A spring-based resonance coupling for hugely enhancing the performance of triboelectric nanogenerators for harvesting low-frequency vibration energy

Microelectromechanical systems (MEMS) are a fast-growing field in microelectronics. MEMS are commonly used as actuators and sensors with a wide variety of applications in health care, automotives, and the military. The MEMS production cycle can be classified as three basic steps: (1) design process, (2) manufacturing process, and (3) operating cycle. Several studies have been conducted for steps (1) and (2); however, information regarding operational failure modes in MEMS is lacking. This paper discusses reliability in the context of MEMS functionality. It also presents a brief review of the most relevant failure mechanisms for MEMS.

https://downloads.hindawi.com/archive/2011/820243.pdf

On MEMS Reliability and Failure Mechanisms

External mechanical disturbances including vibration and shock have profound impact on the performance and reliability of MEMS devices. This project seeks to develop generic technologies that protect microsystems against them. Vibration produces undesirable outputs that cannot be corrected with electronics. Tuning fork gyroscopes (TFG), which are known to be relatively immune to linear vibrations, cannot completely eliminate linear vibrations in several special situations. Simulations were conducted using two commonly used TFG designs. Both designs have decoupled sense and drive masses; however, one is anchored at its sense mass (Type-A), whereas the other is anchored at its drive mass (Type-B). The results demonstrate that both designs are affected by linear vibrations. However, Type-B design is more resistant to vibration-induced errors than Type-A (>99% reduction). Shock permanently damages MEMS devices and hard shock stops have been conventionally used to limit the damage. However, hard stops can generate subsequent impacts, which should also be minimized. This goal can be achieved by two novel shock protection technologies using nonlinear springs and soft coatings. Devices integrated with nonlinear spring (silicon) stops and soft coating (Parylene) stops were fabricated and tested together with hard-stop devices. Test results show that both stops provide superior shock protection. The device survival rates of nonlinear spring stops (87%) and soft coating stops (94%) are more than 10x better than hard stops (4%). This project is supported by the Defense Advanced Research Projects Agency HERMIT program under contract number W31P4Q-04-1-R001. Sang Won Yoon, Sangwoo Lee, Noel C. Perkins, and Khalil Najafi

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Vibration Isolation and Shock Protection for MEMS.

This article presents a fundamental investigation in which velocity amplification is employed in non-resonant structures to enhance the power harvested from ambient vibrations. Velocity amplification is achieved utilising sequential collisions between free-moving masses, and the final velocity is proportional to the number of masses and the mass ratios selected. The governing theory is discussed, particularly how the final velocity scales with the number of masses. This article examines n-mass velocity-amplified vibration energy harvesters and examines their performance relative to single-mass harvesters. Electromagnetic energy conversion is chosen as it is fundamental in allowing the free movement of the masses. Experimental results from two- and three-mass prototypes are presented that demonstrate a wider frequency response and a gain in power of 33 times compared to single-mass configurations under wideband random excitation. The volume of the devices was constrained, which resulted in the two-mass sys...

Enhanced vibrational energy harvester based on velocity amplification

This paper presents the design and results of shock experiments conducted to demonstrate the advantages of two novel shock-protection technologies: 1) nonlinear spring shock stops and 2) soft coating shock stops. Both technologies basically employ the conventional idea of hard shock stops to decouple device design from shock-protection design but are specialized to reduce impact force, which is one of the drawbacks of hard stops. In addition, they enable wafer-level and batch fabrication processes compatible with microfabrication techniques. We designed test devices to reflect the effect of impact force and fabricated them using silicon microbeams (nonlinear springs) or Parylene coating (soft coating). After conducting multiple shock tests (up to 2500 g), we demonstrate that the shock-survival rates of test devices are considerably improved in both our novel technologies (nonlinear spring: 88%, soft coating: 94%) compared to conventional hard stops (4%). Moreover, we demonstrated that shock protection is improved by optimizing the design of shock springs. Finally, we analyzed dynamics of flexible beams and identified a new device-fracture mechanism induced by impact force, which is different from conventionally known mechanisms.

Shock-Protection Improvement Using Integrated Novel Shock-Protection Technologies

The major focus of this work is to examine the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop useful machines. For instance low-cost machines based on this principle could be used for detailed, very-high acceleration shock-testing of MEMS devices. A theoretical basis for determining the number and mass of intermediate stages in such a velocity amplifier, based on simple rigid body mechanics, is proposed. The influence of mass ratios and the coefficient of restitution on the optimisation of the system is identified and investigated. In particular, two cases are examined: in the first, the velocity of the final mass in the chain (that would have the object under test mounted on it) is maximised by defining the ratio of adjacent masses according to a power law relationship; in the second, the energy transfer efficiency of the system is maximised by choosing the mass ratios such that all masses except the final mass come to rest following impact. Comparisons are drawn between both cases and the results are used in proposing design guidelines for optimal shock amplifiers. It is shown that for most practical systems, a shock amplifier with mass ratios based on a power law relationship is optimal and can easily yield velocity amplifications of a factor 5-8 times. A prototype shock testing machine that was made using above principles is briefly introduced.

