Edmond Cretu
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- Polymer-based transducer arrays for ultrasound imaging - applications in biomedical imaging, or for non-destructive testing of materials and structures
- Advanced microsystems for inertial navigation
- Sensor microinstrumentation systems
- Signal processing and modern control applied to MEMS transducers
- Wearable body sensor systems
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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
This Ph.D. thesis introduces a technology framework to fabricate MEMS devices using SU-8 photoresist. The lithography process of SU-8 photoresist can directly create polymeric microstructures with good chemical and mechanical stability. This characteristic makes the SU-8 photoresist suitable as the structural material for polymeric MEMS devices. However, there are several issues in the current primary fabrication methods for SU-8 MEMS devices. Existing microfabrication methods suffer from processing complexity, uncertainties, and relatively high cost. This thesis focuses on addressing these issues. Novel methodologies are used to improve the fabrication process of SU-8 out-of-plane and in-plane MEMS devices. Sacrificial-layer-free fabrication of SU-8 out-of-plane MEMS devices has been implemented using an advanced lithography principle, pixel-level dose modulation, resulting in a three-step fabrication method. A novel processing strategy is proposed to implement optimum processing for SU-8 in-plane MEMS fabrication, resulting in a four-step fabrication method. Experimental case studies with test structures have been carried out to evaluate the newly-developed methods. The main features (process predictability, reproducibility, robustness, etc.) of the newly-developed microfabrication processes have been validated after calibration. The research work of this thesis presents two new technological paths to fabricate SU-8 MEMS devices. The technology framework proposed in this thesis validates the feasibility of the two new paths’ methodologies, building up the basis towards a customizable, low-cost, and rapid platform to fabricate polymeric MEMS devices. Meanwhile, the technology framework itself can be used for the rapid fabrication of SU-8 MEMS prototypes, accelerating the application and validation of novel principles and designs for polymeric MEMS devices.
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Conducting polymer actuators offer large strain (> 1%) and high work density, operate at low voltages and can resonate at tens to hundreds of Hertz. Unfortunately, they dry out in air if a solvent-based electrolyte is used, and exchange ions in wet environments, both of which cause their performance to change over time. They also lack a scalable fabrication process through which devices with reproducible performance (especially with fast actuation) are achieved. In this work, we show that a 100 µm poly(styrene-b-isobutylene-b-styrene) encapsulation helps these devices to retain 80% of their stored solvent more than 1000 times longer compared to when there is no encapsulation. The shelf life of the encapsulated device, which is around 4 days when there is no encapsulation, is expected to improve by 600 times with encapsulation. We also developed a new, easily reproducible, and scalable fabrication process through which conducting polymer films as thin as 400 nm can be obtained. High electronic and ionic conductivities of 4 × 10^4 S/m and 4 × 10^-3 S/m, volumetric capacitance of 2.4 × 10^7 F/m3, and strain difference of ~0.65 %, were obtained from thin sprayed films of poly(2,3-dihydro-thieno-1,4-dioxin)-poly(styrene-sulfonate) on porous polyvinylidene fluoride membranes with thicknesses of ~3.5 µm. Using this technique, we showed that 10 mm long, 2 mm wide and 0.125 mm thick trilayers with a steady state peak to peak displacement of ~4.5 mm, and cut off frequency of ~2 Hz, produce a ~0.5 mm displacement up to 50 Hz.In this work, we also modified the already developed 2D transmission line model of trilayer conducting polymer actuators to take into account the effect of contact electrodes and the non-uniform charge-induced strain throughout the volume of the conducting polymer layers. Based on this model, we created a web-based graphical user-interface tool, named ActuaTool, to facilitate the design and modeling of trilayer conducting polymer actuators.This work is paving the way to employ fast conducting polymer actuators in real applications through developing a new fabrication process, their encapsulation and creating a design optimization tool.
