Wideband Energy Harvesting from Low Frequency Vibrations using Mechanical Impact based Frequency Up-conversion Mechanism : Wideband Energy Harvesting from Low Frequency Vibrations using Mechanical Impact based Frequency Up-conversion Mechanism

미아 광운대학교 2016 국내박사

Extracting energy from mechanical vibration has drawn much attention over the last few decades due to its abundance in nature and unlimited lifetime. Different vibration sources, such as human and machine motion, water and wind flow, rotary motion etc. generate vibrations of different frequencies and amplitudes, but most vibrations are of low frequencies and large amplitudes with various cyclic movements in different directions. Widely used techniques for harvesting energy from mechanical vibrations are piezoelectric, electromagnetic and electrostatic mechanisms. Generally, these mechanisms are employed by spring-mass systems using cantilever beam structures having a specific resonant frequency. Harvested energy is maximum when harvester’s resonant frequency matches the applied vibration frequency. But, vibration energy harvesters are capable of generating significant amount of power at higher frequencies rather than generating at low frequencies. Resonant energy harvesters cannot efficiently generate significant power, especially at low frequencies below 30 Hz. Moreover, the electromechanical coupling is poor at low frequency vibrations. Since low frequency vibrations (1 Hz to 30 Hz) around the ambient environment are discursive in nature, resonance based power generators are limited to use within this low frequency range. Therefore, a wideband energy harvester is desirable. In order to address the low frequency environmental energy harvesting challenges, wideband energy harvesting method has been proposed by using mechanical impact based frequency up-conversion mechanism that allows the transduction element (in the form of a spring-mass system) to actuate at its own resonant frequency (considerably high) by a low frequency oscillatory system that responds to the external low frequency vibration, no matter how low it is. Due to their higher energy densities, piezoelectric (35 mJcm-3) and electromagnetic (25 mJcm-3) transduction mechanisms have been chosen. The proposed approaches for mechanical frequency up-conversion include: single-degree-of-freedom (SDOF) piecewise linear impact (piezoelectric), impact-enhanced dynamic magnifier (piezoelectric), direct mechanical impact by freely movable spherical ball (electromagnetic), and transverse mechanical impact by freely movable spherical ball (electromagnetic). Design, modeling, and experimental results for all those approaches have been demonstrated. Results show, the first approach offers maximum 7.5 Hz bandwidth (7 Hz to 14.5 Hz) and 734 μW peak power; the second approach offers 15 Hz bandwidth (9 Hz to 24 Hz) and 449 μW peak power. The last two approached were intended for energy harvesting from human-body-induced vibration, especially from hand-shaking. Both devices show non-resonant behavior. These two devices are capable of generating maximum 2.15 mW and 103.55 μW average power while excited by hand-shaking vibration of 5.17 Hz and 5.8 Hz frequencies, respectively. However, even though proper design of frequency up-converted wideband energy harvesters address the challenges of low frequency vibration energy harvesting, a number of research issues (such as efficient power management, compact design, mico-fabrication, reliability, etc.) still need to be addressed before its commercialization.

Study of single point incremental sheet forming with low-frequency vibration : 저주파 진동을 이용한 단점 점진 판재 성형에 관한 연구

