Edmond Cretu

Associate Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Design, microfabrication, and characterization of a moulded PDMS/SU-8 inkjet dispenser for a lab-on-a-printer platform technology (2018)

No abstract available.

Capacitance readout circuits based on weakly-coupled resonators (2016)

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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Design, modelling, self-testing and self-calibration of MEMS accelerometers with adaptive and non-linear digital control (2013)

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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Exploring multiple-mode vibrations of capacitive micromachined ultrasonic transducers (CMUTs) (2013)

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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Nonlinear amplification techniques for inertial MEMS sensors (2013)

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 (2010 - 2018)
Scalable smart transducers network using Power-over-Ethernet : towards smarter, safer and more controllable buildings (2017)

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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Characterization of ionic polymers : towards applications as soft sensors in medicine (2016)

A phenomenon termed the piezoionic effect is described and characterized in various ionic polymers including polymer networks containing aqueous electrolytes (hydrogels) and organic electrolytes. Initial observations suggest that when an ion containing polymer is compressed, a concentration gradient is induced by the pressure differential, leading to an electrical potential difference detectable at electrodes placed at compressed and uncompressed portions of the polymer. The work focuses on the fundamental characterization of the nature of the piezoionic transduction to probe the effects of relative mobilities of the ions present in the system. The effective ion radii due to ion-solvent interactions and electrostatic ion-polymer interactions have been investigated for their contribution in dictating the piezoionic behavior by NMR measurements of the self-diffusion coefficients. The results are qualitatively correlated to the voltage response to mechanical compression of the polymer samples. Following the experiments, a numerical model is developed which incorporates a number of contributing events believed to be taking place in a concerted manner to cause the piezoionic effect. The deformation induced solvent flow is modeled by means of Biot’s constitutive equations on poroelasticity, a combination of thermodynamic equilibrium and Darcy’s law. The Darcy’s flow induced is then used as the input to model transport of dilute species. Here, the convective factor is being continuously modulated by Darcy’s flow, while Fickian diffusion concurrently takes place. The ionic species experience different displacements due to Stokes' drag experienced by the solvation spheres of the ionic species and solvent molecules and the electrostatic interactions between the charged polymer chains and the mobile ions. Furthermore, this non-homogeneous ionic charge distribution yields a voltage distribution via the Poisson’s equation. This voltage distribution is used to account for the migration of ionic species. The following chapter is dedicated to a novel electrochemical method and modelling approach designed to probe various ionic polymers, some electronically conductive and others interpenetrated, to determine the phase-wise contributions to ionic conductivities. Finally, potential applications of the piezoionic polymers as soft sensors in medicine, particularly in unobtrusive and longitudinal monitoring of physical parameters, are discussed and some preliminary prototypes are introduced and ultimate feasibility is assessed.

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Position-weighted template matching for measuring in-plane dynamics of microdevices (2016)

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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Design and characterization of polymeric strain gauges for biomedical applications (2015)

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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Concussion balance and postural stability assessment system using kinetic data analysis (2014)

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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Evaporation cast thin film carbon nanotube strain gauges (2013)

This work describes the research performed on synthesising and measuring the gauge factor of evaporation cast thin film carbon nanotube strain gauges. The main characteristics pursued of the strain gauges are inexpensive, easily manufactured and reasonably sensitive. Carbon nanotubes have exhibited a high gauge factor due to their intrinsic piezoresistivity and were incorporated into evaporation cast films to try to take advantage of the high sensitivity. Another direction taken to improve the sensitivity is alignment of carbon nanotubes in the thin film.Previous work produced an evaporation cast carbon nanotube strain gauge with a relatively high gauge factor. However, it was not reproducible and the research encompassed extends from the previous work. A number of ink compositions with different carbon nanotube and surfactant loadings were used to synthesise thin films of carbon nanotubes on a polyimide substrate. Variations of evaporation casting were used to decrease the evaporation rate in attempts of carbon nanotube alignment through a self-organising liquid crystal phase during evaporation. Other methods of inkjet printing and air flow evaporation casting were also attempted to achieve alignment. Electrical connections using a conductive polymer and metal wires were fabricated onto the samples for electrical measurements. A four-point probe resistance measurement under the application of strain was used to elicit the gauge factors.The strain gauge design was modified from previous work for more reliable electrical connections and for higher applied strains. A procedure for electrical measurements coupled with the application of strain was devised and the gauge factors achieved varied between 0.1 and 4.0 with a median of 1.1 ±0.1. The median gauge factor was reproducible and exhibited by several samples fabricated with different types of evaporation casting. The decrease in evaporation rate did not result in either alignment or relatively high gauge factors. In general, alignment was not achieved with the other methods of air flow evaporation and inkjet printing.

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A new method for removing Electrical Vestibular Stimulation induced artifacts from Electro-Encephalographic recordings (2012)

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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Leveraging eigenvalue veering for improved sensing with microelectromechanical systems (2012)

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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A PolyMUMPs Capacitive Micromachined Ultrasonic Transducer (2011)

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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Design and analysis of capacitive micromachined ultrasound transducer (2010)

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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Inkjet printing of transducers (2010)

No abstract available.

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