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Title: Towards ultra-high resolution mode-localised MEMS sensors
Author: Pandit, Milind Narasimha
ISNI:       0000 0004 7972 9610
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Sensors employing mode localisation in weakly coupled resonators have been increasingly viewed as an alternative to resonant frequency shift based sensing. Much theory has been proposed highlighting the advantages of these sensors including the increased sensitivity and the promise of common mode rejection to first order environmental variations. This has led to the development of proof-of-concept sensors to sense physical quantities such as displacement, charge, mass, and acceleration. However, practical aspects of developing a sensor starting from design of a closed-loop implementation to understanding different operating regions with the aim of resolution analysis and noise optimisation have yet to be explored in depth. This work delves into these practical aspects of developing ultra-high resolution mode-localised MEMS sensors. First, the mechanical sensor is integrated with a prototype closed-loop oscillator along with the interface electronics on a printed circuit board. Key aspects of sensors such as stability, noise floor, and bandwidth are analysed using this integrated sensor system. A critical observation is made on the improvement of stability of the amplitude ratio output metric over its frequency shift counterpart at large integration times therefore, highlighting the advantage of common mode rejection to environmental factors. The common mode rejection abilities of both mechanically and electrically coupled devices are next studied at different operating regions. These are then compared to the state-of-the-art differential frequency measurements. Amplitude ratio measurements in an electrically coupled device showed an order of magnitude better rejection to temperature variations over a mechanically coupled device. Furthermore, amplitude ratio measurements in the electrically coupled device were on par with the rejection offered by the differential frequency output in the same device. This result highlights the advantage of amplitude ratio measurements that are able to achieve the same common mode rejection with the help of a single oscillator instead of the two oscillators required in differential frequency output measurements. The resolution of the mode-localised sensor is then explored with the purpose of optimising operating regions to achieve the best noise figure. A detailed theoretical analysis is first undertaken to optimise the amplitude ratio noise in different noise dominant regimes. It is predicted that the resonator-based noise (such as thermo-mechanical noise) can be optimised be operating at an amplitude ratio of √2 and the electronic sourced noises can be optimised at an amplitude ratio of √1.5 in a single ended resonator drive configuration. Additionally, both sources of noise are predicted to decrease with the decrease of the coupling stiffness. This result is then validated using experimental data to verify the claim. A further noise reduction is sought by operating the coupled resonators in the nonlinear domain with interesting observations on the variations of the amplitude ratio output metric. The phase filtering offered by the bifurcation points in the nonlinear domain is utilised to further improve the noise by 4 times. Finally, a mode-localised accelerometer design is proposed that employs a novel differential amplitude ratio output metric. Noise optimisation techniques are then used to optimise this novel output metric. A noise floor of 3 μg√Hz with a stability of 3 μg is achieved thus, benchmarking the mode-localised accelerometer favourably with respect to other high-end commercial MEMS accelerometers. Additionally, their potential is demonstrated with a measurement of seismic activity. This measurement is then compared to reference data sourced from an accelerometer from the British Geological Survey. Lastly, suggestions are made to further optimise the resolution in the accelerometer to push the limits of amplitude ratio sensing thereby, putting mode-localised accelerometers at par with the best resonant accelerometers till date.
Supervisor: Seshia, Ashwin Sponsor: Innovate UK ; Natural Environment Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Mode-localisation ; Accelerometer ; Resolution