以荷姆霍茲共振器為基礎的線上蔬果體積分級系統之研製

Abstract

本研究乃利用聲學原理中的荷姆霍茲共振器研製一套線上蔬果體積分級系統。本系統為乾式、非接觸式的量測系統，除可用於蔬果的體積量測，也適用於其他不可浸水的物體。本系統的量測原理，是利用共振器在置入外物時，腔內氣體的共振頻率會因而改變，並依此特性推算置入物體積。本系統利用機械敲擊產生的衝擊響應訊號來激起共振器共振，並在時域中以奇異值分解法(Singular Value Decomposition, SVD)分析系統的共振頻率。本系統的敲擊機構是以三個固定角度差的凸輪與壓縮彈簧的撞針來製造三個固定時間差的衝擊訊號，並以奇異值分解法分析共振訊號的平均值來預測體積，以增加系統強健性。在輸送蔬果方面，因共振器的量測系統在非開口處必須保持氣密，故本研究以皮帶輪建構輸送系統，讓蔬果在量測過程受到輸送系統的影響降至最低。在分級的部分，本研究利用直線運動的氣壓缸與滑軌、平板結合，利用氣壓缸伸長時將平板推轉進而將移動中的蔬果導入收集槽中。本系統的分級水準，對於規則形蔬果(珍珠芭樂、富士蘋果)以及不規則形蔬果(紫心地瓜、釋迦)分四級的成功率皆可達85%以上，與現有分級機械的90%成功率相近，已接近實用標準。本系統的分級速度，最高為每分鐘35顆，尚有改善空間。 This research is aimed at developing a fruits as well as vegetables volume sorting system, which is based on a Helmholtz resonator. This system is a dry type and non-contact measurement system. In addition, it can be used to measure the volume of water-soaking-forbidden objects. The principle of Helmholtz resonator is based on the fact that its acoustic resonant frequency is a function of cavity volume. When an alien object is inserted into the cavity, the total air volume inside the cavity decreases. As a result, its acoustic resonant frequency increases. An automatic striking mechanism composed of three hammers was designed to induce the acoustic impulse responses, and a Singular Values Decomposition (SVD) algorithm was applied to extract these resonant frequencies consecutively. The averaged resonant frequency was then used as the measured resonant frequency. An air-sealed conveyor system was fabricated to transport each fruit for measuring its resonant frequency. For grading purposes, a mechanical-electrical system with air-cylinder, wood-slider, and a Programmable Logic Controller (PLC) was also integrated. The averaged successful rate of sorting fruits reaches 85% for both regular-shape fruits (guava and apple) and irregular-shape fruits (yam and custard apple), very close to the requirement of 90% successful rate for a commercial fruit grading system. The top speed of the developed system is 35 fruits per minute.