多種加速度開關與微結構快速疲勞測試法之開發

Abstract

本論文研製多種加速度開關，並研製一種可應用於微結構快速疲勞測試法。於加速度開關部分為脈衝觸發式與連續觸發式等兩大方向加以研究，在脈衝觸發式加速度開關的部分，提出一高加速度閾值(高G值)加速度開關，採懸臂樑結構並結合懸臂樑自由端之工字樑設計之可撓性機構之設計，透過高剛性之懸臂結構、微間距電極距離及高撓性之工字樑結構，以達成高G值加速度開關閾值之需求，並縮短開關反應時間和延長電極間接觸時間，增加訊號連續輸出之時間長度。高G值加速度開關延長電極間相接觸時間並避免電極不連續接觸(skip contact)現象，需要搭配單可動撓性懸浮電極結構才能實現，主要原因為雙可動撓性懸浮電極結構碰撞時，會因振動週期不同而產生skip contact現象，縮短訊號連續輸出之時間長度，經實驗發現，單可動撓性懸浮電極結構可以有效提升電極相接觸時間至40μs，而採用雙可動撓性懸浮電極結構會因skip contact現象，導致電極相接觸時間縮短至8μs~17μs，此外高G值加速度開關閾值約為450G、反應時間為125μs及電極相接觸時間為40μs，已符合軍用碰炸開關之需求。 在連續觸發式加速度開關部分，包含兩類：一次性連續觸發式加速度開關與可重複使用之連續觸發式加速度開關；在一次性連續觸發式加速度開關的部分，提出高強健性之加速度開關，其是利用金屬鎳電開關本體、空間限制之概念及機械卡榫結構，達成開關具有低接觸阻值、超高強健性及不需電池記錄加速度閾值之能力，且採用空間限制懸臂樑位移使加速度開關具備的強健性，可以抵抗設計閾值154倍的反向慣性力，而讓開關持續保持觸發之狀態，此概念可以解決一般雙穩態或微卡榫式加速度開關強健性不足，造成無法忠實的記錄加速度閾值之情況。 在可重複使用之連續觸發式加速度開關的部分，提出可回復連續觸發式磁液珠開關，其可無線感測、無線回復的連續觸發式磁液珠開關，是利用電感電容迴路與3D列印之頸縮結構元件整合而成，可感測約0.78G值的慣性力，當此磁液珠開關受到超過設計閾值之慣性力，磁液珠移動至不同的感應位置並維持其狀態，而此狀態會影響迴路之共振頻率，因此可無線讀取之，進而得知開關是否遭受超過閾值之慣性力，而當讀取完畢後，更可透過外部磁鐵之磁場對磁液珠進行驅動穿過頸縮結構，使磁液珠開關回復至初始狀態，便可進行下一次慣性力記錄，重複運作，可應用於封閉環境加速度之監控；在此磁液珠分別可穩定停留於感測端及非感測端，以記錄其開關狀態，當磁液珠開關觸發時電感電容迴路之共振頻率會出現於10.1 MHz，當磁液珠開關回復後，10.1 MHz之電感電容迴路之共振頻率隨之消失，而此實驗總共進行了10次加速度閾值記錄，成功驗證無線讀取、無線回復之可回復連續觸發式磁液珠開關的可行性。 於開發微結構之快速疲勞測試法之部分，使用單晶矽材料製做懸臂樑作為疲勞測試試片，並利用外部壓電片激振單晶矽懸臂樑結構，其方法採用之概念為利用一外部振動源激振結構，使結構產生共振，利用共振效應放大結構振幅，使結構能夠達到疲勞測試時所需之最大應力，並由理論推導出於疲勞測試所需之最大應力下，其結構阻尼比最大容許值(ξupper bound)，當結構阻尼比小於ξupper bound即可進行疲勞測試，而試片之阻尼比則透過空氣擠壓阻力(squeeze force) 氣體流動阻力(airflow) 材料內部摩擦力(internal friction) 和支承耗損 (support loss)與結構幾何尺寸之關係，設計出小於ξupper bound之阻尼比試片，本研究提出2種構型之設計：有質量塊之懸臂樑(共振頻率約16kHz)，無質量塊之懸臂樑(共振頻率約72kHz)，進行疲勞試驗，而此試片只需5.2hr即可測試1.35x10^9次，並測得單晶矽懸樑疲勞限約為1.33 GPa。 本論文分別在加速度開關與微結構快速疲勞測試法等兩大方向做出貢獻，實現了高G值加速度開關(450G)，以及超高強健性之加速度開關，更實現了第一個液態式無線電感電容感測與致動之設計，成功開發出可無線讀取、無線回復、可重複使用之可回復連續觸發式磁液珠開關，此外於微結構疲勞測試，成功實現外部致動、結構簡單且測試時間短之新式微結構快速疲勞測試法。