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This thesis presents a novel concept, lab-on-a-printer, whereby microfluidic modules are directly integrated into an inkjet dispenser. To enable this concept, a novel inkjet dispenser that can be integrated with microfluidic modules is designed, fabricated, and characterized.To limit the risk of cross contamination which is critical to many targeted applications, the inkjet dispenser is designed to have a modular structure that enables reusing its actuation unit and the disposal of its microfluidic chip. Furthermore, a low-cost fabrication process for the disposable microfluidic chip, mainly based on simple Polydimethylsiloxane (PDMS) moulding and SU-8 epoxy-based negative photoresist casting processes, is developed to reduce its cost. The use of PDMS in the fabrication of the microfluidic chip creates a path for its integration with pre-existing PDMS-based microfluidic modules.The fabricated inkjet dispensers are characterized to understand their limitations and identify their potential applications. For instance, droplet-to-droplet variations and maximum printable ink viscosity are used as characterization metrics. Diameters of more than 50,000 droplets per tested device are found to have a coefficient of variation (CV) in a range of 0.8% - 2.5% for each of 10 tested devices. A water and glycerol mixture with a viscosity of ~19 mP·s is identified as the mixture with maximum printable viscosity.Numerical simulations are employed to identify the key design parameters for the inkjet dispenser. These simulation results, combined with manufacturability constraints, are used to derive techniques for improving performance, hence broadening the potential applications of the technology. This resulted in inkjet dispensers with improved performance, where the maximum printable viscosity is doubled, meeting and surpassing commercial devices.Finally, a novel approach to integrate microfluidics with the inkjet dispenser is presented. This approach is successfully implemented to integrate the inkjet dispenser with a microfluidic mixer to demonstrate the capability of the integrated lab-on-a-printer platform concept, specifically the capability of printing patterns with a configurable ink composition. The presented lab-on-a-printer concept has potential applications in multiple scientific fields including biology, chemistry, and printable electronics.
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Ultrasound imaging is the most widely used medical imaging modality in the world. Modern ultrasound systems still rely on the same piezoelectric-based technology since their creation in the 1930s. Despite their mature technology, they are expensive to fabricate, difficult to create 2D arrays and cannot be miniaturized. Capacitive Micromachined Ultrasonic Transducers (CMUTs) are considered the replacement of piezoelectric transducers given their high bandwidth, ease of integration with electronics and miniaturization. The main focus of this dissertation involves the simulation, fabrication and characterization of polymer-based CMUTs (polyCMUTs). A new fabrication process involving inexpensive polymer materials and minimum fabrication steps was developed. The fabrication procedure allows the creation of biocompatible ultrasound chips in a few hours and with costs well below $100 USD, having a performance comparable to current commercial devices.The fabricated polyCMUTs exhibit a phenomenon termed “pre-biasing”, which allowed the operation of polyCMUTs as passive devices (no external power needed). The first B-mode ultrasound image in the world created using polyCMUTs is also presented. As a future plan, the development of a low-cost wearable ultrasound health monitoring system is conceived.
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Capacitive sensors and their associated readout circuits are well known and have been used in many measurement applications in different industries. Improving the sensitivity, resolution and accuracy of measuring small capacitance changes has always been one of the important research topics, especially in recent years that sensors are becoming smaller in size with lower associated capacitance values. This thesis focuses proposes a new method for implementing capacitance readout circuits with higher sensitivity. This is the first time, to our knowledge, that this method has ever been applied directly in electrical domain for capacitance measurement applications.The proposed method, which is based on weakly-coupled-resonators (WCRs) concept, can achieve considerably (orders of magnitudes) higher sensitivity while simplifying the analog front end circuitry and reducing the cost. For comparison, capacitance-to-frequency conversion readout circuits were chosen, which are one of the most reliable and best performing designs and also the closest to our WCR method since both involve shift in natural modes due to capacitance changes. Analysis and SPICE simulations followed by experiments proved the concept. The experimental results have shown almost two orders of magnitude higher relative sensitivity for our two-degree-of-freedom (2DOF) WCR-based system. In the next step we proposed a novel (named hybrid) method to reduce the measurement error considerably (4 to 6 times lower). Hybrid method is robust and insensitive to variations in excitation frequency, which is one of the main sources for errors. We have also analyzed the use of active inductors in our coupled resonators. The analyses and simulations proved the concept. This opens an avenue towards implementation of WCR-based readout in integrated circuits; specifically applicable for micro-electro-mechanical systems (MEMS) devices, and even integrating both MEMS sensors and the readout circuit in the same integrated circuit (IC) package. Another route on this research was to exploit the insensitivity and robustness of three-degree-of freedom (3DOF) weakly-coupled resonators to resonant frequency deviations. Analyses, followed by simulations, proved that applying 3DOF WCR in sensing differential capacitance changes does not require frequency tracking, yet has the same sensitivity achieved in 2DOF-based readout circuits.
