Edmond Cretu

Behavioural analysis of the pull-in dynamic transition

A careful analysis of the dynamics of the pull-in displacement reveals a metastable transient interval for devices with a Q factor lower than 1.2. The duration of this metastable regime could be up to 20 ms for the structure used in this work, depending on the damping. For typical device dimensions this regime dominates pull-in dynamics. This paper explicitly focuses on the metastable regime. The results of numerical simulations are confirmed with measurement results with the purpose of providing a better understanding of the underlying mechanisms. This may contribute to both improved actuator design and enhanced sensitivity of pressure sensors and accelerometers operating on pull-in time interval measurement. The sensitivity of the pull-in time to external accelerations is 6 ? 10?2 s/ms?2 (~0.6 ms mg?1) for current devices and can be increased by design.

Squeezed film damping measurements on a parallel-plate MEMS in the free molecule regime

This paper provides the first experimental validation of the predictions by two recently introduced models for free molecule squeezed film damping. Measurements were carried out using a parallel-plate microstructure with a 2.29 µm gap operated at air pressures from 105 down to 101 Pa (corresponding to Knudsen numbers from 0.03 to 300). Experiments are in good agreement with the modeling based on molecular dynamics at low pressures. The results also indicate that modeling based on the modified Reynolds equation including inertia effects underestimates the damping due to end effects, but correctly predicts the trend at lower Knudsen numbers reaching into the transitional regime.

Development and validation of an objective balance error scoring system

INTRODUCTION Limited access to sophisticated technology and the unreliability of simple tools prevent accurate and reliable human standing balance assessments outside research laboratory settings. The goal of this study was to develop and validate a simple objective balance assessment tool that provides an accurate, reliable, and affordable alternative to currently available laboratory and clinical methods. METHODS Thirty healthy subjects were filmed performing the Balance Error Scoring System (BESS) while wearing inertial measurement units (IMU) measuring linear accelerations and angular velocities from seven locations of the body: forehead, sternum, waist, right and left wrist, and right and left shin. Each video was scored by four experienced BESS raters, whose mean scores were used to develop an algorithm computing objective BESS (oBESS) scores solely from IMU data. Interrater reliability and accuracy of oBESS scores were assessed using intraclass correlations (ICC). RESULTS Raters displayed low variability in scoring (ICC3,1 = 0.91). The oBESS was able to produce scores with accurate fit to raters (ICC3,1 = 0.92) and predicted individual BESS scores (ICC3,1 = 0.90) using data from one IMU placed at the forehead. oBESS was unable to produce accurate scores (ICC3,1 = 0.68) when using IMU data from the subset of conditions (firm surface only) used in popular concussion identification protocols. CONCLUSION The oBESS can reliably predict total BESS scores in healthy subjects. Pending further validation, oBESS could represent a valid tool to assess balance by offering an objective and reliable alternative to the current scoring methods of the BESS.

Ring Resonator Optical Gyroscopes—Parameter Optimization and Robustness Analysis

Semiconductor ring resonators as the core components of instruments and devices have found applications in the areas of telecommunications and sensors. In this study, the feasibility of using ring resonators as solid-state angular rate sensors (gyroscopes) based on the Sagnac effect is assessed, and an extensive analysis of the optimal values of resonator length, coupling, and detuning is carried out, for the drop port of a double-bus ring, and for an all-pass, single-bus ring, for different values of propagation losses, consistent with different available technologies. We show that for both the all-pass and the drop-port configurations, optimally undercoupled rings show larger extinction ratios and, thus, better resolutions than critically coupled rings of the same length, contrary to our initial intuitive assumption that critically coupled rings should offer the best resolutions. Our analysis also shows that the ring resonator gyroscopes require a technology-constrained optimization, with the propagation loss as the main factor that hinders the resolution, and that determines the optimum values of all other parameters, namely length, coupling, and resonance detuning. According to our model, standard-chip-sized racetrack gyroscopes are suitable for rate- and tactical-grade applications for selected, currently feasible low propagation loss waveguides.

A Digitally Controlled MEMS Gyroscope With Unconstrained Sigma-Delta Force-Feedback Architecture

In this paper we describe the system architecture and prototype measurements of a MEMS gyroscope with a resolution of 0.055°/s/√Hz. Two innovations are presented. The first is the complete migration of control and demodulation tasks to the digital domain. For this purpose, interfacing circuits based on ΣΔ techniques are introduced for both primary and secondary mode. The advantage is that complex analog electronics for tracking the resonant frequency, stabilizing the amplitude of the primary mode oscillation and phase-sensitive demodulation can be replaced by their digital counterpart. A second innovation relates to the ΣΔ force-feedback loop. In previously reported structures a compensation filter is introduced for stabilizing the loop [ 1– 3]. Unfortunately, the compensation filter introduces extra poles and influences the noise-shaping characteristic, which makes the loop difficult to design and optimize. We demonstrate the possibility of obtaining a stable ΣΔ force-feedback loop without an explicit compensation filter.

