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Future Blog Post
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Sensor performance enhancement using modal interactions
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Minaturization enabled fitting a phone into our pocket, bringing the prosperity of our current digital age. However, the scaling law of miniaturization does not make it easy for us. The objective of this research is to address this challenge using modal interactions within micro-devices.
Blog Post number 4
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Blog Post number 3
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Blog Post number 2
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Blog Post number 1
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portfolio
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publications
A Force Sensor Based on Three Weakly Coupled Resonators with Ultrahigh Sensitivity
Published in Sensors and Actuators A: Physical, 2015
Abstract
A proof-of-concept force sensor based on three degree-of-freedom (DoF) weakly coupled resonators was fabricated using a silicon-on-insulator (SOI) process and electrically tested in 20 μTorr vacuum. Compared to the conventional single resonator force sensor with frequency shift as output, by measuring the amplitude ratio of two of the three resonators, the measured force sensitivity of the 3DoF sensor was 4.9 × 106/N, which was improved by two orders magnitude. A bias stiffness perturbation was applied to avoid mode aliasing effect and improve the linearity of the sensor. The noise floor of the amplitude ratio output of the sensor was theoretically analyzed for the first time, using the transfer function model of the 3DoF weakly coupled resonator system. It was shown based on measurement results that the output noise was mainly due to the thermal–electrical noise of the interface electronics. The output noise spectral density was measured, and agreed well with theoretical estimations. The noise floor of the force sensor output was estimated to be approximately 1.39nN for an assumed 10 Hz bandwidth of the output signal, resulting in a dynamic range of 74.8 dB.
[Elsevier]
A Force Sensor Based on Three Weakly Coupled Resonators with Ultrahigh Sensitivity
Published in Sensors and Actuators A: Physical, 2015
Abstract
A proof-of-concept force sensor based on three degree-of-freedom (DoF) weakly coupled resonators was fabricated using a silicon-on-insulator (SOI) process and electrically tested in 20 μTorr vacuum. Compared to the conventional single resonator force sensor with frequency shift as output, by measuring the amplitude ratio of two of the three resonators, the measured force sensitivity of the 3DoF sensor was 4.9 × 106/N, which was improved by two orders magnitude. A bias stiffness perturbation was applied to avoid mode aliasing effect and improve the linearity of the sensor. The noise floor of the amplitude ratio output of the sensor was theoretically analyzed for the first time, using the transfer function model of the 3DoF weakly coupled resonator system. It was shown based on measurement results that the output noise was mainly due to the thermal–electrical noise of the interface electronics. The output noise spectral density was measured, and agreed well with theoretical estimations. The noise floor of the force sensor output was estimated to be approximately 1.39nN for an assumed 10 Hz bandwidth of the output signal, resulting in a dynamic range of 74.8 dB.
[Elsevier]
A Three Degree-of-Freedom Weakly Coupled Resonator Sensor With Enhanced Stiffness Sensitivity
Published in IEEE Journal of Microelectromechanical Systems, 2016
Abstract
This paper reports a three degree-of-freedom (3DoF) microelectromechanical systems (MEMS) resonant sensing device consisting of three weakly coupled resonators with enhanced sensitivity to stiffness change. If one resonator of the system is perturbed by an external stimulus, mode localization occurs, which can be detected by a change of modal amplitude ratio. The perturbation can be, for example, a change in stiffness of one resonator. A detailed theoretical investigation revealed that a mode aliasing effect, along with the thermal noise floor of the sensor and the associated electrical system ultimately limit the dynamic range of the sensor. The nonlinearity of the 3DoF sensor was also analyzed theoretically. The 3DoF resonator device was fabricated using a silicon on insulator process. Measurement results from a prototype device agreed well with the predictions of the analytical model. A significant, namely 49 times, improvement in sensitivity to stiffness change was evident from the fabricated 3DoF resonator sensor compared with the existing state-of-the-art 2DoF resonator sensors, while the typical nonlinearity was smaller than ±2% for a wide span of stiffness change. In addition, measurements indicate that a dynamic range of at least 39.1 dB is achievable, which could be further extended by decreasing the noise of the device and the interface electronics.