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The full abstract for this thesis is available in the body of the thesis, and will be available when the embargo expires
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Capacitive Micromachined Ultrasonic Transducers (CMUTs) are considered advantageous over piezoelectric transducers for ultrasound imaging for the high bandwidth, ease of integration with electronics and miniaturization. Research efforts over the past two decades have been focusing on manufacturing and system integration of CMUTs to achieve comparable and better performance than the piezoelectric counterparts, while the uniqueness of the CMUT structure and physics is barely exploited.This thesis explores the complex behavior of CMUTs from a mode superposition perspective, and demonstrates imaging applications using CMUTs' multi-modal operation. The operation of CMUTs is first analytically modeled as a coupled electro-mechano-acoustical system using plate vibration theory. As the simplest case, the first symmetric and asymmetric modes of vibration can be excited simultaneously via asymmetric electrostatic actuation, resulting in a vibration profile with a shifted center. Finite element modeling (FEM) is used to verify the theoretical calculation, and an equivalent circuit consisting of two sub-circuits for the symmetric and asymmetric vibration modes is built to show the possibility of fast simulation of complex CMUT array behavior. Experimental characterization of fabricated CMUT chips show that asymmetric vibration can be achieved with multi-electrode CMUTs.Two imaging applications using the multi-modal operation of CMUTs are proposed. The first concept, tiltable transducers, explores the benefits of orienting each transducer element toward the focal point to concentrate the acoustic energy and reduce grating lobes and side lobes. Imaging simulation shows the grating lobes can be reduced by 20dB while the main lobe energy is preserved. FEM simulation demonstrates that CMUTs capable of asymmetric vibration can be a viable candidate as tiltable transducers with careful design of the cell dimension and central frequency. The second imaging application takes advantage of the ringing response of a CMUT to off-axis acoustic sources to achieve super-resolution imaging with low computational cost. The differential responses across all CMUT cells form a more decorrelated pattern than the regular average responses, which leads to better estimation performance of the proposed super-resolution algorithm. While only preliminary experimental results for the proposed applications are presented, the multi-modal operation concept shows potential in improving several aspects of ultrasound imaging.