Xiao, Xiao 경북대학교 대학원 2022 국내박사

Single point incremental sheet metal forming is a new type of sheet metal plastic forming technology. A specific tool is used to locally load the sheet, resulting in overall cumulative deformation under a die-less and unconstrained condition, and finally, the target shape is obtained. It integrates the design and manufacture, has the advantages of flexible, green, fast, and low cost. However, due to the local loading characteristics of the tool, the sheet is prone to instability during the forming process, leading to large springback, and ultimately making the forming accuracy difficult to meet the requirements of use. In addition, due to the uneven deformation of the material and the complex evolution of the structure, as well as the interaction of the forming parameters on the forming accuracy, the control of the forming accuracy becomes more difficult. This article attempts to introduce low-frequency vibration into single point incremental sheet forming technology to improve the quality problems of formed products. In this research, a low-frequency vibration-assisted single point incremental forming method is proposed. The unit-body in the contact area between the tool and the sheet is analyzed, and the mechanical equation is established. According to the assumption of the relationship between tensile stress and shear stress in sheet plastic forming, an approximate solution to the mechanical equation is obtained. A finite element model of the low-frequency vibration single point incremental sheet forming is established in which the low-frequency vibration is applied to the forming tool. Through the explicit and implicit numerical simulation of the whole forming process, the effects of low-frequency vibration on equivalent stress, equivalent strain, thickness distribution, forming force, residual stress, and springback are studied. The results showed that the low-frequency vibration assisted single point incremental forming could effectively reduce the equivalent stress and forming force. At the same time, the reduction of residual stress significantly reduced springback and improved the accuracy of formed products. A set of low-frequency vibration single point incremental sheet forming systems was developed. After that, through a series of experiments, the influence of low-frequency vibration on formability, forming force, surface quality, and forming accuracy was analyzed. In addition, based on the RSM analysis, the main parameters in the forming process of low-frequency vibration were analyzed and optimized. The results showed that the low-frequency vibration significantly reduces the forming force and improves the formed product's surface quality and forming accuracy. At the same time, there is no coupling relationship between low-frequency vibration and other main parameters, which is suitable for popularization and use in actual production. 단점 점진 판재 성형공정은 금형이 없이 간단한 공구를 사용하여 판재의 국부적인 소성변형을 가하여 점진적으로 성형하는 새로운 금속 소성성형 기술이다. 동 기술은 시제품이나 다품종 소량 제품의 빠른 생산을 위해 특화된 기술로 기존의 프레스 현장에서와 대용량의 대형 프레스와 상하 금형을 사용하지 않는다. 그 대신에 공구의 경로를 제어할 수 있는 기능을 갖는 CNC 머신이나 로봇 등을 통해 제품의 외부 형상의 궤적을 따라 이동하고 가압하면서 국부적인 소성을 유도하는 것에 의해 제품을 축차적으로 장출성형하는 혁신적인 기술이다. 그러나, 공구의 국부적인 하중 특성과 금형이 없기 때문에 성형 공정 중에 판재의 불안정 현상이 쉽게 발생하고, 이에 따라 상대적으로 큰 스프링 백을 발생하여 제품의 성형 정확도가 떨어진다. 또한, 가공 공구 경로로 인해 성형품의 표면 품질 저하, 큰 가공력이 필요하다는 문제들이 존재하여 실제 산업 분야에서 단점 점진 판재 성형 기술을 적용하는 것이 제한적이다. 본 연구에서 기존 단점 점진 판재 성형에 성형품의 품질 문제를 개선하기 위해 단점 점진 판재 성형 공정에 저주파 진동 보조 기술을 도입하고자 한다. 우선, 단점 점진 판재 성형공정의 메커니즘을 분석하여 단점 점진 판재 공정의 성형 특성을 기반으로 기계적 해석 모델을 구성하였다. 이에 따라 저주파 진동을 포함한 SPIF 과정의 역학 해석 모델을 도출하였다. 이어서 공구가 저주파 진동을 가할 수 있는 단점 점진 판재 성형 유한 요소 모델을 구성하였다. 명시적 및 암시적 수치 시뮬레이션을 통해 전체 성형 공정에 저주파 진동은 등가 응력, 등가 변형률, 두께 분포, 가공력, 잔류 응력 및 스프링 백에 미치는 영향을 해석하였다. 결과적으로 저주파 진동으로 인해 등가 응력과 가공력을 효과적으로 줄일 수 것을 확인되었고 특히 잔류 응력이 감소함으로 인해 스프링백의 발생이 억제되며 성형품의 성형 정밀도가 크게 향상되었다. 저주파 진동에 대한 실험적 연구를 수행하기 위해 저주파 진동 단점 점진 판재 성형 시스템을 개발하였고 실제 실험을 통해서 가공력 및 성형 정확도에 대한 저주파 진동의 시뮬레이션 결과를 검증하였다. 특히 저주파 진동으로 인해 판재와 공구 사이의 마찰 조건을 효과적으로 개선되며 윤활제의 사용을 크게 줄일 수 있다는 결과를 확인되었다. 또한, 응답표면법을 사용하여 저주파 진동 단점 점진 판재 성형 공정의 주요 공정변수를 분석하여 최적화하였다. 결과적으로 저주파 진동으로 인해 성형 공정에 필요한 가공력이 크게 감소하며 성형품의 표면 품질과 성형 정확도가 크게 향상된다는 것을 확인되었다.