[IEEE]
Mass Sensor Utilising the Mode-localisation Effect in An Electrostatically-coupled MEMS Resonator Pair Fabricated Using An SOI Process
Published in Microelectronic Engineering, 2016
Abstract
The change in the mass, achieved with focused ion beam (FIB) milling, of one of a pair of electrostatically-coupled microelectromechanical systems (MEMS) resonators has been detected utilising the mode-localisation effect. It has been demonstrated that the shift in the amplitude ratio of the coupled-resonators at the in-phase mode-frequency, in response to a mass change, is five orders of magnitude greater than the equivalent resonant frequency shift of a single resonator device. The device has been fabricated using a silicon-on-insulator (SOI) based process, which allows for high-yield and stiction-free fabrication. In addition, the design of the resonators has been created to have a larger surface area than previously reported designs, in order to facilitate future biological functionalisation. The mass sensitivity has been compared to current state-of-the-art mode-localised mass sensors and a 5.4 times increase in the amplitude ratio response to a given mass change has been demonstrated for the device in this work.
[Elsevier] [Researchgate full texts]
A Closed-Loop Readout Configuration for Mode-Localized Resonant MEMS Sensors
Published in IEEE Journal of Microelectromechanical Systems, 2016
Abstract
This paper systematically investigates the characteristics of different output metrics for a weakly coupled three degree-of-freedom microelectromechanical systems resonant sensor. The key figures-of-merit examined are sensitivity and linear range. The four main output metrics investigated are mode frequency shift, amplitude difference, amplitude ratio, and eigenstate shift. It is shown from theoretical considerations, equivalent RLC circuit model simulations and electrical measurements, that there is a strong tradeoff between sensitivity and linear range. For instance, the amplitude difference has the best sensitivity but the worst linear range, whereas frequency shift has the widest linear range but the lowest sensitivity. We also show that using the vibrational amplitude ratio as an output metric provides the best balance between sensitivity and linear range.
[IEEE][Researchgate full texts]
An Investigation of Structural Dimension Variation in Electrostatically Coupled MEMS Resonator Pairs Using Mode Localization
Published in IEEE Sensors Journal, 2016
Abstract
If a pair of MEMS resonators is electrostatically coupled together, the vibration amplitude ratios at the resonant frequencies of the resulting coupled system are sensitive to stiffness perturbation. An imbalance between the two resonators causes the confinement of vibration energy when the system is resonating, an effect known as mode localization. The degree of localization can be determined by extracting the amplitude ratio of the resonators through capacitive transduction. In this paper, we have fabricated MEMS devices, using a dicing-free silicon-on-insulator process, consisting of pairs of closely spaced microresonators. Each resonator consists of a clamped-clamped beam with a wider section in the middle, which is the location of the electrostatic coupling, instituted through the dc biasing of the resonators. Several devices have been fabricated, with the length of the anchor beams being varied, which influences the frequency of resonance. Stiffness imbalance between the resonators has been introduced through electrostatic spring softening, with the sensitivity of the amplitude ratio of the resonant-mode shape being greater for the higher frequency, shorter anchor devices. The sensitivities of the devices in this paper have been found to be nine times greater than the state-of-the-art two-degree-of-freedom mode-localized sensors.
[IEEE]
A Closed-Loop Readout Configuration for Mode-Localized Resonant MEMS Sensors
Published in IEEE Journal of Microelectromechanical Systems, 2017
Abstract
This letter presents the first experimental results on the closed-loop characterization of a mode-localized microelectromechanical resonator system. Comparisons between the closed-loop oscillator approach and the open-loop frequency sweep approach show good agreement of output metrics including the amplitude ratios and mode frequencies. This new approach enables real-time measurements using emerging mode-localized resonant sensors and represents an important step toward realizing sensors based on this measurement principle.