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Inertial sensors, specifically MEMS gyroscopes, suffer in performance withdown scaling. Non linear amplification techniques, such as parametric resonance,can be employed in many resonant structures to alleviate this degradationin performance, improve sensitivity and Signal to Noise Ratio (SNR).In this thesis the application of parametric resonance amplification anddamping to both modes of a vibratory gyroscope is carried out using specializedcombs. Gap-varying combs, which are usually used for the sensingmode are known for producing electrostatic spring modulations. They areused in this thesis to achieve parametric modulation in sense mode, for increasingspectral selectivity and to reduce the equivalent input noise angularrate (from 0.0046 deg/s/√Hz to 0.0026 deg/s/√Hz , for a parametric gainof 5). Additionally, novel shaped combs were used for performing parametricmodulation of the driven mode of a resonant gyroscope as well. Analyticalmodes for both types of parametric amplification are derived and experimentallyverified. In order to study the effect of parametric modulation for largesignal operation, the dynamic pull-in process is analyzed and modeled in inertialMEMS sensors. The dynamic analytical model is derived and experimentallyverified for parametric amplification. The dependence of dynamicpull-in voltage amplitudes on the values of externally-induced accelerations(e.g. Coriolis accelerations in the case of vibratory gyroscopes) is experimentally. The measurements indicate that the dynamic pull-in voltagesreduce from 100 V to 56 V for a designed and fabricated MEMS gyroscope(device A) and from 21.77 V to 17.3 V for a MEMS accelerometer (deviceB), for an equivalent input acceleration signal of 0.319 ms-2, when thestructures are actuated at their resonance frequency. In order to further analyze the fundamental limitations of sensing at microscale, a separate noise analysis of MEMS resonant sensors is performed. The frequency-dependentdamping theory is used to suggest new optimization methods for the designof MEMS vibratory gyroscopes.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Microparticle manipulation means actively controlling the position and dynamics of micro-/nano-particles in a microfluidic environment, which is of great interest to many fields, especially in biomedical science, for example, bioanalysis, disease diagnosis, and drug delivery. Many mechanisms such as acoustophoresis, electrical forces, thermophoresis, magnetophoresis, and optical forces have been used to solve this task. The acoustophoresis method is an effective non-contact solution, which makes use of the acoustic streaming force and the acoustic radiation force to gather and migrate microparticles. Normally, acoustophoresis is achieved through the generation of standing acoustic waves. This requires a piezoelectric substrate and a pair of IDTs (interdigitated transducers) at the ends of the microfluidic channels for one-dimensional manipulation, or two pairs of IDTs for two-dimensional (in-plane) manipulation.This thesis proposes a novel technique of creating a patterned pressure field, by using a CMUT (Capacitive Micromachined Ultrasonic Transducer) array to manipulate microparticles in two-dimensional space. CMUTs are MEMS-based ultrasound transducers, that can generate and receive ultrasound signals. By digitally controlling the amplitude and phase of the acoustic beam generated by each element in the CMUT array, the acoustic field can be adjusted to form a desired pressure pattern on a target plane. Microparticles will be moved to the target locations due to acoustophoresis effects in the field. CMUT arrays are chosen to accomplish this task because CMUTs can be easily fabricated into large arrays at a low cost. In addition, the CMUT array is reconfigurable and can be controlled in real-time. This provides the feasibility of real-time closed-loop control of microparticles.In this thesis, we derive the equations of the acoustophoresis effects. Then create the numerical model using COMSOL Multiphysics® software to verify the feasibility of manipulating microparticles using a CMUT array. Finally, we discuss the potential physical experiment setup and the closed-loop system to manipulate microparticles in real-time.
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No abstract available.
Sleep is an essential process needed by the body; as a result, the effects of sleep deprivation are severe and include increased risk of heart diseases and dementia. It is estimated that 40% of Canadians suffer from sleep disorders; nevertheless, most of them are unaware of their condition. Approximately 75–80% of cases of sleep apnea (SA), the most common sleep disorder, are still undiagnosed and thus lacking treatment. However, demand for sleep studies is unmet by the available spaces—high operating costs and expensive equipment limits their availability and the access to diagnosis. Thus, the necessity of inexpensive, yet adequate alternatives for the monitoring of sleep parameters is evident.We present the development of a respiratory effort (RE), body position (BP), and heart rate (HR) monitoring systems for sleep using inertial measurement units (IMU) comprising a 3D accelerometer, gyroscope, and magnetometer. These parameters are required by the American Academy of Sleep Medicine (AASM), and necessary for diagnosis of SA.The chest’s angle variations due to breathing are tracked using all three sensors in the IMU and an extended Kalman filter (EKF) as a data fusion method, thus obtaining the RE in supine and lateral recumbent positions, as well as monitoring BP by tracking gravity vector orientation relative to the body. The system is self-contained, wireless, battery operated and real time. Thus, we improve over previous works that only measure RE limited to a supine position of the body.The HR is estimated by detecting cardiac induced vibrations from a single axis angular rate measuring channel. Using a simple algorithm with an adaptive threshold for peak detection, we obtain good instantaneous HR readings. Although its simplicity sacrifices some accuracy, it is amenable for real-time implementation in a low cost, wireless microcontroller, such as the one utilized for RE/BP.Both tests show promising results, with a high correlation value for RE (e.g. 96.26%) and low error values for HR (e.g. mean 0.35bpm) with respect the reference signals. Furthermore, preliminary tests show the possibility of obtaining all three signals using a single IMU device, suggesting a viable alternative to current technologies.