Measuring and interpreting the mechanical-thermal noise spectrum in a MEMS

The meta-stability of the pull-in displacement of an electrostatically operated parallel plate micromechanical structure is used for the capacitive measurement of the mechanical–thermal noise spectrum in a MEMS. Pull-in time depends on force and is not affected by the input-referred noise of the readout circuit. Repeatedly bringing the microstructure to pull-in while measuring the pull-in time followed by FFT enables the measurement of the mechanical noise spectrum with a non-mechanical noise level set primarily by the resolution of the time measurement. The white noise level is found to be in agreement with the theory on damping. The 1/f noise spectrum is found to be independent of ambient gas pressure with a 1/f noise–white noise cross-over frequency at 0.007 Hz for a 1 bar gas pressure and is reproducible for devices fabricated in the same process and the same run.

Micromechanical voltage reference using the pull-in of a beam

The pull-in voltage of a single-side anchored freestanding beam, under lateral deflection, has been investigated for application as a DC voltage reference. Two sets of electrodes, along side the tip, are used for parallel-plate type of electrostatic actuation of the 200 /spl mu/m long beam in the plane of the wafer. Another set of buried electrodes is aligned with the plate electrode at the free-standing tip and is used as a differential capacitor for the simultaneous detection of the displacement, with the purpose to determine the stability border and thus the pull-in voltage. The single-end clamping ensures that the pull-in voltage is insensitive to technology-induced stresses. A 2D energy-based analytical model for the static pull-in is compared with measurements. Bifurcation diagrams are computed numerically, based on a local continuation method. Devices have been designed and fabricated in an epi-poly process. Measurements are in agreement with modeling and confirm a pull-in voltage in the 9.1-9.5 V range. Reproducibility is limited by hysteresis and charging of the dielectric layer in between the electrodes. The device can be operated in feedback or as a seesaw, by using the two sets of electrodes.

Ultrasensitive resonant MEMS transducers with tuneable coupling

This paper introduces a novel ultrasensitive resonant MEMS transducer with tunable electrostatic coupling, in order to measure micro-displacements induced stiffness perturbations. Enhanced sensitivity is achieved based on the principle of energy localization in eigenvalue veering phenomena, resulting from a symmetry breaking in coupled resonator systems. Experimental results from a coupled two-resonators MEMS device are compared with both analytical calculations, and Simulink model simulations. Modal vector sensitivity is shown to be an order of magnitude higher than resonant frequency sensitivity under ambient conditions. Decreasing the coupling strength between the two resonators, using tunable electrostatic spring, is shown to enhance sensitivity, albeit in a narrowed range of perturbations.

Tiltable Ultrasonic Transducers: Concept, Beamforming Methods and Simulation

This paper investigates the concept of tiltable ultrasonic transducers, and their application in focusing and steering in a transducer array. Results of simulated imaging processes suggest that physical focusing and steering with tiltable transducers is promising in reducing grating lobe and side lobe artifacts, and preserving the beam power, especially when steering to large angles. We propose that one embodiment of the tiltable transducers can be adaptive capacitive micromachined ultrasonic transducers (CMUTs) modeled as clamped plate radiators. By applying different levels of electrical field on their split electrodes, one can adjust the shape and orientation of the adaptive CMUTs adaptively and dynamically, creating a tilted effect in the beam direction. The feasibility of using adaptive CMUTs to implement tiltable transducers is studied using finite element modeling (FEM) and analytical modeling. Experimental measurements of the tilted behavior of fabricated adaptive CMUTs are also provided. Possible applications of the tiltable transducers, including spatial compounding, high intensity focused ultrasound (HIFU), adaptive imaging, and harmonic imaging, are discussed.

Response to sensory uncertainty in Parkinson’s disease: a marker of cerebellar dysfunction?

Motor performance is profoundly influenced by sensory information, yet sensory input can be noisy and uncertain. The basal ganglia and the cerebellum are important in processing sensory uncertainty, as the basal ganglia incorporate the uncertainty of predictive reward cues to reinforce motor programs, and the cerebellum and its connections mitigate the effect of ambiguous sensory input on motor performance through the use of forward models. Although Parkinson’s disease (PD) is classically considered a primary disease of the basal ganglia, alterations in cerebellar activation are also observed, which may have consequences for the processing of sensory uncertainty. The aim of this study was to investigate the effect of visual uncertainty on motor performance in 15 PD patients and ten age‐matched control subjects. Subjects performed a visually guided tracking task, requiring large‐amplitude arm movements, by tracking with their index finger a moving target along a smooth trajectory. To induce visual uncertainty, the target position randomly jittered about the desired trajectory with increasing amplitudes. Tracking error was related to target ambiguity to a significantly greater degree in PD subjects off medication compared with control subjects, indicative of susceptibility to visual uncertainty in PD. l‐Dopa partially ameliorated this deficit. We interpret our findings as suggesting an inability of PD subjects to create adequate forward models and/or de‐weight less informative visual input. As these computations are normally associated with the cerebellum and connections, we suggest that alterations in normal cerebellar functioning may be a significant contributor to altered motor performance in PD.