[IEEE][Researchgate full texts]
A New Electrode Design Method in Piezoelectric Vibration Energy Harvesters to Maximize Output Power
Published in Sensors and Actuators A: Physical, 2017
Abstract
A resonant vibration energy harvester typically comprises of a clamped anchor and a vibrating shuttle with a proof mass. Piezoelectric materials are embedded in locations of high strain in order to transduce mechanical deformation into electrical charge. Conventional design for piezoelectric vibration energy harvesters (PVEH) usually utilizes piezoelectric materials and metal electrode layers covering the entire surface area of the cantilever with no consideration provided to examine the trade-off involved with respect to maximize output power. This paper reports on the theory and experimental verification underpinning optimization of the active electrode area in order to maximize output power. The calculations show that, in order to maximize the output power of a PVEH, the electrode should cover the piezoelectric layer from the peak strain area to a position, where the strain is a half of the average strain in all the previously covered area. With the proposed electrode design, the output power can be improved by 145% and 126% for a cantilever and a clamped-clamped beam, respectively. MEMS piezoelectric harvesters are fabricated to experimentally validate the theory.
[Elsevier] [Researchgate full texts]
Real-world Evaluation of A Self-startup SSHI Rectifier for Piezoelectric Vibration Energy Harvesting
Published in Sensors and Actuators A: Physical, 2017
Abstract
This paper presents an enhanced SSHI (synchronized switch harvesting on inductor) rectifier with startup circuit and representative environment validation using real world vibration data collected from a tram. Compared to a conventional SSHI rectifier, the proposed rectifier dynamically monitors the working status of the circuit and restarts it when necessary. The proposed rectifier is designed in a 0.35 μm HV CMOS process and its performance is experimentally evaluated. With a 500-s real-world collected vibration data, the conventional and the proposed SSHI rectifiers record average power performance improvements by 9.2× and 22× respectively, compared to a passive full-bridge rectifier. As the startup circuit helps restart the SSHI rectifier several times, it is able to extract energy in an increased excitation range and its average power output performance is 2.4× higher than a conventional SSHI rectifier.
[Elsevier] [Researchgate full texts]
Parametric Noise Reduction in A High-order Nonlinear MEMS Resonator Utilizing Its Bifurcation Points
Published in IEEE Journal of Microelectromechanical Systems, 2017
Abstract
An electrostatically actuated non-linear microelectromechanical systems (MEMS) resonator can describe double hysteresis behavior in the measured frequency response due to the interplay between electrical and mechanical non-linearities in the system. This paper provides the first experimental mapping of the stable and unstable branches of the frequency response of a MEMS resonator describing a double hysteretic frequency response using a closed-loop phase feedback oscillator. Furthermore, the frequency stability of the oscillator is compared for varying amplitude and phase feedback conditions, and it is experimentally demonstrated that parametric noise up-conversion can be suppressed in such a system by suitably biasing the resonator at one of the four bifurcation points in such a system. This result is qualitatively consistent with theoretical prediction and demonstrates that improved frequency stability in a non-linear MEMS oscillator is possible by suitably biasing the resonator using simultaneous amplitude and phase feedback.
[IEEE] [Researchgate full texts]
Experimental Observation of Noise Reduction in Weakly Coupled Nonlinear MEMS Resonators
Published in IEEE Journal of Microelectromechanical Systems, 2017
Abstract
In this paper, we present the strongly nonlinear behavior of a 2-degree-of-freedom weakly coupled microelectromechanical systems (MEMS) resonator system in a mixed nonlinear regime, using a closed-loop phase feedback oscillator approach. Three out of four nonlinear bifurcation points within a strongly nonlinear coupled resonator system, with both electrical and mechanical nonlinearities, were revealed. Furthermore, we are able to study the amplitude and frequency stabilities of the resulting system when biased at the bifurcation points. Specifically, we discover that, as compared with the linear case, orders of magnitude improvement in amplitude and frequency signal resolution can be observed at the nonlinear bifurcation points, demonstrating that coupled nonlinear MEMS resonators can be useful for enhancing the amplitude and frequency stability for relevant applications.