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In the age of rapid technological advancement, many telecommunication applications are being integrated into our lives, including smart phones and Internet of Things (IoT). Smart buildings (and houses) use these technologies to reduce energy consumption and increase safety. The need for these buildings is growing as urbanization continues and resources dwindle. According to a report by the United Nations, by 2050 66% of the world’s population will live in cities. This will cause the size and number of megacities to expand drastically in the near future. Complex communication networks, controls and other services will allow us to build smart cities to manage and improve the public’s quality of life. The necessity of smart buildings and, eventually, cities, has stimulated growth of sensor networks for these purposes. This thesis discusses the research on creating a smart transducer network architecture concept that uses Power over Ethernet (PoE) as a method for transferring data and power over a single medium, together with principles of decentralized and remote computing for data processing. The concept prototype had its power supplied by a Cisco Catalyst 4507R+E switch and utilized cloud computing to provide an easily scalable and adaptable architecture, able to easily adapt to a wide array of applications and fit the demands of new trends or integration into other systems. The set-up was tested on RaspberryPi and BeagleBone microcontroller boards as sensor hubs, and used DigitalOcean as the cloud computing service of choice. The server in this implementation acts as user interface, front end, and as the console unit back end. The architecture has demonstrated the feasibility of the concept of uniting PoE and IoT to create a flexible architecture. The system has also demonstrated fast communication times, below 200ms, in a cross-continental setting and the ability to provide fast processing times of under 1s. This shows particular promise for the use of the architecture within the context of green and smart housing equipped with a low-cost sensor network, where local power is partially provided via renewable energy harvesting, and the majority of short-range power transmission is DC-centered.
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The measurement of in-plane dynamics of microdevices is crucial to analyzing their dynamic characteristics under certain excitations. It has become more and more important to enable precise measurements and visual means to characterize dynamic microstructures, as the designs of moving micro-electro-mechanical systems (MEMS) are rapidly becoming more and more complex. And the visualization and measurement of the dynamics of MEMS structures are of considerable significance to the development of more effective and advanced microdevices. This thesis investigates the problem of visualizing, measuring and analyzing the in-plane dynamics of microdevices. We propose a novel object position tracking algorithm, called position-weighted template matching, improving the traditional template matching technique. The newly proposed algorithm effectively addresses the position "jump" problem that typically happens for object tracking in planar microdevices, where similar sub-patterns may exist in a single structure. We have incorporated the parabola fitting interpolation technique into our algorithm to achieve a higher, sub-pixel resolution level. We have implemented our proposed methods into a software module, associated with a LabVIEW Graphical User Interface (GUI). Several comparative experiments were carried out to demonstrate the effectiveness of our algorithm. In addition, the procedure was also used for performing a system identification on a fabricated MEMS resonator. Our implemented LabVIEW GUI can be potentially interfaced with low-cost hardware to enable visualization and measurement of in-plane motion of microdevices.
