| 摘要： 本論文主要是探討如何設計一套以荷姆霍茲共振器預測待測物體積的量測系統，以及評估實驗結果，研究目標則是希望藉此發展一套可量測重量、體積與密度的整合式量測系統。 現有體積量測的方法各有特色，但遠比重量量測複雜。本文受到了日本西津貴久教授研究的啟發，選用聲學中的荷姆霍茲共振器做為自行發展體積量測系統的基礎。本體積量測系統具有下列優點：(1)乾式，可應用於不可浸水之受測物；(2)屬於非侵入式，不會損害受測物；(3)低價位，易為市場所接受。 以荷姆霍茲共振器量測體積的原理是利用待測物置入腔體後，因原腔體內部空氣的總體積減少造成其聲學共振頻率提高的反向關係，藉此反推置入物的體積。在整個量測過程中，估算共振頻率是最關鍵的一環。傳統上以掃頻實驗法測量系統的共振頻率需要較多的時間，本文則利用圓擬合法之特性，自撰可有效減少量測時間的演算法。實驗結果顯示，與傳統掃頻法相較，可降低50%以上的量測時間，精確度與傳統動態頻譜分析儀相比誤差僅0.1%。體積量測精確度方面，根據多種蔬果的實驗結果顯示，適用的置入物體積宜為空腔體積的7%~30%，量測誤差百分比的平均值可在1%左右，標準差則在5%~7%附近，已接近實用範圍。This study focuses on designing a volume measuring system using Helmholtz resonator, and assessing how the system performs. The ultimate objective is to develop an integrated system capable of measuring weight, volume, and density. The existing volume measuring systems are quite different in many aspects, but definitely much more complicated than weight measuring systems. Inspired by Prof. T. Nishizu’s research, this study develops its own volume measuring system using Helmholtz resonator, which is also an application based on acoustics. The developed system has three features: dry-type, non-invasive, and low cost. The principle of Helmholtz resonator is based on the fact that its acoustic resonant frequency is a function of cavity volume. When an alien material is inserted into the cavity, the total air volume inside the cavity reduces. As a result, its acoustic resonant frequency rises. This relationship leads to the estimation of alien material volume if the resonant frequency can be calculated experimentally. Compared with conventional Sine Swept method in frequency domain, the algorithm developed in this thesis can save more than 50% time for finding resonant frequency in conjunction with Curve Fitting method. As for the accuracy of volume measuring, the experimental results of a few fruits and vegetables show that the averaged relative errors can be as low as 1% and standard deviations around 5% to 7%, if the volumes of fruits or vegetables are between 7% and 30% of the cavity. |
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