[IEEE] [Researchgate full texts]
A Review on MEMS Coupled Resonators for Sensing Applications Utilizing Mode Localization
Published in Sensors and Actuators A: Physical, 2018
Abstract
In this paper, we review a recent technology development based on coupled MEMS resonators that has the potential of fundamentally transforming MEMS resonant sensors. Conventionally MEMS resonant sensors use only a single resonator as the sensing element, and the output of the sensor is typically a frequency shift caused by the external stimulus altering the mechanical properties, i.e. the mass or stiffness, of the resonator. Recently, transduction techniques utilizing additional coupled resonators have emerged. The mode-localized resonant sensor is one example of such a technique. If the mode localization effect is utilized, the vibrational amplitude pattern of the resonators changes as a function of the quantity to be measured. Compared to using frequency shift as an output signal, the sensitivity can be improved by several orders of magnitude. Another feature of the mode-localized sensors is the common mode rejection abilities due to the differential structure. These advantages have opened doors for new sensors with unprecedented sensitivity.
[Elsevier] [Researchgate full texts]
A Mass Sensor Based on 3-DOF Mode Localized Coupled Resonator under Atmospheric Pressure
Published in Sensors and Actuators A: Physical, 2018
Abstract
In this paper, for the first time, the mass sensitivity of a 3-DoF mode localized electrostatically coupled resonator is investigated and characterized under atmospheric pressure. A reversible method is used in which nanoparticles are added on and removed from one resonator of the 3-DOF coupled resonator system. Furthermore, a comparison of three mass sensitivity characterization methods was carried out: resonance frequency shift, resonance vibration amplitude change and resonance vibration amplitude ratio. MATLAB/SIMULINK and COMSOL Multiphysics models for the 3-DoF coupled resonator system are presented. The simulation results and theoretical calculations are in good agreement with the experimental data. The results show that a 3-DOF mode localized coupled resonator has potential to be employed for biosensing applications.
[Elsevier] [Researchgate full texts]
On the Noise Optimization of Resonant MEMS Sensors Utilizing Vibration Mode Localization
Published in Applied Physics Letters, 2018
Abstract
In this letter, we experimentally demonstrate the existence of an optimal operating region in a resonant microelectromechanical systems sensor utilizing the principle of vibration mode localization, to achieve ultra-high resolution amplitude ratio measurements. We have shown analytically that the coupling strength and stiffness mismatch between the two resonators are primary parameters that influence the resolution of the amplitude ratio measurements within such a sensor. Using this optimization strategy, a minimum noise spectral density equivalent to 16 ppb/Hz1∕2 in terms of normalized stiffness perturbation is experimentally demonstrated. This is by far the best resolution achieved in this type of sensor. These results can be used to aid the design of future high resolution sensor employing mode localization.
[AIP][Researchgate full texts]
Closed-Loop Characterization of Noise and Stability in a Mode-Localized Resonant MEMS Sensor
Published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2018
Abstract
This paper presents results from the closed-loop characterization of an electrically coupled mode-localized sensor topology including measurements of amplitude ratios over a long duration, stability, noise floor, and the bandwidth of operation. The sensitivity of the prototype sensor is estimated to be -5250 in the linear operation regime. An input-referred stability of 84 ppb with respect to normalized stiffness perturbations is achieved at 500 s. When compared to frequency shift sensing within the same device, amplitude ratio sensing provides higher resolution for long-term measurements due to the intrinsic common-mode rejection properties of a mode-localized system. A theoretical framework is established to quantify noise floor associated with measurements validated through numerical simulations and experimental data. In addition, the operating bandwidth of the sensor is found to be 3.5 Hz for 3-dB flatness.
[IEEE][Researchgate full texts]
An Ultra-high Resolution Resonant MEMS Accelerometer
Published in IEEE Conference on Microelectromechanical Systems, 2019, 2019
Abstract
This paper shows the first implementation of a micro-fabricated resonant accelerometer with a noise floor of 17.8ng/Hz1/2 and a bias stability of 17.5ng with a differential frequency readout configuration. The differential readout scheme provides for rejection of common mode environmental effects to first order. This prototype sensor demonstrates the best bias stability and noise floor of all MEMS resonant accelerometer configurations reported till date.
[(Link not yet available, stay tuned for the updates on the paper url!)]
