Ji-Tzuoh Lin

The magnetic coupling of a piezoelectric cantilever for enhanced energy harvesting efficiency

It is shown that the energy harvesting capabilities of a piezoelectric cantilever can be enhanced through coupling to a static magnetic field. A permanent magnet is fixed to the end of a piezoelectric cantilever, causing it to experience a non-linear force as it moves with respect to a stationary magnet. The magnetically coupled cantilever responds to vibration over a much broader frequency range than a standard cantilever, and exhibits non-periodic or chaotic motion. While the off-resonance response is substantially increased compared to that of a standard cantilever, no reduction in the response at the resonant frequency is observed, as long as a symmetric magnetic force is applied. The magnetically coupled cantilever motion is analyzed using a simple driven harmonic oscillator model with a non-linear magnetic force term. The results show that magnetic coupling can be used to increase the amount of power scavenged from environments containing multi-mode, or random vibration sources.

Viscous damping of microresonators for gas composition analysis

The damping effect of various gas environments on a silicon, lateral microresonator implemented with piezoresistive detection is investigated in this study. The resonant frequency of the cantilever shifts due to viscous damping by an amount that is directly determined by the molar mass of the gas, thereby providing a method to determine the composition of the gas environment. In addition, the microresonator demonstrates the ability to perform CO2 composition analysis using this nonreaction based detection method. The advantages of this gas analysis method are that it is simple, repeatable, reversible and not limited to reactive gases.

Enhancement of Energy Harvested from a Random Vibration Source by Magnetic Coupling of a Piezoelectric Cantilever

It is demonstrated experimentally that the energy harvested from a random noise source by a piezoelectric cantilever can be substantially enhanced by introducing a magnetic coupling force. The coupled cantilever responds to a 1/f vibration spectrum (‘pink noise’) with chaotic motion that on average has larger amplitude than the non-chaotic motion of an uncoupled cantilever. A 50% increase in output voltage was observed in the coupled cantilever compared to the uncoupled cantilever. Calculations show that the magnetic force transforms the quadratic spring potential of the cantilever into a double valley of two potential wells. Fluctuations between the two potential minima increase the amplitude of the cantilever motion over a range of vibration frequencies.

Design and development of a MEMS capacitive bending strain sensor

The design, modeling, fabrication and testing of a MEMS-based capacitive bending strain sensor utilizing a comb drive is presented. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist with the diagnosis of spinal fusion. ABAQUS/CAE finite-element analysis (FEA) software was used to predict sensor actuation, capacitance output and avoid material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 ?m sidewalls with 25 ?m spacing between the adjacent fingers. The sensor was adhered to a steel beam and subjected to four-point bending to mechanically change the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers. At the unstrained state, the capacitive output was 7.56 pF and increased inversely to 17.04 pF at 1571 ?? of bending strain. The FEA and analytical models were comparable with the largest differential of 0.65 pF or 6.33% occurring at 1000 ??. Advantages of this design are a dice-free process without the use of expensive silicon-on-insulator (SOI) wafers.

Design, modeling, fabrication and testing of a MEMS capacitive bending strain sensor

Presented herein are the design, modelling, fabrication and testing of a MEMSbased capacitive bending strain sensor utilizing a comb drive. This sensor is designed to be integrated with a telemetry system that will monitor changes in bending strain to assist orthopaedic surgeons with the diagnosis of spinal fusion. ABAQUS/CAE version 6.5 finite element analysis (FEA) modelling software was used to predict sensor actuation, capacitance output and the avoidance of material failure. Highly doped boron silicon wafers with a low resistivity were fabricated into an interdigitated finger array employing deep reactive ion etching (DRIE) to create 150 µm sidewalls with 25 µm spacing between the adjacent fingers. For testing, the sensor was adhered to a steel beam, which was subjected to four-point bending. This mechanically changed the spacing between the interdigitated fingers as a function of strain. As expected, the capacitance output increased as an inverse function of the spacing between the interdigitated fingers, beginning with an initial capacitance of 7.56 pF at the unstrained state and increasing inversely to 17.04 pF at 1571 µe of bending strain. The FEA and analytical models were comparable with experimental data. The largest differential of 0.65 pF or 6.33% occurred at 1000 µe.