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The Dynamics of Multiple Pair-Wise Collisions in a Chain for Designing Optimal Shock Amplifiers

Recent advances in micro-electro-mechanical system (MEMS) fabrication technology have resulted in proliferation of micro-scale mechanical devices, some of which are applied in environments with severe levels of shock. The objective of this paper was to investigate the use of experimental and numerical methods in quantifying the behaviour of representative MEMS devices subject to high-g impact stimuli. Micro-cantilevers were analysed under vibration and shock on a modified Hopkinson pressure bar and vibration table to determine the mechanical properties of single crystal silicon (SCS). The characteristic dimensions of the beams were of 100 μm in height/width with beam lengths ranging from 5 to 7 mm. The experimental approach allowed non-invasive in situ monitoring of the micro-cantilevers upon impact through accelerometry and high-speed imaging. An investigation of the shock response of micro-cantilever beams indicates that orientation plays a significant role in their sensitivity to shock because of the planarity of the fabrication technique. Scanning electron microscopy identified octahedral cleavage of SCS as the dominant failure mechanism of the micro-cantilevers. Finite element analysis in conjunction with in situ high-speed imaging proved to be a viable non-invasive inverse technique to determine the loci and amplitude of tensile stress within generic micro-scale devices.

The Failure Mechanisms of Micro‐Scale Cantilevers Under Shock and Vibration Stimuli

Contemporary shock testing of micro-devices is carried out in controlled test environments where test parameters can be monitored with current metrology techniques. Due to demanding environments and limited scope of design rules, the reliability of micro devices has become a concern. A modified Hopkinson pressure bar (HPB) is used to investigate failure mechanisms of single crystal silicon (SCS) micro-cantilever devices under high-g accelerations. Response upon impact is monitored using high speed imaging (HSI) to ascertain the cause of failure. White light interferometry (WLI) and scanning electron microscopy (SEM) are used as post analysis techniques to investigate cause of failure and fracture topography. The modified HPB method in conjunction with high speed imaging allowed valid prediction of modal and temporal failure information of the micro cantilevers. WLI investigated the effects of deep reactive ion etching (DRIE) etching on crack instigation. SEM identified octahedral cleavage of SCS as the dominant failure mechanism of the micro-cantilevers.

https://hal.archives-ouvertes.fr/docs/00/27/76/68/PDF/dtip08002.pdf

The failure mechanisms of micro-scale cantilevers in shock and vibration stimuli

This theme of this paper is the design and characterisation of a velocity amplifier (VAMP) machine for high-acceleration shock testing of micro-scale devices. The VAMP applies multiple sequential impacts to amplify velocity through a system of three progressively smaller masses constrained to move in the vertical axis. Repeatable, controlled, mechanical shock pulses are created through the metal-on-metal impact between pulse shaping test rods, which form part of the penultimate and ultimate masses. The objectives are to investigate the controllable parameters that affect the shock pulses induced on collision, namely; striker and incident test rod material; test rod length; pul se shaping mechanisms; and impact velocity. The optimum VAMP configuration was established as a 60 mm long titanium striker test rod and a 120 mm long titanium incident rod. This configuration exhibited an acceleration magnitude and a primary pulse duration range of 5,800-23,400 g and 28.0-44.0 µs respectively. It was illustrated that the acceleration sp ectral content can be manipulated through control of the test rod material and length. This is critical in the context of pr actical applications, where it is postulated that the accel eration signal can be controlled to effectively excite specific components in a multi-component assembly affixed to the VAMP incident te st rod.

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Shock pulse shaping in a small-form factor velocity amplifier

This paper examines the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop guidelines for the design of useful machines. For instance, shock-testing machines based on this principle could be extremely versatile: low-cost; providing a very large range of shocks for testing; and a safer and more precise alternative to ballistic testing for the very-high acceleration shock-testing of inherently rugged emerging electronic devices like MEMS, nano-optics devices, and sensors. A theoretical basis for determining the number and mass of intermediate stages in such a velocity amplifier, modeled as a horizontal chain or a vertical stack of monotonically decreasing masses arranged such that impacts between masses are pairwise, is proposed. The analysis (presented in detail in [1]) is based on simple but adequate rigid body mechanics (that which governs momentum transfer and energy dissipation during impacts between two point masses or rigid spheres) and similar in spirit to that developed in [2,3,4,5]. The influence of mass ratios and the coefficient of restitution on the optimization of the system is identified and investigated. Two cases are examined in particular: (1) The velocity of the final mass in the chain (that would have the object under test mounted on it) is maximized by defining the ratio of adjacent masses according to a power law relationship. It is shown that such a configuration leads to maximum velocity gain (MVG).. (2) The energy transfer efficiency of the system is maximised by choosing the mass ratios such that all masses except the final mass come to rest following impact. It is shown that such a configuration leads to maximum energy transfer (MET).

Michael Sheehy

Papers

The Dynamics of Multiple Pair-Wise Collisions in a Chain for Designing Optimal Shock Amplifiers

The Failure Mechanisms of Micro‐Scale Cantilevers Under Shock and Vibration Stimuli

The failure mechanisms of micro-scale cantilevers in shock and vibration stimuli

Shock pulse shaping in a small-form factor velocity amplifier

The dynamics of shock amplification through multiple impacts