunneling current). It is found that the ECB tunneling current is sufficient to explain experimental gate leakage current results reported in the literature for MOSFETs with low substrate doping concentration. However, for the tunneling current strain sensor with a higher substrate doping concentration reported here, a model using both ECB and EVB tunneling current is required. The model fits our experiments. During both DC and AC measurements, the MOS tunneling current is found to drift with time. The drift could arise from the trap states within the oxide. The current drift makes it difficult to obtain an absolute measurement of the strain. Combining the tunneling current strain sensor with a resonant sensor may be a good choice because it measures changes in the mechanical resonant frequency, independent of a drift of the tunneling current amplitude