Study on Non-resonant Piezoelectric Cantilever Energy Harvesters : Study on Non-resonant Piezoelectric Cantilever Energy Harvesters

박현수 광운대학교 2014 국내석사

The development of low-power wireless sensor networks has recently opened up a possibility that energy harvesters can be used to power self-sustainable autonomous sensor nodes instead of traditional electrochemical batteries. Ambient vibration energy has great potential as an energy harvesting source and a piezoelectric power conversion mechanism has been actively studied because of the simple geometry and high power density in micro scale. Multi-dimensional, low-frequency, and non-peoriodic vibration energy harvesting dvices are necessary to efficiently convert ambient arbitrary vibrations into electrical energy. Micro electromechanical system (MEMS) energy harvesters have been developed to reduce their sizes and to possilby integrate with other integrated circuits (ICs) and components. First, a hair-cell structure based MEMS piezoelectric energy harvester was designed, fabricated, and characterized for three dimensional ambient vibration energy harvesting. The hair-cell structure was a curled and elongated cantilever that was comprised of a SiNx/PZT layer and NiFe proof mass. The structural geometry was newly designed for harvesting three dimensional vibrations effectively. The output voltage of the fabricated device was maximized to 15 mV at its resonance frequency of 116 Hz under vertically induced vibrations with an acceleration of 50 m/s2. The fabricated device successfully generated the output voltage of 33 mV and 10 mV under longitudinally and horizontally induced vibrations at its resonance frequency of 116 Hz. It was expected to power wireless sensor nodes for reducing the cost of maintenance and installation. Second, a piezoelectric non-resonant vibration energy harvester was designed, fabricated, and characterized to scavenge low frequency, large amplitude, and non-periodic random vibrations. The proposed energy harvester was comprised of a hair-cell structure based resonant vibration energy harvester and a springless spherical permanent magnet. The piezoelectric energy harvester is stayed in its original position, since no stress is induced on the piezoelectric thin film when the spherical magnet is placed at the end of the cylindrical channel. However, the piezoelectric energy harvester starts to generate electrical power at any random vibraions, since the NiFe alloy proof mass is displaced at any time due to the magnetic attraction force caused by the springless spherical magnet. The fabricated device generated the output voltage of 6 mV under acceleration of 30 m/s2 at low frequencies ranging from 5 to 20 Hz. Moreover, it also generated the output voltage of 3.2 mV at the cantilever resonance frequency of 110 Hz.

Development of Ultrahigh Output Triboelectric-Electromagnetic Biomechanical Energy Harvesters for Wearable Self-Powered Sensors and Smart Electronics

