Shahrzad Towfighian

A large-stroke electrostatic micro-actuator

Voltage-driven parallel-plate electrostatic actuators suffer from an operation range limit of 30% of the electrostatic gap; this has restrained their application in microelectromechanical systems. In this paper, the travel range of an electrostatic actuator made of a micro-cantilever beam above a fixed electrode is extended quasi-statically to 90% of the capacitor gap by introducing a voltage regulator (controller) circuit designed for low-frequency actuation. The voltage regulator reduces the actuator input voltage, and therefore the electrostatic force, as the beam approaches the fixed electrode so that balance is maintained between the mechanical restoring force and the electrostatic force. The low-frequency actuator also shows evidence of high-order superharmonic resonances that are observed here for the first time in electrostatic actuators.

Analysis of a Chaotic Electrostatic Micro-Oscillator

The closed-loop dynamics of a chaotic electrostatic microbeam actuator are presented. The actuator was found to be an asymmetric two-well potential system with two distinct chaotic attractors: one of which occurs predominantly in the lower well and a second that visits a lower-well orbit and a two-well orbit. Bifurcation diagrams obtained by sweeping the ac voltage amplitudes and frequency are presented. Period doubling, reverse period doubling, and the one-well chaos through period doubling are observed in amplitude sweep. In frequency sweep, period doubling, one-well, and two-well chaos, superharmonic resonances and on and off chaotic oscillations are found. DOI: 10.1115/1.4002086

A Review of Locomotion Systems for Capsule Endoscopy

Wireless capsule endoscopy for gastrointestinal (GI) tract is a modern technology that has the potential to replace conventional endoscopy techniques. Capsule endoscopy is a pill-shaped device embedded with a camera, a coin battery, and a data transfer. Without a locomotion system, this capsule endoscopy can only passively travel inside the GI tract via natural peristalsis, thus causing several disadvantages such as inability to control and stop, and risk of capsule retention. Therefore, a locomotion system needs to be added to optimize the current capsule endoscopy. This review summarizes the state-of-the-art locomotion methods along with the desired locomotion features such as size, speed, power, and temperature and compares the properties of different methods. In addition, properties and motility mechanisms of the GI tract are described. The main purpose of this review is to understand the features of GI tract and diverse locomotion methods in order to create a future capsule endoscopy compatible with GI tract properties.

Static and Dynamic Analysis of a Bistable Micro-Actuator

The static response of an electrostatic micro-catilever beam has been obtained by using Galerkin’s method. To make the system bi-stable, a controller has been added and the static response profile is presented using a multi-mode model for the beam. The number of mode shapes leading to convergence has been studied. The softening effect of adding more mode shapes has been investigated along with the effect of changing the system parameters on the static response. Decreasing the controller gain has been found to widen the voltage range of the bi-stability region and increasing the sensor amplification factor is shown to push the upper equilibrium point away from pull-in. Properly choosing these parameters can adjust the range of voltage for bi-stability. By doing a linearization about the stable fixed points, we also found the two natural frequencies for each stable equilibrium point. Finally, we have found the dynamic response of the bistable system using one- and three-mode-models. The basins of attraction for each stable fixed point and the exchange of energy between the two potential energy wells (equilibrium points), are demonstrated.Copyright

Nonlinear Dynamic Behavior of a Bi-Axial Torsional MEMS Mirror with Sidewall Electrodes

Nonlinear dynamic responses of a Micro-Electro-Mechanical Systems (MEMS) mirror with sidewall electrodes are presented that are in close agreement with previously-reported experimental data. An analysis of frequency responses reveals softening behavior, and secondary resonances originated from the dominant quadratic nonlinearity. The quadratic nonlinearity is an electromechanical coupling effect caused by the electrostatic force. This effect is reflected in our mathematical model used to simulate the dynamic response of the micro-mirror. The effects of increased forcing and decreased damping on the frequency response are investigated as the mirrors are mostly used in vacuum packages. The results can predict MEMS mirror behaviors in optical devices better than previously-reported models.