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The market need for organic materials to be used in sensor design has increased with the growing interest in organic printed electronics. Therefore, it is important to find and investigate the piezoelectric and piezoresistive properties of organic materials through the use of alternative rapid fabrication techniques. Poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate), commonly known as PEDOT:PSS, a conductive polymer widely used in organic electronics, can be possibly used as piezoresistive element to measure the strain on flexible substrate electronics. Using PEDOT:PSS and other metallic inks such as silver, the goal of this work is use alternative microfabrication technologies to deposit PEDOT:PSS on flexible substrates and then to use these methods to design strain gauges. The targeted biomedical applications of the designed strain gauges vary from rehabilitation devices to smart biomedical monitoring systems. In this work, PEDOT:PSS strain gauges are initially designed using aerosol jet deposition on a flexible polyamide substrate. The technology has proved to be very powerful in depositing lines with thickness less than 1um. In order to reduce the initial resistance of the strain gauges, it is desirable to increase the thickness of the structure. For this reason, laser micromachining etching is used to fabricate PEDOT:PSS strain gauges. The designed structures have been tested mechanically and electrically in order to measure their gauge factors to longitudinal and transversal mechanical strains. The resultant longitudinal gauge factor varied in the range of -1 and 2, while little change in the resistance was noticed for transversal characterization. Using the same fabrication method, silver paint strain gauges are designed and characterized to have a high longitudinal gauge factor approximated to be higher than 10. The silver paint gauge factor barely responded to transversal actuation. While the variability of the PEDOT:PSS strain gauges results seemed to be an issue, the reproducibility of silver ink strain gauges proved the viability of the technological fabrication process presented in this work.
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In current scientific literature, there are numerous approaches that clinicians can use to assess the static postural stability of patients. Among them, the Balance Error Scoring System is a notable method with merits such as cost-effectiveness and portability. Traditional measurement of errors made by patients in BESS test experiment relies on the manual inspection of sophisticated clinicians to the whole experiment process. A new avenue of detecting errors with wireless sensor network and signal processing technique can eliminate the instability from subjective evaluation in traditional method. This thesis present a reliable analytical system that can provide accurate evaluation on errors in BESS test of patient with concussion to assist clinicians to investigate their standing postural stability.In this research, the kinetic signal data is collected by wearable WSN equipment consisting of seven sensors embedded with accelerometer and gyroscope fixed on body of patients while they are completing BESS experiment. We use experimental data of 30 subjects to train back-propagation neural network and test the performance of neural network with testing data set. In this procedure, statistical technique such as principal component analysis and independent component analysis are applied in the step of signal pre-processing. Meanwhile, feature extraction is an alternative pre-processing technique for kinetic signal and the feature data serves as input data to train the neural network. With regard to target training data, the standard error information are acquired from the analysis of a group of researchers on video of the conducted experiment and we present them with Gaussian curve signal indicating the possibility of the error event. By testing the neural network, the technique of feature extraction in combination with back-propagation neural network is confirmed to account for the most optimal assessment of the postural error in BESS test. Furthermore, we can confirm the type of each detected error from six possible types of postural errors with neural network classification technique. Each type of error is corresponding to a certain unstable posture according to “BESS Protocol”. Ultimately, the presented error detecting system is convinced to supply reliable evaluation of the static postural stability of patients with concussion problem.
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In this work, we present a new method for removing artifacts from scalp Electro-Encephalography (EEG) signals recorded during Electrical Vestibular Stimulation (EVS). Using EVS, we stimulate the vestibular nerves, which can affect different regions in the brain via the interconnection of the vestibular system with some regions in the brain. As a result, some of the brain functions can be altered during the EVS application. Throughout its long history, EVS has been found as an interesting research tool in physiology and neurology. Various applications of EVS have been implemented in the health-care industry and also in other industries such as entertainment. Although there have been many advances in the EVS applications, it still remains a challenging problem to understand how the EVS stimulus acts as an input to the brain and how the brain responds. In this study, we monitored and recorded the brain activities during the application of EVS, using EEG. The recorded EEG data during EVS application, contain the information that elicit the EVS induced responses. However, the distribution of the EVS current throughout the scalp generates an artifact on the EEG signals. To analyze the EEG and study the brain functions during EVS, we have to eliminate this artifact. We developed a method to remove this artifact by estimating the contribution of the EVS current in the EEG signals at each electrode. The proposed method is a hybrid method, which combines time series regression and wavelet decomposition methods to estimate the artifact and remove it. Wavelet transform was employed to project the recorded EEG signal into various frequency bands and then the regression method was used to estimate the EVS current distribution in each frequency band separately. We optimized the proposed method using simulated data. Then we assessed the performance of our method and compared it to the other well accepted artifact removal methods, using both simulated and real data. The results show that the proposed method has better performance compared to the others, in terms of achieving higher signal to artifact ratio and introducing less distortion to the original EEG signals.