A High Gauge Factor Capacitive Strain Sensor and its Telemetry Application in Biomechanics

A highly sensitive strain sensing system has been developed using a capacitive MEMS bending strain sensor for telemetry application in biomechanics such as spinal fusion monitoring. This telemetry sensor system is capable of detection with a linear gauge factor as high as 249 in frequency domain. The task is accomplished by converting the capacitive strain to frequency using a low power capacitance-frequency converter circuit that modulates the 125 kHz magnetic carrier source from the interrogating reader. The reader demodulates the 125 kHz signal and recovers the strain information from the sensor. Experimentally, various situation tests were performed with loads on a material test system (MTS) machine up to 1000 micro- strains to simulate corpectomy model on a stainless rod. Strain measurements were proved reliable within 10 cm range.

Total-Ionizing-Dose Effects in Piezoresistive Micromachined Cantilevers

We evaluate the response of T-shaped, asymmetric, piezoresistive, micromachined cantilevers fabricated on p-type Si to 10-keV X-ray irradiation. The resonant frequency decreases by 25 ppm at 2.1 Mrad(SiO2), and partially recovers during post-irradiation annealing. An explanation of the results is proposed that is based on radiation-induced acceptor depassivation. This occurs because radiation-generated holes release hydrogen from previously passivated acceptors, causing the carrier concentration to increase, especially near the surface. Increased carrier concentration decreases Young’s modulus, resulting in a decrease in the cantilever resonant frequency. Finite element simulations show that the effect of a decreasing Young’s modulus in the surface region is consistent with the measured decrease in resonant frequency in the irradiated devices.

Axial asymmetry for improved sensitivity in MEMS piezoresistors

The strain induced resistance change is compared for asymmetric, symmetric and diffused piezoresistive elements. Finite element analysis is used to simulate the performance of a T-shaped piezoresistive MEMS cantilever, including a lumped parameter model to show the effect of geometric asymmetry on the piezoresistor sensitivity. Asymmetric piezoresistors are found to be much more sensitive to applied load than the typical symmetric design producing about two orders of magnitude higher resistance change. This is shown to be due to the difference in the stress distribution in the symmetric and asymmetric geometries resulting in less resistance change cancellation in the asymmetric design. Although still less sensitive than diffused piezoresistors, asymmetric piezoresistors are sensitive enough for many applications, and are much easier to fabricate and integrate into MEMS devices.

Enhanced stochastic, subharmonic, and ultraharmonic energy harvesting

Nonlinear bistable systems have been shown to provide improved efficiency for harvesting energy from random and broadband vibration sources. This article explores the distinct frequency response in the broadened spectrum of a particular nonlinear energy harvester, a piezoelectric cantilever with magnetic coupling. The cantilever response evolves dynamically with frequency around the main cantilever resonance. Both stochastic and multifrequency vibration responses are observed and account for some of the improved efficiency. In addition, subharmonics and ultraharmonics of the main resonance, along with various combinations of these, appear. An analytical model of the bistable dynamics produces results consistent with those observed experimentally. Overall, four distinct types of efficiency improvements appear, in which the signal is amplified above the noncoupled cantilever response: (a) ultraharmonic amplification below resonance frequency, (b) stochastic amplifications in multifrequency and multiamplitude oscillations, (c) ultrasubharmonic amplification, and (d) subharmonic amplification. Taken together, the stochastic, subharmonic, and ultraharmonic responses produce an average of threefold to fivefold increase in voltage production.

Dose-Rate Effects on the Total-Ionizing-Dose Response of Piezoresistive Micromachined Cantilevers

Total-ionizing-dose-induced resonance frequency shifts in piezoresistive micromachined cantilevers are experimentally shown to be dose-rate dependent. Devices were irradiated to 1 Mrad(SiO2) at rates from 5.4 to 30.3 krad(SiO2)/min, with lower rate exposures producing up to four-times more negative frequency shifts than higher rate exposures. Devices that were hydrogenated in a steam bath for 1 h showed shifts similar to those of control (not hydrogenated) devices at higher dose rates, and larger shifts than control devices at lower rates. All devices recovered to levels close to preirradiation after several hours of post-irradiation annealing. The dose-rate dependence is attributed to differences in carrier concentration caused by varying efficiencies of the depassivation of boron by hydrogen at higher and lower dose rates and/or surface charging effects, and the subsequent differences in Young’s modulus that occur as a result. Many of these processes are similar to effects that lead to ELDRS in linear bipolar transistors, emphasizing the need to include low-dose-rate testing of microelectromechanical systems devices when considering them for use in space systems.