소헬 광운대학교 일반대학원 2023 국내박사

The forthcoming biomechanical energy harvester has extensive possibilities for wearable self-powered sensors and smart electronics applications owing to an important renewable energy source. However, poor sensing abilities, low power outputs, and battery dependency have greatly constrained their advancement. Developing a biomechanically driven energy harvester with high output performance such as power density, sensitivity, stability, and fast charging as a sustainable energy source for extended practical applications is a significant challenge. To overcome these limitations, a number of novels, highly miniaturized, and high-output energy harvesting devices were designed and developed based on triboelectric nanogenerators (TENGs) and electromagnetic generators (EMGs) to harvest biomechanical energy from low-frequency vibrations induced by the human body in order to realize self-powered sensors, smart electronics, and IoT (Internet of Things) systems. Firstly, an ultra-flexible, highly sensitive, sustainable, single-electrode contactless double-layer TENG (CDL-TENG) was newly designed developed, optimized, and demonstrated by using metal-organic framework based Cobalt nanoporous carbon (Co-NPC)/Ecoflex with MXene/Ecoflex nanocomposite layer for contactless self-powered sensor applications. The porous structure of the Co-NPC provides a high surface area of the nanocomposite and the charge storage layer of the MXene/Ecoflex nanocomposite accumulates more negative charge to improve the functionality two and three times of the CDL-TENG, respectively. Compared with Ecoflex film-based TENGs, the CDL-TENG exhibited an 8 times slower decay rate due to charge trapping characteristics. In addition, the CDL-TENG showed excellent humidity and acceleration sensitivity of about 0.3 V/% and 2.06Vs2/m. The CDL-TENG delivers 20 cm non-contact position detection and it was integrated with mobile vehicles and an intelligent robot to detect obstacles and detect human motion. Finally, a contactless door lock password authentication mechanism performs. Subsequently, I designed and fabricated a stretchable and humidity-resistant triboelectric nanogenerator (SHR-TENG) using a metal-organic framework (MOF-525)@Ecoflex with a layer of Ecoflex@cobalt nanoporous carbon (Co-NPC)/MXene for self-powered biomotion and tactile sensing application. The MOF-525 improved TENG’s performance four times due to high charge accumulation. When the Ecoflex@Co-NPC@MXene nanocomposite was added as an intermediate layer, it captured triboelectric charges from the charge-generating layer and accumulated more negative charges, improving the SHR-TENG performance by 13 times. The Co-NPC promotes high charge-trapping capability, while the MXene improves transport-ions inside the nanocomposite. The as-prepared SHR-TENG showed an excellent power density of 25.7 W/m2. By exploiting the outstanding stretchability (245%) and its ultra-high sensitivity (149 V/KPa), practical applications were successfully demonstrated in biomotion monitoring, high-precision character recognition, self-powered tactile sensor, and virtual-reality car games control by tracing the finger. Secondly, an electro-spun nanofiber-based TENG using PVDF-TrFE/MXene nanocomposite material with superior dielectric constant and high surface charge density was designed and fabricated, investigated theoretically and experimentally. The fabricated MXene nanocomposite-based TENG (MNB-TENG) exhibited a maximum power density of 4.02 W/m2. The PVDF-TrFE/MXene nanocomposite improved the output performance of MNB-TENG fourfold. The TENG successfully powered an electronic stopwatch and thermo-hygrometer by harvesting energy from human finger tapping. Moreover, it was utilized in smart home applications as a self-powered switch for controlling electrical home appliances, including fire alarms, fans, and smart doors. Subsequently, a strong electron-donating poly(diallyldimethylammonium chloride) (poly-DADMAC)/nylon-11 composite nanofiber is proposed as a highly positive tribo-layer for boosting the TENG performance. Through the dielectric modulation of the nylon-11 nanofiber achieved by incorporating cationic poly-DADMAC, the output performance of the poly-DADMAC functionalized composite-based TENG (PDFCB-TENG) was significantly enhanced by the dielectric constant, which is attributed to the three-fold increase in the surface charge trapping capability of the composite materials. The fabricated TENG exhibited excellent power density (7.6 W/m2) and was demonstrated as a self-powered pressure sensor with an ultra-high sensitivity of 1.01 V/KPa. The PDFCB-TENG was also adopted as a self-powered wireless motion sensor and demonstrated to successfully monitor human motion using a smartphone. Thirdly, an innovative hand motion-driven hybrid nanogenerator (HMD-HN) was designed, developed, optimized, and demonstrated based on the combination of electromagnetic and triboelectric mechanisms for charging multifunctional smart electronics by scavenging mechanical energy from human hand motions. A dual Halbach magnet array arrangement was introduced in the EMG instead of an alternative polarity magnet array, increasing the output power density eightfold. To improve the TENGs' output performance, an innovative phase inversion method was used to construct a microstructure as a tribo-positive material (nylon/11 film). To further boost the TENG output, a nanowire-like surface-modified polytetrafluoroethylene (PTFE) film was used as a tribo-negative material. Moreover, an EMG and the TENG were combined into a hybridized nanogenerator, to increase the overall output performance with the same mechanical input. The HMD-HN’s performance was studied from different standpoints, revealing a maximum output power density of 185 W/m2 across an ideal load resistance of 1.6 kΩ at 5 Hz frequency under 20 m/s2 acceleration. Owing to its high-power density, the HMD-HN can allow a user to charge a wide variety of electronic devices. Additionally, the hybridized nanogenerator could power a Bluetooth mouse without any external power source. Subsequently, I designed and fabricated a novel human body induced vibration driven hybrid nanogenerator (HBIVD-HN) to harvest biomechanical energy from low-frequency vibrations using Halbach-magnet-array-based EMG and freestanding-mode-based TENG. The Halbach magnet array in the EMG was used to concentrate more magnetic flux in the coil and extensively enhance the EMG performance, while novel PEO nanofibers boost the triboelectric performance. By implementing a mechanical spring-mass model, simulation, optimization, and rational integration of electromagnetic and triboelectric generators, the HBIVD-HN could deliver an ultrahigh output power of 1.02W at a frequency of 5 Hz. The HBIVD-HN was successfully demonstrated to charging of Li-ion batteries, smartphones, smartwatches, budlives, iTags, healthcare, and motion monitoring systems via customized power management circuits.