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Energy localization in nearly periodic microsystems can be leveraged to create a new sensing paradigm that is orders of magnitude more sensitive than current resonant-frequency based systems. In this thesis, the theory which supports this claim is independently developed from a mathematical description of a two degree-of-freedom resonant system.A novel proof-of-concept microelectromechanical system (MEMS) was also designed and fabricated to support the theoretical claims. The system employed a unique resonator design with two different approaches to inducing asymmetry in the system which in turn leads to the localization of energy in one of the resonators. The system proved the resonant frequency dependence on disorder in the system and also showed that the eigenvector sensitivity to disorder was at least an order of magnitude greater than the frequency sensitivity. However, the eigenvector sensitivity could not be matched with theory. This was likely due to the time-varying nature of the coupling spring stiffness (up to a 300% change in magnitude). The coupling spring stiffness was time-varying due to the inverse cubic relationship to coupling gap distance. The gap distance changes with time since it is practically impossible to excite only the common mode, leading to a superposition with the anti-phase mode. This was partially due to the input signal displaying non harmonic tendencies. At the same time, energy localization in the system leads to different amplitudes of vibration for each resonator which will also lead to gap distance modulation.A three degree-of-freedom system was also examined theoretically with different approaches to stiffness perturbation and the resultant sensitivity expressions which can be leveraged for improved sensors were developed. The analysis shows that three degree-of-freedom systems can yield a 250% improvement over two degree-of-freedom systems which themselves are practically able to provide three to four order of magnitude improvements in sensitivity over resonant-frequency based sensors of the same size.The tools and insight needed to design for higher degree-of-freedom system are also provided in the form of the eigen-derivatives approach to calculating eigenvalue and eigenvector sensitivity to disorder in a symmetric system.
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This work presents the design of Capacitive Micromachined Ultrasonic Transducers (CMUTs) with one and multiple bottom electrodes and their fabrication using the PolyMUMPs technique provided by MEMSCAP Inc. It also reports a new behavioral model of the CMUTs written in VHDL-AMS, complemented by a comparison between finite element analysis, behavioral simulations and experimental measurements on the newly fabricated CMUT arrays. As an improvement on a previously developed VHDL-AMS CMUT behavioral model [1], where the CMUT was treated as a movable rigid plate capacitor, a mode decomposition approach was used in the present work to better approximate the dynamics of the CMUT membrane. Besides the frequency responses, time responses and electro-mechanical conversion efficiency, the simulation results also showed the electrostatic spring softening effect, and the optimization of the DC/AC voltage ratio that leads to a maximum transmitted acoustic power. The CMUT membrane capacitance variation predicted by the model compares favorably with results from the finite element analysis, with better matching than the previously developed models. Polytec Micro System Analyzer (Polytec MSA-500) using Laser Doppler Vibrometry was used for the experimental characterization, in which the pull-in voltage, vibration modes and their respective resonant frequencies were determined. Characterization results were compared with the ones from the finite element analysis and the behavioral model simulations, and excellent agreement was shown.