Magnetoelastic beam with extended polymer for low frequency vibration energy harvesting

Ambient energy in the form of mechanical kinetic energy is mostly considered waste energy. The process of scavenging and storing such energy is known as energy harvesting. Energy harvesting from mechanical vibration is performed using resonant energy harvesters (EH) with two major goals: enhancing the power scavenged at low frequency sources of vibrations, and increasing the efficiency of scavenging energy by increasing the bandwidth near the resonant frequency. Toward such goals, we propose a piezoelectric EH of a composite cantilever beam with a tip magnet facing another magnet at a distance. The composite cantilever consists of a piezoelectric bimorph with an extended polymer material. With the effect of the nonlinearity of the magnetic force, higher amplitude can be achieved because of the generated bi-stability oscillations of the cantilever beam under harmonic excitation. The contribution of the this paper is to demonstrate lowering the achieved resonant frequency down to 17 Hz compared to 100 Hz for the piezoelectric bimorph beam without the extended polymer. Depending on the magnetic distance, the beam responses are divided to mono and bi-stable regions, for which we investigate static and dynamic behaviors. The dynamics of the system and the frequency and voltage responses of the beam are obtained using the shooting method.

Parametrically Excited Electrostatic MEMS Cantilever Beam With Flexible Support

Parametric resonators that show large amplitude of vibration are highly desired for sensing applications. In this paper, a microelectromechanical system (MEMS) parametric resonator with a flexible support that uses electrostatic fringe fields to achieve resonance is introduced. The resonator shows a 50% increase in amplitude and a 50% decrease in threshold voltage compared with a fixed support cantilever model. The use of electrostatic fringe fields eliminates the risk of pull-in and allows for high amplitudes of vibration. We studied the effect of decreasing boundary stiffness on steady-state amplitude and found that below a threshold chaotic behavior can occur, which was verified by the information dimension of 0.59 and Poincar e maps. Hence, to achieve a large amplitude parametric resonator, the boundary stiffness should be decreased but should not go below a threshold when the chaotic response will appear. The resonator described in this paper uses a crab-leg spring attached to a cantilever beam to allow for both translation and rotation at the support. The presented study is useful in the design of mass sensors using parametric resonance (PR) to achieve large amplitude and signal-to-noise ratio. [DOI: 10.1115/1.4034954]

Static Modeling of a Bi-Axial Micro-Mirror With Sidewall Electrodes

The static modeling of a bi-axial torsional micro-mirror with sidewall electrodes is investigated. The mirror and sidewall electrodes experience different voltages. As a result of the potential difference, the mirror rotates about x and y axes with the torques generated from electrostatic forces by the sidewall and bottom electrodes. The rotations about two axes are made possible using a gimbal frame and serpentine springs. An analytical model is presented that is a simplified model of a previous study based on independent excitation of the two rotation angles. The simplified model enables prediction of the rotation angles with good accuracy and faster computation time.Copyright

Finite element modeling of low speed reaming vibrations with reamer geometry modifications

Reaming is a finishing process used to remove a small amount of material from a predrilled hole. In low speed cutting processes, it is the formation of lobed or multi-cornered holes that is of concern, rather than tool chatter, which occurs at high speed near the natural frequency of the tool. Using a quasi-static model in the characteristic form for the reaming process, a finite element modeling for the low speed reaming process, based on the Euler–Bernoulli beam model, was developed. Cutting and rubbing forces were applied as concentrated and distributed forces on a variable engagement length of the reamer. The variable engagement length is considered to simulate the actual applied forces length as the reamer advances to the workpiece. The time dependant changes in the bending stiffness of the reamer were included in the governing equation of the equilibrium of the reamer, and its stability analysis was performed at different time steps. Using this model, the vibration damping effect of uneven spacing of reamer teeth was investigated. The results demonstrate that uneven spacing of reamer teeth reduces the tool vibration, and therefore leads to a more stable condition. Finally, the optimum configuration of uneven tooth pitch angles for a six-flute reamer, in order to have the highest vibration decay rate during the reaming, was presented.

Performance of a Bi-Stable Resonator With Random Input Vibrations

By converting ambient mechanical energy to electricity, vibration energy harvesting, enable powering of low-power remote sensors. However, realistic ambient vibrations are random and spread over a wide frequency spectrum, which means linear resonators fail to perform effectively because of their narrow frequency bandwidth. Hence, there is a need for thorough investigation of performance of nonlinear resonators with Gaussian random vibration. This article presents a simulation study on the use of magnets to improve a nonlinear oscillator for energy harvesting from broadband low frequency random excitation. The resonator response to Gaussian distribution random input is investigated using root mean square value and power spectral density of voltage. The obtained results show that in a broadband low frequency spectrum the nonlinear system performs better than linear resonance. The optimal performance is found when the distance between two magnets is near the mono-stable to bi-stable transition regime.