Utilizing Energy Localization in Weakly Coupled Nonlinear Resonators for Sensing Applications
Published in IEEE Journal of Microelectromechanical Systems, 2019
Abstract
In this paper, energy localization within weakly coupled nonlinear MEMS resonators has been utilized for sensing applications. The sensitivity and the noise floor of the weakly coupled resonators operating beyond the critical amplitude have been characterized at the boundaries of the unstable branch, termed as bifurcation points in this paper. The measurement results show that such measurements can provide beneficial gains in resolution when compared with linear operation, despite the reduced sensitivity at the top bifurcation point. A minimum input-referred noise floor of 18 ppb/Hz1/2 has been achieved through this sensing scheme which is a 40% betterment in comparison to its linear counterpart.
[IEEE]
A Passive Design Scheme to Increase the Rectified Power of Piezoelectric Energy Harvesters
Published in IEEE Transactions on Industrial Electronics, 2019
Abstract
Piezoelectric vibration energy harvesting is becoming a promising solution to power wireless sensors and portable electronics. While miniaturizing energy harvesting systems, rectified power efficiencies from miniaturized piezoelectric transducers (PTs) are usually decreased due to insufficient voltage levels generated by the PTs. In this paper, a monolithic PT is split into several regions connected in series. The raw electrical output power is kept constant for different connection configurations, as theoretically predicted. However, the rectified power following a full-bridge rectifier (FBR), or a synchronized switch harvesting on an inductor (SSHI) rectifier, is significantly increased due to the higher voltage/current ratio of series connections. This is an entirely passive design scheme without introducing any additional quiescent power consumption, and it is compatible with most of the state-of-the-art interface circuits. Detailed theoretical derivations are provided to support the theory, and the results are experimentally evaluated using a custom microelectromechanical system PT and a complementary metal-oxide-semiconductor rectification circuit. The results show that, while a PT is split into eight regions connected in series, the performance while using an FBR and an SSHI circuit is increased by 2.3× and 5.8×, respectively, providing an entirely passive approach to improving energy conversion efficiency.
[IEEE] [Researchgate full texts]
A Fully Integrated Split-Electrode SSHC Rectifier for Piezoelectric Energy Harvesting
Published in IEEE Journal of Solid-State Circuits, 2019
Abstract
In order to efficiently extract power from piezoelectric vibration energy harvesters, various active rectifiers have been proposed in the past decade, which include synchronized switch harvesting on inductor (SSHI), synchronous electric charge extraction (SECE), and so on. Although reported active rectifiers show good performance improvements compared to full-bridge rectifiers (FBRs), large off-chip inductors are typically required and the system volume is inevitably increased as a result, counter to the requirement for system miniaturization. In this paper, a fully integrated split-electrode synchronized switch harvesting on capacitors (SSHC) rectifier is proposed, which achieves significant performance enhancement without employing any off-chip components. The proposed circuit is designed and fabricated in a 0.18-μm CMOS process and it is co-integrated with a custom microelectromechanical systems (MEMS) piezoelectric transducer with its electrode layer equally split into four regions. The measured results show that the proposed rectifier can provide up to 8.2× and 5.2× boost, using on-chip and off-chip diodes, respectively, in harvested power compared to an FBR under low excitation levels and the peak rectified output power achieves 186 μW.
[IEEE]
A Resonant MEMS Accelerometer With 56ng Bias Stability and 98ng/Hz1/2 Noise Floor
Published in IEEE Journal of Microelectromechanical Systems, 2019
Abstract
This letter presents a high-performance resonant MEMS accelerometer comprising of a single force-sensitive vibrating beam element sandwiched between two inertial masses. The accelerometer demonstrates a noise floor of 98 ng/Hz1/2 and a bias stability of 56 ng under ambient conditions, corresponding to a frequency noise floor of 0.77 ppb/Hz1/2 and a frequency bias stability of 0.43 ppb. These are the best results achieved for a MEMS accelerometer employing the resonant sensing paradigm to-date.