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Capacitive Micromachined Ultrasound Transducers (CMUTs) have been recently introduced as a viable substitute to piezoelectric transducers in medical ultrasound imaging. CMUT possesses advantages such as allowing high frequency, having wide bandwidth, high sensitivity, low cost, CMOS compatibility and being easy to fabricate. This thesis is motivated by movement towards a better detection of breast tumors using ultrasound imaging techniques, which CMUTs have promised to achieve. Therefore, CMUTs were designed to fulfill requirements of this application in terms of resonant frequency, pull-in voltage and geometrical dimensions. The entire design and analysis were performed considering that the CMUTs are to be fabricated using PolyMUMPs technology, for this technology being precise, accurate and well established in the micro-electromechanical systems (MEMS) community. CMUTs were first analytically modeled and designed by exploiting the parallel-plate capacitor equations. A behavioral model was developed in VHDL-AMS, which, unlike previous models, incorporates the non-linear electromechanical relations of the CMUT. The behavioral model has the advantage of being more time efficient than finite element models (FEM) and more accurate than analytical models. Prior to fabrication, a 3D FEM was developed in COMSOL Multiphysics® software. Resonant frequency analysis determined the frequency response and eigenfrequencies of the CMUT, which could not be determined using previous models. Parametric analysis determined the pull-in voltage, the spring constant and spring softening effect, the variation in capacitance and the electromechanical efficiency of the CMUT. The CMUT resonated at 5.868MHz frequency and the collapse voltage was determined at 275V using FEM results, which were close to analytical modeling results and in excellent agreement with behavioral modeling results. The thickness and the radius of the circular CMUT membrane were found to be 1.5μm and 32μm, respectively. The air/vacuum gap distance was 0.75μm and the insulation layer was 0.6μm.The CMUTs were fabricated in cell and array form. An array of 128 elements each containing 118 cells were fabricated to be compatible with existing ultrasound probes. Unfortunately, due to mal-fabrication by the company, which was experimentally proved, the experimental results were not as successful.
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In the past few years, inkjet printing has been emerging as a cost effective, environment friendly, net-shape microfabrication technique. This non-contact deposition technique facilitated the deposition of metallic and polymeric inks, biological proteins, and cells. The present work investigates the inkjet printing of microtransducers, with a focus on stress-sensing and movable microstructures. Piezoresistive and interdigitated capacitor based strain gauges were printed and tested. The inexpensive conductive polymer, poly(3,4-ethylenedioxythiophene) oxidized with poly(styrenesulfonate) (PEDOT:PSS), was used as base material. We have performed measurements on several test structures to show that PEDOT:PSS does preserve its piezoresistive properties after printing. As we were relying further on PEDOT:PSS as a base material for printed transducers, the mechanical and electrical properties of this commercially available ink were comprehensively investigated. A dedicated experimental setup, which was used for the mechanical and electrical characterization of test structures, and micro-topography measurements were combined in order to extract the parameters of the PEDOT:PSS thin film: a zero-stress electrical conductivity of G=201 S/cm and gauge factor of 3.63. The longitudinal and transversal piezoresistive coefficients were estimated to be, [formula omitted] and [formula omitted] respectively, which denote a piezoresistive material in a similar range of piezoresistivity as n-doped silicon and conventionally fabricated PEDOT:PSS. A second explored direction was using inkjet microprinting technology for the fabrication of movable microstructures. An inkjet-printed CMUT (capacitive micromachined ultrasound transducer) was the target device, using ZnO as sacrificial layer and PEDOT:PSS as structural layer. The printed ZnO sacrificial layer was too rough, non-uniform and with a high porosity, so that printing a conductive membrane on top of it was unsuccessful. An alternative solution approach used kapton tape, a polyimide, as movable membrane; experimental characterization has shown that the structure is not properly clamped along its rim, yielding a vibraion at a frequency of 1.1 KHz, when actuated, compared to a resonant frequency of 9.3 KHz achieved by finite element analysis of the CMUT structure. The approach shows enough promise for further investigations, along directions suggested at the end of the thesis.
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