This dissertation focuses on the development of acceleration switches and a novel rapid fatigue testing method for microstructure. An active acceleration switch for high-G monitoring is presented. This high-G acceleration switch consists of cantilever beams with flexible electrode to enhance the contact time. Short response time is also achieved through utilizing high-stiffness beam and fine electrode gap. In addition, the skip contact, which is dynamic contact process of the electrodes (contact time about 8μs~17μs), can be avoid due to the designed flexible electrode. As a high-G acceleration about 450 G is applied, the switch is found to have response time and contact time as 125μs and 40μs, respectively. For an irreversible passive acceleration switch, a novel robust micro mechanical-latch shock switch with low contact resistance is presented. The robust micro shock switch is made by nickel metal to reach low contact resistance. As the robust micro shock switch suffers acceleration over than threshold, the suspended beam latches on fixed electrode. Since the suspended beam remains contacted with the fixed electrode, the switch can record the threshold without any active power supplying, that is, passively. Furthermore, in order to enhance robustness of switch, the space limitation design is proposed. This design can withstand 154 times opposite inertial force without changing the latching state. For a resettable passive acceleration switch, a novel resettable passive switch using magnetic droplet switch with LC circuit for wireless signal transmission is presented. The switch can record the happening of object falling down during object delivery or transportation, even the object is recovered to its initial state after falling down. A magnetic droplet is used in the switch with a converging microchannel to control the desired threshold acceleration. When the inertia force exceeds the designed threshold acceleration (0.78G), the magnetic droplet will pass through the converging microchannel to a deep cavity to become a different state. By connecting the switch with the sensing LC circuit, the state of the recorder can be wirelessly identified through the LC resonant frequency. After triggering, this switch can be remotely reset to initial state by an external magnetic field. A proper hydrophobic surface treatment is employed to facilitate the movement of magnetic droplet and reduce the variation on threshold acceleration. The capabilities including switching, recording, wireless reading and resetting of the proposed gravity force recorder are successfully designed, fabricated, and verified with ten recording-reset tests. For rapid fatigue testing of microstructures, a novel method is proposed and performed, where the test sample is excited at its resonant frequency by an external piezoelectric actuator. Due to the scaling effect, the micro cantilever beam owns high resonant frequency, therefore, the testing time can be greatly reduced. Also, no complicated built-in micro actuator is needed due to the off-chip excitation source. Single-crystal-silicon (SCS) beams are fabricated to demonstrate the proposed method here with two types of beams. A notch is designed near the root to have stress concentration effect. The input voltage of the piezoelectric patch is adjusted to excite the cantilever beams at its resonant frequency to induce different amplitudes to find the corresponding fatigue life. A Laser Doppler Vibrometer is used to monitor displacements of the cantilever beam to control the stress amplitude. Then, the stress-life (S/N) curve of silicon beam can be obtained accordingly. It is found that the fatigue limit of the SCS beam is about 1.33 GPa, and it takes only about 5.2 hours to test 1.35x109 cycles at resonant frequency of 72 kHz. Comparing to previous fatigue test methods with an external probe, testing 10^9 cycles at frequency 20 Hz will need 1.58 years theoretically. This indicates that the proposed rapid fatigue test method is particularly suitable for high-cycle fatigue test of micro structure. Novel designs and methods for performance enhancement and fast reliability testing are presented in this dissertation. The high-G acceleration switch with flexible electrodes is proposed and found to enhance the contact time successfully. The robust micro mechanical-latch shock switch with proper space limitation is also realized to own robustness. Furthermore, the resettable passive shock switch with wireless reading and wireless resetting capabilities is achieved through LC circuits and external magnetic field. For rapid fatigue testing of microstructures, a novel method is proposed and performed through resonant excitation. This testing method is found to be 2662 times faster than traditional probing method.