[IEEE] [Researchgate full texts]
Practical Limits to Common Mode Rejection in Mode Localized Weakly Coupled Resonators
Published in IEEE Sensor Journal, 2019
Abstract
The amplitude ratio output metric in mode localized weakly coupled resonators has been established to show superior rejection of first order temperature and pressure variations over frequency shift output metric. This work for the first time documents a holistic study on the temperature dependence of amplitude ratio and frequency shift output metric over various operating points of a mode localized resonator while comparing different modes of operation and different coupling schemes. The results show that both modes of an electrically coupled resonator system exhibit ≈ 10× improvement in rejecting temperature fluctuations over the modes of similar mechanically coupled resonators. Both mechanically coupled and electrically coupled resonators showed an improvement of 2-3 orders of magnitude common mode rejection as compared to the frequency shift output. For the first time, a comparison was made between amplitude ratio and differential frequency measurements and the results showed similar rejection capabilities between the two output metrics. The various mechanisms leading to the temperature dependence of the three output metrics are discussed in detail thus highlighting the advantages of using each of them for sensing and timing applications.
[IEEE]
A High Resolution Differential Mode-localized MEMS Accelerometer
Published in IEEE Journal of Microelectromechanical Systems, 2019
Abstract
A novel differential configuration of a highresolution mode-localized resonant SOI-MEMS accelerometer is presented in this paper. An effective maximum scale factor of 11/g and a bias-stability of 2.96µg is demonstrated. A minimum noise floor of 3µg/Hz1/2 is shown for device operation in an optimum working region. A large bandwidth of 350Hz is reported and methods to electronically tune the working bandwidth are described. These results benchmark mode-localized accelerometers favourably with respect to other high-end low-g commercial MEMS accelerometers.
[(Link not yet available, stay tuned for the updates on the paper url!)]
Towards High-resolution Inertial Sensors Employing Parametric Modulation in Coupled Micro-mechanical Resonators
Published in Physical Review Applied, 2019
Abstract
Highly accurate MEMS inertial sensors have a wide range of potential applications, including inertial navigation and seismometry. Conventional approaches to the implementation of inertial sensors reply on transducers that convert the external acceleration into changes in displacement of a proof mass or shifts in resonant frequencies. Recently, it has been demonstrated that inertial forces can also be measured through monitoring spatial energy distribution between two coupled micro-resonators. To extend this approach, we show that a weak dynamic coupling can be established through periodic modulation of the stiffness for a mechanically coupled microresonator system integrated as part of an accelerometer, enhancing the scale factor and resolution of the accelerometer. The resulting capability of parametric modulation also enabled the tuning of the operating point of the accelerometer through the modulation frequency. Utilizing this technique, we show that the scale factor of the accelerometer can be enhanced by a factor of 188, and a factor of 25 improvement in sensor resolution is demonstrated. Dynamic tuning of the sensor scale factor and inherent noise filtering is also demonstrated.
[(Link not yet available, stay tuned for the updates on the paper url!)]
talks
Talk-1 on MEMS Resonant Sensors: A A potential pathway to future high performance sensing technologies
Published:
In this talk, potential pathways to improve the resolution and long-term stability for MEMS resonant sensors, MEMS resonant accelerometers in particular are discussed.
Tutorial-1 on Mode-localized Resonant Accelerometers
Published:
In this tutorial, the concept of emerging mode-localized accelerometers are explained. Additionally, the advantages and disadvantages, as well as practical design considerations of mode-localized sensors are discussed. Finally, future pathways for developing the technology are offered.
Talk-2 on Recent Progress of MEMS Sensors based on Coupled Resonators
Published:
In this talk, recent progress on MEMS sensors based on coupled resonators are dicussed. In particular, the concept and technical advantages (in terms of improved sensitivity and long-term stability) of MEMS mode-localized sensors are presented. Nonlinear effects and modal interactions are proposed to enhance the sensor performance. Future outlook in this area is also discussed.
teaching
Teaching experience 1
Undergraduate course, University 1, Department, 2014
This is a description of a teaching experience. You can use markdown like any other post.
Teaching experience 2
Workshop, University 1, Department, 2015
This is a description of a teaching experience. You can use markdown like any other post.